Relaxin: a Missing Link in the Pathomechanisms of Systemic Lupus Erythematosus?

http://informahealthcare.com/mor ISSN 1439-7595 (print), 1439-7609 (online)

REVIEW ARTICLE Relaxin: A missing link in the pathomechanisms of Systemic Lupus Erythematosus?

Madhusoothanan Bhagavathi Perumal1 and Saranya Dhanasekaran 2

1 Queensland Brain Institute, The University of Queensland, Brisbane, Australia and 2 Chhatrapati Shahuji Maharaj Hospital, Lucknow, India Downloaded from https://academic.oup.com/mr/article/24/4/547/6303646 by guest on 01 October 2021

Abstract Keywords Systemic Lupus Erythematosus (SLE) is an autoimmune rheumatic disease which predominantly Autoimmunity , Endometrium , Macrophages , aﬀ ects women of reproductive age. Despite signiﬁ cant progress in recent years to elucidate Relaxin , Systemic Lupus Erythematosus many potential mechanisms involved in the generation of autoimmunity the factors behind the high incidence among women, the relapsing– remitting clinical course and pregnancy-related History complications in SLE remain unclear. In this review, we hypothesize a potential role for uterine Received 5 February 2013 endometrium through its production of relaxin, a peptide hormone, as a “ missing-link ” to explain Accepted 15 May 2013 this female predominance, variable clinical course and obstetric complications operating in SLE. Published online 16 October 2013

Background In contrast, M2 macrophages are important in tissue repair and wound healing. They express high levels of anti-infl ammatory Systemic Lupus Erythematosus (SLE) is an autoimmune disease cytokines (such as IL10) as well as growth factors such as vascular which can aff ect any part of the body leading to a myriad of clini- endothelial growth factor (VEGF) and basic Fibroblast growth fac- cal manifestations. Characteristic features include the generation tor (bFGF). M2 macrophages also express high levels of mannose- of autoantibodies against host antigens, such as double stranded binding lectins (MBL) receptor and C1q complement receptors DNA, and a predilection for women of reproductive age with a which help in the effi cient and safe clearance of apoptotic cells relapsing – remitting clinical course [1]. A number of diff erent [4]. Therefore, factors which regulate the balance of production triggers for autoimmunity in SLE have been proposed includ- of these diff erent functional phenotypes of these innate immune ing impaired apoptotic cell clearance, genetic, hormonal and cells may infl uence the generation of autoimmune diseases [5]. A environmental factors. Not all of these factors, however, readily similar process operates in cancer cells which generate a variety of explain the fact that 90% of patients are women and attention factors to evade host immune response by promoting production has focused upon the role of sex hormones. The mechanisms of the M2 macrophage phenotype. Many cancers express immu- however, linking these hormones with generation of autoimmu- nomodulatory factors such as relaxin peptides to regulate the func- nity and disease exacerbations, remain unclear. Therefore, other tion of innate immune cells [6]. These immunomodulatory eff ects factors should be considered and in this article we will discuss may also be important in tissues with physiological high cellular the potential eff ects of a hormone called relaxin upon uterine turnover such as the uterine endometrium and thus relevant to the endometrium leading to generation of recurrent antigenic load pathogenesis of SLE. and thus disease pathogenesis.

Physiological factors infl uencing autoimmunity Relaxin peptides Apoptosis is a physiological process of programed cell death occur- Relaxin peptides are insulin-like growth factors expressed by three ring in multi-cellular organisms throughout their lifetime to ensure genes in humans [7]. H1 relaxin and H2 relaxin share 95% sequence the orderly removal of cells from the body. Macrophages are the homology and are expressed in the uterine decidua, placenta, corpus major scavengers of dying cells and it is increasingly recognized luteum, endometrium and mammary glands in women and by pros- that they can acquire extremes of functional phenotypes broadly tate and seminal vesicles in men. They are believed to have similar divided as M1 and M2 [2]. M1 macrophages are pro-infl ammatory physiological eff ects [8, 9], which are described below. Other major cells which combat infections, express high levels of Interleukin sites of H1 and H2 expression are skin, myocardium, kidney and 1 β (IL1 β), generate infl ammatory reactions against apoptotic cells lung. H3 relaxin is predominantly expressed in the brain and it is and present host antigens to adaptive immune cells, which may considered as a neuropeptide associated with stress response [10]. serve as the initial trigger of autoimmunity [3]. Four G-protein coupled receptors (GPCR) have been described as targets for these peptides [11]. Leucine-rich GPCR, Relaxin Correspondence to: Dr Madhusoothanan Bhagavathi Perumal, Level 5-2- Family Peptide Receptors 1 (RXFP1) (also known as LGR7) and 10, Queensland Brain Institute, St Lucia, The University of Queensland, RXFP2 (also known as LGR8) are widely expressed through- ϩ Brisbane 4067, Australia. Tel: 61(0)470410706. E-mail: sudhanbk@ out the body especially by immune cells. These GPCRs show gmail.com, [email protected] high affi nity for H2 relaxin. Small peptide-associated GPCR This is an open-access article distributed under the terms of the CC-BY-NC-ND 3.0 License which permits users to download and share the RXFP3 (also known as GPCR135) and RXFP4 (also known as article for non-commercial purposes, so long as the article is reproduced in GPCR142), are expressed in the brain and are the main target the whole without changes, and provided the original source is credited. receptors for H3 relaxin. Since H2 relaxin is the main member 548 M. B. Perumal & S. Dhanasekaran Mod Rheumatol, 2014; 24(4): 547–551

