Cell type-specific localization of sphingosine kinase 1a in human tissues

Sphingosine kinase 1 is involved in triglyceride breakdown by maintaining lysosomal integrity in brown adipocytes

Sphingosine 1-phosphate (S1P) has been implicated in brown adipose tissue (BAT) formation and energy consumption; however, the mechanistic role of sphingolipids, including S1P, in BAT remains unclear. Here, we showed that, in mice, BAT activation by cold exposure upregulated mRNA and protein expression of the S1P-synthesizing enzyme sphingosine kinase 1 (SphK1) and S1P production in BAT. Treatment of wild-type brown adipocytes with exogenous S1P or S1P receptor subtype-selective agonists stimulated triglyceride (TG) breakdown only marginally, compared with noradrenaline. However, genetic deletion of Sphk1 resulted in hypothermia and diminished body weight loss upon cold exposure, suggesting that SphK1 is involved in thermogenesis through mechanisms different from receptor-mediated, extracellular action of S1P. In BAT of wild-type mice, SphK1 was localized largely in the lysosomes of brown adipocytes. In the brown adipocytes of Sphk1-/- mice, the number of lysosomes was reduced and lysosomal function, including proteolytic activity, acid esterase activity, and motility, was impaired. Concordantly, nuclear translocation of transcription factor EB, a master transcriptional regulator of lysosome biogenesis, was reduced, leading to decreased mRNA expression of the lysosome-related genes in Sphk1-/- BAT. Moreover, BAT of Sphk1-/- mice showed greater TG accumulation with dominant larger lipid droplets in brown adipocytes. Inhibition of lysosomes with chloroquine resulted in a less extent of triglyceride accumulation in Sphk1-/- brown adipocytes compared with wild-type brown adipocytes, suggesting a reduced lysosome-mediated TG breakdown in Sphk1-/- mice. Our results indicate a novel role of SphK1 in lysosomal integrity, which is required for TG breakdown and thermogenesis in BAT.

Stimulating the expression of sphingosine kinase 1 (SphK1) is beneficial to reduce acrylamide-induced nerve cell damage

Sphingosine kinase 1 (SphK1) is an important signaling molecule for cell proliferation and survival. However, the role of SphK1 in acrylamide (ACR)-induced nerve injury remains unclear. The purpose of this study was to investigate the role and potential mechanism of SphK1 in ACR-induced nerve injury. Liquid chromatography triple quadrupole tandem mass spectrometry (LC-MS/MS) and reverse transcription-quantitative PCR (RT-qPCR) were used to detect sphingosine 1-phosphate (S1P) content in serum and SphK1 content in whole blood from an occupational work group exposed to ACR compared to a non-exposed group. For in vitro experiments, SphK1 in human SH-SY5Y neuroblastoma cells was activated using SphK1-specific activator phorbol 12-myristate 13-acetate (PMA). Our research also utilized cell viability assays, flow cytometry, western blots, RT-qPCR and related protein detection to assess activity of the mitogen activated protein kinase (MAPK) signaling pathway. The results of the population study showed that the contents of SphK1 and S1P in the ACR-exposed occupational contact group were lower than in the non-exposed group. The results of in vitro experiments showed that expression of SphK1 decreased with the increase in ACR concentration. Activating SphK1 improved the survival rate of SH-SY5Y cells and decreased the apoptosis rate. Activating SphK1 in SH-SY5Y cells also regulated MAPK signaling, including enhancing the phosphorylation of extracellular signal-regulated protein kinases (ERK) and inhibiting the phosphorylation of c-Jun N-terminal kinase (JNK) and p38. These results suggest that activating SphK1 can protect against nerve cell damage caused by ACR.

Sphingolipids in Hematopoiesis: Exploring Their Role in Lineage Commitment

Sphingolipids, associated enzymes, and the sphingolipid pathway are implicated in complex, multifaceted roles impacting several cell functions, such as cellular homeostasis, apoptosis, cell differentiation, and more through intrinsic and autocrine/paracrine mechanisms. Given this broad range of functions, it comes as no surprise that a large body of evidence points to important functions of sphingolipids in hematopoiesis. As the understanding of the processes that regulate hematopoiesis and of the specific characteristics that define each type of hematopoietic cells is being continuously refined, the understanding of the roles of sphingolipid metabolism in hematopoietic lineage commitment is also evolving. Recent findings indicate that sphingolipid alterations can modulate lineage commitment from stem cells all the way to megakaryocytic, erythroid, myeloid, and lymphoid cells. For instance, recent evidence points to the ability of de novo sphingolipids to regulate the stemness of hematopoietic stem cells while a substantial body of literature implicates various sphingolipids in specialized terminal differentiation, such as thrombopoiesis. This review provides a comprehensive discussion focused on the mechanisms that link sphingolipids to the commitment of hematopoietic cells to the different lineages, also highlighting yet to be resolved questions.

Mammalian sphingosine kinase (SphK) isoenzymes and isoform expression: challenges for SphK as an oncotarget

The various sphingosine kinase (SphK) isoenzymes (isozymes) and isoforms, key players in normal cellular physiology, are strongly implicated in cancer and other diseases. Mutations in SphKs, that may justify abnormal physiological function, have not been recorded. Nonetheless, there is a large and growing body of evidence demonstrating the contribution of gain or loss of function and the imbalance in the SphK/S1P rheostat to a plethora of pathological conditions including cancer, diabetes and inflammatory diseases. SphK is expressed as two isozymes SphK1 and SphK2, transcribed from genes located on different chromosomes and both isozymes catalyze the phosphorylation of sphingosine to S1P. Expression of each SphK isozyme produces alternately spliced isoforms. In recent years the importance of the contribution of SpK1 expression to treatment resistance in cancer has been highlighted and, additionally, differences in treatment outcome appear to also be dependent upon SphK isoform expression. This review focuses on an exciting emerging area of research involving SphKs functions, expression and subcellular localization, highlighting the complexity of targeting SphK in cancer and also comorbid diseases. This review also covers the SphK isoenzymes and isoforms from a historical perspective, from their first discovery in murine species and then in humans, their role(s) in normal cellular function and in disease processes, to advancement of SphK as an oncotarget.

Localization of Sphingosine Kinase-1 in Mouse Sperm Acrosomes

Sphingosine kinase (SPHK) catalyzes sphingosine phosphorylation to form a bioactive lipid mediator, sphingosine-1-phosphate (S1P). In the current study, we report the presence of SPHK-1 in mouse spermatozoa. SPHK-1 was localized to the acrosomes of spermatozoa, and its expression was proven by RT-PCR and Western blot analysis. SPHK activity of mouse spermatozoa was 18.1 pmol/min/mg protein. Furthermore, we identified the presence of the S1P receptors S1P1, S1P2, S1P3, and S1P5, in mouse spermatozoa by RT-PCR. These results suggest that S1P produced by SPHK-1 would play a role in the acrosomal reaction through S1P receptors.

Anti-S1P Antibody as a Novel Therapeutic Strategy for VEGFR TKI-Resistant Renal Cancer

PURPOSE

VEGFR2 tyrosine kinase inhibition (TKI) is a valuable treatment approach for patients with metastatic renal cell carcinoma (RCC). However, resistance to treatment is inevitable. Identification of novel targets could lead to better treatment for patients with TKI-naïve or -resistant RCC.

EXPERIMENTAL DESIGN

In this study, we performed transcriptome analysis of VEGFR TKI-resistant tumors in a murine model and discovered that the SPHK-S1P pathway is upregulated at the time of resistance. We tested sphingosine-1-phosphate (S1P) pathway inhibition using an anti-S1P mAb (sphingomab), in two mouse xenograft models of RCC, and assessed tumor SPHK expression and S1P plasma levels in patients with metastatic RCC.

RESULTS

Resistant tumors expressed several hypoxia-regulated genes. The SPHK1 pathway was among the most highly upregulated pathways that accompanied resistance to VEGFR TKI therapy. SPHK1 was expressed in human RCC, and the product of SPHK1 activity, S1P, was elevated in patients with metastatic RCC, suggesting that human RCC behavior could, in part, be due to overproduction of S1P. Sphingomab neutralization of extracellular S1P slowed tumor growth in both mouse models. Mice bearing tumors that had developed resistance to sunitinib treatment also exhibited tumor growth suppression with sphingomab. Sphingomab treatment led to a reduction in tumor blood flow as measured by MRI.

