CAMP Does Not Inhibit Convulxin-Induced Tyrosyl-Phosphorylation of Human Platelet Proteins, Including PLCγ2, But Completely Blocks the Integrin αIIbβ3-Dependent Dephosphorylation Step: Comparisons with RGDS Peptide, Cytochalasin D, and Phenylarsine Oxide

Abstract

Convulxin (Cvx) isolated fromCrotalus durissus terrificusvenom, induces platelet aggregation, phospholipase C (PLC) activation, and tyrosyl-phosphorylation (PTP) of multiple proteins, including PLCγ2 by a mechanism independent of integrin αIIbβ3. However, PTP induced by Cvx is followed by a dephosphorylation step in a platelet aggregation-dependent manner. Here we show that increasing intraplatelet content of cAMP with forskolin is associated with the inhibition of Cvx-induced platelet aggregation, ATP secretion, and inositol-phosphates production. However, the early onset of Cvx-induced PTP is not sensitive to cAMP (including PLCγ2), and it also occurs in the presence of integrin αIIbβ3-antagonist (RGDS peptide, RGDS) or inhibitors of actin polymerization (cytochalasin D, CD) and tyrosine-phosphatases (phenylarsine oxide, PAO). However, forskolin, RGDS, and CD prevented the dephosphorylation step together with inhibition of platelet aggregation, whereas in the presence of phenylarsine oxide (PAO) the dephosphorylation step was replaced by an increase in the number and intensity of tyrosyl-phosphorylated proteins. Our data provide evidence to conclude that (i) cAMP inhibits platelet aggregation at a downstream site to PLCγ2 tyrosyl-phosphorylation; (ii) Cvx-induced PTP is independent on integrin αIIbβ3engagement, actin polymerization, and tyrosine-phosphatases activation; (iii) integrin αIIbβ3mediates the dephosphorylation step in a platelet aggregation-dependent manner; and (iv) Cvx and collagen stimulate platelets by a similar signal transduction pathway.