INSULIN-LIKE GROWTH-FACTOR-I STIMULATION OF CELLS INDUCES FORMATION OF COMPLEXES CONTAINING PHOSPHATIDYLINOSITOL-3-KINASE, GUANOSINE TRIPHOSPHATASE-ACTIVATING PROTEIN (GAP), AND P62 GAP-ASSOCIATED PROTEIN

The insulin-like growth factor-1 (IGF-1) receptor is structurally related to the insulin receptor and shares common features in receptor signaling. These features include receptor autophosphorylation, phosphorylation of insulin receptor substrate-1, and activation of Ras and phosphatidylinositol-3-kinase (PI3K). Previously, we reported that after insulin treatment of rat HTC cells expressing human insulin receptors, a unique insulin receptor signaling complex was formed that contained the insulin receptor, the p85 subunit of PI3K, GTPase-activating protein (GAP), and p62 GAP-associated protein. In the present study, using wild type HTC cells, we investigated whether the activated IGF-1 receptor also forms a similar signaling complex. To study the proteins present in IGF-1 receptor signaling complexes, we used immunoprecipitation and Western blotting analysis with appropriate antibodies. In response to IGF-1, insulin receptor substrate-1 was tyrosine phosphorylated and formed a complex with the PI3K heterodimer that consists of a p85 regulatory subunit and a p110 catalytic subunit. In addition, a separate complex was formed, consisting of p85, p62 GAP-associated protein and GAP. The p62 in this complex was tyrosine phosphorylated. These studies suggest, therefore, that the IGF-1 receptor, like the insulin receptor, induces the formation of multiple signaling complexes that most likely mediate the proliferative effects of these receptors.