Current Status and Perspectives in Ceramide-Targeting Molecular Medicine

N-Metallocenoylsphingosines as targeted ceramidase inhibitors: Syntheses and antitumoral effects

Three N-metallocenoylsphingosines with variance in the central metal (Fe, Co, Ru), the charge (neutral or cationic), and the arene ligands (Cp₂, Cp*Ph) were synthesized from serine and metallocene carboxylic acids as substrate-analogous inhibitors of human acid ceramidase (AC). Their inhibitory potential was examined using the recombinant full length ASAH1 enzyme, expressed and secreted from High Five insect cells, and the fluorescent substrate Rbm14-12. All complexes inhibited AC, most strongly so ruthenium(II) complex 13a. Some antitumoral effects of the complexes, such as the interference with the microtubular and F-actin cytoskeleton of cancer cells, were correlated to their AC-inhibition, whereas others, e.g. their cytotoxicity and their induction of caspase-3/-7 activity in cancer cells, were not. All complexes accumulated preferentially in the lysosomes of cancer cells like their target AC, arrested the cells in G1 phase of the cell cycle, and displayed cytotoxicity with mostly single-digit micromolar IC₅₀ values while inducing cancer cell apoptosis.

Acid Sphingomyelinase Down-regulation Alleviates Vascular Endothelial Insulin Resistance in Diabetic Rats

Insulin resistance in endothelial cells contributes to the development of cardiovascular disease in patients with type 2 diabetes. Acid sphingomyelinase (ASM) is a soluble glycoprotein which plays a vital role in the development and progression of various diseases such as cardiovascular and metabolic diseases. However, it remains unknown if ASM regulates insulin resistance in vascular endothelial cells in type 2 diabetes. ASM down-regulation with gene silencing and selective inhibitor amitriptyline was used in the rat aortic endothelial cells (RAECs) treated with palmitic acid (PA), a common saturated free fatty acid, which is thought to be the major cause of insulin resistance. It was shown that ASM down-regulation increased glucose uptake and glucose transporter-4 (Glut4) expression and reversed the phosphorylation of pIRS-1-ser307 and AKT-ser473 via ceramide, consequently resulting in the decrease of the production of endothelial nitric oxide synthase (eNOS) and nitric oxide in PA-induced RAECs. We further found that ASM down-regulation blocked the Nox2- and Nox4-dependent superoxide (O₂ ^-· ) generation, which regulated glucose metabolism in RAECs during PA stimulation. In vivo, amitriptyline relieved the vasodilatory response to acetylcholine and restored the level of ceramide, Nox2 and Nox4 in the aorta endothelium of high-fat diet-fed rats following an injection of streptozotocin. Taken together, these results suggest that ASM down-regulation can improve endothelial insulin resistance which is attributed to inhibiting redox signalling in RAECs. Thus, these data support the idea that ASM is a promising clinical biomarker and potential therapeutic target for diabetic vascular complication.

Regulation of Sphingolipid Biosynthesis by the Morphogenesis Checkpoint Kinase Swe1

Sphingolipid (SL) biosynthesis is negatively regulated by the highly conserved endoplasmic reticulum-localized Orm family proteins. Defective SL synthesis in Saccharomyces cerevisiae leads to increased phosphorylation and inhibition of Orm proteins by the kinase Ypk1. Here we present evidence that the yeast morphogenesis checkpoint kinase, Swe1, regulates SL biosynthesis independent of the Ypk1 pathway. Deletion of the Swe1 kinase renders mutant cells sensitive to serine palmitoyltransferase inhibition due to impaired sphingoid long-chain base synthesis. Based on these data and previous results, we suggest that Swe1 kinase perceives alterations in SL homeostasis, activates SL synthesis, and may thus represent the missing regulatory link that controls the SL rheostat during the cell cycle.

Adiposopathy: how do diet, exercise and weight loss drug therapies improve metabolic disease in overweight patients?

