Peroxisome proliferator-activated receptor-γ (PPAR-γ) ligands as potential therapeutic agents to treat arthritis

Pharmacological Research 60 (2009) 160–169

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Pharmacological Research journal homepage: www.elsevier.com/locate/yphrs

Review

Peroxisome proliferator-activated receptor-␥ (PPAR-␥) ligands as potential therapeutic agents to treat arthritis Costas Giaginis, Athina Giagini, Stamatios Theocharis ∗ Department of Forensic Medicine and Toxicology, Medical School, University of Athens, Athens 11527, Greece

a r t i c l e

i n f o

Article history: Received 6 December 2008 Received in revised form 7 February 2009 Accepted 8 February 2009 Keywords: PPAR-␥ ligands Arthritis Osteoarthritis Rheumatoid arthritis Inflammation

a b s t r a c t Peroxisome proliferator-activated receptor-␥ (PPAR-␥) has already been considered as an attractive molecular target for the treatment of human metabolic disorders. Pleiotropic functions beyond this limit, such as anti-inflammatory and anti-proliferative effects against several pathological states, including atherosclerosis, osteoporosis and cancer, are currently being explored. Several natural and synthetic PPAR-␥ ligands have been the focus of extensive research effort as potent anti-inflammatory agents in diverse disease states. In the last decade, accumulative experimental evidence has further suggested that PPAR-␥ is involved in several inflammatory signaling pathways associated with arthritis. PPAR-␥ appears to be expressed by major cell populations in joints, such as chondrocytes, synoviocytes, fibroblasts and endothelial cells. PPAR-␥ ligands have also been shown to inhibit major inflammatory signaling pathways, reducing the synthesis of cartilage catabolic factors responsible for articular cartilage degradation in arthritis. In the present review the crucial role of PPAR-␥ ligands in arthritis and the underlying mechanisms participating in essential inflammatory signaling pathways are summarized. Taking into consideration the data so far, PPAR-␥ ligands seem to represent potential therapeutic agents in the aim to reduce mainly the inflammation implicated in arthritis. However, the precise molecular mechanisms through which PPAR-␥ ligands exert their actions are strongly recommended to be clarified, as both receptor-dependent and -independent actions were shown to be elicited. © 2009 Elsevier Ltd. All rights reserved.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Specific forms of arthritis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Therapeutic targets for arthritis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PPAR-␥ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. PPAR-␥ function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. PPAR-␥ ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. PPAR-␥ in inflammation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PPAR-␥ and arthritis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. PPAR-␥ expression and arthritis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. PPAR-␥ polymorphisms and arthritis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of PPAR-␥ ligands on arthritis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Osteoarthritis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Rheumatoid arthritis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Psoriatic (PsA) and gouty arthritis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conflict of interest statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author at: Department of Forensic Medicine and Toxicology, Medical School, National and Kapodistrian University of Athens, 75M. Asias str., Goudi, GR11527 Athens, Greece. Tel.: +30 210 7462413; fax: +30 210 7716098. E-mail address: [email protected] (S. Theocharis). 1043-6618/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.phrs.2009.02.005

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1. Introduction 1.1. Specific forms of arthritis Arthritis is a group of conditions involving damage to the joints of the body, and includes five distinct forms. Osteoarthritis (OA), also known as degenerative arthritis, is the most common form of arthritis and a major cause of disability and impaired quality of life in the elderly [1]. OA is characterized by degeneration of articular cartilage, limited intra-articular inflammation with synovitis, and alterations in peri-articular and subchondral bone. Multiple factors are involved in the pathogenesis of OA, including mechanical influences, the effects of aging on cartilage matrix composition and structure, and genetic factors. Inflammatory response genes, such as proteinases, cyclooxygenase, and cytokines are implicated in its pathogenesis [2]. Rheumatoid arthritis (RA) is a chronic, systemic autoimmune disorder in which both genetic and environmental factors are involved [3]. RA is mostly known as polyarticular joint disease characterized by synovial proliferation in the joint, infiltration of the synovial stroma by B-cells, CD4+ helper T-cells and macrophages. Additional histopathological features include hypervascularisation, increased osteoclast activity and pannus formation consisting of a mass of synovium, inflammatory cells and fibroblasts causing destruction and ossification [4]. RA pathology is mediated by a number of inflammatory mediators, such as the cytokines tumor necrosis factor (TNF)-␣, interleukin (IL)-1, -6, -17 and interferon (IFN)-␥ the chemokines monocytes chemoattractant protein (MCP)-1, -4, and CC chemokine ligand 18 (CCL18), the cell adhesion molecules intracellular cellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1, matrix metalloproteinases (MMPs) and the metabolic proteins cyclooxygenase (COX)-1, -2 and inducible nitric oxide synthase (iNOS) [5]. Septic arthritis is a result of hematogenous seeding, direct introduction, or extension from a contiguous focus of infection. The pathogenesis of septic arthritis is multifactorial and depends on the interaction of the host immune response and the adherence factors, toxins, and immunoavoidance strategies of the invading pathogen [6]. Psoriatic arthritis (PsA) is a complex, chronic, multisystem inflammatory disease with both skin and joint involvement. Its pathophysiology comprises a dysfunctional stromal-immune cell and cytokine network leading to inflammation of skin, entheses and joints. The pro-inflammatory cytokines TNF-␣, IL-12/IL-23 and a variety of co-stimulatory molecules have been identified as critical factors for its progression [7]. Gouty arthritis is the most common form of inflammatory arthritis affecting people in the elderly. Gouty arthritis is a disease caused by the deposition of monosodium urate monohydrate (MSU) crystals. A number of studies indicated that MSU crystals can function as effective molecular signal, which resembles exogenous adjuvants, while toll-like receptor (TLR)-mediated or MyD88-dependent IL-1 receptor pathways are further involved in acute gout [8]. 1.2. Therapeutic targets for arthritis Several biological pathways related to inflammation have been considered as potential therapeutic targets for the treatment of arthritis. In this context, mitogen-activated protein kinases (MAPKs) have been implicated in the production of proinflammatory cytokines and downstream signaling events leading to joint inflammation and destruction and thus they constitute rational targets of drug design for novel therapies in arthritis [9]. Several MAPK inhibitors have been advanced in clinical trials; however, they exhibited serious side effects, as the multiple isoforms of MAPKs, such as extracellular signal regulated-kinase (ERK) 1–8, c-jun N-terminal kinase (JNK) 1–2 and p38-␣, -␤, -

