Cloning and expression of cystolic phospholipase A2 (cPLA2) and a naturally occurring variant. Phosphorylation of Ser505 of recombinant cPLA2 by p42 mitogen-activated protein kinase results in an increase in specific activity

Loss of PTEN stabilizes the lipid modifying enzyme cytosolic phospholipase A₂α via AKT in prostate cancer cells

Aberrant increase in pAKT, due to a gain-of-function mutation of PI3K or loss-of-function mutation or deletion of PTEN, occurs in prostate cancer and is associated with poor patient prognosis. Cytosolic phospholipase A₂α (cPLA₂α) is a lipid modifying enzyme by catalyzing the hydrolysis of membrane arachidonic acid. Arachidonic acid and its metabolites contribute to survival and proliferation of prostate cancer cells. We examined whether AKT plays a role in promoting cPLA₂α action in prostate cancer cells. We found a concordant increase in pAKT and cPLA₂α levels in prostate tissue of prostate epithelial-specific PTEN-knockout but not PTEN-wide type mice. Restoration of PTEN expression or inhibition of PI3K action decreased cPLA₂α expression in PTEN-mutated or deleted prostate cancer cells. An increase in AKT by Myr-AKT elevated cPLA₂α protein levels, which could be diminished by inhibition of AKT phosphorylation without noticeable change in total AKT levels. pAKT levels had no influence on cPLA₂α at mRNA levels but reduced cPLA₂α protein degradation. Anti-AKT antibody co-immunoprecipitated cPLA₂α and vice versa. Hence, AKT plays a role in enhancing cPLA₂α protein stability in PTEN-null prostate cancer cells, revealing a link between oncogenic pathway and lipid metabolism.

A stochastic signaling network mediates the probabilistic induction of cerebellar long-term depression

Many cellular processes involve a small number of molecules and undergo stochastic fluctuations in their levels of activity. Cerebellar long-term depression (LTD) is a form of synaptic plasticity expressed as a reduction in the number of synaptic AMPA receptors (AMPARs) in Purkinje cells. We developed a stochastic model of the LTD signaling network, including a PKC-ERK-cPLA(2) positive feedback loop and mechanisms of AMPAR trafficking, and tuned the model to replicate calcium uncaging experiments. The signaling network activity in single synapses switches between two discrete stable states (LTD and non-LTD) in a probabilistic manner. The stochasticity of the signaling network causes threshold dithering and allows at the macroscopic level for many different and stable mean magnitudes of depression. The probability of LTD occurrence in a single spine is only modulated by the concentration and duration of the signal used to trigger it, and inputs with the same magnitude can give rise to two different responses; there is no threshold for the input signal. The stochasticity is intrinsic to the signaling network and not mostly dependent on noise in the calcium input signal, as has been suggested previously. The activities of the ultrasensitive ERK and of cPLA(2) undergo strong stochastic fluctuations. Conversely, PKC, which acts as a noise filter, is more constantly activated. Systematic variation of the biochemical population size demonstrates that threshold dithering and the absence of spontaneous LTD depend critically on the number of molecules in a spine, indicating constraints on spine size in Purkinje cells.

Phospholipase A2 activation as a therapeutic approach for cognitive enhancement in early-stage Alzheimer disease

RATIONALE

Alzheimer disease (AD) is the leading cause of dementia in the elderly and has no known cure. Evidence suggests that reduced activity of specific subtypes of intracellular phospholipases A2 (cPLA2 and iPLA2) is an early event in AD and may contribute to memory impairment and neuropathology in the disease.

OBJECTIVE

The objective of this study was to review the literature focusing on the therapeutic role of PLA2 stimulation by cognitive training and positive modulators, or of supplementation with arachidonic acid (PLA2 product) in facilitating memory function and synaptic transmission and plasticity in either research animals or human subjects.

METHODS

MEDLINE database was searched (no date restrictions) for published articles using the keywords Alzheimer disease (mild, moderate, severe), mild cognitive impairment, healthy elderly, rats, mice, phospholipase A(2), phospholipid metabolism, phosphatidylcholine, arachidonic acid, cognitive training, learning, memory, long-term potentiation, protein kinases, dietary lipid compounds, cell proliferation, neurogenesis, and neuritogenesis. Reference lists of the identified articles were checked to select additional studies of interest.

