Suppression of insulin receptor activation by overexpression of the protein-tyrosine phosphatase LAR in hepatoma cells.

Protein-tyrosine phosphatases (PTPases) play an essential role in the regulation of reversible tyrosine phosphorylation of cellular proteins that mediate insulin action. In order to explore the potential role of the transmembrane PTPase (LAR) in insulin receptor signal transduction, we overexpressed the full-length LAR protein in McA-RH7777 rat hepatoma cells and found that modest increases in the abundance of LAR protein expression downregulated a number of insulin-stimulated cellular responses closely related to the activation of the receptor kinase. An increase in LAR protein of 2.4-fold over the level in control cells caused a 40% reduction in insulin receptor autophosphorylation in intact cells, without an alteration in insulin receptor mass or a change in the insulin-stimulated receptor kinase activity measured with partially purified receptors in vitro. In addition, insulin-stimulated tyrosine phosphorylation of the endogenous insulin receptor substrates IRS-1 and Shc were decreased to 57% and 73% of control, respectively, and IRS-1 associated phosphatidylinositol 3'-kinase activity was reduced to 47% of control of the cells overexpressing LAR. The present results, taken with our recent data demonstrating that reducing the abundance of LAR by expression of antisense mRNA enhances insulin receptor signal transduction (Kulas D. T., et al. J. Biol. Chem. 270:2435, 1995), supports the hypothesis that LAR acts as a physiological modulator of insulin action in insulin-sensitive hepatoma cells.