Transcriptional responses to oxidative stress: Pathological and toxicological implications

Pharmacology & Therapeutics 125 (2010) 376–393

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Pharmacology & Therapeutics j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p h a r m t h e r a

Associate editor: L.H. Lash

Transcriptional responses to oxidative stress: Pathological and toxicological implications Qiang Ma ⁎ Receptor Biology Laboratory, Toxicology and Molecular Biology Branch, Health Effects Laboratory Division, National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, United States Department of Biochemistry, West Virginia University School of Medicine, Morgantown, WV 26505, United States

a r t i c l e

i n f o

Keywords: Reactive oxygen species Oxidative stress Transcription factor Gene regulation Disease Chemical toxicity

a b s t r a c t The utilization of molecular oxygen as the terminal electron acceptor for energy production has in many ways shaped the evolution of complex life, physiology, and certain disease processes. The generation of reactive oxygen species (ROS), either as by-products of O2 metabolism or by specialized enzymes, has the potential to damage cellular components and functions. Exposure to a variety of exogenous toxicants also promotes ROS production directly or through indirect means to cause toxicity. Oxidative stress activates the expression of a wide range of genes that mediate the pathogenic effect of ROS or are required for the detection and detoxification of the oxidants. In many cases, these are mediated by specific transcription factors whose expression, structure, stability, nuclear targeting, or DNA-binding affinity is regulated by the level of oxidative stress. This review examines major transcription factors that mediate transcriptional responses to oxidative stress, focusing on recent progress in the signaling pathways and mechanisms of activation of transcription factors by oxidative stress and the implications of this regulation in the development of disease and chemical toxicity. Published by Elsevier Inc.

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . 2. ROS metabolism . . . . . . . . . . . . . . . . . . . 3. Major pathways and transcription factors mediating the 4. Oxidative stress and disease . . . . . . . . . . . . . 5. Chemical toxicity associated with oxidative stress . . . 6. Conclusion and perspective. . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Abbreviations: AP-1, activator protein 1; Akt, homologue of viral oncogene v-Akt, protein kinase B; ARE, antioxidant response element; ASK, apoptosis signal-regulating kinase; Cul, cullin; ERK, extracellular signal-regulated kinase; FOXO, forkhead protein class ‘O’; GSH, glutathione; HIF, hypoxia-inducible factor; HSF, heat shock factor; GP, glutathione peroxidase; JNK, c-Jun N-terminal kinase; MAPK, mitogen-activated protein kinase; MT, metallothionein; MTF-1, metal-activated transcription factor 1; NF-κB, nuclear factor κB; NOX, NADPH oxidase; Nrf2, nuclear factor erythroid 2-related factor 2; p53, protein 53; PD, Parkinson's disease; PI3K, phosphoinositide 3-kinase; Prx, peroxyredoxin; Ref-1, redox factor-1; ROS, reactive oxygen species; RNS, reactive nitrogen species; SIRT1, sirtuin 1; SOD, superoxide dismutase; Trx, thioredoxin; UCP, uncoupling protein. ⁎ Mailstop 3014, 1095 Willowdale Rd., Morgantown, WV 26505, United States. Tel.: 304 285 6241; fax: 304 285 5708. E-mail address: [email protected]. 0163-7258/$ – see front matter. Published by Elsevier Inc. doi:10.1016/j.pharmthera.2009.11.004

Aerobic organisms use molecular oxygen (O2) to generate chemical energy in the form of adenosine 5′-triphosphate (ATP) that transforms structure to function in cells. Due to its rising abundance in the atmosphere and favorable thermodynamic properties, O2 was selected during the course of evolution as the terminal electron acceptor for the reduction of carbon-based fuels to generate ATP by oxidative phosphorylation (Berner et al., 2007). At the same time, the utilization of O2 shaped the evolution of complex life and mammalian physiology with regard to organismal size, multicellularity, placentation, development, aging, and disease processes (Falkowski et al., 2005; Raymond & Segre,

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2006). Additionally, O2 is critical in O2-dependent biosynthesis of many molecules, including tyrosine, nicotinic acid, sterols, polyunsaturated fatty acids, hydoxyproline, hydroxylysine, and retinal, which are essential for many cellular structures and processes (Goldfine, 1965). Although aerobic respiration and O2-dependent biosynthesis have significant advantages for life, the use of O2 in aerobes has a price to pay. The generation of reactive oxygen species (ROS), either as by-products of O2 metabolism or by specialized enzymes, potentially damages cellular components. ROS, such as superoxide anion (O2•−) and hydroxyl • radical ( OH), avidly interact with proteins, lipids, and nucleic acids and, thereby, irreversibly destroy or alter the function of target molecules. In fact, ROS have been increasingly recognized as major contributors to various pathological processes in nearly all biological organisms that use O2. Fifty years ago, Harman proposed the “free radical theory” of aging, a fundamental life process, speculating that endogenous oxygen radicals are generated in cells and result in a pattern of cumulative damage leading to aging (Harman, 1956). Despite substantial gaps and unknowns that persist, aerobic metabolism and the corresponding generation of ROS remain the most widely accepted cause of aging and aging-related chronic diseases in humans, such as neurodegeneration, cardiovascular disease, and cancer (Balaban et al., 2005). Exogenous pathogenic agents, such as microbes, environmental carcinogens, and toxic food ingredients, can also induce ROS production in the body, either by damaging the mitochondria or by promoting endogenous processes, such as inflammation, that produce ROS. In turn, oxidative stress contributes to the pathogenesis of the diseases caused by the agents. In addition to recognized deleterious effects of ROS, cumulative evidence reveals that ROS serve “useful purposes” in the body (Thannickal, 2009). In this more comprehensive view of the

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biology of oxidants (Fig. 1), ROS is generated in an endogenous process or is induced by exogenous agents. ROS generation within certain boundaries is essential for maintaining homeostasis. In this respect, the ROS-generating NADPH-oxidase enzyme in phagocytic cells (NOX2, gp91phox) is a critical host defense against invading microbes (Bedard & Krause, 2007). ROS also function as specific signaling molecules to trigger the activation of certain signaling pathways. Some of the pathways transmit the effect of ROS on cellular functions, such as regulating the proliferative response at low levels of oxidative stress, whereas other pathways represent the cellular strategies for detoxification of ROS and thus, are essential for survival of living organisms exposed to high levels of ROS (Finkel, 1998; Ma, 2008; Nemoto et al., 2000; Nishikawa et al., 2000). Regardless of the origin of ROS, increased ROS production or oxidative stress has two consequences: activation of specific signal transduction pathways and damage to cellular components, both of which significantly impact on physiology and the development of disease. Mechanistically, many of these effects involve activation of specific transcription factors to control the transcription of a range of target genes. These genes encode specific proteins/enzymes to mediate biological responses to oxidative stress. In recent years, significant advances have been made in understanding the interaction between oxidative stress and the transcriptional machinery, in particular, the molecular mechanism of such interaction and the implication of the interaction in human disease. The purpose of this review is to analyze major transcription factors that mediate gene regulation in response to oxidative stress. Molecular recognition of oxidants/antioxidants and the signaling pathways of activation of transcription factors will be examined. Finally, how

Fig. 1. Metabolism and biological effects of ROS. ROS production derives from normal metabolism and pathological processes endogenously or is stimulated by exogenous toxic agents. A complex enzymatic and non-enzymatic antioxidant defense system combats against ROS and thus maintains the oxidant/antioxidant homeostasis in the cell. ROS within a physiological range is required for normal function, whereas over-production of ROS leads to oxidative stress that contributes to aging and development of diseases.

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the transcriptional responses to oxidative stress impact on the pathogenesis and therapy of human disease and chemical toxicity are discussed. 2. ROS metabolism 2.1. ROS production ROS consist of a variety of oxygen-derived small molecules with diverse structures, including oxygen radicals, such as superoxide • anion (O2•−), hydroxyl radical ( OH), peroxyl radical (RO2• ), and alkoxyl radical (RO•), and certain nonradicals that are either oxidizing agents and/or are easily converted into radicals, such as hypochlorous acid (HOCl), ozone (O3), singlet oxygen (1O2), and hydrogen peroxide (H2O2). Some of these species, such as superoxide and hydroxyl radicals, are extremely unstable, whereas others, like hydrogen peroxide, are relatively long-lived and freely diffusible. Nitrogencontaining oxidants, such as nitric oxide (NO•) and the strong oxidant peroxynitrite anion (ONOO−), are called reactive nitrogen species (RNS). RNS also play important roles in oxidative physiology and pathology in the body (Droge, 2002). ROS can be generated in multiple compartments and by multiple enzymes in cells. In fact, most, if not all, enzymes that are capable of metabolizing oxygen are also capable of generating ROS either intentionally or accidentally. Mitochondria consume about 90% of the body's oxygen to generate ATP by oxidative phosphorylation. In vitro evidence indicates that 1–2% of the oxygen molecules consumed are converted to superoxide anions in mitochondria (Boveris & Chance, 1973). Even though the in vivo rate of mitochondrial superoxide production is likely to be much less than this number (St-Pierre et al., 2002; Staniek & Nohl, 2000), the majority of intracellular ROS can be traced back to mitochondria. Oxidative phosphorylation in mitochondria uses controlled oxidation of NADH or FADH2 to generate a potential energy for protons across the mitochondrial inner membrane. This potential energy is then used to phosphorylate ADP by the F1–F0 ATPase. Along the respiratory chain, electrons derived from NADH or FADH2 can directly react with oxygen and generate free radicals. Production of superoxide radicals in mitochondria occurs primarily in complex I (NADH dehydrogenase) and complex III (ubiquinonecytochrome c reductase), with the latter being the major site of ROS production under normal metabolic conditions (Fig. 2A) (Turrens, 1997). Cytoplasmic enzymes contribute to oxidative stress, including the expanding family of ROS-generating NADPH oxidases (NOX), such as NOX1, NOX2, NOX3, NOX4, NOX5, DUOX1, and DUOX2 (Bedard & Krause, 2007). The NOX enzymes share the capacity to transport electrons across the plasma membrane and to generate superoxide and other downstream ROS. NOX2 (gp91phox) is the prototype of NOX enzymes and is found mainly in neutrophils and other phagocytic cells. Superoxide radicals generated by NOX2 are critical in defense against microbes. Loss of function of the NOX2 system is responsible for chronic granulomatous disease, a human genetic disorder characterized by diminished bactericidal capacity of phagocytes (Baehner & Nathan, 1967). Non-phagocytic NOXs generate superoxide and other radicals that may trigger cellular transformation or replicative senescence (Bedard & Krause, 2007). These findings support the notion that, in addition to stochastically damaging macromolecules, ROS are purposely used in normal cellular signaling and homeostasis. Additional sources of cytoplasmic ROS production include cytochrome P450s, lipoxygenases, and one-electron reduction of quinones by NADPH:cytochrome P450 reductase. In this latter case, semiquinone radicals (Q•−) formed by enzymatic one-electron reduction cycle back to quinones and, at the same time, pass electrons to O2 resulting in the formation of superoxide anion radical. This futile cycling between quinone and

semiquinone radical with concomitant ROS production contributes to the toxicities of many chemicals that have quinone moieties, such as doxorubicin and menadione (Fig. 2B) (Enster, 1986; O'Brien, 1991). Toxic metals can induce the production of ROS by directly acting as catalytic centers for redox reactions with molecular oxygen or other endogenous oxidants, or by promoting the irondependent Fenton reaction (Fig. 2C) (Kasprzak, 2002). 2.2. Antioxidant defense Given the potentially deleterious effects of ROS, it is likely that protective mechanisms have evolved to limit the production and release of ROS. In mitochondria, metabolic uncoupling is one mechanism regulating ROS production (Skulachev, 1996). In metabolic uncoupling, oxygen consumption is uncoupled from ATP generation but results in the production of heat. This thermogenesis is mediated through a family of uncoupling proteins (UCP-1, UCP-2, and UCP-3). An increase in uncoupling reduces mitochondrial ROS production, whereas deletion of UCP-3 in mice increases mitochondrial ROS levels (Vidal-Puig et al., 2000). The intricate antioxidant defense system is perhaps the major mechanism by which cells counteract ROS production. The system includes enzymatic scavengers, such as superoxide dismutase (SOD), catalase, and glutathione peroxidase (Fig. 3). Two SOD enzymes exist in the cell: SOD1 (Cu/ZnSOD) is a copper- and zinc-containing enzyme primarily localized in the cytoplasm and SOD2 (MnSOD) is a manganese-dependent enzyme in the mitochondrial matrix. SOD catalyzes the conversion of superoxide anions to hydrogen peroxide, whereas catalase and glutathione peroxidase convert hydrogen peroxide to water. Recently, a new family of peroxide scavengers termed peroxiredoxins was identified (Chae et al., 1999). Peroxiredoxins reduce peroxides in the presence of thioredoxins. Myeloperoxidase is found in the granules of neutrophils and catalyzes the conversion of H2O2 and Cl− to more reactive hypochlorous acid (OCl−), which is important for the bactericidal activity of neutrophils. In addition to these enzymes and proteins, a number of non-enzymatic and small molecules are important in scavenging ROS; these include glutathione, vitamins (C and E), pyruvate, flavonoids, carotinoids, urate, and many plant-derived antioxidants. Glutathione is likely the most important antioxidant of low molecular mass, because it is present in millimolar concentrations in cells. Metallothioneins (MT) I and II are small proteins rich in cysteine thiols. MTs are highly inducible by metals and oxidants and thus, are critical in protection against exogenous oxidants, such as the carcinogenic metals cadmium and arsenic (Palmiter, 1998). 2.3. Oxidative stress An imbalance between oxidants and antioxidants resulting from increased production of oxidants and/or reduction in the amounts of antioxidants generates a state of stress in the cell termed oxidative stress. Clearly, oxidative stress encompasses a wide variety of physiological and pathophysiological, endogenous and exogenous processes that directly or indirectly affect the cellular oxidant/ antioxidant balance. This is well illustrated in the case of metalinduced oxidative damage. Toxic metals, such as cadmium and chromium, induce oxidative stress in a variety of target cells via multiple mechanisms that include directly damaging mitochondrial respiration, increased ROS production via the Fenton reaction, lipid peroxidation, and depletion of intracellular antioxidants, such as GSH (He et al., 2007, 2008; Kasprzak, 2002; Valko et al., 2005). Oxidative stress is a double-edged sword: within a physiological range, it is necessary for proliferative stimulation and perhaps the removal of aged cellular components, whereas extensive oxidative stress damages the structure and function of tissues. Consequences of oxidative stress include modifications of cellular proteins, lipids, and

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Fig. 2. ROS production. (A) ROS production during mitochondrial respiratory electron transfer. Complextwo-electron reduction of quinones and quinone-semiquinone radical cycling. (C) The Fenton reaction (upper reaction) and Fenton-like reaction (lower reaction) where M is a transition metal ion, such as copper and cobalt.

DNA. Modification of proteins leads to the formation of carbonyl derivatives by direct oxidation of certain amino acid side chains and oxidation-induced peptide cleavage (Stadtman, 1992). Modification of lipids by ROS and other radicals results in lipid peroxidation. The hydroxyl radical is the principal player in producing oxidative DNA

damage, altering purine and pyrimidine bases and deoxyribose sugar as well as cleaving the phosphodiester DNA backbone to create DNA strand breaks. Due to its proximity to the main source of ROS generated and a limited DNA repair capacity, mitochondrial DNA is more sensitive to oxidative stress than nuclear DNA. Damaged mitochondria release more ROS and set in motion a vicious cycle in which increasing DNA damage leads to increased ROS production that in turn leads to more DNA damage. This vicious nature of oxidative damage may explain in part why oxidative stress is commonly associated with chronic diseases, such as neurodegeneration, chronic inflammatory disorders, and various cancers. 3. Major pathways and transcription factors mediating the transcriptional response to oxidative stress

Fig. 3. Enzymatic elimination of ROS. SOD, superoxide dismutase; Prx, peroxyredoxin; GP, glutathione peroxidase.

