The sphingosine 1-phosphate receptor 2/4 antagonist JTE-013 elicits off-target effects on sphingolipid metabolism

Sphingosine 1-phosphate (S1P) is a signaling lipid that has broad roles, working either intracellularly through various protein targets, or extracellularly via a family of five G-protein coupled receptors. Agents that selectively and specifically target each of the S1P receptors have been sought as both biological tools and potential therapeutics. JTE-013, a small molecule antagonist of S1P receptors 2 and 4 (S1P2 and S1P4) has been widely used in defining the roles of these receptors in various biological processes. Indeed, our previous studies showed that JTE-013 had anti-acute myeloid leukaemia (AML) activity, supporting a role for S1P2 in the biology and therapeutic targeting of AML. Here we examined this further and describe lipidomic analysis of AML cells that revealed JTE-013 caused alterations in sphingolipid metabolism, increasing cellular ceramides, dihydroceramides, sphingosine and dihydrosphingosine. Further examination of the mechanisms behind these observations showed that JTE-013, at concentrations frequently used in the literature to target S1P2/4, inhibits several sphingolipid metabolic enzymes, including dihydroceramide desaturase 1 and both sphingosine kinases. Collectively, these findings demonstrate that JTE-013 can have broad off-target effects on sphingolipid metabolism and highlight that caution must be employed in interpreting the use of this reagent in defining the roles of S1P2/4.

Dihydroceramide Desaturase Functions as an Inducer and Rectifier of Apoptosis: Effect of Retinol Derivatives, Antioxidants and Phenolic Compounds

Dihydroceramide desaturase (Degs1) catalyses the introduction of a 4,5-trans double bond into dihydroceramide to form ceramide. We show here that Degs1 is polyubiquitinated in response to retinol derivatives, phenolic compounds or anti-oxidants in HEK293T cells. The functional predominance of native versus polyubiquitinated forms of Degs1 appears to govern cytotoxicity. Therefore, 4-HPR or celecoxib appear to stimulate the de novo ceramide pathway (with the exception of C24:0 ceramide), using native Degs1, and thereby promote PARP cleavage and LC3B-I/II processing (autophagy/apoptosis). The ubiquitin-proteasomal degradation of Degs1 is positively linked to cell survival via XBP-1s and results in a concomitant increase in dihydroceramides and a decrease in C24:0 ceramide levels. However, in the case of 4-HPR or celecoxib, the native form of Degs1 functionally predominates, such that the apoptotic programme is sustained. In contrast, 4-HPA or AM404 do not produce apoptotic ceramide, using native Degs1, but do promote a rectifier function to induce ubiquitin-proteasomal degradation of Degs1 and are not cytotoxic. Therefore, Degs1 appears to function both as an ‘inducer’ and ‘rectifier’ of apoptosis in response to chemical cellular stress, the dynamic balance for which is dependent on the nature of chemical stress, thereby determining cytotoxicity. The de novo synthesis of ceramide or the ubiquitin-proteasomal degradation of Degs1 in response to anti-oxidants, retinol derivatives and phenolic compounds appear to involve sensors, and for rectifier function, this might be Degs1 itself.

Dihydroceramide desaturase directs the switch between exosome production and autophagy for neuronal maintenance

ABSTRACTExosomes play important roles in the nervous system. Mutations in the human dihydroceramide desaturase gene, DEGS1, are recently linked to severe neurological disorders, but the cause remains unknown. Here, we show that Ifc is required for the morphology and function of Drosophila photoreceptor neurons and not in the surrounding glia, but the degeneration of ifc-KO eyes can be rescued by glial expression of ifc, possibly mediated by exosomes. We develop an in vivo assay using Drosophila eye imaginal discs and show that the level and activity of Ifc correlates with the detection of exosome-like vesicles. While ifc overexpression and autophagy inhibition both enhances exosome production, combining the two had no additive effect. Moreover, ifc-KO reduces the density of the exosome precursor intraluminal vesicles (ILVs) in vivo, and DEGS1 promotes ILV formation in vitro. In conclusion, dihydroceramide desaturase promotes exosome formation and prevents its autophagic degradation in the nervous system.

