Aryl hydrocarbon receptor and experimental autoimmune arthritis
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- Nguyen, N.T., Nakahama, T. & Kishimoto, T. Semin Immunopathol (2013) 35: 637. doi:10.1007/s00281-013-0392-6
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Aryl hydrocarbon receptor (Ahr) is thought to be a crucial factor that regulates immune responses. Many Ahr-mediated immune regulatory mechanisms have been discovered, which will likely enhance our understanding of the molecular pathogenesis of autoimmune inflammation including rheumatoid arthritis (RA). RA is a systemic inflammatory disease that affects approximately 1 % of the population and is characterized by chronic inflammation of the synovium and subsequent joint destruction. Recent findings showed that Ahr plays critical roles in the development of Th17 cells, which are key effector T cells in a variety of human autoimmune diseases including RA. Consistent with these findings, our previous study demonstrated that Ahr in T cells is important for the development of collagen-induced arthritis, a widely used murine model of human RA, possibly via the induction of Th17 cells. In addition, Ahr is an attractive molecule because tobacco smoke is a well-known environmental risk factor for RA development and Ahr agonists, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin, 3-methyl cholanthrene, and benzo[a]pyrene, are major toxic components in cigarettes. This review summarizes recent findings on Ahr functions in immune cells in the context of RA pathogenesis during stimulation with smoking-derived ligands. We also discuss the potential link between Ahr and novel factors, such as microRNAs, in the development of RA, thereby providing further mechanistic insight into this autoimmune disorder.
KeywordsDioxin receptorRheumatoid arthritis
Rheumatoid arthritis (RA) is the most common type of inflammatory and autoimmune arthritis [1, 2]. However, the pathogenic mechanisms of RA remain largely unknown. Multiple factors such as genetics, bacterial and viral infection, environmental pollution, and cigarette smoking may exacerbate the symptoms of RA [1, 2]. The disease primarily affects the synovium, resulting in synovial inflammation. Bone and cartilage destruction also occur via osteoclasts and fibroblast-like synoviocytes. The role of the immune system in the pathogenesis of RA, particularly T cells, macrophages, dendritic cells (DCs), B cells as well as pro- and anti-inflammatory cytokines (IL-6, TNF-α, IL-1, IL-17, IL-10, IL-23, and IFN-γ), autoantibodies, growth and differentiation factors (GM-CSF, M-CSF, and RANKL), and chemokines have been carefully studied [3–13]. Many T cell subsets, including IL-17-producing T helper (Th17) and regulatory T (Treg) cells, have been shown to play important roles in pathogenesis of RA. Th17 cells induced by TGF-β plus IL-6 or IL-21 secret IL-17 isoforms that in association with other cytokines, such as IL-1 or TNF-α, synergistically activate synovial fibroblasts to produce matrix metalloproteinases. On the other hand, Treg cells, induced by TGF-β signaling, exert suppressive functions that are important for immunologic tolerance. The Th17/Treg cell balance is a new paradigm for autoimmunity [14, 15]. Moreover, DCs and macrophages are potent antigen presenting cells that contribute to RA development because they are present in the synovium, stimulate T lymphocytes and regulate the cytokine and chemokine milieu.
To date, the inhibition of inflammatory cytokines such as IL-6 and TNF-α have been promising therapies for RA [16–24]. Treatment strategies targeting key cytokines that reduce inflammation have been examined, including IL-6 (tocilizumab), TNF-α (infliximab and etanercept), and IL-1 (anakinra).
Accumulating data from recent studies have shown that aryl hydrocarbon receptor (Ahr), known as dioxin receptor, plays a key role in regulating the differentiation of T cell subsets, including Th17 and Treg cells, as well as altering the functions of DCs and macrophages that consequently effect the development of several autoimmune diseases including RA [25–34]. Cigarette smoke-derived chemical substances such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), benzo[a]pyrene (BaP), and 3-methylcholanthrene (3-MC) are well-known environmental risk factors for RA. These factors are also strong Ahr agonists, suggesting that Ahr is closely associated with RA pathogenesis. Several reports have indicated that Ahr plays a role in exacerbating RA in smokers [35–38].