RESULTS

Overall, the data suggest that PLA2 activation is induced in the healthy brain during learning and memory. Furthermore, learning seems to regulate endogenous neurogenesis, which has been observed in AD brains. Finally, PLA2 appears to be implicated in homeostatic processes related to neurite outgrowth and differentiation in both neurodevelopmental processes and response to neuronal injury.

CONCLUSION

The use of positive modulators of PLA2 (especially of cPLA2 and iPLA2) or supplementation with dietary lipid compounds (e.g., arachidonic acid) in combination with cognitive training could be a valuable therapeutic strategy for cognitive enhancement in early-stage AD.

Conditioning training and retrieval increase phospholipase A(2) activity in the cerebral cortex of rats

In rats, phospholipase A(2) (PLA(2)) activity was found to be increased in the hippocampus immediately after training and retrieval of a contextual fear conditioning paradigm (step-down inhibitory avoidance [IA] task). In the present study we investigated whether PLA(2) is also activated in the cerebral cortex of rats in association with contextual fear learning and retrieval. We observed that IA training induces a rapid (immediately after training) and long-lasting (3 h after training) activation of PLA(2) in both frontal and parietal cortices. However, immediately after retrieval (measured 24 h after training), PLA(2) activity was increased just in the parietal cortex. These findings suggest that PLA(2) activity is differentially required in the frontal and parietal cortices for the mechanisms of contextual learning and retrieval. Because reduced brain PLA(2) activity has been reported in Alzheimer disease, our results suggest that stimulation of PLA(2) activity may offer new treatment strategies for this disease.

A positive feedback signal transduction loop determines timing of cerebellar long-term depression

Synaptic activity produces short-lived second messengers that ultimately yield a long-term depression (LTD) of cerebellar Purkinje cells. Here, we test the hypothesis that these brief second messenger signals are translated into long-lasting biochemical signals by a positive feedback loop that includes protein kinase C (PKC) and mitogen-activated protein kinase. Histochemical "epistasis" experiments demonstrate the reciprocal activation of these kinases, and physiological experiments--including the use of a light-activated protein kinase--demonstrate that such reciprocal activation is required for LTD. Timed application of enzyme inhibitors reveals that this positive feedback loop causes PKC to be active for more than 20 min, allowing sufficient time for LTD expression. Such regenerative mechanisms may sustain other long-lasting forms of synaptic plasticity and could be a general mechanism for prolonging signal transduction networks.

High glucose inhibits fructose uptake in renal proximal tubule cells: involvement of cAMP, PLC/PKC, p44/42 MAPK, and cPLA2

The precise signal that regulates fructose transport in renal proximal tubule cells (PTCs) under high glucose conditions is not yet known although fructose has been recommended as a substitute for glucose in the diets of diabetic people. Thus, we investigated that effect of high glucose on fructose uptake and its signaling pathways in primary cultured rabbit renal PTCs. Glucose inhibited the fructose uptake in a time- and dose-dependent manner. A maximal inhibitory effect of glucose on fructose uptake was observed at 25 mM glucose after 48 h, while 25 mM mannitol and l-glucose did not affect fructose uptake. Indeed, 25 mM glucose for 48 h decreased GLUT5 protein level. Thus, the treatment of 25 mM glucose for 48 h was used for this study. Glucose-induced (25 mM) inhibition of fructose uptake was blocked by pertussis toxin (PTX), SQ-22536 (an adenylate cyclase inhibitor), and myristoylated amide 14-22 (a protein kinase A inhibitor). Indeed, 25 mM glucose increased the intracellular cAMP content. Furthermore, 25 mM glucose-induced inhibition of fructose uptake was prevented by neomycin or U-73122 (phospholipase C inhibitors) and staurosporine or bisindolylmaleimide I (protein kinase C inhibitors). In fact, 25 mM glucose increased the total PKC activity and translocation of PKC from the cytosolic to membrane fraction. In addition, PD 98059 (a p44/42 mitogen-activated protein kinase (MAPK) inhibitor) but not SB 203580 (a p38 MAPK inhibitor) and mepacrine or AACOCF3 (phospholipase A2 inhibitors) blocked 25 mM glucose-induced inhibition of fructose uptake. Results of Western blotting using the p44/42 MAPK and GLUT5 antibodies were consistent with the results of uptake experiments. In conclusion, high glucose inhibits the fructose uptake through cAMP, PLC/PKC, p44/42 MAPK, and cytosolic phospholipase A2 (cPLA2) pathways in the PTCs.

