CoA-independent transacylase has characteristics distinct from those of PLA2 enzymes

Evidence that 85 kDa phospholipase A2 is not linked to CoA-independent transacylase-mediated production of platelet-activating factor in human monocytes

Platelet-activating factor (PAF) production is carefully controlled in inflammatory cells. The specific removal of arachidonate (AA) from 1-O-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine (GPC), thought to be mediated by CoA-independent transacylase (CoA-IT), is required to generate the PAF precursor 1-O-alkyl-2-lyso-GPC in human neutrophils. Exposure of A23187-stimulated human monocytes to the CoA-IT inhibitors SK&F 98625 and SK&F 45905 inhibited PAF formation (IC50s of 10 and 12 microM, respectively), indicating that these cells also need CoA-IT activity for PAF production. Because CoA-IT activity transfers arachidonate to a 2-lyso phospholipid substrate, its activity is obligated to an sn-2 acyl hydrolase to form the 2-lyso phospholipid substrate. SB 203347, an inhibitor of 14 kDa phospholipase A2 (PLA2), and AACOCF3, an inhibitor of 85 kDa PLA2, both inhibited AA release from A23187-stimulated human monocytes. However, AACOCF3 had no effect on A23187-induced PAF formation at concentrations as high as 3 microM. Further, depletion of 85 kDa PLA2 using antisense (SB 7111, 1 microM) had no effect on PAF production, indicating a lack of a role of 85 kDa PLA2 in PAF biosynthesis. Both SB 203347 and the 14 kDa PLA2 inhibitor scalaradial blocked PAF synthesis in monocytes (IC50s of 2 and 0.5 microM, respectively), suggesting a key role of 14 kDa PLA2 in this process. Further, A23187-stimulated monocytes produced two forms of PAF: 80% 1-O-alkyl-2-acetyl-GPC and 20% 1-acyl-2-acetyl-GPC, which were both equally inhibited by SB 203347. In contrast, inhibition of CoA-IT using SK&F 45905 (20 microM) had a greater effect on the production of 1-O-alkyl (-80%) than of 1-acyl (-14%) acetylated material. Finally, treatment of U937 cell membranes with exogenous human recombinant (rh) type II 14 kDa PLA2, but not rh 85 kDa PLA2, induced PAF production. Elimination of membrane CoA-IT activity by heat treatment impaired the ability of 14 kDa PLA2 to induce PAF formation. Taken together, these results suggest that a 14 kDa PLA2-like activity, and not 85 kDa PLA2, is coupled to monocyte CoA-IT-induced PAF production.

Fatty acid and phospholipid selectivity of different phospholipase A2 enzymes studied by using a mammalian membrane as substrate

Previous studies using phospholipid mixed vesicles have demonstrated that several types of phospholipase A2 (PLA2) enzymes exhibit different selectivity for fatty acids at the sn-2 position, for the type of chemical bond at the sn-1 position or for the phosphobase moiety at the sn-3 position of phospholipids. In the present study, we have utilized natural mammalian membranes from U937 monocytes to determine whether two purified 14 kDa PLA2 isoenzymes (Type I, Type II) and a partially purified 110 kDa PLA2 exhibit substrate selectivity for certain fatty acids or phospholipids. In these studies, arachidonic acid (AA) release from membranes was measured under conditions where the remodelling of AA mediated by CoA-independent transacylase (CoA-IT) activity has been eliminated. In agreement with the mixed-vesicle models, AA was the major unsaturated fatty acid hydrolysed from membranes by the 110 kDa PLA2, suggesting that this PLA2 is selective in releasing AA from natural membranes. By contrast, Type I and Type II PLA2s were less selective in releasing AA from phospholipids and released a variety of unsaturated fatty acids at molar ratios that were proportional to the ratios of these fatty acids in U937 microsomal membranes. Examination of AA release from phospholipid classes indicated that all three enzymes released AA from the major AA-containing phospholipid classes (phosphatidylethanolamine, phosphatidylcholine, and phosphatidylinositol) of U937 membranes. The 110 kDa PLA2 released AA from phospholipid subclasses in ratios that were proportional to the AA content within phospholipid classes and subclasses of U937 membranes. These data suggested that the 110 kDa PLA2 shows no preference either for the sn-1 linkage or for the sn-3 phosphobase moiety of phospholipids. By contrast, Type I and Type II PLA2s preferentially released AA from ethanolamine-containing phospholipids and appeared to prefer the 1-acyl-linked subclass. Taken together, these data indicate that the 110 kDa PLA2 selectively releases AA from U937 membranes, whereas Type I and Type II PLA2 release a variety of unsaturated fatty acids. Furthermore, the 110 kDa PLA2 releases the same molar ratios of AA from all major phospholipid subclasses, whereas Type I and Type II PLA2s show some specificity for phosphatidylethanolamine when these enzymes are incubated with a complex mammalian membrane substrate.

