Relaxin and estrogen synergistically accelerate growth and development in the uterine cervix of prepubertal pigs

The relaxin family peptide receptor 1 (RXFP1): An emerging player in human health and disease

Background

Relaxin/relaxin family peptide receptor 1 (RXFP1) signaling is important for both normal physiology and disease. Strong preclinical evidence supports relaxin as a potent antifibrotic molecule. However, relaxin‐based therapy failed in clinical trial in patients with systemic sclerosis. We and others have discovered that aberrant expression of RXFP1 may contribute to the abnormal relaxin/RXFP1 signaling in different diseases. Reduced RXFP1 expression and alternative splicing transcripts with potential functional consequences have been observed in fibrotic tissues. A relative decrease in RXFP1 expression in fibrotic tissues—specifically lung and skin—may explain a potential insensitivity to relaxin. In addition, receptor dimerization also plays important roles in relaxin/RXFP1 signaling.

Methods

This review describes the tissue specific expression, characteristics of the splicing variants, and homo/heterodimerization of RXFP1 in both normal physiological function and human diseases. We discuss the potential implications of these molecular features for developing therapeutics to restore relaxin/RXFP1 signaling and to harness relaxin's potential antifibrotic effects.

Results

Relaxin/RXFP1 signaling is important in both normal physiology and in human diseases. Reduced expression of RXFP1 in fibrotic lung and skin tissues surrenders both relaxin/RXFP1 signaling and their responsiveness to exogenous relaxin treatments. Alternative splicing and receptor dimerization are also important in regulating relaxin/RXFP1 signaling.

Conclusions

Understanding the molecular mechanisms that drive aberrant expression of RXFP1 in disease and the functional roles of alternative splicing and receptor dimerization will provide insight into therapeutic targets that may restore the relaxin responsiveness of fibrotic tissues.

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.

The effect of estradiol on COX-2, EP2, and EP4 mRNA expression and the extracellular matrix in the cervix of the hypogonadotrophic, ovariectomized ewe

There is a degree of cervical relaxation in the ewe at estrus that is regulated by changes in prostaglandin synthesis, prostaglandin receptor expression, and changes in the cervical extracellular matrix. It is likely that these are regulated by changes in periovulatory hormones, particularly estradiol. This study determined the effect of estradiol benzoate on the mRNA expression of cyclooxygenase-2 (COX-2) and the prostaglandin E receptors EP(2) and EP(4), the concentration of cervical hyaluronan, and the proportion of smooth muscle and collagen in the cervix of the hypogonadotrophic ovariectomized ewe (Ovis aries). Ovariectomized hypogonadotrophic ewes were given 100 microg estradiol benzoate, and their cervices were collected 0, 24, and 48 h thereafter to determine the expression of cervical COX-2, EP(2), and EP(4) mRNA by in situ hybridization, the concentration of hyaluronan by ELISA, and the proportion of smooth muscle and collagen by Masson's trichrome staining. Estradiol benzoate increased the mRNA expression of COX-2 and EP(4) within 24h after treatment (P<0.05), whereas EP(2) mRNA, hyaluronan, and the ratio of smooth muscle to collagen did not change within 48 h after treatment. The COX-2, EP(2), and EP(4) mRNA expression were greatest in the smooth muscle layers (P<0.05) and least in the luminal epithelium (P<0.05). In conclusion, we inferred that estradiol regulates cervical COX-2 and EP(4) mRNA expression and may regulate cervical relaxation via the synthesis of prostaglandin E(2) and activation of the PGE(2) receptors EP(2) and EP(4).

Characterization of Novel Splice Variants of LGR7 and LGR8 Reveals That Receptor Signaling Is Mediated by Their Unique Low Density Lipoprotein Class A Modules

