Phospholipase D activity in subcellular membranes of rat ventricular myocardium

The substrate specificities of sunflower and soybean phospholipases D using transphosphatidylation reaction

BACKGROUND

Phospholipase D (PLD) belongs to a lipolytic enzyme subclass which catalyzes the hydrolysis and transesterification of glycerophospholipids at the terminal phosphodiester bond.

RESULTS

In this work, we have studied the substrate specificity of PLDs from germinating sunflower seeds and cultured-soybean cells, using their capacity of transphosphatidylation. In the presence of a nucleophilic acceptor, such as [¹⁴C]ethanol, PLD catalyzes the production of phosphatidyl-[¹⁴C]-ethanol. The resulting product is easily identified since it is well separated from the other lipids by thin-layer chromatography. The main advantage of this assay is that the phospholipid used as substrate does not need to be radiolabelled and thus allow us a large choice of polar heads and fatty acids. In vitro, we observed that sunflower and soybean cell PLD show the following decreasing order of specificity: phosphatidylcholine, phosphatidylethanolamine and phosphatidylglycerol; while phosphatidylserine and phosphatidylinositol are utilized much less efficiently.

CONCLUSIONS

The substrate specificity is modulated by the fatty acid composition of the phosphatidylcholine used as well as by the presence of other charged phospholipids.

Phospholipid-mediated signaling and heart disease

Cardiac hypertrophy, congestive heart failure, diabetic cardiomyopathy and myocardial ischemia-reperfusion injury are associated with a disturbance in cardiac sarcolemmal membrane phospholipid homeostasis. The contribution of the different phospholipases and their related signaling mechanisms to altered function of the diseased myocardium is not completely understood. Resolution of this issue is essential for both the understanding of the pathophysiology of heart disease and for determining if components of the phospholipid signaling pathways could serve as appropriate therapeutic targets. This review provides an outline of the role of phospholipase A2, C and D and subsequent signal transduction mechanisms in different cardiac pathologies with a discussion of their potential as targets for drug development for the prevention/treatment of heart disease.

Phospholipid-mediated signaling systems as novel targets for treatment of heart disease

The phospholipases associated with the cardiac sarcolemmal (SL) membrane hydrolyze specific membrane phospholipids to generate important lipid signaling molecules, which are known to influence normal cardiac function. However, impairment of the phospholipases and their related signaling events may be contributory factors in altering cardiac function of the diseased myocardium. The identification of the changes in such signaling systems as well as understanding the contribution of phospholipid-signaling pathways to the pathophysiology of heart disease are rapidly emerging areas of research in this field. In this paper, I provide an overview of the role of phospholipid-mediated signal transduction processes in cardiac hypertrophy and congestive heart failure, diabetic cardiomyopathy, as well as in ischemia-reperfusion. From the cumulative evidence presented, it is suggested that phospholipid-mediated signal transduction processes could serve as novel targets for the treatment of the different types of heart disease.

Diacylglycerols activate mitochondrial cationic channel(s) and release sequestered Ca(2+)

Mitochondria contribute to cytosolic Ca(2+) homeostasis through several uptake and release pathways. Here we report that 1,2-sn-diacylglycerols (DAG's) induce Ca(2+) release from Ca(2+)-loaded mammalian mitochondria. Release is not mediated by the uni-porter or the Na(+)/Ca(2+) exchanger, nor is it attributed to putative catabolites. DAG's-induced Ca(2+) efflux is biphasic. Initial release is rapid and transient, insensitive to permeability transition inhibitors, and not accompanied by mitochondrial swelling. Following initial rapid release of Ca(2+) and relatively slow reuptake, a secondary progressive release of Ca(2+) occurs, associated with swelling, and mitigated by permeability transition inhibitors. The initial peak of DAG's-induced Ca(2+) efflux is abolished by La(3+) (1 mM) and potentiated by protein kinase C inhibitors. Phorbol esters, 1,3-diacylglycerols and 1-monoacylglycerols do not induce mitochondrial Ca(2+) efflux. Ca(2+)-loaded mitoplasts devoid of outer mitochondrial membrane also exhibit DAG's-induced Ca(2+) release, indicating that this mechanism resides at the inner mitochondrial membrane. Patch clamping brain mitoplasts reveal DAG's-induced slightly cation-selective channel activity that is insensitive to bongkrekic acid and abolished by La(3+). The presence of a second messenger-sensitive Ca(2+) release mechanism in mitochondria could have an important impact on intracellular Ca(2+) homeostasis.

Differential changes in phospholipase D and phosphatidate phosphohydrolase activities in ischemia–reperfusion of rat heart

Phospholipase D (PLD2) produces phosphatidic acid (PA), which is converted to 1,2 diacylglycerol (DAG) by phosphatidate phosphohydrolase (PAP2). Since PA and DAG regulate Ca(2+) movements, we examined PLD2 and PAP2 in the sarcolemma (SL) and sarcoplasmic reticular (SR) membranes from hearts subjected to ischemia and reperfusion (I-R). Although SL and SR PLD2 activities were unaltered after 30 min ischemia, 5 min reperfusion resulted in a 36% increase in SL PLD2 activity, whereas 30 min reperfusion resulted in a 30% decrease in SL PLD2 activity, as compared to the control value. SR PLD2 activity was decreased (39%) after 5 min reperfusion, but returned to control levels after 30 min reperfusion. Ischemia for 60 min resulted in depressed SL and SR PLD2 activities, characterized with reduced V(max) and increased K(m) values, which were not reversed during reperfusion. Although the SL PAP2 activity was decreased (31%) during ischemia and at 30 min reperfusion (28%), the SR PAP2 activity was unchanged after 30 min ischemia, but was decreased after 5 min reperfusion (25%) and almost completely recovered after 30 min reperfusion. A 60 min period of ischemia followed by reperfusion caused an irreversible depression of SL and SR PAP2 activities. Our results indicate that I-R induced cardiac dysfunction is associated with subcellular changes in PLD2 and PAP2 activities.

