Stimulation and binding of myocardial phospholipase C by phosphatidic acid

Inhibition of angiotensin converting enzyme induces mechanical allodynia through increasing substance P expression in mice

Although emerging evidence shows that angiotensin converting enzyme (ACE) is associated with pain, it is not clear whether inhibition of ACE could affect to nociceptive transmission and which mediators are involved in this process. Here we investigated whether administration of the ACE inhibitors, captopril and enalapril increases the expression of substance P (SP) and whether this increase contributes to the induction of mechanical allodynia in mice. ACE was expressed in the lumbar dorsal root ganglion (DRG) and the superficial dorsal horn (SDH) region of the spinal cord in mice. Either intraperitoneal or intrathecal administration of the ACE inhibitors, captopril and enalapril for 10 days significantly increased the paw withdrawal frequency to innocuous mechanical stimuli and the levels of SP in both the lumbar DRG and the SDH region of the spinal cord dorsal horn. In addition, intraperitoneal administration of the SP receptor (neurokinin-1 receptor) antagonist, L-733,060 suppressed mechanical allodynia that was induced by pretreatment of captopril and enalapril. Intraplantar administration of SP for 3 days induces mechanical allodynia, and this effect was reduced by exogenous ACE administration. These findings demonstrate that inhibition of ACE increases the levels of SP in both the lumbar DRG and spinal cord dorsal horn, ultimately contributing to the induction of mechanical allodynia in mice.

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Hepatic gluconeogenesis is enhanced by phosphatidic acid which remains uninhibited by insulin in lipodystrophic Agpat2-/- mice

In this study we examined the role of phosphatidic acid (PA) in hepatic glucose production (HGP) and development of hepatic insulin resistance in mice that lack 1-acylglycerol-3-phosphate O-acyltransferase 2 (AGPAT2). Liver lysophosphatidic acid and PA levels were increased ∼2- and ∼5-fold, respectively, in male Agpat2(-/-) mice compared with wild type mice. In the absence of AGPAT2, the liver can synthesize PAs by activating diacylglycerol kinase or phospholipase D, both of which were elevated in the livers of Agpat2(-/-) mice. We found that PAs C16:0/18:1 and C18:1/20:4 enhanced HGP in primary WT hepatocytes, an effect that was further enhanced in primary hepatocytes from Agpat2(-/-) mice. Lysophosphatidic acids C16:0 and C18:1 failed to increase HGP in primary hepatocytes. The activation of HGP was accompanied by an up-regulation of the key gluconeogenic enzymes glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. This activation was suppressed by insulin in the WT primary hepatocytes but not in the Agpat2(-/-) primary hepatocytes. Thus, the lack of normal insulin signaling in Agpat2(-/-) livers allows unrestricted PA-induced gluconeogenesis significantly contributing to the development of hyperglycemia in these mice.

Phosphatidic acid-mediated signaling

Phosphatidic acid (PA) is recognized as an important class of lipid messengers. The cellular PA levels are dynamic; PA is produced and metabolized by several enzymatic reactions, including different phospholipases, lipid kinases, and phosphatases. PA interacts with various proteins and the interactions may modulate enzyme catalytic activities and/or tether proteins to membranes. The PA-protein interactions are impacted by changes in cellular pH and other effectors, such as cations. PA is involved in a wide range of cellular processes, including vesicular trafficking, cytoskeletal organization, secretion, cell proliferation, and survival. Manipulations of different PA production reactions alter cellular and organismal response to a wide range of abiotic and biotic stresses. Further investigations of PA's function and mechanisms of action will advance not only the understanding of cell signaling networks but also may lead to biotechnological and pharmacological applications.

Phospholipase C as a potential target for cardioprotection during oxidative stressThis review is one of a selection of papers published in a Special Issue on Oxidative Stress in Health and Disease

Cardiac dysfunction due to ischemia–reperfusion (I/R) is associated with marked changes in membrane function and subsequent Ca2+-handling abnormalities in cardiomyocytes. The membrane abnormalities in hearts subjected to I/R arise primarily from oxidative stress as a consequence of increased formation of reactive oxygen species and other oxidants, as well as reduced antioxidant defenses. Little is known, however, about the nature and mechanisms of the sarcolemmal membrane changes with respect to phospholipase C (PLC)-related signaling events. In addition, the mechanisms involved in protection of the postischemic myocardium and in ischemic preconditioning with respect to PLC function need to be established. Accordingly, this article reviews the historical and current information on PLC-mediated signal transduction mechanisms in I/R, as well as outlining future directions that should be addressed. Such information will extend our knowledge of ischemic heart disease and help improve its therapy.