An increase in bodyweight is generally associated with an increased risk of excessive fat-related metabolic diseases (EFRMD), including Type 2 diabetes mellitus, hypertension and dyslipidemia. However, not all patients who are overweight have EFRMD, and not all patients with EFRMD are significantly overweight. The adipocentric paradigm provides the basis for a unifying, pathophysiological process whereby fat gain in susceptible patients leads to fat dysfunction ('sick fat'), and wherein pathological abnormalities in fat function (adiposopathy) are more directly related to the onset of EFRMD than increases in fat mass (adiposity) alone. But just as worsening fat function worsens EFRMD, improved fat function improves EFRMD. Peroxisome proliferator-activated receptor-gamma agonists increase the recruitment, proliferation and differentiation of preadipocytes ('healthy fat') and cause apoptosis of hypertrophic and dysfunctional (including visceral) adipocytes resulting in improved fat function and improved metabolic parameters associated with EFRMD. Weight loss interventions, such as a hypocaloric diet and physical exercise, in addition to agents such as orlistat, sibutramine and cannabinoid receptor antagonists, may have favorable effects upon fat storage (lipogenesis and fat distribution), nutrient metabolism (such as free fatty acids), favorable effects upon adipose tissue factors involved in metabolic processes and inflammation, and enhanced 'cross-talk' with other major organ systems. In some cases, weight loss therapeutic agents may even affect metabolic parameters and adipocyte function independently of weight loss alone, suggesting that the benefit of these agents in improving EFRMD may go beyond their efficacy in weight reduction. This review describes how adiposopathy interventions may affect fat function, and thus improve EFRMD.

Consumption of a high-fat diet, but not regular endurance exercise training, regulates hypothalamic lipid accumulation in mice

Obesity is characterised by increased storage of fatty acids in an expanded adipose tissue mass and in peripheral tissues such as the skeletal muscle and liver, where it is associated with the development of insulin resistance. Insulin resistance also develops in the central nervous system with high-fat feeding. The capacity for hypothalamic cells to accumulate/store lipids, and the effects of obesity remain undefined. The aims of this study were (1) to examine hypothalamic lipid content in mice with increased dietary fat intake and in obese ob/ob mice fed a low-fat diet, and (2) to determine whether endurance exercise training could reduce hypothalamic lipid accumulation in high-fat fed mice. Male C57BL/6 mice were fed a low- (LFD) or high-fat diet (HFD) for 12 weeks; ob/ob mice were maintained on a chow diet. HFD-exercise (HFD-ex) mice underwent 12 weeks of high-fat feeding with 6 weeks of treadmill exercise training (increasing from 30 to 70 min day(-1)). Hypothalamic lipids were assessed by unbiased mass spectrometry. The HFD increased body mass and hepatic lipid accumulation, and induced glucose intolerance, while the HFD-ex mice had reduced body weight and improved glucose tolerance. A total of 335 lipid molecular species were identified and quantified. Lipids known to induce insulin resistance, including ceramide (22%↑), diacylglycerol (25%↑), lysophosphatidylcholine (17%↑), cholesterol esters (60%↑) and dihexosylceramide (33%↑), were increased in the hypothalamus of HFD vs. LFD mice. Hypothalamic lipids were unaltered with exercise training and in the ob/ob mice, suggesting that obesity per se does not alter hypothalamic lipids. Overall, hypothalamic lipid accumulation is regulated by dietary lipid content and is refractory to change with endurance exercise training.

Neutral sphingomyelinase 2 deficiency increases hyaluronan synthesis by up-regulation of Hyaluronan synthase 2 through decreased ceramide production and activation of Akt

Fibroblasts from the fro/fro mouse, with a deletion in the Smpd3 gene coding for the active site of neutral sphingomyelinase 2 (NSMase2), secreted increased amounts of hyaluronan (HA). This was reversed by transfection with the Smpd3 gene, suggesting a connection between sphingolipid and glycosaminoglycan metabolism. The deficiency of NSMase2 resulted in storage of sphingomyelin (SM) and cholesterol with a 50% reduction in ceramides (Cer). RT-PCR and Western blot analysis showed that increased HA secretion resulted from increased hyaluronan synthase 2 (HAS2) activity localized to sphingolipid-enriched lipid rafts. Although cholesterol levels were also elevated in lipid rafts from mouse fibroblasts deficient in lysosomal acid SMase activity (deletion of the Smpd1(-/-) gene), there was no increase in HA secretion. We then showed that in fro/fro fibroblasts, the reduced ceramide was associated with decreased phosphorylation of protein phosphatase 2A (PP2A) and increased phosphorylation of its substrate Akt-p, together with PI3K, PDK1, mTOR (mammalian target of rapamycin), and p70S6K, although PTEN was unaffected. Exogenous ceramide, as well as inhibitors of Akt (Akt inhibitor VIII), PI 3-kinase (LY294002 and wortmannin), and mTOR (rapamycin) reduced secretion of HA, whereas the NSMase2 inhibitor GW4869 increased HA synthesis and secretion. We propose that NSMase2/Cer are the key mediators of the regulation of HA synthesis, via microdomains and the Akt/mTOR pathway.

