Inhibitory effects of relaxin on human basophils activated by stimulation of the Fc epsilon receptor. The role of nitric oxide

Characterization of relaxin 3 and its receptors in chicken: Evidence for relaxin 3 acting as a novel pituitary hormone

Mammalian relaxin (RLN) family peptides binding their receptors (RXFPs) play a variety of roles in many physiological processes, such as reproduction, stress, appetite regulation, and energy balance. In birds, although two relaxin family peptides (RLN3 and INSL5) and four receptors (RXFP1, RXFP2, RXFP2-like, and RXFP3) were predicated, their sequence features, signal properties, tissue distribution, and physiological functions remain largely unknown. In this study, using chickens as the experimental model, we cloned the cDNA of the cRLN3 gene and two receptor (cRXFP1 and cRXFP3) genes. Using cell-based luciferase reporter assays, we demonstrate that cRLN3 is able to activate both cRXFP1 and cRXFP3 for downstream signaling. cRXFP1, rather than cRXFP3, is a cognate receptor for cRLN3, which is different from the mammals. Tissue distribution analyses reveal that cRLN3 is highly expressed in the pituitary with lower abundance in the hypothalamus and ovary of female chicken, together with the detection that cRLN3 co-localizes with pituitary hormone genes LHB/FSHB/GRP/CART and its expression is tightly regulated by hypothalamic factors (GnRH and CRH) and sex steroid hormone (E2). The present study supports that cRLN3 may function as a novel pituitary hormone involving female reproduction.

Relaxin as an anti-fibrotic treatment: Perspectives, challenges and future directions

Fibrosis refers to the scarring and hardening of tissues, which results from a failed immune system-coordinated wound healing response to chronic organ injury and which manifests from the aberrant accumulation of various extracellular matrix components (ECM), primarily collagen. Despite being a hallmark of prolonged tissue damage and related dysfunction, and commonly associated with high morbidity and mortality, there are currently no effective cures for its regression. An emerging therapy that meets several criteria of an effective anti-fibrotic treatment, is the recombinant drug-based form of the human hormone, relaxin (also referred to as serelaxin, which is bioactive in several other species). This review outlines the broad anti-fibrotic and related organ-protective roles of relaxin, mainly from studies conducted in preclinical models of ageing and fibrotic disease, including its ability to ameliorate several aspects of fibrosis progression and maturation, from immune cell infiltration, pro-inflammatory and pro-fibrotic cytokine secretion, oxidative stress, organ hypertrophy, cell apoptosis, myofibroblast differentiation and ECM production, to its ability to facilitate established ECM degradation. Studies that have compared and/or combined these therapeutic effects of relaxin with current standard of care medication have also been discussed, along with the main challenges that have hindered the translation of the anti-fibrotic efficacy of relaxin to the clinic. The review then outlines the future directions as to where scientists and several pharmaceutical companies that have recognized the therapeutic potential of relaxin are working towards, to progress its development as a treatment for human patients suffering from various fibrotic diseases.

Changes in DNA methylation from pre- to post-adolescence are associated with pubertal exposures

BACKGROUND

Adolescence is a period characterized by major biological development, which may be associated with changes in DNA methylation (DNA-M). However, it is unknown to what extent DNA-M varies from pre- to post-adolescence, whether the pattern of changes is different between females and males, and how adolescence-related factors are associated with changes in DNA-M.

METHODS

Genome-scale DNA-M at ages 10 and 18 years in whole blood of 325 subjects (n = 140 females) in the Isle of Wight (IOW) birth cohort was analyzed using Illumina Infinium arrays (450K and EPIC). Linear mixed models were used to examine DNA-M changes between pre- and post-adolescence and whether the changes were gender-specific. Adolescence-related factors and environmental exposure factors were assessed on their association with DNA-M changes. Replication of findings was attempted in the comparable Avon Longitudinal Study of Parents and Children (ALSPAC) cohort.