A Novel Selective Sphingosine Kinase 2 Inhibitor, HWG-35D, Ameliorates the Severity of Imiquimod-Induced Psoriasis Model by Blocking Th17 Differentiation of Naïve CD4 T Lymphocytes

Sphingosine kinases (SK) catalyze the phosphorylation of sphingosine to generate sphingosine-1-phosphate. Two isoforms of SK (SK1 and SK2) exist in mammals. Previously, we showed the beneficial effects of SK2 inhibition, using ABC294640, in a psoriasis mouse model. However, ABC294640 also induces the degradation of SK1 and dihydroceramide desaturase 1 (DES1). Considering these additional effects of ABC294640, we re-examined the efficacy of SK2 inhibition in an IMQ-induced psoriasis mouse model using a novel SK2 inhibitor, HWG-35D, which exhibits nM potency and 100-fold selectivity for SK2 over SK1. Topical application of HWG-35D ameliorated IMQ-induced skin lesions and normalized the serum interleukin-17A levels elevated by IMQ. Application of HWG-35D also decreased skin mRNA levels of interleukin-17A, K6 and K16 genes induced by IMQ. Consistent with the previous data using ABC294640, HWG-35D also blocked T helper type 17 differentiation of naïve CD4+ T cells with concomitant reduction of SOCS1. Importantly, HWG-35D did not affect SK1 or DES1 expression levels. These results reaffirm an important role of SK2 in the T helper type 17 response and suggest that highly selective and potent SK2 inhibitors such as HWG-35D might be of therapeutic use for the treatment of psoriasis.

Dihydroceramide desaturase regulates the compartmentalization of Rac1 for neuronal oxidative stress

SummaryDisruption of sphingolipid homeostasis has been shown to cause neurological disorders. How specific sphingolipid species modulate the pathogenesis remains unknown. The last step of sphingolipid de novo synthesis is the conversion of dihydroceramide to ceramide catalyzed by dihydroceramide desaturase (human DEGS1; Drosophila Ifc). Loss of ifc leads to dihydroceramide accumulation and oxidative stress, resulting in photoreceptors degeneration, while DEGS1 variants were associated with leukodystrophy and neuropathy. Here, we demonstrated that ifc regulates Rac1 compartmentalization in fly photoreceptors and further showed that dihydroceramide alters the association of active Rac1 to membranes mimicking specific organelles. We also revealed that the major source of ROS originated from Rac1 and NADPH oxidase (NOX) in the cytoplasm, as the NOX inhibitor apocynin ameliorated the oxidative stress and functional defects in both fly ifc-KO photoreceptors and human neuronal cells with disease-associated variant DEGS1H132R. Therefore, DEGS1/ifc deficiency causes dihydroceramide accumulation, resulting in Rac1 translocation and NOX-dependent neurodegeneration.Graphical AbstractADEGS1/ifc converts dihydroceramide to ceramide in neuronal cells, and the endolysosomal NOX complex is not activated.B Dihydroceramide accumulates without functional DEGS1/ifc and causes alterations in membrane microdomains and recruits active Rac1 to endolysosomes. The activation of endolysosomal Rac1-NOX complex elevates cytosolic ROS levels, causing neurodegeneration.In Brief (eTOC blurb)Deficiency in dihydroceramide desaturase causes oxidative stress-mediated neurological disorders. Tzou and Su et al. show that increased dihydroceramide causes the relocalization of active Rac1, whilst inhibition of the Rac1-NOX ameliorates the oxidative stress and neuronal defects. NOX inhibitor apocynin may provide new direction of treatments for patients with DEGS1 variants.HighlightsDeficiency in dihydroceramide (dhCer) desaturase induces cytoplasmic ROS elevationdhCer alters the binding of active Rac1 to reconstituted organelle membranesActive Rac1 is enriched in endolysosomes in ifc-KO neurons for ROS genesisRac1-NADPH oxidase elicits ROS, degenerating leukodystrophy-related neuronal cells