Human homologues of LAG1 reconstitute Acyl-CoA-dependent ceramide synthesis in yeast

Structure and mechanism of a eukaryotic ceramide synthase complex

Ceramide synthases (CerS) catalyze ceramide formation via N-acylation of a sphingoid base with a fatty acyl-CoA and are attractive drug targets for treating numerous metabolic diseases and cancers. Here, we present the cryo-EM structure of a yeast CerS complex, consisting of a catalytic Lac1 subunit and a regulatory Lip1 subunit, in complex with C26-CoA substrate. The CerS holoenzyme exists as a dimer of Lac1-Lip1 heterodimers. Lac1 contains a hydrophilic reaction chamber and a hydrophobic tunnel for binding the CoA moiety and C26-acyl chain of C26-CoA, respectively. Lip1 interacts with both the transmembrane region and the last luminal loop of Lac1 to maintain the proper acyl chain binding tunnel. A lateral opening on Lac1 serves as a potential entrance for the sphingoid base substrate. Our findings provide a template for understanding the working mechanism of eukaryotic ceramide synthases and may facilitate the development of therapeutic CerS modulators.

Toxoplasma ceramide synthases: Gene duplication, functional divergence, and roles in parasite fitness

Toxoplasma gondii is an obligate, intracellular apicomplexan protozoan parasite of both humans and animals that can cause fetal damage and abortion and severe disease in the immunosuppressed. Sphingolipids have indispensable functions as signaling molecules and are essential and ubiquitous components of eukaryotic membranes that are both synthesized and scavenged by the Apicomplexa. Ceramide is the precursor for all sphingolipids, and here we report the identification, localization and analyses of the Toxoplasma ceramide synthases TgCerS1 and TgCerS2. Interestingly, we observed that while TgCerS1 was a fully functional orthologue of the yeast ceramide synthase (Lag1p) capable of catalyzing the conversion of sphinganine to ceramide, in contrast TgCerS2 was catalytically inactive. Furthermore, genomic deletion of TgCerS1 using CRISPR/Cas-9 led to viable but slow-growing parasites indicating its importance but not indispensability. In contrast, genomic knock out of TgCerS2 was only accessible utilizing the rapamycin-inducible Cre recombinase system. Surprisingly, the results demonstrated that this "pseudo" ceramide synthase, TgCerS2, has a considerably greater role in parasite fitness than its catalytically active orthologue (TgCerS1). Phylogenetic analyses indicated that, as in humans and plants, the ceramide synthase isoforms found in Toxoplasma and other Apicomplexa may have arisen through gene duplication. However, in the Apicomplexa the duplicated copy is hypothesized to have subsequently evolved into a non-functional "pseudo" ceramide synthase. This arrangement is unique to the Apicomplexa and further illustrates the unusual biology that characterize these protozoan parasites.

The formation of migrasomes is initiated by the assembly of sphingomyelin synthase 2 foci at the leading edge of migrating cells

The migrasome is an organelle of migrating cells with diverse physiological functions. How migrasome formation is initiated is unknown. We found that sphingomyelin is enriched in migrasomes and identified sphingomyelin synthase 2 (SMS2) as an essential protein for migrasome biogenesis. SMS2 assembles into immobile foci that adhere on the basal membrane at the leading edge. When cells migrate away, the SMS2 foci 'move' out of cells and into retraction fibres, where they become migrasome formation sites and eventually grow into migrasomes. Mechanistically, SMS2 foci seed migrasomes by converting ceramide to sphingomyelin, which is essential for migrasome formation. Furthermore, CerS5, which is required for the synthesis of long-chain ceramide, and CERT, which transports ceramide from the endoplasmic reticulum to Golgi, are both required for migrasome formation. Our data reveal the essential role of ceramide and sphingomyelin in migrasome formation and suggest that SMS2 forms basal membrane-surface-connecting structures that pre-determine where migrasomes will grow.

Role of Omega-Hydroxy Ceramides in Epidermis: Biosynthesis, Barrier Integrity and Analyzing Method

Attached to the outer surface of the corneocyte lipid envelope (CLE), omega-hydroxy ceramides (ω-OH-Cer) link to involucrin and function as lipid components of the stratum corneum (SC). The integrity of the skin barrier is highly dependent on the lipid components of SC, especially on ω-OH-Cer. Synthetic ω-OH-Cer supplementation has been utilized in clinical practice for epidermal barrier injury and related surgeries. However, the mechanism discussion and analyzing methods are not keeping pace with its clinical application. Though mass spectrometry (MS) is the primary choice for biomolecular analysis, method modifications for ω-OH-Cer identification are lacking in progress. Therefore, finding conclusions on ω-OH-Cer biological function, as well as on its identification, means it is vital to remind further researchers of how the following work should be done. This review summarizes the important role of ω-OH-Cer in epidermal barrier functions and the forming mechanism of ω-OH-Cer. Recent identification methods for ω-OH-Cer are also discussed, which could provide new inspirations for study on both ω-OH-Cer and skin care development.

Genetic Tools for Studying the Roles of Sphingolipids in Viral Infections

Sphingolipids are a critical family of membrane lipids with diverse functions in eukaryotic cells, and a growing body of literature supports that these lipids play essential roles during the lifecycles of viruses. While small molecule inhibitors of sphingolipid synthesis and metabolism are widely used, the advent of CRISPR-based genomic editing techniques allows for nuanced exploration into the manners in which sphingolipids influence various stages of viral infections. Here we describe some of these critical considerations needed in designing studies utilizing genomic editing techniques for manipulating the sphingolipid metabolic pathway, as well as the current body of literature regarding how viruses depend on the products of this pathway. Here, we highlight the ways in which sphingolipids affect viruses as these pathogens interact with and influence their host cell and describe some of the many open questions remaining in the field.

