Relaxin signalling links tyrosine phosphorylation to phosphodiesterase and adenylyl cyclase activity

Rhynchosia volubilis Promotes Cell Survival via cAMP-PKA/ERK-CREB Pathway

Rhynchosia volubilis, a small black bean, has been used as a traditional remedy to treat diseases and maintain health in East Asia, but its cellular effects and molecular mechanisms are not fully understood. The purpose of this study was to investigate the effect of ethanol extract from Rhynchosia volubilis (EERV) on cell survival and to elucidate the biochemical signaling pathways. Our results showed that EERV stimulated the cyclic AMP (cAMP) signal revealed by a fluorescent protein (FP)-based intensiometric sensor. Using a Förster resonance energy transfer (FRET)-based sensor, we further revealed that EERV could activate PKA and ERK signals, which are downstream effectors of cAMP. In addition, we reported that EERV could induce the phosphorylation of CREB, a key signal for cell survival. Thus, our results suggested that EERV protects against apoptosis by activating the cell survival pathway through the cAMP-PKA/ERK-CREB pathway.

Human Recombinant Relaxin (Serelaxin) as Anti-fibrotic Agent: Pharmacology, Limitations and Actual Perspectives

Relaxin (recombinant human relaxin-2 hormone; RLX-2; serelaxin) had raised expectations as a new medication for fibrotic diseases. A plethora of in vitro and in vivo studies have offered convincing demonstrations that relaxin promotes remodelling of connective tissue extracellular matrix mediated by inhibition of multiple fibrogenic pathways, especially the downstream signalling of transforming growth factor (TGF)-β1, a major pro-fibrotic cytokine, and the recruitment and activation of myofibroblast, the main fibrosis-generating cells. However, all clinical trials with relaxin in patients with fibrotic diseases gave inconclusive results. In this review, we have summarized the molecular mechanisms of fibrosis, highlighting those which can be effectively targeted by relaxin. Then, we have performed a critical reappraisal of the clinical trials performed to-date with relaxin as anti-fibrotic drug, in order to highlight their key points of strength and weakness and to identify some future opportunities for the therapeutic use of relaxin, or its analogues, in fibrotic diseases and pathologic scarring which, in our opinion, deserve to be investigated.

Emerging roles for the relaxin/RXFP1 system in cancer therapy

A role for the hormone relaxin in cancer was described well before the receptor was identified. Relaxin predominantly increases the growth and invasive potential in cancers of different origins. However, relaxin was also shown to promote cell differentiation and to act in a dose-and time-dependent manner in different cancer cell models used. Following the discovery of the relaxin like family peptide receptor 1 (RXFP1) as the cellular receptor for RLN1 and RLN2, research has focussed on the ligand interaction with the large extracellular domain of RXFP1 and resulting molecular signaling mechanisms. RXFP1 activation mediates anti-apoptotic functions, angiogenesis and chemoresistance in cancer cells. This minireview summarizes the known biological functions of RXFP1 activation in different cancer entities in-vitro and in-vivo and outlines possible mechanisms to therapeutically address the relaxin-RXFP1 system in cancer cells.

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.

Cardioprotective actions of relaxin

Relaxin is a hormone of pregnancy first discovered for its ability to induce ligament relaxation in nonpregnant guinea pig and is important for softening of the birth canal during parturition, decidualization, implantation, nipple development and increased maternal renal perfusion, glomerular filtration, and cardiac output. Subsequently, relaxin has been shown to exert multiple beneficial cardiovascular effects during pathological events such as hypertension, atrial fibrillation, heart failure and myocardial infarction, including suppression of arrhythmia and inflammation, and reversal of fibrosis. Despite extensive studies, the mechanisms underlying relaxin's effects are not well understood. Relaxin signals primarily through its G protein coupled receptor, the relaxin family peptide receptor-1, to activate multiple signaling pathways and this review summarizes our understanding of these pathways as they relate to the cardioprotective actions of relaxin, focusing on relaxin's anti-fibrotic, anti-arrhythmic and anti-inflammatory properties. Further, this review includes a brief overview of relaxin in clinical trials for heart failure and progress in the development of relaxin mimetics.

Male Seminal Relaxin Contributes to Induction of the Post-mating Cytokine Response in the Female Mouse Uterus

The hormone relaxin is important in female reproduction for embryo implantation, cardiovascular function, and during labor and lactation. Relaxin is also synthesized in males by organs of the male tract. We hypothesized that relaxin might be one component of seminal plasma responsible for eliciting the female cytokine response induced in the uterus at mating. When recombinant relaxin was injected into the uterus of wild-type (Rln^+/+) mice at estrus, it evoked the production of Cxcl1 mRNA and its secreted protein product CXCL1 in four of eight animals. Mating experiments were then conducted using mice with a null mutation in the relaxin gene (Rln^−/− mice). qRT-PCR analysis of mRNA expression in wild-type females showed diminished uterine expression of several cytokine and chemokine genes in the absence of male relaxin. Similar differences were also noted comparing Rln^−/− and Rln^+/+ females mated to wild-type males. Quantification of uterine luminal fluid cytokine content confirmed that male relaxin provokes the production of CXCL10 and CSF3 in Rln^+/+ females. Differences were also seen comparing Rln^−/− and Rln^+/+ females mated with Rln^−/− males for CXCL1, CSF3, and CCL5, implying that endogenous relaxin in females might prime the uterus to respond appropriately to seminal fluid at coitus. Finally, pan-leukocyte CD45 mRNA was increased in wild-type matings compared to other combinations, implying that male and female relaxin may trigger leukocyte expansion in the uterus. We conclude that male and/or female relaxin may be important in activating the uterine cytokine/chemokine network required to initiate maternal immune adaptation to pregnancy.

Human relaxins (RLNH1, RLNH2), their receptor (RXFP1) and fetoplacental growth

Relaxin, a systemic and placental hormone, has potential roles in fetoplacental growth. Human placenta expresses two RLN genes, RLNH1 and RLNH2 Maternal obesity is common and is associated with abnormal fetal growth. Our aims were to relate systemic and cord blood RLNH2, placental RLNs and their receptor (RXFP1) with fetoplacental growth in context of maternal body mass index, and associations with insulin-like growth factor 2 (IGF2) and vascular endothelial growth factor A (VEGFA) in the same placentas. Systemic, cord blood and placental samples were collected prior to term labor, divided by prepregnancy body mass index: underweight/normal (N = 25) and overweight/obese (N = 44). Blood RLNH2 was measured by ELISA; placental RLNH2, RLNH1, RXFP1, IGF2 and VEGFA were measured by quantitative immunohistochemistry and mRNAs were measured by quantitative reverse transcription PCR. Birthweight increased with systemic RLNH2 only in underweight/normal women (P = 0.036). Syncytiotrophoblast RLNH2 was increased in overweight/obese patients (P = 0.017) and was associated with placental weight in all subjects (P = 0.038). RLNH1 had no associations with birthweight or placental weight, but was associated with increased trophoblast and endothelial IGF2 and VEGFA, due to female fetal sex. Thus, while systemic RLNH2 may be involved in birthweight regulation in underweight/normal women, placental RLNH2 in all subjects may be involved in placental weight. A strong association of trophoblast IGF2 with birthweight and placental weight in overweight/obese women suggests its importance. However, an association of only RLNH1 with placental IGF2 and VEGFA was dependent upon female fetal sex. These results suggest that both systemic and placental RLNs may be associated with fetoplacental growth.

