Inhibition of the metabolism of phosphatidylethanol and phosphatidic acid, and stimulation of insulin release, by propranolol in intact pancreatic islets

Characterization of Lysophospholipase D Activity in Mammalian Cell Membranes

Lysophosphatidic acid (LPA) is a lipid mediator that binds to G-protein-coupled receptors, eliciting a wide variety of responses in mammalian cells. Lyso-phospholipids generated via phospholipase A2 (PLA2) can be converted to LPA by a lysophospholipase D (lyso-PLD). Secreted lyso-PLDs have been studied in more detail than membrane-localized lyso-PLDs. This study utilized in vitro enzyme assays with fluorescent substrates to examine LPA generation in membranes from multiple mammalian cell lines (PC12, rat pheochromocytoma; A7r5, rat vascular smooth muscle; Rat-1, rat fibroblast; PC-3, human prostate carcinoma; and SKOV-3 and OVCAR-3, human ovarian carcinoma). The results show that membranes contain a lyso-PLD activity that generates LPA from a fluorescent alkyl-lyso-phosphatidylcholine, as well as from naturally occurring acyl-linked lysophospholipids. Membrane lyso-PLD and PLD activities were distinguished by multiple criteria, including lack of effect of PLD2 over-expression on lyso-PLD activity and differential sensitivities to vanadate (PLD inhibitor) and iodate (lyso-PLD inhibitor). Based on several lines of evidence, including siRNA knockdown, membrane lyso-PLD is distinct from autotaxin, a secreted lyso-PLD. PC-3 cells express GDE4 and GDE7, recently described lyso-PLDs that localize to membranes. These findings demonstrate that membrane-associated lyso-D activity, expressed by multiple mammalian cell lines, can contribute to LPA production.

Phosphatidylethanol Detects Moderate-to-Heavy Alcohol Use in Liver Transplant Recipients

BACKGROUND

Alcohol-dependent liver transplantation (LT) patients who resume alcohol consumption are at risk for a number of alcohol-related problems including liver injury and liver failure. Post-LT patients are strongly advised to remain abstinent. However, we do not know how well this population complies due to a lack of valid methods (self-report and/or biomarkers) to identify alcohol use. Studies suggest as many as 50% resume alcohol use within 5 years. Phosphatidylethanol (PEth) is a new cell-membrane phospholipid biomarker to identify alcohol use in the past 28 days. This prospective study followed 213 LT recipients at 2 U.S. liver transplant centers.

METHODS

Sample included 213 LT subjects; 70.9% (n = 151/213) had a history of alcohol dependence prior to transplantation and 29.1% (n = 62/213) served as non-alcohol-dependent controls. Subjects participated in face-to-face interviews to assess alcohol use using a 30-day calendar. The protocol called for collecting blood samples at baseline, 6-, and 12-month follow-up.

RESULTS

Seventy percent (149/213) who reported no alcohol use had consistently negative PEth levels (<8 ng/ml). A total of 26.4% (57/213), 44 alcohol-dependent patients and 13 controls, had a positive PEth test of >8 ng/ml either at baseline and/or during the follow-up period. Alcohol-dependent subjects (23.8%; n = 36/151) and 16.1% (n = 10/62) controls reported no alcohol use but had at least 1 positive PEth test. Of the 11.2% (24/213) post-LT subjects who reported recent alcohol use, over half (11/24) had a positive PEth. The 13 self-reported alcohol users with a negative PEth level reported insufficient drinking to trigger PEth formation.

CONCLUSIONS

Adoption of PEth as part of routine posttransplant care of LT recipients will enable early identification of patients at risk of alcohol use and facilitate abstinence in patients with a history of alcohol dependence and alcohol-related liver damage.

