- Email: [email protected]
Epigenetics, sirtuins and osteoarthritis
Joint Bone Spine 79 (2012) 570–573
Available online at
www.sciencedirect.com
Review
Epigenetics, sirtuins and osteoarthritis Odile Gabay a,∗ , Christelle Sanchez b a b
Cartilage Biology and Orthopedics Branch, National Institute of Arthritis, Musculoskeletal and Skin Disease, NIH, Bldg 50, Bethesda, MD 20892, USA Bone and Cartilage Research Unit, Institute of Pathology B23, University of Liege, Liege, Belgium
a r t i c l e
i n f o
Article history: Accepted 11 April 2012 Available online 26 June 2012 Keywords: Chromatin-modifying enzymes Sirt1 OA Cartilage Sirtuins
a b s t r a c t Epigenetics, modifications of the DNA other than changes on the DNA sequences, is frequently studied in cancer research and aging. DNA methylation, mi-RNA, and histones deacetylation are investigated in different pathologies, including inflammatory diseases and age-related diseases such as osteoarthritis (OA). In this review, we focus on the chromatin-modifying enzymes in arthritic pathologies, and more particularly on Sirtuins. We also review the role of Sirt1 in OA, which has been highlighted in recent publications, and examine the possible protective role Sirt1 could play in this disease. Moreover, we discuss the possible therapeutic target of such a protein, reviewing the potential inhibitors/activators of this enzyme and their properties. © 2012 Société franc¸aise de rhumatologie. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction Osteoarthritis (OA) is a complex multifactorial disease with a tissular inflammatory phase, leading to cartilage degeneration and ultimately to joint destruction. Age and obesity are the main risk factors [1]. One risk factor recently focused on is genetic. A region on chromosome 7q22 has been associated with OA susceptibility [2]. Already a decade ago, studies in twins have shown showing a genetic component in OA disease [3–5]. Now a new field of investigation, research is focused on epigenetics, changes occurring on the chromatin and histones. Epigenetics, which can be defined as: “heritable changes in gene expression or cellular phenotypes caused by mechanisms others than changes on the DNA sequences”, is intensively studied today. Originally limited to the area of cancer research, epigenetics has become an actively developed topic in the study of arthritis and osteoarthritis. Modifications of the chromatin include DNA methylation, micro-RNA and histones deacetylation [6]. These molecules are known as chromatin-modifying enzymes. Briefly, DNA methylation seems to be influenced by environmental factors. It is mediated by DNA methyl transferases (DNMT1, -3a, -3b). A methyl group (CH3) is added to the DNA near the promoters of genes and suppresses the gene expression or silences it (Fig. 1A). To date, little is known in cartilage about the importance of this chromatin modification. Recently, a total DNA methylation was performed by Sesselmann et al. and the analysis in control versus OA cartilage by chromatography shows no differences [7].
∗ Corresponding author. Tel.: +1 301 443 5406. E-mail address: [email protected] (O. Gabay).
However, a fine downregulation of DNA methylation on matrix metalloproteinase (MMP)-3, -9, -13 and a desintegrin and a metalloproteinase with thrombospondin motif (ADAMTS)-4 promoters has been previously shown in OA compared to normal cartilage, possibly associated with change in their expression [8]. MicroRNA or miRNA are small, noncoding RNA consisting of 20–23 nucleotides single strand, regulating gene expression, recently discovered. They are exported from the nucleus and play major roles in the regulation of development or various pathologies (Fig. 1B). The first miRNA of interest in cartilage and widely studied is miR140. MiR140 regulates insulin-like growth factor binding protein (IGFBP)-5, ADAMTS-5 and MMP-13 genes [9], and modulates interleukin (IL)-1 beta response increasing aggrecan expression [10]. MiR140 targets histone deacetylase (HDAC) 4, which controls chondrocyte hypertrophy during skeletogenesis [11]. It appears to play a role in cartilage homeostasis [12] and to protect against cartilage damage. Today, along with miR140, a panel of other miRNA is studied in cartilage and OA, as miR453, miR221, mi-R323-3p (unpublished data, 2011). A recent paper by Iliopoulos et al. studied the potential involvement and role of 16 miRNA in OA, including miR-16, -22, -23b, -25, -26a, -29a, -30b, -103, -210, -223, -337, -373, -377, -483, -509 [13,14]. Acetylation is mediated by histones acetyl transferases (HATs), allowing access to the transcriptional machinery. There are two types of deacetylases: histones deacetylases (HDACs), which use a zinc-catalyzed mechanism of deacetylation; and Sirtuins deacetylases, which are NAD+ dependent [14] (Fig. 1C). Numerous activators/repressors are now studied as well, such as HDAC inhibitors, HDACi [15]. We will focus this review on Sirtuins deacetylases that are NAD+ dependent.
