Vascular actions of relaxin: nitric oxide and beyond

Relaxin receptor deficiency promotes vascular inflammation and impairs outward remodeling in arteriovenous fistulas

The pathophysiology of arteriovenous fistula (AVF) maturation failure is not completely understood but impaired outward remodeling (OR) and intimal hyperplasia are thought to be contributors. This adverse vascular response after AVF surgery results from interplay between vascular smooth muscle cells (VSMCs), the extracellular matrix (ECM), and inflammatory cells. Relaxin (RLN) is a hormone that acts on the vasculature via interaction with RLN/insulin-like peptide family receptor 1 (RXFP1), resulting in vasodilatation, ECM remodeling, and decreased inflammation. In the present study, we evaluated the consequences of RXFP1 knockout ( Rxfp1-/-) on AVF maturation in a murine model of AVF failure. Rxfp1-/- mice showed a 22% decrease in vessel size at the venous outflow tract 14 d after AVF surgery. Furthermore, a 43% increase in elastin content was observed in the lesions of Rxfp1-/- mice and coincided with a 41% reduction in elastase activity. In addition, Rxfp1-/- mice displayed a 6-fold increase in CD45+ leukocytes, along with a 2-fold increase in monocyte chemoattractant protein 1 (MCP1) levels, when compared with wild-type mice. In vitro, VSMCs from Rxfp1-/- mice exhibited a synthetic phenotype, as illustrated by augmentation of collagen, fibronectin, TGF-β, and platelet-derived growth factor mRNA. In addition, VSMCs derived from Rxfp1-/- mice showed a 5-fold increase in cell migration. Finally, RXFP1 and RLN expression levels were increased in human AVFs when compared with unoperated cephalic veins. In conclusion, RXFP1 deficiency hampers elastin degradation and results in induced vascular inflammation after AVF surgery. These processes impair OR in murine AVF, suggesting that the RLN axis could be a potential therapeutic target for promoting AVF maturation.-Bezhaeva, T., de Vries, M. R., Geelhoed, W. J., van der Veer, E. P., Versteeg, S., van Alem, C. M. A., Voorzaat, B. M., Eijkelkamp, N., van der Bogt, K. E., Agoulnik, A. I., van Zonneveld, A.-J., Quax, P. H. A., Rotmans, J. I. Relaxin receptor deficiency promotes vascular inflammation and impairs outward remodeling in arteriovenous fistulas.

Peptide hormone relaxin: from bench to bedside

The peptide hormone relaxin has numerous roles both within and independent of pregnancy and is often thought of as a “pleiotropic hormone.” Relaxin targets several tissues throughout the body, and has many functions associated with extracellular matrix remodeling and the vasculature. This review considers the potential therapeutic applications of relaxin in cervical ripening, in vitro fertilization, preeclampsia, acute heart failure, ischemia-reperfusion, and cirrhosis. We first outline the animal models used in preclinical studies to progress relaxin into clinical trials and then discuss the findings from these studies. In many cases, the positive outcomes from preclinical animal studies were not replicated in human clinical trials. Therefore, the focus of this review is to evaluate the various animal models used to develop relaxin as a potential therapeutic and consider the limitations that must be addressed in future studies. These include the use of human relaxin in animals, duration of relaxin treatment, and the appropriateness of the clinical conditions being considered for relaxin therapy.