Alteration of Sphingolipids in Biofluids: Implications for Neurodegenerative Diseases

Sphingolipids (SL) modulate several cellular processes including cell death, proliferation and autophagy. The conversion of sphingomyelin (SM) to ceramide and the balance between ceramide and sphingosine-1-phosphate (S1P), also known as the SL rheostat, have been associated with oxidative stress and neurodegeneration. Research in the last decade has focused on the possibility of targeting the SL metabolism as a therapeutic option; and SL levels in biofluids, including serum, plasma, and cerebrospinal fluid (CSF), have been measured in several neurodegenerative diseases with the aim of finding a diagnostic or prognostic marker. Previous reviews focused on results from diseases such as Alzheimer's Disease (AD), evaluated total SL or species levels in human biofluids, post-mortem tissues and/or animal models. However, a comprehensive review of SL alterations comparing results from several neurodegenerative diseases is lacking. The present work compiles data from circulating sphingolipidomic studies and attempts to elucidate a possible connection between certain SL species and neurodegeneration processes. Furthermore, the effects of ceramide species according to their acyl-chain length in cellular pathways such as apoptosis and proliferation are discussed in order to understand the impact of the level alteration in specific species. Finally, enzymatic regulations and the possible influence of insulin resistance in the level alteration of SL are evaluated.

Advances in determining signaling mechanisms of ceramide and role in disease

Ceramide is a critical bioactive lipid involved in diverse cellular processes. It has been proposed to regulate cellular processes by influencing membrane properties and by directly interacting with effector proteins. Advances over the past decade have improved our understanding of ceramide as a bioactive lipid. Generation and characterization of ceramide-metabolizing enzyme KO mice, development of specific inhibitors and ceramide-specific antibodies, use of advanced microscopy and mass spectrometry, and design of synthetic ceramide derivatives have all provided insight into the signaling mechanisms of ceramide and its implications in disease. As a result, the role of ceramide in biological functions and disease are now better understood, with promise for development of therapeutic strategies to treat ceramide-regulated diseases.

Changes in ceramide metabolism are essential in Madin-Darby canine kidney cell differentiation

Ceramides (Cers) and complex sphingolipids with defined acyl chain lengths play important roles in numerous cell processes. Six Cer synthase (CerS) isoenzymes (CerS1-6) are the key enzymes responsible for the production of the diversity of molecular species. In this study, we investigated the changes in sphingolipid metabolism during the differentiation of Madin-Darby canine kidney (MDCK) cells. By MALDI TOF TOF MS, we analyzed the molecular species of Cer, glucosylceramide (GlcCer), lactosylceramide (LacCer), and SM in nondifferentiated and differentiated cells (cultured under hypertonicity). The molecular species detected were the same, but cells subjected to hypertonicity presented higher levels of C24:1 Cer, C24:1 GlcCer, C24:1 SM, and C16:0 LacCer. Consistently with the molecular species, MDCK cells expressed CerS2, CerS4, and CerS6, but with no differences during cell differentiation. We next evaluated the different synthesis pathways with sphingolipid inhibitors and found that cells subjected to hypertonicity in the presence of amitriptyline, an inhibitor of acid sphingomyelinase, showed decreased radiolabeled incorporation in LacCer and cells did not develop a mature apical membrane. These results suggest that hypertonicity induces the endolysosomal degradation of SM, generating the Cer used as substrate for the synthesis of specific molecular species of glycosphingolipids that are essential for MDCK cell differentiation.

Making Sense of the Yeast Sphingolipid Pathway

Sphingolipids (SL) and their metabolites play key roles both as structural components of membranes and as signaling molecules. Many of the key enzymes and regulators of SL metabolism were discovered using the yeast Saccharomyces cerevisiae, and based on the high degree of conservation, a number of mammalian homologs were identified. Although yeast continues to be an important tool for SL research, the complexity of SL structure and nomenclature often hampers the ability of new researchers to grasp the subtleties of yeast SL biology and discover new modulators of this intricate pathway. Moreover, the emergence of lipidomics by mass spectrometry has enabled the rapid identification of SL species in yeast and rendered the analysis of SL composition under various physiological and pathophysiological conditions readily amenable. However, the complex nomenclature of the identified species renders much of the data inaccessible to non-specialists. In this review, we focus on parsing both the classical SL nomenclature and the nomenclature normally used during mass spectrometry analysis, which should facilitate the understanding of yeast SL data and might shed light on biological processes in which SLs are involved. Finally, we discuss a number of putative roles of various yeast SL species.

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.

Chemogenetic E-MAP in Saccharomyces cerevisiae for Identification of Membrane Transporters Operating Lipid Flip Flop

While most yeast enzymes for the biosynthesis of glycerophospholipids, sphingolipids and ergosterol are known, genes for several postulated transporters allowing the flopping of biosynthetic intermediates and newly made lipids from the cytosolic to the lumenal side of the membrane are still not identified. An E-MAP measuring the growth of 142'108 double mutants generated by systematically crossing 543 hypomorphic or deletion alleles in genes encoding multispan membrane proteins, both on media with or without an inhibitor of fatty acid synthesis, was generated. Flc proteins, represented by 4 homologous genes encoding presumed FAD or calcium transporters of the ER, have a severe depression of sphingolipid biosynthesis and elevated detergent sensitivity of the ER. FLC1, FLC2 and FLC3 are redundant in granting a common function, which remains essential even when the severe cell wall defect of flc mutants is compensated by osmotic support. Biochemical characterization of some other genetic interactions shows that Cst26 is the enzyme mainly responsible for the introduction of saturated very long chain fatty acids into phosphatidylinositol and that the GPI lipid remodelase Cwh43, responsible for introducing ceramides into GPI anchors having a C26:0 fatty acid in sn-2 of the glycerol moiety can also use lyso-GPI protein anchors and various base resistant lipids as substrates. Furthermore, we observe that adjacent deletions in several chromosomal regions show strong negative genetic interactions with a single gene on another chromosome suggesting the presence of undeclared suppressor mutations in certain chromosomal regions that need to be identified in order to yield meaningful E-map data.

All living cells define their boundaries by lipid-containing membranes, which are impermeable to ions and water-soluble metabolic intermediates, and thus allow maintaining constant conditions inside the cells and stopping metabolic intermediates from diffusing away. Membranes are formed by amphiphilic lipids that have a hydrophilic and a hydrophobic component. Such lipids form flat double-layered sheets (bilayers) wherein the hydrophilic components of the constituent lipids are directed towards the aqueous surroundings, the hydrophobic ones populate the center of the bilayer. Membranes grow when enzymes resident in the bilayer synthesize new amphiphilic lipids. These enzymes have their active site on one side of the membrane and insert the newly made lipids in only one of the two layers. To ensure symmetric growth of membranes, cells need flippases catalyzing the transfer of lipids from one into the other layer. To identify unknown flippases we performed a chemogenetic interaction screen able to bring to light functions of unknown proteins through their genetic interaction with genes of known function. The data point to Flc proteins as potential lipid flippases of the endoplasmic reticulum, reveal novel lipid modifying activities of Cst26 and Cwh43 and suggest that undeclared suppressor mutations in certain chromosomal regions can generate false genetic interactions.