Synthetic non-peptide low molecular weight agonists of the relaxin receptor 1

Relaxin is a small heterodimeric peptide hormone of the insulin/relaxin superfamily produced mainly in female and male reproductive organs. It has potent antifibrotic, vasodilatory and angiogenic effects and regulates the normal function of various physiological systems. Preclinical studies and recent clinical trials have shown the promise of recombinant relaxin as a therapeutic agent in the treatment of cardiovascular and fibrotic diseases. However, there are the universal drawbacks of peptide-based pharmacology that apply to relaxin: a short half-life in vivo requires its continuous delivery, and there are high costs of production, storage and treatment, as well as the possibility of immune responses. All these issues can be resolved by the development of low non-peptide MW agonists of the relaxin receptors which are stable, bioavailable, easily synthesized and specific. In this review, we describe the discovery and characterization of the first series of such compounds. The lead compound, ML290, binds to an allosteric site of the relaxin GPCR, RXFP1. ML290 shows high activity and efficacy, measured by cAMP response, in cells expressing endogenous or transfected RXFP1. Relaxin-like effects of ML290 were shown in various functional cellular assays in vitro. ML290 has excellent absorption, distribution, metabolism and excretion properties and in vivo stability. The identified series of low MW agonists does not activate rodent RXFP1 receptors and thus, the production of a RXFP1 humanized mouse model is needed for preclinical studies. The future analysis and clinical perspectives of relaxin receptor agonists are discussed.

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.

International Union of Basic and Clinical Pharmacology. XCV. Recent advances in the understanding of the pharmacology and biological roles of relaxin family peptide receptors 1-4, the receptors for relaxin family peptides

Relaxin, insulin-like peptide 3 (INSL3), relaxin-3, and INSL5 are the cognate ligands for the relaxin family peptide (RXFP) receptors 1-4, respectively. RXFP1 activates pleiotropic signaling pathways including the signalosome protein complex that facilitates high-sensitivity signaling; coupling to Gα(s), Gα(i), and Gα(o) proteins; interaction with glucocorticoid receptors; and the formation of hetero-oligomers with distinctive pharmacological properties. In addition to relaxin-related ligands, RXFP1 is activated by Clq-tumor necrosis factor-related protein 8 and by small-molecular-weight agonists, such as ML290 [2-isopropoxy-N-(2-(3-(trifluoromethylsulfonyl)phenylcarbamoyl)phenyl)benzamide], that act allosterically. RXFP2 activates only the Gα(s)- and Gα(o)-coupled pathways. Relaxin-3 is primarily a neuropeptide, and its cognate receptor RXFP3 is a target for the treatment of depression, anxiety, and autism. A variety of peptide agonists, antagonists, biased agonists, and an allosteric modulator target RXFP3. Both RXFP3 and the related RXFP4 couple to Gα(i)/Gα(o) proteins. INSL5 has the properties of an incretin; it is secreted from the gut and is orexigenic. The expression of RXFP4 in gut, adipose tissue, and β-islets together with compromised glucose tolerance in INSL5 or RXFP4 knockout mice suggests a metabolic role. This review focuses on the many advances in our understanding of RXFP receptors in the last 5 years, their signal transduction mechanisms, the development of novel compounds that target RXFP1-4, the challenges facing the field, and current prospects for new therapeutics.

RLN2 Is a Positive Regulator of AKT-2-Induced Gene Expression Required for Osteosarcoma Cells Invasion and Chemoresistance

The aim of the study was to determine the effect of H2 relaxin (RLN2) on invasion, migration, and chemosensitivity to cisplatin in human osteosarcoma U2-OS and MG-63 cells and then to investigate the effect of RLN2 on the AKT/NF-κB signaling pathway. The expression of RLN2, p-AKT (Ser473), and p-ERK1/2 (Phospho-Thr202/Tyr204) proteins was detected by western blot in OS tissues from 21 patients with pulmonary metastatic disease, and the correlation between RLN2 and p-AKT or RLN2 and p-ERK1/2 expression was investigated. RLN2 expression was inhibited by RLN2 siRNA transfection in the MG-63 cells. RLN2 was overexpressed in the U2-OS cells by treatment with recombinant relaxin. The results showed that positive relation was found between RLN2 and p-AKT expression in tissues of OS. Silencing RLN2 inhibited cell migratory and invasive ability and angiogenesis formation and increased the chemosensitivity to cisplatin in MG-63 cells. RLN2 overexpression promoted migratory and invasive ability and angiogenesis and increased the chemoresistance to cisplatin in U2-OS cells. Silencing RLN2 inhibited the activity of AKT/NF-κB signaling pathway in MG-63 cells, and vice versa. Blockage of both pathways by specific inhibitors abrogated RLN2-induced survival and invasion of OS cells, and vice versa. Our results indicated RLN2 confers to migratory and invasive ability, angiogenesis, and chemoresistance to cisplatin via modulating the AKT/NF-κB signaling pathway in vitro.

Bioactivity assay of porcine relaxin based on cAMP accumulation in THP-1 cells quantified by LC-MS/MS

This study describes a reliable bioactivity assay of porcine relaxin (pRLX) based on cyclic adenosine 3',5'-monophosphate (cAMP) accumulation in the human monocyte cell-line quantified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). As a result, the LC-MS/MS was based on a positive selected reaction monitoring of cAMP with a stable internal standard, 8-Br-cAMP and a protein precipitation procedure by HClO4. The standard curve of cAMP was linear from 5.0 ng mL(-1) to 992.0 ng mL(-1), with lower limits of detection and quantification of 0.5 ng mL(-1) and 5.0 ng mL(-1), respectively. The satisfactory validation data including stability assay were obtained. When measured by the LC-MS/MS, the pRLX sample showed a time- and dose-dependent stimulation of cAMP with the concentration for 50% of the maximal effect (EC50) of 40.6 ng mL(-1). The developed method indicated higher precision and selectivity than the commercial enzyme linked immunosorbent assay kits, which showed EC50 of 66.6 ng mL(-1).

Regulation of the reproductive cycle and early pregnancy by relaxin family peptides

The relaxin family of peptide hormones are structurally closely related to one another sharing a heterodimeric A-B structure, like that of insulin. They may also be active as unprocessed B-C-A pro-forms. Relaxin has been shown to pay a key role within the ovary, being involved in follicle growth, and ovulation. Relaxin is produced in large amounts also by the corpus luteum where it acts as an endocrine hormone positively affecting implantation, placentation and vascularization during the all-important first trimester phase of pregnancy establishment. Relaxin exerts its functions via the receptor RXFP1. Insulin-like peptide 3 (INSL3) in contrast acts through the related receptor RXFP2, and plays an essential role in the production of androgens within growing antral follicles. INSL3 is also produced in large amounts by the male fetus shortly after sex determination, where it controls the first transabdominal phase of testicular descent. However, this fetal INSL3 is also able to influence placental and maternal physiology, indicating associations with later preeclampsia and/or fetal growth restriction. Other members of this relaxin-like family of peptides, such as INSL4, INSL5 and INSL6 are less well studied, though all suggest modulatory roles in ovarian and/or placental function.