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␥, -␦ have been implicated in a plethora of essential cellular responses [9,10]. Nuclear factor-␬B (NF-␬B) pathway constitutes another therapeutic target for arthritis. NF-␬B activation results in the transactivation of a multitude of responsive genes which contribute to the inflammatory phenotype of arthritis, including TNF-␣ from macrophages, MMPs from synovial fibroblasts and chemokines that recruit immune cells to the inflamed pannus [11]. Small molecule inhibitors that target NF-␬B-driven genes, mainly TNF-␣, IL-1 and IL-6, have currently been explored in clinical trials; however, such treatments are expensive and the investigated drugs are injected rather than orally administrated [11,12]. In addition, selective Cox-2 inhibitors (coxibs) are currently approved for the relief of acute pain and symptoms of chronic inflammatory conditions, such as OA and RA; however, two coxibs were withdrawn from the market, since their long-term use was associated with an increased risk for cardiovascular adverse events [13]. MMP inhibitors are also expected to be useful for the treatment of OA and RA. A large number of MMP inhibitors have been developed in recent years; however, they failed in clinical trials due to their low specificity. Thus, further research efforts have been directed to the design of more specific MMP inhibitors with small molecular weight and different Zn-binding groups [14]. In view of the above considerations, there is strong demand for novel pharmacological agents against the different forms of arthritis. In the last few years, a gradually increasing number of studies have rendered several peroxisome proliferator-activated receptor␥ (PPAR-␥) ligands potential therapeutic agents to treat OA and RA and, to a lesser extent, other forms of arthritis. We therefore aimed to review the available data so far concerning the expression of PPAR-␥ in the major cell populations of joints, as well as the effect of natural and synthetic PPAR-␥ ligands on inflammatory conditions associated with specific forms of arthritis.

2. PPAR-␥ 2.1. PPAR- function Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors, initially described as molecular targets for compounds, which induce peroxisomal proliferation [15,16]. To date, three different isotypes of PPARs have been identified in various species: PPAR-␣, -␤/␦ and -␥ [15,16]. PPAR-␥, the most extensively studied amongst the three PPAR subtypes, is mainly expressed in brown and white adipose tissue and, to a lesser extent, in other tissues and cells throughout the body and plays a central role in the process of adipocyte differentiation, peripheral glucose utilization and insulin sensitization [17–20]. Alternative splicing and promoter usage result in the formation of two isoforms PPAR-␥1 and -␥2. In particular, PPAR-␥2 differs from PPAR-␥1 only by 30 additional amino acids at the N-terminal extremity [21]. PPAR-␥ is a ligand-activated transcription factor that binds to specific DNA sequences, known as peroxisome proliferator response elements (PPREs), in the promoter of the target genes only as an heterodimer with the Retinoid X Receptor (RXR) [15,16,22–24]. A PPRE usually consists of an almost perfect direct repeat of the sequence TGACCT, spaced by a single base pair and it has been mainly identified in the upstream regulatory sequences of genes related to metabolic pathways [15,16,22–24]. In addition, PPAR-␥ can regulates gene expression independently of PPRE, either by suppressing growth hormone protein-1 (GHP-1), a transcription factor involved in pituitary specific gene expression, or by interfering with the function of activator protein (AP)-1, signal transducer and activator of transcription (STAT)-1 and NF-␬B [24–27].

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2.2. PPAR- ligands A wide range of natural and synthetic compounds can function as PPAR-␥ ligands by binding selectively to and activating their receptor. The long chain polyunsaturated fatty acids and their oxidized derivatives, such as eicosanoids 8-S-hydroxyeicosatetraenoic acid (8S-HETE) and leukotriene B4 (LTB4) selectively bind to and activate all three PPARs [24,28]. PPAR-␥ can selectively be activated by several prostanoids, such as 15-deoxy-12,14-prostaglandin J2 (15d-PGJ2) and 15-hydroxy-eicosatetraenoic acid (15-HETE), which are derivatives of arachidonic acid synthesized through the lipoxygenase pathway, as well as by nitrolinoleic acid derivatives [24,28,29]. Thiazolidinediones (TZDs) and tyrosine derivatives constitute the most well known synthetic ligands [30,31], while PPAR-␥ affinity for some non-steroidal anti-inflammatory drugs (NSAIDs) was reported [32]. TZDs represent a promising class of oral anti-diabetic agents, some of which are already marketed drugs (pioglitazone and rosiglitazone) for the treatment of type II diabetes mellitus [33]. Interestingly, a large number of studies have revealed a wide spectrum of action for PPAR-␥ ligands beyond the treatment of metabolic disorders, including anti-inflammatory and antineoplastic properties [34,35]. PPAR-␥ has further been considered as a therapeutic target in serious pathological states such as atherosclerosis and osteoporosis [36–38]. Recently, a new generation of dual- or pan-action PPAR ligands, such as muraglitazar, tesaglitazar and farglitazar, are being developed to activate both PPAR-␣ and PPAR-␥ in order to combine their anti-diabetic actions with reducing diabetic complications, exerting further therapeutic benefits beyond diabetes [39]. In the last few years, the design, synthesis and biological evaluation of novel PPAR-␥ ligands, either TZD or non-TZD derivatives, has been the focus of extensive research effort in order to enhance their anti-diabetic therapeutic potential, minimizing possible side effects. Recent studies based on quantitative structure–activity relationships (QSAR) in the chemical space of PPAR-␥ ligands have also been appeared in the literature in order to provide important information for further design of molecules with improved bioavailability characteristics and enhanced chance for success in clinical development [40,41]. 2.3. PPAR- in inflammation There is accumulating evidence which supports that PPAR-␥ plays a crucial role in the control of inflammatory response by inhibiting pro-inflammatory gene expression induced in response to inflammatory cell activation [42]. Currently, the cell-specific expression of PPAR-␥ isotype with respect to inflammatory cells has been well-established by both in vitro and in vivo studies. In fact, PPAR-␥ expression appears to be enhanced during the differentiation of monocytes into macrophages in response to proinflammatory mediators [12]. In addition, PPAR-␥ expression in endothelial, vascular smooth muscle and dendritic cells, as well as T- and B-cells and platelets was associated with inflammatory processes [43,44]. PPAR-␥ ligand treatment was shown to reduce a wide variety of inflammatory markers in several animal models of OA, RA, sepsis, pancreatitis, atherosclerosis, ulcerative colitis, chronic asthma, as well as Parkinson and Alzheimer’s disease [43,44]. Substantial clinical evidence has also well documented the crucial role of PPAR-␥ in several disease states related to inflammatory processes. PPAR-␥ expression was considerably up-regulated in several inflammatory diseases and PPAR-␥ ligand therapy reduced the levels of acute phase proteins, as well as other inflammatory mediators, such as MMPs, TNF-␣ and soluble CD40L, which in turn attenuated the degree of inflammation, thus improving patients’ clinical outcome [43,44]. Although the large body of evidence clearly rendered PPAR-␥ ligands as anti-inflammatory

