Inhibitors Of Sphingolipid Metabolism Research Articles

Structure Revision of Halisphingosine A via Total Synthesis and Bioactivity Studies.

Sphingoid bases are important bioactive lipids found in a variety of organisms, serving as the backbone of sphingolipids, which regulate essential physiological processes. Here we describe the total synthesis and structure revision of halisphingosine A, a sphingoid base initially isolated from marine sponges. To address inconsistencies in the NMR interpretation of this natural product, we developed a synthetic route involving a late-stage enantioselective Henry reaction that allows access to multiple stereoisomers of the proposed halisphingosine core structure. Our library of 32 fully characterized synthetic stereoisomers enabled us to rectify the structure of halisphingosine A as (2R,3R,8R,Z)-2-aminooctadec-9-ene-1,3,8-triol, and to pursue further structure-activity relation (SAR) studies regarding their antimicrobial and cytotoxic potential. In summary, our study offers a yet unreported compound library along with validated analytical datasets of marine sphingoid base derivatives, which significantly affects future ecometabolomic marine research and will facilitate the identification of inhibitors of sphingolipid metabolism or antagonists of sphingolipid base-sensing receptors.

The Manifold Roles of Sphingolipids in Viral Infections.

Sphingolipids are essential components of eukaryotic cells. In this review, we want to exemplarily illustrate what is known about the interactions of sphingolipids with various viruses at different steps of their replication cycles. This includes structural interactions during entry at the plasma membrane or endosomal membranes, early interactions leading to sphingolipid-mediated signal transduction, interactions with internal membranes and lipids during replication, and interactions during virus assembly and budding. Targeted interventions in sphingolipid metabolism – as far as they can be tolerated by cells and organisms – may open novel possibilities to support antiviral therapies. Human immunodeficiency virus type 1 (HIV-1) infections have intensively been studied, but for other viral infections, such as influenza A virus (IAV), measles virus (MV), hepatitis C virus (HCV), dengue virus, Ebola virus, and severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2), investigations are still in their beginnings. As many inhibitors of sphingolipid metabolism are already in clinical use against other diseases, repurposing studies for applications in some viral infections appear to be a promising approach.

Identification and distribution of gene clusters required for synthesis of sphingolipid metabolism inhibitors in diverse species of the filamentous fungus Fusarium

BackgroundSphingolipids are structural components and signaling molecules in eukaryotic membranes, and many organisms produce compounds that inhibit sphingolipid metabolism. Some of the inhibitors are structurally similar to the sphingolipid biosynthetic intermediate sphinganine and are referred to as sphinganine-analog metabolites (SAMs). The mycotoxins fumonisins, which are frequent contaminants in maize, are one family of SAMs. Due to food and feed safety concerns, fumonisin biosynthesis has been investigated extensively, including characterization of the fumonisin biosynthetic gene cluster in the agriculturally important fungi Aspergillus and Fusarium. Production of several other SAMs has also been reported in fungi, but there is almost no information on their biosynthesis. There is also little information on how widely SAM production occurs in fungi or on the extent of structural variation of fungal SAMs.ResultsUsing fumonisin biosynthesis as a model, we predicted that SAM biosynthetic gene clusters in fungi should include a polyketide synthase (PKS), an aminotransferase and a dehydrogenase gene. Surveys of genome sequences identified five putative clusters with this three-gene combination in 92 of 186 Fusarium species examined. Collectively, the putative SAM clusters were distributed widely but discontinuously among the species. We propose that the SAM5 cluster confers production of a previously reported Fusarium SAM, 2-amino-14,16-dimethyloctadecan-3-ol (AOD), based on the occurrence of AOD production only in species with the cluster and on deletion analysis of the SAM5 cluster PKS gene. We also identified SAM clusters in 24 species of other fungal genera, and propose that one of the clusters confers production of sphingofungin, a previously reported Aspergillus SAM.ConclusionOur results provide a genomics approach to identify novel SAM biosynthetic gene clusters in fungi, which should in turn contribute to identification of novel SAMs with applications in medicine and other fields. Information about novel SAMs could also provide insights into the role of SAMs in the ecology of fungi. Such insights have potential to contribute to strategies to reduce fumonisin contamination in crops and to control crop diseases caused by SAM-producing fungi.