The relaxin and insulin-like peptide 3 receptors, LGR7 and LGR8, respectively, are unique members of the leucine-rich repeat-containing G-protein-coupled receptor (LGR) family, because they possess an N-terminal motif with homology to the low density lipoprotein class A (LDLa) modules. By characterizing several LGR7 and LGR8 splice variants, we have revealed that the LDLa module directs ligand-activated cAMP signaling. The LGR8-short variant encodes an LGR8 receptor lacking the LDLa module, whereas LGR7-truncate, LGR7-truncate-2, and LGR7-truncate-3 all encode truncated secreted proteins retaining the LGR7 LDLa module. LGR8-short and an engineered LGR7 variant missing its LDLa module, LGR7-short, bound to their respective ligands with high affinity but lost their ability to signal via stimulation of intracellular cAMP accumulation. Conversely, secreted LGR7-truncate protein with the LDLa module was able to block relaxin-induced LGR7 cAMP signaling and did so without compromising the ability of LGR7 to bind to relaxin or be expressed on the cell membrane. Although the LDLa module of LGR7 was N-glycosylated at position Asn-14, an LGR7 N14Q mutant retained relaxin binding affinity and cAMP signaling, implying that glycosylation is not essential for optimal LDLa function. Using real-time PCR, the expression of mouse LGR7-truncate was detected to be high in, and specific to, the uterus of pregnant mice. The differential expression and evolutionary conservation of LGR7-truncate further suggests that it may also play an important role in vivo. This study highlights the essential role of the LDLa module in LGR7 and LGR8 function and introduces a novel model of GPCR regulation.

Relaxin's physiological roles and other diverse actions

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

Regional differences in water content, collagen content, and collagen degradation in the cervix of nonpregnant cows

The cow could be a suitable model for studies concerning functional changes of the cervix. However, as in many species, the bovine cervix becomes softer in texture during the follicular phase of the estrous cycle compared to the luteal phase. In the present study, we explored if changes in the collagen network take place that could be responsible for this phenomenon and if regional differences in water content, collagen content, and collagen degradation along the cross-sectional and longitudinal axes of the cervix were present. Two groups of nonpregnant animals with different progesterone status were studied. One group (n = 11) was under high progesterone influence, and the other group (n = 12) was under low progesterone influence. The water content was derived from the weight of the samples before and after lyophilization. The collagen content and the ratio of collagenous to noncollagenous proteins (hydroxyproline:proline ratio) were determined by performing amino acid analysis on hydrolyzed samples using high-performance liquid chromatography. Collagen denaturation was quantified with a colorimetric assay by determining the amount of hydroxyproline released from samples treated with alpha-chymotrypsine. The water content of the superficial layer of the submucosa was always significantly (P < 0.01) higher than the water content of the deep layer in the vaginal, mid, and uterine segments, but this was unrelated to the progesterone status of the animals. No effect of the tissue layers or of the progesterone status of the animals on the collagen content was observed, but an effect of segment was noted. The collagen content (mug/mg dry wt) in the vaginal segment of the cervix was significantly higher than in the mid (P < 0.05) and the uterine (P < 0.01) segments. The hydroxyproline:proline ratio showed the same pattern as the collagen content. The percentage of collagen denaturation in the superficial layer was always significantly (P < 0.01) higher than that in the deep layer, but no effect of the progesterone status or of the segment along the longitudinal axis was seen. It is concluded that regional differences in collagen biochemistry are present in the cervix of nonpregnant cows, which may account for the difference in firmness of different parts along the circular or the longitudinal axis of the cervix. However, differences in texture of the cervix between the two groups of cows could not be explained by differences in the collagen content, percentage of collagen denaturation, or water content.

Relaxin increases secretion of matrix metalloproteinase-2 and matrix metalloproteinase-9 during uterine and cervical growth and remodeling in the pig

Matrix metalloproteinases are proteolytic enzymes that degrade the extracellular matrix and are essential for tissue remodeling. Uterine and cervical growth require remodeling of structural barriers to cell invasion and matrix metalloproteinase-2 and -9 degrade type IV collagen, the major component of basement membranes. Relaxin stimulates uterine and cervical growth and remodeling, which includes remodeling of support elements such as basement membranes. The objective of this study was to determine whether relaxin alters the production and/or activity of matrix metalloproteinase-2 and -9 in the uterus or cervix of the pig. The growth-promoting effects of relaxin were elicited by administering relaxin to prepubertal gilts every 6 h for 54 h. The expression of matrix metalloproteinase-2 and matrix metalloproteinase-9 was characterized by gel zymography, and proteins were quantified by immunoblotting. Total enzyme activity was measured using matrix metalloproteinase-specific fluorescent substrate assays. In both uterine and cervical tissues, immunoreactive matrix metalloproteinase-2 and matrix metalloproteinase-9 protein expression was similar in relaxin-treated and control animals. However, tissue-associated gelatinase activity was attenuated by relaxin (P < 0.05). In contrast, relaxin significantly increased the secretion of active matrix metalloproteinase-2 and -9 protein into uterine fluid (P < 0.05). Given the importance of matrix metalloproteinases in extracellular matrix degradation, the observation that relaxin promotes uterine secretion of matrix metalloproteinase-2 and -9 supports the concept that relaxin facilitates the growth and remodeling of reproductive tissues by increasing extracellular proteolysis in the pig reproductive tract.

