Role of enhanced inositol phospholipid metabolism in neutrophil activation

Soy Phospholipids Exert a Renoprotective Effect by Inhibiting the Nuclear Factor Kappa B Pathway in Macrophages

Complications associated with chronic kidney disease (CKD), which involves kidney inflammation, are a major health problem. Soy protein isolate (SPI) reportedly inhibits CKD exacerbation; however, its detailed action mechanism remains obscure. Therefore, the role of the polar lipid component of SPI in suppressing inflammation was investigated. Zucker fatty rats were divided into three groups and fed a diet containing casein, SPI, or casein + SPI ethanol extract (SPIEE) for 16 weeks. The isoflavones and phospholipids of SPIEE were evaluated for their anti-inflammatory effects. Rats in the SPI and casein + SPIEE groups showed reduced levels of the urinary N-acetyl-β-d-glucosaminidase and renal IL-1β mRNA (an inflammatory marker) compared with those in the casein group. In proximal tubular cells, genistein significantly inhibited monocyte chemoattractant protein-1 (MCP-1) expression induced by an IL-1β stimulus. In macrophages, soybean phospholipids suppressed lipopolysaccharide-induced IL-1β gene expression by inhibiting the phosphorylation of inhibitor κB and p65. Phosphatidylinositol (PI) was found to be essential for inhibition of IL-1β expression. SPIEE inhibited the exacerbation of kidney disease. Genistein and soybean phospholipids, especially soybean-specific phospholipids containing PI, effectively inhibited the inflammatory spiral in vitro. Hence, daily soybean intake may be effective for inhibiting chronic inflammation and slowing kidney disease progression.

Structural organization of the neutrophil NADPH oxidase: phosphorylation and translocation during priming and activation

The reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is part of the microbicidal arsenal used by human polymorphonuclear neutrophils (PMNs) to eradicate invading pathogens. The production of a superoxide anion (O2-) into the phagolysosome is the precursor for the generation of more potent products, such as hydrogen peroxide and hypochlorite. However, this production of O2- is dependent on translocation of the oxidase subunits, including gp91phox, p22phox, p47phox, p67phox, p40phox, and Rac2 from the cytosol or specific granules to the plasma membrane. In response to an external stimuli, PMNs change from a resting, nonadhesive state to a primed, adherent phenotype, which allows for margination from the vasculature into the tissue and chemotaxis to the site of infection upon activation. Depending on the stimuli, primed PMNs display altered structural organization of the NADPH oxidase, in that there is phosphorylation of the oxidase subunits and/or translocation from the cytosol to the plasma or granular membrane, but there is not the complete assembly required for O2- generation. Activation of PMNs is the complete assembly of the membrane-linked and cytosolic NADPH oxidase components on a PMN membrane, the plasma or granular membrane. This review will discuss the individual components associated with the NADPH oxidase complex and the function of each of these units in each physiologic stage of the PMN: rested, primed, and activated.

Oxidative status of plasma and muscle in rabbits supplemented with dietary vitamin E

Thirty New Zealand white rabbits, mean weight 2 kg, were divided into three equal groups balanced for body weight and randomly assigned to a diet containing 60 (C), 150 (T1) or 375 (T2) mg/kg of all-rac-alpha-tocopheryl acetate. After 29 days, the animals were slaughtered. alpha-Tocopherol was assayed in muscle (longissimus dorsi) and plasma; triglycerides and cholesterol (total, high density lipoprotein, low density lipoprotein) were analysed in plasma; reactive oxygen metabolites (ROMs) were analysed in serum; and thiobarbituric acid-reactive substances (TBARS) were analysed in muscle. There were no body weight and food intake differences between the groups. The plasma vitamin E and vitamin E:lipid ratio were significantly higher in groups T1 and T2 than in C, but increases were not linearly related to dietary levels. Muscle alpha-tocopherol concentrations in the treated groups were significantly higher than in C, and linearly related (R =.67) to the vitamin E:lipid ratio. ROM and vitamin E levels in blood were inversely related (R =.74), with ROMs significantly lower in the treated groups than in C. The 60-mg/kg dose of C recommended by the National Research Council was unable to control ROM production. Lipid oxidation in muscle was significantly lower in T2 than in the other groups, and TBARS correlated significantly with muscle vitamin E (R =.61) and serum ROM (R =.73). These data suggest that vitamin E supplemented at 375 mg/kg diet can effectively control ROM production and improve muscle lipostability. ROM assay provides a useful indirect estimate of the oxidative status of muscle in vivo.

