Sphingolipid signaling and redox regulation

Limited therapeutic effect of N-acetylcysteine on hepatic insulin resistance in an experimental model of alcohol-induced steatohepatitis

BACKGROUND

Alcohol-related steatohepatitis is associated with increased oxidative stress, DNA damage, lipotoxicity, and insulin resistance in liver. As inflammation and oxidative stress can promote insulin resistance, effective treatment with antioxidants, for example, N-acetylcysteine (NAC), may restore ethanol-impaired insulin signaling in the liver.

METHODS

Adult male Sprague-Dawley rats were fed for 130 days with liquid diets containing 0 or 37% ethanol by caloric content, and simultaneously treated with vehicle or NAC. Chow-fed controls were studied in parallel. Liver tissues were used for histopathology, cytokine activation, and insulin/IGF-1 signaling assays.

RESULTS

We observed significant positive trends of increasing severity of steatohepatitis (p = 0.016) with accumulation of neutral lipid (p = 0.0002) and triglycerides (p = 0.0004) from chow to control, to the ethanol diet, irrespective of NAC treatment. In ethanol-fed rats, NAC reduced inflammation, converted the steatosis from a predominantly microvesicular to a mainly macrovesicular histological pattern, reduced pro-inflammatory cytokine gene expression, ceramide load, and acid sphingomyelinase activity, and increased expression of IGF-1 receptor and IGF-2 in liver. However, NAC did not abrogate ethanol-mediated impairments in signaling through insulin/IGF-1 receptors, IRS-1, Akt, GSK-3β, or p70S6K, nor did it significantly reduce pro-ceramide or GM3 ganglioside gene expression in liver.

CONCLUSIONS

Antioxidant treatments reduce the severity of chronic alcohol-related steatohepatitis, possibly because of the decreased expression of inflammatory mediators and ceramide accumulation, but they do not restore insulin/IGF-1 signaling in liver, most likely due to persistent elevation of GM3 synthase expression. Effective treatment of alcohol-related steatohepatitis most likely requires dual targeting of oxidative stress and insulin/IGF resistance.

Role for Sit4p-dependent mitochondrial dysfunction in mediating the shortened chronological lifespan and oxidative stress sensitivity of Isc1p-deficient cells

Saccharomyces cerevisiae cells lacking Isc1p, an orthologue of mammalian neutral sphingomyelinase 2, display a shortened lifespan and an increased sensitivity to oxidative stress. A lipidomic analysis revealed specific changes in sphingolipids that accompanied the premature ageing of Isc1p-deficient cells under severe calorie restriction conditions, including a decrease of dihydrosphingosine levels and an increase of dihydro-C(26) -ceramide and phyto-C(26) -ceramide levels, the latter raising the possibility of activation of ceramide-dependent protein phosphatases. Consequently, deletion of the SIT4 gene, which encodes for the catalytic subunit of type 2A ceramide-activated protein phosphatase in yeast, abolished the premature ageing and hydrogen peroxide sensitivity of isc1Δ cells. SIT4 deletion also abolished the respiratory defects and catalase A deficiency exhibited by isc1Δ mutants. These results are consistent with catabolic derepression associated with the loss of Sit4p. The overall results show that Isc1p is an upstream regulator of Sit4p and implicate Sit4p activation in mitochondrial dysfunction leading to the shortened chronological lifespan and oxidative stress sensitivity of isc1Δ mutants.

Programmed necrosis, not apoptosis, in the heart

It is well known that apoptosis is an actively mediated cell suicide process. In contrast, necrosis, a morphologically distinct form of cell death, has traditionally been regarded as passive and unregulated. Over the past decade, however, experiments in Caenorhabditis elegans and mammalian cells have revealed that a significant proportion of necrotic death is, in fact, actively mediated by the doomed cell. Although a comprehensive understanding of necrosis is still lacking, some key molecular events have come into focus. Cardiac myocyte apoptosis and necrosis are prominent features of the major cardiac syndromes. Accordingly, the recognition of necrosis as a regulated process mandates a reexamination of cell death in the heart. This review discusses pathways that mediate programmed necrosis, how they intersect with apoptotic pathways, roles of necrosis in heart disease, and new therapeutic opportunities that the regulated nature of necrosis presents.

Cell death signaling and anticancer therapy

For a long time, it was commonly believed that efficient anticancer regimens would either trigger the apoptotic demise of tumor cells or induce a permanent arrest in the G₁ phase of the cell cycle, i.e., senescence. The recent discovery that necrosis can occur in a regulated fashion and the increasingly more precise characterization of the underlying molecular mechanisms have raised great interest, as non-apoptotic pathways might be instrumental to circumvent the resistance of cancer cells to conventional, pro-apoptotic therapeutic regimens. Moreover, it has been shown that some anticancer regimens engage lethal signaling cascades that can ignite multiple oncosuppressive mechanisms, including apoptosis, necrosis, and senescence. Among these signaling pathways is mitotic catastrophe, whose role as a bona fide cell death mechanism has recently been reconsidered. Thus, anticancer regimens get ever more sophisticated, and often distinct strategies are combined to maximize efficacy and minimize side effects. In this review, we will discuss the importance of apoptosis, necrosis, and mitotic catastrophe in the response of tumor cells to the most common clinically employed and experimental anticancer agents.

Acid sphingomyelinase in macrophage biology

Macrophages play a central role in innate immune responses, in disposal of cholesterol, and in tissue homeostasis and remodeling. To perform these vital functions macrophages display high endosomal/lysosomal activities. Recent studies have highlighted that acid sphingomyelinase (ASMase), which generates ceramide from sphingomyelin, is involved in modulation of membrane structures and signal transduction in addition to its metabolic role in the lysosome. In this review, we bring together studies on ASMase, its different forms and locations that are necessary for the macrophage to accomplish its diverse functions. We also address the importance of ASMase to several disease processes that are mediated by activated macrophages.

Probiotic sonicates selectively induce mucosal immune cells apoptosis through ceramide generation via neutral sphingomyelinase

Background

Probiotics appear to be beneficial in inflammatory bowel disease, but their mechanism of action is incompletely understood. We investigated whether probiotic-derived sphingomyelinase mediates this beneficial effect.

Methodology/Principal Findings

Neutral sphingomyelinase (NSMase) activity was measured in sonicates of the probiotic L. brevis (LB) and S. thermophilus (ST) and the non-probiotic E. coli (EC) and E. faecalis (EF). Lamina propria mononuclear cells (LPMC) were obtained from patients with Crohn's disease (CD) and Ulcerative Colitis (UC), and peripheral blood mononuclear cells (PBMC) from healthy volunteers, analysing LPMC and PBMC apoptosis susceptibility, reactive oxygen species (ROS) generation and JNK activation. In some experiments, sonicates were preincubated with GSH or GW4869, a specific NSMase inhibitor. NSMase activity of LB and ST was 10-fold that of EC and EF sonicates. LB and ST sonicates induced significantly more apoptosis of CD and UC than control LPMC, whereas EC and EF sonicates failed to induce apoptosis. Pre-stimulation with anti-CD3/CD28 induced a significant and time-dependent increase in LB-induced apoptosis of LPMC and PBMC. Exposure to LB sonicates resulted in JNK activation and ROS production by LPMC. NSMase activity of LB sonicates was completely abrogated by GW4869, causing a dose-dependent reduction of LB-induced apoptosis. LB and ST selectively induced immune cell apoptosis, an effect dependent on the degree of cell activation and mediated by bacterial NSMase.

