Postmenopausal osteoporosis in rheumatoid arthritis

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Author's Personal Copy Bone 103 (2017) 102–115

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Review Article

Postmenopausal osteoporosis in rheumatoid arthritis: The estrogen deficiency-immune mechanisms link Rony Sapir-Koren a, Gregory Livshits a,b,⁎ a b

Human Population Biology Research Group, Department of Anatomy and Anthropology, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel Lilian and Marcel Pollak Chair of Biological Anthropology, Department of Anatomy and Anthropology, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel

a r t i c l e

i n f o

Article history: Received 26 March 2017 Revised 13 June 2017 Accepted 26 June 2017 Available online 27 June 2017 Keywords: Postmenopausal osteoporosis Postmenopausal rheumatoid arthritis Inflammatory cytokines CD + T-cells RANKL ACPA

a b s t r a c t Rheumatoid arthritis (RA) is characterized, among other factors, by systemic bone loss, reaching ~50% prevalence of osteoporosis in postmenopausal women. This is roughly a doubled prevalence in comparison with agematched non-RA women. Postmenopausal RA women are more likely to be sero-positive for the anticitrullinated peptide antibody (ACPA). Our extensive review of recent scientific literature enabled us to propose several mechanisms as responsible for the accelerated bone loss in ACPA(+) RA postmenopausal women. Menopause-associated estrogen deficiency plays a major role in these pathological mechanisms, as follows: 1) Estrogen withdrawal causes immune dysregulation manifested in a skewed distribution of T helper-cell subsets, and enhanced reactivity of T helper-17 (Th17) cells. This results in a shift toward elevated levels of inflammatory cytokines, especially TNFα, IL-17, and RANKL, as well as accelerated net bone loss. 2) The proposed interaction between estrogen deficiency and RA-genetic risk alleles promotes enhanced Th17cell autoreactivity, manifested by ACPA(+) RA. Such interactions exacerbate the inflammatory conditions and cause massive bone destruction. 3) TNFα and IL-17 play a dual role in RA because they stimulate bone resorption and inhibit bone formation. 4) An RA-unique factor, the pathogenic appearance of ACPA, promotes an inflammation independent-mechanism, resulting in direct osteoclastogenesis and bone resorption. © 2016 Elsevier Inc. All rights reserved.

Contents 1. 2. 3. 4. 5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Estrogen deficiency and RA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 RA and bone loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Aging-related estrogen deficiency as a common cause for postmenopausal osteoporosis . . . . . . . . . . . . . . . . . . . . . . . . . 103 Mechanisms underlying the development of postmenopausal osteoporosis in RA patients — an overview . . . . . . . . . . . . . . . . . . 104 Estrogen deficiency associated with OP and with ACPA(+) RA — a link for proinflammatory cytokines promoting enhanced T-cell autoreactivity and osteoclastogenesis 104 6.1. RANKL in RA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 7. Inflammatory cytokines shared by postmenopausal estrogen deficiency and by RA, as culprits for RA-RANKL-excessive risk for bone destruction . . 105 7.1. RA pathogenesis and TNFα . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 7.2. RA pathogenesis and IL-17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 8. Osteoblast-mediated bone formation is inhibited in RA: the role of IL-17 and TNFα . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 9. ACPA, micro RNA (miRNA), and Myostatin as agents of bone destruction in RA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 10. The effects of glucocorticoid (GC) treatment on bone loss in RA patients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 11. Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Conflict of interests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

⁎ Corresponding author at: Department of Anatomy and Anthropology, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv 69978, Israel. E-mail addresses: [email protected] (R. Sapir-Koren), [email protected] (G. Livshits).

http://dx.doi.org/10.1016/j.bone.2017.06.020 8756-3282/© 2016 Elsevier Inc. All rights reserved.

Author's Personal Copy R. Sapir-Koren, G. Livshits / Bone 103 (2017) 102–115

1. Introduction Rheumatoid arthritis (RA) is a systemic autoimmune disease of unclear complex etiology. RA is the most common inflammatory arthritis, with an estimated global prevalence in 2010 (from 5 to 100 years of age) of 0.24%; it was about two times higher in females than males [1]. RA global prevalence remained stable from 1990 to 2010, with the major peak of RA prevalence occurring in older ages. The previous estimates for RA were based on a small number of studies that primarily collected data from Western Europe and North America, reporting a prevalence of between 0.3% and 1% [1]. RA is a chronic and progressive symmetrical polyarthritis disorder in which arthritis, particularly the small joints of the hands and feet, cause significant functional impairment. The characteristic features of the disease are synovial inflammation and proliferation, accompanied by cartilage erosion and bone loss. It causes significant morbidity, a reduced life span, and loss of work productivity [2–4]. The two known antibodies, produced against autoantigens, which are widely expressed inside and outside the joints, are the rheumatoid factor (RF) and the anti-citrullinated peptide antibody (ACPA) [5]. The citrullinated peptides are found in many matrix proteins such as fibrinogen, filaggrin, keratin, alpha-enolase, and vimentin. This implies that post-translational citrullination might expose some cryptic epitopes, which eventually results in loss of tolerance in RA pathogenesis [6]. 2. Estrogen deficiency and RA RA peak incidence in women coincides with a perimenopausal period, suggesting a relationship between estrogen deficiency and the development of RA [7]. Indeed, early menopause is a risk factor for developing RA: those women who reached menopause before 45 years of age had a higher risk for RA than did women who reached menopause in a later age [8–10]. Postmenopausal women with an earlier age of menopause are more likely to be seropositive (for RF or ACPA) and have worse patient-reported pain and global assessment scores than those with the usual age of menopause [8]. Thus, earlier menopause, together with the fact that women with RA have been reported to experience a later menarche [11], represents a shorter duration up to menopause for the bone tissue to be exposed to estrogenic protection against a negative balance between bone formation and resorption, which results in bone loss. Data gathered from both animal and human-epidemiologic studies support this notion of the distinct beneficial effects of estrogens on arthritis. Female mice subjected to ovariectomy (OVX), and therefore having reduced levels of estrogens, display a higher frequency and an increased severity of collagen-induced arthritis (CIA), as compared with OVX mice treated with estrogen or sham-operated mice with intact levels of estrogen [12]. Epidemiologic studies revealed that other estrogen-related factors, such as longer durations of breast feeding have appeared protective, whereas parity and oral contraceptive use have been neutral [10,13,14]. Although the use of hormonal replacement therapy (HRT) does not appear to influence the risk of developing RA [15], HRT has been shown to improve both the symptoms and the progression of the disease, with decreased joint destruction, reduced inflammation, increased bone density, and better patient health assessment [16–19]. Importantly, women who had used HRT were less likely to have ACPA, and the longer duration of HRT conferred greater odds of being seronegative [18]. HRT also reduced the risk for ACPA positivity conferred by having RA-high risk HLA alleles [17]. Thus, female sex hormone exposure can modify seropositivity and may also be involved in

