Insulin regulation of growth hormone receptor gene expression. Evidence for a transcriptional mechanism of down-regulation in rat hepatoma cells.

The role of insulin in regulating responsiveness to growth hormone (GH) remains unclear. Continuous insulin treatment reduces GH binding, which suggests that insulin may effect growth hormone receptor (GHR) levels. The present study used rat hepatoma cells to examine the effects of insulin and GH on GHR gene expression. Prolonged insulin treatment (greater than 3h) significantly reduced GHR mRNA, and removal of insulin led to a gradual recovery. This effect of insulin occurred at physiologic concentrations, occurred many hours before the insulin-regulated decrease in GHR protein, and was mediated by reduction of GHR transcription. GH treatment dramatically reduced GHR protein, but caused only a modest reduction in GHR mRNA. These findings indicate that the heterologous reduction of GHR by insulin occurs via transcriptional downregulation, and the homologous reduction of GHR by GH occurs via a different mechanism. Furthermore, with insulin, extended time of exposure may be necessary for appreciable reduction of GHR.