Conclusions

These results suggest that induction of immune cell apoptosis is a mechanism of action of some probiotics, and that NSMase-mediated ceramide generation contributes to the therapeutic effects of probiotics.

Role for peroxynitrite in sphingosine-1-phosphate-induced hyperalgesia in rats

Sphingosine-1-phosphate (S1P) is an important mediator of inflammation recently shown in in vitro studies to increase the excitability of small-diameter sensory neurons, at least in part, via activation of the S1P(1) receptor subtype. Activation of S1PR(1) has been reported to increase the formation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-derived superoxide (O(2)(·-)) and nitric oxide synthase (NOS)-derived nitric oxide (NO). This process favors the formation of peroxynitrite (ONOO(-) [PN]), a potent mediator of hyperalgesia associated with peripheral and central sensitization. The aims of our study were to determine whether S1P causes peripheral sensitization and thermal hyperalgesia via S1PR(1) activation and PN formation. Intraplantar injection of S1P in rats led to a time-dependent development of thermal hyperalgesia that was blocked by the S1PR(1) antagonist W146, but not its inactive enantiomer W140. The hyperalgesic effects of S1P were mimicked by intraplantar injection of the well-characterized S1PR(1) agonist SEW2871. The development of S1P-induced hyperalgesia was blocked by apocynin, a NADPH oxidase inhibitor; N(G)-nitro-l-arginine methyl ester, a nonselective NOS inhibitor; and by the potent PN decomposition catalysts (FeTM-4-PyP(5+) and MnTE-2-PyP(5+)). Our findings provide mechanistic insight into the signaling pathways engaged by S1P in the development of hyperalgesia and highlight the contribution of the S1P(1) receptor-to-PN signaling in this process. Sphingosine-1-phosphate (S1P)-induced hyperalgesia is mediated by S1P1 receptor activation and mitigated by inhibition or decomposition of peroxynitrite, providing a target pathway for novel pain management strategies.

The extracellular matrix protein CCN1 dictates TNFα and FasL cytotoxicity in vivo

It has long been appreciated that the apoptotic activity of TNFα is context-dependent, and requires inhibition of NFκB signaling or de novo protein synthesis to be manifested in most normal cells in culture. Recent studies have uncovered an unexpected pro-apoptotic synergism between TNF cytokines and the CCN family of extracellular matrix proteins, which are dynamically expressed at sites of injury repair and inflammation. The presence of CCN1, CCN2, or CCN3 allows TNFα to induce apoptosis with high efficacy without perturbation of NFκB signaling or de novo protein synthesis, thus converting TNFα from a proliferation-promoting protein into an apoptotic inducer. CCN proteins also enhance the cytotoxicity of other TNF family cytokines including LTα, FasL, and TRAIL. CCN proteins synergize with TNF cytokines through binding to integrin α₆β₁ and the heparan sulfate proteoglycan (HSPG) syndecan-4 to induce reactive oxygen species (ROS) accumulation. Knockin mice that express a CCN1 mutant defective for binding α₆β₁-HSPG are severely blunted in TNFα- and Fas-mediated apoptosis, indicating that CCN1 is a physiologic regulator of these processes. Thus, CCN proteins in the extracellular matrix microenvironment can provide the contextual cues for the cytotoxicity of TNFα and related cytokines, and profoundly influence their activity.

Programmed necrosis from molecules to health and disease

During the past decade, cell death researchers have witnessed a gradual but deep conceptual revolution: it has been unequivocally shown that necrosis, which for long had been considered as a purely accidental cell death mode, can also be induced by finely regulated signal transduction pathways. In particular, when caspases are inhibited by pharmacological or genetic means, the ligation of death receptors such as the tumor necrosis factor receptor 1 (TNFR1) can lead to the assembly of a supramolecular complex containing the receptor-interacting protein kinases 1 and 3 (RIP1 and RIP3) that delivers a pronecrotic signal. Such complex has recently been dubbed necrosome and mediates the execution of a specific instance of regulated necrosis, necroptosis. Soon, it turned out that programmed necrosis occurs in nonmammalian model organisms and that it is implicated in human diseases including ischemia and viral infection. In this review, we first describe the historical evolution of the concept of programmed necrosis and the molecular mechanisms that underlie necroptosis initiation and execution. We then provide evidence suggesting that necroptosis represents an ancient and evolutionarily conserved cell death modality that may be targeted for drug development.

The anti-apoptotic action of 5-hydroxyindole: protection of mitochondrial integrity

5-Hydroxyindole (5HI), a metabolite of tryptophan, is involved in learning and memory, central neuron system regulation, and anti-oxidant activity. However, its protective action in mitochondrial function is not clear. Here, we tested whether 5HI protects against tert-butylhydroperoxide (t-BHP)-induced oxidative damage and mitochondrial dysfunction in human fibroblast cells. 5HI significantly suppressed t-BHP-induced cytotoxicity as determined by intracellular reactive species generation, lipid peroxidation, glutathione depletion, and peroxynitrite (ONOO(-)) generation. In addition, 5HI reduced t-BHP-induced DNA condensation. Pretreatment with 5HI significantly restored mitochondrial membrane potential (Deltapsim), suggesting that it protected cells against t-BHP-induced apoptosis. Western blot analysis also revealed that 5HI markedly inhibited cytochrome c release and caspase-3 activation, but not caspase-9 activation. Our data suggest that 5HI protects cells by attenuating oxidative stress and consequently protects against mitochondrial dysfunction.

Molecular mechanisms of necroptosis: an ordered cellular explosion

For a long time, apoptosis was considered the sole form of programmed cell death during development, homeostasis and disease, whereas necrosis was regarded as an unregulated and uncontrollable process. Evidence now reveals that necrosis can also occur in a regulated manner. The initiation of programmed necrosis, 'necroptosis', by death receptors (such as tumour necrosis factor receptor 1) requires the kinase activity of receptor-interacting protein 1 (RIP1; also known as RIPK1) and RIP3 (also known as RIPK3), and its execution involves the active disintegration of mitochondrial, lysosomal and plasma membranes. Necroptosis participates in the pathogenesis of diseases, including ischaemic injury, neurodegeneration and viral infection, thereby representing an attractive target for the avoidance of unwarranted cell death.