Ceramide kinase mediates cytokine- and calcium ionophore-induced arachidonic acid release

Despite the importance of prostaglandins, little is known about the regulation of prostanoid synthesis proximal to the activation of cytosolic phospholipase A2, the initial rate-limiting step. In this study, ceramide-1-phosphate (C-1-P) was shown to be a specific and potent inducer of arachidonic acid (AA) and prostanoid synthesis in cells. This study also demonstrates that two well established activators of AA release and prostanoid synthesis, the cytokine, interleukin-1beta (IL-1beta), and the calcium ionophore, A23187, induce an increase in C-1-P levels within the relevant time-frame of AA release. Furthermore, the enzyme responsible for the production of C-1-P in mammalian cells, ceramide kinase, was activated in response to IL-1beta and A23187. RNA interference targeted to ceramide kinase specifically down-regulated ceramide kinase mRNA and activity with a concomitant decrease of AA release in response to IL-1beta and A23187. Down-regulation of ceramide kinase had no effect on AA release induced by exogenous C-1-P. Collectively, these results indicate that ceramide kinase, via the formation of C-1-P, is an upstream modulator of phospholipase A2 activation. This study identifies previously unknown roles for ceramide kinase and its product, C-1-P, in AA release and production of eicosanoids and provides clues for potential new targets to block inflammatory responses.

Role of cyclooxygenase 2, p38 and p42/44 MAPK in the secretion of prostacyclin induced by epidermal growth factor, endothelin-1 and angiotensin II in rat ventricular cardiomyocytes

We studied the respective roles of cyclooxygenases (COX) isoforms as well as the p38 and p42/44 MAP kinase cascades in angiotensin II (AngII)-, endothelin-1 (ET-1)- and epidermal growth factor (EGF)-induced prostacyclin (PGI(2)) secretion in neonatal rat ventricular cardiomyocytes. Exposure of these cells for 1 h to 100 nM AngII, ET-1 or EGF resulted in an increase in prostacyclin formation which was abolished by the COX-2 specific inhibitor NS-398 (1 microM), while the COX-1 inhibitor valeryl salicylate (5 microM) had no effect. Agonist-induced prostacyclin secretion was also abolished in the presence of cycloheximide (10 microg/ml), indicating that newly synthesized proteins are necessary for this response. In this context, the COX-2 protein amount was significantly increased following 1 h incubation of cardiomyocytes, with AngII, ET-1 and EGF. These results indicate that in cardiomyocytes AngII, ET-1 and EGF induce both the synthesis and the activity of COX-2. Investigating the role of MAPK in the stimulation of prostacyclin induced by these three agonists, we found that both the p42/44 MAPK inhibitor PD 98059 (50 microM) and the p38 MAPK blocker SB 203580 (5 microM) prevented agonist-induced PGI(2) secretion without affecting COX-2 activity or synthesis. Our results show that p42/44 and p38 MAPK activation is at the basis of AngII-, ET-1- and EGF-induced prostacyclin secretion in cardiomyocytes. They further suggest that these MAPK act on a target(s) located upstream of COX-2.

Phospholipase A2 enzymes

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate arachidonic acid (AA), a precursor of eicosanoids including prostaglandins and leukotrienes. The same reaction also produces lysophosholipids, which represent another class of lipid mediators. So far, at least 19 enzymes that possess PLA2 activity have been identified and cloned in mammals. The secretory PLA2 (sPLA2) family, in which 10 isozymes have been identified, consists of low-molecular weight, Ca2+-requiring secretory enzymes that have been implicated in a number of biological processes, such as modification of eicosanoid generation, inflammation, and host defense. The cytosolic PLA2 (cPLA2) family consists of three enzymes, among which cPLA2alpha has been paid much attention by researchers as an essential component of the initiation of AA metabolism. The activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains two enzymes and may play a major role in phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family contains four enzymes that exhibit unique substrate specificity toward PAF and/or oxidized phospholipids. Degradation of these bioactive phospholipids by PAF-AHs may lead to the termination of inflammatory reaction and atherosclerosis.

