Identification of Relaxin-3/INSL7 as a Ligand for GPCR142

Abstract

We have recently identified the insulin-like peptide relaxin-3 (aka INSL7) as the endogenous ligand for an orphan G-protein-coupled receptor, GPCR135 (aka somatostatin-and angiotensin-like peptide receptor). Analysis of possible receptors related to GPCR135 revealed a single orphan receptor, GPCR142. Thus, we tested whether GPCR142 could also respond to relaxin-3 or related insulin-like molecules. Surprisingly, GPCR142 was activated by nanomolar concentrations of relaxin-3 but was completely unresponsive to all other known insulin-like peptides. We evaluated by reverse transcriptase-PCR the expression of GPCR142 mRNA in a variety of human tissues and found expression in brain, kidney, testis, thymus, placenta, prostate, salivary gland, thyroid, and colon. In an analysis of other species, we were able to find a full-length mouse homolog of GPCR142, but were unable to detect any complete GPCR142 transcripts in rat. With respect to intracellular signaling, GPCR142 is similar to GPCR135 in that it potently inhibits adenylate cyclase and stimulates 35 S-GTP␥S incorporation in response to relaxin-3. However, whereas GPCR135 signaling could be converted to calcium mobilization using a G qi5 or G␣ 16 G-proteins, GPCR142 was only capable of functioning in the presence of G␣ 16 . In the accompanying article (Liu, C., Eriste, E., Sutton, S., Chen, J., Roland, B., Kuei, C., Farmer, N., Jö rnvall, H., Sillard, R., and Lovenberg, T. W. (2003) J. Biol. Chem. 278, 50754 -50764), we present the case that relaxin-3, which has previously been shown to bind to the relaxin receptor LGR7, is most likely the endogenous ligand for GPCR135. In this report, we show an additional receptor, GPCR142, which is also selectively activated by relaxin-3. However, the anatomical localization of GPCR142 suggests that GPCR142 may have different physiological functions.Relaxin-3 (1) is a member of the insulin superfamily, where each member consists of two peptide subunits arranged by three disulfide bridges. Recently, two leucine-rich repeat-containing G-protein-coupled receptors (LGRs), 1 LGR7 and LGR8 (2, 3), have been identified as the receptors for relaxin (3-7).LGR7 and LGR8 belong to the type III hormone receptor family (follicle stimulating hormone (FSH), luteinizing hormone (LH), and thyroid stimulating hormone (TSH) etc. that stimulate adenylate cyclase). Relaxin-3 was recently demonstrated to be an additional ligand for the relaxin receptor LGR7, but not LGR8 (8). Instead, the ligand INSL3 (9), another member of the insulin/relaxin family of peptides, has been shown to be an additional ligand for LGR8 (10).As part of a directed effort to identify ligands for orphan G-protein-coupled receptors (GPCRs), we established a systematic program to create tissue extracts as a source of possible GPCR ligands. In the accompanying article (23), we report the purification and identification of the peptide ligand, relaxin-3, as an endogenous ligand for orphan receptor GPCR135, aka SALPR (somatostatin-and angiotensin-like peptide receptor).In that report...

Relaxin receptors in hepatic stellate cells and cirrhotic liver

“…Two additional receptors for relaxin-3 have been identified, known as GPCR135 (RXFP3) and GPCR142 (RXFP4) [27,28]. Neither of these receptors is activated by relaxin or InsL3.…”

The A-chain of Human Relaxin Family Peptides Has Distinct Roles in the Binding and Activation of the Different Relaxin Family Peptide Receptors

“…The determinants for H3 relaxin activity on RXFP3 and RXFP4 are probably located in the B-chain alone, since synthetic S-reduced H3 relaxin B-chain is an RXFP3 and RXFP4 agonist (12,13). It has recently been demonstrated that key residues in the H3 relaxin B-chain are responsible for both binding affinity and cAMP-inhibitory activity (14).…”

Bradykinin specificity and signaling at GPR100 and B2 kinin receptors

“…Thus, GPR100 could potentially be a kinin receptor subtype and BK a GPR100 ligand. However, this theory was questioned when Liu et al (2003) almost simultaneously proposed that insulin-like peptide relaxin 3 is a ligand for GPR100 since it binds to and stimulates GTPgS binding and inhibits AC through this receptor at nanomolar concentrations. Consequently, this led to the question: Is GPR100 a BK or relaxin receptor or both?…”