Relaxin as a Therapeutic Target for the Cardiovascular Complications of Diabetes

Cardiovascular complications are the major cause of mortality in patients with diabetes. This is closely associated with both macrovascular and microvascular complications of diabetes, which lead to organ injuries in diabetic patients. Previous studies have consistently demonstrated the beneficial effects of relaxin treatment for protection of the vasculature, with evidence of antioxidant and anti-remodeling actions. Relaxin enhances nitric oxide, prostacyclin and endothelium-derived hyperpolarization (EDH)-type-mediated relaxation in various vascular beds. These effects of relaxin on the systemic vasculature, coupled with its cardiac actions, reduce pulmonary capillary wedge pressure and pulmonary artery pressure. This results in an overall decrease in systemic and pulmonary vascular resistance in heart failure patients. The anti-fibrotic actions of relaxin are well established, a desirable property in the context of diabetes. Further, relaxin ameliorates diabetic wound healing, with accelerated angiogenesis and vasculogenesis. Relaxin-mediated stimulation of vascular endothelial growth factor (VEGF) and stromal cell-derived factor 1-α, as well as regulation of metalloproteinase expression, ameliorates cardiovascular fibrosis in diabetic mice. In the heart, relaxin is a cardioprotective molecule in several experimental animal models, exerting anti-fibrotic, anti-hypertrophy and anti-apoptotic effects in diabetic pathologies. Collectively, these studies provide a foundation to propose the therapeutic potential for relaxin as an adjunctive agent in the prevention or treatment of diabetes-induced cardiovascular complications. This review provides a comprehensive overview of the beneficial effects of relaxin, and identifies its therapeutic possibilities for alleviating diabetes-related cardiovascular injury.

Targeting the Relaxin Pathway for Liver Disease Treatment

Hepatic fibrosis is a progressive disease with few treatment options outside of transplantation. Relaxin is a member of the insulin/relaxin superfamily of peptide hormones. Originally known for its roles in pregnancy, relaxin promotes reproductive tissue remodelling and regulates vascular changes, including increased arterial compliance and reduced vascular resistance. Outside of pregnancy, relaxin plays a major role in the protection of organs from excess extracellular matrix accumulation, as demonstrated by the relaxin-null mouse, which develops widespread fibrosis with ageing. Relaxin reduces scarring due to excess collagen deposition by inhibiting collagen production while simultaneously promoting its degradation and can reduce established fibrosis in several animal models of extracellular matrix-associated disease, including liver fibrosis. Treatment with relaxin reduces the myofibroblastic phenotype of activated hepatic stellate cells, the major hepatic collagen-producing cell in fibrosis and cirrhosis. Relaxin also has haemodynamic effects, including vasodilation, and can reduce portal hypertension associated with cirrhosis. In this review, a brief overview of hepatic fibrosis and the role of the hepatic stellate cell will be presented, followed by an introduction to relaxin and its actions. The use of relaxin to treat preclinical models of fibrotic diseases, including liver diseases, will also be discussed. Finally, the completed, current, and ongoing clinical trials of relaxin in human disease will be described, followed by the limitations and future directions for the use of relaxin for disease treatment.

Heart Disease and Relaxin: New Actions for an Old Hormone

The hormone relaxin has long been recognized for its involvement in maternal adaptation during pregnancy. However, discoveries during the past two decades on the mechanism of action of relaxin, its family of receptors, and newly described roles in attenuating ischemia/reperfusion (I/R) injury, inflammation, and arrhythmias have prompted vast interest in exploring its therapeutic potential in cardiovascular disease. These observations inspired recently concluded clinical trials in patients with acute heart failure. This review discusses our current understanding of the protective signaling pathways elicited by relaxin in the heart, and highlights important new breakthroughs about relaxin signaling that may pave the way to more carefully designed future trials.

The Vasculature in Prediabetes

The frequency of prediabetes is increasing as the prevalence of obesity rises worldwide. In prediabetes, hyperglycemia, insulin resistance, and inflammation and metabolic derangements associated with concomitant obesity cause endothelial vasodilator and fibrinolytic dysfunction, leading to increased risk of cardiovascular and renal disease. Importantly, the microvasculature affects insulin sensitivity by affecting the delivery of insulin and glucose to skeletal muscle; thus, endothelial dysfunction and extracellular matrix remodeling promote the progression from prediabetes to diabetes mellitus. Weight loss is the mainstay of treatment in prediabetes, but therapies that improved endothelial function and vasodilation may not only prevent cardiovascular disease but also slow progression to diabetes mellitus.