Preparation of (13)C-labeled ceramide by acetic acid bacteria and its incorporation in mice

We prepared 2-hydroxypalmitoyl-sphinganine (dihydroceramide) labeled with a stable isotope by culturing acetic acid bacteria with (13)C-labeled acetic acid. The GC/MS spectrum of the trimethylsilyl derivative of (13)C-labeled dihydroceramide gave molecular ions with an increased mass of 12-17 Da over that of nonlabeled dihydroceramide. The fragment ions derived from both sphinganine base and 2-hydroxypalmitate were confirmed to be labeled with the stable isotope in the spectrum. Therefore, (13)C-labeled dihydroceramide can be an extremely useful tool for analyzing sphingolipid metabolism. The purified [(13)C]dihydroceramide was administered orally to mice for 12 days, and the total sphingoid base fractions in various tissues were analyzed by GC/MS. The spectrum patterns specific to (13)C-labeled sphingoids were detected in the tissues tested. Sphinganine pools in skin epidermis, liver, skeletal muscle, and synapse membrane in brain were replaced by [(13)C]sphinganine at about 4.5, 4.0, 1.0, and 0.3%, respectively. Moreover, about 1.0% of the sphingosine pool in the liver was replaced by [(13)C]sphingosine, implying that exogenous dihydroceramide can be converted to sphingosine. These results clearly indicate that ingested dihydroceramide can be incorporated into various tissues, including brain, and metabolized to other sphingolipids.

Improved synthesis of a fluorogenic ceramidase substrate

Substantial interest has focused on the roles of sphingolipid metabolizing enzymes in a variety of hyperproliferative and inflammatory diseases. A key family of enzymes involved in these pathologies is the ceramidases. Ceramidases cleave the pro-apoptotic lipid ceramide into a long-chain fatty acid and sphingosine, which can then be further metabolized to the mitogenic and inflammatory lipid sphingosine 1-phosphate. Consequently, development of ceramidase inhibitors would provide useful pharmacologic probes for further studies of sphingolipid metabolism, as well as lead compounds for drug development. This effort has been hampered by the lack of in vitro and cellular ceramidase assays that are amenable to high-throughput screening. Recently, a fluorogenic ceramide analog has been described as a substrate for use in ceramidase assays. The synthesis of this compound has now been substantially improved in terms of both the required effort and the overall yield of the process. Key improvements include: reduction in number of required steps, use of a hydroboration reaction; incorporation of a Mitsunobu reaction; improved acylation by the addition of triethylamine; together providing a fourfold increase in the overall yield. In addition, it has been demonstrated that the ceramide analog can be used in high-throughput assays to identify ceramidase inhibitors. Overall, the improved efficiency in the preparation of this ceramidase substrate should accelerate discovery efforts relating to sphingolipid metabolism.

Are the new moisturizers more effective?

PURPOSE OF REVIEW

The authors discuss the role of moisturizers in the daily skin care regimen of healthy skin and in atopic dermatitis, as well as the efficacy of newer products.

RECENT FINDINGS

Recent findings have shown that the affected skin of atopic individuals is deficient in ceramides, an integral lipid component of the stratum corneum. Several manufacturers have developed topical products with ceramides in an attempt to replace the missing molecules. These products, though more expensive, have shown moderate efficacy improvements over traditional moisturizers.

SUMMARY

Moisturizers are recommended to nearly every atopic patient, and should be part of the daily skin care of these patients, as they have the potential to reduce the quantity of topical steroids necessary to keep disease quiescent. The vast array of choices available to consumers is overwhelming, and most patients would like their physicians to recommend specific products. We attempt to provide a review of the newer products available, with special emphasis on those products that contain ceramides.

The expression of the Goodpasture antigen-binding protein (ceramide transporter) in adult rat brain

The Goodpasture antigen-binding protein (GPBP) plays a critical role in brain development. Knockdown of GPBP leads to loss of myelinated tracts in the central nervous system and to extensive apoptosis in the brain during early embryogenesis. GPBP was initially identified as a protein associated with the autoantigen in Goodpasture autoimmune syndrome, where it was shown to be a kinase that regulates type IV collagen organization. GPBP isoforms bind and transport ceramide from the endoplasmic reticulum to the Golgi apparatus and are therefore also known as ceramide transporters (CERT). Ceramide dysregulation is involved in autoimmunity and neurodegenerative disorders. In order to analyze the possible role of GPBP in neuroinflammation and neurodegeneration we studied the basal GPBP expression in normal rat brain. High levels of immunoreactivity were detected in neurons of the cerebral cortex, hippocampal formation, the basal ganglia, the olfactory bulb and nuclei of the thalamus, the hypothalamus and the septal area. Lower expression levels of GPBP were observed widely throughout the brain, suggesting that GPBP plays an important role in central nervous system neuron function.