Oxidative stress triggers a range of physiological, pathological, and adaptive responses in cells either as a result of cellular damage or through specific signaling molecules. These responses ultimately modulate

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transcriptional outputs to influence cell fate and disease processes (Fig. 1). In the past two decades, a number of transcription factors and signaling pathways have been identified and delineated to mediate critical transcriptional responses to oxidative stress. These examples demonstrate the importance as well as the complexity of how alterations in intracellular ROS are converted into discrete and reproducible alterations in gene expression, and ultimately disease outcomes. 3.1. MAPKs, PI3K, and PKC 3.1.1. MAPKs It is well known that many oxidative stress signals activate protein kinase pathways that modulate gene transcription. Mitogen-activated protein kinases (MAPKs) encompass a large number of serine/threonine kinases critically involved in the regulation of proliferation, differentiation, stress adaptation, and apoptosis. MAPKs include three structurally divergent subfamilies: the extracellular signal-regulated kinases (ERK), the c-Jun N-terminal kinases (JNK), and the p38 kinases. ERK, JNK, and p38 are activated via independent, but sometimes overlapping, pathways to transduce signals to effect proteins, most notably transcription factors that regulate gene transcription. Activation of MAP kinases may involve a cascade of kinase reactions from MAPKKK to MAPKK to MAPK (Chang & Karin, 2001). The ERK pathway lies at the heart of many signal transduction processes and transduces proliferative signals from growth factor receptors to the nucleus. Oxidative stress leads to substantial activation of ERK. Growth factor receptors, such as EGFR, PDGFR, and T-cell receptor complex, play important roles in mediating this effect. Oxidative insults, such as hydrogen peroxide, asbestos, UVC, and arsenite, induce phosphorylation and activation of the receptors, whereas interference of phosphorylation attenuates ERK activation in response to oxidative stress (Chen et al., 1998; Sachsenmaier et al., 1994). In this scenario, oxidants may mimic ligand–receptor interaction to activate the receptors perhaps by modifying critical cysteine residues in the receptors (Chen et al., 1998). Activation of growth factor signaling pathways by oxidants is consistent with the fact that low concentrations of hydrogen peroxide are mitogenic (Burdon, 1995). JNK and p38 are more commonly related to stress response signals, such as cytokines, radiation, osmotic shock, mechanical injury, heat stress, and oxidative damage, hence, the name stress-activated protein kinases (SAPK) (Kyriakis & Avruch, 2001). Cellular redox may be key to oxidant-induced activation of the SAPK pathways (Adler et al., 1999). For instance, the redox regulatory protein thioredoxin (Trx) binds and inhibits the apoptosis signal-regulating kinase 1 (ASK1), a MAPKKK involved in both JNK and p38 activation. Oxidative stress dissociates the Trx-ASK1 complex leading to activation of JNK and p38 to regulate apoptosis (Saitoh et al., 1998). In mouse embryonic fibroblasts with JNK1 and JNK2 double knockout, UVC-induced apoptosis is completely ablated (Tournier et al., 2000), whereas ASK1 deletion eliminates JNK activation in response to hydrogen peroxide and renders the cell resistant to apoptosis by the oxidants (Tobiume et al., 2001). The p38 kinase may be required for apoptosis induced by singlet oxygen, but not that by hydrogen peroxide (Zhuang et al., 2000). p38 also participates in mitotic arrest under conditions of low level oxidative stress (Kurata, 2000). Despite identification of numerous targets of MAPK phosphorylation, determination of those that mediate the biological effects remains a challenging question. For cell survival, c-Jun and p53 are important targets of MAPKs. c-Jun is phosphorylated via the JNK pathway, and like JNK, c-Jun functions in a manner that is cell-type and agent specific for both pro- and anti-apoptotic functions (Bossy-Wetzel et al., 1997). The tumor suppressor p53 constitutes a potential target of pro-apoptotic signaling by JNK and p38. Both JNK and p38 are capable of phosphorylating p53 and both have been implicated in regulating p53 expression by stabilizing the p53 protein under conditions of oxidative stress (Bulavin et al., 1999; Fuchs et al., 1998). In the case of p38,

inhibition of its activity markedly reduces UVC-induced apoptosis in a p53-dependent manner (Bulavin et al., 1999).

3.1.2. PI3K/Akt Akt is a serine/threonine kinase and, like ERK, plays an important role in integrating cellular responses to growth factors and other extracellular signals (Kandel & Hay, 1999). Akt is activated via a phosphoinositide 3-kinase (PI3K) pathway in which PI3K-mediated generation of 3′-phosphorylated phosphoinositide leads to the recruitment of Akt to the cell membrane for phosphorylation by kinases such as the phosphoinositide-dependent kinase-1 (PDK-1) (Toker & Cantley, 1997). Akt is an important anti-apoptotic protein and is activated in response to oxidant injury as well as stresses known to induce oxidative stress and toxicity. Activation of Akt by hydrogen peroxide in HeLa cells requires EGFR, whereas activation of the kinase by peroxynitrite relies on PDGFR (Klotz et al., 2000; Wang et al., 2000). The PI3K/Akt pathway transduces survival signals through phosphorylation dependent-suppression of apoptotic factors including forkhead transcription factors and NF-κB suppressor IKKα (Kandel & Hay, 1999). In another example, Akt-mediated phosphorylation of ASK1 prevents activation of JNK by ASK1 and its downstream target transcription factor ATF2, thereby protecting cells against hydrogen peroxide-induced apoptosis (Kim et al., 2001). In this scenario, the PI3K/Akt and JNK pathways interact to regulate transcription and apoptosis.

3.1.3. PKC Protein kinase C (PKC) is a family of phospholipid-dependent serine/ threonine kinases that are involved in a variety of pathways regulating cell growth, death, and stress responses. PKCs are divided into three categories depending on their cofactors: (1) conventional PKCs that are calcium-dependent and stimulated by diacylglycerol; (2) novel PKCs that are calcium-independent but are stimulated by diacylglycerol; and (3) atypical PKCs that do not require either calcium or diacylglycerol for optimal activity. PKCs are the major cellular target for activation by tumor-promoter phorbol esters. Phorbol esters mimic the action of the second messenger diacylglycerol for PKC activation; however, activation by diacylglycerol is transient, whereas that by phorbol esters is persistent. PKCs regulate various cellular processes including mitogenesis, cell adhesion, apoptosis, angiogenesis, invasion, and metastasis (Nishizuka, 1992). PKCs contain unique structural features that are susceptible to oxidative modification (Gopalakrishna & Jaken, 2000). The N-terminal regulatory region contains zinc-binding, cysteine-rich motifs readily oxidizable by peroxides. Oxidation of the region inhibits the autoinhibitory function of the regulatory domain, resulting in elevated PKC activity. The C-terminal catalytic domain contains several reactive cysteines that are targets for chemopreventive antioxidants, such as seleno-compounds, the polyphenolic agent curcumin, and vitamin E analogues. Modification of the cysteines decreases PKC activity. Therefore, the two domains of PKC respond differently to oxidants and antioxidants, which may account for the fact that PKCs are important targets for both tumor promotion by oxidants and for tumor prevention by antioxidants. Individual PKC isoforms mediate distinct cellular responses, further complicating the role of PKCs in oxidative stress and related diseases. Like other protein kinases, such as p38, JNK, ERK, and PI3K, activated PKCs phosphorylate transcription factors, such as NF-κB, AP-1, and STAT3, to modulate gene transcription in oxidative response. For instance, selenite and other redox-active seleno-compounds inhibit phorbol ester-induced transformation of JB6 epidermal cells and induction of ornithine decarboxylase (Gopalakrishna et al., 1997). In this process, selenium-induced inhibition of PKC contributes to selenium-mediated inhibition of AP-1 and NF-κB transactivation.

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3.2. p53 p53 (protein 53) is critical in the regulation of expression of genes involved in growth arrest and apoptosis (Burns & El-Deiry, 1999). As a tumor suppressor, p53 functions as a universal sensor of genotoxic stress in multicellular organisms, conserving genomic stability by preventing genome mutations (Sionov & Haupt, 1999). The p53 protein possesses seven domains critical for its function and regulation. Activation domain 1 (AD1) at the N-terminus mediates transcription activation. AD1 is followed by AD2 and a proline-rich domain, both of which are important for apoptotic activity. The central DNA-binding domain (DBD) with one zinc atom mediates DNA-binding and is frequently mutated in tumor cells. Juxtaposed and C-terminal to DBD are the nuclear localization signal and the homooligomerization domain (OD). Tetramerization is essential for p53 activity in vivo. The C-terminal end is involved in down-regulation of DBD function. In addition to DNA damage, p53 is activated by a variety of stresses including oxidative stress, osmotic shock, ribonucleotide depletion, and oncogene expression. Activation of p53 occurs largely through posttranslational mechanisms that enhance its protein stability and increase its DNA-binding activity. This complex process involves multiple phosphorylation, acetylation, and ubiquitination events (Sionov & Haupt, 1999). In normal cells, the p53 protein is kept at a low level through Mdm2-dependent ubiquitination and proteasomal degradation. Many stress signals activate p53 by inhibiting the Mdm2 pathway to stabilize p53 (Colman et al., 2000) (Table 1). The N-terminal AD1 contains a large number of phosphorylation sites representing the primary target for protein kinases that transduce stress signals. Another prominent feature of p53 regulation derives from the fact that major p53 regulators and target gene products often form delicate positive and negative feedback loops that fine-tune the p53 protein level and function. ROS are involved in p53 signaling at multiple levels. Damaging DNA by ROS is a direct signal for p53 activation. ROS modulate the redox status of a critical cysteine residue in the DNA-binding domain of p53 to affect its DNA-binding capacity (Meplan et al., 2000). p53 can also be activated by ROS through crosstalk with other signaling pathways. For instance, both p38 and JNK can

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be activated by ROS and activated p38 and JNK phosphorylate the AD1 of p53 to activate the protein (Bulavin et al., 1999; Buschmann et al., 2001). In another example, p53 is up-regulated in response to hydrogen peroxide in T cells in an NF-κB-dependent manner (Dumont et al., 1999). NQO1, which is transcriptionally regulated by oxidants/ antioxidants through Nrf2, regulates p53 by inhibiting its degradation (Asher et al., 2001). Activation of p53 induces growth arrest, activation of DNA repair proteins, or initiation of apoptosis (Table 1). Activated p53 binds DNA and activates expression of p21 (important in G1/S arrest by binding and inhibiting G1-S/CDK and S/CDK complexes) and GADD45 and 14-3-3σ (important in G2/M arrest) (Chen et al., 2000; Taylor & Stark, 2001). p53 and p21 are believed to be important in mediating hydrogen peroxide-induced growth arrest and replicative senescence (Chen et al., 2000). Growth arrest will allow DNA repair proteins to be activated and to fix the DNA damage before continuing the cell cycle. In the case that DNA damage proves irreparable, activated p53 initiates programmed cell death of damaged cells to avoid cancerous mutations. A large number of p53 target genes have been implicated in p53-dependent apoptotic effects (Martindale & Holbrook, 2002). These include Bax (a pro-apoptotic Bcl-2 family member), several mitochondrial proteins (Noxa, p53AIP1, and PUMA), and several genes associated with death receptor-mediated apoptosis (Fas, Killer/ DR5, and PIDD). It is likely that not a single, but several, p53 target genes are responsible for either growth arrest or apoptosis functions of p53. p53 directly modulates ROS homeostasis. Activation of p53 results in the generation of ROS, suggesting an important consequence of oxidant-induced activation of p53 is a further increase in the level of oxidative stress. This positive feedback loop may help to achieve a critical threshold of ROS for inducing apoptosis of the cell (Johnson et al., 1996). As expected, p53 regulates the expression of a large number of genes that are related to oxidative stress. PIG3 is upregulated by p53 and is involved in the perpetuation of ROS associated with cell death (Flatt et al., 2000). In addition, p53 suppresses the expression of SOD2, and SOD2 reciprocally down-regulates p53 transcription, forming a negative regulatory loop between p53 and SOD2 (Drane et al., 2001). On the other hand, p53 activates the

Table 1 Comparison of transcriptional response to oxidants/antioxidants by p53, NF-κB, AP-1, and Nrf2.

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transcription of glutathione peroxidase, which is an early event following p53 activation (Tan et al., 1999). Thus, p53 plays a role in regulating cellular redox state, and small shifts in the redox state may influence the biological responses of p53 depending on the cell type and the extent and duration of the stress and damage in the cell.

3.3. NF-κB Nuclear factor κB (NF-κB) consists of homodimers or heterodimers of Rel proteins that share a Rel homology domain in their N-termini. As a transcription factor, NF-κB regulates a large number of genes related to immune function, inflammation, apoptosis, cell proliferation, and synaptic plasticity (Pahl, 1999; Perkins, 2007). NF-κB is activated in response to a variety of stimuli including oxidative stress, cytokines, free radicals, UV irradiation, and infection. ROS and

oxidative stress activate NF-κB, whereas many antioxidants effectively block NF-κB activation (Li & Karin, 1999) (Table 1). In unstimulated cells, NF-κB dimers are sequestered in the cytoplasm by a family of inhibitors, IκBs, among which IκBα is the best studied. Various stimuli lead to phosphorylation of IκBα at serine residues 32 and 36 (human IκBα). Phosphorylation initiates the ubiquitination and proteasomal degradation of IκBα through the Cul1-dependent SCF E3 complex (Fig. 4A). NF-κB dimers are then freed from inhibition by IκBs and enter the nucleus to activate gene transcription (Perkins, 2007). A number of kinases have been reported to phosphorylate IκBs at the serine residues, including IκB kinase (IKK, a complex of IKKα, IKKβ, and Nemo), NF-κB-inducing kinase (NIK), double-stranded RNA-activated serine–threonine protein kinase (PKR), p90RSK, MEKK1, and Akt. These kinases likely serve as targets of oxidative signals for activation of NF-κB and as points of cross-talk between NF-κB and the signaling pathways of the kinases in

Fig. 4. Regulation of NF-κB, Nrf2, and HIF1α by oxidants/antioxidants via cullin-dependent ubiquitination/26S proteasomal pathways. (A) Oxidants induce phosphorylation of IκBα leading to the ubiquitination and proteasomal degradation of IκBα through the Cul1-dependent SCF E3 complex and thereby, free NF-κB from inhibition by IκB. (B) Antioxidants, ROS, and electrophiles bind to critical cysteine thiol groups of Keap1 and Nrf2 resulting in the inhibition of ubiquitination–proteasomal degradation of Nrf2 through the Keap1/Cul3 E3 complex. Stabilized Nrf2 enters the nucleus and mediates gene transcription. (C) Under normaxia, two proline residues of HIF1α are converted to hydroxyproline that is recognized by VHL. VHL targets HIF1α for ubiquitination–proteasomal degradation via the Cul2-dependent ECS E3 complex. Hypoxia reduces HIF1α proline hydroxylation and inhibits the ubiquitination–proteasomal degradation of HIF1α. This process may involve production of ROS.

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response to oxidative and other stress (Gilmore, 2006). Hydrogen peroxide, UVC, and ionizing irradiation have all been observed to stimulate degradation of IκB (Zhang et al., 2001). Hydrogen peroxide and hypoxia/reoxygenation can also lead to phosphorylation of IκBα at tyrosine 42, which displaces IκBα from NF-κB and thereby subjects IκBα to digestion by calpain proteases (Schoonbroodt et al., 2000). Nearly all steps in the NF-κB cascade contain redox-sensitive proteins whose activities are modulated by oxidative stress (Janssen-Heininger et al., 2000). NF-κB itself must be in a reduced form to exhibit DNAbinding activity. Reducing agents enhance NF-κB DNA-binding, whereas oxidizing agents inhibit DNA-binding. Thioredoxin may regulate NF-κB at two steps but in an opposite fashion: it blocks degradation of IκB to inhibit NF-κB activation in the cytoplasm, but enhances NF-κB DNAbinding in the nucleus (Hirota et al., 1999). Many natural or synthetic antioxidants exhibit potent anti-inflammatory activities that correlate with their abilities to block NF-κB activation and expression of inflammatory cytokines, such as TNFα, IL1, and IL6. The phenolic antioxidant tert-butylhydroquinone (tBHQ) was observed to inhibit LPS-induced expression of TNFα in macrophages by inhibiting the DNA-binding activity of NF-κB (Ma & Kinneer, 2002). Inhibition of NF-κB involves redox cycling of the antioxidant, suggesting a redox-sensitive factor important for the binding of NF-κB to DNA as a target of the antioxidant (Ma et al., 2003). Antioxidant enzymes and mimics also block NF-κB activation by various stimuli. Overexpression of peroxiredoxin or thioredoxin blocks NF-κB activation by H2O2, whereas overexpression of SOD or GSH peroxidase abolishes NF-κB activation by preventing the degradation of IκB after stimulation with TNFα (Kang et al., 1998; KretzRemy et al., 1996; Manna et al., 1998; Schenk et al., 1994). NF-κB is involved in regulating many aspects of cellular functions, in particular, immune function, inflammation, apoptosis, and cell proliferation (Gilmore, 2006; Perkins, 2007). Examples of NF-κB regulated genes include: IL-2, TAP1, and MHC molecules in immune response; IL-1, IL-6, TNFα, and leukocyte adhesion molecules (E-selectin, VCAM-1, and ICAM-1) in inflammatory response; c-myc, ras, and p53 for cell growth and differentiation; and TRAF1, TRAF2, CIAPs, SOD2, the A20 zinc finger protein, and the Bcl-2 family proteins Bf1-1/A1 and Bcl-XL for anti-apoptotic effects. Thus, both activation of NF-κB by oxidative stress and inhibition of NF-κB by antioxidants have important implications in the pathogenesis, therapy, and prevention of many diseases, including cancer, chronic inflammatory disorders, and microbe infection, as discussed in more detail later in the review. 3.4. AP-1 and redox regulation by Ref-1 Activator protein 1 (AP-1) is a transcription factor of the bZip family. AP-1 is composed of heterodimeric proteins that belong to the c-Fos, c-Jun, ATF, and JDP families (Table 1). AP-1 regulates the induction of a variety of genes in response to a host of stimuli, such as oxidative and other stresses, cytokines, growth factors, and infections, and thereby controls a number of cellular processes including differentiation, proliferation, and apoptosis (Hess et al., 2004). As discussed earlier, JNK and other MAPKs phosphorylate c-Jun and thereby transmit oxidative signals to AP-1. AP-1 is also regulated by redox mechanisms. Oxidation of conserved cysteine residues at the DNA-binding domains of AP-1 proteins reduces DNA-binding and, hence, the overall biological activity of AP-1. Mutation of a conserved cysteine residue in c-Fos increased DNA-binding, but redox-dependent binding of c-Fos was abrogated, whereas the ability of the protein to induce transformation of cells was enhanced (Okuno et al., 1993). Redox regulation of DNA-binding of AP-1 is mediated through redox factor-1 (Ref-1, Ape1, or APEX1) (Xanthoudakis et al., 1992). Ref-1 is a bifunctional enzyme: the highly conserved C-terminal region functions as a DNA repair enzyme in the repair of apyrimidinic/ apurinic nucleotides, whereas the N-terminal region contains the redox regulatory domain characterized by 2 critical cysteines, C65 and C93 (Xanthoudakis et al., 1992, 1994). In addition to c-Fos and c-Jun,