agents, a few studies have supported that they can also exert proinflammatory actions [45,46]. With respect to the mode of action through which PPAR-␥ exerts anti-inflammatory effects, it is considered mainly to transrepress pro-inflammatory genes in a DNA-binding-dependent manner. Three potential mechanisms for PPAR-␥-ligand-dependent transrepression have been reported. First, upon ligand activation, PPAR-␥/RXR heterodimers may bind to co-activators that are crucial for pro-inflammatory gene induction by other transcription factors. Indeed, PPAR-␥ was shown to bind to several transcriptional co-activators, such as cAMP-responsive-elementbinding-factor (CREB)-binding protein (CBP), p300, steroid receptor coactivator-1 (SRC-1), transcription intermediary factor 2 (TIF2), amplified in breast cancer-1 (AIB-1), thyroid hormone receptorassociated protein (TRAP220) and vitamin D receptor-interacting protein (DRIP)205 [44,47]. Thus, the transcriptional transactivation of several transcription factors, including AP-1, STAT-1, NF-␬B and nuclear factor of activated T cells (NFAT), which need the recruitment of the aforementioned co-activators, can considerably be reduced [44,47]. The second mechanism involves direct binding of PPAR-␥/RXR heterodimers to transcription factors. This mechanism was reported for p50 and p65 subunits of the NF-␬B family, as well as NFAT [48]. A third mechanism suggests that the ligand activated PPAR-␥/RXR heterodimers may regulate negatively MAPK cascade. In this aspect, a study carried out in PPAR-␥-deficient (PPAR-␥+/− ) heterozygous mice revealed that the phosphorylation of JNK and p38 protein was significantly reduced when compared with wildtype littermates [49].

3. PPAR-␥ and arthritis 3.1. PPAR- expression and arthritis PPAR-␥ was reported to be expressed at both mRNA and protein levels by major cell populations in joints [50–52]. PPAR-␥ was present in rat chondrocytes, at both mRNA and protein levels [50]. Interestingly, its transcript was found to be weakly expressed in rat cartilage compared to adipose tissue, whereas mRNA levels were not significantly different than those determined in liver tissue [50]. In support of this view, the presence of PPAR-␥ was evident in human articular chondrocytes either cultured, in vitro, or embedded in the cartilage unit [51,52]. The cellular pattern of PPAR-␥ distribution was mainly nuclear and only occasionally cytoplasmic [52]. Immunohistochemical analysis also confirmed the presence of PPAR-␥ localized to the distal femoral growth plate cartilage of neonatal rats [53]. PPAR-␥ mRNA was detected in both normal and OA synovial cells [54]. Importantly, Fahmi et al. and Kalajdzic et al. showed that human synovial cells from patients with OA expressed functional PPAR-␥ [51,55]. Immunohistochemical analysis further revealed that PPAR-␥ was located in the superficial zone of cartilage and that the levels of PPAR-␥ protein expression were significantly lower in OA compared to normal cartilage [56]. Additionally, PPAR-␥1 mRNA levels were about 10-fold higher than PPAR-␥2 mRNA levels and PPAR-␥1 but not -␥2 was 2.4-fold lower in OA compared to normal cartilage, supporting evidence for a selective down-regulation of PPAR-␥1 in OA cartilage [56]. In addition, it was shown that OA patients exhibited increased PPAR-␥ mRNA levels in the adipocytes of the bone marrow [57]. PPAR-␥ was reported to be expressed in both normal and RA synovial cells [54,58]. Enhanced PPAR-␥ mRNA and protein levels in macrophages and in the synovial lining layer, fibroblasts, and endothelial cells of RA patients were also noted [59]. In addition, RA patients exhibited increased PPAR-␥ mRNA levels in the adipocytes of the bone marrow [57].

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3.2. PPAR- polymorphisms and arthritis In a French-Canadian population (172 OA patients and 212 healthy controls), genotype and allele frequencies for the polymorphisms Pro12Ala and C1431T in the PPAR-␥ gene did not differ significantly between patients with OA and controls. Moreover, no significant differences were observed after stratification of patients according to age at disease onset and radiological or functional severity. In addition, haplotype analysis of both polymorphisms in the PPAR-␥ gene showed no association of any haplotype with susceptibility to or severity of OA [60]. In a study of 474 Korean RA patients and 400 controls, the associations of PPAR-␥2 genotypes with the risk and the severity of RA were also explored. The genetic polymorphism of PPAR-␥2, Pro12Ala, was not found to play significant role in the susceptibility to RA among Koreans [61]. In contrast, in a Caucasian population, an association between PsA and the known coding single-nucleotide polymorphism, Pro12Ala, of the PPAR-␥ gene was noted [62]. 4. Effects of PPAR-␥ ligands on arthritis 4.1. Osteoarthritis Several studies supported substantial evidence that PPAR-␥ activation can inhibit major signaling pathways of inflammation and reduce the synthesis of cartilage catabolic factors responsible for articular cartilage degradation in OA. The whole data concerning the effects of PPAR-␥ ligands on OA are summarized in Table 1. In fact, 15d-PGJ2 and rosiglitazone inhibited IL-1␤ induction of both NO and MMP-13 in human OA chondrocytes. This inhibition occurred, at least in part, at the transcriptional level through a PPAR␥-dependent pathway, probably by interfering with the activation of AP-1 and NF-␬B [51,63]. Rosiglitazone and 15d-PGJ2 also inhibited MMP-1 gene expression, suggesting that they may be useful