Sphingomyelinase stimulates oxidant signaling to weaken skeletal muscle and promote fatigue

Sphingomyelinase (SMase) hydrolyzes membrane sphingomyelin into ceramide, which increases oxidants in nonmuscle cells. Serum SMase activity is elevated in sepsis and heart failure, conditions where muscle oxidants are increased, maximal muscle force is diminished, and fatigue is accelerated. We tested the hypotheses that exogenous SMase and accumulation of ceramide in muscle increases oxidants in muscle cells, depresses specific force of unfatigued muscle, and accelerates the fatigue process. We also anticipated that the antioxidant N-acetylcysteine (NAC) would prevent SMase effects on muscle function. We studied the responses of C2C12 myotubes and mouse diaphragm to SMase treatment in vitro. We observed that SMase caused a 2.8-fold increase in total ceramide levels in myotubes. Exogenous ceramide and SMase elevated oxidant activity in C2C12 myotubes by 15-35% (P < 0.05) and in diaphragm muscle fiber bundles by 58-120% (P < 0.05). The SMase-induced increase in diaphragm oxidant activity was prevented by NAC. Exogenous ceramide depressed diaphragm force by 55% (P < 0.05), while SMase depressed maximal force by 30% (P < 0.05) and accelerated fatigue--effects opposed by treatment with NAC. In conclusion, our findings suggest that SMase stimulates a ceramide-oxidant signaling pathway that results in muscle weakness and fatigue.

Tumor necrosis factor-mediated cell death: to break or to burst, that's the question

In this review, we discuss the signal-transduction pathways of three major cellular responses induced by tumor necrosis factor (TNF): cell survival through NF-kappaB activation, apoptosis, and necrosis. Recruitment and activation of caspases plays a crucial role in the initiation and execution of TNF-induced apoptosis. However, experimental inhibition of caspases reveals an alternative cell death pathway, namely necrosis, also called necroptosis, suggesting that caspases actively suppress the latter outcome. TNF-induced necrotic cell death crucially depends on the kinase activity of receptor interacting protein serine-threonine kinase 1 (RIP1) and RIP3. It was recently demonstrated that ubiquitination of RIP1 determines whether it will function as a pro-survival or pro-cell death molecule. Deeper insight into the mechanisms that control the molecular switches between cell survival and cell death will help us to understand why TNF can exert so many different biological functions in the etiology and pathogenesis of human diseases.

The role of the kinases RIP1 and RIP3 in TNF-induced necrosis

Tumor necrosis factor (TNF) is a pleiotropic molecule with a crucial role in cellular stress and inflammation during infection, tissue damage, and cancer. TNF signaling can lead to three distinct outcomes, each of which is initiated by different signaling complexes: the gene induction or survival mode, the apoptosis mode, and the necrosis mode. The kinases receptor-interacting protein 1 (RIP1) and RIP3 are key signaling molecules in necrosis and are regulated by caspases and ubiquitination. Moreover, TNF stimulation induces the formation of a necrosome in which RIP3 is activated and interacts with enzymes that control glycolytic flux and glutaminolysis. The necrosome induces mitochondrial complex I-mediated production of reactive oxygen species (ROS) and cytotoxicity, which suggest a functional link between increased bioenergetics and necrosis. In addition, other effector mechanisms also contribute to TNF-induced necrosis, such as recruitment of NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate) oxidases and subsequent ROS production at the membrane-associated TNF receptor complex I; calcium mobilization; activation of phospholipase A(2), lipoxygenases, and acid sphingomyelinases; and lysosomal destabilization. However, the link between RIP1 and RIP3 and these subcellular events remains to be established. The regulation of RIP1 and RIP3 and their downstream signaling cascades opens new therapeutic avenues for treatment of pathologies associated with cell loss, such as ischemia-reperfusion damage and neurodegeneration, and ways to stimulate alternative immunogenic cell death pathways in cancer.

Therapeutic strategies for diabetes and complications: a role for sphingolipids?

Diabetes is a debilitating chronic disease that has no cure and can only be managed by pharmaceutical or nutritional interventions. Worldwide, the incidence of diabetes and diabetic complications is dramatically increasing. This may reflect the incomplete knowledge base underlying the role of inflammatory or nutritional stresses to exacerbate diabetic complications. Despite the knowledge that hyperlipidemia is a cardinal feature of both Type 1 and 2 diabetes, the actual lipid species that contribute to complications such as diabetic nephropathy, retinopathy, neuropathy and cardiovascular disease have not been well defined, or have not elucidated new treatment strategies. Sphingolipids comprise only a fraction of total lipids but a body of evidence has now identified dysfunctional sphingolipid metabolism and/or generation of specific sphingolipid metabolites as contributors to diabetic complications. This review suggests that pharmacological therapies that target dysfunctional sphingolipid metabolism and/or signaling may prove beneficial in decreasing the chronic pathology of hyperglycemia and hyperlipidemia. Moreover, the review suggests that these treatment options may also prove beneficial to ameliorate or delay pancreatic beta cell failure.

Cancer treatment strategies targeting sphingolipid metabolism

Ceramide and sphingosine-1-phosphate are related sphingolipid metabolites that can be generated through a de novo biosynthetic route or derived from the recycling of membrane sphingomyelin. Both these lipids regulate cellular responses to stress, with generally opposing effects. Sphingosine-1-phosphate functions as a growth and survival factor, acting as a ligand for a family of G protein-coupled receptors, whereas ceramide activates intrinsic and extrinsic apoptotic pathways through receptor-independent mechanisms. A growing body of evidence has implicated ceramide, sphingosine-1-phosphate and the genes involved in their synthesis, catabolism and signaling in various aspects of oncogenesis, cancer progression and drug- and radiation resistance. This may be explained in part by the finding that both lipids impinge upon the PI3K/ AKT pathway, which represses apoptosis and autophagy. In addition, sphingolipids influence cell cycle progression, telomerase function, cell migration and stem cell biology. Considering the central role of ceramide in mediating physiological as well as pharmacologically stimulated apoptosis, ceramide can be considered a tumor-suppressor lipid. In contrast, sphingosine-1-phosphate can be considered a tumor-promoting lipid, and the enzyme responsible for its synthesis functions as an oncogene. Not surprisingly, genetic mutations that result in reduced ceramide generation, increased sphingosine-1-phosphate synthesis or which reduce steady state ceramide levels and increase sphingosine-1-phosphate levels have been identified as mechanisms of tumor progression and drug resistance in cancer cells. Pharmacological tools for modulating sphingolipid pathways are being developed and represent novel therapeutic strategies for the treatment of cancer.

