Insulin regulation of growth hormone receptor gene expression: involvement of both the PI-3 kinase and MEK/ERK signaling pathways.

The mechanism(s) of insulin's effects on growth hormone receptor (GHR) gene expression are poorly understood. Using rat hepatoma cells, we have previously shown that insulin treatment reduces GHR mRNA and protein in a time- and concentration-dependent manner, at least in part via down-regulation of GHR transcription. The present study determines whether the phosphatidylinositol-3 kinase (PI-3 kinase) and mitogen activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) pathways are involved in mediating these effects of insulin. Inhibition of the PI-3 kinase pathway partially blocked insulin's reduction of GHR mRNA, as did inhibition of the MEK/ERK pathway, resulting in higher GHR mRNA levels. Inhibition of both pathways was necessary to completely block insulin effects. Similar results were obtained for GHR protein. Collectively, these data suggest that insulin signaling via either the PI-3 kinase or MEK/ERK pathway may result in partial reduction of GHR gene expression, whereas signaling via both pathways may be required to achieve the full insulin effect.