Nonoxidative ethanol metabolism in humans-from biomarkers to bioactive lipids

Ethanol is a widely used psychoactive drug whose chronic abuse is associated with organ dysfunction and disease. Although the prevalent metabolic fate of ethanol in the human body is oxidation a smaller fraction undergoes nonoxidative metabolism yielding ethyl glucuronide, ethyl sulfate, phosphatidylethanol and fatty acid ethyl esters. Nonoxidative ethanol metabolites persist in tissues and body fluids for much longer than ethanol itself and represent biomarkers for the assessment of ethanol intake in clinical and forensic settings. Of note, the nonoxidative reaction of ethanol with phospholipids and fatty acids yields bioactive compounds that affect cellular signaling pathways and organelle function and may contribute to ethanol toxicity. Thus, despite low quantitative contributions of nonoxidative pathways to overall ethanol metabolism the resultant ethanol metabolites have important biological implications. In this review we summarize the current knowledge about the enzymatic formation of nonoxidative ethanol metabolites in humans and discuss the implications of nonoxidative ethanol metabolites as biomarkers of ethanol intake and mediators of ethanol toxicity. © 2016 IUBMB Life, 68(12):916-923, 2016.

Phosphatidylethanol: normalization during detoxification, gender aspects and correlation with other biomarkers and self-reports

Phosphatidylethanol (PEth) is a direct ethanol metabolite, and has recently attracted attention as biomarker of ethanol intake. The aims of the current study are: (1) to characterize the normalization time of PEth in larger samples than previously conducted; (2) to elucidate potential gender differences; and (3) to report the correlation of PEth with other biomarkers and self-reported alcohol consumption. Fifty-seven alcohol-dependent patients (ICD 10 F 10.25; 9 females, 48 males) entering medical detoxification at three study sites were enrolled. The study sample was comprised of 48 males and 9 females, with mean age 43.5. Mean gamma glutamyl transpeptidase (GGT) was 209.61 U/l, average mean corpuscular volume (MCV) was 97.35 fl, mean carbohydrate deficient transferrin (%CDT) was 8.68, and mean total ethanol intake in the last 7 days was 1653 g. PEth was measured in heparinized whole blood with a high-pressure liquid chromatography method, while GGT, MCV and %CDT were measured using routine methods. PEth levels at day 1 of detoxification ranged between 0.63 and 26.95 micromol/l (6.22 mean, 4.70 median, SD 4.97). There were no false negatives at day 1. Sensitivities for the other biomarkers were 40.4% for MCV, 73.1% for GGT and 69.2% for %CDT, respectively. No gender differences were found for PEth levels at any time point. Our data suggest that PEth is (1) a suitable intermediate term marker of ethanol intake in both sexes; and (2) sensitivity is extraordinary high in alcohol dependent patients. The results add further evidence to the data that suggest that PEth has potential as a candidate for a sensitive and specific biomarker, which reflects longer-lasting intake of higher amounts of alcohol and seemingly has the above mentioned certain advantages over traditional biomarkers.

Phosphatidylethanol in Human Organs and Blood: A Study on Autopsy Material and Influences by Storage Conditions

OBJECTIVE

Phosphatidylethanol (PEth) is an abnormal phospholipid that is formed and accumulated in mammalian cells that have been exposed to ethanol. PEth has been proposed as a marker of ethanol abuse. This study was conducted to investigate the concentration of PEth in blood and organs obtained during the autopsy of alcoholics. In addition, we performed experiments on rat tissues and human blood to evaluate the effect of various storage conditions on PEth concentrations.

METHODS

Human tissues and blood from alcoholics and controls were obtained at autopsy and frozen at -20 degrees C until extraction. Blood from healthy donors was incubated with ethanol for 24 hr and thereafter either extracted directly or stored at room temperature, stored at 4 degrees C, frozen at -20 degrees C, or frozen in liquid nitrogen and stored at -80 degrees C before extraction. Rats were given intraperitoneal injections of ethanol and then killed, either while still intoxicated or when sober. Rat organs were homogenized and extracted directly, after a period of storage, and/or after freezing at -20 degrees C. PEth concentration was analyzed using HPLC and verified by mass spectrometry.