Relaxin Deficiency Leads to Uterine Artery Dysfunction During Pregnancy in Mice

The uterine vasculature undergoes profound adaptations in response to pregnancy. Augmentation of endothelial vasodilator function and reduced smooth muscle reactivity are factors contributing to uterine artery adaptation and are critical for adequate placental perfusion. The peptide hormone relaxin has an important role in mediating the normal maternal renal vascular adaptations during pregnancy through a reduction in myogenic tone and an increase in flow-mediated vasodilation. Little is known however about the influence of endogenous relaxin on the uterine artery during pregnancy. We tested the hypothesis that relaxin deficiency increases myogenic tone and impairs endothelial vasodilator function in uterine arteries of late pregnant relaxin deficient (Rln^−/−) mice. Reactivity of main uterine arteries from non-pregnant and late pregnant wild-type (Rln^+/+) and Rln^−/− mice was studied using pressure and wire myography and changes in gene expression explored using PCR. Myogenic tone was indistinguishable in arteries from non-pregnant mice. In late pregnancy uterine artery myogenic tone was halved in Rln^+/+ mice (P < 0.0001), an adaptation that failed to occur in arteries from pregnant Rln^−/− mice. The role of vasodilator prostanoids in the regulation of myogenic tone was significantly reduced in arteries of pregnant Rln^−/− mice (P = 0.02). Agonist-mediated endothelium-dependent vasodilation was significantly impaired in non-pregnant Rln^−/− mice. With pregnancy, differences in total endothelial vasodilator function were resolved, although there remained an underlying deficiency in the role of vasodilator prostanoids and alterations to the contributions of calcium-activated K⁺ channels. Fetuses of late pregnant Rln^−/− mice were ~10% lighter (P < 0.001) than those of Rln^+/+ mice. In conclusion, relaxin deficiency is associated with failed suppression of uterine artery myogenic tone in pregnancy, which likely contributes to reduced uteroplacental perfusion and fetal growth restriction.

Relaxin influences ileal muscular activity through a dual signaling pathway in mice

AIM

To investigate the signaling pathways involved in the relaxin (RLX) effects on ileal preparations from mice through mechanical and electrophysiological experiments.

METHODS

For mechanical experiments, ileal preparations from female mice were mounted in organ baths containing Krebs-Henseleit solution. The mechanical activity was recorded via force-displacement transducers, which were coupled to a polygraph for continuous recording of isometric tension. Electrophysiological measurements were performed in current- and voltage-clamp conditions by a microelectrode inserted in a single smooth muscle cell (SMC) of the ileal longitudinal layer. Both the membrane passive properties and inward voltage-dependent L-type Ca²⁺ currents were recorded using suitable solutions and voltage stimulation protocols.

RESULTS

Mechanical experiments showed that RLX induced a decay of the basal tension and a reduction in amplitude of the spontaneous contractions. The effects of RLX were partially reduced by 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxalin-1-one (ODQ) or 9-cyclopentyladenine mesylate (9CPA), inhibitors of guanylate cyclase (GC) and adenylate cyclase (AC), respectively, and were abolished in the concomitant presence of both drugs. Electrophysiological experiments demonstrated that RLX directly influenced the biophysical properties of ileal SMCs, decreasing the membrane conductance, hyperpolarizing the resting membrane potential, reducing the L-type calcium current amplitude and affecting its kinetics. The voltage dependence of the current activation and inactivation time constant was significantly speeded by RLX. Each electrophysiological effect of RLX was reduced by ODQ or 9CPA, and abolished in the concomitant presence of both drugs as observed in mechanical experiments.

CONCLUSION

Our new findings demonstrate that RLX influences ileal muscle through a dual mechanism involving both GC and AC.