CONCLUSIONS

Our findings suggest that S1P inhibition may be a novel therapeutic strategy in patients with treatment-naïve RCC and also in the setting of resistance to VEGFR TKI therapy.

Campylobacter fetus subsp. testudinum subsp. nov., isolated from humans and reptiles

A polyphasic study was undertaken to determine the taxonomic position of 13 Campylobacter fetus -like strains from humans (n = 8) and reptiles (n = 5). The results of matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) MS and genomic data from sap analysis, 16S rRNA gene and hsp60 sequence comparison, pulsed-field gel electrophoresis, amplified fragment length polymorphism analysis, DNA–DNA hybridization and whole genome sequencing demonstrated that these strains are closely related to C. fetus but clearly differentiated from recognized subspecies of C. fetus . Therefore, this unique cluster of 13 strains represents a novel subspecies within the species C. fetus , for which the name Campylobacter fetus subsp. testudinum subsp. nov. is proposed, with strain 03-427T ( = ATCC BAA-2539T = LMG 27499T) as the type strain. Although this novel taxon could not be differentiated from C. fetus subsp. fetus and C. fetus subsp. venerealis using conventional phenotypic tests, MALDI-TOF MS revealed the presence of multiple phenotypic biomarkers which distinguish Campylobacter fetus subsp. testudinum subsp. nov. from recognized subspecies of C. fetus .

Sphingosine kinase 1 and cancer: a systematic review and meta-analysis

BACKGROUND

Sphingosine kinase 1 (SK1) is a key regulator of the dynamic ceramide/sphingosine 1-phosphate rheostat balance and important in the pathological cancer genesis, progression, and metastasis processes. Many studies have demonstrated SK1 overexpressed in various cancers, but no meta-analysis has evaluated the relationship between SK1 and various cancers.

METHODS

We retrieved relevant articles from the PubMed, EBSCO, ISI, and OVID databases. A pooled odds ratio (OR) was used to assess the associations between SK1 expression and cancer; hazard ratios (HR) were used for 5-year and overall survival. Review Manager 5.0 was used for the meta-analysis, and publication bias was evaluated with STATA 12.0 (Egger's test).

RESULTS

Thirty-four eligible studies (n=4,673 patients) were identified. SK1 positivity and high expression were significantly different between cancer, non-cancer, and benign tissues. SK1 mRNA and protein expression levels were elevated in the cancer tissues, compared with the normal tissues. SK1 positivity rates differed between various cancer types (lowest [27.3%] in estrogen receptor-positive breast cancer and highest [82.2%] in tongue squamous cell carcinoma). SK1 positivity and high expression were associated with 5-year survival; the HR was 1.86 (95% confidence interval [CI], 1.18-2.94) for breast cancer, 1.58 (1.08-2.31) for gastric cancer, and 2.68 (2.10-3.44) for other cancers; the total cancer HR was 2.21 (95% CI, 1.83-2.67; P < 0.00001). The overall survival HRs were 2.09 (95% CI, 1.35-3.22), 1.56 (1.08-2.25), and 2.62 (2.05-3.35) in breast, gastric, and other cancers, respectively. The total effect HR was 2.21 (95% CI, 1.83-2.66; P < 0.00001).

CONCLUSIONS

SK1 positivity and high expression were significantly associated with cancer and a shorter 5-year and overall survival. SK1 positivity rates vary tremendously among the cancer types. It is necessary to further explore whether SK1 might be a predictive biomarker of outcomes in cancer patients.

Sphingosine kinase/sphingosine 1-phosphate axis: a new player for insulin-like growth factor-1-induced myoblast differentiation

Background

Insulin-like growth factor-1 (IGF-1) is the most important physiological regulator of skeletal muscle progenitor cells, which are responsible for adult skeletal muscle regeneration. The ability of IGF-1 to affect multiple aspects of skeletal muscle cell biology such as proliferation, differentiation, survival and motility is well recognized, although the molecular mechanisms implicated in its complex biological action are not fully defined. Since sphingosine 1-phosphate (S1P) has recently emerged as a key player in skeletal muscle regeneration, we investigated the possible involvement of the sphingosine kinase (SK)/S1P receptor axis on the biological effects of IGF-1 in murine myoblasts.

Methods

RNA interference, chemical inhibition and immunofluorescence approaches were used to assess the role of the SK/S1P axis on the myogenic and mitogenic effects of IGF-1 in C2C12 myoblasts.

Results

We show that IGF-1 increases SK activity in mouse myoblasts. The effect of the growth factor does not involve transcriptional regulation of SK1 or SK2, since the protein content of both isoforms is not affected; rather, IGF-1 enhances the fraction of the active form of SK. Moreover, transactivation of the S1P₂ receptor induced by IGF-1 via SK activation appears to be involved in the myogenic effect of the growth factor. Indeed, the pro-differentiating effect of IGF-1 in myoblasts is impaired when SK activity is pharmacologically inhibited, or SK1 or SK2 are specifically silenced, or the S1P₂ receptor is downregulated. Furthermore, in this study we show that IGF-1 transactivates S1P₁/S1P₃ receptors via SK activation and that this molecular event negatively regulates the mitogenic effect elicited by the growth factor, since the specific silencing of S1P₁ or S1P₃ receptors increases cell proliferation induced by IGF-1.

Conclusions

We demonstrate a dual role of the SK/S1P axis in response to myoblast challenge with IGF-1, that likely is important to regulate the biological effect of this growth factor. These findings add new information to the understanding of the mechanism by which IGF-1 regulates skeletal muscle regeneration.

Sphingosine kinase-1/S1P1 signalling axis negatively regulates mitogenic response elicited by PDGF in mouse myoblasts

PDGF is known to be critically implicated in skeletal muscle repair; however its molecular mechanism of action has been only marginally investigated. In this study we show that in mouse myoblasts PDGF transactivates S1P(1) receptor via sphingosine kinase (SK)-1 activation and that this molecular event exerts a negative regulation of the mitogenic effect elicited by this growth factor. Indeed, pharmacological inhibition of S1P(1), or its specific silencing increased PDGF-dependent cell proliferation, whereas S1P(1) overexpression diminished the biological effect. Moreover, the mitogenic response to PDGF was enhanced by pharmacological inhibition of SK activity as well as specific silencing of SK1 but not SK2. Furthermore, ERK1/2 signalling pathway was found to be upstream of the observed attenuation of PDGF-induced cell proliferation. Interestingly, PDGF-directed engagement of S1P(1) exerted also a positive modulatory action of the growth factor-dependent cell motility. The here highlighted dual role of S1P(1)-mediated signalling in response to myoblast challenge with PDGF is likely important to guarantee the fine control of the biological response to this growth factor, finalized to efficient repopulation of skeletal muscle after damage, where a tight balance between proliferation and migration of tissue progenitor cells is required.

The expression of sphingosine kinase-1 in head and neck carcinoma

Sphingosine kinase-1 (SPHK1) modulates the proliferation, apoptosis and differentiation of keratinocytes through the regulation of ceramide and sphingosine-1-phosphate levels. However, studies on the expression of SPHK1 in human head and neck squamous cell carcinoma (HNSCC) specimens are lacking. Therefore, the aim of the present work was to evaluate SPHK1 expression in human primary HNSCCs and to correlate the results with clinical and anatomopathological parameters. We investigated the expression of this protein by immunohistochemistry performed in tissue microarrays of HNSCC and in an independent cohort of 37 paraffin-embedded specimens. SPHK1 expression was further validated by real-time PCR performed on laser capture-microdissected tissue samples. The positive rate of SPHK1 protein in the cancerous tissues was significantly higher (74%) than that in the nontumor oral tissues (23%), and malignant tissues showed stronger immunoreactivity for SPHK1 than normal matching samples. These results were confirmed by real-time PCR quantification of SPHK1 mRNA. Interestingly, the positive expression of SPHK1 was associated with shorter patient survival time (Kaplan-Meier survival curves) and with the loss of p21 expression. Taken together, these results demonstrate that SPHK1 is upregulated in HNSCC and provide clues of the role SPHK1 might play in tumor progression.