Figure 1. Immune modulation by relaxin. Summary of potential immunonodulatory eff ects of relaxin upon monocytes leading to diff erentiation into M2 macrophages and Th cells. VEGF, Vascular- induced growth factor; bFGF, basic fi broblast growth factor; MMP, Matrix metalloproteinase. Downloaded from https://academic.oup.com/mr/article/24/4/547/6303646 by guest on 01 October 2021 of this family of peptides in the female reproductive tract and expression of relaxin and its receptor RXFP1 at the uterine its physiological functions are extensively studied, all further endometrium. Estrogens promote proliferation of endometrial reference to relaxin in this article will be synonymous with H2 cells during the first half of the menstrual cycle and up-regulate relaxin. expression of uterine RXFP1 receptors [18]. Ovulation is then followed by formation of the corpus luteum which secretes Immune modulation by relaxin progesterone during the luteal phase. Progesterone promotes endometrial cell maturation, secretion as well as survival, and In humans, relaxin peptides act mainly in paracrine or autocrine up-regulates relaxin expression by the uterine endometrium fashion and circulating levels are negligible. Findings from cancer [19], (Figure 2). research [12] suggest that relaxin has three crucial eff ects on the In the absence of fertilization, prolactin levels peak during the innate immune cells: to recruit monocytes, to promote diff erentia- latter half of luteal phase and in conjunction with progesterone tion into the M2 macrophage phenotype and to inhibit migration of promote relaxin expression [20]. Prolactin also increases MCP-1 diff erentiated macrophages (Figure 1). expression in the endothelial cells [21]. Relaxin levels peak on the Recruitment of monocytes by relaxin depends on monocyte 8th day after ovulation – in conjunction with a prolactin surge – and chemo attractant peptide (MCP-1) levels. In vivo, MCP-1 is may then enhance endometrial recruitment and diff erentiation into expressed by monocytes, macrophages and endothelial cells lead- the M2 macrophage phenotype to aid clearance of the increased ing to marginalization of monocytes before migration into the number of apoptotic endometrial cells present at the end of the tissues [13]. It up-regulates expression of cell surface adhesion luteal phase. By this mechanism, relaxin helps to replenish the tis- molecules in monocytes to form cell clusters and diff erentiation sue resident scavenger macrophage system during each menstrual to macrophages. It inhibits IL1 β expression and promotes M2 cycle to clear apoptotic cells. phenotype acquisition. Furthermore, IL1 β has been shown to In addition, relaxin also promotes expression of Glycodelin, be suppressed in monocytes pre-treated with relaxin even after a lipocalin and immune regulator in the uterine endometrium exposure to pro-infl ammatory stimuli such as Lipopolysacchride [22]. Glycodelin induces apoptosis of pro-infl ammatory mono- (LPS) [12]. In addition, relaxin inhibits migration of diff erentiated cytes and natural killer cells and cytotoxic T cells. In addition, macrophages. Relaxin also induces expression of VEGF, bFGF it also suppresses B cell diff erentiation [23]. Therefore, loss of and matrix metallo proteinases (MMPs) by the monocytes and the anti-infl ammatory immunosuppressive eff ects of relaxin may macrophages [14]. conceivably contribute to disease pathogenesis in women of Few studies have addressed the eff ect of relaxin on other reproductive age. immune cells. On T helper (Th) cells, it has been shown to promote expression of interferon γ without altering IL4 expression [15] and suppress the release of proteases from neutrophils and basophils Endometrium and female predilection of SLE [16,17]. Overall, relaxin has been shown to generate a potential Much interest has focused upon whether sex hormone fl uctua- anti-infl ammatory immune environment and promote apoptotic tions contribute to SLE pathogenesis and disease activity [24]. cell-clearance capacity by recruiting and modulating monocytes ’ The frequency of disease fl ares have been shown to decrease after function in cancer cells. Therefore, it is possible that relaxin may menopause or ovarian failure [25,26]. Clinical trials however, have signifi cant immunomodulatory eff ects upon apoptotic cell which have examined whether estrogen (Hormone replacement or clearance in healthy cells with high physiological turnover such as combined oral contraceptive) therapy exacerbates disease activity the uterine endometrium. in patients with SLE, have not found such a direct relationship. Relaxin and the endometrium These studies did not fi nd any diff erence in severe disease fl are rates between the estrogen and placebo-treated groups, although High levels of auto antibodies may be generated in patients mild/moderate disease fl ares were signifi cantly increased in the with SLE through exposure to increased amounts of potentially hormone-replacement group [27,28]. more immunogenic antigen from impaired clearance of apop- Given that uterine endometrium is a site of physiological cel- totic cells. Therefore, regulation of this process is particularly lular turnover, under the infl uence of sex hormones it may provide important in endometrial tissue with high cyclical cellular a potential link between sex hormones and SLE. In a retrospective turnover in women of reproductive age. Sex hormones regulate case control study, the eff ect of hysterectomy among 1006 premeno- the cellular proliferation and survival of the endometrial cells pausal women with SLE were assessed for onset, disease activity and during menstrual cycle and pregnancy. They also influence incidence of nephritis compared with age and ethnically matched DOI 10.3109/14397595.2013.844297 A paracrine perspective on autoimmunity 549

