Sphingolipid Rheostat Research Articles

Expression of the Follicular Lymphoma Variant Translocation 1 Gene in Diffuse Large B-Cell Lymphoma Correlates With Subtype and Clinical Outcome

Sphingolipids serve an important role as effector molecules in signaling pathways bearing on apoptosis and cell survival. The balance between proapoptotic ceramide and prosurvival sphingosine-1-phosphate, sometimes termed the "sphingolipid rheostat," has received particular attention. Less well studied is the role of the follicular lymphoma variant translocation 1 (FVT1) gene, which was identified through its involvement in an atypical follicular lymphoma translocation and which encodes an enzyme in the synthetic pathway of ceramide. We investigated the expression of FVT1 in a variety of B-cell non-Hodgkin lymphomas and found that FVT1 is significantly underexpressed by germinal center-type diffuse large B-cell lymphoma (DLBCL) when compared with non-germinal center-type DLBCL, follicular lymphoma, and normal tonsil control samples. Increased expression of FVT1 correlated with decreased survival, suggesting that changes in the expression of FVT1 and in the concentrations of bioactive sphingolipids may be important in the pathogenesis and treatment of some types of DLBCL.

Implications of sphingosine kinase 1 expression level for the cellular sphingolipid rheostat: relevance as a marker for daunorubicin sensitivity of leukemia cells

We recently reported increased sphingosine kinase 1 (SPHK1) and decreased neutral sphingomyelinase 2 (NSMase2) gene expression in myelodysplastic syndromes and acute leukemia. This alteration is supposed to change the cellular sphingolipid metabolites; however, positive correlations were observed between daunorubicin (DA)-IC50 and the SPHK1 message but not between DA-IC50 and NSMase2 messages, when 16 different leukemia cell lines were used to analyze the relationship between gene expressions and chemosensitivity against DA. Using two cell lines with either the highest or lowest SPHK1 expression, cellular ceramides and sphingosine 1-phosphate (S1P) were quantified by liquid chromatography/mass spectrometry. Increased ceramide was observed in DA-sensitive, but not in DA-resistant cell lines treated with low doses of DA. Upon DA treatment, S1P decreased more in the sensitive cell lines than in resistant cell lines. A SPHK inhibitor recovered the DA sensitivity of DA-resistant cells. The modulation of SPHK1 gene expression by either overexpression or using siRNA affected the DA sensitivity of representative cell lines. Results clearly show that SPHK1 is both a good marker to predict the DA sensitivity of leukemia cells and a potential therapeutic target for leukemia with high SPHK1 expression, and suggest that the sphingolipid rheostat plays a significant role in DA-induced cytotoxicity.

Microarray analysis of altered sphingolipid metabolism reveals prognostic significance of sphingosine kinase 1 in breast cancer

Beside their structural role for the cell membrane the family of sphingolipids act as effector molecules in signal transduction with links to various aspects of cancer initiation, progression and treatment response. The "sphingolipid rheostat" balances between apoptosis inducing ceramid and growth promoting sphingosine-1-phosphate. We analyzed gene expression of 43 proteins from this pathway in different subtypes of breast cancer using microarray data of 1,269 tumor samples (test set n=171; validation sets n=1098) and observed significant differences for several genes. Sphingosine kinase 1 (SPHK1), ceramide galactosyltransferase (UGT8), and Ganglioside GD3-Synthase (ST8SIA1) displayed higher expression among ER negative tumors. In contrast, glucosylceramidsynthase (GCS), dihydroceramidsynthases (LASS4, LASS 6) and acid ceramidase (ASAH1) were higher expressed in ER positive samples. Survival analysis revealed a worse outcome of patients with high SPHK1 expression. To avoid a confounding effect of the ER status we also restricted the analysis to 750 patients with ER positive tumors. Again a worse outcome was observed for tumors displaying high SPHK1 expression. While 75.8+/-1.9% of the patients with tumors low in SPHK1 expression were free of metastasis at 5 years, this was the case for only 64.9+/-3.6% of patients with tumors displaying high SPHK1 expression (P=0.008). Immunohistochemistry identified the carcinoma cells as the major source of SPHK1 expression in the tumor. The correlation of SPHK1 with a poor prognosis as well as its high expression among ER negative tumors are in line with the antiapoptotic and proliferative properties of its product sphingosine-1-phosphate. Targeting of the sphingolipid rheostat may thus open new treatment options.

