Serine palmitoyltransferase: role in apoptotic de novo ceramide synthesis and other stress responses

Cannabinoids and ceramide: two lipids acting hand-by-hand

Cannabinoids, the active components of Cannabis sativa (marijuana) and their endogenous counterparts, exert their effects by binding to specific G-protein-coupled receptors that modulate adenylyl cyclase and ion channels. Recent research has shown that the CB1 cannabinoid receptor is also coupled to the generation of the lipid second messenger ceramide via two different pathways: sphingomyelin hydrolysis and ceramide synthesis de novo. Sustained ceramide accumulation in tumor cells mediates cannabinoid-induced apoptosis, as evidenced by in vitro and in vivo studies. This effect seems to be due to the impact of ceramide on key cell signalling systems such as the extracellular signal-regulated kinase cascade and the Akt pathway. These findings provide a new conceptual view on how cannabinoids act, and raise interesting physiological and therapeutic questions.

Mammalian Lass6 and its related family members regulate synthesis of specific ceramides

The Lass (longevity-assurance homologue) family members, which are highly conserved among eukaryotes, function in ceramide synthesis. In the mouse, there are at least five Lass family members, Lass1, Lass2, Lass4, Lass5 and the hitherto uncharacterized Lass6. To investigate specific roles for each Lass member in ceramide synthesis, we cloned these five mouse proteins. Overproduction of any Lass protein in cultured cells resulted in an increase in cellular ceramide, but the ceramide species produced varied. Overproduction of Lass1 increased C18:0-ceramide levels preferentially, and overproduction of Lass2 and Lass4 increased levels of longer ceramides such as C22:0- and C24:0-ceramides. Lass5 and Lass6 produced shorter ceramide species (C14:0- and C16:0-ceramides); however, their substrate preferences towards saturated/unsaturated fatty acyl-CoA differed. In addition to differences in substrate preferences, we also demonstrated by Northern blotting that Lass family members are differentially expressed among tissues. Additionally, we found that Lass proteins differ with regard to glycosylation. Of the five members, only Lass2, Lass5 and Lass6 were N-glycosylated, each at their N-terminal Asn residue. The occurrence of N-glycosylation of some Lass proteins provides topological insight, indicating that the N-termini of Lass family members probably face the luminal side of the endoplasmic reticulum membrane. Furthermore, based on a proteinase K digestion assay, we demonstrated that the C-terminus of Lass6 faces the cytosolic side of the membrane. From these data we propose topology for the conserved Lag1 motif in Lass family members, namely that the N-terminal region faces the luminal side and the C-terminal region the cytosolic side of the endoplasmic reticulum membrane.

Integration of glycosphingolipid metabolism and cell-fate decisions in cancer and stem cells: review and hypothesis

The metabolism of glycosphingolipids is strictly regulated during the mitotic cell cycle. Before the G1-to-S transition, the ceramide and glucosylceramide concentration is elevated. Ceramide induces apoptosis synergistically with the pro-apoptotic protein prostate apoptosis response 4 (PAR-4) that may be asymmetrically inherited during cell division. Only one daughter cell dies shortly after mitosis, a mechanism we suggested to regulate the number of neural stem cells during embryonic development. The progeny cells, however, may protect themselves by converting ceramide to glucosylceramide and other glycosphingolipids. In particular, complex gangliosides have been found to sustain cell survival and differentiation. The cell cycle may thus be a turning point for (glyco)sphingolipid metabolism and explain rapid changes of the sphingolipid composition in cells that undergo mitotic cell-fate decisions. In the proposed model termed "Shiva cycle", progression through the cell cycle, differentiation, or apoptosis may rely on a delicate balance of (glyco)sphingolipid second messengers that modulate the retinoblastoma-dependent G1-to-S transition or caspase-dependent G1-to-apoptosis program. Ceramide-induced cell cycle delay at G0/G1 is either followed by ceramide-induced apoptosis or by conversion of ceramide to glucosylceramide, a proposed key regulatory rheostat that rescues cells from re-entry into a life/death decision at G1-to-S. We propose a mechanistic model for sphingolpid-induced protein scaffolds ("slip") that regulate cell-fate decisions and will discuss the biological consequences and pharmacological potential of manipulating the (glyco)sphingolipid-dependent cell fate program in cancer and stem cells.