Activation of human neutrophil NADPH oxidase by phosphatidic acid or diacylglycerol in a cell-free system. Activity of diacylglycerol is dependent on its conversion to phosphatidic acid

The superoxide-generating neutrophil NADPH oxidase can be activated in cell-free reconstitution systems by several agonists, most notably arachidonic acid and the detergent sodium dodecyl sulfate. In this study, we show that both phosphatidic acids and diacylglycerols can serve separately as potent, physiologic activators of NADPH oxidase in a cell-free system. Stimulation of superoxide generation by these lipids was dependent upon both Mg(2+) and agonist concentration. Activation of NADPH oxidase by phosphatidic acids did not appear to require their conversion to corresponding diacylglycerols by phosphatidate phosphohydrolase, since diacylglycerols were much slower than phosphatidic acids to activate the system and required the presence of ATP. Stimulation of the oxidase by dioctanoylglycerol proved to be by a means other than the activation of protein kinase C. Instead, dioctanoylglycerol was converted to dioctanoylphosphatidic acid by an endogenous diacylglycerol kinase present in the cell-free reaction system. This conversion was sensitive to the diacylglycerol kinase inhibitor R59949 and explains the markedly slower kinetics of activation and the novel ATP requirement seen with dioctanoylglycerol. The level of dioctanoylphosphatidic acid formed was suboptimal for NADPH oxidase activation but could synergize with the unmetabolized dioctanoylglycerol to activate superoxide generation.

Activation of polymorphonuclear leukocytes in oleic acid-induced lung injury

OBJECTIVE

Oleic acid (OA) can produce a lung injury similar to the adult respiratory distress syndrome (ARDS). Elastase and superoxides are thought to have an effect in ARDS. However, the effect that elastase and superoxide have in OA lung injury is unclear. To examine their involvement in OA lung injury, we tested the effects of methoxysuccinyl-alanyl-alanyl-prolyl-valyl chloromethyl ketone (MAAPVCK), an elastase inhibitor, and N-acetyl-L-cysteine (NAC), an active oxygen scavenger, on the increase in pulmonary vascular permeability caused by OA. We also examined whether OA stimulated elastase and/or superoxide release from polymorphonuclear leukocytes (PMNs).

DESIGN

Prospective trial.

SETTING

University laboratory.

INTERVENTIONS

(1) Guinea pigs were anesthetized. MAAPVCK (2.5 mg/ kg) or NAC (150 mg/kg) was infused over OA (15 microl/kg) injection. Evans blue was used to measure vascular permeability. (2) PMNs were isolated from the blood of guinea pigs and rats. Elastase release was measured with MeO-Suc-Ala-Ala-Pro-Val-7-amino-4-methylcoumarin. Superoxide production was measured by the ferricytochrome c reduction method.

MEASUREMENTS AND RESULTS

OA caused pulmonary hemorrhage and an increase in vascular permeability. MAAPVCK and NAC significantly attenuated the increase in vascular permeability in distal bronchus and trachea, respectively. OA induced superoxide production from PMNs in guinea pigs, but elastase release from PMNs was not detected.

CONCLUSIONS

These results suggest that elastase and superoxide are involved in OA lung injury.

Inhibitory effects of the new anti-platelet agent KBT-3022 and its metabolite on rabbit neutrophil function in vitro

