Sarah A. Marshall

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.

Relaxin deficiency attenuates pregnancy-induced adaptation of the mesenteric artery to angiotensin II in mice.

Pregnancy is associated with reduced peripheral vascular resistance, underpinned by changes in endothelial and smooth muscle function. Failure of the maternal vasculature to adapt correctly leads to serious pregnancy complications, such as preeclampsia. The peptide hormone relaxin regulates the maternal renal vasculature during pregnancy; however, little is known about its effects in other vascular beds. This study tested the hypothesis that functional adaptation of the mesenteric and uterine arteries during pregnancy will be compromised in relaxin-deficient (Rln(-/-)) mice. Smooth muscle and endothelial reactivity were examined in small mesenteric and uterine arteries of nonpregnant (estrus) and late-pregnant (day 17.5) wild-type (Rln(+/+)) and Rln(-/-) mice using wire myography. Pregnancy per se was associated with significant reductions in contraction to phenylephrine, endothelin-1, and ANG II in small mesenteric arteries, while sensitivity to endothelin-1 was reduced in uterine arteries of Rln(+/+) mice. The normal pregnancy-associated attenuation of ANG II-mediated vasoconstriction in mesenteric arteries did not occur in Rln(-/-) mice. This adaptive failure was endothelium-independent and did not result from altered expression of ANG II receptors or regulator of G protein signaling 5 (Rgs5) or increases in reactive oxygen species generation. Inhibition of nitric oxide synthase with l-NAME enhanced ANG II-mediated contraction in mesenteric arteries of both genotypes, whereas blockade of prostanoid production with indomethacin only increased ANG II-induced contraction in arteries of pregnant Rln(+/+) mice. In conclusion, relaxin deficiency prevents the normal pregnancy-induced attenuation of ANG II-mediated vasoconstriction in small mesenteric arteries. This is associated with reduced smooth muscle-derived vasodilator prostanoids.

The Role of Relaxin in Normal and Abnormal Uterine Function During the Menstrual Cycle and Early Pregnancy

The hormone relaxin is a 6-kDa peptide with high structural similarity to insulin. It is primarily produced by the corpus luteum during pregnancy but is also synthesized by other reproductive organs such as the uterus, decidua, and placenta. Relaxin binds to its receptor RXFP1, which has been localized to a wide variety of reproductive and nonreproductive tissues. The peptide’s many uterotropic effects include stimulating uterine growth and vascularization, remodeling extracellular matrix components, and regulating vascular endothelial growth factor in preparation for implantation. Evidence also supports a role for relaxin in the systemic maternal vascular adaptations required for a healthy pregnancy. Diminished relaxin levels in early pregnancy are linked with increased risks of miscarriage and the development of preeclampsia. In addition to pregnancy, relaxin may also play a functional role in the uterus during the menstrual cycle, and modified relaxin activity may contribute to gynecological disorders such as uterine fibrosis and endometriosis. Despite over 75 years of research, we still have a limited understanding of relaxin’s broad roles in the uterus, particularly as there are significant species differences in its synthesis and activity, which restricts the use of animal models for human-centric questions. Here, we review current knowledge regarding relaxin actions in the human uterus during the menstrual cycle and in early pregnancy, with a focus on its potential roles in various gynecological disorders, as well as the pregnancy disorders such as preeclampsia, recurrent miscarriage, and early pregnancy loss.

Relaxin treatment reduces angiotensin II-induced vasoconstriction in pregnancy and protects against endothelial dysfunction

ABSTRACT The peptide relaxin has gained considerable attention as a new vasoactive drug, largely through its beneficial therapeutic effects in cardiovascular disease. In this study, we tested the hypothesis that relaxin treatment alleviates systemic vascular dysfunction characteristic of hypertensive diseases of pregnancy. We investigated vascular effects and mechanisms of relaxin action in (i) pregnant relaxin-deficient (Rln -/-) mice with enhanced responses to angiotensin II (AngII) and (ii) arteries pre-incubated ex vivo in trophoblast conditioned media (TCM) to induce endothelial dysfunction. Pregnant Rln-/- mice received 0.5 µg/h recombinant human H2 relaxin (rhRLX: n = 5) or placebo (20 nM sodium acetate; n = 7) subcutaneously via osmotic minipumps for 5 days prior to gestational day 17.5. This treatment protocol significantly reduced AngII-mediated contraction of mesenteric arteries and increased plasma 6-keto prostaglandin F1α. These vascular effects were endothelium independent and likely involve smooth muscle-derived vasodilator prostanoids. In the second study, mesenteric arteries were incubated ex vivo for 24 h at 37°C in TCM, which contained high levels of soluble Flt-1 (>20 ng/ml) and soluble Eng (>1 ng/ml). TCM incubation caused significant reduction in endothelium-dependent relaxation and increased sensitivity to AngII. Coincubation of arteries with rhRLX for 24 h (n = 6–16/treatment) prevented endothelial dysfunction but had no effect on AngII-mediated contraction. In conclusion, relaxin treatment prevents and/or reverses vascular dysfunction in mesenteric arteries, but acts through different vascular pathways depending on duration of relaxin treatment and type of vascular dysfunction. Overall, our data suggest that relaxin is a potential therapeutic to alleviate maternal systemic vascular dysfunction associated with hypertensive diseases in pregnant women. Summary Sentence Relaxin treatment reduces the vasoconstriction of the mesenteric artery to angiotensin II and protects the vasculature from developing endothelial dysfunction.

Evidence of proteinuria, but no other characteristics of pre-eclampsia, in relaxin-deficient mice.