RESULTS

In all rat organs studied, PEth was formed during freezing at -20 degrees C with ethanol present. PEth concentrations of 9 to 205 mumol/liter were observed in the blood obtained at autopsy. The highest value was found in the case with the highest blood alcohol concentration (114 mmol/liter) at the time of death. In the experiments on human blood stored with ethanol present, PEth concentrations were not affected after 72 hr at 4 degrees C or after freezing in liquid nitrogen and storage at -80 degrees C for up to 144 hr but were slightly elevated after 24 hr at room temperature and at -20 degrees C. PEth was found in all organs obtained from the cadavers of alcoholics. Storage of organs at 4 degrees C for 24 hr with ethanol present had no effect on the PEth concentration. The PEth concentration was unaffected when no ethanol was present at the time of freezing.

CONCLUSIONS

The rat experiments indicated that the very high PEth concentrations found in the organs of the alcoholics were probably largely formed while the organs were frozen at -20 degrees C. Our data suggest that tissue material from bodies that were exposed to ethanol must be stored properly to obtain reliable results from subsequent analysis for PEth. Tissue should not be frozen at -20 degrees C but instead stored refrigerated until extraction, preferably within hours of autopsy, or frozen in liquid nitrogen and stored at -80 degrees C. Blood samples that contain ethanol can be stored refrigerated for up to 72 hr or frozen in liquid nitrogen and stored at -80 degrees C without affecting PEth levels.

Studies of insulin secretory responses and of arachidonic acid incorporation into phospholipids of stably transfected insulinoma cells that overexpress group VIA phospholipase A2 (iPLA2beta ) indicate a signaling rather than a housekeeping role for iPLA2beta

A cytosolic 84-kDa group VIA phospholipase A(2) (iPLA(2)beta) that does not require Ca(2+) for catalysis has been cloned from several sources, including rat and human pancreatic islet beta-cells and murine P388D1 cells. Many potential iPLA(2)beta functions have been proposed, including a signaling role in beta-cell insulin secretion and a role in generating lysophosphatidylcholine acceptors for arachidonic acid incorporation into P388D1 cell phosphatidylcholine (PC). Proposals for iPLA(2)beta function rest in part on effects of inhibiting iPLA(2)beta activity with a bromoenol lactone (BEL) suicide substrate, but BEL also inhibits phosphatidate phosphohydrolase-1 and a group VIB phospholipase A(2). Manipulation of iPLA(2)beta expression by molecular biologic means is an alternative approach to study iPLA(2)beta functions, and we have used a retroviral construct containing iPLA(2)beta cDNA to prepare two INS-1 insulinoma cell clonal lines that stably overexpress iPLA(2)beta. Compared with parental INS-1 cells or cells transfected with empty vector, both iPLA(2)beta-overexpressing lines exhibit amplified insulin secretory responses to glucose and cAMP-elevating agents, and BEL substantially attenuates stimulated secretion. Electrospray ionization mass spectrometric analyses of arachidonic acid incorporation into INS-1 cell PC indicate that neither overexpression nor inhibition of iPLA(2)beta affects the rate or extent of this process in INS-1 cells. Immunocytofluorescence studies with antibodies directed against iPLA(2)beta indicate that cAMP-elevating agents increase perinuclear fluorescence in INS-1 cells, suggesting that iPLA(2)beta associates with nuclei. These studies are more consistent with a signaling than with a housekeeping role for iPLA(2)beta in insulin-secreting beta-cells.

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.

Sequential actions of phospholipase D and phosphatidic acid phosphohydrolase 2b generate diglyceride in mammalian cells