1297-319X/$ – see front matter © 2012 Société franc¸aise de rhumatologie. Published by Elsevier Masson SAS. All rights reserved. doi:10.1016/j.jbspin.2012.04.005
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2. The Sirtuin family and regulation We are just beginning to understand the mechanisms related to Sirtuins and our comprehension remains rudimentary. Fifteen years ago, the silent information regulator 2 (Sir2) gene was shown to extend the lifespan in yeast by repressing genome instability. This discovery led to questions about its potential role in longevity and aging [16]. We know now that Sirtuins are found in all organisms and play key roles in survival and aging. Sir2 is a member of a large family of conserved genes. In mammals, Sirt1 is the homolog of Sir2. In contrast to lower eukariotes, seven Sirtuins isoforms-Sirt1 to Sirt7, characterized by their highly conserved central NAD+ binding and catalytic domain- have been identified in mammals [17]. These seven isoforms range in size from less than 40 kD to more than 100 kD. They share a 300-amino-acids-long conserved catalytic domain. Sirtuins display mono-ADP-ribosyltransferase activity and protein deacetylase activity [18]. Cellular localization differs, depending on the isoform: for Sirt1, 6, and 7, it is mainly nuclear; for Sirt2, it is mainly cytoplasmic; and for Sirt 3, 4, and 5, it is mitochondrial [19]. Sirt1 activity is directed to histones proteins, and more specifically to acetylated lysine residues and other, non-histone targets. Sirtuins de-acetylate on modified lysine residues through a unique mechanism requiring NAD+ cleavage with each reaction cycle. The Sirtuins’ activity has a specific link to the state of the cell, in contrast to HDACs that hydrolize acetyl lysine residues. The reaction begins by a cleavage of NAD+ and formation of nicotinamide (NAM) and ADPribose. Then, it forms O-acetyl ADP-ribose and the deacetylated substrate (Fig. 2). The biosynthetic pathways generating NAD+ are critical in Sirtuins regulation. Stress status and cell type may regulate the Sirtuin cellular localization. Recent publications have shown that Sirt1 is cleaved in chondrocytes under stress condition by TNF-alpha stimulation and exported from the nucleus to the cytoplasm [20,21]. Sirtuins inhibitors/activators, such as deleted in breast cancer 1 (DBC1) are actively studied.
Fig. 2. Mechanism of deacetylation by Sirtuins NAD+ dependant. Sirt 1 deacetylases Lysine residues (removing the CH3) on the DNA (or other substrate) by a chemical process coupled to the conversion of nicotinamide adenine dinucleotide, NAD+ contained into the cell, into Nam (Nicotinamide).
