Acylation of naturally occurring and synthetic 1-deoxysphinganines by ceramide synthase. Formation of N-palmitoyl-aminopentol produces a toxic metabolite of hydrolyzed fumonisin, AP1, and a new category of ceramide synthase inhibitor

DNA damage in astrocytes exposed to fumonisin B1

Fumonisins are a group of toxic metabolites mainly produced by Fusarium moniliforme and Fusarium proliferatum, fungi that commonly occur on corn throughout the world. Fumonisin B1 (FB1), structurally resembling sphingoid bases, is an inhibitor of ceramide synthase, a key enzyme involved in de novo sphingolipid biosynthesis and in the reacylation of free sphingoid bases derived from sphingolipid turnover. This inhibitory effect leads to accumulation of free sphinganine (SA) and sphingosine (SO), inducing cell death. However, little is known on the down stream effectors activated by these sphingolipids in the cell death signaling pathway. We exposed rat astrocytes to FB1 with the aim of evaluating the involvement of oxygen free radicals and of some other biochemical pathways such as caspase-3 activity and DNA damage. Our results indicate that FB1 treatment (48, 72 h and 6 days in vitro, DIV, and 10, 50, 100 microM) does not affect cell viability. Conversely, after 72 h of treatment, FB1 (50 and 100 microM) induced DNA damage and an enhancement of caspase-3 activity compared to controls. In addition, FB1 increased the expression of HSP70 at 10 and 50 microM at 48, 72 h, and 6 DIV of treatment. We conclude that DNA damage of apoptotic type in rat astrocytes is caused by FB1 and that the genotoxic potential of FB1 has probably been underestimated and should be reconsidered.

DNA damage in human fibroblasts exposed to fumonisin B(1)

Fumonisins are mycotoxins produced by several Fusarium species (Fusarium verticilloides and F. proliferatum) that infest corn and other cereals. Fumonisin B(1) (FB(1)), structurally resembling sphingoid bases, is an inhibitor of ceramide synthetase, a key enzyme involved in de novo sphingolipid biosynthesis and in the reacylation of free sphingoid bases derived from sphingolipid turnover. This inhibitory effect leads to accumulation of free sphinganine and sphingosine and subsequent induction of cell death. However, the downstream effectors activated by these sphingolipids in the cell death-signalling pathway are little known. The aim of this study was to evaluate, in FB(1)-exposed human fibroblasts, the involvement of oxygen free radicals and of some other biochemical pathways, caspase-3 activity, poly(ADP-ribose)polymerase (PARP) cleavage and DNA damage evaluated by comet assay. Our results indicate that FB(1) treatment (48, 72 h and 10, 50, 100 microM) does not affect cellular viability. Conversely, after 72 h of treatment, FB(1) (50 and 100 microM) induced DNA damage, an enhancement of caspase-3-activity and cleavage of PARP compared to controls. In addition, FB(1) increased the expression of HSP70 in a concentration and time-dependent manner. Our results indicate that DNA damage of apoptotic type in human fibroblasts is caused by exposure to FB(1) at high concentrations and for a prolonged time and that the genotoxic potential of FB(1) has probably been underestimated and should be reconsidered.

Instability of N-acetylated fumonisin B1 (FA1) and the impact on inhibition of ceramide synthase in rat liver slices

Fumonisin B1 (FB1) is a mycotoxin produced by Fusarium verticillioides. It inhibits ceramide synthase, which is a proposed underlying mechanism responsible for the myriad of toxic endpoints observed. We previously reported that N-acetylation of FB1 prevents ceramide synthase inhibition, but cautioned that impure preparations of FA1 can contain a contaminant with the ability to inhibit ceramide synthase. We now report that FA1 spontaneously rearranges to O-acetylated analogs. These rearrangement products are putative inhibitors of ceramide synthase. Rat liver slices exposed to impure FA1 containing O-acetylated FB1 had sphinganine/sphingosine (Sa:So) ratios of 1.15-1.64. Control slices had Sa:So ratios of 0.07-0.24. Clean-up to remove the O-acetylated FB1 yielded purified FA1, which produced Sa:So ratios in liver slices of 0.08-0.18. After storage for approximately 1 year as either a dry powder in a desiccator, or as a dried film at 4 degrees C, the purified FA1 again contained O-acetylated FB1, and was capable of ceramide synthase inhibition. FA1 was most stable in neutral solution, but in acidic solution the equilibrium shifted towards the O-acetylated forms. FA1 in solid form also rearranged, but more slowly than in acid solution. As FA1 is considerably less cytotoxic than FB1, these results provide additional support for the conclusion that a primary amino group is necessary for both ceramide synthase inhibition and toxicity.

