Tài liệu Báo cáo khoa học: Molecular aspects of rheumatoid arthritis: role of transcription factors ppt

MINIREVIEW Molecular aspects of rheumatoid arthritis: role of transcription factors Hiroshi Okamoto 1 , Thomas P. Cujec 2 , Hisashi Yamanaka 1 and Naoyuki Kamatani 1 1 Institute of Rheumatology, Tokyo Women’s Medical University, Japan 2 Ambrx, Inc. La Jolla, CA, USA The central dogma of molecular biology is that DNA produces RNA, which, in turn, produces protein. In the process of transcription, RNA is produced from the DNA and this conversion is an essential element in gene expression. The central role of transcription in the process of gene expression makes it an attractive control process for regulating the expression of genes in particular cell types or in response to a particular signal, such as a cytokine. To study this control mech- anism, the DNA sequences within individual genes that are essential for basal or regulated gene expression have been extensively studied. In most eukaryotic genes a TATA box is found upstream of the site of transcriptional initiation, although this element is lack- ing in housekeeping genes and in some tissue-specific genes. In the genes without a TATA box, a sequence known as the initiator element, which is located over the start site of transcription, appears to play a critical role in determining the initiation point and acts as a minimal promoter capable of producing basal levels of transcription. In TATA-less promoters, the weak acti- vity of the promoter is dramatically increased by other elements located upstream of the proximal promoter region. These elements are found in a wide variety of Keywords NFAT; NF-jB; rheumatoid arthritis; transcription factors Correspondence H. Okamoto, Institute of Rheumatology, Tokyo Women’s Medical University, 10-22 Kawada-cho, Shinjuku, Tokyo 162-0054, Japan Fax: +81 3 5269 1726 Tel: +81 3 5269 1725 E-mail: hokamoto@ior.twmu.ac.jp (Received 14 March 2008, accepted 22 May 2008) doi:10.1111/j.1742-4658.2008.06582.x Rheumatoid arthritis is a multifactorial disease characterized by chronic inflammation of the joints. Both genetic and environmental factors are involved in the pathogenesis leading to joint destruction and ultimately dis- ability. In the inflamed RA joint the synovium is highly infiltrated by CD4 + T cells, B cells and macrophages, and the intimal lining becomes hyperplastic owing to the increased number of macrophage-like and fibro- blast-like synoviocytes. This hyperplastic intimal synovial lining forms an aggressive front, called pannus, which invades cartilage and bone struc- tures, leading to the destruction and compromised function of affected joints. This process is mediated by a number of cytokines (tumor necrosis factor-a, interleukin-1, interleukin-6, interleukin-17 interferon-c, etc.), chemokines (monocyte chemoattractant protein-1, monocyte chemo- attractant protein-4 CCL18, etc.), cell adhesion molecules (intercellular adhesion molecule-1, vascular cell adhesion molecule-1, etc.) and matrix metalloproteinases. Expression of these molecules is controlled at the tran- scription level and activation of a limited number of transcription factors is involved in this process. Abbreviations AGE, advanced glycation end-product; AP-1, activator protein-1; FLIP, Fas-associated death domain-like interleukin 1b-converting enzyme- inhibitory protein; FLS, fibroblast-like synoviocytes; GM-CSF, granulocyte–macrophage colony-stimulating factor; IKK, IjB kinase; IL, interleukin; IjB, inhibitor of NF-jB proteins; MMP, matrix metalloproteinase; NFAT, nuclear factor for activation of T cells; NF-jB, nuclear factor-jB; PPAR, peroxisome proliferator-activated receptor; PPRE, peroxisome proliferator response element; RA, rheumatoid arthritis; RAGE, receptor for advanced glycation end-products; RANKL, receptor activation of NF-jB ligand; SAA, serum amyloid A; TNF-a, tumor necrosis factor-a. FEBS Journal 275 (2008) 4463–4470 ª 2008 The Authors Journal compilation ª 2008 FEBS 4463 genes and play a role in stimulating the constitutive activity of promoters. In addition, the presence of spe- cific DNA sequences that can bind particular proteins will confer on a specific gene