Stimulation of the Phosphorylation of Cytoskeletal 350-kDa and 300-kDa Proteins by Insulin-Like Growth Factor-I, Platelet-Derived Growth Factor and Phorbol Ester in Rat 3Y1 Cells

Insulin-like growth factor-I (IGF-I) stimulated the phosphorylation of cytoskeletal 350-kDa and 300-kDa proteins which were immunoprecipitated with antibodies against brain high molecular weight microtubule-associated proteins in quiescent rat 3Y1 cells. The data on the effective concentrations of IGF-I and ¹²⁵I-labeled IGF-I binding indicated that type I IGF receptors mediate this IGF-I effect. Platelet-derived growth factor (PDGF) as well as phorbol ester (TPA) also stimulated the phosphorylation of these proteins. These proteins, whether immunoprecipitated from cells stimulated by insulin, IGF-I, TPA, PDGF, or epidermal growth factor, produced very similar phosphopeptide mapping patterns irrespective of the stimulant. The results suggest the possibility that these growth factors and phorbol esters may activate a common protein kinase which is responsible for the phosphorylation of the 350-kDa and 300-kDa proteins in cells.