Mouse sphingosine kinase isoforms SPHK1a and SPHK1b differ in enzymatic traits including stability, localization, modification, and oligomerization

The role of sphingosine-1-phosphate in the development and progression of Parkinson's disease

Parkinson's disease (PD) could be viewed as a proteinopathy caused by changes in lipids, whereby modifications in lipid metabolism may lead to protein alterations, such as the accumulation of alpha-synuclein (α-syn), ultimately resulting in neurodegeneration. Although the loss of dopaminergic neurons in the substantia nigra is the major clinical manifestation of PD, the etiology of it is largely unknown. Increasing evidence has highlighted the important role of lipids in the pathophysiology of PD. Sphingosine-1-phosphate (S1P), a signaling lipid, has been suggested to have a potential association with the advancement and worsening of PD. Therefore, better understanding the mechanisms and regulatory proteins is of high interest. Most interestingly, S1P appears to be an important target to offers a new strategy for the diagnosis and treatment of PD. In this review, we first introduce the basic situation of S1P structure, function and regulation, with a special focus on the several pathways. We then briefly describe the regulation of S1P signaling pathway on cells and make a special focused on the cell growth, proliferation and apoptosis, etc. Finally, we discuss the function of S1P as potential therapeutic target to improve the clinical symptoms of PD, and even prevent the progression of the PD. In the context of PD, the functions of S1P modulators have been extensively elucidated. In conclusion, S1P modulators represent a novel and promising therapeutic principle and therapeutic method for PD. However, more research is required before these drugs can be considered as a standard treatment option for PD.

Determining the structure of protein-bound ceramides, essential lipids for skin barrier function

Protein-bound ceramides, specialized ceramides covalently bound to corneocyte surface proteins, are essential for skin permeability barrier function. However, their exact structure and target amino acid residues are unknown. Here, we found that epoxy-enone (EE) ceramides, precursors of protein-bound ceramides, as well as their synthetic analog, formed stable conjugates only with Cys among nucleophilic amino acids. NMR spectroscopy revealed that the β-carbon of the enone was attached by the thiol group of Cys via a Michael addition reaction. We confirmed the presence of Cys-bound EE ceramides in mouse epidermis by mass spectrometry analysis of protease-digested epidermis samples. EE ceramides were reversibly released from protein-bound ceramides via sulfoxide elimination. We found that protein-bound ceramides with reversible release properties accounted for approximately 60% of total protein-bound ceramides, indicating that Cys-bound EE ceramides are the predominant protein-bound ceramides. Our findings provide clues to the molecular mechanism of skin barrier formation by protein-bound ceramides.

Metabolism of sphingadiene and characterization of the sphingadiene-producing enzyme FADS3

Of the long-chain bases (LCBs) that comprise the ceramides (CERs) present in mammals, only 4,14-sphingadiene (sphingadiene; SPD) has a cis double bond (at C14). Because of this unique structure, the metabolism of SPD may differ from that of other LCBs, but whether this is the case remains unclear. FADS3 is responsible for introducing the cis double bond in SPD. However, the substrate specificity of FADS3 and cofactors involved in the FADS3-catalyzed reaction are also unknown. In the present study, a cell-based assay using a ceramide synthase inhibitor and an in vitro experiment showed that FADS3 is active toward sphingosine (SPH)-containing CERs (SPH-CERs) but not toward free SPH. FADS3 exhibits specificity with respect to the chain length of the SPH moiety of SPH-CERs (active toward C16-20), but not that of the fatty acid moiety. Furthermore, FADS3 is active toward straight-chain and isobranched-chain SPH-containing CERs but not toward anteiso-branched forms. In addition to SPH-CERs, FADS3 also shows activity toward dihydrosphingosine-containing CERs, but this activity is approximately half of that toward SPH-CERs. It uses either NADH or NADPH as an electron donor, and the electron transfer is facilitated by cytochrome b₅. The metabolic flow of SPD to sphingomyelin is predominant over that to glycosphingolipids. In the metabolic pathway from SPD to fatty acids, the chain length of the SPD is reduced by two carbons and the trans double bond at C4 is saturated. This study thus elucidates the enzymatic properties of FADS3 and the metabolism of SPD.

Sphingosine-1-phosphate related signalling pathways manipulating virus replication

Viruses can create a unique cellular environment that facilitates replication and transmission. Sphingosine kinases (SphKs) produce sphingosine-1-phosphate (S1P), a bioactive sphingolipid molecule that performs both physiological and pathological effects primarily by activating a subgroup of the endothelial differentiation gene family of G-protein coupled cell surface receptors known as S1P receptors (S1PR1-5). A growing body of evidence indicates that the SphK/S1P axis is crucial for regulating cellular activities in virus infections like respiratory viruses, enteroviruses, hepatitis viruses, herpes viruses, and arboviruses replicate. Depending on the type of virus, pro- or anti-viral activities of the SphK/S1P axis sometimes rely on the host immune system and sometimes directly through intracellular signalling pathways or cell proliferation. Recent research has shown novel roles of S1P and SphK in viral replication. Sphingosine kinase isoforms (SphK1 and SphK2) levels can be manipulated by several viruses to promote the effects that are expected. Regulation of cellular signalling pathways plays a significant role in the mechanism. The purpose of this review is to provide insight of the characters played by the SphK/S1P axis throughout diverse viral infection processes. We then assess potential therapeutic methods that are based on S1P signalling and metabolism during viral infections.

Incomplete Elongation of Ultra-long-chain Polyunsaturated Acyl-CoAs by the Fatty Acid Elongase ELOVL4 in Spinocerebellar Ataxia Type 34

Spinocerebellar ataxias (SCAs) are autosomal dominant diseases characterized by cerebellar atrophy and ataxia. The SCA subtype SCA34 is caused by specific mutations in the gene ELOVL4, which encodes a fatty acid (FA) elongase that synthesizes ultra-long-chain (ULC; ≥C26) FAs. However, the pathogenesis and molecular mechanism that confers dominant inheritance remains unknown. Here, a cell-based assay demonstrated that each of the five known SCA34 mutants produced shorter ULC polyunsaturated FA-containing phosphatidylcholines (ULC-PCs) than wild-type protein, in the following order of severity: Q180P and T233M > W246G > I171T and L168F. Next, we generated knock-in mouse embryonic stem cells that contained heterozygous Q180P, heterozygous W246G, or homozygous W246G mutations. Neuronal differentiation-dependent production of ULC-PCs was reduced in heterozygous Q180P and homozygous W246G cells relative to control cells, and we observed shortening of the FA moiety in all mutant cells. This FA shortening was consistent with our prediction that amino acid residues substituted by SCA34 mutations are located in the transmembrane helices that interact with the ω-end region of the FA moiety of the substrate acyl-CoA. Hence, reduced levels and shortening of ULC-PCs in neurons may cause SCA34, and incomplete elongation of ULC polyunsaturated acyl-CoAs by mutated ELOVL4 may induce dominant inheritance.

Sphingosine-1-Phosphate (S1P) and S1P Signaling Pathway Modulators, from Current Insights to Future Perspectives

Sphingosine-1-phosphate (S1P) and S1P receptors (S1PR) are bioactive lipid molecules that are ubiquitously expressed in the human body and play an important role in the immune system. S1P-S1PR signaling has been well characterized in immune trafficking and activation in both innate and adaptive immune systems. Despite this knowledge, the full scope in the pathogenesis of autoimmune disorders is not well characterized yet. From the discovery of fingolimod, the first S1P modulator, until siponimod, the new molecule recently approved for the treatment of secondary progressive multiple sclerosis (SPMS), there has been a great advance in understanding the S1P functions and their involvement in immune diseases, including multiple sclerosis (MS). Modulation on S1P is an interesting target for the treatment of various autoimmune disorders. Improved understanding of the mechanism of action of fingolimod has allowed the development of the more selective second-generation S1PR modulators. Subtype 1 of the S1PR (S1PR1) is expressed on the cell surface of lymphocytes, which are known to play a major role in MS pathogenesis. The understanding of S1PR1’s role facilitated the development of pharmacological strategies directed to this target, and theoretically reduced the safety concerns derived from the use of fingolimod. A great advance in the MS treatment was achieved in March 2019 when the Food and Drug Association (FDA) approved Siponimod, for both active secondary progressive MS and relapsing–remitting MS. Siponimod became the first oral disease modifying therapy (DMT) specifically approved for active forms of secondary progressive MS. Additionally, for the treatment of relapsing forms of MS, ozanimod was approved by FDA in March 2020. Currently, there are ongoing trials focused on other new-generation S1PR1 modulators. This review approaches the fundamental aspects of the sphingosine phosphate modulators and their main similarities and differences.

