INCREASED SYNTHESIS AND SECRETION OF A 14-KDA PHOSPHOLIPASE-A(2) BY GUINEA-PIG ALVEOLAR MACROPHAGES - DISSOCIATION FROM ARACHIDONIC-ACID LIBERATION AND MODULATION BY DEXAMETHASONE

The occurrence of a 14-kDa secretory phospholipase A2 (PLA2) in guinea pig alveolar macrophages (AM) and its relationship with the release o f arachidonic acid (AA) were investigated. Freshly collected AM showed no detectable PLA2 activity as measured by the in vitro hydrolysis of phosphatidic acid. However, the PLA2 activity increased progressively when AM were maintained in culture to reach a level 60- to 100-fold g reater than basal values within 20 h, with a parallel secretion into t he incubation medium. By contrast, the activities of other phospholipi d-hydrolyzing enzymes (platelet-activating factor acetylhydrolase and lysophospholipase) were modified only marginally. Both intra- and extr acellular increases of PLA2 activity were abrogated with actinomycin D or cycloheximide. The enhanced PLA2 activity preferentially hydrolyze d negatively charged phospholipids in the order phosphatidic acid > ph osphatidylglycerol > phosphatidylethanolamine > phosphatidylcholine, h ad an optimum pH of 7.5, and required a millimolar Ca2+ concentration for optimal activity and an apparent molecular mass of 14 kDa. Taken t ogether, these results suggest that cultured AM elaborate an enzyme si milar to the group II PLA2. On the other hand, our results show that A M hydrolyzed exogenous 2-arachidonoyl phosphatidylcholine and released AA and metabolites on FMLP stimulation. However, in contrast to the i ncrease observed in the activity of the 14-kDa PLA2, the enzymatic act ivity involved in the hydrolysis of 2-arachidonoyl phosphatidylcholine and AA release remained constant with the culture duration of AM. Fin ally, dexamethasone markedly inhibited the increase of PLA2 activity, but only marginally inhibited the release of AA and metabolites from F MLP-stimulated AM. We conclude that guinea pig AM elaborate a 14-kDa P LA2 similar to the group II PLA2 through RNA-and protein synthesis-dep endent processes. This elaboration appears to be induced by the adhesi on of AM and is clearly dissociated from the liberation of AA.