The effects of the new anti-platelet agent KBT-3022, ethyl 2-[4,5-bis(4-methoxyphenyl)-thiazol-2-yl]pyrrol-1-ylacetate, and its metabolite desethyl KBT-3022 on rabbit neutrophil function were investigated in comparison with the effects of acetylsalicylic acid (ASA), ticlopidine hydrochloride (TP), cilostazol (CIL) and indomethacin (IM). The adhesion and migration of neutrophils induced by formyl-methionyl-leucyl-phenylalanine (fMLP) were inhibited by all the compounds tested, their rank order of potency being KBT-3022 = desethyl KBT-3022 > TP = CIL = IM > ASA. KBT-3022, desethyl KBT-3022, CIL and IM all suppressed fMLP-induced increases in the intracellular free Ca2+ concentration ([Ca2+]i) in neutrophils, their potencies correlating with their inhibitory effects on fMLP-induced adhesion and migration. KBT-3022 (1 microM), desethyl KBT-3022 (1-10 microM) and CIL (10 microM) but not IM significantly inhibited both neutrophil migration and the increase in [Ca2+]i induced by leukotriene B4 (LTB4). KBT-3022 (1 microM) and desethyl KBT-3022 (1 microM) suppressed the increase in [Ca2+]i induced by complement C5a. Although KBT-3022 and desethyl KBT-3022 did not influence [3H]LTB4 and [125I]C5a specific binding, [3H]fMLP specific binding was inhibited by desethyl KBT-3022 (IC50: 1.9 microM). Neutrophil adhesion and superoxide anion production stimulated by phorbol 12-myristate 13-acetate were partially inhibited by KBT-3022 (1 microM) and desethyl KBT-3022 (1-10 microM). These results suggest that KBT-3022 and desethyl KBT-3022 have a wider spectrum of action and are more potent inhibitors of neutrophil activation than ASA, TP, CIL and IM.

Inhibitory effect of vitamins C and E on the oxygen free radical production in human polymorphonuclear leucocytes

Beneficial effects of dietary supplementation with antioxidant vitamins are attributed mainly to the influence upon lipid metabolism, endothelial and vascular functions. Their effect upon leucocyte oxygen free radical producing capacity has not been investigated. In 13 healthy volunteers we examined the influence of oral supplementation with vitamins C and E (aa 600 mg per day for 14 days) upon leucocyte oxygen free radical production estimated by lucigenin-amplified chemiluminescence in isolated leucocytes stimulated with arachidonic acid. After supplementation with vitamins, significant increase in serum content of ascorbic acid and tocopherol was concomitant with significant (P < 0.001) decrease of leucocyte chemiluminescent response (mean 63.2 + 23.0 SD, % of initial values) and lowering of serum lipid peroxides (P < 0.05). These findings suggest that suppression of leucocyte capacity to produce oxygen free radicals as shown in this study, may contribute to vasoprotective action of vitamins C and E.

Chemotactic peptide stimulation of arachidonic acid release in HL60 cells, an interaction between G protein and phospholipase C mediated signal transduction

The mechanism of phospholipase A2 activation by chemotactic peptide was investigated in human promyelocytic HL60 cells. N-Formyl-methionyl-leucyl-phenylalanine (fMetLeuPhe) and the non-hydrolyzable GTP analogue guanosine 5'-[gamma-thio]triphosphate (GTP[S]) induced arachidonic acid release in permeabilized and metabolically inhibited HL60 cells, a preparation in which calcium was buffered and inositol phospholipid hydrolysis was inhibited. Inositol phosphate generation and arachidonic acid were shown to be temporally dissociated. These results suggest that receptor-dependent phospholipase C activity is not required for fMetLeuPhe to induce arachidonic acid release. However, fMetLeuPhe effects were highly calcium-dependent and inhibition of phospholipase C reduced fMetLeuPhe stimulation of arachidonic acid release even in the permeabilized cell preparation. We conclude that although phospholipase A2 activation is linked to the fMetLeuPhe receptor independent of phospholipase C, actions of phospholipase C to mobilize calcium and release diacylglycerol may be important to phospholipase A2 activation in the intact cell.

Phosphatidic acid as a second messenger in human polymorphonuclear leukocytes. Effects on activation of NADPH oxidase