Agonist-specific cross talk between ERKs and p38(mapk) regulates PGI(2) synthesis in endothelium

We have examined the mechanisms regulating prostacyclin (PGI(2)) synthesis after acute exposure of human umbilical vein endothelial cells (HUVEC) to interleukin-1 alpha (IL-1 alpha). IL-1 alpha evoked an early (30 min) release of PGI(2) and [(3)H]arachidonate that was blocked by the cytosolic phospholipase A(2)alpha (cPLA(2)alpha) inhibitor arachidonyl trifluoromethyl ketone. IL-1 alpha-mediated activation of extracellular signal-regulated kinase 1/2 (ERK1/2; p42/p44(mapk)) coincided temporally with phosphorylation of cPLA(2)alpha and with the onset of PGI(2) synthesis. The mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitors, PD-98059 and U-0126, blocked IL-1 alpha-induced ERK activation and partially attenuated cPLA(2)alpha phosphorylation and PGI(2) release, suggesting that ERK-dependent and -independent pathways regulate cPLA(2)alpha phosphorylation. SB-203580 treatment enhanced IL-1 alpha-induced MEK, p42/44(mapk), and cPLA(2)alpha phosphorylation but reduced thrombin-stimulated MEK and p42/44(mapk) activation. IL-1 alpha, but not thrombin, activated Raf-1 as assessed by immune-complex kinase assay, as did SB-203580 alone. These results show that IL-1 alpha causes an acute upregulation of PGI(2) generation in HUVEC, establish a role for the MEK/ERK/cPLA(2)alpha pathway in this early release, and provide evidence for an agonist-specific cross talk between p38(mapk) and p42/44(mapk) that may reflect receptor-specific differences in the signaling elements proximal to MAPK activation.

Protein kinase C-alpha coordinately regulates cytosolic phospholipase A(2) activity and the expression of cyclooxygenase-2 through different mechanisms in mouse keratinocytes

Transgenic mice (K5-PKC alpha) in which the keratin 5 promoter directs the expression of protein kinase C-alpha (PKC alpha) to epidermal keratinocytes display a 10-fold increase in PKC alpha protein in their epidermis and alterations in phorbol ester-induced cutaneous inflammation [J Cell Science 1999;112:3497-3506]. In the current study, we have used these K5-PKC alpha mice to examine the role of PKC alpha in keratinocyte phospholipid metabolism/eicosanoid production and cutaneous inflammation. Primary keratinocytes from wild-type and transgenic mice were prelabeled in culture with [(3)H]arachidonic acid (AA) and subsequently treated with TPA. Compared with wild-type keratinocytes, K5-PKC alpha keratinocytes displayed a 2-fold increase in AA release. TPA treatment resulted in the phosphorylation of cPLA(2). PKC inhibitors GF-109203X or H7, but not mitogen-activated protein/extracellular signal-regulated protein kinase (MEK) inhibitor PD 98059, could inhibit phosphorylation and AA release. Topical 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment of K5-PKC alpha mice resulted in a 5-fold increase in epidermal COX-2 induction and a 2- to 3-fold increase in prostaglandin (PG) E(2) levels above that observed in TPA-treated wild-type mice. PD 98059, GF-109203X, or H7 could block cyclooxygenase-2 (COX-2) induction by TPA. Because C/EBP beta, a basic leucine zipper transcription factor, can be activated via a PKC alpha/mitogen-activated protein kinase pathway and can influence COX-2 expression, we examined whether C/EBP beta is involved in TPA-induced epidermal COX-2 expression. TPA-induced COX-2 expression was similar in C/EBP beta nullizygous and wild-type mice. In summary, our results indicate that epidermal PKC alpha coordinately regulates cPLA(2) activity and COX-2 expression resulting in increased levels of AA and PGE(2). Furthermore, PKC alpha-induced AA release and cPLA(2) phosphorylation are independent of MEK, whereas PKC alpha-induced COX-2 expression and PGE(2) production are MEK-dependent and C/EBP beta-independent events.

