Identification of genetic and epigenetic similarities of SPHK1/Sphk1 in mammals

Importin α5 Regulates Anxiety through MeCP2 and Sphingosine Kinase 1

Importins mediate transport from synapse to soma and from cytoplasm to nucleus, suggesting that perturbation of importin-dependent pathways should have significant neuronal consequences. A behavioral screen on five importin α knockout lines revealed that reduced expression of importin α5 (KPNA1) in hippocampal neurons specifically decreases anxiety in mice. Re-expression of importin α5 in ventral hippocampus of knockout animals increased anxiety behaviors to wild-type levels. Hippocampal neurons lacking importin α5 reveal changes in presynaptic plasticity and modified expression of MeCP2-regulated genes, including sphingosine kinase 1 (Sphk1). Knockout of importin α5, but not importin α3 or α4, reduces MeCP2 nuclear localization in hippocampal neurons. A Sphk1 blocker reverses anxiolysis in the importin α5 knockout mouse, while pharmacological activation of sphingosine signaling has robust anxiolytic effects in wild-type animals. Thus, importin α5 influences sphingosine-sensitive anxiety pathways by regulating MeCP2 nuclear import in hippocampal neurons.

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Reduced expression of importin α5 in hippocampal neurons decreases anxiety

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Importin α5 is required for nuclear localization of MeCP2 in hippocampal neurons

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Importin α5 knockout increases expression of Sphk1, an MeCP2-regulated gene

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Pharmacological modulation of Sphk1 and the S1P receptor affects anxiety

Regulation of tissue-specific expression of renal organic anion transporters by hepatocyte nuclear factor 1 α/β and DNA methylation

We have reported previously that the kidney- and liver-specific expression of transporters in mice involves the coordinated regulation by hepatocyte nuclear factor 1 (HNF1) and DNA methylation. The present study was aimed at investigating the role of this cascade in the transcriptional regulation of renal organic anion transporters (OATs) yet to be characterized in human and mouse. Luciferase assays and electrophoretic mobility-shift assays demonstrated that HNF1α/β enhances the promoter activity of OAT4/SLC22A11 via binding to the HNF1 motif located near the transcriptional start site (TSS). DNA methylation profiles of human OAT1, OAT3, OAT4, and urate transporter 1 (URAT1) were determined in human liver and kidney cortex by bisulfite sequencing. Most of the CpG dinucleotides around the TSSs of OAT1 and OAT3 were highly methylated in the liver compared with kidney cortex, being consistent with their tissue specificity, whereas the difference in the DNA methylation status was less remarkable between the two tissues for OAT4 and URAT1. Mouse Oat1 gene also contained CpG dinucleotides hypomethylated in the kidney and hypermethylated in the liver downstream its TSS, whereas two of the seven CpG dinucleotides around the TSS of mouse Oat3 were significantly methylated in the liver compared with the kidney. Taken together, these findings underscored the central role of HNF1α/β in the transcriptional regulation of OATs and highlighted DNA methylation-dependent gene silencing as one of the mechanisms underlying the tissue-specific transactivation by this master regulator.

Impact of DNA methylation on trophoblast function

The influence of epigenetics is evident in many fields of medicine today. This is also true in placentology, where versatile epigenetic mechanisms that regulate expression of genes have shown to have important influence on trophoblast implantation and placentation. Such gene regulation can be established in different ways and on different molecular levels, the most common being the DNA methylation. DNA methylation has been shown today as an important predictive component in assessing clinical prognosis of certain malignant tumors; in addition, it opens up new possibilities for non-invasive prenatal diagnosis utilizing cell-free fetal DNA methods. By using a well known demethylating agent 5-azacytidine in pregnant rat model, we have been able to change gene expression and, consequently, the processes of trophoblast differentiation and placental development. In this review, we describe how changes in gene methylation effect trophoblast development and placentation and offer our perspective on use of trophoblast epigenetic research for better understanding of not only placenta development but cancer cell growth and invasion as well.

Interleukin-1 regulates the expression of sphingosine kinase 1 in glioblastoma cells

Chronic inflammation and inflammatory cytokines have recently been implicated in the development and progression of various types of cancer. In the brain, neuroinflammatory cytokines affect the growth and differentiation of both normal and malignant glial cells, with interleukin 1 (IL-1) shown to be secreted by the majority of glioblastoma cells. Recently, elevated levels of sphingosine kinase 1 (SphK1), but not SphK2, were correlated with a shorter survival prognosis for patients with glioblastoma multiforme. SphK1 is a lipid kinase that produces the pro-growth, anti-apoptotic sphingosine 1-phosphate, which can induce invasion of glioblastoma cells. Here, we show that the expression of IL-1 correlates with the expression of SphK1 in glioblastoma cells, and neutralizing anti-IL-1 antibodies inhibit both the growth and invasion of glioblastoma cells. Furthermore, IL-1 up-regulates SphK1 mRNA levels, protein expression, and activity in both primary human astrocytes and various glioblastoma cell lines; however, it does not affect SphK2 expression. The IL-1-induced SphK1 up-regulation can be blocked by the inhibition of JNK, the overexpression of the dominant-negative c-Jun(TAM67), and the down-regulation of c-Jun expression by small interference RNA. Activation of SphK1 expression by IL-1 occurs on the level of transcription and is mediated via a novel AP-1 element located within the first intron of the sphk1 gene. In summary, our results suggest that SphK1 expression is transcriptionally regulated by IL-1 in glioblastoma cells, and this pathway may be important in regulating survival and invasiveness of glioblastoma cells.