Deadly liaisons: fatal attraction between CCN matricellular proteins and the tumor necrosis factor family of cytokines

Recent studies have revealed an unexpected synergism between two seemingly unrelated protein families: CCN matricellular proteins and the tumor necrosis factor (TNF) family of cytokines. CCN proteins are dynamically expressed at sites of injury repair and inflammation, where TNF cytokines are also expressed. Although TNFα is an apoptotic inducer in some cancer cells, it activates NFκB to promote survival and proliferation in normal cells, and its cytotoxicity requires inhibition of de novo protein synthesis or NFκB signaling. The presence of CCN1, CCN2, or CCN3 overrides this requirement and unmasks the apoptotic potential of TNFα, thus converting TNFα from a proliferation-promoting protein into an apoptotic inducer. These CCN proteins also enhance the cytotoxicity of other TNF cytokines, including LTα, FasL, and TRAIL. Mechanistically, CCNs function through integrin α₆β₁ and the heparan sulfate proteoglycan (HSPG) syndecan-4 to induce reactive oxygen species (ROS) accumulation, which is essential for apoptotic synergism. Mutant CCN1 proteins defective for binding α₆β₁-HSPGs are unable to induce ROS or apoptotic synergism with TNF cytokines. Further, knockin mice that express an α₆β₁-HSPG-binding defective CCN1 are blunted in TNFα- and Fas-mediated apoptosis, indicating that CCN1 is a physiologic regulator of these processes. These findings implicate CCN proteins as contextual regulators of the inflammatory response by dictating or enhancing the cytotoxicity of TNFα and related cytokines.

The role of ceramide in major depressive disorder

Major depression is a severe mood disorder with a lifetime prevalence of more than 10%. The pharmacokinetic hypothesis claims that a slow accumulation of antidepressant drugs by acid trapping mainly into lysosomes is responsible for the therapeutic latency and that a lysosomal target mediates the antidepressant effects. The lysosomal lipid metabolizing enzyme acid sphingomyelinase (ASM) cleaves sphingomyelin into ceramide and phosphorylcholine. In a pilot study, the activity of this enzyme was increased in peripheral blood cells of patients with major depressive disorder (MDD), making the ASM an interesting molecular target of antidepressant drugs. Indeed, several antidepressant drugs functionally inhibit ASM. The ASM/ceramide pathway might be a missing link unifying independent findings in neurobiology and the treatment of MDD such as therapeutic latency, oxidative stress, immune activation and increased risk of cardiovascular disease.

Increased acid sphingomyelinase activity in peripheral blood cells of acutely intoxicated patients with alcohol dependence

BACKGROUND

Acid sphingomyelinase (ASM; EC 3.1.4.12) hydrolyses membrane sphingomyelin into the bioactive lipid ceramide and is thus involved in different cellular processes such as differentiation, immunity, or cell death. Activation of ASM has been reported in particular in conjunction with the cellular stress response to several external stimuli, and increased ASM activity was observed in a variety of human diseases. Ethanol-induced activation of ASM has been observed in different cell culture systems, thus raising the question about the effect of alcohol intoxication in human subjects on ASM activity in vivo.

METHODS

We determined ASM activity in peripheral blood mononucleated cells of 27 patients suffering from alcohol dependence. Patients were classified according to their blood alcohol concentration at admission, and ASM activity was determined repeatedly from all patients during alcohol withdrawal.

RESULTS

Acutely intoxicated patients displayed significantly higher ASM activity than patients in early abstinence (Mann-Whitney U test: Z = - 2.6, p = 0.009). ASM activity declined in acutely intoxicated patients to normal values with the transition from the intoxicated state to early abstinence (Wilcoxon test: Z = -2.7, p = 0.007). At the end of withdrawal, ASM activity was significantly increased again compared to the early phase of abstinence in both patient groups (Wilcoxon test: Z = -2.691, p = 0.007 and Z = -2.275, p = 0.023, respectively).

CONCLUSIONS

Alcohol-induced activation of ASM occurs in human subjects and might be responsible for deleterious effects of ethanol intoxication. Chronic alcohol abuse may induce deregulation of sphingomyelin metabolism in general, and this impairment may cause side effects during withdrawal from alcohol.

Doxorubicin induces ceramide and diacylglycerol accumulation in rat hepatocytes through independent routes

Doxorubicin (DOX) is a potent anticancer drug, whose clinical use is limited due to its toxicity. This toxicity has been associated with free radicals generated during the drug metabolism. We previously found that DOX increased the intracellular diacylglycerol (DAG) levels at 1h in isolated rat hepatocytes, probably by mobilizing choline-enriched phospholipids. In this work, we studied the effects of DOX on oxidative stress markers, and the possible contribution of ceramide metabolism to DAG accumulation. Other possible routes of DAG production, such as impairment of triacylglycerol (TAG) synthesis, and their connection with oxidative stress were also investigated. Time-course experiments revealed that DOX decreased intracellular GSH at 2h, but did not affect cell viability, ATP or malondialdehyde (MDA) levels at any time. DOX did not modify the intracellular levels of [(3)H]-ceramide during the first 90 min of exposure, but increased it significantly at 2h. [(3)H]-Sphingomyelin remained unchanged during the whole period. These results indicate that ceramide metabolism is not involved in the early DAG response to DOX. The drug markedly increased the incorporation of [(3)H]-oleate into intracellular DAG from 60 min. In contrast, DOX reduced the incorporation of [(3)H]-oleate into intracellular phospholipids and TAG. DOX inhibited TAG synthesis at the DAG acyltransferase step. These results suggest that DOX increases the intracellular levels of the lipid messengers, ceramide and DAG, by independent mechanisms. Activation of the de novo synthesis of ceramide is probably involved in the sphingolipid accumulation, while inhibition of TAG synthesis contributes to DAG accumulation, this response being independent of oxidative damage.

Ceramide and mitochondria in ischemia/reperfusion

A hallmark of tissue injury in various models of ischemia/reperfusion (IR) is mitochondrial dysfunction and the release of mitochondrial proapoptotic proteins leading to cell death. Although IR-induced mitochondrial injury has been extensively studied and key mitochondrial functions affected by IR are chiefly characterized, the nature of the molecule that causes loss of mitochondrial integrity and function remains obscure. It has become increasingly clear that ceramide, a membrane sphingolipid and a key mediator of cell stress responses, could play a critical role in IR-induced mitochondrial damage. Emerging data point to excessive ceramide accumulation in tissue and, specifically, in mitochondria after IR. Exogenously added to isolated mitochondria, ceramide could mimic some of the mitochondrial dysfunctions occurring in IR. The recent identification and characterization of major enzymes in ceramide synthesis is expected to contribute to the understanding of molecular mechanisms of ceramide involvement in mitochondrial damage in IR. This review will examine the experimental evidence supporting the important role of ceramide in mitochondrial dysfunction in IR to highlight potential targets for pharmacological manipulation of ceramide levels.