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modifying the genetic mechanism by which ACPA is produced. However, HRT is no longer recommended for long-term therapy due to the risk of serious side effects. 3. RA and bone loss A major pathological RA manifestation is the reorganization of the synovial architecture, with immune cells infiltrating into the synovium, fibroblast-like synoviocytes (FLS) proliferation, synovial inflammation, and the pannus formation [20]. The FLS proliferation is induced by the inflammatory microenvironment and pro-inflammatory cytokines. The proliferated FLS consequently promote chronic damage to joint cartilage as well as bone loss [20,21]. The associated local-joint inflammatory state and the systemic one that follows, result in three radiographically identified forms of altered extra-articular-skeletal remodeling in RA patients [22–26]: a) periarticular bone loss, mainly caused by the proinflammatory cytokines from the inflamed synovium, which is in direct contact with bone. This varies with the disease severity; b) erosion of the subchondral bone, accompanied by mechanisms commonly associated with periarticular osteopenia; c) systemic/generalized osteopenia or osteoporosis (OP) involving the axial and appendicular skeleton. Bone loss occurs in the very early course of RA, with the most significant rate of loss appearing early after disease onset. It has been shown that ~ 25% of patients with early RA show signs of osteopenia at the spine or hip before the beginning of therapy, and 10% have generalized OP, which is twice as high in comparison with the general population [27–31]. Each postmenopausal woman experiences physiological estrogen deficiency; however, the proportion of OP among these women has reached about 30% in the USA and Europe [32]. However, the prevalence of concurrent RA-OP in postmenopausal women is much higher than this number, reaching ~50% [24,33,34]. Interestingly, postmenopausal RA women were found to have a high frequency of OP (55.7%) in comparison with a relatively low frequency of OP in RA premenopausal women (18%) [33]. Several other studies are in agreement with these observations [24,34–38]. Moreover, the occurrence of hip and vertebral fractures is roughly doubled in postmenopausal RA, as compared with age-matched controls [39,40]. This may reflect, at least partially, a shared multifactorial etiology, with an added influence of factors unique to RA. The important question that arises in this connection is the nature of the one or more pathological mechanisms that cause such a dramatic elevation in OP prevalence in postmenopausal RA. This paper proposes potential mechanisms for explaining this phenomenon, in later sections. 4. Aging-related estrogen deficiency as a common cause for postmenopausal osteoporosis Aging-related bone loss and osteoporosis affect millions of people worldwide [41] and in general, it is associated with substantial structural alterations in the skeleton, including a reduction in the trabecular bone volume, density, and strength [42]. With aging the number of osteoblasts decreases owing to a reduced number of their stem cells, defective proliferation and differentiation, a diversion of these progenitors toward the adipocyte lineage, as well as to increased apoptosis. Aging also significantly increases stromal/osteoblastic cell-induced osteoclastogenesis and promotes an expansion of the osteoclast precursor pool [43]. Advancing age is also associated with an initially subclinical proinflammatory state that has been termed ‘inflamm-aging’ [44], which is accompanied by an increase in oxidative stress. The hallmark of ‘inflamm-aging’ is the overproduction of proinflammatory (and often bone-resorbing) cytokines by macrophages. Over time, this

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impairs the antioxidant and repair potential of cells and promotes the generation of factors called AGEs (advanced glycation end products), a process that further stimulates the microenvironment to undergo a shift toward elevated proinflammatory cytokine levels in bone marrow and the serum [45–47]. In women, aging is accompanied by estrogen deficiency, as the result of menopause, which can be defined as starting after 1 year of amenorrhea [48]. Under premenopausal conditions estrogens exert beneficial effects on bone mass [7,49]. In contrast, estrogen deficiency plays a specific and major role in contributing to the inflammatory bone-microenvironment state; this causes an impaired regulation of osteoclastogenesis and osteoblastogenesis, which results in a net bone loss and induces osteoporotic fractures [50,51]. In summary, in postmenopausal women, bone homeostasis is dysregulated by aging and hormonal deficiency, leading to enhanced bone turnover, and hence, increased bone formation and even greater rates of bone resorption, resulting in a net bone loss. 5. Mechanisms underlying the development of postmenopausal osteoporosis in RA patients — an overview In postmenopausal RA, the general consensus is that the pathogenic process of bone loss in RA patients is due to the abundance of the bone resorbing cells, osteoclasts, and a resulting imbalance associated with the activities of bone-forming osteoblasts [52,53]. Osteoclastogenesis in RA is considered as the manifestation of elevated levels of joint-inflammatory-derived cytokines, which enter the systemic circulation. This process is further boosted by unique risk factors associated with the disease, in particular, genetic predisposition, low body mass index (BMI), and lack of mobility, as well as applied therapy such as a glucocorticoid prescription, which can trigger significant bone loss [39,40, 52,54–58]. In this review it is assumed that the postmenopausal ACPA(+) RAassociated genetic risk factors interact with estrogen deficiency,

resulting in modulation of the physiological immune responses [59], elevated levels of inflammatory cytokines, which lead to accelerated and doubled bone loss in postmenopausal RA-OP. Fig. 1 schematically presents the main ideas discussed here. First, we will attempt to present up-to date data suggesting a key role for estrogen deficiency as a common risk factor for both postmenopausal OP and RA. We will argue that such hormone deprivation, associated with postmenopausal status, causes elevated circulating levels of inflammatory cytokines, especially TNFα, IL-17, and RANKL, by a mechanism leading to an excess of osteoclastogenesis and bone resorption as a major pathogenic factor for OP in RA. Second, we will attempt to clarify the question whether RA-specific factors also contribute to the increased net bone loss. In this connection, we will discuss the data on a) RA-inhibited bone formation, and b) the presence of ACPA and other factors involved in RA (such as elevated levels of DKKI, myostatin), as unique factors contributing to RA bone loss. We believe that focusing on the potential biological mechanisms associated with the double occurrence of OP in postmenopausal RA is of paramount importance for deciphering the underlying etiology and for developing therapy strategies associated with the co-existence of OP-RA. 6. Estrogen deficiency associated with OP and with ACPA(+) RA — a link for proinflammatory cytokines promoting enhanced T-cell autoreactivity and osteoclastogenesis Currently the main bulk of the data on the relations between estrogen deficiency and the immune system comes from animal-model studies, whereas direct evidence from human studies is limited. Under OVXestrogen deficiency, osteoporotic animals exhibit features of an immune inflammatory disease involving enhanced T-cell reactivity and a deranged distribution of T-cell subsets to a pool of self and foreign antigens, physiologically present in healthy animals [60]. In both women with postmenopausal osteoporosis as well as with mouse models, the

Fig. 1. Schematic presentation of the main proposed pathogenic mechanisms causing bone loss in postmenopausal ACPA(+)RA women. A key role is suggested for estrogen deficiency as a common pathogenic factor for both postmenopausal OP and RA, which stimulate immune dysregulation and induce elevated circulating levels of inflammatory cytokines, especially TNFα, IL-17, and RANKL. This inflammatory condition results in an excess of osteoclastogenesis and bone resorption. In PM RA, owing to RA-risk alleles, estrogen deficiency interactions and massive bone destruction occur. In addition, RA-inhibited bone formation increases the net bone loss, and the presence of RA-ACPA itself contributes to bone loss.


