BBA - Molecular and Cell Biology of Lipids (v.1585, #2-3)

Lipids and apoptosis: foreword by Thierry Levade; Yusuf Hannun; Sarah Spiegel (51).

Transbilayer phospholipid movement and the clearance of apoptotic cells by Patrick Williamson; Robert A Schlegel (53-63).
When lymphocytes (and other cells) die by apoptosis, they orchestrate their own orderly removal by macrophages, and thereby prevent the inflammation that would otherwise attend cell lysis. As part of their demise, apoptotic cells disrupt the normal asymmetric distribution of phospholipids across their plasma membranes, an asymmetry normally maintained by an aminophospholipid translocase. This disruption of asymmetry, mediated by an activity known as the scramblase, generates ligands on the cell surface that trigger phagocytosis of the dying cell before lysis can occur. This crucial alteration of the plasma membrane is not dependent on caspase-mediated proteolysis, but quite unexpectedly, it is required both on the apoptotic target cell and on the phagocyte that engulfs it. At least in the phagocyte, this rearrangement may depend on the activity of an ABC ATPase, termed ABC1 in mammals and ced-7 in C. elegans.
Keywords: Phospholipid; Apoptotic cell; Lymphocyte;

ABCA1 and the engulfment of apoptotic cells by Yannick Hamon; Olivier Chambenoit; Giovanna Chimini (64-71).
Programmed cell death is one of the major devices controlling cellular homeostasis. However, the generation of cell debris that follows the execution phase of apoptosis has to be backed up by their efficient removal by phagocyte. This highly dynamic process requires the concerted action of a number of surface molecules able to recognize early signals of membrane modifications on the apoptotic prey. Among those, the loss of phospholipid asymmetry and exposure of phosphatidylserine on the prey to be is determinant to engage phagocyte receptors and trigger the removal of corpses. A loss of membrane lipid asymmetry occurs also on the phagocyte determining its efficiency as an undertaker. Here we will discuss how, in our mind, the ATP binding cassette transporter, ABCA1, by its action on the arrangement of lipids at the phagocyte membrane, may actively promote their competence to engulf.
Keywords: ABC transporter; Phosphatidylserine; Scavenger receptor; ABCA1; Flippase;

Phospolipase A2 and apoptosis by Makoto Mark Taketo; Masahiro Sonoshita (72-76).
Phospolipase A2 (PLA2) is the esterase activity that cleaves the sn-2 ester bond in glycerophospholipids, releasing free fatty acids and lysophospholipids. The PLA2 activity is found in a variety of enzymes which can be divided in several types based on their Ca2+ dependence for their activity; Ca2+-dependent secretory phosholipases (sPLA2s) and cytosolic phospholipases (cPLA2s), and Ca2+-independent phospholipase A2s (iPLA2s). These enzymes also show diverse size and substrate specificity (i.e., in the fatty acid chain length and extent of saturation). Among the fatty acids released by PLA2, arachidonic acid (AA) is of particular biological importance, because it is subsequently converted to prostanoids and leukotrienes by cyclooxygenases (COX) and lipoxygenases (LOX), respectively. Free AA may also stimulate apoptosis through activation of sphingomyelinase. Alternatively, it is suggested that oxidized metabolites generated from AA by LOX induce apoptosis. Although the precise mechanisms remain to be elucidated, changes are observed in glycerolipid metabolism during apoptotic processes. In some cells induced to undergo apoptosis, AA is released concomitant with loss of cell viability, caspase activation and DNA fragmentation. Such AA releases appear to be mediated by activation of cPLA2 and/or iPLA2. For example, tumor necrosis factor-α (TNF-α)-induced cell death is mediated by cPLA2, whereas Fas-induced apoptosis appears to be mediated by iPLA2. Some discrepancies among early experimental results were probably caused by differences in the experimental conditions such as the serum concentration, inhibitors used that are not necessarily specific to a single-type enzyme, or differential expression of each PLA2 in cells employed in the experiments. Recent studies eliminated such problems, by carefully defining the experimental conditions, and using multiple inhibitors that show different specificities. Accordingly, more convincing data are available that demonstrate involvement of some PLA2s in the apoptotic processes. In addition to cPLA2 and iPLA2, sPLA2s were recently found to play roles in apoptosis. Moreover, new proteins that appear to control PLA2s are being discovered. Here, the roles of PLA2s in apoptosis are discussed by reviewing recent reports.
Keywords: Cytosolic phospholipase A2; Secretory phospholipase A2; Calcium-independent phospholipase A2; Arachidonic acid; Prostanoid;