Phosphatidylcholine (PC) is a major source of lipid-derived second messenger molecules that function as both intracellular and extracellular signals. PC-specific phospholipase D (PLD) and phosphatidic acid phosphohydrolase (PAP) are two pivotal enzymes in this signaling system, and they act in series to generate the biologically active lipids phosphatidic acid (PA) and diglyceride. The identity of the PAP enzyme involved in PLD-mediated signal transduction is unclear. We provide the first evidence for a functional role of a type 2 PAP, PAP2b, in the metabolism of PLD-generated PA. Our data indicate that PAP2b localizes to regions of the cell in which PC hydrolysis by PLD is taking place. Using a newly developed PAP2b-specific antibody, we have characterized the expression, posttranslational modification, and localization of endogenous PAP2b. Glycosylation and localization of PAP2b appear to be cell type and tissue specific. Biochemical fractionation and immunoprecipitation analyses revealed that PAP2b and PLD2 activities are present in caveolin-1-enriched detergent-resistant membrane microdomains. We found that PLD2 and PAP2b act sequentially to generate diglyceride within this specialized membrane compartment. The unique lipid composition of these membranes may provide a selective environment for the regulation and actions of enzymes involved in signaling through PC hydrolysis.

Studies of the role of group VI phospholipase A2 in fatty acid incorporation, phospholipid remodeling, lysophosphatidylcholine generation, and secretagogue-induced arachidonic acid release in pancreatic islets and insulinoma cells

An 84-kDa group VI phospholipase A2 (iPLA2) that does not require Ca2+ for catalysis has been cloned from Chinese hamster ovary cells, murine P388D1 cells, and pancreatic islet beta-cells. A housekeeping role for iPLA2 in generating lysophosphatidylcholine (LPC) acceptors for arachidonic acid incorporation into phosphatidylcholine (PC) has been proposed because iPLA2 inhibition reduces LPC levels and suppresses arachidonate incorporation and phospholipid remodeling in P388D1 cells. Because islet beta-cell phospholipids are enriched in arachidonate, we have examined the role of iPLA2 in arachidonate incorporation into islets and INS-1 insulinoma cells. Inhibition of iPLA2 with a bromoenol lactone (BEL) suicide substrate did not suppress and generally enhanced [3H]arachidonate incorporation into these cells in the presence or absence of extracellular calcium at varied time points and BEL concentrations. Arachidonate incorporation into islet phospholipids involved deacylation-reacylation and not de novo synthesis, as indicated by experiments with varied extracellular glucose concentrations and by examining [14C]glucose incorporation into phospholipids. BEL also inhibited islet cytosolic phosphatidate phosphohydrolase (PAPH), but the PAPH inhibitor propranolol did not affect arachidonate incorporation into islet or INS-1 cell phospholipids. Inhibition of islet iPLA2 did not alter the phospholipid head-group classes into which [3H]arachidonate was initially incorporated or its subsequent transfer from PC to other lipids. Electrospray ionization mass spectrometric measurements indicated that inhibition of INS-1 cell iPLA2 accelerated arachidonate incorporation into PC and that inhibition of islet iPLA2 reduced LPC levels by 25%, suggesting that LPC mass does not limit arachidonate incorporation into islet PC. Gas chromatography/mass spectrometry measurements indicated that BEL but not propranolol suppressed insulin secretagogue-induced hydrolysis of arachidonate from islet phospholipids. In islets and INS-1 cells, iPLA2 is thus not required for arachidonate incorporation or phospholipid remodeling and may play other roles in these cells.

Diacylglycerol--when is it an intracellular messenger?

Distinct, structurally different forms of sn-1,2-diacylglycerol are found in cells, these are polyunsaturated, mono- or di-unsaturated and saturated. The pathways that generate or metabolise sn-1, 2-diacylglycerol are reviewed. The evidence that it is the polyunsaturated forms of sn-1,2-diacylglycerol, but the more saturated forms of phosphatidate which function as intracellular signals is considered.

