Fumonisins: fungal toxins that shed light on sphingolipid function

Fumonisins are sphinganine analogues produced by Fusarium moniliforme and related fungi. They inhibit ceramide synthase and block the biosynthesis o f complex sphingolipids, promoting accumulation o f sphinganine and sphinganine 1 phosphate. Disruption o f sphingolipid metabolism by fumonisin B(1) alters cell-cell interactions, the behaviour o f cell-surface proteins, the activity o f protein kinases, the metabolism of other lipids, and cell growth and viability. This multitude of effects probably accounts for the toxicity and carcinogenicity of these mycotoxins. Naturally occurring inhibitors o f sphingolipid metabolism such as fumonisins are proving to be powerful tools for studying the diverse roles of sphingolipids in cell regulation and disease.     

Inhibition of sphingolipid biosynthesis by fumonisins. Implications for diseases associated with Fusarium moniliforme

Culture materials and grains contaminated with certain isolates of Fusarium moniliforme cause equine leucoencephalomalacia, porcine pulmonary edema syndrome, and liver cancer in rats. The causative agents are thought to be a family of compounds called fumonisins, which bear considerable structural similarity to the long-chain (sphingoid) base backbones of sphingolipids. Incubation of rat hepatocytes with fumonisins inhibited incorporation of [14C]serine into the sphingosine moiety of cellular sphingolipids with an IC50 of 0.1 microM for fumonisin B1. In contrast, fumonisin B1 increased the amount of the biosynthetic intermediate sphinganine, which suggests that fumonisins inhibit the conversion of [14C]sphinganine to N-acyl-[14C]sphinganines, a step that is thought to precede introduction of the 4,5-trans double bond of sphingosine (Merrill, A.H., Jr. and Wang, E. (1986) J. Biol. Chem. 261, 3764-3769). In agreement with this mechanism, fumonisin B1 inhibited the activity of sphingosine N-acyltransferase (ceramide synthase) in rat liver microsomes with 50% inhibition at approximately 0.1 microM and reduced the conversion of [3H]sphingosine to [3H]ceramide by intact hepatocytes. As far as we are aware, this is the first discovery of a naturally occurring inhibitor of this step of sphingolipid metabolism. These findings suggest that disruption of the de novo pathway of sphingolipid biosynthesis may be a critical event in the diseases that have been associated with consumption of fumonisins.     

High-performance liquid chromatographic determination of sphinganine and sphingosine in serum and urine of subjects from an endemic nephropathy area in Croatia

Endemic nephropathy (EN) is a chronic renal disease present as an endemic in Brodska Posavina, Croatia. The aim of the study was to assess the possible role of fumonisins, i.e., mycotoxins produced by Fusarium moniliforme, as causative agents for EN. Fumonisins inhibit ceramide synthase, the enzyme of de novo synthesis of sphingolipids, which leads to an increase in the sphinganine/sphingosine ratio. In the present study, a modified method has been used for the determination of the sphinganine/sphingosine ratio in human serum and urine of healthy subjects and EN patients from the endemic area. Free sphingoid bases, sphinganine and sphingosine, were obtained by base hydrolysis. Afterwards, precolumn ortho-phthaldialdehyde derivatisation, HPLC separation and quantification by fluorescence detection were performed. The results thus obtained pointed to a sphingolipid metabolism impairment, which may have been induced by fumonisins or fumonisin-like mycotoxins. As statistically significant differences were recorded in the subjects not yet affected with EN, an impairment in the metabolism of sphingolipids might be considered as an early indicator of EN.     

Mechanism of action of sphingolipids and their metabolites in the toxicity of fumonisin B1

Fumonisins are a group of mycotoxins produced primarily by Fusarium moniliforme. Several fumonisins have been isolated through out the years but only fumonisin B1, B2 and B3 are the ones present in naturally contaminated foods, with B1 being the most toxic between them. The structural similarity between sphinganine and fumonisin B1 suggests that the mechanism of action of this mycotoxin is mainly via disruption of sphingolipid metabolism, this is an important step in the cascade of events leading to altered cell growth, differentiation and cell injury. Sphingolipids are a second type of lipid found in cell membranes, particularly nerve cells and brain tissues. Toxicity of fumonisin B1 is given via inhibition of ceramide synthase that catalyzes the formation of dihydroceramide from sphingosine.