Sarcoplasmic reticulum and cardiac oxidative stress: an emerging target for heart disease

The sarcoplasmic reticulum (SR) is a major player in maintaining cardiac function, as it is intimately involved in the regulation of Ca2+-movements on a beat-to-beat basis. SR dysfunction due to abnormalities in SR protein content has been reported in different cardiac diseases such as ischaemic heart disease, myocardial infarction, congestive heart failure and various cardiomyopathies; thus the genes expressing the SR Ca2+-pump, Ca2+-channels, calsequestrin, phospholamban and other regulatory proteins are considered important targets for drug development. In our experience, ischaemic preconditioning (IP) and pharmacological therapies, such as anti-oxidants, beta-adrenergic receptor blockers, angiotensin receptor (AT-1) blockers, angiotensin converting enzyme inhibitors (ACE-I) and angiotensin receptor blockers are effective therapies that improve cardiac performance in the failing heart by improving SR function. Accordingly, this paper is intended to shed light on the knowledge in the field of cardiac therapy targeted to improve and protect SR function.

Expression of phospholipase D isozymes in scar and viable tissue in congestive heart failure due to myocardial infarction

The phospholipase D (PLD) associated with the cardiac sarcolemmal (SL) membrane hydrolyses phosphatidylcholine to produce phosphatidic acid, an important phospholipid signaling molecule known to influence cardiac function. The present study was undertaken to examine PLD isozyme mRNA expression, protein contents and activities in congestive heart failure (CHF) subsequent to myocardial infarction (MI). MI was induced in rats by occlusion of the left anterior descending coronary artery. At 8 weeks after the surgical procedure, hemodynamic assessment revealed that these experimental rats were at a moderate stage of CHF. Semi-quantitative reverse transcriptase-polymerase chain reaction revealed that PLD1 and PLD2 mRNA amounts were unchanged in viable left ventricular (LV) tissue of the failing heart. Furthermore, this technique demonstrated the presence of PLD1 and PLD2 mRNA in the scar tissue. While SL PLD1 and PLD2 protein contents were elevated in the viable LV tissue of the failing heart, SL PLD1 activity was significantly decreased, whereas SL PLD2 activity was significantly increased. On the other hand, although PLD1 protein was undetectable, PLD2 protein and activity were detected in the scar tissue. Our findings suggest that differential changes in PLD isozymes may contribute to the pathophysiology of CHF and may also be involved in the processes of scar remodeling.

Defective phosphatidic acid–phospholipase C signaling in diabetic cardiomyopathy

The effects of exogenous phosphatidic acid (PA) on Ca2+ transients and contractile activity were studied in cardiomyocytes isolated from chronic streptozotocin-induced diabetic rats. In control cells, 25 microM PA induced a significant increase in active cell shortening and Ca2+ transients. PA increased IP3 generation in the control cardiomyocytes and its inotropic effects were blocked by a phospholipase C inhibitor. In cardiomyocytes from diabetic rats, PA induced a 25% decrease in active cell shortening and no significant effect on Ca2+ transients. Basal and PA-induced IP3 generation in diabetic rat cardiomyocytes was 3-fold lower as compared to control cells. Sarcolemmal membrane PLC activity was impaired. Insulin treatment of the diabetic animals resulted in a partial recovery of PA responses. Our results, therefore, identify an important defect in the PA-PLC signaling pathway in diabetic rat cardiomyocytes, which may have significant implications for heart dysfunction during diabetes.

Depressed phosphatidic acid-induced contractile activity of failing cardiomyocytes

The effects of phosphatidic acid (PA), a known inotropic agent, on Ca(2+) transients and contractile activity of cardiomyocytes in congestive heart failure (CHF) due to myocardial infarction were examined. In control cells, PA induced a significant increase (25%) in active cell shortening and Ca(2+) transients. The phospholipase C (PLC) inhibitor, 2-nitro-4-carboxyphenyl N,N-diphenylcarbonate, blocked the positive inotropic action induced by PA, indicating that PA induces an increase in contractile activity and Ca(2+) transients through stimulation of PLC. Conversely, in failing cardiomyocytes there was a loss of PA-induced increase in active cell shortening and Ca(2+) transients. PA did not alter resting cell length. Both diastolic and systolic [Ca(2+)] were significantly elevated in the failing cardiomyocytes. In vitro assessment of the cardiac sarcolemmal (SL) PLC activity revealed that the impaired failing cardiomyocyte response to PA was associated with a diminished stimulation of SL PLC activity by PA. Our results identify an important defect in the PA-PLC signaling pathway in failing cardiomyocytes, which may have significant implications for the depressed contractile function during CHF.