Accumulating evidence has recognized phospholipase D (PLD) as an important element in signal transduction of cell responses, including proliferation and differentiation, However, its role in pro-apoptotic, anti-apoptotic or pro-survival signaling is not well-understood. Involvement of PLD in these signaling mechanisms is considered to differ depending on the cell type and the extracellular stimulus.
Keywords: Phospholipase D (PLD); Apoptosis; Survival; Lipid signaling;

Phosphatidylcholine and cell death by Zheng Cui; Martin Houweling (87-96).
Phosphatidylcholine (PC) constitutes a major portion of cellular phospholipids and displays unique molecular species in different cell types and tissues. Inhibition of the CDP–choline pathway in most mammalian cells or overexpression of the hepatic phosphatidylethanolamine methylation pathway in hepatocytes leads to perturbation of PC homeostasis, growth arrest or even cell death. Although many agents that perturb PC homeostasis and induce cell death have been identified, the signaling pathways that mediate this cell death have not been well defined. This review summarizes recent progress in understanding the relationship between PC homeostasis and cell death.
Keywords: Cell death; Apoptosis; Phosphatidylcholine; Lipid homeostasis; The CDP–choline pathway; Phosphatidylethanolamine methylation;

Cardiolipin and apoptosis by Jeanie B McMillin; William Dowhan (97-107).
Cardiolipin (CL) is recognized to be an essential phospholipid in eukaryotic energy metabolism so that physiological and pathological perturbations in its synthetic and catabolic pathways play key roles in maintaining mitochondrial structure and function, and ultimately cell survival. This review describes potential regulatory mechanisms in CL synthesis and the effects of de-acylation pathways on steady state levels of CL and its interaction with cytochrome c. The latter interaction is significant in the initiation of programmed cell death. Physiological factors that modify CL acylation include ageing, dietary influences and ischemia/reperfusion where the terminal events may be either necrosis or apoptosis. In various pathologies, phospholipase activity increases in response to production of peroxidized CL. The cell may use lysosomal or mitochondrial pathways for CL degradation. However, the manner by which CL and cytochrome c leave the mitochondria is not well understood. The lipid (CL)-bound form of cytochrome c is thought to initiate apoptosis via a lipid transfer step involving mitochondrially targeted Bid. A direct relationship between CL loss and cytochrome c release from the mitochondria has been identified as an initial step in the pathway to apoptosis. An absolute requirement for CL in the function of crucial mitochondrial proteins, e.g., cytochrome oxidase and the adenine nucleotide translocase, are likely additional factors impacting apoptosis and cellular energy homeostasis. This is reflected in the occurrence of both oncotic and apoptotic events in ischemia and reperfusion injury. Other potential clinical manifestations of perturbations of CL synthesis are discussed with particular emphasis on Barth Syndrome where a primary defect can be attributed to CL metabolism and is associated with dilated cardiomyopathy. Finally, the model of fatty acid induced apoptosis is used as a paradigm to our understanding of the temporal relationship between decreased mitochondrial CL, release of cytochrome c, and initiation of apoptosis.
Keywords: Apoptosis; Cardiolipin; Phosphatidylglycerol; Cytochrome c; Cardiomyopathy; Fatty acid; Membrane permeability pore; Ischemia; Mitochondrion;