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.

Constitutive formation of an RXFP1-signalosome: a novel paradigm in GPCR function and regulation

The classical second messenger cAMP is important in diverse physiological processes, where its spatial and temporal compartmentalization allows precise control over multiple cellular events. Within this context, G-protein-coupled receptors (GPCRs) govern specialized pools of cAMP, which are functionally specific for the unique cellular effects attributed to a particular system. The relaxin receptor, RXFP1, is a GPCR that exerts pleiotropic physiological effects including a potent anti-fibrotic response, increased cancer metastases, and has efficacy as a vasodilator in heart failure. On a cellular level, relaxin stimulation of RXFP1 results in the activation of multiple G-protein pathways affecting cAMP accumulation. Specificity and diversity in the cAMP signal generated by RXFP1 is controlled by differential G-protein coupling dependent upon the background of cellular expression, and cAMP compartmentalization. Further complexity in cAMP signalling results from the constitutive assembly of an RXFP1-signalosome, which specifically responds to low concentrations of relaxin, and activates a distinct cAMP pathway. The RXFP1-signalosome is a higher-order protein complex that facilitates receptor sensitivity to attomolar concentration of peptide, exhibits constitutive activity and dual coupling to G-proteins and β-arrestins and reveals a concentration-biased agonism mediated by relaxin. The specific and directed formation of GPCR-centered signalosomes allows an even greater spatial and temporal control of cAMP, thus rationalizing the considerable physiological scope of this ubiquitous second messenger.

Relaxin signaling activates peroxisome proliferator-activated receptor gamma

Relaxin is a polypeptide hormone that triggers multiple signaling pathways through its receptor RXFP1 (relaxin family peptide receptor 1). Many of relaxin's functions, including vascular and antifibrotic effects, are similar to those induced by activation of PPARgamma. In this study, we tested the hypothesis that relaxin signaling through RXFP1 would activate PPARgamma activity. In cells overexpressing RXFP1 (HEK-RXFP1), relaxin increased transcriptional activity through a PPAR response element (PPRE) in a concentration-dependent manner. In cells lacking RXFP1, relaxin had no effect. Relaxin increased both the baseline activity and the response to the PPARgamma agonists rosiglitazone and 15d-PGJ(2), but not to agonists of PPARalpha or PPARdelta. In HEK-RXFP1 cells infected with adenovirus expressing PPARgamma, relaxin increased transcriptional activity through PPRE, and this effect was blocked with an adenovirus expressing a dominant-negative PPARgamma. Knockdown of PPARgamma using siRNA resulted in a decrease in the response to both relaxin and rosiglitazone. Both relaxin and rosiglitazone increased expression of the PPARgamma target genes CD36 and LXRalpha in HEK-RXFP1 and in THP-1 cells naturally expressing RXFP1. Relaxin did not increase PPARgamma mRNA or protein levels. Treatment of cells with GW9662, an inhibitor of PPARgamma ligand binding, effectively blocked rosiglitazone-induced PPARgamma activation, but had no effect on relaxin activation of PPARgamma. These results suggest that relaxin activates PPARgamma activity, and increases the overall response in the presence of PPARgamma agonists. This activation is dependent on the presence of RXFP1. Furthermore, relaxin activates PPARgamma via a ligand-independent mechanism. These studies represent the first report that relaxin can activate the transcriptional activity of PPARgamma.

Roles of the receptor, the ligand, and the cell in the signal transduction pathways utilized by the relaxin family peptide receptors 1-3

The relaxin-like peptides produce their effects by acting at four G-protein-coupled receptors (GPCRs) RXFP1 to 4. RXFP1 and 2 are characterized by large extracellular domains containing leucine-rich repeats, whereas RXFP3 and 4 closely resemble small-peptide-liganded GPCRs. Studies with mutant RXFP1 receptors established that the final 10 amino acids of the C-terminus and Arg(752) in particular are obligatory for the second phase of cAMP signaling. Examination of the importance of cell type revealed different patterns of cAMP signaling related to the types of G-proteins expressed in these cells. Studies of RXFP3 signaling using reporter genes revealed that both relaxin and relaxin-3 activated the receptor but displayed different patterns of signaling. The studies suggest that the functional domains of the receptor, the cell type in which it is expressed, and the ligand used to activate the receptor all have important roles in determining the functional response observed.

Relaxin family peptide receptor-1 protects against airway fibrosis during homeostasis but not against fibrosis associated with chronic allergic airways disease

Endogenous relaxin has recently been demonstrated to protect the airway/lung against age-related fibrosis and against inflammation-associated airway fibrosis in animal models of allergic airways disease (AAD). In the current study, we examined the contribution of the primary relaxin receptor, relaxin family peptide receptor-1 (RXFP1), in mediating these effects of relaxin. Lung tissues from healthy aging RXFP1 gene-knockout (Rxfp1(-/-)) and wild-type (Rxfp1(+/+)) mice and from 8- to 10-wk-old Rxfp1(-/-) and Rxfp1(+/+) mice subjected to a mouse model of AAD were assessed for various markers of airway fibrosis and remodeling. Male and female Rxfp1(-/-) mice demonstrated an age-related progression of airway/lung fibrosis. Saline-treated Rxfp1(-/-) mice had significantly increased myofibroblast differentiation and lung collagen deposition (both P < 0.05), decreased matrix metalloproteinase (MMP)-9 expression and activity (P < 0.05), but equivalent levels of MMP-2 and tissue inhibitor of metalloproteinases (TIMPs) to that measured in saline-treated Rxfp1(+/+) mice. As expected, ovalbumin (OVA)-treated Rxfp1(+/+) mice developed markedly increased lung myofibroblast differentiation and collagen deposition (both P < 0.01 vs saline-treated Rxfp1(+/+) mice), significantly decreased lung MMP-2 and MMP-9 expression and activity and increased TIMP-1 expression (all P < 0.05 vs. respective measurements from saline-treated Rxfp1(+/+) mice). Surprisingly, however, OVA-treated Rxfp1(-/-) animals had equivalent levels of airway fibrosis and gelatinase activity but increased TIMP-1 expression (P < 0.05) compared with OVA-treated Rxfp1(+/+) mice. These combined findings demonstrate that RXFP1 is involved in mediating relaxin's effects on airway fibrosis during homeostasis but not during inflammation-induced fibrosis associated with chronic AAD.

