Fibrin Clot Retraction by Human Platelets Correlates with αIIbβ3Integrin-dependent Protein Tyrosine Dephosphorylation

We have analyzed tyrosine phosphorylation associated with retraction of the fibrin clot by washed platelets in purified fibrinogen. Retraction was dependent on integrin alpha(IIb)beta(3), based on absence of retraction of alpha(IIb)beta(3)-deficient thrombasthenic platelets. However, only a subset of alpha(IIb)beta(3)-blocking antibodies or peptides were able to inhibit retraction, suggesting a differential engagement of alpha(IIb)beta(3) in fibrin clot retraction versus aggregation. Immunoblotting demonstrated a phosphorylated protein pattern comparable with aggregation at early time points. However, as opposed to aggregation, tyrosine phosphorylation decreased rapidly in parallel to retraction (up to 60 min). Dephosphorylation was alpha(IIb)beta(3)-dependent, since it was blocked by alpha(IIb)beta(3)-specific inhibitors and was absent in thrombasthenic platelets. Inhibition of platelet clot retraction by phenyl-arsine oxide and peroxovanadate, suggested a role for tyrosine phosphatases. Cytochalasin D and E (5 microm) blocked fibrin clot retraction and tyrosine dephosphorylation, suggesting regulation by actin cytoskeleton assembly. Tyrosine phosphatase activities were found associated with clot retraction using the "in-gel" tyrosine phosphatase assay; however, none were alpha(IIb)beta(3)-dependent. An 85-kDa protein and to a lesser degree "Src" showed the closest dose-dependent correlation between inhibition of tyrosine dephosphorylation and inhibition of retraction. We thus postulate that alpha(IIb)beta(3) engagement in fibrin clot retraction drives, in an actin cytoskeleton-dependent manner, the interaction of tyrosine phosphatases and of the tyrosine-phosphorylated substrates 85-kDa protein and Src, the dephosphorylation of which regulates the force generation and/or transmission required for full contraction of the fibrin matrix.