Phosphorylation and calcium influx are not sufficient for the activation of cytosolic phospholipase A2 in U937 cells: requirement for a Gi alpha-type G-protein

Close teamwork between Nrf2 and peroxiredoxins 1 and 6 for the regulation of prostaglandin D2 and E2 production in macrophages in acute inflammation

Inflammation is a complex biological self-defense reaction triggered by tissue damage or infection by pathogens. Acute inflammation is regulated by the time- and cell type-dependent production of cytokines and small signaling molecules including reactive oxygen species and prostaglandins. Recent studies have unveiled the important role of the transcription factor Nrf2 in the regulation of prostaglandin production through transcriptional regulation of peroxiredoxins 1 and 6 (Prx1 and Prx6) and lipocalin-type prostaglandin D synthase (L-PGDS). Prx1 and Prx6 are multifunctional proteins important for cell protection against oxidative stress, but also work together to facilitate production of prostaglandins E2 and D2 (PGE2 and PGD2). Prx1 secreted from cells under mild oxidative stress binds Toll-like receptor 4 and induces NF-κB activation, important for the expression of cyclooxygenase-2 and microsomal PGE synthase-1 (mPGES-1) expression. The activated MAPKs p38 and ERK phosphorylate Prx6, leading to NADPH oxidase-2 activation, which contributes to production of PGD2 by hematopoietic prostaglandin D synthase (H-PGDS). PGD2 and its end product 15-deoxy-∆(12,14)-prostaglandin J2 (15d-PGJ2) activate Nrf2 thereby forming a positive feedback loop for further production of PGD2 by L-PGDS. Maintenance of cellular glutathione levels is an important role of Nrf2 not only for cell protection but also for the synthesis of prostaglandins, as mPGES-1 and H-PGDS require glutathione for their activities. This review is aimed at describing the functions of Prx1 and Prx6 in the regulation of PGD2 and PGE2 production in acute inflammation in macrophages and the importance of 15d-PGJ2 as an intrinsic Nrf2 activator.

Histamine H3-receptor signaling in cardiac sympathetic nerves: Identification of a novel MAPK-PLA2-COX-PGE2-EP3R pathway

We hypothesized that the histamine H(3)-receptor (H(3)R)-mediated attenuation of norepinephrine (NE) exocytosis from cardiac sympathetic nerves results not only from a Galpha(i)-mediated inhibition of the adenylyl cyclase-cAMP-PKA pathway, but also from a Gbetagamma(i)-mediated activation of the MAPK-PLA(2) cascade, culminating in the formation of an arachidonate metabolite with anti-exocytotic characteristics (e.g., PGE(2)). We report that in Langendorff-perfused guinea-pig hearts and isolated sympathetic nerve endings (cardiac synaptosomes), H(3)R-mediated attenuation of K(+)-induced NE exocytosis was prevented by MAPK and PLA(2) inhibitors, and by cyclooxygenase and EP(3)-receptor (EP(3)R) antagonists. Moreover, H(3)R activation resulted in MAPK phosphorylation in H(3)R-transfected SH-SY5Y neuroblastoma cells, and in PLA(2) activation and PGE(2) production in cardiac synaptosomes; H(3)R-induced MAPK phosphorylation was prevented by an anti-betagamma peptide. Synergism between H(3)R and EP(3)R agonists (i.e., imetit and sulprostone, respectively) suggested that PGE(2) may be a downstream effector of the anti-exocytotic effect of H(3)R activation. Furthermore, the anti-exocytotic effect of imetit and sulprostone was potentiated by the N-type Ca(2+)-channel antagonist omega-conotoxin GVIA, and prevented by an anti-Gbetagamma peptide. Our findings imply that an EP(3)R Gbetagamma(i)-induced decrease in Ca(2+) influx through N-type Ca(2+)-channels is involved in the PGE(2)/EP(3)R-mediated attenuation of NE exocytosis elicited by H(3)R activation. Conceivably, activation of the Gbetagamma(i) subunit of H(3)R and EP(3)R may also inhibit Ca(2+) entry directly, independent of MAPK intervention. As heart failure, myocardial ischemia and arrhythmic dysfunction are associated with excessive local NE release, attenuation of NE release by H(3)R activation is cardioprotective. Accordingly, this novel H(3)R signaling pathway may ultimately bear therapeutic significance in hyper-adrenergic states.

Late-Gestation Rat Myometrial Cells Express Multiple Isoforms of Phospholipase A2 That Mediate PCB 50-Induced Release of Arachidonic Acid with Coincident Prostaglandin Production

Previous reports have shown that ortho-substituted polychlorinated biphenyls (PCBs) are uterotonic and activate phospholipase A2 to release arachidonic acid (AA) from membrane phospholipids. AA serves as the precursor to various eicosanoids, which, in addition to AA itself, are capable of modulating uterine function. To examine whether PCBs stimulate phospholipase A2 (PLA2) to mobilize arachidonic acid from late-gestation rat uterus, primary cultures of gestation day 20 (gd20) rat myometrial cells (RMC) were labeled with 0.5 microCi 3H-AA prior to a 10-, 20-, or 30-min exposure to 2,2',4,6-tetrachlorobiphenyl (PCB 50) (1-50 microM) or 0.1% DMSO (solvent control). PCB 50 stimulated the release of 3H-AA from gd20 RMC in concentration- and time-dependent manners (p < 0.05). PCB 50 stimulation of RMC was attenuated with ethylene glycol bis(2-aminoethyl ether)-N,N,N'N'-tetraacetic acid (EGTA) and nifedipine, suggesting that AA release was dependent on the influx of extracellular calcium through L-type voltage-operated calcium channels. PCB 50-induced release of AA from RMC was also attenuated with the PLA2-specific inhibitors methyl arachidonyl fluorophosphonate (MAFP), bromoenol lactone (BEL), and manoalide (p < 0.05). Stimulation of PLA2 enzymes in response to PCB exposure occurred via p38 mitogen activated protein kinase (MAPK) activation as indicated by the significant attenuation of PCB 50-induced AA release from RMC in the presence of SB 202190. In addition to stimulating AA release, PCB 50 induced a significant production of prostaglandins from gd20 RMC compared with controls (p < 0.05). These results suggest that myometrial cells express multiple PLA2 isoforms that may serve as a target and effector for ortho-substituted PCB-mediated stimulation of uterine function through arachidonic acid and prostaglandin release.