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Ref-1 is believed to be important in the redox-dependent regulation of NF-κB, p53, ATF/CREB, HIF1α, and HIF-like factors (Fritz et al., 2003; Tell et al., 2005) (Table 1). A general theme is observed in the action of Ref-1: oxidized forms of the transcription factors have reduced or absent DNA-binding activity and Ref-1 appears to mediate specific reduction of these transcription factors. Furthermore, critical cysteine residues in c-Fos, c-Jun, and other Ref-1-regulated transcription factors appear to be surrounded by a stretch of basic amino acid residues that may facilitate the reactivity of the cysteines, thereby distinguishing “redox-sensitive” and “reactive” cysteine moieties from non-reactive cysteines found in any given protein structures. Whether Ref-1 acts as a direct redox sensor remains elusive. Ref-1 was found to bind the more ubiquitous thiol-containing redox protein thioredoxin (Hirota et al., 1997). Moreover, stimuli that activate AP-1 promote nuclear accumulation of thioredoxin where it binds to Ref-1 and augments AP-1 activity. Thus, redox regulation of transcription factors by Ref-1 may require thioredoxin and other unidentified factors. It is now clear that many transcription factors contain one or more critical cysteine residues within their DNA-binding domains and many appear to require Ref-1 for optimal activity. In this manner, redox regulation via Ref-1 intersects multiple pathways in oxidative transcriptional responses (Fritz et al., 2003; Tell et al., 2005). 3.5. Nrf2 and antioxidant response Nrf2 (nuclear factor erythroid 2-related factor 2) belongs to a group of specialized transcription factors termed xenobiotic-activated receptors (XARs) (Ma, 2008). XARs recognize specific xenobiotics—small chemicals derived from environmental, occupational, therapeutic, and dietary sources—and coordinate the transcription of batteries of genes. Enzymes/proteins encoded by the genes combat against the chemical insults by way of metabolism and disposition as well as antagonistic actions to maintain the chemical homeostasis in the cell. As a XAR, Nrf2 plays critical roles in the defense against oxidant and electrophilic chemical insults (Kensler et al., 2007; Kobayashi et al., 2004b; Ma, 2008; Talalay, 2005). Loss of Nrf2 function in mice is associated with increased susceptibility to a range of diseases and chemical toxicity in which oxidative stress is an important component of pathogenesis (He et al., 2007, 2008, 2009; Hu et al., 2006; Hubbs et al., 2007; Ma et al., 2006; Ramos-Gomez et al., 2001). On the other hand, boosting Nrf2 function by chemoprotective agents protects the body from cancer and a range of toxicities (Dinkova-Kostova et al., 2005, 2006; Fahey et al., 2002) (Table 1). Nrf2 was cloned as a result of its similarity in sequence and DNAbinding activity to NF-E2, a transcription factor critical for the regulation of developmental expression of the beta globin protein in hematopoietic cells (Moi et al., 1994). Structurally, this group of transcription factors (NF-E2, Nrf1, Nrf2, Nrf3, Bach1, and Bach2) are characterized by a shared CNC motif (named for sequence homology to the Drosophila protein Cap ‘N’ Collar) preceding the bZip DBD domain at the C-terminal region of the protein. Another common feature among this group of proteins stems from the fact that they all heterodimerize with a small Maf protein (Maf G or K) for specific DNA-binding (Kensler et al., 2007; Ma, 2008). The Nrf2-Maf dimer binds to a specific DNA-recognition sequence called the “antioxidant response element” (ARE)—so named because it mediates the induction of phase II detoxification enzymes by antioxidants, such as the phenolic antioxidant tBHQ (Nguyen et al., 2003). Typical target genes of Nrf2 include two sets of enzymes/proteins: (1) drug-metabolizing enzymes and transporters, such as NAD(P)H: quinone oxidoreductase 1 (NQO1), glutathione S-transferases (GST), UDP-glucuronyltransferases (UGT), microsomal epoxide hydrolase (mEH), CYP2A5, P-glycoprotein (P-gp, MDR1), and multi-drug resistanceassociated proteins MRP2, MRP3, and MRP4; and (2) antioxidant proteins/enzymes, such as heme oxygenase 1 (HO1), γ-glutamylcysteine synthetase (γ-GCS), thioredoxin, and thioredoxin reductase 1 (TXNRD1) (Ma, 2008). Additionally, Nrf2 regulates an increasing number of genes in

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a cell-type and inducer-dependent manner. One example is the expression and induction of FOXO3 mRNA in the ovary by the ovotoxicant 4-vinylcyclohexine diepoxide (VCD); FOXO3 mRNA expression and induction are lost in Nrf2 KO ovaries (Hu et al., 2006), revealing Nrf2-dependence of the induction. Increasing evidence reveals that Nrf2dependent protection against oxidants and electrophiles goes beyond xenobiotic defense, but also includes safeguarding the body from numerous endogenous oxidants and toxicants that are implicated in the development of many diseases and cancers. Nrf2 mRNA is widely expressed in animal tissues. However, under basal conditions, the Nrf2 protein is rapidly turned over through a specific ubiquitin–26S proteasome pathway with a t1/2 of ∼20 min (He et al., 2006). Only barely detectable activities of Nrf2 are expressed to maintain basal expression of ARE-dependent genes in many cell types. Ubiquitination of Nrf2 is controlled by the Keap1/Cul3-dependent ubiquitin ligase (E3) in which Keap1 binds and brings Nrf2 into the E3 complex for ubiquitination of Nrf2 (Cullinan et al., 2004; He et al., 2006; Hong et al., 2005; Kobayashi et al., 2004a; Ma et al., 2004; Zhang et al., 2004) (Fig. 4B). Silencing of Nrf2 is further achieved by anchoring the protein in the cytoplasm through a Keap1-cytoskeleton interaction (Kang et al., 2004). In the presence of an inducer, Nrf2 protein is stabilized and t1/2 extended to a magnitude longer (He et al., 2006). Activated Nrf2 translocates into the nucleus with Keap1 and is deubiquitinated; Keap1 may assist Nrf2 for nuclear translocation and subsequent signaling events through a currently unclear mechanism (He et al., 2006). In human cells, activation of Nrf2 by antioxidants and toxic metals is accompanied by phosphorylation of the transcription activation domain by Casein kinase II (CK2); phosphorylated Nrf2 is preferentially localized in the nucleus and phosphorylation is required for its transcription activation activity (Apopa et al., 2008). An essential question that remains elusive is how Nrf2 senses and is activated by oxidant and electrophilic inducers.Keap1 contains ∼25 cysteine residues and binds inducers avidly. Extensive analyses of inducer-Keap1 cysteine thiol interaction in several laboratories using mass spectrometry and mutagenesis have identified a number of cysteine residues that are highly reactive to inducers and are important for repression of Nrf2 activity under basal conditions or activation of Nrf2 in the presence of an inducer. Notably, highly reactive cysteines were consistently found in the linker region of Keap1 (mouse Keap1 amino acid residues 178 to 321) including C273, C288, and C297 (Dinkova-Kostova et al., 2002; Eggler et al., 2005; Hong et al., 2005; He and Ma, 2009a). Mutation of Keap1 cysteines revealed that C151, C273, and C288 are critical residues for Nrf2 regulation in cell-based functional studies. Transgenic expression of mutants C151S, C273A, or C288A in Keap1 null mice further identified C273 and C288 as required for suppression of Nrf2, and C151 important in facilitating Nrf2 activation (Yamamoto et al., 2008). Recently, eleven Nrf2-activating compounds were evaluated for interaction with Keap1 cysteine thiols in a zebrafish embryo model in which fish Nrf2 and Keap1 (a and b forms) were expressed by injecting mRNAs into the embryo to reconstitute Nrf2 regulation. The study confirmed that different inducers modify different sets of cysteine residues in Keap1, leading to a proposal that distinct sets of Keap1 cysteines function as “cysteine codes” for sensing and transducing disparate inducing signals (Kobayashi et al., 2009). Nrf2 itself contains 7 cysteines that are conserved across species from chicken to mouse, rat, and human (He & Ma, 2009c). FlAsH and PAO, two potent Nrf2 activators, bind to Nrf2 and binding is reversed by tBHQ and arsenic. Mutation of Nrf2 cysteines enhanced Nrf2-Keap1 association, ubiquitination of Nrf2, and proteasomal degradation of Nrf2, resulting in markedly shortened t1/2. Arsenic failed to activate Nrf2 mutants or induce Nrf2 target gene Nqo1 in Nrf2 knockout (KO) cells that express the cysteine mutants. The study demonstrated multiple and critical roles of Nrf2 cysteine residues that include inducer-sensing, Keap1-dependent ubiquitination and degradation of Nrf2, and transactivation by Nrf2. Therefore, both Nrf2 and Keap1

contribute to oxidant/electrophile sensing through their corresponding cysteine residues (He & Ma, 2009a, 2009c). 3.6. SIRT1 and NAD+/NADH-binding transcription factors Increasing amounts of evidence reveal that redox regulation of gene transcription extends beyond modulation of cysteine residues in oxidative stress. A number of transcription regulating proteins are sensitive to redox pairs, such as NAD+/NADH, NADP+/NADPH, and GSSG/GSH, major redox buffers that determine the redox status in the cell. Sirtuin 1 (SIRT1, or silent mating type information regulation 2 homolog S. cerevasiae) is a NAD-dependent deacetylase. The yeast enzyme Sir2 and the mammalian enzyme SIRT1 contribute to cellular regulation in longevity and response to stresses including oxidative stress (Blander & Guarente, 2004). The ratio of oxidized-to-reduced nitotinamide adenine dinucleotides can regulate the activity of SIRT1 (Fulco et al., 2003). In addition to modulating chromatin dynamics, SIRT1 regulates a number of transcription factors under oxidative stress. For instance, SIRT1 and FOXO transcription factor FOXO3 forms a complex in cells in response to oxidative stress (H2O2, menadione, and heat shock) (Brunet et al., 2004). SIRT1 deacetylates FOXO3 both in vitro and in vivo. A dual effect on FOXO3 by SIRT1 is observed: it increases the ability FOXO3 to induce cell cycle arrest and resistance to oxidative stress, but inhibits its ability to induce cell death. Thus, one way Sir2 proteins increase organismal longevity is to tip FOXO-dependent responses away from apoptosis but toward stress resistance. The transcription repressor C-terminal-binding protein (CtBP) is also dependent on nicotinamide adenine dinucleotide for its transcription repression function (Chinnadurai, 2003). CtBP binds to E1A, a strong viral oncoprotein, resulting in the inhibition of transcription by E1A. CtBP also mediates transcriptional repression of a number of other transcription partners (Chinnadurai, 2003; Zhang et al., 2002). Thus, as a redox sensor, CtBP potentially mediates oxidative response by modulating the activities of transcription factors. Transcriptional regulation of circadian rhythm offers another example in which transcriptional activity is coupled to cellular redox status. Circadian transcription factors include Clock, NPAS2, and BMAL1. DNA-binding of both Clock-BMAL1 and NPAS2-BMAL1 heterodimers is sensitive to the NAD(P)+/NAD(P)H ratios (Rutter et al., 2001). In this way, circadian rhythms are regulated by both environmental cues and energy production in the body (Rutter et al., 2002). 3.7. FOXO proteins FOXO proteins are a subgroup of the Forkhead family of transcription factors (Carter & Brunet, 2007). This family is characterized by a conserved DNA-binding domain called “Forkhead Box” or FOX. Members of the Class ‘O’ share the characteristic of being regulated by the insulin/PI3K/Akt signaling pathway. Mammalian FOXOs include FOXO1, 3, 4, and 6. FOXO factors are evolutionarily conserved mediators of insulin and growth factor signal transduction (Accili & Arden, 2004). FOXO proteins are at the interface of important cellular processes, such as apoptosis, cell cycle progression, and oxidative stress resistance (Accili & Arden, 2004; Greer & Brunet, 2005). For apoptotic function, FOXO factors activate transcription of FasL, the ligand for the Fas-dependent cell death pathway, and Bim, a member of the proapoptotic Bcl-2 family. To promote cell cycle arrest, FOXO proteins upregulate cell cycle inhibitor p27kip1 to induce G1 arrest, or GADD45 to induce G2 arrest. In oxidative stress, FOXO up-regulates catalase and SOD2 and thereby enhances detoxification of ROS (Kops et al., 2002). FOXO also regulates DNA repair by up-regulating GADD45 and DDB1 (Carter & Brunet, 2007). A number of mechanisms have been implicated in the activation of FOXO transcription factors by oxidative signals (Furukawa-Hibi et al., 2005; Brunet et al., 2004). Under conditions of oxidative stress, JNK or

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Mst1 phosphorylates FOXO proteins to cause nuclear translocation and activation of the proteins. This is opposite to Akt-mediated phosphorylation of FOXO in response to insulin, in which phosphorylation results in the export of FOXO factors from the nucleus to the cytoplasm and consequently, inhibition of FOXO-dependent transcription. Oxidative stress promotes the interaction of FOXO with protein acetylases including p300 and p300/CBP-associated factor (PCAF). Acetylation inhibits its transcription activity. As mentioned before, FOXO proteins can be deacetylated by SIRT1 in response to oxidative stress, giving rise to enhanced DNA-binding of FOXO proteins (Brunet et al., 2004). FOXO proteins are also monoubiquitinated under oxidative stress and this increases transcription activity. Lastly, poly-ubiquitination results in proteasomal degradation of FOXO proteins that is also influenced by oxidative signals. 3.8. HIF Hypoxia-inducible factors (HIF) are transcription factors that respond to low oxygen or hypoxia in cells (Maxwell, 2005; Wang et al., 1995). HIF is a heterodimer composed of α and β subunits. Both the α and β subunits belong to the Per-Arnt-Sim (PAS) subfamily of the basic-helix–loop–helix (bHLH) family of transcription factors. The α subunit is responsible for oxygen sensing, whereas the β subunit (also termed aryl hydrocarbon receptor nuclear translocator or ARNT) is a constitutively expressed nuclear protein. In addition to dimerization with HIFα to mediate hypoxia response, ARNT participates in xenobiotic response by heterodimerizing with the aryl hydrocarbon receptor (AhR) (Ma, 2001). The prototype of HIF is HIF1, consisting of HIF1α and β. HIF proteins mediate the effects of hypoxia including angiogenesis, glucose uptake, and metabolism. Typical target genes of HIF in hypoxia response include erythropoietin (EPO), VEGF, Glut1, and phosphoglycerate kinase 1 (Li et al., 1996; Maxwell, 2005). The principle mechanism of low oxygen sensing and activation of HIF1 is hydroxylation of critical proline residues (P403 and P564) in HIF1α via HIF prolyl hydroxylases (Schofield & Ratcliffe, 2004). In the presence of molecular oxygen, the prolyl hydroxylases add one atom of oxygen to the prolines, converting them to 4-hydroxyproline. The von Hippel–Lindau (VHL) protein recognizes the hydroxylated prolines by specifically binding to the residues and subsequently targets HIF1α for ubiquitination via the Cul2 and VHL-dependent ECS E3 complex (Fig. 4C). Ubiquitinated HIF1α is rapidly degraded by the 26S proteasomes. Under hypoxia conditions, the prolyl hydroxylases are unable to hydroxylate the proline residues due to the lack of O2, thus freeing HIF1α from ubiquitination and proteasomal degradation. Stabilized HIF1α translocates into the nucleus and dimerizes with Arnt for activation of gene transcription. The HIF activity can be modulated by oxidative stress in several ways. Hydroxylation of the prolyl residues by dioxygenases is irondependent and oxoglutarate-dependent. The enzymes use superoxide as a catalytic intermediate (Pugh et al., 2001). Thus, redox agents that are known to influence HIF functions may act by modulating the enzyme activity. In many tissues, hypoxia conditions generate ROS and oxidative stress. Hypoxic preconditioning is protective in neurons. Protection involves activation of HIF1 and induction of a number of HIF1 target genes. However, this neuroprotection by hypoxic preconditioning is lost in SOD1 transgenic mice, suggesting that oxidative stress that occurs during hypoxic preconditioning is involved in neuroprotection (Liu et al., 2005). HIF1α is also regulated by Ref-1. Ref-1 binds and activates HIF1α to mediate hypoxic induction of vascular endothelial growth factor (Ziel et al., 2005). This response requires hypoxia-induced ROS production. These oxidants modify the 3′ guanine within the HIF1-DNArecognition sequence. ROS-induced formation of an apyrimidinic/ apurinic site within this region of DNA then facilitates recruitment of a complex containing both HIF1 and Ref-1. In this example, the DNA repair and redox functions of Ref-1 are physiologically integrated in hypoxia and oxidative responses.