Ceramide synthase 4 is highly expressed in involved skin of patients with atopic dermatitis

BACKGROUND

Ceramide is a crucial lipid in the stratum corneum (SC) which maintains the barrier function and hydration of the skin. In atopic dermatitis (AD) patients who have defective skin barrier function, ceramide levels are altered. We previously reported that although the amount of total ceramide was lower in involved skin compared with uninvolved skin of AD patients and with healthy control skin, the amounts of smaller ceramide species of Cer[NS] (<40 total carbons, which are total carbons of both sphingoid base and amide-linked fatty acid), especially Cer[NS] with 34 total carbons (C34-Cer[NS]), were higher. However, the enzyme(s) that produces the higher levels of smaller ceramide species in involved skin of AD patients was unclear.

OBJECTIVE

To identify the enzyme(s) that produces higher levels of smaller ceramide species of Cer[NS] in the involved skin of AD patients.

METHODS

Eight female Caucasian subjects who were diagnosed with AD on their arms (age range: 21-45 years) were enroled in this study. We compared ceramide levels in the SC and the expression levels of enzymes involved in ceramide metabolism using real-time PCR and immunohistochemistry between involved and uninvolved skin of AD patients.

RESULTS

Level of mRNA encoding ceramide synthase 4 (CERS4), which is one of the enzymes that synthesize ceramide from a sphingoid base and an amide-linked fatty acid, was significantly higher in involved skin than in uninvolved skin (P < 0.01). Additionally, the protein expression level of CERS4 in the epidermis was also higher in involved skin compared with uninvolved skin. The expression level of CERS4 correlated with the amount of C34-Cer[NS] (P < 0.01) and the skin hydration value (P < 0.05).

CONCLUSIONS

The elevated expression level of CERS4 contributes to the increase of C34-Cer[NS] and the impaired SC barrier function in involved skin of AD patients.

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.

Ceramide signals for initiation of yeast mating-specific cell cycle arrest

Sphingolipids are major constituents of membranes. A number of S. cerevisiae sphingolipid intermediates such as long chains sphingoid bases (LCBs) and ceramides act as signaling molecules regulating cell cycle progression, adaptability to heat stress, and survival in response to starvation. Here we show that S. cerevisiae haploid cells must synthesize ceramide in order to induce mating specific cell cycle arrest. Cells devoid of sphingolipid biosynthesis or defective in ceramide synthesis are sterile and harbor defects in pheromone-induced MAP kinase-dependent transcription. Analyses of G1/S cyclin levels indicate that mutant cells cannot reduce Cln1/2 levels in response to pheromone. FACS analysis indicates a lack of ability to arrest. The addition of LCBs to sphingolipid deficient cells restores MAP kinase-dependent transcription, reduces cyclin levels, and allows for mating, as does the addition of a cell permeable ceramide to cells blocked at ceramide synthesis. Pharmacological studies using the inositolphosphorylceramide synthase inhibitor aureobasidin A indicate that the ability to synthesize and accumulate ceramide alone is sufficient for cell cycle arrest and mating. Studies indicate that ceramide also has a role in PI(4,5)P2 polarization during mating, an event necessary for initiating cell cycle arrest and mating itself. Moreover, our studies suggest a third role for ceramide in localizing the mating-specific Ste5 scaffold to the plasma membrane. Thus, ceramide plays a role 1) in pheromone-induced cell cycle arrest, 2) in activation of MAP kinase-dependent transcription, and 3) in PtdIns(4,5)P2 polarization. All three events are required for differentiation during yeast mating.

Mitochondria to nucleus signaling and the role of ceramide in its integration into the suite of cell quality control processes during aging

Mitochondria to nucleus signaling has been the most extensively studied mode of inter-organelle communication. The first signaling pathway in this category of information transfer to be discovered was the retrograde response, with its own set of signal transduction proteins. The finding that this pathway compensates for mitochondrial dysfunction to extend the replicative lifespan of yeast cells has generated additional impetus for its study. This research has demonstrated crosstalk between the retrograde response and the target of rapamycin (TOR), small GTPase RAS, and high-osmolarity glycerol (HOG) pathways in yeast, all of which are key players in replicative lifespan. More recently, the retrograde response has been implicated in the diauxic shift and survival in stationary phase, extending its operation to the yeast chronological lifespan as well. In this capacity, the retrograde response may cooperate with other, related mitochondria to nucleus signaling pathways. Counterparts of the retrograde response are found in the roundworm, the fruit fly, the mouse, and even in human cells in tissue culture. The exciting realization that the retrograde response is embedded in the network of cellular quality control processes has emerged over the past few years. Most strikingly, it is closely integrated with autophagy and the selective brand of this quality control process, mitophagy. This coordination depends on TOR, and it engages ceramide/sphingolipid signaling. The yeast LAG1 ceramide synthase gene was the first longevity gene cloned as such, and its orthologs hyl-1 and hyl-2 determine worm lifespan. Thus, the involvement of ceramide signaling in quality control gives these findings cellular context. The retrograde response and ceramide are essential components of a lifespan maintenance process that likely evolved as a cytoprotective mechanism to defend the organism from diverse stressors.

Characterisation of detergent-insoluble membranes in pollen tubes of Nicotiana tabacum (L.)

Pollen tubes are the vehicle for sperm cell delivery to the embryo sac during fertilisation of Angiosperms. They provide an intriguing model for unravelling mechanisms of growing to extremes. The asymmetric distribution of lipids and proteins in the pollen tube plasma membrane modulates ion fluxes and actin dynamics and is maintained by a delicate equilibrium between exocytosis and endocytosis. The structural constraints regulating polarised secretion and asymmetric protein distribution on the plasma membrane are mostly unknown. To address this problem, we investigated whether ordered membrane microdomains, namely membrane rafts, might contribute to sperm cell delivery. Detergent insoluble membranes, rich in sterols and sphingolipids, were isolated from tobacco pollen tubes. MALDI TOF/MS analysis revealed that actin, prohibitins and proteins involved in methylation reactions and in phosphoinositide pattern regulation are specifically present in pollen tube detergent insoluble membranes. Tubulins, voltage-dependent anion channels and proteins involved in membrane trafficking and signalling were also present. This paper reports the first evidence of membrane rafts in Angiosperm pollen tubes, opening new perspectives on the coordination of signal transduction, cytoskeleton dynamics and polarised secretion.