Lysophosphatidic acid as a novel cell survival/apoptotic factor by Xiaoqin Ye; Isao Ishii; Marcy A Kingsbury; Jerold Chun (108-113).
Lysophosphatidic acid (LPA) activates its cognate G protein-coupled receptors (GPCRs) LPA1–3 to exert diverse cellular effects, including cell survival and apoptosis. The potent survival effect of LPA on Schwann cells (SCs) is mediated through the pertussis toxin (PTX)-sensitive Gi/o/phosphoinositide 3-kinase (PI3K)/Akt signaling pathways and possibly enhanced by the activation of PTX-insensitive Rho-dependent pathways. LPA promotes survival of many other cell types mainly through PTX-sensitive Gi/o proteins. Paradoxically, LPA also induces apoptosis in certain cells, such as myeloid progenitor cells, hippocampal neurons, and PC12 cells, in which the activation of the Rho-dependent pathways and caspase cascades has been implicated. The effects of LPA on both cell survival and apoptosis underscore important roles for this lipid in normal development and pathological processes.
Keywords: LPA; LPA receptor; Schwann cell; Survival; Apoptosis;

Ceramide in apoptosis: an overview and current perspectives by Benjamin J. Pettus; Charles E. Chalfant; Yusuf A. Hannun (114-125).
Recent years have witnessed significant advances in the understanding of the role of ceramide in apoptosis. This review summarizes these recent findings and discusses insights from studies of ceramide metabolism, topology, and effector actions. The recent identification of several genes for enzymes of ceramide metabolism, the development of mass spectrometric methods for ceramide analysis, and the increasing molecular and pharmacological tools to probe ceramide metabolism and function promise an accelerated phase in defining the molecular and biochemical details of the role of ceramide in apoptosis.
Keywords: Ceramide; Apoptosis; Topology; Metabolism; Analysis; Mass spectrometry; Binding effector; Ceramide-activated protein phosphatase;

Sphingomyelin hydrolysis during apoptosis by Nathalie Andrieu-Abadie; Thierry Levade (126-134).
Sphingolipid breakdown products are now being recognized as important players in apoptosis. Ceramide, which is considered to serve as second messenger, is mainly generated by hydrolysis of the membrane sphingophospholipid sphingomyelin (SM) through the action of a sphingomyelinase (SMase). However, little is known about the localization and regulation of this phenomenon. Here, we summarize the current knowledge on the function of SM hydrolysis in apoptosis signaling. In particular, the present review focuses on the role of neutral sphingomyelinase (N-SMase) in the generation of the proapoptotic ceramide. This enzyme is regulated by several mechanisms, including the tumor necrosis factor (TNF) receptor-associated protein FAN (for factor associated with N-SMase activation) and oxidative stress. These observations place SMase activation and SM hydrolysis as early events in the apoptosis signaling cascade.
Keywords: Sphingomyelin; Sphingomyelinase; Apoptosis; Ceramide;

Premature ovarian failure and infertility are well-known side-effects observed in young girls and reproductive-age women treated for cancer. Although the need for tumor eradication in these patients is clear, the long-term consequences of chemotherapy and radiation on non-target tissues, such as the ovaries where large numbers of germ cells (oocytes) are also killed off, are substantial. Unfortunately, the mechanism mediating the undesirable toxicity of cancer therapies in the female gonads has only recently been explored. Nevertheless, some important insights into the role of ceramide and sphingosine-1-phosphate (S1P) as a mediator and suppressor, respectively, of cancer therapy-induced oocyte apoptosis have emerged over the past few years. Such findings are exciting in that a better understanding of the crime — how radiation and chemotherapy kill off this irreplaceable population of innocent cells in the ovaries — may finally allow for the development of novel lipid-based strategies to combat infertility and premature menopause in female cancer patients.
Keywords: Ceramide; Sphingosine-1-phosphate; Apoptosis; Cell death; Ovary; Oocyte; Fertility;

