Involvement of de novo ceramide biosynthesis in tumor necrosis factor-alpha/cycloheximide-induced cerebral endothelial cell death

Emerging Roles for Sphingolipids in Cardiometabolic Disease: A Rational Therapeutic Target?

Cardiovascular disease is a leading cause of morbidity and mortality. New research elucidates increasingly complex relationships between cardiac and metabolic health, giving rise to new possible therapeutic targets. Sphingolipids are a heterogeneous class of bioactive lipids with critical roles in normal human physiology. They have also been shown to play both protective and deleterious roles in the pathogenesis of cardiovascular disease. Ceramides are implicated in dysregulating insulin signalling, vascular endothelial function, inflammation, oxidative stress, and lipoprotein aggregation, thereby promoting atherosclerosis and vascular disease. Ceramides also advance myocardial disease by enhancing pathological cardiac remodelling and cardiomyocyte death. Glucosylceramides similarly contribute to insulin resistance and vascular inflammation, thus playing a role in atherogenesis and cardiometabolic dysfunction. Sphingosing-1-phosphate, on the other hand, may ameliorate some of the pathological functions of ceramide by protecting endothelial barrier integrity and promoting cell survival. Sphingosine-1-phosphate is, however, implicated in the development of cardiac fibrosis. This review will explore the roles of sphingolipids in vascular, cardiac, and metabolic pathologies and will evaluate the therapeutic potential in targeting sphingolipids with the aim of prevention and reversal of cardiovascular disease in order to improve long-term cardiovascular outcomes.

Ceramides are fuel gauges on the drive to cardiometabolic disease

Ceramides are signals of fatty acid excess that accumulate when a cell's energetic needs have been met and its nutrient storage has reached capacity. As these sphingolipids accrue, they alter the metabolism and survival of cells throughout the body including in the heart, liver, blood vessels, skeletal muscle, brain, and kidney. These ceramide actions elicit the tissue dysfunction that underlies cardiometabolic diseases such as diabetes, coronary artery disease, metabolic-associated steatohepatitis, and heart failure. Here, we review the biosynthesis and degradation pathways that maintain ceramide levels in normal physiology and discuss how the loss of ceramide homeostasis drives cardiometabolic pathologies. We highlight signaling nodes that sense small changes in ceramides and in turn reprogram cellular metabolism and stimulate apoptosis. Finally, we evaluate the emerging therapeutic utility of these unique lipids as biomarkers that forecast disease risk and as targets of ceramide-lowering interventions that ameliorate disease.

The Impact of Phytochemicals in Obesity-Related Metabolic Diseases: Focus on Ceramide Metabolism

The prevalence of obesity and related metabolic diseases has increased dramatically worldwide. As obesity progresses, various lipid species accumulate in ectopic tissues. Amongst them, ceramides-a deleterious sphingolipid species-accumulate and cause lipotoxicity and metabolic disturbances. Dysregulated ceramide metabolism appears to be a key feature in the pathogenesis of obesity-related metabolic diseases. Notably, dietary modification might have an impact on modulating ceramide metabolism. Phytochemicals are plant-derived compounds with various physiological properties, which have been shown to protect against obesity-related metabolic diseases. In this review, we aim to examine the impact of a myriad of phytochemicals and their dietary sources in altering ceramide deposition and ceramide-related metabolism from in vitro, in vivo, and human clinical/epidemiological studies. This review discusses how numerous phytochemicals are able to alleviate ceramide-induced metabolic defects and reduce the risk of obesity-related metabolic diseases via diverse mechanisms.

Sphingolipids and Atherosclerosis: The Dual Role of Ceramide and Sphingosine-1-Phosphate

Sphingolipids are bioactive molecules that play either pro- and anti-atherogenic roles in the formation and maturation of atherosclerotic plaques. Among SLs, ceramide and sphingosine-1-phosphate showed antithetic properties in regulating various molecular mechanisms and have emerged as novel potential targets for regulating the development of atherosclerosis. In particular, maintaining the balance of the so-called ceramide/S1P rheostat is important to prevent the occurrence of endothelial dysfunction, which is the trigger for the entire atherosclerotic process and is strongly associated with increased oxidative stress. In addition, these two sphingolipids, together with many other sphingolipid mediators, are directly involved in the progression of atherogenesis and the formation of atherosclerotic plaques by promoting the oxidation of low-density lipoproteins (LDL) and influencing the vascular smooth muscle cell phenotype. The modulation of ceramide and S1P levels may therefore allow the development of new antioxidant therapies that can prevent or at least impair the onset of atherogenesis, which would ultimately improve the quality of life of patients with coronary artery disease and significantly reduce their mortality.

Contribution of specific ceramides to obesity-associated metabolic diseases

Ceramides are a heterogeneous group of bioactive membrane sphingolipids that play specialized regulatory roles in cellular metabolism depending on their characteristic fatty acyl chain lengths and subcellular distribution. As obesity progresses, certain ceramide molecular species accumulate in metabolic tissues and cause cell-type-specific lipotoxic reactions that disrupt metabolic homeostasis and lead to the development of cardiometabolic diseases. Several mechanisms for ceramide action have been inferred from studies in vitro, but only recently have we begun to better understand the acyl chain length specificity of ceramide-mediated signaling in the context of physiology and disease in vivo. New discoveries show that specific ceramides affect various metabolic pathways and that global or tissue-specific reduction in selected ceramide pools in obese rodents is sufficient to improve metabolic health. Here, we review the tissue-specific regulation and functions of ceramides in obesity, thus highlighting the emerging concept of selectively inhibiting production or action of ceramides with specific acyl chain lengths as novel therapeutic strategies to ameliorate obesity-associated diseases.

The Role of Obesity, Inflammation and Sphingolipids in the Development of an Abdominal Aortic Aneurysm

Abdominal aortic aneurysm (AAA) is a local dilatation of the vessel equal to or exceeding 3 cm. It is a disease with a long preclinical period commonly without any symptoms in its initial stage. Undiagnosed for years, aneurysm often leads to death due to vessel rupture. The basis of AAA pathogenesis is inflammation, which is often associated with the excess of adipose tissue, especially perivascular adipose tissue, which synthesizes adipocytokines that exert a significant influence on the formation of aneurysms. Pro-inflammatory cytokines such as resistin, leptin, and TNFα have been shown to induce changes leading to the formation of aneurysms, while adiponectin is the only known compound that is secreted by adipose tissue and limits the development of aneurysms. However, in obesity, adiponectin levels decline. Moreover, inflammation is associated with an increase in the amount of macrophages infiltrating adipose tissue, which are the source of matrix metalloproteinases (MMP) involved in the degradation of the extracellular matrix, which are an important factor in the formation of aneurysms. In addition, an excess of body fat is associated with altered sphingolipid metabolism. It has been shown that among sphingolipids, there are compounds that play an opposite role in the cell: ceramide is a pro-apoptotic compound that mediates the development of inflammation, while sphingosine-1-phosphate exerts pro-proliferative and anti-inflammatory effects. It has been shown that the increase in the level of ceramide is associated with a decrease in the concentration of adiponectin, an increase in the concentration of TNFα, MMP-9 and reactive oxygen species (which contribute to the apoptosis of vascular smooth muscle cell). The available data indicate a potential relationship between obesity, inflammation and disturbed sphingolipid metabolism with the formation of aneurysms; therefore, the aim of this study was to systematize the current knowledge on the role of these factors in the pathogenesis of abdominal aortic aneurysm.

You aren't IMMUNE to the ceramides that accumulate in cardiometabolic disease

Obesity leads to persistent increases in immune responses that contribute to cardiometabolic pathologies such as diabetes and cardiovascular disease. Pro-inflammatory macrophages infiltrate the expanding fat mass, which leads to increased production of cytokines such as tumor necrosis factor-alpha. Moreover, saturated fatty acids enhance signaling through the toll-like receptors involved in innate immunity. Herein we discuss the evidence that ceramides-which are intermediates in the biosynthetic pathway that produces sphingolipids-are essential intermediates that link these inflammatory signals to impaired tissue function. We discuss the mechanisms linking these immune insults to ceramide production and review the numerous ceramide actions that alter cellular metabolism, induce oxidative stress, and stimulate apoptosis. Lastly, we evaluate the correlation of ceramides in humans with inflammation-linked cardiometabolic disease and discuss preclinical studies which suggest that ceramide-lowering interventions may be an effective strategy to treat or prevent such maladies.

