Depletion of human monocyte 85-kDa phospholipase A2 does not alter leukotriene formation

Ionophore A23187-induced leukotriene biosynthesis in equine granulocytes-neutrophils, but not eosinophils require exogenous arachidonic acid

Equine granulocyte suspensions, mainly consisting of neutrophils, failed to produce detectable amounts of leukotrienes when stimulated with calcium ionophore A23187 alone, whereas leukotrienes were dose-dependently formed in control incubations with human granulocytes. In contrast, ionophore A23187 initiated synthesis of leukotrienes B4 and C4 in equine granulocytes when added in combination with low concentrations of exogenous arachidonic acid. Similarly, ionophore A23187 provoked leukotriene biosynthesis when added alone to human whole blood, whereas addition of exogenous arachidonic acid was a prerequisite for ionophore A23187-induced leukotriene formation in equine whole blood. Leukotriene biosynthesis was provoked by A23187 alone after addition of homologous platelets to equine granulocyte suspensions. After separation of equine neutrophils and eosinophils, purified eosinophil suspensions produced LTC4 after stimulation with ionophore A23187 alone, whereas exogenous arachidonic acid was required for ionophore-induced LTB4 formation in purified neutrophil suspensions. Leukotriene synthesis in both eosinophils and neutrophils was suppressed by the 5-lipoxygenase activating protein (FLAP) inhibitor, MK-886. Exogenous arachidonic acid was needed for ionophore-induced leukotriene synthesis also in bovine granulocytes, but was not a prerequisite for the production of leukotrienes in porcine granulocytes or in rat and rabbit white blood cell suspensions. The results indicate differences in the mechanisms regulating leukotriene synthesis in equine neutrophils, as compared to human granulocytes or equine eosinophils, and suggest that elevation of intracellular calcium is an insufficient stimulus to provoke utilisation of endogenous arachidonic acid for leukotriene synthesis in equine neutrophils.

Segregated Coupling of Phospholipases A2, Cyclooxygenases, and Terminal Prostanoid Synthases in Different Phases of Prostanoid Biosynthesis in Rat Peritoneal Macrophages

We examined herein the functional linkage of enzymes regulating the initial, intermediate, and terminal steps of PG biosynthesis to provide PGs in rat peritoneal macrophages stimulated with LPS and/or A23187. Quiescent cells stimulated with A23187 produced thromboxane B2 (TXB2) in marked preference to PGE2 within 30 to 60 min (constitutive immediate response), which was mediated by preexisting cytosolic phospholipase A2 (cPLA2), cyclooxygenase-1 (COX-1), and TX synthase. Cells treated with LPS predominantly produced PGE2 during culture for 3 to 24 h (delayed response), where cPLA2 and secretory PLA2 functioned cooperatively with inducible COX-2, which was, in turn, coupled with inducible PGE2 synthase. Cells primed for 12 h with LPS and stimulated for 30 min with A23187 produced PGE2 in marked preference to TXB2 (induced immediate response), in which three inducible enzymes, cPLA2, COX-2, and PGE2 synthase, were functionally linked. Preferred coupling of the two inducible enzymes, COX-2 and PGE2 synthase, was further confirmed by the ability of LPS-treated cells to convert exogenous arachidonic acid to PGE2 optimally at a time when both enzymes were simultaneously induced. These results suggest that distinct PG biosynthetic enzymes display segregated functional coupling following different transmembrane stimulation events even when enzymes that catalyze similar reactions in vitro coexist in the same cells.

Cytosolic phospholipase A2 is required for cytokine-induced expression of type IIA secretory phospholipase A2 that mediates optimal cyclooxygenase-2-dependent delayed prostaglandin E2 generation in rat 3Y1 fibroblasts

Activation of rat fibroblastic 3Y1 cells with interleukin-1 beta (IL-1 beta) and tumor necrosis factor alpha (TNF alpha) induced delayed prostaglandin (PG) E2 generation over 6-48 h, which occurred in parallel with de novo induction of type IIA secretory phospholipase A2 (sPLA2) and cyclooxygenase (COX)-2, without accompanied by changes in the constitutive expression of type IV cytosolic PLA2 (cPLA2) and COX-1. Types V and IIC sPLA2s were barely detectable in these cells. Studies using an anti-type IIA sPLA2 antibody, sPLA2 inhibitors, and a type IIA sPLA2-specific antisense oligonucleotide revealed that IL-1 beta/TNF alpha-induced delayed PGE2 generation by these cells was largely dependent on inducible type IIA sPLA2, which was functionally linked to inducible COX-2. Delayed PGE2 generation was also suppressed markedly by the cPLA2 inhibitor arachidonoyl trifluoromethyl ketone (AACOCF3), which attenuated induction of type IIA sPLA2, but not COX-2, expression. AACOCF3 inhibited the initial phase of cytokine-stimulated arachidonic acid release, and supplementing AACOCF3-treated cells with exogenous arachidonic acid partially restored type IIA sPLA2 expression. These results suggest that certain metabolites produced by the cPLA2-dependent pathway are crucial for the subsequent induction of type IIA sPLA2 expression and attendant delayed PGE2 generation. Some lipoxygenase-derived products might be involved in this event, since IL-1 beta/TNF alpha-induced type IIA sPLA2 induction and PGE2 generation were reduced markedly by lipoxygenase, but not COX, inhibitors. In contrast, Ca2+ ionophore-stimulated immediate PGE2 generation was regulated predominantly by the constitutive enzymes cPLA2 and COX-1, even when type IIA sPLA2 and COX-2 were maximally induced after IL-1 beta/TNF alpha treatment, revealing functional segregation of the constitutive and inducible PG biosynthetic enzymes.