RESULTS

In the IOW cohort, after controlling for technical variation and cell compositions at both pre- and post-adolescence, 15,532 cytosine-phosphate-guanine (CpG) sites (of 400,825 CpGs, 3.88%) showed statistically significant DNA-M changes from pre-adolescence to post-adolescence invariant to gender (false discovery rate (FDR) = 0.05). Of these 15,532 CpGs, 10,212 CpGs (66%) were replicated in the ALSPAC cohort. Pathway analysis using Ingenuity Pathway Analysis (IPA) identified significant biological pathways related to growth and development of the reproductive system, emphasizing the importance of this period of transition on epigenetic state of genes. In addition, in IOW, we identified 1179 CpGs with gender-specific DNA-M changes. In the IOW cohort, body mass index (BMI) at age 10 years, age of growth spurt, nonsteroidal drugs use, and current smoking status showed statistically significant associations with DNA-M changes at 15 CpGs on 14 genes such as the AHRR gene. For BMI at age 10 years, the association was gender-specific. Findings on current smoking status were replicated in the ALSPAC cohort.

CONCLUSION

Adolescent transition is associated with changes in DNA-M at more than 15K CpGs. Identified pathways emphasize the importance of this period of transition on epigenetic state of genes relevant to cell growth and immune system development.

Understanding relaxin signalling at the cellular level

The peptide hormone relaxin mediates many biological actions including anti-fibrotic, vasodilatory, angiogenic, anti-inflammatory, anti-apoptotic, and organ protective effects across a range of tissues. At the cellular level, relaxin binds to the G protein-coupled receptor relaxin family peptide receptor 1 (RXFP1) to activate a variety of downstream signal transduction pathways. This signalling cascade is complex and also varies in diverse cellular backgrounds. Moreover, RXFP1 signalling shows crosstalk with other receptors to mediate some of its physiological functions. This review summarises known signalling pathways induced by acute versus chronic treatment with relaxin across a range of cell types, it describes RXFP1 crosstalk with other receptors, signalling pathways activated by other ligands targeting RXFP1, and it also outlines physiological relevance of RXFP1 signalling outputs. Comprehensive understanding of the mechanism of relaxin actions in fibrosis, vasodilation, as well as organ protection, will further support relaxin's clinical potential.

Relaxin and extracellular matrix remodeling: Mechanisms and signaling pathways

Fibrosis is associated with accumulation of excess fibrillar collagen, leading to tissue dysfunction. Numerous processes, including inflammation, myofibroblast activation, and endothelial-to-mesenchymal transition, play a role in the establishment and progression of fibrosis. Relaxin is a peptide hormone with well-known antifibrotic properties that result from its action on numerous cellular targets to reduce fibrosis. Relaxin activates multiple signal transduction pathways as a mechanism to suppress inflammation and myofibroblast activation in fibrosis. In this review, the general mechanisms underlying fibrotic diseases are described, along with the current state of knowledge regarding cellular targets of relaxin. Finally, an overview is presented summarizing the signaling pathways activated by relaxin and other relaxin family peptide receptor agonists to suppress fibrosis.

The actions of relaxin on the human cardiovascular system

The insulin-like peptide relaxin, originally identified as a hormone of pregnancy, is now known to exert a range of pleiotropic effects including vasodilatory, anti-fibrotic, angiogenic, anti-apoptotic and anti-inflammatory effects in both males and females. Relaxin produces these effects by binding to a cognate receptor RXFP1 and activating a variety of signalling pathways including cAMP, cGMP and MAPKs as well as by altering gene expression of TGF-β, MMPs, angiogenic growth factors and endothelin receptors. The peptide has been shown to be effective in halting or reversing many of the adverse effects including fibrosis in animal models of cardiovascular disease including ischaemia/reperfusion injury, myocardial infarction, hypertensive heart disease and cardiomyopathy. Relaxin given to humans is safe and produces favourable haemodynamic changes. Serelaxin, the recombinant form of relaxin, is now in extended phase III clinical trials for the treatment of acute heart failure. Previous clinical studies indicated that a 48 h infusion of relaxin improved 180 day mortality, yet the mechanism underlying this effect is not clear. This article provides an overview of the cellular mechanism of effects of relaxin and summarizes its beneficial actions in animal models and in the clinic. We also hypothesize potential mechanisms for the clinical efficacy of relaxin, identify current knowledge gaps and suggest new ways in which relaxin could be useful therapeutically.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Serelaxin for the treatment of acute heart failure: a review with a focus on end-organ protection