higher T-cell (and monocyte) activity compared with healthy controls results in the increased production of inflammatory bone-resorbing cytokines, especially RANKL and TNFα; the level of production is sufficient to promote osteoclast differentiation of bone marrow macrophages (osteoclastogenesis), and to increase osteoclast activity and life span. This contributes to the increased estrogen deficiency-induced bone loss [51,61–64]. Of the CD4 + T-cells subsets, the newly identified Th17 cells are considered to be responsible for the physiological clearance of extracellular bacteria and fungi, and for promoting B-cell production of IgG antibodies [65]. Under estrogen deprivation, the Th17 subset is up-regulated, and it plays a role as the most potent inducer of osteoclastogenesis, whereas the functions of CD4+ regulatory T-cells (Tregs) in suppressing osteoclast differentiation and bone resorption in vitro are down regulated [66,67]. The suggested major role played by increased osteoclastogenesis in postmenopausal OP is supported by several lines of evidence as follows: Eghbali-Fatourechi et al. [68] convincingly demonstrated that RANKL expression was amplified in early postmenopausal women as compared with premenopausal women as well as with estrogen-treated postmenopausal women. These authors showed that the surface concentration of RANKL per cell was increased by two- to threefold on T-cells, Bcells, and pre-osteoblastic mesenchymal stem cells, and on total RANKL-expressing cells. Additional parameters for increased bone resorption – a higher number of activated osteoclasts, and increased osteoclast survival – were shown to result in prolonged bone loss, deeper resorption cavities, and trabecular perforation, consequently increasing the bone fragility [69,70]. Indeed, for several years, denosumab, a human monoclonal IgG2 antibody against RANKL, has been available for treating osteoporosis [71]. Circulating levels of the natural soluble decoy receptor for RANKL, osteoprotegerin (OPG), [known to inhibit osteoclastogenesis [72]], was found to be higher in postmenopausal osteoporotic women than in healthy controls [61,73,74]; the elevated serum OPG level is considered as a compensation for the persistent bone loss after menopause in osteoporotic women. Taken together, these data indicate that the immune system is involved in RANKL production in postmenopausal women, and hence, bone resorption. 6.1. RANKL in RA In RA, abnormal activation of the immune system leads to up-regulated local production of RANKL by activated T-lymphocytes, monocytes, and FLS in the synovium of inflamed joints. This results in RANKL-dependent osteoclast differentiation, and increased numbers of active mature osteoclasts at the site [75]; it ultimately leads to bone (and joint) destruction in RA, mediated by a large number of inflammatory cytokines produced by the above-mentioned cells [39,76]. RANKL may be produced either as a membrane-bound protein or it may be cleaved by metalloproteinase into a soluble molecule (sRANKL) [77]. The available data suggest that T-cell- and FLS-derived RANKL within the joints of RA patients is the main contributor to enhanced osteoclastogenesis [78,79–81]. Foxp3 + T-cell-derived Th17 cells were found to be very capable of supporting osteoclastogenesis [82]. These cells indeed express RANKL, but their major contribution to osteoclastogenesis is not a direct effect, but rather, an indirect one through their ability to upregulate RANKL expression on FLS, primarily due to the action of IL-17 secretion [82–86]. The axis of IL17-induced upregulation of RANKL expression, indicating a synergy between Th17 cells and FLS in arthritis-related bone loss, is further discussed in section 7 (under ‘RA pathogenesis and IL-17’). Danks et al. [87] recently attempted to quantify the contribution of collagen-specific CD4 + T-cells vs FLS to RANKL production in CIA male mice. The authors demonstrated that RANKL's expression on FLS, but not on T-cells, is the major factor responsible for the formation of osteoclasts and for bone destruction during inflammatory arthritis.

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Nevertheless, the contribution of T-cell-derived RANKL vs FLS to osteoclast differentiation in arthritic joints remains to be determined in vivo, and to be further supported in humans, especially in postmenopausal RA women, which is the focus of this review. The reports on RANKL's effects on bone destruction in RA differ between a complete RANKL dependency, since RANKL-deficient mice with experimental arthritis were completely protected from bone erosions [88], and between reported evidence supporting that it is TNFα that may directly promote pre-osteoclast differentiation [89]. In early untreated RA, the ratio of circulating RANKL/OPG is a predictor of the subsequent destruction of the bone component of the joint [90]. The RA condition also induces the mass production of soluble RANKL, thus indicating sharing, at least partially, of mechanisms having a similar pathology, for both local joint-bone destruction and for the entire systemic skeletal involvement in RA [20,91]. Accordingly, use of denosumab (an anti-RANKL antibody) in patients with RA in a controlled study was effective at reduction of bone damage; however, there was no clinical benefit for arthritis symptoms such as inflammation or joint-space narrowing [92]. These results were supported by a recent phase II study [93] showing that denosumab treatment in RA patients on methotrexate markedly suppresses bone erosion and increases bone mineral density. However, there was no obvious evidence of the treatment's effect on joint space narrowing. These results emphasize the central role of RANKL-induced osteoclastogenesis in bone loss in RA, but not in cartilage loss. 7. Inflammatory cytokines shared by postmenopausal estrogen deficiency and by RA, as culprits for RA-RANKL-excessive risk for bone destruction As concluded, ample data suggest that the immune system is critically involved in RANKL production. An arsenal of pro-inflammatory cytokines (e.g., TNFα, IL-1β, IL-6, and IL-17) is abundant in the synovial tissue and fluid. They play pivotal roles in regulating RANKL expression and production, primarily by T-cells and FLS. In this way, these cytokines are indirectly responsible for the activation and survival of osteoclasts as well as bone loss [94]. Moreover, these cytokines can directly induce osteoclast differentiation [88]. Numerous studies using cytokine blockers, such as anti-IL-6 [95], anti-TNFα [96], as well as OPG [97] consistently demonstrated the inhibition of osteoclast formation and bone resorption. Thus, the major mechanism underlying increased bone resorption seems to be the excess of cytokines, which promotes osteoclastogenesis through RANKL stimulation; in addition, this process is accompanied by a deficiency of cytokines hampering such processes (IFN-γ, IFN-α, IL-4) [98]. The menopausal status appears to influence the production of the aforementioned proinflammatory cytokines TNFα, IL-1β, IL-6, IL-17, and of the macrophage-colony-stimulating factor (M-CSF). Estrogen deprivation results physiologically in a shift toward increased production of these cytokines, by immune and bone cells. It is recognized that increased osteoclastogenesis, accompanied by increased bone loss, in response to estrogen deficiency, is cytokine driven [60,99– 101]. Functional block of these cytokines is likely to afford effective skeletal protection post-OVX [102–105]. It is worth mentioning that elevated inflammatory markers were shown to be prognostic for fractures [106], and that estrogen therapy restores these cytokine-serum levels [7,107–112]. The existing data strongly suggest that the proinflammatory cytokines TNFα, IL-1β, IL-6, IL-17, and RANKL also appear as RA-derived factors, probably with increased doses, as compared with non-RA OP. Next, we will consider in more detail the involvement of each of the crucial inflammatory cytokines (TNFα, IL-1β, IL-6, and IL-17) in bone loss as associated, first, with estrogen deficiency, and then, with RA. The outline of the possible pathways integrating immune cells and cytokines upon postmenopausal estrogen deficiency is schematically shown in Fig. 2. The figure is divided into two interrelated sections: A and B.



