Characterisation of Rac activation in thrombin- and collagen-stimulated human blood platelets

In this study, we characterised the mechanisms of Rac GTPase activation in human platelets stimulated by two physiological agonists, either thrombin, acting through membrane receptors coupled to heterotrimeric G-proteins, or collagen which is known to mobilise a tyrosine kinase-dependent pathway. Bo th agonists induced a rapid activation of Rac that was not significantly af fected by the inhibition of integrin alpha (11b)beta (3) engagement. Using pharmacological inhibitors, we found that phospholipase C activation and ca lcium mobilisation were essential for platelet Rac activation by either thr ombin or collagen whereas protein kinase C inhibition was without effect. I n contrast to Rac, Cdc42 activation was independent of phospholipase C acti vation, indicating that the two GTPAses are differently regulated. We also found that phosphoinositide 3-kinase was not required for Rac activation in response to thrombin but was involved in its activation by collagen. (C) 2 001 Federation of European Biochemical Societies. Published by Elsevier Sci ence B.V. All rights reserved.