the ability to respond to particular stimuli. Such binding proteins are transcrip- tional factors. In this review, we focus on the role of transcriptional factors on the pathology of rheumatoid arthritis (RA). Nuclear factor-jB The nuclear factor-jB (NF-jB) proteins are a family of ubiquitously expressed transcription factors that play an essential role in most immune and inflamma- tory responses. In mammals, the NF-jB family con- sists of five members: RelA (p65), RelB, c-Rel, NF- jB1 (p50 and its precursor p105) and NF-jB2 (p52 and its precursor p100). They form a variety of homodimers and heterodimers, each of which activates its own characteristic set of genes, and share a 300- amino acid domain (designated the Rel homology domain) that mediates their DNA binding, dimeriza- tion and nuclear translocation [1–3]. Although, the most prevalent activated form is the heterodimer RelA (p65) and p50, different dimers can bind to the same or distinct sites in NF-jB-dependent promoters, thus regulating the transcription of response genes in a cell-type and stimulus-type specific manner [4,5]. The NF-jB proteins are retained in an inactive form in the cytoplasm through their interaction with inhibitor of NF-jB proteins (IjB). Cellular stimulation, by cyto- kines such as tumor necrosis factor-a (TNF-a) and interleukin (IL)-1b, activate the inhibitor of NF-jB kinase [IjB kinase (IKK) complex] and then this com- plex phosphorylates IjB, which leads to its ubiquitina- tion and subsequent proteosomal degradation. Degradation of IjB enables NF-jB to translocate to the nucleus, leading to stimulation of the transcription of genes containing the consensus jB sequence 5¢-GGGPuNNPyPyCC-3¢ (where Pu denotes a purine and Py denotes a pyrimidine). The genes containing the jB sequence include cytokine and chemokine genes [TNF, IL-1, IL-2, IL-6, macrophage inflammatory pro- tein-1b, macrophage inflammatory protein-2, regulated on activation, normal, T-cell expressed, and secreted (RANTES), etc.], adhesion molecule genes (E-selectin, intercellular adhesion molecule-1, vascular cell adhe- sion molecule-1, etc.), anti-apoptosis genes [XIAP, c-IAPs, c-Fas-associated death domain-like interleukin 1b-converting enzyme-inhibitory protein (c-FLIP), survivine, bcl-2, bcl-x L , etc.], NF-jB family genes (p52 ⁄ p100, p50 ⁄ p105, c-Rel, IjBa, etc.), cell prolifera- tion-associated genes [cyclin D1, c-Myc, bone morpho- genetic protein-2 (BMP-2), etc.], viral genes [HIV-1, simian immunodeficiency virus, Epstein–Barr virus, etc.) and others [matrix metalloproteinases (MMPs), vascular endothelial growth factor, inducible nitric oxide synthase, cyclooxygenase-2, etc.). Some of these genes have been reported to have important roles in the pathogenesis of RA [1]. In addition, the NF-jB family of genes has been reported to be highly expressed and activated in RA-affected tissues, and several interventions, such as dominant-negative IKK and antisense NF-jB oligonucleotides, have effectively prevented the expression of cytokines and the develop- ment of arthritis in vitro and in animal models. Fur- thermore, NF-jB has been reported to contribute to the fierce proliferation of synovial cells. Several lines of evidence suggest that RA synovial cells proliferate as fiercely as tumor cells and that this aggressive pro- liferation plays an important role in the pathogenesis of RA. Synovial hyperproliferation has been reported to be caused, at least in part, by impaired apoptosis of synovial cells and deficient apoptosis of synovial cells resulting from the upregulation of anti-apoptotic mole- cules such as bcl-2 and FLIP [6,7]. Thus, NF-jB con- tributes to the hyperproliferation of synovial cells in RA by regulating the gene expression of FLIP and bcl-2. Nuclear factor-jB is activated by various inducers, including cytokines (TNF-a, IL-1b, IL-2, IL-17, etc.), mitogens [B-lymphocyte activating factor (BAFF), CD40 ligand, etc.] and stess ⁄ cartinogens (ultraviolet light, hypoxia, 4b-phorbol 12-myristate 13-acetate, etc.). Another inducer is the serum amyloid A (SAA) protein, an acute-phase protein produced by hepatocytes in response to pro-inflammatory cytokines, and its expres- sion is up-regulated