Rafts and sphingolipid biosynthesis in the kinetoplastid parasitic protozoa

Summary relatively large rafts are a feature of activated mammalian cells. These studies allow us to consider the functional role of lipid rafts in kinetoplastid parasites, which are particularly rich in lipid-anchored surface molecules. Morphological, biochemical and genetic studies indicate that lipid rafts (and sphingolipid biosythesis) are important in the differentiation of extracellular Leishmania to mammalian-infective metacyclic promastigotes, perhaps orchestrating the clearly observable reorganization of the plasma membrane during this process that leads to an activated metacyclic primed for invasion. However, the first step in the sphingolipid biosynthetic pathway (mediated by serine palmitoyltransferase), and at least regulated, de novo sphingoid base and ceramide synthesis, are not essential for the pathogenesis of intramacrophage Leishmania amastigotes.

Sphingolipid-free Leishmania are defective in membrane trafficking, differentiation and infectivity

Sphingolipids are structural components of the eukaryotic plasma membrane that are involved, together with cholesterol, in the formation of lipid microdomains (rafts). Additionally, sphingolipid metabolites have been shown to modulate a wide variety of cellular events, including differentiation and apoptosis. To investigate the role of de novo sphingolipid biosynthesis in Leishmania, we have focused on serine palmitoyltransferase (SPT), which catalyses the first, rate-limiting step in the synthetic pathway. Genetic ablation of one SPT subunit, LmLCB2, yields viable null parasites that can no longer synthesize ceramide and sphingolipids de novo. Unexpectedly, LmLCB2 expression (and sphingolipid biosynthesis) is stage regulated in Leishmania, being undetectable in intramacrophage parasites. As expected from this observation, the LmLCB2 null mutants maintain infectivity in vivo. However, they are compromised in their ability to form infective extracellular parasites, correlating with a defect in association of the virulence factor, leishmanolysin or GP63, with lipid rafts during exocytosis and an observed relocalization of a second virulence factor, lipophosphogycan, during differentiation. Thus, de novo sphingolipid biosynthesis is critical for membrane trafficking events in extracellular Leishmania but has at best a minor role in intracellular pathogenesis.

Fatty acid synthase expression is induced by the Epstein-Barr virus immediate-early protein BRLF1 and is required for lytic viral gene expression

The Epstein-Barr virus (EBV) immediate-early (IE) protein BRLF1 (R) is a transcription factor that induces the lytic form of EBV infection. R activates certain early viral promoters through a direct binding mechanism but induces transcription of the other EBV IE gene, BZLF1 (Z), indirectly through cellular factors binding to a CRE motif in the Z promoter (Zp). Here we demonstrate that R activates expression of the fatty acid synthase (FAS) cellular gene through a p38 stress mitogen-activated protein kinase-dependent mechanism. B-cell receptor engagement of Akata cells also increases FAS expression. The FAS gene product is required for de novo synthesis of the palmitate fatty acid, and high-level FAS expression is normally limited to liver, brain, lung, and adipose tissue. We show that human epithelial tongue cells lytically infected with EBV (from oral hairy leukoplakia lesions) express much more FAS than uninfected cells. Two specific FAS inhibitors, cerulenin and C75, prevent R activation of IE (Z) and early (BMRF1) lytic EBV proteins in Jijoye cells. In addition, cerulenin and C75 dramatically attenuate IE and early lytic gene expression after B-cell receptor engagement in Akata cells and constitutive lytic viral gene expression in EBV-positive AGS cells. However, FAS inhibitors do not reduce lytic viral gene expression induced by a vector in which the Z gene product is driven by a strong heterologous promoter. In addition, FAS inhibitors do not reduce R activation of a naked DNA reporter gene construct driven by the Z promoter (Zp). These results suggest that cellular FAS activity is important for induction of Z transcription from the intact latent EBV genome, perhaps reflecting the involvement of lipid-derived signaling pathways or palmitoylated proteins. Furthermore, using FAS inhibitors may be a completely novel approach for blocking the lytic form of EBV replication.

Activity of partially inhibited serine palmitoyltransferase is sufficient for normal sphingolipid metabolism and viability of HSN1 patient cells

Hereditary sensory neuropathy type I (HSN1) is a common degenerative disorder of peripheral sensory neurons. HSN1 is caused by mutations in the gene, encoding the long chain base 1 of serine palmitoyltransferase (SPT) [Nat. Genet. 27 (2001) 309]. Here, we show a 44% reduction of SPT activity in transformed lymphocytes from HSN1 patients with mutation T399G in the SPTLC1 gene. However, the decrease in SPT activity had no effect on de novo sphingolipid biosynthesis, cellular sphingolipid content, cell proliferation and death (apoptosis and necrosis). The removal of extracellular sphingolipids did not affect viability of HSN1 cells. We also found no significant difference in whole blood counts, viability, and permeability to Triton X-100 of primary lymphocytes from HSN1 patients. These results suggest that, despite the inhibition of mutant allele, the activity of nonmutant allele of STP may be sufficient for adequate sphingolipid biosynthesis and cell viability. Therefore, the neurodegeneration in HSN1 is likely to be caused by subtler and rather long-term effect(s) of these mutations such as loss of a cell-type selective facet of sphingolipid metabolism and/or function, or perhaps accumulation of toxic species, including abnormal protein(s) as in other neurodegenerations.

