Dihydroceramide desaturase activity in tumors

Dihydroceramide desaturase activity in the transplantable mouse hepatoma-22, rat hepatoma-27, M1 sarcoma, and RS1 rat cholangiocellular carcinoma has been investigated. It was found that the dihydroceramide desaturase activity in mouse hepatoma-22 is lower than that in normal mouse liver. However, the activity of this enzyme in subcutaneously and intrahepatically transplanted rat hepatoma-27 is increased compared to normal value. Dihydroceramide desaturase activity in subcutaneously and intrahepatically transplanted M1 sarcoma as well as in hepatoma-27 is dependent on the tumor microenvironment. The enzyme activity in RS1 tumor was not revealed. The data indicate that dihydroceramide desaturase activity depends on the tumor type and its microenvironment.     

Dihydroceramide desaturase inhibitors induce autophagy via dihydroceramide-dependent and independent mechanisms

BackgroundAutophagy consists on the delivery of cytoplasmic material and organelles to lysosomes for degradation. Research on autophagy is a growing field because deciphering the basic mechanisms of autophagy is key to understanding its role in health and disease, and to paving the way to discovering novel therapeutic strategies. Studies with chemotherapeutic drugs and pharmacological tools support a role for dihydroceramides as mediators of autophagy. However, their effect on the autophagy outcome (cell survival or death) is more controversial.MethodsWe have examined the capacity of structurally varied Des1 inhibitors to stimulate autophagy (LC3-II analysis), to increase dihydroceramides (mass spectrometry) and to reduce cell viability (SRB) in T98G and U87MG glioblastoma cells under different experimental conditions.ResultsThe compounds activity on autophagy induction took place concomitantly with accumulation of dihydroceramides, which occurred by both stimulation of ceramide synthesis de novo and reduction of Des1 activity. However, autophagy was also induced by the test compounds after preincubation with myriocin and in cells with a reduced capacity to produce dihydroceramides (U87DND). Autophagy inhibition with 3-methyladenine in the de novo dihydroceramide synthesis competent U87MG cells increased cytotoxicity, while genetic inhibition of autophagy in U87DND cells, poorly efficient at synthesizing dihydroceramides, augmented resistance to the test compounds.ConclusionDihydroceramide desaturase 1 inhibitors activate autophagy via both dihydroceramide-dependent and independent pathways and the balance between the two pathways influences the final cell fate.General significanceThe cells capacity to biosynthesize dihydroceramides must be taken into account in proautophagic Des1 inhibitors-including therapies.     

Sphingolipid players in the leukemia arena

Sphingolipids function as bioactive mediators of different cellular processes, mostly proliferation, survival, differentiation and apoptosis, besides being structural components of cellular membranes. Involvement of sphingolipid metabolism in cancerogenesis was demonstrated in solid tumors as well as in hematological malignancies. Herein, we describe the main biological and clinical aspects of leukemias and summarize data regarding sphingolipids as mediators of apoptosis triggered in response to anti-leukemic agents and synthetic analogs as inducers of cell death as well. We also report the contribution of molecules that modulate sphingolipid metabolism to development of encouraging strategies for leukemia treatment. Finally we address how deregulation of sphingolipid metabolism is associated to occurrence of therapy resistance both in vitro and in vivo. Sphingolipids can be considered promising therapeutic tools alone or in combination with other compounds, as well as valid targets in the attempt to eradicate leukemia and overcome drug resistance.     

Sphingolipid players in the leukemia arena

Sphingolipids function as bioactive mediators of different cellular processes, mostly proliferation, survival, differentiation and apoptosis, besides being structural components of cellular membranes. Involvement of sphingolipid metabolism in cancerogenesis was demonstrated in solid tumors as well as in hematological malignancies. Herein, we describe the main biological and clinical aspects of leukemias and summarize data regarding sphingolipids as mediators of apoptosis triggered in response to anti-leukemic agents and synthetic analogs as inducers of cell death as well. We also report the contribution of molecules that modulate sphingolipid metabolism to development of encouraging strategies for leukemia treatment. Finally we address how deregulation of sphingolipid metabolism is associated to occurrence of therapy resistance both in vitro and in vivo. Sphingolipids can be considered promising therapeutic tools alone or in combination with other compounds, as well as valid targets in the attempt to eradicate leukemia and overcome drug resistance.     