Structural Determinants for Phosphatidic Acid Regulation of Phospholipase C-β1

Signaling from G protein-coupled receptors to phospholipase C-beta (PLC-beta) is regulated by coordinate interactions among multiple intracellular signaling molecules. Phosphatidic acid (PA), a signaling phospholipid, binds to and stimulates PLC-beta(1) through a mechanism that requires the PLC-beta(1) C-terminal domain. PA also modulates Galpha(q) stimulation of PLC-beta(1). These data suggest that PA may have a key role in the regulation of PLC-beta(1) signaling in cells. The present studies addressed the structural requirements and the mechanism for PA regulation of PLC-beta(1). We used a combination of enzymatic assays, PA-binding assays, and circular dichroism spectroscopy to evaluate the interaction of PA with wild-type and mutant PLC-beta(1) proteins and with fragments of the Galpha(q) binding domain. The results identify a region that includes the alphaA helix and flexible loop of the Galpha(q)-binding domain as necessary for PA regulation. A mutant PLC-beta(1) with multiple alanine/glycine replacements for residues (944)LIKEHTTKYNEIQN(957) was markedly impaired in PA regulation. The high affinity and low affinity component of PA stimulation was reduced 70% and PA binding was reduced 45% in this mutant. Relative PLC stimulation by PA increased with PLC-beta(1) concentration in a manner suggesting cooperative binding to PA. Similar concentration dependence was observed in the PLC-beta(1) mutant. These data are consistent with a model for PA regulation of PLC-beta(1) that involves cooperative interactions, probably PLC homodimerization, that require the flexible loop region, as is consistent with the dimeric structure of the Galpha(q)-binding domain. PA regulation of PLC-beta(1) requires unique residues that are not required for Galpha(q) stimulation or GTPase-activating protein activity.

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.

Phospholipase D

Phospholipase D catalyses the hydrolysis of the phosphodiester bond of glycerophospholipids to generate phosphatidic acid and a free headgroup. Phospholipase D activities have been detected in simple to complex organisms from viruses and bacteria to yeast, plants, and mammals. Although enzymes with broader selectivity are found in some of the lower organisms, the plant, yeast, and mammalian enzymes are selective for phosphatidylcholine. The two mammalian phospholipase D isoforms are regulated by protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families. Mammalian and yeast phospholipases D are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. This review discusses the identification, characterization, structure, and regulation of phospholipase D. Genetic and pharmacological approaches implicate phospholipase D in a diverse range of cellular processes that include receptor signaling, control of intracellular membrane transport, and reorganization of the actin cytoskeleton. Most ideas about phospholipase D function consider that the phosphatidic acid product is an intracellular lipid messenger. Candidate targets for phospholipase-D-generated phosphatidic acid include phosphatidylinositol 4-phosphate 5-kinases and the raf protein kinase. Phosphatidic acid can also be converted to two other lipid mediators, diacylglycerol and lyso phosphatidic acid. Coordinated activation of these phospholipase-D-dependent pathways likely accounts for the pleitropic roles for these enzymes in many aspects of cell regulation.

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.

Molecular modeling of the membrane targeting of phospholipase C pleckstrin homology domains

