Fumonisins: fungal toxins that shed light on sphingolipid function

Host-selective toxins as agents of cell death in plant-fungus interactions

Summary Host-selective toxins are known determinants of compatibility in plant-fungus interactions and provide a powerful model for understanding the specificity of these associations. The identification of genes required for toxin biosynthesis has shown that the genes are unique to the toxin producing species and are clustered in complex loci. These loci may have been acquired by horizontal gene transfer. Many, if not all, host-selective toxins act by disrupting biochemical processes and in several cases the resulting cell death has the characteristics of programmed cell death. This ability to make dead tissue from living has enabled these facultative saprophytic fungi to become plant pathogens.

Determination of N-(carboxymethyl)fumonisin B(1) in corn products by liquid chromatography/electrospray ionization--mass spectrometry

It is well-known that fumonisin B(1) (FB(1)) in corn meal decreases during baking, frying, and cooking, but it is still not exactly clear how heating affects the formation of N-(carboxymethyl)fumonisin B(1) (NCM-FB(1)), the reaction product of FB(1) and reducing sugars. In model experiments corn grits were spiked with FB(1) (2 mg/kg) and D-glucose (50 g/kg) or sucrose (50 g/kg) and manufactured into extrusion products at various temperatures (160--180 degrees C) and moisture levels (16--20%). A liquid chromatography/electrospray ionization-mass spectrometry method using isotopically labeled fumonisin FB(1)-d(6) as an internal standard was developed for the determination of NCM-FB(1). For sample cleanup solid-phase C18 cartridges were used. The detection limit achieved with this method was 10 ng/g (signal-noise ratio = 3:1) using the protonated molecule [M + H](+) signal of NCM-FB(1) (m/z 780) in the selected ion monitoring mode. Low concentrations of NCM-FB(1) (29-97 ng/g) were detected in all samples spiked with D-glucose and FB(1), whereas those spiked with FB(1) and sucrose showed only NCM-FB(1) in samples produced at 180 degrees C (NCM-FB(1) = 27 ng/g). Various corn-containing food samples from the German market were analyzed for the presence of NCM-FB(1), FB(1), and hydrolyzed fumonisin B(1) (HFB(1)). All samples were contaminated with FB(1) (22--194 ng/g) and HFB(1) (5--247 ng/g). Six of nine samples contained NCM-FB(1) in low concentrations ranging from 10 to 76 ng/g. From these data and the low toxicity of NCM-FB(1) it can be concluded that the significance of NCM-FB(1) in food seems to be a minor one.

Regulation of fumonisin B(1) biosynthesis and conidiation in Fusarium verticillioides by a cyclin-like (C-type) gene, FCC1

Fumonisins are a group of mycotoxins produced in corn kernels by the plant-pathogenic fungus Fusarium verticillioides. A mutant of the fungus, FT536, carrying a disrupted gene named FCC1 (for Fusarium cyclin C1) resulting in altered fumonisin B(1) biosynthesis was generated. FCC1 contains an open reading frame of 1,018 bp, with one intron, and encodes a putative 319-amino-acid polypeptide. This protein is similar to UME3 (also called SRB11 or SSN8), a cyclin C of Saccharomyces cerevisiae, and contains three conserved motifs: a cyclin box, a PEST-rich region, and a destruction box. Also similar to the case for C-type cyclins, FCC1 was constitutively expressed during growth. When strain FT536 was grown on corn kernels or on defined minimal medium at pH 6, conidiation was reduced and FUM5, the polyketide synthase gene involved in fumonisin B(1) biosynthesis, was not expressed. However, when the mutant was grown on a defined minimal medium at pH 3, conidiation was restored, and the blocks in expression of FUM5 and fumonisin B(1) production were suppressed. Our data suggest that FCC1 plays an important role in signal transduction regulating secondary metabolism (fumonisin biosynthesis) and fungal development (conidiation) in F. verticillioides.

Molecular dynamics simulations on the mycotoxin fumonisin B1

The solution conformational properties of the mycotoxin fumonisin B(1) have been studied using molecular dynamics methodology. Fumonisins have been shown to inhibit sphinganine (sphingosine) N-acyltransferase (ceramide synthase) and show a wide range of toxic effects in many animals. This study of the solution properties of fumonisin B(1) attempts to add to the structural models necessary for the understanding of the binding and activity properties. The computational method uses a box with periodic boundaries, filled with explicit TIP3P water molecules, the substrate fumonisin B(1), and selected counterions for charge neutrality. The starting structure of fumonisin B(1) is added to the box by excluding water molecules. The explicit image method using 12-A cutoffs is applied to the system and molecular dynamics are carried out on different starting conformations at 300 K in 100-picosecond (ps) steps. Examination of the resulting equilibrated conformations suggests that the structure is relatively extended and that previous computational studies in vacuo, showing a compact folded structure, may not be consistent with the solution structure.

Identification of differentially expressed genes following treatment of monkey kidney cells with the mycotoxin fumonisin B(1)

Fumonisin B(1) (FB(1)) is a mycotoxin produced by the phytopathogenic fungus Fusarium moniliforme, which structurally resembles sphingoid bases. FB(1) perturbs sphingolipid synthesis by inhibiting the activity of ceramide synthase. Depending on the host, ingestion of FB(1) causes equine leukoencephalomalacia or porcine pulmonary edema. It is also carcinogenic to rats and may play a role in certain human cancers. Previous studies showed that FB(1) repressed specific isoforms of protein kinase C and cyclin-dependent kinase 2 (CDK2) activity. Conversely, FB(1) induced expression of CDK inhibitors, p21(Waf1/Cip1), p27(Kip1), and p57(Kip2) in monkey kidney cells (CV-1). Consequently, FB(1) treatment of CV-1 cells leads to cell-cycle arrest and apoptosis. The baculovirus IAP gene (inhibitor of apoptosis), which blocks tumor necrosis factor (TNF)-induced apoptosis, protects several fibroblast cell types from apoptosis, suggesting the TNF pathway is important for FB(1)-induced apoptosis. To identify genes that are induced by FB(1), we used a PCR-based subtraction approach. Eight genes that showed high similarity (> 90%) to known mammalian genes were identified. These genes included: tumor necrosis factor type 1 receptor associated protein 2 (TRAP2), human leukemia virus receptor (GLVR1), human Scaffold attachment factor A (SAF-A) also called heterogeneous nuclear ribonucleoprotein U (hnRNP-U), human protein kinase C-binding protein (RACK7), human oligosaccharyl transferase STT3 subunit, mouse WW-domain binding protein 2 (WBP2), human fibronectin, and an unknown human clone. The ability of FB(1) to alter gene expression and signal transduction pathways may be necessary for its carcinogenic and toxic effects.

Effect of nixtamalization (Alkaline cooking) on fumonisin-contaminated corn for production of masa and tortillas

Studies were undertaken to determine the fate of the mycotoxins, fumonisins, during the process of alkaline cooking (nixtamalization), using normal-appearing corn that was naturally contaminated with fumonisin B(1) (FB(1)) at 8.79 ppm. Corn was processed into tortillas, starting with raw corn that was cooked with lime and allowed to steep overnight; the steeped corn (nixtamal) was washed and ground into masa, which was used to make tortillas. Calculations to determine how much of the original fumonisin remained in the finished products took into consideration that FB(1) will be converted to hydrolyzed fumonisin B(1) (HFB(1)) by the process of alkaline cooking. All fractions, including steeping and washing water, were weighed, and percent moisture and fumonisin content were determined. Tortillas contained approximately 0.50 ppm of FB(1), plus 0.36 ppm of HFB(1), which represented 18.5% of the initial FB(1) concentration. Three-fourths of the original amount of fumonisin was present in the liquid fractions, primarily as HFB(1). Nixtamalization significantly reduced the amount of fumonisin in maize.