3. Sirtuin and age-related diseases Aging increases susceptibility to a variety of diseases, including cardiovascular and neurodegenerative diseases, cancer, diabetes, and inflammation. However, mechanisms underlying age-related diseases and aging are not yet fully understood. The Sirtuins members are interesting proteins and their role in age-related diseases is now actively studied. Sirtuins play a key role in the regulation of metabolism: they maintain homeostasis energy status, regulate energy intake and oxidative stress, storage and expenditure. Fluctuations are regulated by a fine balance between them [22]. Sirtuins are at the heart of a debate concerning cancer, their role in tumorigenesis and cell proliferation. For example, Sirt1 plays a dual role in cell survival and apoptosis that can be modulated by different stimuli [23,24]. Cardiovascular diseases are the number one cause of death in the world. A major function of Sirt1 is protecting the cardiovascular system by suppressing inflammation. Its role in inducing nitric oxide signaling, eNOs, is as well atheroprotective. The vasoprotective and anti-inflammatory effects of Sirt1 make this Sirtuin a potential drug target for treatment of cardiovascular diseases [22]. Sirt1 plays a protective role in cholesterol metabolism [25] could play a neuroprotective role [26] and an anti-inflammatory role in different tissues, by its modulation of the NF-kB pathway [27–29]. Sirt1 has been shown to prevent senescence of aortic tissues [30] and protects against nonalcoholic fatty liver diseases. Sirt1 is involved in the regulation of cell aging [31]. Recently, studies in macrophages has shown that Sirt1 suppresses the transcription factor activator protein 1, AP1, cyclo-oxygenase 2 (COX)2 expression [32] and inhibits inflammatory pathways [33]. 4. Sirtuins and osteoarthritis
Fig. 1. A. Schematic mechanism of DNA methylation, showing the transcription factors fixed on the DNA permitting gene transcription, then the DNA methyl transferases, allowing the CH3 methyl group on the compacted chromatin, not permitting gene transcription anymore. B. Schematic miRNA biogenesis and post-transcriptionnal gene silencing. C. Schematic mechanism of histone acetylation/deacetylation showing the condensed chromatin and gene silencing, then the expanded chromatin with gene transcription.
Chondrocytes histone modification analysis has led to better knowledge and understanding about osteoarthritis mechanisms. Because the “arthritic joint” is now seen as an organ, all its tissues - such as cartilage, synovium, and subchondral bone - are involved in the disease’s progression. In OA cartilage, Sirt1 appears to play a predominant regulatory role. Fujita et al. have shown a potential involvement of Sirt1 in the pathogenesis of OA through the modulation of chondrocytes’ gene expressions [34]. Sirt1 has been shown to be essential in chondrocytes’ survival by enhancing IGF signaling to deactivate p53 [35]. A
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new mechanism shows its export to the cytoplasm and link to the mitochondria, leading to a 75 kD fragment able to counteract the apoptosome and Caspase 3 activation [21]. Sirt1 regulates apoptosis in human chondrocytes [36]. Sirt1 also regulates cartilage-specific gene expression such as aggrecan, type II and IX collagen, and cartilage oligomeric matrix protein (COMP), partially via deacetylation of SOX9 [37] and by inhibiting ADAMTS-5 expression [34]. Moreover, when inhibited by p38 Mapkinase, ionizing radiations induce cellular senescence of articular chondrocytes via the negative regulation of Sirt1 [38]. Some new molecular mechanisms have been recently shown, such as the cleavage and inactivation by cathepsin B and export of Sirt1 from the nucleus to the cytoplasm under TNF alpha stimulation, recreating an inflammatory context in chondrocytes, bringing some new insight of the protein functions [20]. Huber et al., studying the synovium in patients with RA, demonstrated a balance between HAT and HDACs activity, compared with normal synovium or OA synovium [39]. Sirt1 overexpression in RA synovium seems to play an opposite role compared with RA cartilage, on cytokines production and apoptosis resistance [37,40]. We can learn about Sirt 1role in OA by studying mutant mice. In 2003, McBurney et al. created a strain of mice carrying a null allele of the Sir2 gene and then created animals carrying the two null alleles for Sir2 [41]. These animals were smaller size, exhibited retardation in certain developmental processes, and died quickly after birth. In an outbred background, (129/J) the animals can live to adulthood but are sterile. Two organs consistently affected in the mutants were the lung and pancreas. They ultimately showed that Sirt1 null mice develop an autoimmune-like condition and develop tumors at normal rates but are poorly protected by resveratrol [42,43]. Following this group, another group created a new strain of Sirt1 null mice having a mutant Sirt1 that lacked part of the catalytic domain [44]. These mice have the same phenotype, are smaller, and present cardiac and development process defects not previously described. Finally, another group created a transgenic strain of mice overexpressing Sirt1 [45]. These mice present phenotypes resembling calorie restriction. Recently, Seifert et al. created mice with a point mutation on the Sirt1 catalytic domain and showed that catalytic activity is required for metabolic homeostasis and male fertility [46]. A recent work shows a cartilage phenotype in Sirt 1 heterozygous mice with an increase of apoptosis in cartilage [47]. A strong phenotype in Sirt1 null mice or in the transgenic strain with a point mutation is shown. These mice develop cartilage degradation and OA-like disease faster than their littermates, with an increase of apoptosis and decrease of cartilage matrix proteins [47].
