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.