Activation of rat splenic macrophage and lymphocyte functions by fumonisin B1

Fumonisins represent a family of toxic, structurally related metabolites produced by fungi that are found in corn worldwide. We investigated the effects of the mycotoxin, fumonisin B(1), on rat splenic macrophage and lymphocyte functions. Pretreatment (24 h) of resident macrophages with fumonisin B(1) (1, 10, and 100 microg/ml) significantly (p<0.01) stimulated nitric oxide production (0.48, 2. 60, and 4.40 nmol nitrite/well, respectively), compared with the response of untreated macrophages (no nitrite detected), after 72 h of culture. Fumonisin B(1) (1 and 10 microg/ml) and IFN-gamma acted in an additive manner to activate nitric oxide production. The response of IFN-gamma (50 U/ml)-activated macrophages (1.68 nmol nitrite/well) was potentiated (3.52, 4.96, and 4.44 nmol nitrite/well) by fumonisin B(1) (1, 10, and 100 microg/ml, respectively). In addition, fumonisin B(1) significantly (p<0.05) potentiated Con A (1.25 to 5 microg/ml) (1.46- to 2.62-fold increases)- and antiTCR, IL-2 or antiTCR+IL-2 (1.72- to 2.60-fold increases)-induced proliferation of splenic cells in the presence of the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine (NMA). These results show two distinct and separate effects of fumonisin B(1): it induces nitric oxide production by macrophages and it stimulates T cell proliferation.

Sindbis virus entry into cells triggers apoptosis by activating sphingomyelinase, leading to the release of ceramide

Sindbis virus (SV) causes acute encephalomyelitis by infecting and inducing the death of neurons. Induction of apoptosis occurs during virus entry and involves acid-induced conformational changes in the viral surface glycoproteins and sphingomyelin (SM)-dependent fusion of the virus envelope with the endosomal membrane. We have studied neuroblastoma cells to determine how this entry process triggers cell death. Acidic sphingomyelinase was activated during entry followed by activation of neutral sphingomyelinase, SM degradation, and a sustained increase in ceramide. Ceramide-induced apoptosis and SV-induced apoptosis could be inhibited by treatment with Z-VAD-fmk, a caspase inhibitor, and by overexpression of Bcl-2, an antiapoptotic cellular protein. Acid ceramidase, expressed in a recombinant SV, decreased intracellular ceramide and protected cells from apoptosis. The data suggest that acid-induced SM-dependent virus fusion initiates the apoptotic cascade by inducing SM degradation and ceramide release.

The increased sensitivity of neurons with elevated glucocerebroside to neurotoxic agents can be reversed by imiglucerase

We have recently demonstrated that incubation of cultured rat hippocampal neurons with conduritol beta-epoxide (CBE), an inhibitor of glucocerebrosidase, the enzyme defective in Gaucher disease, results in changes in intracellular morphology and in functional calcium stores. Changes in levels of functional calcium stores are directly related to neuronal cell death. We now show that neurons incubated with either CBE or a non-hydrolysable analogue of GlcCer (glucosylthioceramide), are more sensitive to the toxic effects of high concentrations of glutamate and of a variety of metabolic inhibitors. A linear relationship exists between level of accumulation of GlcCer and the extent of neuronal cell death. The deleterious effects of elevated GlcCer levels can be completely reversed by addition of human glucocerebrosidase (imiglucerase) to the culture medium. Imiglucerase is internalized to lysosomes, where it presumably degrades excess GlcCer. This suggests that the limited success of enzyme replacement therapy in neuronopathic forms of Gaucher disease is not due to lack of efficacy of glucocerebroside in degrading GlcCer in neurons of the central nervous system, and adds impetus to attempts to develop ways to efficiently deliver glucocerebrosidase to the brains of neurologically compromised Gaucher disease patients.

Structure determination of soybean and wheat glucosylceramides by tandem mass spectrometry

Glucosylceramide (GluCer) is a major sphingolipid of plant tissue and, thus, abundant in nature and in dietary food sources. The lipid backbones of mammalian GluCer (sphingosine, d18:1(delta4), and ceramide) induce cell death (apoptosis) and inhibit colon carcinogenesis, it is critical to know the structures of GluCer present in plants as a first step toward understanding this potential link between diet and cancer. This study characterized the molecular species of GluCer from soybean and wheat by low-resolution, high-resolution and tandem mass spectrometry. Soybean GluCer was comprised primarily (>95%) of ceramide with 4,8-sphingadiene (d18:2(delta4,delta8)) and alpha-hydroxypalmitic acid (h16:0); the remainder had the same backbone with h18:0, h20:0, h22:0 and h24:0 fatty acids. Wheat GluCer had three major ceramide, d18:2(delta4,delta8) with h16:0, d18:1(delta8) with h16:0 and d18: 2(delta4,delta8) with h20:0, and smaller amounts of other homologs. These backbones differ from those of mammalian sphingolipids, which often have a delta4-double bond (but rarely a delta8-double bond), and have alpha-hydroxy fatty acids in only some cases. Previously unexplained fragmentations that were diagnostic for the type of sphingoid base backbone (i.e. by homolytic cleavage of the doubly allylic C-6-C-7 bond to yield a stable distonic allylic radical cation and an allylic radical neutral) were also identified. Hence this method should be useful in the identification of double bonds in sphingolipids, and structure-function relationships between sphingolipids and colon carcinogenesis.

Brave little yeast, please guide us to thebes: sphingolipid function in S. cerevisiae

Sphingolipids typically cover the exoplasmic leaflet of the plasma membrane of eukaryotic cells. They differ from the more abundant glycerophospholipids in that they contain ceramide instead of diacylglycerol as a hydrophobic anchor. Why did nature choose to invent this complex class of lipids, and why do eukaryotic cells follow elaborate remodelling pathways in order to generate dozens to hundreds of different molecular species of sphingolipid, depending on cell type? Yeast may, once again, serve as a model to dissect sphingolipid function at various levels. Almost the complete pathway for sphingolipid synthesis in yeast has been uncovered during the past two decades. More recently, key enzymes in sphingolipid degradation and signalling have been identified. Together with a wealth of genetic data obtained from the characterization of various suppressor mutants, this information now allows for an unprecedented analysis of sphingolipid function in this organism. This overview summarizes recent data on sphingolipid function in cell signalling, their role in the heat-stress response and Ca(2+) homeostasis, and addresses their function in transport of glycosylphosphatidylinositol-anchored proteins.

Regulation of cytochrome P450 expression by sphingolipids

Sphingolipids modulate many aspects of cell function, including the expression of cytochrome P450, a superfamily of heme proteins that participate in the oxidation of a wide range of compounds of both endogenous (steroid hormones and other lipids) and exogenous (e.g. alcohol, drugs and environmental pollutants) origin. Cytochrome P450-2C11 (CYP 2C11) is down-regulated in response to interleukin-1beta (IL-1beta), and this response involves the hydrolysis of sphingomyelin to ceramide as well as ceramide to sphingosine, and phosphorylation of sphingosine to sphingosine 1-phosphate. Activation of ceramidase(s) are a key determinant of which bioactive sphingolipid metabolites are formed in response to IL-1beta. Ceramidase activation also appears to account for the loss of expression of CYP 2C11 when hepatocytes are placed in cell culture, and the restoration of expression when they are plated on Matrigel; hence, this pathway is influenced by, and may mediate, interactions between hepatocytes and the extracellular matrix. Recent studies using inhibitors of sphingolipid metabolism have discovered that sphingolipids are also required for the induction of CYP1A1 by 3-methylcholanthrene, however, in this case, the requirement is for de novo sphingolipid biosynthesis rather than the turnover of complex sphingolipids. These findings illustrate how changes in sphingolipid metabolism can influence the regulation of at least several isoforms of cytochrome P450.