Recent advances in the formulation of sphingolipid anticancer therapeutics

The bioactive sphingolipids such as ceramide, sphingosine, and sphingosine-1-phosphate are of special interest as key regulators of cellular proliferation or apoptosis signaling. The balance between ceramide and sphingosine-1-phosphate is critical for the selection of proliferative or apoptotic signaling as well as for therapeutic efficacy against cancer cells. A number of drug candidates designed to regulate the balance between ceramide and sphingosine-1-phosphate have been developed and several (i.e., safingol, ABC294640, Sonepcizumab, and fingolimod) are under clinical investigation as anti-cancer drugs. Various nanotherapeutic challenges combined with the use of anticancer drugs and/or phospholipid metabolism inhibitors and short chain ceramide has been investigated to increase the cellular accumulation of ceramide, which acts as apoptotic signaling molecules. Many sphingolipid-containing nanotherapeutics have shown promising results as cancer treatments. These sphingolipid nanotherapeutics can include a synergistic combination of short chain ceramide and ceramide-forming anticancer agents or sphingolipid metabolism inhibitors. Sphingolipid nanotherapeutics could also be used to treat multidrug-resistant cancer cells as well as sensitive cancer cells.

Use of Acid Ceramidase and Sphingosine Kinase Inhibitors as Antiviral Compounds Against Measles Virus Infection of Lymphocytes in vitro.

As structural membrane components and signaling effector molecules sphingolipids influence a plethora of host cell functions, and by doing so also the replication of viruses. Investigating the effects of various inhibitors of sphingolipid metabolism in primary human peripheral blood lymphocytes (PBL) and the human B cell line BJAB we found that not only the sphingosine kinase (SphK) inhibitor SKI-II, but also the acid ceramidase inhibitor ceranib-2 efficiently inhibited measles virus (MV) replication. Virus uptake into the target cells was not grossly altered by the two inhibitors, while titers of newly synthesized MV were reduced by approximately 1 log (90%) in PBL and 70–80% in BJAB cells. Lipidomic analyses revealed that in PBL SKI-II led to increased ceramide levels, whereas in BJAB cells ceranib-2 increased ceramides. SKI-II treatment decreased sphingosine-1-phosphate (S1P) levels in PBL and BJAB cells. Furthermore, we found that MV infection of lymphocytes induced a transient (0.5–6 h) increase in S1P, which was prevented by SKI-II. Investigating the effect of the inhibitors on the metabolic (mTORC1) activity we found that ceranib-2 reduced the phosphorylation of p70 S6K in PBL, and that both inhibitors, ceranib-2 and SKI-II, reduced the phosphorylation of p70 S6K in BJAB cells. As mTORC1 activity is required for efficient MV replication, this effect of the inhibitors is one possible antiviral mechanism. In addition, reduced intracellular S1P levels affect a number of signaling pathways and functions including Hsp90 activity, which was reported to be required for MV replication. Accordingly, we found that pharmacological inhibition of Hsp90 with the inhibitor 17-AAG strongly impaired MV replication in primary PBL. Thus, our data suggest that treatment of lymphocytes with both, acid ceramidase and SphK inhibitors, impair MV replication by affecting a number of cellular activities including mTORC1 and Hsp90, which alter the metabolic state of the cells causing a hostile environment for the virus.

Abstract 3603: Sphingomyelin synthase inhibition in sarcomas results in ceramide accumulation and apoptosis

Abstract Sphingolipid metabolism is critical to cellular homeostasis especially as cells undergo mitosis. It is, therefore, unsurprising that the sphingolipid pathway is upregulated in a number of a cancers. Sphingoid bases such as sphingosine and ceramide are rapidly synthesized to provide the building blocks of other sphingolipids. Inhibition of downstream enzymes that convert sphingosine and ceramide into sphingolipids can lead to an increase of ceramide, which results in cellular toxicity and initiation of apoptosis. Using the marine natural product Jaspine B and chemically synthesized analogues, we investigated the potency of inhibiting sphingomyelin synthase in sarcoma cell lines. Sphingomyelin synthase is a downstream enzyme that converts ceramide into sphingomyelin by adding the polar head group phosphorylcholine to ceramide. Jaspine B and its analogues exhibited submicromolar potency against several human sarcoma cell lines. It was determined that the inhibition of sphingomyelin synthase resulted in an increase in ceramide and sphingosine levels and that sphingomyelinase enzymatic activity increased. While the mitochondria mass remained the same, the membrane potential of the mitochondria was significantly affected by the presence of the inhibitors. In conclusion, inhibitors of sphingolipid metabolism are viable therapeutic options with high untapped potential to treat cancer. Citation Format: Jared J. Barrott, Trevor Meldrum, Farjana Afrin, Srinath Pashikanti. Sphingomyelin synthase inhibition in sarcomas results in ceramide accumulation and apoptosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3603.