Relaxin family peptide receptor (RXFP1) coupling to G(alpha)i3 involves the C-terminal Arg752 and localization within membrane Raft Microdomains

The relaxin family peptide receptors (RXFP) 1 and 2 are targets for the relaxin family peptides relaxin and insulin-like peptide 3 (INSL3), respectively. Although both receptors and peptides share a high degree of sequence identity, the cAMP signaling pathways activated by the two systems are quite distinct. Relaxin activation of RXFP1 initially results in accumulation of cAMP via G(alpha)(s), but this is modulated by inhibition of cAMP through G(alpha)(oB). Over time, RXFP1 recruits coupling to G(alpha)(i3), causing additional cAMP accumulation via a G(alpha)(i3)-Gbetagamma-phosphoinositide 3-kinase (PI3K)-protein kinase C (PKC)zeta pathway. In contrast, INSL3 activation of RXFP2 results in accumulation of cAMP only via G(alpha)(s), modulated by cAMP inhibition through G(alpha)(oB). Thus, the aim of this study was to identify the cause of differential G-protein coupling between these highly similar receptors. Construction of chimeric receptors revealed that G(alpha)(i3) coupling is dependent upon the transmembrane region of RXFP1 and independent of the receptor ectodomain or ligand bound. Generation of C-terminal truncated receptors identified the terminal 10 amino acids of the RXFP1 C terminus as essential for G(alpha)(i3) signaling, and point mutations revealed an obligatory role for Arg(752). RXFP1-mediated G(alpha)(i3), but not G(alpha)(s) or G(alpha)(oB), signaling was also found to be dependent upon membrane rafts, and RXFP1 coupled to G(alpha)(i3) after only 3 min of receptor stimulation. Therefore, RXFP1 coupling to the G(alpha)(i3)-Gbetagamma-PI3K-PKCzeta pathway requires the terminal 10 amino acids of the RXFP1 C terminus and membrane raft localization, and the observed delay in this pathway occurs downstream of G(alpha)(i3).

Relaxin signalling in primary cultures of human myometrial cells

In myometrium of pigs and rats, though not humans, relaxin appears to mediate an inhibition of spontaneous and oxytocin-induced contractility, presumably acting through a G-protein coupled receptor (RXFP1) to generate cAMP. In humans, circulating relaxin is highest in the first trimester, including the time of implantation, when transitory uterine quiescence could help a blastocyst to implant. We investigated whether relaxin can activate adenylate cyclase in primary human myometrial cells from non-pregnant tissue, and we show that relaxin is able to stimulate the generation of cAMP in a manner, which is dependent upon a tyrosine phosphorylation activity, as in the endometrium. We identified transcripts for the relaxin receptor RXFP1 as full-length variants, though a minor splice variant missing exon 2 was also present in low amounts. These cells also express transcripts encoding RXFP2, the receptor for the closely related hormone, INSL3. Although able to respond to relaxin at high concentrations, this receptor does not appear to function by contributing to the cAMP production in human myometrial cells, nor does INSL3 act as a functional agonist or antagonist of relaxin action. In conclusion, the inability of relaxin to inhibit contractility in human myometrial cells would appear to be due to events downstream of simple cAMP generation.

Effect of amino acids and dipeptides on the acrosome reaction and accumulation of ammonia in porcine spermatozoa

Aim:  The present study was designed to investigate the effect of amino acids and their dipeptides in the medium related to the urea cycle on the motility, viability, acrosome reaction (AR) and accumulation of ammonia in the medium over different incubation periods in porcine spermatozoa and to assess the utilization of glucose. Methods:  Porcine spermatozoa were washed, swim-up and incubated at 37°C for 0-4 h in mTALP medium supplemented with 75-600 µmol/L ammonia. Amino acids (1.0 mmol) or their dipeptides (2.0 mmol) were added individually to the mTALP medium containing either no ammonia or 300 µmol/L of ammonia. The viability and AR of porcine spermatozoa were assessed using the triple-staining technique and the accumulation of ammonia in the medium was measured using the indophenol method. Results:  The motility, viability and AR were adversely affected (P < 0.05) by concentrations of ammonia ≥300 µmol/L compared with the control. Supplementation of l-alanyl-l-glutamine (AlaGln), l-glycyl-l-glutamine (GlyGln) and AlaGln + GlyGln in the presence of 300 µmol/L ammonia significantly increase (P < 0.05) the rate of motility, viability, AR, incorporation, accumulation of ammonia and oxidation of ¹⁴C(U)-glucose compared with the ammonia supplement control. Conclusion:  AlaGln and GlyGln in mTALP medium were more stable and effective than the individual amino acids in reducing the accumulation of ammonia, and subsequently increasing the rate of AR and the utilization of glucose in porcine spermatozoa. (Reprod Med Biol 2008; 7: 123-131).

Diverse signalling mechanisms used by relaxin in natural cells and tissues: the evolution of a "neohormone"

The small peptide hormone relaxin is a member of a rapidly evolving family of hormones and growth factors, whose mode of action appears to be particularly adapted to purely mammalian physiology. It is representative of a new category of hormones, referred to as neohormones, which appear to have evolved specifically to accommodate the needs of viviparity, lactation and wound repair. The mechanism of receptor signalling has also evolved in this family, with older members using receptor tyrosine kinases and new members such as relaxin adopting 7-transmembrane G-protein coupled receptors. Although relaxin primarily generates cAMP as second messenger, studies of relaxin signalling show that this does not conform to a classic G-protein dependent activation of adenylate cyclase: it requires additional cytoplasmic components, it can involve further coupling to PI3-kinase and PKCzeta and it is absolutely dependent on a tyrosine kinase activity linked closely to the relaxin receptor. Relaxin may also independently activate glucocorticoid receptors. This diversity of signalling leads to a broad range of possible downstream transcriptional effects. Finally, in tissues where relaxin is known to be effective, there is often also local relaxin induction, amplifying the effects of the endocrine hormone.

Studies of the molecular mechanisms of action of relaxin on the adenylyl cyclase signaling system using synthetic peptides derived from the LGR7 relaxin receptor

The peptide hormone relaxin produces dose-dependent stimulation of adenylyl cyclase activity in rat tissues (striatum, cardiac and skeletal muscle) and the muscle tissues of invertebrates, i.e., the bivalve mollusk Anodonta cygnea and the earthworm Lumbricus terrestris, adenylyl cyclase stimulation being more marked in the rat striatum and cardiac muscle. Our studies of the type of relaxin receptor involved in mediating these actions of relaxin involved the first synthesis of peptides 619-629, 619-629-Lys(Palm), and 615-629, which are derivatives of the primary structure of the C-terminal part of the third cytoplasmic loop of the type 1 relaxin receptor (LGR7). Peptides 619-629-Lys(Palm) and 615-629 showed competitive inhibition of adenylyl cyclase stimulation by relaxin in rat striatum and cardiac muscle but had no effect on the action of relaxin in rat skeletal muscle or invertebrate muscle, which is evidence for the tissue and species specificity of their actions. On the one hand, this indicates involvement of the LGR7 receptor in mediating the adenylyl cyclase-stimulating action of relaxin in rat striatum and cardiac muscle and, on the other, demonstrates the existence of other adenylyl cyclase signal mechanisms for the actions of relaxin in rat skeletal muscle and invertebrate muscle, not involving LGR7 receptors. The adenylyl cyclase-stimulating effect of relaxin in the striatum and cardiac muscles was found to be decreased in the presence of C-terminal peptide 385-394 of the alpha(s) subunit of the mammalian G protein and to be blocked by treatment of membranes with cholera toxin. These data provide evidence that in the striatum and cardiac muscle, relaxin stimulates adenylyl cyclase via the LGR7 receptor, this being functionally linked with G(s) protein. It is also demonstrated that linkage of relaxin-activated LGR7 receptor with the G(s) protein is mediated by interaction of the C-terminal half of the third cytoplasmic loop of the receptor with the C-terminal segment of the alpha(s) subunit of the G protein.