Phospholipases C (PLCs) reversibly associate with membranes to hydrolyze phosphatidylinositol-4, 5-bisphosphate (PI[4,5]P(2)) and comprise four main classes: beta, gamma, delta, and epsilon. Most eukaryotic PLCs contain a single, N-terminal pleckstrin homology (PH) domain, which is thought to play an important role in membrane targeting. The structure of a single PLC PH domain, that from PLCdelta1, has been determined; this PH domain binds PI(4,5)P(2) with high affinity and stereospecificity and has served as a paradigm for PH domain functionality. However, experimental studies demonstrate that PH domains from different PLC classes exhibit diverse modes of membrane interaction, reflecting the dissimilarity in their amino acid sequences. To elucidate the structural basis for their differential membrane-binding specificities, we modeled the three-dimensional structures of all mammalian PLC PH domains by using bioinformatic tools and calculated their biophysical properties by using continuum electrostatic approaches. Our computational analysis accounts for a large body of experimental data, provides predictions for those PH domains with unknown functions, and indicates functional roles for regions other than the canonical lipid-binding site identified in the PLCdelta1-PH structure. In particular, our calculations predict that (1). members from each of the four PLC classes exhibit strikingly different electrostatic profiles than those ordinarily observed for PH domains in general, (2). nonspecific electrostatic interactions contribute to the membrane localization of PLCdelta-, PLCgamma-, and PLCbeta-PH domains, and (3). phosphorylation regulates the interaction of PLCbeta-PH with its effectors through electrostatic repulsion. Our molecular models for PH domains from all of the PLC classes clearly demonstrate how a common structural fold can serve as a scaffold for a wide range of surface features and biophysical properties that support distinctive functional roles.

Effects of hyperoxia and acrylonitrile on the phospholipase C isozyme protein levels in rat heart and brain

We previously showed that hyperoxia exerts oxidative stress on the rat cerebral cortex, and the protein levels of phospholipase C (PLC) -beta1 and -delta1, but not PLC-gamma1, were changed. Acrylonitrile (ACN) appears to induce astrocytomas through induction of oxidative stress on the rat brain selectively. This study compared hyperoxia or ACN treatments of rats with respect to lipid peroxidation and PLC levels in the heart and cerebral cortex. Treatment of rats with ACN promoted lipid peroxidation in the heart and cerebral cortex, the percent increase above control being greater in the cortex than heart. Hyperoxia did not cause significant increases in lipid peroxidation in the cerebral cortex or heart. In the ACN-treated cerebral cortex, significant increases in the PLC-beta1 and -delta1 in the cytosol, and PLC-gamma1 in the cytosolic and particulate fractions, and lysate were observed. In the rat heart, in which PLC-beta1 could not be detected, PLC-gamma1 and -delta1 were increased and decreased in the cytosolic and particulate fractions, respectively, by hyperoxia. In addition, the expression level of PLC-gamma1 was decreased in the lysate by the treatment. In the heart treated with ACN, there was no change in the level of PLC-gamma1, while PLC-delta1 was elevated in all fractions. These findings suggested that the expression levels of PLC isozymes are altered by hyperoxia and ACN, but there are apparent differences in these altered levels between the different levels of oxidative stress, and between the organs.

Modification of intracellular free calcium in cultured A10 vascular smooth muscle cells by exogenous phosphatidic acid

Exogenous phosphatidic acid (PA) was observed to produce a concentration-dependent increase in [Ca(2+)](i) in cultured A10 vascular smooth muscle cells. Preincubation of cells with sarcoplasmic reticulum Ca(2+)-ATPase inhibitors (cyclopiazonic acid and thapsigargin), a phospholipase C inhibitor (2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate), inositol 1,4,5-trisphosphate receptor antagonists (2-aminoethoxydiphenyl borate and xestospongin), and an activator of protein kinase C (PKC) (phorbol 12-myristate 13-acetate) depressed the PA-evoked increase in [Ca(2+)](i). Although EGTA, an extracellular Ca(2+) chelator, decreased the PA-induced increase in [Ca(2+)](i), sarcolemmal Ca(2+)-channel blockers (verapamil or diltiazem) did not alter the action of PA. On the other hand, inhibitors of PKC (bisindolylmaleimide I) and G(i)-protein (pertussis toxin) potentiated the increase in [Ca(2+)](i) evoked by PA significantly. These results suggest that the PA-induced increase in [Ca(2+)](i) in vascular smooth muscle cells may occur upon the activation of phospholipase C and the subsequent release of Ca(2+) from the inositol 1,4,5-trisphosphate-sensitive Ca(2+) pool in the sarcoplasmic reticulum. This action of PA may be mediated through the involvement of PKC.

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.