Sphingomyelin synthase inhibition in sarcomas results in ceramide accumulation and apoptosis

Sphingolipid metabolism is critical to cellular homeostasis especially as cells undergo mitosis. It is, therefore, unsurprising that the sphingolipid pathway is upregulated in a number of a cancers. Sphingoid bases such as sphingosine and ceramide are rapidly synthesized to provide the building blocks of other sphingolipids. Inhibition of downstream enzymes that convert sphingosine and ceramide into sphingolipids can lead to an increase of ceramide, which results in cellular toxicity and initiation of apoptosis. Using the marine natural product Jaspine B and chemically synthesized analogues, we investigated the potency of inhibiting sphingomyelin synthase in sarcoma cell lines. Sphingomyelin synthase is a downstream enzyme that converts ceramide into sphingomyelin by adding the polar head group phosphorylcholine to ceramide. Jaspine B and its analogues exhibited submicromolar potency against several human sarcoma cell lines. It was determined that the inhibition of sphingomyelin synthase resulted in an increase in ceramide and sphingosine levels and that sphingomyelinase enzymatic activity increased. While the mitochondria mass remained the same, the membrane potential of the mitochondria was significantly affected by the presence of the inhibitors. In conclusion, inhibitors of sphingolipid metabolism are viable therapeutic options with high untapped potential.Support or Funding InformationIdaho State University Start UpThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Interdiction of Sphingolipid Metabolism Revisited: Focus on Prostate Cancer.

Sphingolipid metabolism is known to play a role in cell death, survival, and therapy resistance in cancer. Sphingolipids, particularly dihydroceramide and ceramide, are associated with antiproliferative or cell death responses, respectively, and are central to effective cancer therapy. Within the last decade, strides have been made in elucidating many intricacies of sphingolipid metabolism. New information has emerged on the mechanisms by which sphingolipid metabolism is dysregulated during malignancy and how cancer cells survive and/or escape therapeutic interventions. This chapter focuses on three main themes: (1) sphingolipid enzymes that are dysregulated in cancer, particularly in prostate cancer; (2) inhibitors of sphingolipid metabolism that antagonize prosurvival responses; and (3) sphingolipid-driven escape mechanisms that allow cancer cells to evade therapies. We explore clinical and preclinical approaches to interdict sphingolipid metabolism and provide a rationale for combining strategies to drive the generation of antiproliferative ceramides with prevention of ceramide clearance.

The protein kinase Sch9 is a key regulator of sphingolipid metabolism inSaccharomyces cerevisiae

The Saccharomyces cerevisiae protein kinase Sch9 is an in vitro and in vivo effector of sphingolipid signaling. This study examines the link between Sch9 and sphingolipid metabolism in S. cerevisiae in vivo based on the observation that the sch9Δ mutant displays altered sensitivity to different inhibitors of sphingolipid metabolism, namely myriocin and aureobasidin A. Sphingolipid profiling indicates that sch9Δ cells have increased levels of long-chain bases and long-chain base-1 phosphates, decreased levels of several species of (phyto)ceramides, and altered ratios of complex sphingolipids. We show that the target of rapamycin complex 1-Sch9 signaling pathway functions to repress the expression of the ceramidase genes YDC1 and YPC1, thereby revealing, for the first time in yeast, a nutrient-dependent transcriptional mechanism involved in the regulation of sphingolipid metabolism. In addition, we establish that Sch9 affects the activity of the inositol phosphosphingolipid phospholipase C, Isc1, which is required for ceramide production by hydrolysis of complex sphingolipids. Given that sphingolipid metabolites play a crucial role in the regulation of stress tolerance and longevity of yeast cells, our data provide a model in which Sch9 regulates the latter phenotypes by acting not only as an effector but also as a regulator of sphingolipid metabolism.

