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

Both group IB and group IIA secreted phospholipases A2 are natural ligands of the mouse 180-kDa M-type receptor

Snake venom and mammalian secreted phospholipases A2 (sPLA2s) have been associated with toxic (neurotoxicity, myotoxicity, etc.), pathological (inflammation, cancer, etc.), and physiological (proliferation, contraction, secretion, etc.) processes. Specific membrane receptors (M and N types) for sPLA2s have been initially identified with snake venom sPLA2s as ligands, and the M-type 180-kDa receptor was cloned from different animal species. This paper addresses the problem of the endogenous ligands of the M-type receptor. Recombinant group IB and group IIA sPLA2s from human and mouse species have been prepared and analyzed for their binding properties to M-type receptors from different animal species. Both mouse group IB and group IIA sPLA2s are high affinity ligands (in the 1-10 nM range) for the mouse M-type receptor. These two sPLA2s are expressed in the mouse tissues where the M-type receptor is also expressed, making it likely that both types of sPLA2s are physiological ligands of the mouse M-type receptor. This conclusion does not hold for human group IB and IIA sPLA2s and the cloned human M-type receptor. The two mouse sPLA2s have relatively high affinities for the mouse M-type receptor, but they can have much lower affinities for receptors from other animal species, indicating that species specificity exists for sPLA2 binding to M-type receptors. Caution should thus be exerted in avoiding mixing sPLA2s, cells, or tissues from different animal species in studies of the biological roles of mammalian sPLA2s associated with an action through their membrane receptors.

Functional coupling between various phospholipase A2s and cyclooxygenases in immediate and delayed prostanoid biosynthetic pathways

Several distinct phospholipase A2s (PLA2s) and two cyclooxygenases (COXs) were transfected, alone or in combination, into human embryonic kidney 293 cells, and their functional coupling during immediate and delayed prostaglandin (PG)-biosynthetic responses was reconstituted. Signaling PLA2s, i.e. cytosolic PLA2 (cPLA2) (type IV) and two secretory PLA2s (sPLA2), types IIA (sPLA2-IIA) and V (sPLA2-V), promoted arachidonic acid (AA) release from their respective transfectants after stimulation with calcium ionophore or, when bradykinin receptor was cotransfected, with bradykinin, which evoked the immediate response, and interleukin-1 plus serum, which induced the delayed response. Experiments on cells transfected with either COX alone revealed subtle differences between the PG-biosynthetic properties of the two isozymes in that COX-1 and COX-2 were favored over the other in the presence of high and low exogenous AA concentrations, respectively. Moreover, COX-2, but not COX-1, could turn on endogenous AA release, which was inhibited by a cPLA2 inhibitor. When PLA2 and COX were coexpressed, AA released by cPLA2, sPLA2-IIA and sPLA2-V was converted to PGE2 by both COX-1 and COX-2 during the immediate response and predominantly by COX-2 during the delayed response. Ca2+-independent PLA2 (iPLA2) (type VI), which plays a crucial role in phospholipid remodeling, failed to couple with COX-2 during the delayed response, whereas it was linked to ionophore-induced immediate PGE2 generation via COX-1 in marked preference to COX-2. Finally, coculture of PLA2 and COX transfectants revealed that extracellular sPLA2s-IIA and -V, but neither intracellular cPLA2 nor iPLA2, augmented PGE2 generation by neighboring COX-expressing cells, implying that the heparin-binding sPLA2s play a particular role as paracrine amplifiers of the PG-biosynthetic response signal from one cell to another.

Interleukin-1beta regulation of inducible nitric oxide synthase and cyclooxygenase-2 involves the p42/44 and p38 MAPK signaling pathways in cardiac myocytes

The genes encoding inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2, also known as prostaglandin-endoperoxide synthase-2) are induced in many types of cells in response to proinflammatory cytokines. We have previously shown that interleukin-1beta (IL) stimulates iNOS and COX-2 mRNA in cardiac myocytes. Because IL has been shown to activate mitogen-activated protein kinase (MAPK) signaling pathways in many different cells, we tested whether the p42/44 and p38 MAPK pathways were involved in IL stimulation of iNOS and COX-2, using a specific inhibitor of p42/44 activation, PD98059 (PD), and the p38 inhibitor SB205380 (SB). Nitrites were measured using the Griess reagent, prostaglandin PGE2 by an enzyme immunoassay, iNOS and COX-2 protein by Western blot analysis, and iNOS mRNA by Northern blot analysis. Tested separately, the p38 kinase and MAPK inhibitors partially reduced IL stimulation of nitrite, iNOS protein, and iNOS mRNA; used together, they completely abolished the effect of IL. SB and PD inhibited IL-stimulated COX-2 protein by 60% and 80%, respectively, and IL-stimulated COX-2 protein was totally prevented by the combination of inhibitors. PGE2 production was inhibited more than 99% by either drug alone, suggesting a posttranslational effect on enzyme activity. To test whether this posttranslational effect involved the cytosolic phospholipase A2 (cPLA2) isoform, Western blots were probed for cPLA2 protein. Results indicated that IL stimulated cPLA2 activity and synthesis, which was inhibited by SB but not PD. These data indicate that (1) IL induction of iNOS synthesis depends on both the p42/44 and p38 signaling pathways, acting primarily at the level of transcriptional regulation; and (2) IL regulation of COX-2 synthesis involves the p42/44 and p38 signaling pathways, with an additional level of regulation occurring posttranslationally, perhaps at the level of activation of the cPLA2 isoform, which may be involved in intracellular signaling, as well as regulation of arachidonic acid release for COX-2 activity.

