Summary
Effect of human recombinant thrombopoietin (TPO) on platelet activation in vitro was
studied. Although TPO by itself did not cause platelet aggregation, it upregulated
ADP-induced aggregation, especially the second wave of aggregation. This effect was
dose-dependent for up to 5 ng/ml of TPO. When platelets were activated by epinephrine,
collagen, or α-thrombin, similar effect was observed. However, TPO did not affect
A23187- or PMA-induced aggregation, suggesting that TPO may have modulated the signal
transduction pathway upstream of inositol 1,4,5-trisphosphate and diacylglycerol production.
TPO also upregulated thrombin-induced α-granule secretion. To clarify the involvement
of protein tyrosine phosphorylation, platelets were activated by TPO and/or suboptimal
concentration of ADP, then tyrosine phosphorylation was detected by immunoblot analysis,
using anti-phosphotyrosine monoclonal antibody. TPO by itself caused significant tyrosine
phosphorylation of 146,130,122,108, 97,94, and 88 kDa proteins. Further, by using
antibodies against signal transduction molecules for immunoprecipitation, we observed
the significant tyrosine phosphorylation in Jak2 and Tyk2 molecules after TPO-stimulation.
The results of the present experiment clearly indicate that TPO directly activated
platelets and modulated intracellular signal transduction pathway.