Sphingoid base 1-phosphate phosphatase: a key regulator of sphingolipid metabolism and stress response

Early Life to Adult Brain Lipidome Dynamic: A Temporospatial Study Investigating Dietary Polar Lipid Supplementation Efficacy

The lipid composition of the brain is well regulated during development, and the specific temporospatial distribution of various lipid species is essential for the development of optimal neural functions. Dietary lipids are the main source of brain lipids and thus contribute to the brain lipidome. Human milk is the only source of a dietary lipids for exclusively breastfed infant. Notably, it contains milk fat globule membrane (MFGM) enriched in polar lipids (PL). While early life is a key for early brain development, the interplay between dietary intake of polar lipids and spatial dynamics of lipid distribution during brain development is poorly understood. Here, we carried out an exploratory study to assess the early postnatal temporal profiling of brain lipidome between postnatal day (PND) 7 and PND 50 using matrix-assisted laser desorption ionization as a mass spectrometry imaging (MALDI-MSI) in an in vivo preclinical model. We also assessed the effect of chronic supplementation with PL extracted from alpha-lactalbumin-enriched whey protein concentrate (WPC) containing 10% lipids, including major lipid classes found in the brain (37% phospholipids and 15% sphingomyelin). MALDI-MSI of the spatial and temporal accretion of lipid species during brain development showed that the brain lipidome is changing heterogeneously along time during brain development. In addition, increases in 400+ PL supplement-dependent lipids were observed. PL supplementation had significant spatial and temporal effect on specific fatty esters, glycerophosphocholines, glycerophosphoethanolamines, and phosphosphingolipids. Interestingly, the average levels of these lipids per brain area tended to be constant in various brain structures across the age groups, paralleling the general brain growth. In contrast, other lipids, such as cytidine diphosphate diacylglycerol, diacylglycerophosphates, phosphocholines, specific ether-phosphoethanolamines, phosphosphingolipids, glycerophosphoinositols, and glycerophosphoserines showed clear age-dependent changes uncoupled from the general brain growth. These results suggest that the dietary PL supplementation may preferentially provide the building blocks for the general brain growth during development. Our findings add to the understanding of brain-nutrient relations, their temporospatial dynamics, and potential impact on neurodevelopment.

Identification of two lipid phosphatases that regulate sphingosine-1-phosphate cellular uptake and recycling

Sphingosine-1-phosphate (S1P) is a sphingolipid metabolite that serves as a potent extracellular signaling molecule. Metabolic regulation of extracellular S1P levels impacts key cellular activities through altered S1P receptor signaling. Although the pathway through which S1P is degraded within the cell and thereby eliminated from reuse has been previously described, the mechanism used for S1P cellular uptake and the subsequent recycling of its sphingoid base into the sphingolipid synthesis pathway is not completely understood. To identify the genes within this S1P uptake and recycling pathway, we performed a genome-wide CRISPR/Cas9 KO screen using a positive-selection scheme with Shiga toxin, which binds a cell-surface glycosphingolipid receptor, globotriaosylceramide (Gb3), and causes lethality upon internalization. The screen was performed in HeLa cells with their sphingolipid de novo pathway disabled so that Gb3 cell-surface expression was dependent on salvage of the sphingoid base of S1P taken up from the medium. The screen identified a suite of genes necessary for S1P uptake and the recycling of its sphingoid base to synthesize Gb3, including two lipid phosphatases, PLPP3 (phospholipid phosphatase 3) and SGPP1 (S1P phosphatase 1). The results delineate a pathway in which plasma membrane–bound PLPP3 dephosphorylates extracellular S1P to sphingosine, which then enters cells and is rephosphorylated to S1P by the sphingosine kinases. This rephosphorylation step is important to regenerate intracellular S1P as a branch-point substrate that can be routed either for dephosphorylation to salvage sphingosine for recycling into complex sphingolipid synthesis or for degradation to remove it from the sphingolipid synthesis pathway.

The Ceramide Synthase Subunit Lac1 Regulates Cell Growth and Size in Fission Yeast

Cell division produces two viable cells of a defined size. Thus, all cells require mechanisms to measure growth and trigger cell division when sufficient growth has occurred. Previous data suggest a model in which growth rate and cell size are mechanistically linked by ceramide-dependent signals in budding yeast. However, the conservation of mechanisms that govern growth control is poorly understood. In fission yeast, ceramide synthase is encoded by two genes, Lac1 and Lag1. Here, we characterize them by using a combination of genetics, microscopy, and lipid analysis. We showed that Lac1 and Lag1 co-immunoprecipitate and co-localize at the endoplasmic reticulum. However, each protein generates different species of ceramides and complex sphingolipids. We further discovered that Lac1, but not Lag1, is specifically required for proper control of cell growth and size in Schizosaccharomyces pombe. We propose that specific ceramide and sphingolipid species produced by Lac1 are required for normal control of cell growth and size in fission yeast.

Membrane Contact Sites in Yeast: Control Hubs of Sphingolipid Homeostasis

Sphingolipids are the most diverse class of membrane lipids, in terms of their structure and function. Structurally simple sphingolipid precursors, such as ceramides, act as intracellular signaling molecules in various processes, including apoptosis, whereas mature and complex forms of sphingolipids are important structural components of the plasma membrane. Supplying complex sphingolipids to the plasma membrane, according to need, while keeping pro-apoptotic ceramides in check is an intricate task for the cell and requires mechanisms that tightly control sphingolipid synthesis, breakdown, and storage. As each of these processes takes place in different organelles, recent studies, using the budding yeast Saccharomyces cerevisiae, have investigated the role of membrane contact sites as hubs that integrate inter-organellar sphingolipid transport and regulation. In this review, we provide a detailed overview of the findings of these studies and put them into the context of established regulatory mechanisms of sphingolipid homeostasis. We have focused on the role of membrane contact sites in sphingolipid metabolism and ceramide transport, as well as the mechanisms that prevent toxic ceramide accumulation.

Lipid metabolism has been good to me

My career in research has flourished through hard work, supportive mentors, and outstanding mentees and collaborators. The Carman laboratory has contributed to the understanding of lipid metabolism through the isolation and characterization of key lipid biosynthetic enzymes as well as through the identification of the enzyme-encoding genes. Our findings from yeast have proven to be invaluable to understand regulatory mechanisms of human lipid metabolism. Several rewarding aspects of my career have been my service to the Journal of Biological Chemistry as an editorial board member and Associate Editor, the National Institutes of Health as a member of study sections, and national and international scientific meetings as an organizer. I advise early career scientists to not assume anything, acknowledge others’ accomplishments, and pay it forward.