Mechanisms of the priming effect of lipopolysaccharides on the biosynthesis of leukotriene B4 in chemotactic peptide-stimulated human neutrophils

The goal of this study was to explain the priming effect of lipopolysaccharides (LPS) in human polymorphonuclear leukocytes on leukotriene B4 (LTB4) biosynthesis after stimulation with the receptor-mediated agonist formyl-methionyl-leucyl-phenylalanine (fMLP). This priming effect for LTB4 biosynthesis was maximal after a 30 min preincubation with LPS but was lost when incubations were extended to 90 min or longer. Priming with LPS resulted in an enhanced maximal activation of 5-lipoxygenase (5- to15-fold above unprimed cells) as well as a prolonged activation of the enzyme after stimulation with fMLP compared to that measured in unprimed cells. The activation of 5-lipoxygenase was associated with its translocation to the nuclear fraction of the cell after stimulation of LPS-primed cells but not of unprimed cells. Priming of cells with LPS also resulted in an enhanced capacity (fivefold increase) for arachidonic acid (AA) release after stimulation with fMLP compared to unprimed cells as measured by mass spectrometry. This release of AA was very efficiently blocked in a dose-dependent manner by the 85 kDa cytosolic phospholipase A2 (PLA2) inhibitor MAFP (IC50=10nM) but not by the 14 kDa secretory PLA2 inhibitor SB 203347 (up to 5 microM), indicating that the 85 kDa cPLA2 is the PLA2 responsible for AA release in response to receptor-mediated agonists. In accord with inhibitor studies, the LPS-mediated phosphorylation of cPLA2 followed the same kinetics as the priming for AA release, and a measurable fMLP-induced translocation of cPLA2 was observed only in primed cells. As with AA release and LTB4 biosynthesis, both the phosphorylation and capacity to translocate cPLA2 were reversed when the preincubation period with LPS was extended to 120 min. These results explain some of the cellular events responsible for the potentiation and subsequent decline of functional responses of human polymorphonuclear leukocytes recruited to inflammatory foci.

Induction of Cyclooxygenase-2 by Secretory Phospholipases A2 in Nerve Growth Factor-Stimulated Rat Serosal Mast Cells Is Facilitated by Interaction with Fibroblasts and Mediated by a Mechanism Independent of Their Enzymatic Functions

Mast cells exhibit a biphasic (immediate and delayed) eicosanoid-biosynthetic response after stimulation with particular cytokines or FcεRI (high affinity receptor for IgE) cross-linking. Treatment of rat serosal connective tissue mast cells (CTMC) with nerve growth factor (NGF) induced only the delayed phase of PGD2 generation that depended on inducible cyclooxygenase-2 (COX-2), but not constitutive COX-1, even though the subcellular distributions of these isoforms were similar. Experiments using several phospholipase A2 (PLA2) isozyme-specific probes and inhibitors suggested that both constitutive cytosolic PLA2 and inducible type IIA secretory PLA2 (sPLA2) are involved in NGF-initiated, COX-2-dependent, delayed PGD2 generation in rat CTMC. A type IIA sPLA2 inhibitor, but neither cytosolic PLA2 nor COX inhibitors, reduced, while adding exogenous type IIA sPLA2 augmented, NGF-induced COX-2 expression and its attendant PGD2 generation, indicating that the sPLA2-mediated increase in delayed PGD2 generation was attributable mainly to enhanced COX-2 expression. Type IIA sPLA2 and its close relative type V sPLA2 associated with fibroblastic cell surfaces increased NGF-induced COX-2 expression more efficiently than the soluble enzymes, revealing a particular juxtacrine sPLA2 presentation route. Surprisingly, catalytically inactive type IIA sPLA2 mutants, which were incapable of promoting arachidonic acid release from cytokine-primed cells, retained the ability to enhance COX-2 expression in CTMC, indicating that the COX-2-inducing activities of sPLA2 are independent of their catalytic functions.