Fig. 2. Schematic presentation of possible RA-estrogen deficiency pathways, integrating immune cells, monocytes/macrophages, RA-fibroblast-like synoviocytes, and inflammatory cytokine production. Section A focuses on the activation of CD4 + T-cells, which results in elevated levels of inflammatory cytokines, which also exhibit reciprocal effects. These cytokines stimulate RANKL production, osteoclastogenesis, and bone destruction. Section B focuses on Th17-IL-17 production as a major factor for RANKL stimulation and osteoclastogenesis. Under estrogen deficiency conditions, naïve CD4 + T-cells are primed to give rise to increased numbers and the increased activity of Th17 cells. These cells secrete increased amounts of IL-17, driving inflammatory cascades that also create a feedback loop for stimulation of Th17 cells. Consequently, IL-17-elevated levels drive RANKL production and bone loss. All these pathways are exacerbated in postmenopausal RA, which results in stimulation of autoreactive Th17 cells that promote a worse inflammatory condition, and ACPA production (not shown).

I. TNFα. We start with postmenopausal estrogen deficiency-activated CD4 + T-cells, which in turn secrete TNFα (Fig. 2A). The latter stimulates RANKL production, osteoclast development, and it functions directly as well as indirectly by increasing the production of M-CSF by bone marrow stromal cells. This also decreases the release of OPG by osteoblasts [113]. Enhanced TNFα production by antigen-activated T-cells appears to be a key mechanism by which estrogen deficiency induces RANKL-stimulated osteoclastogenesis and bone loss in vivo. This was concluded from a series of experiments with TNFα knockout mice that do not develop osteoporosis after OVX [114]. Also in agreement with these findings are human data showing that osteoporotic postmenopausal women have an elevated production of TNFα and RANKL triggered by activated T-cells [61,115]. Under estrogen deficiency, TNFα directly enhances the activity of mature osteoclasts, and indirectly activates osteoclast formation by stimulating the secretion of RANKL by osteoblasts and stromal cells [51,116]. However, the intimate mechanism underlying estrogen deficiency-induced bone loss in humans remains mostly vague because human studies do not provide an adequate means to reproduce the findings in OVX animals. 7.1. RA pathogenesis and TNFα This cytokine is a key molecule in RA, and its overexpression can stimulate systemic bone loss, whereas RANKL permissively affects the osteoclastogenesis effect of TNFα [117]. TNFα is responsible for monocytes/macrophages' high level production of downstream mediators, the proinflammatory cytokines IL-6 and IL-1β, which in turn, promote Th17-IL-17 production [118–121]. In RA patients receiving an antiTNFα therapy (infliximab or adalimumab) the RANKL levels are reduced dramatically, in parallel with a diminution of the disease activity score, and they benefit BMD changes [122]. The significant advantage in the treatment of RA [123] may lie in an indirect mechanism that

protects RA patients from severe inflammation by blocking Th17 cell differentiation and inhibition of IL-17 [118,124–126], which is discussed next. II. IL-6. Under estrogen deprivation, circulating levels of IL-6 are elevated [95,127] (Fig. 2A, top). IL-6 is considered among the ‘classic’ pro-inflammatory and pro-osteoclastogenic cytokines that lead to bone loss, and it is thought to be the most abundant and effective cytokine in blood [69,95,99,128]. In a populationbased study, variations within the low levels of inflammatory markers, especially IL-6, can predict bone loss and resorption [129]. Under estrogen deficiency, elevated serum IL-6 levels are recognized as a potent stimulator of enhanced RANKL production by osteoblasts; thus, IL-6 levels can predict bone loss [61,130– 132]. Another study has shown that mice deficient in IL-6 do not develop OVX-induced bone loss [133]. IL-6 frequently cooperates with TNFα to promote bone loss [134,135]. The soluble IL-6 receptor increases after menopause, but this increase can be prevented and reversed with HRT [136]. Such prevention was recently also reported in women with postmenopausal RA [37]. III. IL-1β. This cytokine directly enhances the ability of osteoclasts to resorb bone [137], and it also serves as a mediator of TNFα-induced osteoclastogenesis [138]. Its production by mononuclear cells is also increased by estrogen deficiency [127]. In mice overexpressing TNFα, but lacking IL-1β, no deleterious bone effect has been observed and no bone loss can be measured [139]. Moreover, blockade of TNFα or IL-1β reduces bone resorption markers in healthy postmenopausal women [140]. The combined relative effects of IL-6, IL-1β, and TNFα may be used to explain the clinical observations of the heterogeneity of joint destruction severity and bone involvement among RA patients.


