The objective of this project is to investigate the regulation of cellular responsiveness to insulin and insulin-like growth factors (IGFs) in established rat and human hepatoma cell lines in tissue culture. The induction of tyrosine aminotransferase, stimulation of amino acid transport and Na/K pump activity, and stimulation of glycogen synthesis will be used as markers of insulin and IGF action. Incubation of HTC rat hepatoma cells with insulin causes a virtually complete resistance to the further actions of this hormone. Regulation of insulin responsiveness can be dissociated from the insulin-induced down-regulation of insulin receptor number, and is mediated by post-binding events. Insulin also induces unresponsiveness to the actions of IGFs, which are mediated by specific IGF receptors. Since insulin does not down-regulate IGF binding, we conclude that insulin affects a post-binding step which might be common to the actions of both insulin and the insulin-like growth factors. In this project we plan to characterize both the insulin- and IGF-induced desensitization to insulin and IGF action in HTC and H-35 rat hepatoma cells and in Hep G2 human hepatoma cells. We will investigate the mechanisms by which these peptides induce desensitization using both biochemical and genetic approaches. A major thrust of this proposal is the isolation and characterization of variant hepatoma lines resistant to insulin and IGFs. We will utilize these variants to study the mechanism of insulin and IGF action, and to elucidate the mechanism of regulation of hormonal responsiveness. Alteration in insulin responsiveness is a major abnormality in noninsulin-dependent diabetes mellitus, and post-binding defects are responsible for a significant component of the insulin resistance in this disease. Thus, an understanding of the regulation of insulin and IGF responsiveness is intimately related to an understanding of the pathophysiology of NIDDM and other diseases associated with hormone resistance.