5. Targeting deacetylases in rheumatology Since the discovery of the importance of histone deacteylases and Sirtuins, the activators and inhibitors of these enzymes have been identified, studied, and targeted. Studies on histones modifications in RA are focused on histones acetylation and particularly on HDACs inhibitors. A number of these studies has shown a beneficial effect of HDACs inhibitors, HDACi, in animal models of RA [48]. In bone, Nakamura et al. studied the effects of HDACi in rat adjuvant arthritis and showed that they inhibited osteoclastogenesis and bone destruction by inducing IFNbeta production [49]. Shakibaei et al. have confirmed the inhibition of osteoclastogenesis by Sirt-1 resveratrol-mediated interactions with p300 modulating RANKL activation of NF-kB signaling in bone in vitro [50]. HDACi can block cytokine-induced proteoglycan release and cartilage degradation in explants, as well as cytokine-induced MMPs in chondrocytes. Moreover, HDACi appears to modulate NFkB signaling [51,52].
In the synovium, however, the histone deacetylases inhibitors appear to play a protective role. In a mouse model of arthritis induced by auto-antibody, intravenous perfusion of HDACi reduced the symptoms of arthritis and the expression of pro-inflammatory cytokines, TNF-alpha and IL1-beta, in the synovium [53]. Sirt1 has demonstrated anti-apoptotic properties and survival enhancing by deacetylation of p53. Modulation of p53 by Sirt1 has been shown to be under the control of Sirt1- interacting proteins enhancing or repressing its deacetylase activity. Interaction of Sirt1 with tumor repressor DBC1 leads to the inhibition of Sirt1 catalytic activity [54,55]. The role of Sirtuins has been established in aging processes and neurodegenerative diseases. Upregulation of Sirt1-by resveratrol, for example, has been beneficial in different models in rodents [56]. The evidence of beneficial anti-aging properties linking Sir2 and its hortologs (Sirt1) enhanced the search for Sirtuins-activators. Resveratrol quickly showed its activation of Sir2/Sirt1 in vitro [57]. In mice, chronic administration of resveratrol prolongs lifespan [58]. However, these results with resveratrol and Sirt1 activation have been largely debated and criticized [59]. Because resveratrol is poorly bioavailable, some resveratrol derivatives have been looked for. The prenylated resveratrol derivative (tetrahydroxystylbene), issued from fungus-infected peanuts, inhibited lipopolysaccharide-induced COX2 expression. However, resveratrol has shown some limitations: in order to obtain a pharmacological effect, relatively high concentrations are needed. Among the other Sirt1 activators, three new molecules have been identified: SRT1460, SRT1720, and SRT2183. These molecules display one thousand increased potency compared to resveratrol and have shown their ability to normalize glucose homeostasis in rodent animal studies [60]. However, no clear mechanism has yet been shown for their activity. Sirtuins inhibitors such as Sirtinol, Sirtinol derivatives, Splitomycin, Cambinol, Salermide, Chalcone derivative, and Tenovin have been studied in cancer research [15]. Among the various Sirtuins modulators actively studied, only resveratrol and derivatives have been tested in human for treating several cancers and metabolic and neurodegenerative diseases. However, in OA, Sirtinol and Sirtinol derivatives are used in in vitro models. 6. Conclusion In this review, we tried to take account of and summarize the recent discoveries in epigenetics in arthritis and more particularly in OA. Recent data suggest that epigenetic regulation plays an important role in the pathophysiology of the disease, even as the complex mechanisms are not completely understood. More extended studies with Sirtuins are needed, dealing with cartilage, synovium, and bone as the entire joint. Because Sirtuins seem to display a protective effect on cartilage, the therapeutic goal will be to stimulate them in order to slow down or to prevent OA. The other avenues of epigenetic research, such as DNA methylation and miRNA could complete this mechanism. Targeting potential Sirtuins modulators seems to be now a new therapeutic approach in treating this age-related disease. Disclosure of interest The authors declare that they have no conflicts of interest concerning this article. Acknowledgements This work was supported in part by the Intramural Research Program of the National Institutes of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health. The authors
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Review
Epigenetics, sirtuins and osteoarthritis Odile Gabay a,∗ , Christelle Sanchez b a b