Phospholipase A2 activating protein (PLAA) is required for 1alpha,25(OH)2D3 signaling in growth plate chondrocytes

Phospholipase A2 (PLA2) is pivotal in the rapid membrane-mediated actions of 1,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3]. Microarray analysis indicated that PLA2 activating protein (PLAA) mRNA is upregulated 6-fold before rat growth plate cells exhibit 1alpha,25(OH)2D3-dependent protein kinase C (PKC) increases, suggesting that it plays an important role in 1alpha,25(OH)2D3's mechanism of action. PLAA mRNA was confirmed in 1alpha,25(OH)2D3-responsive growth zone (prehypertrophic and upper hypertrophic cell zones) chondrocytes by RT-PCR and Northern blot in vitro and by in situ hybridization in vivo. PLAA protein was shown by Western blot and immunohistochemistry. PLAAs role in 1alpha,25(OH)2D3 signaling was evaluated in growth zone cell cultures using PLAA peptide. Arachidonic acid release was increased as was PLA2-specific activity in plasma membranes and matrix vesicles. PKCalpha, but not PKCbeta, PKCepsilon, or PKCzeta, was increased. PLAAs effect was comparable to that of 1alpha,25(OH)2D3 and was additive with 1alpha,25(OH)2D3. PLA2 inhibitors quinacrine and AACOCF3, and cyclooxygenase inhibitor indomethacin blocked the effect of PLAA peptide on PKC, indicating arachidonic acid and its metabolites were involved. This was confirmed using exogenous arachidonic acid. Prostaglandin acted via EP1 based on inhibition by SC19220 and not via EP2 since AH6809 had no effect. Like 1alpha,25(OH)2D3, PLAA peptide also increased activity of phospholipase C-specific activity via beta-1 and beta-3 isoforms, but not delta-1 or gamma-1; the effect of PLAA was via lysophospholipid but not via arachidonic acid. PLAA peptide decreased [3H]-thymidine incorporation to 50% of the decrease caused by 1alpha,25(OH)2D3. In contrast, PLAA peptide increased alkaline phosphatase-specific activity and proteoglycan production in a manner similar to 1alpha,25(OH)2D3. This indicates that PLAA is a specific activator of PLA2 in growth plate chondrocytes, and suggests that it mediates the membrane effect of 1alpha,25(OH)2D3, thereby modulating physiological response.

Amyloid β(1–42) and its β(25–35) fragment induce activation and membrane translocation of cytosolic phospholipase A2 in bovine retina capillary pericytes

We investigated changes in cytosolic phospholipase A(2) (cPLA(2)) and calcium-independent PLA(2) (iPLA(2)) activities in bovine retina capillary pericytes after stimulation with 50 microM amyloid-beta (Abeta) (1-42) and its (25-35) fragment, over 24 h (mild, sublethal model of cell damage). In the presence of Abeta peptides, we found that cPLA(2) activity was increased and translocated from the cytosolic fraction to the membrane system, particularly in the nuclear region. Reversed-sequence Abeta(35-25) peptide did not stimulate or induce cPLA(2) translocation. Exposure to both Abeta peptides had no significant effect on cPLA(2) protein content as tested by Western immunoblot analysis. The addition of Abetas to quiescent pericytes was followed by phosphorylation of cPLA(2) and arachidonic acid release. Treatment with inhibitors (AACOCF(3), staurosporine and cycloheximide) resulted in a sharp decrease in basal and stimulated cPLA(2) activity. Inactivating effects of bromoenol lactone (BEL), inhibitor of iPLA(2), demonstrated that the stimulation of total PLA(2) activity by Abetas was mediated by both PLA(2) enzymes. Taken together with our previous observations that both Abeta peptides may induce hydrolysis of phosphatidylcholine, the present results provide evidence that this process is cooperatively mediated by cPLA(2) activation/translocation and iPLA(2) activation. The effect is very likely triggered by a mild prooxidant mechanism which was not able to divert the cell to degeneration. The data confirm the hypothesis that pericytes could be a target of potential vascular damage and reactivity during processes involving amyloid accumulation.

Fcgamma RI-triggered generation of arachidonic acid and eicosanoids requires iPLA2 but not cPLA2 in human monocytic cells

Aggregation of receptors for immunoglobulin G (FcgammaRs) on myeloid cells activates a series of events that are key in the inflammatory response and that can ultimately lead to targeted cell killing by antibody-directed cellular cytotoxicity. Generation of lipid-derived proinflammatory mediators is an important component of the integrated cellular response mediated by receptors for the constant region of immunoglobulins (Fc). We have demonstrated previously that, in interferon-gamma-primed U937 cells, the high affinity receptor for IgG, FcgammaRI, is coupled to a novel intracellular signaling pathway that involves the sequential activation of phospholipase D, sphingosine kinase, calcium transients, and protein kinase C isoforms, leading to the activation of the NADPH-oxidative burst. Here, we investigate the nature of the phospholipase that regulates arachidonic acid and eicosanoid production. Our data show that FcgammaRI couples to iPLA(2)beta for the release of arachidonic acid and the generation of leukotriene B(4) and prostaglandin E(2). Activation of iPLA(2)beta was protein kinase C-dependent; on the other hand, platelet-activating factor triggered cPLA(2)alpha by means of the mitogen-activated protein kinase pathway. These studies demonstrate that intracellular PLA(2)s can be selectively regulated by different stimuli and suggest a critical role for iPLA(2)beta in the intracellular signaling cascades initiated by FcgammaRI and its functional role in the generation of key inflammatory mediators.

G-protein alpha-subunit levels in hippocampus and entorhinal cortex of brains staged for Alzheimer's disease neurofibrillary and amyloid pathologies

G-protein alpha-subunits (Galphao, Galphai, Galphas, Galphaq) and adenylyl cyclase (AC) I and II isoforms were quantified in hippocampus and entorhinal cortex from 22 cases staged for Alzheimer's disease (AD) pathologies according to Braak and Braak. Hippocampal Galphai levels declined significantly with neurofibrillary staging, whereas AC I levels in this region increased. Significant amyloid stage-related reductions of Galphai were seen in both the hippocampus and entorhinal cortex. The hippocampus also showed a significant reduction of Galphao with amyloid staging. It is concluded that levels of inhibitory G-protein subunits Galphao, and in particular Galphai, decrease in parallel to the extent of AD pathology.