In this review, we summarize recent findings suggesting that Ahr regulates immune responses that lead to the development of autoimmune arthritis. Furthermore, we discuss the function of potential Ahr agonists and antagonists as well as novel Ahr-targeting approaches that might be valuable in the development of curative and/or preventive therapeutics for autoimmune arthritis.
IL-6, an inducer of Ahr under Th17-polarizing conditions, plays a key role in RA development
RA is characterized by inflammation of the synovial membrane and joint destruction, which leads to disability. The pathogenesis of RA involves dysregulation of both innate and adaptive immune responses. Many cytokines are associated with RA pathogenesis, including IL-6, TNF-α, IL-10, IL-17, and IL-23 [9, 39]. RA can be induced by several factors including genetics, obesity, infection and environmental risks . Activation of innate immunity plays an important role in RA. It is known that cytokines such as IL-6 and TNF-α are strong therapeutic targets for RA and biologic therapies that target these cytokines have revolutionized the treatment of RA in the past decade. Thus, blocking pro-inflammatory cytokines such as IL-6 and TNF-α is a promising treatment for RA [16–18, 41–44].
IL-6 is a pleiotropic cytokine that regulates many biological processes, including immune responses in several autoimmune diseases such as RA, Castleman's disease, Crohn's disease, and multiple sclerosis (MS) [45–50]. IL-6 acts on a variety of cells, including B cells, T cells, and hematopoietic progenitor cells. High concentrations of IL-6 have been detected in the serum and joint fluids of RA patients . A humanized antibody against the 80-kd IL-6R (tocilizumab) was shown to significantly affect Castleman's disease, juvenile idiopathic arthritis, Crohn's disease, and psoriatic arthritis [20, 44, 51, 52]. Furthermore, tocilizumab is also efficacious in RA patients who are refractory to conventional therapy or anti-TNF-α therapy, demonstrating the clinical benefits of IL-6 blockade [22, 23].
More recent studies using animal models have demonstrated that Th17 cells play a key role in autoimmune diseases such as RA . Interestingly, IL-6, combined with TGF-β in mice and IL-1β in humans, induces Th17 differentiation [54, 55]. Consistent with these findings, anti-IL-6 receptor antibody (MR-16-1) treatment or an IL-6 deficiency in mouse models of RA suppresses Th17 cell differentiation and disease development [56–58]. Fujimoto et al.  showed that blocking IL-6 signaling in experimental arthritis is effective in the early phase of collagen-induced arthritis (CIA; day 0) instead of the late phase (day 14). The induction of Th17 cells is significantly suppressed by anti-IL-6 receptor monoclonal antibodies on day 0 but not on day 14, indicating that blocking IL-6 signaling inhibits the differentiation of Th17 cells . In a human study, inhibiting IL-6 signaling with Tocilizumab regulated the Th17/Treg cell balance in RA patients . Samson et al.  showed that Th17 cells are increased while Treg cells are decreased in the peripheral blood of RA patients, suggesting that both Th17 and Treg cells are important in the development of RA and that targeting IL-6 to control the Th17/Treg cell balance is an effective therapy. In this study, tocilizumab treatment respectively decreased and increased the percentage of Th17 and Treg cells in the peripheral blood of patients with active RA . The frequency of Treg cells did not increase, however, with anti-IL-6R mAb treatment on day 0 in mice . These results suggest that Treg cell populations may be distinct in humans and mice. Of note, natural Foxp3+ Treg cells derived from the thymus and inducible Foxp3+ cells in experimental arthritis studies and in RA patients have not been well defined.
Recently, we and other groups identified a transcription factor, Ahr that is induced by IL-6 and TGF-β and is responsible for Th17 cell generation [25, 28, 30]. Ahr is a ligand-activated transcription factor that regulates the differentiation of several T cell subsets and alters the function of DCs and macrophages. We will discuss in detail the functions of Ahr in the following sections.
Taken together, targeting IL-6 is one of the most effective practices to control autoimmune arthritis by regulating the development of T cell subsets, including Th17 cells.
