Synthesis of Cyclopropene Analogues of Ceramide and Their Effect on Dihydroceramide Desaturase

Targeting dihydroceramide desaturase 1 (Des1): Syntheses of ceramide analogues with a rigid scaffold, inhibitory assays, and AlphaFold2-assisted structural insights reveal cyclopropenone PR280 as a potent inhibitor

Dihydroceramide desaturase 1 (Des1) catalyzes the formation of a CC double bond in dihydroceramide to furnish ceramide. Inhibition of Des1 is related to cell cycle arrest and programmed cell death. The lack of the Des1 crystalline structure, as well as that of a close homologue, hampers the detailed understanding of its inhibition mechanism and difficults the design of new inhibitors, thus making Des1 a strategic target. Based on previous structure-activity studies, different ceramides containing rigid scaffolds were designed. The synthesis and evaluation of these compounds as Des1 inhibitors allowed the identification of PR280 as a better Des 1 inhibitor in vitro (IC50 = 700 nM) than GT11 and XM462, the current reference inhibitors. This cyclopropenone ceramide was obtained in a 6-step synthesis with a 24 % overall yield. The highly confident 3D structure of Des1, recently predicted by AlphaFold2, served as the basis for conducting docking studies of known Des1 inhibitors and the ceramide derivatives synthesized by us in this study. For this purpose, a complete holoprotein structure was previously constructed. This study has allowed a better knowledge of key ligand-enzyme interactions for Des1 inhibitory activity. Furthermore, it sheds some light on the inhibition mechanism of GT11.

Pharmacological Elevation of Cellular Dihydrosphingomyelin Provides a Novel Antiviral Strategy against West Nile Virus Infection

The flavivirus life cycle is strictly dependent on cellular lipid metabolism. Polyphenols like gallic acid and its derivatives are promising lead compounds for new therapeutic agents as they can exert multiple pharmacological activities, including the alteration of lipid metabolism. The evaluation of our collection of polyphenols against West Nile virus (WNV), a representative medically relevant flavivirus, led to the identification of N,N'-(dodecane-1,12-diyl)bis(3,4,5-trihydroxybenzamide) and its 2,3,4-trihydroxybenzamide regioisomer as selective antivirals with low cytotoxicity and high antiviral activity (half-maximal effective concentrations [EC₅₀s] of 2.2 and 0.24 μM, respectively, in Vero cells; EC₅₀s of 2.2 and 1.9 μM, respectively, in SH-SY5Y cells). These polyphenols also inhibited the multiplication of other flaviviruses, namely, Usutu, dengue, and Zika viruses, exhibiting lower antiviral or negligible antiviral activity against other RNA viruses. The mechanism underlying their antiviral activity against WNV involved the alteration of sphingolipid metabolism. These compounds inhibited ceramide desaturase (Des1), promoting the accumulation of dihydrosphingomyelin (dhSM), a minor component of cellular sphingolipids with important roles in membrane properties. The addition of exogenous dhSM or Des1 blockage by using the reference inhibitor GT-11 {N-[(1R,2S)-2-hydroxy-1-hydroxymethyl-2-(2-tridecyl-1-cyclopropenyl)ethyl]octanamide} confirmed the involvement of this pathway in WNV infection. These results unveil the potential of novel antiviral strategies based on the modulation of the cellular levels of dhSM and Des1 activity for the control of flavivirus infection.

Application of pyrrolo-protected amino aldehydes in the stereoselective synthesis of anti-1,2-amino alcohols

Herein, we demonstrate the applicability of the 2,5-dimethylpyrrolo unit as a complementary N-protecting group in the highly diastereoselective synthesis of more than 20 different anti-amino alcohols (63-90% yields with up to 20 : 1 dr). Cleavage of the pyrrolo-N-protecting group was accomplished, e.g. in the presence of NH₂OH under microwave conditions with yields exceeding 80%. The applicability of the protecting groups was further demonstrated by a short total synthesis of the sphinganine-like natural product clavaminol A. The introduction of the N-pyrrolo protecting group also offers the possibility to analyse product mixtures by NMR measurements due to the absence of conformational isomers, which are otherwise common for N-protecting groups.

Identification of TRAMs as sphingolipid-binding proteins using a photoactivatable and clickable short-chain ceramide analog

Ceramide is a lipid molecule that regulates diverse physiological and pathological reactions in part through inverting the topology of certain transmembrane proteins. This topological inversion is achieved through regulated alternative translocation (RAT), which reverses the direction by which membrane proteins are translocated across the endoplasmic reticulum during translation. However, owing to technical challenges in studying protein-ceramide interaction, it remains unclear how ceramide levels are sensed in cells to trigger RAT. Here, we report the synthesis of pac-C₇-Cer, a photoactivatable and clickable short-chain ceramide analog that can be used as a probe to study protein-ceramide interactions. We demonstrate that translocating chain-associated membrane protein 2 (TRAM2), a protein known to control RAT of transmembrane 4 L6 subfamily member 20, and TRAM1, a homolog of TRAM2, interacted with molecules derived from pac-C₇-Cer. This interaction was competed by naturally existing long-chain ceramide molecules. We showed that binding of ceramide and its analogs to TRAM2 correlated with their ability to induce RAT of transmembrane 4 L6 subfamily member 20. In addition to probing ceramide-TRAM interactions, we provide evidence that pac-C₇-cer could be used for proteome-wide identification of ceramide-binding proteins. Our study provides mechanistic insights into RAT by identifying TRAMs as potential ceramide-binding proteins and establishes pac-C₇-Cer as a valuable tool for future study of ceramide-protein interactions.

