Costimulation of the Gi‐coupled ADP receptor and the Gq‐coupled TXA 2 receptor is required for ERK2 activation in collagen‐induced platelet aggregation

The stimulation of platelets by low doses of collagen induces extracellular signal‐regulated kinase 2 (ERK2) activation. In this report, we demonstrate that collagen‐induced ERK2 activation depends on thromboxane A 2 (TXA 2 ) formation and ADP release. The collagen‐induced ERK2 activation was inhibited by indomethacin (88%) and by AR‐C69931MX (70%), a specific antagonist of P2Y12, a Gi‐coupled ADP receptor. AR‐C69931MX (10 μM) inhibition was overcome by epinephrine (1 μM), an agonist of the Gi‐coupled α 2A ‐adrenergic receptor, suggesting that the Gi‐coupled receptor was necessary for ERK2 activation by collagen. By contrast, MRS 2179 (10 μM), a specific antagonist of P2Y1, a Gq‐coupled ADP receptor, did not affect collagen‐induced ERK2 activation. Little or no ERK2 activation was observed with ADP alone (10 μM). By contrast, U46619 (10 μM), a stable analog of TXA 2 , induced ERK2 activation in an ADP‐dependent manner, via the P2Y12 receptor. These results suggest that the Gi‐dependent signaling pathway, stimulated by ADP or epinephrine, was not the only pathway required for ERK2 activation by collagen. Costimulation of the specific G 12/13 ‐coupled TXA 2 receptor with a low dose of U46619 (10 nM) and of Gi‐ and Gq‐coupled ADP receptor (10 μM) induced very low levels of ERK2 activation, similar to those observed with ADP alone, suggesting that G 12/13 is not involved or not sufficient to induce the additional pathway necessary for ERK2 activation. The Gq‐coupled TXA 2 receptor was required for ERK2 activation by U46619 (10 μM) and low doses of collagen, clearly showing that a coordinated pathway through both Gq from TXA 2 and Gi from ADP was necessary for ERK2 activation. Finally, we demonstrate that ERK2 activation is involved in collagen‐induced aggregation and secretion.