Cartilage Biology and Orthopedics Branch, National Institute of Arthritis, Musculoskeletal and Skin Disease, NIH, Bldg 50, Bethesda, MD 20892, USA Bone and Cartilage Research Unit, Institute of Pathology B23, University of Liege, Liege, Belgium
a r t i c l e
i n f o
Article history: Accepted 11 April 2012 Available online 26 June 2012 Keywords: Chromatin-modifying enzymes Sirt1 OA Cartilage Sirtuins
a b s t r a c t Epigenetics, modifications of the DNA other than changes on the DNA sequences, is frequently studied in cancer research and aging. DNA methylation, mi-RNA, and histones deacetylation are investigated in different pathologies, including inflammatory diseases and age-related diseases such as osteoarthritis (OA). In this review, we focus on the chromatin-modifying enzymes in arthritic pathologies, and more particularly on Sirtuins. We also review the role of Sirt1 in OA, which has been highlighted in recent publications, and examine the possible protective role Sirt1 could play in this disease. Moreover, we discuss the possible therapeutic target of such a protein, reviewing the potential inhibitors/activators of this enzyme and their properties. © 2012 Société franc¸aise de rhumatologie. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction Osteoarthritis (OA) is a complex multifactorial disease with a tissular inflammatory phase, leading to cartilage degeneration and ultimately to joint destruction. Age and obesity are the main risk factors [1]. One risk factor recently focused on is genetic. A region on chromosome 7q22 has been associated with OA susceptibility [2]. Already a decade ago, studies in twins have shown showing a genetic component in OA disease [3–5]. Now a new field of investigation, research is focused on epigenetics, changes occurring on the chromatin and histones. Epigenetics, which can be defined as: “heritable changes in gene expression or cellular phenotypes caused by mechanisms others than changes on the DNA sequences”, is intensively studied today. Originally limited to the area of cancer research, epigenetics has become an actively developed topic in the study of arthritis and osteoarthritis. Modifications of the chromatin include DNA methylation, micro-RNA and histones deacetylation [6]. These molecules are known as chromatin-modifying enzymes. Briefly, DNA methylation seems to be influenced by environmental factors. It is mediated by DNA methyl transferases (DNMT1, -3a, -3b). A methyl group (CH3) is added to the DNA near the promoters of genes and suppresses the gene expression or silences it (Fig. 1A). To date, little is known in cartilage about the importance of this chromatin modification. Recently, a total DNA methylation was performed by Sesselmann et al. and the analysis in control versus OA cartilage by chromatography shows no differences [7].
∗ Corresponding author. Tel.: +1 301 443 5406. E-mail address: [email protected] (O. Gabay).
However, a fine downregulation of DNA methylation on matrix metalloproteinase (MMP)-3, -9, -13 and a desintegrin and a metalloproteinase with thrombospondin motif (ADAMTS)-4 promoters has been previously shown in OA compared to normal cartilage, possibly associated with change in their expression [8]. MicroRNA or miRNA are small, noncoding RNA consisting of 20–23 nucleotides single strand, regulating gene expression, recently discovered. They are exported from the nucleus and play major roles in the regulation of development or various pathologies (Fig. 1B). The first miRNA of interest in cartilage and widely studied is miR140. MiR140 regulates insulin-like growth factor binding protein (IGFBP)-5, ADAMTS-5 and MMP-13 genes [9], and modulates interleukin (IL)-1 beta response increasing aggrecan expression [10]. MiR140 targets histone deacetylase (HDAC) 4, which controls chondrocyte hypertrophy during skeletogenesis [11]. It appears to play a role in cartilage homeostasis [12] and to protect against cartilage damage. Today, along with miR140, a panel of other miRNA is studied in cartilage and OA, as miR453, miR221, mi-R323-3p (unpublished data, 2011). A recent paper by Iliopoulos et al. studied the potential involvement and role of 16 miRNA in OA, including miR-16, -22, -23b, -25, -26a, -29a, -30b, -103, -210, -223, -337, -373, -377, -483, -509 [13,14]. Acetylation is mediated by histones acetyl transferases (HATs), allowing access to the transcriptional machinery. There are two types of deacetylases: histones deacetylases (HDACs), which use a zinc-catalyzed mechanism of deacetylation; and Sirtuins deacetylases, which are NAD+ dependent [14] (Fig. 1C). Numerous activators/repressors are now studied as well, such as HDAC inhibitors, HDACi [15]. We will focus this review on Sirtuins deacetylases that are NAD+ dependent.
