Low-density lipoprotein activates the small GTPases Rap1 and Ral in human platelets

Physiological concentrations of low-density lipoprotein (LDL) sensitize blo od platelets to alpha-thrombin- and collagen-induced secretion, and after p rolonged contact trigger secretion independent of other agonists. Here we r eport that LDL activates the small GTPases Rap1 and Ra1 but not Ras, as ass essed by specific precipitation of the GTP-bound enzymes. In unstirred susp ensions, the inhibitor SB203580 blocks Rap1 activation by 60-70% suggesting activation via p38 mitogen-activated protein kinase and a second, unidenti fied route. Inhibitors of cyclooxygenase (indomethacin) and the thromboxane A(2) (TxA(2)) receptor (SQ30741) induce complete inhibition, indicating th at Rap1 activation is the result of TxA(2) formation. Stirring reveals a se cond, TxA(2)-independent Rap1 activation, which correlates quantitatively w ith a slow induction of dense granule secretion. Both pathways are unaffect ed by inhibitors of ligand binding to integrin alpha(IIb)beta(3). The resul ts suggest that Rap1 and Ra1, but not Ras, may take part in signalling rout es initiated by LDL that initially enhance the sensitivity of platelets to other agonists and later trigger LDL-dependent secretion.