Differential Activation of Cytosolic Phospholipase A (cPLA) by Thrombin and Thrombin Receptor Agonist Peptide in Human Platelets. EVIDENCE FOR ACTIVATION OF cPLA2 INDEPENDENT OF THE MITOGEN-ACTIVATED PROTEIN KINASES ERK1/2

• Published in: The Journal of biological chemistry 1995-06, Vol.270 (24), p.14816
• Main Authors: Ruth M. Kramer, Edda F. Roberts, Paul A. Hyslop, Barbara G. Utterback, Kwan Y. Hui, Joseph A. Jakubowski
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.270.24.14816

The thrombin receptor agonist peptide SFLLRN was less effective than thrombin in eliciting the liberation of arachidonic acid and the generation of thromboxane A 2 by human platelets. We found that while SFLLRN evokes an initial transient increase in cytosolic free calcium concentration ([Ca ] ) of similar magnitude as that caused by thrombin, the SFLLRN-induced elevation of [Ca ] declines more rapidly to near resting levels than that evoked by thrombin, suggesting that disparate levels of [Ca ] may contribute to the attenuated arachidonic acid release. Furthermore, we observed that SFLLRN is less effective than thrombin in mediating the “activating” phosphorylation of cytosolic phospholipase A 2 (cPLA 2 ). Both thrombin and SFLLRN rapidly and transiently activated kinases that phosphorylate the 21-residue synthetic peptide Thr derived from the epidermal growth factor receptor, but the maximal activation of proline-directed kinases by SFLLRN was less pronounced than that by thrombin. MonoQ chromatography and immunoblot analysis of extracts from stimulated platelets revealed that while thrombin induced a prominent activation of the mitogen-activated protein kinases ERK1 and ERK2, SFLLRN completely failed to do so. On the other hand, SFLLRN, like thrombin, stimulated the activity of a proline-directed kinase distinct from ERK1/2, but the activation of this kinase was less pronounced following stimulation of platelets with SFLLRN compared with thrombin. We conclude 1) that the partial activation of cPLA 2 and the subsequent attenuated mobilization of arachidonic acid in response to SFLLRN may be the consequence of a less prolonged elevation of [Ca ] and insufficient activation of proline-directed kinase(s) by SFLLRN and 2) that the ability of SFLLRN to mediate the activating phosphorylation of cPLA 2 in the absence of ERK1/2 stimulation suggests that, at least in human platelets, proline-directed kinases other than ERK1/2 may phosphorylate and activate cPLA 2 .