Mass spectrometric evidence that agents that cause loss of Ca2+ from intracellular compartments induce hydrolysis of arachidonic acid from pancreatic islet membrane phospholipids by a mechanism that does not require a rise in cytosolic Ca2+ concentration

Stimulation of pancreatic islets with glucose induces phospholipid hydrolysis and accumulation of nonesterified arachidonic acid, which may amplify the glucose-induced Ca2+ entry into islet beta-cells that triggers insulin secretion. Ca2+ loss from beta-cell intracellular compartments has been proposed to induce both Ca2+ entry and events dependent on arachidonate metabolism. We examine here effects of inducing Ca2+ loss from intracellular sequestration sites with ionophore A23187 and thapsigargin on arachidonate hydrolysis from islet phospholipids. A23187 induces a decline in islet arachidonate-containing phospholipids and release of nonesterified arachidonate. A23187-induced arachidonate release is of similar magnitude when islets are stimulated in Ca2+-replete or in Ca2+-free media or when islets loaded with the intracellular Ca2+ chelator BAPTA are stimulated in Ca2+-free medium, a condition in which A23187 induces no rise in beta-cell cytosolic [Ca2+]. Thapsigargin also induces islet arachidonate release under these conditions. A23187- or thapsigargin-induced arachidonate release is prevented by a bromoenol lactone (BEL) inhibitor of a beta-cell phospholipase A2 (iPLA2), which does not require Ca2+ for catalytic activity and which is negatively modulated by and physically interacts with calmodulin by Ca2+-dependent mechanisms. Agents that cause Ca2+ loss from islet intracellular compartments thus induce arachidonate hydrolysis from phospholipids by a BEL-sensitive mechanism that does not require a rise in cytosolic [Ca2+], and a BEL-sensitive enzyme-like iPLA2 or a related membranous activity may participate in sensing Ca2+ compartment content.

Lipid composition of glucose-stimulated pancreatic islets and insulin-secreting tumor cells

The effect of glucose stimulation (25 mM for 5 min) on the phospholipid and neutral lipid composition of isolated pancreatic islets was studied to find out whether there is a change in the mass of potential lipid mediators or modulators of insulin secretion. For comparison, the lipid compositions of homogenates and subcellular fractions from RINm5F insulin-secreting tumor cells and of glucose-stimulated streptozotocin/nicotinamide-induced islet cell tumors were analyzed. After separation of the lipid extract into a neutral and an acidic fraction by anion-exchange chromatography, lipids were separated by high-performance thin-layer chromatography and quantitated by in situ densitometry of the cupric sulfate-charred bands. In glucose-stimulated islets, the molar percentages of phosphatidic acid (PA) and of phosphatidylinositol were significantly increased (3.1 vs. 4.7 mol% and 8.6 vs. 11.8 mol%), while those of all other phospholipids and neutral lipids, including 1,2-diacylglycerol, were not significantly changed. In stimulated islet cell tumors, an increase of PA was visible in the microsomal fraction, and there was an increase of lysophosphatidylcholine in the mitochondrial fraction. However, in both tumoral tissues, particularly in RINm5F cells, the lipid distribution pattern showed abnormalities which can be regarded as a loss of differentiation and which limit the usefulness of these tissues for the study of the physiological regulation of lipid metabolism during glucose stimulation. In conclusion, the data are in accordance with a role of PA early in stimulus-secretion coupling. The well-known stimulation of phospholipid synthesis in pancreatic islets during glucose-induced insulin secretion does not result in an increase in the total phospholipid mass.

Agents that increase phosphatidic acid inhibit the LH-induced testosterone production

The results of the present study point to phosphatidic acid (PtdOH) as a possible intracellular messenger, which might be involved in local modulation of testicular testosterone production in vivo. Propranolol (27-266 microM) induced an increased level of [3H]PtdOH in isolated rat Leydig cells, prelabeled with [3H]myristate, and at the same time a strong dose-dependent inhibition of the acute testosterone production stimulated by luteinizing hormone (LH). The inhibition was not bypassed by the addition of dibutyryl-cAMP but was overcome, when 22(R)-hydroxycholesterol was added as a direct substrate for cytochrome P-450 side chain cleavage enzyme. Thus, the inhibition appears to be exerted at a point distal to cAMP-generation but before the first enzyme in the testosterone synthetic pathway. Treatment with other agents (4 beta-phorbol 12-myristate 13-acetate (PMA), A23187, and sphingosine) giving rise to increases in the PtdOH-level resulted in the inhibition of the LH-induced testosterone formation as well, thus indicating a connection between the two effects. Furthermore, we were able to demonstrate a highly significant correlation between the PtdOH-increase and the inhibition of the LH-stimulated testosterone production. This may suggest a causal relationship between these two parameters.