Sphingosine kinase mediates resistance to the synthetic retinoid N-(4-hydroxyphenyl)retinamide in human ovarian cancer cells

A2780 human ovarian carcinoma cells respond to treatment with the synthetic retinoid N-(4-hydroxyphenyl)retinamide (HPR) with the production of dihydroceramide and with a concomitant reduction of cell proliferation and induction of apoptosis. The derived HPR-resistant clonal cell line, A2780/HPR, is less responsive to HPR in terms of dihydroceramide generation. In this report, we show that the production of sphingosine 1-phosphate (S1P) is significantly higher in A2780/HPR versus A2780 cells due to an increased sphingosine kinase (SK) activity and SK-1 mRNA and protein levels. Treatment of A2780 and A2780/HPR cells with a potent and highly selective pharmacological SK inhibitor effectively reduced S1P production and resulted in a marked reduction of cell proliferation. Moreover, A2780/HPR cells treated with a SK inhibitor were sensitized to the cytotoxic effect of HPR, due to an increased dihydroceramide production. On the other hand, the ectopic expression of SK-1 in A2780 cells was sufficient to induce HPR resistance in these cells. Challenge of A2780 and A2780/HPR cells with agonists and antagonists of S1P receptors had no effects on their sensitivity to the drug, suggesting that the role of SK in HPR resistance in these cells is not mediated by the S1P receptors. These data clearly demonstrate a role for SK in determining resistance to HPR in ovarian carcinoma cells, due to its effect in the regulation of intracellular ceramide/S1P ratio, which is critical in the control of cell death and proliferation.

Transforming growth factor-beta1 induces transdifferentiation of myoblasts into myofibroblasts via up-regulation of sphingosine kinase-1/S1P3 axis

The pleiotropic cytokine transforming growth factor (TGF)-beta1 is a key player in the onset of skeletal muscle fibrosis, which hampers tissue repair. However, the molecular mechanisms implicated in TGFbeta1-dependent transdifferentiation of myoblasts into myofibroblasts are presently unknown. Here, we show that TGFbeta1 up-regulates sphingosine kinase (SK)-1 in C2C12 myoblasts in a Smad-dependent manner, and concomitantly modifies the expression of sphingosine 1-phosphate (S1P) receptors (S1PRs). Notably, pharmacological or short interfering RNA-mediated inhibition of SK1 prevented the induction of fibrotic markers by TGFbeta1. Moreover, inhibition of S1P(3), which became the highest expressed S1PR after TGFbeta1 challenge, strongly attenuated the profibrotic response to TGFbeta1. Furthermore, downstream of S1P(3), Rho/Rho kinase signaling was found critically implicated in the profibrotic action of TGFbeta1. Importantly, we demonstrate that SK/S1P axis, known to play a key role in myogenesis via S1P(2), consequently to TGFbeta1-dependent S1PR pattern remodeling, becomes responsible for transmitting a profibrotic, antidifferentiating action. This study provides new compelling information on the mechanism by which TGFbeta1 gives rise to fibrosis in skeletal muscle, opening new perspectives for its pharmacological treatment. Moreover, it highlights the pleiotropic role of SK/S1P axis in skeletal myoblasts that, depending on the expressed S1PR pattern, seems capable of eliciting multiple, even contrasting biological responses.

S1P3-mediated cardiac fibrosis in sphingosine kinase 1 transgenic mice involves reactive oxygen species

AIMS

Sphingosine kinase 1 (SPHK1), its product sphingosine-1-phosphate (S1P), and S1P receptor subtypes have been suggested to play protective roles for cardiomyocytes in animal models of ischaemic preconditioning and cardiac ischaemia/reperfusion injury. To get more insight into roles for SPHK1 in vivo, we have generated SPHK1-transgenic (TG) mice and analysed the cardiac phenotype.

METHODS AND RESULTS

SPHK1-TG mice overexpressed SPHK1 in diverse tissues, with a nearly 20-fold increase in enzymatic activity. The TG mice grew normally with normal blood chemistry, cell counts, heart rate, and blood pressure. Unexpectedly, TG mice with high but not low expression levels of SPHK1 developed progressive myocardial degeneration and fibrosis, with upregulation of embryonic genes, elevated RhoA and Rac1 activity, stimulation of Smad3 phosphorylation, and increased levels of oxidative stress markers. Treatment of juvenile TG mice with pitavastatin, an established inhibitor of the Rho family G proteins, or deletion of S1P3, a major myocardial S1P receptor subtype that couples to Rho GTPases and transactivates Smad signalling, both inhibited cardiac fibrosis with concomitant inhibition of SPHK1-dependent Smad-3 phosphorylation. In addition, the anti-oxidant N-2-mercaptopropyonylglycine, which reduces reactive oxygen species (ROS), also inhibited cardiac fibrosis. In in vivo ischaemia/reperfusion injury, the size of myocardial infarct was 30% decreased in SPHK1-TG mice compared with wild-type mice.

CONCLUSION

These results suggest that chronic activation of SPHK1-S1P signalling results in both pathological cardiac remodelling through ROS mediated by S1P3 and favourable cardioprotective effects.

Sphingosine kinase 1 is essential for proteinase-activated receptor-1 signalling in epithelial and endothelial cells

There is accumulating evidence that activation of sphingosine kinase 1 (SPHK1) is an important element in intracellular signalling cascades initiated by stimulation of multiple receptors, including certain growth factor, cytokine, and also G-protein coupled receptors. We here report that stimulation of the lung epithelial cell line A549 by thrombin leads to transient increase of SPHK1 activity and elevation of intracellular sphingosine-1-phosphate (S1P); abrogation of this stimulation by SPHK1-specific siRNA, pharmacological inhibition, or expression of a dominant-negative SPHK1 mutant blocks the response to thrombin, as measured by secretion of MCP-1, IL-6, IL-8, and PGE(2). Using selective stimulation of proteinase-activated receptors (PARs) a specific involvement of SPHK1 in the PAR-1 induced responses in A549 cell, including activation of NFkappaB, was evident, while PAR-2 and PAR-4 responses were independent of SPHK1. Moreover, PAR-1 or thrombin-induced cytokine production and adhesion factor expression of human umbilical vein endothelial cells was also seen to depend on SPHK1. Using dermal microvascular endothelial cells from SPHK1-deficient mice, we showed that absence of the enzyme abrogates MCP-1 production induced in these cells upon treatment with thrombin or PAR-1 activating peptide. We propose SPHK1 inhibition as a novel way to block PAR-1 mediated signalling, which could be useful in treatment of a number of diseases, in particular in atherosclerosis.

Role for sphingosine kinase 1 in colon carcinogenesis

Sphingosine kinase 1 (SphK1) phosphorylates sphingosine to form sphingosine-1-phosphate (S1P) and is a critical regulator of sphingolipid-mediated functions. Cell-based studies suggest a tumor-promoting function for the SphK1/S1P pathway. Also, our previous studies implicated the SphK1/S1P pathway in the induction of the arachidonic acid cascade, a major inflammatory pathway involved in colon carcinogenesis. Therefore, we investigated whether the SphK1/S1P pathway is necessary for mediating carcinogenesis in vivo. Here, we report that 89% (42/47) of human colon cancer samples stained positively for SphK1, whereas normal colon mucosa had negative or weak staining. Adenomas had higher expression of SphK1 vs. normal mucosa, and colon cancers with metastasis had higher expression of SphK1 than those without metastasis. In the azoxymethane (AOM) murine model of colon cancer, SphK1 and S1P were significantly elevated in colon cancer tissues compared to normal mucosa. Moreover, blood levels of S1P were higher in mice with colon cancers than in those without cancers. Notably, SphK1(-/-) mice subjected to AOM had significantly less aberrant crypt foci (ACF) formation and significantly reduced colon cancer development. These results are the first in vivo evidence that the SphK1/S1P pathway contributes to colon carcinogenesis and that inhibition of this pathway is a potential target for chemoprevention.