Ceramide from sphingomyelin hydrolysis induces neuronal differentiation, whereas de novo ceramide synthesis and sphingomyelin hydrolysis initiate apoptosis after NGF withdrawal in PC12 Cells

Background

Ceramide, important for both neuronal differentiation and dedifferentiation, resides in several membranes, is synthesized in the endoplasmic reticulum, mitochondrial, and nuclear membranes, and can be further processed into glycosphingolipids or sphingomyelin. Ceramide may also be generated by hydrolysis of sphingomyelin by neutral or acidic sphingomyelinases in lysosomes and other membranes. Here we asked whether the differing functions of ceramide derived from different origins.

Methods

We added NGF to PC12 cells and to TrkA cells. These latter overexpress NGF receptors and are partially activated to differentiate, whereas NGF is required for PC12 cells to differentiate. We differentiated synthesis from hydrolysis by the use of appropriate inhibitors. Ceramide and sphingomyelin were measured by radiolabeling.

Results

When NGF is added, the kinetics and amounts of ceramide and sphingomyelin indicate that the ceramide comes primarily from hydrolysis but, when hydrolysis is inhibited, can also come from neosynthesis. When NGF is removed, the ceramide comes from both neosynthesis and hydrolysis.

Conclusion

We conclude that the function of ceramide depends heavily on its intracellular location, and that further understanding of its function will depend on resolving its location during changes of cell status.

Graphical Abstract

The effect of dihydroceramide desaturase 1 inhibition on endothelial impairment induced by indoxyl sulfate

Protein-bound uremic toxins (PBUTs) have adverse effects on vascular function, which is imperative in the progression of cardiovascular and renal diseases. The role of sphingolipids in PBUT-mediated vasculo-endothelial pathophysiology is unclear. This study assessed the therapeutic potential of dihydroceramide desaturase 1 (Des1) inhibition, the last enzyme involved in de novo ceramide synthesis, to mitigate the vascular effects of the PBUT indoxyl sulfate (IS). Rat aortic rings were isolated and vascular reactivity was assessed in organ bath experiments followed by immunohistochemical analyses. Furthermore, cultured human aortic endothelial cells were assessed for phenotypic and mechanistic changes. Inhibition of Des1 by a selective inhibitor CIN038 (0.1 to 0.3 μM) improved IS-induced impairment of vasorelaxation and modulated immunoreactivity of oxidative stress markers. Des1 inhibition also reversed IS-induced reduction in endothelial cell migration (1.0 μM) by promoting the expression of angiogenic cytokines and reducing inflammatory and oxidative stress markers. These effects were associated with a reduction of TIMP1 and the restoration of Akt phosphorylation. In conclusion, Des1 inhibition improved vascular relaxation and endothelial cell migration impaired by IS overload. Therefore, Des1 may be a suitable intracellular target to mitigate PBUT-induced adverse vascular effects.

Ceramides and other sphingolipids as drivers of cardiovascular disease

Increases in calorie consumption and sedentary lifestyles are fuelling a global pandemic of cardiometabolic diseases, including coronary artery disease, diabetes mellitus, cardiomyopathy and heart failure. These lifestyle factors, when combined with genetic predispositions, increase the levels of circulating lipids, which can accumulate in non-adipose tissues, including blood vessel walls and the heart. The metabolism of these lipids produces bioactive intermediates that disrupt cellular function and survival. A compelling body of evidence suggests that sphingolipids, such as ceramides, account for much of the tissue damage in these cardiometabolic diseases. In humans, serum ceramide levels are proving to be accurate biomarkers of adverse cardiovascular disease outcomes. In mice and rats, pharmacological inhibition or depletion of enzymes driving de novo ceramide synthesis prevents the development of diabetes, atherosclerosis, hypertension and heart failure. In cultured cells and isolated tissues, ceramides perturb mitochondrial function, block fuel usage, disrupt vasodilatation and promote apoptosis. In this Review, we discuss the body of literature suggesting that ceramides are drivers - and not merely passengers - on the road to cardiovascular disease. Moreover, we explore the feasibility of therapeutic strategies to lower ceramide levels to improve cardiovascular health.

Unwrapping the mechanisms of ceramide and fatty acid-initiated signals leading to immune-inflammatory responses in obesity

Obesity is considered a global epidemic developed in part as a consequence of the overconsumption of high fat diets. One of the main negative outcomes of obesity is the development of low-grade chronic systemic inflammation, induced by dysregulated immune responses, which can lead to multiple obesity-related diseases. Ceramides are a group of bioactive lipids known to be elevated in obesity and obesity-associated conditions, including cardiovascular disease and type II diabetes. Ceramides may be key players in promoting an obesity-induced inflammatory environment due to their ability to activate key pathways such as Toll-like receptor 4 (TLR4) and NLR pyrin domain containing receptor 3 (Nlrp3), while studies have shown that inhibition of ceramide synthesis gives rise to an anti-inflammatory environment. N-3 polyunsaturated fatty acids (n-3 PUFA) have been of interest due to their anti-inflammatory actions and shown to have beneficial effects in obesity-related diseases. This review will highlight the impact of ceramides in promoting an obesity-induced inflammatory microenvironment and discuss how n-3 PUFA could potentially counteract these responses and have a regulatory effect promoting immune homeostasis.

Heat stress impairs oocyte maturation through ceramide-mediated apoptosis in pigs

Heat stress (HS) is an emerging issue that greatly impairs the reproductive performance of animals and humans. In particular, disruption of oocyte maturation due to HS is considered a major cause of impaired reproductive performance. HS is known to induce ceramide generation, which causes reactive oxygen species (ROS) production and mitochondrial dysfunction, thereby inducing apoptosis. Therefore, we investigated whether inhibition of ceramide generation ameliorates HS-induced apoptosis in porcine cumulus-oocyte complexes (COCs) using specific inhibitors of the de novo (fumonisin B1, FB1) and hydrolytic pathways (desipramine, Des) of ceramide formation. We investigated the effects of FB1 and Des supplementation under HS conditions (41.5 °C for 44 h) on in vitro maturation (IVM) of porcine COCs. After IVM, HS significantly reduced proportion of COCs exhibiting fully expanded cumulus cells and the rate of metaphase II in oocytes. After parthenogenetic activation (PA), HS significantly reduced the rates of cleavage and blastocyst formation with a lower total cell number and a higher percentage of apoptosis in blastocysts. However, FB1 or Des supplementation under HS avoided detrimental effects of HS on expansion of cumulus cells, nuclear maturation of oocytes, and embryonic development after PA including the rates of cleavage and blastocyst formation, total cell number, and the percentage of apoptosis in blastocysts. Furthermore, FB1 or Des addition under HS, compared with HS alone, significantly decreased ceramide generation, ROS production, cytochrome C expression, and apoptosis and increased mitochondrial membrane potential in COCs, reaching levels comparable with those of the control. Taken together, our results indicate that HS impaired oocyte maturation through ceramide-mediated apoptosis.

Adipocyte Ceramides-The Nexus of Inflammation and Metabolic Disease

Adipose depots are heterogeneous tissues that store and sense fuel levels. Through the secretion of lipids, cytokines, and protein hormones (adipokines), they communicate with other organ systems, informing them of the organism's nutritional status. The adipose tissues include diverse types of adipocytes (white, beige, and brown) distinguished by the number/size of lipid droplets, mitochondrial density, and thermogenic capacity. Moreover, they include a spectrum of immune cells that modulate metabolic activity and tissue remodeling. The unique characteristics and interplay of these cells control the production of ceramides, a class of nutrient signals derived from fat and protein metabolism that modulate adipocyte function to regulate glucose and lipid metabolism. The excessive accumulation of ceramides contributes to the adipose tissue inflammation and dysfunction that underlies cardiometabolic disease. Herein we review findings on this important class of lipid species and discuss their role at the convergence point that links overnutrition/inflammation to key features of the metabolic syndrome.

