Regulation of c-Jun-NH2 terminal kinase and extracellular-signal regulatedkinase in human platelets

Platelets are an interesting model for studying the relationship between ad hesion and mitogen-activated protein (MAP) kinase activation. We have recen tly shown that in platelets, ERK2 was activated by thrombin and downregulat ed by alpha(llb)beta(3) integrin engagement. Here we focused our attention on the c-Jun NH54-terminal kinases (JNKs) and their activation in condition s of platelet aggregation. We found that JNK1 was present in human platelet s and was activated after thrombin induction. JNK1 phosphorylation was dete cted with low concentrations of thrombin (0.02 U/mL) and after 1 minute of thrombin-induced platelet aggregation. JNK1 activation was increased (fivef old) when fibrinogen binding to alpha(llb)beta(3) integrin was inhibited by the Arg-Gly-Asp-Ser (RGDS) peptide or (Fab')(2) fragments of a monoclonal antibody specific for alpha(llb)beta(3) demonstrating that, like ERK2, alph a(llb)beta(3) integrin engagement negatively regulates JNK1 activation. Com parison of JNK1 activation by thrombin in stirred and unstirred platelets i n the presence of RGDS peptide showed a positive regulation by stirring its elf, independently of alpha(llb)beta(3) integrin engagement, which was conf irmed in a thrombasthenic patient lacking platelet alpha(llb)beta(3) The sa me positive regulation by stirring was found for ERK2. These results sugges t that MAP kinases (JNK1 and ERK2) are activated positively by thrombin and stirring. In conclusion, we found that JNK1 is present in platelets and ca n be activated after thrombin induction. Moreover, this is the first report showing that two different MAP kinases (ERK2 and JNK1) are regulated negat ively by alpha(llb)beta(3) engagement and positively by mechanical forces i n platelets. (C) 1999 by The American Society of Hematology.