Glutathione dysregulation and the etiology and progression of human diseases

Glutathione (GSH) plays an important role in a multitude of cellular processes, including cell differentiation, proliferation, and apoptosis, and as a result, disturbances in GSH homeostasis are implicated in the etiology and/or progression of a number of human diseases, including cancer, diseases of aging, cystic fibrosis, and cardiovascular, inflammatory, immune, metabolic, and neurodegenerative diseases. Owing to the pleiotropic effects of GSH on cell functions, it has been quite difficult to define the role of GSH in the onset and/or the expression of human diseases, although significant progress is being made. GSH levels, turnover rates, and/or oxidation state can be compromised by inherited or acquired defects in the enzymes, transporters, signaling molecules, or transcription factors that are involved in its homeostasis, or from exposure to reactive chemicals or metabolic intermediates. GSH deficiency or a decrease in the GSH/glutathione disulfide ratio manifests itself largely through an increased susceptibility to oxidative stress, and the resulting damage is thought to be involved in diseases, such as cancer, Parkinson's disease, and Alzheimer's disease. In addition, imbalances in GSH levels affect immune system function, and are thought to play a role in the aging process. Just as low intracellular GSH levels decrease cellular antioxidant capacity, elevated GSH levels generally increase antioxidant capacity and resistance to oxidative stress, and this is observed in many cancer cells. The higher GSH levels in some tumor cells are also typically associated with higher levels of GSH-related enzymes and transporters. Although neither the mechanism nor the implications of these changes are well defined, the high GSH content makes cancer cells chemoresistant, which is a major factor that limits drug treatment. The present report highlights and integrates the growing connections between imbalances in GSH homeostasis and a multitude of human diseases.

Fas-mediated apoptosis is regulated by the extracellular matrix protein CCN1 (CYR61) in vitro and in vivo

Although Fas ligand (FasL) is primarily expressed by lymphoid cells, its receptor Fas (CD95/Apo-1) is broadly expressed in numerous nonlymphoid tissues and can mediate apoptosis of parenchymal cells upon injury and infiltration of inflammatory cells. Here we show that CCN1 (CYR61) and CCN2 (CTGF), matricellular proteins upregulated at sites of inflammation and wound repair, synergize with FasL to induce apoptosis by elevating cellular levels of reactive oxygen species (ROS). CCN1 acts through engagement of integrin alpha(6)beta(1) and cell surface heparan sulfate proteoglycans, leading to ROS-dependent hyperactivation of p38 mitogen-activated protein kinase in the presence of FasL to enhance mitochondrial cytochrome c release. We show that CCN1 activates neutral sphingomyelinase, which functions as a key source of CCN1-induced ROS critical for synergism with FasL. Furthermore, Fas-dependent hepatic apoptosis induced by an agonistic monoclonal anti-Fas antibody or intragastric administration of alcohol is severely blunted in knock-in mice expressing an apoptosis-defective Ccn1 allele. These results demonstrate that CCN1 is a physiologic regulator of Fas-mediated apoptosis and that the extracellular matrix microenvironment can modulate Fas-dependent apoptosis through CCN1 expression.

Reactive nitrogen species: molecular mechanisms and potential significance in health and disease

Reactive nitrogen species (RNS) are various nitric oxide-derived compounds, including nitroxyl anion, nitrosonium cation, higher oxides of nitrogen, S-nitrosothiols, and dinitrosyl iron complexes. RNS have been recognized as playing a crucial role in the physiologic regulation of many, if not all, living cells, such as smooth muscle cells, cardiomyocytes, platelets, and nervous and juxtaglomerular cells. They possess pleiotropic properties on cellular targets after both posttranslational modifications and interactions with reactive oxygen species. Elevated levels of RNS have been implicated in cell injury and death by inducing nitrosative stress. The aim of this comprehensive review is to address the mechanisms of formation and removal of RNS, highlighting their potential cellular targets: lipids, DNA, and proteins. The specific importance of RNS and their paradoxic effects, depending on their local concentration under physiologic conditions, is underscored. An increasing number of compounds that modulate RNS processing or targets are being identified. Such compounds are now undergoing preclinical and clinical evaluations in the treatment of pathologies associated with RNS-induced cellular damage. Future research should help to elucidate the involvement of RNS in the therapeutic effect of drugs used to treat neurodegenerative, cardiovascular, metabolic, and inflammatory diseases and cancer.

Oxidative impairment of hippocampal long-term potentiation involves activation of protein phosphatase 2A and is prevented by ketone bodies

Previous studies have shown that ketone bodies (KB) exert antioxidant effects in experimental models of neurological disease. In the present study, we explored the effects of the KB acetoacetate (ACA) and beta-hydroxybutyrate (BHB) on impairment of hippocampal long-term potentiation (LTP) in rats by hydrogen peroxide (H(2)O(2)) using electrophysiological, fluorescence imaging, and enzyme assay techniques. We found that: 1) a combination of ACA and BHB (1 mM each) prevented impairment of LTP by H(2)O(2) (200 microM); 2) KB significantly lowered intracellular levels of reactive oxygen species (ROS)--measured with the fluorescent indicator carboxy-H(2)DCFDA (carboxy-2',7'-dichlorodihydrofluorescein diacetate)--in CA1 pyramidal neurons exposed to H(2)O(2); 3) the effect of KB on LTP was replicated by the protein phosphatase 2A (PP2A) inhibitor fostriecin; 4) KB prevented impairment of LTP by the PP2A activator C(6) ceramide; 5) fostriecin did not prevent the increase in ROS levels in CA1 pyramidal neurons exposed to H(2)O(2), and C(6) ceramide did not increase ROS levels; 6) PP2A activity was enhanced by both H(2)O(2) and rotenone (a mitochondrial complex I inhibitor that increases endogenous superoxide production); and 7) KB inhibited PP2A activity in protein extracts from brain tissue treated with either H(2)O(2) or ceramide. We propose that oxidative impairment of hippocampal LTP is associated with PP2A activation, and that KB prevent this impairment in part by inducing PP2A inhibition through an antioxidant mechanism.

Propofol pretreatment reduces ceramide production and attenuates intestinal mucosal apoptosis induced by intestinal ischemia/reperfusion in rats

BACKGROUND

Apoptosis has been shown to be a major mode of intestinal epithelial cell death caused by intestinal ischemia/reperfusion (II/R), a condition that is associated with increased oxidative stress. Ceramide has been proposed as a messenger of apoptosis. We investigated if pretreatment with propofol, an anesthetic with antioxidant properties, could reduce ceramide production, and consequently, mucosal epithelial apoptosis induced by II/R in rats.

METHODS

Rat II/R injury was produced by clamping the superior mesenteric artery for 1 h followed by 3 h of reperfusion. Thirty rats were randomly allocated into control, injury (II/R) and propofol (pretreatment) groups (n = 10 per group). In the propofol group, propofol 50 mg/kg, a dose that has been shown to cause the loss of reflex responses to a painful stimulus while remaining sensitive to skin incision in rats, was administered intraperitoneally 30 min before inducing intestinal ischemia, while animals in control and untreated injury groups received an equal volume of intralipid. Intestinal mucosal epithelial apoptosis was detected via electron microscopy and TUNEL analysis. Lipid oxidation product malondialdehyde and the activities of superoxide dismutase were assessed by colorimetric analyses. Ceramide generation and sphingomyelinase mRNA expression in intestinal mucosa were determined by high performance thin layer chromatography and reverse transcriptase polymerase chain reaction, respectively.

RESULTS

II/R caused intestinal mucosal epithelial apoptosis and over-production of ceramide accompanied by up-regulation of sphingomyelinase mRNA expression and increases in lipid oxidation (all P < 0.01 versus control). Propofol pretreatment significantly attenuated these changes (all P < 0.01, propofol versus injury).