Disruption of Retinoic Acid Receptor Alpha Reveals the Growth Promoter Face of Retinoic Acid

BackgroundRetinoic acid (RA), the bioactive derivative of Vitamin A, by epigenetically controlling transcription through the RA-receptors (RARs), exerts a potent antiproliferative effect on human cells. However, a number of studies show that RA can also promote cell survival and growth. In the course of one of our studies we observed that disruption of RA-receptor alpha, RARα, abrogates the RA-mediated growth-inhibitory effects and unmasks the growth-promoting face of RA (Ren et al., Mol. Cell. Biol., 2005, 25:10591). The objective of this study was to investigate whether RA can differentially govern cell growth, in the presence and absence of RARα, through differential regulation of the “rheostat” comprising ceramide (CER), the sphingolipid with growth-inhibitory activity, and sphingosine-1-phosphate (S1P), the sphingolipid with prosurvival activity.Methodology/Principal FindingsWe found that functional inhibition of endogenous RARα in breast cancer cells by using either RARα specific antagonists or a dominant negative RARα mutant hampers on one hand the RA-induced upregulation of neutral sphingomyelinase (nSMase)-mediated CER synthesis, and on the other hand the RA-induced downregulation of sphingosine kinase 1, SK1, pivotal for S1P synthesis. In association with RA inability to regulate the sphingolipid rheostat, cells not only survive, but also grow more in response to RA both in vitro and in vivo. By combining genetic, pharmacological and biochemical approaches, we mechanistically demonstrated that RA-induced growth is, at least in part, due to non-RAR-mediated activation of the SK1-S1P signaling.Conclusions/SignificanceIn the presence of functional RARα, RA inhibits cell growth by concertedly, and inversely, modulating the CER and S1P synthetic pathways. In the absence of a functional RARα, RA–in a non-RAR-mediated fashion–promotes cell growth by activating the prosurvival S1P signaling. These two distinct, yet integrated processes apparently concur to the growth-promoter effects of RA.

Chasing sphingosine-1-phosphate, a lipid mediator for cardiomyocyte survival

See article by Tao et al. [5] (pages 56–63) in this issue. Sphingosine-1-phosphate (S1P) is a sphingolipid metabolite that regulates many essential biological processes in various cells and tissues. S1P may act as an extracellular ligand to specific G protein-coupled S1P receptors (S1P1–5) or as an intracellular second messenger. Sphingosine is derived from cleavage of ceramide by ceramidases in the sphingolipid degradative pathway. It can be catalyzed by sphingosine kinases (SphK1 and 2) into S1P, which is then degraded by a S1P lyase and S1P phosphohydrolases. S1P is most abundant in platelets, but it can also be synthesized in various cells such as cardiomyocytes. A “sphingolipid rheostat” hypothesis suggests that the relative levels of these lipids are important determinants of cell fate [1]. In fact, both ceramide and S1P are known to be involved in cardiomyocyte survival and apoptosis. It has been shown that ceramide induces cardiomyocyte apoptosis in the heart [2], and it has been proposed that it is a mediator of cardiomyocyte death in myocardial ischemic–reperfusion injury [3]. On the contrary, S1P exerts an anti-apoptosis effect in rat cultured neonatal cardiomyocytes [4]. … *Tel.: +1 404 756 5056; fax: +1 404 742 1042. Email address: qyang{at}msm.edu