Alterations of sarcolemmal phospholipase D and phosphatidate phosphohydrolase in congestive heart failure

Phospholipase D 2 (PLD2) is the major PLD isozyme associated with the cardiac sarcolemmal (SL) membrane. Hydrolysis of SL phosphatidylcholine (PC) by PLD2 produces phosphatidic acid (PA), which is then converted to 1,2 diacylglycerol (DAG) by the action of phosphatidate phosphohydrolase type 2 (PAP2). In view of the role of both PA and DAG in the regulation of Ca(2+) movements and the association of abnormal Ca(2+) homeostasis with congestive heart failure (CHF), we examined the status of both PLD2 and PAP2 in SL membranes in the infarcted heart upon occluding the left coronary artery in rats for 1, 2, 4, 8 and 16 weeks. A time-dependent increase in both SL PLD2 and PAP2 activities was observed in the non-infarcted left ventricular tissue following myocardial infarction (MI); however, the increase in PAP2 activity was greater than that in PLD2 activity. Furthermore, the contents of both PA and PC were reduced, whereas that of DAG was increased in the failing heart SL membrane. Treatment of the CHF animals with imidapril, an angiotensin-converting enzyme (ACE) inhibitor, attenuated the observed changes in heart function, SL PLD2 and PAP2 activities, as well as SL PA, PC and DAG contents. The results suggest that heart failure is associated with increased activities of both PLD2 and PAP2 in the SL membrane and the beneficial effect of imidapril on heart function may be due to its ability to prevent these changes in the phospholipid signaling molecules in the cardiac SL membrane.

Depressed responsiveness of phospholipase C isoenzymes to phosphatidic acid in congestive heart failure

The cardiac sarcolemmal membrane cis -unsaturated fatty acid-sensitive phospholipase D hydrolyzes phosphatidylcholine to form phosphatidic acid. The functional significance of phosphatidic acid is indicated by its ability to increase [Ca(2+)](i)and augment cardiac contractile performance via the activation of phospholipase C. Accordingly, we tested the hypothesis that a defect occurs in the membrane level of phosphatidic acid and/or the responsiveness of cardiomyocytes to phosphatidic acid in congestive heart failure due to myocardial infarction. Myocardial infarction was produced in rats by ligation of the left coronary artery while sham-operated animals served as control. At 8 weeks after surgery, the experimental animals were at a stage of moderate congestive heart failure. Compared to sham controls, phosphatidic acid-mediated increase in [Ca(2+)](i), as determined by the fura 2-AM technique, was significantly reduced in failing cardiomyocytes. Immunoprecipitation of sarcolemmal phospholipase C isoenzymes using specific monoclonal antibodies revealed that the stimulation of phospholipase C gamma(1)and delta(1)phosphatidylinositol 4,5-bisphosphate hydrolyzing activities by phosphatidic acid was decreased in the failing heart. Although the activity of phospholipase C beta(1)in the failing heart was higher than the control, phosphatidic acid did not stimulate this isoform in control sarcolemma, and produced an inhibitory action in the failing heart preparation. Furthermore, the specific binding of phosphatidic acid to phospholipase C gamma(1)and delta(1)isoenzymes was decreased, whereas binding to phospholipase beta(1)was absent in the failing heart. A reduction in the intramembranal level of phosphatidic acid derived via cis -unsaturated fatty acid-sensitive phospholipase D was also seen in the failing heart. These findings suggest that a defect in phosphatidic acid-mediated signal pathway in sarcolemma may represent a novel mechanism of heart dysfunction in congestive heart failure.

A HPLC-fluorescence detection method for determination of cardiac phospholipase D activity in vitro

A nonradioactive assay for the investigation of phospholipase D (PLD) activity in cardiac membranes has been developed. A fluorescent derivative of phosphatidylcholine [2-decanoyl-1-(O-(11-(4,4-difluoro-5,7-dimethyl-4-bora-3a, 4a-diaza-s-indacene-3proprionyl)amino) undecyl) sn-glycero-3-phosphocholine] was utilized as substrate in an in vitro PLD-catalyzed transphosphatidylation reaction utilizing ethanol as second substrate. Unreacted phosphatidylcholine and the products of phospholipase activity (PEtOH, phosphatidylethanol; PA, phosphatidic acid; DAG, diacylglycerol) were separated by a binary gradient HPLC system and detected by fluorometry. The detection limit of this assay is approximately 0.6 pmol PEtOH. The reaction proceeded at a linear rate for up to 45 min and increased linearly with increasing amounts of rat cardiac membrane protein in a range of 0.625 microg up to at least 25 microg. In the presence of potassium fluoride, formation of fluorescent PA increased at the expense of DAG generation, demonstrating the presence of PA phosphohydrolase activity in rat cardiac membranes. PEtOH formation was unchanged in the presence of the PA phosphohydrolase inhibitor, indicating that the phosphatidylalcohol is not subject to further metabolism by this enzyme. Activation of protein kinase C by phorbol ester significantly increased PLD activity in cardiac membranes. This assay proved to be sensitive for accurate and rapid assessment of PLD activity in cardiac membranes permitting further characterization of the regulation of PLD signal transduction in the heart.