This mechanism of action may explain the wide variety of health effects observed when this mycotoxin is ingested like high rate of human oesophageal cancer and promotion of primary liver cancer.     

Fumonisin- and AAL-Toxin-Induced Disruption of Sphingolipid Metabolism with Accumulation of Free Sphingoid Bases

Fumonisins (FB) and AAL-toxin are sphingoid-like compounds produced by several species of fungi associated with plant diseases. In animal cells, both fumonisins produced by Fusarium moniliforme and AAL-toxin produced by Alternaria alternata f. sp. lycopersici inhibit ceramide synthesis, an early biochemical event in the animal diseases associated with consumption of F. moniliforme-contaminated corn. In duckweed (Lemna pausicostata Heglem. 6746), tomato plants (Lycopersicon esculentum Mill), and tobacco callus (Nicotiana tabacum cv Wisconsin), pure FB1 or AAL-toxin caused a marked elevation of phytosphingosine and sphinganine, sphingoid bases normally present in low concentrations. The relative increases were quite different in the three plant systems. Nonetheless, disruption of sphingolipid metabolism was clearly a common feature in plants exposed to FB1 or AAL-toxin. Resistant varieties of tomato (Asc/Asc) were much less sensitive to toxin-induced increases in free sphinganine. Because free sphingoid bases are precursors to plant "ceramides," their accumulation suggests that the primary biochemical lesion is inhibition of de novo ceramide synthesis and reacylation of free sphingoid bases. Thus, in plants the disease symptoms associated with A. alternata and F. moniliforme infection may be due to disruption of sphingolipid metabolism.     

Screening of fungal species for fumonisin production andfumonisin-like disruption of sphingolipid biosynthesis

Fumonisins are mycotoxins produced by several species of Fusaria. They are found on corn and in corn-based products, can cause fatal illnesses in some animals and are suspected human esophageal carcinogens. Fumonisins are believed to cause toxicity by blocking ceramide synthase, a key enzyme in sphingolipid biochemistry which converts sphinganine (or sphingosine) and fatty acyl CoA to ceramide. Relatively fewfungal species have been evaluated for their ability to produce fumonisins. Fewer have been studied to determine if they produce ceramide synthase inhibitors, whether fumonisin-like structures or not, therefore potentially having toxicity similar to fumonisins. We analyzed corn cultures of 49 isolates representing 32 diversespecies of fungi for their ability to produce fumonisins. We also evaluated the culture extracts for ceramide synthase activity. Only cultures prepared with species reported previously to produce fumonisins – Fusarium moniliforme and F. proliferatum – tested positive for fumonisins. Extracts of these cultures inhibited ceramide synthase, as expected. None of the other fungal isolates we examined produced fumonisins or other compounds capable of inhibiting ceramide synthase. Although the fungi we selected for these studies represent only a few ofthe thousands of species that exist, they share the commonality that they are frequently associated with cereal grasses, including corn, either as pathogens or as asymptomatic endophytes. Thus,these results should be encouraging to those attempting to find ways to genetically manipulate fumonisin-producing fungi, tomake corn more resistant, or to develop biocontrol measures because it appears that only a relatively few fungal contaminants of corn can produce fumonisins. This revised version was published online in June 2006 with corrections to the Cover Date.     

In vitro toxicology of fumonisins and the mechanistic implications

The effects of fumonisins B₁FB₁, B₂(FB{2}), and the backbone of fumonisin B₁ remaining after hydrolysis of the tricarballylic groups with base (HFB₁) on sphingolipid biosynthesis were studied in both primary rat hepatocytes and pig kidney epithelial cells (LLC-PK₁). Fumonisins were potent inhibitors of sphingolipid biosynthesis in hepatocytes (IC₅₀ of FB₁=0.1 M), but overt toxicity was not observed. In renal cells, fumonisins also inhibited sphingosine biosynthesis (IC₅₀ for FB₁=35 M), and caused decreased cell proliferation as well. Higher doses (70 M) killed renal cells after exposure for 3 days. The inhibition of de novo sphingolipid biosynthesis was specific, and appeared to be at the site of ceramide synthase, which catalyzes the formation of dihydroceramide or ceramide by the addition of the amide-linked fatty acid to sphinganine or sphingosine. These results may account for the ability of fumonisins to cause equine leucoencephalomalacia and to promote tumor formation.     