Regulation and functions of sphingosine kinases in the brain

It has long been known that sphingolipids, especially sphingomyelin, a principal component of myelin, are highly enriched in the central nervous system and are structural components of all eukaryotic cell membranes. In the last few years, substantial evidence has accumulated from studies of many types of cells demonstrating that in addition to their structural roles, their breakdown products form a new class of signaling molecules with potent and myriad regulatory effects on essentially every cell in the body. While the sphingolipid metabolites sphingosine and its precursor ceramide have been associated with cell growth arrest and apoptosis, sphingosine-1-phosphate (S1P) enhances proliferation, differentiation, and cell survival as well as regulates many physiological and pathological processes. The relative levels of these three interconvertible sphingolipid metabolites, and thus cell fate, are strongly influenced by the activity of sphingosine kinases, of which there are two isoforms, designated SphK1 and SphK2, the enzymes that phosphorylate sphingosine to produce S1P. Not much is yet known of the importance of S1P in the central nervous system. Therefore, this review is focused on current knowledge of regulation of SphK1 and SphK2 on both transcriptional and post-translational levels and the functions of these isozymes and their product S1P and its receptors in the central nervous system.

A new class of tissue-specifically methylated regions involving entire CpG islands in the mouse

CpG islands, which have higher GC content and CpG frequencies compared to the genome as a whole, are generally believed to be unmethylated in tissues except at promoters of genes undergoing X chromosome inactivation or genomic imprinting. Recent studies, however, have shown that CpG islands at promoters of a number of genes contain tissue-dependent, differentially methylated regions (T-DMRs). In general, the tissue-specific methylation is restricted to a part of the promoter CpG island, with hypomethylation of the remaining sequence. In the current study, using comparison between Restriction Landmark Genomic Scanning (RLGS) and in silico RLGS, we identified ten sperm-specific unmethylated NotI sites, T-DMRs located in CpG islands that were hypomethylated in sperm but near-completely methylated in the kidney and brain. Unusually, these T-DMRs involve the whole CpG island at each of these loci. We characterized one of these genes, adenine nucleotide translocator 4 (Ant4), which is expressed in germ cells. Using a promoter assay, we demonstrated that expression of Ant4 gene is controlled by DNA methylation at the CpG island sequences within the promoter region. Ant4 and other sperm-specific hypomethylated loci represent a new class of CpG islands that become completely methylated in different cell lineages. T-DMRs at CpG islands are functionally important gene regulatory elements that may now be categorized into two classes: T-DMRs involving a subregion of the CpG island and those that occupy the whole CpG island.

Methylation of a conserved intronic CpG island of mouse SF-1 is associated with cell-specific expression of SF-1 in a culture system but not with tissue-specific expression

The mechanism for the steroidogenic tissue or cell-specific expression of SF-1 has not been well clarified. We examined whether the methylation status of a large CpG island in the first intron of mouse SF-1 gene is associated with the expression level of SF-1 in cultured cells and in tissues. The island consists of three small islands (ICI-1, ICI-2, and ICI-3). In cultured adrenocortical Y-1 cells and in Leydig tumor cells, I-10, that both express high levels of SF-1, the upstream region of ICI-2, ICI-2-1, was clearly hypomethylated compared to cultured mouse bone marrow cells that do not express SF-1. However, this methylation status was not clearly associated with the tissue-specific expression of SF-1, in either adult or during development. These results suggest that methylation of ICI-2-1of SF-1 may partly determine the level of SF-1 expression at the cellular level, but may not be essential at the tissue level.

Cloning and characterization of rat sphingosine kinase 1 with an N-terminal extension

Sphingosine kinase (SK) is an enzyme that converts sphingosine to sphingosine 1-phosphate, a lysophospholipid with various cellular functions. Here, we report cloning and characterization of a novel isoform of rat SK1 (rSK1) with an N-terminal extension (long-rSK1). Long-rSK1 is 458 amino acid-long and has a calculated molecular weight of 49.8kDa. Deduced amino acid sequences of rat and human long-SK1 have high homology (71.3% identity and 78.9% similarity). Long-rSK1 mRNA is widely expressed throughout the tissues including brain, heart, lung, liver, kidney, and spleen, whereas mRNA encoding rSK1 has been shown not to be expressed in heart or liver. When the long-rSK1 was transfected in COS-7 cells, SK activity was 53-fold increased. Substrate specificity and dependency on divalent cations of long-rSK1 were similar to those of rSK1. Taken together, the fact that long-rSK1 with similar enzymatic characteristics to rSK1 displays differential tissue distribution may suggest an additional role of long-rSK1.