Regulation of guanine-nucleotide binding proteins in islet subcellular fractions by phospholipase-derived lipid mediators of insulin secretion

In the accompanying article (Kowluru, A., Rabaglia, M.E., Mose, K.E. and Metz, S.A. (1994) Biochim. Biophys. Acta 1222, 348-359) we identified three specific GTPase activities in islet subcellular fractions; most notably, two of these were enriched in the secretory granules. In the present study, we describe the regulation of GTPase activity in subcellular fractions of normal rat and human islets by insulinotropic lipids with a similar rank order as their insulin-releasing capacity. Arachidonic acid (AA), lysophosphatidylcholine (LPC), or phosphatidic acid (PA) inhibited the GTPase activities significantly (by 60-80%) in islet homogenates; each also selectively inhibited certain GTPases in specific individual fractions. Less insulinotropic fatty acids, such as linoleic acid and oleic acid, inhibited GTPase to a lesser degree, whereas lysophosphatidic acid (LPA), phosphatidylcholine (PC) or palmitic acid, which do not acutely promote secretion, were ineffective. Similar inhibitory effects of these lipids were also demonstrable in fractions of human islets as well as those of transformed beta-cells (HIT cells). The effects of lipids were not attributable to their detergent properties (since several detergents failed to mimic lipid effects) or to inhibition of GTP binding (since they actually increased GTP gamma S binding modestly, and moreover, in reconstituted fractions, they potentiated GDP/GTP exchange activity up to 2-fold). These data indicate that the insulinotropic nature of the lipids might be due, in part, to their ability to maintain G-proteins in their GTP-bound (active) configuration by increasing GTP binding and decreasing its hydrolysis. These studies comprise the first evidence for the regulation by biologically active lipids of endocrine cell G-proteins at a locus distal to plasma membrane events (i.e., on endocrine secretory granules), and provide thereby a possible novel mechanism whereby the activation of islet endogenous phospholipases might culminate in insulin exocytosis.

Agonist-stimulated and basal phosphatidylethanol formation in neutrophils from alcoholics

Phospholipase D has been shown to be a key enzyme in the signal transduction systems involved in neutrophil activation. In the presence of ethanol, the enzyme catalyzes a transphosphatidylation reaction through which phosphatidylethanol is formed instead of the normal product phosphatidic acid. The effects of ethanol on the formation of phosphatidylethanol and phosphatidic acid was studied in neutrophils from human alcoholics in vitro. Neutrophils were isolated and cellular lipids were labeled with [3H]oleate, whereafter the cells were preincubated with cytochalasin B. Subsequently, cells were stimulated with the chemotactic peptide formyl-Met-Leu-Phe in the presence of ethanol concentration ranging from 0 to 200 mM. In the presence of ethanol, both neutrophils from alcoholics and controls produced phosphatidylethanol, with a concomitant reduction of the production of phosphatidic acid. The amounts of phosphatidyl-ethanol and phosphatidic acid formed were dependent on the concentration of ethanol. In neutrophils from alcoholics, a higher apparent Km for the phospholipase D-mediated transphosphatidylation reaction was noted (58 mM ethanol compared with 28 mM in controls). The in vivo mass of phosphatidylethanol in recently drinking alcoholics was also analyzed in neutrophils. Measurable phosphatidyl-ethanol levels (average 5.6 pmol/10(8) neutrophils) were found in alcoholics up to 23 hr after the last intake of ethanol. Thus, in addition to the ethanol-induced changes in the normal production of phosphatidic acid, phosphatidylethanol accumulated in vivo in alcoholics may be expected to influence neutrophil function.