Metabolomic profiling of sphingolipids in human glioma cell lines by liquid chromatography tandem mass spectrometry.

Sphingolipids participate in membrane structure and signaling in neuronal cells, and an emerging strategy for control of gliomas is to inhibit growth and/or induce apoptosis using ceramide and ceramide analogs. Nonetheless, some sphingolipids (ceramides and sphingosine) induce and others (sphingosine 1-phosphate) inhibit apoptosis; therefore, when testing putative anti-cancer agents, it is critical to obtain precise knowledge of the types and quantities of not only the test compounds, but also their effects on endogenous species. Combination of liquid chromatography and tandem mass spectrometry affords a "metabolomic" profile of all of the intermediates of ceramide biosynthesis (3-ketosphinganine, sphinganine and dihydroceramides) and the direct products of ceramide metabolism (sphingomyelins and monohexosylceramides as well as sphingosine and sphingosine 1-phosphate). This method has been applied to four human glioma cell lines (LN18, LN229, LN319 and T98G), and differences in the amounts and types of sphingolipids were found. For example, LN229 and LN319 have approximately twice the sphingosine 1-phosphate of LN18 and T98G; LN229 and LN319 have more monohexosylceramides than lactosylceramides, whereas the opposite is the case for LN18 and T98G; and the fatty acyl chain distributions of the sphingolipids differ among the cell lines. The ability to obtain this type of "metabolomic" profile allows studies of how anti-cancer agents (especially sphingolipids and sphingolipid analogs) affect the amounts of these bioactive species, and may lead to a better understanding of the abnormal phenotypes of gliomas.     

Alteration in sphingolipid metabolism: bioassays for fumonisin- and ISP-I-like activity in tissues, cells and other matrices

The first discovered naturally occurring inhibitor of de novo sphingolipid biosynthesis was fumonisin B1. There are now 11 identified fungal inhibitors of ceramide synthase or 'fumonisin B1-like' compounds. With the exception of the australifungins, all other fungal ceramide synthase inhibitors are structurally sphingoid-like. There are several recently discovered fungal inhibitors of another enzyme in the de novo sphingolipid biosynthesis pathway: serine palmitoyltransferase (SPT). One of the SPT inhibitors is named ISP-I. While ceramide synthase inhibitors are toxic to animals, plants and fungi, the SPT inhibitors are not known to cause animal or plant disease, but are potent inhibitors of fungal growth. Very little is known about their toxicity in animals. There are at least 24 fungal SPT inhibitors produced by a variety of fungi. Given that the fungal inhibitors of sphingolipid biosynthesis are chemically and biologically diverse, two bioassays have been developed to screen for fumonisin-like or ISP-I-like activity in naturally contaminated products or fungal culture materials. These bioassays are based on the changes in free sphingoid base concentration that occur when the ceramide synthase or SPT are inhibited. The bioassays have the advantage that they are functionally rather than chemically specific and thus will detect ceramide synthase and SPT inhibitors regardless of their chemical structure.     

Cancer and sphingolipid storage disease therapy using novel synthetic analogs of sphingolipids

