The effects of an Inhibitor of Diglyceride Lipase on Collagen-induced Pltelet Activation

Human platelet activation by collagen occurs in a dose-dependent manner. High concentrations of collagen bind to a pair of receptors, the α2ß1 integrin and GPVI/FcRγ, which stimulate a cascade of events including Syk, LAT, Btk , Gads and phospholipase Cγ2, leading to calcium release and protein kinase C (PKC) activation. Calcium and PKC are responsible for a range of platelet responses including exocytosis and aggregation, as well as the cytosolic phospholipase A2 (cPLA2)-mediated release of arachidonic acid which is converted to thromboxane (Tx) A2. In contrast low concentrations of collagen are acutely aspirin sensitive, and calcium release and aggregation are TxA2–dependent. Under these conditions cPLA2 is not involved and it has been suggested that phospholipase C generates diglyceride (DG) from which arachidonic acid is liberated by diacylglycerol lipase (DGL). Here a novel DGL blocker (OMDM-188), inhibited collagen-, but not arachidonic acid-, induced aggregation and TxA2 synthesis. Furthermore OMDM-188 inhibited collagen-induced arachidonic acid release. Finally OMDM-188 inhibited collagen-induced p38MAPK, but not ERK, phosphorylation, with no effect on the phosphorylation of either enzyme in response to arachidonic acid. Taken together these data suggest a role for a pathway involving PLC liberating DG from membrane phospholipids in response to minimallyactivate concentrations of collagen. The DG serves as a substrate for DGL, potentially under the regulations of p38MAPK, to release arachidonic acid which is subsequently converted to TxA2 which mediates the final platelet response.