The Insulin-like Growth Factor Type 1 and Insulin-like Growth Factor Type 2/Mannose-6-phosphate Receptors Independently Regulate ERK1/2 Activity in HEK293 Cells

Insulin-like growth factor types 1 and 2 (IGF-1; IGF-2) and insulin-like peptides are all members of the insulin superfamily of peptide hormones but bind to several distinct classes of membrane receptor. Like the insulin receptor, the IGF-1 receptor is a heterotetrameric receptor tyrosine kinase, whereas the IGF-2/ mannose 6-phosphate receptor is a single transmembrane domain protein that is thought to function primarily as clearance receptors. We recently reported that IGF-1 and IGF-2 stimulate the ERK1/2 cascade by triggering sphingosine kinase-dependent "transactivation" of G protein-coupled sphingosine-1-phosphate receptors. To determine which IGF receptors mediate this effect, we tested seven insulin family peptides, IGF-1, IGF-2, insulin, and insulin-like peptides 3, 4, 6, and 7, for the ability to activate ERK1/2 in HEK293 cells. Only IGF-1 and IGF-2 potently activated ERK1/2. Although IGF-2 was predictably less potent than IGF-1 in activating the IGF-1 receptor, they were equipotent stimulators of ERK1/2. Knockdown of IGF-1 receptor expression by RNA interference reduced the IGF-1 response to a greater extent than the IGF-2 response, suggesting that IGF-2 did not signal exclusively via the IGF-1 receptor. In contrast, IGF-2 receptor knockdown markedly reduced IGF-2-stimulated ERK1/2 phosphorylation, with no effect on the IGF-1 response. As observed previously, both the IGF-1 and the IGF-2 responses were sensitive to pertussis toxin and the sphingosine kinase inhibitor, dimethylsphingosine. These data indicate that endogenous IGF-1 and IGF-2 receptors can independently initiate ERK1/2 signaling and point to a potential physiologic role for IGF-2 receptors in the cellular response to IGF-2.

Inappropriate activation of androgen receptor by relaxin via β-catenin pathway

We have previously demonstrated that human H2-relaxin can mediate androgen-independent growth of LNCaP through a mechanism that involves the activation of the androgen receptor (AR) signaling pathway. The goal of the current study is to elucidate the mechanism(s) by which H2-relaxin causes activation of the AR pathway. Our data indicate that there is cross-talk between AR and components of the Wnt signaling pathway. Addition of H2-relaxin to LNCaP cells resulted in increased phosphorylation of protein kinase B (Akt) and inhibitory phosphorylation of glycogen synthase kinase-3beta (GSK-3beta) with subsequent cytoplasmic accumulation of beta-catenin. Immunoprecipitation and immunocytochemical studies demonstrated that the stabilized beta-catenin formed a complex with AR, which was then translocated into the nucleus. Chromatin immunoprecipitation analysis determined that the AR/beta-catenin complex binds to the proximal region of the prostate-specific antigen promoter. Inhibition of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, using LY294002, prevented both H2-relaxin-mediated phosphorylation of Akt and GSK-3beta and translocation of beta-catenin/AR into the nucleus. Knockdown of beta-catenin levels using a beta-catenin-specific small interfering RNA inhibited H2-relaxin-induced AR activity. The combined data demonstrate that PI3K/Akt and components of the Wnt pathway can facilitate H2-relaxin-mediated activation of the AR pathway.

Relaxin signalling in THP-1 cells uses a novel phosphotyrosine-dependent pathway

The heterodimeric peptide hormone relaxin acts through the novel G-protein coupled receptor LGR7 to elicit the production of cAMP in the human monocyte cell line THP-1. The very small number of receptors on the cell surface, and the lack of response in cell membranes imply the involvement of a cytoplasmic signal amplification process. Here we show that this process comprises a novel and specific tyrosine kinase activity close to the receptor, and involves neither protein kinase A, mitogen-activated protein kinase, nor phosphoinositide-3 kinase activities as major upstream components. Furthermore, this novel involvement of a tyrosine kinase activity is cell-type dependent, being largely absent from LGR7-transfected HEK293T cells, and receptor-dependent; vasoactive intestinal peptide or isoproterenol signalling in the same cells does not require this tyrosine kinase activity.

Cyclooxygenase enzyme expression and E series prostaglandin receptor signalling are enhanced in heavy menstruation

BACKGROUND

Although the mechanisms underlying the causes of heavy menstrual blood loss (MBL) remain to be elucidated, prostaglandins have been previously implicated. This study was initiated to elucidate a pattern of expression of the various components of the cyclooxygenase (COX)-prostaglandin signalling pathways present in the endometrium of women with normal and heavy MBLs.

METHODS

Endometrial biopsies were collected at different stages of the menstrual cycle from women who underwent measurement of MBL. Tissue was divided for either examination of gene expression by quantitative RT-PCR analysis or in vitro culture experimentation.

RESULTS

Analysis of gene expression demonstrated a significant elevation in expression of COX-1 and COX-2 mRNA in endometrium obtained from women with heavy MBL when compared with endometrium obtained from women with normal MBL. Tissue culture with PGE(2) stimulation caused a significantly elevated production of cyclic AMP (cAMP) by endometrium of women with heavy MBL when compared with normal MBL. Expression of phosphodiesterase 4B, an enzyme involved in cAMP breakdown, was reduced in these same endometrial samples obtained from women with heavy MBL.

CONCLUSIONS

These data identify the E series prostaglandin receptors and their signalling pathways as potential therapeutic targets in the treatment of heavy menstruation.

Relaxin family peptide receptors--former orphans reunite with their parent ligands to activate multiple signalling pathways

The relaxin family peptides, although structurally closely related to insulin, act on a group of four G protein-coupled receptors now known as Relaxin Family Peptide (RXFP) Receptors. The leucine-rich repeat containing RXFP1 and RXFP2 and the small peptide-like RXFP3 and RXFP4 are the physiological targets for relaxin, insulin-like (INSL) peptide 3, relaxin-3 and INSL5, respectively. RXFP1 and RXFP2 have at least two binding sites--a high-affinity site in the leucine-rich repeat region of the ectodomain and a lower-affinity site in an exoloop of the transmembrane region. Although they respond to peptides that are structurally similar, RXFP3 and RXFP4 demonstrate distinct binding properties with relaxin-3 being the only peptide that can recognize these receptors in addition to RXFP1. Activation of RXFP1 or RXFP2 causes increased cAMP and the initial response for both receptors is the resultant of Gs-mediated activation and G(oB)-mediated inhibition of adenylate cyclase. With RXFP1, an additional delayed increase in cAMP involves betagamma subunits released from G(i3). In contrast, RXFP3 and RXFP4 inhibit adenylate cyclase and RXFP3 causes ERK1/2 phosphorylation. Drugs acting at RXFP1 have potential for the treatment of diseases involving tissue fibrosis such as cardiac and renal failure, asthma and scleroderma and may also be useful to facilitate embryo implantation. Activators of RXFP2 may be useful to treat cryptorchidism and infertility and inhibitors have potential as contraceptives. Studies of the distribution and function of RXFP3 suggest that it is a potential target for anti-anxiety and anti-obesity drugs.