The role of sphingosine kinase and sphingosine-1-phosphate in the regulation of endothelial cell biology

Sphingolipids, in particular sphingosine kinase (SphK) and its product sphingosine-1-phosphate (S1P), are now recognized to play an important role in regulating many critical processes in endothelial cells. Activation of SphK1 is essential in mediating the endothelial proinflammatory effects of inflammatory cytokines such as tumor necrosis factor (TNF). In addition, S1P regulates the survival and proliferation of endothelial cells, as well as their ability to undergo cell migration, all essential components of angiogenesis. Thus the inflammatory and angiogenic potential of the endothelium is in part regulated by intracellular components including the activity of SphK1 and levels of S1P. Herein a review of the sphingomyelin pathway with a particular focus on its relevance to endothelial cell biology is presented.

Purkinje cell subtype specification in the cerebellar cortex: early B-cell factor 2 acts to repress the zebrin II-positive Purkinje cell phenotype

The mammalian cerebellar cortex is highly compartmentalized. First, it is subdivided into four transverse expression domains: the anterior zone (AZ), the central zone (CZ), the posterior zone (PZ), and the nodular zone (NZ). Within each zone, the cortex is further subdivided into a symmetrical array of parasagittal stripes. The most extensively studied compartmentation antigen is zebrin II/aldolase c, which is expressed by a subset of Purkinje cells forming parasagittal stripes. Stripe phenotypes are specified early in cerebellar development, in part through the action of early B-cell factor 2 (Ebf2), a member of the atypical helix-loop-helix transcription factor family Collier/Olf1/EBF. In the murine cerebellum, Ebf2 expression is restricted to the zebrin II-immunonegative (zebrin II-) Purkinje cell population. We have identified multiple cerebellar defects in the Ebf2 null mouse involving a combination of selective Purkinje cell death and ectopic expression of multiple genes normally restricted to the zebrin II- subset. The nature of the cerebellar defect in the Ebf2 null is different in each transverse zone. In contrast to the ectopic expression of genes characteristic of the zebrin II+ Purkinje cell phenotype, phospholipase Cbeta4 expression, restricted to zebrin II- Purkinje cells in control mice, is well maintained, and the normal number of stripes is present. Taken together, these data suggest that Ebf2 regulates the expression of genes associated with the zebrin II+ Purkinje cell phenotype and that the zebrin II- Purkinje cell subtype is specified independently.

Dimethylsphingosine and FTY720 inhibit the SK1 form but activate the SK2 form of sphingosine kinase from rat heart

Fractionation of cytosolic sphingosine kinase (SKase) activity by gel filtration chromatography gave rise to a 96-kDa peak that contained only the SK2 form of SKase (by Western analysis) and a broad ca. 46 kDa peak that contained only SK1 forms. SK2 appeared to have a bound accessory protein. When tested with the classic SKase inhibitor dimethylsphingosine (DMS), SK1 was extensively inhibited; however, SK2 was not inhibited but unexpectedly was activated. Activation of SK2 was the result of DMS enhancing the affinity of the enzyme for sphingosine, and, at low concentrations of ATP and sphingosine, activated by more than 100%. Activation of SK2 could be demonstrated in the cytosolic fraction indicating it was unrelated to the purification step. The immunomodulator FTY720 also activated SK2 (although to a lesser extent), but was a potent inhibitor of SK1. SK2 from rat liver and spleen was also not inhibited by DMS. L-Sphingosine and to a lesser extent dihydrosphingosine and phytosphingosine were effective inhibitors of both forms.

Distribution of sphingosine kinase activity and mRNA in rodent brain

Sphingosine-1-phosphate (S1P) is a lipid mediator that exerts multiple cellular functions through activation of a subfamily of G-protein-coupled receptors. Although there is evidence that S1P plays a role in the developing and adult CNS, little is known about the ability of brain parenchyma to synthesize this lipid. We have therefore analyzed the brain distribution of the enzymatic activity of the S1P synthesizing enzyme, sphingosine kinase (SPHK) [EC:2.7.1.91], as well as mRNA distribution for one of the two isoforms of this enzyme, sphingosine kinase 2. SPHK activity, measured by the conversion of [(3)H]sphingosine to [(3)H]S1P, is highest in cerebellum, followed by cortex and brainstem. Lowest activities were found in striatum and hippocampus. Sensitivity to 0.1% Triton-X suggests that this activity is accounted for by SPHK2. RT-PCR and in situ hybridization studies show that mRNA for this isoform has a distribution similar to that of SPHK activity. In vivo and in vitro ischemia increase SPHK activity and SPHK2 mRNA levels. These results indicate that SPHK2 is the predominant S1P-synthesizing isoform in normal brain parenchyma. Its heterogeneous distribution, in particular laminar distribution in cortex, suggests a neuronal localization and a possible role in cortical and cerebellar functions, in normal as well as ischemic brain.

Maternal and Embryonic Control of Uterine Sphingolipid-Metabolizing Enzymes During Murine Embryo Implantation1

During early gestation in invasively implanting species, the uterine stromal compartment undergoes dramatic remodeling, defined by the differentiation of stromal fibroblast cells into decidual cells. Lipid signaling molecules from a number of pathways are well-established functional components of this decidualization reaction. Because of a correlation in the events that transpire in the uterus during early implantation with known functions of bioactive sphingolipid metabolites established from studies in other organ systems, we hypothesized that uterine sphingolipid metabolism would change during implantation. By a combination of Northern blot, Western blot, and immunohistochemical analyses, we establish that enzymes at each of the major catalytic steps in the sphingolipid cascade become transcriptionally up-regulated in the uterus during decidualization. Each of the enzymes analyzed was up-regulated from Days of Pregnancy (DOP) 4.5-7.5. When comparing embryo-induced decidualization (decidual) with mechanically induced decidualization (deciduomal), sphingomyelin phosphodiesterase 1 (Smpd1) mRNA and sphingosine kinase 1 (SPHK1) protein were shown to be dually regulated in the endometrium by both maternal and embryonic factors. As measured by the diacyl glycerol kinase assay, ceramide levels rose in parallel with Smpd1 gene expression, suggesting that elevated transcription of sphingolipid enzymes results in heightened catalytic activity of the pathway. Altogether, these findings place sphingolipids on a growing list of lipid signaling molecules that become increasingly present at the maternal-embryonic interface.

RET signaling-induced SPHK1 gene expression plays a role in both GDNF-induced differentiation and MEN2-type oncogenesis

RET, the receptor of glial cell line-derived neurotrophic factor (GDNF) family ligands, is important for the development of kidney and peripheral neurons. GDNF promotes survival and differentiation of neurons. Mutation of RET leads to the constitutive signal activation causing papillary thyroid carcinoma and multiple endocrine neoplasia type 2 (MEN2). In this study, we report that GDNF/RET signaling up-regulates sphingosine kinase (SPHK) enzyme activity, SPHK1 protein and SPHK1 message in TGW human neuroblastoma cells. Silencing of SPHK1 using siRNA inhibited GDNF-induced neurite formation, GAP43 expression, and cell growth, suggesting the important role of SPHK1 in GDNF signal transduction. Furthermore, NIH3T3 cells transfected with MEN2A type mutated RET but not c-RET demonstrated the up-regulation of SPHK activity, SPHK1 protein and SPHK1 message compared with NIH3T3 cells. The cell growth and anchorage-independent colony formation of MEN2A-NIH3T3 was inhibited with siRNA of SPHK1, while no effect of scramble siRNA was observed. These results suggest the oncogenic role of SPHK1 in MEN2A type tumor. Promoter analysis showed that activator protein 2 and specificity protein 1 binding motif of the 5' promoter region of SPHK1 gene is important for its induction by GDNF. Furthermore, we demonstrated that ERK1/2 and PI3 kinase are involved in GDNF-induced SPHK1 transcription by using specific inhibitors.