De novo synthesis of sphingolipids is required for cell survival by down-regulating c-Jun N-terminal kinase in Drosophila imaginal discs

Mitogen-activated protein kinase (MAPK) is a conserved eukaryotic signaling factor that mediates various signals, cumulating in the activation of transcription factors. Extracellular signal-regulated kinase (ERK), a MAPK, is activated through phosphorylation by the kinase MAPK/ERK kinase (MEK). To elucidate the extent of the involvement of ERK in various aspects of animal development, we searched for a Drosophila mutant which responds to elevated MEK activity and herein identified a lace mutant. Mutants with mild lace alleles grow to become adults with multiple aberrant morphologies in the appendages, compound eye, and bristles. These aberrations were suppressed by elevated MEK activity. Structural and transgenic analyses of the lace cDNA have revealed that the lace gene product is a membrane protein similar to the yeast protein LCB2, a subunit of serine palmitoyltransferase (SPT), which catalyzes the first step of sphingolipid biosynthesis. In fact, SPT activity in the fly expressing epitope-tagged Lace was absorbed by epitope-specific antibody. The number of dead cells in various imaginal discs of a lace hypomorph was considerably increased, thereby ectopically activating c-Jun N-terminal kinase (JNK), another MAPK. These results account for the adult phenotypes of the lace mutant and suppression of the phenotypes by elevated MEK activity: we hypothesize that mutation of lace causes decreased de novo synthesis of sphingolipid metabolites, some of which are signaling molecules, and one or more of these changes activates JNK to elicit apoptosis. The ERK pathway may be antagonistic to the JNK pathway in the control of cell survival.

Elevation of intracellular glucosylceramide levels results in an increase in endoplasmic reticulum density and in functional calcium stores in cultured neurons

Gaucher disease is a glycosphingolipid storage disease caused by defects in the activity of the lysosomal hydrolase, glucocerebrosidase (GlcCerase), resulting in accumulation of glucocerebroside (glucosylceramide, GlcCer) in lysosomes. The acute neuronopathic type of the disease is characterized by severe loss of neurons in the central nervous system, suggesting that a neurotoxic agent might be responsible for cellular disruption and neuronal death. We now demonstrate that upon incubation with a chemical inhibitor of GlcCerase, conduritol-B-epoxide (CBE), cultured hippocampal neurons accumulate GlcCer. Surprisingly, increased levels of tubular endoplasmic reticulum elements, an increase in [Ca(2+)](i) response to glutamate, and a large increase in [Ca(2+)](i) release from the endoplasmic reticulum in response to caffeine were detected in these cells. There was a direct relationship between these effects and GlcCer accumulation since co-incubation with CBE and an inhibitor of glycosphingolipid synthesis, fumonisin B(1), completely antagonized the effects of CBE. Similar effects on endoplasmic reticulum morphology and [Ca(2+)](i) stores were observed upon incubation with a short-acyl chain, nonhydrolyzable analogue of GlcCer, C(8)-glucosylthioceramide. Finally, neurons with elevated GlcCer levels were much more sensitive to the neurotoxic effects of high concentrations of glutamate than control cells; moreover, this enhanced toxicity was blocked by pre-incubation with ryanodine, suggesting that [Ca(2+)](i) release from ryanodine-sensitive intracellular stores can induce neuronal cell death, at least in neurons with elevated GlcCer levels. These results may provide a molecular mechanism to explain neuronal dysfunction and cell death in neuronopathic forms of Gaucher disease.

Sphingolipid depletion increases formation of the scrapie prion protein in neuroblastoma cells infected with prions

Sphingolipid-rich rafts play an essential role in the posttranslational (Borchelt, D. R., Scott, M., Taraboulos, A., Stahl, N., and Prusiner, S. B. (1990) J. Cell Biol. 110, 743-752)) formation of the scrapie prion protein PrP(Sc) from its normal conformer PrP(C) (Taraboulos, A., Scott, M., Semenov, A., Avrahami, D., Laszlo, L., Prusiner, S. B., and Avraham, D. (1995) J. Cell Biol. 129, 121-132). We investigated the importance of sphingolipids in the metabolism of the PrP isoforms in scrapie-infected ScN2a cells. The ceramide synthase inhibitor fumonisin B(1) (FB(1)) reduced both sphingomyelin (SM) and ganglioside GM1 in cells by up to 50%, whereas PrP(Sc) increased by 3-4-fold. Whereas FB(1) profoundly altered the cell lipid composition, the raft residents PrP(C), PrP(Sc), caveolin 1, and GM1 remained insoluble in Triton X-100. Metabolic radiolabeling demonstrated that PrP(C) production was either unchanged or slightly reduced in FB(1)-treated cells, whereas PrP(Sc) formation was augmented by 3-4-fold. To identify the sphingolipid species the decrease of which correlates with increased PrP(Sc), we used two other reagents. When cells were incubated with sphingomyelinase for 3 days, SM levels decreased, GM1 was unaltered, and PrP(Sc) increased by 3-4-fold. In contrast, the glycosphingolipid inhibitor PDMP reduced PrP(Sc) while increasing SM. Thus, PrP(Sc) seems to correlate inversely with SM levels. The effects of SM depletion contrasted with those previously obtained with the cholesterol inhibitor lovastatin, which reduced PrP(Sc) and removed it from detergent-insoluble complexes.

Effect of aromatic short-chain analogues of ceramide on axonal growth in hippocampal neurons

A series of D-erythro- and L-threo-ceramide analogues was synthesized and investigated for their ability to reverse the inhibitory effects of fumonisin B(1) (FB(1)) on axonal growth in hippocampal neurons. The analogues contained either a C(7) side chain or a phenyl group substituted for the C(13) residue present in naturally occurring ceramides, while the N-acyl chain length was reduced from C(16)-C(24) to C(2)-C(8). D-erythro-Ceramide 18a with a C(7) side chain and an N-acetyl residue and D-erythro-ceramide 20c with an aromatic side chain and an N-hexanoyl residue were most active in reversing the inhibitory effects of FB(1) on axonal growth, although the mechanism remains unclear.

Influence of kernel age on fumonisin B1 production in maize by Fusarium moniliforme

Production of fumonisins by Fusarium moniliforme on naturally infected maize ears is an important food safety concern due to the toxic nature of this class of mycotoxins. Assessing the potential risk of fumonisin production in developing maize ears prior to harvest requires an understanding of the regulation of toxin biosynthesis during kernel maturation. We investigated the developmental-stage-dependent relationship between maize kernels and fumonisin B1 production by using kernels collected at the blister (R2), milk (R3), dough (R4), and dent (R5) stages following inoculation in culture at their respective field moisture contents with F. moniliforme. Highly significant differences (P </= 0.001) in fumonisin B1 production were found among kernels at the different developmental stages. The highest levels of fumonisin B1 were produced on the dent stage kernels, and the lowest levels were produced on the blister stage kernels. The differences in fumonisin B1 production among kernels at the different developmental stages remained significant (P </= 0.001) when the moisture contents of the kernels were adjusted to the same level prior to inoculation. We concluded that toxin production is affected by substrate composition as well as by moisture content. Our study also demonstrated that fumonisin B1 biosynthesis on maize kernels is influenced by factors which vary with the developmental age of the tissue. The risk of fumonisin contamination may begin early in maize ear development and increases as the kernels reach physiological maturity.