Protective effects of protein kinase C during myocardial ischemia require activation of phosphatidyl-inositol specific phospholipase C

BACKGROUND

Protein kinase C (PKC) activation during myocardial ischemia is thought to be cardioprotective. However, the mechanism of ischemia-induced PKC activation remains unclear. We hypothesized that ischemic PKC activation occurs through activation of phosphatidyl-inositol specific phospholipase C (PI-PLC) and protects the heart from ischemic injury.

METHODS

Isolated rabbit hearts were subjected to 20 minutes of normothermic ischemia and reperfusion. The PI-PLC inhibitor U73122 (0.5 micromol/L), its inactive analogue U73343 (0.5 micromol/L), or the PKC inhibitor chelerythrine (2 micromol/L) were given just before ischemia. Another group received U73122 plus the direct PKC activator phorbol 12-myristate-13-acetate (PMA, 10 pmol/L). Measurements included contractile function, intracellular calcium, PI-PLC activity, and translocation of PKC isoforms.

RESULTS

PI-PLC activity increased during myocardial ischemia and was inhibited by U73122. PI-PLC inhibition prevented the ischemic translocation of PKC-alpha, PKC-epsilon, and PKC-eta, and impaired cardiac recovery and cytosolic calcium regulation without significant changes in energy metabolism. PMA restored both contractile function and PKC translocation pattern in U73122-treated hearts. Direct PKC inhibition with chelerythrine mimicked the effects of U73122.

CONCLUSIONS

PI-PLC mediates PKC translocation during myocardial ischemia. Inhibition of PI-PLC or PKC activation, or both, during ischemia significantly impairs postischemic myocardial recovery.

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.

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.

Regulation of phospholipase C-beta(1) activity by phosphatidic acid

The role of phosphatidic acid (PA) in regulating phospholipase C-beta(1) (PLC-beta(1)) activity was determined. PA promoted the binding of PLC-beta(1) to sucrose-loaded unilamellar vesicles (SLUV) containing phosphatidylcholine. PA increased enzymatic activity over a range of Ca(2+) concentrations and reduced the Ca(2+) concentration required for half-maximal stimulation of activity. PA did not affect the apparent K(m) for phosphatidylinositol 4, 5-bisphosphate. Lysophosphatidic acid also enhanced the binding of PLC-beta(1) to SLUV but was less effective in stimulating enzymatic activity. Diacylglycerol, phosphatidylserine, and oleic acid had little effect on activity. Anionic and neutral detergents did not stimulate activity. PA stimulation was relatively independent of acyl chain length. Dipalmitoyl-PA (16:0) was comparable to PA from egg lecithin and dimyristoyl-PA (C14:0) in stimulating activity, while dilauroyl-PA (C12:0) was slightly less effective. A 100 kDa catalytic fragment of PLC-beta(1) lacking amino acid residues C-terminal to His(880) did not bind to PA and was insensitive to stimulation by 7-15 mol % PA. Stimulation of 100 kDa enzymatic activity required 30 mol % PA. PA increased receptor-G protein stimulation of PLC-beta(1) activity in membranes. These results demonstrate that PA stimulates basal and receptor-G protein-regulated PLC-beta(1) activity. PA stimulation occurs through both a C-terminal-dependent and an independent mechanism. The C-terminal-mediated mechanism for stimulation may constitute an important pathway for conferring specific regulation of PLC-beta(1) in response to increases in cellular PA levels.

Phosphatidic acid increases inositol-1,4,5,-trisphosphate and [Ca2+]i levels in neonatal rat cardiomyocytes