Oxidative Stress and Renal Fibrosis: Recent Insights for the Development of Novel Therapeutic Strategies

Chronic kidney disease (CKD) is a significant worldwide healthcare problem. Regardless of the initial injury, renal fibrosis is the common final pathway leading to end stage renal disease. Although the underlying mechanisms are not fully defined, evidence indicates that besides inflammation, oxidative stress plays a crucial role in the etiology of renal fibrosis. Oxidative stress results from an imbalance between the production of free radicals that are often increased by inflammation and mitochondrial dysfunction, and reduced anti-oxidant defenses. Several studies have demonstrated that oxidative stress may occur secondary to activation of transforming growth factor β1 (TGF-β1) activity, consistent with its role to increase nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox) activity. A number of other oxidative stress-related signal pathways have also been identified, such as nuclear factor erythroid-2 related factor 2 (Nrf2), the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP)-cGMP-dependent protein kinase 1-phosphodiesterase (cGMP-cGK1-PDE) signaling pathway, and the peroxisome proliferator-activated receptor gamma (PPARγ) pathway. Several antioxidant and renoprotective agents, including cysteamine bitartrate, epoxyeicosatrienoic acids (EETs), and cytoglobin (Cygb) have demonstrated ameliorative effects on renal fibrosis in preclinical or clinical studies. The mechanism of action of many traditional Chinese medicines used to treat renal disorders is based on their antioxidant properties, which could form the basis for new therapeutic approaches. This review focuses on the signaling pathways triggered by oxidative stress that lead to renal fibrosis and provides an update on the development of novel anti-oxidant therapies for CKD.

Recent progress in the understanding of relaxin family peptides and their receptors

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.

Luteal and hypophyseal expression of the canine relaxin (RLN) system during pregnancy: Implications for luteotropic function

By acting through its receptors (RXFP1, RXFP2), relaxin (RLN) exerts species-specific effects during pregnancy; possible luteotropic effects through stimulation of prolactin (PRL) release have been suggested. In the domestic dog (Canis lupus familiaris) serum PRL increases in pregnant bitches shortly after RLN appears in the circulation, and a possible functional relationship between the RLN and the PRL systems in regulating progesterone secretion has been implied. Therefore, here (Study 1) the luteal expression and localization of the RLN system was investigated by immunohistochemistry using custom-made antibodies and semi-quantitative PCR, at selected time points during gestation: pre-implantation (d. 8–12), post-implantation (d. 18–25), mid-gestation (d. 35–40) and at normal and antigestagen-induced luteolysis. Further, (Study 2) hypophyseal expression of the RLN system and its spatial association with PRL was assessed. Luteal expression of RLN, but not of its receptors, was time-dependent: it increased significantly following implantation towards mid-gestation and decreased at prepartum. Antigestagen treatment resulted in downregulation of RLN and RXFP2. Whereas RLN was localized in steroidogenic cells, RXFP1 and RXFP2 also stained strongly in macrophages and vascular endothelial cells. The RLN system was detected in the canine adenohypophysis and was co-localized with PRL in hypophyseal lactotrophs. The intraluteal RLN seems to be involved in regulating the canine corpus luteum (CL) in a time-dependent manner. The presence of RLN family members in the adenohypophysis implies their possible involvement in regulating the availability of PRL and other pituitary hormones.

Divergence of insulin superfamily ligands, receptors and Igf binding proteins in marine versus freshwater stickleback: Evidence of selection in known and novel genes

Three-spine stickleback (Gasterosteus aculeatus) is a teleost model for understanding genetic, physiological and morphological changes accompanying freshwater (FW) adaptation. There is growing evidence that the insulin superfamily plays important roles in traits involved in marine and FW adaptation. We performed a candidate gene analysis to look for evidence of selection on 33 insulin superfamily ligand-receptor genes and insulin-like growth factor binding proteins (Igfbp's) in stickleback. Using genotype data from 11 marine and 10 FW populations, we calculated the number of SNPs per site in regulatory and intronic regions, the number of synonymous and nonsynonymous mutations in coding regions, Wright's fixation index (Fst), and performed t-tests to identify SNPs with divergent genotype frequencies between marine/FW versus Atlantic/Pacific populations. Next, we analysed genome-wide transcriptome data from eight tissues to assess differential gene expression. Two Igfbp's (Igfbp2a and Igfbp5a) show evidence of divergent adaptation between life-history types, and a cluster of nonsynonymous mutations in Igfbp5a exhibit high Fst in exons apparently alternatively spliced in gill. We find evidence of selection on the relaxin family ligand-receptor gene pair, Insl3-Rxfp2, known to be involved in male spermatogenesis and bone metabolism, and in the 5' regulatory region of Igf2. We also confirmed the gene and coding sequence of two unannotated relaxin family ligands. These analyses underscore the utility of candidate gene studies and indicate directions for further exploration of the function of insulin superfamily genes in FW adaptation.