Small Molecule Inhibitors Targeting Biosynthesis of Ceramide, the Central Hub of the Sphingolipid Network

Ceramides are composed of a sphingosine and a single fatty acid connected by an amide linkage. As one of the major classes of biologically active lipids, ceramides and their upstream and downstream metabolites have been implicated in several pathological conditions including cancer, neurodegeneration, diabetes, microbial pathogenesis, obesity, and inflammation. Consequently, tremendous efforts have been devoted to deciphering the dynamics of metabolic pathways involved in ceramide biosynthesis. Given that several distinct enzymes can produce ceramide, different enzyme targets have been pursued depending on the underlying disease mechanism. The main objective of this review is to provide a comprehensive overview of small molecule inhibitors reported to date for each of these ceramide-producing enzymes from a medicinal chemistry perspective.

Vaticanol C, a phytoalexin, induces apoptosis of leukemia and cancer cells by modulating expression of multiple sphingolipid metabolic enzymes

Resveratrol (RSV) has recently attracted keen interest because of its pleiotropic effects. It exerts a wide range of health-promoting effects. In addition to health-promoting effects, RSV possesses anti-carcinogenic activity. However, a non-physiological concentration is needed to achieve an anti-cancer effect, and its in vivo bioavailability is low. Therefore, the clinical application of phytochemicals requires alternative candidates that induce the desired effects at a lower concentration and with increased bioavailability. We previously reported a low IC₅₀ of vaticanol C (VTC), an RSV tetramer, among 12 RSV derivatives (Ito T. et al, 2003). However, the precise mechanism involved remains to be determined. Here, we screened an in-house chemical library bearing RSV building blocks ranging from dimers to octamers for cytotoxic effects in several leukemia and cancer cell lines and their anti-cancer drug-resistant sublines. Among the compounds, VTC exhibited the highest cytotoxicity, which was partially inhibited by a caspase 3 inhibitor, Z-VAD-FMK. VTC decreased the expression of sphingosine kinase 1, sphingosine kinase 2 and glucosylceramide synthase by transcriptional or post-transcriptional mechanisms, and increased cellular ceramides/dihydroceramides and decreased sphingosine 1-phosphate (S1P). VTC-induced sphingolipid rheostat modulation (the ratio of ceramide/S1P) is thought to be involved in cellular apoptosis. Indeed, exogenous S1P addition modulated VTC cytotoxicity significantly. A combination of SPHK1, SPHK2, and GCS chemical inhibitors induced sphingolipid rheostat modulation, cell growth suppression, and cytotoxicity similar to that of VTC. These results suggest the involvement of sphingolipid metabolism in VTC-induced cytotoxicity, and indicate VTC is a promising prototype for translational research.

The anti-cancer drug ABTL0812 induces ER stress-mediated cytotoxic autophagy by increasing dihydroceramide levels in cancer cells

ABTL0812 is a first-in-class small molecule with anti-cancer activity, which is currently in clinical evaluation in a phase 2 trial in patients with advanced endometrial and squamous non-small cell lung carcinoma (NCT03366480). Previously, we showed that ABTL0812 induces TRIB3 pseudokinase expression, resulting in the inhibition of the AKT-MTORC1 axis and macroautophagy/autophagy-mediated cancer cell death. However, the precise molecular determinants involved in the cytotoxic autophagy caused by ABTL0812 remained unclear. Using a wide range of biochemical and lipidomic analyses, we demonstrated that ABTL0812 increases cellular long-chain dihydroceramides by impairing DEGS1 (delta 4-desaturase, sphingolipid 1) activity, which resulted in sustained ER stress and activated unfolded protein response (UPR) via ATF4-DDIT3-TRIB3 that ultimately promotes cytotoxic autophagy in cancer cells. Accordingly, pharmacological manipulation to increase cellular dihydroceramides or incubation with exogenous dihydroceramides resulted in ER stress, UPR and autophagy-mediated cancer cell death. Importantly, we have optimized a method to quantify mRNAs in blood samples from patients enrolled in the ongoing clinical trial, who showed significant increased DDIT3 and TRIB3 mRNAs. This is the first time that UPR markers are reported to change in human blood in response to any drug treatment, supporting their use as pharmacodynamic biomarkers for compounds that activate ER stress in humans. Finally, we found that MTORC1 inhibition and dihydroceramide accumulation synergized to induce autophagy and cytotoxicity, phenocopying the effect of ABTL0812. Given the fact that ABTL0812 is under clinical development, our findings support the hypothesis that manipulation of dihydroceramide levels might represents a new therapeutic strategy to target cancer.Abbreviations: 4-PBA: 4-phenylbutyrate; AKT: AKT serine/threonine kinase; ATG: autophagy related; ATF4: activating transcription factor 4; Cer: ceramide; DDIT3: DNA damage inducible transcript 3; DEGS1: delta 4-desaturase, sphingolipid 1; dhCer: dihydroceramide; EIF2A: eukaryotic translation initiation factor 2 alpha; EIF2AK3: eukaryotic translation initiation factor 2 alpha kinase 3; ER: endoplasmic reticulum; HSPA5: heat shock protein family A (Hsp70) member 5; MAP1LC3B: microtubule associated protein 1 light chain 3 beta; MEF: mouse embryonic fibroblast; MTORC1: mechanistic target of rapamycin kinase complex 1; NSCLC: non-small cell lung cancer; THC: Δ⁹-tetrahydrocannabinol; TRIB3: tribbles pseudokinase 3; XBP1: X-box binding protein 1; UPR: unfolded protein response.