Heat stress alters serum lipid metabolism of Chinese indigenous broiler chickens-a lipidomics study

Heat stress (HS) by high-temperature environment reduced the production performance of poultry and caused losses to the breeding industry. The present study was conducted to investigate the effects of HS on serum lipidomics in Chinese indigenous slow-growing broiler chickens (Huaixiang chickens). A total of 40 8-week-old female Huaixiang chickens were randomly allocated to two groups, including normal temperature (NT, fed basal diet) and HS (fed basal diet), and each group consisted of five replicates with four birds per replicate. NT and HS groups were exposed to 21.3 ± 1.2 °C and 32.5 ± 1.4 °C for 4 weeks, respectively. Serum lipidomics in broilers was determined by liquid chromatography-mass spectrometry (LC-MS)-based metabolomics. The results indicated that there were significant differences in metabolic spectra between the groups, and a total of 17 differential metabolites were screened. Compared with NT group, HS group reduced the serum ceramide (cer) (d18:1/22:0), cer (d18:1/24:1), cer (d20:2/22:2), lyso-phosphatidylcholine (LPC) (18:0), phosphatidylcholine (PC) (18:0/20:4), PC (15:0/23:4), PC (18:0/22:6), PC (18:2/18:2), phosphatidylethanolamine (PE) (18:1/18:1), polyethylene terephthalate (PEt) (37:3/8:0), phosphatidylglycerol (PG) (32:1/16:2), phosphatidyl methyl ethanolamine (PMe) (19:3/13:0), PMe (26:1/9:0), sphingomyelin (SM) (d16:0/18:1), triglycerides (TG) (18:0/18:1/18:2), and TG (19:4/21:6/21:6) levels [variable importance in the projection (VIP > 1 and P < 0.05)], while HS group increased serum PC (17:0/17:0) content (VIP > 1 and P < 0.05). Also, metabolic pathway analysis showed that the pathways of glycerolphospholipid, linoleic acid and α-linolenic acid metabolism, and glycosylphosphatidylinositol (GPI)-anchored biosynthesis were changed (P < 0.05). In conclusion, HS led to the disorders of serum lipid metabolism in broilers, and mainly downregulated serum content of phospholipids. These findings provide novel insights into the effects of HS on serum lipidomics in indigenous slow-growing chickens.

The influence of different polymer types of microplastics on adsorption, accumulation, and toxicity of triclosan in zebrafish

Although the combined effects of microplastics (MPs) and other organic pollutants have raised increasing attention, the impacts of polymer types on the biological effects (e.g., bioaccumulation and toxicity) of the mixtures are still unclear. This study aimed to evaluate the influence of different polymer types of MPs including polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) on the adsorption, accumulation, and toxic effects of triclosan (TCS) in zebrafish. As a result, all three types of MPs could adsorb TCS and PP-MPs has the highest adsorption capacity for TCS (1.18 mg/g). Compared with the TCS alone, MPs changed the distribution of TCS in tissues and increased the accumulation of TCS in the liver and gut following the order of TCS + PP > TCS + PVC > TCS + PE. Compared with individual TCS and PP-MPs, after co-exposed for 28 days, TCS + PP significantly aggravated oxidative stress and lipid peroxidation in the liver as well as enhanced neurotoxicity in the brain. Moreover, TCS + PP disturbed the metabolism in the liver and MPs contributed more to the metabolic disorders. The upregulated lipid metabolites (e.g., sphingosine and L-palmitoylcarnitine) and downregulated carbohydrate metabolites (e.g., sucrose) could be potential targets for future risk assessment of MPs combined with other pollutants.

Sphingosine-1-phosphate: From insipid lipid to a key regulator

It is a great honor to be asked to write a "Reflections" article by one of the true icons of biochemistry, Herb Tabor. I felt humbled, especially since it follows many written by biochemists I admire and whose contributions have shaped major advances in biochemistry and molecular biology in the last century. Here I present my personal reflections on my adventure with the bioactive sphingolipid metabolite sphingosine-1-phosphate intertwined with those of my family life as a wife, mother, and grandmother. These reflections brought back many memories of events in my early career that played significant roles in determining the path I have taken for more than 40 years and that brought much fun and satisfaction into my life. It has been an exciting journey so far, with many surprises along the way, that still continues.

The dynamics and role of sphingolipids in eukaryotic organisms upon thermal adaptation

All living beings have an optimal temperature for growth and survival. With the advancement of global warming, the search for understanding adaptive processes to climate changes has gained prominence. In this context, all living beings monitor the external temperature and develop adaptive responses to thermal variations. These responses ultimately change the functioning of the cell and affect the most diverse structures and processes. One of the first structures to detect thermal variations is the plasma membrane, whose constitution allows triggering of intracellular signals that assist in the response to temperature stress. Although studies on this topic have been conducted, the underlying mechanisms of recognizing thermal changes and modifying cellular functioning to adapt to this condition are not fully understood. Recently, many reports have indicated the participation of sphingolipids (SLs), major components of the plasma membrane, in the regulation of the thermal stress response. SLs can structurally reinforce the membrane or/and send signals intracellularly to control numerous cellular processes, such as apoptosis, cytoskeleton polarization, cell cycle arresting and fungal virulence. In this review, we discuss how SLs synthesis changes during both heat and cold stresses, focusing on fungi, plants, animals and human cells. The role of lysophospholipids is also discussed.

Sphingolipids and the link between alcohol and cancer

Epidemiological evidence underscores alcohol consumption as a strong risk factor for multiple cancer types, with liver cancer being most commonly associated with alcohol intake. While mechanisms linking alcohol consumption to malignant tumor development are not fully understood, the likely players in ethanol-induced carcinogenesis are genotoxic stress caused by formation of acetaldehyde, increased oxidative stress, and altered nutrient metabolism, including the impairment of methyl transfer reactions. Alterations of sphingolipid metabolism and associated signaling pathways are another potential link between ethanol and cancer development. In particular, ceramides are involved in the regulation of cellular proliferation, differentiation, senescence, and apoptosis and are known to function as important regulators of malignant transformation as well as tumor progression. However, to date, the cross-talk between ceramides and alcohol in cancer disease is largely an open question and only limited data are available on this subject. Most studies linking ceramide to cancer considered liver steatosis as the underlying mechanism, which is not surprising taking into consideration that ceramide pathways are an integral part of the overall lipid metabolism. This review summarizes the latest studies pointing to ceramide as an important mediator of cancer-promoting effects of chronic alcohol consumption and underscores the necessity of understanding the role of sphingolipids and lipid signaling in response to alcohol in order to prevent and/or successfully manage diseases caused by alcohol.

Pho85 and PI(4,5)P2 regulate different lipid metabolic pathways in response to cold

Lipid homeostasis allows cells to adjust membrane biophysical properties in response to changes in environmental conditions. In the yeast Saccharomyces cerevisiae, a downward shift in temperature from an optimal reduces membrane fluidity, which triggers a lipid remodeling of the plasma membrane. How changes in membrane fluidity are perceived, and how the abundance and composition of different lipid classes is properly balanced, remain largely unknown. Here, we show that the levels of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2], the most abundant plasma membrane phosphoinositide, drop rapidly in response to a downward shift in temperature. This change triggers a signaling cascade transmitted to cytosolic diphosphoinositol phosphate derivatives, among them 5-PP-IP4 and 1-IP7, that exert regulatory functions on genes involved in the inositol and phospholipids (PLs) metabolism, and inhibit the activity of the protein kinase Pho85. Consistent with this, cold exposure triggers a specific program of neutral lipids and PLs changes. Furthermore, we identified Pho85 as playing a key role in controlling the synthesis of long-chain bases (LCBs) via the Ypk1-Orm2 regulatory circuit. We conclude that Pho85 orchestrates a coordinated response of lipid metabolic pathways that ensure yeast thermal adaptation.