Differential regulation of sphingomyelin synthesis and catabolism in oligodendrocytes and neurons

Neurons (both primary cultures of 3-day rat hippocampal neurons and embryonic chick neurons) rapidly converted exogenous NBD-sphingomyelin (SM) to NBD-Cer but only slowly converted NBD-Cer to NBD-SM. This was confirmed by demonstrating low in vitro sphingomyelin synthase (SMS) and high sphingomyelinase (SMase) activity in neurons. Similar results were observed in a human neuroblastoma cell line (LA-N-5). In contrast, primary cultures of 3-day-old rat oligodendrocytes only slowly converted NBD-SM to NBD-Cer but rapidly converted NBD-Cer to NBD-SM. This difference was confirmed by high in vitro SMS and low SMase activity in neonatal rat oligodendrocytes. Similar results were observed in a human oligodendroglioma cell line. Mass-Spectrometric analyses confirmed that neurons had a low SM/Cer ratio of (1.5 : 1) whereas oligodendroglia had a high SM/Cer ratio (9 : 1). Differences were also confirmed by [(3)H]palmitate-labeling of ceramide, which was higher in neurons compared with oligodendrocytes. Stable transfection of human oligodendroglioma cells with neutral SMase, which enhanced the conversion of NBD-SM to NBD-Cer and increased cell death, whereas transfection with SMS1 or SMS2 enhanced conversion of NBD-Cer to NBD-SM and was somewhat protective against cell death. Thus, SMS rather than SMases may be more important for sphingolipid homeostasis in oligodendrocytes, whereas the reverse may be true for neurons.

Recent advances on the molecular mechanisms involved in the drug resistance of cancer cells and novel targeting therapies

This review summarizes the recent knowledge obtained on the molecular mechanisms involved in the intrinsic and acquired resistance of cancer cells to current cancer therapies. We describe the cascades that are often altered in cancer cells during cancer progression that may contribute in a crucial manner to drug resistance and disease relapse. The emphasis is on the implication of ATP-binding cassette (ABC) multidrug efflux transporters in drug disposition and antiapoptotic factors, including epidermal growth factor receptor cascades and deregulated enzymes in ceramide metabolic pathways. The altered expression and activity of these signaling elements may have a critical role in the resistance of cancer cells to cytotoxic effects induced by diverse chemotherapeutic drugs and cancer recurrence. Of therapeutic interest, new strategies for reversing the multidrug resistance and developing more effective clinical treatments against the highly aggressive, metastatic, and recurrent cancers, based on the molecular targeting of the cancer progenitor cells and their further differentiated progeny, are also described.

The membrane and lipids as integral participants in signal transduction: lipid signal transduction for the non-lipid biochemist

Reviews of signal transduction have often focused on the cascades of protein kinases and protein phosphatases and their cytoplasmic substrates that become activated in response to extracellular signals. Lipids, lipid kinases, and lipid phosphatases have not received the same amount of attention as proteins in studies of signal transduction. However, lipids serve a variety of roles in signal transduction. They act as ligands that activate signal transduction pathways as well as mediators of signaling pathways, and lipids are the substrates of lipid kinases and lipid phosphatases. Cell membranes are the source of the lipids involved in signal transduction, but membranes also constitute lipid barriers that must be traversed by signal transduction pathways. The purpose of this review is to explore the magnitude and diversity of the roles of the cell membrane and lipids in signal transduction and to highlight the interrelatedness of families of lipid mediators in signal transduction.

Ceramide induces apoptosis in human lung adenocarcinoma A549 cells through mitogen-activated protein kinases

AIM

To provide experimental data for further research on the signal transduction of apoptosis in lung adenocarcinoma cells, we examined the effects of exogenous C2-ceramide administration on several members of the mitogen-activated protein kinase (MAPK) superfamily and caspase-3 in A549 cells.

METHODS

Cell viability and apoptosis were analyzed by cell counting kit-8 assay and flow cytometry. Various MAPK and caspase-3 proteins were detected by Western blotting.