CONCLUSION

The findings indicate that propofol pretreatment attenuates II/R-induced intestinal epithelial apoptosis, which might be attributable to its antioxidant property modulating the ceramide pathway.

The neuroprotective properties of calorie restriction, the ketogenic diet, and ketone bodies

Both calorie restriction and the ketogenic diet possess broad therapeutic potential in various clinical settings and in various animal models of neurological disease. Following calorie restriction or consumption of a ketogenic diet, there is notable improvement in mitochondrial function, a decrease in the expression of apoptotic and inflammatory mediators and an increase in the activity of neurotrophic factors. However, despite these intriguing observations, it is not yet clear which of these mechanisms account for the observed neuroprotective effects. Furthermore, limited compliance and concern for adverse effects hamper efforts at broader clinical application. Recent research aimed at identifying compounds that can reproduce, at least partially, the neuroprotective effects of the diets with less demanding changes to food intake suggests that ketone bodies might represent an appropriate candidate. Ketone bodies protect neurons against multiple types of neuronal injury and are associated with mitochondrial effects similar to those described during calorie restriction or ketogenic diet treatment. The present review summarizes the neuroprotective effects of calorie restriction, of the ketogenic diet and of ketone bodies, and compares their putative mechanisms of action.

Fatty acids as modulators of the cellular production of reactive oxygen species

Long-chain nonesterified ("free") fatty acids (FFA) and some of their derivatives and metabolites can modify intracellular production of reactive oxygen species (ROS), in particular O(2)(-) and H(2)O(2). In mitochondria, FFA exert a dual effect on ROS production. Because of slowing down the rate of electron flow through Complexes I and III of the respiratory chain due to interaction within the complex subunit structure, and between Complexes III and IV due to release of cytochrome c from the inner membrane, FFA increase the rate of ROS generation in the forward mode of electron transport. On the other hand, due to their protonophoric action on the inner mitochondrial membrane ("mild uncoupling effect"), FFA strongly decrease ROS generation in the reverse mode of electron transport. In the plasma membrane of phagocytic neutrophils and a number of other types of cells, polyunsaturated FFA stimulate O(2)(-) generation by NADPH oxidase. These effects of FFA can modulate signaling functions of ROS and be, at least partly, responsible for their proapoptotic effects in several types of cells.

Ceramide elevates 12-hydroxyeicosatetraenoic acid levels and upregulates 12-lipoxygenase in rat primary hippocampal cell cultures containing predominantly astrocytes

We report, exogenous addition of ceramide significantly increases 12-hydroxyeicosatetraenoic acid [12-(S)-HETE] levels, in a dose-dependent manner. 12-(S)-HETE levels, in 20, 30 and 40microM ceramide exposed rat primary hippocampal cell cultures containing predominantly astrocytes and few neurons and other glial cells (the cultured hippocampal cells were predominantly astrocytes amounting to over 99% of total cells with few neurons and other glial cells) amounted to 207, 260 and 408% of the controls, respectively. However, dihydroceramide, an inactive analog of ceramide did not alter the levels of 12-(S)-HETE. Ceramide also increased the mRNA and protein expression, and activity of 12-lipoxygease (12-LOX) needed for the synthesis of 12(S)-HETE. These results indicate a possible link between ceramide and 12-LOX pathway. However, ceramide did not alter expression of 5-lipoxygenase (5-LOX), another member of the lipoxygenase family. However, ceramide upregulated expression of cytosolic phospholipase-A(2) (cPLA(2)) and cyclooxygenase-2 (COX-2). Further, ceramide caused a significant increase in the levels of reactive oxygen species (ROS). Ceramide-mediated generation of ROS was inhibited by baicalien but not by indomethacin. In addition, ceramide treated cells exhibited increased mRNA expression of DNA damage induced transcript3 (Ddit3). This report which demonstrate induction of pro-carcinogenic 12-LOX pathway by an anticancer ceramide, may be relevant to cancer biologists studying drug resistant tumors and devising potent anticancer therapeutic strategies to treat drug resistant tumors. These results indicate possibility of 12-LOX involvement in ceramide-mediated generation of ROS and cellular oxidative stress. Induction of 12-LOX pathway by ceramide may have implications in understanding pathophysiology of neurodegenerative diseases involving ROS generation and inflammation.

Effect of probiotic Lactobacillus rhamnosus GG intervention on global serum lipidomic profiles in healthy adults

AIM: To investigate the effect of three weeks’ intervention with a probiotic Lactobacillus rhamnosus GG (LGG) bacteria on global serum lipidomic profiles and evaluate whether the changes in inflammatory variables (CRP, TNF-α and IL-6) are reflected in the global lipidomic profiles of healthy adults.

METHODS: We performed UPLC/MS-based global lipidomic platform analysis of serum samples (n = 26) in a substudy of a randomised, double-blind, placebo-controlled 3-wk clinical intervention trial investigating the immunomodulatory effects of probiotics in healthy adults.

RESULTS: A total of 407 lipids were identified, corresponding to 13 different lipid classes. Serum samples showed decreases in the levels of lysophosphatidylcholines (LysoGPCho), sphingomyelins (SM) and several glycerophosphatidylcholines (GPCho), while triacylglycerols (TAG) were mainly increased in the probiotic LGG group during the intervention. Among the inflammatory variables, IL-6 was moderately associated by changes in global lipidomic profiles, with the top-ranked lipid associated with IL-6 being the proinflammatory LysoGPCho (20:4). There was a weak association between the lipidomic profiles and the two other inflammatory markers, TNF-α and CRP.

CONCLUSION: This was the first study to investigate the effects of probiotic intervention on global lipidomic profiles in humans. There are indications that probiotic LGG intervention may lead to changes in serum global lipid profiles, as reflected in decreased GPCho, LysoGPCho and SM as well as mainly increased TAG.

Ceramide induces p38 MAPK and JNK activation through a mechanism involving a thioredoxin-interacting protein-mediated pathway

Ceramide, a tumor-suppressor lipid, is generated by sphingomyelin hydrolysis or by de novo synthesis when cells are activated by various stress stimuli as well as when cancer cells are subjected to genotoxic chemotherapy. Ceramide may modulate apoptotic signaling pathways; however, its transcription-dependent effects remain unclear. Our data showed that actinomycin D partially inhibited ceramide-induced apoptosis. Using microarray analysis, we found that ceramide up-regulated a tumor suppressor gene called thioredoxin-interacting protein (Txnip). Similarly, the chemotherapeutic agent etoposide induced Txnip expression en route to apoptosis, which was blocked by inhibitors of ceramide production. Txnip colocalized with thioredoxin and reduced its activity, which caused dissociation of thioredoxin from apoptosis signal-regulating kinase 1 (ASK1). Cells expressing ASK1 siRNA were more resistant to ceramide-induced apoptosis. Ceramide caused ASK1-regulated p38 mitogen-activated protein kinase (MAPK) and JNK activation, as well as activation of the endoplasmic reticulum (ER) stress cascade, and pharmacologic or siRNA-mediated inhibition of p38 MAPK or JNK partially reduced ceramide-induced mitochondria-mediated apoptosis. Furthermore, ceramide-induced ASK1, p38, and JNK phosphorylation and cell apoptosis were inhibited by Txnip siRNA transfection. Taken together, we show that ceramide exhibits a mechanism of transcriptional regulation involving up-regulation of Txnip expression, also induced by etoposide, which results in ASK1 activation, ER stress, and p38 and JNK phosphorylation, all leading to apoptosis.

