1,3-Dioctanoylglycerol (1,3-DiC8) Is as Effective as 1,2-Dioctanoylglycerol (1,2-DiC8) in Priming Phospholipase A2 Activation in Human Platelets and Neutrophils

In the present study, we investigated the effects of different diacylglycerols in comparison with phorbol 12-myristate 13-acetate (PMA) on eicosanoid-independent phospholipase A₂ (PLA₂) activation in human platelets and neutrophils. Eicosanoid-independent PLA₂ activation was measured under conditions where both cyclooxygenase and lipoxygenases were blocked by BW755C. In the presence of PMA (50 nM), the amount of mass arachidonic acid (AA) released represented 400 and 257% of control (without PMA) in A23187-stimulated platelets and neutrophils, respectively, while 1,2-dioctanoylglycerol (1,2-DiC₈) and 1-oleoyl-2-acetyl-sn-glycerol (OAG) had increased the eicosanoid-independent AA release by 150 and 117-134% of control, in platelets and neutrophils, respectively. Our results further demonstrate that 1,3-dioctanoylglycerol (1,3-DiC₈), a poor activator of protein kinase C (PKC), is nearly as effective as diacylglycerols, such as OAG and 1,2-DiC₈ (activators of PKC) in priming PLA₂ activation, but is less effective than PMA as a priming agent. However, all three diacylglycerols were less effective than PMA as priming agents. Furthermore, diacylglycerols including 1,3-DiC₈ exerted a much greater effect on PLA₂ activation in platelets than in neutrophils. Neither 1,3-DiC₈ nor 1,2-DiC₈ and OAG had any significant priming effect on the accumulation of palmitic and stearic acids, while PMA caused a substantial accumulation of these fatty acids in platelets, but not in neutrophils. We also found that exogenously added OAG underwent significant hydrolysis even in unstimulated platelets, but not in neutrophils, suggesting that exogenously added OAG may be readily accessible for diacylglycerol (DAG) lipase/PLA₁ in platelets. It is possible that the priming of PLA₂ by diacylglycerols in both cell types may involve a PKC-independent mechanism, whereas that by PMA may involve both PKC-dependent and PKC-independent mechanisms. The differential effects of PMA and diacylglycerols on PLA₂/DAG lipase activation observed between platelets and neutrophils may stem from the differences in the predominance of certain enzyme isoforms, requiring specific factors such as cytosolic/exogenous Ca²⁺, receptor-agonist interaction, enzyme-diacylglycerol interactions, and PKC and tyrosine kinase mediated phosphorylations.