Early platelet activation by low density lipoprotein via p38MAP kinase

Low Density Lipoprotein (LDL) is known to sensitize platelets for physiological agonists. To clarify the basis of this sensitization, we investigated the involvement of p38(MAP) Kinase(p38(MAPK)). As dual phosphorylation on Thr(180) and Tyr(182) of P38(MAPK) is the trigger for activation of the kinase, p38(MAPK)-activity was measured with an antibody that recognizes the dual-phosphorylated sequence. LDL induced a rapid and dose dependent activation of p38(MAPK). Th, activation was not inhibited by a wide variety of inhibitors of platelet signalling, including TxA(2)-formation, Phospholipase C-activation, Ca2+-mobilization and ERK 1/2-activation. Only a slight reduction in p38(MAPK)-activation was observed when protein kinase C was inhibited. Activation of p38(MAPK) was strongly inhibited by a rise in cAMP. Thus, p38(MAPK)-activation was upstream of most signalling pathways and close to the LDL-receptor. A number of platelet receptors was screened with the use of antibodies. Integrins alpha(IIb)beta(3) and alpha(2)beta(1), as well as the Fc gamma RII-receptor, CD36 (platelet glycoprotein IV), CD68 (gp110) and Low Density Lipoprotein-receptor related protein (LRP) were not implicated in LDL-induced p38(MAPK)-activation. Inhibition of LDL binding by modification of apo B100 lysines reduced p38(MAPK)-activation by 80 %. Activation of p38(MAPK) resulted in an increase in release of arachidonic acid, the precursor for thromboxane A(2) synthesis. In conclusion, activation of p38(MAPK) might be the first step in platelet sensitization by LDL, leading to formation of arachidonate metabolites and increased aggregation and secretion responses to physiological agonists.