Ahr regulates autoimmune arthritis development
Ahr activation by smoking-derived ligands linked to autoimmune arthritis
Ahr is a ligand-dependent transcription factor of the basic helix–loop–helix Per-Arnt-Sim family. Ahr is activated by various ligands, including exogenous ligands such as TCDD, BaP, and polychlorinated biphenyls and endogenous ligands such as 2-(1'H-indole-3'-carbonyl)-thiazole-4-carboxylic acid methyl ester, UV photoproducts of tryptophan, and tryptophan metabolites (FICZ) . Ligand binding to Ahr causes the complex to translocate from the cytoplasm into the nucleus where the ligand-activated receptor forms a heterodimer with aryl hydrocarbon receptor translocator (Arnt). Ahr/Arnt heterodimers interact with a specific sequence, designated the xenobiotic responsive element, which causes a variety of toxicological effects. In addition to the aforementioned classical Ahr/Arnt pathway, it has also been reported that Ahr nuclear translocation caused by TCDD or forskolin treatment is involved in the regulation of IL-8. This represents an alternative pathway of Ahr regulation that may be partly regulated by the activation of protein kinase A and an interaction with RelB, but not Arnt . Furthermore, a recent study demonstrated that Ahr is a ligand-dependent E3 ubiquitin ligase, implying that Ahr has dual functions in regulating protein levels, serving both as a transcription factor that promotes the expression of target proteins and as a ligand-dependent E3 ubiquitin ligase that regulates selective protein degradation [62, 63]. Ahr agonists, such as TCDD, BaP, and 3-MC contained in cigarette smoke, are the most established environmental risk factors for RA . They adversely affect organs and systems in the body, leading to carcinogenesis and immunosuppression. Furthermore, Ahr has multiple functions in regulating the toxic effects of dioxin and modulating sex steroid hormone signaling [62, 63].
Although experimental models of RA have furthered our understanding of the pathogenesis and development of RA, the etiology of RA remains unclear. Many studies have suggested that infectious agents such as Epstein–Barr virus, parvovirus B19, rubella, human T cell leukemia virus type 1, erysipelothrix, proteus, mycobacteria, and Candida albicans may trigger RA. However, epidemiological studies are not consistent with these possibilities. On the other hand, it was demonstrated that tobacco smoking is a strong environmental risk factor and these findings are supported by epidemiological studies. Cigarette smoking is associated with rheumatoid factor (RF) seropositivity, radiographic erosions and nodules, and may adversely affect RA severity in a dose-dependent fashion. In addition, it was also reported that smoking affects the disease risk of RF-seropositive RA that is associated with classic genetic risk factors for immune-mediated diseases (the shared epitope of HLA-DR) in the population at risk. Although evidence supports that smoking influences RA disease susceptibility and severity, the precise mechanisms by which smoking contributes to RA are largely unknown.
Although Ahr intracellular signaling upon stimulation by various ligands remains unclear for certain inflammatory disorders, several studies have presented evidence that links Ahr and RA. For instance, Ahr is highly expressed in synovial tissue isolated from RA patients and in inguinal lymph node cells isolated from CIA-treated mice. Tamaki et al.  demonstrated that Ahr agonists, such as 3-MC, BaP, and TCDD, upregulate IL-1β mRNA in a human-like synoviocyte line whereas α-napthoflavone, an Ahr antagonist, inhibits the effects of 3-MC.
Taken together, these findings indicate that cigarette smoking containing Ahr ligands positively correlates with RA induction and progression. Additionally, Ahr is highly expressed in humans with RA and in mice with CIA, suggesting that Ahr is a critical factor for RA.