Activation of innate immunity system during aging: NF-kB signaling is the molecular culprit of inflamm-aging

Innate and adaptive immunity are the major defence mechanisms of higher organisms against inherent and environmental threats. Innate immunity is present already in unicellular organisms but evolution has added novel adaptive immune mechanisms to the defence armament. Interestingly, during aging, adaptive immunity significantly declines, a phenomenon called immunosenescence, whereas innate immunity seems to be activated which induces a characteristic pro-inflammatory profile. This process is called inflamm-aging. The recognition and signaling mechanisms involved in innate immunity have been conserved during evolution. The master regulator of the innate immunity is the NF-kB system, an ancient signaling pathway found in both insects and vertebrates. The NF-kB system is in the nodal point linking together the pathogenic assault signals and cellular danger signals and then organizing the cellular resistance. Recent studies have revealed that SIRT1 (Sir2 homolog) and FoxO (DAF-16), the key regulators of aging in budding yeast and Caenorhabditis elegans models, regulate the efficiency of NF-kB signaling and the level of inflammatory responses. We will review the role of innate immunity signaling in the aging process and examine the function of NF-kB system in the organization of defence mechanisms and in addition, its interactions with the protein products of several gerontogenes. Our conclusion is that NF-kB signaling seems to be the culprit of inflamm-aging, since this signaling system integrates the intracellular regulation of immune responses in both aging and age-related diseases.

Isc1p plays a key role in hydrogen peroxide resistance and chronological lifespan through modulation of iron levels and apoptosis

The inositolphosphosphingolipid phospholipase C (Isc1p) of Saccharomyces cerevisiae belongs to the family of neutral sphingomyelinases that generates the bioactive sphingolipid ceramide. In this work the role of Isc1p in oxidative stress resistance and chronological lifespan was investigated. Loss of Isc1p resulted in a higher sensitivity to hydrogen peroxide that was associated with an increase in oxidative stress markers, namely intracellular oxidation, protein carbonylation, and lipid peroxidation. Microarray analysis showed that Isc1p deficiency up-regulated the iron regulon leading to increased levels of iron, which is known to catalyze the production of the highly reactive hydroxyl radicals via the Fenton reaction. In agreement, iron chelation suppressed hydrogen peroxide sensitivity of isc1Delta mutants. Cells lacking Isc1p also displayed a shortened chronological lifespan associated with oxidative stress markers and aging of parental cells was correlated with a decrease in Isc1p activity. The analysis of DNA fragmentation and caspase-like activity showed that Isc1p deficiency increased apoptotic cell death associated with oxidative stress and aging. Furthermore, deletion of Yca1p metacaspase suppressed the oxidative stress sensitivity and premature aging phenotypes of isc1Delta mutants. These results indicate that Isc1p plays an important role in the regulation of cellular redox homeostasis, through modulation of iron levels, and of apoptosis.

Apoptosis induced by capsaicin in prostate PC-3 cells involves ceramide accumulation, neutral sphingomyelinase, and JNK activation

Numerous studies have recently focused on the anticarcinogenic, antimutagenic, or chemopreventive activities of the main pungent component of red pepper, capsaicin (N-vanillyl-8-methyl-1-nonenamide). We have previously shown that, in the androgen-independent prostate cancer PC-3 cells, capsaicin inhibits cell growth and induces apoptosis through reactive oxygen species (ROS) generation [Apoptosis 11 (2006) 89-99]. In the present study, we investigated the signaling pathways involved in the antiproliferative effect of capsaicin. Here, we report that capsaicin apoptotic effect was mediated by ceramide generation which occurred by sphingomyelin hydrolysis. Using siRNA, we demonstrated that N-SMase expression is required for the effect of capsaicin on prostate cell viability. We then investigated the role of MAP kinase cascades, extracellular signal-regulated protein kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK, in the antiproliferative effect of capsaicin, and we confirmed that capsaicin could activate ERK and JNK but not p38 MAPK. Pharmacological inhibition of JNK kinase, as well as inhibition of ROS by the reducing agent N-acetylcysteine, prevented ceramide accumulation and capsaicin-induced cell death. However, inhibition of ceramide accumulation by the SMase inhibitor D609 did not modify JNK activation. These data reveal JNK as an upstream regulator of ceramide production. Capsaicin-promoted activation of ERK was prevented with all the inhibitors tested. We conclude that capsaicin induces apoptosis in PC-3 cells via ROS generation, JNK activation, ceramide accumulation, and second, ERK activation.

Sphingolipid therapy in myocardial ischemia-reperfusion injury

Sphingolipids are known to play a significant physiological role in cell growth, cell differentiation, and critical signal transduction pathways. Recent studies have demonstrated a significant role of sphingolipids and their metabolites in the pathogenesis of myocardial ischemia-reperfusion injury. Our laboratory has investigated the cytoprotective effects of N,N,N-trimethylsphingosine chloride (TMS), a stable N-methylated synthetic sphingolipid analogue on myocardial and hepatic ischemia-reperfusion injury in clinically relevant in vivo murine models of ischemia-reperfusion injury. TMS administered intravenously at the onset of ischemia reduced myocardial infarct size in the wild-type and obese (ob/ob) mice. Following myocardial I/R, there was an improvement in cardiac function in the wild-type mice. Additionally, TMS also decreased serum liver enzymes following hepatic I/R in wild-type mice. The cytoprotective effects did not extend to the ob/ob mice following hepatic I/R or to the db/db mice following both myocardial and hepatic I/R. Our data suggest that although TMS is cytoprotective following I/R in normal animals, the cytoprotective actions of TMS are largely attenuated in obese and diabetic animals which may be due to altered signaling mechanisms in these animal models. Here we review the therapeutic role of TMS and other sphingolipids in the pathogenesis of myocardial ischemia-reperfusion injury and their possible mechanisms of cardioprotection.