Similar plasma lipidomic profile in people living with HIV treated with a darunavir-based or an integrase inhibitor-based antiretroviral therapy

Cardiovascular disease is an important cause of morbidity and mortality in people living with HIV (PLWH), who commonly experience lipid disturbances. The aim of this study was to determine whether the plasma lipidomic profile differs between PLWH receiving a darunavir-based ART and those receiving integrase inhibitor-based ART. This was a cross-sectional study of unselected patients for whom metabolomic analysis was performed using ultra-high-performance liquid chromatography coupled to mass spectrometry. Data for the two subgroups were compared by calculating the log2 of the fold change for each metabolite and then grouping these into the main lipid families. Sixty-two PLWH aged 49.3 ± 8.6 years (82% men) were included: 12 patients (19.4%) had hypertension, 8 (12.9%) had type 2 diabetes, 25 (41.0%) had dyslipidaemia and 9 (14.5%) were taking statins, without significant differences in all these variables between the two groups. Twenty-five (40.3%) received darunavir-based ART and 37 (59.7%) integrase inhibitor-based ART. Although the differences were not statistically significant, patients treated with darunavir-based ART had higher concentrations of total cholesterol (211 mg/dL vs 194 mg/dL), LDL-cholesterol (132 mg/dL vs 117 mg/dL) and triglycerides (155 mg/dL vs 122 mg/dL), and lower HDL-cholesterol concentration (50 mg/dL vs 52 mg/dL). The main lipid families and metabolites differed slightly between groups (log2-fold change; P-value): ceramides (-0.07; 0.49), phosphatidylinositols (-0.05; 0.63), diacylglycerols (0.10; 0.64), phosphatidylethanolamines (0.03; 0.78), triacylglycerols (0.27; 0.18) and lysophosphatidylethanolamines (0.03; 0.83). In the integrase inhibitor-based group, the use of tenofovir alafenamide fumarate significantly increases the majority of lipid fractions, when compared with tenofovir disoproxil fumarate. The lipidomic profile did not differ between PLWH treated with darunavir-based or integrase inhibitor-based ART. This was especially true for ceramides, which are involved in cardiovascular disease. Further studies are needed to study the impact of ART in lipidomic profile.

Plasma Sphingolipids are Associated With Gait Parameters in the Mayo Clinic Study of Aging

Background

Disrupted gait has been associated with an increased risk of frailty, disability, and death, but the causal molecular pathways are not well understood. Sphingolipids, including ceramides, are associated with multiple age-related diseases. Ceramides promote atrophy, necrosis, and proteolysis in cellular and animal models, and ceramide C16:0 levels are negatively correlated with muscle mass in men. However, there is a paucity of evidence examining sphingolipids and physical function.

Methods

We examined the cross-sectional association between plasma ceramides, sphingosine-1-phosphate (S1P), and ceramide/S1P ratios and gait, a robust measure of physical function, in 340 clinically normal participants aged 70 years and older enrolled in the Mayo Clinic Study of Aging. GAITRite® instrumentation was used to measure gait speed, cadence, step width, double support time, and intra-individual stride time variability. Based on previous studies, we hypothesized that higher plasma levels of ceramide C16:0 would be associated with worse gait.

Results

Multivariable adjusted linear regression models revealed that higher levels of ceramide C16:0 were associated with slower gait speed, decreased cadence, and increased double support time.

Conclusions

These results suggest an association between plasma ceramide C16:0 and physical function. Longitudinal studies are needed to determine whether elevated ceramide C16:0 can be utilized as a prognostic marker for functional decline.

Inhibition of de novo ceramide biosynthesis affects aging phenotype in an in vitro model of neuronal senescence

Although aging is considered to be an unavoidable event, recent experimental evidence suggests that the process can be counteracted. Intracellular calcium (Ca²⁺_i) dyshomeostasis, mitochondrial dysfunction, oxidative stress, and lipid dysregulation are critical factors that contribute to senescence-related processes. Ceramides, a pleiotropic class of sphingolipids, are important mediators of cellular senescence, but their role in neuronal aging is still largely unexplored. In this study, we investigated the effects of L-cycloserine (L-CS), an inhibitor of thede novoceramide biosynthesis, on the aging phenotype of cortical neurons cultured for 22 days, a setting employed as anin vitromodel of senescence. Our findings indicate that, compared to control cultures, ‘aged’ neurons display dysregulation of [Ca²⁺]_ilevels, mitochondrial dysfunction, increased generation of reactive oxygen species (ROS), altered synaptic activity as well as the activation of neuronal death-related molecules. Treatment with L-CS positively affected the senescent phenotype, a result associated with recovery of neuronal [Ca²⁺]_isignaling and reduction of mitochondrial dysfunction and ROS generation. The results suggest that thede novoceramide biosynthesis represents a critical intermediate in the molecular and functional cascade leading to neuronal senescence and identify ceramide biosynthesis inhibitors as promising pharmacological tools to decrease age-related neuronal dysfunctions.

Effect of combined testing of ceramides with high-sensitive troponin T on the detection of acute coronary syndrome in patients with chest pain in China: a prospective observational study

OBJECTIVE

Ceramides are associated with coronary plaque vulnerability. We aim to investigate the potential diagnostic value of ceramides for acute coronary syndrome (ACS) in Chinese patients with chest pain.

DESIGN

Prospective observational survey.

SETTING

Shanghai, China, 2016-2017.

PARTICIPANTS

A total of 2773 patients with chest pain from four hospitals in Shanghai, China, between August 2016 and October 2017.

MAIN OUTCOME MEASURES

Performance of metabolites model in detection of ACS cases including ST-elevation myocardial infarction (STEMI), non-STEMI (NSTEMI) and unstable angina.

RESULTS

Plasma levels of 12 ceramide molecules and corresponding ratios were compared between patients diagnosed with ACS and those without. Cer(d18:1/24:1(15Z))/Cer(d18:1/24:0) ratio, Cer(d18:1/14:0) and Cer(d18:1/22:0) were independent predictors of ACS after adjustment of traditional risk factors and high-sensitivecardiac troponin T. Receiver operating characteristic curve analysis showed a significant improvement in detecting ACS in the multivariable model with ceramides compared with that without (0.865 (0.840 to 0.889) vs 0.808 (0.776 to 0.841), p<0.001).

CONCLUSION

Distinct plasma ceramides are independent diagnostic predictors of ACS among patients with chest pain. Ceramides together with high-sensitive troponin and traditional factors showed great potential in identifying ACS among patients with chest pain.

Contribution of Adipose Tissue Inflammation to the Development of Type 2 Diabetes Mellitus

The objective of this comprehensive review is to summarize and discuss the available evidence of how adipose tissue inflammation affects insulin sensitivity and glucose tolerance. Low-grade, chronic adipose tissue inflammation is characterized by infiltration of macrophages and other immune cell populations into adipose tissue, and a shift toward more proinflammatory subtypes of leukocytes. The infiltration of proinflammatory cells in adipose tissue is associated with an increased production of key chemokines such as C-C motif chemokine ligand 2, proinflammatory cytokines including tumor necrosis factor α and interleukins 1β and 6 as well as reduced expression of the key insulin-sensitizing adipokine, adiponectin. In both rodent models and humans, adipose tissue inflammation is consistently associated with excess fat mass and insulin resistance. In humans, associations with insulin resistance are stronger and more consistent for inflammation in visceral as opposed to subcutaneous fat. Further, genetic alterations in mouse models of obesity that reduce adipose tissue inflammation are-almost without exception-associated with improved insulin sensitivity. However, a dissociation between adipose tissue inflammation and insulin resistance can be observed in very few rodent models of obesity as well as in humans following bariatric surgery- or low-calorie-diet-induced weight loss, illustrating that the etiology of insulin resistance is multifactorial. Taken together, adipose tissue inflammation is a key factor in the development of insulin resistance and type 2 diabetes in obesity, along with other factors that likely include inflammation and fat accumulation in other metabolically active tissues. © 2019 American Physiological Society. Compr Physiol 9:1-58, 2019.

Plasma ceramides predict cardiovascular death in patients with stable coronary artery disease and acute coronary syndromes beyond LDL-cholesterol

Aims

The aim was to study the prognostic value of plasma ceramides (Cer) as cardiovascular death (CV death) markers in three independent coronary artery disease (CAD) cohorts.

Methods and results

Corogene study is a prospective Finnish cohort including stable CAD patients (n = 160). Multiple lipid biomarkers and C-reactive protein were measured in addition to plasma Cer(d18:1/16:0), Cer(d18:1/18:0), Cer(d18:1/24:0), and Cer(d18:1/24:1). Subsequently, the association between high-risk ceramides and CV mortality was investigated in the prospective Special Program University Medicine—Inflammation in Acute Coronary Syndromes (SPUM-ACS) cohort (n = 1637), conducted in four Swiss university hospitals. Finally, the results were validated in Bergen Coronary Angiography Cohort (BECAC), a prospective Norwegian cohort study of stable CAD patients. Ceramides, especially when used in ratios, were significantly associated with CV death in all studies, independent of other lipid markers and C-reactive protein. Adjusted odds ratios per standard deviation for the Cer(d18:1/16:0)/Cer(d18:1/24:0) ratio were 4.49 (95% CI, 2.24–8.98), 1.64 (1.29–2.08), and 1.77 (1.41–2.23) in the Corogene, SPUM-ACS, and BECAC studies, respectively. The Cer(d18:1/16:0)/Cer(d18:1/24:0) ratio improved the predictive value of the GRACE score (net reclassification improvement, NRI = 0.17 and ΔAUC = 0.09) in ACS and the predictive value of the Marschner score in stable CAD (NRI = 0.15 and ΔAUC = 0.02).