Amplification mechanisms of inflammation: paracrine stimulation of arachidonic acid mobilization by secreted phospholipase A2 is regulated by cytosolic phospholipase A2-derived hydroperoxyeicosatetraenoic acid

In macrophages and other major immunoinflammatory cells, two phospholipase A(2) (PLA(2)) enzymes act in concert to mobilize arachidonic acid (AA) for immediate PG synthesis, namely group IV cytosolic phospholipase A(2) (cPLA(2)) and a secreted phospholipase A(2) (sPLA(2)). In this study, the molecular mechanism underlying cross-talk between the two PLA(2)s during paracrine signaling has been investigated. U937 macrophage-like cells respond to Con A by releasing AA in a cPLA(2)-dependent manner, and addition of exogenous group V sPLA(2) to the activated cells increases the release. This sPLA(2) effect is abolished if the cells are pretreated with cPLA(2) inhibitors, but is restored by adding exogenous free AA. Inhibitors of cyclooxygenase and 5-lipoxygenase have no effect on the response to sPLA(2). In contrast, ebselen strongly blocks it. Reconstitution experiments conducted in pyrrophenone-treated cells to abolish cPLA(2) activity reveal that 12- and 15-hydroperoxyeicosatetraenoic acid (HPETE) are able to restore the sPLA(2) response to levels found in cells displaying normal cPLA(2) activity. Moreover, 12- and 15-HPETE are able to enhance sPLA(2) activity in vitro, using a natural membrane assay. Neither of these effects is mimicked by 12- or 15-hydroxyeicosatetraenoic acid, indicating that the hydroperoxy group of HPETE is responsible for its biological activity. Collectively, these results establish a role for 12/15-HPETE as an endogenous activator of sPLA(2)-mediated phospholipolysis during paracrine stimulation of macrophages and identify the mechanism that connects sPLA(2) with cPLA(2) for a full AA mobilization response.

Calcium-independent phospholipase A2 is required for lysozyme secretion in U937 promonocytes

As a part of their surveillance functions in the immune system, monocytes/macrophages secrete large amounts of the bactericidal enzyme lysozyme to the extracellular medium. We report here that lysozyme secretion in activated U937 promonocytes depends on a functional calcium-independent phospholipase A(2) (iPLA(2)). Inhibition of the enzyme by bromoenol lactone or by treatment with a specific antisense oligonucleotide results in a diminished capacity of the cells to secrete lysozyme to the extracellular medium. Calcium-independent PLA(2) is largely responsible for the maintenance of the steady state of lysophosphatidylcholine (lysoPC) levels within the cells, as manifested by the marked decrease in the levels of this metabolite in cells deficient in iPLA(2) activity. Reconstitution experiments reveal that lysoPC efficiently restores lysozyme secretion in iPLA(2)-deficient cells, whereas other lysophospholipids, including lysophosphatidic acid, lysophosphatidylserine, and lysophosphatidylethanolamine, are without effect. Arachidonic acid mobilization in activated U937 cells is under control of cytosolic phospholipase A(2) (cPLA(2)). Selective inhibition of cPLA(2) results in a complete abrogation of the arachidonate mobilization response, but has no effect on lysozyme secretion. These results identify iPLA(2)-mediated lysoPC production as a necessary component of the molecular machinery leading to lysozyme secretion in U937 cells and rule out a role for cPLA(2) in the response. Collectively, the results demonstrate distinct roles in inflammatory cell signaling for these two intracellular phospholipases.

1alpha,25(OH)2D3 causes a rapid increase in phosphatidylinositol-specific PLC-beta activity via phospholipase A2-dependent production of lysophospholipid

1alpha,25(OH)(2)D(3) activates protein kinase C (PKC) in rat growth plate chondrocytes via mechanisms involving phosphatidylinositol-specific phospholipase C (PI-PLC) and phospholipase A(2) (PLA(2)). The purpose of this study was to determine if 1alpha,25(OH)(2)D(3) activates PI-PLC directly or through a PLA(2)-dependent mechanism. We determined which PLC isoforms are present in the growth plate chondrocytes, and determined which isoform(s) of PLC is(are) regulated by 1alpha,25(OH)(2)D(3). Inhibitors and activators of PLA(2) were used to assess the inter-relationship between these two phospholipid-signaling pathways. PI-PLC activity in lysates of prehypertrophic and upper hypertrophic zone (growth zone) cells that were incubated with 1alpha,25(OH)(2)D(3), was increased within 30s with peak activity at 1-3 min. PI-PLC activity in resting zone cells was unaffected by 1alpha,25(OH)(2)D(3). 1beta,25(OH)(2)D(3), 24R,25(OH)(2)D(3), actinomycin D and cycloheximide had no effect on PLC in lysates of growth zone cells. Thus, 1alpha,25(OH)(2)D(3) regulation of PI-PLC enzyme activity is stereospecific, cell maturation-dependent, and nongenomic. PLA(2)-activation (mastoparan or melittin) increased PI-PLC activity to the same extent as 1alpha,25(OH)(2)D(3); PLA(2)-inhibition (quinacrine, oleyloxyethylphosphorylcholine (OEPC), or AACOCF(3)) reduced the effect of 1alpha,25(OH)(2)D(3). Neither arachidonic acid (AA) nor its metabolites affected PI-PLC. In contrast, lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE) activated PI-PLC (LPE>LPC). 1alpha,25(OH)(2)D(3) stimulated PI-PLC and PKC activities via Gq; GDPbetaS inhibited activity, but pertussis toxin did not. RT-PCR showed that the cells express PLC-beta1a, PLC-beta1b, PLC-beta3 and PLC-gamma1 mRNA. Antibodies to PLC-beta1 and PLC-beta3 blocked the 1alpha,25(OH)(2)D(3) effect; antibodies to PLC-delta and PLC-gamma did not. Thus, 1alpha,25(OH)(2)D(3) regulates PLC-beta through PLA(2)-dependent production of lysophospholipid.

Cytosolic phospholipase A2 mediates arachidonoyl phospholipid hydrolysis in immortalized rat brain endothelial cells stimulated by oxidized LDL

