A study on the functional interaction between the GH/PRL family of polypeptides with their receptors in zebrafish: Evidence against GHR1 being the receptor for somatolactin

The growth hormone (GH)/prolactin (PRL) family of polypeptide hormones plays important roles in many aspects of vertebrate physiology. In fish, there is an additional member in this family called somatolactin (SL). Specifically, zebrafish contains five ligands (GH, SLα, SLβ, PRL1 and PRL2) and four cognate receptors including two GH receptors (GHR1 and GHR2) and two PRL receptors (PRLR1 and PRLR2). There is much controversy regarding whether one of the two GHRs in teleosts is in fact the receptor of SL. A multitude of different assay methods were employed to study the functional interaction among these ligands and their receptors in zebrafish. These include assessment of the binding between the ligands and the extracellular domains of the receptors using His-tag pulldown assays, activation of receptor-evoked promoter activities by treatment of the receptor-transfected cells with the recombinant hormones, and phosphorylation of post-receptor signaling factors by treatment of receptor-transfected cells with the recombinant hormones. The results showed that the zebrafish GH can only interact with the GHRs and the zebrafish PRLs can only interact with the PRLRs. The zebrafish SLs, found to be biologically active in another assay, were found to be ineffective in interacting with the zebrafish GHRs and PRLRs. Our data argue against the hypothesis that GHR1 is the SL receptor.