Anti-inflammatory action of lipid nanocarrier-delivered myriocin: therapeutic potential in cystic fibrosis

BackgroundSphingolipids take part in immune response and can initiate and/or sustain inflammation. Various inflammatory diseases have been associated with increased ceramide content, and pharmacological reduction of ceramide diminishes inflammation damage in vivo. Inflammation and susceptibility to microbial infection are two elements in a vicious circle. Recently, sphingolipid metabolism inhibitors were used to reduce infection. Cystic fibrosis (CF) is characterized by a hyper-inflammation and an excessive innate immune response, which fails to evolve into adaptive immunity and to eradicate infection. Chronic infections result in lung damage and patient morbidity. Notably, ceramide content in mucosa airways is higher in CF mouse models and in patients than in control mice or healthy subjects. MethodsThe therapeutic potential of myriocin, an inhibitor of the sphingolipid de novo synthesis rate limiting enzyme (Serine Palmitoyl Transferase, SPT),was investigated in CF cells and mice models. ResultsWe treated CF human respiratory epithelial cells with myriocin, This treatment resulted in reduced basal, as well as TNFα-stimulated, inflammation. In turn, TNFα induced an increase in SPT in these cells, linking de novo synthesis of ceramide to inflammation. Furthermore, myriocin-loaded nanocarrier, injected intratrachea prior to P. aeruginosa challenge, enabled a significant reduction of lung infection and reduced inflammation. ConclusionsThe presented data suggest that de novo ceramide synthesis is constitutively enhanced in CF mucosa and that it can be envisaged as pharmacological target for modulating inflammation and restoring effective innate immunity against acute infection. General significanceMyriocin stands as a powerful immunomodulatory agent for inflammatory and infectious diseases.

Sphingolipid Metabolism Cooperates with BAK and BAX to Promote the Mitochondrial Pathway of Apoptosis

Mitochondria are functionally and physically associated with heterotypic membranes, yet little is known about how these interactions impact mitochondrial outer-membrane permeabilization (MOMP) and apoptosis. We observed that dissociation of heterotypic membranes from mitochondria inhibited BAK/BAX-dependent cytochrome c (cyto c) release. Biochemical purification of neutral sphingomyelinases that correlated with MOMP sensitization suggested that sphingolipid metabolism coordinates BAK/BAX activation. Using purified lipids and enzymes, sensitivity to MOMP was achieved by in vitro reconstitution of the sphingolipid metabolic pathway. Sphingolipid metabolism inhibitors blocked MOMP from heavy membrane preparations but failed to influence MOMP in the presence of sphingolipid-reconstituted, purified mitochondria. Furthermore, the sphingolipid products, sphingosine-1-PO(4) and hexadecenal, cooperated specifically with BAK and BAX, respectively. Sphingolipid metabolism was also required for cellular responses to apoptosis. Our studies suggest that BAK/BAX activation and apoptosis are coordinated through BH3-only proteins and a specific lipid milieu that is maintained by heterotypic membrane-mitochondrial interactions.

Modulation of the activity of cytosolic phospholipase A2α (cPLA2α) by cellular sphingolipids and inhibition of cPLA2α by sphingomyelin

We examined the effect of the cellular sphingolipid level on the release of arachidonic acid (AA) and activity of cytosolic phospholipase A2alpha (cPLA2alpha) using two Chinese hamster ovary (CHO)-K1-derived mutants deficient in sphingolipid synthesis: LY-B cells defective in the LCB1 subunit of serine palmitoyltransferase for de novo synthesis of sphingolipid species, and LY-A cells defective in the ceramide transfer protein CERT for SM synthesis. When LY-B and LY-A cells were cultured in Nutridoma medium and the sphingolipid level was reduced, the release of AA stimulated by the Ca(2+) ionophore A23187 increased 2-fold and 1.7-fold, respectively, compared with that from control cells. The enhancement in LY-B cells was decreased by adding sphingosine and treatment with the cPLA2alpha inhibitor. When CHO cells were treated with an acid sphingomyelinase inhibitor to increase the cellular SM level, the release of AA induced by A23187 or PAF was decreased. In vitro studies were then conducted to test whether SM interacts directly with cPLA2alpha. Phosphatidylcholine vesicles containing SM reduced cPLA2alpha activity. Furthermore, SM disturbed the binding of cPLA2alpha to glycerophospholipids. These results suggest that SM at the biomembrane plays important roles in regulating the cPLA2alpha-dependent release of AA by inhibiting the binding of cPLA2alpha to glycerophospholipids.