during the course of the inflamma- tory process [8]. Although a wealth of information concerning the diagnosis and pathogenesis of AA amy- loidosis has accumulated, the biological role(s) of SAA in the pathogenesis of RA is still not fully understood. Mullan et al. [9] reported that acute-phase SAA was as effective at increasing the time-dependent and dose- dependent expression of intercellular adhesion mole- cule-1 and vascular cell adhesion molecule as IL-1b and TNF-a, and that their expression was partially mediated by NF-jB signaling. The accumulation of advanced glycation end-products (AGEs), S100A12 and high-mobility-group-box chromosomal protein 1 (HMGB1) has been associated with joint inflammation in RA. The receptor for these proteins, termed recep- tor for AGEs (RAGE) has been reported to be highly expressed in synovial tissue macrophages from RA patients [10]. RAGE has also been reported to be a receptor for the amyloidogenic form of SAA [11]. Role of transcription factors H. Okamoto et al. 4464 FEBS Journal 275 (2008) 4463–4470 ª 2008 The Authors Journal compilation ª 2008 FEBS From these findings, we hypothesized that acute-phase SAA could bind to RAGE on the surface of synovial cells, thereby resulting in NF-jB signaling and the active promotion of RA-mediated joint inflammation. To study the biological implication of SAA expression in RA joints, we further analyzed the in vitro effects of SAA. We studied the effects of SAA on cytokine pro- duction from fibroblast-like synoviocytes (FLS) and found that SAA induced expression of the pro-inflam- matory cytokines IL-6 and IL-8 in a dose-dependent manner. Serum amyloid A stimulated the transcrip- tional activation by NF-jB in a dose-dependent manner in a reporter gene assay in 293T cells transfect- ed with p4xjB-Luc plasmid. We studied the effects of SAA on NF-jB activation and found that SAA induced the degradation of IjBa as well as IL-1b (10 ngÆmL )1 ). In order to study whether the effect of SAA on NF-jB activation is mediated through the binding of SAA to RAGE on synovial cells, we pre- incubated SAA with various concentrations of soluble recombinant RAGE protein before adding it to the FLS. We observed a dose-dependent inhibition of SAA-induced IjBa degradation. By immunofluores- cent studies, we also found that SAA stimulation promoted nuclear translocation of NF-jB, whereas pre-incubation of SAA with RAGE inhibited nuclear translocation [12]. These data suggested that SAA of RA joints is actively involved in the pathogenesis of RA through the SAA–RAGE–NF-jB signaling path- way (Fig. 1). We found that angiotensin II is also an inducer of NF-jB activation in FLS. We have shown that angio- tensin II activated NF-jB in synovial cells to induce the monocyte chemoattractant protein-1 and that the angiotensin receptor blocker inhibited this activation [13]. It is noteworthy that some anti-RA drugs, including corticosteroids, have been shown to block the NF-jB activation cascade (Fig. 2). Among the drugs currently used for the treatment of diseases other than RA, such as diabetes, hyperlipidemia and hypertension, there are some drugs that have the potential to inhibit NF-jB activation. To seek other candidate compounds for use in an anti-RA strategy, we studied several drugs that have pleiotropic actions on the NF-jB activation cas- cade. Ligands for peroxisome proliferator-activated receptors (PPARs) are such examples. Peroxisome proliferator-activated receptors are members of the nuclear hormone receptor family, the largest family of transcription factors [14]. Three distinct members of the PPAR subfamily have been reported: a, d (also called b, NUC-1) and c, all of them being activated by naturally occurring fatty acids or fatty acid derivatives. Peroxisome proliferator-activated receptors heterodi- merize with the retinoid X receptor and regulate tran- scription of target genes through binding to specific peroxisome proliferator response elements (PPREs), which consist of a direct repeat of the nuclear receptor hexameric DNA core recognition motif spaced by one nucleotide. In addition to the regulation of gene tran- scription via PPREs, PPARs modulate gene expression in a