Summary
The mechanism of human platelet activation by thrombopoietin (TPO) was investigated
in vitro. We found that rHuTPO stimulated thromboxane A2 formation and serotonin secretion,
despite the absence of shape change and aggregation. Blockade of the arachidonic acid
pathway did not inhibit rHuTPO-induced platelet secretion. rHuTPO stimulated the tyrosine
phosphorylation of 64, 80/85, 95, 130 and 140 kDa proteins, but phosphoproteins of
100-105 and 125 kDa obtained when platelets aggregated in the presence of thrombin
were absent. rHuTPO stimulated and potentiated the thrombin-induced tyrosine phosphorylation
of a 80 kDa protein identified as the cortical actin-associated protein, p80/85 cortactin.
When platelets were aggregated in the presence of rHuTPO and fibrinogen, cortactin
phosphorylation was enhanced as compared to rHuTPO alone. Treatment with RGDS or cytochalasin
D respectively reduced or abolished cortactin tyrosine phosphorylation. This confirms
the existence of fibrinogen binding-dependent and independent pools of phosphorylated
cortactin, both requiring intact actin polymerization. Cytoskeleton-binding proteins
may be implicated in in vitro platelet activation by rHuTPO.