The MAP kinase inhibitors, PD098059, UO126 and SB203580, inhibit IL-1beta-dependent PGE(2) release via mechanistically distinct processes

1. In common with human bronchial epithelial cells, pulmonary A549 cells release prostaglandin (PG) E(2) in response to pro-inflammatory cytokines. We have therefore used these cells to examine the effect of the selective mitogen activated protein (MAP) kinase inhibitors; PD098059, a mitogen activated and extracellular regulated kinase kinase (MEK) 1 inhibitor, UO126, a dual MEK1 & MEK2 inhibitor, and SB203580, a p38 MAP kinase inhibitor in the IL-1beta-dependent release of PGE(2). 2. Following IL-1beta treatment the extracellular regulated kinases (ERKs) and the p38 MAP kinases were rapidly phosphorylated. 3. PD09059, UO126 and SB203580 prevented IL-1beta-induced PGE(2) release at doses that correlated closely with published IC(50) values. Small or partial effects at the relevant doses were observed on induction of cyclo-oxygenase (COX) activity or COX-2 protein suggesting that the primary effects were at the level of arachidonate availability. 4. Neither PD098059 nor SB203580 showed any effect on IL-1beta-induced arachidonate release. We therefore speculate that the MEK1/ERK and p38 kinase cascades play a role in the functional coupling of arachidonate release to COX-2. 5. In contrast, UO126 was highly effective at inhibiting IL-1beta-dependent arachidonate release, implicating MEK2 in the activation of the PLA(2) that is involved in IL-1beta-dependent PGE(2) release. 6. We conclude that the MEK1, MEK2 and p38 MAP kinase inhibitors, PD098059, UO126 and SB203580, are highly potent in respect of inflammatory PG release. Finally, we conclude that these inhibitors act via mechanistically distinct processes, which may have anti-inflammatory benefits.

Cytokine-mediated cPLA(2) phosphorylation is regulated by multiple MAPK family members

Cytosolic phospholipase A(2) (cPLA(2)) plays a critical role in various neutrophil functions including the generation of leukotrienes and platelet-activating factor release. Enzyme activity is regulated both by translocation to the membrane in a Ca(2+)-dependent manner and serine phosphorylation by members of the mitogen-activated protein kinase (MAPK) family. In this report, we have investigated the role of granulocyte/macrophage colony-stimulating factor (GM-CSF)-mediated signalling pathways in the regulation of cPLA(2). GM-CSF-induced cPLA(2) phosphorylation was not affected by pharmacological inhibition of p38 MAPK, phosphatidylinositol 3-kinase or Src. However, inhibition of extracellular signal-regulated kinase (ERK) MAPK activation resulted in a partial inhibition of cPLA(2) phosphorylation, revealed in a slower onset of phosphorylation. A cell line stably transfected with the GM-CSF receptor was used to further analyze GM-CSF-mediated cPLA(2) phosphorylation. Mutation of tyrosine residues 577 and 612 resulted in a delayed cPLA(2) phosphorylation similar to the pharmacological ERK inhibition. Furthermore, inhibition of p38 MAPK in cells bearing the double mutant betac577/612 completely abrogated GM-CSF-induced cPLA(2) phosphorylation. We conclude that GM-CSF can mediate cPLA(2) phosphorylation through the redundant activation of both p38 and ERK MAP kinases.

Mitogen-activated protein kinase: conservation of a three-kinase module from yeast to human

Mitogen-activated protein kinases (MAPK) are serine-threonine protein kinases that are activated by diverse stimuli ranging from cytokines, growth factors, neurotransmitters, hormones, cellular stress, and cell adherence. Mitogen-activated protein kinases are expressed in all eukaryotic cells. The basic assembly of MAPK pathways is a three-component module conserved from yeast to humans. The MAPK module includes three kinases that establish a sequential activation pathway comprising a MAPK kinase kinase (MKKK), MAPK kinase (MKK), and MAPK. Currently, there have been 14 MKKK, 7 MKK, and 12 MAPK identified in mammalian cells. The mammalian MAPK can be subdivided into five families: MAPKerk1/2, MAPKp38, MAPKjnk, MAPKerk3/4, and MAPKerk5. Each MAPK family has distinct biological functions. In Saccharomyces cerevisiae, there are five MAPK pathways involved in mating, cell wall remodelling, nutrient deprivation, and responses to stress stimuli such as osmolarity changes. Component members of the yeast pathways have conserved counterparts in mammalian cells. The number of different MKKK in MAPK modules allows for the diversity of inputs capable of activating MAPK pathways. In this review, we define all known MAPK module kinases from yeast to humans, what is known about their regulation, defined MAPK substrates, and the function of MAPK in cell physiology.

