'Relaxin' the stiffened heart and arteries: the therapeutic potential for relaxin in the treatment of cardiovascular disease

Relaxin inhibits renal myofibroblast differentiation via RXFP1, the nitric oxide pathway, and Smad2

The hormone relaxin inhibits renal myofibroblast differentiation by interfering with TGF-beta1/Smad2 signaling. However, the pathways involved in the relaxin-TGF-beta1/Smad2 interaction remain unknown. This study investigated the signaling mechanisms by which human gene-2 (H2) relaxin regulates myofibroblast differentiation in vitro by examining its effects on mixed populations of fibroblasts and myofibroblasts propagated from injured rat kidneys. Cultures containing approximately 60-70% myofibroblasts were used to determine which relaxin receptors, G-proteins, and signaling pathways were involved in the H2 relaxin-mediated regulation of alpha-smooth muscle actin (alpha-SMA; a marker of myofibroblast differentiation). H2 relaxin only inhibited alpha-SMA immunostaining and collagen concentration in the presence of relaxin family peptide receptor 1 (RXFP1). H2 relaxin also induced a transient rise in cAMP in the presence of G(i/o) inhibition, and a sustained increase in extracellular signal-regulated kinase (ERK)-1/2 phosphorylation. Furthermore, inhibition of neuronal nitric oxide synthase (nNOS), NO, and cGMP significantly blocked the inhibitory effects of relaxin on alpha-SMA and Smad2 phosphorylation, while the NO inhibitor, L-nitroarginine methyl ester (hydrochloride) (L-NAME) significantly blocked the inhibitory actions of relaxin on collagen concentration in vivo. These findings suggest that relaxin signals through RXFP1, and a nNOS-NO-cGMP-dependent pathway to inhibit Smad2 phosphorylation and interfere with TGF-beta1-mediated renal myofibroblast differentiation and collagen production.

Relaxin as a natural agent for vascular health

Hypertension, atherothrombosis, myocardial infarction, stroke, peripheral vascular disease, and renal failure are the main manifestations of cardiovascular disease (CVD), the leading cause of death and disability in developed countries. Continuing insight into the pathophysiology of CVD can allow identification of effective therapeutic strategies to reduce the occurrence of death and/or severe disabilities. In this context, a healthy endothelium is deemed crucial to proper functioning and maintenance of anatomical integrity of the vascular system in many organs. Of note, epidemiologic studies indicate that the incidence of CVD in women is very low until menopause and increases sharply thereafter. The loss of protection against CVD in post-menopausal women has been chiefly attributed to ovarian steroid deficiency. However, besides steroids, the ovary also produces the peptide hormone relaxin (RLX), which provides potent vasoactive effects which render it the most likely candidate as the elusive physiological shield against CVD in fertile women. In particular, RLX has a specific relaxant effect on peripheral and coronary vasculature, exerted by the stimulation of endogenous nitric oxide (NO) generation by cells of the vascular wall, and can induce angiogenesis. Moreover, RLX inhibits the activation of inflammatory leukocytes and platelets, which play a key role in CVD. Experimental studies performed in vascular and blood cell in vitro and in animal models of vascular dysfunction, as well as pioneer clinical observations, have provided evidence that RLX can prevent and/or improve CVD, thus offering background to clinical trials aimed at exploring the broad therapeutic potential of human recombinant RLX as a new cardiovascular drug.

Functional and histopathological improvement of the post-infarcted rat heart upon myoblast cell grafting and relaxin therapy

Although the myocardium contains progenitor cells potentially capable of regenerating tissue upon lethal ischaemic injury, their actual role in post-infarction heart healing is negligible. Therefore, transplantation of extra-cardiac stem cells is a promising therapeutic approach for post-infarction heart dysfunction. Paracrine cardiotropic factors released by the grafted cells, such as the cardiotropic hormone relaxin (RLX), may beneficially influence remodelling of recipient hearts. The current study was designed to address whether grafting of mouse C2C12 myoblasts, genetically engineered to express green fluorescent protein (C2C12/GFP) or GFP and RLX (C2C12/RLX), are capable of improving long-term heart remodelling in a rat model of surgically induced chronic myocardial infarction. One month after myocardial infarction, rats were treated with either culture medium (controls), or C2C12/GFP cells, or C2C12/RLX cells plus exogenous RLX, or exogenous RLX alone. The therapeutic effects were monitored for 2 further months. Cell transplantation and exogenous RLX improved the main echocardiographic parameters of cardiac function, increased myocardial viability (assessed by positron emission tomography), decreased cardiac sclerosis and myocardial cell apoptosis and increased microvascular density in the post-infarction scar tissue. These effects were maximal upon treatment with C2C12/RLX plus exogenous RLX. These functional and histopathological findings provide further experimental evidence that myoblast cell grafting can improve myocardial performance and survival during post-infarction heart remodelling and dysfunction. Further, this study provides a proof-of-principle to the novel concept that genetically engineered grafted cells can be effectively employed as cell-based vehicles for the local delivery of therapeutic cardiotropic substances, such as RLX, capable of improving adverse heart remodelling.

