Sphingolipid base phosphorylation by cell-free preparations from Tetrahymena pyriformis

Two forms of membrane-bound sphingosine kinase in Tetrahymena and activity changes during growth and the cell cycle

Sphingosine kinase is responsible for the formation of sphingosine-1-phosphate, a sphingolipid mediator with important roles in numerous physiological processes. The sphingosine kinase activity of Tetrahymena pyriformis was recovered predominantly in the particulate fraction and it could be solubilised in 1% beta-octylglucoside. Anion-exchange chromatography resolved the beta-octylglucoside-solubilised sphingosine kinase activity into two peaks corresponding to proteins of Mr 140,000 and 80,000 respectively, as determined by subsequent size exclusion chromatography on Superdex 200. N,N-dimethylsphingosine did not inhibit the sphingosine kinase activity in either fraction, whereas D,L-threo-dihydrosphingosine inhibited sphingosine phosphorylation by the Mr 80,000 kinase but had no effect on the Mr 140,000 kinase activity. The activities also showed different stimulatory responses to Triton X-100 or NaCl. Overall, the results suggest the existence in Tetrahymena of two distinct membrane-associated sphingosine kinases. The kinase activity determined at the different culture stages showed a transient elevation at the mid-logarithmic phase. Further, the sphingosine kinase activity was examined during the synchronous cell division induced by cyclic heat treatments in T. pyriformis. We report for the first time that the sphingosine kinase activity greatly increased at 30 to 45 min after the end of heat treatment prior to the synchronous cell division (75 min), suggesting that the activity changes were associated with the cell cycle and that the up-regulated sphingosine kinase activity would be required for the initiation of the oncoming synchronous cell division in Tetrahymena cells.

CpG island of rat sphingosine kinase-1 gene: tissue-dependent DNA methylation status and multiple alternative first exons

It is generally recognized that CpG islands are not methylated in normal tissues. SPHK1 is a key enzyme catalyzing the production of sphingosine 1-phosphate, a novel signaling molecule for the proliferation and differentiation of various cells, including neural cells. Sequencing of genomic DNA and cDNA reveals that rat Sphk1a consists of six exons encoding 383 amino acids. Furthermore, we identified six alternative first exons for mRNA subtypes (Sphk1a, -b, -c, -d, -e, and -f) within a 3.7-kb CpG island. The CpG island contains a tissue-dependent, differentially methylated region (T-DMR; approximately 200 bp), which is located - 800 bp upstream of the first exon of Sphk1a. T-DMR is hypomethylated in the adult brain where Sphk1a is expressed, whereas it is hypermethylated in the adult heart where the gene is not expressed. In fetal tissues, hypomethylation of T-DMR is not associated with expression of Sphk1a, which suggests that differential availability of transcription factors is also likely to be involved in the mechanism of its expression. Here, we identify rat Sphk1, using multiple alternative first exons for the subtypes, and demonstrate that there is a CpG island bearing T-DMR.

Subcellular study of sphingoid base phosphorylation in rat tissues: evidence for multiple sphingosine kinases

The enzymatic phosphorylation of sphingoid bases was analysed in rat tissues, using D-erythro-[4,5-(3)H]sphinganine as substrate. After optimisation of the assay, taking care to block sphingosine-phosphate lyase and sphingosine phosphatase, highest ATP-dependent kinase activities were present in testis, followed by kidney, and intestinal mucosa. Approximately two thirds of the kidney activity were membrane bound, the remaining being cytosolic. Classical cell fractionation studies of kidney and liver did not allow to identify unequivocally the subcellular site of the membrane bound kinase. Separation of a particulate fraction from kidney homogenates by Percoll gradient and sucrose density gradient centrifugation revealed that kinase activities are associated with vesicles derived from the endoplasmic reticulum and the plasma membrane. Based on indirect data, such as the effect of detergents and divalent ions, the cytosolic and both membrane bound activities appear to reside in different proteins. N,N-Dimethylsphingenine was inhibitory to all three different kinases, which were mainly active towards the D-erythro isomers of sphingenine and sphinganine.

Sphingolipid transport in eukaryotic cells

Sphingolipids constitute a sizeable fraction of the membrane lipids in all eukaryotes and are indispensable for eukaryotic life. First of all, the involvement of sphingolipids in organizing the lateral domain structure of membranes appears essential for processes like protein sorting and membrane signaling. In addition, recognition events between complex glycosphingolipids and glycoproteins are thought to be required for tissue differentiation in higher eukaryotes and for other specific cell interactions. Finally, upon certain stimuli like stress or receptor activation, sphingolipids give rise to a variety of second messengers with effects on cellular homeostasis. All sphingolipid actions are governed by their local concentration. The intricate control of their intracellular topology by the proteins responsible for their synthesis, hydrolysis and intracellular transport is the topic of this review.