Whole picture of human stratum corneum ceramides, including the chain-length diversity of long-chain bases

Ceramides are essential lipids for skin permeability barrier function, and a wide variety of ceramide species exist in the stratum corneum (SC). Although ceramides with long-chain bases (LCBs) of various lengths have been identified in the human SC, a quantitative analysis that distinguishes ceramide species with different LCB chain lengths has not been yet published. Therefore, the whole picture of human SC ceramides remains unclear. Here, we conducted LC/MS/MS analyses to detect individual ceramide species differing in both the LCB and FA chain lengths and quantified 1,327 unbound ceramides and 254 protein-bound ceramides: the largest number of ceramide species reported to date. Ceramides containing an LCB whose chain length was C16–26 were present in the human SC. Of these, C18 (28.6%) was the most abundant, followed by C20 (24.8%) and C22 (12.8%). Each ceramide class had a characteristic distribution of LCB chain lengths and was divided into five groups according to this distribution. There was almost no difference in FA composition between the ceramide species containing LCBs of different chain lengths. Furthermore, we demonstrated that one of the serine palmitoyltransferase (SPT) complexes, SPTLC1/SPTLC3/SPTSSB, was able to produce C16–24 LCBs. The expression levels of all subunits constituting the SPT complexes increased during keratinocyte differentiation, resulting in the observed chain-length diversity of LCBs in the human SC. This study provides a molecular basis for elucidating human SC ceramide diversity and the pathogenesis of skin disorders.

Formation of fatty alcohols-components of meibum lipids-by the fatty acyl-CoA reductase FAR2 is essential for dry eye prevention

Various lipids (mainly meibum lipids secreted by the meibomian glands) are present in the tear film lipid layer and play important roles in tear stability and the health of the cornea and conjunctiva. Many meibum lipids contain fatty alcohols (FAls) with chain lengths ≥C24, but the fatty acyl-CoA reductases (FARs) that produce them remain unclear. Here, using cell-based assays, we found that the two FAR isozymes (FAR1 and FAR2) show different substrate specificities: FAR1 and FAR2 are involved in the production of C16-C18 and ≥C20 FAls, respectively. Next, we generated Far2 knockout (KO) mice and examined their dry eye phenotype and meibum lipid composition. These mice showed a severe dry eye phenotype, characterized by plugged meibomian gland orifices, corneal damage, and tear film instability. The plugging was attributed to an increase in the melting point of the meibum lipids. Liquid chromatography coupled with tandem mass spectrometry revealed that FAl-containing meibum lipids (wax monoesters and types 1ω, 2α, and 2ω wax diesters) with a hydroxyl group at position 1 were almost completely absent in Far2 KO mice. The levels of di-unsaturated (O-acyl)-ω-hydroxy fatty acids were higher in Far2 KO mice than in wild type mice, but those of tri-unsaturated ones were comparable, suggesting the presence of two synthesis pathways for type 1ω wax diesters. These results indicate the importance of FAl-containing meibum lipids in the formation of a functional tear film lipid layer. In addition, our study provides clues to the molecular mechanism of the biosynthesis of meibum lipids.

Lysophosphatidate Promotes Sphingosine 1-Phosphate Metabolism and Signaling: Implications for Breast Cancer and Doxorubicin Resistance

Lysophosphatidate (LPA) and sphingosine 1-phosphate (S1P) promote vasculogenesis, angiogenesis, and wound healing by activating a plethora of overlapping signaling pathways that stimulate mitogenesis, cell survival, and migration. As such, maladaptive signaling by LPA and S1P have major effects in increasing tumor progression and producing poor patient outcomes after chemotherapy and radiotherapy. Many signaling actions of S1P and LPA are not redundant; each are vital in normal physiology and their metabolisms differ. In the present work, we studied how LPA signaling impacts S1P metabolism and signaling in MDA-MB-231 and MCF-7 breast cancer cells. LPA increased sphingosine kinase-1 (SphK1) synthesis and rapidly activated cytosolic SphK1 through association with membranes. Blocking phospholipase D activity attenuated the LPA-induced activation of SphK1 and the synthesis of ABCC1 and ABCG2 transporters that secrete S1P from cells. This effect was magnified in doxorubicin-resistant MCF-7 cells. LPA also facilitated S1P signaling by increasing mRNA expression for S1P₁ receptors. Doxorubicin-resistant MCF-7 cells had increased S1P₂ and S1P₃ receptor expression and show increased LPA-induced SphK1 activation, increased expression of ABCC1, ABCG2 and greater S1P secretion. Thus, LPA itself and LPA-induced S1P signaling counteract doxorubicin-induced death of MCF-7 cells. We conclude from the present and previous studies that LPA promotes S1P metabolism and signaling to coordinately increase tumor growth and metastasis and decrease the effectiveness of chemotherapy and radiotherapy for breast cancer treatment.

Recent Insights into the Interplay of Alpha-Synuclein and Sphingolipid Signaling in Parkinson's Disease

Molecular studies have provided increasing evidence that Parkinson’s disease (PD) is a protein conformational disease, where the spread of alpha-synuclein (ASN) pathology along the neuraxis correlates with clinical disease outcome. Pathogenic forms of ASN evoke oxidative stress (OS), neuroinflammation, and protein alterations in neighboring cells, thereby intensifying ASN toxicity, neurodegeneration, and neuronal death. A number of evidence suggest that homeostasis between bioactive sphingolipids with opposing function—e.g., sphingosine-1-phosphate (S1P) and ceramide—is essential in pro-survival signaling and cell defense against OS. In contrast, imbalance of the “sphingolipid biostat” favoring pro-oxidative/pro-apoptotic ceramide-mediated changes have been indicated in PD and other neurodegenerative disorders. Therefore, we focused on the role of sphingolipid alterations in ASN burden, as well as in a vast range of its neurotoxic effects. Sphingolipid homeostasis is principally directed by sphingosine kinases (SphKs), which synthesize S1P—a potent lipid mediator regulating cell fate and inflammatory response—making SphK/S1P signaling an essential pharmacological target. A growing number of studies have shown that S1P receptor modulators, and agonists are promising protectants in several neurological diseases. This review demonstrates the relationship between ASN toxicity and alteration of SphK-dependent S1P signaling in OS, neuroinflammation, and neuronal death. Moreover, we discuss the S1P receptor-mediated pathways as a novel promising therapeutic approach in PD.

Divergence of Intracellular Trafficking of Sphingosine Kinase 1 and Sphingosine-1-Phosphate Receptor 3 in MCF-7 Breast Cancer Cells and MCF-7-Derived Stem Cell-Enriched Mammospheres

Breast cancer MCF-7 cell-line-derived mammospheres were shown to be enriched in cells with a CD44+/CD24- surface profile, consistent with breast cancer stem cells (BCSC). These BCSC were previously reported to express key sphingolipid signaling effectors, including pro-oncogenic sphingosine kinase 1 (SphK1) and sphingosine-1-phosphate receptor 3 (S1P3). In this study, we explored intracellular trafficking and localization of SphK1 and S1P3 in parental MCF-7 cells, and MCF-7 derived BCSC-enriched mammospheres treated with growth- or apoptosis-stimulating agents. Intracellular trafficking and localization were assessed using confocal microscopy and cell fractionation, while CD44+/CD24- marker status was confirmed by flow cytometry. Mammospheres expressed significantly higher levels of S1P3 compared to parental MCF-7 cells (p < 0.01). Growth-promoting agents (S1P and estrogen) induced SphK1 and S1P3 translocation from cytoplasm to nuclei, which may facilitate the involvement of SphK1 and S1P3 in gene regulation. In contrast, pro-apoptotic cytokine tumor necrosis factor α (TNFα)-treated MCF-7 cells demonstrated increased apoptosis and no nuclear localization of SphK1 and S1P3, suggesting that TNFα can inhibit nuclear translocation of SphK1 and S1P3. TNFα inhibited mammosphere formation and induced S1P3 internalization and degradation. No nuclear translocation of S1P3 was detected in TNFα-stimulated mammospheres. Notably, SphK1 and S1P3 expression and localization were highly heterogenous in mammospheres, suggesting the potential for a large variety of responses. The findings provide further insights into the understanding of sphingolipid signaling and intracellular trafficking in BCs. Our data indicates that the inhibition of SphK1 and S1P3 nuclear translocation represents a novel method to prevent BCSCs proliferation.

