DIFFERENTIAL REGULATION OF INTEGRIN-MEDIATED PROPLATELET FORMATION AND MEGAKARYOCYTE SPREADING

Guinea pig bone marrow megakaryocytes were cultured on a type I rat tail collagen gel which stimulated proplatelet formation. Proplatelet formation was inhibited by monoclonal antibody LM609 to the alpha(v) beta(3) integrin (VnR), but not by monoclonal antibodies to the alpha(5), alpha(6), beta(1), or IIb beta(3(GPIIb-IIIa)) integrin proteins. Megakaryocytes cultured on a plastic surface and stimulated with thrombin undergo a spreading and an adhesion reaction. This reaction is blocked in a dose-dependent manner by the tetrapeptide RGDS and by the monoclonal antibody PG2 to the GPIIb-IIIa integrin, but not by the monoclonal antibody LM609 to the VnR. Immunoprecipitation and affinity chromatography experiments demonstrate that guinea pig megakaryocytes have distinct GPIIb-IIIa and VnR integrins with similar electrophoretic mobility. Spreading was significantly inhibited in a dose-dependent fashion by drugs which elevate cellular cyclic AMP, including forskolin, dibutyryl cAMP, and isobutylmethylxanthine. In contrast to spreading, megakaryocyte proplatelet formation was stimulated by these agents in a dose-dependent manner. Megakaryocyte spreading was stimulated by the protein kinase C(PKC) activator phorbol myristate acetate (PMA) and inhibited by the PKC inhibitors Calphostin C and K5720 in a dose-dependent manner. PKC inhibitors did not inhibit megakaryocyte proplatelet formation. These results demonstrate that the closely related VnR and GPIIb-IIIa integrins regulate different aspects of megakaryocyte morphological change and appear to be associated with different second messenger systems. (C) 1995 Wiley-Liss, Inc.