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in reducing joint tissue destruction. Interestingly, the inhibition of MMP-1 production was associated with a reduction in AP-1 binding activity [63]. Furthermore, Boyault et al. showed that PPAR-␥ expression was decreased by IL-1␤ in human OA chondrocytes [52]. 15d-PGJ2, but not troglitazone, was highly potent to counteract IL-1␤-induced COX-2 and iNOS expression, as well as NO production and the decrease of proteoglycan synthesis. Western blot and gel-shift assay analyses further demonstrated that 15d-PGJ2 inhibited NF-␬B activation, while troglitazone was ineffective. The absence or the low effect of troglitazone suggested that 15d-PGJ2 action in human chondrocytes is mainly PPAR-␥-independent [52]. In a more recent study by Afif et al., IL-1 reduced PPAR-␥1 expression in cultured chondrocytes from OA patients, in a dose- and time-dependent manner [56]. TNF-␣, IL-17 and prostaglandin E2 (PGE2), which are involved in the pathogenesis of OA were shown to downregulate PPAR-␥1 expression. Specific inhibitors of MAPKs p38 (SB203580) and JNK (SP600125) but not ERK (PD98059) suppressed IL-1-induced down-regulation of PPAR-␥1 expression. Thus, the pro-inflammatory cytokine IL-1 was assumed to be responsible for PPAR-␥1 down-regulation via a mechanism involving activation of MAPKs (p38 and JNK) and NF-␬B signaling pathways [56]. Moreover, it was shown that Ox-LDL/LOX-1 system up-regulated VEGF expression in articular cartilage, at least in part, through activation of PPAR-␥ [64]. These data deserve special attention as accumulating in vitro and in vivo studies have provided extensive evidence that PPAR-␥ ligands can function as modulators of the angiogenic signaling cascade targeting mainly VEGF signaling pathway [65,66]. 15d-PGJ2 may also play important role in the pathogenesis of arthritic joint destruction via regulation of chondrocyte apoptosis. In fact, the study by Shan et al. showed that 15d-PGJ2 was released by human articular chondrocytes, being located in the joint synovial fluids obtained from OA patients [67]. Pro-inflammatory cytokines, such as IL-1␤ and TNF-␣, up-regulated chondrocyte release of

Table 1 Effects of PPAR-␥ ligands on osteoarthritis. PPAR-␥ ligand

Organisms/type of cells

15d-PGJ2

In vitro Human chondrocytes

Human synoviocytes

Rosiglitazone

Troglitazone

In vitro Human chondrocytes

In vitro Human chondrocytes Human synoviocytes

Pioglitazone

In vivo Dogs

Guinea pigs

Effect

Ref.

NO and MMP-13↓ MMP-1↓ COX-2, iNOS and NO ↓ Proteoglycan synthesis↓ Apoptosis induction↑ NF-␬B activation↓ p38 MAPK activation↓

[51] [63] [52]

TNF-␣ and IL-1␤↓ NF-␬B activation↓ PGE2, mPGES-1↓ Egr-1 DNA-binding activity↓

[54]

NO and MMP-13↓ MMP-1↓

[51] [63]

COX-2, iNOS and NO ↔ Proteoglycan synthesis↔

[52]

PGE2, mPGES-1↓ Egr-1 DNA-binding activity↓

[68]

Cartilage lesions↓ MMP-1, ADAMTS-5 and iNOS synthesis↓ ERK-1/2 and p38 MAPKs activation↓ NF-␬B activation↓

[70]

Cartilage lesions↓ MMP-13 and IL-1␤↓

[71]

[67]

[68]

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Fig. 1. Molecular mediators in signaling pathways implicated in the pathogenesis of arthritis which have been considered as potential targets for PPAR-␥ ligands.

15d-PGJ2. 15d-PGJ2-induced apoptosis of chondrocytes from OA arthritis patients, as well as control nonarthritic subjects in a timeand dose-dependent manner through a PPAR-␥-dependent mechanism. PPAR-␥ expression was up-regulated by IL-1␤ and TNF-␣. Inhibition of NF-␬B, and the activation of p38 MAPK were also found to be involved in 15d-PGJ2-induced chondrocyte apoptosis. These signal pathways led to the activation of the downstream pro-apoptotic molecule p53 and caspase cascades [67]. In OA synoviocytes, the induction of inflammatory cytokine mRNA expression, such as TNF-␣ and IL-1␤ was significantly inhibited by 15d-PGJ2. Both troglitazone and 15d-PGJ2 markedly inhibited TNF-␣-induced NF-␬B activation [54]. Moreover, 15dPGJ2 and troglitazone dose-dependently suppressed IL-1␤-induced PGE2 production, as well as membrane-associated prostaglandin PGE2 synthase-1 (mPGES-1) protein and mRNA expression, which catalyzes the conversion of PGH2 to PGE2. Pretreatment of OA synoviocytes with GW9662, a PPAR-␥ antagonist, relieved the suppressive effect of both PPAR-␥ ligands on mPGES-1 protein expression, suggesting that the inhibition of mPGES-1 expression was a receptor-mediated effect. Interestingly, both PPAR-␥ ligands suppressed Egr-1-mediated induction of the activities of the mPGES-1 promoter and of a synthetic reporter construct containing three tandem repeats of an Egr-1 binding site. Electrophoretic mobility shift and supershift assays for Egr-1 binding sites in the mPGES-1 promoter showed that both PPAR-␥ ligands inhibited IL-1␤-induced DNA-binding activity of Egr-1 [68]. Cultivation of synoviocytes isolated from OA patients with IFN-␥, TNF-␣ or IL1␤ inhibited the expression of PPAR-␥, as well as CCAAT/enhancer binding protein (C/EBP) nuclear activity and thus suppressed adipocyte-like cell differentiation, in vitro [69]. Several in vivo studies supported evidence that PPAR-␥ ligands can inhibit a number of essential biological pathways responsible for the structural changes that occur in OA. More to the point, in an experimental dog model of OA, pioglitazone (15 or 30 mg/kg/day) reduced the development of cartilage lesions in a dose-dependent manner, with the highest dosage producing a statistically significant change. In addition, pioglitazone significantly reduced the synthesis of the key OA mediators MMP-1, disintegrin-like and metallopeptidase (reprolysin type) with thrombospondin type 1 motif (ADAMTS)-5, and iNOS and, at the same time, inhibited the activation of the signaling pathways for MAPKs ERK-1/2, p38, and NF-␬B [70]. Pioglitazone (2 and 20 mg/kg/day) also dose-dependently reduced the severity of experimental OA guinea pigs through a reduction in the levels of MMP-13 and IL-1␤, which are known to