Interleukin-1-mediated phospholipid breakdown and arachidonic acid release in human synovial cells

OBJECTIVE

Interleukin-1 (IL-1), an important mediator contributing to joint destruction in rheumatoid arthritis, is known to stimulate the release of arachidonic acid (AA) and prostaglandin E2 (PGE2) from adherent synoviocytes. To study the intracellular pathways involved in these functions, we stimulated cultures of human synovial cells with recombinant IL-1 beta.

METHODS

AA liberation was measured after labeling synovial cells with 3H-AA, and PGE2 levels were determined by high performance liquid chromatography or radioimmunoassay. Identification of 3H-AA-labeled phospholipids was performed by thin layer chromatography. Cell-associated phospholipase A2 (PLA2) enzymatic activity was determined by an assay with cell-free systems and exogenous substrates.

RESULTS

Stimulation of synovial cells with recombinant IL-1 beta induced a decrease in phosphatidylcholine (PC), phosphatidylinositol (PI), and phosphatidylethanolamine (PE), and a marked increase in cell-associated PLA2 activity as compared with controls. In the presence of either quinacrine, an inhibitor of PLA2 pathway activation, or neomycin, which binds to PI mono- and biphosphate thus blocking their degradation by phospholipases, AA and PGE2 secretion were reduced in a dose-dependent manner. Kinetic studies revealed that quinacrine had little blocking activity on the IL-1-mediated AA release after 1 hour of stimulation but completely abolished it after 5 or 8 hours. In contrast, neomycin exerted a partial but significant inhibitory effect from the first hour of stimulation onward. Addition of quinacrine was also demonstrated to abolish the IL-1-induced hydrolysis of PC and PE but not PI, indicating that PC and PE are the preferred substrates for PLA2 enzymatic activity in human synovial cells.

CONCLUSION

Our findings strongly suggest that AA and PGE2 production by IL-1-triggered synoviocytes are largely dependent upon PLA2-mediated hydrolysis of PC and PE and to a lesser extent upon the earlier degradation of PI.

Porcine aortic endothelial cell membranes contain a LPAF: CoA-independent transacylase

Membranes isolated from porcine aortic endothelial cells (PAEC) contain a CoA-independent transacylase enzyme (CoA-IT). CoA-IT, an integral membrane protein, transfers an acyl moiety to added [3H]alkylhydroxyglycerophosphocholine (LPAF) to generate [3H]alkylacylglycerophosphocholine (alkylacyl-GPC). This enzyme exhibits an apparent Km of 0.7 microM and a Vmax of 0.8 nmol/min per mg for the transfer of an acyl group to added [3H]LPAF. The addition of the nonionic detergent Triton X-100 (TX-100) (0.5 mg/ml), the sulfhydryl reagents N-ethylmaleimide (NEM) (200 microM) or thimerosal (200 microM), or pre-incubating the membranes at 95 degrees C for 10 min all decreased LPAF: CoA-IT activity by more than 95%. The inhibitory action of NEM or thimerosal suggests that sulfhydryl group(s) are involved in or are close to the catalytic site of LPAF: CoA-IT.

Evidence for different mechanisms involved in the formation of lyso platelet-activating factor and the calcium-dependent release of arachidonic acid from human neutrophils

Recent studies suggest that the first step in platelet-activating factor (PAF) biosynthesis, 1-alkyl-2-lyso-GPC (lyso PAF) formation, may be initiated by the selective transfer of arachidonate from 1-alkyl-2-arachidonoyl-GPC to an acceptor lyso phospholipid by a CoA-independent transacylase activity (CoA-IT). The present study was designed to determine whether the formation of 1-alkyl-2-lyso-GPC and the release of arachidonic acid can occur by different mechanisms. These experiments examined both the formation of 1-[3H]alkyl-2-lyso-GPC from 1-[3H]alkyl-2-acyl-GPC and the release of arachidonic acid from membrane phospholipids as determined by GC/MS in neutrophil homogenates under various conditions. The addition of unlabelled lyso phospholipids to neutrophil homogenates stimulated the time-dependent formation of 1-[3H]alkyl-2-lyso-GPC from 1-[3H]alkyl-2-acyl-GPC. Without exogenous lyso phospholipids, little 1-[3H]alkyl-2-lyso-GPC was formed in this reaction. The activity which catalyzed the formation of 1-[3H]alkyl-2-lyso-GPC had characteristics identical to CoA-IT as indicated by the fact that both reactions were: independent of Ca2+, Mg2+, CoA and CoA fatty acids, located in microsomal fractions, and stable in 10 mM dithiothreitol. In sharp contrast to the aforementioned reaction, addition of lyso phospholipids did not affect the quantity of arachidonic acid released from membrane phospholipids. Furthermore, there was a Ca(2+)-independent release of arachidonic acid from membrane phospholipid that was increased 4 to 5-fold after the addition of 5 mM Ca2+. Finally, Ca(2+)-dependent arachidonic acid release was inhibited by putative phospholipase A2 inhibitors, aristolochic acid and scalaradial, at concentrations where neither the production of 1-[3H]alkyl-2-lyso-GPC nor Ca(2+)-independent arachidonic acid release was altered. Together these data imply that there may be different mechanisms involved in the formation of 1-alkyl-2-lyso-GPC and arachidonic acid from membrane phospholipids.