Receptor-mediated agonists, such as FMLP, induce an early, phospholipase D (PLD)-mediated accumulation of phosphatidic acid (PA) which may play a role in the activation of NADPH oxidase in human PMN. We have determined the effect of changes in PA production on O2 consumption in intact PMN and the level of NADPH oxidase activity measured in a cell-free assay. Pretreatment of cells with various concentrations of propranolol enhanced (less than or equal to 200 microM) or inhibited (greater than 300 microM) PLD-induced production of PA (mass and radiolabel) in a manner that correlated with enhancement or inhibition of O2 consumption in PMN stimulated with 1 microM FMLP in the absence of cytochalasin B. The concentration-dependent effects of propranolol on FMLP-induced NADPH oxidase activation was confirmed by direct assay of the enzyme in subcellular fractions. In PA extracted from cells pretreated with 200 microM propranolol before stimulation with 1 microM FMLP, phospholipase A1 (PLA1)-digestion for 90 min, followed by quantitation of residual PA, showed that a minimum of 44% of PA in control (undigested) sample was diacyl-PA; alkylacyl-PA remained undigested by PLA1. Propranolol was also observed to have a concentration-dependent enhancement of mass of 1,2-DG formed in PMN stimulated with FMLP. DG levels reached a maximum at 300 microM propranolol and remained unchanged up to 500 microM propranolol. However, in contrast to PA levels, the level of DG produced did not correlate with NADPH oxidase activation. Exogenously added didecanoyl-PA activated NADPH oxidase in a concentration-dependent manner (1-300 microM) in a reconstitution assay using membrane and cytosolic fractions from unstimulated PMN. In addition, PA synergized with SDS for oxidase activation. Taken together, these results indicate that PA plays a second messenger role in the activation of NADPH oxidase in human PMN and that regulation of phospholipase D is a key step in the activation pathway.

Effects of adenosine on polymorphonuclear leucocyte function, cyclic 3': 5'-adenosine monophosphate, and intracellular calcium

1. Inhibition of human polymorphonuclear leucocyte (PMN) function by adenosine was studied with respect to effects of adenosine on intracellular cyclic AMP and calcium during the PMN respiratory burst. 2. The adenosine analogue 5'-N-ethylcarboxamide-adenosine (NECA) and L-N6-phenyl-isopropyl-adenosine (L-PIA) inhibited PMN oxygen metabolite generation with relative potencies (NECA greater than adenosine greater than L-PIA) characteristic of an A2 receptor. 3. The respiratory burst was inhibited by adenosine when PMN were activated by calcium ionophore or chemotactic peptide but not when cells where activated by oleoyl-acetyl-glycerol (OAG). 4. Adenosine increased intracellular cyclic AMP during the PMN respiratory burst regardless of whether cells were stimulated by ionophore, chemotactic peptide or OAG. 5. To determine whether the differences in cell inhibition by adenosine were related to differences in intracellular calcium mobilization by each activating agent, calcium was evaluated with the fluorescent probe, indo-1. Adenosine suppressed the increase in intracellular calcium following PMN activation by calcium ionophore or chemotactic peptide. In contrast, calcium did not increase in PMN activated by OAG and adenosine did not affect intracellular calcium changes following this stimulus. 6. These results demonstrate that physiological concentrations of adenosine inhibit the PMN respiratory burst in association with an increase in intracellular cyclic AMP and reduction of intracellular calcium.

Mass determination of receptor-mediated accumulation of phosphatidate and diglycerides in human neutrophils measured by Coomassie blue staining and densitometry

Quantitation of 1,2-diacylglycerol (AAG), 1-0-alkyl-2-acylglycerol (EAG) and phosphatidic acid (PA) was conducted in polymorphonuclear leukocytes (PMN) labeled with 1-0-[3H]alkyl-2-acyl-GPC following stimulation with 1 microM fMLP using Coomassie blue staining and densitometry. At 5s AAG and PA increased by 80% and 107%, respectively, over controls. The accumulation of PA, which reached a maximum by 30s, was higher than AAG by 302% at 5s, and 550% at 30s. EAG accumulation was delayed by 15s following stimulation of PMN. These results show that AAG accumulates before EAG and support the role of AAG in cellular activation, perhaps, via the stimulation of protein kinase C (PKC). EAG may serve to counter the effects of AAG or may itself elicit responses. The high concentrations of PA which accumulate early suggest that PA may be generated by the activation of phospholipase D in PMN stimulated with fMLP.

The role of free oxygen radicals in the expulsion of primary infections of Nippostrongylus brasiliensis

The many and varied pathological, immunological and physiological manifestations of infection with Nippostrongylus brasiliensis may be unified by considering the role of leukocyte-generated free oxygen radicals in worm expulsion. Aside from directly damaging the adult stage of N.brasiliensis and possibly leading to its elimination from the small intestine, free radicals may also damage intestinal cells, thereby contributing to the gut pathology characteristic of infection. gamma-Interferon (and possibly tumour necrosis factor) may be involved in the initiation of free radical generation in response to N. brasiliensis and may also contribute to various side effects of infection such as hypertriglyceridaemia and cachexia. gamma-Interferon may initiate free radical generation via the agency of protein kinase C, an enzyme that can induce various additional responses including lysosomal enzyme and amine secretion and arachidonic acid metabolism. The possible interactions between these mediators and free radicals are subtle and diverse and may profoundly affect the course of infection.

