p38 Mitogen-activated Protein Kinase Phosphorylates Cytosolic Phospholipase A 2 (cPLA 2 ) in Thrombin-stimulated Platelets

The Ca 2+ -sensitive 85-kDa cytosolic phospholipase A 2 (cPLA 2 ) is responsible for thrombin-stimulated mobilization of arachidonic acid for the synthesis of thromboxane A 2 in human platelets. We have previously shown that thrombin activates p38 kinase, a recently discovered new member of the mitogen-activated protein kinase family (Kramer, R. M., Roberts, E. F., Strifler, B. A., and Johnstone, E. M. (1995) J. Biol. Chem. 270, 27395-27398) and also induces phosphorylation of cPLA 2, thereby increasing its intrinsic catalytic activity. In the present study we have examined the role of p38 kinase in the phosphorylation and activation of cPLA 2 in stimulated platelets. We have observed that activation of p38 kinase accompanies receptor-mediated events in platelets and coincides with cPLA 2 phosphorylation. Furthermore, in the presence of inhibitors of p38 kinase, the proline-directed phosphorylation of cPLA 2 was completely blocked in platelets stimulated with the thrombin receptor agonist peptide SFLLRN and was suppressed during the early (up to 2 min) phase of platelet stimulation caused by thrombin. Unexpectedly, we found that prevention of proline-directed phosphorylation of cPLA 2 in stimulated platelets did not attenuate its ability to release arachidonic acid from platelet phospholipids. We conclude that: 1) cPLA 2 is a physiological target of p38 kinase; 2) p38 kinase is involved in the early phosphorylation of cPLA 2 in stimulated platelets; and 3) proline-directed phosphorylation of cPLA 2 is not required for its receptor-mediated activation.