Activation of phospholipase D in CHO cells transfected with the human epidermal growth factor (EGF) receptor: differential effects of protein kinase C activation and EGF

Multiple intracellular signal transduction pathways, including phospholipases A2 and D, can be activated by epidermal growth factor (EGF) in both a protein kinase C (PKC)-dependent and -independent manner. We investigated the activation of phospholipase D (PLD) by a PKC activator, phorbol myristate acetate (PMA) and by EGF in CHO cells transfected with the full-length EGF receptor. In cells labelled with arachidonic acid or linoleic acid, PMA activated a PLD, determined by formation of the transphosphatidylation product phosphatidylethanol in the presence of ethanol. A basal PLD activity was seen in linoleic acid-labelled cells but not in cells labelled with arachidonic acid. This basal activity was augmented by the protein phosphotyrosine phosphatase inhibitor vanadate and reduced by tyrosine kinase inhibition and was contributed to by PKC, as activity could not be elicited following prolonged exposure to phorbol ester, known to down-regulate some PKC isoforms. By contrast, EGF failed to stimulate formation of phosphatidylethanol in cells labelled with either fatty acid species. It is proposed that in the basal condition PKC-dependent PLD activation and protein tyrosine kinase phosphorylation are linked (possibly by a phospholipase C (PLC)-mediated formation of diacylglycerol); EGF which activated a phospholipase A2 (PLA2) but which failed to elicit PLC activation in these cells is without further effect on PLD.

Evidence for coupling of Clostridium perfringens alpha-toxin-induced hemolysis to stimulated phosphatidic acid formation in rabbit erythrocytes

When rabbit erythrocytes were exposed to low concentrations of Clostridium perfringens alpha-toxin, hot-cold hemolysis was observed. The toxin induced production of phosphatidic acid (PA) in a dose-dependent manner when incubated with erythrocytes at 37 degrees C. When erythrocyte membranes were incubated with the toxin and [gamma-32P]ATP in the presence or absence of ethanol, [32P]PA formation was maximal within 30 s, then sharply decreased, and began again after 5 min of incubation. Ethanol had no effect on the early appearance (at approximately 5 min) of PA formation induced by the toxin but significantly inhibited formation of PA over 10 min of incubation. Treatment of erythrocyte membranes with alpha-toxin resulted in the biphasic formation of 1,2-diacylglycerol and PA as well as an increase of inositol-1,4,5-trisphosphate (IP3) and decrease of phosphatidylinositol-4,5-bisphosphate (PIP2) within 30 s. Neomycin inhibited the toxin-induced increase in turbidity of egg yolk suspensions but did not inhibit the toxin-induced hemolysis of intact erythrocytes. On the other hand, neomycin inhibited the toxin-induced hemolysis of saponin-treated erythrocytes. In addition, neomycin inhibited PA formation induced by the toxin in erythrocyte membranes. IP3 was released by incubation of PIP2 with erythrocyte membranes but not by incubation of PIP2 with the toxin. The toxin stimulated the membrane-induced release of IP3 from PIP2. These data suggest that the toxin-induced hemolysis is dependent on the action of phospholipase C in erythrocyte membranes.

Receptor-mediated phospholipase D activity in primary astroglial cultures

Phospholipase D, an enzyme involved in signal transduction cascades, catalyses the formation of phosphatidic acid and, when ethanol is present, the formation of phosphatidylethanol. In the present study we demonstrate that stimulation of muscarinic acetylcholine receptors as well as P2-purinergic receptors induces activation of phospholipase D in primary cultures of astroglial cells. Both the hydrolysis and the transphosphatidylation reactions were stimulated by receptor agonists. Carbachol and ATP induced a rapid increase in the amount of [3H]phosphatidic acid in astroglial cells prelabelled with [3H]oleic acid. When ethanol (150 mM) was present, phosphatidylethanol was formed. Furthermore, the receptor-mediated increase in the concentration of phosphatidic acid was inhibited by ethanol, indicating that the phosphatidic acid production was indeed mediated by phospholipase D. The formation of phosphatidylethanol was concentration dependent, with a half-maximal effective concentration of 5 x 10(-5) M for carbachol and 10(-5) M for ATP. The carbachol-induced response was almost completely inhibited by atropine, indicating activation of phospholipase D via muscarinic receptors. The purinergic response is most probably mediated via P2-receptors since ADP was almost as efficient as ATP in inducing phosphatidylethanol formation, whereas AMP was significantly less potent. We conclude that astroglial cells in primary culture display muscarinic and purinergic receptors coupled to phospholipase D. The relationship to cell function needs to be further investigated.