Ceramide-Rubusoside Nanomicelles, a Potential Therapeutic Approach to Target Cancers Carrying p53 Missense Mutations

Ceramide (Cer) is an active cellular sphingolipid that can induce apoptosis or proliferation-arrest of cancer cells. Nanoparticle-based delivery offers an effective approach for overcoming bioavailability and biopharmaceutics issues attributable to the pronounced hydrophobicity of Cer. Missense mutations of the protein p53, which have been detected in approximately 42% of cancer cases, not only lose the tumor suppression activity of wild-type p53, but also gain oncogenic functions promoting tumor progression and drug resistance. Our previous works showed that cellular Cer can eradicate cancer cells that carry a p53 deletion-mutation by modulating alternative pre-mRNA splicing, restoring wild-type p53 protein expression. Here, we report that new ceramide-rubusoside (Cer-RUB) nanomicelles considerably enhance Cer in vivo bioavailability and restore p53-dependent tumor suppression in cancer cells carrying a p53 missense mutation. Natural RUB encapsulated short-chain C₆-Cer so as to form Cer-RUB nanomicelles (∼32 nm in diameter) that substantially enhanced Cer solubility and its levels in tissues and tumors of mice dosed intraperitoneally. Intriguingly, Cer-RUB nanomicelle treatments restored p53-dependent tumor suppression and sensitivity to cisplatin in OVCAR-3 ovarian cancer cells and xenograft tumors carrying p53 R248Q mutation. Moreover, Cer-RUB nanomicelles showed no signs of significant nonspecific toxicity to noncancerous cells or normal tissues, including bone marrow. Furthermore, Cer-RUB nanomicelles restored p53 phosphorylated protein and downstream function to wild-type levels in p53 R172H^/+ transgenic mice. Altogether, this study, for the first time, indicates that natural Cer-RUB nanomicelles offer a feasible approach for efficaciously and safely targeting cancers carrying p53 missense mutations.

Serine-Dependent Sphingolipid Synthesis Is a Metabolic Liability of Aneuploid Cells

Aneuploidy disrupts cellular homeostasis. However, the molecular mechanisms underlying the physiological responses and adaptation to aneuploidy are not well understood. Deciphering these mechanisms is important because aneuploidy is associated with diseases, including intellectual disability and cancer. Although tumors and mammalian aneuploid cells, including several cancer cell lines, show altered levels of sphingolipids, the role of sphingolipids in aneuploidy remains unknown. Here, we show that ceramides and long-chain bases, sphingolipid molecules that slow proliferation and promote survival, are increased by aneuploidy. Sphingolipid levels are tightly linked to serine synthesis, and inhibiting either serine or sphingolipid synthesis can specifically impair the fitness of aneuploid cells. Remarkably, the fitness of aneuploid cells improves or deteriorates upon genetically decreasing or increasing ceramides, respectively. Combined targeting of serine and sphingolipid synthesis could be exploited to specifically target cancer cells, the vast majority of which are aneuploid.

The role of sphingosine-1-phosphate in skeletal muscle: Physiology, mechanisms, and clinical perspectives

Sphingolipids were discovered more than a century ago and were simply considered as a class of cell membrane lipids for a long time. However, after the discovery of several intracellular functions and their role in the control of many physiological and pathophysiological conditions, these molecules have gained much attention. For instance, the sphingosine-1-phosphate (S1P) is a circulating bioactive sphingolipid capable of triggering strong intracellular reactions through the family of S1P receptors (S1PRs) spread in several cell types and tissues. Recently, the role of S1P in the control of skeletal muscle metabolism, atrophy, regeneration, and metabolic disorders has been widely investigated. In this review, we summarized the knowledge of S1P and its effects in skeletal muscle metabolism, highlighting the role of S1P/S1PRs axis in skeletal muscle regeneration, fatigue, ceramide accumulation, and insulin resistance. Finally, we discussed the physical exercise role in S1P/S1PRs signaling in skeletal muscle cells, and how this nonpharmacological strategy may be prospective for future investigations due to its ability to increase S1P levels.

Ceramide Signaling and p53 Pathways

Ceramides, important players in signal transduction, interact with multiple cellular pathways, including p53 pathways. However, the relationship between ceramide and p53 is very complex, and mechanisms underlying their coregulation are diverse and not fully characterized. The role of p53, an important cellular regulator and a transcription factor, is linked to its tumor suppressor function. Ceramides are involved in the regulation of fundamental processes in cancer cells including cell death, proliferation, autophagy, and drug resistance. This regulation, however, can be pro-death or pro-survival depending on cancer type, the balance between ceramide species, the rate of their synthesis and utilization, and the availability of a specific array of downstream targets. This chapter highlights the central role of ceramide in sphingolipid metabolism, its role in cancer, specific effectors in ceramide pathways controlled by p53, and coregulation of ceramide and p53 signaling. We discuss the recent studies, which underscore the function of p53 in the regulation of ceramide pathways and the reciprocal regulation of p53 by ceramide. This complex relationship is based on several molecular mechanisms including the p53-dependent transcriptional regulation of enzymes in sphingolipid pathways, the activation of mutant p53 through ceramide-mediated alternative splicing, as well as modulation of the p53 function through direct and indirect effects on p53 coregulators and downstream targets. Further insight into the connections between ceramide and p53 will allow simultaneous targeting of the two pathways with a potential to yield more efficient anticancer therapeutics.

Nuclear Translocation of SGPP-1 and Decrease of SGPL-1 Activity Contribute to Sphingolipid Rheostat Regulation of Inflammatory Dendritic Cells

A balanced sphingolipid rheostat is indispensable for dendritic cell function and survival and thus initiation of an immune response. Sphingolipid levels are dynamically maintained by the action of sphingolipid enzymes of which sphingosine kinases, S1P phosphatases (SGPP-1/2) and S1P lyase (SGPL-1), are pivotal in the balance of S1P and sphingosine levels. In this study, we present that SGPP-1 and SGPL-1 are regulated in inflammatory dendritic cells and contribute to S1P fate. TLR-dependent activation caused SGPL-1 protein downregulation with subsequent decrease of enzymatic activity by two-thirds. In parallel, confocal fluorescence microscopy revealed that endogenous SGPP-1 was expressed in nuclei of naive dendritic cells and was translocated into the cytoplasmatic compartment upon inflammatory stimulation resulting in dephosphorylation of S1P. Mass spectrometric determination showed that a part of the resulting sphingosine was released from the cell, increasing extracellular levels. Another route of diminishing intracellular S1P was possibly taken by its export via ATP-binding cassette transporter C1 which was upregulated in array analysis, while the S1P transporter, spinster homolog 2, was not relevant in dendritic cells. These investigations newly describe the sequential expression and localization of the endogenous S1P regulators SGPP-1 and SGPL-1 and highlight their contribution to the sphingolipid rheostat in inflammation.