Native and Polyubiquitinated Forms of Dihydroceramide Desaturase Are Differentially Linked to Human Embryonic Kidney Cell Survival

There is controversy concerning the role of dihydroceramide desaturase (Degs1) in regulating cell survival, with studies showing that it can both promote and protect against apoptosis. We have therefore investigated the molecular basis for these opposing roles of Degs1. Treatment of HEK293T cells with the sphingosine kinase inhibitor SKi [2-(p-hydroxyanilino)-4-(p-chlorophenyl)thiazole] or fenretinide, but not the Degs1 inhibitor GT11 {N-[(1R,2S)-2-hydroxy-1-hydroxymethyl-2-(2-tridecyl-1-cyclopropenyl)ethyl]octan-amide}, induced the polyubiquitination of Degs1 (M _r = 40 to 140 kDa) via a mechanism involving oxidative stress, p38 mitogen-activated protein kinase (MAPK), and Mdm2 (E3 ligase). The polyubiquitinated forms of Degs1 exhibit "gain of function" and activate prosurvival pathways, p38 MAPK, c-Jun N-terminal kinase (JNK), and X-box protein 1s (XBP-1s). In contrast, another sphingosine kinase inhibitor, ABC294640 [3-(4-chlorophenyl)-adamantane-1-carboxylic acid (pyridin-4-ylmethyl)amide], at concentrations of 25 to 50 μM failed to induce formation of the polyubiquitinated forms of Degs1. In contrast to SKi, ABC294640 (25 μM) promotes apoptosis of HEK293T cells via a Degs1-dependent mechanism that is associated with increased de novo synthesis of ceramide. These findings are the first to demonstrate that the polyubiquitination of Degs1 appears to change its function from proapoptotic to prosurvival. Thus, polyubiquitination of Degs1 might provide an explanation for the reported opposing functions of this enzyme in cell survival/apoptosis.

Novel Sphingolipid-Based Cancer Therapeutics in the Personalized Medicine Era

Sphingolipids are bioactive lipids that participate in a wide variety of biological mechanisms, including cell death and proliferation. The myriad of pro-death and pro-survival cellular pathways involving sphingolipids provide a plethora of opportunities for dysregulation in cancers. In recent years, modulation of these sphingolipid metabolic pathways has been in the forefront of drug discovery for cancer therapeutics. About two decades ago, researchers first showed that standard of care treatments, e.g., chemotherapeutics and radiation, modulate sphingolipid metabolism to increase endogenous ceramides, which kill cancer cells. Strikingly, resistance to these treatments has also been linked to altered sphingolipid metabolism, favoring lipid species that ultimately lead to cell survival. To this end, many inhibitors of sphingolipid metabolism have been developed to further define not only our understanding of these pathways but also to potentially serve as therapeutic interventions. Therefore, understanding how to better use these new drugs that target sphingolipid metabolism, either alone or in combination with current cancer treatments, holds great potential for cancer control. While sphingolipids in cancer have been reviewed previously (Hannun & Obeid, 2018; Lee & Kolesnick, 2017; Morad & Cabot, 2013; Newton, Lima, Maceyka, & Spiegel, 2015; Ogretmen, 2018; Ryland, Fox, Liu, Loughran, & Kester, 2011) in this chapter, we present a comprehensive review on how standard of care therapeutics affects sphingolipid metabolism, the current landscape of sphingolipid inhibitors, and the clinical utility of sphingolipid-based cancer therapeutics.

From the configurational preference of dihydroceramide desaturase-1 towards Δ6-unsaturated substrates to the discovery of a new inhibitor

Dihydroceramide desaturase 1 (Des1) catalyzes the last step of ceramide synthesis de novo, thus regulating the physiologically relevant balance between dihydrosphingolipids and sphingolipids. Here we report on the configurational preference of Des1 towards isomeric Δ⁶-unsaturated dihydroceramide analogs and the discovery of a potent Des1 inhibitor.

Substrate reduction therapy for Krabbe's disease

Krabbe's disease (KD) is a lysosomal storage disorder in which galactosylceramide, a major glycosphingolipid of myelin, and psychosine (galactose-sphingosine) cannot be adequately metabolized because of a deficiency in galactosylceramidase. Substrate reduction therapy (SRT) has been tested in preclinical studies. The premise of SRT is to reduce the synthesis of substrates that are not adequately digested so that the substrate burden is lowered, resulting in less accumulation of unmetabolized material. SRT is used for Gaucher's disease, in which inhibitors of the terminal biosynthetic step are used. Unfortunately, an inhibitor for the final step of galactosylceramide biosynthesis, i.e., UDP glycosyltransferase 8 (a.k.a. UDP-galactose ceramide galactosyltransferase), has not been found. Approaches that inhibit an earlier biosynthetic step or that lessen the substrate burden by other means, such as genetic manipulations, have been tested in the twitcher mouse model of KD. Either as a stand-alone therapy or in combination with other approaches, SRT slowed the disease course, indicating that this approach has potential therapeutic value. For instance, in individuals with adult-onset disease, SRT theoretically could lessen the production of substrates so that residual enzymatic activity could adequately manage the lower substrate burden. In more severe forms of disease, SRT theoretically could be part of a combination therapy. However, SRT has the potential to impair normal function by reducing the synthesis of galactosylceramide to levels that impede myelin function, or SRT could have other deleterious effects. Thus, multiple issues need to be resolved before this approach is ready for testing in humans. © 2016 Wiley Periodicals, Inc.

Gamma-tocotrienol profoundly alters sphingolipids in cancer cells by inhibition of dihydroceramide desaturase and possibly activation of sphingolipid hydrolysis during prolonged treatment

Vitamin E gamma-tocotrienol (γTE) is known to have anticancer effects, but mechanisms underlying these actions are not clear. Here using liquid chromatography tandem mass spectrometry, we show that γTE induced marked changes of sphingolipids including rapid elevation of dihydrosphingosine and dihydroceramides (dhCers) in various types of cancer cells. The elevation of dihydrosphingolipids coincided with increased cellular stress, as indicated by JNK phosphorylation, and was prior to any sign of induction of apoptosis. Chemically blocking de novo synthesis of sphingolipids partially counteracted γTE-induced apoptosis and autophagy. Experiments using ¹³C₃, ¹⁵N-labeled l-serine together with enzyme assays indicate that γTE inhibited cellular dihydroceramide desaturase (DEGS) activity without affecting its protein expression or de novo synthesis of sphingolipids. Unlike the effect on dhCers, γTE decreased ceramides (Cers) after 8-h treatment but increased C_18:0-Cer and C_16:0-Cer after 16 and 24 h, respectively. The increase of Cers coincides with γTE-induced apoptosis and autophagy. Since γTE inhibits DEGS and decreases de novo Cer synthesis, elevation of Cers during prolonged γTE treatment is likely caused by sphingomeylinase-mediated hydrolysis of sphingomyelin. This idea is supported by the observation that an acid sphingomeylinase inhibitor partially reversed γTE-induced cell death. Our study demonstrates that γTE altered sphingolipid metabolism by inhibiting DEGS activity and possibly by activating SM hydrolysis during prolonged treatment in cancer cells.