Conclusions

Distinct plasma ceramide ratios are significant predictors of CV death both in patients with stable CAD and ACS, over and above currently used lipid markers. This may improve the identification of high-risk patients in need of more aggressive therapeutic interventions.

Protective effects of fish oil on pre-diabetes: a lipidomic analysis of liver ceramides in rats

n-3 PUFA (EPA/DHA 1 : 1) from fish oil modified the ceramide profile of the liver and reduced their total content in pre-diabetic rats.

Ceramide metabolism regulates autophagy and apoptotic cell death induced by melatonin in liver cancer cells

Autophagy is a process that maintains homeostasis during stress, although it also contributes to cell death under specific contexts. Ceramides have emerged as important effectors in the regulation of autophagy, mediating the crosstalk with apoptosis. Melatonin induces apoptosis of cancer cells; however, its role in autophagy and ceramide metabolism has yet to be clearly elucidated. This study was aimed to evaluate the effect of melatonin administration on autophagy and ceramide metabolism and its possible link with melatonin-induced apoptotic cell death in hepatocarcinoma (HCC) cells. Melatonin (2 mm) transiently induced autophagy in HepG2 cells through JNK phosphorylation, characterized by increased Beclin-1 expression, p62 degradation, and LC3II and LAMP-2 colocalization, which translated in decreased cell viability. Moreover, ATG5 silencing sensitized HepG2 cells to melatonin-induced apoptosis, suggesting a dual role of autophagy in cell death. Melatonin enhanced ceramide levels through both de novo synthesis and acid sphingomyelinase (ASMase) stimulation. Serine palmitoyltransferase (SPT) inhibition with myriocin prevented melatonin-induced autophagy and ASMase inhibition with imipramine-impaired autophagy flux. However, ASMase inhibition partially protected HepG2 cells against melatonin, while SPT inhibition significantly enhanced cell death. Findings suggest a crosstalk between SPT-mediated ceramide generation and autophagy in protecting against melatonin, while specific ASMase-induced ceramide production participates in melatonin-mediated cell death. Thus, dual blocking of SPT and autophagy emerges as a potential strategy to potentiate the apoptotic effects of melatonin in liver cancer cells.

Molecular Mechanism for the Control of Eukaryotic Elongation Factor 2 Kinase by pH: Role in Cancer Cell Survival

Acidification of the extracellular and/or intracellular environment is involved in many aspects of cell physiology and pathology. Eukaryotic elongation factor 2 kinase (eEF2K) is a Ca(2+)/calmodulin-dependent kinase that regulates translation elongation by phosphorylating and inhibiting eEF2. Here we show that extracellular acidosis elicits activation of eEF2K in vivo, leading to enhanced phosphorylation of eEF2. We identify five histidine residues in eEF2K that are crucial for the activation of eEF2K during acidosis. Three of them (H80, H87, and H94) are in its calmodulin-binding site, and their protonation appears to enhance the ability of calmodulin to activate eEF2K. The other two histidines (H227 and H230) lie in the catalytic domain of eEF2K. We also identify His108 in calmodulin as essential for activation of eEF2K. Acidification of cancer cell microenvironments is a hallmark of malignant solid tumors. Knocking down eEF2K in cancer cells attenuated the decrease in global protein synthesis when cells were cultured at acidic pH. Importantly, activation of eEF2K is linked to cancer cell survival under acidic conditions. Inhibition of eEF2K promotes cancer cell death under acidosis.

Homocysteine induces cerebral endothelial cell death by activating the acid sphingomyelinase ceramide pathway

Homocysteine (Hcy) levels may rise after a stroke, but the mechanism of Hcy-induced cerebral endothelial cell (CEC) dysfunction has not been explored. In this study we examined the role of the acid sphingomyelinase (Asm)-ceramide pathway in the molecular mechanism of Hcy-induced CEC dysfunction. Murine CECs were prepared from fresh mouse brains. CECs were treated with 50-500 μM Hcy and 30-100 μM C2-ceramide for 48 h. Sphingomyelinase assays were performed to determine Asm activity. Quantitative assessments of cell survival and death by the MTT reduction and LDH release were conducted. Treatment of murine CECs with Hcy and ceramide caused cell death in a dose-dependent manner as determined by LDH and MTT assays. 250 μM Hcy and 50 μM C2-ceramide caused 50% cell death. Hcy induced murine CEC death also occurred in a time-dependant manner with substantial cell death noted as early as 24h after Hcy exposure. C2-ceramide-induced murine CEC death occurred earlier than Hcy-induced cell death by about 18h. Hcy treatment increased Asm activity and intracellular ceramide accumulation. This study demonstrated that Hcy and C2-ceramide can cause murine CEC death. Hcy induces CEC death possibly by activating the Asm-ceramide pathway.

Influence of fish oil on skeletal muscle mitochondrial energetics and lipid metabolites during high-fat diet

Omega-3 polyunsaturated fatty acids (n-3 PUFAs) enhance insulin sensitivity and glucose homeostasis in rodent models of insulin resistance. These beneficial effects have been linked with anti-inflammatory properties, but emerging data suggest that the mechanisms may also converge on mitochondria. We evaluated the influence of dietary n-3 PUFAs on mitochondrial physiology and muscle lipid metabolites in the context of high-fat diet (HFD) in mice. Mice were fed control diets (10% fat), HFD (60% fat), or HFD with fish oil (HFD+FO, 3.4% kcal from n-3 PUFAs) for 10 wk. Body mass and fat mass increased similarly in HFD and HFD+FO, but n-3 PUFAs attenuated the glucose intolerance that developed with HFD and increased expression of genes that regulate glucose metabolism in skeletal muscle. Despite similar muscle triglyceride levels in HFD and HFD+FO, long-chain acyl-CoAs and ceramides were lower in the presence of fish oil. Mitochondrial abundance and oxidative capacity were similarly increased in HFD and HFD+FO compared with controls. Hydrogen peroxide production was similarly elevated in HFD and HFD+FO in isolated mitochondria but not in permeabilized muscle fibers, likely due to increased activity and expression of catalase. These results support a hypothesis that n-3 PUFAs protect glucose tolerance, in part by preventing the accumulation of bioactive lipid mediators that interfere with insulin action. Furthermore, the respiratory function of skeletal muscle mitochondria does not appear to be a major factor in sphingolipid accumulation, glucose intolerance, or the protective effects of n-3 PUFAs.

Interdiction of sphingolipid metabolism to improve standard cancer therapies

Non-surgical therapies for human malignancies must negotiate complex cell signaling pathways to impede cancer cell growth, ideally promoting death of cancer cells while sparing healthy tissue. For most of the past half century, medical approaches for treating cancer have relied primarily on cytotoxic chemotherapeutics that interfere with DNA replication and cell division, susceptibilities of rapidly dividing cancer cells. As a consequence, these therapies exert considerable cell stress, promoting the generation of ceramide through de novo synthesis and recycling of complex glycosphingolipids and sphingomyelin into apoptotic ceramide. Radiotherapy of cancer exerts similar geno- and cytotoxic cell stresses, and generation of ceramide following ionizing radiation therapy is a well-described feature of radiation-induced cell death. Emerging evidence now describes sphingolipids as mediators of death in response to newer targeted therapies, cementing ceramide generation as a common mechanism of cell death in response to cancer therapy. Many studies have now shown that dysregulation of ceramide accumulation-whether by reduced generation or accelerated metabolism-is a common mechanism of resistance to standard cancer therapies. The aims of this chapter will be to discuss described mechanisms of cancer resistance to therapy related to dysregulation of sphingolipid metabolism and to explore clinical and preclinical approaches to interdict sphingolipid metabolism to improve outcomes of standard cancer therapies.