Fumonisin B1, a mycotoxin contaminant of cereal grains, and inducer of apoptosis via the tumour necrosis factor pathway and caspase activation

Fumonisins are mycotoxins produced by Fusarium moniliforme, a prevalent fungus which infects corn or other cereal grains. Fumonisin B1 (FB1) is the most common mycotoxin produced by F. moniliforme, suggesting that it has toxicological significance. The structure of FB1 resembles sphingoid bases and it inhibits ceramide synthase. As sphingoid bases regulate cell growth, differentiation, transformation and apoptosis, it is reasonable to hypothesize that FB1 can also regulate these activities. Previous studies concluded that FB1 induced apoptosis or cell-cycle arrest in CV-1 cells (African green monkey kidney fibroblasts). In this study, we have identified genes that inhibit FB1-induced apoptosis in CV-1 cells and in two primary human cell types (lung fibroblasts and neonatal kidney cells). A baculovirus gene. inhibitor of apoptosis (IAP), protected CV-1 and the human cells from apoptosis. IAP blocks apoptosis which is induced by the tumour necrosis factor (TNF) pathway. Inhibition of interleukin converting enzymes (ICE proteases or caspases) by the baculovirus gene p35 also inhibited FB1-induced apoptosis. FB1 treatment led to cleavage of Rb (retinoblastoma protein) at its C-terminus in CV-1 or human lung cells. As the C-terminus of Rb is cleaved by ICE proteases during apoptosis, this supports an active role for ICE proteases in FB1-induced apoptosis. The tumour suppressor gene p53 was not required for FB1-induced apoptosis because p53-/- primary mouse embryo fibroblasts underwent apoptosis following FB1 treatment. Furthermore, Bcl-2 was not an effective inhibitor of FB1-induced apoptosis in CV-1 or IMR-90 cells. In summary, these results demonstrate that the TNF pathway and caspases plays an important role in FB1-induced apoptosis.

Histopathology and gene expression changes in rat liver during feeding of fumonisin B1, a carcinogenic mycotoxin produced by Fusarium moniliforme

Fumonisin B1 (FB1) is a carcinogenic mycotoxin produced by the fungus Fusarium moniliforme in corn. Feeding of FB1 to rats causes acute liver injury, chronic liver injury progressing to cirrhosis, and sometimes terminates in hepatocellular carcinoma or cholangiocarcinoma. This study describes the histolopathology and changes in gene expression in the rat liver during short-term feeding of FB1. Male Fischer rats were fed either FB1 250 mg/kg or control diet, and were killed weekly for 5 weeks. FB1 caused a predominantly zone 3 'toxic' liver injury, with hepatocyte death due to necrosis and apoptosis. Hepatocyte injury and death were mirrored by hepatic stellate cell proliferation and marked fibrosis, with progressive disturbance of architecture and formation of regenerative nodules. Despite ongoing hepatocyte mitotic activity, oval cell proliferation was noted from week 2, glutathione S-transferase pi-positive hepatic foci and nodules developed and, at later time points, oval cells were noted inside some of the 'atypical' nodules. Northern blot (mRNA) analysis of liver specimens from weeks 3 to 5 showed a progressive increase in gene expression for alpha-fetoprotein, hepatocyte growth factor, transforming growth factor alpha (TGF-alpha) and especially TGF-beta1 and c-myc. Immunostaining with LC(1-30) antibody demonstrated a progressive increase in expression of mature TGF-beta1 protein by hepatocytes over the 5 week feeding period. The overexpression of TGF-beta1 may be causally related to the prominent apoptosis and fibrosis seen with FB1-induced liver injury. Increased expression of c-myc may be involved in the cancer promoting effects of FB1.

The mycotoxin fumonisin B1 transcriptionally activates the p21 promoter through a cis-acting element containing two Sp1 binding sites

Fumonisin B1 (FB1) is a food-borne mycotoxin produced by Fusarium moniliforme. Structurally FB1 resembles sphingoid bases, and ingestion of FB1 causes several animal diseases. FB1 will cause hepatic carcinoma in rats and is implicated as a cofactor in esophageal or hepatic carcinoma. Previous studies concluded that FB1 repressed cyclin-dependent kinase 2 (CDK2) activity but induced CDK inhibitors p21(Waf1/Cip1), p27(Kip1), and p57(Kip2) in monkey kidney cells (CV-1). In contrast, CV-1 cells transformed by simian virus 40 are resistant to the antiproliferative or apoptotic effects of FB1. Consequently, FB1 treatment of CV-1 cells leads to cell cycle arrest and apoptosis. In this study, we demonstrate that FB1 transcriptionally activates the p21 promoter. Functional analysis of the p21 promoter by reporter gene assays mapped the FB1-responsive region to -124 to -47. DNase I footprinting analysis revealed two protected motifs that span the FB1-responsive region, -124 to -101 (footprint II) and -89 to -67 (footprint III). Further studies demonstrated that DNA sequences from -124 to -101 were sufficient for FB1 stimulation. DNA sequences from -124 to -101 contain two Sp1 binding sites, and gel shift assays provided evidence that nuclear factors specifically bind to this region. Disruption of the two Sp1 binding sites abrogated the binding of nuclear proteins and prevented activation by FB1. Taken together, these results suggest that Sp1 or Sp1-related proteins mediate FB1-induced activation of the p21 promoter.

Up-regulation of neutral glycosphingolipid synthesis upon long term inhibition of ceramide synthesis by fumonisin B1

In a previous study we observed that long term (5 days) incubation with fumonisin B1 (FB1), an inhibitor of acylation of sphingoid long chain bases to (dihydro)ceramide, resulted in morphological and biochemical changes in 3T3 fibroblasts (Meivar-Levy, I., Sabanay, H., Bershadsky, A. D., and Futerman, A. H. (1997) J. Biol. Chem. 272, 1558-1564). Among these were changes in the profile of synthesis of sphingolipids (SLs) and glycosphingolipids (GSLs). Whereas [3H]globotriaosylceramide ([3H]Gb3) comprised 1.9% of the total [3H]SLs and [3H]GSLs synthesized in control cells, it comprised 16. 5% in FB1-treated cells. We now demonstrate by in vitro analysis that inhibition of ceramide synthesis by FB1 for 5 days results in up-regulation of the activities of three enzymes in the pathway of Gb3 synthesis, namely glucosylceramide, lactosylceramide, and Gb3 synthases; up-regulation is due to an increase in Vmax, with no change in Km values toward lipid substrates. Moreover, molecular analysis (reverse transcriptase-polymerase chain reaction) of glucosylceramide synthase indicated that this enzyme is up-regulated at the transcriptional level. No changes in either the Vmax or Km values of sphingomyelin or of GM3 synthase were detected after FB1 treatment. Analysis of SL and GSL synthesis in cultured cells using [4,5-3H]sphinganine as a metabolic precursor demonstrated that at low substrate concentrations, Gb3 synthesis is favored over GM3 synthesis and glucosylceramide synthesis is favored over sphingomyelin synthesis, whereas the opposite is true at high substrate concentrations. These data demonstrate that GSL synthesis and in particular Gb3 synthesis are tightly regulated in fibroblasts, presumably so as to maintain constant levels of Gb3 on the cell surface.