Acute heart failure (AHF) is a complex clinical syndrome characterized by fluid overload and haemodynamic abnormalities (short-term clinical consequences) and the development of end-organ damage (long-term consequences). Current therapies for the treatment of AHF, such as loop diuretics and vasodilators, help to relieve haemodynamic imbalance and congestion, but have not been shown to prevent (and may even contribute to) end-organ damage, or to provide long-term clinical benefit. Serelaxin is the recombinant form of human relaxin-2, a naturally occurring hormone involved in mediating haemodynamic changes during pregnancy. Preclinical and clinical studies have investigated the effects mediated by serelaxin and the suitability of this agent for the treatment of patients with AHF. Data suggest that serelaxin acts via multiple pathways to improve haemodynamics at the vascular, cardiac, and renal level and provide effective congestion relief. In addition, this novel agent may protect the heart, kidneys, and liver from damage by inhibiting inflammation, oxidative stress, cell death, and tissue fibrosis, and stimulating angiogenesis. Serelaxin may therefore improve both short- and long-term outcomes in patients with AHF. In this review, we examine the unique mechanisms underlying the potential benefits of serelaxin for the treatment of AHF, in particular, those involved in mediating end-organ protection.

Relaxin: a missing link in the pathomechanisms of Systemic Lupus Erythematosus?

Systemic Lupus Erythematosus (SLE) is an autoimmune rheumatic disease which predominantly affects women of reproductive age. Despite significant progress in recent years to elucidate 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 complications in SLE remain unclear. In this review, we hypothesize a potential role for uterine endometrium through its production of relaxin, a peptide hormone, as a "missing-link" to explain this female predominance, variable clinical course and obstetric complications operating in SLE.

Relaxin family peptides and their receptors

There are seven relaxin family peptides that are all structurally related to insulin. Relaxin has many roles in female and male reproduction, as a neuropeptide in the central nervous system, as a vasodilator and cardiac stimulant in the cardiovascular system, and as an antifibrotic agent. Insulin-like peptide-3 (INSL3) has clearly defined specialist roles in male and female reproduction, relaxin-3 is primarily a neuropeptide involved in stress and metabolic control, and INSL5 is widely distributed particularly in the gastrointestinal tract. Although they are structurally related to insulin, the relaxin family peptides produce their physiological effects by activating a group of four G protein-coupled receptors (GPCRs), relaxin family peptide receptors 1-4 (RXFP1-4). Relaxin and INSL3 are the cognate ligands for RXFP1 and RXFP2, respectively, that are leucine-rich repeat containing GPCRs. RXFP1 activates a wide spectrum of signaling pathways to generate second messengers that include cAMP and nitric oxide, whereas RXFP2 activates a subset of these pathways. Relaxin-3 and INSL5 are the cognate ligands for RXFP3 and RXFP4 that are closely related to small peptide receptors that when activated inhibit cAMP production and activate MAP kinases. Although there are still many unanswered questions regarding the mode of action of relaxin family peptides, it is clear that they have important physiological roles that could be exploited for therapeutic benefit.

Relaxin promotes clustering, migration, and activation states of mononuclear myelocytic cells

Monocytes are leukocytic precursors of macrophages, dendritic cells, and osteoclasts, with critical roles in inflammation and tumor biology. Tumors can elicit signals that activate monocytes to extravasate, infiltrate tumors, and differentiate into tumor-associated macrophages (TAMs), which can modulate host immune surveillance. In order to assess whether relaxin can influence monocyte activation status, we assessed its ability to alter cell-cell clustering and cytokine expression of the monocytic cell line THP-1. Here we report that relaxin can induce time- and substrate-dependent homotypic cell-cell clustering of monocytes. In addition, we demonstrate that relaxin can suppress macrophage migration in an adenylate cyclase-independent, nitric oxide synthase-dependent fashion. We confirm relaxin-induced upregulation of vascular endothelial growth factor expression and regulation of M1/M2 cytokine profiles. By stimulating monocyte activation and modulating inflammatory cytokine expression and migratory activity of resulting macrophages in response to endotoxin exposure, relaxin may be a critical regulator of the macrophage activation state that regulates the TAM phenotype.

