Expression of insulin-like growth factor I in cultured rat hepatocytes: effects of insulin and growth hormone.

The effects of GH and insulin the accumulation of insulin-like growth factor I (IGF-I) RNA transcripts and the secretion of immunoreactive IGF-I protein were studied in rat liver hepatocytes cultured in serum-free medium. GH at concentrations of 10 ng/ml or greater stimulated the accumulation of IGF-I RNA transcripts relative to actin transcripts in poly(A)+ RNA isolated from cultured hepatocytes. The time course of IGF-I transcript accumulation in response to GH appeared to be biphasic. The transcript levels rose dramatically during the first 2 h after exposure of the cultures to GH, declined between 3 and 6 h, but reaccumulated by 24 h after exposure to GH. The presence of insulin did not influence the effect of GH on the accumulation of IGF-I RNA transcripts, although insulin did elevate IGF-I transcript levels in the absence of GH. Analysis of RNA pulse-labeled with thiouridine followed by purification of the thiol-labeled RNA using mercurated agarose indicated that GH probably acts by increasing IGF-I transcription. Insulin also affected the release of immunoreactive IGF-I into the culture medium. In the presence of insulin, immunoreactive IGF-I accumulated in the culture medium to approximately the same extent over a 24-h period regardless of whether GH was also present. In the absence of insulin, immunoreactive IGF-I accumulated in the medium only if GH was present. The results suggest that insulin may play an important role in both IGF-I transcript accumulation and the secretion of IGF-I from cultured hepatocytes.