Differential involvement of tyrosine and serine/threonine kinases in platelet integrin αIIbβ3 exposure

The relative contributions of protein tyrosine kinases (PTKs) and protein kinase C isoenzymes (PKCs), a family of serine/threonine kinases, in integrin alpha(IIb)beta(3) (glycoprotein IIb/lIIa) 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 I-125-fibrinogen binding to alpha(IIb)beta(3) and on aggregation/secretion induced by different agonists. Dose-response studies show ed complete inhibition of alpha(IIb)beta(3) exposure by 30 mu mol/L (ADP stimulation) and 35 to 40 mu mol/L (alpha-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 approximate to 80% (for alpha-thrombin). Studies with a submaximal dose of herbimycin A (approximate to 50% inhibition of the ADP-response) and a maximal dose of bisindolylmaleimide I showed that optical aggregation had a similar sensitivity to the inhibitors as alpha(IIb)beta(3) exposure with minimal interference by secreted ADP. Thus, the relative contributions of tyrosine and serine/threonine kinases in alpha(IIb)beta(3) 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 alpha-thrombin.