We tested the hypothesis that oxidized low-density lipoprotein (oxLDL), administered in sublethal doses to the culture medium of immortalized rat brain endothelial cells (ECs, GP8.39), acts as a prooxidant signal to stimulate peroxidation processes and membrane phospholipid hydrolysis. ECs were grown at confluence in a medium with or without native LDL (nLDL) or oxLDL (1.5 mg/dish; up to 350-450 nmol hydroperoxides/mg protein) for two temporally distinct phases (short incubation period up to 1 h, or long incubation period spanning 24 h). Peroxidation parameters (conjugated dienes, MDA, hydroperoxides and LDH release) and arachidonic acid (AA) release were determined. Cell lysates and subcellular fractions were assayed for cPLA(2) while the cytotoxic effect and apoptosis were monitored by morphological changes, trypan blue dye exclusion, MTT reduction test, caspase-3 activity, COMET and laser confocal fluorescence microscopy (LCFM) analyses. Effects of alpha-tocopherol and 85-kDa PLA(2) inhibitor (AACOCF(3)), alone or in combination, were also tested. Immunoblot analysis of cPLA(2) was carried out on cell fraction proteins. After incubation for 1 or 24 h, oxLDL (100-200 microM hydroperoxides), but not nLDL, markedly increased lipid peroxidation, cPLA(2) activity and AA release in a dose-dependent manner. AACOCF(3) and antioxidant alpha-tocopherol (1 mM) strongly inhibited the prooxidant-stimulated AA release. Long-term exposure (24 h) to oxLDL (100 microM) had no effect on the cPLA(2) protein content as tested by Western immunoblot analysis, while showing a sharp cytotoxic effect on the cells. Caspase-3 activity and LCFM analysis indicated that oxLDL (100/200 microM) were able to trigger an apoptotic process. The results suggest that (i) ECs may be the target of extensive oxidative damage caused by oxLDL; (ii) activation of cPLA(2) mediates liberation of AA; (iii) cPLA(2) expression was not stimulated by long-term exposure to oxLDL; (iv) oxidized specific constituents of oxLDL, acting as regulatory signals, increase the ability of ECs to degrade membrane phospholipids, end products of which are linked to the development of atherosclerotic lesions.

Roles of various phospholipases A2 in providing lysophospholipid acceptors for fatty acid phospholipid incorporation and remodelling

In the present study the lysophospholipid sources for arachidonic (AA) and eicosapentaenoic acid (EPA) incorporation into and redistribution within the phospholipids of phorbol-ester-differentiated U937 cells was investigated. Initially, AA incorporated primarily into choline glycerophospholipids (PC), whereas EPA incorporated mainly into ethanolamine glycerophospholipids (PE). Bromoenol lactone (BEL), an inhibitor of the Group VI Ca2+-independent phospholipase A2 (iPLA2), diminished both lysophosphatidylcholine levels and the incorporation of AA into phospholipids. However BEL had little effect on EPA incorporation. In concanavalin A-activated cells, EPA, but not AA, incorporation was also affected by methyl arachidonyl fluorophosphonate (MAFP), suggesting an additional role for the group IV cytosolic phospholipase A2. In the activated cells AA and EPA did not compete with each other for incorporation, indicating that the pathways for AA and EPA incorporation are partially different. The AA and EPA initially incorporated into PC slowly moved to PE in a process that took several hours. The transfer of AA and EPA from PC to PE was not inhibited by BEL, MAFP or LY311727 [3-(3-acetamide 1-benzyl-2-ethylindolyl-5-oxy)propanesulphonic acid], raising the possibility that an as-yet-undetermined phospholipase A2 may be involved in fatty acid phospholipid remodelling. A strong candidate to be involved in these reactions is a novel Ca2+-independent phospholipase A2 that, unlike all known iPLA2s, is resistant to inhibition by BEL and also to MAFP and LY311727. The enzyme activity cleaves both PC and PE and is thus able to provide the lysoPC and lysoPE acceptors required for the fatty acid acylation reactions.

Cytosolic phospholipase A2 shows burst kinetics consistent with the slow, reversible formation of a dead-end complex

Cytosolic phospholipase A2 catalyzes the hydrolysis of the sn-2 ester of arachidonate-containing phospholipids. In the present research, a "burst" of arachidonate which precedes a somewhat slower, linear rate (upsilon) of product formation was observed and characterized using covesicles of 1,2-dimyristoyl-sn-glycero-3-phosphomethanol (DMPM) containing <10 mol% 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine as substrate. The magnitude of the burst (pi) was enzyme dependent, in both the presence and absence of glycerol. Upon subsequent addition of enzyme after the primary burst was complete, a second burst of arachidonate production was observed. This is consistent with the effect resulting from an enzyme effect and not from changes in the substrate. The use of 1,2-dioleoyl-sn-glycero-3-phosphomethanol as the carrier phospholipid instead of DMPM greatly reduced the rate of hydrolysis without a large effect on the pi/upsilon ratio, consistent with the burst not being the result of limitations in the lateral diffusion rate of phospholipids within the covesicles. When the assay is performed in the presence of glycerol, the burst phenomenon was also observed with the monoarachidonoyl glycerol transacylase product which shows that the effect occurs through a common mechanism. The burst and subsequent linear rate of hydrolysis are highly temperature dependent, with a pronounced increase in the pi/upsilon ratio as the temperature is increased from 35 to 45 degrees C. A mechanism in which a slow equilibrium between an active and less active (inactive) state of substrate-bound enzyme is proposed. This may provide a means by which the enzyme is switched off after a few hundred turnovers in order to prevent unabated phospholipid hydrolysis in cells which may be deleterious to membrane integrity.

Helicobacter pylori-induced prostaglandin E(2) synthesis involves activation of cytosolic phospholipase A(2) in epithelial cells

Helicobacter pylori initiates an inflammatory response and gastric diseases, which are more common in patients infected with H. pylori strains carrying the pathogenicity island, by colonizing the gastric epithelium. In the present study we investigated the mechanism of prostaglandin E(2) (PGE(2)) synthesis in response to H. pylori infection. We demonstrate that H. pylori induces the synthesis of PGE(2) via release of arachidonic acid predominately from phosphatidylinositol. In contrast to H. pylori wild type, an isogenic H. pylori strain with a mutation in the pathogenicity island exerts only weak arachidonic acid and PGE(2) synthesis. The H. pylori-induced arachidonic acid release was abolished by phospholipase A(2) (PLA(2)) inhibitors and by pertussis toxin (affects the activity of G alpha(i)/G alpha(o)). The role of phospholipase C, diacylglycerol lipase, or phospholipase D was excluded by using specific inhibitors. An inhibitor of the stress-activated p38 kinase (SB202190), but neither inhibitors of protein kinase C nor an inhibitor of the extracellular-regulated kinase pathway (PD98059), decreased the H. pylori-induced arachidonic acid release. H. pylori-induced phosphorylation of p38 kinase and cytosolic PLA(2) was blocked by SB202190. These results indicate that H. pylori induces the release of PGE(2) from epithelial cells by cytosolic PLA(2) activation via G alpha(i)/G alpha(o) proteins and the p38 kinase pathway.

Localization and regulation of cytosolic phospholipase A(2)

Liberation of arachidonic acid by cytosolic phospholipase A(2) (cPLA(2)) upon cell activation is often the initial and rate-limiting step in leukotriene and prostaglandin biosynthesis. This review discusses the essential features of cPLA(2) isoforms and addresses intriguing insights into the catalytic and regulatory mechanisms. Gene expression, posttranslational modification and subcellular localization can regulate these isoforms. Translocation of cPLA(2)alpha from the cytosol to the perinuclear region in response to calcium transients is critical for the immediate arachidonic acid release. Therefore, particular emphasis is placed on the mechanism of the translocation and the role of the proteins and lipids implicated in this process. The regional distribution and cellular localization of cPLA(2) may help to better understand its function as an arachidonic acid supplier to downstream enzymes and as a regulator of specific cellular processes.