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3.9. MTF1 Metal-activated transcription factor 1 (MTF1) is a member of the zinc finger family (Ma, 2008; Palmiter, 1994; Radtke et al., 1993). MTF1 contains six zinc fingers of the Cys2His2 type in its N-terminal half as the signature DNA-binding motif and three separable regions in the C-terminal half as the transcription activation domain. MTF1 is evolutionally conserved from Drosophila, to pufferfish, mouse, and human both structurally and functionally (Balamurugan et al., 2004). MTF1 plays critical roles in liver development as knockout of MTF1 in mice is embryonic lethal due to liver decay (Wang et al., 2004). In adult animals, MTF1 is the major transcription factor mediating the induction of metallothioneins 1 and 2 (MT1, MT2) through the metal response element (MRE) in response to toxic metals, oxidative and other stresses (Andrews, 2000; Giedroc et al., 2001; Palmiter, 1998; Wang et al., 2004). MTs are rich in cysteines. In addition to chelating toxic metals, MTs provide a zinc reserve and quench ROS and other free radicals. MTF1 also regulates a number of other genes including antioxidant proteins selenoprotein W and muscle 1 gene, and N-myc downstream regulated gene 1 (NDRG1) (Wimmer et al., 2005). Activation of MTF1 involves translocation of the protein into the nucleus where it binds to metal response element (MRE) for gene transcription. MTF1 is activated by a dozen toxic metals such as zinc, cadmium, arsenic, and nickel, ROS such as H2O2, antioxidants such as tBHQ, and hypoxia. Zinc may activate MTF1 by binding to the zinc fingers; however, binding of zinc to the six zinc fingers and the roles of the fingers in MTF1 activation may vary. It has been proposed that other transition metals, such as cadmium, and H2O2, activate MTF1 by displacing zinc from protein-bound zinc pools in the cell, such as metallothionein, to increase intracellular free zinc concentrations (Zhang et al., 2003). Free zinc then binds to the zinc fingers to activate MTF1. Indeed, antioxidants such as tBHQ have been shown to mobilize intracellular zinc and thereby increase free zinc concentration and activate MTF1 (Bi et al., 2004). Oxidative signals (tBHQ, H2O2) induce binding of MTF1 to MRE (Dalton et al., 1996). Recent studies reveal that MTF1 contains a stretch of five cysteine residues at its C-terminal end that appear to be well conserved through evolution. These cysteine residues are essential for binding to metals and for induction of MTs by arsenic and other metal inducers in intact cells (He & Ma, 2009b). In this manner, the recurrent theme of regulating transcription factors by modulating critical cysteine thiols by oxidants and electrophiles is again observed in MTF1-mediated response to oxidative stress. 3.10. HSF1 The heat shock transcription factor 1 (HSF1) is the major transcription factor mediating the induction of heat shock proteins (Hsp) in response to heat shock and a variety of other stresses including oxidative stress (Pirkkala et al., 2001). Induction of Hsp constitutes the most ubiquitous and evolutionarily conserved stress response known to the living world. Hsp comprises of a group of related proteins including Hsp100, 90, 70, 60, and 40, and small heat shock proteins. Hsp proteins function as molecular cheparones aiding in the assembly, folding, and translocation of proteins within the cell. Under stress, Hsp is believed to be critical for preventing misfolding and aggregation of proteins as well as for facilitating refolding and removal of damaged proteins (Jolly & Morimoto, 2000). ROS have been implicated in the activation of HSF1. Many oxidizing agents induce Hsps, whereas treatment with antioxidants prior to stresses, such as heat shock, attenuates the inductive response (Gorman et al., 1999). Protein damage may be a common signal for HSF activation and Hsp induction during stresses. In the case of oxidative stress, oxidative damage to proteins is perhaps an activating signal. However, the exact mechanisms by which oxidants activate

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HSF and antioxidants inhibit Hsp induction remain elusive at present. Direct oxidative damage by hydrogen peroxide or hypoxia/reperfusion injury, as well as other stresses in which generation of oxidative stress is implicated in cytotoxicity, such as chemotherapeutic agents, heat stress, and cytokines, induce Hsp70 and small heat shock proteins to enhance cell survival and prevent apoptosis (Baek et al., 2000; Chen et al., 1999; Wong et al., 1998).

This strong connection between aging and chronic diseases indicates organismal aging and many age-related chronic diseases share a common underlying mechanism in which ROS produced from mitochondria fuel both processes. This notion raises the importance of treating aging in the prevention and treatment of chronic diseases as compared with targeting specific, individual disease processes.

4. Oxidative stress and disease

4.2. Cancer

4.1. Aging

ROS are tumorigenic by virtue of their ability to increase cell proliferation, migration, and survival, and by inducing DNA damage, all contributing to tumor initiation, promotion, and metastasis (Klaunig & Kamendulis, 2004). Mice deficient in antioxidant capacity, such as peroxiredoxin-1 deficiency or heterozygous deleted SOD2, have increased spontaneous tumor formation (Neumann et al., 2003; Van Remmen et al., 2003). Carcinoma cells produce ROS at elevated rates in vitro, some of which are mediated through growth factorinduced ROS burst and signaling pathways (Szatrowski & Nathan, 1991). On the other hand, many tumors appear to be resistant to oxidative stress, which may be advantageous for tumor cells to survive and proliferate. Tumor oncogenes and tumor suppressors can directly modulate the intracellular redox status, and vice versa, oxidative signals regulate their activities. For instance, p53, Myc, ATM, and Ras all alter intracellular ROS levels and the alteration is linked to the ability of Ras and Myc to induce cellular transformation and genomic instability (Storz, 2005; Vafa et al., 2002; Woo & Poon, 2004). In the case of Ras, overexpression of the protein induces senescence in normal diploid cells, but promotes transformation in immortalized cells, both of which involve Rasinduced ROS production (Irani et al., 1997; Serrano et al., 1997). These Ras-mediated effects may require seladin-1 that translocates into the nucleus and stabilizes p53 upon exposure to ROS (Wu et al., 2004). Activated p53 in turn induces the phenotypes. ROS regulates the levels of tumor-secreted angiogenic growth factors by activating transcription factor JunD through a redox-dependent pathway (Gerald et al., 2004). ROS generated through the NADPH oxidases have also been implicated in tumor angiogenesis (Arbiser et al., 2002). ROS may regulate tumor invasiveness by modulating the expression and activity of matrix metalloproteinase in a redox-dependent manner (Nelson & Melendez, 2004). Evidence obtained from animal studies indicates that increasing the activity of Nrf2 by using chemoprotective agents such as oltipraz and sulphoraphane significantly reduces the incidence of B[a]P-induced forestomach tumors; the protection is greatly reduced or lost in Nrf2 KO mice (Fahey et al., 2002; Ramos-Gomez et al., 2001). Thus, Nrf2 is an effective target of chemoprevention. In this regard, Nrf2 regulates the expression of genes that both metabolize the carcinogens and defend against oxidative stress (Ma, 2008). On the other hand, an increasing number of studies reveal an elevated rate of mutations of the Nrf2 suppressor protein, Keap1, in many tumors, resulting in increased activities of Nrf2 in tumor cells (Ohta et al., 2008). It is believed that elevated Nrf2 function is advantageous to tumors for resistance to ROS and chemotherapeutic agents. Therefore, Nrf2 appears to be a doubleedged sword in tumorigenesis: in the initiation phase, it is anticarcinogenic, whereas in the late stage of cancer, it promotes tumor formation.

The “free radical theory of aging” remains the leading contender to explain the basis of aging since it was proposed more than fifty years ago (Harman, 1956). Mitochondrial respiration is likely a major source of ROS generation during aging. Mitochondrial integrity and function decline as a function of age (Shigenaga et al., 1994). The formation of 8-oxo-2′-deoxyguanosine (oxo8dG), a biomarker of oxidative DNA damage, is significantly higher in mitochondrial DNA than in genomic DNA in postmitotic tissues such as brain (Richter et al., 1988). Increased sensitivity of mitochondrial DNA to oxidative damage supports the concept of a “vicious cycle” between mitochondrial ROS production and mitochondrial damage during aging. A number of whole animal models with altered life spans exhibit changes in mitochondrial metabolism, ROS generation, and oxidative stress resistance as their primary alterations, supporting a strong underlying relationship between oxidative stress and aging. Mice with targeted disruption of the p66shc gene live 30% longer than the controls and also exhibit increased resistance to oxidative stress and decreased basal and stress-induced levels of ROS production (Nemoto & Finkel, 2002). p66shc is an adaptor protein that regulates protein–protein interactions for a number of cell surface receptors. p66shc appears to regulate mammalian Forkhead transcription factors to increase the expression of catalase and SOD (Kops et al., 2002; Nemoto & Finkel, 2002). A fraction of the protein localizes in the mitochondria and thus, may also directly regulate mitochondrial oxidant production (Orsini et al., 2004). Knock-in mice that express a proofreading-deficient form of a nuclear encoded mitochondrial DNA polymerase have significantly higher numbers of point mutations and deletions in mitochondrial DNA (Trifunovic et al., 2004). The mice exhibit shortened life span and appearance of aging, including hair loss, kyphosis, and reduced fertility. Thus, damaged mitochondrial DNA accelerates aging. Finally, an outbred strain of mice with higher metabolic rates and oxygen consumption live longer than animals with lower metabolic rates (Speakman et al., 2004). These mice have significant increases in metabolic uncoupling, suggesting that these mice may decrease ROS production even with increased oxygen consumption by way of increased mitochondrial uncoupling. Observations that direct oxidant challenge mimics many of the cellular and transcriptional changes seen with aging also strengthen the link between the level of ROS and the rate of aging. Lowering the ambient oxygen concentration significantly extends the life span of primary cells in culture (Packer & Fuehr, 1977). Increasing the antioxidant levels can also achieve prolongation of cellular life span. Augmenting the level of SOD extends the life span of primary fibroblasts, whereas knockdown of SOD with RNAi induces senescence (Blander et al., 2003; Serra et al., 2003). In this later case, induction of p53 is required for senescence. Expression of activated Ras can also induce senescence in primary fibroblasts, which is likely p53-dependent and involves increased ROS production (Serrano et al., 1997). Major chronic diseases, such as neurodegeneration, cancer, and cardiovascular disorders, increase rapidly (almost exponentially) in both incidence and mortality as a function of age (Finkel, 2005).

4.3. Neurodegeneration A characteristic feature of Alzheimer's disease is the formation of extracellular amyloid plaques in the patient's brain (Bossy-Wetzel et al., 2004). The plaques contain cleavage products (amyloid-β or Aβ) of amyloid precursor protein (APP). Moreover, mutations in APP or in gene products that are involved in APP processing have been linked to inherited forms of Alzheimer's disease. Evidence reveals that a number

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of mechanisms link the formation of Aβ to oxidative stress (Behl et al., 1994). The Aβ peptide can directly induce oxidative stress, possibly through interaction with the mitochondrial alcohol dehydrogenase (Lustbader et al., 2004). Aβ may also trigger the release of excitatory amino acids, such as glutamate, from surrounding glial cells. Glutamate then stimulates NMDA receptors that in turn induce the formation of nitric oxide (NO•). NO• interacts with superoxide (O2•−) to produce peroxynitrite (ONOO−) (Aleman et al., 2003). On the other hand, the cellular redox conditions and aging itself can influence the rate of protein aggregation. Protein residues such as arginine, proline, and lysine can undergo carbonylation via metal-catalyzed oxidation. The levels of carbonylated proteins increase dramatically with age (Nystrom, 2005). Oxidative modification of proteins seems to significantly accelerate protein aggregation and crosslinking (Grune et al., 2004). Thus, ROS may function in a potential positive feedback loop to contribute to neurodegerative diseases that are characterized by protein aggregation. Additionally, oxidative stress may contribute to the progression of the diseases by inhibiting the normal repair processes. Parkinson's disease (PD) is linked to oxidative stress in several ways. At least five separate genes associated with PD have been identified: αsynuclein, parkin, ubiquitin C-terminal hydrolase-1, DJ1, and PTENinduced kinase-1 (PINK1) (Dawson & Dawson, 2003). All five proteins can trigger an increase in neuronal ROS formation. In the brain of PD patients, α-synuclein is modified by oxidative and nitrosative stress. Inhibition of mitochondrial function increases α-synuclein aggregation that in turn further impairs mitochondrial function (Dawson & Dawson, 2003; Hsu et al., 2000). This positive feedback loop between oxidative stress and α-synuclein is similar to that of Aβ aggregate formation seen in Alzheimer's. Mice deficient in parkin show mitochondrial defects, and neurons lacking DJ1 have increased sensitivity to oxidative stress (Martinat et al., 2004; Palacino et al., 2004). Similarly, PINK1 can localize in mitochondria and function as a kinase that is required for mitochondrial function (Valente et al., 2004). Finally, chemical inhibitors of mitochondrial complex I induce PD phenotypes in clinical patients (Bossy-Wetzel et al., 2004; Dawson & Dawson, 2003; Rosen et al., 1993). Amyotrophic lateral sclerosis (ALS) is characterized by motorneuron degeneration and progressive muscle wasting (Bossy-Wetzel et al., 2004). A fraction of inherited ALS results from mutations in SOD1 (Rosen et al., 1993). Although how mutations in SOD1 cause ALS remains unclear, it provides a strong connection between ROS metabolism and ALS pathogenesis. 4.4. Cardiovascular and metabolic diseases Besides aging, a number of risk factors have been identified for cardiovascular diseases, including hypercholesterolemia, hypertension, cigarette smoking, and diabetes. For metabolic diseases, such as type 2 diabetes, obesity and lack of activity are apparent risk factors. In both animals and humans, these risk factors induce a pro-oxidant environment in vessel walls and other tissues (Brownlee, 2001; Madamanchi et al., 2005). Therefore, ROS appear to have an important role in the development of cardiovascular and metabolic diseases. In the development of atherosclerotic plaques, the deposition of oxidized low-density lipoprotein (LDL) in blood vessel walls appears to be the initial event. The source of ROS that oxidize LDL remains unspecified, but the plasma membrane-bound NADPH-oxidase systems have been implicated (Madamanchi et al., 2005). Knockout of NADPH oxidase in mice reduces vascular-oxidant production and atherosclerosis (Barry-Lane et al., 2001; Hsich et al., 2000). Mitochondrial ROS production also contributes to vascular lesions. Mice with heterozygous SOD2 (−/+) have increased rates of atherosclerotic-plaque formation (Ballinger et al., 2002). A genetic study of a family whose members suffer from hypomagnesmia, hypertension, and hypercholesterolemia reveals that the disease has a maternal inheritance and the mutation is in a mitochondria-encoded tRNA, providing a direct link between mitochondrial function and hypertension (Wilson et al., 2004).

