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Expression of Insulin-Like Growth Factor I in Cultured Rat Hepatocytes: Effects of Insulin and Growth Hormone

Department of Developmental Genetics and Anatomy, School of Medicine and the Developmental Biology Center, Case Western Reserve University Cleveland, Ohio 44106
Department of Medicine, Division of Oncology, School of Medicine and the Developmental Biology Center, Case Western Reserve University Cleveland, Ohio 44106
Division of Rheumatology, School of Medicine and the Developmental Biology Center, Case Western Reserve University Cleveland, Ohio 44106

Address requests for reprints to: Dr. Thomas R. Johnson, Department of Developmental Genetics and Anatomy, Case Expression of IGF-I Western Reserve University, 2119 Abington Road, Cleveland, Ohio 44106.

Abstract

The effects of GH and insulin on 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

FOOTNOTES

This work was supported by NIH Grants AR-20618 (to the Northeast Ohio Multipurpose Arthritis Center), CA-43703, and HD-25004; the American Diabetes Association; and the Henry M. and Lillian Stratton Foundation.

Received for publication April 26, 1988. Revision received December 8, 1988. Accepted for publication December 10, 1988.

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