Comparison of alkylacylglycerol vs. diacylglycerol as activators of mitogen-activated protein kinase and cytosolic phospholipase A(2) in human neutrophil priming

In human neutrophils, the choline-containing phosphoglycerides contain almost equal amounts of alkylacyl- and diacyl-linked subclasses. In contrast to phosphatidylinositol hydrolysis which yields diacylglycerol, hydrolysis of choline-containing phosphoglycerides by phospholipase D coupled with phosphohydrolase yields both alkylacyl- and diacylglycerol. While diacylglycerol activates protein kinase C, alkylacylglycerol does not, and its role is unclear. Yet previous studies have shown that exogenous alkylacyl- and diacylglycerols can prime for the release of radiolabeled arachidonic acid (AA) in intact neutrophils stimulated by formyl-methionyl-leucyl-phenyl alanine. We have now examined the effects of both diacylglycerol (1-oleoyl-2-acetylglycerol; OAG) and alkylacylglycerol (1-0-hexadecyl-2-acetylglycerol; EAG) on the activation of mitogen-activated protein (MAP) kinase and the 85-kDa cytosolic phospholipase A(2) (cPLA(2)) in human neutrophils. We observed that while OAG could effectively activate p42 and p44 MAP kinases along with cPLA(2) in a time and concentration-dependent manner, EAG could not. A novel p40 MAP kinase isoform is also present and activated in response to OAG treatment; the behavior of this MAP kinase isoform is discussed. The activation of cPLA(2) and MAP kinase by 20 mu M OAG could be inhibited by pretreatment with 1 mu M GF-109203X, a selective inhibitor of protein kinase C. Although only OAG activated cPLA(2), both OAG and EAG primed for the release of AA mass as determined by gas chromatography/mass spectrometry. The priming of AA release by OAG may be explained by the phosphorylation of cPLA(2) through the activation of protein kinase C linked to MAP kinase. However, priming by EAG appears to involve a separate mechanism that is dependent on a different PLA(2). Our results support a role for phospholipase D-derived products modulating the activation of cPLA(2), further supporting the idea of cross-talk among various phospholipases. (C) 1997 Elsevier Science B.V.