PROTEIN-TYROSINE PHOSPHORYLATION AND P72(SYK) ACTIVATION IN HUMAN PLATELETS STIMULATED WITH COLLAGEN IS DEPENDENT UPON GLYCOPROTEIN IA IIA AND ACTIN POLYMERIZATION/

In human platelets treated with acetylsalicylic acid, collagen induced protein-tyrosine-phosphorylation of several proteins. The major 75 kD a band included cortactin and autophosphorylated p72(syk). p72(syk) ac tivity rapidly increased upon collagen stimulation. whereas p60(c-src) activation was below detectable levels. A combination of inhibitors t o remove the effects of extracellular and intracellular Ca2+, released ADP, and fibrinogen binding to GPIIb/IIIa delayed and attenuated the major 75 kDa band. By contrast, p72(syk) activation was not inhibited by these treatments. Cytochalasin D completely inhibited protein tyros ine phosphorylation and p72(syk) activation. It also potently inhibite d aggregation and [Ca2+](i) elevation. Anti-GPIa/IIa MoAb in a concent ration-dependent manner partially attenuated protein tyrosine phosphor ylation and p72(syk) activation. Its inhibitory effects on intracellul ar Ca2+ mobilization, release of intracellular granule contents, and a ggregation also were partial. No tyrosine kinase activity was coprecip itated with GPIa/IIa. These results suggest that p72(syk) activation l ies upstream of protein tyrosine phosphorylation, Ca2+ mobilization, A DP release, thromboxane A(2) production and aggregation. GPIa/IIa play s a key role in p72(syk) activation induced by collagen, but other col lagen receptors may work in synergy to fully activate p72(syk). Actin polymerization is a prerequisite for both p72(syk) activation and othe r intracellular signal transduction pathways.