Vitamin E metabolite 13'-carboxychromanols inhibit pro-inflammatory enzymes, induce apoptosis and autophagy in human cancer cells by modulating sphingolipids and suppress colon tumor development in mice

Vitamin E forms are substantially metabolized to various carboxychromanols including 13'-carboxychromanols (13'-COOHs) that are found at high levels in feces. However, there is limited knowledge about functions of these metabolites. Here we studied δT-13'-COOH and δTE-13'-COOH, which are metabolites of δ-tocopherol and δ-tocotrienol, respectively. δTE-13'-COOH is also a natural constituent of a traditional medicine Garcinia Kola. Both 13'-COOHs are much stronger than tocopherols in inhibition of pro-inflammatory and cancer promoting cyclooxygenase-2 (COX-2) and 5-lipoxygenase (5-LOX), and in induction of apoptosis and autophagy in colon cancer cells. The anticancer effects by 13'-COOHs appeared to be partially independent of inhibition of COX-2/5-LOX. Using liquid chromatography tandem mass spectrometry, we found that 13'-COOHs increased intracellular dihydrosphingosine and dihydroceramides after short-time incubation in HCT-116 cells, and enhanced ceramides while decreased sphingomyelins during prolonged treatment. Modulation of sphingolipids by 13'-COOHs was observed prior to or coinciding with biochemical manifestation of cell death. Pharmaceutically blocking the increase of these sphingolipids partially counteracted 13'-COOH-induced cell death. Further, 13'-COOH inhibited dihydroceramide desaturase without affecting the protein expression. In agreement with these mechanistic findings, δTE-13'-COOH significantly suppressed the growth and multiplicity of colon tumor in mice. Our study demonstrates that 13'-COOHs have anti-inflammatory and anticancer activities, may contribute to in vivo anticancer effect of vitamin E forms and are promising novel cancer prevention agents.

Inhibitors of dihydroceramide desaturase 1: Therapeutic agents and pharmacological tools to decipher the role of dihydroceramides in cell biology

Dihydroceramide desaturase (Des1) is the last enzyme in the de novo synthesis of ceramides (Cer). It catalyzes the insertion of a double bond into dihydroceramides (dhCer) to convert them to Cer, both of which are further metabolized to more complex (dihydro) sphingolipids. For many years dhCer have received poor attention, mainly due to their supposed lack of biological activity. It was not until about ten years ago that the concept that dhCer might have regulatory roles in biology emerged for the first time. Since then, multiple publications have established that dhCer are implicated in a wide spectrum of biological processes. Physiological and pathophysiological functions of dhCer have been recently reviewed. In this review we will focus on the biochemical features of Des1 and on its inhibition by different compounds with presumably different modes of action.

Inhibition of dihydroceramide desaturase activity by the sphingosine kinase inhibitor SKI II

Sphingosine kinase inhibitor (SKI) II has been reported as a dual inhibitor of sphingosine kinases (SKs) 1 and 2 and has been extensively used to prove the involvement of SKs and sphingosine-1-phosphate (S1P) in cellular processes. Dihydroceramide desaturase (Des1), the last enzyme in the de novo synthesis of ceramide (Cer), regulates the balance between dihydroceramides (dhCers) and Cers. Both SKs and Des1 have interest as therapeutic targets. Here we show that SKI II is a noncompetitive inhibitor (Ki = 0.3 μM) of Des1 activity with effect also in intact cells without modifying Des1 protein levels. Molecular modeling studies support that the SKI II-induced decrease in Des1 activity could result from inhibition of NADH-cytochrome b5 reductase. SKI II, but not the SK1-specific inhibitor PF-543, provoked a remarkable accumulation of dhCers and their metabolites, while both SKI II and PF-543 reduced S1P to almost undetectable levels. SKI II, but not PF543, reduced cell proliferation with accumulation of cells in the G0/G1 phase. SKI II, but not PF543, induced autophagy. These overall findings should be taken into account when using SKI II as a pharmacological tool, as some of the effects attributed to decreased S1P may actually be caused by augmented dhCers and/or their metabolites.

C22:0- and C24:0-dihydroceramides confer mixed cytotoxicity in T-cell acute lymphoblastic leukemia cell lines

We previously reported that fenretinide (4-HPR) was cytotoxic to acute lymphoblastic leukemia (ALL) cell lines in vitro in association with increased levels of de novo synthesized dihydroceramides, the immediate precursors of ceramides. However, the cytotoxic potentials of native dihydroceramides have not been defined. Therefore, we determined the cytotoxic effects of increasing dihydroceramide levels via de novo synthesis in T-cell ALL cell lines and whether such cytotoxicity was dependent on an absolute increase in total dihydroceramide mass versus an increase of certain specific dihydroceramides. A novel method employing supplementation of individual fatty acids, sphinganine, and the dihydroceramide desaturase-1 (DES) inhibitor, GT-11, was used to increase de novo dihydroceramide synthesis and absolute levels of specific dihydroceramides and ceramides. Sphingolipidomic analyses of four T-cell ALL cell lines revealed strong positive correlations between cytotoxicity and levels of C22:0-dihydroceramide (ρ = 0.74–0.81, P ≤ 0.04) and C24:0-dihydroceramide (ρ = 0.84–0.90, P ≤ 0.004), but not between total or other individual dihydroceramides, ceramides, or sphingoid bases or phosphorylated derivatives. Selective increase of C22:0- and C24:0-dihydroceramide increased level and flux of autophagy marker, LC3B-II, and increased DNA fragmentation (TUNEL assay) in the absence of an increase of reactive oxygen species; pan-caspase inhibition blocked DNA fragmentation but not cell death. C22:0-fatty acid supplemented to 4-HPR treated cells further increased C22:0-dihydroceramide levels (P ≤ 0.001) and cytotoxicity (P ≤ 0.001). These data demonstrate that increases of specific dihydroceramides are cytotoxic to T-cell ALL cells by a caspase-independent, mixed cell death mechanism associated with increased autophagy and suggest that dihydroceramides may contribute to 4-HPR-induced cytotoxicity. The targeted increase of specific acyl chain dihydroceramides may constitute a novel anticancer approach.