DNA-binding-independent manner. Peroxisome proliferator-activated receptor-a is highly expressed in liver, heart, muscle, kidney and cells of the arterial wall and it is activated by fibrate, fatty acids and eico- sanoids. Peroxisome proliferator-activated receptor-a ligands inhibit IL-1-induced production of IL-6 and prostaglandin and inhibit the expression of cyclooxy- genase-2 by negatively interfering with NF-jB tran- scriptional activity. Peroxisome proliferator-activated receptor-a ligands are thought to inhibit NF-jB activ- ity by inducing IjBa, which, in turn, inhibits NF-jB signaling. Peroxisome proliferator-activated receptor-c is expressed at high levels in adipose tissue, is a critical regulator of adipocyte differentiation and reportedly plays a role in glucose homeostasis and insulin sensi- tivity. In addition, PPAR-c has been suggested to be an important immunomodulatory factor that is expressed in cells of the immune system, specifically in the spleen, monocytes, bone-marrow precursors and helper T cells [15]. Peroxisome proliferator-activated receptor-c ligands also reportedly inhibit disease pro- gression of inflammatory bowel diseases, ischemic heart diseases, experimental autoimmune encephalomy- elitis and RA [16]. These PPAR-c ligands inhibit gene expression by preventing the phosphorylation of IKK, which, in turn, reduces the activity of the transcription SAA Chondrocytes synovial cells SAA blood circulation RAGE P P Ubiquitination and (Cytoplasm) p65 Phosphorylation of IκBα degradation of IκBα p50 IκBα IκBα at Ser 32/36 p65 p50 p50 (Nucleus) IL-6, IL-8, MMPs, etc. Target gene SAA SAA p65 Fig. 1. SAA in RA joints binds to RAGE on synovial cells and acti- vates the NF-jB signaling pathway in these cells. H. Okamoto et al. Role of transcription factors FEBS Journal 275 (2008) 4463–4470 ª 2008 The Authors Journal compilation ª 2008 FEBS 4465 factor, NF-jB [17]. Taken together, these findings sug- gest that PPAR-a and PPAR-c may negatively regu- late the inflammatory processes in RA. To examine the induction of IL-6, IL-8 and granulocyte–macrophage colony-stimulating factor (GM-CSF), FLS obtained from RA patients were stimulated with 10 ng ÆmL )1 of IL-1b. Interleukin-6, IL-8 and GM-CSF production from FLS were suppressed in a dose-dependent man- ner in the presence of PPAR-c ligands and a PPAR-a ligand, fenofibrate. Neither PPAR-a nor PPAR-c ligands inhibit basal level expression of these cyto- kines, and these compounds are not toxic to FLS. Next, we examined whether PPAR-a and PPAR-c ligands inhibit nuclear translocation of NF-jBinan immunohistochemical assay. As shown in Fig. 3A, FLS were incubated in the presence of 10 ngÆmL )1 of IL-1b in order to stimulate NF-jB nuclear transloca- tion. As expected, without IL-1b stimulation, NF-jB remained localized in the cytoplasm. However, after 30 min of stimulation with IL-1b, NF-jB was mainly localized in the nucleus. In the presence of 100 lm pioglitazone, or fenofibrate, nuclear localization of NF-jB was inhibited. These results are consistent with the PPAR-induced suppression of cytokine expression described above and indicate that this suppression is caused by the inhibition of NF-jB nuclear trans- location in FLS. To investigate further the anti-NF-jB effects of these compounds, we performed western γ Aspirin and glucocorticoids I κB α [Nature (2000) 403, 103–108] [ J Clin Invest ( 1998 ) 101 1163–1174] IL-1 receptor (Cytoplasm) NIK PPAR- α ligands [J Biol Chem (2000) 275, 36703–36707] PPAR-α ligands, aspirin, salicylate, sulindac [Nature (1998) 396, 77–80, Sulfasalazine [ J Clin Invest ( 1998 ) 101, 1163 1174] NIK IKK Complex β α IκBα P P Ubiquitination and degradation of IκBα J Biol Chem (1999) 274, 27307–27314] p65 p50 IκBα Phosphorylation of IκBα at Ser 32/36 IκBα p65 p50 PPAR- α I d ti f I B (Nucleus) p50 p65 Target gene Co-activators In d uction o f IκBα TNF- α, Cox-2, IL-1, IL-6, IL-8, MMPs, etc. Glucocorticoids [ Science (1995) 270, 286–290, Science (1994) 265, 956–959, Science (1994) 270, 283–286] [ Mol Cell Biol (1995) 15, 943–953] Fig. 2. NF-jB activation pathway and the site of inhibition of various compounds. PLC l d Ca 2+ PLC coupled receptor CRAC (cytoplasm) Ca 2+ Calmodulin