Human sphingosine kinase: molecular cloning, functional characterization and tissue distribution

Sphingosine-1-phosphate (SPP), the product of sphingosine kinase, is an important signaling molecule with intra- and extracellular functions. The cDNA for the mouse sphingosine kinase has recently been reported. In this paper we describe the cloning, expression and characterization of the human sphingosine kinase (huSPHK1). Sequence analysis comparison revealed that this kinase is evolutionarily very conserved, having a high degree of homology with the murine enzyme, and presenting several conserved regions with bacteria, yeast, plant, and mammalian proteins. Expressed huSPHK1 cDNA specifically phosphorylates D-erythro-sphingosine and, to a lesser extent, D, L-erythro-dihydrosphingosine, and not at all the 'threo' isoforms of dihydrosphingosine; hydroxy-ceramide or non-hydroxy-ceramide; diacylglycerol (DAG); phosphatidylinositol (PI); phosphatidylinositol-4-phosphate (PIP); or phosphatidylinositol-4, 5-bisphosphate (PIP(2)). huSPHK1 shows typical Michaelis-Menten kinetics (V(max)=56microM and K(m)=5microM). The kinase is inhibited by D,L-threo-dihydrosphingosine (K(i)=3microM), and by N, N-dimethyl-sphingosine (K(i)=5microM). Northern blots indicate highest expression in adult lung and spleen, followed by peripheral blood leukocyte, thymus and kidney, respectively. It is also expressed in brain and heart. In addition, database searches with the stSG2854 sequence indicate that huSPHK1 is also expressed in endothelial cells, retinal pigment epithelium, and senescent fibroblasts.

Purification and characterization of rat kidney sphingosine kinase

Sphingosine kinase catalyzes the formation of the bioactive sphingolipid metabolite sphingosine 1-phosphate, which plays important roles in numerous physiological processes, including growth, survival, and motility. We have purified rat kidney sphingosine kinase 6 x 10(5)-fold to apparent homogeneity. The purification procedure involved ammonium sulfate precipitation followed by chromatography on an anion exchange column. Partially purified sphingosine kinase was found to be stabilized by the presence of high salt, and thus, a scheme was developed to purify sphingosine kinase using sequential dye-ligand chromatography steps (since the enzyme bound to these matrices even in the presence of salt) followed by EAH-Sepharose chromatography. This 385-fold purified sphingosine kinase bound tightly to calmodulin-Sepharose and could be eluted in high yield with EGTA in the presence of 1 M NaCl. After concentration, the calmodulin eluate was further purified by successive high pressure liquid chromatography separations on hydroxylapatite, Mono Q, and Superdex 75 gel filtration columns. Purified sphingosine kinase has an apparent molecular mass of approximately 49 kDa under denaturing conditions on SDS-polyacrylamide gel, which is similar to the molecular mass determined by gel filtration, suggesting that the active form is a monomer. Sphingosine kinase shows substrate specificity for D-erythro-sphingosine and does not catalyze the phosphorylation of phosphatidylinositol, diacylglycerol, ceramide, DL-threo-dihydrosphingosine, or N,N-dimethylsphingosine. However, the latter two sphingolipids were potent competitive inhibitors. With sphingosine as substrate, the enzyme had a broad pH optimum of 6.6-7.5 and showed Michaelis-Menten kinetics, with Km values of 5 and 93 microM for sphingosine and ATP, respectively. This study provides the basis for molecular characterization of a key enzyme in sphingolipid signaling.

Effect of acidic phospholipids on sphingosine kinase

Sphingosine-1-phosphate (SPP) is a unique sphingolipid metabolite involved in cell growth regulation and signal transduction. SPP is formed from sphingosine in cells by the action of sphingosine kinase, an enzyme whose activity can be stimulated by growth factors. Little is known of the mechanisms by which sphingosine kinase is regulated. We found that acidic phospholipids, particularly phosphatidylserine, induced a dose-dependent increase in sphingosine kinase activity due to an increase in the apparent Vmax of the enzyme. Other acidic phospholipids, such as phosphatidylinositol, phosphatidic acid, phosphatidylinositol bisphosphate, and cardiolipin stimulated sphingosine kinase activity to a lesser extent than phosphatidylserine, whereas neutral phospholipids had no effect. Diacylglycerol, a structurally similar molecule which differs from phosphatidic acid in the absence of the phosphate group, failed to induce any changes in sphingosine kinase activity. Our results suggest that the presence of negative charges on the lipid molecules is important for the potentiation of sphingosine kinase activity, but the effect does not directly correlate with the number of negative charges. These results also support the notion that the polar group confers specificity in the stimulation of sphingosine kinase by acidic glycerophospholipids. The presence of a fatty acid chain in position 2 of the glycerol backbone was not critical since lysophosphatidylserine also stimulated sphingosine kinase, although it was somewhat less potent. Dioleoylphosphatidylserine was the most potent species, including a fourfold stimulation, whereas distearoyl phosphatidylserine was completely inactive. Thus, the degree of saturation of the fatty acid chain of the phospholipids may also play a role in the activation of sphingosine kinase.

Partial purification and properties of diglyceride kinase from Escherichia coli

Diglyceride kinase (diacylglycerol kinase, E.C. 2.7.1.-), an enzyme localized in the inner membrane of Escherichia coli, has been purified about 600-fold. The purified enzyme exhibits an absolute requirement for magnesium ion; its activity toward both lipid and nucleotide substrates is stimulated by diphosphatidylglycerol or other phospholipids. Adenine nucleotides are much better substrates for the enzyme than are other purine or pyrimidine nucleotides. The purified enzyme preparation catalyzes the phosphorylation of a number of lipids, including ceramide and several ceramide and diacylglycerol-like analogs. The broad lipid substrate specificity of diglyceride kinase suggests that this enzyme may function in vivo for the phosphorylation of an acceptor other than diacylglycerol.