play an important role in the pathophysiology of OA lesions [71]. In Fig. 1, the involvement of PPAR-␥ in inflammatory signaling pathways implicated in the pathogenesis of arthritis, including OA is depicted. 4.2. Rheumatoid arthritis Substantial evidence has indicated that PPAR-␥ activation can inhibit major signaling pathways of inflammation and reduce the synthesis of cartilage catabolic factors responsible for articular cartilage degradation in RA. The whole data concerning the effects of PPAR-␥ ligands on RA are summarized in Table 2. PPAR-␥ activation by 15d-PGJ2 inhibited the expression of inflammatory cytokines, such as TNF-␣ and IL-1␤ in RA synoviocytes. Both troglitazone and 15d-PGJ2 markedly inhibited TNF-␣-induced NF-␬B activation at 30 ␮M. Furthermore, PPAR-␥ activation induced apoptosis by itself and augments TRAIL/Apo2L-induced apoptosis in human monocytic THP-1 cells [54]. 15d-PGJ2 and ciglitazone down-regulate TNF-␣-mediated osteoclast differentiation in human monocytes, in part via suppression of the action of MCP-1, a chemokine commonly found at the site of bone degradation in RA [72]. In addition, CLX-090717, a novel synthetic PPAR-␥ agonist, significantly inhibited spontaneous TNF-␣ release by RA synovial membrane cells, as well as LPS-induced TNF-␣ release from human and murine monocytic cells. Inhibition of TNF-␣ in monocytes was mediated partially through a NF-␬B-dependent pathway, as judged by sustained levels of I␬B␣ in cytosolic extracts and a reduced level of LPS-induced NF-␬B activity in nuclear extracts [73]. Ishino et al. also showed that 15d-PGJ2 suppressed IL-1␤-induced PGE2 synthesis in RA synoviocytes. Treatment of mouse osteoblastic cells, MC3T3-E1 with 15d-PGJ2 led to a significant increase in IL-1␣-induced COX-2 expression and PGE2 production in a dose dependent manner. The effect of 15d-PGJ2 was stronger than that of troglitazone; however, the cell viability of MC3T3-E1 cells remained unaltered. This study reinforced the notion that 15dPGJ2 may exert a positive feedback regulation of the arachidonate cascade of PGE2 in osteoblastic cells, providing important information about the pathogenesis and treatment of bone resorption in a variety of diseases, including RA [74]. Accordingly, in a previous study by Tsubouchi et al., 15d-PGJ2, but not troglitazone and other prostanoids, suppressed IL-1␤-induced PGE2 synthesis in RA synovial fibroblasts through the inhibition of COX-2 and cytosolic phospholipase A2 (cPLA2) expression [75]. However, the inhibition of COX-2 and cPLA2 expression was not affected by

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Table 2 Effects of PPAR-␥ ligands on rheumatoid arthritis. PPAR-␥ ligand

Organisms/type of cells

Effect

Ref.

15d-PGJ2

In vitro Rat chondrocytes

PGE2and mPGES-1↓ NF-␬B binding and I␬B␣ sparing↓ Prostaglandins↓ NF-␬B activation↓

[77]

Human chondrocytes

Apoptosis induction↑

[67]

Human synoviocytes

TNF-␣ and IL-1␤↓ NF-␬B activation↓ Apoptosis induction↔ PGE2 synthesis↓ PGE2 synthesis↓ COX-2 and cPLA2↓ Apoptosis induction↑ Apoptosis induction↑

[54]

[76]

Human monocytic THP-1 cells Human monocytes Mouse osteoblastic MC3T3-E1 cells

IL-1, IL-6 and TNF-␣↓ MCP-1↓ COX-2 and PGE2↓

[78] [72] [74]

Pannus formation↓ Mononuclear cell infiltration↓

[59]

PGE2, mPGES-1↔ IL-1, IL-6 and TNF-␣↔ iNOS, COX-2, ICAM-1 and nitrotyrosine formation↓ NF-␬B activation↓

[77] [78] [84]

In vitro Rat chondrocytes Human monocytic THP-1 cells Murine murine macrophage-like cell RAW 264 cells

Rat

In vitro Rat chondrocytes Human synoviocytes

iNOS↓

[84]

Cartilage lesions↓ TNF-␣, IL-1␤ and bFGF↓

[85]

Prostaglandins↔ NF-␬B activation↔

[79]

NF-␬B activation↓ Apoptosis induction↔ PGE2 synthesis↓ COX-2 and cPLA2↓ Apoptosis induction↑ Apoptosis induction↑

[54] [75] [59] [58]

Human fibroblast-like synovial cells

TNF-␣, IL-6, IL-8 and MMP-3↓ Apoptosis induction↔ NF-␬B activation↓

[82]

Rat synovial fibroblasts

TNF-␣ and iNOS↓ COX-2 and IL-1␤↔ NF-␬B DNA-binding activity↔ AP-1 DNA-binding ↑

[76]

Human monocytic THP-1 cells

Apoptosis induction↑ IL-1, IL-6 and TNF-␣↔

[54] [78]

Pannus formation↓ Mononuclear cell infiltration↓

[59]

Apoptosis induction↑ IL-1, IL-6 and TNF-␣↔

[54] [78]

iNOS, COX-2, ICAM-1 and nitrotyrosine formation↓

[84]

iNOS↓

[84]

Cartilage lesions↓ TNF-␣, IL-1␤ and bFGF↓

[85]

In vivo Rats

Pioglitazone

[59] [58]

COX-2 and iNOS↓ TNF-␣ and IL-1␤↓ NF-␬B and AP-1 DNA-binding activity ↓

In vivo Mice

Troglitazone

[74] [75]

Rat synovial fibroblasts

In vivo Rats

Rosiglitazone

[79]

In vitro Human monocytic THP-1 cells Murine murine macrophage-like cell RAW 264 cells In vivo Mice Rat

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Table 2 (Continued ) PPAR-␥ ligand

Organisms/type of cells

Effect

Ref.