Oxidants depress the synthesis of phosphatidylinositol 4,5-bisphosphate in heart sarcolemma

Phosphatidylinositol 4,5-bisphosphate (PtdIns 4,5-P2) is the substrate for phosphoinositide-phospholipase C (PLC) and is required for the function of several cardiac cell plasma membrane (sarcolemma, SL) proteins. PtdIns 4,5-P2 is synthesized in the SL membrane by coordinated and successive actions of PtdIns 4-kinase and PtdIns 4-phosphate 5-kinase. These kinases and the generation of PtdIns 4,5-P2 may be a factor in the cardiac dysfunction during pathophysiological conditions of oxidative stress. Therefore, we examined the effects of different reactive oxygen species (ROS) on the kinases' activities and subsequent generation of PtdIns 4,5-P2. Exposure to the xanthine-xanthine oxidase-ROS generating system significantly reduced both SL kinase activities. Superoxide dismutase did not prevent this inhibition; however, catalase significantly prevented the xanthine-xanthine oxidase induced inhibition. Treatment of SL with hydrogen peroxide (H2O2) resulted in inhibition of both the kinases, which was prevented by catalase and dithiothreitol (DTT). Hypochlorous acid also inhibited both the kinases, which was prevented by DTT. Deferoxamine (an iron chelator) and mannitol (an *OH scavenger) did not modify the H2O2-induced depression of the kinases, eliminating any role of *OH. Furthermore, the IC50 of H2O2 on PtdIns 4-kinase and PtdIns 4-P 5-kinase was 27 and 81 microM, respectively. In addition, inclusion of reduced glutathione in the assay of the kinases in the absence of H2O2 did not affect the activities of the kinases; however, oxidized glutathione induced a significant depression. Also, a significant decline of the PtdIns 4-kinase and PtdIns 4-P 5-kinase activities due to changing of the redox ratio was observed. Thiol modifiers (N-ethylmaleimide, methyl methanethiosulfonate, or p-chloromercuriphenylsulfonic acid) were detected to depress the kinases' activities, which were substantially prevented by DTT. The results suggest that functionally critical thiol groups may be associated with PtdIns 4-kinase and PtdIns 4-P 5-kinase and that changes of their redox state by ROS can impair their activities, which may be an important factor in the oxidant-induced cardiac dysfunction.

Cardiac phospholipase D2 localizes to sarcolemmal membranes and is inhibited by alpha-actinin in an ADP-ribosylation factor-reversible manner

Myocardial phospholipase D (PLD) has been implicated in the regulation of Ca(2+) mobilization and contractile performance in the heart. However, the molecular identity of this myocardial PLD and the mechanisms that regulate it are not well understood. Using subcellular fractionation and Western blot analysis, we found that PLD2 is the major myocardial PLD and that it localizes primarily to sarcolemmal membranes. A 100-kDa PLD2-interacting cardiac protein was detected using a protein overlay assay employing purified PLD2 and then identified as alpha-actinin using peptide-mass fingerprinting with matrix-assisted laser desorption/ionization mass spectroscopy. The direct association between PLD2 and alpha-actinin was confirmed using an in vitro binding assay and localized to PLD2's N-terminal 185 amino acids. Purified alpha-actinin potently inhibits PLD2 activity (IC(50) = 80 nm) in an interaction-dependent and ADP-ribosylation factor-reversible manner. Finally, alpha-actinin co-localizes with actin and with PLD2 in the detergent-insoluble fraction from sarcolemmal membranes. These results suggest that PLD2 is reciprocally regulated in sarcolemmal membranes by alpha-actinin and ARF1 and accordingly that a major role for PLD2 in cardiac function may involve reorganization of the actin cytoskeleton.

Localization and possible functions of phospholipase D isozymes

The activation of PLD is believed to play an important role in the regulation of cell function and cell fate by extracellular signal molecules. Multiple PLD activities have been characterized in mammalian cells and, more recently, several PLD genes have been cloned. Current evidence indicates that diverse PLD activities are localized in most, if not all, cellular organelles, where they are likely to subserve different functions in signal transduction, membrane vesicle trafficking and cytoskeletal dynamics.

The subcellular localization of phospholipase D activities in rat Leydig cells

Rat Leydig cells contain a phospholipase D (PLD), which can be activated by vasopressin and phorbol ester. In order to clarify which Leydig cell organelles that express PLD activity, the subcellular localization of two differently regulated PLD activities was investigated by subcellular fractionation on a 40% (v/v) self-generating Percoll gradient. PLD activities in broken cells were estimated using radiolabeled didecanoylphosphatidylcholine as a substrate. Initial experiments revealed the presence of an oleate Mg2+ -activated PLD and a phosphatidylinositol 4,5-bisphosphate-activated PLD (PIP2-PLD) in the microsomal fraction of Leydig cells. The latter activity could be further stimulated by recombinant nonmyristoylated ADP ribosylating factor 1 (ARF1) plus GTPgammaS. The peak of oleate Mg2+ -PLD activity colocalized with the plasma membrane marker, whereas the highest specific activity of the PIP2-PLD activity was found in fractions with a slightly lower density than those containing the plasma membrane and trans-Golgi marker enzymes. In order to localize phorbol ester-stimulated PLD activity in intact Leydig cells, the cells were prelabeled with [14C]-palmitate and then stimulated for 15 min with 100 nM 4-beta-phorbol-12-myristate-13-acetate (PMA) in the presence of ethanol or butanol. The PLD product [14C]-phosphatidylethanol, expressed as the percentage of total labeled phospholipids in the fraction, was slightly increased in all Percoll fractions and showed a prominent peak in the fractions containing plasma membrane, trans-Golgi, and fractions of slightly lower density. The PMA-induced formation of [14C]-phosphatidylbutanol could be inhibited dose-dependently with brefeldin A suggesting that the activation of PLD by the phorbol ester was mediated by ARF.

