Mechanisms of Growth Hormone Signal Transduction - Studies on the insulin-like metabolic effects in primary rat adipocytes

Abstract: Growth hormone (GH) exerts growth-promoting and metabolic effects in mammals. The metabolic effects of GH in adipose tissue are insulin-antagonistic or insulin-like. Cells that just have been exposed to GH are insensitive to the hormone. This study addresses the question of how GH exerts its insulin-like metabolic effects, using primary rat adipocytes as a model system. It is shown that GH increases the phosphorylation of a 46 kDa plasma membrane protein enriched by hGH-Sepharose chromatography and that this phosphopeptide contains phosphotyrosine, suggesting a GH-induced tyrosine kinase activation. We could later immunoprecipitate a 114 kDa phosphopolypeptide that was identified as the GH receptor. This protein was rapidly phosphorylated on tyrosine in response to GH (in GH-sensitive cells) in a dose-dependent manner. GH also induced the tyrosine phosphorylation of the 130 kDa cytosolic protein Janus tyrosine kinase-2 (JAK2). It was also found that GH in GH-sensitive cells induces a rapid turnover of phospholipids which most probably represents the activation of phospholipase C. A low Km cAMP phosphodiesterase (PDE3) has been implicated as a mediator of the metabolic effects of insulin. We used a selective inhibitor of PDE3 to completely block the antilipolytic effects of insulin and GH in rat adipocytes, while the lipogenic effects of the hormones were largely unaffected. The effects of OPC3911 could not be attributed to excessive stimulation of cAMP-dependent protein kinase. In summary the study emphasises the similarity of the mechanisms involved in the regulation of the metabolic effects of insulin and GH in rat adipocytes, where both hormones induce intracellular tyrosine phosphorylations involved in receptor-postreceptor signaling, affect phospholipid turnover and appear to activate a phosphodiesterase that reduces intracellular cAMP.