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Antisense-mediated suppression of the transmembrane protein-tyrosine phosphatase (PTPase) LAR has been shown previously to increase insulin-dependent phosphatidylinositol 3-kinase (PI 3-kinase) activation by greater than 300% in the rat hepatoma cell line McA-RH7777. Here, insulin-dependent insulin receptor tyrosine kinase activation was examined with recombinant insulin receptor substrate 1 (IRS-1) as the substrate and shown to be 3-fold greater in cells with suppressed LAR levels. Consistent with a receptor level effect, in vivo insulin-dependent tyrosine phosphorylation of both IRS-1 and Shc was increased by a similar 3-fold with LAR suppression. These increases in IRS-1 and Shc phosphorylation were paralleled by increases in insulindependent PI 3-kinase association with IRS-1 and activation of the MAP kinase pathway.Reduced LAR levels also resulted in increases of over 300% and 250% in epidermal growth factor (EGF)-and hepatocyte growth factor (HGF)-dependent receptor autophosphorylation, respectively, as well as a severalfold increase in substrate tyrosine phosphorylation. In a post-receptor response, EGF-and HGF-dependent MAP kinase activation was increased by 300% and 350%, respectively, with LAR suppression. Similarly, growth factor-dependent PI 3-kinase activation was increased in LAR antisense expressing cells when compared to null vector expressing cells. These results demonstrate that the transmembrane PTPase LAR modulates liganddependent activation of at least three receptor tyrosine kinases.Although the pathways that become activated by receptor tyrosine kinases have been characterized extensively, the mechanisms by which these receptors can be modulated have not been well studied. A significant number of in vitro studies have demonstrated that protein-tyrosine phosphatases (PTPases) 1 might function as important regulators of receptor tyrosine kinases (reviewed in Ref. 1). We and others have shown that overexpression of PTPases within intact cells results in a blunting of ligand-mediated receptor tyrosine kinase autophosphorylation (2-5). These studies suggest that cellular responses to receptor tyrosine kinase activation might be dependent upon receptor-selective PTPase activity. A logical extension of this work would be to identify physiological PTPases capable of regulating receptor tyrosine kinase activity. Our recent work involving antisense suppression of the PTPase LAR within intact hepatoma cells has implicated this particular transmembrane PTPase as an important modulator of insulin receptor signaling (6). The insulin receptor is a heterotetrameric protein complex. Upon binding insulin, the intrinsic tyrosine kinase activity of the transmembrane ␤ subunit increases, allowing it to phosphorylate itself as well as intracellular substrates (reviewed in Ref. 7). Two well characterized substrates of the insulin receptor include insulin receptor substrate 1 (IRS-1) and . When phosphorylated, IRS-1 can bind to a number of effector proteins, including phosphatidylinositol 3-kinase (PI 3-kinase), ...

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