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

ATP-binding cassette ABCC1 is involved in the release of sphingosine 1-phosphate from rat uterine leiomyoma ELT3 cells and late pregnant rat myometrium

Sphingosine 1-phosphate (S1P), a bioactive lipid generated by sphingosine kinases (SphK1/2), initiates different signalling pathways involved in physiological and pathological processes. We previously demonstrated that in rat myometrium at late (day 19) gestation, SphK1 increases the expression of COX2 via S1P generation and release. In rat uterine leiomyoma cells (ELT3), SphK1/S1P axis controls survival and proliferation. In the present study we demonstrate that PDBu activates SphK1 but not SphK2. SphK1 activation requires PKC and MAPK ERK1/2. S1P produced by PDBu is released in the medium. PDBu-induced S1P export is abolished by Ro-318220 and BIM (PKC inhibitors), by U0126 and PD98059 (MEK inhibitors), SKI-II (SphKI/2 inhibitor) and SphK1-siRNA, suggesting the involvement of PKC, ERK and SphK1 respectively. The release of S1P is insensitive to inhibitors of ATP Binding Cassette (ABC)A1 and ABCB1 transporters, but is abolished when ABCC1 transporters are inhibited by MK571 or down-regulated by ABCC1-siRNA. PDBu increases COX2 expression that is blocked by the inhibition of PKC, ERK1/2, SphK1, and when cells are treated with MK571 or transfected with ABCC1-siRNA. The induction of COX2 by the S1P release due to PDBu or by exogenous S1P involves S1P2 receptors coupled to Gi. In myometrium from rat at late gestation, the release of S1P is also strongly reduced when SphK and ABCC1 are inhibited. The data reveal that in rat leiomyoma cells and late pregnant rat myometrium, the release of S1P involves a similar signalling pathway and occurs through ABCC1.

Sphingosine kinase as an oncogene: autocrine sphingosine 1-phosphate modulates ML-1 thyroid carcinoma cell migration by a mechanism dependent on protein kinase C-alpha and ERK1/2

Sphingosine 1-phosphate (S1P) induces migration of the human thyroid follicular carcinoma cell line ML-1 by activation of S1P(1) and S1P(3) receptors, G(i) proteins, and the phosphatidylinositol 3-kinase-Akt pathway. Because sphingosine kinase isoform 1 (SK) recently has been implicated as an oncogene in various cancer cell systems, we investigated the functions of SK in the migration, proliferation and adhesion of the ML-1 cell line. SK overexpressing ML-1 cells show an enhanced secretion of S1P, which can be attenuated, by inhibiting SK activity and a multidrug-resistant transport protein (ATP-binding cassette transporter). Furthermore, overexpression of SK enhances serum-induced migration of ML-1 cells, which can be attenuated by blocking ATP-binding cassette transporter and SK, suggesting that the migration is mediated by autocrine signaling through secretion of S1P. Inhibition of protein kinase C alpha, with both small interfering RNA (siRNA) and small molecular inhibitors attenuates migration in SK overexpressing cells. In addition, SK-overexpressing cells show an impaired adhesion, slower cell growth, and an up-regulation of ERK1/2 phosphorylation, as compared with cells expressing a dominant-negative SK. Taken together, we present evidence suggesting that SK enhances migration of ML-1 cells by an autocrine mechanism and that the S1P-evoked migration is dependent on protein kinase C alpha, ERK1/2, and SK.

Sphingosine kinase isoforms regulate oxaliplatin sensitivity of human colon cancer cells through ceramide accumulation and Akt activation

The relationship between sphingosine kinase (SPHK), cellular ceramide concentration and chemosensitivity was investigated in human colon cancer cell lines. Among nine colon cancer cell lines, SPHK1 and SPHK2 activity and protein expression was highest in RKO cells and lowest in HCT116 cells. A viability assay revealed that HCT116 cells were sensitive to the effects of oxaliplatin (l-OHP), whereas RKO cells were resistant to those of l-OHP. Treatment with 5microg/ml l-OHP induced a marked time-dependent increase in various ceramides (C16, C24, C24:1) in HCT116 cells but not in RKO cells, as indicated by liquid chromatography/mass spectrometry. The increase in ceramide and caspase activation induced by l-OHP in the sensitive HCT116 cells was abolished by pretreatment with a neutral sphingomyelinase inhibitor, suggesting that the ceramide formation was due to the activation of neutral, rather than acid, sphingomyelinase. In contrast, in l-OHP-resistant RKO cells, treatment with an SPHK inhibitor or SPHK1 and SPHK2 silencing by RNA interference suppressed cell viability and increased caspase activity and cellular ceramide formation after l-OHP treatment. The elevated ceramide formation induced by SPHK inhibition and l-OHP was inhibited by fumonisin B1 but not myriocin, suggesting that ceramide formation was through the salvage pathway. Endogenous phosphorylated Akt levels were much higher in the resistant RKO cells than in the sensitive HCT116 cells. Either SPHK1 or SPHK2 silencing in RKO cells decreased phosphorylated Akt levels and increased p53 and p21 protein levels as well as poly(ADP-ribose) polymerase cleavage in response to l-OHP treatment. These findings indicate that SPHK isoforms and neutral sphingomyelinase contribute to the regulation of chemosensitivity by controlling ceramide formation and the downstream Akt pathway in human colon cancer cells.