Functional defects in adenylyl cyclase signaling mechanisms of insulin and relaxin in skeletal muscles of rat with streptozotocin type 1 diabetes

Functional disturbance in the novel adenylyl cyclase signaling mechanism (ACSM) of insulin and relaxin action in rat streptozotocin (STZ) type I diabetes was studied on the basis of the authors’ conception of molecular defects in hormonal signaling systems as the main causes of endocrine diseases. Studying the functional state of molecular components of the ACSM and the mechanism as a whole, the following changes were found in the skeletal muscles of diabetic rats compared with control animals: 1) increase of insulin receptor binding due to an increase in the number of insulin binding sites with high and low affinity; 2) increase of the basal adenylyl cyclase (AC) activity and the reduction of AC-activating effect of non-hormonal agents (guanine nucleotides, sodium fluoride, forskolin); 3) reduction of ACSM response to stimulatory action of insulin and relaxin; 4) decrease of the insulin-activating effect on the key enzymes of carbohydrate metabolism, glycogen synthase and glucose-6-phosphate dehydrogenase. Hence, the functional activity of GTP-binding protein of stimulatory type, AC and their functional coupling are decreased during experimental type 1 diabetes that leads to the impairment of the transduction of insulin and relaxin signals via ACSM.

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.

Effect of relaxin on acrosome reaction and utilization of glucose in boar spermatozoa

Relaxin is a peptide hormone found in seminal plasma that has a physiological influence on sperm motility in some species. There are no reports on the effect of relaxin on acrosome reaction and utilization of glucose in boar spermatozoa. In this study, to investigate the effects of relaxin on sperm motility, acrosome reaction, and incorporation and oxidation of labeled glucose, boar spermatozoa were washed and preincubated for swim-up and then incubated (0-6 h) with 0, 20, or 40 ng/ml relaxin in mTALP medium. The results indicated that the addition of relaxin stimulated sperm motility significantly (P<0.05) during 1-4 h of incubation. The percentage of acrosome-reacted live spermatozoa was higher (P<0.05) when the spermatozoa were treated with 20 or 40 ng/ml relaxin. The rate of incorporation, and oxidation of glucose were also greater (P<0.05) in the spermatozoa incubated with relaxin compared to the control spermatozoa. The rate of incorporation and oxidation of (14)C-glucose were increased in correlation with acrosome reaction up to 4 h of incubation and then decreased in line with the increasing incubation period. In conclusion, the present study demonstrates that relaxin accelerates not only motility but also the acrosome reaction and utilization of glucose in boar spermatozoa.

Relaxin enhances the oncogenic potential of human thyroid carcinoma cells

The role of members of the insulin-like superfamily in human thyroid carcinoma is primarily unknown. Here we demonstrate the presence of RLN2 relaxin and relaxin receptor LGR7 in human papillary, follicular, and undifferentiated anaplastic thyroid carcinoma suggesting a specific involvement of relaxin-LGR7 signaling in thyroid carcinoma. Stable transfectants of the LGR7-positive human follicular thyroid carcinoma cell lines FTC-133 and FTC-238 that secrete bioactive proRLN2 revealed this hormone to act as a multifunctional endocrine factor in thyroid carcinoma cells. Although RLN2 did not act as a mitogen, it acted as an autocrine/paracrine factor and significantly increased anchorage-independent growth and thyroid carcinoma cell motility and invasiveness through elastin matrices. Suppression of LGR7 expression by LGR7-siRNA abolished the RLN2-mediated accelerated tumor cell motility. The increased elastinolytic activity correlated with enhanced production and secretion of the lysosomal proteinases cathepsin-D (cath-D) and cath-L forms hereby identified as new RLN2 target molecules in human neoplastic thyrocytes. We found the intracellular distribution of procath-L specifically altered in RLN2 transfectants, providing first evidence for selective actions of relaxin on the powerful elastinolytic cath-L production, storage, and secretion in thyroid carcinoma cells. Thus, relaxin enhances the oncogenic potential and acts as novel endocrine modulator of invasiveness in human thyroid carcinoma cells.

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

Although originally characterised as a reproductive hormone, relaxin has emerged as a multi-functional endocrine and paracrine factor that plays a number of important roles in several organs, including the normal and diseased cardiovascular system. The recent discovery of the H3/relaxin-3 gene, and the elusive receptors for relaxin (Relaxin family peptide receptor; RXFP1) and relaxin-3 (RXFP3/RXFP4) have led to the re-classification of a distinct relaxin peptide/receptor family. Additionally, the identification of relaxin and RXFP1 mRNA and/or relaxin binding sites in the heart and blood vessels has confirmed that the cardiovascular system is a target for relaxin peptides. While evidence for the production of relaxins within the cardiovascular system is limited, several studies have established that the relaxin genes are upregulated in the diseased human and rodent heart where they likely act as cardioprotective agents. The ability of relaxin to protect the heart is most likely mediated via its antifibrotic, anti-hypertrophic, anti-inflammatory and vasodilatory actions, but it may also directly stimulate myocardial regeneration and repair. This review describes relaxin and its primary receptor (RXFP1) in relation to the roles and effects of relaxin in the normal and pathological cardiovascular system. It is becoming increasingly clear that relaxin has a number of diverse physiological and pathological roles in the cardiovascular system that may have important therapeutic and clinical implications.

The R273H p53 mutation can facilitate the androgen-independent growth of LNCaP by a mechanism that involves H2 relaxin and its cognate receptor LGR7

Mutations in p53 occur at a rate of approximately 70% in hormone-refractory prostate cancer (CaP), suggesting that p53 mutations facilitate the progression of CaP to androgen-independent (AI) growth. We have previously reported that transfection of p53 gain of function mutant alleles into LNCaP, an androgen-sensitive cell line, allows for AI growth of LNCaP in vitro. We herein confirm the in vivo relevance of those findings by demonstrating that the R273H p53 mutation (p53(R273H)) facilitates AI growth in castrated nude mice. In addition, we demonstrate that H2 relaxin is responsible for facilitating p53(R273H)-mediated AI CaP. H2 relaxin is overexpressed in the LNCaP-R273H subline. Downregulation of H2 relaxin expression results in significant inhibition of AI growth, whereas addition of recombinant human H2 relaxin to parental LNCaP promotes AI growth. Inhibition of AI growth was also achieved by blocking expression of LGR7, the cognate receptor of H2 relaxin. Chromatin immunoprecipitation analysis was used to demonstrate that p53(R273H) binds directly to the relaxin promoter, further confirming a role for H2 relaxin signaling in p53(R273H)-mediated AI CaP. Lastly, we used a reporter gene assay to demonstrate that H2 relaxin can induce the expression of prostate-specific antigen via an androgen receptor-mediated pathway.