Sphingolipid perturbations as mechanisms for fumonisin carcinogenesis

There is a great deal of evidence that altered sphingolipid metabolism is associated with fumonisin-induced animal diseases including increased apoptotic and oncotic necrosis, and carcinogenesis in rodent liver and kidney. The biochemical consequences of fumonisin disruption of sphingolipid metabolism most likely to alter cell regulation are increased free sphingoid bases and their 1-phosphates, alterations in complex sphingolipids, and decreased ceramide (CER) biosynthesis. Because free sphingoid bases and CER can induce cell death, the fumonisin inhibition of CER synthase can inhibit cell death induced by CER but promote free sphingoid base-induced cell death. Theoretically, at any time the balance between the intracellular concentration of effectors that protect cells from apoptosis (decreased CER, increased sphingosine 1-phosphate) and those that induce apoptosis (increased CER, free sphingoid bases, altered fatty acids) will determine the cellular response. Because the balance between the rates of apoptosis and proliferation is important in tumorigenesis, cells sensitive to the proliferative effect of decreased CER and increased sphingosine 1-phosphate may be selected to survive and proliferate when free sphingoid base concentration is not growth inhibitory. Conversely, when the increase in free sphingoid bases exceeds a cell's ability to convert sphinganine/sphingosine to dihydroceramide/CER or their sphingoid base 1-phosphate, then free sphingoid bases will accumulate. In this case cells that are sensitive to sphingoid base-induced growth arrest will die and insensitive cells will survive. If the cells selected to die are normal phenotypes and the cells selected to survive are abnormal, then cancer risk will increase.

Comparison of the metabolism of L-erythro- and L-threo-sphinganines and ceramides in cultured cells and in subcellular fractions

Ceramide (Cer) is a key intermediate in the synthetic and degradative pathways of sphingolipid metabolism, and is also an important second messenger. Natural Cer exists in the D-erythro configuration. Three additional, non-natural stereoisomers exist, but conflicting reports have appeared concerning their metabolism. We now compare the stereospecificity of three enzymes in the sphingolipid biosynthetic pathway, namely dihydroceramide (dihydroCer), sphingomyelin (SM) and glucosylceramide synthases, in subcellular fractions and in cultured cells. The L-erythro enantiomers of sphinganine, dihydroCer and Cer do not act as substrates for any of the three enzymes. In contrast, the diastereoisomer, L-threo-sphinganine, is acylated by dihydroCer synthase, and L-threo-dihydroCer and L-threo-Cer are both metabolized to dihydroSM and SM, respectively, but not to dihydroglucosylceramide and glucosylceramide. No significant difference was detected in the ability of SM synthase to metabolize Cer containing a short (hexanoyl) versus long acyl chain (palmitoyl), demonstrating that short-acyl chain Cers mimic their natural counterparts, at least in the sphingolipid biosynthetic pathway.

Toxicity assessment of fumonisins using the brine shrimp (Artemia salina) bioassay

The Fusarium mycotoxins fumonisin B(1) (FB(1)) (1) and B(2) (FB(2)) (2), their hydrolysed analogues HFB(1) (3) and HFB(2) (4) and the recently discovered fumonisin derivatives N-palmitoyl-HFB(1) (5) and N-carboxymethyl-FB(1) (6) were compared for their toxicity in a short term bioassay using brine shrimp (Artemia salina). The brine shrimp were hatched in artificial sea water and exposed to the fumonisins in microwell plates with a mortality endpoint after 48 hours. LC(50) values were calculated after Probit transformation of the resulting data. Of the substances tested, fumonisin B(1) emerged to be the most toxic whereas its N-carboxymethyl analogue was 100-fold less effective. The hydrolysed fumonisins showed a four- to sixfold reduced toxicity compared to FB(1). N-Palmitoyl-HFB(1) had a higher LC(50) value than its precursor HFB(1). The brine shrimp assay proved to be a convenient and rapid system for toxicity assessment of this group of mycotoxins.