Elevation of ceramide in serum lipoproteins during acute phase response in humans and mice: role of serine-palmitoyl transferase

Recent studies have indicated that ceramide generated in the liver is secreted into the bloodstream as component of very-low-density lipoproteins (VLDL) and low-density lipoproteins (LDL). This manuscript investigates the effect of host acute phase response to inflammation on lipoprotein ceramide levels. In humans, two different patterns of responses were found. One group of volunteers experienced transient increases in serum ceramide at 1.5h after LPS administration. Second group showed prolonged increases that reached up to 10-fold above the basal level and continued for up to 24h. Increases in ceramide were found only in VLDL and LDL particles. LPS administration induced similar increases in mice. These increases were accompanied by activation of secreted sphingomyelinase in serum and serine-palmitoyl transferase in liver. ASMase knockout mice retained LPS-induced increases in serum ceramide, thus suggesting that the elevation of VLDL and LDL ceramide content is attributed at least in part to activation of de novo synthesis of ceramide in the liver.

Generation and metabolism of bioactive sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a bioactive lysosphingophospholipid that has been implicated in the regulation of vital biological processes. Abundant evidence indicates that S1P acts as both an intracellular messenger and an extracellular ligand for a family of five specific G protein-coupled S1P receptors (S1PRs). Cellular levels of S1P are tightly regulated in a spatio-temporal manner through its synthesis catalyzed by sphingosine kinases (SphKs) and degradation by S1P lyase (SPL) and specific S1P phosphohydrolases. Over the past decade, the identification and cloning of genes encoding S1P metabolizing enzymes has increased rapidly. Overexpression and deletion of these enzymes has provided important insights into the intracellular and the "inside-out" functions of S1P. The purpose of this review is to summarize the current knowledge of S1P metabolizing enzymes, their enzymatic properties, and their roles in the control of cellular functions by S1P.

Ceramide Is Involved inR(+)-Methanandamide-Induced Cyclooxygenase-2 Expression in Human Neuroglioma Cells

Cannabinoids have recently been shown to induce the expression of the cyclooxygenase-2 (COX-2) isoenzyme in H4 human neuroglioma cells. Using this cell line, the present study investigates the contribution of the second messenger ceramide to this signaling pathway. Incubation of cells with the endocannabinoid analog R(+)-methanandamide (R(+)-MA) was associated with an increase of intracellular ceramide levels. Enhancement of ceramide formation by R(+)-MA was abolished by fumonisin B1, a ceramide synthase inhibitor, whereas inhibitors of neutral sphingomyelinase (spiroepoxide, glutathione) and serine palmitoyltransferase (l-cycloserine, ISP-1) were inactive in this respect. R(+)-MA caused a biphasic activation of the p38 and p42/44 mitogen-activated protein kinases (MAPKs), with phosphorylation peaks occurring after 15-min and 4- to 8-h treatments, respectively. Inhibition of ceramide synthesis with fumonisin B1 was associated with a suppression of R(+)-MA-induced delayed phosphorylations of p38 and p42/44 MAPKs and subsequent COX-2 expression. The involvement of ceramide in COX-2 expression was corroborated by findings showing that C2-ceramide and neutral sphingomyelinase from Bacillus cereus caused concentration-dependent increases of COX-2 expression that were suppressed in the presence of 4-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl)imidazol (SB203580, a p38 MAPK inhibitor) or 2'-amino-3'-methoxyflavone (PD98059, a p42/44 MAPK activation inhibitor). In contrast, dihydro-C2-ceramide being used as a negative control did not induce MAPK phosphorylation and COX-2 expression. Collectively, our results demonstrate that R(+)-MA induces COX-2 expression in human neuroglioma cells via synthesis of ceramide and subsequent activation of p38 and p42/44 MAPK pathways. Induction of COX-2 expression via ceramide represents a hitherto unknown mechanism by which cannabinoids mediate biological effects within the central nervous system.