Sphingolipid metabolites have become recognized for their participation in cell functions and signaling events that control a wide array of cellular activities. Two main sphingolipids, ceramide and sphingosine-1-phosphate, are involved in signaling pathways that regulate cell proliferation, apoptosis, motility, differentiation, angiogenesis, stress responses, protein synthesis, carbohydrate metabolism, and intracellular trafficking. Ceramide and S1P often exert opposing effects on cell survival, ceramide being pro-apoptotic and S1P generally promoting cell survival. Therefore, the conversion of one of these metabolites to the other by sphingolipid enzymes provides a vast network of regulation and provides a useful therapeutic target. Here we provide a survey of the current knowledge of the roles of sphingolipid metabolites in cancer and in lipid storage disease. We review our attempts to interfere with this network of regulation and so provide new treatments for a range of diseases. We synthesized novel analogs of sphingolipids which inhibit the hydrolysis of ceramide or its conversion to more complex sphingolipids. These analogs caused elevation of ceramide levels, leading to apoptosis of a variety of cancer cells. Administration of a synthetic analog to tumor-bearing mice resulted in reduction and even disappearance of the tumors. Therapies for sphingolipid storage diseases, such as Niemann-Pick and Gaucher diseases were achieved by two different strategies: inhibition of the biosynthesis of the substrate (substrate reduction therapy) and protection of the mutated enzyme (chaperone therapy). Sphingolipid metabolism was monitored by the use of novel fluorescent sphingolipid analogs. The results described in this review indicate that our synthetic analogs could be developed both as anticancer drugs and for the treatment of sphingolipid storage diseases.     

Sphingolipids as bioactive regulators of thrombin generation

Sphingolipids contribute to modulation of two opposing cell processes, cell growth and apoptotic cell death; ceramide and sphingosine promote the latter and sphingosine-1-phosphate triggers the former. Thrombin, a pro-inflammatory protease that is regulated by the blood coagulation cascade, exerts similar effects depending on cell type. Here we report a new mechanism for cross-talk between sphingolipid metabolism and thrombin generation. Sphingosine and sphinganine, but not ceramide or sphingosine-1-phosphate, down-regulated thrombin generation on platelet surfaces (IC(50) = 2.4 and 1.4 microm for sphingosine and sphinganine, respectively) as well as in whole plasma clotting assays. Thrombin generation was also inhibited by glucosylsphingosine, lysosphingomyelin, phytosphingosine, and primary alkylamines with >10 carbons. Acylation of the amino group ablated anticoagulant activities. Factor Va was required for the anticoagulant property of sphingosine because prothrombin activation was inhibited by sphingosine, sphinganine, and stearylamine in the presence but not in the absence of factor Va. Sphingosine did not inhibit thrombin generation when Gla-domainless factor Xa was used in prothrombinase assays, whereas sphingosine inhibited activation of Gla-domainless prothrombin by factor Xa/factor Va in the absence of phospholipids (IC(50) = 0.49 microm). Fluorescence spectroscopy studies showed that sphingosine binds to fluorescein-labeled factor Xa and that this interaction required the Gla domain. These results imply that sphingosine disrupts interactions between factor Va and the Gla domain of factor Xa in the prothrombinase complex. Thus, certain sphingolipids may be bioactive lipid mediators of thrombin generation such that certain sphingolipid metabolites may modulate proteases that affect cell growth and death, blood coagulation, and inflammation.     

Cell cycle progression and cell polarity require sphingolipid biosynthesis in Aspergillus nidulans

Sphingolipids are major components of the plasma membrane of eukaryotic cells and were once thought of merely as structural components of the membrane. We have investigated effects of inhibiting sphingolipid biosynthesis, both in germinating spores and growing hyphae of Aspergillus nidulans. In germinating spores, genetic or pharmacological inactivation of inositol phosphorylceramide (IPC) synthase arrests the cell cycle in G(1) and also prevents polarized growth during spore germination. However, inactivation of IPC synthase not only eliminates sphingolipid biosynthesis but also leads to a marked accumulation of ceramide, its upstream intermediate. We therefore inactivated serine palmitoyltransferase, the first enzyme in the sphingolipid biosynthesis pathway, to determine effects of inhibiting sphingolipid biosynthesis without an accumulation of ceramide. This inactivation also prevented polarized growth but did not affect nuclear division of germinating spores. To see if sphingolipid biosynthesis is required to maintain polarized growth, and not just to establish polarity, we inhibited sphingolipid biosynthesis in cells in which polarity was already established. This inhibition rapidly abolished normal cell polarity and promoted cell tip branching, which normally never occurs. Cell tip branching was closely associated with dramatic changes in the normally highly polarized actin cytoskeleton and found to be dependent on actin function. The results indicate that sphingolipids are essential for the establishment and maintenance of cell polarity via control of the actin cytoskeleton and that accumulation of ceramide is likely responsible for arresting the cell cycle in G(1).     