Tumor metastasis suppressor gene-1 and tumors

Tumor metastasis suppressor gene-1 (TMSG-1) is a newly discovered tumor metastasis suppressor gene that plays important roles in promoting apoptosis and inhibiting invasion and metastasis of tumor cells. The inhibitory function of TMSG-1 in tumor cells may be related to vacuolar H⁺-ATPase and ceramide, but the underlying mechanism remains unknown. This review discusses the discovery of TMSG-1, the structure of TMSG-1 protein, as well as its possible mechanisms of action, expression in tumors and applications.

Regulation of ceramide synthase by casein kinase 2-dependent phosphorylation in Saccharomyces cerevisiae

Complex sphingolipids are important components of eukaryotic cell membranes and, together with their biosynthetic precursors, including sphingoid long chain bases and ceramides, have important signaling functions crucial for cell growth and survival. Ceramides are produced at the endoplasmic reticulum (ER) membrane by a multicomponent enzyme complex termed ceramide synthase (CerS). In budding yeast, this complex is composed of two catalytic subunits, Lac1 and Lag1, as well as an essential regulatory subunit, Lip1. Proper formation of ceramides by CerS has been shown previously to require the Cka2 subunit of casein kinase 2 (CK2), a ubiquitous enzyme with multiple cellular functions, but the precise mechanism involved has remained unidentified. Here we present evidence that Lac1 and Lag1 are direct targets for CK2 and that phosphorylation at conserved positions within the C-terminal cytoplasmic domain of each protein is required for optimal CerS activity. Our data suggest that phosphorylation of Lac1 and Lag1 is important for proper localization and distribution of CerS within the ER membrane and that phosphorylation of these sites is functionally linked to the COP I-dependent C-terminal dilysine ER retrieval pathway. Together, our data identify CK2 as an important regulator of sphingolipid metabolism, and additionally, because both ceramides and CK2 have been implicated in the regulation of cancer, our findings may lead to an enhanced understanding of their relationship in health and disease.

Harnessing the power of yeast to elucidate the role of sphingolipids in metabolic and signaling processes pertinent to psychiatric disorders

The development of therapies for neuropsychiatric disorders is hampered by the lack of understanding of the mechanisms underlying their pathologies. While aberrant sphingolipid metabolism is associated with psychiatric illness, the role of sphingolipids in these disorders is not understood. The genetically tractable yeast model can be exploited in order to elucidate the cellular consequences of sphingolipid perturbation. Hypotheses generated from studies in yeast and tested in mammalian cells may contribute to our understanding of the role of sphingolipids in psychiatric disorders and to the development of new treatments. Here, we compare sphingolipid metabolism in yeast and mammalian cells, discuss studies implicating sphingolipids in psychiatric disorders and propose approaches that utilize yeast in order to elucidate sphingolipid function and identify drugs that target sphingolipid synthesis.

Sorting of GPI-anchored proteins from yeast to mammals--common pathways at different sites?

Glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) are luminal secretory cargos that are attached by a post-translational glycolipid modification, the GPI anchor, to the external leaflet of the plasma membrane. GPI-APs are conserved among eukaryotes and possess many diverse and vital functions for which the GPI membrane attachment appears to be essential. The presence of the GPI anchor and its subsequent modifications along the secretory pathway confer to the anchored proteins unique trafficking properties that make GPI-APs an exceptional system to study mechanisms of sorting. In this Commentary, we discuss the recent advances in the field of GPI-AP sorting focusing on the mechanisms operating at the level of the exit from the ER and from the trans-Golgi network (TGN), which take place, respectively, in yeast and in polarized mammalian cells. By considering the similarities and differences between these two sorting events, we present unifying principles that appear to work at different sorting stations and in different organisms.

Expression of the bacterial type III effector DspA/E in Saccharomyces cerevisiae down-regulates the sphingolipid biosynthetic pathway leading to growth arrest

Erwinia amylovora, the bacterium responsible for fire blight, relies on a type III secretion system and a single injected effector, DspA/E, to induce disease in host plants. DspA/E belongs to the widespread AvrE family of type III effectors that suppress plant defense responses and promote bacterial growth following infection. Ectopic expression of DspA/E in plant or in Saccharomyces cerevisiae is toxic, indicating that DspA/E likely targets a cellular process conserved between yeast and plant. To unravel the mode of action of DspA/E, we screened the Euroscarf S. cerevisiae library for mutants resistant to DspA/E-induced growth arrest. The most resistant mutants (Δsur4, Δfen1, Δipt1, Δskn1, Δcsg1, Δcsg2, Δorm1, and Δorm2) were impaired in the sphingolipid biosynthetic pathway. Exogenously supplied sphingolipid precursors such as the long chain bases (LCBs) phytosphingosine and dihydrosphingosine also suppressed the DspA/E-induced yeast growth defect. Expression of DspA/E in yeast down-regulated LCB biosynthesis and induced a rapid decrease in LCB levels, indicating that serine palmitoyltransferase (SPT), the first and rate-limiting enzyme of the sphingolipid biosynthetic pathway, was repressed. SPT down-regulation was mediated by dephosphorylation and activation of Orm proteins that negatively regulate SPT. A Δcdc55 mutation affecting Cdc55-PP2A protein phosphatase activity prevented Orm dephosphorylation and suppressed DspA/E-induced growth arrest.

The yeast sphingolipid signaling landscape

Sphingolipids are recognized as signaling mediators in a growing number of pathways, and represent potential targets to address many diseases. The study of sphingolipid signaling in yeast has created a number of breakthroughs in the field, and has the potential to lead future advances. The aim of this article is to provide an inclusive view of two major frontiers in yeast sphingolipid signaling. In the first section, several key studies in the field of sphingolipidomics are consolidated to create a yeast sphingolipidome that ranks nearly all known sphingolipid species by their level in a resting yeast cell. The second section presents an overview of most known phenotypes identified for sphingolipid gene mutants, presented with the intention of illuminating not yet discovered connections outside and inside of the field.

