Relaxin has intrigued and confounded researchers since its discovery in 1926. While first characterized as a hormone of pregnancy, and believed to be of interest only in the female, we now know that relaxin is a multi functional hormone with non-reproductive actions in several systems. Relaxin is structurally similar to insulin and thus a part of the insulin superfamily. In total, seven relaxin-like peptides have been identified; relaxin-1, relaxin-2, relaxin-3 and the insulin- like (INSL) peptides INSL3, INSL4, INSL5 and INSL6. Four relaxin-like peptide family receptors have been identified to date, which are GPCRs and unlike the tyrosine kinase insulin receptor. Pharmacological and functional data show that LGR7 is the relaxin receptor and LGR8 the INSL3 receptor. However, complex systems of multiple interactions between the relaxin-like peptides and their receptors are being unraveled. Pharmacological data also show relaxin-3 to be a high affinity agonist for LGR7, as well as GPCR135 and GPCR142, while GPCR142 has a second ligand in INSL5. Surprisingly, LGR7 and LGR8 have invertebrate homologs, are only distantly related to GPCR135 and GPCR142, and have markedly different ectodomains. Structure- function studies are beginning to identify the peptide determinants of the interactions between the relaxin-like peptides and their receptors. Mechanisms for ligand binding and activation are unlikely to be similar between the different receptor types. The relaxin-like peptides have numerous clinical applications, and developing these to their full potential will require a complete understanding of the interplay between the relaxin-like peptides and their receptors.
Keywords: insulin-like peptide, cancer, insulin superfamily, relaxin genes, porcine testis-specific transcript, epil peptides, somatostatin, gpcr superfamily
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