STIMULATION OF MITOGEN-ACTIVATED PROTEIN-KINASE AND NA+ H+ EXCHANGER IN HUMAN PLATELETS DIFFERENTIAL EFFECT OF PHORBOL ESTER AND VASOPRESSIN/

Treatment of human platelets with phorbol 12-myris tate 13-acetate (PM A) and arginine vasopressin (AVP) increase the phosphorylation and act ivation of mitogen-activated protein kinase (MAPK). Electrophoretic re tardation of MAPK mobility on SDS polyacrylamide gels was used for det ermination of MAPK phosphorylation, The activity of MAPK was tested in myelin basic protein (MBP)-containing polyacrylamide gels, In this st udy we compared the PMA and AVP signal transduction pathways leading t o the activation of MAPKs and Na+/H+ exchanger (NHE). Both agonists st imulate MAPK and NHE activities in a similar time frame and concentrat ion dependence, The MAPK and NHE activities induced by PMA were inhibi ted by staurosporine, a potent inhibitor for protein kinase C (PKC), a nd by MAPK kinase (MEK) inhibitor, PD98059, but were not affected by t he tyrosine kinase inhibitor genistein, In contrast, both AVP-induced MAPK and NHE activities were inhibited by genistein and MEK inhibitor but were not affected by staurosporine, Immunoprecipitation studies de monstrate that PMA, but not AVP, enhances the basal phosphorylation of the NHE-1. In this study, MAPKs are suggested to be a part of converg ing signaling leading to NHE activation by PKC-dependent and AVP-tyros ine kinase-dependent pathways. We propose that the MAPK activation of the NHE-1 does not involve phosphorylation of this exchanger protein, On the other hand, PKC can lead to phosphorylation and to additional a ctivation of the NHE-1 through a MAPK-independent pathway.