Identification of the N-linked glycosylation sites of the human relaxin receptor and effect of glycosylation on receptor function

The relaxin receptor, RXFP1, is a member of the leucine-rich repeat-containing G-protein-coupled receptor (LGR) family. These receptors are characterized by a large extracellular ectodomain containing leucine-rich repeats which contain the primary ligand binding site. RXFP1 contains six putative Asn-linked glycosylation sites in the ectodomain at positions Asn-14, Asn-105, Asn-242, Asn-250, Asn-303, and Asn-346, which are highly conserved across species. N-Linked glycosylation is the most common post-translational modification of G-protein-coupled receptors, although its role in modulating receptor function differs. We herein investigate the actual N-linked glycosylation status of RXFP1 and the functional ramifications of these post-translational modifications. Site-directed mutagenesis was utilized to generate single- or multiple-glycosylation site mutants of FLAG-tagged human RXFP1 which were then transiently expressed in HEK-293T cells. Glycosylation status was analyzed by immunoprecipitation and Western blot and receptor function analyzed with an anti-FLAG ELISA, (33)P-H2 relaxin competition binding, and cAMP activity measurement. All of the potential N-glycosylation sites of RXFP1 were utilized in HEK-293T cells, and importantly, disruption of glycosylation at individual or combinations of double and triple sites had little effect on relaxin binding. However, combinations of glycosylation sites were required for cell surface expression and cAMP signaling. In particular, N-glycosylation at Asn-303 of RXFP1 was required for optimal intracellular cAMP signaling. Hence, as is the case for other LGR family members, N-glycosylation is essential for the transport of the receptor to the cell surface. Additionally, it is likely that glycosylation is also essential for the conformational changes required for G-protein coupling and subsequent cAMP signaling.

Cloning, expression, and functional characterization of relaxin receptor (leucine-rich repeat-containing g protein-coupled receptor 7) splice variants from human fetal membranes

The relaxin receptor [leucine-rich repeat-containing G protein-coupled receptor 7 (LGR7)] belongs to the leucine-rich repeat containing G protein-coupled receptors subgroup C. Three new LGR7 splice variants have been cloned from the human fetal membranes and shown to be truncated versions of the full-length receptor, encoded by different lengths of the extracellular domain. The expression of their mRNAs has been confirmed by both qualitative and quantitative PCR and shown to be higher in the chorion and decidua before, compared with after, spontaneous labor. When HEK293 cells were transfected with each LGR7 splice variant, their proteins were retained within the endoplasmic reticulum. However, the protein for the shortest variant was also secreted into the medium. We have characterized the intracellular functions and effects of these LGR7 variants on the function of the wild-type (WT)-LGR7. In coexpression studies, each splice variant interacted directly with the WT-LGR7 and exerted a dominant-negative effect on cAMP accumulation by the WT-LGR7 after relaxin treatment. This interaction resulted in the sequestration of the WT-LGR7 inside the cells by down-regulation of its maturation and cell surface delivery. The constitutive homodimerization of WT-LGR7 has been shown here to take place in the endoplasmic reticulum, and the presence of any one of the splice variants decreased this by the formation of heterodimers with the WT-LGR7, supporting the view that homodimerization is a prerequisite for receptor trafficking to the cell surface. These data suggest that the dominant-negative effects of the LGR7 splice variants expressed in the chorion and decidua could be functionally significant in the peripartal period by inhibiting the function of WT-LGR7 and dampening the responsiveness of these tissues to endogenous relaxin.

