TYROSINE KINASES REGULATE THE CYTOSKELETAL ATTACHMENT OF INTEGRIN ALPHA(IIB)BETA(3) (PLATELET GLYCOPROTEIN IIB/IIIA) AND THE CELLULAR RETRACTION OF FIBRIN

Integrins promote cell-substratum and cell-cell adhesion by acting as transmembrane linker molecules between extracellular adhesion proteins and the actin-rich cytoskeleton. The integrin alpha(IIb)beta(3) (platelet glycoprotein IIb/IIIa) is essential for platelet spreading, aggregation, fibrin clot retraction, and for the transduction of extracellular signals. We examined the effect of the specific tyrosine kinase inhibitor herbimycin A on integrin and cytoskeletal-mediated events in thrombin-stimulated platelets. Incubation of washed platelets for 24 h with herbimycin A (5 mu M) abolished the thrombin-stimulated cytoskeletal enzyme activity of pp60(c-src) in parallel with a reduction in the tyrosine phosphorylation of multiple platelet proteins, as assessed with anti-phosphotyrosine immunoblots. However, thrombin-induced activation of protein kinase C and the production of thromboxane A(2) were not altered by herbimycin A. Despite the absence of cytoskeletal pp60(c-src) enzyme activity, platelet shape change, aggregation, and serotonin release were unaltered following platelet stimulation with thrombin (0.05-1.0 unit/ml). Herbimycin A-treated platelets also demonstrated normal platelet aggregation in response to collagen (5 mu g/ml), ionophore A23187 (2 mu M), and ADP/adrenaline (10 pat each). However, the ability of herbimycin A-treated platelets to retract fibrin gels was significantly reduced. This defect in clot retraction was associated with reduced incorporation of integrin alpha(IIb)beta(3) into the cytoskeletal fraction of thrombin-aggregated platelets. Our studies suggest that tyrosine kinases in platelets regulate the cytoskeletal attachment of alpha(IIb)beta(3), as an essential process for the transmission of cellular contractile forces to fibrin polymers.