Relaxin-2 in Cardiometabolic Diseases: Mechanisms of Action and Future Perspectives

Despite the great effort of the medical community during the last decades, cardiovascular diseases remain the leading cause of death worldwide, increasing their prevalence every year mainly due to our new way of life. In the last years, the study of new hormones implicated in the regulation of energy metabolism and inflammation has raised a great interest among the scientific community regarding their implications in the development of cardiometabolic diseases. In this review, we will summarize the main actions of relaxin, a pleiotropic hormone that was previously suggested to improve acute heart failure and that participates in both metabolism and inflammation regulation at cardiovascular level, and will discuss its potential as future therapeutic target to prevent/reduce cardiovascular diseases.

Serelaxin Treatment Reduces Oxidative Stress and Increases Aldehyde Dehydrogenase-2 to Attenuate Nitrate Tolerance

Background: Glyceryl trinitrate (GTN) is a commonly prescribed treatment for acute heart failure patients. However, prolonged GTN treatment induces tolerance, largely due to increased oxidative stress and reduced aldehyde dehydrogenase-2 (ALDH-2) expression. Serelaxin has several vasoprotective properties, which include reducing oxidative stress and augmenting endothelial function. We therefore tested the hypothesis in rodents that serelaxin treatment could attenuate low-dose GTN-induced tolerance.

Methods and Results: Co-incubation of mouse aortic rings ex vivo with GTN (10 μM) and serelaxin (10 nM) for 1 h, restored GTN responses, suggesting that serelaxin prevented the development of GTN tolerance. Male Wistar rats were subcutaneously infused with ethanol (control), low-dose GTN+placebo or low-dose GTN+serelaxin via osmotic minipumps for 3 days. Aortic vascular function and superoxide levels were assessed using wire myography and lucigenin-enhanced chemiluminescence assay respectively. Changes in aortic ALDH-2 expression were measured by qPCR and Western blot respectively. GTN+placebo infusion significantly increased superoxide levels, decreased ALDH-2 and attenuated GTN-mediated vascular relaxation. Serelaxin co-treatment with GTN significantly enhanced GTN-mediated vascular relaxation, reduced superoxide levels and increased ALDH-2 expression compared to GTN+placebo-treated rats.

Conclusion: Our data demonstrate that a combination of serelaxin treatment with low dose GTN attenuates the development of GTN-induced tolerance by reducing superoxide production and increasing ALDH-2 expression in the rat aorta. We suggest that serelaxin may improve nitrate efficacy in a clinical setting.

Serelaxin as a potential treatment for renal dysfunction in cirrhosis: Preclinical evaluation and results of a randomized phase 2 trial

BACKGROUND

Chronic liver scarring from any cause leads to cirrhosis, portal hypertension, and a progressive decline in renal blood flow and renal function. Extreme renal vasoconstriction characterizes hepatorenal syndrome, a functional and potentially reversible form of acute kidney injury in patients with advanced cirrhosis, but current therapy with systemic vasoconstrictors is ineffective in a substantial proportion of patients and is limited by ischemic adverse events. Serelaxin (recombinant human relaxin-2) is a peptide molecule with anti-fibrotic and vasoprotective properties that binds to relaxin family peptide receptor-1 (RXFP1) and has been shown to increase renal perfusion in healthy human volunteers. We hypothesized that serelaxin could ameliorate renal vasoconstriction and renal dysfunction in patients with cirrhosis and portal hypertension.