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Oxidative stress contributes to the development of diabetes and diabetic complications (Brownlee, 2001). For instance, high glucose promotes ROS production in cardiomyocytes both in vitro and in vivo. Cardiomyocytes from Nrf2 KO hearts show much higher basal and high glucose-induced production of ROS compared with wild-type controls (He et al., 2009). Moreover, the hearts of diabetic Nrf2 KO mice show apparent pathologic lesions and markedly reduced contractility compared with wild-type diabetic hearts. The findings underscore a critical role of Nrf2-mediated gene transcription in protection against diabetic cardiomyopathy. 4.5. Chronic inflammatory diseases Chronic inflammatory diseases encompass a large group of medical conditions characterized by persistent inflammation, including asthma, rheumatoid arthritis, chronic obstructive pulmonary diseases (COPD), inflammatory bowel diseases, skin lesions, such as psoriasis, vascular diseases, such as atherosclerosis and vasculitis, and autoimmune diseases, such as lupus. Oxidative stress contributes to the initiation and progression of chronic inflammatory diseases by promoting cell proliferation, adhesion molecule expression, cytokine and chemokine production, and matrix metalloprotenase expression (Rahman, 2005). In addition to directly damaging tissues, ROS generated in response to pro-inflammatory cytokines, growth factors, chemical exposure and other stresses, serve as important signaling molecules in the activation of key transcription factors and protein kinases to regulate the transcription of genes involved in inflammatory and immune responses. Evidence reveals that oxidative biomarkers are elevated in a number of inflammatory diseases, that diminished antioxidant capacity results in enhanced susceptibility to immune and inflammatory diseases, and that use of antioxidants reduces or prevents inflammatory lesions (Kirkham & Rahman, 2006; Luchoomun et al., 2006). Oxidative signals are capable of regulating the expression of a variety of cytokines and chemokines, such as TNFα, IL-1β, IL-6, IL-8, and MCP-1, and adhesion molecules, such as VCAM-1 and ICAM-1 (Luchoomun et al., 2006; Luster et al., 1999). Matrix metalloproteinases (MMPs) are a family of proteolytic enzymes involved in tissue destruction, an important pathologic feature of destructive inflammatory diseases including rheumatoid arthritis, asthma, and atherosclerosis. Expression of MMPs (MMP-1, MMP-2, and MMP-9) is regulated by redox-sensitive mechanisms. NF-κB is a critical element in the regulation of gene expression in immune and inflammatory responses (Gilmore, 2006; Hayden et al., 2006). NF-κB transduces a wide variety of noxious or inflammatory stimuli into coordinated activation of multiple genes that encode cytokines, cytokine receptors, adhesion molecules, and chemokines. ROS activates NF-κB in response to TNFα, IL-1β, LPS, H2O2, and various cellular stresses. This activation can be specifically inhibited by diverse thiol antioxidants, supporting ROS or thiol-dependent redox mechanisms in the activation of NF-κB and inflammatory gene expression. In addition, ROS play a role in the activation of AP-1 and HIF1α that also contribute to gene regulation in inflammatory diseases. A number of protein kinases are activated by oxidative signals during inflammation and inflammatory diseases (Rahman, 2005). The p38 MAPK pathway regulates the expression of many inflammatory genes, such as MCP-1, IL-1β, IL-6, VCAM-1, and TNFα. Both IL-1β and TNFα stimulate the phosphorylation and activation of p38 and this stimulation can be blocked by thiol antioxidants and SOD. Thus, a positive feedback loop among ROS, p38, and inflammatory cytokines is formed to amplify inflammatory responses. ASK1 is activated by various oxidants and plays important roles in oxidative stress-mediated apoptosis and inflammatory responses. Reduced thioredoxin directly binds to ASK1 and inhibits oxidant- and cytokine-induced activation of ASK1. Other kinases activated by ROS during inflammation include Src, JNK, ERK1, and PKC. Increasing evidence indicates that Nrf2 plays important roles in the protection against a number of inflammatory conditions. As

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discussed above, Nrf2 is activated by oxidants and electrophiles and coordinately regulates the basal and inducible expression of a battery of phase II genes and antioxidant enzymes/proteins. Nrf2 KO mice exhibit increased oxidative stress and decreased antioxidant capacity. Nrf2 KO mice show spontaneous lupus-like autoimmune phenotypes (Ma et al., 2006). When the mice are exposed to hyperoxia and oxidants, enhanced pulmonary injury and increased inflammatory cell infiltration are observed compared with wild-type (Chan & Kan, 1999; Cho et al., 2002). Nrf2 KO mice also have prolonged inflammation in cutaneous wound healing, enhanced bronchial inflammation, and increased susceptibility to cigarette smoke-induced emphysema (Kensler et al., 2007). 5. Chemical toxicity associated with oxidative stress Many chemicals and materials are capable of inducing oxidative stress in the body via direct or indirect mechanisms. It is generally accepted that ROS produced from exposure of the chemicals contribute significantly to the toxicity and carcinogenicity of the chemicals. Discussed below are a few examples of chemicals that directly induce ROS production as a major mechanism of toxicity. 5.1. Metals Many studies have focused on metal-induced generation of ROS in metal toxicity and carcinogenicity, underscoring the significance of oxidative stress in metal action in biological systems (Leonard et al., 2004; Liu et al., 2008; Valko et al., 2005). Metal overload depletes antioxidants in the cell by binding to reduced glutathione, metallothioneins, and thioredoxins. More importantly, metals directly promote ROS production via Fenton or Fenton-like reactions (Fig. 2C). In this scenario, metals induce an increase in superoxide (O2•−)—initial oxidant—that in turn releases iron from iron-containing molecules. Free Fe3+ then participate in Fenton reaction to generate highly reactive hydroxyl radical—secondary ROS (Fe2+ + H2O2 → Fe3+ + •OH +OH−). Many transition metals can directly participate in this reaction by replacing Fe (Mn+ + H2O2 → M(n + 1)+ + •OH + OH−, where M is a transition metal ion, such as iron, copper, chromium, cobalt, and certain other metals).

hydroxyl radical via the Fenton reaction. Cr6+ induces prominent ROS production in cultured cells (He et al., 2007). 5.1.3. Cadmium Cadmium is highly toxic and is a human carcinogen causing cancers in lung, prostate, pancreas, and kidney (WHO, 1992). Cadmium itself may not be capable of generating free radicals directly, but cells exposed to Cd2+ clearly show elevated levels of ROS (He et al., 2008). It has been proposed that cadmium may replace iron and copper; increased free iron and copper then induce oxidative stress via the Fenton reaction. Cadmium is highly reactive to protein thiols, which may contribute to depletion of antioxidants and toxicity in kidneys and other tissues by binding to and inhibiting thiol-dependent enzymes and other proteins. 5.1.4. Arsenic Arsenic causes a wide variety of toxic and carcinogenic effects in humans, including cancers in skin, lung, bladder, kidney, and liver; skin lesions; nerve damage; and cardiovascular lesions such as atherosclerosis (ATSDR, 2005). Arsenic-mediated generation of ROS is a complex process that involves a variety of ROS including superoxide, singlet oxygen, peroxyl radical, nitric oxide, hydrogen peroxide, dimethylarsinic peroxyl radicals, and the dimethylarsinic radical. The mechanism of ROS generation by arsenic remains unclear but may involves the formation of intermediary arsine species, or the oxidation of As3+ to As5+, which results in the formation of H2O2 under physiological conditions (H3AsO3 + H2O + O2 → H3AsO4 + H2O2) (Valko et al., 2005). This reaction is exergonic and spontaneous in cells. Arsenic is also potently thiol-reactive, a property that contributes to many of arsenic's functions and toxicities (He & Ma, 2009b). Many metals directly or indirectly activate transcription factors and other signaling molecules. These include NF-κB, AP-1, p53, HIF-1, MTF1, Nrf2, and various protein kinases (see discussions in Section 3). For instance, chromium activates p53 to induce apoptosis in cells via several mechanisms: by causing DNA damage, via DNA-binding by chromium reduction products, by activation of MAPKs upstream of p53, through direct activation of p53, and by enhancing the effect of other carcinogens co-exposed to the cells (Wang & Shi, 2001). MTF1 controls the induction of metallothioneins I and II in response to a range of metals, such as zinc, cadmium, cobalt, nickel, gold, mercury, bismuth, and arsenic (Ma, 2008; Palmiter, 1994; Radtke et al., 1993). Deletion of both MTs in mice markedly increases the sensitivity of the mice to the toxicities of these metals (Palmiter, 1998). Metal inducers of MTs bind to a stretch of 5 cysteine residues at the C-terminal end of MTF1 and binding is required for activation of MTF1 in intact cells (He & Ma, 2009b). Similarly, metal inducers of ARE-dependent genes activate Nrf2 by binding to the cysteine residues of both Keap1 and Nrf2 (He & Ma, 2009a, 2009c). In Nrf2 KO mice, both basal and metalinduced ROS production and apoptosis are significantly increased in comparison with wild-type controls, demonstrating a critical role of Nrf2 in cellular defense against metal-induced oxidative stress and toxicity (He et al., 2007, 2008).

5.1.1. Iron Both animal experiments and human data reveal a link between increased levels of iron in the body and enhanced risks of vascular diseases, cancer, and certain neurological conditions (Berg et al., 2001; Siah et al., 2005). The genetic hemochromatosis is associated with increased risk of hepatocellular carcinoma (Kowdley, 2004). Exposure to asbestos containing about 30% (by weight) of iron is related to increased risks of asbestosis (Stayner et al., 1996). It is generally accepted that asbestos-induced carcinogenesis is linked to ROS production. Iron may also interact with bile acids, the K vitamins, and oxygen to induce an oncogenic effect in the colon via free radical generation (Valko et al., 2001).

5.2. Particles and fibers

5.1.2. Chromium Cr3+ is a trace element in the body required for regulation of blood glucose levels, whereas Cr6+ is toxic and carcinogenic in humans (Dayan & Paine, 2001; Yao et al., 2008). Workers exposed to hexavalent chromium from air at workplace have much higher rates of lung cancer than workers who were not exposed. Cr6+ has also been implicated in breast and other cancers (Kilic et al., 2004). Cr6+ can actively enter the cells through channels for isoelectric and isostructural anions. It is generally believed that, once inside the cells, Cr6+ is reduced to lower valence forms that cause a wide variety of DNA lesions including Cr-DNA adducts, DNA–protein crosslinks, DNA–DNA crosslinks, and oxidative DNA damage (Valko et al., 2005). Cr5+ can react with H2O2 to form

Airborne particles and fibers (asbestos, silica, coal dust, diesel exhaust, ultrafine particles, and nanoparticles) induce a range of diseases and cancer, mostly within the respiratory system. Many studies have shown that particle/fiber-induced oxidative stress plays a major role in the pathogenesis of the respiratory lesions (Castranova, 2004; Fubini & Hubbard, 2003). Deposition of airborne particles/fibers in the airway and/or alveoli induces inflammatory responses in the lung characterized by influx of inflammatory cells, such as pulmonary macrophages and polymorphonuclear cells (PMN). Both cell types then engulf the particles/fibers and transport them from alveolar epithelium into the lung interstitium. During the process, macrophages and PMNs become reactivated and undergo respiratory burst producing and

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releasing large amounts of ROS and growth factors. These radicals and growth factors stimulate fibroblasts to produce collagen and cause lung fibrosis (Brody et al., 1981). Particles/fibers can also induce the production of ROS via biologically active materials attached on their surface. Iron and other transition metals on the surface of particles/ fibers induce ROS production by catalyzing the Fenton reaction. In the airways, nitric oxide (NO•) plays a key role in the physiological regulation of paracrine and autocrine functions, such as the relaxation of smooth muscle in arterioles, inhibition of platelet aggregation, and inflammation. When stimulated by particles/fibers, activated inflammatory cells also generate NO• that interacts with O2•− to produce strong oxidizing reactive nitrogen species, such as peroxynitrite anions (ONOO−). These peroxynitrite anions are involved in progression of particle/fiberinduced lung fibrosis (Bhattacharya et al., 2007). Similarly to toxic metals that induce pulmonary damage, toxic particles and fibers activate transcription factors and signaling pathways that ultimately result in changes in gene expression (Castranova, 2004). Gene products induced by particles/fibers include pro-inflammatory cytokines, including IL-1, IL-6, and TNFα, prostaglandins, MMP9, and MMP12, that further exacerbate inflammation; PDGF, TGFβ, and TNFα, that directly regulate fibroblast proliferation and extracellular matrix deposition, the hallmark of fibroproliferative lung disease; and antioxidant enzymes and proteins such as HO1 and TXNRD1, that represent adaptive responses to oxidative damage by particles/fibers in the respiratory system (Bhattacharya et al., 2007). Transcription factors activated include NF-κB, AP-1, and Nrf2. Protein kinases are also significantly activated. Activation of p53 is observed in mesothelioma and other cancers induced by asbestos. 5.3. Organic, ROS-generating chemicals 5.3.1. Paraquat, menadione, and doxorubicin Paraquat, menadione, and doxorubicin are typical chemicals that have structural moieties that undergo redox cycling in living cells to induce ROS production (Fig. 2B). Therefore, these chemicals cause apparent toxicities in animals and humans in which oxidative stress is a prominent component of pathogenesis. Paraquat (N′,N′-dimethyl-4,4′-bipyridinium dichloride), one of the most widely used herbicides in the world, produces degenerative lesions in the lung after systemic administration to animals and humans. Cyclic single electron reduction/oxidation of paraquat is critical in the development of pulmonary toxicity (Bus & Gibson, 1984). This cycling leads to two potentially important consequences relevant to the toxicity: (1) generation of ROS including superoxide anion, hydrogen peroxide, and hydroxyl radical, and (2) oxidation and depletion of reducing equivalents (NADPH, GSH, etc); both contribute to the induction of oxidative stress and damage to the tissue. Menadione (2-methylnaphthalene-1,4-dione) is a precursor of vitamin K2. Menadione may cause hemolytic anemia, neonatal brain and liver damage, or neonatal death. Menadione toxicity is closely associated with one-electron reduction/oxidation cycling. Like paraquat, this redox cycling generates ROS and concomitant decrease of NAD(P)H. Antioxidants, such as N-acetyl-L-cysteine, inhibit menadione-induced ROS production and toxicity. NQO1-catalyzed twoelectron reduction protects against menadione redox cycling and consequently, its toxicity (Fig. 2B) (Criddle et al., 2006). Doxorubicin (adriamycin) is an anthracycline antibiotic commonly used in the treatment of a wide range of cancers including hematological malignancies, many types of carcinoma, and soft tissue sarcomas (Singal & Iliskovic, 1998). Like all anthracyclines, doxorubicin intercalates DNA and inhibits topoisomerase II. Doxorubicin induces ROS production, likely due to its quinone moiety that undergoes redox cycling upon oneelectron reduction. ROS production at the site of intercalated DNA further damages the DNA. Doxorubicin can cause a range of toxicities, but the most severe ones are congestive heart failure, dilated cardiomyopathy, and death (Saltiel & McGuire, 1983). Doxorubicin cardiotoxicity is characterized by dose-dependent damage to mito-

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chondrial oxidative phosphorylation and oxidative stress in cardiomyocytes (Kotamraju et al., 2000). 5.3.2. Acetaminophen Acetaminophen (paracetamol) is a widely used over-the-counter analgesic and antipyretic. Acetaminophen toxicity is the foremost cause of acute liver failure in the Western world. Acetaminophen toxicity derives from its metabolite NAPQI (N-acetyl-p-benzoquinone imine)—it's so called reactive intermediate. Formation of NAPQI is catalyzed by microsomal cytochrome P450 (CYP) enzymes CYP2E1, CYP3A4, and CYP1A2 in the liver. NAPQI is eliminated by conjugating with glutathione, glucuronic acid, or sulfate. Acetaminophen overdose leads to accumulation of NAPQI, which damages cell structures, depletes GSH leading to oxidative stress within the cell, inhibits mitochondrial functions, and disrupts calcium homeostasis and redox balances. Treatment with acetaminophen activates Nrf2 in both cell and mouse models. Experiments using Nrf2 KO mice demonstrate that Nrf2-mediated induction of ARE-dependent phase II genes and antioxidant proteins/enzymes are critical for protection against acetaminophen-induced hepatotoxicity and death in mice (Chan et al., 2001). 5.3.3. Troglitazone Troglitazone is an anti-diabetic and anti-inflammatory drug of the thiazolidinedione family. It was withdrawn from the market due to an idiosyncratic reaction leading to drug-induced hepatitis (Chojkier, 2005; Watkins, 2005). The pharmacological effects of troglitazone are related to its agonistic actions on peroxisome proliferator-activated receptors (PPAR) α and γ. Additionally, troglitazone is associated with a decrease of NF-κB, which may be linked to its anti-inflammatory activity. The mechanism by which troglitazone induces hepatotoxicity remains unclear. However, recent studies with heterozygous SOD2+/− mice showed that troglitazone induces increased production of superoxide anion, reduction of aconitase activity and complex I activity in the mitochondria, and increased protein carbonyls (Ong et al., 2007). The findings demonstrate that SOD2 is necessary for protection against troglitazone-induced mitochondrial oxidant stress in mouse liver. 6. Conclusion and perspective The past two decades have witnessed a significant advance in understanding the signal transduction pathways of response to oxidative stress, and of particular interest in this review, those important in mediating transcriptional response to oxidants/antioxidants, at molecular levels. With this has also come an appreciation for the complexity of the responses and awareness that the individual signaling pathways do not act in isolation, but intersect with one another to mediate many of the physiological, pathological, and adaptive effects of ROS in dose and cellular context-dependent manners. In this regard, regulation of gene transcription by ROS has been an evolutionally conserved strategy to convert alterations in the amount of intracellular ROS into discrete and reproducible alterations in gene expression. A number of recurrent themes emerge to account for how cells detect oxidants/antioxidants and activate specific transcription factors. These include activation of specific protein kinase cascades that activate transcription factors by phosphorylating specific residues of the factors, direct oxidation/reduction of critical cysteines of transcription factors, regulation of transcription factors via redox factors, such as Ref-1, and modulation by alterations in redox buffers, such as NAD+/NADH. Other modifications of transcription factors including mono- and poly-ubiquitination, SUMOylation, and acetylation/deacetylation, are also frequently observed in transcriptional responses to ROS. These protein modulations affect various aspects of transcription factor activation and transcriptional activity by modulating the protein structure and stability, subcellular localization, dimerization, DNA-binding, chromatin remodeling, and recruitment