Functional loss of two ceramide synthases elicits autophagy-dependent lifespan extension in C. elegans

Ceramide and its metabolites constitute a diverse group of lipids, which play important roles as structural entities of biological membranes as well as regulators of cellular growth, differentiation, and development. The C. elegans genome comprises three ceramide synthase genes; hyl-1, hyl-2, and lagr-1. HYL-1 function is required for synthesis of ceramides and sphingolipids containing very long acyl-chains (≥C24), while HYL-2 is required for synthesis of ceramides and sphingolipids containing shorter acyl-chains (≤C22). Here we show that functional loss of HYL-2 decreases lifespan, while loss of HYL-1 or LAGR-1 does not affect lifespan. We show that loss of HYL-1 and LAGR-1 functions extend lifespan in an autophagy-dependent manner, as knock down of the autophagy-associated gene ATG-12 abolishes hyl-1;lagr-1 longevity. The transcription factors PHA-4/FOXA, DAF-16/FOXO, and SKN-1 are also required for the observed lifespan extension, as well as the increased number of autophagosomes in hyl-1;lagr-1 animals. Both autophagic events and the transcription factors PHA-4/FOXA, DAF-16, and SKN-1 have previously been associated with dietary restriction-induced longevity. Accordingly, we find that hyl-1;lagr-1 animals display reduced feeding, increased resistance to heat, and reduced reproduction. Collectively, our data suggest that specific sphingolipids produced by different ceramide synthases have opposing roles in determination of C. elegans lifespan. We propose that loss of HYL-1 and LAGR-1 result in dietary restriction-induced autophagy and consequently prolonged longevity.

Mutations in CERS3 cause autosomal recessive congenital ichthyosis in humans

Autosomal recessive congenital ichthyosis (ARCI) is a rare genetic disorder of the skin characterized by abnormal desquamation over the whole body. In this study we report four patients from three consanguineous Tunisian families with skin, eye, heart, and skeletal anomalies, who harbor a homozygous contiguous gene deletion syndrome on chromosome 15q26.3. Genome-wide SNP-genotyping revealed a homozygous region in all affected individuals, including the same microdeletion that partially affects two coding genes (ADAMTS17, CERS3) and abolishes a sequence for a long non-coding RNA (FLJ42289). Whereas mutations in ADAMTS17 have recently been identified in autosomal recessive Weill-Marchesani-like syndrome in humans and dogs presenting with ophthalmologic, cardiac, and skeletal abnormalities, no disease associations have been described for CERS3 (ceramide synthase 3) and FLJ42289 so far. However, analysis of additional patients with non-syndromic ARCI revealed a splice site mutation in CERS3 indicating that a defect in ceramide synthesis is causative for the present skin phenotype of our patients. Functional analysis of patient skin and in vitro differentiated keratinocytes demonstrated that mutations in CERS3 lead to a disturbed sphingolipid profile with reduced levels of epidermis-specific very long-chain ceramides that interferes with epidermal differentiation. Taken together, these data present a novel pathway involved in ARCI development and, moreover, provide the first evidence that CERS3 plays an essential role in human sphingolipid metabolism for the maintenance of epidermal lipid homeostasis.

Autosomal recessive congenital ichthyosis (ARCI) is a heterogeneous group of human keratinization disorders mainly characterized by generalized abnormal scaling of the skin. To date, positional cloning and homozygosity mapping of families with ARCI have identified disease-associated mutations in seven genes: ABCA12, ALOX12B, ALOXE3, CYP4F22, ICHTHYIN, PNPLA1, and TGM1. The reported molecular mechanisms underlying disease development are related to defects in epidermal lipid pathways that interfere with terminal keratinocyte differentiation and skin barrier function. In this study we used genome-wide SNP mapping, which identified homozygous mutations in the CERS3 (ceramide synthase 3) gene that cause a new type of ARCI. Functional analysis of a skin sample and in vitro differentiated keratinocytes from one patient demonstrated that mutated CERS3 impairs the synthesis of ceramides with very long-chain acyl moieties. The defect in sphingolipid metabolism disturbs the epidermal lipid profile, which leads to an abnormal terminal differentiation process. In summary, mutations in CERS3 are causative for ARCI and illustrate the important role of ceramide synthesis in human skin physiology.

Membrane topology of yeast alkaline ceramidase YPC1

Ypc1p (yeast phyto-ceramidase 1) and Ydc1p (yeast dihydroceramidase 1) are alkaline ceramide hydrolases that reside in the ER (endoplasmic reticulum). Ypc1p can catalyse the reverse reaction, i.e. the condensation of non-esterified fatty acids with phytosphingosine or dihydrosphingosine and overexpression of YPC1 or YDC1 can provide enough ceramide synthesis to rescue the viability of cells lacking the normal acyl-CoA-dependent ceramide synthases. To better understand the coexistence of acyl-CoA-dependent ceramide synthases and ceramidases in the ER we investigated the membrane topology of Ypc1p by probing the cysteine residue accessibility of natural and substituted cysteines with membrane non-permeating mass-tagged probes. The N- and C-terminal ends of Ypc1p are oriented towards the lumen and cytosol respectively. Two of the five natural cysteines, Cys27 and Cys219, are essential for enzymatic activity and form a disulfide bridge. The data allow the inference that all of the amino acids of Ypc1p that are conserved in the Pfam PF05875 ceramidase motif and the CREST {alkaline ceramidase, PAQR [progestin and adipoQ (adiponectin) receptor] receptor, Per1 (protein processing in the ER 1), SID-1 (sister disjunction 1) and TMEM8 (transmembrane protein 8)} superfamily are located in or near the ER lumen. Microsomal assays using a lysine residue-specific reagent show that the reverse ceramidase activity can only be blocked when the reagent has access to Ypc1p from the lumenal side. Overall the data suggest that the active site of Ypc1p resides at the lumenal side of the ER membrane.

p53 and Ceramide as Collaborators in the Stress Response

The sphingolipid ceramide mediates various cellular processes in response to several extracellular stimuli. Some genotoxic stresses are able to induce p53-dependent ceramide accumulation leading to cell death. However, in other cases, in the absence of the tumor suppressor protein p53, apoptosis proceeds partly due to the activity of this "tumor suppressor lipid", ceramide. In the current review, we describe ceramide and its roles in signaling pathways such as cell cycle arrest, hypoxia, hyperoxia, cell death, and cancer. In a specific manner, we are elaborating on the role of ceramide in mitochondrial apoptotic cell death signaling. Furthermore, after highlighting the role and mechanism of action of p53 in apoptosis, we review the association of ceramide and p53 with respect to apoptosis. Strikingly, the hypothesis for a direct interaction between ceramide and p53 is less favored. Recent data suggest that ceramide can act either upstream or downstream of p53 protein through posttranscriptional regulation or through many potential mediators, respectively.

