Protein-tyrosine phosphorylation and p72syk 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 kDa band included cortactin and auto-phosphorylated p72syk. p72syk activity rapidly increased upon collagen stimulation, whereas p60c-src activation was below detectable levels. A combination of inhibitors to 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, p72syk activation was not inhibited by these treatments. Cytochalasin D completely inhibited protein tyrosine phosphorylation and p72syk activation. It also potently inhibited aggregation and [Ca2+]i elevation. Anti-GPIa/IIa MoAb in a concentration-dependent manner partially attenuated protein tyrosine phosphorylation and p72syk activation. Its inhibitory effects on intracellular Ca2+ mobilization, release of intracellular granule contents, and aggregation also were partial. No tyrosine kinase activity was coprecipitated with GPIa/IIa. These results suggest that p72syk activation lies upstream of protein tyrosine phosphorylation, Ca2+ mobilization, ADP release, thromboxane A2 production and aggregation. GPIa/IIa plays a key role in p72syk activation induced by collagen, but other collagen receptors may work in synergy to fully activate p72syk. Actin polymerization is a prerequisite for both p72syk activation and other intracellular signal transduction pathways.