Porcine von Willebrand factor and thrombin induce the activation of c-Jun amino-terminal kinase (JNK/SAPK) whereas only thrombin induces activation of extracellular signal-related kinase 2 (ERK2) in human platelets

The interaction of platelets with subendothelial von Willebrand factor (VWF ), especially under high shear stress, is considered to be the first activa tion step which primes platelets for subsequent haemostatic events. The sig nalling cascade which results from the interaction of VWF and its receptor GPIbIX has only been partially defined. Mitogen-activated protein kinases ( MAPKs) are a family of downstream transmembrane signalling serine-threonine kinases and have been demonstrated to be present and functional in platele ts; these include the extracellular signal-related kinases (ERKs), c-Jun am ino-terminal kinases (JNKs) and p38 MAPK. Previously, we showed that p38 MA PK was not required in VWF-induced human platelet activation. It is not kno wn whether VWF-dependent platelet activation involves the activation of the JNK and ERK family of signalling molecules. This report demonstrates that porcine von Willebrand factor (pVWF) induced a sustained and stable JNK act ivation measurable by 1 min after activation. Thrombin also induced JNK act ivation assessed at 1 min after activation. In contrast to thrombin, pVWF d id not induce ERK2 activation at any time point tested. To ensure that ERK activation was unnecessary for pVWF-dependent platelet activation, we funct ionally inhibited ERK-dependent signalling with PD98059, a potent and selec tive inhibitor of the MAP kinase kinase (MEK-1), which is the upstream kina se of ERK1 and ERK2. Although PD98059 inhibited ERK2 activation in platelet s, it had no effect on pVWF- or thrombin-induced platelet alpha or lysozoma l granule release, modulation of membrane glycoprotein CD41, microparticle formation, platelet shape change or platelet agglutination. It is concluded that pVWF and thrombin induced JNK activation, but whereas thrombin induce d ERK2 activation VWF did not; functional ERK2 activity was also not requir ed for pVWF- or thrombin-dependent platelet activation.