1297-319X/$ – see front matter © 2012 Société franc¸aise de rhumatologie. Published by Elsevier Masson SAS. All rights reserved. doi:10.1016/j.jbspin.2012.04.005
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2. The Sirtuin family and regulation We are just beginning to understand the mechanisms related to Sirtuins and our comprehension remains rudimentary. Fifteen years ago, the silent information regulator 2 (Sir2) gene was shown to extend the lifespan in yeast by repressing genome instability. This discovery led to questions about its potential role in longevity and aging [16]. We know now that Sirtuins are found in all organisms and play key roles in survival and aging. Sir2 is a member of a large family of conserved genes. In mammals, Sirt1 is the homolog of Sir2. In contrast to lower eukariotes, seven Sirtuins isoforms-Sirt1 to Sirt7, characterized by their highly conserved central NAD+ binding and catalytic domain- have been identified in mammals [17]. These seven isoforms range in size from less than 40 kD to more than 100 kD. They share a 300-amino-acids-long conserved catalytic domain. Sirtuins display mono-ADP-ribosyltransferase activity and protein deacetylase activity [18]. Cellular localization differs, depending on the isoform: for Sirt1, 6, and 7, it is mainly nuclear; for Sirt2, it is mainly cytoplasmic; and for Sirt 3, 4, and 5, it is mitochondrial [19]. Sirt1 activity is directed to histones proteins, and more specifically to acetylated lysine residues and other, non-histone targets. Sirtuins de-acetylate on modified lysine residues through a unique mechanism requiring NAD+ cleavage with each reaction cycle. The Sirtuins’ activity has a specific link to the state of the cell, in contrast to HDACs that hydrolize acetyl lysine residues. The reaction begins by a cleavage of NAD+ and formation of nicotinamide (NAM) and ADPribose. Then, it forms O-acetyl ADP-ribose and the deacetylated substrate (Fig. 2). The biosynthetic pathways generating NAD+ are critical in Sirtuins regulation. Stress status and cell type may regulate the Sirtuin cellular localization. Recent publications have shown that Sirt1 is cleaved in chondrocytes under stress condition by TNF-alpha stimulation and exported from the nucleus to the cytoplasm [20,21]. Sirtuins inhibitors/activators, such as deleted in breast cancer 1 (DBC1) are actively studied.
Fig. 2. Mechanism of deacetylation by Sirtuins NAD+ dependant. Sirt 1 deacetylases Lysine residues (removing the CH3) on the DNA (or other substrate) by a chemical process coupled to the conversion of nicotinamide adenine dinucleotide, NAD+ contained into the cell, into Nam (Nicotinamide).