Novel chemotherapeutic drugs in sphingolipid cancer research

Sphingolipid-metabolizing enzymes are becoming targets for chemotherapeutic development with an increasing interest in the recent years. In this chapter we introduce the sphingolipid family of lipids, and the role of individual species in cell homeostasis. We also discuss their roles in several rare diseases and overall, in cancer transformation. We follow the biosynthesis pathway of the sphingolipid tree, focusing on the enzymes in order to understand how using small molecule inhibitors makes it possible to modulate cancer progression. Finally, we describe the most used and historically significant inhibitors employed in cancer research, their relationships to sphingolipid metabolism, and some promising results found in this field.

Identification of DES1 as a vitamin A isomerase in Müller glial cells of the retina

Absorption of a light particle by an opsin-pigment causes photoisomerization of its retinaldehyde chromophore. Restoration of light sensitivity to the resulting apo-opsin requires chemical re-isomerization of the photobleached chromophore. This is carried out by a multistep enzyme pathway called the visual cycle. Accumulating evidence suggests the existence of an alternate visual cycle for regenerating opsins in daylight. Here, we identified dihydroceramide desaturase-1 (DES1) as a retinol isomerase and an excellent candidate for isomerase-2 in this alternate pathway. DES1 is expressed in retinal Müller cells where it co-immunoprecipitates with cellular retinaldehyde binding protein (CRALBP). Adenoviral gene therapy with DES1 partially rescued the biochemical and physiological phenotypes in rpe65 ^−/− mice lacking isomerohydrolase (isomerase-1). Knockdown of DES1 expression by RNA-interference concordantly reduced isomerase-2 activity in cultured Müller cells. Purified DES1 possessed very high isomerase-2 activity in the presence of appropriate cofactors, suggesting that DES1 by itself is sufficient for isomerase activity.

Practical multigram-scale synthesis of 4,6- and 4,8-sphingadienes, chemopreventive sphingoid bases

Sphingadienes are chemopreventive agents that act by blocking signaling pathways that are activated in cancer. A practical synthesis of 4,6- and 4,8-sphingadienes on a scale of gram quantities is reported here in order to allow evaluation of the biological properties of these sphingolipids. The key steps in the preparation of 4,6-sphingadiene (1a) are an intramolecular cyclization of N-Boc derivative 5a to oxazolidinone derivative 6a, followed by conversion to carbamate intermediate 7a and base-mediated hydrolysis to afford the product without further purification. 4,8-Sphingadiene (1b) was prepared in a similar fashion; the requisite trans-γ,δ-unsaturated aldehyde 15 was prepared by an ester enolate Ireland-Claisen rearrangement.

3-Deoxy-3,4-dehydro analogs of XM462. Preparation and activity on sphingolipid metabolism and cell fate

Three analogs of the dihydroceramide desaturase inhibitor XM462 are reported. The compounds inhibit both dihydroceramide desaturase and acid ceramidase, but with different potencies depending on the N-acyl moiety. Other enzymes of sphingolipid metabolism, such as neutral ceramidase, acid sphingomyelinase, acid glucosylceramide hydrolase, sphingomyelin synthase and glucosylceramide synthase, are not affected. The effect on the sphingolipidome of the two best inhibitors, namely (R,E)-N-(1-hydroxy-4-(tridecylthio)but-3-en-2-yl)octanamide (RBM2-1B) and (R,E)-N-(1-hydroxy-4-(tridecylthio)but-3-en-2-yl)pivalamide (RBM2-1D), is in accordance with the results obtained in the enzyme assays. These two compounds reduce cell viability in A549 and HCT116 cell lines with similar potencies and both induced apoptotic cell death to similar levels than C8-Cer in HCT116 cells. The possible therapeutic implications of the activities of these compounds are discussed.

Signaling and regulatory functions of bioactive sphingolipids as therapeutic targets in multiple sclerosis

Spingolipids (SLs) are an important component of central nervous system (CNS) myelin sheaths and affect the viability of brain cells (oligodendrocytes, neurons and astrocytes) that is determined by signaling mediated by bioactive sphingoids (lyso-SLs). Recent studies indicate that two lipids, ceramide and sphingosine 1-phosphate (S1P), are particularly involved in many human diseases including the autoimmune inflammatory demyelination of multiple sclerosis (MS). In this review we: (1) Discuss possible sources of ceramide in CNS; (2) Summarize the features of the metabolism of S1P and its downstream signaling through G-protein-coupled receptors; (3) Link perturbations in bioactive SLs metabolism to MS neurodegeneration and (4) Compile ceramide and S1P relationships to this process. In addition, we described recent preclinical and clinical trials of therapies targeting S1P signaling, including 2-amino-2-propane-1,3-diol hydrochloride (FTY720, fingolimod) as well as proposed intervention to specify critical SL levels that tilt balances of apoptotic/active ceramide versus anti-apoptotic/inactive dihydroceramide that may offer a novel and important therapeutic approach to MS.