NFAT P PLC- IP 3 Ca 2+ Ca 2 + Ca 2+ Ca 2+ C 2+ Ca 2+ Ca 2+ Ca 2+ Calcineurin A ER Ca 2 Ca 2+ Ca 2+ Ca 2+ Ca 2+ Ca 2+ Ca 2+ p38 MAP kinase MEKK-1 CsA FK-506 Cyclophilin FKBP (Nucleus) NFAT DYRK Target ge ne Co-activators NFAT IL-2, IL-4, IFN- , GM-CSF, CD40L, TNF-α, αγ etc. Calcineurin B Fig. 3. NFAT activation pathway and the site of inhibition of immunosuppressive drugs CD40 ligand (CD40L), dual-specificity tyrosine-regulated kinase (DYRK), endo- plasmic reticulum (ER), mitogen-activated protein (MAP) kinase, MAPK kinase kinase-1 (MEKK-1). Role of transcription factors H. Okamoto et al. 4466 FEBS Journal 275 (2008) 4463–4470 ª 2008 The Authors Journal compilation ª 2008 FEBS blots to detect IjBa degradation by the IL-1b signal. As demonstrated, the PPAR-c ligand (pioglitazone) and fenofibrate inhibited the IL-1b-stimulated degrada- tion of IjBa. Therefore, PPAR-a and PPAR-c ligands induced NF-jB signaling in FLS, as illustrated in Fig. 2. We tested the effect of PPAR-a and PPAR-c ligands in vivo on the progression and severity of adju- vant-induced arthritis in female Lewis rats and found that pioglitazone and fenofibrate suppressed the pro- gression of clinical arthritis compared with control rats treated with NaCl ⁄ P i , as demonstrated by paw volume and arthritis score. These data suggest that both PPAR-a and PPAR-c ligands have anti-arthritis effects in vivo [18]. Considering the wide array of events under the control of NF-jB, including cytokine and cyclo- oxygenase-2 expression, osteoclast differentiation and apoptosis, and the impact of these events on the path- ogenesis of RA, NF-jB is an efficient and feasible therapeutic target for RA. Therapy with fenofibrate may serve as a new anti-NF-jB strategy for the treat- ment of RA. We have also shown, by case reports, that fenofibrate is useful for the treatment of RA and autoimmune hepatitis [19,20]. Besides its involvement in immunoregulation, NF- jB has also been reported to be associated with the inhibition of programmed cell death and has an impor- tant role in the development and homeostasis of the immune, hepatic and nervous systems. The embryonic lethality of RelA-deficient mice was one of the first indications that NF-jB contributes a crucial anti- apoptotic effect during normal development and this embryonic death was attributed to extensive apoptosis of developing hepatocytes. A similar phenotype is seen in mice lacking both copies of IKK, lacking IKK along with IKK, or lacking the IKK regulator NEMO. In addition, the initial cloning of the NF-jB p50 ⁄ p105 subunit cDNA revealed homology to the cellular homolog (c-Rel) of the oncoprotein (v-Rel) from the avian reticuloendotheliosis virus, suggesting a potential link between NF-jB and oncogenesis. In fact, struc- tural alteration of the NF-jB p52 ⁄ p100 subunit encoded by the NFKB2 gene has been reported in cer- tain T-cell lymphomas, chronic lymphocytic leukemias, myelomas and B-cell lymphomas. Amplification of the c-rel gene has also been reported in several types of B-cell lymphoma. In addition to these genetic observa- tions, several lines of evidence have demonstrated that NF-jB transcription targets are linked to pro- mote the oncogenic phenotype. For example, NF-jB can promote retinoblastoma hyperphosphorylation by binding and activating the cyclin D1 promoter, result- ing in progression into the S phase of the cell cycle, and IKK has been proposed to play a role in cyclin D1 transcription through a T-cell factor site in the promoter. Nuclear factor-jB has also been reported to potentiate cancer cell growth by the NF-jB-associated upregulation of hypoxia-inducible factor-1 and its reg- ulation of c-myc transcription. Resistance to apoptosis is a common feature of cancer cells and is associated with the increased expression of anti-apoptotic factors, such as Bcl-2 or Bcl-x L . Nuclear factor-jB directly reg- ulates a potent anti-apoptotic pathway, and genes regulated by NF-jB that suppress apoptosis, such as Bcl-2 and Bcl-x L , are often expressed in human can- cers. Given the strong association between NF-jB and the regulation of apoptosis, many studies suggest that NF-jB controls the anti-apoptotic mechanisms associ- ated