International Union of Pharmacology LVII: recommendations for the nomenclature of receptors for relaxin family peptides

Although the hormone relaxin was discovered 80 years ago, only in the past 5 years have the receptors for relaxin and three other receptors that respond to related peptides been identified with all four receptors being G-protein-coupled receptors. In this review it is suggested that the receptors for relaxin (LGR7) and those for the related peptides insulin-like peptide 3 (LGR8), relaxin-3 (GPCR135), and insulin-like peptide 5 (LGPCR142) be named the relaxin family peptide receptors 1 through 4 (RXFP1-4). RXFP1 and RXFP2 are leucine-rich repeat-containing G-protein-coupled receptors with complex binding characteristics involving both the large ectodomain and the transmembrane loops. RXFP1 activates adenylate cyclase, protein kinase A, protein kinase C, phosphatidylinositol 3-kinase, and extracellular signaling regulated kinase (Erk1/2) and also interacts with nitric oxide signaling. RXFP2 activates adenylate cyclase in recombinant systems, but physiological responses are sensitive to pertussis toxin. RXFP3 and RXFP4 resemble more conventional peptide liganded receptors and both inhibit adenylate cyclase, and in addition RXFP3 activates Erk1/2 signaling. Physiological studies and examination of the phenotypes of transgenic mice have established that relaxin has roles as a reproductive hormone involved in uterine relaxation (some species), reproductive tissue growth, and collagen remodeling but also in the cardiovascular and renal systems and in the brain. The connective tissue remodeling properties of relaxin acting at RXFP1 receptors have potential for the development of agents effective for the treatment of cardiac and renal fibrosis, asthma, and scleroderma and for orthodontic remodelling. Agents acting at RXFP2 receptors may be useful for the treatment of cryptorchidism and infertility, whereas antagonists may be used as contraceptives. The brain distribution of RXFP3 receptors suggests that actions at these receptors have the potential for the development of antianxiety and antiobesity drugs.

Relaxin family peptide receptors RXFP1 and RXFP2 modulate cAMP signaling by distinct mechanisms

Two orphan leucine-rich repeat-containing G protein-coupled receptors were recently identified as targets for the relaxin family peptides relaxin and insulin-like peptide (INSL) 3. Human gene 2 relaxin is the cognate ligand for relaxin family peptide receptor (RXFP) 1, whereas INSL3 is the ligand for RXFP2. Constitutively active mutants of both receptors when expressed in human embryonic kidney (HEK) 293T cells signal through Galphas to increase cAMP. However, recent studies using cells that endogenously express the receptors revealed greater complexity: cAMP accumulation after activation of RXFP1 involves a time-dependent biphasic pathway with a delayed phase involving phosphoinositide 3-kinase (PI3K) and protein kinase C (PKC) zeta, whereas the RXFP2 response involves inhibition of adenylate cyclase via pertussis toxin-sensitive G proteins. The aim of this study was to compare and contrast the cAMP signaling pathways used by these two related receptors. In HEK293T cells stably transfected with RXFP1, preliminary studies confirmed the biphasic cAMP response, with an initial Galphas component and a delayed response involving PI3K and PKCzeta. This delayed pathway was dependent upon G-betagamma subunits derived from Galphai3. An additional inhibitory pathway involving GalphaoB affecting cAMP accumulation was also identified. In HEK293T cells stably transfected with RXFP2, the cAMP response involved Galphas and was modulated by inhibition mediated by GalphaoB and release of inhibitory G-betagamma subunits. Thus, initially both RXFP1 and RXFP2 couple to Galphas and an inhibitory GalphaoB pathway. Differences in cAMP accumulation stem from the ability of RXFP1 to recruit coupling to Galphai3, release G-betagamma subunits and thus activate a delayed PI3K-PKCzeta pathway to further increase cAMP accumulation.

A Novel, Adenylate Cyclase, Signaling Mechanism of Relaxin H2 Action

For the first time, the adenylate cyclase signaling mechanism (ACSM) of the action of relaxin H2 was revealed and deciphered in human and rat muscle tissues. The comparative study of signaling blocks forming the ACSM of relaxin and insulin (discovered earlier) showed that the postreceptor signaling chain of relaxin coincides with that of insulin. However, the type of relaxin receptor involved in ACSM remains obscure. Currently, the ACSM of relaxin may be represented as a signaling cascade: receptor --> Gi protein (betagamma-dimer) --> phosphatidylinositol 3-kinase (PI3K) --> protein kinase Czeta (PKCzeta) --> Gs protein --> adenylate cyclase.

Receptors for Relaxin Family Peptides

Recent studies have identified four receptors that are the physiological targets for relaxin family peptides. All are class I (rhodopsin like) G-protein-coupled receptors with LGR7 (RXFP1) and LGR8 (RXFP2) being type C leucine-rich repeat-containing receptors, whereas GPCR135 (RXFP3) and GPCR142 (RXFP4) resemble receptors that respond to small peptides such as somatostatin and angiotensin II. The cognate ligands for the receptors have been identified: relaxin for RXFP1; INSL3 for RXFP2; relaxin 3 for RXFP3 and INSL5 for RXFP4. RXFP1 and RXFP2 receptors produce increases in intracellular cAMP levels upon stimulation, although the response is complex and contains a component sensitive to PI-3-kinase inhibitors. There is also evidence that RXFP1 can activate Erk1/2 and nitric oxide synthase, and relaxin has been reported to enter cells and activate glucocorticoid receptors. In contrast, RXFP3 and RXFP4 couple to Gi by a pertussis toxin-sensitive mechanism to cause inhibition of cAMP production. Now that the receptors for relaxin family peptides and their cognate ligands have been identified, we suggest a nomenclature for both the peptides and the receptors that we hope will be helpful to researchers in this rapidly advancing field.

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: antifibrotic properties and effects in models of disease

Fibrosis (progressive scarring) is a leading cause of organ failure worldwide and causes loss of organ function when normal tissue is replaced with excess connective tissue. Several organs are prone to this process regardless of etiology. The pleiotropic hormone, relaxin, is emerging as a novel antifibrotic therapy. Relaxin has been shown to limit collagen production and reorganization, while stimulating increased collagen degradation. It not only prevents fibrogenesis, but also reduces established scarring. This review summarizes (1) the levels at which relaxin inhibits collagen production and existing collagen overexpression in induced models of fibrosis, and (2) the collagen-related phenotypes of relaxin- and LGR7-deficient mice. Recent studies on relaxin-deficient mice have established relaxin as an important, naturally occurring regulator of collagen turnover and provide new insights into the therapeutic potential of relaxin.