Activation of the respiratory burst oxidase in neutrophils: on the role of membrane-derived second messengers, Ca++, and protein kinase C

A major bactericidal mechanism of neutrophils involves activation of the respiratory burst oxidase to generate superoxide (O2-). The oxidase is activated rapidly, often within a minute, in response to extracellular signals such as chemoattractants, inflammatory mediators, and invading microorganisms. Increasing evidence indicates that lipases also respond rapidly, releasing potent regulatory molecules from progenitor lipids. Released molecules include potential regulators of protein kinase C--diacylglycerol (DAG), arachidonate, and sphingosine--and levels of one of these, DAG, frequently correlate with O2- production. In this author's view, the available data implicate DAG and protein kinase C as key factors in the regulation of the respiratory burst. Herein, the array of activating agonists, the generation and function of some lipid-derived mediators, and evidence pertaining to the participation of protein kinase C are reviewed.

Stimulation of arachidonic acid release by guanine nucleotide in saponin-permeabilized neutrophils: evidence for involvement of GTP-binding protein in phospholipase A2 activation

Addition of a guanine nucleotide analog, guanosine 5'-O-(thiotriphosphate) (GTP gamma S)(1-100 microM) induced release of [3H]arachidonic acid from [3H]arachidonate-prelabeled rabbit neutrophils permeabilized with saponin. The chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP)-induced arachidonate release was enhanced by GTP gamma S, Ca2+, or their combination. Ca2+ alone (up to 100 microM) did not effectively stimulate lipid turnover. However, the combination of fMLP plus GTP gamma S elicited greater than additional effects in the presence of resting level of free Ca2+. The addition of 100 microM of GTP gamma S reduced the Ca2+ requirement for arachidonic acid liberation induced by fMLP. Pretreatment of neutrophils with pertussis toxin resulted in the abolition of arachidonate release and diacylglycerol formation. Neomycin (1 mM) caused no significant reduction of arachidonate release. In contrast, about 40% of GTP gamma S-induced arachidonate release was inhibited by a diacylglycerol lipase inhibitor, RHC 80267 (30 microM). These observations indicate that liberation of arachidonic acid is mediated by phospholipase A2 and also by phospholipase C/diacylglycerol lipase pathways. Fluoride, which bypasses the receptor and directly activates G proteins, induced arachidonic acid release and diacylglycerol formation. The fluoride-induced arachidonate release also appeared to be mediated by these two pathways. The loss of [3H]arachidonate was seen in phosphatidylinositol, phosphatidylcholine, and phosphatidylethanolamine. These data indicate that a G protein is involved between the binding of fMLP to its receptor and activation of phospholipase A2, and also that the arachidonic acid release is mediated by both phospholipase A2 and phospholipase C/diacylglycerol lipase.

Mobilization of arachidonic acid from diacyl and ether-linked phospholipids in FMLP stimulated alveolar macrophages

Exposure of [1 14C]AA labeled guinea-pig alveolar macrophages to FMLP for 15 min induced an extensive mobilization of AA from phospholipids. PC and PI mainly contributed to the AA release, and labeled PE remained unchanged. Analysis of ether-linked phospholipids showed a significant breakdown of labeled diacyl and alkyl-acyl PC and an increase in labeled alkenyl-acyl PE.

Relationship of phosphatidylinositol bisphosphate hydrolysis to calcium mobilization and functional activation in fluoride-treated neutrophils

Sodium fluoride (20 mM) effected rapid hydrolysis of phosphatidylinositol bisphosphate (PIP2) in human neutrophils. Intracellular free Ca2+ levels increased after PIP2 hydrolysis but before respiratory burst activation. Both the increase in intracellular free Ca2+ levels and the extent of functional activation were dependent on the availability of extracellular Ca2+. The rate of F(-)-stimulated PIP2 hydrolysis, however, was not affected when the rise in cytosolic Ca2+ was severely limited by depletion of extracellular Ca2+. Fluoride caused the specific hydrolysis of PIP2 in isolated neutrophil plasma membranes. This effect occurred in the presence of low levels of available Ca2+ and was accompanied by the release of inositol phosphates. We conclude that PIP2 hydrolysis is an early event in the response of neutrophils to F-. This response is not Ca2+-regulated but may lead to an influx of Ca2+ from the extracellular medium. Activation of a PIP2-specific phospholipase independent of a change in cytosolic free Ca2+ levels may be the initial event in the stimulus-response pathway triggered by fluoride.