Cytosolic 85-kDa phospholipase A2-mediated release of arachidonic acid is critical for proliferation of vascular smooth muscle cells

Recent evidence suggests that arachidonic acid (AA) may be involved in regulating cellular proliferation. The predominant mechanism of AA release from cellular phospholipids is via phospholipase A2 (PLA2) hydrolysis. The purpose of this study was to examine the roles of the distinct 14-kDa and 85-kDa PLA2 enzymes in human coronary artery vascular smooth muscle cell (hCAVSMC) proliferation. Cultured hCAVSMCs proliferate in the presence of growth medium with a typical doubling time of 30-40 h, grow at a slower proliferative rate upon reaching confluency (day 8), and eventually undergo contact inhibition of growth (day 10). Neither Type II 14-kDa PLA2 activity nor mass changed over a 10-day culture period. In contrast, 85-kDa PLA2 protein activity and mRNA decreased as time in culture progressed. This reduction in 85-kDa PLA2 correlated with reductions in DNA synthesis and suggested a possible association between 85-kDa PLA2 and proliferation. To directly evaluate the role of the 85-kDa PLA2 in proliferation we examined the effects of an 85-kDa PLA2 inhibitor (AACOCF3) and 85-kDa PLA2 antisense oligonucleotides on proliferation. Both reagents dose dependently inhibited proliferation, whereas a 14-kDa PLA2 inhibitor (SB203347), a calcium-independent PLA2 inhibitor (HELSS), an 85-kDa sense oligonucleotide, and a nonrelevant scrambled control oligonucleotide had no effect. The mechanism by which 85-kDa PLA2 influences cellular proliferation remains unclear. Inhibition of 85-kDa PLA2 activity produced neither phase-specific cell cycle arrest nor apoptosis (fluorescence-activated cell sorter analysis). Addition of AA (20 mu M) attenuated the effects of both AACOCF3 and 85-kDa antisense oligonucleotides implicating AA as a key mediator in cellular proliferation. However, although prostaglandin E2 (PGE2) was present in the culture medium, it peaked early (day 3) in culture, and indomethacin had no effect on cellular proliferation indicating that hCAVSMC proliferation was not mediated through PGE2. These data provide the first direct evidence that PLA2 is involved in control of VSMC proliferation and indicate that 85-kDa PLA2-mediated liberation of AA is critical for cellular proliferation.

Differential regulation of cyclo-oxygenase-2 and 5-lipoxygenase-activating protein (FLAP) expression by glucocorticoids in monocytic cells

1. The objective of the present study was to determine the effects of dexamethasone on key constituents of prostaglandin and leukotriene biosynthesis, cyclo-oxygenase-2 (COX-2) and 5-lipoxygenase activating protein (FLAP). The human monocytic cell line THP-1 was used as a model system. mRNA and protein levels of COX-2 and FLAP were determined by Northern and Western blot analyses, respectively. 2. Low levels of COX-2 and FLAP mRNA were expressed in undifferentiated THP-1 cells, but were induced upon differentiation of the cells along the monocytic pathway by treatment with phorbol ester (TPA, 5 nM). Maximal expression was observed after two days. 3. Coincubation of the undifferentiated cells with dexamethasone (10(-9) - 10(-6) M) and phorbol ester prevented induction of COX-2 mRNA, but did not affect the induction of FLAP mRNA. 4. Dexamethasone downregulated COX-2 mRNA and protein in differentiated, monocyte-like THP-1 cells. In contrast, FLAP mRNA and protein were upregulated by dexamethasone in differentiated THP-1 cells. After 24 h, FLAP mRNA levels were increased more than 2 fold. Dexamethasone did not change 5-lipoxygenase mRNA expression. 5. Release of prostaglandin E2 (PGE2) and peptidoleukotrienes was determined in cell culture supernatants of differentiated THP-1 cells by ELISA. Calcium ionophore-dependent PGE2 synthesis was associated with COX-2 expression, whereas COX-1 and COX-2 seemed to participate in arachidonic acid-dependent PGE2 synthesis. Very low levels of peptidoleukotrienes were released from differentiated THP-1 cells upon incubation with ionophore. Treatment with dexamethasone did not significantly affect leukotriene release. 6. These data provide evidence that prostaglandin synthesis is consistently downregulated by glucocorticoids. However, the glucocorticoid-mediated induction of FLAP may provide a mechanism to maintain leukotriene biosynthesis through more efficient transfer of arachidonic acid to the 5-lipoxygenase reaction, in spite of inhibitory effects on other enzymes of the biosynthetic pathway.

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.