Figure 2. Alterations of sex hormone and relaxin levels regulating menstrual cycle. Graphical display of changes in sex hormone and relaxin levels as well as the endometrium during the menstrual cycle. VEGF, vascular endothelial growth factor; bFGF, basic ﬁ broblast growth factor; IFNγ , Interferon gamma; MMPs, Matrix metalloproteinases. Downloaded from https://academic.oup.com/mr/article/24/4/547/6303646 by guest on 01 October 2021

controls with lupus who had not undergone hysterectomy. Patients and M1 macrophage formation may (Figure 3) in the uterine who had undergone hysterectomy had signiﬁ cantly decreased endometrium. disease activity, reduced ANA and improved clinical progression compared with those who had not had a hysterectomy. Interest- Relaxin: a missing link in SLE? ingly, 90% of these women were also reported to be taking hormone replacement therapy, yet had reduced disease activity [29]. To date, no studies have been performed examining the potential Therefore, dysregulation of relaxin, mediated immune modu- relationship between relaxin and pathogenesis of SLE, to conclu- lation in patients with SLE may serve to increase apoptotic load sively answer the question of whether dysregulation of relaxin

Figure 3. Relaxin immune regulations, apoptotic cell clearance and autoimmunity. Summary of monocyte diff erentiation and apoptotic clearance in the presence (A) and absence (B) of relaxin. C1q, complement component; MBL, Mannose-binding lectin; iNOs, infl ammatory Nitric oxide synthase; NK cells, Natural Killer Cells; VEGF, Vascular endothelial growth factor; bFGF, basic Fibroblast Growth Factor; MMPs, Matrix metalloproteinases. 550 M. B. Perumal & S. Dhanasekaran Mod Rheumatol, 2014; 24(4): 547–551 provides a missing link to lupus pathogenesis. Indirect evidence, Acknowledgements however, exists that reduced levels of relaxin may contribute to pregnancy morbidity, gram-negative sepsis and reduced cancer Authors are immensely grateful to Dr Ian Giles, University risk which are all recognized complications of SLE. College London for his guidance and insights. Pre-eclampsia is increased in patients with SLE [30] which occurs due to abnormalities of trophoblastic invasion and spiral Confl ict of interest artery development [31]. In many mammals, relaxin has been None. found to be important in normal trophoblastic invasion [32] and its pro-angiogenic eff ects would be expected to promote spiral References artery formation and improve placental circulation. Reduced 1. Rahman A , Isenberg DA . Systemic lupus erythematosus . N Engl J placental expression of RXFP1 has been reported in otherwise Med. 2008 ; 358(9) : 929 – 39 . healthy patients who develop pre-eclampsia [33]. Direct evi- 2. Mantovani A , Sica A , Locati M . Macrophage polarization comes of dence, however, to link the development of pre-eclampsia in age . Immunity. 2005 ; 23(4) : 344 – 6 . patients with SLE with reduced RXFP1 and/or dysregulation of 3. Murray PJ , Wynn TA . Protective and pathogenic functions of relaxin is lacking. macrophage subsets . 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