Fumonisin content in masa and tortillas from Mexico

Fumonisins, a family of mycotoxins produced by Fusarium moniliforme and Fusarium proliferatum, are found in maize worldwide and have been associated with animal diseases. There is concern that high dietary intake of a maize-based diet may expose people in Mexico and Central America to fumonisins. Nixtamalized maize products from Mexico and the United States were examined to evaluate methods for quantitation of the different forms of fumonisins. The chelating reagent EDTA (exceeding the calcium concentration by a factor of 1. 36) was added to enhance extraction of fumonisins because calcium remained in the samples as a result of processing. It was expected that the majority of the fumonisin detected would be in the hydrolyzed form, yet the highest level of hydrolyzed fumonisin B(1) detected was 0.1 ppm. The amount of fumonisin B(1) was significantly higher in Mexican samples (mean = 0.79 ppm) than in samples purchased in the United States (mean = 0.16 ppm).

Elimination and excretion of a single dose of the mycotoxin fumonisin B2 in a non-human primate

The mycotoxin fumonisin B2 (FB2), which can be present at significant levels in maize infected with the fungus Fusarium moniliforme, was dosed both iv and by gavage to vervet monkeys. It was rapidly eliminated from the plasma of vervet monkeys dosed i.v. with 2 mg FB2/kg body mass. The concentration of FB2 in plasma after the iv dose was characterized by an initial distributional phase and a subsequent elimination phase with a mean half-life of 18 min. When two monkeys were dosed by gavage with a single bolus (7.5 mg/kg body mass), only one showed detectable trace levels of FB2 in plasma (25-40 ng/ml over the 3-5 hr period after dosing). This indicates that, like FB1, FB2 has a limited bioavailability. Urinary excretion of FB2 was extremely low, even after i.v. dosing. In total, a mean of 4.1% of the i.v. dose and 0.2% of the gavage dose was recovered in urine over a 7-day period. The predominant route of excretion was via the faeces, mainly as the unmetabolized toxin or as a partially hydrolysed analogue, with the latter accounting for between 6% and 47% of the dose. Limited amounts (maximum of 1.1%) of the fully hydrolysed aminopolyol backbone of FB2 were recovered in faeces.

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.

Antifungals: mechanism of action and resistance, established and novel drugs

Serious fungal infections, caused mostly by opportunistic species, are increasingly common in immunocompromised and other vulnerable patients. The use of antifungal drugs, primarily azoles and polyenes, has increased in parallel. Yet, established agents do not satisfy the medical need completely: azoles are fungistatic and vulnerable to resistance, whereas polyenes cause serious host toxicity. Drugs in clinical development include echinocandins, pneumocandins, and improved azoles. Promising novel agents in preclinical development include several inhibitors of fungal protein, lipid and cell wall syntheses. Recent advances in fungal genomics, combinatorial chemistry, and high-throughput screening may accelerate the antifungal discovery process.

Characterization of cell-cycle arrest by fumonisin B1 in CV-1 cells

Fusarium moniliforme is a widespread fungal pathogen which primarily infects corn, but can also infect rice or wheat. Fusarium moniliforme produce several mycotoxins, the most prominent of which is called fumonisin B1 (FB1). Epidemiological studies have indicated that ingestion of fumonisins correlates with a higher incidence of oesophageal cancer in Africa and China. Fumonisins also cause a neurodegenerative disease in horses, induce hepatic cancer in rats, are nephrotoxic in rats, or cause pulmonary oedema in swine. Structurally, fumonisins resemble sphingolipids and can alter sphingolipid biosynthesis. suggesting that sphingolipid alterations play a role in disease and carcinogenesis. Previous studies determined that FB1 blocked cell-cycle progression in CV-1 cells but not COS-7 cells. Herein, we have examined the effects that FB1 treatment has on cell-cycle regulatory proteins. Our studies established that FB1 treatment of CV-1 cells, but not COS-7 cells, leads to dephosphorylation of the retinoblastoma (Rb) protein. Cyclin dependent kinase 2 (CDK2) activity was repressed five- to 10-fold and cyclin E protein levels were lower in CV-1 cells after fumonisin treatment. Two CDK inhibitors, Kip1 and Kip2, were induced within 3 hours after fumonisin treatment of CV-1 cells, suggesting these two proteins mediate cell-cycle arrest induced by FB1. This mycotoxin caused large increases in sphinganine within 3 hours after addition of FB1. As sphingoid bases are known to induce Rb phosphorylation, this increase in sphinganinie might be the stimulus for the suppression of cyclin dependent kinase activities via Kip1 and Kip2. The ability of FB1 to accumulate sphingosine or sphinganine and arrest the cell cycle in some cells but not others may play an important role in carcinogenesis or disease.

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

Fumonisin B1 (FB1) is the predominant member of a family of mycotoxins produced by Fusarium moniliforme (Sheldon) and related fungi. Certain foods also contain the aminopentol backbone (AP1) that is formed upon base hydrolysis of the ester-linked tricarballylic acids of FB1. Both FB1 and, to a lesser extent, AP1 inhibit ceramide synthase due to structural similarities between fumonisins (as 1-deoxy-analogs of sphinganine) and sphingoid bases. To explore these structure-function relationships further, erythro- and threo-2-amino, 3-hydroxy- (and 3, 5-dihydroxy-) octadecanes were prepared by highly stereoselective syntheses. All of these analogs inhibit the acylation of sphingoid bases by ceramide synthase, and are themselves acylated with Vmax/Km of 40-125 for the erythro-isomers (compared with approximately 250 for D-erythro-sphinganine) and 4-6 for the threo-isomers. Ceramide synthase also acylates AP1 (but not FB1, under the conditions tested) to N-palmitoyl-AP1 (PAP1) with a Vmax/Km of approximately 1. The toxicity of PAP1 was evaluated using HT29 cells, a human colonic cell line. PAP1 was at least 10 times more toxic than FB1 or AP1 and caused sphinganine accumulation as an inhibitor of ceramide synthase. These studies demonstrate that: the 1-hydroxyl group is not required for sphingoid bases to be acylated; both erythro- and threo-isomers are acylated with the highest apparent Vmax/Km for the erythro-analogs; and AP1 is acylated to PAP1, a new category of ceramide synthase inhibitor as well as a toxic metabolite that may play a role in the diseases caused by fumonisins.

Significance of gangliosides in neuronal differentiation of neuroblastoma cells and neurite growth in tissue culture

Addition of DL-threo-1-phenyl-2-decanolylamine-3-morpholino-1-propanol HCl (PDMP; 7-24 microM) or Fumonisin B1 (FB1; 30-50 microM) to SH-SY5Ytrk-A human neuroblastoma cells results within 4 days in a 40% decrease of the ganglioside content and in a reduction of nerve-growth-factor (NGF)-induced outgrowth of neuritic processes. NGF-induced enhancement of GAP-43 expression was not affected. However, unlike controls, immunostained GAP-43 appeared concentrated in defined areas of cell perikarya and mostly absent from cell processes. Presence of 20-microM exogenous GM1 for 4 days in NGF and PDMP containing cell cultures led to an increase of cell-associated GM1(15-fold), GM2 (10-fold), GM3 (15 fold), GD1a (4-fold), GD2, GD1b, and GT1b (all 3-fold), and partially reversed the PDMP (and FB1) effects on neurite growth and GAP-43 distribution. In a newly developed neuronal tissue culture system, PDMP and FB1 led to a comparable dose-dependent inhibition of neurite outgrowth from embryonic chicken spinal cord explants, which had been embedded into a fibrin matrix. In this system, addition of GM1 led to a further inhibition of neurite growth, probably due to an interaction with growth-promoting components present in the surrounding fibrin matrix.