Phospholipase D in rat myocardium: formation of lipid messengers and synergistic activation by G-protein and protein kinase C

Activation of phospholipase D (PLD) and phosphoinositide-specific phospholipase C (PI-PLC) by fluoride, to stimulate heterotrimeric G-proteins, and by phorbol esters, to stimulate protein kinase C (PKC), was studied in rat atria. Fluoride and 4beta-phorbol-12beta,13alpha-dibutyrate (PDB), in contrast to 4beta-phorbol-13alpha-acetate (PAc), activated PLD, catalyzing the formation of [3H]-phosphatidylethanol ([3H]-PETH), [3H]-phosphatidic acid ([3H]-PA), choline and sn-1,2-diacylglycerol (DAG). Basal PLD activity was resistant to drastic changes in Ca2+ and to Ro 31-8220, a PKC inhibitor, but was decreased by genistein, an inhibitor of tyrosine kinase, and increased by vanadate, a tyrosine phosphatase inhibitor; both effects were, however, very small. Fluoride-evoked PLD activity was resistant to Ro 31-8220 and to genistein, but was Ca2+-dependent. The rate of fluoride-induced PLD activation was maintained for at least 60 min. In contrast, PDB-mediated PLD activity was blocked by Ro 31-8220 and was resistant to extracellular Ca2+-depletion and desensitized within ca. 15 min. PDB markedly potentiated the fluoride-evoked generation of [3H]-phosphatidylethanol and of choline, but inhibited the formation of [3H]-inositol phosphates ([3H]-IP(1-3)). Ethanol (2%) blocked the PDB-evoked generation of both [3H]-phosphatidic acid and of sn-1,2-diacylglycerol, whereas fluoride-evoked responses were reduced only to approximately 50%. In conclusion, the trimeric G-protein-PLD pathway in heart tissue did not enclose PKC activation and was long-lasting and Ca2+-dependent; there was no evidence for an involvement of tyrosine phosphorylation. However, PKC activation modulated G-protein-coupled PLD and PI-PLC activities in opposite directions. PLD activity significantly contributed to the mass production of sn-1,2-diacylglycerol in the heart. The evidence for a pathophysiological role of PLD activation in cardiac hypertrophy and in ischemic preconditioning is discussed.

Cyclosporin A inhibits oxidant and calcium stimulated phospholipase D activity in the rat intestinal mitochondria

Mitochondrial swelling and calcium cycling occurs during oxidative stress and can be prevented by cyclosporin A (CysA). Our earlier work has shown that enterocyte mitochondria contains a phospholipase D (PLD) which can be activated by superoxide or calcium. In this study, we have shown that enterocyte mitochondrial PLD activated by these agents can be inhibited by cyclosporin A. This was clearly shown by the absence of phosphatidic acid (PA) formation and phosphatidylethanolamine (PE) degradation. Since this PLD specifically utilizes PE as substrate, PLD activity was also assessed by ethanolamine formation which was inhibited by CysA. CysA also inhibited the cabbage PLD activity as judged by phosphatidylethanol formation. These results suggest that cyclosporin A is an inhibitor of PLD and this may be one of the mechanism by which CysA protects enterocyte mitochondria from oxidative stress.

Activation of intestinal mitochondrial phospholipase D by polyamines and monoamines

Intestinal mitochondria have a phospholipase D (PLD) activity which was stimulated by polyamines and monoamines resulting in the formation of phosphatidic acid (PA) from endogenous phospholipids. When stimulated by polyamines, mitochondrial PLD utilized endogenous phosphatidylethanolamine (PE) as substrate whereas stimulated by monoamines, both PE and phosphatidylcholine (PC) were hydrolysed. Stimulation of PA formation by spermine was enhanced by the presence of calcium. Since polyamines are known to alter the calcium transport by mitochondria and PA is known to possess an ionophore effect, stimulation of PA formation in mitochondria by polyamines suggests that polyamine-induced alteration in calcium homeostasis might involve a PA related mechanism.

Subcellular alterations in cardiac phospholipase D activity in chronic diabetes

Several studies have suggested that myocardial phospholipase D (PLD) and its hydrolytic product, phosphatidic acid (PtdOH), may regulate Ca2+ movements and contractile performance of the heart. Since abnormal intracellular Ca2+ handling is a major factor of myocardial dysfunction in chronic diabetes, we examined subcellular changes in PLD activity in myocardium from insulin-dependent diabetic rats. Diabetes in rats was induced by a single i.v. injection of streptozotocin (65 mg/kg body wt) and 8 weeks later the ventricular tissue was processed for the isolation of sarcolemma, sarcoplasmic reticulum and mitochondria. Compared to age-matched controls, the sarcolemmal, sarcoplasmic reticular and mitochondrial PLD activities were significantly depressed in the diabetic animals. The depressed sarcolemmal PLD activity was normalized, whereas the sarcoplasmic reticular and mitochondrial enzyme activities were partially reversed upon treating the 6-week diabetic rats with insulin for a period of 2 weeks. These data suggest that the reduction of PLD-derived PtdOH may lead to an impairment in this phospholipid signal transduction pathway and subsequent cardiac dysfunction in chronic diabetes.