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of co-activators and co-repressors of corresponding transcription factors. One caveat in analyzing these transcriptional signaling pathways to ROS is that many of the well-established examples come from analyzing cellular responses to exogenous oxidant/electrophile challenges that often result in robust induction of gene transcription. On the other hand, the moderate and sustained rise in the amount of ROS observed in many disease processes, such as chronic inflammation and diabetic complications, presumably results in smaller but equally regulated alterations in gene transcription in vivo. However, it is reasonable to believe that the lessons learned from examples discussed above can be applied to understanding the more subtle, disease-associated changes in gene transcription induced by oxidative stress in future. Apparently, a number of transcriptional pathways activated by ROS control critical cellular functions, such as cell growth and differentiation, apoptosis, inflammation and immune response, DNA repair, protein folding, xenobiotic metabolism and disposition, and oxidant metabolism. As discussed in the review, evidence from both animal and human data support the notion that activation of these transcription pathways by oxidative signals play critical roles in the development of a variety of diseases and chemical toxicity. Consequently, these transcription factors/signaling molecules can serve as effective targets of drug development for the treatment and prevention of the diseases. Molecular understanding of the interactions between oxidative stress and these transcriptional pathways has already facilitated and will continue to provide a driving force for developing effective therapeutics against ROSassociated diseases, including cancer, aging, neurodegeneration, cardiovascular and metabolic diseases, chronic inflammatory lesions, and chemical toxicity. Acknowledgments The findings and conclusions in this article are those of the author and do not necessarily represent the views of the National Institute for Occupational Safety and Health. References Accili, D., & Arden, K. C. (2004). FoxOs at the crossroads of cellular metabolism, differentiation, and transformation. Cell 117(4), 421−426. Adler, V., Yin, Z., Tew, K. D., & Ronai, Z. (1999). Role of redox potential and reactive oxygen species in stress signaling. Oncogene 18(45), 6104−6111. Aleman, B. M., Raemaekers, J. M., Tirelli, U., Bortolus, R., van 't Veer, M. B., Lybeert, M. L., et al. (2003). Involved-field radiotherapy for advanced Hodgkin's lymphoma. N Engl J Med 348(24), 2396−2406. Andrews, G. K. (2000). Regulation of metallothionein gene expression by oxidative stress and metal ions. Biochem Pharmacol 59(1), 95−104. Apopa, P. L., He, X., & Ma, Q. (2008). Phosphorylation of Nrf2 in the transcription activation domain by casein kinase 2 (CK2) is critical for the nuclear translocation and transcription activation function of Nrf2 in IMR-32 neuroblastoma cells. J Biochem Mol Toxicol 22(1), 63−76. Arbiser, J. L., Petros, J., Klafter, R., Govindajaran, B., McLaughlin, E. R., Brown, L. F., et al. (2002). Reactive oxygen generated by Nox1 triggers the angiogenic switch. Proc Natl Acad Sci U S A 99(2), 715−720. Asher, G., Lotem, J., Cohen, B., Sachs, L., & Shaul, Y. (2001). Regulation of p53 stability and p53-dependent apoptosis by NADH quinone oxidoreductase 1. Proc Natl Acad Sci U S A 98(3), 1188−1193. ATSDR (2005). Toxicological Profile for Arsenic (Update). Atlanta, Georgia: Agency for Toxic Substances and Disease Registry. Baehner, R. L., & Nathan, D. G. (1967). Leukocyte oxidase: defective activity in chronic granulomatous disease. Science 155(764), 835−836. Baek, S. H., Min, J. N., Park, E. M., Han, M. Y., Lee, Y. S., Lee, Y. J., et al. (2000). Role of small heat shock protein HSP25 in radioresistance and glutathione-redox cycle. J Cell Physiol 183(1), 100−107. Balaban, R. S., Nemoto, S., & Finkel, T. (2005). Mitochondria, oxidants, and aging. Cell 120(4), 483−495. Balamurugan, K., Egli, D., Selvaraj, A., Zhang, B., Georgiev, O., & Schaffner, W. (2004). Metalresponsive transcription factor (MTF-1) and heavy metal stress response in Drosophila and mammalian cells: a functional comparison. Biol Chem 385(7), 597−603. Ballinger, S. W., Patterson, C., Knight-Lozano, C. A., Burow, D. L., Conklin, C. A., Hu, Z., et al. (2002). Mitochondrial integrity and function in atherogenesis. Circulation 106(5), 544−549.

Barry-Lane, P. A., Patterson, C., van der Merwe, M., Hu, Z., Holland, S. M., Yeh, E. T., et al. (2001). p47phox is required for atherosclerotic lesion progression in ApoE(−/−) mice. J Clin Invest 108(10), 1513−1522. Bedard, K., & Krause, K. H. (2007). The NOX family of ROS-generating NADPH oxidases: physiology and pathophysiology. Physiol Rev 87(1), 245−313. Behl, C., Davis, J. B., Lesley, R., & Schubert, D. (1994). Hydrogen peroxide mediates amyloid beta protein toxicity. Cell 77(6), 817−827. Berg, D., Gerlach, M., Youdim, M. B., Double, K. L., Zecca, L., Riederer, P., et al. (2001). Brain iron pathways and their relevance to Parkinson's disease. J Neurochem 79(2), 225−236. Berner, R. A., Vandenbrooks, J. M., & Ward, P. D. (2007). Evolution. Oxygen and evolution. Science 316(5824), 557−558. Bhattacharya, K., Alink, G., & Dopp, E. (2007). Oxidative stress and changed gene expression profiles in fiber-/particle-induced carcinogenesis. Int J Hum Genet 7(1), 1−21. Bi, Y., Palmiter, R. D., Wood, K. M., & Ma, Q. (2004). Induction of metallothionein I by phenolic antioxidants requires metal-activated transcription factor 1 (MTF-1) and zinc. Biochem J 380(Pt 3), 695−703. Blander, G., de Oliveira, R. M., Conboy, C. M., Haigis, M., & Guarente, L. (2003). Superoxide dismutase 1 knock-down induces senescence in human fibroblasts. J Biol Chem 278 (40), 38966−38969. Blander, G., & Guarente, L. (2004). The Sir2 family of protein deacetylases. Annu Rev Biochem 73, 417−435. Bossy-Wetzel, E., Bakiri, L., & Yaniv, M. (1997). Induction of apoptosis by the transcription factor c-Jun. EMBO J 16(7), 1695−1709. Bossy-Wetzel, E., Schwarzenbacher, R., & Lipton, S. A. (2004). Molecular pathways to neurodegeneration. Nat Med 10(Suppl), S2−S9. Boveris, A., & Chance, B. (1973). The mitochondrial generation of hydrogen peroxide. General properties and effect of hyperbaric oxygen. Biochem J 134(3), 707−716. Brody, A. R., Hill, L. H., Adkins, B., Jr., & O'Connor, R. W. (1981). Chrysotile asbestos inhalation in rats: deposition pattern and reaction of alveolar epithelium and pulmonary macrophages. Am Rev Respir Dis 123(6), 670−679. Brownlee, M. (2001). Biochemistry and molecular cell biology of diabetic complications. Nature 414(6865), 813−820. Brunet, A., Sweeney, L. B., Sturgill, J. F., Chua, K. F., Greer, P. L., Lin, Y., et al. (2004). Stressdependent regulation of FOXO transcription factors by the SIRT1 deacetylase. Science 303(5666), 2011−2015. Bulavin, D. V., Saito, S., Hollander, M. C., Sakaguchi, K., Anderson, C. W., Appella, E., et al. (1999). Phosphorylation of human p53 by p38 kinase coordinates N-terminal phosphorylation and apoptosis in response to UV radiation. EMBO J 18(23), 6845−6854. Burdon, R. H. (1995). Superoxide and hydrogen peroxide in relation to mammalian cell proliferation. Free Radic Biol Med 18(4), 775−794. Burns, T. F., & El-Deiry, W. S. (1999). The p53 pathway and apoptosis. J Cell Physiol 181 (2), 231−239. Bus, J. S., & Gibson, J. E. (1984). Paraquat: model for oxidant-initiated toxicity. Environ Health Perspect 55, 37−46. Buschmann, T., Potapova, O., Bar-Shira, A., Ivanov, V. N., Fuchs, S. Y., Henderson, S., et al. (2001). Jun NH2-terminal kinase phosphorylation of p53 on Thr-81 is important for p53 stabilization and transcriptional activities in response to stress. Mol Cell Biol 21 (8), 2743−2754. Carter, M. E., & Brunet, A. (2007). FOXO transcription factors. Curr Biol 17(4), R113−R114. Castranova, V. (2004). Signaling pathways controlling the production of inflammatory mediators in response to crystalline silica exposure: role of reactive oxygen/nitrogen species. Free Radic Biol Med 37(7), 916−925. Chae, H. Z., Kang, S. W., & Rhee, S. G. (1999). Isoforms of mammalian peroxiredoxin that reduce peroxides in presence of thioredoxin. Methods Enzymol 300, 219−226. Chan, K., & Kan, Y. W. (1999). Nrf2 is essential for protection against acute pulmonary injury in mice. Proc Natl Acad Sci U S A 96(22), 12731−12736. Chan, K., Han, X. D., & Kan, Y. W. (2001). An important function of Nrf2 in combating oxidative stress: detoxification of acetaminophen. Proc Natl Acad Sci U S A 98(8), 4611−4616. Chang, L., & Karin, M. (2001). Mammalian MAP kinase signalling cascades. Nature 410 (6824), 37−40. Chen, H. C., Guh, J. Y., Tsai, J. H., & Lai, Y. H. (1999). Induction of heat shock protein 70 protects mesangial cells against oxidative injury. Kidney Int 56(4), 1270−1273. Chen, Q. M., Liu, J., & Merrett, J. B. (2000). Apoptosis or senescence-like growth arrest: influence of cell-cycle position, p53, p21 and bax in H2O2 response of normal human fibroblasts. Biochem J 347(Pt 2), 543−551. Chen, W., Martindale, J. L., Holbrook, N. J., & Liu, Y. (1998). Tumor promoter arsenite activates extracellular signal-regulated kinase through a signaling pathway mediated by epidermal growth factor receptor and Shc. Mol Cell Biol 18(9), 5178−5188. Chinnadurai, G. (2003). CtBP family proteins: more than transcriptional corepressors. Bioessays 25(1), 9−12. Cho, H. Y., Jedlicka, A. E., Reddy, S. P., Zhang, L. Y., Kensler, T. W., & Kleeberger, S. R. (2002). Linkage analysis of susceptibility to hyperoxia. Nrf2 is a candidate gene. Am J Respir Cell Mol Biol 26(1), 42−51. Chojkier, M. (2005). Troglitazone and liver injury: in search of answers. Hepatology 41 (2), 237−246. Colman, M. S., Afshari, C. A., & Barrett, J. C. (2000). Regulation of p53 stability and activity in response to genotoxic stress. Mutat Res 462(2–3), 179−188. Criddle, D. N., Gillies, S., Baumgartner-Wilson, H. K., Jaffar, M., Chinje, E. C., Passmore, S., et al. (2006). Menadione-induced reactive oxygen species generation via redox cycling promotes apoptosis of murine pancreatic acinar cells. J Biol Chem 281(52), 40485−40492. Cullinan, S. B., Gordan, J. D., Jin, J., Harper, J. W., & Diehl, J. A. (2004). The Keap1-BTB protein is an adaptor that bridges Nrf2 to a Cul3-based E3 ligase: oxidative stress sensing by a Cul3-Keap1 ligase. Mol Cell Biol 24(19), 8477−8486.

Q. Ma / Pharmacology & Therapeutics 125 (2010) 376–393 Dalton, T. P., Li, Q., Bittel, D., Liang, L., & Andrews, G. K. (1996). Oxidative stress activates metal-responsive transcription factor-1 binding activity. Occupancy in vivo of metal response elements in the metallothionein-I gene promoter. J Biol Chem 271 (42), 26233−26241. Dawson, T. M., & Dawson, V. L. (2003). Molecular pathways of neurodegeneration in Parkinson's disease. Science 302(5646), 819−822. Dayan, A. D., & Paine, A. J. (2001). Mechanisms of chromium toxicity, carcinogenicity and allergenicity: review of the literature from 1985 to 2000. Hum Exp Toxicol 20 (9), 439−451. Dinkova-Kostova, A. T., Holtzclaw, W. D., Cole, R. N., Itoh, K., Wakabayashi, N., & Katoh, Y. (2002). Direct evidence that sulfhydryl groups of Keap1 are the sensors regulating induction of phase 2 enzymes that protect against carcinogens and oxidants. Proc Natl Acad Sci U S A 99(18), 11908−11913. Dinkova-Kostova, A. T., Jenkins, S. N., Fahey, J. W., Ye, L., Wehage, S. L., Liby, K. T., et al. (2006). Protection against UV-light-induced skin carcinogenesis in SKH-1 high-risk mice by sulforaphane-containing broccoli sprout extracts. Cancer Lett 240(2), 243−252. Dinkova-Kostova, A. T., Liby, K. T., Stephenson, K. K., Holtzclaw, W. D., Gao, X., Suh, N., et al. (2005). Extremely potent triterpenoid inducers of the phase 2 response: correlations of protection against oxidant and inflammatory stress. Proc Natl Acad Sci U S A 102 (12), 4584−4589. Drane, P., Bravard, A., Bouvard, V., & May, E. (2001). Reciprocal down-regulation of p53 and SOD2 gene expression-implication in p53 mediated apoptosis. Oncogene 20 (4), 430−439. Droge, W. (2002). Free radicals in the physiological control of cell function. Physiol Rev 82(1), 47−95. Dumont, A., Hehner, S. P., Hofmann, T. G., Ueffing, M., Droge, W., & Schmitz, M. L. (1999). Hydrogen peroxide-induced apoptosis is CD95-independent, requires the release of mitochondria-derived reactive oxygen species and the activation of NF-kappaB. Oncogene 18(3), 747−757. Eggler, A. L., Liu, G., Pezzuto, J. M., van Breemen, R. B., & Mesecar, A. D. (2005). Modifying specific cysteines of the electrophile-sensing human Keap1 protein is insufficient to disrupt binding to the Nrf2 domain Neh2. Proc Natl Acad Sci U S A 102(29), 10070−10075. Enster, L. (1986). Oxygen as an environmental poison. Chem Scr 26, 525−534. Fahey, J. W., Haristoy, X., Dolan, P. M., Kensler, T. W., Scholtus, I., Stephenson, K. K., et al. (2002). Sulforaphane inhibits extracellular, intracellular, and antibiotic-resistant strains of Helicobacter pylori and prevents benzo[a]pyrene-induced stomach tumors. Proc Natl Acad Sci U S A 99(11), 7610−7615. Falkowski, P. G., Katz, M. E., Milligan, A. J., Fennel, K., Cramer, B. S., Aubry, M. P., et al. (2005). The rise of oxygen over the past 205 million years and the evolution of large placental mammals. Science 309(5744), 2202−2204. Finkel, T. (1998). Oxygen radicals and signaling. Curr Opin Cell Biol 10(2), 248−253. Finkel, T. (2005). Radical medicine: treating ageing to cure disease. Nat Rev Mol Cell Biol 6(12), 971−976. Flatt, P. M., Polyak, K., Tang, L. J., Scatena, C. D., Westfall, M. D., Rubinstein, L. A., et al. (2000). p53-dependent expression of PIG3 during proliferation, genotoxic stress, and reversible growth arrest. Cancer Lett 156(1), 63−72. Fritz, G., Grosch, S., Tomicic, M., & Kaina, B. (2003). APE/Ref-1 and the mammalian response to genotoxic stress. Toxicology 193(1–2), 67−78. Fubini, B., & Hubbard, A. (2003). Reactive oxygen species (ROS) and reactive nitrogen species (RNS) generation by silica in inflammation and fibrosis. Free Radic Biol Med 34(12), 1507−1516. Fuchs, S. Y., Adler, V., Pincus, M. R., & Ronai, Z. (1998). MEKK1/JNK signaling stabilizes and activates p53. Proc Natl Acad Sci U S A 95(18), 10541−10546. Fulco, M., Schiltz, R. L., Iezzi, S., King, M. T., Zhao, P., Kashiwaya, Y., et al. (2003). Sir2 regulates skeletal muscle differentiation as a potential sensor of the redox state. Mol Cell 12(1), 51−62. Furukawa-Hibi, Y., Kobayashi, Y., Chen, C., & Motoyama, N. (2005). FOXO transcription factors in cell-cycle regulation and the response to oxidative stress. Antioxid Redox Signal 7(5–6), 752−760. Gerald, D., Berra, E., Frapart, Y. M., Chan, D. A., Giaccia, A. J., Mansuy, D., et al. (2004). JunD reduces tumor angiogenesis by protecting cells from oxidative stress. Cell 118 (6), 781−794. Giedroc, D. P., Chen, X., & Apuy, J. L. (2001). Metal response element (MRE)-binding transcription factor-1 (MTF-1): structure, function, and regulation. Antioxid Redox Signal 3(4), 577−596. Gilmore, T. D. (2006). Introduction to NF-kappaB: players, pathways, perspectives. Oncogene 25(51), 6680−6684. Goldfine, H. (1965). The evolution of oxygen as a biosynthetic reagent. J Gen Physiol 49 (Suppl. 1), 253−274. Gopalakrishna, R., Chen, Z. H., & Gundimeda, U. (1997). Selenocompounds induce a redox modulation of protein kinase C in the cell, compartmentally independent from cytosolic glutathione: its role in inhibition of tumor promotion. Arch Biochem Biophys 348(1), 37−48. Gopalakrishna, R., & Jaken, S. (2000). Protein kinase C signaling and oxidative stress. Free Radic Biol Med 28(9), 1349−1361. Gorman, A. M., Heavey, B., Creagh, E., Cotter, T. G., & Samali, A. (1999). Antioxidantmediated inhibition of the heat shock response leads to apoptosis. FEBS Lett 445(1), 98−102. Greer, E. L., & Brunet, A. (2005). FOXO transcription factors at the interface between longevity and tumor suppression. Oncogene 24(50), 7410−7425. Grune, T., Jung, T., Merker, K., & Davies, K. J. (2004). Decreased proteolysis caused by protein aggregates, inclusion bodies, plaques, lipofuscin, ceroid, and ‘aggresomes’ during oxidative stress, aging, and disease. Int J Biochem Cell Biol 36(12), 2519−2530. Harman, D. (1956). Aging: a theory based on free radical and radiation chemistry. J Gerontol 11(3), 298−300.