Inhibition of sphingolipid synthesis, but not degradation, alters the rate of dendrite growth in cultured hippocampal neurons

Axonal growth can be disrupted by various treatments that inhibit the synthesis of membrane components or their delivery by microtubule-based transport. In cultured hippocampal neurons, a direct correlation exists between the synthesis of sphingolipids, and particularly the simplest glycosphingolipid, glucosylceramide, and the ability of growth factors to stimulate axonal growth [S. Boldin, A.H. Futerman, J. Neurochem. 68 (1997) 882-885]. We now demonstrate that dendritic growth in hippocampal neurons also requires ongoing sphingolipid synthesis. Upon incubation with fumonisin B1 (FB1), an inhibitor of acylation of sphingoid long-chain bases, dendritic growth rates are approximately 25% slower than those of control cells, resulting in neurons with shorter dendritic arbors and less dendritic branch points per cell, and readily apparent differences in morphology compared to control cells after 10-14 days in culture. In contrast, FB1 had no effect on the initial growth of the minor processes, which are destined to become dendrites, even in cells in which FB1 affected the rate of axon growth. Inhibition of sphingolipid degradation, by incubation with conduritol-B-epoxide (an inhibitor of glucosylceramide degradation) had no effect on dendrite or minor process growth at any stage of development, and no aberrant neurite or ectopic dendrite formation was observed. Together, these data demonstrate that normal dendrite growth in hippocampal neurons requires sphingolipid synthesis, although the molecular requirements for sphingolipid synthesis may differ from those in axons.

Induction of ceramide-mediated apoptosis by the anticancer phospholipid analog, hexadecylphosphocholine

The prototype of a new class of antiproliferative phospholipid analogs, hexadecylphosphocholine (HePC), has been shown to inhibit tumor growth and is currently used for the treatment of cutaneous metastases of mammary carcinomas. Although several cellular targets of HePC, e.g. protein kinase C and CTP:phosphocholine cytidylyltransferase, have been proposed, the mechanisms of HePC-induced anticancer activity are still unclear. Considering that the antiproliferative effect of HePC correlates with inhibition of phosphatidylcholine biosynthesis, which is tightly coupled to sphingomyelin biosynthesis, we tested the hypothesis that treatment of cells with the anticancer drug leads to increased cellular ceramide and subsequently to apoptotic cell death. In the present study, we showed that 25 micromol/liter HePC induced apoptosis. In further experiments, we demonstrated that HePC inhibited the incorporation of radiolabeled choline into phosphatidylcholine and at a later time point into sphingomyelin. This was confirmed by metabolic labeling of the lipid backbone using radiolabeled serine, and it was shown that HePC decreased the incorporation of serine into sphingomyelin by 35% and simultaneously increased the incorporation of serine into ceramide by 70%. Determination of the amount of ceramide revealed an increase of 53% in HePC-treated cells compared with controls. In accordance with the hypothesis that elevated ceramide levels may be the missing link between the metabolic effects of HePC and its proapoptotic properties, HePC-induced apoptosis was blocked by fumonisin B1, an inhibitor of ceramide synthesis. Furthermore, we found that membrane-permeable ceramides additively increased the apoptotic effect of HePC.

A GTPase-independent mechanism of p21-activated kinase activation. Regulation by sphingosine and other biologically active lipids

p21-activated kinases (PAKs) are serine/threonine kinases that have been identified as targets for the small GTPases Rac and Cdc42. PAKs have been implicated in cytoskeletal regulation, stimulation of mitogen-activated protein kinase cascades, and in control of the phagocyte NADPH oxidase. Membrane targeting of PAK1 induced increased kinase activity in a GTPase-independent manner, suggesting that other mechanisms for PAK regulation exist. We observed concentration- and time-dependent activation of PAK1 by sphingosine and several related long chain sphingoid bases but not by ceramides or a variety of other lipids. Although phospholipids were generally ineffective, phosphatidic acid and phosphatidylinositol also had stimulatory effects on PAK1. Lipid stimulation induced a similar level of PAK1 activity as did stimulation by GTPases, and the patterns of PAK1 autophosphorylation determined after partial tryptic digestion and two-dimensional peptide analysis were similar with each class of activator. Lipid stimulation of PAK1 activity was dependent upon intact PAK kinase activity, as indicated by studies with a kinase-dead PAK1 mutant. Treatment of COS-7 cells expressing wild type PAK1 with sphingosine, fumonisin B, or sphingomyelinase, all of which are able to elevate the levels of free sphingosine, induced increased activity of PAK1 as determined using a p47(phox) peptide substrate. Studies using PAK1 mutants suggest that lipids act at a site overlapping or identical to the GTPase-binding domain on PAK. The inactive sphingosine derivative N,N-dimethylsphingosine was an effective inhibitor of PAK1 activation in response to either sphingosine or Cdc42. Our results demonstrate a novel GTPase-independent mechanism of PAK activation and, additionally, suggest that PAK(s) may be important mediators of the biological effects of sphingolipids.

Induction of apoptosis by fumonisin B1 in HT29 cells is mediated by the accumulation of endogenous free sphingoid bases

Fumonisin B1 (FB1) and aminopentol (AP1) (which is formed by hydrolysis of FB1) are found in corn contaminated with some strains of Fusarium moniliforme. Incubation of HT29 cells (a human colonic cell line) with FB1 or AP1 caused a significant reduction in cell number; AP1 was less potent, with 50 microM AP1 causing the same reduction (ca. 30% after 24 h) as 10 microM FB1. The reduction in cell number reflected increases in DNA fragmentation and the percentage of apoptotic cells. Both FB1 and AP1 caused the accumulation of sphinganine (25- and 35-fold by 10 microM FB1 and 50 microM AP1, respectively); thus, concentrations of FB1 and AP1 that caused comparable reductions in cell number were also similar with respect to elevation of sphinganine, a compound that is growth inhibitory and cytotoxic. Inhibition of the first step of sphingolipid biosynthesis with ISP-1 prevented the elevation in sphinganine, DNA fragmentation, and apoptosis induced by FB1. Therefore, these effects of FB1 on HT29 cells can be attributed to the accumulation of sphinganine. Since consumption of food contaminated with Fusarium moniliforme (Sheldon) exposes colonic cells to these mycotoxins, the possibility that FB1 and AP1 are toxic for intestinal cells in vivo should be evaluated, especially in the light of the recent report (Bhat et al., Clin. Toxicol. 35, 249, 1997) describing intestinal disturbances in humans after consumption of moldy corn and sorghum containing fumonisins.