A frontal variant of Alzheimer's disease exhibits decreased calcium-independent phospholipase A2 activity in the prefrontal cortex

A frontal variant of Alzheimer's disease (AD) has recently been identified on neuropathological and neuropsychological grounds (Johnson, J.K., Head, E., Kim, R., Starr, A., Cotman, C.W., 1999. Clinical and pathological evidence for a frontal variant of Alzheimer Disease. Arch. Neurol. 56, 1233-1239). Frontal AD differs strikingly from typical AD by the occurrence of neurofibrillary tangle densities in the frontal cortex as high or higher than in the entorhinal cortex. Since cerebrocortical membranes are commonly abnormal in Alzheimer's disease (AD), we assayed frontal AD cases for enzymes regulating membrane phospholipid composition. We specifically measured activity of phospholipase A2s (PLA2s) in dorsolateral prefrontal and lateral temporal cortices of frontal AD cases (n=12), which have respectively high and low densities of neurofibrillary tangles. In neither cortical area was Ca(2+)-dependent PLA2 activity abnormal compared to controls (n=12). In contrast, a significant 42% decrease in Ca(2+)-independent PLA2 activity was found in the dorsolateral prefrontal, but not the lateral temporal, cortex of the frontal AD cases. Similarly, the dorsolateral prefrontal cortex, but not the lateral temporal cortex of the frontal AD cases suffered a 42% decrease in total free fatty acid content, though neither that decrease nor those in any one species of free fatty acid was significant. The observed biochemical changes probably occurred in neurons given (a) our finding that PLA2 activity of cultured human NT2 neurons is virtually all Ca(2+)-independent and (b) the finding of others that nearly all Ca(2+)-independent PLA2 in brain gray matter is neuronal. The decrease in Ca(2+)-independent PLA2 activity is not readily attributable to Group VI or VIII iPLA2s since neither NT2N neurons nor our brain homogenates were greatly inhibited by drugs potently suppressing those iPLA2s. Decreased Ca(2+)-independent PLA2 activity in frontal AD may reflect a compensatory response to pathologically accelerated phospholipid metabolism early in the disorder. That could cause an early elevation of prefrontal free fatty acids, which can stimulate polymerization of tau and thus promote the prefrontal neurofibrillary tangle formation characteristic of frontal AD.

Electrospray ionization/mass spectrometric analyses of human promonocytic U937 cell glycerolipids and evidence that differentiation is associated with membrane lipid composition changes that facilitate phospholipase A2 activation

Upon differentiation, U937 promonocytic cells gain the ability to release a large fraction of arachidonate esterified in phospholipids when stimulated, but the mechanism is unclear. U937 cells express group IV phospholipase A(2) (cPLA(2)), but neither its level nor its phosphorylation state increases upon differentiation. A group VI PLA(2) (iPLA(2)) that is sensitive to a bromoenol lactone inhibitor catalyzes arachidonate hydrolysis from phospholipids in some cells and facilitates arachidonate incorporation into glycerophosphocholine (GPC) lipids in others, but it is not known whether U937 cells express iPLA(2). We confirm that ionophore A23187 induces substantial [(3)H]arachidonate release from differentiated but not control U937 cells, and electrospray ionization mass spectrometric (ESI/MS) analyses indicate that differentiated cells contain a higher proportion of arachidonate-containing GPC species than control cells. U937 cells express iPLA(2) mRNA and activity, but iPLA(2) inhibition impairs neither [(3)H]arachidonate incorporation into nor release from U937 cells. Experiments with phosphatidate phosphohydrolase (PAPH) and phospholipase D (PLD) inhibitors coupled with ESI/MS analyses of PLD-PAPH products indicate that differentiated cells gain the ability to produce diacylglycerol (DAG) via PLD-PAPH. DAG promotes arachidonate release by a mechanism that does not require DAG hydrolysis, is largely independent of protein kinase C, and requires cPLA(2) activity. This may reflect DAG effects on cPLA(2) substrate state.

Inflammatory cytokines enhance muscarinic-mediated arachidonic acid release through p38 mitogen-activated protein kinase in A2058 cells

The human melanoma cell line A2058 expresses the Gq-coupled M5 subtype of muscarinic receptor. Stimulation with the cholinergic agonist, carbachol, induces a dose-dependent increase in arachidonic acid release. The carbachol-induced arachidonate release is potentiated two- to threefold by pretreatment of A2058 cells with either of the inflammatory cytokines, tumor necrosis factor-alpha or interleukin-1beta . Cytokine-induced enhancement of muscarinic-mediated arachidonic acid release peaks near 1 h. Western analysis suggests that both cytokines are capable of activating the nuclear factor-kappaB (NF-kappaB) and p38 mitogen-activated protein kinase (MAPK) pathways. Anisomycin (1 microM) treatment mimics the cytokine-induced enhancement of arachidonic acid production and activates the p38 MAPK pathway, but does not activate the NF-kappaB pathway. Furthermore, pre-treatment of A2058 cells with the putative p38 MAPK inhibitor, SB202190, ablates the cytokine-dependent augmentation without interfering with the muscarinic-mediated arachidonic acid release in untreated cells. Moreover, cytokine treatment does not affect other M5-coupled pathways (e.g., phospholipase C activity or intracellular Ca2+ mobilization), suggesting that p38 MAPK activation principally modulates muscarinic-mediated phospholipase A2 activity. Finally, in primary cultures of cells taken from rat cerebellum, key aspects of this finding are repeated in cultures enriched for glia, but not in cultures enriched for granule neurons.