Inhibition of Ca2+-independent phospholipase A2 by bromoenol lactone attenuates prostaglandin generation induced by interleukin-1 beta and dibutyryl cAMP in rat mesangial cells

Cytokine-induced prostaglandin generation in rat mesangial cells has been suggested to be dependent on the expression of secretory phospholipase A2 (sPLA2). In the present study, we investigated the possible involvement of Ca2+-independent phospholipase A2 (iPLA2) in the generation. The results showed that bromoenol lactone, a relatively selective iPLA2 inhibitor, significantly attenuated prostaglandin E2 generation induced by interleukin-1beta and dibutyryl cAMP in parallel with the inhibition of iPLA2 activity. However, the inhibitor did not affect sPLA2 release upon stimulation, activities of sPLA2 or cytosolic phospholipase A2, or Ca2+ ionophore-induced arachidonic acid liberation. These results suggest that prostaglandin E2 generation upon stimulation may be partially mediated by iPLA2 in addition to sPLA2.

The functions of five distinct mammalian phospholipase A2S in regulating arachidonic acid release. Type IIa and type V secretory phospholipase A2S are functionally redundant and act in concert with cytosolic phospholipase A2

We examined the relative contributions of five distinct mammalian phospholipase A2 (PLA2) enzymes (cytosolic PLA2 (cPLA2; type IV), secretory PLA2s (sPLA2s; types IIA, V, and IIC), and Ca2+-independent PLA2 (iPLA2; type VI)) to arachidonic acid (AA) metabolism by overexpressing them in human embryonic kidney 293 fibroblasts and Chinese hamster ovary cells. Analyses using these transfectants revealed that cPLA2 was a prerequisite for both the calcium ionophore-stimulated immediate and the interleukin (IL)-1- and serum-induced delayed phases of AA release. Type IIA sPLA2 (sPLA2-IIA) mediated delayed AA release and, when expressed in larger amounts, also participated in immediate AA release. sPLA2-V, but not sPLA2-IIC, behaved in a manner similar to sPLA2-IIA. Both sPLA2s-IIA and -V, but not sPLA2-IIC, were heparin-binding PLA2s that exhibited significant affinity for cell-surface proteoglycans, and site-directed mutations in residues responsible for their membrane association or catalytic activity markedly reduced their ability to release AA from activated cells. Pharmacological studies using selective inhibitors as well as co-expression experiments supported the proposal that cPLA2 is crucial for these sPLA2s to act properly. The AA-releasing effects of these sPLA2s were independent of the expression of the M-type sPLA2 receptor. Both cPLA2, sPLA2s-IIA, and -V were able to supply AA to downstream cyclooxygenase-2 for IL-1-induced prostaglandin E2 biosynthesis. iPLA2 increased the spontaneous release of fatty acids, and this was further augmented by serum but not by IL-1. Finally, iPLA2-derived AA was not metabolized to prostaglandin E2. These observations provide evidence for the functional cross-talk or segregation of distinct PLA2s in mammalian cells in regulating AA metabolism and phospholipid turnover.

Fas-induced arachidonic acid release is mediated by Ca2+-independent phospholipase A2 but not cytosolic phospholipase A2, which undergoes proteolytic inactivation

Fas-mediated apoptosis of human leukemic U937 cells was accompanied by increased arachidonic acid (AA) and oleic acid release from membrane glycerophospholipids, indicating phospholipase A2 (PLA2) activation. During apoptosis, type IV cytosolic PLA2 (cPLA2), a PLA2 isozyme with an apparent molecular mass of 110 kDa critical for stimulus-coupled AA release, was converted to a 78-kDa fragment with concomitant loss of catalytic activity. Cleavage of cPLA2 correlated with increased caspase-3-like protease activity in apoptotic cells and was abrogated by a caspase-3 inhibitor. A mutant cPLA2 protein in which Asp522 was replaced by Asn, which aligns with the consensus sequence of the caspase-3 cleavage site (DXXD downward arrowX), was resistant to apo-ptosis-associated proteolysis. Moreover, a COOH-terminal deletion mutant of cPLA2 truncated at Asp522 comigrated with the 78-kDa fragment and exhibited no enzymatic activity. Thus, caspase-3-mediated cPLA2 cleavage eventually leads to destruction of a catalytic triad essential for cPLA2 activity, thereby terminating its AA-releasing function. In contrast, the activity of type VI Ca2+-independent PLA2 (iPLA2), a PLA2 isozyme implicated in phospholipid remodeling, remained intact during apoptosis. Inhibitors of iPLA2, but neither cPLA2 nor secretory PLA2 inhibitors, suppressed AA release markedly and, importantly, delayed cell death induced by Fas. Therefore, we conclude that iPLA2-mediated fatty acid release is facilitated in Fas-stimulated cells and plays a modifying although not essential role in the apoptotic cell death process.