Differential involvement of tyrosine and serine/threonine kinases in platelet integrin α(IIb)β3 exposure

Abstract

The relative contributions of protein tyrosine kinases (PTKs) and protein kinase C isoenzymes (PKCs), a family of serine/threonine kinases, in integrin α(IIb)β3 (glycoprotein IIb/IIIa) exposure are the subject of much controversy. In the present study we measured the effect of the PTK inhibitor herbimycin A and the PKC inhibitor bisindolylmaleimide I on 125(I)- fibrinogen binding to α(IIb)β3 and on aggregation/secretion induced by different agonists. Dose-response studies showed complete inhibition of α(IIb)β3 exposure by 30 μmol/L (ADP stimulation) and 35 to 40 μmol/L (α-thrombin stimulation) herbimycin A. In contrast, inhibition of exposure by bisindolylmaleimide I varied from none (for ADP and epinephrine), to 30% (for platelet-activating factor), and to κ80% (for α-thrombin). Studies with a submaximal dose of herbimycin A (κ50% inhibition of the ADP-response) and a maximal dose of bisindolylmaleimide I showed that optical aggregation had a similar sensitivity to the inhibitors as α(IIb)β3 exposure with minimal interference by secreted ADP. Thus, the relative contributions of tyrosine and serine/threonine kinases in α(IIb)β3 exposure and aggregation differ among the different agonists, with an exclusive role for PTKs in ADP- and epinephrine-induced responses and a role for both PTKs and PKCs in responses induced by platelet-activating factor and α-thrombin.