TPA inhibits the tyrosine kinase activity of the neu protein in vivo and in vitro.

P185 is a receptor-like protein encoded by the neu/erbB-2 proto-oncogene. A point mutation in the transmembrane domain renders this protein oncogenic. We report here that incubation of cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulates the phosphorylation of the normal neu protein (p185) and the oncogenic neu protein (p185*). The increased phosphorylation occurs mainly on serine and threonine residues. Phosphate labeling experiments showed that TPA causes a reduction of basal phosphotyrosine in p185 but not p185*. Immunoblotting with antiphosphotyrosine antibody yielded similar results. TPA also inhibited tyrosine phosphorylation of p185* in an in vitro immune complex kinase assay. These data suggest that protein kinase C, the receptor for TPA, regulates p185 function through serine or threonine phosphorylation.