Cadmium-induced ceramide formation triggers calpain-dependent apoptosis in cultured kidney proximal tubule cells

A major target of cadmium (Cd2+) toxicity is the kidney proximal tubule (PT) cell. Cd2+-induced apoptosis of PT cells is mediated by sequential activation of calpains at 3–6 h and caspases-9 and -3 after 24-h exposure. Calpains also partly contribute to caspase activation, which emphasizes the importance of calpains for PT apoptosis by Cd2+. Upstream processes underlying Cd2+-induced calpain activation remain unclear. We describe for the first time that 10–50 μM Cd2+ causes a significant increase in ceramide formation by ∼22% (3 h) and ∼72% (24 h), as measured by diacylglycerol kinase assay. Inhibition of ceramide synthase with fumonisin B1 (3 μM) prevents ceramide formation at 3 h and abolishes calpain activation at 6 h, which is associated with significant attenuation of apoptosis at 3–6 h with Hoechst 33342 nuclear staining and/or 3(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2 H-tetrazolium bromide (MTT) death assays. This indicates that Cd2+ enhances de novo ceramide synthesis and that calpains are a downstream target of ceramides in apoptosis execution. Moreover, addition of C6-ceramide to PT cells increases cytosolic Ca2+ and activates calpains. Apoptosis mediated by C6-ceramide at 24 h is significantly reduced by caspase-3 inhibition, which supports cross talk between calpain- and caspase-dependent apoptotic pathways. We conclude that Cd2+-induced apoptosis of PT cells entails endogenous ceramide elevation and subsequent Ca2+-dependent calpain activation, which propagates kidney damage by Cd2+.

The sensitivity of the Ewing's sarcoma family of tumours to fenretinide-induced cell death is increased by EWS-Fli1-dependent modulation of p38MAPK activity

The Ewing's sarcoma family of tumours (ESFT) are small round cell tumours characterized by the non-random EWS-ETS gene rearrangements. We have previously demonstrated that ESFT are highly sensitive to fenretinide-induced death, effected in part through a reactive oxygen species (ROS)-dependent pathway. Here, we demonstrate for the first time that the sensitivity of ESFT cells to fenretinide-induced cell death is decreased following downregulation of the oncogenic fusion protein EWS-Fli1; siRNA targeting EWS-Fli1 attenuated fenretinide-induced cell death in cell lines expressing EWS-Fli1, but not EWS-ERG. This decrease in cell death was independent of the level of ROS produced following exposure to fenretinide, but was effected through EWS-Fli1-dependent modulation of p38(MAPK) activity. Furthermore, inhibition of p38(MAPK) activity and knockdown of EWS-Fli1 reduced fenretinide-induced mitochondrial permeabilization, cytochrome c release, caspase and PARP cleavage, consistent with the hypothesis that p38(MAPK) is critical for activation of the death cascade by fenretinide in ESFT cells. These data demonstrate that expression of EWS-Fli1 enhances fenretinide-induced cell death in ESFT and that this is effected at least in part through modulation of p38(MAPK) activity.

Activation of de novo synthetic pathway of ceramides is responsible for the initiation of hydrogen peroxide-induced apoptosis in HL-60 cells

Sphingolipid metabolites in HL-60 cells were analyzed to gain an understanding of their roles in early events underlying hydrogen peroxide (H2O2)-induced apoptosis. Incubation of cells with H2O2 increased the intracellular levels of ceramides and sphinganine, but decreased those of ceramide 1-phosphates (ceramide 1-P) and sphingosine. The levels of sphingomyelins and sphingomyelinase (SMase) activities were not affected by H2O2 treatment. These results were similar to the profiles induced by daunorubicin, an activator of serine palmitoyl CoA transferase (SPT), suggesting that H2O2 stimulated the de novo synthetic pathway of ceramides. L-cycloserine and fumonisin B1 (FB1), specific inhibitors of de novo ceramide biosynthesis, suppressed the elevation of ceramides and sphinganine induced by H2O2, which consequently reduced apoptotic cell death. Collectively, these results demonstrated that H2O2 increased the intracellular concentrations of ceramides via activation of a de novo biosynthetic pathway, and the enhanced ceramides might initiate apoptosis in HL-60 cells.

Acid sphingomyelinase and its redox amplification in formation of lipid raft redox signaling platforms in endothelial cells

This study examined the role of acid sphingomyelinase (ASM) and its redox amplification in mediating the formation of lipid raft (LR) redox signaling platforms in coronary arterial endothelial cells (CAECs). Using small interference RNA (siRNA) of ASM, Fas ligand (FasL)-induced increase in ASM activity, production of ceramide, and LR clustering in CAECs were blocked, and clustered Fas was also substantially reduced in detergent-resistant membrane fractions of CAECs. LR clustering, gp91(phox) aggregation, and p47(phox) translocation to the LR clusters induced by FasL were also blocked in ASM-siRNA transfected CAECs. Corresponding to this reduction of LR clustering with NAD(P)H oxidase subunits in ASM-siRNA transfected CAECs, superoxide (O(2)(-*)) production was significantly decreased as measured by either ESR or fluorescent spectrometry. Interestingly, superoxide dismutase (SOD) not only scavenged (O(2)(-*)), but also markedly attenuated LR clustering. Xanthine/xanthine oxidase, an exogenous (O(2)(-*)) generating system, dramatically increased ASM activity and LR clustering in EC membrane and enhanced FasL-induced LR clustering, which were blocked by SOD. These results suggest that that ASM activates LR clustering to form redox signaling platforms, where (O(2)(-*)) production enhances ASM activity, and thereby results in a forwarding amplification of LR and redox signaling. This ASM-mediated feedforwarding mechanism may be critical for an efficient transmembrane signaling through LRs.

Altered lipid parameters in hepatic subcellular membrane fractions induced by fumonisin B1

Alteration of lipid constituents of cellular membranes has been proposed as a possible mechanism for cancer promotion by fumonisin B(1 )(FB(1)). To further investigate this hypothesis a dietary dosage which initiates and promotes liver cancer (250 mg FB(1)/kg) was fed to male Fischer rats for 21 days and the lipid composition of plasma, microsomal, mitochondrial and nuclear subcellular fractions determined. The effect of FB(1) on the cholesterol, phosphatidylcholine (PC) and phosphatidylethanolamine (PE), as well as sphingomyelin (SM) and the phospholipids-associated fatty acid (FA) profiles, were unique for each subcellular membrane fraction. PE was significantly increased in the microsomal, mitochondrial and plasma membrane fractions, whereas cholesterol was increased in both the microsomal and nuclear fraction. In addition SM was decreased and increased in the mitochondrial and nuclear fractions, respectively. The decreased PC/PE and polyunsaturated/saturated (P/S) FA ratio in the different membrane fractions suggest a more rigid membrane structure. The decreased levels in polyunsaturated fatty acids in PC together with a pronounced increase in C18:1omega9 and C18:2omega6 were indicative of an impaired delta-6 desaturase. The increased omega6/omega3 ratio and decreased C20:4omega6 PC/PE ratio due to an increase in C20:4omega6 in PE relatively to PC in the different subcellular fractions suggests a shift towards prostanoid synthesis of the E2 series. Changes in the PE and C20:4omega6 parameters in the plasma membrane could alter key growth regulatory and/or other cell receptors in lipid rafts known to be altered by FB(1). An interactive role between C20:4omega6 and ceramide in the mitochondria, is suggested to regulate the balance between proliferation and apoptosis in altered initiated hepatocytes resulting in their selective outgrowth during cancer promotion effected by FB(1).