Ahr in T cells but not macrophages is important for the development of experimental arthritis
Kimura et al.  found that, under Th17-polarizing conditions with TGF-β and IL-6, Th17 cell development is enhanced via Ahr induction as a result of inhibited STAT1 activation. IL-6 inhibits TGF-β-induced Foxp3+ Treg cell differentiation and initiates Th17 cell generation; these effects shift the balance of Th17 and Treg cells and contribute to the pathogenesis of several animal models of autoimmune diseases, including experimental autoimmune encephalomyelitis (EAE) and CIA. Interestingly, specific ligands and cell types may affect the regulatory role of Ahr. Veldhoen et al.  demonstrated that Ahr activation by FICZ enhances Th17 cell development and promotes the development of EAE. On the other hand, Quintana et al.  demonstrated that Ahr regulates Treg and Th17 cell differentiation in a ligand-dependent manner. It has been shown that FICZ has higher affinity than TCDD for Ahr and is rapidly metabolized. On the other hand, TCDD also has a high affinity for Ahr, but it is a non-metabolized Ahr ligand. The differences in the stability of TCDD and FICZ may explain the different effects of these Ahr ligands.
Recent studies have shown that Ahr is involved in the production of IL-22, a member of the IL-10-related cytokine family, from Th17 cells and γδ T cells [30, 64]. It was shown that IL-22 has pro-inflammatory roles in CIA . This result is consistent with the findings of Nakahama et al., who showed that an Ahr deficiency in T cells but not macrophages inhibits Th17 cell generation and the onset of CIA. In this study, CIA development was significantly abrogated in Ahr-deficient mice. An Ahr deficiency blocks cartilage destruction and reduces MMP-3 levels. Importantly, T cell-specific, but not macrophage-specific, Ahr-deficient mice are resistant to CIA. These results suggest that the pro-inflammatory effects of Ahr in mice with CIA can be mediated via Th17 cells and IL-22.
Resveratrol, an Ahr antagonist, improves the outcome of RA
Resveratrol (3,5,4'-trihydroxystilbene), a phytoalexin derived from a variety of plants is a competitive inhibitor of Ahr (Ahr antagonist) [66–68]. Several research groups demonstrated that resveratrol inhibits the enzymatic activity of CYP1A1 induced by TCDD and BaP [66, 68]. In addition, resveratrol prevents the TCDD-induced transformation of cytosolic Ahr to its nuclear DNA-binding form . Casper et al.  demonstrated that resveratrol administration suppresses CYP1A1 expression induced by two Ahr ligands, BaP, and 7,12-dimethylbenz(a)anthracene, which are present at high concentrations in cigarette smoke. The effects of resveratrol have been demonstrated in various experimental autoimmune diseases, such as CIA, EAE, and DSS-induced colitis [69–72]. For CIA, it was shown that resveratrol inhibits Th17 cell differentiation and autoantibody production by B cells, thus suppressing disease development . Xuzhu et al.  also showed that resveratrol prevents the production of pro-inflammatory cytokines such as IL-6, TNF-α, IL-17, and IFN-γ in CIA mice. On the other hand, resveratrol can ameliorate EAE via inducing Ahr-mediated apoptosis in T cells . Although many studies have supported the benefits of resveratrol treatment for autoimmune diseases, further detailed investigations are required.
The link between Ahr and microRNAs: a novel approach to control autoimmune diseases including RA
Recently, microRNAs (miRs) have emerged as a new class of negative regulators of gene expression. miRs base pair with binding sites in the 3′-untranslated region of target mRNAs, leading to translational inhibition and/or mRNA degradation of target genes . miRs are non-coding RNA molecules of 20–22 nucleotides that regulate many biological systems, including immune systems . Recent studies have suggested that miRs are important in RA, and abnormal miR expression seems to contribute to disease development [76–78]. It was reported that various miRs, such as miR-223, miR-155, miR-146a, miR-132, miR-124a, and miR-16, exhibit dysregulated expression during the development of RA in patients. miRs can be involved in the differentiation of various Th subsets and thereby regulate autoimmune inflammation. It was previously shown that miR-326 promotes the development of Th17 cells by targeting Ets-1, a negative regulator of Th17 cell differentiation . Du et al.  demonstrated that miR-326 is increased during active disease relapses in MS patients. The authors used an EAE model and found that miR-326 overexpression exacerbated EAE symptoms via increasing Th17 cell differentiation, whereas inhibition of miR-326 improved the clinical outcomes via decreased Th17 cell differentiation . In addition, several other miRs have been shown to regulate Th17 cells as well as other T cell subsets in models of autoimmune diseases. For instance, O'Connell et al.  reported that miR-155-null mice are highly resistant to EAE because miR-155 positively regulates Th17 cell differentiation via intrinsic regulation. Furthermore, miR-155 is also required for optimum cytokine production by DCs to promote Th17 cells. Thus, miR-155 seems to be an important pro-inflammatory factor for immune responses during autoimmune diseases. In contrast to the pro-inflammatory function of miR-155, miR-146a has anti-inflammatory roles in certain diseases. miR-146a expression inhibits osteoclastogenesis and overexpression of miR-146a prevents joint destruction in CIA mice . Furthermore, Tang et al.  showed that miR-146a directly represses transactivation downstream of type I interferon via targeting interferon regulator factor 5 and STAT1, leading to reduced SLE severity in patients. More recently, it was demonstrated that miR-301a enhances the development of Th17 cells by targeting the IL-6/IL-23-STAT3 pathway, which is likely mediated by changes in the expression of PIAS3, a potent inhibitor of the STAT3 pathway .