Sphingolipid Metabolism of a Sea Anemone Is Altered by the Presence of Dinoflagellate Symbionts

In host-microbe interactions, signaling lipids function in interpartner communication during both the establishment and maintenance of associations. Previous evidence suggests that sphingolipids play a role in the mutualistic cnidarian-Symbiodinium symbiosis. Exogenously applied sphingolipids have been shown to alter this partnership, though endogenous host regulation of sphingolipids by the sphingosine rheostat under different symbiotic conditions has not been characterized. The rheostat regulates levels of pro-survival sphingosine-1-phosphate (S1P) and pro-apoptotic sphingosine (Sph) through catalytic activities of sphingosine kinase (SPHK) and S1P phosphatase (SGPP). The role of the rheostat in recognition and establishment of cnidarian-Symbiodinium symbiosis was investigated in the sea anemone Aiptasia pallida by measuring gene expression, protein levels, and sphingolipid metabolites in symbiotic, aposymbiotic, and newly recolonized anemones. Comparison of two host populations showed that symbiotic animals from one population had lower SGPP gene expression and Sph lipid concentrations compared to aposymbiotic animals, while the other population had higher S1P concentrations than their aposymbiotic counterparts. In both populations, the host rheostat trended toward host cell survival in the presence of symbionts. Furthermore, upregulation of both rheostat enzymes on the first day of host recolonization by symbionts suggests a role for the rheostat in host-symbiont recognition during symbiosis onset. Collectively, these data suggest a regulatory role of sphingolipid signaling in cnidarian-Symbiodinium symbiosis and symbiont uptake.

Induction of Sphk1 activity in obese adipose tissue macrophages promotes survival

During obesity, adipose tissue macrophages (ATM) are increased in concert with local inflammation and insulin resistance. Since the levels of sphingolipid (SLs) in adipose tissue (AT) are altered during obesity we investigated the potential impact of SLs on ATMs. For this, we first analyzed expression of SL metabolizing genes in ATMs isolated from obese mice. A marked induction of sphingosine kinase 1 (Sphk1) expression was observed in obese ATM when compared to lean ATM. This induction was observed in both MGL-ve (M1) and MGL1+ve (M2) macrophages from obese WAT. Next, RAW264.7 cells were exposed to excessive palmitate, resulting in a similar induction of Sphk1. This Sphk1 induction was also observed when cells were treated with chloroquine, a lysosomotropic amine impacting lysosome function. Simultaneous incubation of RAW cells with palmitate and the Sphk1 inhibitor SK1-I promoted cell death, suggesting a protective role of Sphk1 during lipotoxic conditions. Interestingly, a reduction of endoplasmic reticulum (ER) stress related genes was detected in obese ATM and was found to be associated with elevated Sphk1 expression. Altogether, our data suggest that lipid overload in ATM induces Sphk1, which promotes cell viability.

The role of sphingosine-1-phosphate in the tumor microenvironment and its clinical implications

Elucidating the interaction between cancer and non-cancer cells, such as blood vessels, immune cells, and other stromal cells, in the tumor microenvironment is imperative in understanding the mechanisms underlying cancer progression and metastasis, which is expected to lead to the development of new therapeutics. Sphingosine-1-phosphate is a bioactive lipid mediator that promotes cell survival, proliferation, migration, angiogenesis/lymphangiogenesis, and immune responsiveness, which are all factors involved in cancer progression. Sphingosine-1-phosphate is generated inside cancer cells by sphingosine kinases and then exported into the tumor microenvironment. Although sphingosine-1-phosphate is anticipated to play an important role in the tumor microenvironment and cancer progression, determining sphingosine-1-phosphate levels in the tumor microenvironment has been difficult due to a lack of established methods. We have recently developed a method to measure sphingosine-1-phosphate levels in the interstitial fluid that bathes cancer cells in the tumor microenvironment, and reported that high levels of sphingosine-1-phosphate exist in the tumor interstitial fluid. Importantly, sphingosine-1-phosphate can be secreted from cancer cells and non-cancer components such as immune cells and vascular/lymphatic endothelial cells in the tumor microenvironment. Furthermore, sphingosine-1-phosphate affects both cancer and non-cancer cells in the tumor microenvironment promoting cancer progression. Here, we review the roles of sphingosine-1-phosphate in the interaction between cancer and non-cancer cells in tumor microenvironment, and discuss future possibilities for targeted therapies against sphingosine-1-phosphate signaling for cancer patients.

Genome-wide analysis suggests divergent evolution of lipid phosphotases/phosphotransferase genes in plants

Genes of the LPPT (lipid phosphatase/phosphotransferase) family play important roles in lipid phosphorous transfer and triacylglycerol accumulation in plants. To provide overviews of the plant LPPT family and their overall relationships, here we carried out genome-wide identifications and analyses of plant LPPT family members. A total of 643 putative LPPT genes were identified from 48 sequenced plant genomes, among which 205 genes from 14 plants were chosen for further analyses. Plant LPPT genes belonged to three distinctive groups, namely the LPT (lipid phosphotransfease), LPP (lipid phosphatase), and pLPP (plastidic lipid phosphotransfease) groups. Genes of the LPT group could be further partitioned into three groups, two of which were only identified in terrestrial plants. Genes in the LPP and pLPP groups experienced duplications in early stages of plant evolution. Among 17 Zea mays LPPT genes, divergence of temporal-spatial expression patterns was revealed based on microarray data analysis. Peptide sequences of plant LPPT genes harbored different conserved motifs. A test of Branch Model versus One-ratio Model did not support significant selective pressures acting on different groups of LPPT genes, although quite different nonsynonymous evolutionary rates and selective pressures were observed. The complete picture of the plant LPPT family provided here should facilitate further investigations of plant LPPT genes and offer a better understanding of lipid biosynthesis in plants.

The sphingosine rheostat is involved in the cnidarian heat stress response but not necessarily in bleaching

Sphingolipids play important roles in mitigating cellular heat and oxidative stress by altering membrane fluidity, receptor clustering and gene expression. Accumulation of signaling sphingolipids that comprise the sphingosine rheostat, pro-apoptotic sphingosine (Sph) and pro-survival sphingosine-1-phosphate (S1P), is key to determining cell fate. Reef-building corals and other symbiotic cnidarians living in shallow tropical waters can experience elevated seawater temperature and high UV irradiance, two stressors that are increasing in frequency and severity with climate change. In symbiotic cnidarians, these stressors disrupt the photosynthetic machinery of the endosymbiont and ultimately result in the collapse of the partnership (dysbiosis), known as cnidarian bleaching. In a previous study, exogenously applied sphingolipids altered heat-induced bleaching in the symbiotic anemone Aiptasia pallida, but endogenous regulation of these lipids is unknown. Here, we characterized the role of the rheostat in the cnidarian heat stress response (HSR) and in dysbiosis. Gene expression of rheostat enzymes sphingosine kinase (AP-SPHK) and S1P phosphatase (AP-SGPP), and concentrations of sphingolipids were quantified from anemones incubated at elevated temperatures. We observed a biphasic HSR in A. pallida. At early exposure, rheostat gene expression and lipid levels were suppressed while gene expression of a heat stress biomarker increased and 40% of symbionts were lost. After longer incubations at the highest temperature, AP-SGPP and then Sph levels both increased. These results indicate that the sphingosine rheostat in A. pallida does not participate in initiation of dysbiosis, but instead functions in the chronic response to prolonged heat stress that promotes host survival.