IV. IL-17 (Fig. 2B). Naïve CD4 + T-cells, primed by the combined effect of TGFβ and IL-6, give rise to Th17 cells; continued IL-23 signaling is essential for their survival and expansion [141,142]. Other cytokines, such as IL-1β, IL-21, or TNFα might contribute to these processes in both mice and humans [143]. The combination of four cytokines, namely, IL-23, IL-1β, IL-6, and TGFβ, is commonly used for generating human Th17 cells in vitro. This cocktail induces the expression of the transcription factor, nuclear receptor RORγt, which is essential for Th17 cell differentiation [144]. In addition, TGFβ was shown to be required for inducing pathogenic Th17 cells in vivo, because local, but not systemic administration of anti–TGFβ inhibits Th17 cell generation [145]. Furthermore, the development of pathogenic vs nonpathogenic Th17 cells may be contingent on the type of TGFβ present during the initial stage of Th17 differentiation, since it was demonstrated that TGFβ-3, but not TGFβ-1, is the critical TGFβ isoform required for inducing pathogenic Th17 cells [146]. Recently, it was proposed that the human T follicular helper (TFH) cell subset shares properties with Th17 cells (TFH represents more efficient B cell helpers than do other subsets) [147]. Both TFH and Th17 cells co-emerge in many human autoimmune diseases, including RA [148] and TGFβ, which is abundantly expressed at inflammatory sites in human autoimmune diseases, might contribute to their generation [149]. The signature cytokine produced by Th17 cells is the key pro-inflammatory cytokine, IL-17. Increases in the number of IL-17-producing CD4 + T-cells and in the circulating IL-17 levels were observed in postmenopausal women as compared with premenopausal women, and in osteoporotic postmenopausal women, as compared with healthy controls. In addition, the osteoporotic postmenopausal women had higher serum concentrations of TNFα, IL-6, and OPG than did the postmenopausal women with normal BMD measurements (healthy controls); whereas the circulating levels of RANKL were not statistically different between the osteoporotic and healthy groups [73,108,150,151]. This may imply that upregulated cellular IL-17 production by CD4 + T-cells may play an important role in osteoporosis in postmenopausal women; thus, IL-17 mediates estrogen deficiency effects manifested by bone loss and osteoporosis. Such relations between estrogen deficiency-IL-17 elevated levels and bone loss were frequently reported in animal studies. The increment of IL-17 production-mediated bone loss and osteoporosis acts as a direct enhancer of RANKL expression on osteoblasts [109,152–154]. DeSelm et al. [152] found that mice lacking the principal IL-17 receptor (IL17RA) or its effector, Act1, were protected from the skeletal effects of OVX-induced bone loss, and that such bone loss was also prevented by an antibody targeting the IL-17 cytokine. Tyagi et al. [109] have shown that the OVX mouse-induced bone loss was dependent on the increased expression of the transcription factors (RORγt and RORα), which are essential for promoting Th17 cell differentiation and proliferation. A recent research study by Tyagi et al. [155] examined how the potential neutralization/inhibition of anti-TNFα, antiRANKL, or anti-IL-17 antibody, administered to estrogen-deficient (OVX) adult mice affects the skeletal parameters. The superior immunoprotective effects of anti-IL-17 (over anti-RANKL or antiTNFa therapy) were demonstrated. Anti-IL-17 antibody better protected against OVX-induced osteopenia by suppressing osteoclast function, promoting osteoblast survival and differentiation, and reversing immune-senescent proinflammatory changes triggered by OVX. Anti-IL-17 had the strongest effect by effectively reducing proinflammatory cytokine production, inducing Tregs, and inhibiting osteoblast apoptosis. Since functional blocking of IL-17 was shown to be more potent than blocking of TNFα or RANKL, it can be concluded that anti-IL-17 is translated to better skeletal preservation. Furthermore, IL-17 has the capacity to stimulate the production of TNFα and IL-6 in the context of estrogen deficiency [109,153,154]. Given

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that TNFα is critical for IL-17-regulated bone loss, and that elevated levels of IL-6 are thought to favor Th17 cell differentiation, the elevated circulating levels of TNFα, IL-6, and IL-17 may act concomitantly to mediate postmenopausal bone loss. Taken together, the data gathered from postmenopausal women and OVX mouse studies support the concept of a key role played by IL-17 as an important mediator of the pathogenesis of estrogen deficiency-induced bone loss. 7.2. RA pathogenesis and IL-17 Both human and animal model studies of arthritis suggest a key role of IL-17 in the pathogenesis of RA, and in the associated bone destruction [85,143,156–158]. Elevated synovial and serum IL-17 levels are found in RA patients, which are correlated with the degree of disease activity and severity, and with disease markers such as ACPA [159]. In murine models of RA, IL-17-producing cells were shown to be restricted to the inflamed synovium and IL-17 was shown to play a central role in bone loss and joint destruction. Treatment with IL-17 blockade has beneficial effects [85,160,161]. IL-17 activates inflammatory cell recruitment and acts at the top of RA-related inflammatory cytokine cascades [156,162]. It stimulates joint FLS and macrophages to produce more proinflammatory mediators, such as IL-1β, IL-6, and TNFα as well as various chemokines; this forms a positive feedback loop that amplifies the differentiation of Th17 cells [157,158,163,164], all of which, in turn, govern bone erosion by up-regulating osteoclastogenesis [119, 120,165]. IL-17 upregulated RANKL expression on RA-FLS and its blockade decreased osteoclast formation by 80% [165]. This axis is critical for arthritis-related bone loss and destruction, indicating a synergy between Th17 cells and FLS [85]. Furthermore, the IL-17-secreting Th17 cells also directly produce RANKL [166,167]. Andersson et al. [168] characterized the effects of estrogen on Th17 cells in OVX mice with experimental RA. The authors demonstrated that estrogen influenced Th17 cells' migratory pathways during the development of arthritis, by increasing Th17 in lymph nodes in early arthritis (in an ERα-dependent manner) and by decreasing joint Th17 in established arthritis. Importantly, this was completely reversed by estrogen supplementation. A recent study by Mansoori et al. [173] supports an important role for increased Th17-producing IL-17 cells along with a decrease in Treg cells (Foxp3+ cells), regarding RA pathology, under estrogen-deficient conditions. Using OVX mice, they demonstrated the relations between the pro-inflammatory IL-18 cytokine and its natural antagonist IL18BP, along with the Th17/Treg cell ratio and BMD. Upregulation of IL18 in RA was previously shown to increase the production of key osteoclastogenic regulators from human FLS [174]; and in mice, IL-18 was shown to play a role in the pathogenesis of CIA, whereas IL-18BP plays a protective role in these mice [175]. Mansoori et al. [173] suggested the immunoprotective and osteoprotective role of IL-18BP administration in the estrogen-deficient Ovx mouse model, as follows: estrogen deficiency up-regulates IL-18 production, which promotes Th17 responses, leading to an increase in IL-17 production. IL-17 would then induce osteoclastogenesis and bone loss. This effect is inhibited by IL-18BP, which antagonizes IL-18 and increases the Treg/ Th17 ratio, leading to inhibition of osteoclastogenesis and hence, reduced bone loss. This is supported by the authors' observations of a significant elevation in serum IL-18 levels in osteoporotic postmenopausal women, whereas the serum IL-18BP levels are reduced. Biological agents that inhibit inflammatory cytokines have been used in the treatment of RA. However, there is a constant need for alternative therapies, particularly among patients who have failed treatment with previous therapies, including TNF inhibitors [169,170]. Treatment with an anti-IL-17A monoclonal antibody, ixekizumab, in a phase I study of patients with RA who were naive to biologic therapies, significantly improved the signs and symptoms of RA with no significant safety findings [171]. In a continuation of this research, a phase II doseranging study was conducted to confirm the efficacy of ixekizumab in