METHODS AND FINDINGS

To establish preclinical proof of concept, we developed two independent rat models of cirrhosis that were characterized by progressive reduction in renal blood flow and glomerular filtration rate and showed evidence of renal endothelial dysfunction. We then set out to further explore and validate our hypothesis in a phase 2 randomized open-label parallel-group study in male and female patients with alcohol-related cirrhosis and portal hypertension. Forty patients were randomized 1:1 to treatment with serelaxin intravenous (i.v.) infusion (for 60 min at 80 μg/kg/d and then 60 min at 30 μg/kg/d) or terlipressin (single 2-mg i.v. bolus), and the regional hemodynamic effects were quantified by phase contrast magnetic resonance angiography at baseline and after 120 min. The primary endpoint was the change from baseline in total renal artery blood flow. Therapeutic targeting of renal vasoconstriction with serelaxin in the rat models increased kidney perfusion, oxygenation, and function through reduction in renal vascular resistance, reversal of endothelial dysfunction, and increased activation of the AKT/eNOS/NO signaling pathway in the kidney. In the randomized clinical study, infusion of serelaxin for 120 min increased total renal arterial blood flow by 65% (95% CI 40%, 95%; p < 0.001) from baseline. Administration of serelaxin was safe and well tolerated, with no detrimental effect on systemic blood pressure or hepatic perfusion. The clinical study's main limitations were the relatively small sample size and stable, well-compensated population.

CONCLUSIONS

Our mechanistic findings in rat models and exploratory study in human cirrhosis suggest the therapeutic potential of selective renal vasodilation using serelaxin as a new treatment for renal dysfunction in cirrhosis, although further validation in patients with more advanced cirrhosis and renal dysfunction is required.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01640964.

Serelaxin treatment reverses vascular dysfunction and left ventricular hypertrophy in a mouse model of Type 1 diabetes

Serelaxin prevents endothelial dysfunction in the mouse aorta ex vivo and inhibits apoptosis in cardiomyocytes under acute hyperglycaemia. Less is known about the effects of serelaxin in an in vivo mouse model of diabetes. Therefore, we tested the hypothesis in streptozotocin (STZ)-treated mice that serelaxin is able to reverse diabetes-induced vascular dysfunction and cardiac remodelling. Mice were divided into citrate buffer + placebo, STZ + placebo and STZ + serelaxin (0.5 mg/kg/d, 2 weeks) groups. After 12 weeks of diabetes, sensitivity to the endothelium-dependent agonist acetylcholine (ACh) was reduced in the mesenteric artery. This was accompanied by an enhanced vasoconstrictor prostanoid contribution and a decrease in endothelium-derived hyperpolarisation (EDH)-mediated relaxation. Serelaxin restored endothelial function by increasing nitric oxide (NO)-mediated relaxation but not EDH. It also normalised the contribution of vasoconstrictor prostanoids to endothelial dysfunction and suppressed diabetes-induced hyper-responsiveness of the mesenteric artery to angiotensin II. Similarly, diabetes reduced ACh-evoked NO-mediated relaxation in the aorta which was reversed by serelaxin. In the left ventricle, diabetes promoted apoptosis, hypertrophy and fibrosis; serelaxin treatment reversed this ventricular apoptosis and hypertrophy, but had no effect on fibrosis. In summary, serelaxin reversed diabetes-induced endothelial dysfunction by enhancing NO-mediated relaxation in the mouse vasculature and attenuating left ventricular hypertrophy and apoptosis.

Vascular actions of relaxin: nitric oxide and beyond

The peptide hormone relaxin regulates the essential maternal haemodynamic adaptations in early pregnancy through direct actions on the renal and systemic vasculature. These vascular actions of relaxin occur mainly through endothelium-derived NO-mediated vasodilator pathways and improvements in arterial compliance in small resistance-size arteries. This work catalysed a plethora of studies which revealed quite heterogeneous responses across the different regions of the vasculature, and also uncovered NO-independent mechanisms of relaxin action. In this review, we first describe the role of endogenous relaxin in maintaining normal vascular function, largely referring to work in pregnant and male relaxin-deficient animals. We then discuss the diversity of mechanisms mediating relaxin action in different vascular beds, including the involvement of prostanoids, VEGF, endothelium-derived hyperpolarisation and antioxidant activity in addition to the classic NO-mediated vasodilatory pathway. We conclude the review with current perspectives on the vascular remodelling capabilities of relaxin.

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.