Extracellular ATP-mediated phospholipase A(2) activation in rat thyroid FRTL-5 cells: regulation by a G(i)/G(o) protein, Ca(2+), and mitogen-activated protein kinase

We investigated the mechanism of phospholipase A(2) (PLA(2)) activation in response to the P2 receptor agonist ATP in rat thyroid FRTL-5 cells. The PLA(2) activity was determined by measuring the release of [(3)H]-arachidonic acid (AA) from prelabeled cells. ATP evoked a dose- and time-dependent AA release. This release was totally inhibited by pertussis toxin (PTX) treatment, indicating the involvement of a G(i)/G(o) protein. The AA release was also diminished by chelating extracellular Ca(2+) with EGTA or by inhibiting influx of Ca(2+) using Ni(2+). Although the activation of protein kinase C (PKC) by 12-phorbol 13-myristate acetate (PMA) alone did not induce any AA release, the ATP-evoked AA release was significantly reduced when PKC was inhibited by GF109203X or by a long incubation with PMA to downregulate PKC. Both the ATP-evoked AA release and the mitogen-activated protein kinase (MAP kinase) phosphorylation were decreased by the MAP kinase kinase (MEK) inhibitor PD98059. Furthermore, the ATP-evoked MAP kinase phosphorylation was also inhibited by GF109203X and by downregulation of PKC, suggesting a PKC-mediated activation of MAP kinase. Inhibiting Src-like kinases by PP1 attenuated both the MAP kinase phosphorylation and the AA release. These results suggest that these kinases are involved in the regulation of MAP kinase and PLA(2) activation. Elevation of intracellular cAMP by TSH or by dBucAMP did not induce a phosphorylation of MAP kinase. Furthermore, neither the ATP-evoked AA release nor the MAP kinase phosphorylation were attenuated by TSH or dBucAMP. Taken together, our results suggest that ATP regulates the activation of PLA(2) by a G(i)/G(o) protein-dependent mechanism. Moreover, Ca(2+), PKC, MAP kinase, and Src-like kinases are also involved in this regulatory process.

Potential role for triglycerides in signal transduction

We previously reported that endothelin-1 or platelet-derived growth factor promoted in aortic smooth muscle cells a rapid hydrolysis of 1-O-alkyl-2-acyl-sn-glycero-3-phosphoethanolamine (alkyl-PE) which was immediately converted into 1-O-alkyl-2,3-diacyl-sn-glycerol (alkyl-TG) within 5 s or 60 s respectively [C. Comminges et al. (1996) Biochem. Biophys. Res. Commun. 220, 1008-1013 and C. Comminges et al. (1997) Biochim. Biophys. Acta 1355, 69-80]. In this study, we show that this alkyl-PE hydrolysis is triggered by a transient activation of a specific phospholipase C (PLC) regulated by pertussis toxin-sensitive heterotrimeric G-proteins. Moreover, this PLC can be triggered through a Ca2+ influx depending on L-type Ca2+ channel activation, as suggested by the use of a specific 'activator' S(-)-BayK 8644 and of selective inhibitors such as nimodipine. Interestingly, low concentrations (10(-8)-10(-7)M) of alkyl-TG block the opening of L-type Ca2+ channels, whereas identical concentrations of DG do not alter L-type Ca2+ channels. This study thus unravels a hitherto unrecognized signaling pathway generating alkyl-TG as a novel lipid second messenger, potentially acting as a negative feedback regulator of L-type Ca2+ channels.

Primitive Myeloid Cells Express High Levels of Phospholipase A2 Activity in the Absence of Leukotriene Release: Selective Regulation by Stem Cell Factor Involving the MAP Kinase Pathway

The activation of phospholipase A2 (PLA2) with release of eicosanoids and prostanoids in mature myeloid cells and the augmentation (priming) of this activity by cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) are central to the inflammatory process. Yet, there are few data concerning PLA2 activity and its regulation by growth factors in primary hematopoietic cells. We therefore analyzed the PLA2activity of mobilized human CD34 antigen-positive (CD34+) stem cells by quantitation of the extracellular release of3H-arachidonate. The PLA2 activity of CD34+ cells stimulated with calcium ionophore (A23187) was of similar magnitude to that of mature neutrophils and monocytes. Preincubation of CD34+ cells with stem cell factor (SCF) before A23187-stimulation resulted in primed PLA2 activity, whereas interleukin-3 (IL-3), GM-CSF, and tumor necrosis factor  had no significant effect. When CD34+ cells were induced to differentiate, PLA2 activity remained responsive to SCF for several days, but after 8 days, at which stage morphological and functional evidence of maturation was occurring, priming of PLA2 by SCF could no longer be elicited, whereas responses to GM-CSF and IL-3 had developed. The further metabolism of arachidonic acid to eicosanoids by CD34+ cells was not detected by either thin-layer chromatography, enzyme immunoassay, or differential spectroscopy. SCF stimulated the rapid but transient activation of ERK2 (p42 MAP kinase) in CD34+ cells, and we used the MAP kinase kinase inhibitor, PD 098059, which at 30 μmol/L blocks ERK2 activation in CD34+ cells, to investigate whether SCF-mediated priming of arachidonate release was mediated by this kinase. PD 098059 only partially inhibited A23187-stimulated PLA2 activity primed by SCF, suggesting the involvement of ERK2 and possibly a further signal transduction pathway. Methyl arachidonyl fluorophosphonate (5 μmol/L), a dual inhibitor of i and cPLA2 isoforms, completely inhibited arachidonate release without affecting ERK2 activation, demonstrating the lack of cellular toxicity. These data provide the first evidence that primitive myeloid cells have the capacity to release arachidonate, which is regulated by an early acting hematopoietic growth factor important for the growth and survival of these cells.

Primitive Myeloid Cells Express High Levels of Phospholipase A2 Activity in the Absence of Leukotriene Release: Selective Regulation by Stem Cell Factor Involving the MAP Kinase Pathway

The activation of phospholipase A2 (PLA2) with release of eicosanoids and prostanoids in mature myeloid cells and the augmentation (priming) of this activity by cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) are central to the inflammatory process. Yet, there are few data concerning PLA2 activity and its regulation by growth factors in primary hematopoietic cells. We therefore analyzed the PLA2activity of mobilized human CD34 antigen-positive (CD34+) stem cells by quantitation of the extracellular release of3H-arachidonate. The PLA2 activity of CD34+ cells stimulated with calcium ionophore (A23187) was of similar magnitude to that of mature neutrophils and monocytes. Preincubation of CD34+ cells with stem cell factor (SCF) before A23187-stimulation resulted in primed PLA2 activity, whereas interleukin-3 (IL-3), GM-CSF, and tumor necrosis factor  had no significant effect. When CD34+ cells were induced to differentiate, PLA2 activity remained responsive to SCF for several days, but after 8 days, at which stage morphological and functional evidence of maturation was occurring, priming of PLA2 by SCF could no longer be elicited, whereas responses to GM-CSF and IL-3 had developed. The further metabolism of arachidonic acid to eicosanoids by CD34+ cells was not detected by either thin-layer chromatography, enzyme immunoassay, or differential spectroscopy. SCF stimulated the rapid but transient activation of ERK2 (p42 MAP kinase) in CD34+ cells, and we used the MAP kinase kinase inhibitor, PD 098059, which at 30 μmol/L blocks ERK2 activation in CD34+ cells, to investigate whether SCF-mediated priming of arachidonate release was mediated by this kinase. PD 098059 only partially inhibited A23187-stimulated PLA2 activity primed by SCF, suggesting the involvement of ERK2 and possibly a further signal transduction pathway. Methyl arachidonyl fluorophosphonate (5 μmol/L), a dual inhibitor of i and cPLA2 isoforms, completely inhibited arachidonate release without affecting ERK2 activation, demonstrating the lack of cellular toxicity. These data provide the first evidence that primitive myeloid cells have the capacity to release arachidonate, which is regulated by an early acting hematopoietic growth factor important for the growth and survival of these cells.