a larger population of biologic-naive patients with moderate to severe RA, and to evaluate the effectiveness of IL-17 neutralization in TNF–inadequate responder (TNF-IR) patients [172]. The overall data presented in this section seem to suggest that physiological-estrogen levels, like in premenopausal women, negatively control the immune mechanisms responsible for chronic inflammatory conditions. In particular, the physiological-estrogen levels negatively regulate Th17 differentiation, thereby reducing IL-17 cytokine production and subsequently, bone loss. It seems likely that in postmenopausal women estrogen deficiency per se is a major contributor to bone loss, by activating those immune mechanisms responsible for upregulating the pathogenic axis: Th17 – IL-17 – inflammatory milieu – RANKL – osteoclastogenesis – bone loss – osteoporosis. In this system, the effects of IL-17 signaling as an essential component governing bone loss are enabled due to a decreased ratio of Tregs/Th17 cells, which is achieved by controlling the expression of the corresponding transcription factors [59]. However, in non-RA postmenopausal women the magnitude of inflammation is most likely much lower than that attended by the RA inflammatory state. In RA, the above axis is controlled by similar immune mechanisms, which, however, are exacerbated by immune functions associated with the interaction of estrogen deficiency and the genetic risk alleles of RA, leading to the production of ACPA [59]. Thus, an exaggerated production of IL-17 and the other bone-resorbing cytokines considered above appear to be one of the major mechanisms causing massive postmenopausal RA-associated bone resorption and doubled osteoporosis prevalence. Although it seems that the secretion of IL-17 by a subset of CD4+ Th17 cells is a central mediator of ACPA(+) RA pathology, other subsets of CD4+ T-cells may play additional key effector functions, such as activating B-cells to produce ACPA autoantibodies. Very recently, Rao et al. [176] have revealed a novel subset of CD4+ T-cells, with a unique combination of features. This subset (PD-1hiCXCR5−CD4 + T-cells) is reported to be a markedly expanded population in the synovium of RA patients, which accounts for almost a quarter of all CD4 + T-cells in the synovium of ACPA(+) RA; the cells were over fivefold higher in ACPA seropositive RA-synovial fluid as compared with synovial fluid from patients with seronegative inflammatory arthritides. These cells were defined as ‘peripheral helper’ T (TPH)-cells that express factors enabling B-cell help, such as IL-21 and others, and were distinct from T follicular helper (TFH) cells. The authors suggest that the abundance of TPH cells in RA-inflamed synovium indicates tissue-localized T–B-cell interactions that promote B-cell responses and antibody production. However, the authors did not explore the relations between these cells and estrogen deficiency in RA. 8. Osteoblast-mediated bone formation is inhibited in RA: the role of IL-17 and TNFα Inflammation in RA, associated with estrogen deficiency, as discussed above, stimulates RANKL over-expression and osteoclast-mediated articular bone erosion as well as systemic bone loss. The question arises whether the RA - estrogen deficiency milieu exerts an additional non-catabolic contribution to the increased rate of bone mass reduction in postmenopausal RA women. Accumulating evidence indicates that unlike non-RA bone loss, the RA-associated pro-inflammatory cytokines, in particular, TNFα, IL-1β, and IL-17 also exert anti-anabolic effects on bone by inhibiting the differentiation of osteoblasts and hence the rate of bone formation [177–179]. Thus, this process complements the massive bone resorption, resulting in an increased net loss of bone mass. The histopathology of the synovial tissue of RA patients has revealed that the abundance of osteoclasts in bone erosions is associated with a small number or even the absence of mature osteoblasts [180]. These authors also measured the impact of inflammation on osteoblast function within eroded arthritic bones in a murine model of RA. They found that within arthritic bones, the extent of mineralized bone

formation was reduced at the bone surface compared with the bone surface adjacent to normal marrow. Therefore, whereas the enhanced osteoclastogenesis in osteoporosis is partly compensated by stimulation of osteoblast-mediated bone formation [181], in postmenopausal RA, bone formation appears to be blocked. The systemic bone loss in RA shows no apparent signs of repaired erosions, even after the inflammatory process has been clinically controlled [55,182–184]. This implies that beyond estrogen deficiency and the inflammatory factors, which are shared by postmenopausal OP and by postmenopausal RA etiology, with their exerted catabolic effects, there might operate additional RAspecific pathways that provide an anti-anabolic contribution to the accelerated bone loss. I. IL-17-mediated inhibition of bone formation in RA. The mechanisms by which inflammation inhibits osteoblast function to limit bone formation and impede the healing of articular erosions have not been fully elucidated. IL-17 and TNFα appear to be the major inflammatory factors that are involved in both increased osteoclastogenesis and bone resorption, and in preventing osteoblast activity and bone formation in RA. Kim et al. [177] examined the effect of IL-17 on osteogenesis in rats, both in vitro and in vivo. IL-17 exerted a negative effect on the osteogenesis of rat calvarial osteoblast precursor cells in vitro. IL-17 also inhibited osteoblast differentiation and function in vivo, since it inhibited the filling of calvarial defects. Several other studies support this IL-17-mediated anti-anabolic role, in addition to its catabolic effects [109,155]. The study of Tyagi et al. [155] showed that anti-IL-17 antibody treatment in OVX mice appeared to be translated to better skeletal preservation, owing to the inhibition of osteoblast apoptosis, the increased number of bone lining cells, and increased Wnt10b expression. Thus, the mechanism underlying IL-17 action, whose production is negatively regulated by estrogen, is suggested to adversely impact bone loss through two different pathways: augmenting osteoclast production and bone resorption, as well as by suppressing the function of osteoblastic bone formation. Importantly, however, estrogen can reverse these effects of IL-17. II. TNFα affects bone formation in RA. TNFα was found to play a double role in RA, which includes both bone resorption and the inhibition of bone formation. It was reported to increase the number of bone-resorbing osteoclasts, with a simultaneous decrease in the number of bone-forming osteoblasts, ultimately leading to an overall bias toward bone resorption [185–186]. Transgenic mice overexpressing human TNFα (TNFtg) develop systemic bone loss and osteoporosis in addition to erosive arthritis due to a higher degree of osteoclastogenesis and the inhibition of bone formation [187–189]. TNFα inhibits bone formation in RA by antagonizing both bone morphogenetic protein (BMP) and Wnt signaling pathways (via the overexpression of DKK1, discussed next). In rheumatic diseases, activation or inhibition of the Wnt and BMP signaling pathways, both essential pathways for the differentiation and function of osteoblasts, results in very different outcomes for bones. The Wnt [190] and BMP [191] pathways, known to regulate skeletal development and organogenesis, are also critical pathways that regulate osteoblast differentiation. It has been shown that TNFα plays a role as an indirect inhibitor of RA-associated osteoblastic bone formation, by affecting both the BMP and Wnt signaling pathways, as follows: In the BMP signaling pathway, osteoblast differentiation and function depend on the expression of BMP 2, 4, and 7. These factors are crucial for the osteogenic differentiation of mesenchymal precursor cells, and thus for maintaining adult bone mass. BMP-induced signaling and its induced osteogenic responses are negatively regulated by secreted antagonists, including a unique member of the BMP family, BMP3 [192,193]. BMP3, also known as osteogenin, was shown to be upregulated late in the course of inflammatory arthritis by osteoblast-lining