391

Hayden, M. S., West, A. P., & Ghosh, S. (2006). NF-kappaB and the immune response. Oncogene 25(51), 6758−6780. He, X., & Ma, Q. (2009a). Critical cysteine residues of Kelch-like ECH-associated protein 1 in arsenic-sensing and suppression of nuclear factor erythroid 2-related factor 2. JPET Fast Forward. Published on Oct. 6, 2009 as doi:10.1124/jpet.109.160465. He, X., & Ma, Q. (2009b). Induction of Metallothionein I by Arsenic via Metal-activated Transcription Factor 1: critical role of C-terminal cysteine residues in arsenic sensing. J Biol Chem 284(19), 12609−12621. He, X., & Ma, Q. (2009c). Nrf2 cysteine residues are critical for oxidant/electrophilesensing, Kelch-like ECHassociated protein-1-dependent ubiquitination-proteasomal degradation, and transcription activation. Mol Pharmacol 76(6), 1265−1278. He, X., Chen, M. G., Lin, G. X., & Ma, Q. (2006). Arsenic induces NAD(P)H-quinone oxidoreductase I by disrupting the Nrf2•Keap1•Cul3 complex and recruiting Nrf2•Maf to the antioxidant response element enhancer. J Biol Chem 281(33), 23620−23631. He, X., Chen, M. G., & Ma, Q. (2008). Activation of Nrf2 in defense against cadmiuminduced oxidative stress. Chem Res Toxicol 21(7), 1375−1383. He, X., Kan, H., Cai, L., & Ma, Q. (2009). Nrf2 is critical in defense against high glucoseinduced oxidative damage in cardiomyocytes. J Mol Cell Cardiol 46(1), 47−58. He, X., Lin, G. X., Chen, M. G., Zhang, J. X., & Ma, Q. (2007). Protection against chromium (VI)-induced oxidative stress and apoptosis by Nrf2. Recruiting Nrf2 into the nucleus and disrupting the nuclear Nrf2/Keap1 association. Toxicol Sci 98(1), 298−309. Hess, J., Angel, P., & Schorpp-Kistner, M. (2004). AP-1 subunits: quarrel and harmony among siblings. J Cell Sci 117(Pt 25), 5965−5973. Hirota, K., Matsui, M., Iwata, S., Nishiyama, A., Mori, K., & Yodoi, J. (1997). AP-1 transcriptional activity is regulated by a direct association between thioredoxin and Ref-1. Proc Natl Acad Sci U S A 94(8), 3633−3638. Hirota, K., Murata, M., Sachi, Y., Nakamura, H., Takeuchi, J., Mori, K., et al. (1999). Distinct roles of thioredoxin in the cytoplasm and in the nucleus. A two-step mechanism of redox regulation of transcription factor NF-kappaB. J Biol Chem 274 (39), 27891−27897. Hong, F., Sekhar, K. R., Freeman, M. L., & Liebler, D. C. (2005). Specific patterns of electrophile adduction trigger Keap1 ubiquitination and Nrf2 activation. J Biol Chem 280(36), 31768−31775. Hsich, E., Segal, B. H., Pagano, P. J., Rey, F. E., Paigen, B., Deleonardis, J., et al. (2000). Vascular effects following homozygous disruption of p47(phox): an essential component of NADPH oxidase. Circulation 101(11), 1234−1236. Hsu, L. J., Sagara, Y., Arroyo, A., Rockenstein, E., Sisk, A., Mallory, M., et al. (2000). alphasynuclein promotes mitochondrial deficit and oxidative stress. Am J Pathol 157(2), 401−410. Hu, X., Roberts, J. R., Apopa, P. L., Kan, Y. W., & Ma, Q. (2006). Accelerated ovarian failure induced by 4-vinyl cyclohexene diepoxide in Nrf2 null mice. Mol Cell Biol 26(3), 940−954. Hubbs, A. F., Benkovic, S. A., Miller, D. B., O'Callaghan, J. P., Battelli, L., Schwegler-Berry, D., et al. (2007). Vacuolar leukoencephalopathy with widespread astrogliosis in mice lacking transcription factor Nrf2. Am J Pathol 170(6), 2068−2076. Irani, K., Xia, Y., Zweier, J. L., Sollott, S. J., Der, C. J., Fearon, E. R., et al. (1997). Mitogenic signaling mediated by oxidants in Ras-transformed fibroblasts. Science 275(5306), 1649−1652. Janssen-Heininger, Y. M., Poynter, M. E., & Baeuerle, P. A. (2000). Recent advances towards understanding redox mechanisms in the activation of nuclear factor kappaB. Free Radic Biol Med 28(9), 1317−1327. Johnson, T. M., Yu, Z. X., Ferrans, V. J., Lowenstein, R. A., & Finkel, T. (1996). Reactive oxygen species are downstream mediators of p53-dependent apoptosis. Proc Natl Acad Sci U S A 93(21), 11848−11852. Jolly, C., & Morimoto, R. I. (2000). Role of the heat shock response and molecular chaperones in oncogenesis and cell death. J Natl Cancer Inst 92(19), 1564−1572. Kandel, E. S., & Hay, N. (1999). The regulation and activities of the multifunctional serine/threonine kinase Akt/PKB. Exp Cell Res 253(1), 210−229. Kang, S. W., Chae, H. Z., Seo, M. S., Kim, K., Baines, I. C., & Rhee, S. G. (1998). Mammalian peroxiredoxin isoforms can reduce hydrogen peroxide generated in response to growth factors and tumor necrosis factor-alpha. J Biol Chem 273(11), 6297−6302. Kang, M. I., Kobayashi, A., Wakabayashi, N., Kim, S. G., & Yamamoto, M. (2004). Scaffolding of Keap1 to the actin cytoskeleton controls the function of Nrf2 as key regulator of cytoprotective phase 2 genes. Proc Natl Acad Sci U S A 101(7), 2046−2051. Kasprzak, K. S. (2002). Oxidative DNA and protein damage in metal-induced toxicity and carcinogenesis. Free Radic Biol Med 32(10), 958−967. Kensler, T. W., Wakabayashi, N., & Biswal, S. (2007). Cell survival responses to environmental stresses via the Keap1-Nrf2-ARE pathway. Annu Rev Pharmacol Toxicol 47, 89−116. Kilic, E., Saraymen, R., Demiroglu, A., & Ok, E. (2004). Chromium and manganese levels in the scalp hair of normals and patients with breast cancer. Biol Trace Elem Res 102 (1–3), 19−25. Kim, A. H., Khursigara, G., Sun, X., Franke, T. F., & Chao, M. V. (2001). Akt phosphorylates and negatively regulates apoptosis signal-regulating kinase 1. Mol Cell Biol 21(3), 893−901. Kirkham, P., & Rahman, I. (2006). Oxidative stress in asthma and COPD: antioxidants as a therapeutic strategy. Pharmacol Ther 111(2), 476−494. Klaunig, J. E., & Kamendulis, L. M. (2004). The role of oxidative stress in carcinogenesis. Annu Rev Pharmacol Toxicol 44, 239−267. Klotz, L. O., Schieke, S. M., Sies, H., & Holbrook, N. J. (2000). Peroxynitrite activates the phosphoinositide 3-kinase/Akt pathway in human skin primary fibroblasts. Biochem J 352(Pt 1), 219−225. Kobayashi, A., Kang, M. I., Okawa, H., Ohtsuji, M., Zenke, Y., Chiba, T., et al. (2004a). Oxidative stress sensor Keap1 functions as an adaptor for Cul3-based E3 ligase to regulate proteasomal degradation of Nrf2. Mol Cell Biol 24(16), 7130−7139.

392

Q. Ma / Pharmacology & Therapeutics 125 (2010) 376–393

Kobayashi, M., Li, L., Iwamoto, N., Nakajima-Takagi, Y., Kaneko, H., Nakayama, Y., et al. (2009). The antioxidant defense system Keap1-Nrf2 comprises a multiple sensing mechanism for responding to a wide range of chemical compounds. Mol Cell Biol 29 (2), 493−502. Kobayashi, A., Ohta, T., & Yamamoto, M. (2004b). Unique function of the Nrf2-Keap1 pathway in the inducible expression of antioxidant and detoxifying enzymes. Methods Enzymol 378, 273−286. Kops, G. J., Dansen, T. B., Polderman, P. E., Saarloos, I., Wirtz, K. W., Coffer, P. J., et al. (2002). Forkhead transcription factor FOXO3a protects quiescent cells from oxidative stress. Nature 419(6904), 316−321. Kotamraju, S., Konorev, E. A., Joseph, J., & Kalyanaraman, B. (2000). Doxorubicininduced apoptosis in endothelial cells and cardiomyocytes is ameliorated by nitrone spin traps and ebselen. Role of reactive oxygen and nitrogen species. J Biol Chem 275(43), 33585−33592. Kowdley, K. V. (2004). Iron, hemochromatosis, and hepatocellular carcinoma. Gastroenterology 127(5 Suppl 1), S79−S86. Kretz-Remy, C., Mehlen, P., Mirault, M. E., & Arrigo, A. P. (1996). Inhibition of I kappa B-alpha phosphorylation and degradation and subsequent NF-kappa B activation by glutathione peroxidase overexpression. J Cell Biol 133(5), 1083−1093. Kurata, S. (2000). Selective activation of p38 MAPK cascade and mitotic arrest caused by low level oxidative stress. J Biol Chem 275(31), 23413−23416. Kyriakis, J. M., & Avruch, J. (2001). Mammalian mitogen-activated protein kinase signal transduction pathways activated by stress and inflammation. Physiol Rev 81(2), 807−869. Leonard, S. S., Harris, G. K., & Shi, X. (2004). Metal-induced oxidative stress and signal transduction. Free Radic Biol Med 37(12), 1921−1942. Li, H., Ko, H. P., & Whitlock, J. P. (1996). Induction of phosphoglycerate kinase 1 gene expression by hypoxia. Roles of Arnt and HIF1alpha. J Biol Chem 271(35), 21262−21267. Li, N., & Karin, M. (1999). Is NF-kappaB the sensor of oxidative stress? FASEB J 13(10), 1137−1143. Liu, J., Goyer, R. A., & Waalkes, M. P. (2008). Toxic effects of metals. In C. D. Klaassen (Ed.), Casarett and Doull's Toxicology. The Basic Science of Poisons (pp. 931−979)., 7 ed New York: McGraw-Hill Medical. Liu, J., Narasimhan, P., Yu, F., & Chan, P. H. (2005). Neuroprotection by hypoxic preconditioning involves oxidative stress-mediated expression of hypoxia-inducible factor and erythropoietin. Stroke 36(6), 1264−1269. Luchoomun, J., Sinibaldi, D., & Chen, X. -L. (2006). Antioxidant therapy for chronic inflammatory diseases. In H. V. Panglossi (Ed.), Leading Edge Antioxidants Research (pp. 145−177)., 1st ed. Hauppauge, NY: Nova Publishers. Lustbader, J. W., Cirilli, M., Lin, C., Xu, H. W., Takuma, K., Wang, N., et al. (2004). ABAD directly links Abeta to mitochondrial toxicity in Alzheimer's disease. Science 304 (5669), 448−452. Luster, M. I., Simeonova, P. P., Gallucci, R., & Matheson, J. (1999). Tumor necrosis factor alpha and toxicology. Crit Rev Toxicol 29(5), 491−511. Ma, Q. (2001). Induction of CYP1A1. The AhR/DRE paradigm: transcription, receptor regulation, and expanding biological roles. Curr Drug Metab 2(2), 149−164. Ma, Q. (2008). Xenobiotic-activated receptors: from transcription to drug metabolism to disease. Chem Res Toxicol 21(9), 1651−1671. Ma, Q., & Kinneer, K. (2002). Chemoprotection by phenolic antioxidants. Inhibition of tumor necrosis factor alpha induction in macrophages. J Biol Chem 277(4), 2477−2484. Ma, Q., Battelli, L., & Hubbs, A. F. (2006). Multiorgan autoimmune inflammation, enhanced lymphoproliferation, and impaired homeostasis of reactive oxygen species in mice lacking the antioxidant-activated transcription factor Nrf2. Am J Pathol 168(6), 1960−1974. Ma, Q., Kinneer, K., Bi, Y., Chan, J. Y., & Kan, Y. W. (2004). Induction of murine NAD(P)H: quinone oxidoreductase by 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin requires the CNC (cap ‘n’ collar) basic leucine zipper transcription factor Nrf2 (nuclear factor erythroid 2-related factor 2): cross-interaction between AhR (aryl hydrocarbon receptor) and Nrf2 signal transduction. Biochem J 377(Pt 1), 205−213. Ma, Q., Kinneer, K., Ye, J., & Chen, B. J. (2003). Inhibition of nuclear factor {kappa}B by phenolic antioxidants: interplay between antioxidant signaling and inflammatory cytokine expression. Mol Pharmacol 64(2), 211−219. Madamanchi, N. R., Vendrov, A., & Runge, M. S. (2005). Oxidative stress and vascular disease. Arterioscler Thromb Vasc Biol 25(1), 29−38. Manna, S. K., Zhang, H. J., Yan, T., Oberley, L. W., & Aggarwal, B. B. (1998). Overexpression of manganese superoxide dismutase suppresses tumor necrosis factor-induced apoptosis and activation of nuclear transcription factor-kappaB and activated protein-1. J Biol Chem 273(21), 13245−13254. Martinat, C., Shendelman, S., Jonason, A., Leete, T., Beal, M. F., Yang, L., et al. (2004). Sensitivity to oxidative stress in DJ-1-deficient dopamine neurons: an ES-derived cell model of primary Parkinsonism. PLoS Biol 2(11), e327. Martindale, J. L., & Holbrook, N. J. (2002). Cellular response to oxidative stress: signaling for suicide and survival. J Cell Physiol 192(1), 1−15. Maxwell, P. H. (2005). Hypoxia-inducible factor as a physiological regulator. Exp Physiol 90(6), 791−797. Meplan, C., Richard, M. J., & Hainaut, P. (2000). Redox signalling and transition metals in the control of the p53 pathway. Biochem Pharmacol 59(1), 25−33. Moi, P., Chan, K., Asunis, I., Cao, A., & Kan, Y. W. (1994). Isolation of NF-E2-related factor 2 (Nrf2), a NF-E2-like basic leucine zipper transcriptional activator that binds to the tandem NF-E2/AP1 repeat of the beta-globin locus control region. Proc Natl Acad Sci U S A 91(21), 9926−9930. Nelson, K. K., & Melendez, J. A. (2004). Mitochondrial redox control of matrix metalloproteinases. Free Radic Biol Med 37(6), 768−784. Nemoto, S., & Finkel, T. (2002). Redox regulation of forkhead proteins through a p66shc-dependent signaling pathway. Science 295(5564), 2450−2452.