Synthetic, non-natural sphingolipid analogs inhibit the biosynthesis of cellular sphingolipids, elevate ceramide and induce apoptotic cell death

Numerous studies have demonstrated the participation of sphingolipids in signal transduction and regulation of cell growth. Several cellular stress agents have been shown to elevate ceramide, the basic precursor of all sphingolipids, initiating a cascade of events leading to arrest of the cell cycle, apoptosis and cell death. Aiming at inhibiting metabolic pathways of sphingolipid metabolism that might lead to an increase of cellular ceramide, we have synthesized non-natural analogs of ceramide, sphingosine and trimethylsphingosine. When the respective analogs were applied to HL60 human myeloid leukemic cells they inhibited the biosynthesis of sphingomyelin (SPM) and glycosphingolipids and induced apoptosis that led to cell death. A fluorescent procedure which has been developed for quantifying the biosynthesis of cellular ceramide indicated an increase in the ceramide content following an incubation with the synthetic analogs. These results suggest that the newly synthesized sphingolipid analogs might be valuable for potential application as a therapeutic modality in leukemia and other malignancies.     

Altered sphingolipid metabolism in human endometrial cancer

There is a growing body of evidence indicating that bioactive sphingolipids play a key role in cancer development, progression and metastasis. However, sphingolipid metabolism in malignant tumors is poorly investigated. Therefore, the aim of the present study was to examine the content of selected intermediates of ceramide metabolism and the activity of key enzymes of ceramide de novo synthesis and sphingosine-1-phosphate (S1P) production in the endometrial cancer. The specimens of cancer tissue and healthy endometrium were obtained from women undergoing surgery because of the cancer (n = 23) and because of myomas (n = 18), respectively. The content of sphinganine, dihydroceramide, ceramide, sphingosine and S1P was measured using high pressure liquid chromatography. The activity of the enzymes was determined using radioactive substrates. It has been found that the content of each examined sphingolipid was markedly elevated in the cancer tissue compared with the healthy endometrium. Namely, sphinganine, sphingosine and dihydroceramide by 3–4.6-fold, ceramide and S1P by 1.9- and 1.6-fold, respectively. Interestingly, the ratio of S1P to ceramide remained stable. The activity of serine palmitoyltransferase and sphingosine kinase 1 was increased by 2.3- and 2.6-fold, respectively. We conclude that endometrial carcinoma is characterized by profound changes in sphingolipid metabolism that likely contribute to its progression and chemoresistance.     

Fumonisin-induced blockade of ceramide synthase in sphingolipid biosynthetic pathway alters aortic input impedance spectrum of pigs

The sphingolipid signaling pathway appears to play an important role in regulating vascular tone. We examined the effect of fumonisin B(1), a fungal toxin in corn that blocks ceramide synthase in the sphingolipid signaling pathway, on the ascending aortic impedance spectrum of pigs. Sixteen pigs were fed culture material containing fumonisin B(1) (20 mg/kg body wt) (n = 7) or a control diet (n = 9) daily for 3 days and then instrumented under alpha-chloralose anesthesia for measurement of ascending aortic pressure and flow. Fumonisin ingestion increased serum sphinganine and sphingosine concentrations. Fumonisin ingestion also decreased cardiac output and characteristic impedance and increased the frequency of the first minimum impedance modulus, systemic vascular resistance, and the terminal, first, and second harmonic reflection coefficients, without changing mean arterial pressure. Thus blockade of ceramide synthase is accompanied by decreased vascular tone in systemic conduit arteries and increased vascular tone in systemic resistance vessels. The results indicate that the sphingolipid signaling pathway influences vascular tone in alpha-chloralose-anesthetized pigs.     