erosion sites, in murine models of RA [194]. The involvement of TNFα in the BMP pathway is related to its inhibitory function in BMP-induced bone formation [195]. Moreover, TNFα (but not IL-1β or IL-17) also induces the osteoblastic expression of BMP3 [196]. This suggests the potential unique contribution of the TNFα - BMP3 axis for RAinflammatory bone loss, owing to the impairment of osteoblast-mediated bone formation and erosion repair. As mentioned, TNFα also interacts with the Wnt signaling pathway by suppressing one of the major bone-anabolic signals, canonical Wnt signaling (also called Wnt/β-catenin signaling) in osteoblasts [197]. It has been repeatedly shown that diminished bone repair in RA appears to be caused, at least in part, by inhibition of Wnt signaling [198,199], specifically through the Wnt/β-catenin signaling pathway [200,201]. Power negative regulators of Wnt/β-catenin anabolic signaling in osteoblasts include sclerostin and DKK1. Importantly, several members of the DKK family were reported to be up-regulated in RA synovium [180,202]. DKK1 expression was found in RA-synovial fibroblasts, microvessels, and chondrocytes; its serum levels are higher in RA patients than in healthy subjects, and correlate with the extent of bone erosion and osteoporosis [138,198,199,203–206]. RA patients carrying risk alleles of genetic variants in DKK1 were found to have higher serum levels of functional DKK1 and more progressive joint destruction over time [207]. Inhibition of DKK1 in an animal model of RA increased OPG expression and reversed the phenotype from a pattern of bone destruction to a pattern of bone formation in the inflamed peripheral joints [202]. This suggests that DKK1 is an important agent for osteoblast differentiation and osteoclast dysregulation in RA. The inhibitory role of TNFα (whose levels, as discussed, are known to rise under estrogen deficiency conditions) in bone-forming osteoblasts is mostly achieved through stimulation of the significant over-expression of DKK1 in synoviocytes [208]. TNFα-antagonist treatment significantly preserved BMD in RA patients [122,209] as well as in the RA mouse model [96], and it reduced DKK1 serum levels in RA patients [160]. In an overview, postmenopausal OP-RA appears to suffer, simultaneously, from at least three major pathogenic processes: cartilage loss, bone resorption, and inhibited bone formation. Basic research has not, so far, suggested or pointed toward a specific recommended drug use. On the contrary, it seems that there is an essential need for a combination of therapies for resolving several aspects of OP-RA pathology. For the purpose of bridging the gap between mouse and human studies, humanized models for studying bone formation and erosion might be useful. Therefore, new options to be developed in the future for the treatment of postmenopausal OP-RA may be successful in stimulating osteoblast activity and in repairing damaged bones, after controlling the inflammatory destructive process. This might involve modulating the Wnt signaling and the TGF-β/BMP pathway; in addition, the balance between OPG, TNF, IL-23, IL-17, and IL-22 might be a promising way to proceed, according to the study of Yago et al. [210]. According to this study, IL-23 is an important cytokine that induces human osteoclastogenesis via IL-17 in vitro; also, anti-IL-23 antibody attenuates collageninduced arthritis in rats. 9. ACPA, micro RNA (miRNA), and Myostatin as agents of bone destruction in RA The pathogenic scenario of postmenopausal OP-RA is even much more complicated, since some new factors are also involved in the process of RA-bone destruction. These agents include autoantibodies of the IgG isotype, microRNAs, and myostatin, which will be considered below in more detail. I. ACPA-immune complexes. Autoantibodies of the IgG isotype, which have been found in the synovial fluid of the joint space, include ACPA, whose presence is one of the strongest risk factors for RA [211–213]. Postmenopausal estrogen deficiency has been recently proposed to interact with an RA-predisposing genetic background

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associated with CD4 + T-cell immune functions [59]. The production of the autoimmune antibodies, ACPA in postmenopausal RA, results from this suggested combined effect; the RA-genetic risk factors include HLA-DRB1 SE alleles and specific alleles of the PAD4, PTPN22, and CCR6 genes. In short, estrogen deprivation was suggested to be involved in determining the fate of CD4 + T-cells, probably by favoring the production of the RA pathogenic IL-17 + exFoxp3 Th17 cells, and by increasing the Th17/Treg ratio imbalance. The consequence of the proposed estrogen deficiency-risk alleles' interaction was postulated to lead to a burst of an inflammatory response, and at the same time, to result in the defective suppression of this inflammatory condition in RA. The affected molecular pathways would govern immune dysregulation, breached-tolerance, and ACPA-autoantibody production in RA [59]. These ideas are supported by the experiments of Luckey et al. [214], who addressed the question about IL17 production in mice carrying HLA alleles not associated with RA risk. Indeed, mice carrying the RA-resistant HLA allele - DRB1*⁄ 0402 were found to be not defective in producing IL-17, as compared with DRB1*⁄0401. The authors suggested that although both alleleexpressed molecules are capable of producing IL-17 (and IL-10), a difference in the kinetics of the response may govern their ability to clear infections and their association with autoreactivity. Whereas DRB1*/0401 has autoreactive memory cells, which, with molecular mimicry, can be activated and can produce IL-17, DRB1*/0402 does not produce any autoreactive response. It has been shown that in ACPA-positive individuals without arthritis, structural bone damage could be observed before clinical disease onset [215,216]; however, such non-RA individuals exhibit signs of bone loss, whereas ACPA-negative healthy individuals do not [217]. Bugatti et al. [218] have found that ACPA is associated with systemic bone loss starting from the earliest stages of RA, and high levels of RF further increase the risk. They suggested that ACPA-RF-positive patients with early RA should therefore be carefully monitored for the development of generalized osteoporosis beyond the assessment of traditional risk factors. Since synovitis may require some time to destroy bones to an extent that is clinically detectable, it might be preceded by one or more other causative factors. Such a candidate might be ACPA, which can be detected in serum years before the clinical onset of the disease [219–221]. The displayed bone damage that precedes the clinical onset might be caused through mechanisms independent of inflammation, thus challenging the concept that bone damage in RA is exclusively caused by inflammation [215]. This alternative pathogenic relevance to ACPA suggests that bone loss starts as early as when an individual has already experienced a break of immune tolerance against citrullinated self-proteins. To date, the molecular pathways involved are poorly understood. ACPA might serve as a mediating factor for the transition from autoimmunity to inflammation. Animal models showed that the passive transfer of mouse-ACPA into mice with pre-existing low levels of synovial inflammation results in ACPA-mediated disease progression [222]. In addition, the passive transfer of purified human IgG or ACPA from RA patients induced arthritis in mice [223,224]. Furthermore, ACPA was reported to directly contribute to bone loss by binding to the surface of osteoclast precursors, thus stimulating osteoclastogenesis, possibly by stimulation of TNFα production [217,225]. Hensvold et al. [226] reported that in a cohort of newly diagnosed untreated RA patients the RANKL levels in serum and synovium were elevated in ACPApositive as compared with ACPA-negative patients, and were associated with erosive disease. Both ACPA and RANKL levels fell following methotrexate treatment. At present it is not clear how the break in immune tolerance occurs, when the first autoimmune plasma cells start producing ACPA, and where the population of plasma cells producing ACPA is localized. Since citrullination of proteins may emerge at sites distant from the joints, like the lung epithelium and gingival mucosa, Kleyer et al. [215] proposed a dual hit hypothesis for RA. The process begins with an initial



