R59022, A DIACYLGLYCEROL (DG) KINASE INHIBITOR POTENTIATES THROMBIN-INDUCED PLATELET AGGREGATION AND GRANULE RELEASE
R59022 is a recently described inhibitor of the enzyme DG kinase [1], which converts DG to phosphatidic acid. While R59002 inhibits DG conversion in platelets resulting in enhanced protein kinase C (PrkC) activation [1], little is known on its effect on other platelet responses. In this study, we have examined the effect of R59022 on agonist-induced platelet aggregation and [14C]-5-hydroxytryptamine (5HT) release using washed human platelets. With a sub-maximal concentration of thrombin (T, 0.05U/ml) R59022 (10-30μM) significantly potentiated T-induced platelet aggregation and [14C]-5HT release eg % [14C]-5HT release:- 0.05U/ml T-52±5,30μM R59022+T-76±8. Removal of external Ca2+ (ImM) using EGTA (5mM) reduced T-induced 5HT release but not the potentiation of it by R59022 eg EGTA+ 0.05U/ml T-36±6%, EGTA+R59022+T- 72±5%. These results show that in the presence of EGTA and R59022 the increased DG levels can compensate for the diminished rise in T-induced Ca/2+ mobilisation thus re-emphasizing the importance of DG in promoting granule secretion. In addition to inhibiting DG phosphorylation, R59022 also inhibits the phosphorylation of the DG analogue 1-oleoyl 2-acetylglycerol (OAG) [1]. OAG (63μM) with pre-incubation times of 10-60 sec, significantly potentiated threshold T (0.03U/ml)-induced [l4C]-5HT release, though with longer incubation times, this potentiatory effect was gradually lost eg 0.03U/ml T-l±0.3%, OAG+T (10 sec)- 33±4%, OAG+T (1 min)-11±3%, 0AG+veh.-0%. However, in the presence of R59022 (30μM), OAG retained its potentiatory effect for longer periods eg R59022+0AG+T (1 min)-45+10%, R59022+T-2±l%. With incubation times > 5 min the potentiatory effects of OAG were lost even in the presence of R59022. This is possibly due to the metabolism of OAG by DG lipase. Our results demonstrate that R59022, which has been reported to inhibit DG kinase leading to enhanced PrkC activation, also enhances agonist-induced platelet aggregation and 5HT release. It may therefore be a useful compound in elucidating further the role of DG in terms of both stimulatory and inhibitory effects on platelet activation.[1]. de Chaffoy de Coucelles, D. et al (1985) J Biol Chem 260, 15762.