Sphingolipids and response to chemotherapy

Chemotherapy is frequently used to treat primary or metastatic cancers, but intrinsic or acquired drug resistance limits its efficiency. Sphingolipids are important regulators of various cellular processes including proliferation, apoptosis, differentiation, angiogenesis, stress, and inflammatory responses which are linked to various aspects of cancer, like tumor growth, neoangiogenesis, and response to chemotherapy. Ceramide, the central molecule of sphingolipid metabolism, generally mediates antiproliferative and proapoptotic functions, whereas sphingosine-1-phosphate and other derivatives have opposing effects. Among the variety of enzymes that control ceramide generation, acid or neutral sphingomyelinases and ceramide synthases are important targets to allow killing of cancer cells by chemotherapeutic drugs. On the contrary, glucosylceramide synthase, ceramidase, and sphingosine kinase are other targets driving cancer cell resistance to chemotherapy. This chapter focuses on ceramide-based mechanisms leading to cancer therapy sensitization or resistance which could have some impacts on the development of novel cancer therapeutic strategies.

Sphingosine mediates TNFα-induced lysosomal membrane permeabilization and ensuing programmed cell death in hepatoma cells

Normally, cell proliferation and death are carefully balanced in higher eukaryotes, but one of the most important regulatory mechanisms, apoptosis, is upset in many malignancies, including hepatocellular-derived ones. Therefore, reinforcing cell death often is mandatory in anticancer therapy. We previously reported that a combination of tumor necrosis factor-α (TNF) and cycloheximide (CHX) efficiently kill HTC cells, a rat hepatoma line, in an apoptosis-like mode. Death is actively mediated by the lysosomal compartment, although lysosomal ceramide was previously shown not to be directly implicated in this process. In the present study, we show that TNF/CHX increase lysosomal ceramide that is subsequently converted into sphingosine. Although ceramide accumulation does not significantly alter the acidic compartment, the sphingosine therein generated causes lysosomal membrane permeabilization (LMP) followed by relocation of lysosomal cathepsins to the cytoplasm. TNF/CHX-induced LMP is effectively abrogated by siRNAs targeting acid sphingomyelinase or acid ceramidase, which prevent both LMP and death induced by TNF/CHX. Taken together, our results demonstrate that lysosomal accumulation of ceramide is not detrimental per se, whereas its degradation product sphingosine, which has the capacity to induce LMP, appears responsible for the observed apoptotic-like death.

Ceramide synthases at the centre of sphingolipid metabolism and biology

Sphingolipid metabolism in metazoan cells consists of a complex interconnected web of numerous enzymes, metabolites and modes of regulation. At the centre of sphingolipid metabolism reside CerSs (ceramide synthases), a group of enzymes that catalyse the formation of ceramides from sphingoid base and acyl-CoA substrates. From a metabolic perspective, these enzymes occupy a unique niche in that they simultaneously regulate de novo sphingolipid synthesis and the recycling of free sphingosine produced from the degradation of pre-formed sphingolipids (salvage pathway). Six mammalian CerSs (CerS1-CerS6) have been identified. Unique characteristics have been described for each of these enzymes, but perhaps the most notable is the ability of individual CerS isoforms to produce ceramides with characteristic acyl-chain distributions. Through this control of acyl-chain length and perhaps in a compartment-specific manner, CerSs appear to regulate multiple aspects of sphingolipid-mediated cell and organismal biology. In the present review, we discuss the function of CerSs as critical regulators of sphingolipid metabolism, highlight their unique characteristics and explore the emerging roles of CerSs in regulating programmed cell death, cancer and many other aspects of biology.

Honokiol traverses the blood-brain barrier and induces apoptosis of neuroblastoma cells via an intrinsic bax-mitochondrion-cytochrome c-caspase protease pathway

Neuroblastomas, an embryonic cancer of the sympathetic nervous system, often occur in young children. Honokiol, a small-molecule polyphenol, has multiple therapeutic effects and pharmacological activities. This study was designed to evaluate whether honokiol could pass through the blood-brain barrier (BBB) and induce death of neuroblastoma cells and its possible mechanisms. Primary cerebral endothelial cells (CECs) prepared from mouse brain capillaries were cultured at a high density for 4 days, and these cells formed compact morphologies and expressed the ZO-1 tight-junction protein. A permeability assay showed that the CEC-constructed barrier obstructed the passing of FITC-dextran. Analyses by high-performance liquid chromatography and the UV spectrum revealed that honokiol could traverse the CEC-built junction barrier and the BBB of ICR mice. Exposure of neuroblastoma neuro-2a cells and NB41A3 cells to honokiolinduced cell shrinkage and decreased cell viability. In parallel, honokiol selectively induced DNA fragmentation and cell apoptosis rather than cell necrosis. Sequential treatment of neuro-2a cells with honokiol increased the expression of the proapoptotic Bax protein and its translocation from the cytoplasm to mitochondria. Honokiol successively decreased the mitochondrial membrane potential but increased the release of cytochrome c from mitochondria. Consequently, honokiol induced cascade activation of caspases-9, -3, and -6. In comparison, reducing caspase-6 activity by Z-VEID-FMK, an inhibitor of caspase-6, simultaneously attenuated honokiol-induced DNA fragmentation and cell apoptosis. Taken together, this study showed that honokiol can pass through the BBB and induce apoptotic insults to neuroblastoma cells through a Bax-mitochondrion-cytochrome c-caspase protease pathway. Therefore, honokiol may be a potential candidate drug for treating brain tumors.

Resveratrol attenuates oxidized LDL-evoked Lox-1 signaling and consequently protects against apoptotic insults to cerebrovascular endothelial cells

Cerebrovascular endothelial cells (CECs) are crucial components of the blood-brain barrier. Our previous study showed that oxidized low-density lipoprotein (oxLDL) induces apoptosis of CECs. This study was designed to further evaluate the effects of resveratrol on oxLDL-induced CEC insults and its possible molecular mechanisms. Resveratrol decreased the oxidation of LDL into oxLDL. Additionally, the oxLDL-caused oxidative stress and cell damage were attenuated by resveratrol. Exposure of CECs to oxLDL induced cell shrinkage, DNA fragmentation, and cell apoptosis, but resveratrol defended against such injuries. Application of Lox-1 small interference (si)RNA into CECs reduced the translation of this membrane receptor, and simultaneously increased resveratrol protection from oxLDL-induced cell apoptosis. By comparison, overexpression of Lox-1 attenuated resveratrol protection. Resveratrol inhibited oxLDL-induced Lox-1 mRNA and protein expressions. Both resveratrol and Lox-1 siRNA decreased oxLDL-enhanced translocation of proapoptotic Bcl-2-associated X protein (Bax) from the cytoplasm to mitochondria. Sequentially, oxLDL-induced alterations in the mitochondrial membrane potential, cytochrome c release, and activities of caspases-9, -3, and -6 were decreased by resveratrol. Pretreatment with Z-VEID-FMK (benzyloxycarbonyl-Leu-Glu-His-Asp-fluoromethyl ketone) synergistically promoted resveratrol's protection against DNA fragmentation and cell apoptosis. Therefore, this study shows that resveratrol can protect CECs from oxLDL-induced apoptotic insults via downregulating Lox-1-mediated activation of the Bax-mitochondria-cytochrome c-caspase protease pathway.

Andrographolide enhances nuclear factor-kappaB subunit p65 Ser536 dephosphorylation through activation of protein phosphatase 2A in vascular smooth muscle cells

Recent studies have demonstrated that transcription factor nuclear factor (NF)-κB inhibition may contribute to the protective anti-inflammatory actions of andrographolide, an abundant component of plants of the genus Andrographis. However, the precise mechanism by which andrographolide inhibits NF-κB signaling remains unclear. We thus investigated the mechanism involved in andrographolide suppression of NF-κB signaling in rat vascular smooth muscle cells (VSMCs) exposed to proinflammatory stimuli, LPS, and IFN-γ. Andrographolide was shown to suppress LPS/IFN-γ-induced inducible nitric-oxide synthase and matrix metalloprotease 9 expression in rat VSMCs. Andrographolide also inhibited LPS/IFN-γ-induced p65 nuclear translocation, DNA binding activity, p65 Ser(536) phosphorylation, and NF-κB reporter activity. However, IKK phosphorylation and downstream inhibitory κBα phosphorylation and degradation were not altered by the presence of andrographolide in LPS/IFN-γ-stimulated VSMCs. These andrographolide inhibitory actions could be prevented by selective inhibition of neutral sphingomyelinase and protein phosphatase 2A (PP2A). Furthermore, andrographolide was demonstrated to increase ceramide formation and PP2A activity in VSMCs and to inhibit neointimal formation in rat carotid injury models. These results suggest that andrographolide caused neutral sphingomyelinase-mediated ceramide formation and PP2A activation to dephosphorylate p65 Ser(536), leading to NF-κB inactivation and subsequent inducible nitric-oxide synthase down-regulation in rat VSMCs stimulated by LPS and IFN-γ.