Ceramide and cell death receptor clustering by Erich Gulbins; Heike Grassmé (139-145).
Acid sphingomyelinase (ASM) has been shown to be activated by a variety of receptor molecules and stimuli including CD95, the tumor necrosis factor receptor (TNF-R), CD40, CD28, LFA-1, CD5, during development, irradiation, heat shock, UV light or bacterial and viral infections. The central role of ASM-released ceramide in the response to those stimuli is confirmed by several genetic studies. ASM and ceramide might mediate their biological effects by the activation of several intracellular signaling molecules including cathepsin D, phospholipase A2 or the kinase suppressor of Ras. In addition, recent fluorescence microscopy studies indicate that distinct, small membrane domains, termed rafts, are modified by ceramide to form larger domains, which serve to cluster receptor molecules. The generation of a high receptor density might be required for initiation of receptor-specific signaling and explain the function of the ASM and ceramide in multiple signaling pathways.
Keywords: Ceramide; Acid sphingomyelinase; Cell death;

Serine palmitoyltransferase is the first and rate-limiting enzyme of sphingolipid synthesis. As such, it is a central control point in the synthesis of bioactivate sphingolipids, and it plays an important role in mediating cellular stress responses. In this review, its role in mediating these responses is discussed within the context of de novo ceramide synthesis. Furthermore, a discussion is provided of its regulation as discerned from both yeast and mammalian studies.
Keywords: Serine palmitoyltransferase; Ceramide; Stress response;

Sphingosine in apoptosis signaling by Olivier Cuvillier (153-162).
The sphingolipid metabolites ceramide, sphingosine, and sphingosine 1-phosphate contribute to controlling cell proliferation and apoptosis. Ceramide and its catabolite sphingosine act as negative regulators of cell proliferation and promote apoptosis. Conversely, sphingosine 1-phosphate, formed by phosphorylation of sphingosine by a sphingosine kinase, has been involved in stimulating cell growth and inhibiting apoptosis. As the phosphorylation of sphingosine diminishes apoptosis, while dephosphorylation of sphingosine 1-phosphate potentiates it, the role of sphingosine as a messenger of apoptosis is of importance. Herein, the effects of sphingosine on diverse signaling pathways implicated in the apoptotic process are reviewed.
Keywords: Sphingosine; Ceramide; Ceramidase; Sphingosine kinase; Sphingosine 1-phosphate; Apoptosis;

Yeast sphingolipids: metabolism and biology by Lina M Obeid; Yasuo Okamoto; Cungui Mao (163-171).
Sphingolipids have recently emerged as important bioactive molecules in addition to being critical structural components of cellular membranes. These molecules have been implicated in regulating cell growth, differentiation, angiogenesis, apoptosis, and senescene. To study sphingolipid mediated biology, it is necessary to investigate sphingolipid metabolism and its regulation. The yeast Saccharomyces cerevisiae has allowed such studies to take place as the sphingolipid metabolic and regulatory pathways appear conserved across species. Using yeast genetic approaches most enzymes of sphingolipid metabolism have been identified and cloned which has led to identification of their mammalian homologues. Many of the yeast enzymes are targets of fungal toxins thus underscoring the importance of this pathway in yeast cell regulation. This review focuses on the yeast sphingolipid metabolic pathway and its role in regulation of yeast biology. Implication of the insights gained from yeast to mammalian cell regulation are discussed.
Keywords: Yeast; Sphingolipid; Cell regulation;

Glucosylceramide synthase and apoptosis by Richard J. Bleicher; Myles C. Cabot (172-178).
Glucosylceramide synthase (GCS) is an enzyme inherent to ceramide metabolism. The enzyme catalyzes the transfer of glucose to ceramide, the first committed step in glycolipid biosynthesis. Known for many years as a branch point enzyme directing synthesis of cerebrosides and gangliosides, GCS has recently been implicated in the cytotoxic response of cancer cells to chemotherapy. With ceramide now occupying a central role in the signaling mechanisms of apoptosis, the position of GCS as sentry is perhaps not unexpected. In particular, it has been recognized that the toxic response of cells to chemotherapy is impaired when GCS activity is elevated and heightened when GCS activity is blocked. Herein we review the control points of ceramide metabolism with special regard to GCS and the cytotoxic response.
Keywords: Glucosylceramide synthase; Glucosylceramide; Apoptosis; Ceramide; Drug resistance;