Endogenous relaxin does not affect chronic pressure overload-induced cardiac hypertrophy and fibrosis

The effect of endogenous relaxin on the development of cardiac hypertrophy, dysfunction, and fibrosis remains completely unknown. We addressed this question by subjecting relaxin-1 deficient (Rln1-/-) and littermate control (Rln1+/+) mice of both genders to chronic transverse aortic constriction (TAC). The extent of left ventricular (LV) remodeling and dysfunction were studied by serial echocardiography over an 8-wk period and by micromanometry. The degree of hypertrophy was estimated by LV weight, cardiomyocyte size, and expression of relevant genes. Cardiac fibrosis was determined by hydroxyproline assay and quantitative histology. Expression of endogenous relaxin during the course of TAC was also examined. In response to an 8-wk period of pressure overload, TAC mice of both genotypes developed significant LV hypertrophy, fibrosis, hypertrophy related gene profile, and signs indicating congestive heart failure when compared with respective sham controls. The severity of these alterations was not statistically different between the two genotypes of either gender. Relaxin mRNA expression was up-regulated, whereas that of its receptor was unchanged in the hypertrophic myocardium of wild-type mice. Collectively, the extent of pressure overload-induced LV hypertrophy, fibrosis, and dysfunction were comparable between Rln1+/+ and Rln1-/- mice. Thus, although up-regulated in its expression, endogenous relaxin had no significant effect on the progression of cardiac maladaptation and dysfunction in the setting of chronic pressure overload.

Paracrine effects of transplanted myoblasts and relaxin on post-infarction heart remodelling

In the post-infarcted heart, grafting of precursor cells may partially restore heart function but the improvement is modest and the mechanisms involved remain to be elucidated. Here, we explored this issue by transplanting C2C12 myoblasts, genetically engineered to express enhanced green fluorescent protein (eGFP) or eGFP and the cardiotropic hormone relaxin (RLX) through coronary venous route to swine with experimental chronic myocardial infarction. The rationale was to deliver constant, biologically effective levels of RLX at the site of cell engraftment. One month after engraftment, histological analysis showed that C2C12 myoblasts selectively settled in the ischaemic scar and were located around blood vessels showing an activated endothelium (ICAM-1-,VCAM-positive). C2C12 myoblasts did not trans-differentiate towards a cardiac phenotype, but did induce extracellular matrix remodelling by the secretion of matrix metalloproteases (MMP) and increase microvessel density through the expression of vascular endothelial growth factor (VEGF). Relaxin-producing C2C12 myoblasts displayed greater efficacy to engraft the post-ischaemic scar and to induce extracellular matrix re-modelling and angiogenesis as compared with the control cells. By echocardiography, C2C12-engrafted swine showed improved heart contractility compared with the ungrafted controls, especially those producing RLX. We suggest that the beneficial effects of myoblast grafting on cardiac function are primarily dependent on the paracrine effects of transplanted cells on extracellular matrix remodelling and vascularization. The combined treatment with myoblast transplantation and local RLX production may be helpful in preventing deleterious cardiac remodelling and may hold therapeutic possibility for post-infarcted patients.

Re-modelling 'hostile' milieu of diseased myocardium via paracrine function of transplanted cells or relaxin

While the approaches of regenerating cardiac muscle remain undetermined, recent evidence indicates that paracrine function of transplanted cells contributes significantly to the beneficial effects of cell therapy. Combination of such paracrine function of grafted cells with extracellular matrix remodelling by relaxin represents a promising complement to cell-based therapy for cardiac repair and muscle regeneration.

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.

Reversal by relaxin of altered ileal spontaneous contractions in dystrophic (mdx) mice through a nitric oxide-mediated mechanism

