Sphingolipid metabolites have emerged as critical players in a number of fundamental biological processes. Among them, sphingosine-1-phosphate (S1P) promotes cell survival and proliferation, in contrast to ceramide and sphingosine, which induce cell growth arrest and apoptosis. These sphingolipids with opposing functions are interconvertible inside cells, suggesting that a finely tuned balance between them can determine cell fate. Sphingosine kinases (SphKs), which catalyze the phosphorylation of sphingosine to S1P, are critical regulators of this balance. Of the two identified SphKs, sphingosine kinase type 1 (SphK1) has been shown to regulate various processes important for cancer progression and will be the focus of this review, since much less is known of biological functions of SphK2, especially in cancer. SphK1 is overexpressed in various types of cancers and upregulation of SphK1 has been associated with tumor angiogenesis and resistance to radiation and chemotherapy. Many growth factors, through their tyrosine kinase receptors (RTKs), stimulate SphK1 leading to a rapid increase in S1P. This S1P in turn can activate S1P receptors and their downstream signaling. Conversely, activation of S1P receptors can induce transactivation of various RTKs. Thus, SphK1 may play important roles in S1P receptor RTK amplification loops. Here we review the role of SphK1 in tumorigenesis, hormonal therapy, chemotherapy resistance, and as a prognostic marker. We will also review studies on the effects of SphK inhibitors in cells in vitro and in animals in vivo and in some clinical trials and highlight the potential of SphK1 as a new target for cancer therapeutics.

The potent bioactive sphingolipid mediator, sphingosine-1-phosphate (S1P), is produced by 2 sphingosine kinase isoenzymes, SphK1 and SphK2. Expression of SphK1 is up-regulated in cancers, including leukemia, and associated with cancer progression. A screen of sphingosine analogs identified (2R,3S,4E)-N-methyl-5-(4-pentylphenyl)-2-aminopent-4-ene-1,3-diol, designated SK1-I (BML-258), as a potent, water-soluble, isoenzymespecific inhibitor of SphK1. In contrast to pan-SphK inhibitors, SK1-I did not inhibit SphK2, PKC, or numerous other protein kinases. SK1-I decreased growth and survival of human leukemia U937 and Jurkat cells, and enhanced apoptosis and cleavage of Bcl-2. Lethality of SK1-I was reversed by caspase inhibitors and by expression of Bcl-2. SK1-I not only decreased S1P levels but concomitantly increased levels of its proapoptotic precursor ceramide. Conversely, S1P protected against SK1-I-induced apoptosis. SK1-I also induced multiple perturbations in activation of signaling and survival-related proteins, including diminished phosphorylation of ERK1/2 and IntroductionSphingosine-1-phosphate (S1P), a potent lipid mediator produced from sphingosine by sphingosine kinases (SphKs), regulates many processes important for cancer progression, including cell growth and survival. 1 In contrast to S1P, its precursors, sphingosine and ceramide, are associated with growth arrest and induction of apoptosis. 2 Thus, the balance between these interconvertible sphingolipid metabolites has been viewed as a cellular rheostat determining cell fate. 3 Numerous studies have shown that perturbations in the S1P/ceramide rheostat are involved in the regulation of resistance to chemotherapy and radiation therapy of neoplastic cells, including those of hematopoietic origin. 2,4,5 Two SphK isoenzymes, SphK1 and SphK2, have been described that, although sharing many features, 6,7 exhibit distinct functions. SphK1 promotes cell growth and survival, [8][9][10][11] whereas SphK2, when overexpressed, has opposite effects. 12,13 SphK1 is a key enzyme that regulates the S1P/ceramide rheostat. 12,14,15 Indeed, S1P and SphK1 have long been implicated in resistance of both primary leukemic cells and leukemia cell lines to apoptosis induced by commonly used cytotoxic agents. 3,[16][17][18] Non-isoenzyme-specific inhibitors of SphKs, such as L-threo-dihydrosphingosine (safingol) and N,N-dimethylsphingosine (DMS), are cytotoxic to leukemia cells. 18,19 Interestingly, multidrugresistant HL-60 myelogenous leukemia cells were more sensitive to DMS than the parental cells. 18 Moreover, SphK1 activity was lower in HL-60 cells sensitive to doxorubicin or etoposide than in MDR (multidrug resistance protein)-1-or MRP1 (multidrug resistance protein 1)-positive HL-60 cells. Enforced expression of SphK1 in sensitive HL-60 cells blocked apoptosis, whereas down-regulation of SphK1 overcame chemoresistance by inducing mitochondria-dependent apoptosis. 10 These observations take on added significance in light of evidence that MDR expression is a strong p...