Extracellular nucleotides induce migration of renal mesangial cells by upregulating sphingosine kinase-1 expression and activity

BACKGROUND AND PURPOSE

Extracellular nucleotides act as potent mitogens for renal mesangial cells (MC). In this study we determined whether extracellular nucleotides trigger additional responses in MCs and the mechanisms involved.

EXPERIMENTAL APPROACH

MC migration was measured after nucleotide stimulation in an adapted Boyden-chamber. Sphingosine kinase-1 (SK-1) protein expression was detected by Western blot analysis and mRNA expression quantified by real-time PCR. SK activity was measured by an in vitro kinase assay using sphingosine as substrate.

KEY RESULTS

Nucleotide stimulation caused biphasic activation of SK-1, but not SK-2. The first peak occurred after minutes of stimulation and was followed by a second delayed peak after 4-24 h of stimulation. The delayed activation of SK-1 is due to increased SK-1 mRNA steady-state levels and de novo synthesis of SK-1 protein, and depends on PKC and the classical MAPK cascade. To see whether nucleotide-stimulated cell responses require SK-1, we selectively depleted SK-1 from cells by using small-interference RNA (siRNA). MC migration is highly stimulated by ATP and UTP; this is mimicked by exogenously added S1P. Depletion of SK-1 by siRNA drastically reduced the effect of ATP and UTP on cell migration but not on cell proliferation. Furthermore, MCs isolated from SK-1-deficient mice were completely devoid of nucleotide-induced migration.

CONCLUSIONS AND IMPLICATIONS

These data show that extracellular nucleotides besides being mitogenic also trigger MC migration and this cell response critically requires SK-1 activity. Thus, pharmacological intervention of SK-1 may have impacts on situations where MC migration is important such as during inflammatory kidney diseases.

Sphingosine kinase 1–mediated inhibition of Fas death signaling in rheumatoid arthritis B lymphoblastoid cells

OBJECTIVE

It is becoming increasingly apparent that B cells play an important role in the pathogenesis of rheumatoid arthritis (RA). Due to the scarcity of B cells in RA, it has been technically difficult to functionally characterize B cell apoptosis in this disease. As a necessary first step to identify candidate aberrations, we investigated Fas-mediated signaling events in immortalized peripheral blood B lymphoblastoid cell lines (LCLs) from patients with RA and controls.

METHODS

Cell death was determined by the MTS assay, and apoptosis was detected by the TUNEL assay and DNA laddering. Proteolytic activation of caspase 3 was determined by immunoblotting, and its enzymatic activity was determined by a fluorometric technique. Messenger RNA (mRNA) expression was quantified by real-time polymerase chain reaction (PCR) analysis. The functional role of sphingosine kinase (SPHK) was determined by measuring its enzymatic activity, by quantifying the levels of its product, sphingosine 1-phosphate (S1P), and by investigating the ability of the SPHK inhibitor N,N-dimethylsphingosine and isozyme-specific small interfering RNA (siRNA) oligonucleotides to reverse signaling aberrations.

RESULTS

LCLs from patients with RA displayed disease-specific Fas-mediated signal transduction impairment with consequent resistance to cell death. RA LCLs displayed high constitutive SPHK activity and increased levels of S1P. Real-time PCR analysis showed higher SPHK-1 mRNA expression levels in RA patients compared with paired controls. Increased SPHK-1 (but not SPHK-2) mRNA levels were observed in synovial tissue from RA patients. Competitive inhibitors of SPHK reversed the resistance of RA LCLs to Fas-induced apoptosis. Additionally, resistance to Fas-mediated signaling was reversed by siRNA oligonucleotides specific for SPHK-1 but not by oligonucleotides specific for SPHK-2.

CONCLUSION

These findings demonstrate disease-specific resistance to Fas-mediated death signaling in patients with RA and implicate increased SPHK-1 activity as the cause of this aberration.

Mouse sphingosine kinase isoforms SPHK1a and SPHK1b differ in enzymatic traits including stability, localization, modification, and oligomerization

Sphingosine kinases catalyze the production of the bioactive lipid molecule sphingosine 1-phosphate. Mice have two isoforms of sphingosine kinase type 1, SPHK1a and SPHK1b. In addition to the previously reported difference in their enzyme activities, we have found that these isoforms differ in several enzymatic characteristics. First, SPHK1b is unstable, whereas SPHK1a is highly stable. Degradation of SPHK1b occurs at the membrane and is inhibited by a proteasome inhibitor. Second, only SPHK1b exhibits abnormal mobility on SDS-PAGE, probably due to its SDS-resistant structure. Third, SPHK1a and SPHK1b are predominantly detected in the soluble and membrane fractions, respectively, when their degradation is inhibited. Fourth, only SPHK1b is modified with lipid, on its unique Cys residues (Cys-4 and Cys-5). Site-directed mutagenesis at these Cys residues resulted in increased sphingosine kinase activity, suggesting that the modification is inhibitory to the enzyme. Finally, SPHK1b tends to form homo-oligomers, whereas most SPHK1a is presented as monomers. We have also determined that the lipid modification of SPHK1b is involved in its homo-oligomerization. Thus, although these two proteins differ only in a few N-terminal amino acid residues, their enzymatic traits are extremely different.