Accumulation of long-chain bases in yeast promotes their conversion to a long-chain base vinyl ether

Long-chain bases (LCBs) are the precursors to ceramide and sphingolipids in eukaryotic cells. They are formed by the action of serine palmitoyl-CoA transferase (SPT), a complex of integral membrane proteins located in the endoplasmic reticulum. SPT activity is negatively regulated by Orm proteins to prevent the toxic overaccumulation of LCBs. Here we show that overaccumulation of LCBs in yeast results in their conversion to a hitherto undescribed LCB derivative, an LCB vinyl ether. The LCB vinyl ether is predominantly formed from phytosphingosine (PHS) as revealed by conversion of odd chain length tracers C17-dihydrosphingosine and C17-PHS into the corresponding LCB vinyl ether derivative. PHS vinyl ether formation depends on ongoing acetyl-CoA synthesis, and its levels are elevated when the LCB degradative pathway is blocked by deletion of the major LCB kinase, LCB4, or the LCB phosphate lyase, DPL1. PHS vinyl ether formation thus appears to constitute a shunt for the LCB phosphate- and lyase-dependent degradation of LCBs. Consistent with a role of PHS vinyl ether formation in LCB detoxification, the lipid is efficiently exported from the cells.

A method for analysis and design of metabolism using metabolomics data and kinetic models: Application on lipidomics using a novel kinetic model of sphingolipid metabolism

We present a model-based method, designated Inverse Metabolic Control Analysis (IMCA), which can be used in conjunction with classical Metabolic Control Analysis for the analysis and design of cellular metabolism. We demonstrate the capabilities of the method by first developing a comprehensively curated kinetic model of sphingolipid biosynthesis in the yeast Saccharomyces cerevisiae. Next we apply IMCA using the model and integrating lipidomics data. The combinatorial complexity of the synthesis of sphingolipid molecules, along with the operational complexity of the participating enzymes of the pathway, presents an excellent case study for testing the capabilities of the IMCA. The exceptional agreement of the predictions of the method with genome-wide data highlights the importance and value of a comprehensive and consistent engineering approach for the development of such methods and models. Based on the analysis, we identified the class of enzymes regulating the distribution of sphingolipids among species and hydroxylation states, with the D-phospholipase SPO14 being one of the most prominent. The method and the applications presented here can be used for a broader, model-based inverse metabolic engineering approach.

Intramolecular Diels-Alder (IMDA) Studies toward the Synthesis of Australifungin. Stereocontrol in the Acetate Aldol Reaction of β,β'-Branched Aldehydes

Studies of australifungin illustrate an enantiocontrolled synthesis of the trans-decalin core 28 via an intramolecular [4π + 2π] cycloaddition. This strategy utilizes the nitroalkene dienophile of 27 as a surrogate ketene equivalent. Stereocontrol at C-2 is critically important for an effective intramolecular Diels-Alder (IMDA) process. Our studies report high asymmetric induction using a nonracemic Duthaler titanium enolate in the acetate aldol reaction with β,β'-branched aldehyde 13 to introduce the required C-2 chirality.

Lipid phosphate phosphatases and their roles in mammalian physiology and pathology

Lipid phosphate phosphatases (LPPs) are a group of enzymes that belong to a phosphatase/phosphotransferase family. Mammalian LPPs consist of three isoforms: LPP1, LPP2, and LPP3. They share highly conserved catalytic domains and catalyze the dephosphorylation of a variety of lipid phosphates, including phosphatidate, lysophosphatidate (LPA), sphingosine 1-phosphate (S1P), ceramide 1-phosphate, and diacylglycerol pyrophosphate. LPPs are integral membrane proteins, which are localized on plasma membranes with the active site on the outer leaflet. This enables the LPPs to degrade extracellular LPA and S1P, thereby attenuating their effects on the activation of surface receptors. LPP3 also exhibits noncatalytic effects at the cell surface. LPP expression on internal membranes, such as endoplasmic reticulum and Golgi, facilitates the metabolism of internal lipid phosphates, presumably on the luminal surface of these organelles. This action probably explains the signaling effects of the LPPs, which occur downstream of receptor activation. The three isoforms of LPPs show distinct and nonredundant effects in several physiological and pathological processes including embryo development, vascular function, and tumor progression. This review is intended to present an up-to-date understanding of the physiological and pathological consequences of changing the activities of the different LPPs, especially in relation to cell signaling by LPA and S1P.

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.

Functional analysis of lipid metabolism genes in wine yeasts during alcoholic fermentation at low temperature

Wine produced by low-temperature fermentation is mostly considered to have improved sensory qualities. However few commercial wine strains available on the market are well-adapted to ferment at low temperature (10 - 15°C). The lipid metabolism of Saccharomyces cerevisiae plays a central role in low temperature adaptation. One strategy to modify lipid composition is to alter transcriptional activity by deleting or overexpressing the key genes of lipid metabolism. In a previous study, we identified the genes of the phospholipid, sterol and sphingolipid pathways, which impacted on growth capacity at low temperature. In the present study, we aimed to determine the influence of these genes on fermentation performance and growth during low-temperature wine fermentations. We analyzed the phenotype during fermentation at the low and optimal temperature of the lipid mutant and overexpressing strains in the background of a derivative commercial wine strain. The increase in the gene dosage of some of these lipid genes, e.g., PSD1, LCB3, DPL1 and OLE1, improved fermentation activity during low-temperature fermentations, thus confirming their positive role during wine yeast adaptation to cold. Genes whose overexpression improved fermentation activity at 12°C were overexpressed by chromosomal integration into commercial wine yeast QA23. Fermentations in synthetic and natural grape must were carried out by this new set of overexpressing strains. The strains overexpressing OLE1 and DPL1 were able to finish fermentation before commercial wine yeast QA23. Only the OLE1 gene overexpression produced a specific aroma profile in the wines produced with natural grape must.

TORC2-dependent protein kinase Ypk1 phosphorylates ceramide synthase to stimulate synthesis of complex sphingolipids

Plasma membrane lipid composition must be maintained during growth and under environmental insult. In yeast, signaling mediated by TOR Complex 2 (TORC2)-dependent protein kinase Ypk1 controls lipid abundance and distribution in response to membrane stress. Ypk1, among other actions, alleviates negative regulation of L-serine:palmitoyl-CoA acyltransferase, upregulating production of long-chain base precursors to sphingolipids. To explore other roles for TORC2-Ypk1 signaling in membrane homeostasis, we devised a three-tiered genome-wide screen to identify additional Ypk1 substrates, which pinpointed both catalytic subunits of the ceramide synthase complex. Ypk1-dependent phosphorylation of both proteins increased upon either sphingolipid depletion or heat shock and was important for cell survival. Sphingolipidomics, other biochemical measurements and genetic analysis demonstrated that these modifications of ceramide synthase increased its specific activity and stimulated channeling of long-chain base precursors into sphingolipid end-products. Control at this branch point also prevents accumulation of intermediates that could compromise cell growth by stimulating autophagy.