Sphingosine kinase activity is required for myogenic differentiation of C2C12 myoblasts

Sphingosine kinase (SphK) is a conserved lipid kinase that catalyzes the formation of sphingosine 1-phosphate (S1P), an important lipid mediator, which regulates fundamental biological processes. Here, we provide evidence that SphK is required for the achievement of cell growth arrest as well as myogenic differentiation of C2C12 myoblasts. Indeed, SphK activity, SphK1 protein content and S1P formation were found to be enhanced in myoblasts that became confluent as well as in differentiating cells. Enforced expression of SphK1 reduced the myoblast proliferation rate, enhanced the expression of myogenic differentiation markers and anticipated the onset of differentiated muscle phenotype. Conversely, down-regulation of SphK1 by specific silencing by RNA interference or overexpression of the catalytically inactive SphK1, significantly increased cell growth and delayed the beginning of myogenesis; noticeably, exogenous addition of S1P rescued the biological processes. Importantly, stimulation of myogenesis in SphK1-overexpressing myoblasts was abrogated by treatment with short interfering RNA specific for S1P(2) receptor. This is the first report of the role of endogenous SphK1 in myoblast growth arrest and stimulation of myogenesis through the formation of S1P that acts as morphogenic factor via the engagement of S1P(2).

Endothelin-1 inhibits apoptosis through a sphingosine kinase 1-dependent mechanism in uterine leiomyoma ELT3 cells

Uterine leiomyomas, or fibroids, are the most common tumors of the myometrium. The ELT3 cell line, derived from Eker rat leiomyoma, has been successfully used as a model for the study of leiomyomas. We have demonstrated previously the potent mitogenic properties of the peptidic hormone endothelin (ET)-1 in this cell line. Here we investigated the antiapoptotic effect of ET-1 in ELT3 cells. We found that 1) serum starvation of ELT3 cells induced an apoptotic process characterized by cytochrome c release from mitochondria, caspase-3/7 activation, nuclei condensation and DNA fragmentation; 2) ET-1 prevented the apoptotic process; and 3) this effect of ET-1 was fully reproduced by ETB agonists. In contrast, no antiapoptotic effect of ET-1 was observed in normal myometrial cells. A pharmacological approach showed that the effect of ET-1 on caspase-3/7 activation in ELT3 cells was not dependent on phosphatidylinositol 3-kinase, ERK1/2, or phospholipase D activities. However, inhibitors of sphingosine kinase-1 (SphK1), dimethylsphingosine and threo-dihydrosphingosine, reduced the effect of ET-1 by about 50%. Identical results were obtained when SphK1 expression was down-regulated in ELT3 cells transfected with SphK1 small interfering RNA. Furthermore, serum starvation induced a decrease in SphK1 activity that was prevented by ET-1 without affecting the level of SphK1 protein expression. Finally, sphingosine 1-phosphate, the product of SphK activity, was as efficient as ET-1 in inhibiting serum starvation-induced caspase-3/7 activation. Together, these results demonstrate that ET-1 possesses a potent antiapoptotic effect in ELT3 cells that involves sphingolipid metabolism through the activation of SphK1.

Novel role of sphingosine kinase 1 as a mediator of neurotrophin-3 action in oligodendrocyte progenitors

We had found previously that neurotrophin-3 (NT-3) is a potent stimulator of cAMP-response element binding protein (CREB) phosphorylation in cultured oligodendrocyte progenitors. Here, we show that CREB phosphorylation in these cells is also highly stimulated by sphingosine-1-phosphate (S1P), a sphingolipid metabolite that is known to be a potent mediator of numerous biological processes. Moreover, CREB phosphorylation in response to NT-3 involves sphingosine kinase 1 (SphK1), the enzyme that synthesizes S1P. Immunocytochemistry and confocal microscopy indicated that NT-3 induces translocation of SphK1 from the cytoplasm to the plasma membrane of oligodendrocytes, a process accompanied by increased SphK1 activity in the membrane fraction where its substrate sphingosine resides. To examine the involvement of SphK1 in NT-3 function, SphK1 expression was down-regulated by treatment with SphK1 sequence-specific small interfering RNA. Remarkably, the capacity of NT-3 to protect oligodendrocyte progenitors from apoptotic cell death induced by growth factor deprivation was abolished by down-regulating the expression of SphK1, as assessed by terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay. Altogether, these results suggest that SphK1 plays a crucial role in the stimulation of oligodendrocyte progenitor survival by NT-3, and demonstrate a functional link between NT-3 and S1P signaling, adding to the complexity of mechanisms that modulate neurotrophin function and oligodendrocyte development.

Sphingosine kinase 1 is up-regulated in colon carcinogenesis

Sphingosine kinase 1 (SK1) phosphorylates sphingosine to form sphingosine 1-phosphate (S1P), which has the ability to promote cell proliferation and survival and stimulate angiogenesis. The SK1/S1P pathway also plays a critical role in regulation of cyclooxygenase-2 (COX-2), a well-established pathogenic factor in colon carcinogenesis. Therefore, we examined the expression of SK1 and COX-2 in rat colon tumors induced by azoxymethane (AOM) and the relationship of these two proteins in normal and malignant intestinal epithelial cells. Strongly positive SK1 staining was found in 21/28 (75%) of rat colon adenocarcinomas induced by AOM, whereas no positive SK1 staining was observed in normal mucosa. The increase in SK1 and COX-2 expression in AOM-induced rat colon adenocarcinoma was confirmed at the level of mRNA by real-time RT-PCR. In addition, it was found that 1) down-regulation of SK1 in HT-29 human colon cancer cells by small interfering RNA (siRNA) decreases COX-2 expression and PGE2 production; 2) overexpression of SK1 in RIE-1 rat intestinal epithelial cells induces COX-2 expression; and 3) S1P stimulates COX-2 expression and PGE2 production in HT-29 cells. These results suggest that the SK1/S1P pathway may play an important role in colon carcinogenesis, in part, by regulating COX-2 expression and PGE2 production.

Overexpression of sphingosine kinase 1 is an oncogenic event in erythroleukemic progression

The erythroleukemia developed by spi-1/PU.1-transgenic mice is a model of multistage oncogenic process. Isolation of tumor cells representing discrete stages of leukemic progression enables the dissection of some of the critical events required for malignant transformation. To elucidate the molecular mechanisms of multistage leukemogenesis, we developed a microarray transcriptome analysis of nontumorigenic (HS1) and tumorigenic (HS2) proerythroblasts from spi-1-transgenic mice. The data show that transcriptional up-regulation of the sphingosine kinase gene (SPHK1) is a recurrent event associated with the tumorigenic phenotype of these transgenic proerythroblasts. SPHK1 is an enzyme of the metabolism of sphingolipids, which are essential in several biologic processes, including cell proliferation and apoptosis. HS1 erythroleukemic cells engineered to overexpress the SPHK1 protein exhibited growth proliferative advantage, increased clonogenicity, and resistance to apoptosis in reduced serum level by a mechanism involving activation of the extracellular signal-related kinases 1/2 (ERK1/2) and phosphatidylinositol 3-kinase (PI3K)/AKT pathways. In addition, SPHK1-overexpressing HS1 cells acquired tumorigenicity when engrafted in vivo. Finally, enforced expression of a dominant-negative mutant of SPHK1 in HS2 tumorigenic cells or treatment with a pharmacologic inhibitor reduced both cell growth and apoptosis resistance. Altogether, these data suggest that overexpression of the sphingosine kinase may represent an oncogenic event during the multistep progression of an erythroleukemia. (Blood. 2005;106:1808-1816)

Transcription factor specificity protein 1 (Sp1) is the main regulator of nerve growth factor-induced sphingosine kinase 1 gene expression of the rat pheochromocytoma cell line, PC12

Sphingosine kinase (SPHK) is known to exert an anti-apoptic role in various cells and cell lines. We previously reported that human brain is rich in SPHK1 (Murate et al. 2001). After showing a high expression of SPHK1 in rat brain, we examined the gene expression mechanism using nerve growth factor (NGF)-stimulated rat PC12 cells. With RT-PCR, we found that both rat brain and PC12 utilized exon 1d mostly out of eight untranslated first exons. NGF induced an increase in SPHK enzyme activity and protein about double those in PC12 cells, and NGF-induced SPHK1 mRNA was three times higher than in the control. The minimal 5' promoter was determined, and TrkA specific inhibitor K252a inhibited the NGF-induced promoter activity of SPHK1. The truncation or mutation of putative transcription factor-binding motifs revealed that one specificity protein 1 (Sp1) binding motif of the 5' region of exon 1d is prerequisite. Electrophoresis mobility shift assay confirmed the promoter analysis, indicating increased Sp1 protein binding to this motif after NGF treatment. Chromatin immunoprecipitation assay also showed the binding of Sp1 and the promoter region in vivo. These results suggest the signal transduction pathway from NGF receptor TrkA to transcription factor Sp1 protein binding to the promoter Sp1-like motif in NGF-induced rat SPHK1 gene expression.