Altered nitric oxide (NO) production/release is involved in gastrointestinal motor disorders occurring in dystrophic (mdx) mice. Since the hormone relaxin (RLX) can upregulate NO biosynthesis, its effects on spontaneous motility and NO synthase (NOS) expression in the ileum of dystrophic (mdx) mice were investigated. Mechanical responses of ileal preparations were recorded in vitro via force-displacement transducers. Evaluation of the expression of NOS isoforms was performed by immunohistochemistry and Western blot. Normal and mdx mice were distributed into three groups: untreated, RLX pretreated, and vehicle pretreated. Ileal preparations from the untreated animals showed spontaneous muscular contractions whose amplitude was significantly higher in mdx than in normal mice. Addition of RLX, alone or together with l-arginine, to the bath medium depressed the amplitude of the contractions in the mdx mice, thus reestablishing a motility pattern typical of the normal mice. The NOS inhibitor N(G)-nitro-L-arginine (L-NNA) or the guanylate cyclase inhibitor ODQ reversed the effects of RLX. In RLX-pretreated mdx mice, the amplitude of spontaneous motility was reduced, thus resembling that of the normal mice, and NOS II expression in the muscle coat was increased in respect to the vehicle-pretreated mdx animals. These results indicate that RLX can reverse the altered ileal motility of mdx mice to a normal pattern, likely by upregulating NOS II expression and NO biosynthesis in the ileal smooth muscle.

A novel, simple bioactivity assay for relaxin based on inhibition of platelet aggregation

In humans, the relaxin hormone family includes H1, H2 and H3 isoforms and insulin-like peptides 3 to 6. The ever-increasing interest in relaxin as potential new drug requires reliable methods to compare bioactivity of different relaxins. The existing bioassays include in vivo or ex vivo methods evaluating the organ-specific responses to relaxin and in vitro methods based on measurement of cAMP increase in relaxin receptor-bearing cells. We previously demonstrated that relaxin dose-dependently inhibits platelet aggregation. On this basis, we have developed a simple, reliable bioassay for relaxin used to compare purified porcine relaxin, assumed as reference standard, with two recombinant human H2 relaxins, H3 relaxin, insulin-like peptides 3 and 5. Pre-incubation of platelets with relaxins (3, 10, 30,100, 300 ng/ml; 10 min.) caused the inhibition of ADP-induced platelet aggregation. Within the 10-100 ng/ml range, porcine relaxin showed the highest effects and a nearly linear dose-response correlation. Lower peptide concentrations were ineffective, as were insulin-like peptides 3 and 5 at any concentration assayed. Platelet inhibition was mediated by specific RXFP1 relaxin receptor and cGMP, whose intracellular levels dose-dependently increased upon relaxin. For comparison, we stimulated THP-1 cells, a relaxin receptor-bearing cell line, with porcine relaxin, human H2 and H3 relaxins at the above concentrations (15 min.). We observed a dose-related increase of intracellular cAMP similar to the trend of platelet inhibition. Insulin like peptide 5 was ineffective. In conclusion, this study shows that inhibition of platelet aggregation may be used to assess bioactivity of relaxin preparations for experimental and clinical purposes.

Relaxin antagonizes hypertrophy and apoptosis in neonatal rat cardiomyocytes

The pregnancy hormone relaxin has recently been shown to be cardio-protective. Despite its well-established antifibrotic actions in the heart, the effects of relaxin on cardiomyocytes (CM) remain to be determined. We investigated effects of isoform 2 of the human relaxin (H2-relaxin) on CM hypertrophy and apoptosis. In cultured neonatal rat CM, phenylephrine (50 microM) and cardiac fibroblast-conditioned medium were used respectively to induce CM hypertrophy. The degree of hypertrophy was indicated by increased cell size, protein synthesis and gene expression of atrial natriuretic peptide. Although H2-relaxin (16.7 nM) alone failed to suppress hypertrophy induced by phenylephrine, it repressed the cardiac fibroblast-conditioned medium-induced increase in protein synthesis by 24% (P<0.05) and reversed the increase in cell size (P<0.001) and atrial natriuretic peptide expression (P<0.01). We further studied the effect of H2-relaxin on CM apoptosis induced by H2O2 (200 microM). Studies of DNA laddering and nuclear staining demonstrated that H2-relaxin treatment reduced H2O2-induced DNA fragmentation. Real-time PCR and Western blot analysis revealed a significant increase in the Bcl2/Bax ratio in H2-relaxin-treated CM. Further analysis showed that activation of Akt (1.8-fold, P<0.001) and ERK (2.0-fold, P<0.01) were involved in the antiapoptotic action of H2-relaxin in CM, and that Gi/o coupling of relaxin receptors was associated with the H2-relaxin-induced Akt activation in CM. In conclusion, these results extend our current knowledge of the cardiac actions of relaxin by demonstrating that H2-relaxin indirectly inhibits CM hypertrophy and directly protects CM from apoptosis.