3. Sirtuin and age-related diseases Aging increases susceptibility to a variety of diseases, including cardiovascular and neurodegenerative diseases, cancer, diabetes, and inflammation. However, mechanisms underlying age-related diseases and aging are not yet fully understood. The Sirtuins members are interesting proteins and their role in age-related diseases is now actively studied. Sirtuins play a key role in the regulation of metabolism: they maintain homeostasis energy status, regulate energy intake and oxidative stress, storage and expenditure. Fluctuations are regulated by a fine balance between them [22]. Sirtuins are at the heart of a debate concerning cancer, their role in tumorigenesis and cell proliferation. For example, Sirt1 plays a dual role in cell survival and apoptosis that can be modulated by different stimuli [23,24]. Cardiovascular diseases are the number one cause of death in the world. A major function of Sirt1 is protecting the cardiovascular system by suppressing inflammation. Its role in inducing nitric oxide signaling, eNOs, is as well atheroprotective. The vasoprotective and anti-inflammatory effects of Sirt1 make this Sirtuin a potential drug target for treatment of cardiovascular diseases [22]. Sirt1 plays a protective role in cholesterol metabolism [25] could play a neuroprotective role [26] and an anti-inflammatory role in different tissues, by its modulation of the NF-kB pathway [27–29]. Sirt1 has been shown to prevent senescence of aortic tissues [30] and protects against nonalcoholic fatty liver diseases. Sirt1 is involved in the regulation of cell aging [31]. Recently, studies in macrophages has shown that Sirt1 suppresses the transcription factor activator protein 1, AP1, cyclo-oxygenase 2 (COX)2 expression [32] and inhibits inflammatory pathways [33]. 4. Sirtuins and osteoarthritis
Fig. 1. A. Schematic mechanism of DNA methylation, showing the transcription factors fixed on the DNA permitting gene transcription, then the DNA methyl transferases, allowing the CH3 methyl group on the compacted chromatin, not permitting gene transcription anymore. B. Schematic miRNA biogenesis and post-transcriptionnal gene silencing. C. Schematic mechanism of histone acetylation/deacetylation showing the condensed chromatin and gene silencing, then the expanded chromatin with gene transcription.
Chondrocytes histone modification analysis has led to better knowledge and understanding about osteoarthritis mechanisms. Because the “arthritic joint” is now seen as an organ, all its tissues - such as cartilage, synovium, and subchondral bone - are involved in the disease’s progression. In OA cartilage, Sirt1 appears to play a predominant regulatory role. Fujita et al. have shown a potential involvement of Sirt1 in the pathogenesis of OA through the modulation of chondrocytes’ gene expressions [34]. Sirt1 has been shown to be essential in chondrocytes’ survival by enhancing IGF signaling to deactivate p53 [35]. A
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new mechanism shows its export to the cytoplasm and link to the mitochondria, leading to a 75 kD fragment able to counteract the apoptosome and Caspase 3 activation [21]. Sirt1 regulates apoptosis in human chondrocytes [36]. Sirt1 also regulates cartilage-specific gene expression such as aggrecan, type II and IX collagen, and cartilage oligomeric matrix protein (COMP), partially via deacetylation of SOX9 [37] and by inhibiting ADAMTS-5 expression [34]. Moreover, when inhibited by p38 Mapkinase, ionizing radiations induce cellular senescence of articular chondrocytes via the negative regulation of Sirt1 [38]. Some new molecular mechanisms have been recently shown, such as the cleavage and inactivation by cathepsin B and export of Sirt1 from the nucleus to the cytoplasm under TNF alpha stimulation, recreating an inflammatory context in chondrocytes, bringing some new insight of the protein functions [20]. Huber et al., studying the synovium in patients with RA, demonstrated a balance between HAT and HDACs activity, compared with normal synovium or OA synovium [39]. Sirt1 overexpression in RA synovium seems to play an opposite role compared with RA cartilage, on cytokines production and apoptosis resistance [37,40]. We can learn about Sirt 1role in OA by studying mutant mice. In 2003, McBurney et al. created a strain of mice carrying a null allele of the Sir2 gene and then created animals carrying the two null alleles for Sir2 [41]. These animals were smaller size, exhibited retardation in certain developmental processes, and died quickly after birth. In an outbred background, (129/J) the animals can live to adulthood but are sterile. Two organs consistently affected in the mutants were the lung and pancreas. They ultimately showed that Sirt1 null mice develop an autoimmune-like condition and develop tumors at normal rates but are poorly protected by resveratrol [42,43]. Following this group, another group created a new strain of Sirt1 null mice having a mutant Sirt1 that lacked part of the catalytic domain [44]. These mice have the same phenotype, are smaller, and present cardiac and development process defects not previously described. Finally, another group created a transgenic strain of mice overexpressing Sirt1 [45]. These mice present phenotypes resembling calorie restriction. Recently, Seifert et al. created mice with a point mutation on the Sirt1 catalytic domain and showed that catalytic activity is required for metabolic homeostasis and male fertility [46]. A recent work shows a cartilage phenotype in Sirt 1 heterozygous mice with an increase of apoptosis in cartilage [47]. A strong phenotype in Sirt1 null mice or in the transgenic strain with a point mutation is shown. These mice develop cartilage degradation and OA-like disease faster than their littermates, with an increase of apoptosis and decrease of cartilage matrix proteins [47].