Multiple sclerosis

Multiple sclerosis (MS) is a complex genetic disease associated with inflammation in the central nervous system (CNS) white matter and is thought to be mediated by autoimmune processes. Clonal expansion of B cells, their antibody products, and T cells, hallmarks of inflammation in the CNS, are found in MS. The association of the disease with major histocompatibility complex genes, the inflammatory white matter infiltrates, similarities with animal models, and the observation that MS can be treated with immunomodulatory and immunosuppressive therapies support the hypothesis that autoimmunity plays a major role in the disease pathology. This review discusses the immunopathology of MS with particular focus given to regulatory T cells and the role of B cells and antibodies, immunomodulatory therapeutics, and finally new directions in MS research, particularly new methods to define the molecular pathology of human disease with high-throughput examination of germline DNA haplotypes, RNA expression, and protein structures that will allow the generation of a new series of hypotheses that can be tested to develop better understandings and therapies for this disease.

Activation of the SPHK/S1P signalling pathway is coupled to muscarinic receptor-dependent regulation of peripheral airways

Background

In peripheral airways, acetylcholine induces contraction via activation of muscarinic M2-and M3-receptor subtypes (M₂R and M₃R). Cholinergic hypersensitivity is associated with chronic obstructive pulmonary disease and asthma, and therefore the identification of muscarinic signaling pathways are of great therapeutic interest. A pathway that has been shown to be activated via MR and to increase [Ca²⁺]_i includes the activation of sphingosine kinases (SPHK) and the generation of the bioactive sphingolipid sphingosine 1-phosphate (S1P). Whether the SPHK/S1P signaling pathway is integrated in the muscarinic control of peripheral airways is not known.

Methods

To address this issue, we studied precision cut lung slices derived from FVB and M₂R-KO and M₃R-KO mice.

Results

In peripheral airways of FVB, wild-type, and MR-deficient mice, SPHK1 was mainly localized to smooth muscle. Muscarine induced a constriction in all investigated mouse strains which was reduced by inhibition of SPHK using D, L-threo-dihydrosphingosine (DHS) and N, N-dimethyl-sphingosine (DMS) but not by N-acetylsphingosine (N-AcS), a structurally related agent that does not affect SPHK function. The initial phase of constriction was nearly absent in peripheral airways of M₃R-KO mice when SPHK was inhibited by DHS and DMS but was unaffected in M₂R-KO mice. Quantitative RT-PCR revealed that the disruption of the M₂R and M₃R genes had no significant effect on the expression levels of the SPHK1-isoform in peripheral airways.

Conclusion

These results demonstrate that the SPHK/S1P signaling pathway contributes to cholinergic constriction of murine peripheral airways. In addition, our data strongly suggest that SPHK is activated via the M₂R. Given the important role of muscarinic mechanisms in pulmonary disease, these findings should be of considerable therapeutic relevance.

Immunohistochemical Distribution of Sphingosine Kinase 1 in Normal and Tumor Lung Tissue

Sphingosine kinase 1 (SK1) is a key enzyme critical to the sphingolipid metabolic pathway responsible for catalyzing the formation of the bioactive lipid sphingosine-1-phosphate. SK1-mediated production of sphingosine-1-phosphate has been shown to stimulate such biological processes as cell growth, differentiation, migration, angiogenesis, and inhibition of apoptosis. In this study, cell type–specific immunolocalization of SK1 was examined in the bronchus/terminal bronchiole of the lung. Strong immunopositive staining was evident at the apical surface of pseudostratified epithelial cells of the bronchus and underlying smooth muscle cells, submucosal serous glands, immature chondrocytes, type II alveolar cells, foamy macrophages, endothelial cells of blood vessels, and neural bundles. Immunohistochemical screening for SK1 expression was performed in 25 samples of normal/tumor patient matched non–small-cell lung cancer tissue and found that 25 of 25 tumor samples (carcinoid [5 samples], squamous [10 samples], and adenocarcinoma tumors [10 samples]), exhibited overwhelmingly positive immunostaining for SK1 as compared with patient-matched normal tissue. In addition, an approximately 2-fold elevation of SK1 mRNA expression was observed in lung cancer tissue versus normal tissue, as well as in several other solid tumors. Taken together, these findings define the localization of SK1 in lung and provide clues as to how SK1 may play a role in normal lung physiology and the pathophysiology of lung cancer.

Synthesis and evaluation of sphingoid analogs as inhibitors of sphingosine kinases

Sphingosine 1-phosphate (S1P), a product of sphingosine kinases (SphK), mediates diverse biological processes such as cell differentiation, proliferation, motility, and apoptosis. In an effort to search and identify specific inhibitors of human SphK, the inhibitory effects of synthetic sphingoid analogs on kinase activity were examined. Among the analogs tested, we found two, SG12 and SG14, that have specific inhibitory effects on hSphK2. N,N-Dimethylsphingosine (DMS), a well-known SphK inhibitor, displayed inhibitory effects for both SphK1 and SphK2, as well as protein kinase C. In contrast, SG12 and SG14 exhibited selective inhibitory effects on hSphK2. Furthermore, SG14 did not affect PKC. In isolated platelets, SG14 blocked the conversion of sphingosine into sphingosine 1-phosphate significantly. This is the first report on the identification of a hSphK2-specific inhibitor, which may provide a useful tool for studying the biological functions of hSphK2.

Sphingosine kinase signalling in immune cells

1. Sphingolipids are potent second messengers modulating biochemical intracellular events and acting as ligands to mediate extracellular systems. Sphingosine kinase (SPHK) is the enzyme that phosphorylates sphingosine into sphingosine-1-phosphate (S1P), a potent bioactive sphingolipid. 2. The fact that SPHK is highly conserved from protozoa to mammals and is ubiquitous in living tissues reveals important roles of the SPHK pathway for the maintenance of health maintenance. This is also supported by comprehensive reviews on features of its main product, S1P, as having intracellular as well as extracellular roles, inducing a wide range of physiological responses from triggering Ca2+ release from internal stores to promoting growth and cell motility. 3. Immune cell activities have been shown to be modulated by the dynamic balance between ceramide, sphingosine and S1P, conceptualized as a rheostat. Cell proliferation, differentiation, motility and survival have been attributed to the regulatory actions of S1P. The properties of SPHK activity in immune cells are linked to the functions of triggered growth and survival factors, phorbol esters, hormones, cytokines and chemokines, as well as antigen receptors, such as FcgammaRI and FcepsilonRI. 4. Mechanisms of the SPHK signalling pathway are explored as new targets for drug development to suppress inflammation and other pathological conditions.

Sphingosine-1-phosphate lyase SPL is an endoplasmic reticulum-resident, integral membrane protein with the pyridoxal 5'-phosphate binding domain exposed to the cytosol

Sphingosine-1-phosphate (S1P) is a sphingolipid metabolite that functions as a bioactive lipid molecule. S1P is degraded either by S1P lyase or by S1P phosphohydrolase. The gene encoding mammalian S1P lyase, SPL, has been identified. Here, we characterize the SPL protein in its expression, localization, and topology. The expression levels of the SPL protein correlated well with the dihydrosphingosine-1-phosphate (DHS1P) lyase activity in most tissues. However, liver and heart exhibited high DHS1P lyase activities compared to their SPL protein levels. The SPL mRNA expression was temporally regulated during mouse embryonal development. Immunofluorescence microscopy demonstrated that SPL is localized at the endoplasmic reticulum. Proteinase K digestion studies revealed that the large hydrophilic domain, containing the active site, faces the cytosol. This active site orientation is opposite to that of S1P phosphohydrolase, indicating that the degradation of S1P by two S1P-degrading enzymes occurs in spatially separated sides of the endoplasmic reticulum.