DOI:http://dx.doi.org/10.7554/eLife.03779.001

Cells are enclosed by a plasma membrane that separates and protects each cell from its environment. These membranes are made of a variety of proteins and fatty molecules called lipids, which are carefully organized throughout the membrane. When cells experience stresses such as heat or excessive pressure, the plasma membrane changes to help protect the cell. In particular, more of a group of lipids called sphingolipids are incorporated into the membrane under stress conditions.

In yeast cells, a protein called Ypk1 plays an important role in protecting the cell from stress. Ypk1 controls the activity of a number of proteins that are responsible for balancing the amounts of different types of lipids in cell membranes. The combined action of these Ypk1-dependent proteins leads to the remodelling of the cell membrane to protect against stress. While several proteins that work with Ypk1 are known, some of the changes that serve to protect the plasma membrane cannot be explained by the action of these proteins alone.

To provide a more comprehensive picture of how Ypk1 helps cells to respond to changes in the environment, Muir et al. developed a new approach that combines biochemical, genetic and bioinformatics techniques to survey the yeast genome for proteins that could be Ypk1 targets. Muir et al. first produced a list of potential candidate proteins by searching for proteins with features similar to known Ypk1 targets, and then considered those that are known to be involved in processes that also involve Ypk1. To filter the potential targets further, Muir et al. performed experiments in yeast cells to see which proteins prevented normal cell growth if they were over-produced. Further experiments investigating which of these proteins interact with Ypk1 when purified identified 12 new proteins that are most likely targets of the Ypk1 protein.

Two of these newly identified Ypk1 target proteins form part of an enzyme complex called ceramide synthase, which produces a family of waxy lipid molecules from which more complex sphingolipids are built. Muir et al. discovered that during stress, Ypk1 enhances the activity of the ceramide synthase enzyme, which increases lipid production and the amount of sphingolipid deposited in the cell membrane. If this process is interrupted at any stage, cells struggle to survive under stress conditions.

The other candidate proteins identified by Muir et al. remain to be validated and characterized as Ypk1 targets. Nevertheless, the techniques used have conclusively identified some new Ypk1 targets and could also be applied to similar searches for proteins targeted in other biological processes.

DOI:http://dx.doi.org/10.7554/eLife.03779.002

Sphingolipids and mitochondrial function, lessons learned from yeast

Mitochondrial dysfunction is a hallmark of several neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease, but also of cancer, diabetes and rare diseases such as Wilson's disease (WD) and Niemann Pick type C1 (NPC). Mitochondrial dysfunction underlying human pathologies has often been associated with an aberrant cellular sphingolipid metabolism. Sphingolipids (SLs) are important membrane constituents that also act as signaling molecules. The yeast Saccharomyces cerevisiae has been pivotal in unraveling mammalian SL metabolism, mainly due to the high degree of conservation of SL metabolic pathways. In this review we will first provide a brief overview of the major differences in SL metabolism between yeast and mammalian cells and the use of SL biosynthetic inhibitors to elucidate the contribution of specific parts of the SL metabolic pathway in response to for instance stress. Next, we will discuss recent findings in yeast SL research concerning a crucial signaling role for SLs in orchestrating mitochondrial function, and translate these findings to relevant disease settings such as WD and NPC. In summary, recent research shows that S. cerevisiae is an invaluable model to investigate SLs as signaling molecules in modulating mitochondrial function, but can also be used as a tool to further enhance our current knowledge on SLs and mitochondria in mammalian cells.

Construction of a Yeast Strain with Regulatable Phospholipid Synthesis for Analysis of the Uptake and Metabolism of Phosphatidylethanolamine with Short Acyl Chains

We constructed a yeast stain, TKY12Ga, in which phosphatidylethanolamine (PE) synthesis can be controlled by means of the carbon source in the medium. When PE synthesis was blocked, its growth was inhibited. However, in the presence of exogenous didecanoyl PE (diC10PE), TKY12Ga grew despite an inability to synthesize PE. Our system, which employs TKY12Ga strain and diC10PE, provides a valuable tool to study the transport and metabolism of PE in yeast.

Yeast phospholipid biosynthesis is linked to mRNA localization

Localization of mRNAs and local translation are universal features in eukaryotes and contribute to cellular asymmetry and differentiation. In Saccharomyces cerevisiae, localization of mRNAs that encode membrane proteins requires the She protein machinery including the RNA-binding protein She2p as well as movement of the cortical endoplasmic reticulum (cER) to the yeast bud. In a screen for ER-specific proteins necessary for directional transport of WSC2 and EAR1 mRNAs, we have identified enzymes of the phospholipid metabolism. Loss of the phospholipid methyltransferase Cho2p, which showed the strongest impact on mRNA localization, disturbs mRNA localization as well as ER morphology and segregation due to an increase in cellular phosphatidylethanolamine (PE). Mislocalized mRNPs containing She2p co-localize with aggregated cER structures suggesting entrapment of mRNA and She2p by the elevated PE level, which is confirmed by elevated binding of She2p to PE-containing liposomes. These findings underscore the importance of ER membrane integrity in mRNA transport.

Lipids of mitochondria

A unique organelle for studying membrane biochemistry is the mitochondrion whose functionality depends on a coordinated supply of proteins and lipids. Mitochondria are capable of synthesizing several lipids autonomously such as phosphatidylglycerol, cardiolipin and in part phosphatidylethanolamine, phosphatidic acid and CDP-diacylglycerol. Other mitochondrial membrane lipids such as phosphatidylcholine, phosphatidylserine, phosphatidylinositol, sterols and sphingolipids have to be imported. The mitochondrial lipid composition, the biosynthesis and the import of mitochondrial lipids as well as the regulation of these processes will be main issues of this review article. Furthermore, interactions of lipids and mitochondrial proteins which are highly important for various mitochondrial processes will be discussed. Malfunction or loss of enzymes involved in mitochondrial phospholipid biosynthesis lead to dysfunction of cell respiration, affect the assembly and stability of the mitochondrial protein import machinery and cause abnormal mitochondrial morphology or even lethality. Molecular aspects of these processes as well as diseases related to defects in the formation of mitochondrial membranes will be described.

Prognostic significance of sphingosine kinase 2 expression in non-small cell lung cancer

Sphingosine kinase 2 (SphK2) as a conserved lipid kinase has not been thoroughly elucidated in non-small cell lung cancer (NSCLC). The aim of the present study was to evaluate the expression of SphK2 in NSCLC tissues and to determine its correlation with clinicopathologic characteristics and its impact on patient prognosis. We assessed the expression of SphK2 and proliferating cell nuclear antigen (PCNA) (as a proliferative index) by immunohistochemistry in 180 NSCLC patient's formalin-fixed paraffin-embedded tissue blocks. Relationship between the expression of SphK2 and PCNA and various clinicopathological features in these patients was evaluated. We detected that expression of SphK2 was gradually upregulated from normal, metaplasia/dysplasia tissues to NSCLC tissues. At the same time, PCNA expression followed a similar pattern. Statistical analysis showed that expression of SphK2 in NSCLC tissues was strongly associated with PCNA expression, histology grade, live vaccine strain invasion, lymph node status, clinical stage, tumors size, and histology type. Patients with SphK2 overexpression in their tissues had lower overall survival (OS) and disease-free survival (DFS) rates than those with low SphK2 expression. Using uni- and multivariate analysis, we found that SphK2 overexpression was an independent prognostic factor for both OS and DFS. The expression of SphK2 parallels the progression of NSCLC, and SphK2 overexpression may represent a novel and potentially independent biomarker for the prognosis of patients with NSCLC.