break of immune tolerance and the formation of ACPA; a second hit is the autoimmunity translation into an inflammatory joint disease characterized by synovitis. The theory of a second hit is supported by evidence that stimulation of macrophages with ACPA from RA patients results in the production of high levels of TNFα by these cells, contributing to osteoclastogenesis [212,227,228]. Recent studies provide evidence that autoantibodies not only bind macrophages — they also bind osteoclast precursors, inducing them to produce TNFα, and thus promoting osteoclastogenesis, resulting in osteopenia [225]. Moreover, Fc receptors present on osteoclast precursors, upon binding autoantibodies (crosslinking of FcγRIV), have been shown to enhance osteoclast differentiation; deletion of FcγRIV decreased osteoclast numbers and bone destruction in arthritic mice [229,230]. These studies imply a mechanistic role for the specific characteristics of the Fc part of the ACPA molecule bound to the Fc receptor on the pre-osteoclast: it induces downstream signals that regulate the process of osteoclastogenesis. Taken together, the data support an early direct pathogenic link between the appearance of ACPA and bone destruction in RA patients. It emphasizes the importance of autoantibodies in mediating bone loss in RA and suggests an additional molecular mechanism by which osteoclastogenesis in RA may be promoted, independent of synovial-derived proinflammatory cytokines. This suggests that in RA patients who are ACPA-positive, its presence could serve as an additional argument when considering osteoporosis therapy to be initiated promptly and probably with a lower threshold. II. Micro RNA (miRNA). MiRNA are small noncoding RNA molecules that serve as key mediators of gene silencing at the post-transcription level in fundamental biological and pathological processes [231]. The evidence linking various cytokines and a specific miRNA to RA remains scarce. For example, IL-34, a newly discovered cytokine, could contribute to the increment of miRNA-21 expression by inducing STAT3 activation, which was found to be associated with disease activity in RA patients [232]. MiRNA-21 was shown to be involved in the imbalance of Th17/Treg cells by regulating STAT3/STAT5 in patients with RA [233]. IL-34 is locally expressed in the synovial tissue and is associated with synovitis severity [234], as well as with inflammatory cytokines before and during therapy in RA; IL-34 was proposed as a potential biomarker for both RA diagnosis and therapy efficiency [235]. This cytokine, in combination with RANKL, promoted osteoclast differentiation, proliferation, and survival, leading to bone resorption in mouse and human cell culture systems [236,237]. A positive correlation between IL-34 levels, IL-17 production, and ACPA titers was also reported [238,239]. The IL-34/ STAT3/miRNA-21 pathway may therefore be crucial for RA synovitis, and the resulting bone loss [232]. III. Myostatin (GDF8) is a member of the TGF-β superfamily, which was previously proposed to play a role in bone remodeling [240,241]. Myostatin might be an additional factor whose expression is higher in RA and in the context of an inflammatory environment. It was suggested that the inflammatory environment of the synovium from people with RA leads to an upregulation of myostatin in synovial cells [242]. This study also identified myostatin as an important direct paracrine and autocrine regulator of RANKL-induced osteoclast development and bone destruction in RA. 10. The effects of glucocorticoid (GC) treatment on bone loss in RA patients GCs are widely used to treat RA, due to their strong anti-inflammatory effect. This treatment is quantified either by daily or cumulative dosage, and it has shown improved outcomes of the disease [243]. Prolonged use of GCs is associated with several adverse side effects such as secondary osteoporosis [244]. Bone loss progresses rapidly

during the first 3 to 6 months of GC therapy, and fractures occur in 30–50% of adult patients receiving long-term GCs [245,246]. For example, GCs use (prednisone equivalent N5 mg daily) by postmenopausal women is associated with vertebral fractures within the first 6 months of treatment [245]. Thus, as described above, the pathogenic process of bone loss in RA patients is mainly attributed to the abundance of osteoclasts (due to inflammatory conditions); however, GC therapy is an additional potential contributor to the bone loss. Aeberli and Schett [247] speculate that there is not only a loss in bone mass but also in bone quality with GC users. The effects of GCs on bone cells include early and quick induction of osteoclastogenesis leading to increased bone resorption. In addition, GCs cause apoptosis of osteoblasts and osteocytes, which lead to the suppression of bone formation, a central feature in their pathogenetic course [248]. Interestingly, a previous study showed that treatment of low-dose prednisone in RA postmenopausal patients caused reduced levels of sex hormones and osteocalcin as well as reduced vertebral bone mass, whereas comparable doses of deflazacort showed only a mild inhibitory effect on sex hormones and osteocalcin, and did not result in any appreciable effect on bone mass; in general, the GC-treated patients showed a direct correlation between vertebral BMD and plasma estradiol levels [249]. Another study [250] compared the effects of short vs long-term GC treatment, with a similar low dose used throughout the study, in female RA patients. It was shown that after 12 months of the trial, there was no statistically significant difference in the bone turnover rate between RA patients treated with low doses of GCs for the first time, and those who carried out their therapy without GCs. In contrast, long-term GC therapy did not provide additional benefits—instead, it elevated the levels of the inflammation markers and further bone resorption was observed. The authors suggested that in RA females, the adverse effects on bone turnover of short-term GC treatment were balanced by the GC anti-inflammatory effect. Hence, careful use of GC treatment in RA female patients, applied for the first time, and as a short-term therapy, can be effective in overcoming inflammation symptoms, without a significant risk of osteoporosis. This suggestion shows the potential benefit of a combined therapy. Indeed, it has been demonstrated that denosumab therapy for 12 months reversed the effects of GC-induced osteoporosis [251]. Denosumab was found to be effective and safe in preventing bone resorption and BMD loss in patients treated with long-term GCs for inflammatory diseases. 11. Summary and conclusions Postmenopausal estrogen deficiency causes a systemic inflammatory condition characterized by elevated circulating levels of proinflammatory cytokines, mainly TNFα, IL-17, and RANKL. These cytokines are involved in osteoclastogenesis, bone resorption, and destruction in postmenopausal OP. In postmenopausal RA, we propose that because of an interaction between estrogen deficiency and the RA genetic risk factors associated with the immune activation of autoreactive CD4 + Th17 cells and down regulation of Tregs, a unique condition of RA arises. This condition consists of elevated levels of circulating TNFα, IL-17, and RANKL as compared with RA-free OP, and in the production of unique RA-ACPA autoantibodies. Both these components stimulate expanded osteoclastogenesis and bone loss. Moreover, the inflammatory cytokines, mainly TNFα and IL-17, are responsible for blocked bone formation. In conclusion, the extent of RANKL production and hence, bone destruction in postmenopausal RA-OP is much higher than in non-RA postmenopausal OP. The inhibited bone formation characterizing postmenopausal RA-OP further contributes to greater net bone loss. This combination may be the culprit for the accelerated and doubled osteoporosis upon postmenopausal RA onset. Future research, focusing on the potential biological mechanisms associated with the double occurrence of OP in postmenopausal RA, might advance our understanding of the hormonal status and the age-


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