Sphingosine kinase 1 is involved in dibutyryl cyclic AMP-induced granulocytic differentiation through the upregulation of extracellular signal-regulated kinase, but not p38 MAP kinase, in HL60 cells

The role of sphingosine kinase (SPHK) in the dibutyryl cyclic AMP (dbcAMP)-induced granulocytic differentiation of HL60 cells was investigated. During differentiation, SPHK activity was increased, as were mRNA and protein levels of SPHK1, but not of SPHK2. Pretreatment of HL60 cells with N,N-dimethylsphingosine (DMS), a potent SPHK inhibitor, completely blocked dbcAMP-induced differentiation. The phosphorylation of mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase 1/2 (ERK1/2), and p38 MAPK was also increased during dbcAMP-induced differentiation. Pretreatment of HL60 cells with the MEK inhibitor, U0126, but not the p38 MAPK inhibitor, SB203580, completely suppressed dbcAMP-induced ERK1/2 activation and granulocytic differentiation, but did not affect the increase in SPHK activity. DMS inhibited dbcAMP-induced ERK1/2 activation, but had little effect on p38 MAPK activation. DMS had no effect on the dbcAMP-induced membrane translocation of protein kinase C (PKC) isozymes, and PKC inhibitors had no significant effect on ERK activation. The overexpression of wild-type SPHK1, but not dominant negative SPHK1, resulted in high basal levels of ERK1/2 phosphorylation and stimulated granulocytic differentiation in HL60 cells. These data show that SPHK1 participates in the dbcAMP-induced differentiation of HL60 cells by activating the MEK/ERK pathway.

Sphingosine kinase protects lipopolysaccharide-activated macrophages from apoptosis

Lipopolysaccharide (LPS) signaling is critical for the innate immune response to gram-negative bacteria. Here, evidence is presented for LPS stimulation of sphingosine kinase (SPK) in the RAW 264.7 murine macrophage cell line and rat primary hepatic macrophages (HMs). LPS treatment of RAW 264.7 cells resulted in a time- and dose-dependent activation of SPK and membrane translocation of SPK1. Further, LPS-induced SPK activation was blocked by SPK1-specific small interfering RNA (siRNA). Overexpression of Toll-like receptor 4 and MD2, the receptor and coreceptor of LPS, in HEK 293 cells activated SPK activity in the absence of LPS treatment. Inhibition of SPK by the pharmacological inhibitor N,N-dimethylsphingosine (DMS) or SPK1-specific siRNA blocked LPS stimulation of extracellular signal-regulated kinase 1/2 and p38 but enhanced LPS-induced c-Jun N-terminal kinase activation. The SPK inhibitor DMS and dominant-negative SPK1 also blocked LPS activation of Elk-1 and NF-kappaB reporters in RAW 264.7 cells. Inhibition of SPK sensitized RAW 264.7 cells and HMs to LPS-induced apoptosis. These data demonstrate the critical role of SPK1 in LPS signaling in macrophages and suggest that SPK1 is a potential therapeutic target to block hyperimmune responses induced by gram-negative bacteria.

Differential transactivation of sphingosine-1-phosphate receptors modulates NGF-induced neurite extension

The process of neurite extension after activation of the TrkA tyrosine kinase receptor by nerve growth factor (NGF) involves complex signaling pathways. Stimulation of sphingosine kinase 1 (SphK1), the enzyme that phosphorylates sphingosine to form sphingosine-1-phosphate (S1P), is part of the functional TrkA signaling repertoire. In this paper, we report that in PC12 cells and dorsal root ganglion neurons, NGF translocates SphK1 to the plasma membrane and differentially activates the S1P receptors S1P₁ and S1P₂ in a SphK1-dependent manner, as determined with specific inhibitors and small interfering RNA targeted to SphK1. NGF-induced neurite extension was suppressed by down-regulation of S1P₁ expression with antisense RNA. Conversely, when overexpressed in PC12 cells, transactivation of S1P₁ by NGF markedly enhanced neurite extension and stimulation of the small GTPase Rac, important for the cytoskeletal changes required for neurite extension. Concomitantly, differentiation down-regulated expression of S1P₂ whose activation would stimulate Rho and inhibit neurite extension. Thus, differential transactivation of S1P receptors by NGF regulates antagonistic signaling pathways that modulate neurite extension.

Compartmentation of the mouse cerebellar cortex by sphingosine kinase

Classic cerebellar anatomy is based on the characteristic array of lobes and lobules. However, there is substantial evidence to suggest that more fundamental architecture is built around arrays of parasagittal stripes, which encompass both the inputs and outputs of the Purkinje cells (PCs). Sphingosine kinase (SPHK) is an enzyme that converts sphingosine (Sph) into sphingosine-1-phosphate (S1P). Recent reports have indicated that ceramide, Sph, and S1P play a role in cell survival, growth, and differentiation in several cell types, including neurons. In this study, we examined the localization of SPHK in the mouse cerebellum by using immunohistochemistry. Anti-SPHK immunoreactivity appeared in the cerebellar molecular layer and the PC membranes. The staining pattern is striped. In the molecular layer, the staining pattern probably reflects dendritic spines and dendrites. By electron microscopy, peroxidase reaction product was deposited within dendrites especially along the plasma membranes near spines. Seen at higher magnification, the staining was in and near the postsynaptic complexes. By double immunostaining, the striped pattern of SPHK expression was shown to be identical to that revealed by anti-zebrin II, although the subcellular distribution within PC's is not. This is the first demonstration of the cerebellar compartmentation of an enzyme related to lipid metabolism, and as such, it provides an insight into the roles of SPHK and formation of S1P. The selective expression of SPHK in the zebrin II-immunoreactive PCs may explain their resistance to cell death when ceramide metabolism is disrupted, as in the acid sphingomyelinase knockout model of Niemann-Pick type A/B disease.

Identification of functional nuclear export sequences in human sphingosine kinase 1

Sphingosine kinase (SPHK) is an enzyme that phosphorylates sphingosine to form sphingosine 1-phosphate (S1P). Human SPHK1 (hSPHK1) was localized predominantly in the cytoplasm when transiently expressed in Cos7 cells. In this study, we have found two functional nuclear export signal (NES) sequences in the middle region of hSPHK1. Deletion and mutagenesis studies revealed that the cytoplasmic localization of SPHK1 depends on its nuclear export, directed by the NES. Furthermore, upon treatment with leptomycin B, a specific inhibitor of the nuclear export receptor CRM1, a marked nuclear accumulation of hSPHK1 was observed, indicating that hSPHK1 shuttles between the cytoplasm and the nucleus. Our results provide the first evidence of the active nuclear export of SPHK1 and suggest it is mediated by a CRM1-dependent pathway.

Characterization of the Drosophila sphingosine kinases and requirement for Sk2 in normal reproductive function

Sphingosine kinase is a highly conserved enzyme that catalyzes the synthesis of sphingosine 1-phosphate and reduces cellular levels of sphingosine and ceramide. Although ceramide is pro-apoptotic and sphingosine is generally growth-inhibitory, sphingosine 1-phosphate signaling promotes cell proliferation, survival, and migration. Sphingosine kinase is thus in a strategic position to regulate important cell fate decisions which may contribute to normal animal development. To facilitate studies examining the potential role of sphingosine kinase and long chain base metabolism in Drosophila development, we characterized two putative Drosophila sphingosine kinase genes, Sk1 and Sk2. Both genes functionally and biochemically complement a yeast sphingosine kinase mutant, express predominantly cytosolic activities, and are capable of phosphorylating a range of endogenous and non-endogenous sphingoid base substrates. The two genes demonstrate overlapping but distinct temporal and spatial expression patterns in the Drosophila embryo, and timing of expression is consistent with observed changes in long chain base levels throughout development. A null Sk2 transposon insertion mutant demonstrated elevated long chain base levels, impaired flight performance, and diminished ovulation. This is the first reported mutation of a sphingosine kinase in an animal model; the associated phenotypes indicate that Sk1 and Sk2 are not redundant in biological function and that sphingosine kinase is essential for diverse physiological functions in this organism.

