Insulin regulation of human hepatic growth hormone receptors: Divergent effects on biosynthesis and surface translocation

Insulin modulates the biological actions of GH, but little is known about i ts effect on human hepatic GH receptors (GHRs). Using the human hepatoma ce ll line HuH7 as a model, we investigated insulin regulation of total, intra cellular, and cell surface GHRs and receptor biosynthesis and turnover. Ins ulin up-regulated total and intracellular GHRs in a concentration-dependent manner. It increased sur face GHRs in a biphasic manner, with a peak respo nse at 10 nmol/L, and modulated GH-induced Janus kinase-2 phosphorylation i n parallel with expression of surface GHRs. The abundance of GHR messenger ribonucleic acid and protein, as assessed by RT-PCR and Western analysis, r espectively, markedly increased with insulin treatment. To examine whether insulin regulates GHRs at the posttranslational level, its effects on recep tor surface translocation and internalization were investigated. Insulin su ppressed surface translocation in a concentration-dependent manner, whereas internalization was unaffected. Moreover, insulin actions on total GHRs an d surface translocation were inhibited by PD98059 and wortmannin, respectiv ely. In conclusion, insulin regulates hepatic GHR biosynthesis and surface translocation in a reciprocal manner, with surface receptor availability th e net result of the divergent effects. The divergent actions of insulin app ear to be mediated by the mitogen-activated protein kinase and phosphatidyl inositol 3-kinase pathways, respectively.