1-Methylthiodihydroceramide, a novel analog of dihydroceramide, stimulates sphinganine degradation resulting in decreased de novo sphingolipid biosynthesis

1-Methylthiodihydroceramide (10 microM) decreased de novo ceramide biosynthesis by about 90% in primary cultured cerebellar neurons. Accordingly, de novo formation of sphingomyelin and of glycosphingolipids, all of which contain ceramide in their backbone, was reduced in a time- and concentration-dependent manner by up to 80%. Complex sphingolipid synthesis was restored upon addition of dihydroceramide or ceramide, in micromolar concentrations, to the culture medium, suggesting that none of the glycosyltransferases involved in glycosphingolipid biosynthesis is inhibited by this analog. Assays of the enzymes catalyzing sphinganine biosynthesis, as well as its N-acylation to form dihydroceramide, revealed that they were also not affected. In contrast, there was a 2.5-fold increase in the activity of sphinganine kinase. Reduction of de novo sphingolipid biosynthesis by 1-methylthiodihydroceramide is therefore due to its ability to deplete cells of newly formed free sphinganine. As a consequence of depletion of sphinganine levels, 1-methylthiodihydroceramide disrupted axonal growth in cultured hippocampal neurons in a manner similar to that reported for direct inhibitors of sphingolipid synthesis; thus, there was essentially no axon growth after incubation with 1-methylthiodihydroceramide between days 2 and 3, and co-incubation with short acyl chain analogs of ceramide (5 microM) antagonized the inhibition of growth. Interestingly, the D-erythro and the L-threo isomere were equally effective, but the corresponding free base as well as other structurally related compounds did not affect either sphingolipid biosynthesis or neuronal growth.

Programmed cell death in plant disease: the purpose and promise of cellular suicide

The interaction of pathogens with plants leads to a disruption in cellular homeostasis, often leading to cell death, in both compatible and incompatible relationships. The mechanistic basis of this cellular disruption and consequent death is complex and poorly characterized, but it is established that host responses to pathogens are dependent on gene expression, involve signal transduction, and require energy. Recent data suggest that in animals, a genetically regulated, signal transduction-dependent programmed cell death process, commonly referred to as apoptosis, is conserved over a wide range of phyla. The basic function of apoptosis is to direct the selective elimination of certain cells during development, but it also is a master template that is involved in host responses to many pathogens. Programmed cell death in plants, while widely observed, has not been studied extensively at either the biochemical or genetic level. Current data suggest that activation or suppression of programmed cell death may underlie diseases in plants as it does in animals. This review describes some of the fundamental characteristics of apoptosis in animals and points to a number of connections to programmed cell death in plants that may lead to both a better understanding of disease processes and novel strategies for engineering disease resistance in plants.

Identification and characterization of Saccharomyces cerevisiae dihydrosphingosine-1-phosphate phosphatase

We have identified the yeast sphingosine resistance gene (YSR2) of Saccharomyces cerevisiae as encoding a protein that specifically dephosphorylates dihydrosphingosine 1-phosphate (DHS-1-P), and we refer to this protein as dihydrosphingosine-1-phosphate phosphatase. Overexpression of YSR2 conferred sphingosine resistance to the dihydrosphingosine-1-P lyase-defective mutant (JS16) of S. cerevisiae, which is hypersensitive to sphingosine. The ysr2Delta deletion mutant of S. cerevisiae accumulated DHS-1-P compared with its wild type strain upon labeling with D-erythro-[4, 5-3H]dihydrosphingosine, whereas overexpression of YSR2 increased dephosphorylation of DHS-1-P. An epitope-tagged fusion protein (YSR2-Flag) was partially purified and found to specifically dephosphorylate DHS-1-P to yield dihydrosphingosine. YSR2 failed to dephosphorylate ceramide 1-phosphate or phosphatidic acid. Functionally, the mutant bearing the ysr2Delta deletion decreased labeling of sphingolipids and increased labeling of glycerolipids dramatically following in vivo labeling with D-erythro-[3H]dihydrosphingosine, but it slightly affected labeling of sphingolipids with inositol. Taken together, these results identify YSR2 as dihydrosphingosine-1-phosphate phosphatase. They also raise the intriguing possibility that phosphorylation followed by dephosphorylation is required for incorporation of exogenous long chain sphingoid bases into sphingolipids.

Mycotoxin-induced elevation of free sphingoid bases in precision-cut rat liver slices: specificity of the response and structure-activity relationships

Fumonisin B1 (FB1) is the predominant member of a family of toxic metabolites produced by several species of Fusarium and is commonly found on corn. FB1 is a potent competitive inhibitor of ceramide synthase, which catalyzes the conversion of sphinganine and sphingosine to ceramide. The resultant accumulation of free sphingoid bases and the disruption of sphingolipid metabolism is believed to be the mechanism of toxicity of the fumonisins. The objectives of this study were to determine the relative potency of analogs of FB1 to inhibit ceramide synthase and to determine whether the inhibition is specific to mycotoxins with fumonisin-like structures. Fumonisins B1, B2, B3, B4, C4, and TA toxin (a structurally similar mycotoxin produced by the tomato pathogen, Alternaria alternata f. sp. lycopersici) were approximately equipotent inhibitors. Hydrolyzed fumonisins B1, B2, and B3, which lack the tricarballylic side chains, were only 30-40% as potent as the parent toxins. N-acetylated FB1 (FA1) did not block ceramide synthase, suggesting that FA1 is nontoxic. Inhibition of ceramide synthase by fumonisin analogs did not appear to be related to the lipophilicity of the compounds, as determined by computer estimation of log P values. The ability of relatively high (10 and 100 microm) doses of other mycotoxins that bear no structural similarity to fumonisins, including aflatoxin B1, cyclopiazonic acid, beauvericin, T-2 toxin, sterigmatocystin, luteoskyrin, verrucarin A, scirpentriol, and zearalenone, to block ceramide synthase was also determined. All of the toxins tested were negative in the bioassay with the exception of fumonisins, indicating that disruption of sphingolipid metabolism is a specific cytotoxic response.

Toxicity of fumonisin B1 to B6C3F1 mice: a 14-day gavage study

Fumonisin B1 (FB1) is a fungal toxin produced by members of the genus Fusarium. Ingestion of FB1 causes species-specific neurotoxic, nephrotoxic, hepatotoxic and pulmonary effects. The clinical, haematological and pathological responses of adult male and female B6C3F1 mice to FB1 were assessed following 14 daily gavage doses ranging from 1 to 75 mg FB1/kg body weight/day. There were no consistent sex-related changes. Although all responses were modest, the most notable effects of FB1 were on the liver, bone marrow, adrenals and kidneys. In the liver, hepatocellular single cell necrosis, mitosis and anisokaryosis were observed, accompanied by elevated serum ALT. In the kidneys, minor histopathological changes were confined to female mice, while mild decreases in ion transport and increases in blood urea nitrogen were seen only in males. Small changes in glutathione levels were observed in the kidneys and livers of male mice. Adrenal cortical cell vacuolation was observed at 15 mg FB1/kg and higher in females and from 35 mg FB1/kg in males. Serum cholesterol was elevated in both male and female mice, possibly due to FB1-induced changes in lipid metabolism in the liver and adrenals. Although bone marrow cell numbers were unchanged, increases in vacuolated myeloid cells and lymphocytes were observed in female mice. In general, the degree of changes observed indicate that mice are not as sensitive a model of FB1 toxicity as rats.