G-protein-mediated signaling in cholesterol-enriched arterial smooth muscle cells. 1. Reduced membrane-associated G-protein content due to diminished isoprenylation of G-gamma subunits and p21ras

Mechanisms contributing to altered heterotrimeric G-protein expression and subsequent signaling events during cholesterol accretion have been unexplored. The influence of cholesterol enrichment on G-protein expression was examined in cultured smooth muscle cells that resemble human atherosclerotic cells by exposure to cationized LDL (cLDL). cLDL, which increases cellular free and esterified cholesterol 2-fold and 10-fold, respectively, reduced the cell membrane content of Galphai-1, Galphai-2, Galphai-3, Gq/11, and Galphas. The following evidence supports the premise that the mechanism by which this occurs is due to reduced isoprenylation of the Ggamma-subunit. First, the inhibitory effect of cholesterol enrichment on the membrane content of Galphai subunits was found to be post-transcriptional, since the mRNA steady-state levels of Galphai(1-3) were unchanged following cholesterol enrichment. Second, the membrane expression of alpha and beta subunits was mimicked by cholesterol and 17-ketocholesterol, both of which inhibit HMG-CoA reductase. Third, inhibition of Galphai and Gbeta expression in cholesterol-enriched cells was overcome by mevalonate, the immediate product of HMG-CoA reductase. Fourth, pulse-chase experiments revealed that cholesterol enrichment did not reduce the degradation rate of membrane-associated Galphai subunits. Fifth, cholesterol enrichment also reduced membrane expression of Ggamma-5, Ggamma-7upper; these gamma subunits are responsible for trafficking of the heterotrimeric G-protein complex to the cell membrane as a result of HMG-CoA reductase-dependent post-translational lipid modification (geranylgeranylation) and subsequent membrane association. Cholesterol enrichment did not alter expression of G-gamma-5 mRNA, as assessed by reverse transcriptase polymerase chain reaction, supporting a post-transcriptional defect in Ggamma subunit expression. Fifth, cholesterol enrichment also reduced the membrane content of p21ras (a low molecular weight G-protein requiring farnesylation for membrane targeting) but did not alter the membrane content of the two proteins that do not require isoprenylation for membrane association&sbd;PDGF-receptor or p60-src. Reduced G-protein content in cholesterol-laden cells was reflected by reduced G-protein-mediated signaling events, including ATP-induced GTPase activity, thrombin-induced inhibition of cyclic AMP accumulation, and MAP kinase activity. Collectively, these results demonstrate that cholesterol enrichment reduces G-protein expression and signaling by inhibiting isoprenylation and subsequent membrane targeting. These results provide a molecular basis for altered G-protein-mediated cell signaling processes in cholesterol-enriched cells.

Phospholipid peroxidation induces cytosolic phospholipase A2 activity: membrane effects versus enzyme phosphorylation

Cytosolic phospholipase A2 (cPLA2) is a signal-responsive enzyme that is highly selective to the nature of phospholipid substrates. A mechanism for cPLA2 activity regulation through a signal transduction pathway has been proposed and this signaling appears to be influenced by oxidants. Oxidant-mediated signaling of PLA2 may serve as an alternative mechanism for enzyme regulation; however, the manner of regulation has yet to be delineated. In this report we demonstrate that there is a direct effect of membrane oxidation on cPLA2 phosphorylation and activity. A simple in vitro system consisting of purified cPLA2 and phospholipid vesicles was used to facilitate protein kinase C (PKC) activity and provide substrates for cPLA2. Using these vesicles we found that the activity of cPLA2 was enhanced twofold when the vesicles contained as little as 5 mol% phosphatidylcholine hydroperoxides (PLPCOOH). The order of hydrolytic preference for fatty acyl species was 20:4 > 18:2 > 18:1 > 16:0, and the presence of PLPCOOH stimulated hydrolysis largely of phosphatidylcholine containing 20:4. The Ca2+ concentrations required for stimulated hydrolytic activity were also twofold lower for oxidized compared to unoxidized vesicles. Using phospholipid micelles as substrates, PKC-mediated phosphorylation of cPLA2 increased hydrolytic activity 71% compared to preparations lacking PKC. Using phospholipid vesicles as substrates, PKC-mediated phosphorylation resulted in an 85% increase in cPLA2 activity compared to preparations without PKC. PKC-mediated phosphorylation of cPLA2, therefore, stimulates catalytic activity toward membrane phospholipids and the extent of activation is enhanced directly by peroxidation of membrane phospholipids and involves a peroxide-induced stimulation of cPLA2 phosphorylation.