NEGATIVE REGULATION OF MITOGEN-ACTIVATED PROTEIN-KINASE ACTIVATION BYINTEGRIN ALPHA(IIB)BETA(3) IN PLATELETS

Activation of the mitogen-activated protein (MAP) kinase pathway in nu cleated cells is dependent on both growth factor receptors and integri ns engaged in cell adhesion, Human platelets are an interesting model for studying cell adhesion and the involvement of integrin engagement on extracellular signal-regulated kinase (ERK) activation, independent ly from the nuclear-DNA signal pathway, Maximal phosphorylation and ac tivity of ERK2 occurred late during thrombin-induced platelet aggregat ion (90 s and later), an alpha(IIb)beta(3) integrin-dependent event, S urprisingly, alpha(IIb)beta(3) inhibition by the RGDS ligand peptide, or (Fab')(2) fragments of the AP-2 monoclonal antibody, resulted in a a-fold enhancement in ERK2 phosphorylation and activity. A similar 2-f old enhancement of ERK2 activation was observed in thrombasthenic plat elets which are defective in alpha(IIb)beta(3) and do not aggregate, T his suggests that ERK2 activation in thrombin-induced platelet aggrega tion is dependent on thrombin rather than on alpha(IIb)beta(3) and is down-regulated by alpha(IIb)beta(3) engaged in ligand (fibrinogen) bin ding and/or aggregation, Finally, in the absence of stirring which all ows fibrinogen binding to alpha(IIb)beta(3) but prevents aggregation, ERK2 was again overactivated. This overactivation appears to be consec utive to inhibition of aggregation itself and to alpha(IIb)beta(3) lig and binding, We conclude that in platelets, alpha(IIb)beta(3) engaged in aggregation down-regulates thrombin-induced ERK2 activation, To our knowledge, this is the first report of a down-regulation of the MAP k inase pathway by integrin engagement.