Demonstration of upregulated H2 relaxin mRNA expression during neuroendocrine differentiation of LNCaP prostate cancer cells and production of biologically active mammalian recombinant 6 histidine-tagged H2 relaxin

Relaxin was recently implicated as a regulator of breast and prostate cancer progression. We characterized upregulated H2 relaxin gene expression during neuroendocrine differentiation of the human prostate cancer model, LNCaP. To examine the impact of relaxin on host cells associated with prostatic adenocarcinomas, we generated recombinant 6 His-tagged relaxin (RLXH) in a mammalian expression system. This immunoreactive and biologically active relaxin preparation was used to screen a variety of cell types for cAMP responsiveness. Of the cell types screened, none was more responsive to RLXH than the well-characterized monocyte/macrophage cell line THP-1.

Relaxin stimulates multiple signaling pathways: activation of cAMP, PI3K, and PKCzeta in THP-1 cells

Relaxin has been shown previously to stimulate cyclic AMP production and the activation of MAPK. We reported that phosphoinositide-3 kinase (PI3K) activity is required for biphasic stimulation of cAMP by relaxin and that relaxin treatment increased PI3K activity in THP-1 cells. A downstream target of PI3K is protein kinase C zeta (PKCzeta). Relaxin stimulated translocation of PKCzeta to the plasma membrane in THP-1, MCF-7, pregnant human myometrial (PHM1-31), and mouse mesangial (MMC) cells. PKCzeta translocation is PI3K dependent and independent of cAMP production. Pharmacological and antisense approaches, utilized to inhibit or knock down PKCzeta, resulted in a 40% inhibition of relaxin-stimulated cAMP production. The stimulation of PKCzeta by relaxin therefore is downstream of PI3K leading to increased cAMP production. To determine the role of PI3K/PKCzeta stimulation by relaxin on downstream-mediated events, we examined the increase in vascular endothelial growth factor (VEGF) gene expression by relaxin. Treatment of THP-1 or MMC cells with the PI3K inhibitor, LY294002, abolished the relaxin-mediated stimulation of VEGF transcript levels. In summary, relaxin has pleiotropic signaling effects in THP-1 cells activating ERK1/2, cAMP, PI3K, and PKCzeta. We have described a novel bifurcated pathway by which relaxin stimulates Gs alpha and PI3K/PKCzeta leading to increased cAMP production and increased VEGF gene expression. Some, but not all, of these pathways are detected in other cell lines which may cause the unique diversity of downstream responses from this interesting hormone.

Relaxin stimulates cAMP production in MCF-7 cells upon overexpression of type V adenylyl cyclase

Relaxin stimulates cAMP production and activation of ERK and PI3K in THP-1 cells. Relaxin also stimulates protein kinase C zeta (PKCzeta) translocation to the plasma membrane in a PI3K-dependent manner in THP-1 and MCF-7 cells. However, relaxin did not increase cAMP production in MCF-7 cells. We overexpressed different adenylyl cyclase (AC) isoforms in MCF-7 cells to examine coupling of endogenous relaxin receptors to cAMP production. Overexpression of types II and IV AC had no effect on cAMP production by relaxin. However, overexpression of type V AC, which is activated by PKCzeta, showed synergistic stimulation of cAMP by relaxin and forskolin.

Insulin-relaxin family peptide signaling and receptors in mouse brain membranes and neuronal cells

Several orphan G-protein-coupled receptors (GPCRs), LGR7 and LGR8, GPCR135 and GPCR142, were recently identified as putative, native receptors for different relaxin-family peptides, and their cell signaling mechanisms were elucidated in stably transfected cell lines. Anatomic studies have demonstrated that discrete populations of neurons in rat brain express relaxin and relaxin-3 mRNA/peptide, relaxin and relaxin-3 binding sites, and LGR7 and GPCR135 mRNAs. Thus, we began to assess the ability of relaxin-family peptides to alter cAMP production in brain and the involvement of the different native receptors. In mouse cortical membranes, a fixed concentration of relaxin peptides (100 nM) inhibited forskolin-induced cAMP production, but further studies in normal and receptor knockout mouse strains are required to assess the specificity of these effects. In addition, whole-cell signaling mechanisms are being investigated in a mouse hypothalamic cell line, GT1-7. Such studies will help to establish the actions of relaxin-family peptides via their different GPCRs in different brain pathways.

New insight into the transcriptional regulation of vascular endothelial growth factor expression in the endometrium by estrogen and relaxin

Increased uterine capillary permeability, which can be induced by both estrogen and relaxin, is required for endometrial growth and implantation. This effect is mediated in both cases by estrogen receptors (ERs), via stimulation of vascular endothelial growth factor (VEGF) expression. The sites on the VEGF promoter through which induction occurs, however, are completely unclear. We have used the technique of chromatin immunoprecipitation in vivo to localize the site of ER action and identify other transcription factors that are involved. We have found that ERa associates with Sp1/Sp3 at a GC-rich region of the promoter. More interesting, however, is the observation that estrogen also induces rapid, transient binding of hypoxia-inducible factor 1 (HIF-1), which mediates VEGF transcription in response to hypoxia, to the promoter. The estrogen-induced HIF-1 binding closely matches the estrogen-induced pattern of VEGF expression in the uterus, suggesting that HIF-1 is involved in that induction, and probably that of many other genes as well (HIF-1 is now known to regulate the expression of more than 40 genes). It is likely that studies now under way will also link relaxin-induced VEGF expression to HIF-1. This is based on the similarities in the effects of the two hormones on VEGF expression and on their shared ability to activate the PI3K and MAPK pathways, both of which can activate HIF-1.

Relaxin receptor activation in the basolateral amygdala impairs memory consolidation

The peptide-hormone relaxin has well-established actions in male and female reproductive tracts, and has functional effects in circumventricular regions of brain involved in neurohormonal secretion. In the current study, we initially mapped the distribution of mRNA encoding the relaxin receptor--leucine-rich repeat-containing G-protein-coupled receptor 7 (LGR7)- and [33P]-human relaxin-binding sites in extra-hypothalamic sites of male Sprague-Dawley rats. The basolateral amygdala (BLA) expressed high levels of LGR7 mRNA and relaxin-binding sites and, although relaxin peptide was not detected in the BLA, several brain regions that send projections to the BLA were found to contain relaxin-expressing neurons. As it is well established that the BLA is involved in regulating the consolidation of memory for emotionally arousing experiences, we investigated whether activation of LGR7 in the BLA modulated memory consolidation for aversively motivated inhibitory avoidance training. Bilateral infusions of human relaxin (10-200 ng in 0.2 microL) into the BLA immediately after inhibitory avoidance training impaired 48-h retention performance in a dose-dependent manner. Delayed infusions of relaxin into the BLA 3 h after training were ineffective, indicating that the retention impairment was due to influences on memory consolidation. Post-training infusions of relaxin into the adjacent central amygdala, which is devoid of LGR7, did not impair retention. These findings suggest a novel function for endogenous relaxin-LGR7 signalling in rat brain involving regulation of memory consolidation.