The retrograde response: when mitochondrial quality control is not enough

Mitochondria are responsible for generating adenosine triphosphate (ATP) and metabolic intermediates for biosynthesis. These dual functions require the activity of the electron transport chain in the mitochondrial inner membrane. The performance of these electron carriers is imperfect, resulting in release of damaging reactive oxygen species. Thus, continued mitochondrial activity requires maintenance. There are numerous means by which this quality control is ensured. Autophagy and selective mitophagy are among them. However, the cell inevitably must compensate for declining quality control by activating a variety of adaptations that entail the signaling of the presence of mitochondrial dysfunction to the nucleus. The best known of these is the retrograde response. This signaling pathway is triggered by the loss of mitochondrial membrane potential, which engages a series of signal transduction proteins, and it culminates in the induction of a broad array of nuclear target genes. One of the hallmarks of the retrograde response is its capacity to extend the replicative life span of the cell. The retrograde signaling pathway interacts with several other signaling pathways, such as target of rapamycin (TOR) and ceramide signaling. All of these pathways respond to stress, including metabolic stress. The retrograde response is also linked to both autophagy and mitophagy at the gene and protein activation levels. Another quality control mechanism involves age-asymmetry in the segregation of dysfunctional mitochondria. One of the processes that impinge on this age-asymmetry is related to biogenesis of the organelle. Altogether, it is apparent that mitochondrial quality control constitutes a complex network of processes, whose full understanding will require a systems approach. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Ceramide synthases: reexamining longevity

The ceramide synthase (CerS) enzymes catalyze the formation of (dihydro) ceramide, and thereby provide critical complexity to all sphingolipids (SLs) with respect to their acyl chain length. This review summarizes the progress in the field of CerS from the time of their discovery more than a decade ago as Longevity assurance (Lass) genes in yeast, until the recent development of CerS-deficient mouse models. Human hereditary CerS disorders are yet to be discovered. However, the recent findings in CerS mutant animals highlight the important physiological role of these enzymes. The fundamental findings with respect to CerS structure, function, localization, and regulation are discussed, as well as CerS roles in maintaining longevity in vivo.

The yeast retrograde response as a model of intracellular signaling of mitochondrial dysfunction

Mitochondrial dysfunction activates intracellular signaling pathways that impact yeast longevity, and the best known of these pathways is the retrograde response. More recently, similar responses have been discerned in other systems, from invertebrates to human cells. However, the identity of the signal transducers is either unknown or apparently diverse, contrasting with the well-established signaling module of the yeast retrograde response. On the other hand, it has become equally clear that several other pathways and processes interact with the retrograde response, embedding it in a network responsive to a variety of cellular states. An examination of this network supports the notion that the master regulator NFκB aggregated a variety of mitochondria-related cellular responses at some point in evolution and has become the retrograde transcription factor. This has significant consequences for how we view some of the deficits associated with aging, such as inflammation. The support for NFκB as the retrograde response transcription factor is not only based on functional analyses. It is bolstered by the fact that NFκB can regulate Myc–Max, which is activated in human cells with dysfunctional mitochondria and impacts cellular metabolism. Myc–Max is homologous to the yeast retrograde response transcription factor Rtg1–Rtg3. Further research will be needed to disentangle the pro-aging from the anti-aging effects of NFκB. Interestingly, this is also a challenge for the complete understanding of the yeast retrograde response.

Sphingolipids and plant defense/disease: the "death" connection and beyond

Sphingolipids comprise a major class of structural materials and lipid signaling molecules in all eukaryotic cells. Over the past two decades, there has been a phenomenal growth in the study of sphingolipids (i.e., sphingobiology) at an average rate of ∼1000 research articles per year. Sphingolipid studies in plants, though accounting for only a small fraction (∼6%) of the total number of publications, have also enjoyed proportionally rapid growth in the past decade. Concomitant with the growth of sphingobiology, there has also been tremendous progress in our understanding of the molecular mechanisms of plant innate immunity. In this review, we (i) cross examine and analyze the major findings that establish and strengthen the intimate connections between sphingolipid metabolism and plant programmed cell death (PCD) associated with plant defense or disease; (ii) highlight and compare key bioactive sphingolipids involved in the regulation of plant PCD and possibly defense; (iii) discuss the potential role of sphingolipids in polarized membrane/protein trafficking and formation of lipid rafts as subdomains of cell membranes in relation to plant defense; and (iv) where possible, attempt to identify potential parallels for immunity-related mechanisms involving sphingolipids across kingdoms.

Ceramide synthases at the centre of sphingolipid metabolism and biology

Sphingolipid metabolism in metazoan cells consists of a complex interconnected web of numerous enzymes, metabolites and modes of regulation. At the centre of sphingolipid metabolism reside CerSs (ceramide synthases), a group of enzymes that catalyse the formation of ceramides from sphingoid base and acyl-CoA substrates. From a metabolic perspective, these enzymes occupy a unique niche in that they simultaneously regulate de novo sphingolipid synthesis and the recycling of free sphingosine produced from the degradation of pre-formed sphingolipids (salvage pathway). Six mammalian CerSs (CerS1-CerS6) have been identified. Unique characteristics have been described for each of these enzymes, but perhaps the most notable is the ability of individual CerS isoforms to produce ceramides with characteristic acyl-chain distributions. Through this control of acyl-chain length and perhaps in a compartment-specific manner, CerSs appear to regulate multiple aspects of sphingolipid-mediated cell and organismal biology. In the present review, we discuss the function of CerSs as critical regulators of sphingolipid metabolism, highlight their unique characteristics and explore the emerging roles of CerSs in regulating programmed cell death, cancer and many other aspects of biology.