Glycerolipid metabolizing enzymes in rat ventricle and in cardiac myocytes

1. The properties and subcellular distribution of phosphatidate phosphohydrolase (PAP) were studied in rat heart. A Mg2(+)-activated activity (PAP1) which was inhibited by N-ethylmaleimide was found mainly in a 105,000 x g soluble fraction. Isolation of the membranes in a medium containing KCl increased the proportion of PAP1 that was associated. Translocation of PAP1 from these membranes occurred on subsequent incubation in a low-ionic strength medium from which KCI was omitted. Incubation of cardiac myocytes with palmitate promoted translocation of PAP activity to cellular membranes. A second activity which was insensitive to N-ethylmaleimide (PAP2) was found in the 105,000 x g membrane fraction. PAP2 was inhibited by concentrations of Mg2+ known to occur in ischaemia. Specific activities of PAP1 and PAP2 in ventricle muscle homogenates were similar. The specific activity of PAP2 in homogenates of cardiac myocytes was only 42% of that in homogenates of ventricle muscle. 2. A glycerolphosphate acyltransferase (GPAT) activity with properties similar to the GPAT found in microsomes from liver or adipose tissue was enriched in the sarcoplasmic reticulum fraction from ventricle muscle. This GPAT had a significantly higher K(m) for glycerol 3-phosphate than the GPAT found in adipose tissue microsomes. The possible physiological significance of this 'high K(m)' GPAT in heart, particularly in ischaemia, is discussed. 3. Comparisons were made of the specific activities of fatty acyl-CoA synthetase, monoacylglycerolphosphate acyltransferase, diacylglycerol acyltransferase and the mitochondrial and microsomal forms of GPAT in homogenates from cardiac myocytes and ventricle muscle.

Kinetics of myocardial phospholipase D

Myocardial phospholipase D (PLD) is located in different subcellular membranes, including sarcolemma (SL) and sarcoplasmic reticulum (SR). In this study, the kinetics of PLD-dependent hydrolytic and transphosphatidylation activities were examined in SL and SR fractions isolated from rat heart by measuring the formation of phosphatidic acid and phosphatidylethanol, respectively. The results showed that, compared to SR PLD, SL PLD had a higher Vmax, i.e. 373 vs. 70 nmol/mg protein/h for the hydrolytic activity and 415 vs. 60 nmol/mg protein/h for the transphosphatidylation activity. In comparison with the SR enzyme, SL PLD had a lower Km value for the hydrolytic activity (0.46 vs. 0.65 mM), buy a higher Km for the transphosphatidylation activity (225 vs. 179 mM). These distinctive kinetic parameters suggest that SL PLD and SR PLD may be isoforms of the enzyme and/or have different membrane domain. Therefore, SL- and SR-localized PLD activities may be under independent control mechanism(s) and play distinct roles in normal conditions and pathological processes.

Enzymology of mammalian phospholipases D: in vitro studies

The existence of multiple forms of phopholipase D was clearly established in a large number of biochemical studies that described and characterized the enzymological properties of the different PLD activities. This review summarizes the in vitro evidence showing differential subcellular localization and chromatographic properties of putative PLD isozymes, their phospholipid and alcohol substrate specificities, their modulation by various divalent cations, small G proteins and protein kinase c isozymes, and the role of phosphatidylinositol 4,5-bisphosphate as a cofactor of phospholipase D.

Involvement of thiol groups in the impairment of cardiac sarcoplasmic reticular phospholipase D activity by oxidants

Considerable phospholipase D (PLD) activity is localized in myocardial sarcoplasmic reticular (SR) membranes, where it may take part in the regulation of Ca2+ movements. In this study, we examined thiol group dependence as a possible regulatory mechanism for SR PLD. SR membranes isolated from rat heart were exposed to four types of thiol group modifiers, which all induced a decrease in SR PLD activity that was prevented by dithiothreitol. Furthermore, since abnormalities in thiol status and Ca2+ homeostasis are characteristic for the myocardial cell damage induced by oxidative stress, we also studied the effects of oxidants on the SR PLD activity. The enzyme was not affected by xanthine-xanthine oxidase, but was depressed by hydrogen peroxide and by hypochlorous acid. These inhibitory effects were prevented by catalase as well as by methionine and dithiothreitol, respectively. Furthermore, reduced glutathione protected against the hydrogen peroxide-induced depression, whereas oxidized glutathione inhibited SR PLD. The results indicate that SR PLD activity is inhibited by nonradical oxidants, hydrogen peroxide and hypochlorous acid, through reversible modification of associated thiol groups. Thus, the enzyme may be controlled by the glutathione redox status of the cardiac cell.

Structure-activity relationship of the effect of cis-unsaturated fatty acids on heart sarcolemmal phospholipase D activity

This study examined the role of fatty acids on the phosphatidylcholine-specific phospholipase D (PLD) function of purified sarcolemmal (SL) membranes isolated from rat hearts. The enzyme's hydrolytic activity was determined by measuring [14C] phosphatidic acid formation from exogenous [14C] phosphatidylcholine (PtdCho) in the absence or presence of the sodium salts of various saturated or unsaturated long-chain fatty acids (FA). In certain experiments the enzyme was also assayed in the transphosphatidylation mode. Cis-unsaturation and free carboxyl groups were structural prerequisites for the stimulatory effect exerted by FA on SL PLD. The most effective compounds were arachidonate and oleate, which maximally activated PLD at 4 and 5 mM concentration, respectively. To verify if a detergent-like mechanism was involved in PLD activation, anionic, zwitterionic and non-ionic detergents were used. Only anionic taurodeoxycholate had a slight effect, which was about 7% of that achieved by arachidonate or oleate. These results suggest that cis-unsaturated FA activate cardiac sarcolemmal PLD by a mechanism(s) which seems to be unrelated to non-specific perturbation of the membrane.

