Regulation of cytosolic PlA₂ activity by PP1/PP2A serine/threonine phosphatases in human platelets

Abstract

Platelet thromboxane A₂ (TXA₂) synthesis is an important pathway of platelet reactivity. We report that in thrombin-stimulated platelets, PP1/PP2A serine/threonine phosphatases regulate phospholipase A₂ (cPLA₂) activity, which is required for TXA₂ synthesis. Two mechanisms are involved: (a) constitutively active PP1/PP2A regulate cPLA₂ phosphorylation, and (b) PP1/PP2A activity mediates agonist-induced increase in cytosolic Ca²⁺ ([Ca²⁺]_i). Inhibition of PP1/PP2A with okadaic acid (OA) induces cPLA₂ phosphorylation but reduces Ca²⁺ responses: release from intracellular stores and influx through the plasma membrane, particularly that mediated by store-mediated Ca²⁺ entry (SMCE). A significant correlation (r = 0.64) exists between OA-regulated [Ca²⁺]_i and TXA₂ synthesis. Okadaic acid-induced decrease in SMCE and the associated TXA₂ synthesis are mediated by a reduction in protein-tyrosine phosphorylation. This reduction is not due to inhibition of tyrosine kinases but rather to an OA-mediated increase in tyrosine phosphatases. This is the first study to report that PP1/PP2A phosphatases are involved in the regulation of the two key elements in eicosanoid synthesis, [Ca²⁺]_i and cPLA₂ phosphorylation. Moreover, PP1/PP2A regulation of [Ca²⁺]_i and tyrosine phosphorylation may be important for other calcium-dependent processes and/or signal transduction mechanisms in platelets.

• Cytosolic PLA2
• serine/threonine phosphatases PP1/PP2A
• platelets
• tyrosine phosphorylation