Dihydroceramide desaturase and dihydrosphingolipids: debutant players in the sphingolipid arena

Sphingolipids are a wide family of lipids that share common sphingoid backbones, including (2S,3R)-2-amino-4-octadecane-1,3-diol (dihydrosphingosine) and (2S,3R,4E)-2-amino-4-octadecene-1,3-diol (sphingosine). The metabolism and biological functions of sphingolipids derived from sphingosine have been the subject of many reviews. In contrast, dihydrosphingolipids have received poor attention, mainly due to their supposed lack of biological activity. However, the reported biological effects of active site directed dihydroceramide desaturase inhibitors and the involvement of dihydrosphingolipids in the response of cells to known therapeutic agents support that dihydrosphingolipids are not inert but are in fact biologically active and underscore the importance of elucidating further the metabolic pathways and cell signaling networks involved in the biological activities of dihydrosphingolipids. Dihydroceramide desaturase is the enzyme involved in the conversion of dihydroceramide into ceramide and it is crucial in the regulation of the balance between sphingolipids and dihydrosphingolipids. Furthermore, given the enzyme requirement for O₂ and the NAD(P)H cofactor, the cellular redox balance and dihydroceramide desaturase activity may reciprocally influence each other. In this review both dihydroceramide desaturase and the biological functions of dihydrosphingolipids are addressed and perspectives on this field are discussed.

Identification of dihydroceramide desaturase as a direct in vitro target for fenretinide

The dihydroceramide desaturase (DES) enzyme is responsible for inserting the 4,5-trans-double bond to the sphingolipid backbone of dihydroceramide. We previously demonstrated that fenretinide (4-HPR) inhibited DES activity in SMS-KCNR neuroblastoma cells. In this study, we investigated whether 4-HPR acted directly on the enzyme in vitro. N-C8:0-d-erythro-dihydroceramide (C(8)-dhCer) was used as a substrate to study the conversion of dihydroceramide into ceramide in vitro using rat liver microsomes, and the formation of tritiated water after the addition of the tritiated substrate was detected and used to measure DES activity. NADH served as a cofactor. The apparent K(m) for C(8)-dhCer and NADH were 1.92 ± 0.36 μm and 43.4 ± 6.47 μm, respectively; and the V(max) was 3.16 ± 0.24 and 4.11 ± 0.18 nmol/min/g protein. Next, the effects of 4-HPR and its metabolites on DES activity were investigated. 4-HPR was found to inhibit DES in a dose-dependent manner. At 20 min, the inhibition was competitive; however, longer incubation times demonstrated the inhibition to be irreversible. Among the major metabolites of 4-HPR, 4-oxo-N-(4-hydroxyphenyl)retinamide (4-oxo-4-HPR) showed the highest inhibitory effect with substrate concentration of 0.5 μm, with an IC(50) of 1.68 μm as compared with an IC(50) of 2.32 μm for 4-HPR. N-(4-Methoxyphenyl)retinamide (4-MPR) and 4-Oxo-N-(4-methoxyphenyl)retinamide (4-oxo-4-MPR) had minimal effects on DES activity. A known competitive inhibitor of DES, C(8)-cyclopropenylceramide was used as a positive control. These studies define for the first time a direct in vitro target for 4-HPR and suggest that inhibitors of DES may be used as therapeutic interventions to regulate ceramide desaturation and consequent function.

Dihydroceramide desaturase inhibition by a cyclopropanated dihydroceramide analog in cultured keratinocytes

Most mammalian sphingolipids contain a 4,5-(E)-double bond. We report on the chemical synthesis of a dihydroceramide derivative that prevents the introduction of the double bond into sphingolipids. Minimal alteration of the parent structure by formally replacing the hydrogen atoms in the 5- and in the 6-position of the sphinganine backbone by a methylene group leads to an inhibitor of dihydroceramide desaturase in cultured cells. In the presence of 10–50 μM of compound (1), levels of biosynthetically formed dihydroceramide and—surprisingly—also of phytoceramide are elevated at the expense of ceramide. The cells respond to the lack of unsaturated sphingolipids by an elevation of mRNAs of enzymes required for sphingosine formation. At the same time, the analysis of proliferation and differentiation markers indicates that the sphingolipid double bond is required to keep the cells in a differentiated state.

Alkaline ceramidase 2 (ACER2) and its product dihydrosphingosine mediate the cytotoxicity of N-(4-hydroxyphenyl)retinamide in tumor cells

Increased generation of dihydrosphingosine (DHS), a bioactive sphingolipid, has been implicated in the cytotoxicity of the synthetic retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) in tumor cells. However, how 4-HPR increases DHS remains unclear. Here we demonstrate that 4-HPR increases the expression of ACER2, which catalyzes the hydrolysis of dihydroceramides to generate DHS, and that ACER2 up-regulation plays a key role in mediating the 4-HPR-induced generation of DHS as well as the cytotoxicity of 4-HPR in tumor cells. Treatment with 4-HPR induced the accumulation of dihydroceramides (DHCs) in tumor cells by inhibiting dihydroceramide desaturase (DES) activity, which catalyzes the conversion of DHCs to ceramides. Treatment with 4-HPR also increased ACER2 expression through a retinoic acid receptor-independent and caspase-dependent manner. Overexpression of ACER2 augmented the 4-HPR-induced generation of DHS as well as 4-HPR cytotoxicity, and 4-HPR-induced death in tumor cells, whereas knocking down ACER2 had the opposite effects. ACER2 overexpression, along with treatment with GT11, another DES inhibitor, markedly increased cellular DHS, leading to tumor cell death, whereas ACER2 overexpression or GT11 treatment alone failed to do so, suggesting that both ACER2 up-regulation and DES inhibition are necessary and sufficient to mediate 4-HPR-induced DHS accumulation, cytotoxicity, and death in tumor cells. Taken together, these results suggest that up-regulation of the ACER2/DHS pathway mediates the cytotoxicity of 4-HPR in tumor cells and that up-regulating or activating ACER2 may improve the anti-cancer activity of 4-HRR and other DHC-inducing agents.