Sphingolipid Metabolism Inhibitors and Cell Function

It has been widely shown that ceramide, ceramide phosphate, sphingosine and sphingosine-1-phosphate are molecules involved in cell proliferation, differentiation and apoptosis. The regulation of the enzymes that control their metabolism has an important role for cellular fate. Many studies in the field have shed light on sphingolipid metabolism inhibition and on its role in cell processes such as gene expression, DNA duplication and RNA transcription. The aim of this review is to collect, in a systematic way, the recent advances in the field of sphingolipid metabolism inhibitors, with special emphasis on their effects on cell function. The name sphingolipid was born at the end of the XIX century on the basis of the molecule structure similar to the riddle of the sphinx. These molecules are complex lipids, localized in animal and plant membranes and in some forms of lower life, which contain sphingoid bases and have a very complex metabolism. The sphingolipid biosynthetic pathway initiates with the condensation of the serine with palmitoyl-CoA to form 3- ketodihydrosphingosine or 3-ketosphinganine, a serine pal- mitoyl transferase (SPT) catalyzed reaction. 3-ketodihy- drosphingosine is reduced to dehydrosphingosine or sphin- ganine by a NADPH dependent 3-ketosphinganine reduc- tase. An acylation in the presence of acyl-CoA produces di- hydroceramide by ceramide synthase (CerS). Dihydrocera- mide is then converted to ceramide through the introduction of the double bond trans-4,-5 by dihydroceramide desatu- rase. The ceramide can be transformed to sphingomyelin (SM) by two different ways: the first one involves the reaction with CDP-choline catalyzed by ceramide choline phos- photransferase and the second, described subsequently, con- sists in phosphocholine transfer from lecithin to ceramide by phosphatidylcholine: ceramide phosphocholine transferase or sphingomyelin synthase (SM-synthase). The synthesized SM can be used as source of phosphocholine to form phosphati- dylcholine in the presence of diacylglycerol by reverse- sphingomyelin-synthase (RSM-synthase) freeing ceramide, or can be degraded to ceramide and phosphocholine by acid or neutral sphingomyelinase (aSMase or N-SMase). The new-produced ceramide can be deacylated by acid or neutral ceramidase (AC or NC) to form sphingosine, which can be phosphorylated by a sphingosine kinase 1 (SphK1) or by sphingosine kinase 2 (SphK2) to sphingosine phosphate, can be phosphorylated by ceramide kinase (CerK) or can be

De novo ceramide biosynthesis is associated with resveratrol-induced inhibition of ornithine decarboxylase activity

Previous studies could demonstrate, that the naturally occuring polyphenol resveratrol inhibits cell growth of colon carcinoma cells at least in part by inhibition of protooncogene ornithine decarboxylase (ODC). The objective of this study was to provide several lines of evidence suggesting that the induction of ceramide synthesis is involved in this regulatory mechanisms. Cell growth was determined by BrdU incorporation and crystal violet staining. Ceramide concentrations were detected by HPLC-coupled mass-spectrometry. Protein levels were examined by Western blot analysis. ODC activity was assayed radiometrically measuring [ 14CO 2]-liberation. A dominant-negative PPARγ mutant was transfected in Caco-2 cells to suppress PPARγ-mediated functions. Antiproliferative effects of resveratrol closely correlate with a dose-dependent increase of endogenous ceramides ( p < 0.001). Compared to controls the cell-permeable ceramide analogues C2- and C6-ceramide significantly inhibit ODC-activity ( p < 0.001) in colorectal cancer cells. C6-ceramide further diminished protein levels of protooncogenes c-myc ( p < 0.05) and ODC ( p < 0.01), which is strictly related to the ability of ceramides to inhibit cell growth in a time- and dose-dependent manner. These results were further confirmed using inhibitors of sphingolipid metabolism, where only co-incubation with a serine palmitoyltransferase (SPT) inhibitor could significantly counteract resveratrol-mediated actions. These data suggest that the induction of ceramide de novo biosynthesis but not hydrolysis of sphingomyelin is involved in resveratrol-mediated inhibition of ODC. In contrast to the regulation of catabolic spermidine/spermine acetyltransferase by resveratrol, inhibitory effects on ODC occur PPARγ-independently, indicating independent pathways of resveratrol-action. Due to our findings resveratrol could show great chemopreventive and therapeutic potential in the treatment of colorectal cancers.

Overcoming resistance to gamma-rays in squamous carcinoma cells by poly-drug elevation of ceramide levels.

