Myocardial necrosis following a transient ischemic episode represents the most common cause of disability and death in the society today. The progression of localized injury initiated by impaired blood flow exhibits characteristic biochemical changes. Increased phospholipid degradation with unesterified fatty acid accumulation and increased cellular calcium concentrations represent three such alterations, in addition to adenosine triphosphate (ATP) depletion and acidosis. The ultimate endpoint is complete loss of cellular membrane integrity, both functional and structural, in the irreversibly injured cell. Any intervention which can prevent the onset of this succession, or delay its rate of progression, would lead to a significant improvement of the patient's prognosis. This chapter provides an overview of myocardial ischemia. It is critical that the specific biochemical aberrations are clearly determined, which occur in ischemic tissue, only then therapeutic interventions can be developed to reduce the debilitating and lasting effects of ischemia.
Phospholipids play a dual role in cells as both essential structural components of the cell membrane and as important metabolic intermediates and regulatory molecules. The wide diversity of phospholipid species found in both eukaryotic and prokaryotic cells suggests a broader spectrum of functions for phospholipids than are currently appreciated. Because of this pleiotropic requirement for normal cell function, it has often been difficult to assign a specific role to a particular phospholipid or group of phospholipids. In most cell types it is difficult to effect systematic and extensive alteration of the normal phospholipid composition by alterations of the growth conditions. Therefore, many earlier studies relied on in vitro biochemical studies without the necessary in vivo confirmation of physiological significance. Utilization of classical genetic approaches has established the essential role of specific phospholipids for cell viability but have not provided a precise understanding at the molecular level for their requirement (Raetz and Dowhan, 1990).
The CDP-ethanolamine pathway is one of the major routes for the biosynthesis of phosphatidylethanolamine in mammalian cells in vivo. In this Chapter we will discuss current knowledge on the enzymes that catalyze the various steps in this pathway. The properties and subcellular organization of these enzymes will be contrasted with those of the corresponding enzymes in the CDP-choline pathway leading to phosphatidylcholine. Although the regulation of phosphatidylethanolamine biosynthesis can take place at multiple sites current evidence suggests that, under most conditions, the supply of CDP-ethanolamine and diacylglycerol are the principal factors controlling the rate of this process. We suggest that reversible binding of phosphoethanolamine cytidylytransferase to the endoplasmic reticulum could contribute to regulating the amount of CDP-ethanolamine in the cell.
Several investigations on the sphingomyelin (SM) cycle offer an initial and important insight into regulated metabolism of sphingolipids and on possible second messenger/cell regulatory roles for sphingolipid derived molecules as exemplified by ceramide. These investigations have two major implications. First, they illustrate the potential role of regulated sphingolipid metabolism in the diverse processes of signal transduction and cell regulation. Accordingly, the SM cycle may serve as a prototype of sphingolipid signal transduction. Investigations conducted over the past several years have already begun implicating sphingosine, sphingosine phosphate, and sphingosine phosphorylcholine as specific regulators of diverse functions (such as calcium mobilization, thymidine uptake, and cell motility). Therefore, an investigation of sphingolipids in a signaling modality may promise the discovery of novel second messengers and the elucidation of important signal transduction pathways. A second implication of these investigations relates to the specific insight that an investigation of the SM/ceramide pathway is beginning to shed on cellular mechanisms involved in growth suppression and apoptosis. Unlike the study of mitogenesis where intensive investigation has identified multiple oncogenes, oncogene products, signal transduction pathways, and transcription factors involved in the mitogenic response, much less insight has been generated in the research of growth inhibitory mechanisms.
Phospholipids are the essential components of all biological membranes which provide the cell with a protective barrier that has selective permeability for cellular metabolism. Membrane proteins and enzymes that provide essential functions for the cells such as metabolite transport and generation of second messengers are contained within the phospholipid bilayer. Hence, the survival of cells depends on the integrity and proper function of the phospholipid bilayer which is underscored by the general lack of inherited disorders in the metabolism of phospholipids. Changes in phospholipid content and acyl composition are detected in cellular injury and under a variety of disease states. These changes may arise from alteration in the intake of precursors, alteration in cellular metabolism which is associated with the biosynthesis and catabolism of the phospholipids, and alteration in the energy status of the cell. Some changes in the content and acyl composition of the membrane phospholipids may be tolerated without noticeable effects on the overall function of the cell.