A new class of lipid desaturase central to sphingolipid biosynthesis and signalling

Sphingolipids and their phosphorylated metabolites play crucial roles in intracellular signalling in animals, and evidence is emerging for analogous situations in fungi and plants. Central to this signalling pathway is the phosphorylation of the sphingoid long chain base, sphingosine, which yields sphingosine-1-phosphate. Until recently, the enzyme responsible for the biosynthesis of sphingosine was unknown, but the Delta(4)-long chain base desaturase that carries out this reaction has now been identified. Orthologues are present in animals, plants and fungi, raising the possibility of using reverse genetics to determine the contribution of sphingosine-1-phosphate to signalling networks.     

Two sphingolipid transfer proteins, CERT and FAPP2: their roles in sphingolipid metabolism

Recent discoveries of two sphingolipid transfer proteins, CERT and FAPP2, have brought the field of sphingolipid metabolism to a more dynamic stage. CERT transfers ceramide from the endoplasmic reticulum (ER) to the Golgi apparatus, a step crucial for sphingomyelin (SM) synthesis. The pleckstrin homology (PH) domain and the FFAT motif of CERT restrict the direction of transfer and destination of ceramide through binding to phosphatidylinositol 4-monophosphate (PI4P) at the Golgi and the ER resident proteins, VAPs, respectively. CERT is regulated by the phosphorylation and dephosphorylation of serine/threonine, in which protein kinase D, possibly casein kinase I, and PP2Cepsilon are involved. On the other hand, FAPP2 transfers glucosylceramide (GlcCer) to appropriate sites for the synthesis of complex glycosphingolipids. Like CERT, FAPP2 contains a PH domain, the binding of which to PI4P is required for its localization to the Golgi. These observations indicate that lipid transfer proteins, CERT and FAPP2, spatially regulate lipid metabolism on the cytosolic side.     

Polarization vision: Drosophila enters the arena

Two new studies introduce the power of Drosophila genetics to polarization vision, revealing distinct photoreceptor inputs to polarotactic behaviour mediated by dorsal and ventral eye regions.     

Identification of a cytochrome b5-fusion desaturase responsible for the synthesis of triunsaturated sphingolipid long chain bases in the marine diatom Thalassiosira pseudonana

Triunsaturated sphingolipid long chain bases (LCBs) have previously been reported in some specialised tissues of marine invertebrates. We report the presence of similar LCBs in the marine diatom Thalassiosira pseudonana and identify the cytochrome b5-fusion desaturase responsible for the introduction of the third double bond at the Δ10 position in d18:3Δ4,8,10. This study extends the catalytic repertoire of the cytochrome b5 fusion desaturase family, also indicating the presence of orthologues in other marine invertebrates. The function of these polyunsaturated sphingolipid LCBs is currently unknown but it was previously suggested that they play an essential role in primitive animals. The identification of the desaturase responsible for their synthesis paves the way for further studies.     

A higher plant delta8 sphingolipid desaturase with a preference for (Z)-isomer formation confers aluminum tolerance to yeast and plants

Three plant cDNA libraries were expressed in yeast (Saccharomyces cerevisiae) and screened on agar plates containing toxic concentrations of aluminum. Nine cDNAs were isolated that enhanced the aluminum tolerance of yeast. These cDNAs were constitutively expressed in Arabidopsis (Arabidopsis thaliana) and one cDNA from the roots of Stylosanthes hamata, designated S851, conferred greater aluminum tolerance to the transgenic seedlings. The protein predicted to be encoded by S851 showed an equally high similarity to Delta6 fatty acyl lipid desaturases and Delta8 sphingolipid desaturases. We expressed other known Delta6 desaturase and Delta8 desaturase genes in yeast and showed that a Delta6 fatty acyl desaturase from Echium plantagineum did not confer aluminum tolerance, whereas a Delta8 sphingobase desaturase from Arabidopsis did confer aluminum tolerance. Analysis of the fatty acids and sphingobases of the transgenic yeast and plant cells demonstrated that S851 encodes a Delta8 sphingobase desaturase, which leads to the accumulation of 8(Z/E)-C(18)-phytosphingenine and 8(Z/E)-C(20)-phytopshingenine in yeast and to the accumulation of 8(Z/E)-C(18)-phytosphingenine in the leaves and roots of Arabidopsis plants. The newly formed 8(Z/E)-C(18)-phytosphingenine in transgenic yeast accounted for 3 mol% of the total sphingobases with a 8(Z):8(E)-isomer ratio of approximately 4:1. The accumulation of 8(Z)-C(18)-phytosphingenine in transgenic Arabidopsis shifted the ratio of the 8(Z):8(E) isomers from 1:4 in wild-type plants to 1:1 in transgenic plants. These results indicate that S851 encodes the first Delta8 sphingolipid desaturase to be identified in higher plants with a preference for the 8(Z)-isomer. They further demonstrate that changes in the sphingolipid composition of cell membranes can protect plants from aluminum stress.     