Pre-eclampsia (PE) is a leading cause of maternal and fetal death, characterised by an imbalance of placental growth factors and hypertension at >20 weeks gestation. Impaired maternal systemic vascular adaptations and fetal growth restriction are features of both PE and pregnant relaxin-deficient (Rln-/-) mice. The aim of the present study was to investigate whether these phenotypes in Rln-/- mice are associated with abnormal placental growth factor expression, increased soluble fms-like tyrosine kinase-1 (sFlt-1), proteinuria and/or hypertension during pregnancy. In addition, we examined relaxin and relaxin receptor (relaxin/insulin like family peptide receptor 1 (RXFP1)) mRNA expression in placentas of women with PE. There was no significant difference in placental vascular endothelial growth factor A (VegfA) and placenta growth factor (Plgf) gene expression between Rln-/- and wild-type mice. Circulating plasma sFlt-1 concentrations in pregnant mice of both genotypes and ages were increased compared with non-pregnant mice but were lower in younger pregnant Rln-/- mice compared with aged-matched Rln+/+ mice. Aged pregnant Rln-/- mice had higher urinary albumin:creatinine ratios compared with age-matched Rln+/+ mice, indicative of proteinuria. Systolic and diastolic blood pressures did not differ between genotypes. In addition, PE in women was not associated with altered placental mRNA expression of RLN2 or RXFP1 at term. Overall, the data demonstrate that pregnant Rln-/- mice do not have the typical characteristics of PE. However, these mice show evidence of proteinuria, but we suggest that this results from systemic renal vascular dysfunction before pregnancy.

Short‐term (48 hours) intravenous serelaxin infusion has no effect on myogenic tone or vascular remodeling in rat mesenteric arteries

Short‐term IV sRLX (recombinant human relaxin‐2) infusion enhances endothelium‐dependent relaxation in mesenteric arteries. This is initially underpinned by increased NO followed by a transition to prostacyclin. The effects of short‐term IV sRLX treatment on pressure‐induced myogenic tone and vascular remodeling in these arteries are unknown. Therefore, we investigated the effects of sRLX infusion on pressure‐induced myogenic tone and passive mechanical wall properties in mesenteric arteries.

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.

B7-33 replicates the vasoprotective functions of human relaxin-2 (serelaxin)

Abstract Recombinant H2 relaxin (serelaxin) has gained considerable attention as a new vasoprotective drug, largely due to its potential therapeutic effects in heart failure and fibrosis. However, serelaxin is laborious and costly to produce. A single‐chain peptidomimetic, B7‐33, has been developed to overcome these problems but little is known about its biological actions in the vascular system. This study first compared the rapid vascular effects of an acute bolus injection of B7‐33 compared with serelaxin. Male Wistar rats received a tail vein injection of placebo (20 mM sodium acetate), B7‐33 (13.3 &mgr;g/kg) or serelaxin (26.6 &mgr;g/kg). Three hours later vascular function in the mesenteric artery, small renal artery and abdominal aorta was assessed by wire myography. B7‐33 and serelaxin selectively enhanced bradykinin‐mediated endothelium‐dependent relaxation in the rat mesenteric artery by increasing endothelium‐derived hyperpolarization, but had no overall effects on relaxation in the small renal artery or aorta. We then compared the actions of B7‐33 and serelaxin in an ex vivo model of vascular disease using virgin female mouse mesenteric arteries pre‐incubated in placental trophoblast conditioned media to induce endothelial dysfunction characteristic of preeclampsia. Co‐incubation of these arteries in trophoblast conditioned media with B7‐33 or serelaxin (15, 30 nM) prevented the development of endothelial dysfunction. In conclusion, equimolar doses of B7‐33 replicated the acute beneficial vascular effects of serelaxin in rat mesenteric arteries and also prevented endothelial dysfunction induced by placental trophoblast conditioned media in mouse mesenteric arteries. Therefore, B7‐33 should be considered as a cost‐effective vasoactive therapeutic in cardiovascular diseases, including preeclampsia.

Abnormal extracellular matrix remodelling in the cervix of pregnant relaxin-deficient mice is not associated with reduced matrix metalloproteinase expression or activity

Relaxin regulates cervical extracellular matrix (ECM) remodelling during pregnancy by modifying collagen and other ECM molecules by unknown mechanisms. We hypothesised that abnormal collagen remodelling in the cervix of pregnant relaxin-deficient (Rln1-/-) mice is due to excessive collagen (Col1a1 and Col3a1) and decreased matrix metalloproteinases (Mmp2, Mmp9, Mmp13 and Mmp7) and oestrogen receptors (Esr1 and Esr2). Quantitative polymerase chain reaction, gelatinase zymography, MMP activity assays and histological staining evaluated changes in ECM in pregnant wildtype (Rln1+/+) and Rln1-/- mice. Cervical Col1a1, Col3a1 and total collagen increased in Rln1-/- mice and were higher at term compared with Rln1+/+ mice. This was not correlated with a decrease in gelatinase (Mmp2, Mmp9) expression or activity, Mmp7 or Mmp13 expression, which were all significantly higher in Rln1-/- mice. In late pregnancy, circulating MMP2 and MMP9 were unchanged. Esr1 expression was highest in Rln1+/+ and Rln1-/- mice in late pregnancy, coinciding with a decrease in Esr2 in Rln1+/+ but not Rln1-/- mice. The relaxin receptor (Rxfp1) decreased slightly in late-pregnant Rln1+/+ mice, but was significantly higher in Rln1-/- mice. In summary, relaxin deficiency results in increased cervical collagen in late pregnancy, which is not explained by a reduction in Mmp expression or activity or decreased Rxfp1. However, an imbalance between Esr1 and Esr2 may be involved.

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.