C16 ceramide is crucial for triacylglycerol-induced apoptosis in macrophages

Triacylglycerol (TG) accumulation caused by adipose triglyceride lipase (ATGL) deficiency or very low-density lipoprotein (VLDL) loading of wild-type (Wt) macrophages results in mitochondrial-mediated apoptosis. This phenotype is correlated to depletion of Ca(2+) from the endoplasmic reticulum (ER), an event known to induce the unfolded protein response (UPR). Here, we show that ER stress in TG-rich macrophages activates the UPR, resulting in increased abundance of the chaperone GRP78/BiP, the induction of pancreatic ER kinase-like ER kinase, phosphorylation and activation of eukaryotic translation initiation factor 2A, the translocation of activating transcription factor (ATF)4 and ATF6 to the nucleus and the induction of the cell death executor CCAAT/enhancer-binding protein homologous protein. C16:0 ceramide concentrations were increased in Atgl-/- and VLDL-loaded Wt macrophages. Overexpression of ceramide synthases was sufficient to induce mitochondrial apoptosis in Wt macrophages. In accordance, inhibition of ceramide synthases in Atgl-/- macrophages by fumonisin B1 (FB1) resulted in specific inhibition of C16:0 ceramide, whereas intracellular TG concentrations remained high. Although the UPR was still activated in Atgl-/- macrophages, FB1 treatment rescued Atgl-/- macrophages from mitochondrial dysfunction and programmed cell death. We conclude that C16:0 ceramide elicits apoptosis in Atgl-/- macrophages by activation of the mitochondrial apoptosis pathway.

Chemical biology approaches to membrane homeostasis and function

The study of membranes is at a turning point. New theories about membrane structure and function have recently been proposed, however, new technologies, combining chemical, physical, and biochemical approaches are necessary to test these hypotheses. In particular, the NCCR in chemical biology aims to visualize and characterize membrane microdomains and determine their function during hormone signaling.

The retrograde response: when mitochondrial quality control is not enough

Mitochondria are responsible for generating adenosine triphosphate (ATP) and metabolic intermediates for biosynthesis. These dual functions require the activity of the electron transport chain in the mitochondrial inner membrane. The performance of these electron carriers is imperfect, resulting in release of damaging reactive oxygen species. Thus, continued mitochondrial activity requires maintenance. There are numerous means by which this quality control is ensured. Autophagy and selective mitophagy are among them. However, the cell inevitably must compensate for declining quality control by activating a variety of adaptations that entail the signaling of the presence of mitochondrial dysfunction to the nucleus. The best known of these is the retrograde response. This signaling pathway is triggered by the loss of mitochondrial membrane potential, which engages a series of signal transduction proteins, and it culminates in the induction of a broad array of nuclear target genes. One of the hallmarks of the retrograde response is its capacity to extend the replicative life span of the cell. The retrograde signaling pathway interacts with several other signaling pathways, such as target of rapamycin (TOR) and ceramide signaling. All of these pathways respond to stress, including metabolic stress. The retrograde response is also linked to both autophagy and mitophagy at the gene and protein activation levels. Another quality control mechanism involves age-asymmetry in the segregation of dysfunctional mitochondria. One of the processes that impinge on this age-asymmetry is related to biogenesis of the organelle. Altogether, it is apparent that mitochondrial quality control constitutes a complex network of processes, whose full understanding will require a systems approach. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Distinct roles of two ceramide synthases, CaLag1p and CaLac1p, in the morphogenesis of Candida albicans

Lag1p and Lac1p catalyse ceramide synthesis in Saccharomyces cerevisiae. This study shows that Lag1 family proteins are generally required for polarized growth in hemiascomycetous yeast. However, in contrast to S. cerevisiae where these proteins are functionally redundant, C. albicans Lag1p (CaLag1p) and Lac1p (CaLac1p) are functionally distinct. Lack of CaLag1p, but not CaLac1p, caused severe defects in the growth and hyphal morphogenesis of C. albicans. Deletion of CaLAG1 decreased expression of the hypha-specific HWP1 and ECE1 genes. Moreover, overexpression of CaLAG1 induced pseudohyphal growth in this organism under non-hypha-inducing conditions, suggesting that CaLag1p is necessary for relaying signals to induce hypha-specific gene expression. Analysis of ceramide and sphingolipid composition revealed that CaLag1p predominantly synthesizes ceramides with C24:0/C26:0 fatty acid moieties, which are involved in generating inositol-containing sphingolipids, whereas CaLac1p produces ceramides with C18:0 fatty acid moieties, which are precursors for glucosylsphingolipids. Thus, our study demonstrates that CaLag1p and CaLac1p have distinct substrate specificities and physiological roles in C. albicans.

Molecular and functional characterization of the ceramide synthase from Trypanosoma cruzi

In this study, we characterized ceramide synthase (CerS) of the protozoan parasite Trypanosoma cruzi at the molecular and functional levels. TcCerS activity was detected initially in a cell-free system using the microsomal fraction of epimastigote forms of T. cruzi, [(3)H]dihydrosphingosine or [(3)H]sphingosine, and fatty acids or acyl-CoA derivatives as acceptor or donor substrates, respectively. TcCerS utilizes both sphingoid long-chain bases, and its activity is exclusively dependent on acyl-CoAs, with palmitoyl-CoA being preferred. In addition, Fumonisin B(1), a broad and well-known acyl-CoA-dependent CerS inhibitor, blocked the parasite's CerS activity. However, unlike observations in fungi, the CerS inhibitors Australifungin and Fumonisin B(1) did not affect the proliferation of epimastigotes in culture, even after exposure to high concentrations or after extended periods of treatment. A search of the parasite genome with the conserved Lag1 motif from Lag1p, the yeast acyl-CoA-dependent CerS, identified a T. cruzi candidate gene (TcCERS1) that putatively encodes the parasite's CerS activity. The TcCERS1 gene was able to functionally complement the lethality of a lag1Δ lac1Δ double deletion yeast mutant in which the acyl-CoA-dependent CerS is not detectable. The complemented strain was capable of synthesizing normal inositol-containing sphingolipids and is 10 times more sensitive to Fumonisin B(1) than the parental strain.

Many ceramides

Intensive research over the past 2 decades has implicated ceramide in the regulation of several cell responses. However, emerging evidence points to dramatic complexities in ceramide metabolism and structure that defy the prevailing unifying hypothesis on ceramide function that is based on the understanding of ceramide as a single entity. Here, we develop the concept that "ceramide" constitutes a family of closely related molecules, subject to metabolism by >28 enzymes and with >200 structurally distinct mammalian ceramides distinguished by specific structural modifications. These ceramides are synthesized in a combinatorial fashion with distinct enzymes responsible for the specific modifications. These multiple pathways of ceramide generation led to the hypothesis that individual ceramide molecular species are regulated by specific biochemical pathways in distinct subcellular compartments and execute distinct functions. In this minireview, we describe the "many ceramides" paradigm, along with the rationale, supporting evidence, and implications for our understanding of bioactive sphingolipids and approaches for unraveling these pathways.