Systemic exacerbation of allergic responses, in which mast cells play a critical role, results in life-threatening anaphylactic shock. Sphingosine-1–phosphate (S1P), a ligand for a family of G protein–coupled receptors, is a new addition to the repertoire of bioactive lipids secreted by activated mast cells. Yet little is known of its role in human mast cell functions and in anaphylaxis. We show that S1P2 receptors play a critical role in regulating human mast cell functions, including degranulation and cytokine and chemokine release. Immunoglobulin E–triggered anaphylactic responses, including elevation of circulating histamine and associated pulmonary edema in mice, were significantly attenuated by the S1P2 antagonist JTE-013 and in S1P2-deficient mice, in contrast to anaphylaxis induced by administration of histamine or platelet-activating factor. Hence, S1P and S1P2 on mast cells are determinants of systemic anaphylaxis and associated pulmonary edema and might be beneficial targets for anaphylaxis attenuation and prophylaxis.

Sphingosine-1-phosphate is a potent sphingolipid mediator of diverse processes important for brain tumors, including cell growth, survival, migration, invasion, and angiogenesis. Sphingosine kinase 1 (SphK1

Glycosyltransferases catalyze the synthesis of glycoconjugates by transferring a properly activated sugar residue to an appropriate acceptor molecule or aglycone for chain initiation and elongation. The acceptor can be a lipid, a protein, a heterocyclic compound, or another carbohydrate residue. A catalytic reaction is believed to involve the recognition of both the donor and acceptor by suitable domains, as well as the catalytic site of the enzyme. To elucidate the structural requirements for substrate recognition and catalytic reactions of glycosyltransferases, we have searched the databases for homologous sequences, identified conserved amino acid residues, and proposed potential domain motifs for these enzymes. Depending on the configuration of the anomeric functional group of the glycosyl donor molecule and of the resulting glycoconjugate, all known glycosyltransferases can be divided into two major types: retaining glycosyltransferases, which transfer sugar residue with the retention of anomeric configuration, and inverting glycosyltransferases, which transfer sugar residue with the inversion of anomeric configuration. One conserved domain of the inverting glycosyltransferases identified in the database is responsible for the recognition of a pyrimidine nucleotide, which is either the UDP or the TDP portion of a donor sugar-nucleotide molecule. This domain is termed "Nucleotide Recognition Domain 1 beta," or NRD1 beta, since the type of nucleotide is the only common structure among the sugar donors and acceptors. NRD1 beta is present in 140 glycosyltransferases. The central portion of the NRD1 beta domain is very similar to the domain that is present in one family of retaining glycosyltransferases. This family is termed NRD1 alpha to designate the similarity and stereochemistry of sugar transfer, and it consists of 77 glycosyltransferases identified thus far. In the central portion there is a homologous region for these two families and this region probably has a catalytic function. A third conserved domain is found exclusively in membrane-bound glycosyltransferases and is termed NRD2; this domain is present in 98 glycosyltransferases. All three identified NRDs are present in archaebacterial, eubacterial, viral, and eukaryotic glycosyltransferases. The present article presents the alignment of conserved NRD domains and also presents a brief overview of the analyzed glycosyltransferases which comprise about 65% of all known sugar-nucleotide dependent (Leloir-type) and putative glycosyltransferases in different databases. A potential mechanism for the catalytic reaction is also proposed. This proposed mechanism should facilitate the design of experiments to elucidate the regulatory mechanisms of glycosylation reactions. Amino acid sequence information within the conserved domain may be utilized to design degenerate primers for identifying DNA encoding new glycosyltransferases.