Advances in the signal transduction of ceramide and related sphingolipids

Recently, the sphingolipid metabolites ceramide, sphingosine, ceramide 1-P, and sphingosine 1-P have been implicated as second messengers involved in many different cellular functions. Publications on this topic are appearing at a rapidly increasing rate and new developments in this field are also appearing rapidly. It is thus important to summarize the results obtained from many different laboratories and from different fields of research to obtain a clearer picture of the importance of sphingolipid metabolites. This article reviews the studies from the last few years and includes the effects of a variety of extracellular agents on sphingolipid signal transduction pathways in different tissues and cells and on the mechanisms of regulation. Sphingomyelin exists in a number of functionally distinct pools and is composed of distinct molecular species. Sphingomyelin metabolites may be formed by many different pathways. For example, the generation of ceramide from sphingomyelin can be catalyzed by at least five different sphingomyelinases. A large variety of stimuli can induce the generation of ceramide, leading to activation or inhibition of various cellular events such as proliferation, differentiation, apoptosis, and inflammation. The effect of ceramide on these physiological processes is due to its many different downstream targets. It can activate ceramide-activated protein kinases and ceramide-activated protein phosphatases. It also activates or inhibits PKCs, PLD, PLA2, PC-PLC, nitric oxide synthase, and the ERK and SAPK/JNK signaling cascades. Ceramide activates or inhibits transcription factors, modulates calcium homeostasis and interacts with the retinoblastoma protein to regulate cell cycle progression. Most of the work in this field has involved the study of ceramide effects, but the roles of the other three sphingomyelin metabolites is now attracting much attention. The complex interactions between signaling components and ceramide and the controls regulating these interactions are now being identified and are presented in this review.

The identification of myriocin-binding proteins

BACKGROUND

Myriocin is a natural product that potently induces apoptosis of a murine cytotoxic T lymphocyte cell line (CTLL-2) and inhibits a serine palmitoyltransferase (SPT) activity that has been detected in cell extracts and is thought to initiate sphingolipid biosynthesis. Because SPT has never been biochemically purified and a comprehensive appraisal of myriocin-binding proteins has not been conducted, we isolated specific targets using myriocin-based affinity chromatography.

RESULTS

Myriocin derivatives were synthesized and evaluated using CTLL-2 proliferation and SPT activity assays. Guided by these results, affinity chromatography matrices were prepared and two specific myriocin-binding proteins were isolated from CTLL-2 lysates. Analyses of these polypeptides establish conclusively that they are murine LCB1 and LCB2, mammalian homologs of two yeast proteins that have been genetically linked to sphingolipid biosynthesis.

CONCLUSION

The ability of myriocin-containing matrices to bind factors that have SPT activity and the exclusive isolation of LCB1 and LCB2 as myriocin-binding proteins demonstrates that the two proteins are directly responsible for SPT activity and that myriocin acts directly upon these polypeptides.

Fumonisin B1 consumption by rats causes reversible, dose-dependent increases in urinary sphinganine and sphingosine

Fumonisin B1 (FB1) is a frequently encountered mycotoxin that inhibits ceramide synthase, the enzyme that acylates sphinganine, sphingosine and other "sphingoid" bases. Exposure of rats, rabbits, pigs and nonhuman primates to fumonisin-contaminated feed elevates sphingoid base amounts in urine; therefore, this study examined the time course and reversibility of these changes. When an AIN-76 diet supplemented with >/=5 microg FB1/g was fed to male Sprague-Dawley rats, there was a significant increase in sphinganine (ca. 50-fold in urine from rats fed 50 microg FB1/g diet) and smaller changes in sphingosine within 5 to 7 d, compared to rats fed the same diet without FB1. No change occurred in sphingoid bases upon feeding 1 microg FB1/g for up to 60 d. When rats were fed FB1 (10 microg FB1/g diet for 10 d), then changed to the same diet minus FB1, urinary sphingoid bases returned to normal within 10 d. However, if the rats were fed 10 microg FB1/g for 10 d, then changed to 1 microg FB1/g, the amounts of sphingoid bases in urine were the same as for rats that were continuously fed 10 microg FB1/g. These results establish that consumption of FB1 causes dose-dependent and reversible elevations in the amounts of urinary sphingoid bases. The finding that 1 microg FB1/g (which does not, alone, alter urinary sphingoid bases) will sustain the elevation caused by previous exposure to 10 microg FB1/g raises the possibility that even low levels of fumonisins could be deleterious when an animal is occasionally exposed to higher amounts.