The finding that hypoxic/ischemic injury leads to plasma membrane bleb formation provided the first evidence that the plasma membrane may serve as a site of expression of hypoxic/ischemic injury. Subsequent studies have focused on the mechanisms by which hypoxic/ischemic injury leads to plasma membrane damage. Numerous studies have demonstrated an association between alterations in plasma membrane phospholipid metabolism and myocardial, renal and hepatic hypoxic/ischemic injury, documented as a temporal correlation between phospholipid degradation and loss of cell viability. Structural and topographical alterations in plasma membrane phospholipid organization and order during hypoxic/ischemic injury have also been observed. Improved survival of hypoxic/ischemic cells and tissues in the presence of phospholipase inhibitors, anti-phospholipase antibodies and acidic intracellular pH (pH.), has been taken as evidence of the involvement of pH-dependent phospholipases in hypoxic/ischemic injury. The recent discovery that: (1) A calcium-independent phospholipase A2 selectively hydrolyzes the plasmalogen molecular species in ischemic myocardium sarcolemma, (2) Hypoxic/ischemic injury in hepatocytes leads to the expression of a group II 14 kDa phospholipase A2, (3 ) Incubation of hepatocytes with antisense oligonucleotides directed against a group II14 KDa phospholipase A2 protects cells against hypoxic injury, and (4) acidotic pH protects cells against hypoxic injury and inhibits phospholipase A2 activity, suggests that pH-dependent phospholipase A2 activity is a critical regulator of hypoxic/ischemic injury.
Elevated serum levels of fatty acids can be seen in most clinically relevant situations of myocardial ischemia. During an episode of mild to moderate ischemia these high levels of fatty acid have been shown to contribute to contractile dysfunction. Recent evidence suggests that high levels of fatty acids also depress the recovery of contractile function following a transient episode of severe ischemia. The mechanisms by which fatty acid are detrimental during reperfusion of severely ischemic hearts appears to differ from the mechanism by which fatty acids enhance injury during ischemia itself. Evidence from our laboratory suggests that a marked depression of glucose oxidation during reperfusion may be primarily responsible for the detrimental effects of high levels of fatty acids. This is supported by the observation that interventions which overcome fatty acid inhibition of glucose oxidation will significantly improve post-ischemic functional recovery. The exact reason(s) why stimulating glucose oxidation during reperfusion is beneficial has yet to be determined. As will be discussed in this paper, we speculate that stimulation of glucose oxidation improves the coupling between glycolysis and glucose oxidation in the immediate reperfusion period. By doing so, a reduction in H+ production from glucose metabolism will occur during the actual reperfusion period. This will result in a lessening of the myocardial H+ load during the critical period in which the heart is recovering from the acidosis that has occurred during the actual ischemic episode. Inhibition of glucose oxidation by fatty acids may not contribute to injury following a period of mild to moderate ischemia, since the decrease in myocardial pH during ischemia is less dramatic. As a result, the heart is not overwhelmed with a high intracellular H+ load during reperfusion.
PGH synthases catalyze the initial reaction common to the formation of prostanoids and are the target sites for most nonsteroidal anti-inflammatory drugs (NSAIDs). Until the recent discovery of a second PGH synthase isozyme (PGH synthase-2; PGHS-2), it was thought that NSAIDs acted by inhibiting a single enzyme, now referred to as PGH synthase-1 (PGHS-1). It is now believed that PGHS-1 produces prostaglandins that are not directly involved in inflammation, but which regulate “housekeeping” activities; this enzyme is constitutively expressed and, thus, available to respond to rapid transient increases in arachidonate brought about by hormone-induced activation of phospholipases. In contrast, PGHS-2 is the enzyme thought to produce prostaglandins involved in inflammation; this enzyme is not expressed normally, but can be induced in immunoregulatory cells, such as macrophages, and in inflamed tissues, such as fibroblasts and synoviacytes. In addition, anti-inflammatory glucocorticoids and anti-inflammatory cytokines inhibit PGHS-2 induction. It now seems likely that the anti-inflammatory effects of NSAIDs are due primarily to inhibition of PGHS-2. Our current hypothesis is that PGHS-1 and PGHS-2 represent two prostaglandin biosynthetic systems which operate independently; that PGHS-1 and PGHS-2 derive substrate from different arachidonate pools and direct products to different extracellular (PGHS-1) and/or intracellular sites (PGHS-2). These two pathways have evolved to carry out specialized functions; the PGHS-1 pathway to produce prostaglandins that act extracellularly as local hormones to coordinate short-term cellular responses to hormonal stimulation and the PGHS-2 pathway to produce prostaglandins to coordinate prolonged physiological events such as inflammation and mitogenesis.