Potentiation of cannabinoid-induced cytotoxicity in mantle cell lymphoma through modulation of ceramide metabolism

Ceramide levels are elevated in mantle cell lymphoma (MCL) cells following treatment with cannabinoids. Here, we investigated the pathways of ceramide accumulation in the MCL cell line Rec-1 using the stable endocannabinoid analogue R(+)-methanandamide (R-MA). We further interfered with the conversion of ceramide into sphingolipids that promote cell growth. Treatment with R-MA led to increased levels of ceramide species C16, C18, C24, and C(24:1) and transcriptional induction of ceramide synthases (CerS) 3 and 6. The effects were attenuated using SR141716A, which has high affinity to cannabinoid receptor 1 (CB1). The CB1-mediated induction of CerS3 and CerS6 mRNA was confirmed using Win-55,212-2. Simultaneous silencing of CerS3 and CerS6 using small interfering RNA abrogated the R-MA-induced accumulation of C16 and C24. Inhibition of either of the enzymes serine palmitoyl transferase, CerS, and dihydroceramide desaturase within the de novo ceramide pathway reversed ceramide accumulation and cell death induced by R-MA treatment. To enhance the cytotoxic effect R-MA, sphingosine kinase-1 and glucosylceramide synthase, enzymes that convert ceramide to the pro-proliferative sphingolipids sphingosine-1-phospate and glucosylceramide, respectively, were inhibited. Suppression of either enzyme using inhibitors or small interfering RNA potentiated the decreased viability, induction of cell death, and ceramide accumulation induced by R-MA treatment. Our findings suggest that R-MA induces cell death in MCL via CB1-mediated up-regulation of the de novo ceramide synthesis pathway. Furthermore, this is the first study were the cytotoxic effect of a cannabinoid is enhanced by modulation of ceramide metabolism.

Nucleophilic carbene catalyzed synthesis of 1,2 amino alcohols via azidation of epoxy aldehydes

We report herein a nucleophilic carbene catalyzed redox azidation of epoxyaldehydes. The intermediate beta-hydroxy acyl azides undergo thermal Curtius rearrangement followed by trapping with excess azide to form carbamoyl azides or, in a complementary sequence, by the hydroxy group to form oxazolidinones. Both products are formed in modest to good yields and diastereoselectivities. The use of an enantioenriched triazolium catalyst leads to modest asymmetric induction.

Chemical tools to investigate sphingolipid metabolism and functions

Sphingolipids comprise an important group of biomolecules, some of which have been shown to play important roles in the regulation of many cell functions. From a structural standpoint, they all share a long 2-amino-1,3-diol chain, which can be either saturated (sphinganine), hydroxylated at C4 (phytosphingosine), or unsaturated at C4 (sphingosine) as in most mammalian cells. N-acylation of sphingosine leads to ceramide, a key intermediate in sphingolipid metabolism that can be enzymatically modified at the C1-OH position to other biologically important sphingolipids, such as sphingomyelin or glycosphingolipids. In addition, both ceramide and sphingosine can be phosphorylated at C1-OH to give ceramide-1-phosphate and sphingosine-1-phosphate, respectively. To better understand the biological and biophysical roles of sphingolipids, many efforts have been made to design synthetic analogues as chemical tools able to unravel their structure-activity relationships, and to alter their cellular levels. This last approach has been thoroughly studied by the development of specific inhibitors of some key enzymes that play an important role in biosynthesis or metabolism of these intriguing lipids. With the above premises in mind, the aim of this review is to collect, in a systematic way, the recent efforts described in the literature leading to the development of new chemical entities specifically designed to achieve the above goals.

Synthesis and biological activity of a novel inhibitor of dihydroceramide desaturase

A novel mechanism-based dihydroceramide desaturase inhibitor (XM462) in which the substrate C5 methylene group is replaced by a sulfur atom is reported. Dihydroceramide desaturase inhibition occurred both in vitro and in cultured cells with IC(50) values of 8.2 and 0.78 microM, respectively, at a substrate concentration of 10 microM. In vitro experiments showed that XM462 produced a mixed-type inhibition (K(i)=2 microM, alpha=0.83). LC-MS analyses showed that accumulation of endogenous dihydroceramides occurred in cells upon treatment with XM462 in serum-free medium, whereas ceramides built up in controls. In addition, XM462 was found to be metabolised to its 1-glucosyl and 1-phosphocholine derivatives, and to the products of N-deacylation and reacylation with palmitoyl and stearoyl groups. In Jurkat A3 cells cultured in serum-free medium, viability, as the percentage of trypan blue unstained cells in total cells, was reduced upon XM462 treatment (5 microM, 24 h), but not in controls. The interest of this compound is discussed.

Involvement of dihydroceramide desaturase in cell cycle progression in human neuroblastoma cells