Resveratrol protects against oxidized LDL-induced breakage of the blood-brain barrier by lessening disruption of tight junctions and apoptotic insults to mouse cerebrovascular endothelial cells

Cerebrovascular endothelial cells (CEC) comprise the blood-brain barrier (BBB). In a previous study, we showed that oxidized LDL (oxLDL) can induce apoptosis of mouse CEC. Resveratrol possesses chemopreventive potential. This study aimed to evaluate the effects of resveratrol on oxLDL-induced insults to mouse CEC and its possible mechanisms. Exposure of mouse CEC to 200 μmol/L oxLDL for 1 h did not cause cell death but significantly altered the permeability and transendothelial electrical resistance of the cell monolayer. However, resveratrol completely normalized such injury. As for the mechanisms, resveratrol completely protected oxLDL-induced disruption of F-actin and microtubule cytoskeletons as well as occludin and zona occludens-1 (ZO-1) tight junctions. The oxLDL-induced decreases in the mitochondrial membrane potential and intracellular ATP levels were normalized by resveratrol. Exposure of mouse CEC to 200 μmol/L oxLDL for 24 h elevated oxidative stress and simultaneously induced cell apoptosis. However, resveratrol partially protected against oxLDL-induced CEC apoptosis. The oxLDL-induced alterations in levels of Bcl-2, Bax, and cytochrome c were completely normalized by resveratrol. Consequently, resveratrol partially decreased oxLDL-induced activation of caspases-9 and -3. Therefore, in this study, we show that resveratrol can protect against oxLDL-induced damage of the BBB through protecting disruption of the tight junction structure and apoptotic insults to CEC.

Apoptosis signal-regulating kinase 1 in peptidoglycan-induced COX-2 expression in macrophages

In this study, we investigated the role of ASK1 in PGN-induced C/EBPbeta activation and COX-2 expression in RAW 264.7 macrophages. The PGN-induced COX-2 expression was attenuated by the DNs of ASK1, JNK1, JNK2, a JNK inhibitor (SP600125), and an AP-1 inhibitor (curcumin). PGN caused ASK1 dephosphorylation time-dependently at Ser967, dissociation from the ASK1-14-3-3 complex, and subsequent ASK1 activation. In addition, PGN activated PP2A and suppression of PP2A by okadaic acid markedly inhibited PGN-induced ASK1 Ser967 dephosphorylation and COX-2 expression. PGN induced the activation of the JNK-AP-1 signaling cascade downstream of ASK1. PGN-increased C/EBPbeta expression and DNA-binding activity were inhibited by the ASK1-JNK-AP-1 signaling blockade. COX-2 promoter luciferase activity induced by PGN was attenuated in cells transfected with the COX-2 reporter construct possessing the C/EBP-binding site mutation. In addition, the ASK1-JNK-AP-1-C/EBPbeta cascade was activated in human peripheral mononuclear cells exposure to PGN. The TLR2 agonist Pam(3)CSK(4) was also shown to induce ASK1 Ser967 dephosphorylation, JNK and c-jun phosphorylation, C/EBPbeta activation, and COX-2 expression in RAW 264.7 macrophages. PGN-induced COX-2 promoter luciferase activity was prevented by selective inhibition of TLR2 and c-Jun in RAW 264.7 macrophages. Our data demonstrate that PGN might activate the TLR2-mediated PP2A-ASK1-JNK-AP-1-C/EBPbeta cascade and subsequent COX-2 expression in RAW 264.7 macrophages.

Triggering role of acid sphingomyelinase in endothelial lysosome-membrane fusion and dysfunction in coronary arteries

The present study determined whether activation of acid sphingomyelinase (ASM) drives membrane proximal lysosomes to fuse to the cell surface, facilitating membrane lipid rafts (LRs) clustering in coronary arterial endothelial cells (CAECs) and leading to endothelial dysfunction. By confocal microscopy, the activators of ASM, phosphatidylinositol (PI), and bis (monoacylglyceryl) phosphate (Bis), and an inducer of ASM, butyrate, were found to increase LRs clustering in bovine CAECs, which was blocked by lysosome fusion inhibitor vacuolin-1. However, arsenic trioxide (Ars), an inducer of de novo synthesis of ceramide, had no such effect. Similarly, vacuolin-1-blockable effects were observed using fluorescence resonance energy transfer detection. Liquid chromatography-electrospray ionization-tandem mass spectrometry analysis demonstrated that all of these treatments, even Ars, increased ceramide production in CAECs. When ASM gene was silenced, all treatments except Ars no longer increased ceramide levels. Furthermore, dynamic fluorescence monitoring in live cells showed that PI and Bis stimulated lysosome-membrane fusion in CAECs. Functionally, PI and Bis impaired endothelium-dependent vasodilation in perfused coronary arteries, which was blocked by vacuolin-1 and a lysosome function inhibitor, bafilomycine. FasL (Fas ligand), a previously confirmed lysosome fusion stimulator as a comparison, also produced a similar effect. It is concluded that ASM activation serves as a triggering mechanism and driving force, leading to fusion of membrane proximal lysosomes into LR clusters on the cell membrane of CAECs, which represents a novel mechanism mediating endothelial dysfunction during death receptor activation or other pathological situation.

Ceramide in stress response

Evidence has consistently indicated that activation of sphingomyelinases and/or ceramide synthases and the resulting accumulation of ceramide mediate cellular responses to stressors such as lipopolysaccharide, interleukin 1beta, tumor necrosis factor alpha, serum deprivation, irradiation and various antitumor treatments. Recent studies had identified the genes encoding most of the enzymes responsible for the generation of ceramide and ongoing research is aimed at characterizing their individual functions in cellular response to stress. This chapter discusses the seminal and more recent discoveries in regards to the pathways responsible for the accumulation of ceramide during stress and the mechanisms by which ceramide affects cell functions. The former group includes the roles of neutral sphingomyelinase 2, serine palmitoyltransferase, ceramide synthases, as well as the secretory and endosomal/lysosomal forms of acid sphingomyelinase. The latter summarizes the mechanisms by which ceramide activate its direct targets, PKCzeta, PP2A and cathepsin D. The ability of ceramide to affect membrane organization is discussed in the light of its relevance to cell signaling. Emerging evidence to support the previously assumed notion that ceramide acts in a strictly structure-specific manner are also included. These findings are described in the context of several physiological and pathophysiological conditions, namely septic shock, obesity-induced insulin resistance, aging and apoptosis of tumor cells in response to radiation and chemotherapy.

Dietary soy protein reduces cardiac lipid accumulation and the ceramide concentration in high-fat diet-fed rats and ob/ob mice

Obesity is an epidemic condition strongly associated with cardiovascular morbidity and mortality. Heart disease secondary to obesity is associated with myocardial steatosis, leading to ceramide synthesis and cell dysfunction in a process known as lipotoxicity. Soy protein has been demonstrated to reduce lipotoxicity in the liver and pancreas in different rodent models of obesity. Thus, our purpose in the present work was to assess the effect of dietary soy protein on cardiac lipid accumulation and ceramide formation during obesity and to evaluate its effect in the following 2 rodent models of obesity: 1) a diet-induced obesity model in Sprague-Dawley rats was produced by feeding rats a control or a high-fat casein or soy protein diet for 180 d; and 2) wild-type and ob/ob mice were fed a casein or soy protein diet for 90 d. Soy protein intake led to lower cholesterol and triglyceride concentrations in the hearts of rats and ob/ob mice in association with a greater PPARalpha mRNA concentration and a lower level of sterol regulatory element binding protein-1 mRNA than those fed casein. The ceramide concentration was also lower in hearts of rats and ob/ob mice that were fed soy protein in association with lower serine palmitoyl transferase (SPT)-1 and tumor necrosis factor-alpha mRNA concentrations. These results indicate that dietary soy protein can reduce the heart ceramide concentration by reducing the expression of SPT-1, a key enzyme in the formation of this sphingolipid in the heart of obese rodents, and by reducing lipid accumulation. Thus, soy protein consumption may be considered as a dietary therapeutic approach for lipotoxic cardiomyopathy prevention.