Activation of rat brain phospholipase D by ADP-ribosylation factors 1,5, and 6: separation of ADP-ribosylation factor-dependent and oleate-dependent enzymes

Two major forms of phospholipase D (PLD) activity, solubilized from rat brain membranes with Triton X-100, were separated by HPLC on a heparin-5PW column with buffer containing octyl glucoside. One form was completely dependent on sodium oleate for activity. The other, which was dramatically activated by the addition of ADP-ribosylation factor (ARF) 1 and guanine 5' [gamma-thio]triphosphate, required the presence of phosphatidylinositol 4,5-bisphosphate in the phosphatidylcholine substrate for demonstration of activity, as described by others. Oleate-dependent activity was unaffected by guanine 5' [gamma-thio]triphosphate, or phosphatidylinositol 4,5-bisphosphate. Both sodium oleate-and ARF-dependent activities catalyzed transphosphatidylation, thus identifying them as PLDs. ARF-dependent PLD was activated by recombinant ARF5 (class II) and ARF6 (class III), as well as ARF1 (class I). Myristoylated recombinant ARFs were more effective than their nonmyristoylated counterparts. ARFs were originally identified as activators of cholera toxin ADP-ribosyltransferase activity. The effects of recombinant ARF proteins from the three classes on cholera toxin activity (assayed under conditions identical to those used to assay PLD activity) did not, however, correlate with those on PLD, consistent with the notion that different aspects of ARF structure are involved in the two functions.

Alcoholic cardiomyopathy

Chronic alcoholism is one of the most important causes of dilated cardiomyopathy, and a large proportion of chronic alcoholics demonstrate impairment of cardiac function. The development of cardiac dysfunction is apparently related to the total lifetime dose of ethanol. Studies in experimental animals have demonstrated that both acute and chronic ethanol administration impair cardiac contractility. However, the relationship, if any, between the acute effects of alcohol and the development of irreversible cardiomyopathy remain to be elucidated.

Effects of ethanol on the contractile function of the heart: a review

Chronic ethanol consumption leads to a number of alterations in the contractile function of the heart and is a leading cause of cardiomyopathy. Ethanol also has an acute negative inotropic effect mediated by direct interaction with cardiac muscle cells, although this action is often masked by indirect actions resulting from enhanced release of catecholamines in vivo. This article reviews the effects of ethanol on the contractile function of the heart. The specific targets affected by ethanol in cardiac muscle cells are discussed in terms of potential mechanisms underlying the depressions of contractility resulting from both acute and chronic actions of ethanol.

Cardiolipin biosynthesis in the isolated heart

The pathway for the biosynthesis of new cardiolipin was investigated in the isolated perfused intact rat heart. Isolated rat hearts were perfused in the Langendorff mode for up to 60 min with Krebs-Henseleit buffer containing 0.1 microM [U-14C]glycerol. Analysis of radioactivity incorporated into phospholipids in the organic phase revealed an increase in radioactivity incorporated into phosphatidylglycerol, cardiolipin and other phospholipids with time of perfusion. This was associated with a loss of radioactivity from phosphatidic acid. In contrast, perfusion of hearts for up to 60 min with 0.1 mM [1,(3)-3H]glycerol in the perfusate revealed an increased radioactivity associated with phosphatidic acid as well as cardiolipin, phosphatidylglycerol and other phospholipids. Perfusion of hearts for up to 60 min with [32P]Pi in the perfusate revealed a time-dependent increase in radioactivity associated with all phospholipids. Perfusion of hearts for up to 60 min with 0.1 microM or 0.1 mM glycerol in the perfusate did not affect the concentration of phosphatidic acid, cardiolipin or phosphatidylglycerol. To determine the rate-limiting step of cardiolipin biosynthesis, hearts were pulsed for 5 min with 0.1 microM [1,(3)-3H]glycerol and chased for up to 60 min with 0.1 microM glycerol in the perfusate. Radioactivity was maximum at the start of the chase in phosphatidic acid (and 1,2-diacylglycerol), and was subsequently chased into phosphatidylinositol, phosphatidylglycerol and cardiolipin (and other phospholipids). Significant radioactivity in phosphatidylglycerol phosphate was not detected. Radioactivity in CDP-sn-1,2-diacylglycerol remained constant throughout the chase. The activities of the enzymes of the Kennedy pathway for cardiolipin biosynthesis in the heart were determined. On the basis of continuous-pulse and pulse-chase labelling studies it is postulated that the cardiac polyglycerophospholipids phosphatidylglycerol and cardiolipin are actively synthesized from newly synthesized phosphatidic acid via the Kennedy pathway. In addition, the results suggest that the rate-limiting step of cardiolipin biosynthesis in the intact heart is probably the conversion of phosphatidic acid into CDP-sn-1,2-diacylglycerol.

Rat brain cytosol contains a factor which reconstitutes guanine-nucleotide-binding-protein-regulated phospholipase-D activation in HL60 cells previously permeabilized with streptolysin O

We report that guanosine 5'-[gamma-thio]triphosphate (GTP[S]) can stimulate phospholipase D (PLD) in HL60 cells acutely permeabilized with streptolysin O. The ability of GTP[S] to stimulate PLD is impaired if the cells are previously permeabilized such that the majority of the cytosol has leaked out. Rat brain and HL60 cytosols were both found to restore GTP[S]-stimulated PLD activity in a reconstitution assay consisting of previously permeabilized HL60 cells. Rat brain cytosol was fractionated on heparin agarose and assayed for reconstitution of GTP[S]-stimulated PLD activity. The active fractions were pooled, concentrated and chromatographed on gel filtration to assess its molecular mass. The molecular mass of the reconstituting factor was found to be 16 kDa. Reconstitution by the cytosolic factor was dependent on GTP[S]. Ca2+ (pCa 5), MgATP and MgCl2 enhanced GTP[S]-dependent reconstitution of PLD activity in the previously permeabilized HL60 cells. These results demonstrate the presence in rat brain cytosol of a factor which is an activator of GTP[S]-stimulated PLD.