Recent strategies to sensitize radioresistant tumours are based on combining gamma-irradiation with inducers of apoptosis. We report that the combination of three inhibitors of sphingolipid metabolism, DL-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol.HCl(DL-PDMP)+imipramine +/- D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol (D-MAPP), with 10-Gy irradiation triggers both mitotic and apoptotic killing in radioresistant SQ20B squamous carcinoma cells. In these cells, apoptosis is defective due to a lack of ceramide generation upstream, which cannot be explained by sphingomyelinase (neutral and acidic) deficiency or rapid derivation to the sphingolipid pathway. We present evidence of a functional transduction death pathway when ceramide generation is restored, which involves the mitochondrial-mediated pathway coupled to alterations in redox status and to executive caspases activation. The poly-drug treatment restored apoptosis to levels similar to those observed in radiosensitive SCC61 squamous carcinoma cells. Simultaneous exposure to gamma-irradiation and poly-drug treatment acted synergistically in SQ20B cells to produce a marked increase in both mitochondrial dysfunction and caspase cleavage, which led to a 7.8-fold increase in apoptosis within 48 h, relative to irradiated cells. Moreover, the results suggest that the ceramide released by irradiation or poly-drug treatment converges upon common cellular targets. Modulation of endogenous ceramide levels by inhibitors of sphingolipid metabolism may represent a new cellular target for the sensitization of radioresistant tumours to gamma-ray therapy.

Increasing endogenous ceramide using inhibitors of sphingolipid metabolism maximizes ionizing radiation-induced mitochondrial injury and apoptotic cell killing.

To enhance the killing effects of ionizing radiation, we amplified the endogenous ceramide signal in Jurkat cell cultures using 3 different inhibitors of sphingolipid metabolism: DL-PDMP, D-MAPP and imipramine. Of the various possible drug combinations, only DL-PDMP (20 microM) + imipramine (20 microM) and DL-PDMP (20 microM) + imipramine (20 microM) + D-MAPP (5 microM) induced a major increase in ceramide levels, reaching 240% and 340% of control values, respectively, after incubation for 48 hr. With these models, we demonstrate that endogenously formed ceramide triggers time- and concentration-dependent apoptosis through induction of mitochondrial injury and activation of the caspase pathway. Cellular dysfunction includes alterations to the cellular redox potential, as assessed by the generation of ROS and total glutathione depletion, and a drop in Delta Psi(m). A parallel elevation of mitochondrial ceramide levels was also observed. The combination of DL-PDMP + imipramine +/- D-MAPP with 10 Gy irradiation produced cumulative effects leading to apoptosis via mitochondrial collapse and activation of the caspase cascade. The association efficiency was confirmed in normal and acid sphingomyelinase-deficient lymphoid cell lines. Taken together, these results suggest that increasing endogenous ceramide levels may potentially be very valuable when combined with ionizing radiation in tumor therapy.

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-1β (IL-1β), 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-1β. 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 CYP 1A1 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 severa1 isoforms of cytochrome P450.

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.

Role of dietary sphingolipids and inhibitors of sphingolipid metabolism in cancer and other diseases.

Sphingolipids are found in all eukaryotic and some prokaryotic organisms and participate in the regulation of cell growth, differentiation, and diverse cell functions including cell-cell communication, cell-substratum interactions and intracellular signal transduction. Nonetheless, the field of nutrition has given scant attention to these compounds so that little is known about the following fundamental questions: What is the fate of sphingolipids that are consumed in food? Does consumption of dietary sphingolipids affect the behavior of cells in the gastrointestinal tract or other organs? How do other factors in the diet affect sphingolipid metabolism? Several recent findings underscore the importance of these questions, for examples: 1) Sphingolipids are digested throughout the GI tract to ceramide and sphingosine, which are highly bioactive compounds that affect cellular regulatory pathways; 2) addition of sphingomyelin to a standard AIN diet (which is essentially devoid of sphingolipids) reduces the appearance of aberrant colonic crypts, and perhaps the number of tumors, in mice treated with a colon carcinogen; and 3) an enzyme of sphingolipid metabolism has been discovered to be the target of a class of toxic and carcinogenic mycotoxins called fumonisins. Given these recent findings, it is possible that some of the confusion that has arisen regarding the relationships between dietary fat and disease might be due to the lack of consideration of the sphingolipids that are also present.