Serine/threonine dephosphorylation may be involved in tyrosine phosphorylation: a new mode of signal transduction in platelets.

Platelet signal transduction involves not only reversible phosphorylation of proteins on both tyrosine and serine/threonine residues, but also mechanisms of cross-talk to coordinate different pathways. We have, therefore, investigated the effect of okadaic acid, a potent inhibitor of serine/threonine protein phosphatases type 1 and type 2A (PP1 and PP2A), to better understand the interplay that must exist between serine/threonine and tyrosine phosphorylations during platelet activation. Okadaic acid drastically inhibits thrombin-induced platelet aggregation, secretion, and thromboxane synthesis. The inhibition is accompanied by a marked increase in the phosphorylation of at least 5 proteins (230, 210, 74, 57, and 50 to 52 kDa). However, protein kinase C activity is not modified because thrombin-and phorbol-12-myristate-13-acetate-induced phosphorylation of pleckstrin is still occurring, although slightly decreased. Inhibition of platelet function and extent of the phosphorylation of the 5 substrates in the presence of okadaic acid are concentration and time dependent, suggesting a relation between the accumulation of one or more phosphoproteins and the inhibitory effect of okadaic acid. Okadaic acid inhibits thrombin-induced tyrosine phosphorylation in a concentration-dependent manner. According to Brautigan and Pinault, the inhibition of protein phosphatases in kidney cells resulted in the activation of a 55-kDa-tyrosine phosphatase and the tyrosine phosphatase activity was synergistically increased when okadaic acid acted in concert with prostaglandin I2 (PGI2). Interestingly, in agreement with these results, the okadaic acid-induced phosphorylation of the 50-kDa substrate, which occurs without a cyclic adenosine monophosphate increase in platelets, has the same molecular weight as the platelet membrane tyrosine phosphatase isolated by Dawicki and Steiner. Furthermore, we also found that thrombin-induced tyrosine phosphorylation was markedly inhibited in the presence of low concentrations of both okadaic acid and PGI2, therefore explaining the synergistic inhibition of platelet aggregation and secretion. The results greatly support the notion of a cross-talk between stimulation of serine/threonine kinases (in response to inhibition of serine/threonine PP) and inhibition of tyrosine phosphorylations and emphasize the role of the 50-kDa substrate in regulating platelet activation.