Phospholipase D and cell signaling

Phospholipase D, which hydrolyzes phospholipids (primarily phosphatidylcholine) to generate phosphatidic acid, has emerged as a critical component in cellular signal transduction. Research during the past year has confirmed and extended the view that phosphatidic acid and its dephosphorylated product, sn-1,2-diacylglycerol, are important intracellular second messengers and that the coupling of phospholipase D to specific receptors occurs through multiple mechanisms involving protein kinase C, protein tyrosine kinase, Ca2+ and GTP-binding proteins.

Phospholipase D: regulation and functional significance

PLD is a major route for hydrolysis of PC in most tissues, consistent with it playing an important role in signal transduction. The enzyme appears to be activated by a variety of different mechanisms in different tissues, suggesting there might be several different isoforms. Little, however, is known at present about its enzymology and molecular biology. There is little direct evidence to indicate the functional significance of PLD activation but an accumulation of indirect evidence links PLD with prolonged changes in cell function. In particular, two areas where there is strong evidence for a role for PLD are mitogenesis and leukocyte hyperresponsiveness. An important area for future work will be the investigation of how products from the PLD pathway exert these effects. Current evidence suggests an important role for Ca(2+)-independent PKC isoforms and probably also for novel cellular targets for the putative second messenger PA.

Depression of cardiac sarcolemmal phospholipase D activity by oxidant-induced thiol modification

Myocardial phospholipase D (PLD) is primarily localized at the sarcolemmal level and selectively hydrolyzes phosphatidylcholine to form phosphatidic acid as part of the signal transduction mechanisms for regulating Ca2+ movements in the heart. Since the myocardial cell damage induced by oxidative stress is associated with abnormalities in Ca2+ homeostasis and thiol status, we examined the thiol group dependence and the effects of oxidant species on this enzyme. Sarcolemmal membranes isolated from rat heart were exposed to several types of thiol group modifiers. Alkylation with N-ethylmaleimide or methyl methanethiosulfonate, mercaptide formation with p-chloromercuriphenylsulfonic acid, and thiol-disulfide exchange with 5,5'-dithio-bis(2-nitrobenzoate) depressed sarcolemmal PLD activity; in all cases the depression was prevented by dithiothreitol. At different concentrations of N-ethylmaleimide the PLD depression correlated well (r = 0.98) with the decrease in total thiol group content of the membrane. The enzyme activity was not affected by xanthine-xanthine oxidase, a superoxide anion-generating system, but was depressed by hydrogen peroxide (H2O2) in a concentration-dependent manner. This inhibitory effect was prevented by catalase as well as by dithiothreitol, but not by D-mannitol. The effect of a hydroxyl radical-generating system (Fenton reaction) could not be assessed because of an interfering direct inhibition by Fe2+. Dithiothreitol was also able to restore PLD activity in H2O2-pretreated membranes and to prevent a severe deactivation of the enzyme by hypochlorous acid (HOCI). Protection by glutathione and inhibition by its oxidized form were also observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipase D signaling in ischemic heart

Phospholipase D (PLD) activity was found to be present in the membrane fraction of rat myocardial cells by in vitro assays (36.7 +/- 4.1 nmol/mg protein per h against 1-palmitoyl-2-arachidonoyl- phosphatidylcholine) and demonstrated in intact cells by the specific transphosphatidylation reaction (in the presence of 0.02% ethanol) quantitated using n-[1-14C]butanol (201.16 +/- 7.1 pmol/min per g dry weight in the whole heart). Both methods showed a significant increase in PLD activity (by 62 and 44%, respectively) in hearts subjected to reversible (30 min) global normothermic ischemia followed by reperfusion (30 min). In hearts prelabeled with [1-14C]arachidonic acid, ischemia/reperfusion induced a significant increase in the amount of radiolabel incorporated into phosphatidic acid (PtdOH) (by 49.6%) and diacylglycerol (DG) (by 259%). DG kinase inhibition by 100 microM dioctanoylethylene glycol did not affect the ischemia/reperfusion DG and PtdOH levels while PtdOH phosphohydrolase inhibition with 40 microM propranolol produced a further increase in PtdOH (to 2.36-fold the baseline level) and a reduction in DG (to only 145% over the baseline levels). Put together, all these results suggest an activation of PLD during myocardial ischemia/reperfusion generating intracellular PtdOH, part of which is converted by PtdOH phosphohydrolase to DG. We further investigated the possible pathophysiological significance of the observed PLD activation. Stimulation of PLD with sodium oleate (20 microM) induced a significant improvement of functional recovery of ischemic hearts during reperfusion (as monitored by coronary flow and left intraventricular pressure measurements) and an attenuation of cellular injury as expressed by lactate dehydrogenase and creatine kinase release in the coronary effluent during reperfusion. These results suggest a PLD-mediated signaling in the ischemic heart which may benefit functional recovery during reperfusion.