Liquid chromatography/electrospray ionisation-mass spectrometry method for the quantification of sphingosine and sphinganine in cell cultures exposed to fumonisins

Fumonisins, mycotoxins produced by Fusarium verticillioides, are potent inhibitors of the de novo sphingolipid biosynthesis via inhibition of the key enzyme ceramide synthase. The cellular response to a fumonisin exposure is obvious as an alteration of the ratio of the sphingoid bases sphingosine (SO) and sphinganine (SA). We developed a new column liquid chromatography/electrospray ionisation-mass spectrometry (LC-ESI-MS) method for the rapid, simultaneous and quantitative determination of these bases in cell cultures of immortalised human kidney epithelial cells (IHKE cells). For sample preparation, cell lysates were only diluted, centrifuged and directly used for LC-MS measurements. Quantification was carried out using phytosphingosine (PSO) as an internal standard. Detecting the protonated molecule [M+H](+) signals of SO (m/z 300) and SA (m/z 302) in the selected ion monitoring (SIM) mode, detection limits of 10 pg for SO (signal-to-noise ratio S/N=3:1) and 25 pg for SA (S/N=3:1) were established. The average recovery for SO and SA was higher than 90% for control IHKE-cells, respectively. The developed LC-ESI-MS method allows the sensitive, selective and rapid monitoring of sphingosine and sphinganine in cell matrices with a drastically reduced time for sample preparation.     

Ablation of Ceramide Synthase 2 Causes Chronic Oxidative Stress Due to Disruption of the Mitochondrial Respiratory Chain

Ceramide is a key intermediate in the pathway of sphingolipid biosynthesis and is an important intracellular messenger. We recently generated a ceramide synthase 2 (CerS2) null mouse that cannot synthesize very long acyl chain (C22-C24) ceramides. This mouse displays severe and progressive hepatopathy. Significant changes were observed in the sphingolipid profile of CerS2 null mouse liver, including elevated C16-ceramide and sphinganine levels in liver and in isolated mitochondrial fractions. Because ceramide may be involved in reactive oxygen species (ROS) formation, we examined whether ROS generation was affected in CerS2 null mice. Levels of a number of anti-oxidant enzymes were elevated, as were lipid peroxidation, protein nitrosylation, and ROS. ROS were generated from mitochondria due to impaired complex IV activity. C16-ceramide, sphingosine, and sphinganine directly inhibited complex IV activity in isolated mitochondria and in mitoplasts, whereas other ceramide species, sphingomyelin, and diacylglycerol were without effect. A fluorescent analog of sphinganine accumulated in mitochondria. Heart mitochondria did not display a substantial alteration in the sphingolipid profile or in complex IV activity. We suggest that C16-ceramide and/or sphinganine induce ROS formation through the modulation of mitochondrial complex IV activity, resulting in chronic oxidative stress. These results are of relevance for understanding modulation of ROS signaling by sphingolipids.     

Biosynthesis of sphinganine-analog mycotoxins

Sphinganine-analog mycotoxins (SAMT) are polyketide-derived natural products produced by a number of plant pathogenic fungi and are among the most economically important mycotoxins. The toxins are structurally similar to sphinganine, a key intermediate in the biosynthesis of ceramides and sphingolipids, and competitive inhibitors for ceramide synthase. The inhibition of ceramide and sphingolipid biosynthesis is associated with several fatal diseases in domestic animals and esophageal cancer and neural tube defects in humans. SAMT contains a highly reduced, acyclic polyketide carbon backbone, which is assembled by a single module polyketide synthase. The biosynthesis of SAMT involves a unique polyketide chain-releasing mechanism, in which a pyridoxal 5'-phosphate-dependent enzyme catalyzes the termination, offloading and elongation of the polyketide chain. This leads to the introduction of a new carbon-carbon bond and an amino group to the polyketide chain. The mechanism is fundamentally different from the thioesterase/cyclase-catalyzed polyketide chain releasing found in bacterial and other fungal polyketide biosynthesis. Genetic data suggest that the ketosynthase domain of the polyketide synthase and the chain-releasing enzyme are important for controlling the final product structure. In addition, several post-polyketide modifications have to take place before SAMT become mature toxins.     