Ruminant relaxin in the pregnant one-humped camel (Camelus dromedarius)

We have determined the cDNA sequence of preprorelaxin in the pregnant one-humped camel by employing reverse transcription- and rapid amplification of cDNA ends-polymerase chain reaction. Camel preprorelaxin consisted of 600 base pairs (bp) encoding a protein of 199 amino acids (aa) with a signal peptide of 25 aa (75 bp), a B domain of 28 aa (84 bp), a C domain of 121 aa (366 bp), and an A domain of 24 aa (72 bp). The N terminus of the C domain of camel prorelaxin contained the unique proline-rich repetitive sequence (-RPAP)(3)-(-K/RPAL-)(2), and within the B domain the classical -GRELVR- receptor binding motif was found. Camel preprorelaxin showed highest homology with porcine (74.6%) and equine (65.4%) relaxin. The ovary and the uteroplacental unit were a dual source of relaxin in the pregnant dromedary. Within the ovary, weak expression of relaxin was detected in large luteal cells of the mature corpus luteum. In the ovarian follicles, immunoreactive relaxin, but not relaxin mRNA, was detected in the granulosa and theca interna cell layer. Beginning at around Day 93 of gestation and coinciding with increasing interdigitation of the fetal villus with the underlying maternal endometrium, uterine luminal epithelial cells in the uteroplacental tissue expressed relaxin. Weak expression of immunoreactive relaxin, but not relaxin mRNA, was observed in villous trophoblast cells. Pseudostratified trophoblast cells at the base of the placental villi and multinucleate giant cells did not express relaxin.

Antiporcine relaxin (antipRLX540) treatment decreases relaxin plasma concentration and disrupts delivery in late pregnant pigs

Antibody against porcine relaxin (antipRLX540; 1:950,000) was produced in sheep and used to determine the effect on relaxin and progesterone secretion, and on parturition in late pregnant pigs. In group 1, Yorkshire gilts with normal estrous cycles were bred on the second observed estrus and fitted with an indwelling jugular cannula and an intraperitoneal cannula on day 100 of pregnancy. Gilts were infused at 6-h intervals with antipRLX540 (n = 10) or PBS (n = 10) beginning on day 103 until parturition. From days 103 to 120, daily blood samples (10 ml) were collected for RIA of relaxin, progesterone, and prolactin. In group 2, bred gilts were randomly assigned to antipRLX540 (n = 11), relaxin (n = 5), and PBS (n = 8) treatment on days 111, 113, and 115. Blood was collected twice daily from day 108 to 120, and every 20 min on days 111, 113, and 115 beginning 60 min before treatment and continuing 180 min. Parturition in gilts given antipRLX540 occurred on day 112.7 compared with day 114.0 in relaxin-treated gilts and day 114.3 in PBS controls (P < 0.05). Duration of delivery from first to last piglet was greatly delayed in antipRLX540 gilts (240 min) compared with PBS controls ([117 min] P < 0.005). Average number of stillborns was greater in antipRLX540- than in PBS-treated controls (2.4 vs. 1.0; P < 0.05). Relaxin concentration in peripheral plasma was lower in antipRLX540-treated gilts from day 105 to 110, but on day 113 the antipRLX540-treated group had a greater relaxin peak release compared with PBS-treated animals (P < 0.05). Plasma progesterone concentrations were similar in antipRLX540- and PBS-treated gilts throughout the period of the study. In group 2, by day 113, progesterone decreased in antipRLX540-treated gilts compared with relaxin- and PBS-treated gilts. Prolactin levels were similar in both antipRLX540- and PBS-treated gilts; however, from 1 to 3 days postpartum the antipRLX540 group had higher prolactin concentration (P < 0.05). The results indicate that antipRLX540 decreased circulating plasma concentrations of unbound or free relaxin during the last 10 days of pregnancy in Yorkshire gilts. AntipRLX540 markedly increased both the duration of delivery of piglets and the average number of stillbirths in this litter-bearing species compared with PBS-treated controls. This study provides strong evidence that increasing circulating concentrations of relaxin during late pregnancy is crucial for unimpaired parturition in the pig.