with oncogenesis, and extensive evidence demon- strates that compounds which block NF-jB activation can serve as an anticancer strategy [21]. In the pathol- ogy of RA, it is widely accepted that the progressive destruction of articular cartilage is reliant on the evo- lution of hyperplastic synovial tissue, and that hyper- plasia of FLS is dependent on dysregulated proliferation and apoptosis [22]. Methotrexate, which is a well-known antitumor agent, is now widely accepted as a standard therapeutic strategy for RA, and the mechanism of action of methotrexate is thought to be its inhibitory effects on the hyperplasia of synovial tissue. Therefore, any compound that could inhibit the fierce hyperplasia of synovial cells has potential as a promising anti-RA strategy. Ligands for PPAR-c have been reported to inhibit arthritis in ani- mal models through the activation of synoviocyte apoptosis [16]. The anti-arthritis effects of ligands for PPAR-a and PPAR-c might be caused by their pro- apoptotic effects through the inhibition of NF-jB sig- naling. Besides anti-NF-jB compounds, some com- pounds that possess anti-proliferation effects in synoviocytes have been reported as potential candi- dates in anti-RA therapeutic strategies. Lipophilic sta- tins, such as fluvastatin, have been reported to induce apoptosis in RA synoviocytes and have potential as novel therapeutic agents for RA [23]. In addition, a cy- clin-dependent kinase inhibitor, p16INK4a, has been shown to suppress synovial cell proliferation, resulting in inhibition of RA pathology in an animal model [24]. Vitamin K2 (menaquinone-4, MK-4) has been reported to induce apoptosis in hepatocellular carcinoma, leuke- mia and MDS cell lines. Thus, we investigated the effect of MK-4 on the proliferation of rheumatoid synovial cells and the development of arthritis in a col- lagen-induced rat model. Our results indicated that MK-4 inhibited the proliferation of cultured synovial fibroblasts and the development of collagen-induced arthritis in a dose-dependent manner. We concluded H. Okamoto et al. Role of transcription factors FEBS Journal 275 (2008) 4463–4470 ª 2008 The Authors Journal compilation ª 2008 FEBS 4467 that MK-4 may represent a new agent for the treat- ment of RA in a combination therapy with other dis- ease-modifying antirheumatic drugs [25,26]. Nuclear factor for activation of T cells Ca 2+ is a sig- naling molecule that functions in a great variety of organs and cells. One of the roles of Ca 2+ is to regulate calcineurin, which in turn dephosphorylates and induces the nuclear localization of the cytoplasmic components of nuclear factor for activation of T cells (NFAT) transcription complexes. In the nucleus, NFAT transcription complexes assemble on target DNA to activate the expression of genes such as IL-2, IL-3, GM-CSF, IL-4, IL-5, IL-13, IFN-c, TNF-a, CD40 ligand and Fas ligand, etc. (Fig. 3). Ligand bind- ing of various receptors results in the activation of phospholipase C (PLC), the release of inositol 1,4,5-tri- phosphate (IP 3 ), and a transient release of Ca 2+ from intracellular stores through IP 3 receptors. This initial release of Ca 2+ is not sufficient to activate NFAT tar- get genes, and an influx of Ca 2+ through Ca (2+) - release-activated Ca (2+) (CRAC) channels is required [27]. Pharmacologic inhibitors of NFAT translocation, such as tacrolimus (FK506) and cyclosporine A, are administered to patients as part of the transplant ther- apy because of their ability to prevent an immune response to transplanted tissue. These compounds bind to two different intracellular proteins, namely FK506- binding protein (FKBP) and cyclophilin, and the drug–protein complex then binds to the interface of the calcineurin A ⁄ B complex and blocks its phosphatase activity by preventing substrate access. Initial evidence showing the importance of NFATs in the pathogenesis of RA is the clinical observation that treatment with cyclosporine A is effective in otherwise refractory RA. Furthermore, tacrolimus (FK506) is now widely used as a treatment of RA [28]. Experimental evidence has shown that NFATs (NFAT1–5) are expressed in the RA synovium and that NFAT1 knockout mice and NFAT1 ⁄ 4 