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

Hypochlorous acid as a precursor of free radicals in living systems

Hypochlorous acid (HOCl) is produced in the human body by the family of mammalian heme peroxidases, mainly by myeloperoxidase, which is secreted by neutrophils and monocytes at sites of inflammation. This review discusses the reactions that occur between HOCl and the major classes of biologically important molecules (amino acids, proteins, nucleotides, nucleic acids, carbohydrates, lipids, and inorganic substances) to form free radicals. The generation of such free radical intermediates by HOCl and other reactive halogen species is accompanied by the development of halogenative stress, which causes a number of socially important diseases, such as cardiovascular, neurodegenerative, infectious, and other diseases usually associated with inflammatory response and characterized by the appearance of biomarkers of myeloperoxidase and halogenative stress. Investigations aimed at elucidating the mechanisms regulating the activity of enzyme systems that are responsible for the production of reactive halogen species are a crucial step in opening possibilities for control of the development of the body's inflammatory response.

Mechanisms of the beneficial actions of ischemic preconditioning on subcellular remodeling in ischemic-reperfused heart

Cardiac function is compromised by oxidative stress which occurs upon exposing the heart to ischemia reperfusion (I/R) for a prolonged period. The reactive oxygen species (ROS) that are generated during I/R incur extensive damage to the myocardium and result in subcellular organelle remodeling. The cardiac nucleus, glycocalyx, myofilaments, sarcoplasmic reticulum, sarcolemma, and mitochondria are affected by ROS during I/R injury. On the other hand, brief periods of ischemia followed by reperfusion, or ischemic preconditioning (IPC), have been shown to be cardioprotective against oxidative stress by attenuating the cellular damage and alterations of subcellular organelles caused by subsequent I/R injury. Endogenous defense mechanisms, such as antioxidant enzymes and heat shock proteins, are activated by IPC and thus prevent damage caused by oxidative stress. Although these cardioprotective effects of IPC against I/R injury are considered to be a consequence of changes in the redox state of cardiomyocytes, IPC is considered to promote the production of NO which may protect subcellular organelles from the deleterious actions of oxidative stress. The article is intended to focus on the I/R-induced oxidative damage to subcellular organelles and to highlight the cardioprotective effects of IPC. In addition, the actions of various endogenous cardioprotective interventions are discussed to illustrate that changes in the redox state due to IPC are cardioprotective against I/R injury to the heart.

Revisited and revised: is RhoA always a villain in cardiac pathophysiology?

The neonatal rat ventricular myocyte model of hypertrophy has provided tremendous insight with regard to signaling pathways regulating cardiac growth and gene expression. Many mediators thus discovered have been successfully extrapolated to the in vivo setting, as assessed using genetically engineered mice and physiological interventions. Studies in neonatal rat ventricular myocytes demonstrated a role for the small G-protein RhoA and its downstream effector kinase, Rho-associated coiled-coil containing protein kinase (ROCK), in agonist-mediated hypertrophy. Transgenic expression of RhoA in the heart does not phenocopy this response, however, nor does genetic deletion of ROCK prevent hypertrophy. Pharmacologic inhibition of ROCK has effects most consistent with roles for RhoA signaling in the development of heart failure or responses to ischemic damage. Whether signals elicited downstream of RhoA promote cell death or survival and are deleterious or salutary is, however, context and cell-type dependent. The concepts discussed above are reviewed, and the hypothesis that RhoA might protect cardiomyocytes from ischemia and other insults is presented. Novel RhoA targets including phospholipid regulated and regulating enzymes (Akt, PI kinases, phospholipase C, protein kinases C and D) and serum response element-mediated transcriptional responses are considered as possible pathways through which RhoA could affect cardiomyocyte survival.

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.

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.

Oxidant-induced pHi/Ca2+ changes in rat aortic smooth muscle cells. The role of atrial natriuretic peptide

The aim of this study was to investigate the effects of oxidative stress on PLD activity, [Ca2+]i and pHi levels and the possible relationship among them. Moreover, since atrial natriuretic peptide (ANP) protects against oxidant-induced injury, we investigated the potential protective role of the hormone in rat aortic smooth muscle (RASM) cells exposed to oxidative stress. Water-soluble 2,2'-Azobis (2-amidinopropane) dihydrochloride (AAPH) was used as free radical generating system, since it generates peroxyl radicals with defined reaction and the half time of peroxyl radicals is longer than other ROS. A significant increase of PLD activity was related to a significant decrease in pHi, while [Ca2+]i levels showed an increase followed by a decrease after cell exposure to AAPH. [Ca2+]i changes and pHi fall induced by AAPH were prevented by cadmium which inhibits a plasma membrane Ca2+ ATPase coupled to Ca2+/H+ exchanger, that operates the efflux of Ca2+ coupled to H+ influx. The involvement of PLD in pHi and [Ca2+]i changes was confirmed by calphostin-c treatment, a potent inhibitor of PLD, which abolished all AAPH-induced effects. Pretreatment of RASM cells with pharmacological concentrations of ANP attenuated the AAPH effects on PLD activity as well as [Ca2+]i and pHi changes, while no effects were observed with physiological ANP concentrations, suggesting a possible role of the hormone as defensive effector against early events of the oxidative stress.