Ciglitazone

In vitro Human monocytes

MCP-1↓

[72]

In vitro Human fibroblast-like synoviocytes Human monocyte derived macrophages

MMP-3↓ IL-6↓

[80] [81]

In vivo Rats

Cartilage lesions↓

[80]

In vitro Human synovial fibroblasts

Apoptosis induction↔

[83]

Cartilage lesions↓ IFN-␥, TNF-␣ and IL-1␤↓ MCP-1 and RANKL↓

[86]

TNF-␣↓ NF-␬B activation↓

[73]

Clinical arthritis score ↓ Paw swelling↓ Anti-type II collagen immune responses↓

[73]

Ajulemic acid

Indomethacin

THR0921

CLX-090717

In vivo Mice

In vitro Synovial membrane cells Human monocytes Murine macrophage-like cell RAW 264 cells In vivo Mice

pre-treatment with anti-PPAR-␥ antibody, which suggested that the anti-inflammatory effect of 15d-PGJ2 for PG synthesis may be PPAR-␥ independent [75]. In addition, 15d-PGJ2 dose-dependently decreased lipopolysaccharide (LPS)-induced COX-2 and iNOS mRNA expression in rat synovial fibroblasts, whereas troglitazone (10 ␮M) only inhibited iNOS mRNA expression [76]. 15d-PGJ2 decreased LPS-induced IL-1␤ and TNF-␣ expression. Interestingly, troglitazone strongly decreased TNF-␣ expression, but had no significant effect on IL-1␤ expression. 15d-PGJ2 was able to inhibit DNAbinding activity of both NF-␬B and AP-1, whereas troglitazone had no effect on NF-␬B activation, but increased LPS-induced AP-1 activation, suggesting different mechanisms of action for 15d-PGJ2 and troglitazone in synovial fibroblasts [76]. The different mechanisms of action between 15d-PGJ2 and TZDs have been reported by several studies. In this context, 15d-PGJ2, but not the high-affinity PPAR-␥ ligand rosiglitazone, decreased almost completely PGE2 synthesis and mPGES-1 expression synthesis in rat chondrocytes stimulated with IL-1␤. The inhibitory potency of 15d-PGJ2 remained unaffected by changes in PPAR-␥ expression and resulted from inhibition of NF-␬B nuclear binding and I␬B␣ sparing, secondary to reduced phosphorylation of IKK␤. Consistently with 15d-PGJ2, being a putative endogenous regulator of the inflammatory reaction if synthesized in sufficient amounts, the present data confirm the variable PPAR-␥dependency of its effects in joint cells, while underlining possible species and cell types specificities [77]. Treatment of phorbol myristate acetate (PMA)-stimulated THP-1 monocytic cells with 15d-PGJ2 resulted in an inhibitory effect on IL-1, IL-6 and TNF-␣ secretion, increasing IL-1 receptor antagonist (IL-1Ra) production. In contrast, rosiglitazone, pioglitazone and troglitazone barely inhibited pro-inflammatory cytokines, but strongly enhanced the production of IL-1Ra from PMA-stimulated THP-1 cells. Unstimulated cells did not respond to TZDs in terms of IL-1Ra production, suggesting that in order to be effective, PPAR-␥ ligands depend on PMA signaling [78]. To this point, it should be noted that in response to inflammatory cytokines, chondrocytes and synovial fibroblasts produce high amounts of prostaglandins, such as 15dPGJ2, which may be considered as a putative endogenous regulator of the inflammatory reaction. In agreement with the previous studies, 15d-PGJ2, in contrast to rosiglitazone, strongly inhibited the

synthesis of prostaglandins in IL-1␤-stimulated rat chondrocytes. This inhibition by 15d-PGJ2 was occurred through inactivation of NF-␬B pathway. Inhibitory effects of 10 ␮M 15d-PGJ2 were neither reduced by PPAR-␥ blockade with GW-9662 nor enhanced by PPAR-␥ overexpression, supporting strong evidence for a PPAR-␥independent mechanism [79]. Likewise, the suppression of MMP-3 secretion from fibroblast-like synoviocytes by ajulemic acid, a nonpsychoactive cannabinoid acid, appeared to be PPAR-␥ independent [80]. The same group also revealed that the addition of ajulemic acid (3–30 ␮M) to human monocyte derived macrophages in vitro reduced steady state levels of IL-6 mRNA and the subsequent secretion of IL-6 from LPS-stimulated cells through PPAR-␥ independent mechanism [81]. Yamasaki et al. showed a negative regulatory function for PPAR␥ on cytokine and MMP production together with inhibition of cytokine-mediated inflammatory responses in RA synovial cells. In fact, incubation of fibroblast-like synovial cells with troglitazone inhibited endogenous production of TNF-␣, IL-6 and IL-8, as well as MMP-3, without inducing apoptosis of the cells. A significant reduction in the DNA binding activity of NF-␬B in troglitazonetreated synoviocytes in response to TNF-␣ or IL-1␤ was also noted. It was therefore assumed that synoviocytes could differentiate into adipocyte-like cells in the presence of proper stimulatory signals including PPAR-␥ [82]. In a more recent study by the same group, cultivation of fibroblast-like synoviocytes isolated from RA patients with IFN-␥, TNF-␣ or IL-1␤ inhibited the expression of PPAR-␥, as well as C/EBP nuclear activity, and thus suppressed adipocyte-like cell differentiation in vitro [69]. 15d-PGJ2 was released by human articular chondrocytes and was found in joint synovial fluids obtained from RA patients, while pro-inflammatory cytokines such as IL-1␤ and TNF-␣ up-regulated chondrocyte release of 15d-PGJ2. Interestingly, 15d-PGJ2-induced apoptosis of chondrocytes from RA patients, as well as control nonarthritic subjects in a time- and dose-dependent manner through a PPAR-␥-mediated mechanism [67]. Kawahito et al. demonstrated that PPAR-␥ activation by 15d-PGJ2 and troglitazone (20 and 40 ␮M, respectively) induced RA synoviocyte apoptosis, in vitro [59]. 15d-PGJ2 and troglitazone reduced the proliferation and induced apoptosis (10 and 30 ␮M, respectively) in RA synovial cells. PPAR-␥ ligands and NSAIDs increased the activation of PPAR-␥ in