Considerable interest in plasmalogens has recently been rekindled because of their important linkage to the production of potent bioactive lipid mediators such as platelet-activating factor (PAF) and the various oxygenated metabolites of arachidonic acid. Moreover, much progress has been made towards attaining a better understanding of the metabolism and functional role of plasmalogens in mammalian cells, including the establishment of the direct metabolic relationship between the ethanolamine- and choline-containing plasmalogens. This chapter reviews the early studies that established the enzymatic reactions leading to the synthesis of the ethanolamine plasmalogens by a Δl-alkyl desaturase and discusses more recent results that have shown the O-alk-1-enyl linkage in choline plasmalogens can be derived from the ethanolamine plasmalogens by enzymatic reactions involving the remodeling of substituents at both the sn-2 and/or n-3 positions. Other topics covered include a critical discussion of criteria and methods required for establishing proof of the plasmalogenic structure isolated from biological materials and an updated view of how plasmalogens participate in the trafficking of arachidonate among membrane phospholipids. A number of studies suggest the ethanolamine plasmalogens serve as a repository for arachidonate and other polyunsaturates in that they appear to be the final destination in the movement of polyenoic acids through the choline and ethanolamine-containing subclasses of glycerophospholipids. Ethanolamine-containing lysoplasmalogens have been shown to be involved in the biosynthesis of PAF by serving as acyl acceptors for the arachidonoyl moiety from alkylarachidonoyl-glycerophosphocholine (a membrane precursor of PAF) in a reaction catalyzed by a CoA-independent transacylase. The lyso-PAF intermediate formed by the transacylase can then be acetylated to form PAF by an acetyl-CoA acetyltransferase. The coupled phospholipase A 2 /transacylase pathway also is closely associated with the production of eicosanoid mediators since arachidonic acid and other polyenoic fatty acids are released from the ethanolamine plasmalogens in the initial hydrolytic step that forms the lyso plasmalogen acceptor.
This chapter is designed to bring together two research areas, phospholipase enzymology and lipid biophysics. A true understanding of phospholipases, their function, and their physiologic and pharmacologie regulation requires both approaches. From the technical point of view, it is important for the investigator to match the goal of the phospholipase study with the proper technical approach for substrate preparation. The best technical approach can be chosen with a basic understanding of the physical as well as the chemical properties of the phospholipid. Here a number of studies on the properties of phospholipids will be cited including some new work from our laboratory that is designed to further explain the basis for methods of phospholipase measurement. More detailed information on these subjects can be seen in works by Small (1986), Marsh (1990), Birdi (1989), Davies and Rideal (1963), Lindblom and Rilfors (1989), de Kruijff, et al. (1985), Chachaty (1987), and Tanford (1980).
Clostridium is a very ancient and phylogenetically deep group of Gram-positive, spore-forming bacteria. Although the polar lipids of relatively few species have been studied, they have already revealed important and often novel information concerning structure, biosynthesis, and functions of these major membrane components. The polar lipids of most of the species that have been examined are characterized by relatively high concentrations of l-(l'-alkenyl) 2-acyl glycerophospholipids (plasmalogens). In addition, some species have unique glycerol acetals and phosphatidylglycerol acetals of the major plasmalogens. Physical studies on these ether lipids have provided information on their conformation and phase behavior. These studies have also revealed the presence of regulatory mechanisms in clostridia that control both membrane fluidity and lipid polymorphism.