RESULTS

C2-ceramide selectively altered the phosphorylation state of members of the MAPK superfamily, causing hyperphosphorylation of mitogen-activated protein kinase kinase (MEK)1/2 and the p38 MAPK, but not affecting the phosphorylation of extracellular signal-regulated kinase 1/2 and the c-Jun N-terminal kinase. SB-203580 (a p38 MAPK inhibitor) and p38 siRNA, but not U0126 (a MEK inhibitor), partially rescued cell death induced by C2-ceramide. C2-ceramide promoted the activation of caspase-3.

CONCLUSION

Exogenous C2-ceramide induced apoptosis in human lung adenocarcinoma A549 cells. The activation of MAPK and caspase-3 were involved in the mechanisms of C2-ceramide-induced apoptosis in A549 cells.

Alterations in lipid homeostasis of mouse dorsal root ganglia induced by apolipoprotein E deficiency: a shotgun lipidomics study

One of the fundamental goals of lipidomics research is to identify the linkage of an individual gene with a given lipidome, thereby revealing the role of that gene in lipid metabolism, transport, and homeostasis. In this study, we have identified four apolipoprotein E (apoE)-induced alterations in the lipidome of mouse dorsal root ganglia (DRG) through utilizing the technology of shotgun lipidomics. First, apoE mediates sulfatide mass content in mouse DRG, which is comparable to its role in the CNS. Second, apoE contributes to galactosylceramide and ceramide homeostasis in mouse DRG. Third, apoE significantly modulates cholesterol levels in mouse DRG. The latter two functions of apoE are distinct from those in the CNS. Finally, mice null for apoE have dramatically less triacylglycerol mass content in DRG which are opposite to the effects observed in the peripheral organs and vascular system. Collectively, this study identifies the specific alterations in the DRG lipidome induced by apoE knockout and suggests the potential roles of apoE in lipid transport and homeostasis in a tissue specific manner, thereby providing insights into the biochemical mechanisms underlying the functions of apoE in the PNS.

Yeast sphingolipids: recent developments in understanding biosynthesis, regulation, and function

Sphingolipids function as required membrane components of virtually all eukaryotic cells. Data indicate that members of the sphingolipid family of lipids, including sphingoid bases, sphingoid base phosphates, ceramides, and complex sphingolipids, serve vital functions in cell biology by both direct mechanisms (e.g., binding to G-protein coupled receptors to transduce an extracellular signal) and indirect mechanisms (e.g., facilitating correct intracellular protein transport). Because of the diverse roles these lipids play in cell biology, it is important to understand not only their biosynthetic pathways and regulation of sphingolipid synthesis, but also the mechanisms by which some sphingolipid species with specific functions are modified or converted to other sphingolipid species with alternate functions. Due to many factors including ease of culture and genetic modification, and conservation of major sphingolipid metabolic pathways, Saccharomyces cerevisiae has served as an ideal model system with which to identify enzymes of sphingolipid biosynthesis and to dissect sphingolipid function. Recent exciting developments in sphingolipid synthesis, transport, signaling, and overall biology continue to fuel vigorous investigation and inspire investigations in mammalian sphingolipid biology.

Sphingolipid players in the leukemia arena

Sphingolipids function as bioactive mediators of different cellular processes, mostly proliferation, survival, differentiation and apoptosis, besides being structural components of cellular membranes. Involvement of sphingolipid metabolism in cancerogenesis was demonstrated in solid tumors as well as in hematological malignancies. Herein, we describe the main biological and clinical aspects of leukemias and summarize data regarding sphingolipids as mediators of apoptosis triggered in response to anti-leukemic agents and synthetic analogs as inducers of cell death as well. We also report the contribution of molecules that modulate sphingolipid metabolism to development of encouraging strategies for leukemia treatment. Finally we address how deregulation of sphingolipid metabolism is associated to occurrence of therapy resistance both in vitro and in vivo. Sphingolipids can be considered promising therapeutic tools alone or in combination with other compounds, as well as valid targets in the attempt to eradicate leukemia and overcome drug resistance.

Sphingolipids as modulators of cancer cell death: potential therapeutic targets

Through modifications in the fine membrane structure, cell-cell or cell-matrix interactions, and/or modulation of intracellular signaling pathways, sphingolipids can affect the tumorigenic potential of numerous cell types. Whereas ceramide and its metabolites have been described as regulators of cell growth and apoptosis, these lipids as well as other sphingolipid molecules can modulate the ability of malignant cells to grow and resist anticancer treatments, and their susceptibility to non-apoptotic cell deaths. This review summarizes our current knowledge on the properties of sphingolipids in the regulation of cancer cell death and tumor development. It also provides an update on the potential perspectives of manipulating sphingolipid metabolism and using sphingolipid analogues in anticancer therapy.