Insulin like growth factor-I induces limited association of phosphatidylinositol 3-kinase to its receptor.

Stimulation by insulin-like growth factor-I (IGF-I) of LISN C4 cells, a mouse fibroblast cell line that overexpresses human IGF-I receptors, led to an increase in the amount of a phosphatidylinositol kinase that could be immunoprecipitated by anti-IGF-I receptor or anti-phosphotyrosine antibodies. The identity of the lipid produced in phosphatidylinositol kinase assays of anti-IGF-I receptor or anti-phosphotyrosine immunoprecipitates indicated that IGF-I selectively increased the amount of immunoprecipitated phosphatidylinositol 3-kinase activity. The amount of immunoprecipitated phosphatidylinositol 3-kinase activity that was increased by IGF-I followed a time course that paralleled the stimulation of IGF-I receptor beta-subunit autophosphorylation. The amount of phosphatidylinositol 3-kinase activity detected in anti-IGF-I receptor immunoprecipitates represented only 2% of that which was immunoprecipitated by anti-phosphotyrosine antibody. Furthermore, phosphatidylinositol 3-kinase activity which was recovered with anti-phosphotyrosine antibody was present in both cytosol and particulate cell fractions at approximately similar levels. Taken together, these results suggest that the stimulation of the IGF-I receptor tyrosine kinase leads to an increase in the amount of phosphatidyl inositol 3-kinase activity immunoprecipitated by antiphosphotyrosine and anti-IGF-I receptor antibodies and to a limited association with the IGF-I receptor itself, even though these cells express very high levels of IGF-I receptors. That the majority of phosphatidylinositol 3-kinase activity does not tightly associate with the IGF-I receptor after IGF-I stimulation suggests that it may be associated with other tyrosine phosphorylated proteins. Alternatively, the kinase itself may become phosphorylated on tyrosine and dissociate from the IGF-I receptor. In this manner, an increase of phosphatidylinositol 3-kinase activity by IGF-I deviates from the activation of phosphatidylinositol 3-kinase by platelet-derived growth factor receptor in that a tight association with the receptor is not produced after stimulation.