Platelet-activating factor (PAF) has been shown to play an important role in both fetal lung maturation and in parturition and it has been demonstrated that part of the PAF present in the amniotic fluid is associated with surfactant and, therefore, of lung origin. PAF receptors are present in type II pneumonocytes and the addition of the autacoid promotes both glycogenolysis and increased surfactant secretion from these cells. Moreover, type II pneumonocytes and a human amnion-derived cell line (WISH) have the capability of metabolizing PAF to ethanolamine phospholipids. It has also been established that PAF will cause intracellular Ca2+ mobilization and promote myosin phosphorylation and increased contractility in myometrial strips. Increased PAF biosynthesis in fetal tissues takes place during the latter stages of pregnancy, simultaneously with a decrease in the activity of PAF-acetylhydrolase (PAF-AH) in maternal plasma. The activity of PAF-AH is decreased by the administration of estrogens in vivo and is increased by glucocorticoids and progestins. A number of agents lower the secretion of PAF-AH by decidual macrophages, including 1,25-(OH)2D3, bacterial lipopolysaccharides, and some cytokines (IL1, TNF-α). This inhibitory action may result in higher levels of PAF and increased myometrial contractility as a consequence of infectious processes. PAF causes a profound intestinal necrosis when administered to rats by intravenous injection. Pretreatment with dexamethasone protects against this disease. The presence of PAF-AH in human milk may explain the protective effect of mother's milk against necrotizing enterocolitis.
Protein purification and molecular cloning approaches have led to the identification of a variety of structurally distinct phospholipase A2 (PLA2) enzymes in mammalian cells. PLA2 enzymes, which degrade phospholipid to free fatty acid and lysophospholipid, function in the processes of membrane repair, digestion of dietary lipid, microbial degradation, and the generation of both extracellular and intracellular lipid mediators and second messengers. The immediate products of PLA2 and their metabolites, the eicosanoids and platelet activating factor, have been implicated as mediators in many diverse tissue and cell functions. In the past decade, mammalian PLA2 enzymes have been identified that play important roles in the pathogenesis of a variety of inflammatory diseases and in signal transduction. This includes low molecular weight secreted PLA2 enzymes, that are structurally similar to the PLA2 enzymes in snake venoms, as well as a heterogeneous group of higher molecular weight, cytosolic PLA2 enzymes that exhibit unique substrate specificities. Current evidence suggests that these PLA2 enzymes are differentially regulated, thus providing alternative pathways for the highly controlled process of phospholipid degradation. In this review the properties, function, and the regulation of mammalian nonpancreatic PLA2 enzymes will be described.
Monocytes/macrophages are a major cellular source of arachidonate. This fatty acid is of considerable biological importance since it serves as the cyclooxygenase and lipoxygenase substrate for production of the eicosanoid inflammatory mediators. It can be released from cells via phospholipase A2(PLA2) hydrolysis of membrane phospholipids. This chapter summarizes the current literature regarding the identification, characterization, and regulation of monocyte/macrophage PLA2s. Additionally, it discusses differential metabolism of the released arachidonate as a function of (a) the stimulus for release, (b) the source and activation state of the macrophages, and (c) the regulation of the cyclooxygenase and lipoxygenase metabolic pathways. Finally, the evidence supporting a role for arachidonate as an autocrine mediator of monocyte/macrophage function is presented. The effects of arachidonate depletion on macrophage trafficking are described in the essential fatty acid deficient mouse and are related to in vitro studies demonstrating an arachidonate requirement for monocyte and macrophage adherence. Additionally, the involvement of arachidonate as an intracellular second messenger is discussed in the context of phagocytosis and membrane fusion.