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.

Oxidative stress and vascular smooth muscle cell function in liver disease

Reactive oxygen species and reactive nitroxy species are now being recognized as regulatory molecules in signaling pathways influencing contractile and noncontractile functions of healthy vascular smooth muscle cells. In liver disease, oxidative stress is a systemic phenomenon, whose extent correlates with the severity of disease. A role for oxidative stress in the development of the hyperdynamic circulation in portal hypertension has been proposed. Evaluation of the limited available data indicates that it is premature to conclude that oxidative stress per se impacts on vascular smooth muscle cell function in liver disease.

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.

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.

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.

Membrane phospholipids and adrenergic receptor function

We have reviewed the effects on adrenergic receptors by membrane phospholipid alterations secondary to oxidative stress and phospholipases' activity. Experimental evidences indicate that the function of both alpha- and beta-adrenoceptors is regulated by their phospholipid microdomain; however, the underlying mechanism is still undefined. No information seems to be available on the influence of phospholipids on alpha 2-adrenoceptors and on all adrenoceptors' subtypes. Thus, further studies are necessary to clarify the role of membrane phospholipids in regulating the function of each member of the adrenergic receptor superfamily.

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.

Oxidative stress modifies the activity of cardiac sarcolemmal phospholipase C

We have examined the direct effects of oxidant metabolites on cardiac sarcolemmal phosphoinositide phospholipase C which transduces signals from various receptors for the modulation of intracellular Ca2+ levels. The enzyme activity in rat cardiac sarcolemmal membranes that had been preincubated (10 min; 37 degrees C) with xanthine-xanthine oxidase, a superoxide anion generating system, was not significantly affected. The addition to this system of superoxide dismutase, which converts superoxide anion to hydrogen peroxide (H2O2), resulted in a significant decrease of the enzyme activity in comparison with control values. Such decrease was fully prevented by catalase. Preincubation of sarcolemma with hypochlorous acid also gave a significant inhibition of phospholipase C, which was counteracted by the synthetic thiol reducer dithiothreitol. H2O2-pretreatment induced a concentration-dependent inhibition of the enzyme which was prevented by catalase but not by the iron chelator deferoxamine. Dithiothreitol was able to protect against, as well as to recover the enzyme activity from the H2O2 effects. These data suggest that superoxide anions and hydroxyl radicals did not interfere with phospholipase C activity, and that the nonradical oxidants, H2O2 and hypochlorous acid, may have acted through oxidation of thiol (SH) groups. The existence of reactive SH groups associated with the enzyme was confirmed by the inhibitory effects of SH modifiers (p-chloromercuriphenylsulfonic acid, 5'5'-dithio-bis(2-nitrobenzoic acid), N-ethylmaleimide and methyl methanethiosulfonate), which were prevented and in some cases also reversed by dithiothreitol. The biological reducer glutathione (GSH) was not able to recover the H2O2-induced inhibition of phospholipase C, whereas its oxidized form (GSSG) decreased the enzyme activity both in control and H2O2-pretreated membranes. The enzyme was active in a wide range of GSH/GSSG redox states, but H2O2 pretreatment narrowed this range. The results showed that oxidative stress changed the redox state of sarcolemmal phospholipase C, and this deactivated the enzyme. The oxidants' concentrations that significantly impaired phospholipase C in this study were compatible with those occurring in vivo during ischemia-reperfusion [Am. J. Med. 91(Suppl. 3C):235, 1991]. This supports the possibility that alteration of the receptor-associated phospholipase C may be a factor in the oxidant-related dysfunction of the ischemic-reperfused heart.

Free radicals and the heart

Because of the molecular configuration, most free radicals are highly reactive and can cause cell injury. Protective mechanisms have evolved to provide defense against free-radical injury. Any time these defense systems are overwhelmed, such as during disease states, cell dysfunction may occur. In this review we discuss cellular sources as well as the significance of free radicals, oxidative stress, and antioxidants. A probable role of oxidative stress in various cardiac pathologies has been also analyzed. Although some methods for the detection of free radicals as well as oxidative stress have been cited, better methods to study the quantity as well as subcellular distribution of free radicals are needed in order to understand fully the role of free radicals in both health and disease.