Regulation of sphingosine kinase 1 gene expression by protein kinase C in a human leukemia cell line, MEG-O1

The prolonged treatment with phorbol 12-myristate 13-acetate (PMA) of a human megakaryoblastic leukemia cell line, MEG-O1, induced increase of sphingosine kinase (SPHK) enzyme activity and SPHK1 protein expression as well as SPHK1 message. Protein kinase C (PKC) inhibitor prevented the PMA-induced SPHK1 gene expression. To elucidate the regulatory mechanism of this gene expression, we examined the promoter area (distal to the first exon) and its binding proteins. Luciferase analyses showed that the area of 300 bp from the first exon was sufficient for PMA-responsiveness, and that specificity protein 1 (Sp1)- and two activator protein 2 (AP-2)-binding motifs within this area were necessary for responsiveness. Inhibitors for PKC and MEK1 decreased this PMA-induced promoter activity. Electrophoresis mobility shift assay (EMSA) showed that Sp1 protein was originally bound to the Sp1 site and that two additional bands bound to the two AP-2 motifs were observed only when stimulated with PMA in MEG-O1 cells. The appearance of these bands resulted from binding to an unknown protein rather than AP-2. These results indicated that PMA up-regulates SPHK1 gene expression through PMA-responsive elements of the 5' promoter area of the gene, and suggested that PMA-mediated SPHK1 gene expression would be mediated via PKC- and ERK-dependent signal transduction pathway by binding the transcription factor to AP-2 motifs.

Distribution of sphingosine kinase activity in mouse tissues: contribution of SPHK1

Sphingosine kinase (SPHK) phosphorylates sphingosine to form a bioactive lipid mediator, sphingosine 1-phosphate (S1P). S1P mediates such diverse biological processes as regulation of cell differentiation, motility, and apoptosis both extracellularly, via S1P (Edg) family receptors, and intracellularly, through unidentified targets. In cells S1P is short-lived, so the synthetic process catalyzed by sphingosine kinase may be important in maintaining the cellular levels of the compound. Thus far, two sphingosine kinases have been reported, with SPHK1 exhibiting the higher activity. However, several studies suggest the existence of unidentified sphingosine kinases. Therefore, to further elucidate the role of SPHK1 in the formation of S1P, we investigated its contribution to the total sphingosine kinase activity in mouse tissues. We found that SPHK1 is a major sphingosine kinase in many tissues, especially in brain, heart, and colon. However, some tissues such as spleen, small intestine, and lung contained sphingosine kinase activity that was not attributable to SPHK1 or SPHK2, as determined by immunodepletion assays. Furthermore, the presence of other sphingosine kinases with different properties, i.e., higher activity toward phytosphingosine and a different subcellular distribution, is suggested.

Sphingosine-1-phosphate lyase is involved in the differentiation of F9 embryonal carcinoma cells to primitive endoderm

Sphingosine 1-phosphate (S1P) is a bioactive lipid molecule that acts both extracellularly and intracellularly. The SPL gene encodes a mammalian S1P lyase that degrades S1P. Here, we have disrupted the SPL gene in mouse F9 embryonal carcinoma cells by gene targeting. This is the first report of gene disruption of mammalian S1P lyase. The SPL-null cells exhibited no S1P lyase activity, and intracellular S1P was increased approximately 2-fold, compared with wild-type cells. Treatment of F9 embryonal carcinoma cells with retinoic acid induces differentiation to primitive endoderm (PrE). An acceleration in this PrE differentiation was observed in the SPL-null cells. This effect was apparently caused by the accumulated S1P, since N,N-dimethylsphingosine, a S1P synthesis inhibitor, had an inhibitory effect on the PrE differentiation. Moreover, F9 cells stably expressing sphingosine kinase also exhibited an acceleration in the differentiation. Exogenous S1P had no effect on differentiation, indicating that intracellular but not extracellular S1P is involved. Moreover, we determined that expression of the SPL protein is up-regulated during the progression to PrE. We also showed that sphingosine kinase activity is increased in PrE-differentiated cells. These results suggest that intracellular S1P has a role in the PrE differentiation and that SPL may be involved in the regulation of intracellular S1P levels during this differentiation.

Ca2+/calmodulin-dependent translocation of sphingosine kinase: role in plasma membrane relocation but not activation

Activation of sphingosine kinase (SPHK), thereby increasing cellular levels of sphingosine 1-phosphate (S1P), may be involved in a variety of intracellular responses including Ca(2+) signaling. This study uses mammalian SPHK1a, tagged with enhanced green fluorescent protein (eGFP), to examine whether translocation of this enzyme is linked with Ca(2+)-mobilizing responses. Real-time confocal imaging of SPHK1a-eGFP in human SH-SY5Y neuroblastoma cells visualized a relocation of the enzyme from the cytosol to the plasma membrane in response to Ca(2+)-mobilizing stimuli (muscarinic M(3)- or lysophosphatidic acid receptor activation, and thapsigargin-mediated store release). This redistribution was preceded by a transient increase in cytosolic SPHK1a-eGFP levels due to liberation of SPHK from localized higher intensity regions. Translocation was dependent on Ca(2+) mobilization from intracellular stores, and was prevented by pretreatment with the Ca(2+)/calmodulin inhibitor W-7, but not W-5 or KN-62. In functional studies, pretreatment with W-7 lowered basal and M(3)-receptor-mediated cellular S1P production. However, this pretreatment did not alter agonist-mediated Ca(2+) responses, and SPHK1a-eGFP activity itself appeared insensitive to Ca(2+)/calmodulin and W-7. These data suggest a role for Ca(2+)/calmodulin in controlling the subcellular distribution but not the activity of SPHK1a.

Sphingosine kinases: a novel family of lipid kinases

Sphingosine kinase (SPHK) catalyzes the formation of sphingosine-1-phosphate (S11). S1P plays an important role in regulation of a variety of biological processes through intracellular and extracellular actions. S1P has recently been shown to be the ligand for the EDG-1 family of G-protein-coupled receptors. To date, seven cloned SPHKs have been reported with confirmed SPHK activity, including human, mouse, yeast, and plant. A computer search of various databases suggests that a new SPHK family is emerging. The cloning and manipulation of SPHK genes will no doubt provide us with important information about the functions of S1P in a wide range of organisms.

The nucleotide-binding site of human sphingosine kinase 1

Sphingosine kinase catalyzes the formation of sphingosine 1-phosphate, a lipid second messenger that has been implicated in a number of agonist-driven cellular responses including mitogenesis, anti-apoptosis, and expression of inflammatory molecules. Despite the importance of sphingosine kinase, very little is known regarding its structure or mechanism of catalysis. Moreover, sphingosine kinase does not contain recognizable catalytic or substrate-binding sites, based on sequence motifs found in other kinases. Here we have elucidated the nucleotide-binding site of human sphingosine kinase 1 (hSK1) through a combination of site-directed mutagenesis and affinity labeling with the ATP analogue, FSBA. We have shown that Gly(82) of hSK1 is involved in ATP binding since mutation of this residue to alanine resulted in an enzyme with an approximately 45-fold higher K(m)((ATP)). We have also shown that Lys(103) is important in catalysis since an alanine substitution of this residue ablates catalytic activity. Furthermore, we have shown that this residue is covalently modified by FSBA. Our data, combined with amino acid sequence comparison, suggest a motif of SGDGX(17-21)K is involved in nucleotide binding in the sphingosine kinases. This motif differs in primary sequence from all previously identified nucleotide-binding sites. It does, however, share some sequence and likely structural similarity with the highly conserved glycine-rich loop, which is known to be involved in anchoring and positioning the nucleotide in the catalytic site of many protein kinases.