Autocrine and paracrine roles of sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid metabolite that has been implicated in many biological processes, including cell migration, survival, proliferation, angiogenesis and immune and allergic responses. S1P levels inside cells are regulated tightly by the balance between its synthesis by sphingosine kinases and degradation by S1P lyases and S1P phosphatases. Activation of sphingosine kinase by any of a variety of agonists increases S1P levels, which in turn can function intracellularly as a second messenger or in an autocrine and/or paracrine fashion to activate and signal through S1P receptors present on the surface of the cell. This review summarizes recent findings on the roles of S1P as a mediator of the actions of cytokines, growth factors and hormones.

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.

Exogenous sphingosine 1-phosphate and sphingosine kinase activated by endothelin-1 induced myometrial contraction through differential mechanisms

Sphingosine 1-phosphate (S1P), a bioactive sphingolipid involved in diverse biological processes, is generated by sphingosine kinase (SphK) and acts via intracellular and/or extracellular mechanisms. We used biochemical, pharmacological, and physiological approaches to investigate in rat myometrium the contractile effect of exogenous S1P and the possible contribution of SphK in endothelin-1 (ET-1)-mediated contraction. S1P stimulated uterine contractility (EC50 = 1 μM and maximal response = 5 μM) by a pertussis toxin-insensitive and a phospholipse C (PLC)-independent pathway. Phosphorylated FTY720, which interacts with all S1P receptors, except S1P2 receptors, failed to mimic S1P contractile response, indicating that the effects of S1P involved S1P2 receptors that are expressed in myometrium. Contraction mediated by S1P and ET-1 required extracellular calcium and Rho kinase activation. Inhibition of SphK reduced ET-1-mediated contraction. ET-1, via ETA receptors coupled to pertussis toxin-insensitive G proteins, stimulated SphK1 activity and induced its translocation to the membranes. Myometrial contraction triggered by ET-1 is consecutive to the sequential activation of PLC, protein kinase C, SphK1 and Rho kinase. Prolonged exposure of the myometrium to S1P downregulated S1P2 receptors and abolished the contraction induced by exogenous S1P. However, in these conditions, the tension triggered by ET-1 was not reduced, indicating that SphK activated by ET-1 contributed to its contractile effect via a S1P2 receptor-independent process. Our findings demonstrated that exogenous S1P and SphK activity regulated myometrial contraction and may be of physiological relevance in the regulation of uterine motility during gestation and parturition.

Thymopentin (TP5), an immunomodulatory peptide, suppresses proliferation and induces differentiation in HL-60 cells

Thymopentin (Arg-Lys-Asp-Val-Tyr, TP5) has shown immuno-regulatory activities in humans. In the present study, we investigated the effects of TP5 on the proliferation and differentiation of a human promyelocyte leukemia cell line, HL-60. It is noteworthy that TP5 displayed concentration-dependent inhibitory effects on the proliferation and colony formation of HL-60 cells. Furthermore, the decrease or even disappearance of AgNORs from nucleoli was observed in HL-60 cells after the treatment with TP5. The suppression induced by TP5 was accompanied by an accumulation of cell cycle in the G0/G1 phase. Moreover, TP5 significantly increased the NBT-reduction activity of HL-60 cells. Cytofluorometric and morphologic analysis indicated that TP5 had induced differentiation along the granulocytes lineage in HL-60 cells. d-tubocurarine (TUB) significantly antagonized the inhibitory effects induced by TP5, whereas atropine did not exhibit such effect. All the results indicated that TP5 was able to significantly inhibit proliferation and induce differentiation in HL-60 cells. Our observations also implied that TP5 not only acted as an immunomodulatory factor in cancer chemotherapy, but is also a potential chemotherapeutic agent in the human leukemia therapy.

Macrophage’s Proinflammatory Response to a Mycobacterial Infection Is Dependent on Sphingosine Kinase-Mediated Activation of Phosphatidylinositol Phospholipase C, Protein Kinase C, ERK1/2, and Phosphatidylinositol 3-Kinase

Previous studies have shown that the ability of Mycobacterium tuberculosis to block a Ca(2+) flux is an important step in its capacity to halt phagosome maturation. This affect on Ca(2+) release results from M. tuberculosis inhibition of sphingosine kinase (SPK) activity. However, these studies did not address the potential role of SPK and Ca(2+) in other aspects of macrophage activation including production of proinflammatory mediators. We previously showed that nonpathogenic Mycobacterium smegmatis and to a lesser extent pathogenic Mycobacterium avium, activate Ca(2+)-dependent calmodulin/calmodulin kinase and MAPK pathways in murine macrophages leading to TNF-alpha production. However, whether SPK functions in promoting MAPK activation upon mycobacterial infection was not defined in these studies. In the present work we found that SPK is required for ERK1/2 activation in murine macrophages infected with either M. avium or M. smegmatis. Phosphoinositide-specific phospholipase C (PI-PLC) and conventional protein kinase C (cPKC) were also important for ERK1/2 activation. Moreover, there was increased activation of cPKC and PI3K in macrophages infected with M. smegmatis compared with M. avium. This cPKC and PI3K activation was dependent on SPK and PI-PLC. Finally, in macrophages infected with M. smegmatis compared with M. avium, we observed enhanced secretion of TNF-alpha, IL-6, RANTES, and G-CSF and found production of these inflammatory mediators to be dependent on SPK, PI-PLC, cPKC, and PI3K. These studies are the first to show that the macrophage proinflammatory response following a mycobacterial infection is regulated by SPK/PI-PLC/PKC activation of ERK1/2 and PI3K pathways.