Competitive, reversible inhibition of cytosolic phospholipase A2 at the lipid-water interface by choline derivatives that partially partition into the phospholipid bilayer

Cytosolic phospholipase A2 (cPLA2) catalyzes the selective release of arachidonic acid from the sn-2 position of phospholipids and is believed to play a key cellular role in the generation of arachidonic acid. When assaying the human recombinant cPLA2 using membranes isolated from [3H]arachidonate-labeled U937 cells as substrate, 2-(2'-benzyl-4-chlorophenoxy)ethyl-dimethyl-n-octadecyl-ammonium chloride (compound 1) was found to inhibit the enzyme in a dose-dependent manner (IC50 = 5 microM). It was over 70 times more selective for the cPLA2 as compared with the human nonpancreatic secreted phospholipase A2, and it did not inhibit other phospholipases. Additionally, it inhibited arachidonate production in N-formyl-methionyl-leucyl-phenylalanine-stimulated U937 cells. To further characterize the mechanism of inhibition, an assay in which the enzyme is bound to vesicles of 1,2-dimyristoyl-sn -glycero-3-phosphomethanol containing 6-10 mol % of 1-palmitoyl-2-[1-14C]arachidonoyl-sn-glycero-3-phosphocholine was employed. With this substrate system, the dose-dependent inhibition could be defined by kinetic equations describing competitive inhibition at the lipid-water interface. The apparent equilibrium dissociation constant for the inhibitor bound to the enzyme at the interface (KI*app) was determined to be 0.097 +/- 0.032 mol % versus an apparent dissociation constant for the arachidonate-containing phospholipid of 0.3 +/- 0.1 mol %. Thus, compound 1 represents a novel structural class of inhibitor of cPLA2 that partitions into the phospholipid bilayer and competes with the phospholipid substrate for the active site. Shorter n-alkyl-chained (C-4, C-6, C-8) derivatives of compound 1 were shown to have even smaller KI*app values. However, these short-chained analogs were less potent in terms of bulk inhibitor concentration needed for inhibition when using the [3H]arachidonate-labeled U937 membranes as substrate. This discrepancy was reconciled by showing that these shorter-chained analogs did not partition into the [3H]arachidonate-labeled U937 membranes as effectively as compound 1. The implications for in vivo efficacy that result from these findings are discussed.

Differentiation of U937 cells enables a phospholipase D-dependent pathway of cytosolic phospholipase A2 activation

Treatment with dibutyryl cyclic AMP (dBcAMP) of the human, premonocytic U937 cell line results in differentiation toward a monocyte/granulocyte-like cell. This differentiation enables the cell to activate cytosolic phospholipase A2 (cPLA2) to release arachidonate upon stimulation. In contrast, undifferentiated cells are unable to release arachidonate even when stimulated with calcium ionophores. In the present research, a role for phospholipase D (PLD) in the regulation of cPLA2 was shown based on a number of observations. First, the ionomycin- and fMLP-stimulated production of arachidonate in differentiated cells was sensitive to ethanol (2% (v/v)). Ethanol acts as an alternate substrate in place of water for PLD producing phosphatidylethanol (PEt) instead of phosphatidic acid. Indeed, ionomycin stimulation of differentiated cells produced a 14-fold increase in PEt levels. Further evidence for the involvement of PLD in the regulation of cPLA2 came from the observation that the stimulated production of diacylglycerol (for which phosphatidic acid is a major source) was greatly diminished in undifferentiated cells as compared to differentiated cells. Moreover, the normally deficient activation of cPLA2 in undifferentiated cells could be stimulated to release arachidonate if the cells were electroporated in the presence of GTP[gamma]S and MgATP. This treatment stimulates phosphatidylinositol-4,5-bisphosphate (PIP2) production which appears to activate PLD and cPLA2 in subsequent steps. The phosphatidic acid (and diacylglycerol derived from phosphatidic acid) appears to greatly regulate the action of cPLA2 by an unknown mechanism, and undifferentiated cells lack the ability to stimulate PLD activity due to a dysfunction of PIP2 production.

The size and curvature of anionic covesicle substrate affects the catalytic action of cytosolic phospholipase A2

Cytosolic phospholipase A2 (cPLA2) is normally located in the cytosol, but in response to cellular activation the enzyme binds to the membrane at the lipid/water interface where it catalyzes the hydrolysis of the sn-2 ester of arachidonate-containing phospholipids. Synthetic phospholipid vesicle systems have been used in kinetic and mechanistic analyses of cPLA2, but these systems result in a rapid loss of enzyme activity. In the present research, covesicles of 1,2-dimyristoyl-sn-glycero-3-phosphomethanol (DMPM) containing </=10 mol% 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (PAPC) as substrate were used to show that this premature cessation of enzyme-catalyzed hydrolysis is dependent on vesicle size with 25-nm-diameter vesicles supporting little activity as compared to 100-, 200-, and 400-nm vesicles. This suggests that the curvature of the vesicle may shift a conformational equilibrium toward an enzyme state which does not support activity. Interestingly, the presence of 30% (v/v) glycerol greatly enhanced the activity of the enzyme, although vesicle size-dependent premature cessation of hydrolysis was still observed. While the premature cessation of hydrolysis in the absence of glycerol is accompanied by enzyme inactivation, little inactivation occured in the presence of glycerol, indicating that premature cessation and inactivation are not absolutely coupled. When using this covesicle substrate system under conditions (6-10 mM CaCl2) where the vesicles are fusing, no premature cessation of hydrolysis has been observed. This is despite a mean vesicle diameter of 400-450 nm under vesicle-fusing conditions, which is comparable to the largest vesicles used under nonfusing conditions (0.5 mM CaCl2) where considerable premature cessation of hydrolysis was observed. Since DMPM has an intrinsic active site dissociation constant at least 330 times larger than that of PAPC, the optimum conditions for conducting kinetic and mechanistic analyses of cPLA2 with this covesicle substrate is one in which cPLA2 is assayed in the presence of glycerol and with fusion-inducing concentrations of calcium. The use of 1,2-dioleoyl-sn-glycero-3-phosphomethanol (DOPM) instead of DMPM in this system supports much less activity and adds the complication of a strong affinity of DOPM for the active site.