Hypoxia-induced neuronal apoptosis is mediated by de novo synthesis of ceramide through activation of serine palmitoyltransferase

Cellular hypoxia can lead to cell death or adaptation and has important effects on development, physiology, and pathology. Here, we investigated the role and regulation of ceramide in hypoxia-induced apoptosis of SH-SY5Y neuroblastoma cells. Hypoxia increased the ceramide concentration; subsequently, we observed biochemical changes indicative of apoptosis, such as DNA fragmentation, nuclear staining, and poly ADP-ribose polymerase (PARP) cleavage. The hypoxic cell death was potently inhibited by a caspase inhibitor, zVAD-fmk (benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone). l-Cycloserine, a serine palmitoyltransferase (SPT) inhibitor, and fumonisin B(1) (FB(1)), a ceramide synthase inhibitor, inhibited the hypoxia-induced increase in ceramide, indicating that the increase occurred via the de novo pathway. Hypoxia increased the activity and protein levels of SPT2, suggesting that the hypoxia-induced increase in ceramide is due to the transcriptional up-regulation of SPT2. Specific siRNA of SPT2 prevented hypoxia-induced cell death and ceramide production. However, hypoxia also increased the cellular level of glucosylceramide, which was inhibited by a glucosylceramide synthase (GCS) inhibitor and specific siRNA, but not a ceramidase inhibitor. The increase in glucosylceramide was accompanied by increases in both PARP cleavage and DNA fragmentation. Together, the current results suggest that both SPT and GCS may regulate the cellular level of ceramide, and thus may be critical enzymes for deciding the fate of the cells exposed to hypoxia.

Relationships among endocrine and signaling-related responses to antidepressants in human monocytic U-937 blood cells: analysis of factors and response patterns

OBJECTIVE

Antidepressants (AD) (desipramine, imipramine, maprotiline, mirtazapine) and corticosteroid (CS) were examined for their effects on gene expression in human monocytic U-937 blood cells. Endocrine and signaling-related response patterns were determined by expression analysis of different factors, comprising endocrine (glucocorticoid receptor [GR], GR-alpha/beta/gamma; mineralocorticoid receptor [MR]) and signaling-related pathways (p105, STAT3, c-jun, c-fos, JNK1, GAPDH, TNF-alpha).

METHODS

A semiquantitative RT-PCR for factor responses after 24 h of treatment was conducted and exploratory multivariate statistical procedures were applied for further analysis.

RESULTS

Compared to controls, significant reduction of mRNA levels of GR-beta under imipramine and of c-jun under desipramine treatment were found. CS treatment significantly reduced mRNA levels of GR-alpha/beta, TNF-alpha, p105 and c-jun compared to controls. Compared to CS treatment, significantly increased mRNA levels were found for JNK1 under imipramine treatment and for GR-alpha after treatment with all AD examined.

DISCUSSION

The multivariate approach meets the requirements of the complex situation of metabolic reactions induced by AD or CS treatment. Our data show that AD affect both, endocrine and signaling-related factors in human monocytic U-937 blood cells, although clearly not in a uniform manner. Hereby, GR is obviously playing a comparably central role. Overall, AD treatment might indeed normalize deviations of cellular endocrine and signaling-related pathways in major depressive disorder via the mechanisms examined.

Sphingolipids, insulin resistance, and metabolic disease: new insights from in vivo manipulation of sphingolipid metabolism

Obesity and dyslipidemia are risk factors for metabolic disorders including diabetes and cardiovascular disease. Sphingolipids such as ceramide and glucosylceramides, while being a relatively minor component of the lipid milieu in most tissues, may be among the most pathogenic lipids in the onset of the sequelae associated with excess adiposity. Circulating factors associated with obesity (e.g., saturated fatty acids, inflammatory cytokines) selectively induce enzymes that promote sphingolipid synthesis, and lipidomic profiling reveals relationships between tissue sphingolipid levels and certain metabolic diseases. Moreover, studies in cultured cells and isolated tissues implicate sphingolipids in certain cellular events associated with diabetes and cardiovascular disease, including insulin resistance, pancreatic beta-cell failure, cardiomyopathy, and vascular dysfunction. However, definitive evidence that sphingolipids contribute to insulin resistance, diabetes, and atherosclerosis has come only recently, as researchers have found that pharmacological inhibition or genetic ablation of enzymes controlling sphingolipid synthesis in rodents ameliorates each of these conditions. Herein we will review the role of ceramide and other sphingolipid metabolites in insulin resistance, beta-cell failure, cardiomyopathy, and vascular dysfunction, focusing on these in vivo studies that identify enzymes controlling sphingolipid metabolism as therapeutic targets for combating metabolic disease.

C18 ceramide analysis in mammalian cells employing reversed-phase high-performance liquid chromatography tandem mass spectrometry

Ceramides play an important role in diverse cellular functions such as differentiation, cell cycle progression, cell-cell adhesion, senescence, and apoptosis. Here we report a method of extracting lipids from mammalian cells and quantifying ceramide, where the assay conditions were optimized for reproducibility, linearity, recovery, and sensitivity. Simultaneous chromatographic separations were carried out by reversed-phase high-performance liquid chromatography coupled to electrospray ionization using a Pursuit 3 Diphenyl column (50 x 2.0 mm) and supported by a mobile phase consisting of acetonitrile plus 0.1% formic acid and 25 mM ammonium acetate. Ceramides were detected in the multiple reaction mode by tandem mass spectrometry in the positive ion mode, and all extracted ion peaks were integrated for quantitative analysis. The limits of detection and quantification achieved were 0.2 and 1.0 pg on column, respectively. Using this method, we successfully quantified and compared differences in C(18) ceramide levels induced by two DNA-damaging agents, mitomycin C and daunorubicin, and two apoptosis-inducing ligands, tumor necrosis factor alpha (TNF-alpha) and TNF-related apoptosis-inducing ligand (TRAIL). This work, therefore, describes a method that will be helpful for investigating how ceramide is regulated by different chemotherapeutic agents and will help us to better understand the mechanisms of signal transduction involving ceramide.

Methylprednisolone protects oligodendrocytes but not neurons after spinal cord injury

Methylprednisolone (MP) is used to treat a variety of neurological disorders involving white matter injury, including multiple sclerosis, acute disseminated encephalomyelitis, and spinal cord injury (SCI). Although its mechanism of action has been attributed to anti-inflammatory or antioxidant properties, we examined the possibility that MP may have direct neuroprotective activities. Neurons and oligodendrocytes treated with AMPA or staurosporine died within 24 h after treatment. MP attenuated oligodendrocyte death in a dose-dependent manner; however, neurons were not rescued by the same doses of MP. This protective effect was reversed by the glucocorticoid receptor (GR) antagonist (11, 17)-11-[4-(dimethylamino)phenyl]-17-hydroxy-17-(1-propynyl)estra-4,9-dien-3-one (RU486) and small interfering RNA directed against GR, suggesting a receptor-dependent mechanism. MP reversed AMPA-induced decreases in the expression of anti-apoptotic Bcl-x(L), caspase-3 activation, and DNA laddering, suggesting anti-apoptotic activity in oligodendrocytes. To examine whether MP demonstrated this selective protection in vivo, neuronal and oligodendrocyte survival was assessed in rats subjected to spinal cord injury (SCI); groups of rats were treated with or without MP in the presence or absence of RU486. Eight days after SCI, MP significantly increased oligodendrocytes (CC-1-immunoreactive cells) after SCI, but neuronal (neuronal-specific nuclear protein-immunoreactive cells) number remained unchanged; RU486 reversed this protective effect. MP also inhibited SCI-induced decreases in Bcl-x(L) and caspase-3 activation. Consistent with these findings, the volume of demyelination, assessed by Luxol fast blue staining, was attenuated by MP and reversed by RU486. These results suggest that MP selectively inhibits oligodendrocyte but not neuronal cell death via a receptor-mediated action and may be a mechanism for its limited protective effect after SCI.

Sphingomyelinase decreases type II collagen expression in bovine articular cartilage chondrocytes via the ERK signaling pathway

OBJECTIVE

Ceramide, a mediator of proinflammatory cytokine signaling, induces cartilage degradation and reduces type II collagen synthesis in articular cartilage. The accumulation of ceramide is associated with arthritis in Farber's disease. The aim of this study was to investigate the mechanism of ceramide-induced down-regulation of type II collagen.