5. Targeting deacetylases in rheumatology Since the discovery of the importance of histone deacteylases and Sirtuins, the activators and inhibitors of these enzymes have been identified, studied, and targeted. Studies on histones modifications in RA are focused on histones acetylation and particularly on HDACs inhibitors. A number of these studies has shown a beneficial effect of HDACs inhibitors, HDACi, in animal models of RA [48]. In bone, Nakamura et al. studied the effects of HDACi in rat adjuvant arthritis and showed that they inhibited osteoclastogenesis and bone destruction by inducing IFNbeta production [49]. Shakibaei et al. have confirmed the inhibition of osteoclastogenesis by Sirt-1 resveratrol-mediated interactions with p300 modulating RANKL activation of NF-kB signaling in bone in vitro [50]. HDACi can block cytokine-induced proteoglycan release and cartilage degradation in explants, as well as cytokine-induced MMPs in chondrocytes. Moreover, HDACi appears to modulate NFkB signaling [51,52].
In the synovium, however, the histone deacetylases inhibitors appear to play a protective role. In a mouse model of arthritis induced by auto-antibody, intravenous perfusion of HDACi reduced the symptoms of arthritis and the expression of pro-inflammatory cytokines, TNF-alpha and IL1-beta, in the synovium [53]. Sirt1 has demonstrated anti-apoptotic properties and survival enhancing by deacetylation of p53. Modulation of p53 by Sirt1 has been shown to be under the control of Sirt1- interacting proteins enhancing or repressing its deacetylase activity. Interaction of Sirt1 with tumor repressor DBC1 leads to the inhibition of Sirt1 catalytic activity [54,55]. The role of Sirtuins has been established in aging processes and neurodegenerative diseases. Upregulation of Sirt1-by resveratrol, for example, has been beneficial in different models in rodents [56]. The evidence of beneficial anti-aging properties linking Sir2 and its hortologs (Sirt1) enhanced the search for Sirtuins-activators. Resveratrol quickly showed its activation of Sir2/Sirt1 in vitro [57]. In mice, chronic administration of resveratrol prolongs lifespan [58]. However, these results with resveratrol and Sirt1 activation have been largely debated and criticized [59]. Because resveratrol is poorly bioavailable, some resveratrol derivatives have been looked for. The prenylated resveratrol derivative (tetrahydroxystylbene), issued from fungus-infected peanuts, inhibited lipopolysaccharide-induced COX2 expression. However, resveratrol has shown some limitations: in order to obtain a pharmacological effect, relatively high concentrations are needed. Among the other Sirt1 activators, three new molecules have been identified: SRT1460, SRT1720, and SRT2183. These molecules display one thousand increased potency compared to resveratrol and have shown their ability to normalize glucose homeostasis in rodent animal studies [60]. However, no clear mechanism has yet been shown for their activity. Sirtuins inhibitors such as Sirtinol, Sirtinol derivatives, Splitomycin, Cambinol, Salermide, Chalcone derivative, and Tenovin have been studied in cancer research [15]. Among the various Sirtuins modulators actively studied, only resveratrol and derivatives have been tested in human for treating several cancers and metabolic and neurodegenerative diseases. However, in OA, Sirtinol and Sirtinol derivatives are used in in vitro models. 6. Conclusion In this review, we tried to take account of and summarize the recent discoveries in epigenetics in arthritis and more particularly in OA. Recent data suggest that epigenetic regulation plays an important role in the pathophysiology of the disease, even as the complex mechanisms are not completely understood. More extended studies with Sirtuins are needed, dealing with cartilage, synovium, and bone as the entire joint. Because Sirtuins seem to display a protective effect on cartilage, the therapeutic goal will be to stimulate them in order to slow down or to prevent OA. The other avenues of epigenetic research, such as DNA methylation and miRNA could complete this mechanism. Targeting potential Sirtuins modulators seems to be now a new therapeutic approach in treating this age-related disease. Disclosure of interest The authors declare that they have no conflicts of interest concerning this article. Acknowledgements This work was supported in part by the Intramural Research Program of the National Institutes of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health. The authors
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