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synovial cells. Furthermore, the ability of NSAIDs and PPAR-␥ ligands to stimulate the activation of PPAR-␥ correlated with their ability to decrease cell viability and to induce DNA fragmentation in synovial cells [58]. Celecoxib suppressed the proliferation of RA synovial fibroblasts through COX-2-independent and PPAR␥-independent induction of apoptosis. Selective COX-2 inhibitors, including indomethacin, which can function as a PPAR-␥ agonist, at concentrations up to 100 ␮M, did not induce apoptosis of RA synovial fibroblasts [83]. In contrast, Ji et al. revealed by the use of a XTT assay and fluorescence activated cell sorter analysis that both troglitazone and 15d-PGJ2 neither reduced proliferation nor induced apoptosis in RA synoviocytes [54]. Several in vivo studies supported evidence that PPAR-␥ ligands can inhibit a number of essential biological pathways responsible for the structural changes that occur in RA. In particular, administration of rosiglitazone or pioglitazone (30 mg/kg/day) significantly inhibited the adjuvant-induced increase in the formation of nitrotyrosine and the expression of iNOS on both ankle and temporomandibular joints in mice with RA. Rosiglitazone also inhibited the adjuvant-induced expression of M30 positive cells, as a marker of apoptosis, in the joint tissues. In addition, treatment with rosiglitazone or pioglitazone (30 ␮M) inhibited LPS plus TNF-␣-induced protein expression of iNOS, COX-2, ICAM-1 and nitrotyrosine formation in RAW 264 cells, a murine macrophagelike cell line. Furthermore, in PPAR-␥-transfected HEK293 cells, rosiglitazone inhibited the TNF-␣-stimulated response using NF␬B-mediated transcription reporter assay, suggesting evidence that PPAR-␥ ligands may possess anti-inflammatory activity against adjuvant-induced arthritis via NF-␬B inactivation [84]. In a model of rat adjuvant-induced arthritis, rosiglitazone (10 mg/kg/day) or pioglitazone (30 mg/kg/day) reduced fever peaks associated with acute or chronic inflammation, respectively, and therefore diminish arthritis severity. At these doses, TZDs reduced synovitis and synovial expression of TNF-␣, IL-1␤ and basic fibroblast growth factor (bFGF) without affecting neovascularization or the expression of VEGF. Both TZDs failed to prevent cartilage lesions and arthritis-induced inhibition of proteoglycan synthesis, aggrecan mRNA level or glycosaminoglycan content in patellar cartilage, but reduced bone erosions and inflammatory bone loss. Both TZDs increased the expression of PPAR-␥ and adiponectin in adipose tissue, confirming that they were activating PPAR-␥ in inflammatory conditions [85]. In another study by Johnson et al., oral administration of ajulemic acid, which binds to and directly activates PPAR-␥, was shown to prevent joint tissue injury in rats with adjuvant arthritis via receptor independent mechanism [80]. In an experimental collagen-induced arthritis mouse model, THR0921, a PPAR-␥ agonist with potent anti-diabetic properties, attenuated collagen-induced arthritis in part by reducing the immune response. In fact, THR0921 (50 mg/kg/day) significantly reduced both clinical disease activity scores and histological scores of joint destruction. Proliferation of isolated spleen cells, as well as circulating levels of IgG antibody to type II collagen, was decreased by THR0921. Moreover, spleen cell production of IFN-␥, TNF-␣ and IL-1␤ in response to exposure to lLPS or type II collagen was reduced by in vivo treatment with THR0921. Steady state mRNA levels of TNF-␣, IL-1␤, MCP-1 and receptor activator of nuclear factor ␬B ligand (RANKL) in isolated joints were all decreased in mice treated with THR0921 [86]. In the same model, CLX-090717 (10 mg/kg/day) reduced clinical signs of arthritis and damage to joint architecture [73]. In fact, CLX-090717 resulted in significant reduction in anti-type II collagen immune responses, while the observed reduction in paw swelling in combination with histological benefit indicated effective control of joint inflammation [73]. Moreover, administration of 15d-PGJ2 (1 mg/kg/day) and troglitazone (100 mg/kg/day) ameliorated adjuvant-induced arthritis with suppression of pannus formation and mononuclear cell infiltra-