Sphingolipids have been suggested to participate in a wide range of cellular processes that are important to growth and differentiation, by functioning as ligands for proteins of the extracellular matrix and cell-cell communication, modulators of the activity of growth factor receptors, lipid second messengers in the action of cytokines and other factors, and as potent activators and inhibitors of cellular regulatory systems such as protein kinase C (as well as other protein kinases and phosphatases), phospholipase C and D, phosphatidic acid phosphatase, and other proteins. For the most part, these roles have been suggested by studies in which specific sphingolipids (e.g., gangliosides, ceramides, sphingosine, and sphingosine 1-phosphate, among others) have been added to cells or enzyme preparations to elicit a response; therefore, the direct involvement of the compounds as regulators of most of these systems in vivo remains to be proven. Nonetheless, a clearer picture is beginning to emerge for three cases, namely: the regulation of the epidermal growth factor receptor by ganglioside G M3 , the participation of a sphingomyelin/ceramide cycle in the action of tumor necrosis factor and a number of other agonists, and the disruption of free long-chain (sphin-goid) base metabolism as the mechanism of action of fumonisins and related mycotoxins. These provide models for additional studies of the roles of sphingolipids in the regulation of cellular growth, differentiation, neoplastic transformation, and diverse biological functions.
Swiss 3T3 cells in culture have recently been shown to incorporate exogenous,[ 3 H]-glycerol-labeled 2-monoacylglycerol into phosphoglycerides. The pathways that form these phosphoglycerides have yet to be characterized in detail, but a monoacylglycerol kinase and a stearoyl-specific transacylase seem to act successively to initiate one of the major pathways that are involved. This pathway is of special interest because its products appear to include sn -1-stearoyl-2-arachidonoyl phosphatidylinositol and sn -1-stearoyl-2-acyl species of phosphatidylethanolamine and phosphatidylserine. These phosphoglyceride species are key components of animal cell membranes that seem not to be formed by the classical pathways that synthesize phosphoglycerides de novo . Their precise function remains to be determined, but phosphoinositides, phosphatidylethanolamine, and phosphatidylserine are known to be present in the cytoplasmic leaflet of the plasma membrane where they contribute to mechanisms of stimulus transduction and serve as binding sites for many intracellular proteins. Therefore, the sn -1-stearoyl-2-acyl phosphoglycerides that the 2-monoacylglycerol incorporation pathway forms may have an important influence on intracellular events.
Plasmalogens are specialized phospholipids which have a unique conformation, possess distinct molecular dynamics, and serve as the major endogenous phospholipid storage depot for arachidonic acid in many mammalian cells. Recently, several novel intracellular phospholipases A 2 have been identified which selectively hydrolyze plasmalogen substrate. Quantitative analysis of phospholipid molecular species in several cell types demonstrate that plasmalogen molecular species containing arachidonic acid are selectively hydrolyzed during cell stimulation and serve as the major source of arachidonic acid mass released during cellular activation. Collectively, these results underscore the importance of plasmalogen hydrolysis as a major mechanism for the release of eicosanoid metabolites during agonist stimulation. Accordingly, this chapter will review recent insights on the de novo synthesis of plasmalogens, address the molecular mechanisms responsible for the enrichment of arachidonic acid in plasmalogen molecular species and, finally, will focus on the mechanisms which mediate accelerated plasmalogen catabolism and the release of arachidonic acid during cellular activation.
Hydrolysis of phosphoinositides by phospholipase C (PLC) plays an important role in signal transduction by a variety of receptors that are expressed in the adult mammalian heart. Hydrolysis of myocardial phosphoinositides is correlated with changes in inotropic function and electrophysiologic properties of cardiac myocytes as well as altered expression of myocardial genes. There is substantial evidence that metabolism of phosphoinositides is altered in ischemic and reperfused myocardium. Thus, an understanding of phosphoinositide metabolism in cardiac tissue will provide important insights into the physiology of the normal and the diseased heart. Cardiovascular investigators face significant challenges in delineating the biochemical events that regulate PLC activity in cardiac myocytes. To characterize myocytic metabolism of phosphoinositides, it is necessary to separate cardiac myocytes from nonmyocytic cells under conditions that preserve the integrity of the myocyte. There are significant methodological problems associated with the quantification of phosphoinositide-derived second messengers in cardiac myocytes. Concepts for regulation of myocardial PLC must consider the molecular heterogeneity of isoenzymes of PLC that are expressed in the adult mammalian heart. Each of these issues will be discussed in terms of future strategies for elucidating mechanisms for regulation of myocardial PLC.