Disruption of sphingolipid metabolism and induction of equine leukoencephalomalacia by Fusarium proliferatum culture material containing fumonisin B(2) or B(3)

Fumonisin B(1), B(2), and B(3) are inhibitors of ceramide synthase, a key enzyme in the pathway for de novo sphingolipid biosynthesis. Corn, naturally contaminated with either predominantly fumonisin B(1) or pure fumonisin B(1), has been shown to cause equine leukoencephalomalacia (ELEM). It has been hypothesized that fumonisin-induced disruption of sphingolipid metabolism is an early event in the development of ELEM. Recently, it was shown that Fusarium proliferatum corn culture diets containing predominantly fumonisin B(2), but not diets which were predominantly fumonisin B(3), at 75 ppm (0.75 mg/kg BW/day) caused hepatotoxicity and ELEM. Analysis of free sphingoid bases and complex sphingolipids in serum, liver, and kidney, revealed that both the fumonisin B(2) and B(3) diets caused significant disruption of sphingolipid metabolism, however, the fumonisin B(2) culture material diet was significantly more effective than the fumonisin B(3) culture material diet at disrupting sphingolipid metabolism and in causing hepatotoxicity and clinical signs indicative of the onset of ELEM. A significant increase in the ratio of free sphinganine to free sphingosine in serum was first evident at day 4 and 11 with the fumonisin B(2) and B(3) diets, respectively. Increase in serum enzymes indicative of liver toxicity was first evident at day 34 in ponies fed the fumonisin B(2) diet and clinical signs (head shaking, gait problems, and muscle tremors) were first observed at day 48. Ponies fed the fumonisin B(3) diets showed no increase in serum enzymes or clinical signs for as long as 65 days when the study with fumonisin B(3) was stopped. The results support the conclusion fumonisin B(2) is more effective than fumonisin B(3) in disrupting sphingolipid metabolism and induction of ELEM and liver injury in ponies.

Distinct roles for ceramide and glucosylceramide at different stages of neuronal growth

Sphingolipids (SLs) are important structural and regulatory components of neuronal plasma membranes. Previous studies using fumonisin B1, an inhibitor of the synthesis of ceramide, the precursor of all SLs, demonstrated that ceramide synthesis is required to sustain axonal growth in hippocampal neurons (; ) and dendritic growth in cerebellar Purkinje cells (). We now show that ceramide plays distinct roles at different stages of neuronal development. (1) During axon growth, ceramide must be metabolized to glucosylceramide (GlcCer) to sustain growth. Thus, whereas D-erythro-ceramide, which is metabolized to GlcCer, is able to antagonize the disruptive effects of fumonisin B1 on axon growth, L-threo-ceramide, which is not metabolized to GlcCer, is ineffective. (2) The formation of minor processes from lamellipodia can be stimulated by incubation with short-acyl chain analogs of ceramide that are active in ceramide-mediated signaling pathways, or by generation of endogenous ceramide by incubation with sphingomyelinase. However, GlcCer synthesis is not required for this initial stage of neuronal development. (3) During minor process formation and during axon growth, incubation with high concentrations of ceramide or sphingomyelinase, but not dihydroceramide, induces apoptosis. Together, these observations are consistent with the possibility that minor process formation and apoptosis can be regulated by ceramide-dependent signaling pathways and that the decision whether to enter these diametrically opposed pathways depends on intracellular ceramide concentrations. In contrast, axonal growth requires the synthesis of GlcCer from ceramide, perhaps to support an intracellular transport pathway.

Importance of sphingolipids and inhibitors of sphingolipid metabolism as components of animal diets

Sphingolipids are highly bioactive compounds that participate in the regulation of cell growth, differentiation, diverse cell functions, and apoptosis. They are present in both plant and animal foods in appreciable amounts, but little is known about their nutritional significance. Recent studies have shown that feeding sphingomyelin to female CF1 mice treated with a colon carcinogen (1,2-dimethylhydrazine) reduced the number of aberrant colonic crypt foci; longer-term feeding also affected the appearance of colonic adenocarcinomas. Therefore, dietary sphingolipids should be considered in studies of the relationships between diet and cancer. Sphingolipids have also surfaced as important factors in understanding the mechanism of action of a recently discovered family of mycotoxins, termed fumonisins. Fumonisins are produced by fungi commonly found on maize and a few related foods, and their consumption can result in equine leukoencephalomalacia, porcine pulmonary edema and a number of other diseases of veterinary animals and, perhaps, humans. A cellular target of fumonisins is the enzyme ceramide synthase, and disruption of sphingolipid metabolism by fumonisins has been established by studies with both cells in culture and animals that have consumed these toxic mycotoxins. These findings underscore the ways in which sphingolipids and agents that affect sphingolipid utilization should be given consideration in selecting animal diets for nutritional and toxicological studies.

Determination of the localization of gangliosides using anti-ganglioside antibodies: comparison of fixation methods

Many studies have examined the localization of gangliosides using anti-ganglioside antibodies, although widely differing conclusions have been reached. We now demonstrate that the apparent localization of gangliosides can be greatly influenced by the fixation method. Using monoclonal antibody (MAb) A2B5 (which reacts with a variety of gangliosides), hippocampal neurons were labeled at the cell surface when incubated with the antibody before fixation, but when incubated after fixation the cells displayed a variety of labeling patterns, depending on the fixation method. Biochemical analysis demonstrated that some of the fixatives (particularly acetone and methanol) significantly reduced or completely depleted cellular gangliosides, implying that the immunoreactivity observed with A2B5, and with other antibodies, was not due to gangliosides. When neurons were incubated with an anti-GD1b antibody prefixation, uniform labeling of the plasma membrane was observed, but after ganglioside depletion using biochemical inhibitors of ganglioside synthesis no cell surface labeling was detected. However, even in cells depleted of gangliosides, labeling of both the cell surface and intracellular compartments was observed when the anti-GD1b antibody was applied after fixation. Moreover, after fixation, antibodies to GM4 and GD2 reacted with hippocampal neurons, although these gangliosides are absent from these neurons. In contrast, the JONES antibody (which reacts with 9-O-acetylated GD3) labeled neurons with a similar pattern, essentially irrespective of the fixation method. These observations demonstrate that great care must be taken in assigning gangliosides to specific cell populations or to intracellular locations solely on the basis of use of anti-ganglioside antibodies, and suggest that optimal fixation conditions must be established for each anti-ganglioside antibody.

The role of sphingolipids in the maintenance of fibroblast morphology. The inhibition of protrusional activity, cell spreading, and cytokinesis induced by fumonisin B1 can be reversed by ganglioside GM3

Previous studies demonstrated that inhibition of sphingolipid synthesis by the mycotoxin fumonisin B1 (FB1) disrupts axonal growth in cultured hippocampal neurons (Harel, R., and Futerman, A. H. (1993) J. Biol. Chem. 268, 14476-14481) by affecting the formation or stabilization of axonal branches (Schwarz, A., Rapaport, E., Hirschberg, K., and Futerman, A.H. (1995) J. Biol. Chem. 270, 10990-10998). We now demonstrate that long term incubation with FB1 affects fibroblast morphology and proliferation. Incubation of Swiss 3T3 cells with FB1 resulted in a decrease in synthesis of ganglioside GM3, the major glycosphingolipid in 3T3 fibroblasts and of sphingomyelin. The projected cell area of FB1-treated cells was approximately 45% less than control cells. FB1 had no affect on the organization of microtubules or intermediate filaments, but fewer actin-rich stress fibers were observed, and there was a loss of actin-rich lamellipodia at the leading edge. Three other processes involving the actin cytoskeleton, cytokinesis, microvilli formation, and the formation of long processes induced by protein kinase inhibitors, were all disrupted by FB1. All the effects of FB1 on cell morphology could be reversed by addition of ganglioside GM3 even in the presence of FB1, whereas the bioactive intermediates, sphinganine, sphingosine, and ceramide, were without effect. Finally, FB1 blocked cell proliferation and DNA synthesis in a reversible manner, although ganglioside GM3 could not reverse the effects of FB1 on cell proliferation. Together, these data suggest that ongoing sphingolipid synthesis is required for the assembly of both new membrane and of the underlying cytoskeleton.