Direct uptake of sphingosine-1-phosphate independent of phospholipid phosphatases

Sphingosine-1-phosphate (S1P) is a lipid mediator that is relatively abundant in plasma and plays an important role in the vascular and immune systems. To date, the only known mechanism for removing S1P from plasma has been dephosphorylation by phospholipid phosphatases (PLPPs) on the surface of cells in contact with the plasma. However, there remains a possibility that PLPP-independent dephosphorylation or direct S1P uptake into cells could occur. To examine these possibilities, here we generated triple KO (TKO) HAP1 cells that lacked all PLPPs (PLPP1–3) present in mammals. In the TKO cells, the intracellular metabolism of externally added deuterium-labeled S1P to ceramide was reduced to 17% compared with the WT cells, indicating that most extracellular S1P is dephosphorylated by PLPPs and then taken up into cells. However, this result also reveals the existence of a PLPP-independent S1P uptake pathway. Tracer experiments using [³²P]S1P showed the existence of a direct S1P uptake pathway that functions without prior dephosphorylation. Overexpression of sphingolipid transporter 2 (SPNS2) or of major facilitator superfamily domain containing 2B (MFSD2B), both known S1P efflux transporters, in TKO cells increased the direct uptake of S1P, whereas KO of MFSD2B in TKO cells reduced this uptake. These results suggest that these are channel-type transporters and capable of not only exporting but also importing S1P. Furthermore, we observed that erythroid cells expressing MFSD2B, exhibited high S1P uptake activity. Our findings describing direct S1P uptake may contribute to the elucidation of the molecular mechanisms that regulate plasma S1P concentration.

Comprehensive stratum corneum ceramide profiling reveals reduced acylceramides in ichthyosis patient with CERS3 mutations

The stratum corneum (SC) of the epidermis acts as a skin permeability barrier, and abnormalities in SC formation lead to several skin disorders. Lipids, especially the epidermis-specific ceramide classes ω-O-acylceramides (acylceramides) and protein-bound ceramides, are essential for skin barrier formation. Ceramide synthase 3 (CERS3) is involved in the synthesis of acylceramides and protein-bound ceramides, and CERS3 mutations cause autosomal recessive congenital ichthyosis. In the present study, we measured ceramide synthase activity and performed comprehensive SC ceramide profiling in an ichthyosis patient with compound heterozygous CERS3 mutations: nonsense mutation p.Arg75* and missense mutation p.Arg229His. The activity of p.Arg75* and p.Arg229His mutant CERS3 proteins was reduced to 4% and 56%, respectively, of the wild-type protein. In the patient's SC, acylceramide levels were greatly reduced, but the levels of protein-bound ceramides remained almost unchanged. Non-acylated ceramide levels were also affected in the patient; in particular, the levels of ceramides composed of sphingosine and non-hydroxy or α-hydroxy fatty acid were substantially higher than in healthy controls. These results suggest that a reduction in acylceramide levels alone leads to ichthyosis. Although protein-bound ceramides are synthesized from acylceramides, levels of acylceramides and protein-bound ceramides are not necessarily correlated.

Sphingosine-1-Phosphate Metabolism and Signaling in Kidney Diseases

In the past few decades, sphingolipids and sphingolipid metabolites have gained attention because of their essential role in the pathogenesis and progression of kidney diseases. Studies in models of experimental and clinical nephropathies have described accumulation of sphingolipids and sphingolipid metabolites, and it has become clear that the intracellular sphingolipid composition of renal cells is an important determinant of renal function. Proper function of the glomerular filtration barrier depends heavily on the integrity of lipid rafts, which include sphingolipids as key components. In addition to contributing to the structural integrity of membranes, sphingolipid metabolites, such as sphingosine-1-phosphate (S1P), play important roles as second messengers regulating biologic processes, such as cell growth, differentiation, migration, and apoptosis. This review will focus on the role of S1P in renal cells and how aberrant extracellular and intracellular S1P signaling contributes to the pathogenesis and progression of kidney diseases.

K27Q/K29Q mutations in sphingosine kinase 1 attenuate high-fat diet induced obesity and altered glucose homeostasis in mice

Obesity and its associated metabolic disorders are increasingly impacting public health worldwide. Sphingosine kinase 1 (Sphk1) is a critical enzyme in sphingolipid metabolism that has been implicated in various metabolic syndromes. In this study, we developed a mouse model constitutively expressing pseudoacetylated mouse Sphk1 (QSPHK1) to study its role in regulating glucose and lipid metabolism. The results showed that QSPHK1 mice gained less body weight than wide type (WT) mice on a high-fat diet, and QSPHK1 mice had improved glucolipid metabolism and insulin. Moreover, QSPHK1 mice had alleviated hepatic triglyceride accumulation and had high-fat-diet-induced hepatic steatosis that occurred as a result of reduced lipogenesis and enhanced fatty acid oxidation, which were mediated by the AMPK/ACC axis and the FGF21/adiponectin axis. Collectively, this study provided evidence that the K27Q/K29Q mutations of Sphk1 could have a protective role in preventing obesity and the related metabolic diseases. Hence, our results contribute to further understanding of the biological functions of Sphk1, which has great pharmaceutical implications.

Production of branched-chain very-long-chain fatty acids by fatty acid elongases and their tissue distribution in mammals

Although most mammalian fatty acids (FAs) are straight-chain, there also exist branched-chain FAs such as iso- and anteiso-FAs, especially in the meibomian glands. Meibum lipids, which are secreted from the meibomian glands and are important for dry eye prevention, contain abundant branched-chain lipids, such as cholesteryl esters and wax esters with chain-lengths of ≥C21 (very-long-chain; VLC). However, the exact tissue distribution of branched-chain lipids or the enzymes involved in the production of branched-chain VLCFAs has remained poorly understood. Here, we revealed that FA elongases ELOVL1, ELOVL3, and ELOVL7, of the seven mammalian ELOVL isozymes, elongated saturated branched-chain acyl-CoAs. ELOVL3 was highly active toward iso-C17:0 and anteiso-C17:0 acyl-CoAs and elongated them up to iso-C23:0 and anteiso-C25:0 acyl-CoAs, respectively. ELOVL1 elongated both iso- and anteiso-C23:0 acyl-CoAs to C25:0 acyl-CoAs. By establishing a liquid chromatography-tandem mass spectrometry method capable of separating branched- and straight-chain lipids, we showed that essentially all of the cholesteryl esters and 88% of the wax esters in the mouse meibomian glands are branched. In Elovl1 mutant mice, the levels of ≥C24:0 branched-chain cholesteryl esters and ≥C25:0 branched-chain wax esters were decreased, indicating that ELOVL1 indeed elongates branched-chain VLC acyl-CoAs in vivo. In addition, substantial amounts of ceramides containing branched-chain FAs were present in the skin, meibomian glands, and liver. Our findings provide new insights into the molecular mechanisms that create FA and lipid diversity.

Lipid polarity gradient formed by ω-hydroxy lipids in tear film prevents dry eye disease

Meibum lipids form a lipid layer on the outermost side of the tear film and function to prevent water evaporation and reduce surface tension. (O-Acyl)-ω-hydroxy fatty acids (OAHFAs), a subclass of these lipids, are thought to be involved in connecting the lipid and aqueous layers in tears, although their actual function and synthesis pathway have to date remained unclear. Here, we reveal that the fatty acid ω-hydroxylase Cyp4f39 is involved in OAHFA production. Cyp4f39-deficient mice exhibited damaged corneal epithelium and shortening of tear film break-up time, both indicative of dry eye disease. In addition, tears accumulated on the lower eyelid side, indicating increased tear surface tension. In Cyp4f39-deficient mice, the production of wax diesters (type 1ω and 2ω) and cholesteryl OAHFAs was also impaired. These OAHFA derivatives show intermediate polarity among meibum lipids, suggesting that OAHFAs and their derivatives contribute to lipid polarity gradient formation for tear film stabilization.

Skin permeability barrier formation by the ichthyosis-causative gene FATP4 through formation of the barrier lipid ω-O-acylceramide

The epidermis-specific lipid acylceramide plays a pivotal role in the formation of the permeability barrier in the skin; abrogation of its synthesis causes the skin disorder ichthyosis. However, the acylceramide synthetic pathway has not yet been fully elucidated: Namely, the acyl-CoA synthetase (ACS) involved in this pathway remains to be identified. Here, we hypothesized it to be encoded by FATP4/ACSVL4, the causative gene of ichthyosis prematurity syndrome (IPS). In vitro experiments revealed that FATP4 exhibits ACS activity toward an ω-hydroxy fatty acid (FA), an intermediate of the acylceramide synthetic pathway. Fatp4 knockout (KO) mice exhibited severe skin barrier dysfunction and morphological abnormalities in the epidermis. The total amount of acylceramide in Fatp4 KO mice was reduced to ∼10% of wild-type mice. Decreased levels and shortening of chain lengths were observed in the saturated, nonacylated ceramides. FA levels were not decreased in the epidermis of Fatp4 KO mice. The expression levels of the FA elongase Elovl1 were reduced in Fatp4 KO epidermis, partly accounting for the reduction and shortening of saturated, nonacylated ceramides. A decrease in acylceramide levels was also observed in human keratinocytes with FATP4 knockdown. From these results, we conclude that skin barrier dysfunction observed in IPS patients and Fatp4 KO mice is caused mainly by reduced acylceramide production. Our findings further elucidate the molecular mechanism governing acylceramide synthesis and IPS pathology.

Biosynthesis of the anti-lipid-microdomain sphingoid base 4,14-sphingadiene by the ceramide desaturase FADS3

Sphingolipids are multifunctional lipids. Among the sphingolipid-component sphingoid bases, 4,14-sphingadiene (SPD) is unique such that it has a cis double bond with a bent structure. Although SPD was discovered half a century ago, its tissue distribution, biosynthesis, and degradation remain poorly understood. Here, we established a specific and quantitative method for SPD measurement and found that SPD exists in a wide range of mammalian tissues. SPD was especially abundant in kidney, where the amount of SPD was ~2/3 of sphingosine, the most abundant sphingoid base in mammals. Although SPD is metabolized to ceramides and SPD 1-phosphate with almost the same efficiency as sphingosine, it is less susceptible to degradation by a cleavage reaction, at least in vitro. We identified the fatty acid desaturase family protein FADS3 as a ceramide desaturase that produces SPD ceramides by desaturating ceramides containing sphingosine. SPD sphingolipids were preferentially localized outside lipid microdomains, suggesting that SPD has different functions compared to other sphingoid bases in the formation of lipid microdomains. In summary, we revealed the biosynthesis and degradation pathways of SPD and its characteristic membrane localization. Our findings contribute to the elucidation of the molecular mechanism underlying the generation of sphingolipid diversity.

Molecular mechanism of the ichthyosis pathology of Chanarin-Dorfman syndrome: Stimulation of PNPLA1-catalyzed ω-O-acylceramide production by ABHD5

BACKGROUND

ABHD5 mutations cause Chanarin-Dorfman syndrome accompanied by ichthyosis. ω-O-Acylceramide (acylceramide) is essential for skin permeability barrier formation. Acylceramide production is impaired in Abhd5 knockout mice. The transacylase PNPLA1 catalyzes the final step of acylceramide production: transfer of linoleic acid in triglyceride to ω-hydroxyceramide.

OBJECTIVE

We aimed to elucidate the role of ABHD5 in acylceramide production and the molecular mechanism of the ichthyosis symptoms of Chanarin-Dorfman syndrome.

METHODS

We investigated how ABHD5 influences acylceramide production using an acylceramide-producing cell system. The effects of ABHD5 and PNPLA1 expression on the morphology of lipid droplets were examined by indirect immunofluorescent microscopy and immunoelectron microscopy.

RESULTS

When ABHD5 was expressed in the acylceramide-producing cell system, acylceramide synthesis by PNPLA1 was enhanced. Dispersed localization of PNPLA1 was observed by immunofluorescent microscopy in HeLa cells under lipid droplet-forming conditions. Co-expression with ABHD5 caused PNPLA1 to localize on the lipid droplet membranes or their periphery. This staining pattern was observed in cells where PNPLA1 and ABHD5 were expressed at low levels. In contrast, lipid droplets disappeared in cells where PNPLA1 and ABHD5 were highly expressed. Immunoelectron microscopic analyses suggested that lipid droplets underwent morphological changes, transforming into vesicles or becoming incorporated into the endoplasmic reticulum. ABHD5 mutations found in Chanarin-Dorfman syndrome patients reduced ABHD5's ability to promote PNPLA1-dependent acylceramide production.

CONCLUSION

ABHD5 enhances PNPLA1-catalyzed acylceramide production. We speculate that ABHD5 retains triglycerides in the endoplasmic reticulum, and presents them to PNPLA1 to promote substrate recognition.

Sphingolipids modulate docking, Ca2+ sensitivity and membrane fusion of native cortical vesicles

Docking, priming, and membrane fusion of secretory vesicles (i.e. regulated exocytosis) requires lipids and proteins. Sphingolipids, in particular, sphingosine and sphingosine-1-phosphate, have been implicated in the modulation of exocytosis. However, the specific exocytotic steps that sphingolipids modulate and the enzymes that regulate sphingolipid concentrations on native secretory vesicle membranes remain unknown. Here we use tightly coupled functional and molecular analyses of fusion-ready cell surface complexes and cortical vesicles isolated from oocytes to assess the role of sphingolipids in the late, Ca²⁺-triggered steps of exocytosis. The molecular changes resulting from treatments with sphingolipid modifying compounds coupled with immunoblotting analysis revealed the presence of sphingosine kinase on native vesicles; the presence of a sphingosine-1-phosphate phosphatase is also indicated. Changes in sphingolipid concentrations on vesicles altered their docking/priming, Ca²⁺-sensitivity, and ability to fuse, indicating that sphingolipid concentrations are tightly regulated and maintained at optimal levels and ratios to ensure efficient exocytosis.

Molecular mechanisms of regulation of sphingosine kinase 1

Within the last 3 decades, there has been intense study of bioactive sphingolipids and the enzymes which metabolize those lipids. One enzyme is the critical lipid kinase sphingosine kinase 1 (SK1), which produces the potent and pleiotropic signaling lipid, sphingosine 1-phosphate (S1P). SK1 and S1P have been implicated in a host of different diseases including cancer, chronic inflammation, and metabolic diseases. However, while there is ample knowledge about the importance of these molecules in the development and progression of disease there is a dearth of knowledge of the molecular mechanisms which regulate SK1 function. In this review, we will cover some of the more recent and exciting findings about the different ways SK1 function can be regulated, from transcriptional regulation to protein stability. Finally, we will delve into recent structural insights into SK1 and how they might relate to function at cell membranes.

Neural symptoms in a gene knockout mouse model of Sjögren-Larsson syndrome are associated with a decrease in 2-hydroxygalactosylceramide

Insulation by myelin lipids is essential to fast action potential conductivity: changes in their quality or amount can cause several neurologic disorders. Sjögren-Larsson syndrome (SLS) is one such disorder, which is caused by mutations in the fatty aldehyde dehydrogenase ALDH3A2. To date, the molecular mechanism underlying SLS pathology has remained unknown. In this study, we found that Aldh3a2 is expressed in oligodendrocytes and neurons in the mouse brain, and neurons of Aldh3a2 knockout (KO) mice exhibited impaired metabolism of the long-chain base, a component of sphingolipids. Aldh3a2 KO mice showed several abnormalities corresponding to SLS symptoms in behavioral tests, including increased paw slips on a balance beam and light-induced anxiety. In their brain tissue, 2-hydroxygalactosylceramide, an important lipid for myelin function and maintenance, was reduced by the inactivation of fatty acid 2-hydroxylase. Our findings provide important new insights into the molecular mechanisms responsible for neural pathogenesis caused by lipid metabolism abnormalities.-Kanetake, T., Sassa, T., Nojiri, K., Sawai, M., Hattori, S., Miyakawa, T., Kitamura, T., Kihara, A. Neural symptoms in a gene knockout mouse model of Sjögren-Larsson syndrome are associated with a decrease in 2-hydroxygalactosylceramide.

LPS and palmitate synergistically stimulate sphingosine kinase 1 and increase sphingosine 1 phosphate in RAW264.7 macrophages

It has been well established that patients with diabetes or metabolic syndrome (MetS) have increased prevalence and severity of periodontitis, an oral infection initiated by bacteria and characterized by tissue inflammation and destruction. To understand the underlying mechanisms, we have shown that saturated fatty acid (SFA), which is increased in patients with type 2 diabetes or MetS, and LPS, an important pathogenic factor for periodontitis, synergistically stimulate expression of proinflammatory cytokines in macrophages by increasing ceramide production. However, the mechanisms by which increased ceramide enhances proinflammatory cytokine expression have not been well understood. Since sphingosine 1 phosphate (S1P) is a metabolite of ceramide and a bioactive lipid, we tested our hypothesis that stimulation of ceramide production by LPS and SFA facilitates S1P production, which contributes to proinflammatory cytokine expression. Results showed that LPS and palmitate, a major SFA, synergistically increased not only ceramide, but also S1P, and stimulated sphingosine kinase (SK) expression and membrane translocation in RAW264.7 macrophages. Results also showed that SK inhibition attenuated the stimulatory effect of LPS and palmitate on IL-6 secretion. Moreover, results showed that S1P enhanced the stimulatory effect of LPS and palmitate on IL-6 secretion. Finally, results showed that targeting S1P receptors using either S1P receptor antagonists or small interfering RNA attenuated IL-6 upregulation by LPS and palmitate. Taken together, this study demonstrated that LPS and palmitate synergistically stimulated S1P production and S1P in turn contributed to the upregulation of proinflammatory cytokine expression in macrophages by LPS and palmitate.

"Dicing and Splicing" Sphingosine Kinase and Relevance to Cancer

Sphingosine kinase (SphK) is a lipid enzyme that maintains cellular lipid homeostasis. Two SphK isozymes, SphK1 and SphK2, are expressed from different chromosomes and several variant isoforms are expressed from each of the isozymes, allowing for the multi-faceted biological diversity of SphK activity. Historically, SphK1 is mainly associated with oncogenicity, however in reality, both SphK1 and SphK2 isozymes possess oncogenic properties and are recognized therapeutic targets. The absence of mutations of SphK in various cancer types has led to the theory that cancer cells develop a dependency on SphK signaling (hyper-SphK signaling) or “non-oncogenic addiction”. Here we discuss additional theories of SphK cellular mislocation and aberrant “dicing and splicing” as contributors to cancer cell biology and as key determinants of the success or failure of SphK/S1P (sphingosine 1 phosphate) based therapeutics.

The 3-hydroxyacyl-CoA dehydratases HACD1 and HACD2 exhibit functional redundancy and are active in a wide range of fatty acid elongation pathways

Differences among fatty acids (FAs) in chain length and number of double bonds create lipid diversity. FA elongation proceeds via a four-step reaction cycle, in which the 3-hydroxyacyl-CoA dehydratases (HACDs) HACD1-4 catalyze the third step. However, the contribution of each HACD to 3-hydroxyacyl-CoA dehydratase activity in certain tissues or in different FA elongation pathways remains unclear. HACD1 is specifically expressed in muscles and is a myopathy-causative gene. Here, we generated Hacd1 KO mice and observed that these mice had reduced body and skeletal muscle weights. In skeletal muscle, HACD1 mRNA expression was by far the highest among the HACDs However, we observed only an ∼40% reduction in HACD activity and no changes in membrane lipid composition in Hacd1-KO skeletal muscle, suggesting that some HACD activities are redundant. Moreover, when expressed in yeast, both HACD1 and HACD2 participated in saturated and monounsaturated FA elongation pathways. Disruption of HACD2 in the haploid human cell line HAP1 significantly reduced FA elongation activities toward both saturated and unsaturated FAs, and HACD1 HACD2 double disruption resulted in a further reduction. Overexpressed HACD3 exhibited weak activity in saturated and monounsaturated FA elongation pathways, and no activity was detected for HACD4. We therefore conclude that HACD1 and HACD2 exhibit redundant activities in a wide range of FA elongation pathways, including those for saturated to polyunsaturated FAs, with HACD2 being the major 3-hydroxyacyl-CoA dehydratase. Our findings are important for furthering the understanding of the molecular mechanisms in FA elongation and diversity.

Mammalian sphingosine kinase (SphK) isoenzymes and isoform expression: challenges for SphK as an oncotarget

The various sphingosine kinase (SphK) isoenzymes (isozymes) and isoforms, key players in normal cellular physiology, are strongly implicated in cancer and other diseases. Mutations in SphKs, that may justify abnormal physiological function, have not been recorded. Nonetheless, there is a large and growing body of evidence demonstrating the contribution of gain or loss of function and the imbalance in the SphK/S1P rheostat to a plethora of pathological conditions including cancer, diabetes and inflammatory diseases. SphK is expressed as two isozymes SphK1 and SphK2, transcribed from genes located on different chromosomes and both isozymes catalyze the phosphorylation of sphingosine to S1P. Expression of each SphK isozyme produces alternately spliced isoforms. In recent years the importance of the contribution of SpK1 expression to treatment resistance in cancer has been highlighted and, additionally, differences in treatment outcome appear to also be dependent upon SphK isoform expression. This review focuses on an exciting emerging area of research involving SphKs functions, expression and subcellular localization, highlighting the complexity of targeting SphK in cancer and also comorbid diseases. This review also covers the SphK isoenzymes and isoforms from a historical perspective, from their first discovery in murine species and then in humans, their role(s) in normal cellular function and in disease processes, to advancement of SphK as an oncotarget.

Sphingosine kinase 1 (SK1) allosteric inhibitors that target the dimerization site

The sphingosine kinase 1 (SK1)/sphingosine-1-phosphate (S1P) signaling pathway is a crucial target for numerous human diseases from cancer to cardiovascular diseases. However, available SK1 inhibitors that target the active site suffer from poor potency, selectivity and pharmacokinetic properties. The selectivity issue of the kinases, which share a highly-conserved ATP-pocket, can be overcome by targeting the less-conserved allosteric sites. SK1 is known to function minimally as a dimer; however, the crystal structure of the SK1 dimer has not been determined. In this study, a template-based algorithm implemented in PRISM was used to predict the SK1 dimer structure and then the possible allosteric sites at the dimer interface were determined via SiteMap. These sites were used in a virtual screening campaign that includes an integrated workflow of structure-based pharmacophore modeling, virtual screening, molecular docking, re-screening of common scaffolds to propose a series of compounds with different scaffolds as potential allosteric SK1 inhibitors. Finally, the stability of the SK1-ligand complexes was analyzed by molecular dynamics simulations. As a final outcome, ligand 7 having a 4,9-dihydro-1H-purine scaffold and ligand 12 having a 2,3,4,9-tetrahydro-1H-β-carboline scaffold were found to be potential selective inhibitors for SK1.

PNPLA1 is a transacylase essential for the generation of the skin barrier lipid ω-O-acylceramide

Lipids are the primary components of the skin permeability barrier, which is the body's most powerful defensive mechanism against pathogens. Acylceramide (ω-O-acylceramide) is a specialized lipid essential for skin barrier formation. Here, we identify PNPLA1 as the long-sought gene involved in the final step of acylceramide synthesis, esterification of ω-hydroxyceramide with linoleic acid, by cell-based assays. We show that increasing triglyceride levels by overproduction of the diacylglycerol acyltransferase DGAT2 stimulates acylceramide production, suggesting that triglyceride may act as a linoleic acid donor. Indeed, the in vitro analyses confirm that PNPLA1 catalyses acylceramide synthesis using triglyceride as a substrate. Mutant forms of PNPLA1 found in patients with ichthyosis exhibit reduced or no enzyme activity in either cell-based or in vitro assays. Altogether, our results indicate that PNPLA1 is directly involved in acylceramide synthesis as a transacylase, and provide important insights into the molecular mechanisms of skin barrier formation and of ichthyosis pathogenesis.

Therapeutic Strategies and Pharmacological Tools Influencing S1P Signaling and Metabolism

During the last two decades the study of the sphingolipid anabolic, catabolic, and signaling pathways has attracted enormous interest. Especially the introduction of fingolimod into market as first p.o. therapeutic for the treatment of multiple sclerosis has boosted this effect. Although the complex regulation of sphingosine-1-phosphate (S1P) and other catabolic and anabolic sphingosine-related compounds is not fully understood, the influence on different (patho)physiological states from inflammation to cytotoxicity as well as the availability of versatile pharmacological tools that represent new approaches to study these states are described. Here, we have summarized various aspects concerning the many faces of sphingolipid function modulation by different pharmacological tools up to clinical candidates. Due to the immense heterogeneity of physiological or pharmacological actions and complex cross regulations, it is difficult to predict their role in upcoming therapeutic approaches. Currently, inflammatory, immunological, and/or antitumor aspects are discussed.

Synthesis and degradation pathways, functions, and pathology of ceramides and epidermal acylceramides

Ceramide (Cer) is a structural backbone of sphingolipids and is composed of a long-chain base and a fatty acid. Existence of a variety of Cer species, which differ in chain-length, hydroxylation status, and/or double bond number of either of their hydrophobic chains, has been reported. Ceramide is produced by Cer synthases. Mammals have six Cer synthases (CERS1-6), each of which exhibits characteristic substrate specificity toward acyl-CoAs with different chain-lengths. Knockout mice for each Cer synthase show corresponding, isozyme-specific phenotypes, revealing the functional differences of Cers with different chain-lengths. Cer diversity is especially prominent in epidermis. Changes in Cer levels, composition, and chain-lengths are associated with atopic dermatitis. Acylceramide (acyl-Cer) specifically exists in epidermis and plays an essential role in skin permeability barrier formation. Accordingly, defects in acyl-Cer synthesis cause the cutaneous disorder ichthyosis with accompanying severe skin barrier defects. Although the molecular mechanism by which acyl-Cer is generated was long unclear, most genes involved in its synthesis have been identified recently. In Cer degradation pathways, the long-chain base moiety of Cer is converted to acyl-CoA, which is then incorporated mainly into glycerophospholipids. This pathway generates the lipid mediator sphingosine 1-phosphate. This review will focus on recent advances in our understanding of the synthesis and degradation pathways, physiological functions, and pathology of Cers/acyl-Cers.

Sphingosine Kinases: Emerging Structure-Function Insights

Sphingosine kinases (SK1 and SK2) catalyse the conversion of sphingosine into sphingosine 1-phosphate and control fundamental cellular processes, including cell survival, proliferation, differentiation, migration, and immune function. In this review, we highlight recent breakthroughs in the structural and functional characterisation of SK1 and these are contextualised by analysis of crystal structures for closely related prokaryotic lipid kinases. We identify a putative dimerisation interface and propose novel regulatory mechanisms governing structural plasticity induced by phosphorylation and interaction with phospholipids and proteins. Our analysis suggests that the catalytic function and regulation of the enzymes might be dependent on conformational mobility and it provides a roadmap for future interrogation of SK1 function and its role in physiology and disease.

Targeting sphingosine kinase 2 (SphK2) by ABC294640 inhibits colorectal cancer cell growth in vitro and in vivo

BACKGROUND

Colorectal cancer (CRC) is a major health problem in China and around the world. It is one of the leading causes of cancer-related deaths. Research groups are thus searching for novel and more efficient anti-CRC agents.

RESULTS

Here we demonstrated that ABC294640, a novel SphK2 inhibitor, induced growth inhibition and apoptosis in transformed and primary CRC cells. The SphK activity was remarkably inhibited by ABC294640, accompanied by sphingosine-1-phosphate (S1P) depletion and ceramide incensement in CRC cells. Exogenously-added S1P inhibited ABC294640-induced HT-29 cell lethality. While C6 ceramide and SphK1 inhibitor SKI-II facilitated ABC294640-induced cytotoxicity against HT-29 cells. ABC294640 inhibited AKT-S6K1, but activated JNK signaling in transformed and primary CRC cells. JNK inhibitors (SP600125 and JNKi-II) alleviated ABC294640-induced CRC cell apoptosis. Moreover, a low concentration of ABC294640 sensitized the activity of 5-FU and cisplatin in vitro. In vivo, ABC294640 oral administration dramatically inhibited HT-29 xenografts growth in nude mice.

CONCLUSIONS

Targeting of SphK2 by ABC294640 potently inhibits CRC cell growth both in vitro and in vivo, ABC294640 could be developed as a novel therapeutic for the treatment of CRC.

Activation of sphingosine kinase 2 by endoplasmic reticulum stress ameliorates hepatic steatosis and insulin resistance in mice

The endoplasmic reticulum (ER) is the principal organelle in the cell for protein folding and trafficking, lipid synthesis, and cellular calcium homeostasis. Perturbation of ER function results in activation of the unfolded protein response (UPR) and is implicated in abnormal lipid biosynthesis and development of insulin resistance. In this study, we investigated whether transcription of sphingosine kinase (Sphk)2 is regulated by ER stress-mediated UPR pathways. Sphk2, a major isotype of sphingosine kinase in the liver, was transcriptionally up-regulated by tunicamycin and lipopolysaccharides. Transcriptional regulation of Sphk2 was mediated by activation of activating transcription factor (ATF)4 as demonstrated by promoter assays, immunoblotting, and small interfering RNA analyses. In primary hepatocytes, adenoviral Sphk2 expression elevated cellular sphingosine 1 phosphate (S1P) and activated protein kinase B phosphorylation, with no alteration of insulin receptor substrate phosphorylation. Hepatic overexpression of Sphk2 in mice fed a high-fat diet (HFD) led to elevated S1P and reduced ceramide, sphingomyelin, and glucosylceramide in plasma and liver. Hepatic accumulation of lipid droplets by HFD feeding was reduced by Sphk2-mediated up-regulation of fatty acid (FA) oxidizing genes and increased FA oxidation in liver. In addition, glucose intolerance and insulin resistance were ameliorated by improved hepatic insulin signaling through Sphk2 up-regulation.

CONCLUSION

Sphk2 is transcriptionally up-regulated by acute ER stress through activation of ATF4 and improves perturbed hepatic glucose and FA metabolism.

Sphingosine kinase 1 isoform-specific interactions in breast cancer

Sphingosine kinase 1 (SK1) is a signaling enzyme that catalyzes the formation of sphingosine-1-phosphate. Overexpression of SK1 is causally associated with breast cancer progression and resistance to therapy. SK1 inhibitors are currently being investigated as promising breast cancer therapies. Two major transcriptional isoforms, SK143 kDa and SK151 kDa, have been identified; however, the 51 kDa variant is predominant in breast cancer cells. No studies have investigated the protein-protein interactions of the 51 kDa isoform and whether the two SK1 isoforms differ significantly in their interactions. Seeking an understanding of the regulation and role of SK1, we used a triple-labeling stable isotope labeling by amino acids in cell culture-based approach to identify SK1-interacting proteins common and unique to both isoforms. Of approximately 850 quantified proteins in SK1 immunoprecipitates, a high-confidence list of 30 protein interactions with each SK1 isoform was generated via a meta-analysis of multiple experimental replicates. Many of the novel identified SK1 interaction partners such as supervillin, drebrin, and the myristoylated alanine-rich C-kinase substrate-related protein supported and highlighted previously implicated roles of SK1 in breast cancer cell migration, adhesion, and cytoskeletal remodeling. Of these interactions, several were found to be exclusive to the 43 kDa isoform of SK1, including the protein phosphatase 2A, a previously identified SK1-interacting protein. Other proteins such as allograft inflammatory factor 1-like protein, the latent-transforming growth factor β-binding protein, and dipeptidyl peptidase 2 were found to associate exclusively with the 51 kDa isoform of SK1. In this report, we have identified common and isoform-specific SK1-interacting partners that provide insight into the molecular mechanisms that drive SK1-mediated oncogenicity.

Involvement of sphingosine 1-phosphate in palmitate-induced insulin resistance of hepatocytes via the S1P2 receptor subtype

AIMS/HYPOTHESIS

Enhanced plasma levels of NEFA have been shown to induce hepatic insulin resistance, which contributes to the development of type 2 diabetes. Indeed, sphingolipids can be formed via a de novo pathway from the saturated fatty acid palmitate and the amino acid serine. Besides ceramides, sphingosine 1-phosphate (S1P) has been identified as a major bioactive lipid mediator. Therefore, our aim was to investigate the generation and function of S1P in hepatic insulin resistance.

METHODS

The incorporation of palmitate into sphingolipids was performed by rapid-resolution liquid chromatography-MS/MS in primary human and rat hepatocytes. The influence of S1P and the involvement of S1P receptors in hepatic insulin resistance was examined in human and rat hepatocytes, as well as in New Zealand obese (NZO) mice.

RESULTS

Palmitate induced an impressive formation of extra- and intracellular S1P in rat and human hepatocytes. An elevation of hepatic S1P levels was observed in NZO mice fed a high-fat diet. Once generated, S1P was able, similarly to palmitate, to counteract insulin signalling. The inhibitory effect of S1P was abolished in the presence of the S1P2 receptor antagonist JTE-013 both in vitro and in vivo. In agreement with this, the immunomodulator FTY720-phosphate, which binds to all S1P receptors except S1P2, was not able to inhibit insulin signalling.

CONCLUSIONS/INTERPRETATION

These data indicate that palmitate is metabolised by hepatocytes to S1P, which acts via stimulation of the S1P2 receptor to impair insulin signalling. In particular, S1P2 inhibition could be considered as a novel therapeutic target for the treatment of insulin resistance.

The role of sphingosine 1-phosphate in inflammation and cancer

The enzymes that catalyze formation of the bioactive sphingolipid, sphingosine 1-phosphate, sphingosine kinase 1 and 2, are predictive markers in inflammatory diseases and cancer as evidenced by data from patients, knockout mice and the use of available molecular and chemical inhibitors. Thus, there is a compelling case for therapeutic targeting of sphingosine kinase. In addition, there are several examples of functional interaction between sphingosine 1-phosphate receptors and sphingosine kinase 1 that can drive malicious amplification loops that promote cancer cell growth. These novel aspects of sphingosine 1-phosphate pathobiology are reviewed herein.

Oncogenic properties of sphingosine kinases in haematological malignancies

The sphingosine kinases (SphKs) have relatively recently been implicated in contributing to malignant cellular processes with particular interest in the oncogenic properties of SPHK1. Whilst SPHK1 has received considerable attention as a putative oncoprotein, SPHK2 has been much more difficult to study, with often conflicting data surrounding its role in cancer. Initial studies focused on non-haemopoietic malignancies, however a growing body of literature on the role of sphingolipid metabolism in haemopoietic malignancies is now emerging. This review provides an overview of the current state of knowledge of the SphKs and the bioactive lipid sphingosine 1-phosphate (S1P), the product of the reaction they catalyse. It then reviews the current literature regarding the roles of S1P and the SphKs in haemopoietic malignancies and discusses the compounds currently available that modulate sphingolipid metabolism and their potential and shortcomings as therapeutic agents for the treatment of haematological malignancies.

Substrate specificity, plasma membrane localization, and lipid modification of the aldehyde dehydrogenase ALDH3B1

The accumulation of reactive aldehydes is implicated in the development of several disorders. Aldehyde dehydrogenases (ALDHs) detoxify aldehydes by oxidizing them to the corresponding carboxylic acids. Among the 19 human ALDHs, ALDH3A2 is the only known ALDH that catalyzes the oxidation of long-chain fatty aldehydes including C16 aldehydes (hexadecanal and trans-2-hexadecenal) generated through sphingolipid metabolism. In the present study, we have identified that ALDH3B1 is also active in vitro toward C16 aldehydes and demonstrated that overexpression of ALDH3B1 restores the sphingolipid metabolism in the ALDH3A2-deficient cells. In addition, we have determined that ALDH3B1 is localized in the plasma membrane through its C-terminal dual lipidation (palmitoylation and prenylation) and shown that the prenylation is required particularly for the activity toward hexadecanal. Since knockdown of ALDH3B1 does not cause further impairment of the sphingolipid metabolism in the ALDH3A2-deficient cells, the likely physiological function of ALDH3B1 is to oxidize lipid-derived aldehydes generated in the plasma membrane and not to be involved in the sphingolipid metabolism in the endoplasmic reticulum.

Usage of sphingosine kinase isoforms in mast cells is species and/or cell type determined

FcεRI engagement in mast cells (MCs) induces the activation of two distinct sphingosine kinase isoforms (SphK1 and SphK2) to produce sphingosine-1-phosphate, a mediator essential for MC responses. Whereas embryonic-derived SphK2-null MCs showed impaired responses to Ag, RNA silencing studies on other MC types indicated a dominant role for SphK1. Given the known functional heterogeneity of MCs, we explored whether the reported differences in SphK1 or SphK2 usage could be reflective of phenotypic differences between MC populations. Using lentiviral-based short hairpin RNA to silence SphK1 or SphK2, we found that SphK2 is required for murine MC degranulation, calcium mobilization, and cytokine and leukotriene production, irrespective of the tissue from which the MC progenitors were derived, the stage of MC granule maturity, or the conditions used for differentiation. This finding was consistent with the lack of a full allergic response in SphK2-null mice challenged to undergo passive cutaneous anaphylaxis. A redundant role for both SphKs was uncovered, however, in chemotaxis toward Ag in all MC types tested and in TNF-α production in certain MC types. In contrast, human MC responses were dependent only on SphK1, associating with a more robust expression of this isoform and a more varied representation of SphK variants relative to murine MCs. The findings show that the function of SphK1 and SphK2 can be interchangeable in MCs; however, an important determinant of SphK isoform usage is the species of origin and an influencing factor, the tissue from which MCs may be derived and/or their differentiation state.

Sphingosine 1-phosphate modulates antigen capture by murine Langerhans cells via the S1P2 receptor subtype

Dendritic cells (DCs) play a pivotal role in the development of cutaneous contact hypersensitivity (CHS) and atopic dermatitis as they capture and process antigen and present it to T lymphocytes in the lymphoid organs. Recently, it has been indicated that a topical application of the sphingolipid sphingosine 1-phosphate (S1P) prevents the inflammatory response in CHS, but the molecular mechanism is not fully elucidated. Here we indicate that treatment of mice with S1P is connected with an impaired antigen uptake by Langerhans cells (LCs), the initial step of CHS. Most of the known actions of S1P are mediated by a family of five specific G protein-coupled receptors. Our results indicate that S1P inhibits macropinocytosis of the murine LC line XS52 via S1P₂ receptor stimulation followed by a reduced phosphatidylinositol 3-kinase (PI3K) activity. As down-regulation of S1P₂ not only diminished S1P-mediated action but also enhanced the basal activity of LCs on antigen capture, an autocrine action of S1P has been assumed. Actually, S1P is continuously produced by LCs and secreted via the ATP binding cassette transporter ABCC1 to the extracellular environment. Consequently, inhibition of ABCC1, which decreased extracellular S1P levels, markedly increased the antigen uptake by LCs. Moreover, stimulation of sphingosine kinase activity, the crucial enzyme for S1P formation, is connected not only with enhanced S1P levels but also with diminished antigen capture. These results indicate that S1P is essential in LC homeostasis and influences skin immunity. This is of importance as previous reports suggested an alteration of S1P levels in atopic skin lesions.

Analysis of substrate specificity of human DHHC protein acyltransferases using a yeast expression system

Twenty-one human DHHC proteins formed acyl intermediates. Seventeen of the proteins exhibited protein acyltransferase activities. DHHC1, 10, 14, and 16 are novel protein acyltransferases. DHHC proteins are classified into three classes based on substrate specificities.

Palmitoylation plays important roles in the regulation of protein localization, stability, and activity. The protein acyltransferases (PATs) have a common DHHC Cys-rich domain. Twenty-three DHHC proteins have been identified in humans. However, it is unclear whether all of these DHHC proteins function as PATs. In addition, their substrate specificities remain largely unknown. Here we develop a useful method to examine substrate specificities of PATs using a yeast expression system with six distinct model substrates. We identify 17 human DHHC proteins as PATs. Moreover, we classify 11 human and 5 yeast DHHC proteins into three classes (I, II, and III), based on the cellular localization of their respective substrates (class I, soluble proteins; class II, integral membrane proteins; class III, lipidated proteins). Our results may provide an important clue for understanding the function of individual DHHC proteins.

The Sjögren-Larsson syndrome gene encodes a hexadecenal dehydrogenase of the sphingosine 1-phosphate degradation pathway

Sphingosine 1-phosphate (S1P) functions not only as a bioactive lipid molecule, but also as an important intermediate of the sole sphingolipid-to-glycerolipid metabolic pathway. However, the precise reactions and the enzymes involved in this pathway remain unresolved. We report here that yeast HFD1 and the Sjögren-Larsson syndrome (SLS)-causative mammalian gene ALDH3A2 are responsible for conversion of the S1P degradation product hexadecenal to hexadecenoic acid. The absence of ALDH3A2 in CHO-K1 mutant cells caused abnormal metabolism of S1P/hexadecenal to ether-linked glycerolipids. Moreover, we demonstrate that yeast Faa1 and Faa4 and mammalian ACSL family members are acyl-CoA synthetases involved in the sphingolipid-to-glycerolipid metabolic pathway and that hexadecenoic acid accumulates in Δfaa1 Δfaa4 mutant cells. These results unveil the entire S1P metabolic pathway: S1P is metabolized to glycerolipids via hexadecenal, hexadecenoic acid, hexadecenoyl-CoA, and palmitoyl-CoA. From our results we propose a possibility that accumulation of the S1P metabolite hexadecenal contributes to the pathogenesis of SLS.

Inhibition kinetics and regulation of sphingosine kinase 1 expression in prostate cancer cells: functional differences between sphingosine kinase 1a and 1b

Sphingosine kinase 1 catalyses the formation of the bioactive lipid, sphingosine 1-phosphate and is a target for anti-cancer agents. We demonstrate here that 2-(p-hydroxyanilino)-4-(p-chlorophenyl)thiazole (SKi, also referred to as SKI-II), FTY720 (Fingolimod), and (S)-FTY720 vinylphosphonate inhibit sphingosine kinase 1 activity with distinct kinetics, indicating that these compounds exhibit different binding modalities with sphingosine kinase 1. Thus, SKi is a mixed inhibitor of sphingosine and ATP binding, whereas FTY720 is competitive with sphingosine and uncompetitive with ATP, and (S)-FTY720 vinylphosphonate is uncompetitive with sphingosine and is a mixed inhibitor with respect to ATP. A novel 'see-saw' model is proposed for the binding of inhibitor to catalytic and allosteric sites, the latter dependent on substrate binding, that provides an explanation for the different inhibitor kinetics. In addition, we demonstrate that the expression level and properties unique to an N-terminal 86 amino-acid isoform variant of sphingosine kinase 1 (SK1b) in prostate cancer cells reduce its sensitivity to SKi-induced proteasomal degradation in comparison to SK1a, i.e. these two N-terminal variants of sphingosine kinase 1 (SK1a and SK1b) have different properties. The reduced sensitivity of SK1b to proteasomal degradation in response to SKi is translated into specific changes in ceramide and S1P levels that leads to apoptosis of androgen-sensitive but not androgen-independent LNCaP prostate cancer cells. Therefore, our proposed 'see-saw' model might be usefully employed in the design of sphingosine kinase inhibitors to promote apoptosis of chemotherapeutic resistant cancer cells.

Sphingosine kinase 1 deficiency exacerbates LPS-induced neuroinflammation

The pathogenesis of inflammation in the central nervous system (CNS), which contributes to numerous neurodegenerative diseases and results in encephalopathy and neuroinflammation, is poorly understood. Sphingolipid metabolism plays a crucial role in maintaining cellular processes in the CNS, and thus mediates the various pathological consequences of inflammation. For a better understanding of the role of sphingosine kinase activation during neuroinflammation, we developed a bacterial lipopolysaccharide (LPS)-induced brain injury model. The onset of the inflammatory response was observed beginning 4 hours after intracerebral injection of LPS into the lateral ventricles of the brain. A comparison of established neuroinflammatory parameters such as white matter rarefactions, development of cytotoxic edema, astrogliosis, loss of oligodendrocytes, and major cytokines levels in wild type and knockout mice suggested that the neuroinflammatory response in SphK1-/- mice was significantly upregulated. At 6 hours after intracerebroventricular injection of LPS in SphK1-/- mice, the immunoreactivity of the microglia markers and astrocyte marker glial fibrillary acidic protein (GFAP) were significantly increased, while the oligodendrocyte marker O4 was decreased compared to WT mice. Furthermore, western blotting data showed increased levels of GFAP. These results suggest that SphK1 activation is involved in the regulation of LPS induced brain injury. RESEARCH HIGHLIGHTS: • Lipopolysaccharide (LPS) intracerebral injection induces severe neuroinflammation. • Sphingosine kinase 1 deletion worsens the effect of the LPS. • Overexpression of SphK1 might be a potential new treatment approach to neuroinflammation.

Sphingosine 1-phosphate signalling in cancer

There is an increasing body of evidence demonstrating a critical role for the bioactive lipid S1P (sphingosine 1-phosphate) in cancer. S1P is synthesized and metabolized by a number of enzymes, including sphingosine kinase, S1P lyase and S1P phosphatases. S1P binds to cell-surface G-protein-coupled receptors (S1P1–S1P5) to elicit cell responses and can also regulate, by direct binding, a number of intracellular targets such as HDAC (histone deacetylase) 1/2 to induce epigenetic regulation. S1P is involved in cancer progression including cell transformation/oncogenesis, cell survival/apoptosis, cell migration/metastasis and tumour microenvironment neovascularization. In the present paper, we describe our research findings regarding the correlation of sphingosine kinase 1 and S1P receptor expression in tumours with clinical outcome and we define some of the molecular mechanisms underlying the involvement of sphingosine kinase 1 and S1P receptors in the formation of a cancer cell migratory phenotype. The role of sphingosine kinase 1 in the acquisition of chemotherapeutic resistance and the interaction of S1P receptors with oncogenes such as HER2 is also reviewed. We also discuss novel aspects of the use of small-molecule inhibitors of sphingosine kinase 1 in terms of allosterism, ubiquitin–proteasomal degradation of sphingosine kinase 1 and anticancer activity. Finally, we describe how S1P receptor-modulating agents abrogate S1P receptor–receptor tyrosine kinase interactions, with potential to inhibit growth-factor-dependent cancer progression.