A large body of evidence has shown that the amyloid b peptide oligomers (Abo) are predominantly responsible for the neurodegeneration/cognitive impairments in Alzheimer's disease (AD). Abo cause mitochondrial dysfunctions leading to an imbalance between pro- and antiapoptotic proteins and finally to neuronal apoptosis. Further, Abo trigger overactivation of microglia followed by enhanced release of proinflammatory cytokines, which exacerbates neurotoxicity of Abo. The above-mentioned alterations are accompanied by disturbed metabolism of prosurvival bioactive sphingolipid, sphingosine-1-phosphate (S1P), and S1P-dependent signalling via specific receptors (S1PR1-5). In the present study, we investigated for the first time the influence of selective - ponesimod (S1PR1), CYM5541 (S1PR3), CYM50308 (S1PR4), A971432 (S1PR4), siponimod (S1PR1,5) - and nonselective - phosphorylated fingolimod/pFTY720 (S1PR1,3-5) - S1P receptor modulators on cell viability, mitochondrial membrane potential (MMP) and expression of genes encoding S1P receptors, pro- and antiapoptotic proteins and proinflammatory cytokines in hippocampal neuronal (HT22) and in microglial (BV2) cell lines treated with 1 µM Abo for 24 hours. A significant reduction in the MMP, cell viability and mRNA levels of Bcl2 and Il18 together with increased Il6 expression was observed in HT22 cells after Abo administration. CYM50308 and A971432 restored the Bcl2 mRNA level to control values (those of Abo-untreated cells) and pFTY720 markedly reduced the Il6 expression. In BV2 cells, Abo induced a significant decrease in the MMP, cell viability and expression of S1pr1, Bad, Bcl2, Tnf and Il18, which was not counteracted by any of the modulators used. In turn, mRNA levels of Il1b, Il6, were markedly increased in microglia after Abo treatment and the administration of studied compounds tended to exacerbate the proinflammatory effect of Abo. In conclusion, the toxic effect of Abo is more pronounced in microglia. S1P receptor modulators may to some extent mitigate proapoptotic and proinflammatory effects of Abo in HT22 cells. In contrast, the same compounds tend to enhance Abo-induced inflammatory changes in BV2 cells.

Disruption of sphingolipid homeostasis and signaling has been implicated in diabetes, cancer, cardiometabolic, and neurodegenerative disorders. Yet, mechanisms governing cellular sensing and regulation of sphingolipid homeostasis remain largely unknown. In yeast, serine palmitoyltransferase, catalyzing the first and rate-limiting step of sphingolipid de novo biosynthesis, is negatively regulated by Orm1 and 2. Lowering sphingolipids triggers Orms phosphorylation, upregulation of serine palmitoyltransferase activity and sphingolipid de novo biosynthesis. However, mammalian orthologs ORMDLs lack the N-terminus hosting the phosphosites. Thus, which sphingolipid(s) are sensed by the cells, and mechanisms of homeostasis remain largely unknown. Here, we identify sphingosine-1-phosphate (S1P) as key sphingolipid sensed by cells via S1PRs to maintain homeostasis. The increase in S1P-S1PR signaling stabilizes ORMDLs, restraining SPT activity. Mechanistically, the hydroxylation of ORMDLs at Pro137 allows a constitutive degradation of ORMDLs via ubiquitin-proteasome pathway, preserving SPT activity. Disrupting S1PR/ORMDL axis results in ceramide accrual, mitochondrial dysfunction, impaired signal transduction, all underlying endothelial dysfunction, early event in the onset of cardio- and cerebrovascular diseases. Our discovery may provide the molecular basis for therapeutic intervention restoring sphingolipid homeostasis.
© 2022 The Authors.

