title: Effects of protein kinase C activation after epidermal growth factor binding on epidermal growth factor receptor phosphorylation.
abstract: The possible role of epidermal growth factor (EGF) receptor phosphorylation at threonine 654 in modulating the protein-tyrosine kinase activity of EGF-treated A431 cells has been studied. It has been suggested that EGF could indirectly activate a protein-serine/threonine kinase, protein kinase C, that can phosphorylate the EGF receptor at threonine 654. Protein kinase C is known to be activated, and threonine 654 is phosphorylated, when A431 cells are exposed to 12-O-tetradecanoylphorbol-13-acetate (TPA). The protein-tyrosine kinase activity of EGF receptors is normally evidenced in EGF-treated cells by phosphorylation of the receptor at tyrosine. This is inhibited when TPA-treated cells are exposed to EGF. We now show that receptor phosphorylation at threonine 654 can also be detected in EGF-treated A431 cells, presumably due to indirect stimulation of protein kinase C or a similar kinase. Some receptor molecules are phosphorylated both at threonine 654 and at tyrosine. Since prior phosphorylation at threonine 654 inhibits autophosphorylation, we propose that protein kinase C can phosphorylate the threonine 654 of autophosphorylated receptors. This provides evidence for models in which protein kinase C activation, consequent upon EGF binding, could reduce the protein-tyrosine kinase activity of the EGF receptor. Indeed, we find that 12-O-tetradecanoylphorbol-13-acetate, added 10 min after EGF, further increases threonine 654 phosphorylation and induces the loss of tyrosine phosphate from A431 cell EGF receptors.
mesh_terms: D002460:Cell Line; D004789:Enzyme Activation / Q000187:drug effects; D004815:Epidermal Growth Factor / Q000378:metabolism / Q000494:pharmacology*; D066246:ErbB Receptors; D006801:Humans; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013912:Threonine / Q000378:metabolism; D014443:Tyrosine / Q000378:metabolism
chemical_list: D011956:Receptors, Cell Surface; D013912:Threonine; D014443:Tyrosine; D004815:Epidermal Growth Factor; D066246:ErbB Receptors; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate