Sphingosine-1-phosphate lyase SPL is an endoplasmic reticulum-resident, integral membrane protein with the pyridoxal 5'-phosphate binding domain exposed to the cytosol

Vascular and Immunobiology of the Circulatory Sphingosine 1-Phosphate Gradient

Vertebrates are endowed with a closed circulatory system, the evolution of which required novel structural and regulatory changes. Furthermore, immune cell trafficking paradigms adapted to the barriers imposed by the closed circulatory system. How did such changes occur mechanistically? We propose that spatial compartmentalization of the lipid mediator sphingosine 1-phosphate (S1P) may be one such mechanism. In vertebrates, S1P is spatially compartmentalized in the blood and lymphatic circulation, thus comprising a sharp S1P gradient across the endothelial barrier. Circulatory S1P has critical roles in maturation and homeostasis of the vascular system as well as in immune cell trafficking. Physiological functions of S1P are tightly linked to shear stress, the key biophysical stimulus from blood flow. Thus, circulatory S1P confinement could be a primordial strategy of vertebrates in the development of a closed circulatory system. This review discusses the cellular and molecular basis of the S1P gradients and aims to interpret its physiological significance as a key feature of the closed circulatory system.

Mechanistic Details of Early Steps in Coenzyme Q Biosynthesis Pathway in Yeast

Coenzyme Q (Q) is a redox lipid that is central for the energetic metabolism of eukaryotes. The biosynthesis of Q from the aromatic precursor 4-hydroxybenzoic acid (4-HB) is understood fairly well. However, biosynthetic details of how 4-HB is produced from tyrosine remain elusive. Here, we provide key insights into this long-standing biosynthetic problem by uncovering molecular details of the first and last reactions of the pathway in the yeast Saccharomyces cerevisiae, namely the deamination of tyrosine to 4-hydroxyphenylpyruvate by Aro8 and Aro9, and the oxidation of 4-hydroxybenzaldehyde to 4-HB by Hfd1. Inactivation of the HFD1 gene in yeast resulted in Q deficiency, which was rescued by the human enzyme ALDH3A1. This suggests that a similar pathway operates in animals, including humans, and led us to propose that patients with genetically unassigned Q deficiency should be screened for mutations in aldehyde dehydrogenase genes, especially ALDH3A1.

Characterization of secreted sphingosine-1-phosphate lyases required for virulence and intracellular survival of Burkholderia pseudomallei

Sphingosine-1-phosphate (S1P), a bioactive sphingolipid metabolite, plays a critical role in the orchestration of immune responses. S1P levels within the mammalian host are tightly regulated, in part through the activity of S1P lyase (S1PL) which catalyses its irreversible degradation. Herein, we describe the identification and characterization of secreted S1PL orthologues encoded by the facultative intracellular bacteria Burkholderia pseudomallei and Burkholderia thailandensis. These bacterial orthologues exhibited S1PL enzymatic activity, functionally complemented an S1PL-deficient yeast strain and conferred resistance to the antimicrobial sphingolipid D-erythro-sphingosine. We report that secretion of these bacterial S1PLs is pH-dependent, and is observed during intracellular infection. S1PL-deficient mutants displayed impaired intracellular replication in murine macrophages (associated with an inability to evade the maturing phagosome) and were significantly attenuated in murine and larval infection models. Furthermore, treatment of Burkholderia-infected macrophages with either S1P or a selective agonist of S1P receptor 1 enhanced bacterial colocalisation with LAMP-1 and reduced their intracellular survival. In summary, our studies confirm bacterial-encoded S1PL as a critical virulence determinant of B. pseudomallei and B. thailandensis, further highlighting the pivotal role of S1P in host-pathogen interactions. In addition, our data suggest that S1P pathway modulators have potential for the treatment of intracellular infection.

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.

Disruption of the Sjögren-Larsson Syndrome Gene Aldh3a2 in Mice Increases Keratinocyte Growth and Retards Skin Barrier Recovery

The fatty aldehyde dehydrogenase (FALDH) ALDH3A2 is the causative gene of Sjögren Larsson syndrome (SLS). To date, the molecular mechanism underlying the symptoms characterizing SLS has been poorly understood. Using Aldh3a2(-/-) mice, we found here that Aldh3a2 was the major FALDH active in undifferentiated keratinocytes. Long-chain base metabolism was greatly impaired in Aldh3a2(-/-) keratinocytes. Phenotypically, the intercellular spaces were widened in the basal layer of the Aldh3a2(-/-) epidermis due to hyperproliferation of keratinocytes. Furthermore, oxidative stress-induced genes were up-regulated in Aldh3a2(-/-) keratinocytes. Upon keratinocyte differentiation, the activity of another FALDH, Aldh3b2, surpassed that of Aldh3a2 As a result, Aldh3a2(-/-) mice were indistinguishable from wild-type mice in terms of their whole epidermis FALDH activity, and their skin barrier function was uncompromised under normal conditions. However, perturbation of the stratum corneum caused increased transepidermal water loss and delayed barrier recovery in Aldh3a2(-/-) mice. In conclusion, Aldh3a2(-/-) mice replicated some aspects of SLS symptoms, especially at the basal layer of the epidermis. Our results suggest that hyperproliferation of keratinocytes via oxidative stress responses may partly contribute to the ichthyosis symptoms of SLS.

Sphingosine 1-phosphate and its carrier apolipoprotein M in human sepsis and in Escherichia coli sepsis in baboons

Sphingosine 1‐phosphate (S1P) is an important regulator of vascular integrity and immune cell migration, carried in plasma by high‐density lipoprotein (HDL)‐associated apolipoprotein M (apoM) and by albumin. In sepsis, the protein and lipid composition of HDL changes dramatically. The aim of this study was to evaluate changes in S1P and its carrier protein apoM during sepsis. For this purpose, plasma samples from both human sepsis patients and from an experimental Escherichia coli sepsis model in baboons were used. In the human sepsis cohort, previously studied for apoM, plasma demonstrated disease‐severity correlated decreased S1P levels, the profile mimicking that of plasma apoM. In the baboons, a similar disease‐severity dependent decrease in plasma levels of S1P and apoM was observed. In the lethal E. coli baboon sepsis, S1P decreased already within 6–8 hrs, whereas the apoM decrease was seen later at 12–24 hrs. Gel filtration chromatography of plasma from severe human or baboon sepsis on Superose 6 demonstrated an almost complete loss of S1P and apoM in the HDL fractions. S1P plasma concentrations correlated with the platelet count but not with erythrocytes or white blood cells. The liver mRNA levels of apoM and apoA1 decreased strongly upon sepsis induction and after 12 hr both were almost completely lost. In conclusion, during septic challenge, the plasma levels of S1P drop to very low levels. Moreover, the liver synthesis of apoM decreases severely and the plasma levels of apoM are reduced. Possibly, the decrease in S1P contributes to the decreased endothelial barrier function observed in sepsis.

Legionella pneumophila S1P-lyase targets host sphingolipid metabolism and restrains autophagy

Autophagy is an essential component of innate immunity, enabling the detection and elimination of intracellular pathogens. Legionella pneumophila, an intracellular pathogen that can cause a severe pneumonia in humans, is able to modulate autophagy through the action of effector proteins that are translocated into the host cell by the pathogen's Dot/Icm type IV secretion system. Many of these effectors share structural and sequence similarity with eukaryotic proteins. Indeed, phylogenetic analyses have indicated their acquisition by horizontal gene transfer from a eukaryotic host. Here we report that L. pneumophila translocates the effector protein sphingosine-1 phosphate lyase (LpSpl) to target the host sphingosine biosynthesis and to curtail autophagy. Our structural characterization of LpSpl and its comparison with human SPL reveals high structural conservation, thus supporting prior phylogenetic analysis. We show that LpSpl possesses S1P lyase activity that was abrogated by mutation of the catalytic site residues. L. pneumophila triggers the reduction of several sphingolipids critical for macrophage function in an LpSpl-dependent and -independent manner. LpSpl activity alone was sufficient to prevent an increase in sphingosine levels in infected host cells and to inhibit autophagy during macrophage infection. LpSpl was required for efficient infection of A/J mice, highlighting an important virulence role for this effector. Thus, we have uncovered a previously unidentified mechanism used by intracellular pathogens to inhibit autophagy, namely the disruption of host sphingolipid biosynthesis.

A facile stable-isotope dilution method for determination of sphingosine phosphate lyase activity

A new technique for quantifying sphingosine phosphate lyase activity in biological samples is described. In this procedure, 2-hydrazinoquinoline is used to convert (2E)-hexadecenal into the corresponding hydrazone derivative to improve ionization efficiency and selectivity of detection. Combined utilization of liquid chromatographic separation and multiple reaction monitoring-mass spectrometry allows for simultaneous quantification of the substrate S1P and product (2E)-hexadecenal. Incorporation of (2E)- d5-hexadecenal as an internal standard improves detection accuracy and precision. A simple one-step derivatization procedure eliminates the need for further extractions. Limits of quantification for (2E)-hexadecenal and sphingosine-1-phosphate are 100 and 50fmol, respectively. The assay displays a wide dynamic detection range useful for detection of low basal sphingosine phosphate lyase activity in wild type cells, SPL-overexpressing cell lines, and wild type mouse tissues. Compared to current methods, the capacity for simultaneous detection of sphingosine-1-phosphate and (2E)-hexadecenal greatly improves the accuracy of results and shows excellent sensitivity and specificity for sphingosine phosphate lyase activity detection.

Inhibitors of sphingosine-1-phosphate metabolism (sphingosine kinases and sphingosine-1-phosphate lyase)

Sphingolipids (SLs) are essential structural and signaling molecules of eukaryotic cells. Among them, sphingosine 1 phosphate (S1P) is a recognized promoter of cell survival, also involved, inter alia, in inflammation and tumorigenesis processes. The knowledge and modulation of the enzymes implicated in the biosynthesis and degradation of S1P are capital to control the intracellular levels of this lipid and, ultimately, to determine the cell fate. Starting with a general overview of the main metabolic pathways involved in SL metabolism, this review is mainly focused on the description of the most relevant findings concerning the development of modulators of S1P, namely inhibitors of the enzymes regulating S1P synthesis (sphingosine kinases) and degradation (sphingosine 1 phosphate phosphatase and lyase). In addition, a brief overview of the most significant agonists and antagonists at the S1P receptors is also addressed.

Sphingosine 1-Phosphate Produced by Sphingosine Kinase 2 Intrinsically Controls Platelet Aggregation In Vitro and In Vivo

RATIONALE

Platelets are known to play a crucial role in hemostasis. Sphingosine kinases (Sphk) 1 and 2 catalyze the conversion of sphingosine to the bioactive metabolite sphingosine 1-phosphate (S1P). Although platelets are able to secrete S1P on activation, little is known about a potential intrinsic effect of S1P on platelet function.

OBJECTIVE

To investigate the role of Sphk1- and Sphk2-derived S1P in the regulation of platelet function.

METHODS AND RESULTS

We found a 100-fold reduction in intracellular S1P levels in platelets derived from Sphk2(-/-) mutants compared with Sphk1(-/-) or wild-type mice, as analyzed by mass spectrometry. Sphk2(-/-) platelets also failed to secrete S1P on stimulation. Blood from Sphk2-deficient mice showed decreased aggregation after protease-activated receptor 4-peptide and adenosine diphosphate stimulation in vitro, as assessed by whole blood impedance aggregometry. We revealed that S1P controls platelet aggregation via the sphingosine 1-phosphate receptor 1 through modulation of protease-activated receptor 4-peptide and adenosine diphosphate-induced platelet activation. Finally, we show by intravital microscopy that defective platelet aggregation in Sphk2-deficient mice translates into reduced arterial thrombus stability in vivo.

CONCLUSIONS

We demonstrate that Sphk2 is the major Sphk isoform responsible for the generation of S1P in platelets and plays a pivotal intrinsic role in the control of platelet activation. Correspondingly, Sphk2-deficient mice are protected from arterial thrombosis after vascular injury, but have normal bleeding times. Targeting this pathway could therefore present a new therapeutic strategy to prevent thrombosis.

Sphingosine-1-phosphate metabolism: A structural perspective

Sphingolipids represent an important class of bioactive signaling lipids which have key roles in numerous cellular processes. Over the last few decades, the levels of bioactive sphingolipids and/or their metabolizing enzymes have been realized to be important factors involved in disease development and progression, most notably in cancer. Targeting sphingolipid-metabolizing enzymes in disease states has been the focus of many studies and has resulted in a number of pharmacological inhibitors, with some making it into the clinic as therapeutics. In order to better understand the regulation of sphingolipid-metabolizing enzymes as well as to develop much more potent and specific inhibitors, the field of sphingolipids has recently taken a turn toward structural biology. The last decade has seen the structural determination of a number of sphingolipid enzymes and effector proteins. In these terms, one of the most complete arms of the sphingolipid pathway is the sphingosine-1-phosphate (S1P) arm. The structures of proteins involved in the function and regulation of S1P are being used to investigate further the regulation of said proteins as well as in the design and development of inhibitors as potential therapeutics.

Sphingolipids in the DNA damage response

Recently, sphingolipid metabolizing enzymes have emerged as important targets of many chemotherapeutics and DNA damaging agents and therefore play significant roles in mediating the physiological response of the cell to DNA damage. In this review we will highlight points of connection between the DNA damage response (DDR) and sphingolipid metabolism; specifically how certain sphingolipid enzymes are regulated in response to DNA damage and how the bioactive lipids produced by these enzymes affect cell fate.

Engineering in vivo gradients of sphingosine-1-phosphate receptor ligands for localized microvascular remodeling and inflammatory cell positioning

Biomaterial-mediated controlled release of soluble signaling molecules is a tissue engineering approach to spatially control processes of inflammation, microvascular remodeling and host cell recruitment, and to generate biochemical gradients in vivo. Lipid mediators, such as sphingosine 1-phosphate (S1P), are recognized for their essential roles in spatial guidance, signaling and highly regulated endogenous gradients. S1P and pharmacological analogs such as FTY720 are therapeutically attractive targets for their critical roles in the trafficking of cells between blood and tissue spaces, both physiologically and pathophysiologically. However, the interaction of locally delivered sphingolipids with the complex metabolic networks controlling the flux of lipid species in inflamed tissue has yet to be elucidated. In this study, complementary in vitro and in vivo approaches are investigated to identify relationships between polymer composition, drug release kinetics, S1P metabolic activity, signaling gradients and spatial positioning of circulating cells around poly(lactic-co-glycolic acid) biomaterials. Results demonstrate that biomaterial-based gradients of S1P are short-lived in the tissue due to degradation by S1P lyase, an enzyme that irreversibly degrades intracellular S1P. On the other hand, in vivo gradients of the more stable compound, FTY720, enhance microvascular remodeling by selectively recruiting an anti-inflammatory subset of monocytes (S1P3(high)) to the biomaterial. Results highlight the need to better understand the endogenous balance of lipid import/export machinery and lipid kinase/phosphatase activity in order to design biomaterial products that spatially control the innate immune environment to maximize regenerative potential.

A gatekeeper helix determines the substrate specificity of Sjögren-Larsson Syndrome enzyme fatty aldehyde dehydrogenase

Mutations in the gene coding for membrane-bound fatty aldehyde dehydrogenase (FALDH) lead to toxic accumulation of lipid species and development of the Sjögren-Larsson Syndrome (SLS), a rare disorder characterized by skin defects and mental retardation. Here, we present the crystallographic structure of human FALDH, the first model of a membrane-associated aldehyde dehydrogenase. The dimeric FALDH displays a previously unrecognized element in its C-terminal region, a 'gatekeeper' helix, which extends over the adjacent subunit, controlling the access to the substrate cavity and helping orientate both substrate cavities towards the membrane surface for efficient substrate transit between membranes and catalytic site. Activity assays demonstrate that the gatekeeper helix is important for directing the substrate specificity of FALDH towards long-chain fatty aldehydes. The gatekeeper feature is conserved across membrane-associated aldehyde dehydrogenases. Finally, we provide insight into the previously elusive molecular basis of SLS-causing mutations.

Dual functions of the trans-2-enoyl-CoA reductase TER in the sphingosine 1-phosphate metabolic pathway and in fatty acid elongation

The sphingolipid metabolite sphingosine 1-phosphate (S1P) functions as a lipid mediator and as a key intermediate of the sole sphingolipid to glycerophospholipid metabolic pathway (S1P metabolic pathway). In this pathway, S1P is converted to palmitoyl-CoA through 4 reactions, then incorporated mainly into glycerophospholipids. Although most of the genes responsible for the S1P metabolic pathway have been identified, the gene encoding the trans-2-enoyl-CoA reductase, responsible for the saturation step (conversion of trans-2-hexadecenoyl-CoA to palmitoyl-CoA) remains unidentified. In the present study, we show that TER is the missing gene in mammals using analyses involving yeast cells, deleting the TER homolog TSC13, and TER-knockdown HeLa cells. TER is known to be involved in the production of very long-chain fatty acids (VLCFAs). A significant proportion of the saturated and monounsaturated VLCFAs are used for sphingolipid synthesis. Therefore, TER is involved in both the production of VLCFAs used in the fatty acid moiety of sphingolipids as well as in the degradation of the sphingosine moiety of sphingolipids via S1P.

Metabolism, physiological role, and clinical implications of sphingolipids in gastrointestinal tract

Sphingolipids in digestive system are responsible for numerous important physiological and pathological processes. In the membrane of gut epithelial cells, sphingolipids provide structural integrity, regulate absorption of some nutrients, and act as receptors for many microbial antigens and their toxins. Moreover, bioactive sphingolipids such as ceramide or sphingosine-1-phosphate regulate cellular growth, differentiation, and programmed cell death-apoptosis. Although it is well established that sphingolipids have clinical implications in gastrointestinal tumorigenesis or inflammation, further studies are needed to fully explore the role of sphingolipids in neoplastic and inflammatory diseases in gastrointestinal tract. Pharmacological agents which regulate metabolism of sphingolipids can be potentially used in the management of colorectal cancer or inflammatory bowel diseases. The aim of this work is to critically the review physiological and pathological roles of sphingolipids in the gastrointestinal tract.

Sphingosine 1-phosphate is a key metabolite linking sphingolipids to glycerophospholipids

The sphingolipid metabolite sphingosine 1-phosphate (S1P) is a well-known lipid mediator. As a lipid mediator, S1P must be present in extracellular space and bind to its cell surface receptors (S1P1-5). However, most S1P, synthesized intracellularly, is metabolized without being released into extracellular space, in other words, without functioning as a lipid mediator in the vast majority of cells except those supplying plasma and lymph S1P such as blood cells and endothelial cells. Instead, intracellular S1P plays an important role as an intermediate of the sole sphingolipid-to-glycerophospholipid metabolic pathway. The degradation of S1P by S1P lyase is the first irreversible reaction (committed step) of this pathway. This metabolic pathway is conserved in eukaryotes from yeast to human, indicating its much older origin than the function of S1P as a lipid mediator, which is found to be present only in vertebrates and chordates. The sphingolipid-to-glycerophospholipid metabolism takes place ubiquitously in mammalian tissues, and its defect causes an aberration of several tissue functions as well as abnormal lipid metabolism. Although this metabolic pathway has been known for over four decades, only recently the precise reactions and enzymes involved in this pathway have been revealed. This review will focus on the recent advances in our understanding of the sphingolipid metabolic pathway via S1P and its physiological and pathological roles. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Increased sphingosine-1-phosphate improves muscle regeneration in acutely injured mdx mice

BACKGROUND

Presently, there is no effective treatment for the lethal muscle wasting disease Duchenne muscular dystrophy (DMD). Here we show that increased sphingosine-1-phoshate (S1P) through direct injection or via the administration of the small molecule 2-acetyl-4(5)-tetrahydroxybutyl imidazole (THI), an S1P lyase inhibitor, has beneficial effects in acutely injured dystrophic muscles of mdx mice.

METHODS

We treated mdx mice with and without acute injury and characterized the histopathological and functional effects of increasing S1P levels. We also tested exogenous and direct administration of S1P on mdx muscles to examine the molecular pathways under which S1P promotes regeneration in dystrophic muscles.

RESULTS

Short-term treatment with THI significantly increased muscle fiber size and extensor digitorum longus (EDL) muscle specific force in acutely injured mdx limb muscles. In addition, the accumulation of fibrosis and fat deposition, hallmarks of DMD pathology and impaired muscle regeneration, were lower in the injured muscles of THI-treated mdx mice. Furthermore, increased muscle force was observed in uninjured EDL muscles with a longer-term treatment of THI. Such regenerative effects were linked to the response of myogenic cells, since intramuscular injection of S1P increased the number of Myf5nlacz/+ positive myogenic cells and newly regenerated myofibers in injured mdx muscles. Intramuscular injection of biotinylated-S1P localized to muscle fibers, including newly regenerated fibers, which also stained positive for S1P receptor 1 (S1PR1). Importantly, plasma membrane and perinuclear localization of phosphorylated S1PR1 was observed in regenerating muscle fibers of mdx muscles. Intramuscular increases of S1P levels, S1PR1 and phosphorylated ribosomal protein S6 (P-rpS6), and elevated EDL muscle specific force, suggest S1P promoted the upregulation of anabolic pathways that mediate skeletal muscle mass and function.

CONCLUSIONS

These data show that S1P is beneficial for muscle regeneration and functional gain in dystrophic mice, and that THI, or other pharmacological agents that raise S1P levels systemically, may be developed into an effective treatment for improving muscle function and reducing the pathology of DMD.

Vitamin-mediated regulation of intestinal immunity

The intestine is exposed continuously to complex environments created by numerous injurious and beneficial non-self antigens. The unique mucosal immune system in the intestine maintains the immunologic homeostasis between the host and the external environment. Crosstalk between immunocompetent cells and endogenous (e.g., cytokines and chemokines) as well as exogenous factors (e.g., commensal bacteria and dietary materials) achieves the vast diversity of intestinal immune functions. In addition to their vital roles as nutrients, vitamins now also are known to have immunologically crucial functions, specifically in regulating host immune responses. In this review, we focus on the immunologic functions of vitamins in regulating intestinal immune responses and their roles in moderating the fine balance between physiologic and pathologic conditions of the intestine.

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.

Sphingosine kinase/sphingosine 1-phosphate signaling in cancer therapeutics and drug resistance

In this chapter, roles of bioactive sphingolipids, specifically sphingosine kinase 1 (SK1) and 2 (SK2) and their product-sphingosine 1-phosphate (S1P)-will be reviewed with respect to regulation of cancer growth, metastasis, chemotherapeutics, and drug resistance. Sphingolipids are known to be key bioeffector molecules that regulate cancer proliferation, angiogenesis, and cell death. Sphingolipid molecules such as ceramide and S1P have been shown to control cancer cell death and proliferation, respectively. Roles of S1P have been described with respect to their intracellular and extracellular pro-survival and drug resistance functions mostly through S1P receptor (S1PR1-5) engagement. Identification of novel intracellular SK/S1P targets has broadened the existing complex regulatory roles of bioactive sphingolipids in cancer pathogenesis and therapeutics. Thus, deciphering the biochemical and molecular regulation of SK/S1P/S1PR signaling could permit development of novel therapeutic interventions to improve cancer therapy and/or overcome drug resistance.

Evidence for a link between histone deacetylation and Ca²+ homoeostasis in sphingosine-1-phosphate lyase-deficient fibroblasts

Embryonic fibroblasts from S1P (sphingosine-1-phosphate) lyase-deficient mice [Sgpl1-/- MEFs (mouse embryonic fibroblasts)] are characterized by intracellular accumulation of S1P, elevated cytosolic [Ca2+]i and enhanced Ca2+ storage. Since S1P, produced by sphingosine kinase 2 in the nucleus of MCF-7 cells, inhibited HDACs (histone deacetylases) [Hait, Allegood, Maceyka, Strub, Harikumar, Singh, Luo, Marmorstein, Kordula, Milstein et al. (2009) Science 325, 1254-1257], in the present study we analysed whether S1P accumulated in the nuclei of S1P lyase-deficient MEFs and caused HDAC inhibition. Interestingly, nuclear concentrations of S1P were disproportionally elevated in Sgpl1-/- MEFs. HDAC activity was reduced, acetylation of histone 3-Lys9 was increased and the HDAC-regulated gene p21 cyclin-dependent kinase inhibitor was up-regulated in these cells. Furthermore, the expression of HDAC1 and HDAC3 was reduced in Sgpl1-/- MEFs. In wild-type MEFs, acetylation of histone 3-Lys9 was increased by the S1P lyase inhibitor 4-deoxypyridoxine. The non-specific HDAC inhibitor trichostatin A elevated basal [Ca2+]i and enhanced Ca2+ storage, whereas the HDAC1/2/3 inhibitor MGCD0103 elevated basal [Ca2+]i without influence on Ca2+ storage in wild-type MEFs. Overexpression of HDAC1 or HDAC2 reduced the elevated basal [Ca2+]i in Sgpl1-/- MEFs. Taken together, S1P lyase-deficiency was associated with elevated nuclear S1P levels, reduced HDAC activity and down-regulation of HDAC isoenzymes. The decreased HDAC activity in turn contributed to the dysregulation of Ca2+ homoeostasis, particularly to the elevated basal [Ca2+]i, in Sgpl1-/- MEFs.

Overexpression of sphingosine kinase 1 prevents ceramide accumulation and ameliorates muscle insulin resistance in high-fat diet-fed mice

The sphingolipids sphingosine-1-phosphate (S1P) and ceramide are important bioactive lipids with many cellular effects. Intracellular ceramide accumulation causes insulin resistance, but sphingosine kinase 1 (SphK1) prevents ceramide accumulation, in part, by promoting its metabolism into S1P. Despite this, the role of SphK1 in regulating insulin action has been largely overlooked. Transgenic (Tg) mice that overexpress SphK1 were fed a standard chow or high-fat diet (HFD) for 6 weeks before undergoing several metabolic analyses. SphK1 Tg mice fed an HFD displayed increased SphK activity in skeletal muscle, which was associated with an attenuated intramuscular ceramide accumulation compared with wild-type (WT) littermates. This was associated with a concomitant reduction in the phosphorylation of c-jun amino-terminal kinase, a serine threonine kinase associated with insulin resistance. Accordingly, skeletal muscle and whole-body insulin sensitivity were improved in SphK1 Tg, compared with WT mice, when fed an HFD. We have identified that the enzyme SphK1 is an important regulator of lipid partitioning and insulin action in skeletal muscle under conditions of increased lipid supply.

Genetic elevation of sphingosine 1-phosphate suppresses dystrophic muscle phenotypes in Drosophila

Duchenne muscular dystrophy is a lethal genetic disease characterized by the loss of muscle integrity and function over time. Using Drosophila, we show that dystrophic muscle phenotypes can be significantly suppressed by a reduction of wunen, a homolog of lipid phosphate phosphatase 3, which in higher animals can dephosphorylate a range of phospholipids. Our suppression analyses include assessing the localization of Projectin protein, a titin homolog, in sarcomeres as well as muscle morphology and functional movement assays. We hypothesize that wunen-based suppression is through the elevation of the bioactive lipid Sphingosine 1-phosphate (S1P), which promotes cell proliferation and differentiation in many tissues, including muscle. We confirm the role of S1P in suppression by genetically altering S1P levels via reduction of S1P lyase (Sply) and by upregulating the serine palmitoyl-CoA transferase catalytic subunit gene lace, the first gene in the de novo sphingolipid biosynthetic pathway and find that these manipulations also reduce muscle degeneration. Furthermore, we show that reduction of spinster (which encodes a major facilitator family transporter, homologs of which in higher animals have been shown to transport S1P) can also suppress dystrophic muscle degeneration. Finally, administration to adult flies of pharmacological agents reported to elevate S1P signaling significantly suppresses dystrophic muscle phenotypes. Our data suggest that localized intracellular S1P elevation promotes the suppression of muscle wasting in flies.

The functional roles of S1P in immunity

The lipid mediator sphingosine-1-phosphate (S1P) is generated within cells from sphingosine by two sphingosine kinases (SPHK1 and SPHK2). Intracellularly synthesized S1P is released into the extracellular fluid by S1P transporters, including SPNS2. Released S1P binds specifically to the G protein-coupled S1P receptors (S1PR1/S1P(1)-S1PR5/S1P(5)), which activate a diverse range of downstream signalling pathways. Recent studies have proposed that one of the central physiological functions of intercellular S1P signalling is in lymphocyte trafficking in vivo because genetic disruption of SPHK1/2, SPNS2 or S1PR1/S1P(1) in mice induces a lymphopenia phenotype. In this review, we discuss the current understanding of intercellular S1P signalling in the context of immunity.

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.

Shaping the landscape: metabolic regulation of S1P gradients

Sphingosine-1-phosphate (S1P) is a lipid that functions as a metabolic intermediate and a cellular signaling molecule. These roles are integrated when compartments with differing extracellular S1P concentrations are formed that serve to regulate functions within the immune and vascular systems, as well as during pathologic conditions. Gradients of S1P concentration are achieved by the organization of cells with specialized expression of S1P metabolic pathways within tissues. S1P concentration gradients underpin the ability of S1P signaling to regulate in vivo physiology. This review will discuss the mechanisms that are necessary for the formation and maintenance of S1P gradients, with the aim of understanding how a simple lipid controls complex physiology. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

PLP-dependent enzymes as entry and exit gates of sphingolipid metabolism

Sphingolipids are membrane constituents as well as signaling molecules involved in many essential cellular processes. Serine palmitoyltransferase (SPT) and sphingosine-1-phosphate lyase (SPL), both PLP (pyridoxal 5'-phosphate)-dependent enzymes, function as entry and exit gates of the sphingolipid metabolism. SPT catalyzes the condensation of serine and a fatty acid into 3-keto-dihydrosphingosine, whereas SPL degrades sphingosine-1-phosphate (S1P) into phosphoethanolamine and a long-chain aldehyde. The recently solved X-ray structures of prokaryotic homologs of SPT and SPL combined with functional studies provide insight into the structure-function relationship of the two enzymes. Despite carrying out different reactions, the two enzymes reveal striking similarities in the overall fold, topology, and residues crucial for activity. Unlike their eukaryotic counterparts, bacterial SPT and SPL lack a transmembrane helix, making them targets of choice for biochemical characterization because the use of detergents can be avoided. Both human enzymes are linked to severe diseases or disorders and might therefore serve as targets for the development of therapeutics aiming at the modulation of their activity. This review gives an overview of the sphingolipid metabolism and of the available biochemical studies of prokaryotic SPT and SPL, and discusses the major similarities and differences to the corresponding eukaryotic enzymes.

Determination of sphingosine-1-phosphate lyase activity by gas chromatography coupled to electron impact mass spectrometry

Sphingosine-1-phosphate lyase (SGPL1) is the last enzyme in the catabolism of sphingolipids. It catalyzes the retroaldolic cleavage of long chain base phosphates into phosphoethanolamine and a fatty aldehyde. In this article we report on an easy and sensitive procedure to determine SPL activity. The assays uses C17-sphinganine-1-phosphate as substrate and the aldehyde product, pentadecanal, is quantified as its pentafluorobenzyloxime derivative by GC/MS. Derivatization of pentadecanal is performed as a one-step reaction, and the oxime product is directly injected for GC/MS analysis without any further purification. Acquisition in selected ion monitoring mode allows very high sensitivity, with a limit of detection of 281fmol. The assay is linear with both protein concentration and incubation time up to 20μg and 40min, respectively. The K(m) value obtained (6μM) is similar to that for the natural substrate sphingosine-1-phosphate. Using this method, FTY720 and deoxypyridoxine phosphate inhibited SPL with similar potencies to those reported.

Pharmacological relevance and potential of sphingosine 1-phosphate in the vascular system

Sphingosine-1-phosphate (S1P) was identified as a crucial molecule for regulating immune responses, inflammatory processes as well as influencing the cardiovascular system. S1P mediates differentiation, proliferation and migration during vascular development and homoeostasis. S1P is a naturally occurring lipid metabolite and is present in human blood in nanomolar concentrations. S1P is not only involved in physiological but also in pathophysiological processes. Therefore, this complex signalling system is potentially interesting for pharmacological intervention. Modulation of the system might influence inflammatory, angiogenic or vasoregulatory processes. S1P activates G-protein coupled receptors, namely S1P(1-5) , whereas only S1P(1-3) is present in vascular cells. S1P can also act as an intracellular signalling molecule. This review highlights the pharmacological potential of S1P signalling in the vascular system by giving an overview of S1P-mediated processes in endothelial cells (ECs) and vascular smooth muscle cells (VSMCs). After a short summary of S1P metabolism and signalling pathways, the role of S1P in EC and VSMC proliferation and migration, the cause of relaxation and constriction of arterial blood vessels, the protective functions on endothelial apoptosis, as well as the regulatory function in leukocyte adhesion and inflammatory responses are summarized. This is followed by a detailed description of currently known pharmacological agonists and antagonists as new tools for mediating S1P signalling in the vasculature. The variety of effects influenced by S1P provides plenty of therapeutic targets currently under investigation for potential pharmacological intervention.

Pyridoxine dependent epilepsy and antiquitin deficiency: clinical and molecular characteristics and recommendations for diagnosis, treatment and follow-up

Antiquitin (ATQ) deficiency is the main cause of pyridoxine dependent epilepsy characterized by early onset epileptic encephalopathy responsive to large dosages of pyridoxine. Despite seizure control most patients have intellectual disability. Folinic acid responsive seizures (FARS) are genetically identical to ATQ deficiency. ATQ functions as an aldehyde dehydrogenase (ALDH7A1) in the lysine degradation pathway. Its deficiency results in accumulation of α-aminoadipic semialdehyde (AASA), piperideine-6-carboxylate (P6C) and pipecolic acid, which serve as diagnostic markers in urine, plasma, and CSF. To interrupt seizures a dose of 100 mg of pyridoxine-HCl is given intravenously, or orally/enterally with 30 mg/kg/day. First administration may result in respiratory arrest in responders, and thus treatment should be performed with support of respiratory management. To make sure that late and masked response is not missed, treatment with oral/enteral pyridoxine should be continued until ATQ deficiency is excluded by negative biochemical or genetic testing. Long-term treatment dosages vary between 15 and 30 mg/kg/day in infants or up to 200 mg/day in neonates, and 500 mg/day in adults. Oral or enteral pyridoxal phosphate (PLP), up to 30 mg/kg/day can be given alternatively. Prenatal treatment with maternal pyridoxine supplementation possibly improves outcome. PDE is an organic aciduria caused by a deficiency in the catabolic breakdown of lysine. A lysine restricted diet might address the potential toxicity of accumulating αAASA, P6C and pipecolic acid. A multicenter study on long term outcomes is needed to document potential benefits of this additional treatment. The differential diagnosis of pyridoxine or PLP responsive seizure disorders includes PLP-responsive epileptic encephalopathy due to PNPO deficiency, neonatal/infantile hypophosphatasia (TNSALP deficiency), familial hyperphosphatasia (PIGV deficiency), as well as yet unidentified conditions and nutritional vitamin B6 deficiency. Commencing treatment with PLP will not delay treatment in patients with pyridox(am)ine phosphate oxidase (PNPO) deficiency who are responsive to PLP only.

The compartmentalization and translocation of the sphingosine kinases: mechanisms and functions in cell signaling and sphingolipid metabolism

Members of the sphingosine kinase (SK) family of lipid signaling enzymes, comprising SK1 and SK2 in humans, are receiving considerable attention for their roles in a number of physiological and pathophysiological processes. The SKs are considered signaling enzymes based on their production of the potent lipid second messenger sphingosine-1-phosphate, which is the ligand for a family of five G-protein-linked receptors. Both SK1 and SK2 are intracellular enzymes and do not possess obvious membrane anchor domains within their primary sequences. The native substrates (sphingosine and dihydrosphingosine) are lipids, as are the corresponding products, and therefore would have a propensity to be membrane associated, suggesting that specific membrane localization of the SKs could affect both access to substrate and localized production of product. Here, we consider the emerging picture of the SKs as enzymes localized to specific intracellular sites, sometimes by agonist-dependent translocation, the mechanism targeting these enzymes to those sites, and the functional consequence of that localization. Not only is the signaling output of the SKs affected by subcellular localization, but the role of these enzymes as metabolic regulators of sphingolipid metabolism may be impacted as well.

Protection of LPS-induced murine acute lung injury by sphingosine-1-phosphate lyase suppression

A defining feature of acute lung injury (ALI) is the increased lung vascular permeability and alveolar flooding, which leads to associated morbidity and mortality. Specific therapies to alleviate the unremitting vascular leak in ALI are not currently clinically available; however, our prior studies indicate a protective role for sphingosine-1-phosphate (S1P) in animal models of ALI with reductions in lung edema. As S1P levels are tightly regulated by synthesis and degradation, we tested the hypothesis that inhibition of S1P lyase (S1PL), the enzyme that irreversibly degrades S1P via cleavage, could ameliorate ALI. Intratracheal instillation of LPS to mice enhanced S1PL expression, decreased S1P levels in lung tissue, and induced lung inflammation and injury. LPS challenge of wild-type mice receiving 2-acetyl-4(5)-[1(R),2(S),3(R),4-tetrahydroxybutyl]-imidazole to inhibit S1PL or S1PL(+/-) mice resulted in increased S1P levels in lung tissue and bronchoalveolar lavage fluids and reduced lung injury and inflammation. Moreover, down-regulation of S1PL expression by short interfering RNA (siRNA) in primary human lung microvascular endothelial cells increased S1P levels, and attenuated LPS-mediated phosphorylation of p38 mitogen-activated protein kinase and I-κB, IL-6 secretion, and endothelial barrier disruption via Rac1 activation. These results identify a novel role for intracellularly generated S1P in protection against ALI and suggest S1PL as a potential therapeutic target.

Synthesis and biosynthetic trafficking of membrane lipids

Eukaryotic cells can synthesize thousands of different lipid molecules that are incorporated into their membranes. This involves the activity of hundreds of enzymes with the task of creating lipid diversity. In addition, there are several, typically redundant, mechanisms to transport lipids from their site of synthesis to other cellular membranes. Biosynthetic lipid transport helps to ensure that each cellular compartment will have its characteristic lipid composition that supports the functions of the associated proteins. In this article, we provide an overview of the biosynthesis of the major lipid constituents of cell membranes, that is, glycerophospholipids, sphingolipids, and sterols, and discuss the mechanisms by which these newly synthesized lipids are delivered to their target membranes.

Sphingosine 1-phosphate in coagulation and inflammation

Sphingosine 1-phosphate (S1P) is a lipid mediator produced from sphingomyelin by the sequential enzymatic actions of sphingomyelinase, ceramidase, and sphingosine kinase. Five subtypes of cell surface G-protein-coupled receptors, S1P(1-5), mediate the actions of S1P in various organs systems, most notably cardiovascular, immune, and central nervous systems. S1P is enriched in blood and lymph but is present at much lower concentrations in interstitial fluids of tissues. This vascular S1P gradient is important for the regulation of trafficking of various immune cells. FTY720, which was recently approved for the treatment of relapsing-remitting multiple sclerosis, potently sequesters lymphocytes into lymph nodes by functionally antagonizing the activity of the S1P(1) receptor. S1P also plays critical roles in the vascular barrier integrity, thereby regulating inflammation, tumor metastasis, angiogenesis, and atherosclerosis. Recent studies have also revealed the involvement of S1P signaling in coagulation and in tumor necrosis factor α-mediated signaling. This review highlights the importance of S1P signaling in these inflammatory processes as well as the contribution of each receptor subtype, which exhibits both cooperative and redundant functions.

Physiological functions and clinical implications of sphingolipids in the gut

Studies of sphingolipids have become one of the most rapidly advancing fields in the last two decades. These highly diverse lipids have been known to have multiple physiological functions and clinical implications in several diseases, including tumorigenesis, inflammation, atherosclerosis and neural degenerative diseases. Unlike other organs, sphingolipids in the intestinal tract are present not only as lipid constituents in the cells but also as dietary compositions for digestion in the lumen. The present review focuses on the presence of sphingolipids and their catalytic enzymes in the gut; the metabolism and the signaling effects of the metabolites and their impacts on barrier functions, cholesterol absorption, inflammatory diseases and tumor development in the gut.

Vascular biology: the role of sphingosine 1-phosphate in both the resting state and inflammation

The vascular and immune systems of mammals are closely intertwined: the individual components of the immune system must move between various body compartments to perform their function effectively. Sphingosine 1-phosphate (S1P), a bioactive lipid mediator, exerts effects on the two organ systems and influences the interaction between them. In the resting state, the vascular S1P gradient contributes to control of lymphocyte recirculation through the blood, lymphoid tissue and lymphatic vasculature. The high level of S1P in blood helps maintain endothelial barrier integrity. During the inflammatory process, both the level of S1P in different immune compartments and S1P receptor expression on lymphocytes and endothelial cells are modified, resulting in functionally important changes in endothelial cell and lymphocyte behaviour. These include transient arrest of lymphocytes in secondary lymphoid tissue, crucial for generation of adaptive immunity, and subsequent promotion of lymphocyte recruitment to sites of inflammation. This review begins with an outline of the basic biochemistry of S1P. S1P receptor signalling is then discussed, followed by an exploration of the roles of S1P in the vascular and immune systems, with particular focus on the interface between them. The latter part concerns crosstalk between S1P and other signalling pathways, and concludes with a look at therapies targeting the S1P-S1P receptor axis.

Role of sphingosine kinase localization in sphingolipid signaling

The sphingosine kinases, SK1 and SK2, produce the potent signaling lipid sphingosine-1-phosphate (S1P). These enzymes have garnered increasing interest for their roles in tumorigenesis, inflammation, vascular diseases, and immunity, as well as other functions. The sphingosine kinases are considered signaling enzymes by producing S1P, and their activity is acutely regulated by a variety of agonists. However, these enzymes are also key players in the control of sphingolipid metabolism. A variety of sphingolipids, such as sphingosine and the ceramides, are potent signaling molecules in their own right. The role of sphingosine kinases in regulating sphingolipid metabolism is potentially a critical aspect of their signaling function. A central aspect of signaling lipids is that their hydrophobic nature constrains them to membranes. Most enzymes of sphingolipid metabolism, including the enzymes that degrade S1P, are membrane enzymes. Therefore the localization of the sphingosine kinases and S1P is likely to be important in S1P signaling. Sphingosine kinase localization affects sphingolipid signaling in several ways. Translocation of SK1 to the plasma membrane promotes extracellular secretion of S1P. SK1 and SK2 localization to specific sites appears to direct S1P to intracellular protein effectors. SK localization also determines the access of these enzymes to their substrates. This may be an important mechanism for the regulation of ceramide biosynthesis by diverting dihydrosphingosine, a precursor in the ceramide biosynthetic pathway, from the de novo production of ceramide.

Structure and function of sphingosine-1-phosphate lyase, a key enzyme of sphingolipid metabolism

Sphingosine-1-phosphate lyase (SPL), a key enzyme of sphingolipid metabolism, catalyzes the irreversible degradation of sphingoid base phosphates. Its main substrate sphingosine-1-phosphate (S1P) acts both extracellularly, by binding G protein-coupled receptors of the lysophospholipid receptor family, and inside the cell, as a second messenger. There, S1P takes part in regulating various cellular processes and its levels are tightly regulated. SPL is a pivotal enzyme regulating S1P intracellular concentrations and a promising drug target for the design of immunosuppressants. We structurally and functionally characterized yeast SPL (Dpl1p) and its first prokaryotic homolog, from Symbiobacterium thermophilum. The Dpl1p structure served as a basis for a very reliable model of Homo sapiens SPL. The above results, together with in vitro and in vivo studies of SPL mutants, reveal which residues are involved in activity and substrate binding and pave the way to studies aimed at controlling the activity of this pivotal enzyme.

Sphingosine kinase localization in the control of sphingolipid metabolism

The sphingosine kinases (sphingosine kinase-1 and -2) have been implicated in a variety of physiological functions. Discerning their mechanism of action is complicated because in addition to producing the potent lipid second messenger sphingosine-1-phosphate, sphingosine kinases, both by producing sphingosine-1-phosphate and consuming sphingosine, have profound effects on sphingolipid metabolism. Sphingosine kinase-1 translocates to the plasma membrane upon agonist stimulation and this translocation is essential for the pro-oncogenic properties of this enzyme. Many of the enzymes of sphingolipid metabolism, including the enzymes that degrade sphingosine-1-phosphate, are membrane bound with restricted subcellular distributions. In the work described here we explore how subcellular localization of sphingosine kinase-1 affects the downstream metabolism of sphingosine-1-phosphate and the access of sphingosine kinase to its substrates. We find, surprisingly, that restricting sphingosine kinase to either the plasma membrane or the endoplasmic reticulum has a negligible effect on the rate of degradation of the sphingosine-1-phosphate that is produced. This suggests that sphingosine-1-phosphate is rapidly transported between membranes. However we also find that cytosolic or endoplasmic-reticulum targeted sphingosine kinase expressed at elevated levels produces extremely high levels of dihydrosphingosine-1-phosphate. Dihydrosphingosine is a proximal precursor in ceramide biosynthesis. Our data indicate that sphingosine kinase can divert substrate from the ceramide de novo synthesis pathway. However plasma membrane-restricted sphingosine kinase cannot access the pool of dihydrosphingosine. Therefore whereas sphingosine kinase localization does not affect downstream metabolism of sphingosine-1-phosphate, localization has an important effect on the pools of substrate to which this key signaling enzyme has access.

Intracellular localization of sphingosine kinase 1 alters access to substrate pools but does not affect the degradative fate of sphingosine-1-phosphate

Sphingosine kinase 1 (SK1) produces sphingosine-1-phosphate (S1P), a potent signaling lipid. The subcellular localization of SK1 can dictate its signaling function. Here, we use artificial targeting of SK1 to either the plasma membrane (PM) or the endoplasmic reticulum (ER) to test the effects of compartmentalization of SK1 on substrate utilization and downstream metabolism of S1P. Expression of untargeted or ER-targeted SK1, but surprisingly not PM-targeted SK1, results in a dramatic increase in the phosphorylation of dihydrosphingosine, a metabolic precursor in de novo ceramide synthesis. Conversely, knockdown of endogenous SK1 diminishes both dihydrosphingosine-1-phosphate and S1P levels. We tested the effects of SK1 localization on degradation of S1P by depletion of the ER-localized S1P phosphatases and lyase. Remarkably, S1P produced at the PM was degraded to the same extent as that produced in the ER. This indicates that there is an efficient mechanism for the transport of S1P from the PM to the ER. In acute labeling experiments, we find that S1P degradation is primarily driven by lyase cleavage of S1P. Counterintuitively, when S1P-specific phosphatases are depleted, acute labeling of S1P is significantly reduced, indicative of a phosphatase-dependent recycling process. We conclude that the localization of SK1 influences the substrate pools that it has access to and that S1P can rapidly translocate from the site where it is synthesized to other intracellular sites.

Disruption of sphingosine 1-phosphate lyase confers resistance to chemotherapy and promotes oncogenesis through Bcl-2/Bcl-xL upregulation

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid metabolite involved in cancer development through stimulation of cell survival, proliferation, migration, and angiogenesis. Irreversible degradation of S1P is catalyzed by S1P lyase (SPL). The human SGPL1 gene that encodes SPL maps to a region often mutated in cancers. To investigate the effect of SPL deficiency on cell survival and transformation, the susceptibility to anticancer drugs of fibroblasts generated from SPL-deficient mouse embryos (Sgpl1(-/-)) was compared with that of cells from heterozygous (Sgpl1(+/-)) or wild-type (Sgpl1(+/+)) embryos. First, loss of SPL caused resistance to the toxic effects of etoposide and doxorubicin. Interestingly, heterozygosity for the Sgpl1 gene resulted in partial resistance to apoptosis. Secondly, doxorubicin-induced apoptotic signaling was strongly inhibited in Sgpl1(-/-) cells (phosphatidylserine externalization, caspase activation, and cytochrome c release). This was accompanied by a strong increase in Bcl-2 and Bcl-xL protein content. Whereas correction of SPL deficiency in Sgpl1(-/-) cells led to downregulation of antiapoptotic proteins, Bcl-2 and Bcl-xL small interfering RNA-mediated knockdown in SPL-deficient cells resulted in increased sensitivity to doxorubicin, suggesting that Bcl-2 upregulation mediates SPL protective effects. Moreover, SPL deficiency led to increased cell proliferation, anchorage-independent cell growth, and formation of tumors in nude mice. Finally, transcriptomic studies showed that SPL expression is downregulated in human melanoma cell lines. Thus, by affecting S1P metabolism and the expression of Bcl-2 members, the loss of SPL enhances cell resistance to anticancer regimens and results in an increased ability of cells to acquire a transformed phenotype and become malignant.

An overview of sphingolipid metabolism: from synthesis to breakdown

Sphingolipids constitute a class of lipids defined by their eighteen carbon amino-alcohol backbones which are synthesized in the ER from nonsphingolipid precursors. Modification of this basic structure is what gives rise to the vast family of sphingolipids that play significant roles in membrane biology and provide many bioactive metabolites that regulate cell function. Despite the diversity of structure and function of sphingolipids, their creation and destruction are governed by common synthetic and catabolic pathways. In this regard, sphingolipid metabolism can be imagined as an array of interconnected networks that diverge from a single common entry point and converge into a single common breakdown pathway. In their simplest forms, sphingosine, phytosphingosine and dihydrosphingosine serve as the backbones upon which further complexity is achieved. For example, phosphorylation of the C1 hydroxyl group yields the final breakdown products and/or the important signaling molecules sphingosine-1-phosphate, phytosphingosine-1-phosphate and dihydrosphingosine-1-phosphate, respectively. On the other hand, acylation of sphingosine, phytosphingosine, or dihydrosphingosine with one of several possible acyl CoA molecules through the action of distinct ceramide synthases produces the molecules defined as ceramide, phytoceramide, or dihydroceramide. Ceramide, due to the differing acyl CoAs that can be used to produce it, is technically a class of molecules rather than a single molecule and therefore may have different biological functions depending on the acyl chain it is composed of. At the apex of complexity is the group of lipids known as glycosphingolipids (GSL) which contain dozens of different sphingolipid species differing by both the order and type of sugar residues attached to their headgroups. Since these molecules are produced from ceramide precursors, they too may have differences in their acyl chain composition, revealing an additional layer of variation. The glycosphingolipids are divided broadly into two categories: glucosphingolipids and galactosphingolipids. The glucosphingolipids depend initially on the enzyme glucosylceramide synthase (GCS) which attaches glucose as the first residue to the C1 hydroxyl position. Galactosphingolipids, on the other hand, are generated from galactosylceramide synthase (GalCerS), an evolutionarily dissimilar enzyme from GCS. Glycosphingolipids are further divided based upon further modification by various glycosyltransferases which increases the potential variation in lipid species by several fold. Far more abundant are the sphingomyelin species which are produced in parallel with glycosphingolipids, however they are defined by a phosphocholine headgroup rather than the addition of sugar residues. Although sphingomyelin species all share a common headgroup, they too are produced from a variety of ceramide species and therefore can have differing acyl chains attached to their C-2 amino groups. Whether or not the differing acyl chain lengths in SMs dictate unique functions or important biophysical distinctions has not yet been established. Understanding the function of all the existing glycosphingolipids and sphingomyelin species will be a major undertaking in the future since the tools to study and measure these species are only beginning to be developed (see Fig 1 for an illustrated depiction of the various sphingolipid structures). The simple sphingolipids serve both as the precursors and the breakdown products of the more complex ones. Importantly, in recent decades, these simple sphingolipids have gained attention for having significant signaling and regulatory roles within cells. In addition, many tools have emerged to measure the levels of simple sphingolipids and therefore have become the focus of even more intense study in recent years. With this thought in mind, this chapter will pay tribute to the complex sphingolipids, but focus on the regulation of simple sphingolipid metabolism.