Ponesimod is a sphingosine 1-phosphate (S1P) receptor (S1PR) modulator that was recently approved for treating relapsing forms of multiple sclerosis (MS). Three other FDA-approved S1PR modulators for MS-fingolimod, siponimod, and ozanimod-share peripheral immunological effects via common S1P1 interactions, yet ponesimod may access distinct central nervous system (CNS) mechanisms through its selectivity for the S1P1 receptor. Here, ponesimod was examined for S1PR internalization and binding, human astrocyte signaling and single-cell RNA-seq (scRNA-seq) gene expression, and in vivo using murine cuprizone-mediated demyelination. Studies confirmed ponesimod's selectivity for S1P1 without comparable engagement to the other S1PR subtypes (S1P2,3,4,5 ). Ponesimod showed pharmacological properties of acute agonism followed by chronic functional antagonism of S1P1 . A major locus of S1P1 expression in the CNS is on astrocytes, and scRNA-seq of primary human astrocytes exposed to ponesimod identified a gene ontology relationship of reduced neuroinflammation and reduction in known astrocyte disease-related genes including those of immediate early astrocytes that have been strongly associated with disease progression in MS animal models. Remarkably, ponesimod prevented cuprizone-induced demyelination selectively in the cingulum, but not in the corpus callosum. These data support the CNS activities of ponesimod through S1P1 , including protective, and likely selective, effects against demyelination in a major connection pathway of the brain, the limbic fibers of the cingulum, lesions of which have been associated with several neurologic impairments including MS fatigue.
© 2022 The Authors. The FASEB Journal published by Wiley Periodicals LLC on behalf of Federation of American Societies for Experimental Biology.

Several vasoprotective functions of high-density lipoproteins (HDL) on the endothelium have been shown to depend on the presence of sphingosine-1-phosphate (S1P) receptors (S1PRs) as well as scavenger receptor class B type 1 (SR-B1). Interference with the presence of S1P or the activity of S1PR1 or S1PR3 mimics many effects seen by the interference with SR-B1. This raises the question on interactions between S1P receptors and SR-B1. We investigated the influence of S1PRs on SR-B1 expression in human aortic endothelial cells. Silencing or pharmacological inhibition of S1PR1 or S1PR3 down-regulated SCARB1 mRNA expression as well as SR-B1 protein abundance. RNA interference with S1PR1 or S1PR3 also decreased cellular association of 125 I-HDL with HAECs. Further mechanistic studies showed that knockdown of S1PR1 or S1PR3 reduced SR-B1 protein by inducing its degradation through deceasing Akt activity. Moreover, silencing of S1PR1 or S1PR3 suppressed SCARB1 mRNA expression by decreasing cellular cAMP levels. In conclusion, we provide evidence for an as yet unappreciated interaction, namely the regulation of SR-B1 abundance by S1PRs on both transcriptional and post-translational levels, suggesting that interactions of S1PRs and SR-B1 regulate signaling functions of HDL as well as uptake of lipoproteins in endothelial cells.

FTY720 (fingolimod) is a FDA-approved sphingosine analog that is phosphorylated in vivo to modulate sphingosine-1-phosphate receptor (S1PR) signaling for immunosuppression in patients with refractory multiple sclerosis. FTY720 also exhibits promising anticancer efficacy in several preclinical models. While FTY720-induced cytotoxicity is not due to S1PR signaling, the mechanism remains unclear and is reported to occur through various cell death pathways. Here, we performed a systematic, mechanistic study of FTY720-induced cell death in acute myeloid leukemia (AML). We found that FTY720 induced cell death in a panel of genetically diverse AML cell lines that was accompanied by rapid phosphatidylserine (PS) externalization. Importantly, FTY720-induced PS exposure was not due to any direct effects on plasma membrane integrity and was independent of canonical signaling by regulated cell death pathways known to activate lipid flip-flop, including caspase-dependent apoptosis/pyroptosis, necroptosis, ferroptosis, and reactive oxygen species-mediated cell death. Notably, PS exposure required cellular vacuolization induced by defects in endocytic trafficking and was suppressed by the inhibition of PP2A and shedding of Annexin V-positive subcellular particles. Collectively, our studies reveal a non-canonical pathway underlying PS externalization and cell death in AML to provide mechanistic insight into the antitumor properties of FTY720.