Relaxin induces mast cell inhibition and reduces ventricular arrhythmias in a swine model of acute myocardial infarction

Resident cardiac mast cells, located mainly around coronary vessels and in the right atrium close to the sinoatrial node, are the main repository of cardiac histamine. Inflammatory activation of cardiac mast cells, as occurs upon acute myocardial infarction, causes the release of histamine and prostanoids. These substances lead to severe tachyarrhythmias, cardiodepressive effects and coronary spasm, thus contributing to myocardial damage and early, lethal outcome. Relaxin, known to inhibit mast cell activation, has been recently validated as a cardiotropic hormone, being produced by the heart and acting on specific heart receptors. In this study, we report on a swine model of heart ischemia/reperfusion, currently used to test cardiotropic drugs, in which human recombinant relaxin (2.5 and 5 microg/kg b.w.), given at reperfusion upon a 30-min ischemia, markedly reduced cardiac injury as compared with the vehicle-treated animals. Evidence is provided that relaxin, at both the assayed doses, causes a clear-cut, significant reduction of plasma histamine, increase in cardiac histamine content and decrease in cardiac mast cell degranulation. This is accompanied by a reduction of oxidative cardiac tissue injury (assessed as tissue malondialdehyde) and of the occurrence of severe ventricular arrhythmias. In conclusion, this study provides further insight into the cardioprotective effects of relaxin, which also involve mast cell inhibition, and confirms the relevance of histamine in the pathophysiology of ischemia-reperfusion-induced cardiac injury and dysfunction. It also offers additional evidence for the potential therapeutic effects of relaxin in animal models of disease involving mast cell activation.

Comparison of Signaling Pathways Activated by the Relaxin Family Peptide Receptors, RXFP1 and RXFP2, Using Reporter Genes

The receptors for H2 relaxin and insulin-like peptide 3, relaxin family peptide receptor (RXFP) 1 and RXFP2, respectively, were recently identified, but their signaling pathways are not yet well characterized. Although previous work has suggested that cAMP is a major signaling pathway activated by these receptors, RXFP1 has also been shown to activate a number of other signaling proteins. To this end, we examined the effect of stimulation of RXFP1 and RXFP2 receptors [expressed in human embryonic kidney (HEK) 293T cells] with human relaxin family peptides on a number of transcription factor-response elements coupled to reporter genes. Hence, reporter gene activity measured by enzyme activity in the cell media is a measure of the activation of a particular signaling pathway. Eight reporter genes were tested at both receptors as a screen to identify other signaling pathways activated by RXFP1 and RXFP2. The cAMP-response element reporter was strongly activated by both receptors. This effect was enhanced by preincubation with pertussis toxin (PTX), suggesting that Gs and inhibitory Gi/Go proteins mediate this response. Only activation of RXFP1 inhibited nuclear factor kappaB transcription, and this was reversed by PTX and the phosphoinositide-3-kinase inhibitor wortmannin. In addition, the glucocorticoid-response element was activated by RXFP1 but not by RXFP2 and was not activated in the parent HEK293T cells. Thus, the use of reporter genes enabled differences in signaling between these two receptors to be revealed and also threw light on the wide range of effects attributed to relaxin.