double- knockout mice developed an asym- metric oligoarthritis [29]. Nuclear factor for activation of T cells has been shown to have roles in the bone destruction of RA. Bone destruction has been shown to be caused by an abnormal activation of the immune system in RA. Osteoclasts are cells of monocyte ⁄ mac- rophage origin and are the key players in the control of bone metabolism. Receptor activation of NF-jB ligand (RANKL) induces osteoclast differentiation in the pres- ence of the macrophage colony-stimulating factor. RANKL activates the TNF receptor-associated factor 6, c-Fos, and calcium signaling pathways, all of which are indispensable for the induction and activa- tion of NFAT1. NFAT1 is the master transcription fac- tor for osteoclast differentiation and regulates many osteoclast-specific genes. Therefore, NFAT plays important roles not only in inflammation but also in osteoclast differentiation, resulting in the bone destruc- tion associated with RA pathology. Activator protein-1 The activator protein-1 (AP-1) transcription factor is composed of members of the Fos, Jun and activating transcription factor families of proteins. While the Fos proteins (Fos, FosB, Fra-1 and Fra-2) can only hetero- dimerize with members of the Jun family, the Jun pro- teins (Jun, JunB and JunD) can both homodimerize and heterodimerize with Fos members to form trans- criptionally active complexes. Activator protein-1 trans- duces extracellular signals to immune cells, resulting in changes in the expression of specific target genes with an AP-1 binding site(s) in their promoter ⁄ enhancer regions. Activator protein-1 can affect the severity of inflammation through several mechanisms, such as (a) activation of cytokine production in co-operation with transcription factors of the NFAT family, (b) regula- tion of naive T-cell differentiation into T helper-1 or T helper-2 cells or (c) interaction and trans-repression of the glucocorticoid receptor. Most cytokine genes are regulated by a transcription factor complex consisting of AP-1 and NFAT, and their co-operation is essential in most of these genes. NFAT and AP-1 have been shown to form highly stable ternary complexes on com- posite DNA-binding sites. As mentioned above, NFAT is a prerequisite for the differentiation of osteoclasts. Fos ⁄ AP-1 is also required for integration of the RANKL and macrophage colony-stimulating factor signals in osteoclast differentiation. Other than osteo- clasts, the expression of MMPs contributes to the bone destruction associated with RA. Expression of MMPs is proposed to be regulated by AP-1, and the upregula- tion of MMPs in the RA synovium correlates with increased DNA-binding activity of AP-1 and increased expression of Fos ⁄ Jun [30]. Thus, AP-1 proteins have significant pathological roles in RA. Other transcription factors Other transcription factors implicated in the patho- genesis of RA are the signal transducer and activator of transcription (STAT) family of proteins, interferon regulatory factors (IRFs), Forkhead (Fox) family pro- teins, T-box transcription factor 21 (TBX21) ⁄ T-box expressed in T cells (T-bet), the CCAAT-enhancer- binding protein family and the Ets transcription factor family [31]. Extensive genetic studies of RA have revealed an association between RA and single nucleo- Role of transcription factors H. Okamoto et al. 4468 FEBS Journal 275 (2008) 4463–4470 ª 2008 The Authors Journal compilation ª 2008 FEBS tide polymorphisms in the Runt-related transcription factor 1 (Runx1)-binding site of the SLC22A4 gene, in the major histocompatibility complex class II trans- activator (MHC2TA) gene, and in the STAT4 gene [32–35]. Concluding remarks Transcription factors play critical roles in the function of immune effector cells, including cytokine ⁄ chemokine expression and also in the control of synovial cell apop- tosis. These cells have prerequisite roles in the pathogen- esis of RA. Growing experimental evidence emphasizes the importance of the NF-jB, NFAT and AP-1 tran- scription factors in RA, and therefore signaling cascades of these transcription factors are feasible targets for a comprehensive anti-RA strategy. 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