CROSS-TALK BETWEEN PHORBOL ESTER-MEDIATED SIGNALING AND TYROSINE KINASE PROTOONCOGENES I - ACTIVATION OF PROTEIN-KINASE-C STIMULATES TYROSINE PHOSPHORYLATION AND ACTIVATION OF ERBB2 AND ERBB3

The tumor-promoting phorbol ester, phorbol 12 myristate 13-acetate (PM A), acutely stimulates the tyrosine phosphorylation of proteins of app roximately 190, 120, and 70 kDa in the well differentiated Fao rat hep atoma cell line, This phosphorylation is dependent on protein kinase C (PKC) and is abolished by down-regulation of PKC or pretreatment with a PKC inhibitor, Purification of the 190-kDa tyrosine phosphorylated protein revealed that it consists of both ErbB2 and ErbB3, Following P MA-induced tyrosine phosphorylation, ErbB2 and ErbB3 were able to asso ciate with the SH2 domains of several signaling proteins including the p85 alpha subunit of phosphatidylinositol S-kinase, Syp, and Grb2, Th e 120-kDa protein phosphorylated in response to PMA consists of at lea st two proteins: focal adhesion kinase that exhibits a minor increase in tyrosine phosphorylation following treatment with PMA, and a major 120-kDa tyrosine-phosphorylated species in PMA-stimulated Fao cells wh ich as yet is unidentified, Similarly, the 70 kDa tyrosine phosphoryla ted protein also appears to represent more than one protein, including paxillin and a second protein of similar mobility which appears to be the major tyrosine phosphorylation in response to PMA, Both ErbB2 and paxillin also exhibit reduced migration on SDS-polyacrylamide gel ele ctrophoresis following PMA treatment, suggesting that they are also ph osphorylated on serine/threonine residues, The mobility shift of both of these proteins is abolished by treatment with inhibitors of PKC or mitogen-activated protein kinase/extracellular signal-related kinase k inase, These results suggest a novel mechanism of cross-talk between t he serine/threonine kinase PKC and tyrosine phosphorylation pathways, The activation of ErbB2 and ErbB3 that is initiated by PMA may contrib ute to the tumor promoting activity of these compounds.