Relaxin—a pleiotropic hormone and its emerging role for experimental and clinical therapeutics

The insulin-related peptide hormone relaxin (Rlx) is known as pregnancy hormone for decades. In the 1980s, researchers began to recognize the highly intriguing fact that Rlx plays a role in a multitude of physiological processes far beyond pregnancy and reproduction. So, Rlx's contribution to the regulation of vasotonus, plasma osmolality, angiogenesis, collagen turnover, and renal function has been established. In addition, the peptide has been demonstrated to represent a mediator of cardiovascular pathology. The ongoing efforts to identify Rlx receptors eventually precipitated the discovery of the G protein-coupled receptors (GPCR) LGR7 and LGR8 as membrane receptors for human Rlx-2 in 2002. This review will summarize the current state of insight into this rapidly evolving field, which has further been expanded by the discovery of GPCR135 and GPCR142 as receptors for Rlx-3. In addition, Rlx has also been shown to activate the human glucocorticoid receptor (GR). There is evidence from Rlx and Rlx receptor knockouts suggesting that LGR7 is the only relevant receptor for mouse Rlx-1 (corresponding to human Rlx-2) in vivo and that insulin-like peptide (INSL)-3 represents the physiological ligand for LGR8. Regarding Rlx signal transduction, the cyclic adenosine monophosphate (cAMP) and nitric oxide (NO) pathways will be characterized as major cascades. Investigation of downstream signaling remains an important field for future research. Finally, the current state of therapeutical strategies using Rlx in animal models as well as in humans is summarized.

Cardiac anaphylaxis: pathophysiology and therapeutic perspectives

Cardiac anaphylaxis refers to the functional and metabolic changes in the heart caused by the anaphylactic release of histamine and vasoactive products of arachidonic acid cascade by mast cells and basophils. As in most type I hypersensitivity-based diseases, histamine plays a key role in the pathophysiology of cardiac anaphylaxis. In the heart, mast cell activation and histamine release are controlled by multiple endogenous mechanisms, including adrenergic neural control, histamine-dependent negative feedback operated through H2 receptors, and the endogenous generation of nitric oxide (NO) and carbon monoxide (CO). All these mechanisms can be targeted by substances that have revealed a clear-cut effect in blunting cardiac anaphylaxis in experimental animal models, and could be developed as potential, novel anti-anaphylactic drugs. In this article, we discuss new findings and significant trends related to this topic.

Relaxin stimulates leukocyte adhesion and migration through a relaxin receptor LGR7-dependent mechanism

Leukocytes are critical effectors of inflammation and tumor biology. Chemokine-like factors produced by such inflammatory sites are key mediators of tumor growth that activate leukocytic recruitment and tumor infiltration and suppress immune surveillance. Here we report that the endocrine peptide hormone, relaxin, is a regulator of leukocyte biology with properties important in recruitment to sites of inflammation. This study uses the human monocytic cell line THP-1 and normal human peripheral blood mononuclear cells to define a novel role for relaxin in regulation of leukocyte adhesion and migration. Our studies indicate that relaxin promotes adenylate cyclase activation, substrate adhesion, and migratory capacity of mononuclear leukocytes through a relaxin receptor LGR7-dependent mechanism. Relaxin-stimulated cAMP accumulation was observed to occur primarily in non-adherent cells. Relaxin stimulation results in increased substrate adhesion and increased migratory activity of leukocytes. In addition, relaxin-stimulated substrate adhesion resulted in enhanced chemotaxis to monocyte chemoattractant protein-1. These responses in THP-1 and peripheral blood mononuclear cells are relaxin dose-dependent and proportional to cAMP accumulation. We further demonstrate that LGR7 is critical for mediating these biological responses by use of RNA interference lentiviral short hairpin constructs. In summary, we provide evidence that relaxin is a novel leukocyte stimulatory agent with properties affecting adhesion and chemomigration.

Relaxin's physiological roles and other diverse actions

Relaxin has vital physiological roles in pregnant rats, mice, and pigs. Relaxin promotes growth and softening of the cervix, thus facilitating rapid delivery of live young. Relaxin also promotes development of the mammary apparatus, thus enabling normal lactational performance. The actions of relaxin on the mammary apparatus vary among species. Whereas relaxin is required for development of the mammary nipples in rats and mice, it is essential for prepartum development of glandular parenchyma in pregnant pigs. During pregnancy relaxin also inhibits uterine contractility and promotes the osmoregulatory changes of pregnancy in rats. Recent studies with male and nonpregnant female rodents revealed diverse therapeutic actions of relaxin on nonreproductive tissues that have clinical implications. Relaxin has been reported to reduce fibrosis in the kidney, heart, lung, and liver and to promote wound healing. Also, probably through its vasodilatory actions, relaxin protects the heart from ischemia-induced injury. Finally, relaxin counteracts allergic reactions. Knowledge of the diverse physiological and therapeutic actions of relaxin, coupled with the recent identification of relaxin receptors, opens numerous avenues of investigation that will likely sustain a high level of research interest in relaxin for the foreseeable future.

Relaxin inhibits the activation of human neutrophils: involvement of the nitric oxide pathway

In animal models of inflammation, the pregnancy hormone relaxin was shown to reduce the recruitment of leukocytes, especially neutrophils, in inflamed tissues. The current study was designed to clarify whether relaxin could inhibit activation of isolated human neutrophils and, if so, whether the nitric oxide (NO) biosynthetic pathway was involved, as occurs in other relaxin targets. Human neutrophils were preincubated with 1, 10, and 100 nmol/liter porcine relaxin for 1 h before activation with N-formyl-Met-Leu-Phe (10 micromol/liter) or phorbol-12-myristate-13-acetate (0.1 micromol/liter). In selected experiments, the NO synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA, 100 micromol/liter) was added to the samples 30 min before relaxin. In other experiments, chemically inactivated relaxin (10 nmol/liter) was substituted for authentic relaxin. Untreated, nonactivated neutrophils were the controls. Relaxin reduced significantly and in a concentration-dependent fashion the expression of the surface activation marker CD11b, as well as the generation of superoxide anion, the rise of intracellular Ca(2+), the release of cytoplasmic granules, and the chemotactic migration. These effects of relaxin were blunted by N(G)-monomethyl-L-arginine and could not be reproduced by inactivated relaxin. Relaxin also increased neutrophil inducible NO synthase expression and NO generation. This study provides evidence that relaxin inhibits the activation of human neutrophils stimulated by different proinflammatory agents. This novel property of relaxin could be of relevance in toning down maternal neutrophil activation during pregnancy, thereby counteracting the occurrence of pregnancy-related disorders such as preeclampsia, which is regarded as an excess maternal inflammatory response to pregnancy.

Carbon monoxide modulates the response of human basophils to FcepsilonRI stimulation through the heme oxygenase pathway

We report the effects of exogenous and endogenous carbon monoxide (CO) on the immunological activation of human basophils. Hemin (1-100 microM), a heme oxygenase substrate analogue, significantly increased the formation of bilirubin from partially purified human basophils, thus indicating that these cells express heme oxygenase. This effect was reversed by preincubating the cells for 30 min with Zn-protoporphyrin IX (100 microM), a heme oxygenase inhibitor. Hemin (100 microM) also decreased immunoglobulin G anti-Fcepsilon (anti-IgE)-induced activation of basophils, measured by the expression of a membrane granule-associated protein, identified as cluster differentiation protein 63 (CD63), and by histamine release. These effects were reversed by Zn-protoporphyrin IX (100 microM), by oxyhemoglobin (HbO(2)), a CO scavenger (100 microM), and by 1H-[1,2,4]oxadiazolo[4,3-a] quinoxalin-1-one (ODQ), an inhibitor of the soluble guanylyl cyclase (100 microM). Exposure of basophils to exogenous CO (10 microM for 30 min) also decreased their activation, while nitrogen (N(2)) was ineffective. HbO(2) and ODQ reversed the inhibition, reversing both membrane protein CD63 expression and histamine release to basal values. Both hemin and exogenous CO significantly raised cGMP levels in basophils and blunted the rise of calcium levels caused by immunological activation. This study suggests that CO increases cGMP formation, which in turn induces a fall in intracellular Ca(2+) concentration, thereby resulting in the inhibition of human basophil activation.