Phosphatidic acid (PA), which can be synthesized de novo, or as a product of phosphatidylcholine hydrolysis and/or phosphorylation of 1,2-diacylglycerol (DAG), mediates diverse cellular functions in various cell types, including cardiomyocytes. We set out to characterize the effect of PA on intracellular free calcium ([Ca2+]i) and inositol-1,4,5-trisphosphate (IP(3)) levels in primary cultures of neonatal rat cardiomyocytes. Addition of PA led to rapid, concentration and time dependent increases in both IP(3) and [Ca2+]i levels in adherent cells. There was strong correlation in the concentration-response relationships between IP(3) and [Ca2+]i increases evoked by PA. Incubation with the sarcoplasmic reticulum (SR) Ca2+ pump inhibitor, cyclopiazonic acid (CPA), significantly attenuated the PA evoked [Ca2+]i increase but had no significant effect on IP(3) accumulation. The phospholipase C (PLC) inhibitor, D-609, attenuated both IP(3) and [Ca2+]i elevations evoked by PA whereas staurosporine (STS), a potent and non-selective PKC inhibitor, had no significant effect on either. Another PLC inhibitor, U73122, but not its inactive analog, U73343, also inhibited PA evoked increases in [Ca2+]i. Depletion of extracellular calcium attenuated both basal and PA evoked increases in [Ca2+]i. The PLA(2) inhibitors, bromophenylacyl-bromide (BPB) and CDP-choline, had no effect on PA evoked [Ca2+]i responses. Neither the DAG analog, dioctanoylglycerol, nor the DAG kinase inhibitor, R59949, affected PA evoked changes in [Ca2+]i. Taken together, these data indicate that PA, in a manner independent of PKC, DAG, or PLA(2), may enhance Ca2+ release from IP(3) sensitive SR Ca(2+) stores via activation of PLC in neonatal rat cardiomyocytes.

Activation of phospholipase C delta1 through C2 domain by a Ca(2+)-enzyme-phosphatidylserine ternary complex

The concentration of free Ca(2+) and the composition of nonsubstrate phospholipids profoundly affect the activity of phospholipase C delta1 (PLCdelta1). The rate of PLCdelta1 hydrolysis of phosphatidylinositol 4,5-bisphosphate was stimulated 20-fold by phosphatidylserine (PS), 4-fold by phosphatidic acid (PA), and not at all by phosphatidylethanolamine or phosphatidylcholine (PC). PS reduced the Ca(2+) concentration required for half-maximal activation of PLCdelta1 from 5.4 to 0.5 microM. In the presence of Ca(2+), PLCdelta1 specifically bound to PS/PC but not to PA/PC vesicles in a dose-dependent and saturable manner. Ca(2+) also bound to PLCdelta1 and required the presence of PS/PC vesicles but not PA/PC vesicles. The free Ca(2+) concentration required for half-maximal Ca(2+) binding was estimated to be 8 microM. Surface dilution kinetic analysis revealed that the K(m) was reduced 20-fold by the presence of 25 mol % PS, whereas V(max) and K(d) were unaffected. Deletion of amino acid residues 646-654 from the C2 domain of PLCdelta1 impaired Ca(2+) binding and reduced its stimulation and binding by PS. Taken together, the results suggest that the formation of an enzyme-Ca(2+)-PS ternary complex through the C2 domain increases the affinity for substrate and consequently leads to enzyme activation.

Phospholipase C-delta3 binds with high specificity to phosphatidylinositol 4,5-bisphosphate and phosphatidic acid in bilayer membranes

In order to acquire an understanding of phospholipase C-delta3 (PLC-delta3) action on substrate localized in lipid membrane we have studied the binding of human recombinant PLC-delta3 to large, unilamellar phospholipid vesicles (LUVs). PLC-delta3 bound weakly to vesicles composed of phosphatidylcholine (PtdCho) or PtdCho plus phosphatidylethanolamine (PtdEtn) or phosphatidylinositol (PtdIns). The enzyme bound strongly to LUVs composed of PtdEtn + PtdCho and phosphatidylinositol 4,5-bisphosphate (PtdInsP2). The binding affinity (molar partition coefficient) of PLC-delta3 to PtdEtn + PtdCho + PtdInsP2 vesicles was 7.7 x 105 m-1. High binding of PLC-delta3 was also observed for LUVs composed of phosphatidic acid (PA). Binding of PLC-delta3 to phosphatidylserine (PtdSer) vesicles was less efficient. Calculated molar partition coefficient for binding of PLC-delta3 to PA and PtdSer vesicles was 1.6 x 104 m-1 and 9.4 x 102 m-1, respectively. Presence of PA in the LUVs containing PtdInsP2 considerably enhanced the binding of PLC-delta3 to the phospholipid membrane. Binding of PLC-delta3 to phospholipid vesicles was not dependent on Ca2+ presence. In the liposome assay PA caused a concentration-dependent increase in activity of PLC-delta3. The stimulatory effect of PA on PLC-delta3 was calcium-dependent. At Ca2+ concentrations lower than 1 microm, no effect of PA on the activity of PLC-delta3 was observed. PA enhanced PLC-delta3 activity by increasing the Vmax and lowering Km for PtdInsP2. As the mol fraction of PA increased from 0-40 mol% the enzyme Vmax increased 2.3-fold and Km decreased threefold. Based on the results presented, we assume that PA supports binding of PLC-delta3 to lipid membranes by interaction with the PH domain of the enzyme. The stimulatory effect of PA depends on calcium-dependent interaction with the C2 domain of PLC-delta3. We propose that binding of PLC-delta3 to PA may serve as a mechanism for dynamic membrane association and modulation of PLC-delta3 activity.