Shooting the Messenger

Ischemia reduces oxygen availability (hypoxia) and can thus lead to tissue damage. However, restoration of blood flow, or reperfusion, has been shown initiate a second phase of injury. Ischemia/reperfusion (I/R) injury has been linked to the generation of reactive oxygen species (ROS), which in turn lead to tissue damage through induction of apoptosis.1 Indeed, I/R injury is reduced by the administration of antioxidants. In addition to nonspecific targets such as lipids and DNA, ROS are becoming appreciated as second messengers involved in many intracellular signaling pathways.1 Still, many of the pathways of ROS-dependent signaling remain to be elucidated. ROS signaling may be mediated in part through sphingolipid metabolites. Sphingolipids are ubiquitous components of the lipid bilayer of eukaryotic cells consisting of a head group attached to ceramide. Ceramide is synthesized de novo or formed by the degradation of sphingolipids, such as sphingomyelin. Ceramide can be further deacylated to sphingosine which can then be phosphorylated by sphingosine kinases (SphK), forming sphingosine-1-phosphate (S1P). Ceramide and S1P are both potent signaling molecules with opposing biological effects. Ceramide is an important regulatory component of stress responses, typically inducing growth arrest and apoptosis.2 In contrast, S1P promotes cell proliferation and survival.3,4 Whereas stresses activate sphingomyelinases, producing ceramide leading to apoptosis, survival factors activate SphK1, resulting in accumulation of S1P and consequent suppression of ceramide-mediated apoptosis.4,5 It has been suggested that the dynamic balance between intracellular S1P and ceramide, the “sphingolipid rheostat (sphingostat)”, and the consequent regulation of opposing signaling pathways, are important factors determining cell fate.5 There is growing evidence that the sphingostat also plays a role in cellular response to oxidative stress. I/R injury leads to ROS generation and apoptotic cell death, and ceramide generation has been implicated in mediating cell death in response to ROS in a …

Toward Solving the Riddle

See related article, pages 468–476 “ It is a riddle wrapped in a mystery inside an enigma .”1 The mythical Greek Sphinx posed riddles to passers-by. Those who could not answer met their demise until Oedipus solved the riddle and the Sphinx self-destructed. Ultimately the terms sphinx and enigma became associated and in 1884 J.L.W. Thudichum, known as the “Father of Neurochemistry,” named the chemical backbone of sphingolipids for their enigmatic “Sphinx-like” properties. Recently, studies of sphingolipid actions have achieved increasing importance in understanding the pathophysiology of acute ischemia/reperfusion injury through studies of sphingosine kinase (SphK) and its end-product, sphingosine 1-phosphate (S1P). Activation of SphK is the final and rate-limiting enzymatic step in the synthesis of S1P, which is an intracellular and extracellular signaling molecule that regulates many important cellular processes including growth, survival, differentiation, cytoskeletal rearrangements, motility, angiogenesis, and calcium mobilization. In contrast, the sphingosine precursor ceramide is a growth-inhibiting lipid implicated in differentiation and apoptosis. These findings have led to the so-called “sphingolipid rheostat” hypothesis, which proposes that the relative levels of these lipids are important determinants of cell fate.2 It is now accepted that many of the actions of S1P are mediated by a family of G protein–coupled S1P receptor isoforms, termed S1P1–5. With regard to the cardiovascular system, platelets store and release S1P, which is known to regulate endothelial function and signaling. There is a substantial gradient between intracellular and extracellular concentrations of S1P. In human serum estimates of S1P concentration range from 0.5 to 1 μmol/L or more, whereas intracellular levels vary among tissues but are much lower possibly due in part to export of S1P or SphK (Figure). Also implicated in the regulation of S1P levels are degradation by both S1P lyase and dephosporylation by S1P phosphohydrolase. About 60% …

Effects of N,N‐Dimethylsphingosine on Sphingosine Kinase

Sphingosine-1-phosphate (S1P) is a bioactive lipid that regulates a myriad of important cellular processes including growth, survival, cytoskeleton rearrangements, motility, angiogenesis, and immunity. There are two distinct sphingosine kinase isoforms, SphK1 and SphK2, which produce S1P. N,N-dimethylsphingosine (DMS), a methyl derivative of sphingosine, is a potent inhibitor of both sphingosine kinases. By inhibiting SphK activity, DMS alters the sphingolipid rheostat shifting the balance towards the pro-apoptotic sphingosine and ceramide, and away from the anti-apoptotic S1P. Though previous experiments show that DMS reduces S1P levels in cells, surprisingly, we have now found that treatment of U937 leukemia cells or Jurkat T lymphoma cells with DMS causes SphK1 protein levels to be drastically increased 24 hours after treatment. In addition, western blotting with a specific anti-SphK1 antibody revealed that the DMS effect was dose-dependent. Yet SphK1 activity remained markedly reduced. Preliminary results suggest that the increase in SphK1 is independent of both translation and transcription as treatment with actinomycin D (a transcription inhibitor) or cycloheximide (a translation inhibitor) did not prevent DMS-induced increases in SphK1. Experiments are now underway to determine how DMS increases SphK1 levels.