GD3 ganglioside and apoptosis by Florence Malisan; Roberto Testi (179-187).
Lipid and glycolipid mediators are important messengers of the adaptive responses to stress, including apoptosis. In mammalian cells, the intracellular accumulation of ganglioside GD3, an acidic glycosphingolipid, contributes to mitochondrial damage, a crucial event during the apoptopic program. GD3 is a minor ganglioside in most normal tissues. Its expression increases during development and in pathological conditions such as cancer and neurodegenerative disorders. Intriguingly, GD3 can mediate additional biological events such as cell proliferation and differentiation. These diverse and opposing effects indicate that tightly regulated mechanisms, including 9-O-acetylation, control GD3 function, by affecting intracellular levels, localization and structure of GD3, and eventually dictate biological outcomes and cell fate decisions.
Keywords: GD3; Apoptosis; Mitochondrion; Sialic acid; Compartmentalization; Shedding;

Fumonisins and fumonisin analogs as inhibitors of ceramide synthase and inducers of apoptosis by Kena Desai; M.Cameron Sullards; Jeremy Allegood; Elaine Wang; Eva M Schmelz; Michaela Hartl; Hans-Ulrich Humpf; D.C Liotta; Qiong Peng; Alfred H Merrill (188-192).
Sphingoid bases are growth inhibitory and pro-apoptotic for many types of cells when added to cells exogenously, and can be elevated to toxic amounts endogenously when cells are exposed to inhibitors of ceramide synthase. An important category of naturally occurring inhibitors are the fumonisins, which inhibit ceramide synthase through structural similarities with both the sphingoid base and fatty acyl-CoA co-substrates. Fumonisins cause a wide spectrum of disease (liver and renal toxicity and carcinogenesis, neurotoxicity, induction of pulmonary edema, and others), and most—possibly all—of the pathophysiologic effects of fumonisins are attributable to disruption of the sphingolipid metabolism. The products of alkaline hydrolysis of fumonisins (which occurs during the preparation of masa flour for tortillas) are aminopentols that also inhibit ceramide synthase, but more weakly. Nonetheless, the aminopentols (and other 1-deoxy analogs of sphinganine) are acylated to derivatives that inhibit ceramide synthase, perhaps as product analogs, elevate sphinganine, and kill the cells. Somewhat paradoxically, fumonisins sometimes stimulate growth and inhibit apoptosis, possibly due to elevation of sphinganine 1-phosphate, which is known to have these cellular effects. These findings underscore the complexity of sphingolipid metabolism and the difficulty of identifying the pertinent mediators unless a full profile of the potentially bioactive species is evaluated.
Keywords: Fumonisin; Aminopentol; Ceramide synthase; Sphinganine; Sphinganine 1-phosphate; Toxicity; Apoptosis; Carcinogenesis;