Coordination of rapid sphingolipid responses to heat stress in yeast

The regulatory roles of sphingolipids in diverse cell functions have been characterized extensively. However, the dynamics and interactions among the different sphingolipid species are difficult to assess, because de novo biosynthesis, metabolic inter-conversions, and the retrieval of sphingolipids from membranes form a complex, highly regulated pathway system. Here we analyze the heat stress response of this system in the yeast Saccharomyces cerevisiae and demonstrate how the cell dynamically adjusts its enzyme profile so that it is appropriate for operation under stress conditions before changes in gene expression become effective. The analysis uses metabolic time series data, a complex mathematical model, and a custom-tailored optimization strategy. The results demonstrate that all enzyme activities rapidly increase in an immediate response to the elevated temperature. After just a few minutes, different functional clusters of enzymes follow distinct activity patterns. Interestingly, starting after about six minutes, both de novo biosynthesis and all exit routes from central sphingolipid metabolism become blocked, and the remaining metabolic activity consists entirely of an internal redistribution among different sphingoid base and ceramide pools. After about 30 minutes, heat stress is still in effect and the enzyme activity profile is still significantly changed. Importantly, however, the metabolites have regained concentrations that are essentially the same as those under optimal conditions.

Sphingolipids play dual roles by serving as components of membrane rafts and by regulating numerous key cell functions. Although sphingolipids have been studied extensively, the details of their functioning are difficult to understand, because their synthesis, pathways of inter-conversion, and utilization constitute a complex, dynamically changing system. We analyze the role of yeast sphingolipids in the response to heat stress. Data show that the profile of these lipids changes almost immediately with the initiation of the stress, but it is a priori unclear how this response is organized. Using experimental data, a sophisticated dynamic model, and a novel optimization strategy, we show how changes in enzyme activities are temporally organized. Intriguingly, the results show that the cells take up as much material as possible in the first few minutes of heat stress and then shut down entry and exit routes of the biosynthetic pathway system. After about 30 minutes, when heat stress is still in effect, the enzyme activity profile is still significantly changed, but the metabolites have regained concentrations that are essentially the same as those under optimal conditions. The results demonstrate how novel insights are achievable with an effective combination of experimental and theoretical research.

Sphingosine-1-phosphate phosphatase 1 regulates keratinocyte differentiation and epidermal homeostasis

Sphingosine 1-phosphate (S1P) is a bioactive lipid whose levels are tightly regulated by its synthesis and degradation. Intracellularly, S1P is dephosphorylated by the actions of two S1P-specific phosphatases, sphingosine-1-phosphate phosphatases 1 and 2. To identify the physiological functions of S1P phosphatase 1, we have studied mice with its gene, Sgpp1, deleted. Sgpp1(-/-) mice appeared normal at birth, but during the 1st week of life they exhibited stunted growth and suffered desquamation, with most dying before weaning. Both Sgpp1(-/-) pups and surviving adults exhibited multiple epidermal abnormalities. Interestingly, the epidermal permeability barrier developed normally during embryogenesis in Sgpp1(-/-) mice. Keratinocytes isolated from the skin of Sgpp1(-/-) pups had increased intracellular S1P levels and displayed a gene expression profile that indicated overexpression of genes associated with keratinocyte differentiation. The results reveal S1P metabolism as a regulator of keratinocyte differentiation and epidermal homeostasis.

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.

Non-genomic progesterone signalling and its non-canonical receptor

The steroid hormone progesterone regulates many critical aspects of vertebrate physiology. The nuclear receptor for progesterone functions as a ligand-activated transcription factor, directly regulating gene expression. This type of signalling is referred to as the 'genomic' pathway. Nevertheless, progesterone also stimulates rapid physiological effects that are independent of transcription. This pathway, termed 'non-genomic', is mediated by the mPRs (membrane progesterone receptors). These mPRs belong to a larger class of membrane receptors called PAQRs (progestin and adipoQ receptors), which include receptors for adiponectin in vertebrates and osmotin in fungi. mPRs have been shown to activate inhibitory G-proteins, suggesting that they act as GPCRs (G-protein-coupled receptors). However, PAQRs do not resemble GPCRs with respect to topology or conserved sequence motifs. Instead, they more closely resemble proteins in the alkaline ceramidase family and they may possess enzymatic activity. In the present paper, we highlight the evidence in support of each model and what is currently known for PAQR signal transduction of this non-canonical receptor.

Degradation of long-chain base 1-phosphate (LCBP) in Arabidopsis: functional characterization of LCBP phosphatase involved in the dehydration stress response

Sphingolipid metabolites, long-chain base 1-phosphates (LCBPs), are involved in ABA signaling pathways. The LCBPs synthesized by long-chain base kinase are dephosphorylated by LCBP phosphatase or degraded by LCBP lyase. Here we show that the At3g58490 gene encodes AtSPP1, a functional LCBP phosphatase. Transient expression of green fluorescent protein fusion in suspension-cultured Arabidopsis cells showed that AtSPP1 is localized in the endoplasmic reticulum. The level of dihydrosphingosine 1-phosphate was increased in loss-of-function mutants (spp1) compared with wild-type (WT) plants, suggesting a role of AtSPP1 in regulating LCBP levels. The rate of decrease in fresh weight of detached aerial parts was significantly slower in spp1 mutants than in WT plants. A stomatal closure bioassay showed that the stomata of spp1 mutants were more sensitive than the WT to ABA, suggesting that AtSPP1 is involved in guard cell signaling. However, spp1 mutants showed decreased sensitivity to ABA with respect to primary root growth but not to seed germination. The response to fumonisin B(1) did not differ between the WT and spp1 mutant. A significant decrease in AtDPL1 (LCBP lyase) transcripts in spp1 mutants was observed. We conclude that AtSPP1 is a functional LCBP phosphatase that may play a role in stomatal responses through LCBP-mediated ABA signaling.

Recent advances in biomarkers and potential targeted therapies in head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC) is a devastating tumor of the upper aerodigestive tract with no significant change in treatment modality or improvement in survival over the last several decades. Biomarkers are important biological molecules that can be utilized in tumor detection, prognosis, and as targeted therapies. There are several important biomarkers and potential targets in the forefront, including biomarkers of tumorigenesis, signal transduction molecules, proteins involved in angiogenesis, and oncogenic viruses. The clinical applications of these biomarkers are in various states from in vitro and in vivo models, phase II and III clinical trials, to accepted modes of treatment in patients with HNSCC. Given the potential improvement in prognosis that biomarkers and their targeted therapies may have on the treatment of HNSCC, their investigation is both important and essential.