Yeast sphingolipid metabolism: clues and connections.

This review of sphingolipid metabolism in the budding yeast Saccharomyces cerevisiae contains information on the enzymes and the genes that encode them, as well as connections to other metabolic pathways. Particular attention is given to yeast homologs, domains, and motifs in the sequence, cellular localization of enzymes, and possible protein-protein interactions. Also included are genetic interactions of special interest that provide clues to the cellular biological roles of particular sphingolipid metabolic pathways and specific sphingolipids.     

Sibling competition arena: selfing and a competition arena can combine to constitute a barrier to gene flow in sympatry

Closely related species coexisting in sympatry provide critical insight into the mechanisms underlying speciation and the maintenance of genetic divergence. Selfing may promote reproductive isolation by facilitating local adaptation, causing reduced hybrid fitness in parental environments. Here, we propose a novel mechanism by which selfing can further impair interspecific gene flow: selfing may act to ensure that nonhybrid progeny systematically co-occur whenever hybrid genotypes are produced. Under a competition arena, the fitness differentials between nonhybrid and hybrid progeny are then magnified, preventing development of interspecific hybrids. We investigate whether this "sibling competition arena" can explain the coexistence in sympatry of closely related species of the plant fungal pathogens (Microbotryum) causing anther-smut disease. The probabilities of intrapromycelial mating (automixis), outcrossing, and sibling competition were manipulated in artificial inoculations to evaluate their contribution to reproductive isolation. We report that both intrapromycelial selfing and sibling competition significantly reduced rates of hybrid infection beyond that expected based solely upon selfing rates and noncompetitive fitness differentials between hybrid and nonhybrid progeny. Our results thus suggest that selfing and a sibling competition arena can combine to constitute a barrier to gene flow and diminish selection for additional barriers to gene flow in sympatry.     

Membranes in balance: mechanisms of sphingolipid homeostasis

Sphingolipids and their metabolites play key cellular roles both as structural components of membranes and as signaling molecules that mediate responses to physiologic cues and stresses. Despite progress during the last two decades in defining the enzymatic machinery responsible for synthesizing and degrading sphingolipids, comparatively little is known about how these enzymes are regulated to ensure sphingolipid homeostasis. Here, we review new insights into how cells sense and control sphingolipid biosynthesis and transport. We also discuss emerging evidence that sphingolipid metabolism is closely coordinated with that of sterols and glycerolipids and with other processes that occur in the secretory pathway. An improved understanding of sphingolipid homeostasis promises to shed light on basic processes in cell biology and disease, including how cells establish and maintain the complex membrane composition and architecture that is a defining feature of eukaryotic cell biology.     

Autophagy regulates sphingolipid levels in the liver

Sphingolipid levels are tightly regulated to maintain cellular homeostasis. During pathologic conditions such as in aging, inflammation, and metabolic and neurodegenerative diseases, levels of some sphingolipids, including the bioactive metabolite ceramide, are elevated. Sphingolipid metabolism has been linked to autophagy, a critical catabolic process in both normal cell function and disease; however, the in vivo relevance of the interaction is not well-understood. Here, we show that blocking autophagy in the liver by deletion of the Atg7 gene, which is essential for autophagosome formation, causes an increase in sphingolipid metabolites including ceramide. We also show that overexpression of serine palmitoyltransferase to elevate de novo sphingolipid biosynthesis induces autophagy in the liver. The results reveal autophagy as a process that limits excessive ceramide levels and that is induced by excessive elevation of de novo sphingolipid synthesis in the liver. Dysfunctional autophagy may be an underlying mechanism causing elevations in ceramide that may contribute to pathogenesis in diseases.     

外泌体：病毒与宿主博弈的另一“角斗场”

外泌体是一类小型的细胞外囊泡,可以包裹蛋白质、核酸等生物活性分子随体液循环到达机体各处,具有广泛的信息传递作用。研究发现,外泌体在病毒感染宿主的过程中也扮演着重要的角色。病毒需要在宿主细胞内完成复制周期并释放子代病毒,而这一过程与外泌体的产生及分泌途径有共通的部分。一方面,病毒可以"挟持"外泌体并将自身成分装入其中,逃避宿主的免疫应答,促进其在细胞间的传播。另一方面,宿主细胞也可利用外泌体传递抗病毒因子以抑制病毒感染。文中旨在从病毒与宿主两方面阐述外泌体在病毒感染宿主过程中的作用,以期为该领域的研究提供新的思路。