REVERSIBLE TRANSLOCATION OF PHOSPHOINOSITIDE 3-KINASE TO THE CYTOSKELETON OF ADP-AGGREGATED HUMAN PLATELETS OCCURS INDEPENDENTLY OF RHO-A AND WITHOUT SYNTHESIS OF PHOSPHATIDYLINOSITOL (3,4)-BISPHOSPHATE

The aim of our study was to evaluate the effect of ADP and the role of cytoskeleton reorganization during re versible and irreversible plate let aggregation induced by ADP and thrombin, respectively, on the hete rodimeric (p85 alpha-p110) phosphoinositide 3-kinase translocation to the cytoskeleton and its activation. Reversible ADP-induced aggregatio n was accompanied by a reversible reorganization of the cytoskeleton a nd an increase in levels of the regulatory subunit p85 alpha in this c ytoskeleton similar to the increase observed in thrombin-activated pla telets. This translocation followed a course parallel to the amplitude of aggregation. No increase in levels of both phosphatidylinositol (3 ,4)-bisphosphate (PtdIns(3,4)P-2) and phosphatidylinositol-(3,4,5)P-3 could, however, be detected even at the maximum aggregation and PI3-ki nase alpha translocation. Moreover, in contrast to the situation for t hrombin stimulation, the GTP-binding protein RhoA was hardly transloca ted to the cytoskeleton when platelets were stimulated with ADP, where as translocation of pp60(c-scr) and focal adhesion kinase did occur. T hese results suggest (i) translocation of signaling enzymes does not n ecessarily imply their activation, (ii) the reversibility of ADP induc ed platelet aggregation may be the cause or the result of a lack of PI 3-kinase activation and hence of PtdIns(3,4)P-2 production, and (iii) RhoA does not seem to be involved in the ADP activation pathway of pl atelets. Whether PtdIns(3,4)P-2 or RhoA may contribute to the stabiliz ation of platelet aggregates remains to be established.