The role of dihydroceramide desaturase as a key enzyme in the de novo pathway of ceramide generation was investigated in human neuroblastoma cells (SMS-KCNR). A novel assay using water-soluble analogs of dihydroceramide, dihydroceramidoids (D-erythro-dhCCPS analogs), was used to measure desaturase activity in situ. Conversion of D-erythro-2-N-[12'-(1''-pyridinium)-dodecanoyl]-4,5-dihydrosphingosine bromide (C(12)-dhCCPS) to its 4,5-desaturated counterpart, D-erythro-2-N-[12'-(1''-pyridinium)dodecanoyl]sphingosine bromide (C(12)-CCPS), was determined by liquid chromatography/mass spectrometry analysis. The validity of the assay was confirmed using C(8)-cyclopropenylceramide, a competitive inhibitor of dihydroceramide desaturase. A human homolog (DEGS-1) of the Drosophila melanogaster des-1 gene was recently identified and reported to have desaturase activity. Transfection of SMS-KCNR cells with small interfering RNA to DEGS-1 significantly blocked the conversion of C(12)-dhCCPS to C(12)-CCPS. The associated accumulation of endogenous dihydroceramides confirmed DEGS-1 as the main active dihydroceramide desaturase in these cells. The partial loss of DEGS-1 inhibited cell growth, with cell cycle arrest at G(0)/G(1). This was accompanied by a significant decrease in the amount of phosphorylated retinoblastoma protein. This hypophosphorylation was inhibited by tautomycin and not by okadaic acid, suggesting the involvement of protein phosphatase 1. Additionally, we found that treatment of SMS-KCNR cells with fenretinide inhibited desaturase activity in a dose-dependent manner. An increase in dihydroceramides (but not ceramides) paralleled this process as measured by liquid chromatography/mass spectrometry. There were no effects on the mRNA or protein levels of DEGS-1, suggesting that fenretinide acts at the post-translational level as an inhibitor of this enzyme. Tautomycin was also able to block the hypophosphorylation of the retinoblastoma protein observed upon fenretinide treatment. These findings suggest a novel biological function for dihydroceramides.

Inhibitors of sphingolipid metabolism enzymes

Sphingolipids are a family of lipids that play essential roles both as structural cell membrane components and in cell signalling. The cellular contents of the various sphingolipid species are controlled by enzymes involved in their metabolic pathways. In this context, the discovery of small chemical entities able to modify these enzyme activities in a potent and selective way should offer new pharmacological tools and therapeutic agents.

Design, synthesis and activity as acid ceramidase inhibitors of 2-oxooctanoyl and N-oleoylethanolamine analogues

The synthesis of novel N-acylethanolamines and their use as inhibitors of the aCDase is reported here. The compounds are either 2-oxooctanamides or oleamides of sphingosine analogs featuring a 3-hydroxy-4,5-hexadecenyl tail replaced by ether or thioether moieties. It appears that, within the 2-oxooctanamide family, the C3-OH group of the sphingosine molecule is required for inhibition both in vitro and in cultured cells. Furthermore, although the (E)-4 double bond is not essential for inhibitory activity, the (E) configuration is required, since the analogue with a (Z)-4 unsaturation was not inhibitory. None of the oleamides inhibited the aCDase in vitro. Conversely, with the exception of N-oleoylethanolamine and its analogs with S-decyl and S-hexadecyl substituents, all the synthesized oleamides inhibited the aCDase in cultured cells, although with a relatively low potency. We conclude that novel aCDase inhibitors can evolve from N-acylation of sphingoid bases with electron deficient-acyl groups. In contrast, chemical modification of the N-oleoylsphingosine backbone does not seem to offer an appropriate strategy to obtain aCDase inhibitors.

Analogs of the dihydroceramide desaturase inhibitor GT11 modified at the amide function: synthesis and biological activities

Dihydroceramide desaturase is the last enzyme in the biosynthesis of ceramide de novo. The cyclopropene-containing sphingolipid GT11 is a competitive inhibitor of dihydroceramide desaturase. The biological effects of chemical modification of the GT11 amide linkage are reported in this article. Either N-methyl substitution or replacement of the amide alpha-carbonyl methylene by oxygen result in inactive compounds. In contrast, both urea (3) and thiourea (4) analogs of GT11, as well as three alpha-ketoamides (5-7), did inhibit the desaturation of N-octanoylsphinganine to N-octanoylsphingosine, although with significantly lower potency than GT11. Furthermore, the alpha-ketoamides 5-7 inhibit the acidic ceramidase with similar potencies (IC50 52-83 microM). Inhibition of the neutral/alkaline ceramidase by these compounds requires around 20-fold higher concentrations. Structure-activity relationships and the biological interest of these compounds are discussed.

New aminocyclitols as modulators of glucosylceramide metabolism

A series of 13 aminocyclitol derivatives belonging to two different families is described. Their configuration is governed by the regio- and stereocontrolled epoxide opening of a suitably protected conduritol-B epoxide. Studies on several glycosyl processing enzymes indicate that some of them are good inhibitors of glucosylceramide hydrolase. A rationale to account for preliminary structure-activity relationships is provided.

Specificity of the dihydroceramide desaturase inhibitor N-[(1R,2S)-2-hydroxy-1-hydroxymethyl-2-(2-tridecyl-1-cyclopropenyl)ethyl]octanamide (GT11) in primary cultured cerebellar neurons

Dihydroceramide desaturase catalyzes the conversion of the innocuous precursor dihydroceramide into a highly bioactive product ceramide. We studied the effect of N-[(1R,2S)-2-hydroxy-1-hydroxymethyl-2-(2-tridecyl-1-cyclopropenyl)ethyl]octanamide (GT11), the first inhibitor of this enzyme, in primary cultured cerebellar neurons. Although desaturase was efficiently inhibited (IC50 of 23 nM), the compound lost its specificity at higher concentrations. From 5 microM upward, GT11 also decreased de novo sphingolipid biosynthesis. Studies with two differentially labeled radioactive analogs of GT11 support that the inhibitor itself and not a downstream metabolic product, interferes with sphingolipid biosynthesis. It is interesting that serine palmitoyltransferase activity decreased in the presence of high concentrations of GT11 in intact cells, but not when added directly into cell homogenates. However, suppression of enzyme transcription could not be detected. But at high concentrations GT11 provoked an accumulation of sphingosine-1-phosphate and especially of dihydrosphingosine-1-phosphate, suggesting a decreased activity of sphingosine-1-phosphate lyase. Enzyme activity measurements indeed supported this assumption. Thus, at higher concentrations, GT11 interferes with lyase activity, inducing an accumulation of sphingoid base phosphates that, in turn, down-regulate serine palmitoyltransferase activity. At low concentrations, however, GT11 is the first specific inhibitor of dihydroceramide desaturase described so far. Considering the proapoptotic and proinflammatory effect of ceramide, GT11 could also turn out to be a novel cell-protective agent.