Nemoto, S., Takeda, K., Yu, Z. X., Ferrans, V. J., & Finkel, T. (2000). Role for mitochondrial oxidants as regulators of cellular metabolism. Mol Cell Biol 20(19), 7311−7318. Neumann, C. A., Krause, D. S., Carman, C. V., Das, S., Dubey, D. P., Abraham, J. L., et al. (2003). Essential role for the peroxiredoxin Prdx1 in erythrocyte antioxidant defence and tumour suppression. Nature 424(6948), 561−565. Nguyen, T., Sherratt, P. J., & Pickett, C. B. (2003). Regulatory mechanisms controlling gene expression mediated by the antioxidant response element. Annu Rev Pharmacol Toxicol 43, 233−260. Nishikawa, T., Edelstein, D., Du, X. L., Yamagishi, S., Matsumura, T., Kaneda, Y., et al. (2000). Normalizing mitochondrial superoxide production blocks three pathways of hyperglycaemic damage. Nature 404(6779), 787−790. Nishizuka, Y. (1992). Intracellular signaling by hydrolysis of phospholipids and activation of protein kinase C. Science 258(5082), 607−614. Nystrom, T. (2005). Role of oxidative carbonylation in protein quality control and senescence. EMBO J 24(7), 1311−1317. O'Brien, P. J. (1991). Molecular mechanisms of quinone cytotoxicity. Chem Biol Interact 80(1), 1−41. Ohta, T., Iijima, K., Miyamoto, M., Nakahara, I., Tanaka, H., Ohtsuji, M., et al. (2008). Loss of Keap1 function activates Nrf2 and provides advantages for lung cancer cell growth. Cancer Res 68(5), 1303−1309. Okuno, H., Akahori, A., Sato, H., Xanthoudakis, S., Curran, T., & Iba, H. (1993). Escape from redox regulation enhances the transforming activity of Fos. Oncogene 8(3), 695−701. Ong, M. M., Latchoumycandane, C., & Boelsterli, U. A. (2007). Troglitazone-induced hepatic necrosis in an animal model of silent genetic mitochondrial abnormalities. Toxicol Sci 97(1), 205−213. Orsini, F., Migliaccio, E., Moroni, M., Contursi, C., Raker, V. A., Piccini, D., et al. (2004). The life span determinant p66Shc localizes to mitochondria where it associates with mitochondrial heat shock protein 70 and regulates trans-membrane potential. J Biol Chem 279(24), 25689−25695. Packer, L., & Fuehr, K. (1977). Low oxygen concentration extends the lifespan of cultured human diploid cells. Nature 267(5610), 423−425. Pahl, H. L. (1999). Activators and target genes of Rel/NF-kappaB transcription factors. Oncogene 18(49), 6853−6866. Palacino, J. J., Sagi, D., Goldberg, M. S., Krauss, S., Motz, C., Wacker, M., et al. (2004). Mitochondrial dysfunction and oxidative damage in parkin-deficient mice. J Biol Chem 279(18), 18614−18622. Palmiter, R. D. (1994). Regulation of metallothionein genes by heavy metals appears to be mediated by a zinc-sensitive inhibitor that interacts with a constitutively active transcription factor, MTF-1. Proc Natl Acad Sci U S A 91(4), 1219−1223. Palmiter, R. D. (1998). The elusive function of metallothioneins. Proc Natl Acad Sci U S A 95(15), 8428−8430. Perkins, N. D. (2007). Integrating cell-signalling pathways with NF-kappaB and IKK function. Nat Rev Mol Cell Biol 8(1), 49−62. Pirkkala, L., Nykanen, P., & Sistonen, L. (2001). Roles of the heat shock transcription factors in regulation of the heat shock response and beyond. FASEB J 15(7), 1118−1131. Pugh, C. W., Gleadle, J., & Maxwell, P. H. (2001). Hypoxia and oxidative stress in breast cancer. Hypoxia signalling pathways. Breast Cancer Res 3(5), 313−317. Radtke, F., Heuchel, R., Georgiev, O., Hergersberg, M., Gariglio, M., Dembic, Z., et al. (1993). Cloned transcription factor MTF-1 activates the mouse metallothionein I promoter. EMBO J 12(4), 1355−1362. Rahman, I. (2005). Oxidative stress in pathogenesis of chronic obstructive pulmonary disease: cellular and molecular mechanisms. Cell Biochem Biophys 43(1), 167−188. Ramos-Gomez, M., Kwak, M. K., Dolan, P. M., Itoh, K., Yamamoto, M., Talalay, P., et al. (2001). Sensitivity to carcinogenesis is increased and chemoprotective efficacy of enzyme inducers is lost in nrf2 transcription factor-deficient mice. Proc Natl Acad Sci U S A 98(6), 3410−3415. Raymond, J., & Segre, D. (2006). The effect of oxygen on biochemical networks and the evolution of complex life. Science 311(5768), 1764−1767. Richter, C., Park, J. W., & Ames, B. N. (1988). Normal oxidative damage to mitochondrial and nuclear DNA is extensive. Proc Natl Acad Sci U S A 85(17), 6465−6467. Rosen, D. R., Siddique, T., Patterson, D., Figlewicz, D. A., Sapp, P., Hentati, A., et al. (1993). Mutations in Cu/Zn superoxide dismutase gene are associated with familial amyotrophic lateral sclerosis. Nature 362(6415), 59−62. Rutter, J., Reick, M., & McKnight, S. L. (2002). Metabolism and the control of circadian rhythms. Annu Rev Biochem 71, 307−331. Rutter, J., Reick, M., Wu, L. C., & McKnight, S. L. (2001). Regulation of clock and NPAS2 DNA binding by the redox state of NAD cofactors. Science 293(5529), 510−514. Sachsenmaier, C., Radler-Pohl, A., Zinck, R., Nordheim, A., Herrlich, P., & Rahmsdorf, H. J. (1994). Involvement of growth factor receptors in the mammalian UVC response. Cell 78(6), 963−972. Saitoh, M., Nishitoh, H., Fujii, M., Takeda, K., Tobiume, K., Sawada, Y., et al. (1998). Mammalian thioredoxin is a direct inhibitor of apoptosis signal-regulating kinase (ASK) 1. EMBO J 17(9), 2596−2606. Saltiel, E., & McGuire, W. (1983). Doxorubicin (adriamycin) cardiomyopathy. West J Med 139(3), 332−341. Schenk, H., Klein, M., Erdbrugger, W., Droge, W., & Schulze-Osthoff, K. (1994). Distinct effects of thioredoxin and antioxidants on the activation of transcription factors NF-kappa B and AP-1. Proc Natl Acad Sci U S A 91(5), 1672−1676. Schofield, C. J., & Ratcliffe, P. J. (2004). Oxygen sensing by HIF hydroxylases. Nat Rev Mol Cell Biol 5(5), 343−354. Schoonbroodt, S., Ferreira, V., Best-Belpomme, M., Boelaert, J. R., Legrand-Poels, S., Korner, M., et al. (2000). Crucial role of the amino-terminal tyrosine residue 42 and the carboxyl-terminal PEST domain of I kappa B alpha in NF-kappa B activation by an oxidative stress. J Immunol 164(8), 4292−4300.

Q. Ma / Pharmacology & Therapeutics 125 (2010) 376–393 Serra, V., von Zglinicki, T., Lorenz, M., & Saretzki, G. (2003). Extracellular superoxide dismutase is a major antioxidant in human fibroblasts and slows telomere shortening. J Biol Chem 278(9), 6824−6830. Serrano, M., Lin, A. W., McCurrach, M. E., Beach, D., & Lowe, S. W. (1997). Oncogenic ras provokes premature cell senescence associated with accumulation of p53 and p16INK4a. Cell 88(5), 593−602. Shigenaga, M. K., Hagen, T. M., & Ames, B. N. (1994). Oxidative damage and mitochondrial decay in aging. Proc Natl Acad Sci U S A 91(23), 10771−10778. Siah, C. W., Trinder, D., & Olynyk, J. K. (2005). Iron overload. Clin Chim Acta 358(1–2), 24−36. Singal, P. K., & Iliskovic, N. (1998). Doxorubicin-induced cardiomyopathy. N Engl J Med 339(13), 900−905. Sionov, R. V., & Haupt, Y. (1999). The cellular response to p53: the decision between life and death. Oncogene 18(45), 6145−6157. Skulachev, V. P. (1996). Role of uncoupled and non-coupled oxidations in maintenance of safely low levels of oxygen and its one-electron reductants. Q Rev Biophys 29(2), 169−202. Speakman, J. R., Talbot, D. A., Selman, C., Snart, S., McLaren, J. S., Redman, P., et al. (2004). Uncoupled and surviving: individual mice with high metabolism have greater mitochondrial uncoupling and live longer. Aging Cell 3(3), 87−95. St-Pierre, J., Buckingham, J. A., Roebuck, S. J., & Brand, M. D. (2002). Topology of superoxide production from different sites in the mitochondrial electron transport chain. J Biol Chem 277(47), 44784−44790. Stadtman, E. R. (1992). Protein oxidation and aging. Science 257(5074), 1220−1224. Staniek, K., & Nohl, H. (2000). Are mitochondria a permanent source of reactive oxygen species? Biochim Biophys Acta 1460(2–3), 268−275. Stayner, L. T., Dankovic, D. A., & Lemen, R. A. (1996). Occupational exposure to chrysotile asbestos and cancer risk: a review of the amphibole hypothesis. Am J Public Health 86 (2), 179−186. Storz, P. (2005). Reactive oxygen species in tumor progression. Front Biosci 10, 1881−1896. Szatrowski, T. P., & Nathan, C. F. (1991). Production of large amounts of hydrogen peroxide by human tumor cells. Cancer Res 51(3), 794−798. Talalay, P. (2005). A fascination with enzymes: the journey not the arrival matters. J Biol Chem 280(32), 28829−28847. Tan, M., Li, S., Swaroop, M., Guan, K., Oberley, L. W., & Sun, Y. (1999). Transcriptional activation of the human glutathione peroxidase promoter by p53. J Biol Chem 274 (17), 12061−12066. Taylor, W. R., & Stark, G. R. (2001). Regulation of the G2/M transition by p53. Oncogene 20(15), 1803−1815. Tell, G., Damante, G., Caldwell, D., & Kelley, M. R. (2005). The intracellular localization of APE1/Ref-1: more than a passive phenomenon? Antioxid Redox Signal 7(3–4), 367−384. Thannickal, V. J. (2009). Oxygen in the evolution of complex life and the price we pay. Am J Respir Cell Mol Biol 40(5), 507−510. Tobiume, K., Matsuzawa, A., Takahashi, T., Nishitoh, H., Morita, K., Takeda, K., et al. (2001). ASK1 is required for sustained activations of JNK/p38 MAP kinases and apoptosis. EMBO Rep 2(3), 222−228. Toker, A., & Cantley, L. C. (1997). Signalling through the lipid products of phosphoinositide-3-OH kinase. Nature 387(6634), 673−676. Tournier, C., Hess, P., Yang, D. D., Xu, J., Turner, T. K., Nimnual, A., et al. (2000). Requirement of JNK for stress-induced activation of the cytochrome c-mediated death pathway. Science 288(5467), 870−874. Trifunovic, A., Wredenberg, A., Falkenberg, M., Spelbrink, J. N., Rovio, A. T., Bruder, C. E., et al. (2004). Premature ageing in mice expressing defective mitochondrial DNA polymerase. Nature 429(6990), 417−423. Turrens, J. F. (1997). Superoxide production by the mitochondrial respiratory chain. Biosci Rep 17(1), 3−8. Vafa, O., Wade, M., Kern, S., Beeche, M., Pandita, T. K., Hampton, G. M., et al. (2002). c-Myc can induce DNA damage, increase reactive oxygen species, and mitigate p53 function: a mechanism for oncogene-induced genetic instability. Mol Cell 9(5), 1031−1044. Valente, E. M., Abou-Sleiman, P. M., Caputo, V., Muqit, M. M., Harvey, K., Gispert, S., et al. (2004). Hereditary early-onset Parkinson's disease caused by mutations in PINK1. Science 304(5674), 1158−1160. Valko, M., Morris, H., & Cronin, M. T. (2005). Metals, toxicity and oxidative stress. Curr Med Chem 12(10), 1161−1208. Valko, M., Morris, H., Mazur, M., Rapta, P., & Bilton, R. F. (2001). Oxygen free radical generating mechanisms in the colon: do the semiquinones of vitamin K play a role in the aetiology of colon cancer? Biochim Biophys Acta 1527(3), 161−166.

393

Van Remmen, H., Ikeno, Y., Hamilton, M., Pahlavani, M., Wolf, N., Thorpe, S. R., et al. (2003). Life-long reduction in MnSOD activity results in increased DNA damage and higher incidence of cancer but does not accelerate aging. Physiol Genomics 16(1), 29−37. Vidal-Puig, A. J., Grujic, D., Zhang, C. Y., Hagen, T., Boss, O., Ido, Y., et al. (2000). Energy metabolism in uncoupling protein 3 gene knockout mice. J Biol Chem 275(21), 16258−16266. Wang, G. L., Jiang, B. H., Rue, E. A., & Semenza, G. L. (1995). Hypoxia-inducible factor 1 is a basic-helix–loop–helix-PAS heterodimer regulated by cellular O2 tension. Proc Natl Acad Sci U S A 92(12), 5510−5514. Wang, S., & Shi, X. (2001). Mechanisms of Cr(VI)-induced p53 activation: the role of phosphorylation, mdm2 and ERK. Carcinogenesis 22(5), 757−762. Wang, X., McCullough, K. D., Franke, T. F., & Holbrook, N. J. (2000). Epidermal growth factor receptor-dependent Akt activation by oxidative stress enhances cell survival. J Biol Chem 275(19), 14624−14631. Wang, Y., Wimmer, U., Lichtlen, P., Inderbitzin, D., Stieger, B., Meier, P. J., et al. (2004). Metalresponsive transcription factor-1 (MTF-1) is essential for embryonic liver development and heavy metal detoxification in the adult liver. Faseb J 18(10), 1071−1079. Watkins, P. B. (2005). Insight into hepatotoxicity: the troglitazone experience. Hepatology 41(2), 229−230. WHO (1992). IPCS Environmental Health Criteria Vol 134. World Health Organization, Geneva: Cadmium. Wilson, F. H., Hariri, A., Farhi, A., Zhao, H., Petersen, K. F., Toka, H. R., et al. (2004). A cluster of metabolic defects caused by mutation in a mitochondrial tRNA. Science 306(5699), 1190−1194. Wimmer, U., Wang, Y., Georgiev, O., & Schaffner, W. (2005). Two major branches of anti-cadmium defense in the mouse: MTF-1/metallothioneins and glutathione. Nucleic Acids Res 33(18), 5715−5727. Wong, H. R., Menendez, I. Y., Ryan, M. A., Denenberg, A. G., & Wispe, J. R. (1998). Increased expression of heat shock protein-70 protects A549 cells against hyperoxia. Am J Physiol 275(4 Pt 1), L836−841. Woo, R. A., & Poon, R. Y. (2004). Activated oncogenes promote and cooperate with chromosomal instability for neoplastic transformation. Genes Dev 18(11), 1317−1330. Wu, C., Miloslavskaya, I., Demontis, S., Maestro, R., & Galaktionov, K. (2004). Regulation of cellular response to oncogenic and oxidative stress by Seladin-1. Nature 432 (7017), 640−645. Xanthoudakis, S., Miao, G., Wang, F., Pan, Y. C., & Curran, T. (1992). Redox activation of Fos-Jun DNA binding activity is mediated by a DNA repair enzyme. EMBO J 11(9), 3323−3335. Xanthoudakis, S., Miao, G. G., & Curran, T. (1994). The redox and DNA-repair activities of Ref-1 are encoded by nonoverlapping domains. Proc Natl Acad Sci U S A 91(1), 23−27. Yamamoto, T., Suzuki, T., Kobayashi, A., Wakabayashi, J., Maher, J., Motohashi, H., et al. (2008). Physiological significance of reactive cysteine residues of Keap1 in determining Nrf2 activity. Mol Cell Biol 28(8), 2758−2770. Yao, H., Guo, L., Jiang, B. H., Luo, J., & Shi, X. (2008). Oxidative stress and chromium(VI) carcinogenesis. J Environ Pathol Toxicol Oncol 27(2), 77−88. Zhang, B., Georgiev, O., Hagmann, M., Gunes, C., Cramer, M., Faller, P., et al. (2003). Activity of metal-responsive transcription factor 1 by toxic heavy metals and H2O2 in vitro is modulated by metallothionein. Mol Cell Biol 23(23), 8471−8485. Zhang, D. D., Lo, S. C., Cross, J. V., Templeton, D. J., & Hannink, M. (2004). Keap1 is a redox-regulated substrate adaptor protein for a Cul3-dependent ubiquitin ligase complex. Mol Cell Biol 24(24), 10941−10953. Zhang, J., Johnston, G., Stebler, B., & Keller, E. T. (2001). Hydrogen peroxide activates NFkappaB and the interleukin-6 promoter through NFkappaB-inducing kinase. Antioxid Redox Signal 3(3), 493−504. Zhang, Q., Piston, D. W., & Goodman, R. H. (2002). Regulation of corepressor function by nuclear NADH. Science 295(5561), 1895−1897. Zhuang, S., Demirs, J. T., & Kochevar, I. E. (2000). p38 mitogen-activated protein kinase mediates bid cleavage, mitochondrial dysfunction, and caspase-3 activation during apoptosis induced by singlet oxygen but not by hydrogen peroxide. J Biol Chem 275 (34), 25939−25948. Ziel, K. A., Grishko, V., Campbell, C. C., Breit, J. F., Wilson, G. L., & Gillespie, M. N. (2005). Oxidants in signal transduction: impact on DNA integrity and gene expression. FASEB J 19(3), 387−394.