Regulation of phospholipase C-delta by GTP-binding proteins-rhoA as an inhibitory modulator

The regulation of Phospholipase C (PLC)delta activity remains obscure. These studies show that PLCdelta1 activity is significantly enhanced by both guanosine thiotriphosphate (GTPgammaS) and Clostridium botulinum exoenzyme C3 (C3) but not by aluminium fluoride. C3 ADP ribosylated a 21-kDa protein in the PLCdelta1 preparation and Western blotting identified rhoA in these samples. RhoA acts as an inhibitory modulator of PLCdelta activity.

Replacements of single basic amino acids in the pleckstrin homology domain of phospholipase C-delta1 alter the ligand binding, phospholipase activity, and interaction with the plasma membrane

The pleckstrin homology (PH) domain of phosphatidylinositol-specific phospholipase C-delta1 (PLC-delta1) binds to both D-myo-inositol 1,4, 5-trisphosphate (Ins(1,4,5)P3) and phosphatidylinositol 4, 5-bisphosphate (PtdIns(4,5)P2) with high affinities. We have previously identified a region rich in basic amino acids within the PH domain critical for ligand binding (Yagisawa, H., Hirata, M., Kanematsu, T., Watanabe, Y., Ozaki, S., Sakuma, K., Tanaka, H., Yabuta, N., Kamata, H., Hirata, H., and Nojima, H. (1994) J. Biol. Chem. 269, 20179-20188; Hirata, M., Kanematsu, T., Sakuma, K., Koga, T., Watanabe, Y., Ozaki, S., and Yagisawa, H. (1994) Biochem. Biophys. Res. Commun. 205, 1563-1571). To investigate the role of these basic residues, we have performed site-directed mutagenesis replacing each of the basic amino acid in the N-terminal 60 residues of PLC-delta1 (Lys24, Lys30, Lys32, Arg37, Arg38, Arg40, Lys43, Lys49, Arg56, Lys57, and Arg60) with a neutral or an acidic amino acid. The effects of these mutations on the PH domain ligand binding properties and their consequence for substrate hydrolysis and membrane interactions of PLC-delta1 were analyzed using several assay systems. Analysis of [3H]-Ins(1,4,5)P3 binding, measurement of the binding affinities, and measurements of phospholipase activity using PtdIns(4,5)P2-containing phospholipid vesicles, demonstrated that residues Lys30, Lys32, Arg37, Arg38, Arg40, and Lys57 were required for these PLC-delta1 functions; in comparison, other mutations resulted in a moderate reduction. A subset of selected mutations was further analyzed for the enzyme activity toward substrate present in cellular membranes of permeabilized cells and for interaction with the plasma membrane after microinjection. These experiments demonstrated that mutations affecting ligand binding and PtdIns(4,5)P2 hydrolysis in phospholipid vesicles also resulted in reduction in the hydrolysis of cellular polyphosphoinositides and loss of membrane attachment. All residues (with the exception of the K43E substitution) found to be critical for the analyzed PLC-delta1 functions are present at the surface of the PH domain shown to contain the Ins(1,4,5)P3 binding pocket.

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.