Sphingosine kinase type 1 inhibition reveals rapid turnover of circulating sphingosine 1-phosphate

S1P (sphingosine 1-phosphate) is a signalling molecule involved in a host of cellular and physiological functions, most notably cell survival and migration. S1P, which signals via a set of five G-protein-coupled receptors (S1P1-S1P5), is formed by the action of two SphKs (sphingosine kinases) from Sph (sphingosine). Interfering RNA strategies and SphK1 (sphingosine kinase type 1)-null (Sphk1-/-) mouse studies implicate SphK1 in multiple signalling cascades, yet there is a paucity of potent and selective SphK1 inhibitors necessary to evaluate the effects of rapid onset inhibition of this enzyme. We have identified a set of submicromolar amidine-based SphK1 inhibitors and report using a pair of these compounds to probe the cellular and physiological functions of SphK1. In so doing, we demonstrate that our inhibitors effectively lower S1P levels in cell-based assays, but we have been unable to correlate SphK1 inhibition with changes in cell survival. However, SphK1 inhibition did diminish EGF (epidermal growth factor)-driven increases in S1P levels and Akt (also known as protein kinase B)/ERK (extracellular-signal-regulated kinase) phosphorylation. Finally, administration of the SphK1 inhibitor to wild-type, but not Sphk1-/-, mice resulted in a rapid decrease in blood S1P levels indicating that circulating S1P is rapidly turned over.

Metabolic link between phosphatidylethanolamine and triacylglycerol metabolism in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae triacylglycerols (TAG) are synthesized by the acyl-CoA dependent acyltransferases Dga1p, Are1p, Are2p and the acyl-CoA independent phospholipid:diacylglycerol acyltransferase (PDAT) Lro1p which uses phosphatidylethanolamine (PE) as a preferred acyl donor. In the present study we investigated a possible link between TAG and PE metabolism by analyzing the contribution of the four different PE biosynthetic pathways to TAG formation, namely de novo PE synthesis via Psd1p and Psd2p, the CDP-ethanolamine (CDP-Etn) pathway and lyso-PE acylation by Ale1p. In cells grown on the non-fermentable carbon source lactate supplemented with 5 mM ethanolamine (Etn) the CDP-Etn pathway contributed most to the cellular TAG level, whereas mutations in the other pathways displayed only minor effects. In cki1∆dpl1∆eki1∆ mutants bearing defects in the CDP-Etn pathway both the cellular and the microsomal levels of PE were markedly decreased, whereas in other mutants of PE biosynthetic routes depletion of this aminoglycerophospholipid was less pronounced in microsomes. This observation is important because Lro1p similar to the enzymes of the CDP-Etn pathway is a component of the ER. We conclude from these results that in cki1∆dpl1∆eki1∆ insufficient supply of PE to the PDAT Lro1p was a major reason for the strongly reduced TAG level. Moreover, we found that Lro1p activity was markedly decreased in cki1∆dpl1∆eki1∆, although transcription of LRO1 was not affected. Our findings imply that (i) TAG and PE syntheses in the yeast are tightly linked; and (ii) TAG formation by the PDAT Lro1p strongly depends on PE synthesis through the CDP-Etn pathway. Moreover, it is very likely that local availability of PE in microsomes is crucial for TAG synthesis through the Lro1p reaction.

Sphingolipid long chain base phosphates can regulate apoptotic-like programmed cell death in plants

Sphingolipids are ubiquitous components of eukaryotic cells and sphingolipid metabolites, such as the long chain base phosphate (LCB-P), sphingosine 1 phosphate (S1P) and ceramide (Cer) are important regulators of apoptosis in animal cells. This study evaluated the role of LCB-Ps in regulating apoptotic-like programmed cell death (AL-PCD) in plant cells using commercially available S1P as a tool. Arabidopsis cell cultures were exposed to a diverse array of cell death-inducing treatments (including Cer) in the presence of S1P. Rates of AL-PCD and cell survival were recorded using vital stains and morphological markers of AL-PCD. Internal LCB-P levels were altered in suspension cultured cells using inhibitors of sphingosine kinase and changes in rates of death in response to heat stress were evaluated. S1P reduced AL-PCD and promoted cell survival in cells subjected to a range of stresses. Treatments with inhibitors of sphingosine kinase lowered the temperature which induced maximal AL-PCD in cell cultures. The data supports the existence of a sphingolipid rheostat involved in controlling cell fate in Arabidopsis cells and that sphingolipid regulation of cell death may be a shared feature of both animal apoptosis and plant AL-PCD.

Increased radiation sensitivity of head and neck squamous cell carcinoma with sphingosine kinase 1 inhibition

BACKGROUND

Sphingosine kinase 1 (SphK1) is an important regulator of apoptosis, survival, and proliferation in cancer cells. SphK1 expression in head and neck squamous cell cancer (HNSCC) cell lines and tumor tissue was assessed, and the efficacy of SphK1 knockdown in increasing tumor radiosensitivity was evaluated in vitro and in vivo.

METHODS

Expression of SphK1 was determined by immunohistochemistry, Western blot, and real-time polymerase chain reaction (RT-PCR) in 34 prospectively collected HNSCC tumor samples. HNSCC cell lines squamous cell carcinoma (SCC)-15 and SCC-25 were treated with SphK1 inhibitor SKI-II and siRNA targeting SphK1 with and without radiation, and the cell viability was assessed. SCC-15 cells with and without transfection of SphK1 siRNA were then injected into athymic nude mice to develop tumor xenografts, and these 2 groups were further divided into 1 group that received radiation and 1 group that did not. Tumor size was measured over 18 days, when the animals were killed and the tumors were evaluated by immunohistochemistry.

RESULTS

SphK1 is found in both HNSCC cell lines and human tumor samples, with higher expression correlated with advanced tumor stage, nodal involvement, and recurrence. In vitro, both SCC-15 and SCC-25 were found to be radioresistant; however, they were sensitized by administration of SKI-II and transfection with siRNA targeting SphK1. In vivo, SphK1-siRNA transfected xenografts were decreased in size compared with both nonradiated control and radiated control mice, whereas mice with both SphK1-siRNA and radiation treatment showed a synergistic reduction in tumor volume. Histopathologic analysis demonstrated a decreased proliferative state in SphK1-siRNA transfected tumors.

CONCLUSION

SphK1 is upregulated in HNSCC, and inhibition of SphK1 sensitizes HNSCC to radiation-induced cytotoxicity.

A view on sphingolipids and disease

Sphingolipid and glycosphingolipid levels and expression of sphingolipid metabolizing enzymes are altered in a variety of diseases or in response to drug treatment. Inherited defects of enzymes and other proteins required for the lysosomal degradation of these lipids lead to human sphingolipidoses. Also genetic defects that affect sphingolipid biosynthesis are known. Although the molecular details are often far from clear, (glyco)sphingolipids have been implicated to play a role in atherosclerosis, insulin resistance, cancer, and infections by pathogens. More general aspects of selected diseases are discussed.