Subcellular localization of enzyme activities involved in the metabolism of platelet-activating factor in rainbow trout leukocytes

The subcellular distribution of an alkyllyso-GPC: acetyl-CoA acetyltransferase (EC 2.3.1.67) and transacylase, two important enzyme activities involved in the remodeling pathway for the biosynthesis of platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine, PAF) have been examined in leukocytes isolated from the pronephros of the rainbow trout, Oncorhynchus mykiss. Contrary to mammalian systems, in which the acetyltransferase is localized to intracellular membranes, the subcellular distribution of an acetyltransferase activity in rainbow trout leukocytes was localized to the plasma membrane. Analysis of the acetyltransferase products by thin-layer chromatography (TLC) and high performance liquid chromatography (HPLC) confirmed synthesis of two subclasses of PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine and 1-acyl-2-acetyl-sn-glycero-3-phosphocholine. The transacylase activity in this study was detected in membrane fractions in two domains of the intermediate density region which also contained the NADH dehydrogenase activity, a marker enzyme for the endoplasmic reticulum. Acylation of lysoPAF (1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine) exhibited approximately 95% specificity for omega-3 fatty acids. Acylation patterns were not significantly different in either domain of the endoplasmic reticulum. A model is proposed herein for the metabolism of PAF in rainbow trout leukocytes.

Differential potentiation of arachidonic acid release by rat alpha2 adrenergic receptor subtypes

CHO transfectants expressing the three subtypes of rat alpha2 adrenergic receptors (alpha2AR): alpha2D, alpha2B, alpha2C were studied to compare the transduction pathways leading to the receptor-mediated stimulation of phospholipase A2 (PLA2) in the corresponding cell lines CHO-2D, CHO-2B, CHO-2C. The alpha2B subtype stimulated the arachidonic acid (AA) release after incubation of the cells with 1 microM epinephrine, whereas alpha2D and alpha2C gave no stimulation. Calcium ionophore A23187 (1 microM) increased the release by a factor of 2-4 in the three strains. When cells were incubated with both epinephrine and Ca2+ ionophore, the AA release differed greatly between cell lines with strong potentiation in CHO-2B (2-3 times greater than Ca2+ ionophore alone), moderate potentiation in CHO-2D, and no potentiation in CHO-2C. The three cell lines each inhibited adenylylcyclase with similar efficiencies when 1 microM epinephrine was used as the agonist. The potentiation depended on both alpha2AR and Gi proteins since yohimbine and pertussis toxin inhibited the process. Pretreatment of CHO-2B cells with MAFP which inhibits both cytosolic and Ca2+-independent PLA2, reduced the release of AA induced by epinephrine+Ca2+ ionophore to basal value, whereas bromoenol lactone, a specific Ca2+-independent PLA2 inhibitor, had no effect. Preincubation of the cells with the intracellular calcium chelator BAPTA gave a dose-dependent inhibition of the arachidonic acid (AA) release. In CHO cells expressing the angiotensin II type 1 receptor, coupled to a Gq protein, the agonist (10-7 M) produced maximal AA release: there was no extra increase when angiotensin and Ca2+ ionophore were added together. There was no increase in the amount of inositol 1,4, 5-triphosphate following stimulation of CHO-2B, -2C, -2D cells with 1 microM epinephrine. Epinephrine led to greater phosphorylation of cPLA2, resulting in an electrophoretic mobility shift for all three cell lines, so inadequate p42/44 MAPKs stimulation was not responsible for the weaker stimulation of cPLA2 in CHO-2C cells. Therefore, the stimulation of cPLA2 by Gi proteins presumably involves another unknown mechanism. The differential stimulation of cPLA2 in these transfectants will be of value to study the actual involvement of the transduction pathways leading to maximal cPLA2 stimulation.

Phospholipase pathway in Alzheimer's disease brains: decrease in Galphai in dorsolateral prefrontal cortex

There is substantial evidence that G-protein-associated signaling pathways in the brain are altered in Alzheimer's disease (AD). Using quantitative immunoblotting we find a significant decrease in Galphai levels in every AD case examined compared to controls (mean Galphai level in AD was 43.5+/-7.4% of control). Galphao levels were slightly decreased, but Galphaq and betagamma were normal. Phospholipase C-beta1, but not gamma1, levels were also decreased. Total phospholipase C activity and ceramide levels were not changed. Thus, in AD, there is impairment in the Galphai-associated signaling pathway in neurons.

A beta-lactam inhibitor of cytosolic phospholipase A2 which acts in a competitive, reversible manner at the lipid/water interface

Cytosolic phospholipase A2 (cPLA2) catalyzes the selective release of arachidonic acid from the sn-2 position of phospholipids and is believed to play a key cellular role in the generation of arachidonic acid. When assaying the human recombinant cPLA2 using membranes isolated from [3H]arachidonate-labeled U937 cells as substrate, 3,3-Dimethyl-6-(3-lauroylureido)-7-oxo-4-thia-1-azabicyclo[3,2,0] heptane-2-carboxylic acid (1) was found to inhibit the enzyme in a dose-dependent manner (IC50 = 72 microM). This beta-lactam did not inhibit other phospholipases, including the human nonpancreatic secreted phospholipase A2. The inhibition of cPLA2 was found not to be time-dependent. This, along with the observation that the degradation of the inhibitor was not catalyzed by the enzyme, demonstrates that the inhibition does not result from the formation of an acyl-enzyme intermediate with the active site serine residue. Moreover, the ring-opened form of 1 is also able to inhibit cPLA2 with near-equal potency. To further characterize the mechanism of inhibition, an assay in which the enzyme is bound to vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphomethanol containing 6-10 mole percent of 1-palmitoyl-2-[1-14C]-arachidonoyl-sn-glycero-3-phosphocholine was employed. With this substrate system, the dose-dependent inhibition was defined by kinetic equations describing competitive inhibition at the lipid/water interface. The apparent dissociation constant for the inhibitor bound to the enzyme at the interface (KI*app) was determined to be 0.5 +/- 0.1 mole% versus an apparent dissociation constant for the arachidonate-containing phospholipid of 0.4 +/- 0.1 mole%. Thus, 1 represents a novel structural class of inhibitors of cPLA2 which partitions into the phospholipid bilayer and competes with the phospholipid substrate for the active site.