METHODS

Bovine articular chondrocytes were stimulated with sphingomyelinase (SMase) to increase levels of endogenous ceramide. Components of the ERK pathway were inhibited by Raf-1 kinase inhibitor and the MEK inhibitor, PD98059. Cell extracts were analyzed by Western blotting for ERK-1/2, SOX9, c-Fos, and type II collagen, and the level of c-fos messenger RNA (mRNA) was analyzed by quantitative polymerase chain reaction. Localization of ERK-1/2, SOX9, and c-Fos was assessed by immunocytochemistry and confocal microscopy.

RESULTS

SMase treatment of chondrocytes caused sustained phosphorylation of ERK-1/2 throughout the cytoplasm and nucleus that was reduced by inhibitors of Raf-1 kinase and MEK-1/2. SMase treatment of chondrocytes also induced translocation of c-Fos to the nucleus and phospho-SOX9 to the cytoplasm and increased expression of c-fos mRNA. Type II collagen expression, which was down-regulated by SMase treatment, was restored by the MEK-1/2 inhibitor, PD98059.

CONCLUSION

SMase down-regulates type II collagen in articular chondrocytes via activation of the ERK signaling cascade, redistribution of SOX9, and recruitment of c-Fos. This new mechanism for cartilage degradation provides potential targets for future treatment of arthritic disease.

Regulation of IGF-I function by proinflammatory cytokines: at the interface of immunology and endocrinology

During the past decade, the immune and endocrine systems have been discovered to interact in controlling physiologic processes as diverse as cell growth and differentiation, metabolism, and even human and animal behavior. The interaction between these two major physiological systems is a bi-directional process. While it has been well documented that hormones, including prolactin (PRL), growth hormone (GH), insulin-like growth factor-I (IGF-I), and thyroid-stimulating hormone (TSH), regulate a variety of immune events, a great deal of data have accumulated supporting the notion that cytokines from the innate immune system also affect the neuroendocrine system. Communication between these two systems coordinates processes that are necessary to maintain homeostasis. Proinflammatory cytokines often act as negative regulatory signals that temper the action of hormones and growth factors. This system of 'checks and balances' is an active, ongoing process, even in healthy individuals. Dysregulation of this process has been implicated as a potential pathogenic factor in the development of co-morbid conditions associated with several chronic inflammatory diseases, including type 2 diabetes, cardiovascular disease, cerebrovascular disease, inflammatory bowel disease, rheumatoid arthritis, major depression, and even normal aging. Over the past decade, research in our laboratory has focused on the ability of the major proinflammatory cytokines, tumor necrosis factor (TNF)alpha and interleukin (IL)-1beta, to induce a state of IGF resistance. This review will highlight these and other new findings by explaining how proinflammatory cytokines induce resistance to the major growth factor, insulin-like growth factor-I (IGF-I). We also highlight that IGF-I can induce resistance or reduce sensitivity to brain TNFalpha and discuss how TNFalpha, IL-1beta, and IGF-I interact to regulate several aspects of behavior and cognition.

Ceramide: a common pathway for atherosclerosis?

Plasma sphingomyelin concentration is correlated with the development of atherosclerosis. It has been found to exist in significantly higher concentrations in aortic plaque. This appears to have clinical relevance as well as it has been shown to be an independent predictor of coronary artery disease. Ceramide, the backbone of sphingolipids, is the key component which affects atherosclerotic changes through its important second-messenger role. This paper sheds light on some of the current literature supporting the significance of ceramide with respect to its interactions with lipids, inflammatory cytokines, homocysteine and matrix metalloproteinases. Furthermore, the potential therapeutic implications of modulating ceramide concentrations are also discussed.

Aging up-regulates expression of inflammatory mediators in mouse adipose tissue

Obesity is a leading risk factor for type 2 diabetes (T2D). Aging is associated with an increase in T2D incidence, which is not totally explained by the much lower prevalence of obesity in the elderly. Low-grade inflammation in adipose tissue (AT) contributes to insulin resistance and T2D. Thus, we determined whether inflammatory responses are up-regulated with age in AT. The results showed that visceral AT from old C57BL mice had significantly higher mRNA expression of the proinflammatory cytokines IL-1beta, IL-6, TNF-alpha, and COX-2 and lower expression of anti-inflammatory PPAR-gamma than those of young mice. We further showed that adipocytes (AD) and not stromal vascular cells including macrophages (Mphi) were the cells responsible for this higher inflammatory state of the aged AT, suggesting that the age-associated increase in AT inflammation is distinguished from that seen in obesity, in which Mphi are the main contributors. However, peritoneal Mphi of either age (young or old) produced more TNF-alpha and IL-6 after incubation in old AD-conditioned medium compared with young AD-conditioned medium. This suggests that in addition to producing more inflammatory cytokines, AD from old mice induce a higher inflammatory response in other cells. Sphingolipid ceramide was higher in old compared with young AD. Reducing ceramide levels or inhibiting NF-kappaB activation decreased cytokine production, whereas the addition of ceramide increased cytokine production in young AD to a level comparable to that seen in old AD, suggesting that ceramide-induced activation of NF-kappaB plays a key role in AT inflammation.

De novo C16- and C24-ceramide generation contributes to spontaneous neutrophil apoptosis

Neutrophils rapidly undergo spontaneous apoptosis following their release from the bone marrow. Although central to leukocyte homeostasis, the mechanisms that regulate neutrophil apoptosis remain poorly understood. We show here that apoptosis of cultured neutrophils is preceded by a substantial increase in the intracellular levels of 16 and 24 carbon atom (C(16)- and C(24))-ceramides, which are lipid second messengers of apoptosis and stress signaling. Treatment of neutrophils with fumonisin B(2), a selective inhibitor of the de novo pathway of ceramide synthesis, prevented accumulation of C(16)- and C(24)-ceramides. Moreover, fumonisin B(2) significantly reduced caspase-3, -8, and -9 activation and apoptosis in these cells. Conversely, 3-O-methylsphingomyelin and fantofarone, which are specific inhibitors of neutral and acid sphingomyelinases, respectively, neither inhibited C(16)- and C(24)-ceramide production nor decreased the apoptosis rate in neutrophils, indicating that in these cells, ceramides are not generated from membrane sphingomyelin. Further experiments showed that increasing endogenous C(16)- and C(24)-ceramide levels by using DL-threo-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol and (1S,2R)-D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol, two inhibitors of ceramide metabolism, enhances caspase-3, -8, and -9 activity and increases neutrophil apoptosis. Similarly, apoptosis was induced rapidly when synthetic C(16)- and/or C(24)-ceramides were added to neutrophil cultures. Finally, GM-CSF, a cytokine that delays neutrophil apoptosis, abrogated C(16)- and C(24)-ceramide accumulation totally in cultured neutrophils, whereas Fas ligation accelerated apoptosis in these cells without affecting de novo ceramide production. We conclude that de novo generation of C(16)- and C(24)-ceramides contributes to spontaneous neutrophil apoptosis via caspase activation and that GM-CSF exerts its antiapoptotic effects on neutrophils, at least partly through inhibition of ceramide accumulation.

Antiphospholipid reactivity against cardiolipin metabolites occurring during endothelial cell apoptosis

We have recently shown that cardiolipin (CL) and its metabolites move from mitochondria to other cellular membranes during death receptor-mediated apoptosis. In this study, we investigate the immunoreactivity to CL derivatives occurring during endothelial apoptosis in patients with antiphospholipid syndrome (APS) and systemic lupus erythematosus (SLE). We compared the serum immunoreactivity to CL with that of its derivatives monolysocardiolipin (MCL), dilysocardiolipin (DCL), and hydrocardiolipin (HCL) by means of both enzyme-linked immunosorbent assay and thin-layer chromatography (TLC) immunostaining. In addition, we investigated the composition of phospholipid extracts from the plasma membrane of apoptotic endothelial cells and the binding of patients' sera to the surface of the same cells by using high-performance TLC and immunofluorescence analysis. The average reactivity to MCL was comparable with that of CL and significantly higher than that for DCL and HCL in patients studied, both in the presence or in the absence of beta₂-glycoprotein I. Of relevance for the pathogenic role of these autoantibodies, immunoglobulin G from patients' sera showed an increased focal reactivity with the plasma membrane of endothelial cells undergoing apoptosis. Interestingly, the phospholipid analysis of these light membrane fractions showed an accumulation of both CL and MCL. Our results demonstrated that a critical number of acyl chains in CL derivatives is important for the binding of antiphospholipid antibodies and that MCL is an antigenic target with immunoreactivity comparable with CL in APS and SLE. Our finding also suggests a link between apoptotic perturbation of CL metabolism and the production of these antibodies.