Sphingosine kinase, sphingosine-1-phosphate, and apoptosis by Michael Maceyka; Shawn G Payne; Sheldon Milstien; Sarah Spiegel (193-201).
The sphingolipid metabolites ceramide (Cer), sphingosine (Sph), and sphingosine-1-phosphate (S1P) play an important role in the regulation of cell proliferation, survival, and cell death. Cer and Sph usually inhibit proliferation and promote apoptosis, while the further metabolite S1P stimulates growth and suppresses apoptosis. Because these metabolites are interconvertible, it has been proposed that it is not the absolute amounts of these metabolites but rather their relative levels that determines cell fate. The relevance of this “sphingolipid rheostat” and its role in regulating cell fate has been borne out by work in many labs using many different cell types and experimental manipulations. A central finding of these studies is that Sph kinase (SphK), the enzyme that phosphorylates Sph to form S1P, is a critical regulator of the sphingolipid rheostat, as it not only produces the pro-growth, anti-apoptotic messenger S1P, but also decreases levels of pro-apoptotic Cer and Sph.Given the role of the sphingolipid rheostat in regulating growth and apoptosis, it is not surprising that sphingolipid metabolism is often found to be disregulated in cancer, a disease characterized by enhanced cell growth, diminished cell death, or both. Anticancer therapeutics targeting SphK are potentially clinically relevant. Indeed, inhibition of SphK has been shown to suppress gastric tumor growth [Cancer Res. 51 (1991) 1613] and conversely, overexpression of SphK increases tumorigenicity [Curr. Biol. 10 (2000) 1527]. Moreover, S1P has also been shown to regulate angiogenesis, or new blood vessel formation [Cell 99 (1999) 301], which is critical for tumor progression. Furthermore, there is intriguing new evidence that S1P can act in an autocrine and/or paracrine fashion [Science 291 (2001) 1800] to regulate blood vessel formation [J. Clin. Invest. 106 (2000) 951]. Thus, SphK may not only protect tumors from apoptosis, it may also increase their vascularization, further enhancing growth. The cytoprotective effects of SphK/S1P may also be important for clinical benefit, as S1P has been shown to protect oocytes from radiation-induced cell death in vivo [Nat. Med. 6 (2000) 1109]. Here we review the growing literature on the regulation of SphK and the role of SphK and its product, S1P, in apoptosis.
Keywords: Sphingosine; Sphingosine-1-phosphate; Sphingosine kinase; Ceramide; Apoptosis;

Lipoapoptosis: its mechanism and its diseases by Roger H Unger; Lelio Orci (202-212).
The balance between cell division and cell death determines the cell population of an organ. When cell death exceeds cell replacement in an organ, a functional deficit is created. A metabolic cause of programmed cell death, lipoapoptosis, has recently been identified to occur in obesity and aging. If nonadipose tissues are exposed to an excess of long-chain fatty acids, unless leptin action increases their oxidation sufficiently, unoxidized fatty acids enter nonoxidative pathways. While initially they are sequestered as harmless neutral fat, ultimately some will enter more toxic pathways. One of these, the de novo ceramide pathway, has been implicated in the lipoapoptosis of β-cells and myocardiocytes of congenitally obese rats in which leptin action is defective. Here we review the mechanisms of lipoapoptosis and the diseases that result from this cause of a diminishing cell population of these organs. We suggest that some of the components of the metabolic syndrome of obese humans and the sarcopenia of aging may be result of failure of leptin liporegulation to prevent lipid overload of lean body mass and lipoapoptosis in certain organ systems.
Keywords: Lipoapoptosis; β Cell; Obesity;

Oxidized low-density lipoprotein-induced apoptosis by Robert Salvayre; Nathalie Auge; Herve Benoist; Anne Negre-Salvayre (213-221).
Cultured cells are able to oxidize low-density lipoproteins (LDL) and oxidized LDL (oxLDL), which are present in atherosclerosis areas, exhibit a variety of biological properties potentially involved in atherogenesis.This review is focused on the toxicity of oxLDL, more precisely on the toxic compounds generated during LDL oxidation, the features and the mechanisms of cell death (apoptosis or necrosis) induced by oxLDL. After internalization, toxic oxidized lipids, namely lipid peroxides, oxysterols and aldehydes, induce modifications of cell proteins, elicit oxidative stress, lipid peroxidation and alter various signaling pathways and gene expression. These events may participate in the toxic effect, and converge to trigger an intense, delayed and sustained calcium peak which elicits either apoptosis or necrosis processes. OxLDL-induced apoptosis involves both mitochondrial and death-receptor (Fas/FasL) apoptotic pathways, thereby activating the classical caspase cascade and subsequent biochemical and morphological apoptotic features. When apoptosis is blocked by overexpression of Bcl-2, oxLDL trigger necrosis through a calcium-dependent pathway.Apoptosis occurring in atherosclerotic areas is potentially involved in endothelial cell lining defects, necrotic core formation and plaque rupture or erosion which may trigger atherothrombotic events. However, the precise role of oxLDL in apoptosis/necrosis occurring in vivo in atherosclerotic plaques remains to be clarified.
Keywords: Oxidized LDL; Apoptosis; Necrosis; Atherosclerosis; Signaling;