Ahr as a potential therapeutic target
To elucidate the novel mechanisms by which IL-6 induces Th17 cells and causes the progression of autoimmune diseases, especially RA, our group screened and identified Ahr as a mediator of IL-6 signaling under Th17-polarizing conditions . At approximately the same time, two independent groups also reported that Ahr is involved in Th17 cell differentiation in EAE, a mouse model of MS [28, 30]. Ahr is highly expressed under Th17-polarizing conditions and contributes to Th17 cell generation via inhibiting the activation of STAT1 and STAT5 and/or the direct interaction with the IL-17 promoter [25, 31, 85]. Consistent with these findings, an Ahr deficiency in T cells, but not macrophages, ameliorated the development of CIA and decreased the frequency of Th17 cells . These findings suggest that reducing Th17 cells in human RA and/or several mouse models of the disease by blocking IL-6 may rely at least in part on the suppression of Ahr expression and that Ahr antagonists may be a promising drug for RA therapy. Resveratrol, a compound present in grape skins and red wine, is a well-known antagonist of Ahr. It is reported that the consumption of resveratrol provides various health benefits, e.g., increased lifespan, reduced inflammation, and cancer prevention [86–90]. Consistent with our finding of Ahr in CIA, a recent study showed that resveratrol treatment suppresses CIA development and reduces the number of Th17 cells . However, in a previous study, leflunomide, an immunomodulatory drug presently used for RA therapy, was shown to have agonistic properties for Ahr , which is not consistent with our previous finding of Ahr in CIA. Given that administering two different Ahr agonists, TCDD and FICZ, had opposite effects on EAE development , several Ahr agonists like leflunomide may also work as a drug for RA. Since each Ahr agonist may cause different effects on the disease, it will be important to determine the detailed mechanisms by which each agonist triggers Ahr signaling.
RA is an inflammatory disease that is triggered by multiple factors. Even with the success of cytokine-targeting therapy such as anti-IL-6R, it is necessary to identify novel factor(s) that would not only increase our current understanding of the pathogenesis of RA but also provide new and efficient treatments for patients. Since Ahr was shown to play important roles in promoting the differentiation of Th17 cells, a crucial T cell subset in RA development, this transcription factor has attracted significant interest and many studies are underway to investigate its function in the development of RA as well as a broad range of other autoimmune diseases. The significant abrogation of experimental arthritis in Ahr-deficient mice as well as in T cell-specific Ahr-deficient mice in the CIA model strongly suggests that Ahr, particularly in T cells, is an indispensable factor for RA development. Unraveling the details of the interaction between Ahr and other factors, such as miRs, may further elucidate the pathogenesis of RA and bring Ahr-based therapy for RA from bench to bedside.
This work was supported by the Program for Promotion of Fundamental Studies in Health Sciences from the National Institute of Biomedical Innovation, and a Grant-in-Aid for Scientific Research (KAKENHI) 24790471 (for N.T.N) from the Japan Society for the Promotion of Science (JSPS).