We have identified a novel human relaxin gene, designated H3 relaxin, and an equivalent relaxin gene in the mouse from the Celera Genomics data base. Both genes encode a putative prohormone sequence incorporating the classic two-chain, three cysteine-bonded structure of the relaxin/insulin family and, importantly, contain the RXXXRXX(I/V) motif in the B-chain that is essential for relaxin receptor binding. A peptide derived from the likely proteolytic processing of the H3 relaxin prohormone sequence was synthesized and found to possess relaxin activity in bioassays utilizing the human monocytic cell line, THP-1, that expresses the relaxin receptor. The expression of this novel relaxin gene was studied in mouse tissues using RT-PCR, where transcripts were identified with a pattern of expression distinct from that of the previously characterized mouse relaxin. The highest levels of expression were found in the brain, whereas significant expression was also observed in the spleen, thymus, lung, and ovary. Northern blotting demonstrated an ϳ1.2-kb transcript present in mouse brain poly(A) RNA but not in other tissues. These data, together with the localization of transcripts in the pars ventromedialis of the dorsal tegmental nucleus of C57BLK6J mouse brain by in situ hybridization histochemistry, suggest a new role for relaxin in neuropeptide signaling processes. Together, these studies describe a third member of the human relaxin family and its equivalent in the mouse.Relaxin is a 6-kDa polypeptide hormone that is secreted by the ovary into the peripheral circulation in highest amounts during pregnancy and has a number of functions in mammals that are generally associated with female reproductive tract physiology (1). To date, only one relaxin gene has been characterized in most mammalian species, with the exception of the human where two separate genes have been described, designated H1 (2) and H2 (3) relaxin. The peptide encoded by the H2 gene is the major stored and circulating form in the human (4). H1 relaxin expression is restricted to the decidua, placenta, and prostate (5); however, the H1 peptide has similar biological activity to that of H2 relaxin in a rat atrial bioassay (6). The actions of relaxin include an ability to inhibit myometrial contractions, to stimulate remodeling of the connective tissue, and to induce softening of the tissues of the birth canal. Additionally, relaxin increases growth and differentiation of the mammary gland and nipple and induces the breakdown of collagen, one of the main components of connective tissue. Relaxin decreases collagen synthesis and increases the release of collagenases (7). These findings were recently confirmed by the establishment of the relaxin gene-knockout mouse (8), which exhibited a number of phenotypic properties associated with pregnancy. Female mice lacking a functionally active relaxin gene failed to relax and elongate the interpubic ligament of the pubic symphysis and could not suckle their pups, who in turn died within 24 h unless cross-fostered t...

Relaxin family peptide 1 (RXFP1) receptor (LGR7) and RXFP2 receptor (LGR8) were recently identified as the receptor targets for H2 relaxin and insulin-like peptide 3 (INSL3), respectively. In this study, we define the pharmacology of these two receptors by using a number of receptor chimeras and relaxin family peptides. We have identified two binding sites on these receptors: one primary, high-affinity site within the ectodomain and a secondary, lower affinity site within the transmembrane region. The primary site was found to dictate receptor binding characteristics, although the lower affinity site also exerts some influence and modulates ligand affinity for the primary site in a manner dependent upon the peptide in question. Not all relaxin peptides were able to bind to the RXFP2 receptor, indicating that the relaxin-RXFP2 receptor interaction is species-specific. INSL3 was found to exhibit characteristics of a partial agonist at the RXFP2 and chimeric RXFP1/2 receptors, with low maximal cAMP responses but high potency in coupling to this pathway. cAMP accumulation studies also revealed that the binding sites couple to cAMP signaling pathways with differing efficiency: the high-affinity site signals with high efficiency, whereas the lower affinity site signals with little to no efficiency. Comparisons between RXFP1, RXFP2, the chimeric receptors, and the truncated receptors revealed that the interaction between receptor sites is critical for optimal ligand binding and signal transduction and that the ectodomain is essential for signaling. Evidence obtained in this study supports a two-stage binding model of receptor activation: binding to the primary site allows a conformational change and interaction with the low-affinity transmembrane site.Relaxin is a two-chain peptide that was discovered after the observation that serum from pregnant guinea pigs caused relaxation of the pubic ligament (Hisaw, 1926). It is structurally closely related to insulin: both peptides have an A and B chain joined by two interchain disulfide bonds, and one intra-A-chain disulfide bond. This discovery established the concept of the insulin-relaxin superfamily (Schwabe and McDonald, 1977). The high degree of similarity between insulin and relaxin precipitated a search for additional members of the insulin-relaxin superfamily with the same structural motif. To date, this peptide family includes insulin-like growth factor-1 and insulin-like growth factor-II (Humbel, 1990), INSL3 (Adham et al., 1993), INSL4 (Koman et al., 1996, INSL5 (Conklin et al., 1999), and INSL6 (Lok et al., 2000).Relaxin itself has many paralogs: in humans, three nonallelic genes produce H1 relaxin (Hudson et al., 1983), H2 relaxin (Hudson et al., 1984, and the recently identified H3 relaxin . H2 relaxin is the major circulating form of relaxin in the human and has equivalent orthologs in other species, including porcine relaxin (Hudson et al., 1981;Haley et al., 1982), rhesus monkey relaxin (Crawford et al., 1989), and rat relaxin (Hudson et al., 1981)...

Relaxin (RLX) is a peptide hormone with known antifibrotic properties. However, its significance in the lung and its role as a therapeutic agent against diseases characterized by pulmonary fibrosis are yet to be established. In this study, we examined age-related structural and functional changes in the lung of relaxin-deficient mice. Lung tissues of male and female RLX knockout (-/-) and RLX wild-type (+/+) mice at various ages were analyzed for changes in collagen expression and content. We demonstrate an age-related progression of lung fibrosis in RLX -/- mice with significantly increased tissue wet weight, collagen content and concentration, alveolar congestion, and bronchiole epithelium thickening. The increased fibrosis was associated with significantly altered peak expiratory flow and lung recoil (lung function) in RLX -/- mice. Treatment of RLX -/- mice with relaxin in early and developed stages of fibrosis resulted in the reversal of collagen deposition. Organ bath studies showed that precontracted lung strips relaxed in the presence of relaxin. Together, these data indicate that relaxin may provide a means to regulate excessive collagen deposition in diseased states characterized by pulmonary fibrosis.

This study supports the concept that relaxin may provide a means to regulate excessive collagen deposition during kidney development and in diseased states characterized by renal fibrosis.