Upstream of growth and differentiation factor 1 (uog1), a mammalian homolog of the yeast longevity assurance gene 1 (LAG1), regulates N-stearoyl-sphinganine (C18-(dihydro)ceramide) synthesis in a fumonisin B1-independent manner in mammalian cells

The longevity assurance gene (LAG1) and its homolog (LAC1) are required for acyl-CoA-dependent synthesis of ceramides containing very long acyl chain (e.g. C26) fatty acids in yeast, and a homolog of LAG1, ASC1, confers resistance in plants to fumonisin B(1), an inhibitor of ceramide synthesis. To understand further the mechanism of regulation of ceramide synthesis, we now characterize a mammalian homolog of LAG1, upstream of growth and differentiation factor-1 (uog1). cDNA clones of uog1 were obtained from expression sequence-tagged clones and sub-cloned into a mammalian expression vector. Transient transfection of human embryonic kidney 293T cells with uog1 followed by metabolic labeling with [4,5-(3)H]sphinganine or L-3-[(3)H]serine demonstrated that uog1 conferred fumonisin B(1) resistance with respect to the ability of the cells to continue to produce ceramide. Surprisingly, this ceramide was channeled into neutral glycosphingolipids but not into gangliosides. Electrospray tandem mass spectrometry confirmed the elevation in sphingolipids and revealed that the ceramides and neutral glycosphingolipids of uog1-transfected cells contain primarily stearic acid (C18), that this enrichment was further increased by FB(1), and that the amount of stearic acid in sphingomyelin was also increased. UOG1 was localized to the endoplasmic reticulum, demonstrating that the fatty acid selectivity and the fumonisin B(1) resistance are not due to a subcellular localization different from that found previously for ceramide synthase activity. Furthermore, in vitro assays of uog1-transfected cells demonstrated elevated ceramide synthase activity when stearoyl-CoA but not palmitoyl-CoA was used as substrate. We propose a role for UOG1 in regulating C18-ceramide (N-stearoyl-sphinganine) synthesis, and we note that not only is this the first case of ceramide formation in mammalian cells with such a high degree of fatty acid specificity, but also that the N-stearoyl-sphinganine produced by UOG1 most significantly impacts neutral glycosphingolipid synthesis.     

Fumonisin contamination of food: progress in development of biomarkers to better assess human health risks.

Fumonisins, fungal toxins produced by Fusarium moniliforme, contaminate maize based foods and feeds throughout the world. They cause liver and kidney toxicity in animals in addition to leukoencephalomalacia in horses and pulmonary edema in pigs. Fumonisin B(1) is carcinogenic in rats and mice. Ecological studies have linked consumption of fumonisin contaminated maize with oesophageal cancer in human populations in South Africa and China. This review discusses the potential health risks for people exposed to the fumonisins, and describes how mechanistic studies of toxicity in animal models have allowed the development of putative biomarkers of fumonisin exposure at the individual level. The requirements for an applicable biomarker include sample availability as well as a high specificity and sensitivity for the exposure of interest. Most environmental toxic insults involve complex exposures both to other toxins and to infections; these confounding factors need to be considered in assessing both the validity of the biomarker and the exposure-disease associations. Fumonisins can be detected in the urine of animals in feeding studies but the sensitivity of the current methodology means only highly exposed people could be monitored. Mechanistic studies indicate that ceramide synthase, an enzyme involved in sphingolipid synthesis, is one cellular target for fumonisin toxicity and carcinogenicity, and this disruption to sphingolipid metabolism increases the ratio of two sphingoid precursors, sphinganine and sphingosine. The altered ratio has been observed in tissues, serum and urine for a number of animal models suggesting it as a good candidate marker of fumonisin exposure. Despite development of analytical methods to measure this biomarker there have been no studies to date correlating it to fumonisin intake in people. Given the toxic effects of fumonisins in animals and the widespread human exposure, which has been calculated to reach 440 micrograms kg(-1) body weight day(-1) in a population consuming high quantities (460 g day(-1)) of contaminated maize, then the development of biomarkers and their application in epidemiological studies should be a priority for research on these toxins.