Effect of sphingomyelin and its metabolites on the activity of human recombinant PLC delta 1

In an attempt to obtain sufficient quantities of pure phospholipase C delta 1 (PLC delta 1) necessary for structural and kinetic studies, human fibroblast PLC delta 1 was cloned in the pPROEX-1 vector, expressed in E. coli cells as a (6xHis) fusion protein and purified to homogeneity. From 11 of E. coli culture 21 mg of pure PLC delta 1 was obtained by a two-step purification procedure, which includes Ni(2+)-NAT agarose and Mono S cation exchange chromatography. Catalytic properties of recombinant PLC delta 1 with respect to activation by spermine and calcium ions and inhibition by sphingomyelin were similar to or identical to PLC delta 1 purified from rat liver. Calcium activation of PLC delta 1 was dependent on the presence of spermine. Half-maximal activity was attained at 250 and 170 nM of free Ca2+ in the presence and absence of spermine, respectively. Sphingomyelin and lysosphingomyelin were mixed type inhibitors with respect to PIP2. Ceramide inhibits PLC delta 1 very weakly. GM1, which is a ceramide bound glucosidically to the oligosaccharide moiety, was a strong non-competitive inhibitor of PLC delta 1. In the absence of spermine, sphingosine and phytosphingosine weakly activated PLC delta 1. The results indicate that the effect of sphingomyelin and its metabolites on PLC delta 1 activity depends on the presence of spermine. It is postulated that, among other factors, in vivo, activity of PLC delta 1 may depend on the turnover of sphingomyelin.

Stimulation of protein synthesis by phosphatidic acid in rat cardiomyocytes

Phosphatidic acid (PA) was observed to stimulate protein synthesis in adult cardiomyocytes in a time- and concentration-dependent manner. The maximal stimulation in protein synthesis (142 +/- 12% vs 100% as the control) was achieved at 10 microM PA within 60 min and was inhibited by actinomycin D (107 +/- 4% of the control) or cycloheximide (105 +/- 6% of the control). The increase in protein synthesis due to PA was attenuated or abolished by preincubation of cardiomyocytes with a tyrosine kinase inhibitor, genistein (94 +/- 9% of the control), phospholipase C inhibitors 2-nitro-4-carboxyphenyl N,N-diphenyl carbamate or carbon-odithioic acid O-(octahydro-4,7-methanol-1H-inden-5-yl (101 +/- 6 and 95 +/- 5% of the control, respectively), protein kinase C inhibitors staurosporine or polymyxin B (109 +/- 3 and 93 +/- 3% of the control), and chelators of extracellular and intracellular free Ca2+ EGTA or BAPTA/AM (103 +/- 6 and 95 +/- 6% of the control, respectively). PA at different concentrations (0.1 to 100 microM) also caused phosphorylation of a cell surface protein of approximately 24 kDa. In addition, mitogen-activated protein kinase was stimulated by PA in a concentration-dependent manner; maximal stimulation (217 +/- 6% of the control) was seen at 10 microM PA. These data suggest that PA increases protein synthesis in adult rat cardiomyocytes and thus may play an important role in the development of cardiac hypertrophy.

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.

Expression, characterization, and crystallization of the catalytic core of rat phosphatidylinositide-specific phospholipase C delta 1

Phosphatidylinositide-specific phospholipase Cs (PI-PLCs) catalyze the calcium-dependent hydrolysis of phosphatidylinositides in response to diverse stimuli in higher eukaryotes. Mammalian PI-PLCs contain divergent regulatory regions, but all share three conserved regions: an N-terminal pleckstrin homology (PH) domain, X, and Y. We report the high-level expression and characterization of a recombinant "catalytic core" of rat PI-PLC delta 1 that contains the catalytically essential X and Y regions, but not the PH domain. The expressed protein, PI-PLC delta delta 1-134, is catalytically active versus phosphatidylinositol 4,5-bisphosphate in deoxycholate micelles with a K(m) of 182 microM and a Vmax of 27 mumol/min/mg. PI-PLC delta delta 1-134 is monomeric and monodisperse as judged by dynamic light scattering. Far-UV CD indicates a structure with approximately 35% alpha-helix. A reversible change in the near-UV CD spectrum is observed on addition of calcium, suggesting that calcium can bind PI-PLC delta delta 1-134 in the absence of phospholipid. Triclinic crystals of PI-PLC delta delta 1-134 have been obtained that diffract beyond 2.4 A resolution under cryogenic conditions. Based on Vm = 2.72 Da/A3 and on the self-rotation function, there are two PI-PLC delta delta 1-134 molecules per asymmetric unit that are related to each other by a noncrystallographic axis of approximate twofold symmetry parallel to a.