167

tion in female Lewis rats [59]. However, a troglitazone dose of 100 mg/kg/day was required to suppress adjuvant-induced arthritis to the same degree as 1 mg/kg/day of 15d-PGJ2 [59]. Thus, in agreement with the aforementioned in vitro data, 15d-PGJ2 appeared to be 5–30 times more potent than TZDs at inducing apoptosis in synoviocytes and inhibiting macrophage-derived cytokines [12]. In Fig. 1, the involvement of PPAR-␥ in signaling pathways implicated in the pathogenesis of arthritis, including RA is depicted. 4.3. Psoriatic (PsA) and gouty arthritis In a pilot study of 10 patients with PsA, pioglitazone appeared to be a promising therapeutic approach. After 12 weeks with a daily dose of 60 mg pioglitazone, six of 10 patients met the psoriatic arthritis response criterion. The mean percentage reduction in psoriasis area and severity index was 38%, with a clinically meaningful psoriasis area and severity index (PASI) 50 response in 2 of 6 patients. However, 3 patients had to be withdrawn from the study due to inefficacy and side effects, such as oedema of the lower extremities and increase in weight [87]. Concerning gouty arthritis, it was shown that MSU crystals rapidly and selectively induced PPAR-␥ expression by monocytes. The induced PPAR-␥ was functional, since troglitazone was able to up-regulate CD36 expression on monocytes. 15d-PGJ2 significantly reduced the crystal-induced production of cytokines by monocytes. Indomethacin inhibited cytokine production only at high concentrations, and troglitazone failed to exert significant effects. Administration of troglitazone and 15d-PGJ2 significantly prevented cellular accumulation in a mouse air-pouch model of MSU crystal-induced acute inflammation. Thus, rapid induction of PPAR-␥ expression on monocytes by MSU crystals may contribute, at least in part, to the spontaneous resolution of acute attacks of gout [88]. 5. Conclusions At present, there is quite a lot of evidence to support that PPAR-␥ is involved in several inflammatory signaling pathways associated with arthritis. PPAR-␥ is expressed by the main cell types of joints. Several natural and synthetic PPAR-␥ ligands proved to be capable of inhibiting major signaling pathways of inflammation, reducing the synthesis of cartilage catabolic factors responsible for articular cartilage degradation in arthritis. In fact, PPAR-␥ ligands were shown to inhibit the production of several pro-inflammatory mediators, such as the cytokines: TNF-␣, IL-1␤, and IL-6, the MMPs: MMP-1, -3 and -13, the chemokines: MCP-1 and IL-8 and the metabolic proteins: COX-2 and iNOS. Two inflammatory signaling pathways have mainly been considered as potential targets for PPAR-␥ ligands. First, MAPKs, which are implicated in the production of pro-inflammatory cytokines and downstream signaling events leading to joint inflammation and destruction, appear to be potential targets for PPAR-␥ ligands. Second, NF␬-B activation, which results in the transactivation of responsive genes that contribute to the inflammatory phenotype of arthritis, including TNF-␣, MMPs and chemokines, seems to be suppressed by PPAR-␥ ligands. There is also substantial evidence that PPAR-␥ ligands can induce apoptosis in both chondrocytes and synovial cells. Alarming enough, both receptor dependent and independent effects were shown to be elicited by PPAR-␥ ligands. Thus, future studies should be orientated to the use of PPAR-␥ antagonists or PPAR-␥ deficient cells or organisms to define more accurately the receptordependent or -independent effects of PPAR-␥ ligands on arthritis. Moreover, the side effects, such fluid retention, oedema and congestive heart failure, which have been reported for both rosiglitazone and pioglitazone, should be taken into careful consideration for the

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chronic clinical exploitation of PPAR-␥ ligands on arthritis. As the available evidence so far is restricted to in vitro experiments and animal studies, evaluation of the clinical outcome of PPAR-␥ ligands in the management of patients with different types of arthritis is also recommended. As inflammation is a multifaceted process involving a wide range of mediators, such as cytokines, chemokines and adhesion molecules, the final outcome of PPAR-␥ ligands seems difficult to be accurately assessed and depends significantly on experimental models and/or treatment conditions. In inflammatory conditions in both bone and cartilage, the commitment of cells belonging to the immune system could modify the final action-outcome of PPAR-␥ ligands in specific cell types, such as chondrocytes, synovial cells, osteoblasts or osteoclasts. In this aspect, the secretion of inflammatory cytokines and endogenous PPAR-␥ ligands such as prostaglandins, both being capable of affecting the differentiation of bone and joint cells and activating the PPAR-␥, should be taken into consideration. This statement further raises the question whether the final effect of PPAR-␥ ligands is completely ascribed to the dose of PPAR-␥ ligand treatment or to the induction of endogenous PPAR-␥ activators, such as 15d-PGJ2. In this aspect, a more stringent approach regarding the concentration range and the duration of exposure could be applied for a more sufficient evaluation of the involvement of PPAR-␥ on arthritis to be obtained. Moreover, in view of the fact that 15d-PGJ2 is a poor PPAR-␥ tool compound and an endogenous bioactive lipid that is not used clinically, the available data so far concerning 15d-PGJ2 needs to be resituated using more specific tools, such as the two clinically relevant PPAR␥ ligands, PGZ and RGZ or the new generation of tyrosine- and indole-based PPAR-␥ ligands. However, the majority of studies so far rendered 15d-PGJ2 as the most potent PPAR-␥ ligands to prevent the articular cartilage degradation in arthritis and therefore it could be considered as a potential lead compound for the design of novel therapeutic agents to treat arthritis. Conflict of interest statement No conflicts of interest. References [1] Abramson SB, Attur M, Yazici Y. Prospects for disease modification in osteoarthritis. Nat Clin Pract Rheumatol 2006;2:304–12. [2] Goldring MB, Goldring SR. Osteoarthritis. J Cell Physiol 2007;213:626–34. [3] Firestein GS. Evolving concepts of rheumatoid arthritis. Nature 2003;423: 356–61. [4] Slezes L, Torocsik D, Nagy L. PPAR␥ in immunity and inflammation: cell types and diseases. Biochim Biophys Acta 2007;1771:1014–30. [5] Muller-Ladner U, Pap T, Gay RE, Neidhart M, Gay S. Mechanisms of disease: the molecular and cellular basis of joint destruction in rheumatoid arthritis. Nat Clin Pract Rheumatol 2005;1:102–10. [6] Shirtliff ME, Mader JT. Acute septic arthritis. Clin Microbiol Rev 2002;15: 527–44. [7] Hueber AJ, McInnes IB. Immune regulation in psoriasis and psoriatic arthritis—recent developments. Immunol Lett 2007;114:59–65. [8] Ene-Stroescu D, Gorbien MJ. Gouty arthritis. A primer on late-onset gout. Geriatrics 2005;60:24–31. [9] Thalhamer T, McGrath MA, Harnett MM. MAPKs and their relevence to arthritis and inflammation. Rheumatology 2008;47:409–14. [10] Raman M, Chen W, Cobb MH. Differential regulation and properties of MAPKs. Oncogene 2007;26:3100–12. [11] Simmonds RE, Foxwell BM. NF-␬B and its relevance to arthritis and inflammation. Rheumatology 2008;47:584–90. [12] Sacre SM, Andreakos E, Taylor P, Feldmann M, Foxwell BM. Molecular therapeutic targets in rheumatoid arthritis. Expert Rev Mol Med 2005;7:1–20. [13] Shi S, Klotz U. Clinical use and pharmacological properties of selective COX-2 inhibitors. Eur J Clin Pharmacol 2008;64:233–52. [14] Tu G, Xu W, Huang H, Li S. Progress in the development of matrix metalloproteinase inhibitors. Curr Med Chem 2008;15:1388–95. [15] Desvergne B, Wahli W. Peroxisome proliferator-activated receptors: nuclear control of metabolism. Endocr Rev 1999;20:649–88. [16] Berger J, Moller DE. The mechanisms of action of PPARs. Annu Rev Med 2002;53:409–35.

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