Unilateral nephrectomy selectively stimulates phospholipase D in the remaining kidney

The activation of phospholipase D in the kidney could be detected in vivo in rats treated with ethanol by the accumulation of phosphatidylethanol. Unilateral nephrectomy stimulated the activity of phospholipase D in the remaining kidney as indicated by an increase in the level of phosphatidylethanol. A significant increase in phosphatidylethanol level was observed as early as 5 min after contralateral nephrectomy and peak accumulation (200% of control) was observed after 15 min. The phosphatidylethanol level decreased again to the basal level after 2 h. The accumulation of phosphatidylethanol was specific for kidney and the product was localized primarily in the cortex. Phospholipase D activity in kidney cortical slices from untreated rats was stimulated in vitro by plasma obtained from unilaterally nephrectomized rats, indicating that circulating factors in the plasma are responsible for the activation of phospholipase D. The phospholipase D activation by plasma from uninephrectomized animals was selectively inhibited by the tyrosine kinase inhibitor genistein, but not by the protein kinase C inhibitor H7. It is concluded that phospholipase D activity is stimulated as an early signal transduction event in compensatory kidney growth.

Phosphatidylethanol affects inositol 1,4,5-trisphosphate levels in NG108-15 neuroblastoma x glioma hybrid cells

Phosphatidylethanol is formed by phospholipase D in animal cells exposed to ethanol. Previous reports have demonstrated that the degradation of phosphatidylethanol is slow, indicating that this lipid may be present in the cells after ethanol itself has disappeared. Accumulation of an abnormal alcohol metabolite may influence cellular functions. In the present study, cultivation of NG108-15 neuroblastoma x glioma hybrid cells in the presence of ethanol resulted in an accumulation of phosphatidylethanol and a simultaneous increase in basal inositol 1,4,5-trisphosphate levels. The direct effects of phosphatidylethanol on the phosphoinositide signal transduction system were examined through incorporation of exogenous phosphatidylethanol into membranes of ethanol-naive cells. An incorporation amounting to 2.8% of cellular phospholipids was achieved after a 5-h incubation with 30 microM phosphatidylethanol. Phosphatidylethanol was found to cause a time- and dose-dependent increase in the basal levels of inositol 1,4,5-trisphosphate. The effects on inositol 1,4,5-trisphosphate levels of exogenously added phosphatidylethanol and ethanol exposure for 2 days were not additive. No effect on bradykinin-stimulated inositol 1,4,5-trisphosphate production could be detected. However, the increase in basal inositol 1,4,5-trisphosphate levels indicates that phosphatidylethanol affects inositol 1,4,5-trisphosphate turnover and emphasizes the importance of considering phosphatidylethanol as a possible mediator of ethanol-induced effects on cellular processes.

Glucose and carbachol synergistically stimulate phosphatidic acid accumulation in pancreatic islets

Phosphatidic acid has been previously implicated as an intracellular mediator of insulin secretion. Very little is known, however, about endogenous phosphatidic acid levels in islets. We now show, for the first time, that glucose and carbachol, at concentrations which stimulate insulin secretion, significantly increase endogenous phosphatidic acid levels in pancreatic islets by 2-fold at 1 min, nearly 3-fold at 2 min, and over 3-fold at 30 min compared to control. Possible mechanisms include de novo synthesis from glucose and/or activation of phospholipase D. Our data, taken together with previous studies, suggest that phosphatidic acid may have a central role in insulin secretion as an intracellular mediator.