Specific roles of Gi protein family members revealed by dissecting somatostatin receptor 5 (SST5) coupling in human pituitary cells

Despite intensive investigation over the past 20 years, the specific role played by individual Gi protein family members in mediating complex cellular effects is still largely unclear. Therefore, we investigated the role of specific Gi proteins in mediating somatostatin (SS) effects in somatotroph cells. Because our previous data showed that SS receptor type 5 (SST5) carrying a spontaneous R240W mutation in the third intracellular loop had a similar ability to inhibit intracellular cAMP levels to the wild-type protein but failed to mediate inhibition of growth hormone (GH) release and cell proliferation, we used this model to check specific receptor-G-protein coupling by a bioluminescent resonance energy transfer analysis. In HEK293 cells, wild-type SST5 stimulated the activation of Gαi1-3 and GαoA, B, whereas R240W SST5 maintained the ability to activate Gαi1-3 and GαoB, but failed to activate the splicing variant GαoA. To investigate the role of the selective deficit in GαoA coupling, we co-transfected human adenomatous somatotrophs with SST5 and a pertussis toxin (PTX)-resistant GαoA (GαoA(PTX-r)) protein. In PTX-treated cells, GαoA(PTX-r) rescued the ability of the selective SST5 analog BIM23206 to inhibit extracellular signal-related kinase 1/2 (ERK1/2) phosphorylation, GH secretion and intracellular cAMP levels. Moreover, we demonstrated that silencing of GαoA completely abolished SST5-mediated inhibitory effects on GH secretion and ERK1/2 phosphorylation, but not on cAMP levels. In conclusion, by analysing the coupling specificity of human SST5 to individual Gαi and Gαo subunits, we identified a crucial role for GαoA signalling in human pituitary cells.