Sphingosine kinase (Sphk) enzymes are important in intracellular sphingolipid metabolism as well as in the biosynthesis of sphingosine 1-phosphate (S1P), an extracellular lipid mediator. Here, we show that Sphk1 is expressed and is required for small intestinal tumor cell proliferation in Apc Min/؉ mice. Adenoma size but not incidence was dramatically reduced in Apc Min/؉ Sphk ؊/؊ mice. Concomitantly, epithelial cell proliferation in the polyps was significantly attenuated, suggesting that Sphk1 regulates adenoma progression. Although the S1P receptors (S1P1R, S1P2R, and S1P3R) are expressed, polyp incidence or size was unaltered in Apc Min/؉ S1p2r ؊/؊ , Apc Min/؉ S1p3r ؊/؊ , and Apc Min/؉ S1p1r ؉/؊ bigenic mice. These data suggest that extracellular S1P signaling via its receptors is not involved in adenoma cell proliferation. Interestingly, tissue sphingosine content was elevated in the adenomas of Apc Min/؉ Sphk1 ؊/؊ mice, whereas S1P levels were not significantly altered. Concomitantly, epithelial cell proliferation and the expression of the G 1 /S cell cycle regulator CDK4 and c-myc were diminished in the polyps of Apc Min/؉ Sphk1 ؊/؊ mice. In rat intestinal epithelial (RIE) cells in vitro, Sphk1 overexpression enhanced cell cycle traverse at the G 1 /S boundary. In addition, RIE cells treated with sphingosine but not C6-ceramide exhibited reduced cell proliferation, reduced retinoblastoma protein phosphorylation, and cyclin-dependent kinase 4 (Cdk4) expression. Our findings suggest that Sphk1 plays a critical role in intestinal tumor cell proliferation and that inhibitors of Sphk1 may be useful in the control of intestinal cancer.The concept that sphingolipid metabolism is an important source of signaling lipids has gained considerable acceptance. For example, sphingomyelin, an abundant membrane phospholipid, is metabolized into lipid mediators such as sphingosine, ceramide, ceramide 1-phosphate, and sphingosine 1-phosphate (S1P) (9). S1P, synthesized by the action of sphingosine kinase (Sphk), is secreted by cells and functions as an extracellular mediator by activating a family of G-protein-coupled receptors termed S1PR1-5 (13, 19, 30). The generality of S1P signaling in vertebrates is underscored by recent findings that S1PRs are needed for a multitude of physiological processes, including heart and vascular development, angiogenesis, and immune cell trafficking (12). However, the physiological and pathological significance of intracellular sphingolipid metabolism in vertebrates is virtually unknown. However, a recent study showed that the inhibition of S1P lyase resulted in alterations in tissue S1P levels, which influenced T-cell trafficking and immune responses (35). The importance of sphingolipid metabolism is underscored by its impact on cell death, stress responses, metabolism, and animal development in unicellular (Saccharomyces cerevisiae and Dictyostelium discoidium) and multicellular (Caenorhabditis elegans and Drosophila melanogaster) eukaryotes (4,5,10,21,26,28).Mammals express two functiona...

Although a number of cellular components of cytokinesis have been identified, little is known about the detailed mechanisms underlying this process. Here, we report that the lipid metabolite psychosine (galactosylsphingosine), derived from galactosylceramide, induced formation of multinuclear cells from a variety of nonadherent and adherent cells due to inhibition of cytokinesis. When psychosine was added to the human myelomonocyte cell line U937, which was the most sensitive among the cell lines tested, cleavage furrow formed either incompletely or almost completely. However, abnormal contractile movement was detected in which the cellular contents of one of the hemispheres of the contracting cell were transferred into its counterpart. Finally, the cleavage furrow disappeared and cytokinesis was reversed. Psychosine treatment also induced giant clots of actin filaments in the cells that probably consisted of small vacuoles with filamentous structures, suggesting that psychosine affected actin reorganization. These observations could account for the formation of multinuclear globoid cells in the brains of patients with globoid cell leukodystrophy, a neurological disorder characterized by the accumulation of psychosine due to galactosylceramidase deficiency.

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ISP-1 (myriocin) is a potent inhibitor of serine palmitoyltransferase, the primary enzyme of sphingolipid biosynthesis, and is a useful tool for studying the biological functions of sphingolipids in both mammals and yeast (Saccharomyces cerevisiae). In a previous study, we cloned yeast multicopy suppressor genes for ISP-1, and one of these, YPK1/SLI2, was shown to encode a serine/threonine kinase which is a yeast homologue of mammalian SGK1 (serum/glucocorticoid-regulated kinase 1). In the present study, another gene, termed SLI1 (YGR212W; GenBank accession number CAA97239.1), was characterized. Sli1p has weak similarity to Atf1p and Atf2p, which are alcohol acetyltransferases. Although a sli1-null strain grew normally, the IC50 of ISP-1 for the growth of this strain was markedly decreased compared with that for the parental strain, indicating that Sli1p is a major contributor to ISP-1 resistance in yeast. On a sli1-null background, the increase in resistance to ISP-1 induced by YPK1 gene transfection was almost abolished. These data indicate that Sli1p co-operates with Ypk1p in mediating resistance to ISP-1 in yeast. Sli1p was found to convert ISP-1 into N-acetyl-ISP-1 in vitro. Furthermore, N-acetyl-ISP-1 did not share the ability of ISP-1 to inhibit the growth of yeast cells, and the serine palmitoyltransferase inhibitory activity of N-acetyl-ISP-1 was much lower than that of ISP-1. These data suggest that Sli1p inactivates ISP-1 due to its N-acetyltransferase activity towards ISP-1.