Selective knockdown of ceramide synthases reveals complex interregulation of sphingolipid metabolism

Mammalian ceramide synthases 1 to 6 (CerS1-6) generate Cer in an acyl-CoA-dependent manner, and expression of individual CerS has been shown to enhance the synthesis of ceramides with particular acyl chain lengths. However, the contribution of each CerS to steady-state levels of specific Cer species has not been evaluated. We investigated the knockdown of individual CerS in the MCF-7 human breast adenocarcinoma cell line by using small-interfering RNA (siRNA). We found that siRNA-induced downregulation of each CerS resulted in counter-regulation of nontargeted CerS. Additionally, each CerS knockdown produced unique effects on the levels of multiple sphingolipid species. For example, downregulation of CerS2 decreased very long-chain Cer but increased levels of CerS4, CerS5, and CerS6 expression and upregulated long-chain and medium-long-chain sphingolipids. Conversely, CerS6 knockdown decreased C16:0-Cer but increased CerS5 expression and caused non-C16:0 sphingolipids to be upregulated. Knockdown of individual CerS failed to decrease total sphingolipids or upregulate sphingoid bases. Treatment with siRNAs targeting combined CerS, CerS2, CerS5, and CerS6, did not change overall Cer or sphingomyelin mass but caused upregulation of dihydroceramide and hexosyl-ceramide and promoted endoplasmic reticulum stress. These data suggest that sphingolipid metabolism is robustly regulated by both redundancy in CerS-mediated Cer synthesis and counter-regulation of CerS expression.

HRAS1 and LASS1 with APOE are associated with human longevity and healthy aging

The search for longevity-determining genes in human has largely neglected the operation of genetic interactions. We have identified a novel combination of common variants of three genes that has a marked association with human lifespan and healthy aging. Subjects were recruited and stratified according to their genetically inferred ethnic affiliation to account for population structure. Haplotype analysis was performed in three candidate genes, and the haplotype combinations were tested for association with exceptional longevity. An HRAS1 haplotype enhanced the effect of an APOE haplotype on exceptional survival, and a LASS1 haplotype further augmented its magnitude. These results were replicated in a second population. A profile of healthy aging was developed using a deficit accumulation index, which showed that this combination of gene variants is associated with healthy aging. The variation in LASS1 is functional, causing enhanced expression of the gene, and it contributes to healthy aging and greater survival in the tenth decade of life. Thus, rare gene variants need not be invoked to explain complex traits such as aging; instead rare congruence of common gene variants readily fulfills this role. The interaction between the three genes described here suggests new models for cellular and molecular mechanisms underlying exceptional survival and healthy aging that involve lipotoxicity.

Mammalian ceramide synthases

In mammals, ceramide, a key intermediate in sphingolipid metabolism and an important signaling molecule, is synthesized by a family of six ceramide synthases (CerS), each of which synthesizes ceramides with distinct acyl chain lengths. There are a number of common biochemical features between the CerS, such as their catalytic mechanism, and their structure and intracellular localization. Different CerS also display remarkable differences in their biological properties, with each of them playing distinct roles in processes as diverse as cancer and tumor suppression, in the response to chemotherapeutic drugs, in apoptosis, and in neurodegenerative diseases.

Antiapoptotic roles of ceramide-synthase-6-generated C16-ceramide via selective regulation of the ATF6/CHOP arm of ER-stress-response pathways

Emerging results suggest that ceramides with different fatty acid chain lengths might play distinct functions in the regulation of tumor growth and therapy. Here we report that de novo-generated C(18)- and C(16)-ceramides by ceramide synthases 1 and 6 (CerS1 and CerS6) play opposing proapoptotic and prosurvival roles, respectively, in human head and neck squamous cell carcinomas (HNSCCs). Unexpectedly, knockdown of CerS6/C(16)-ceramide using small interfering RNA induced endoplasmic reticulum (ER)-stress-mediated apoptosis. Reconstitution of C(16)-ceramide generation by induced expression of wild-type CerS6, but not its catalytically inactive mutant, protected cells from cell death induced by knockdown of CerS6. Moreover, using molecular tools coupled with analysis of sphingolipid metabolism showed that generation of C(16)-ceramide, and not dihydro-C(16)-ceramide, by induced expression of CerS6 rescued cells from ER stress and apoptosis. Mechanistically, regulation of ER-stress-induced apoptosis by CerS6/C(16)-ceramide was linked to the activation of a specific arm, ATF6/CHOP, of the unfolded protein response pathway. Notably, while expression of CerS1/C(18)-ceramide inhibited HNSCC xenograft growth, CerS6/C(16)-ceramide significantly protected ER stress, leading to enhanced tumor development and growth in vivo, consistent with their pro- and antiapoptotic roles, respectively. Thus, these data reveal an unexpected and novel prosurvival role of CerS6/C(16)-ceramide involved in the protection against ER-stress-induced apoptosis and induction of HNSCC tumor growth.

Ceramide biosynthesis in keratinocyte and its role in skin function

The enucleate layer of the epidermis, i.e. the stratum corneum, is responsible for certain critical protective functions, such as epidermal permeability barrier function. Within the epidermal membrane lamella component, ceramides are the dominant lipid class by weight (over 50%) and exhibit the greatest molecular heterogeneity in terms of sphingoid base and fatty acid composition. It is now evermore important to understand how ceramide production and functions are controlled in the epidermis, since decreased epidermal ceramide content has been linked to water loss and barrier dysfunction. During the past several years, critical enzymes in ceramide biosynthesis have been identified, including ceramide synthases (CerS) and ceramide hydroxylase/desaturase. In this review, we describe the molecular heterogeneity of ceramides synthesized in the epidermis and their possible roles in epidermal permeability barrier functions. We also describe recent studies that identified the family of CerS (CerS1-CerS6) in mammals. We further focus on the roles of specific isoforms of these enzymes in synthesizing the epidermal ceramides, especially in relation to chain-length specificity. In addition, we provide experimental information, including our recent findings, as to how applying ceramide or ceramide-containing substances to skin, orally or directly, can benefit skin health.