Translocation of diacylglycerol kinase in response to chemotactic peptide and phorbol ester in neutrophils

When neutrophils were stimulated by the chemotactic peptide, fMLP, a rapid, transient increase in the activity of diacylglycerol(DG) kinase in the membrane fraction was detected. DG kinase in cytosol, on the contrary, showed a transient decrease. The total activity in homogenates was not affected. Tetradecanoylphorbol acetate(TPA) and 1-oleoyl-2-acetylglycerol(OAG) also caused an increase in DG kinase activity in the membrane fraction. Km value of DG kinase in membranes was not changed by the treatment of fMLP or TPA, though Vmax was increased. Considering these results, DG kinase may translocate from cytosol to membranes on stimulation by fMLP, TPA or OAG in neutrophils. The translocation may play important roles in regulation of protein kinase C activity, since DG kinase competes with protein kinase C for DG, which is formed by receptor-activation.

Molecular basis of activation and regulation of the phagocyte respiratory burst

The molecular basis of activation and regulation of the phagocyte respiratory burst is discussed with particular reference to the role of inositol phospholipid hydrolysis, guanine nucleotide coupling proteins, and activation of protein kinase C.

Inhibition of inositol phospholipids metabolism and calcium mobilization by cyclic AMP-increasing agents and phorbol ester in neutrophils

Cyclic AMP-increasing agents such as PGE2 and dibutyryl cAMP inhibited the fMLP-induced inositol phospholipids metabolism mainly through the suppression of the conversion of phosphatidylinositol(PI) to phosphatidylinositol 4,5-bisphosphate(PIP2). A part of this inhibition was found to be caused by the inhibitory effect of cAMP on PI kinase using isolated plasma membranes. On the other hand, 12-O-tetradecanoyl phorbol acetate(TPA) mainly inhibited the conversion of phosphatidylinositol 4-phosphate(PIP) to PIP2 without a significant effect on the fMLP-induced breakdown of PIP2, though direct effect of TPA on PI and PIP kinases was not demonstrated in isolated plasma membranes. Concerning Ca2+ mobilization, both cAMP-increasing agents and TPA inhibited the fMLP-induced second phase of Ca2+ elevation, while they did not affect the first phase of Ca2+ rapid increase. However, Ca2+ ionophore ionomycin-induced Ca2+ elevation was only inhibitable by TPA but not PGE2. These results suggest that cAMP inhibits the fMLP-induced Ca2+ influx, while TPA stimulates Ca2+ removal from cytosol.

Evidence for a catalytic role of phospholipase A in phorbol diester- and zymosan-induced mobilization of arachidonic acid in mouse peritoneal macrophages

Inositol phospholipid degradation and release of phospholipid-bound arachidonic acid was induced in intact peritoneal macrophages by exposure to phorbol myristate acetate (PMA) or zymosan particles. PMA, known to activate protein kinase C, selectively enhanced the deacylation of phosphatidylinositol (i.e., degradation by phospholipase A), while zymosan particles enhanced degradation via both phospholipase A and inositol lipid phosphodiesterase (phospholipase C). The release of arachidonic acid was found to correlate with the degradation of phosphatidylinositol by the phospholipase A pathway and could be dissociated from the phospholipase C-catalyzed cleavage of inositol phospholipids in several experimental situations: (i) when PMA was the stimulus, (ii) by the difference in Ca2+ dependence between the two enzymatic processes when zymosan was the stimulus and (iii) by the parallel inhibition by chlorpromazine of the phospholipase A pathway and arachidonic acid release, but not inositol phospholipid phosphodiesterase. In addition, phloretin, a reported inhibitor of protein kinase C, was found to inhibit arachidonic acid release and the deacylation of phosphatidylinositol. The results are consistent with a model in which arachidonic acid release is mediated by phospholipase(s) A and in which PMA or the phosphodiesterase-catalyzed degradation of phosphoinositides causes activation of the phospholipase A pathway via protein kinase C.