Role of sphingosine-1-phosphate phosphatase 1 in epidermal growth factor-induced chemotaxis

Sphingosine-1-phosphate (S1P) is the ligand for a family of specific G protein-coupled receptors that regulate a wide variety of cellular functions, including cytoskeletal rearrangements and cell motility. Because of the pivotal role of S1P, its levels are low and tightly regulated in a spatial-temporal manner through its synthesis catalyzed by sphingosine kinases and degradation by an S1P lyase and specific S1P phosphatases (SPP). Surprisingly, down-regulation of SPP-1 enhanced migration toward epidermal growth factor (EGF); conversely, overexpression of SPP-1, which is localized in the endoplasmic reticulum, attenuated migration toward EGF. To determine whether the inhibitory effect on EGF-induced migration was because of decreased S1P or increased ceramide as a consequence of acylation of increased sphingosine by ceramide synthase, we used fumonisin B1, a specific inhibitor of ceramide synthase. Although fumonisin B1 blocked ceramide production and increased sphingosine, it did not reverse the negative effect of SPP-1 expression on EGF- or S1P-induced chemotaxis. EGF activated the epidermal growth factor receptor to the same extent in SPP-1-expressing cells, yet ERK1/2 activation was impaired. In agreement, PD98059, an inhibitor of the ERK-activating enzyme MEK, decreased EGF-stimulated migration. We next examined the possibility that intracellularly generated S1P might be involved in activating a G protein-coupled S1P receptor important for EGF-directed migration. Treatment with pertussis toxin to inactivate Galpha(i) suppressed EGF-induced migration. Moreover, expression of regulator of G protein signaling 3, which inhibits S1P receptor signaling and completely prevented ERK1/2 activation mediated by S1P receptors, not only reduced migration toward S1P but also markedly reduced migration toward EGF. Collectively, these results suggest that metabolism of S1P by SPP-1 is important for EGF-directed cell migration.     

Critical involvement of ILK in TGFbeta1-stimulated invasion/migration of human ovarian cancer cells is associated with urokinase plasminogen activator system

The present study investigated the role of integrin-linked kinase (ILK) in TGFbeta1-stimulated invasion/migration of human ovarian cancer cells. We investigated TGFbeta1 regulation of ILK, and effects of ILK knockdown on TGFbeta1-stimulated invasion/migration and the associated proteinase systems, urokinase plasminogen activator (uPA) and matrix metalloproteinases (MMPs) in SKOV3 cells. TGFbeta1 stimulated ILK kinase activity, and had no effect on ILK protein/mRNA levels. Transient transfection of an ILK-specific siRNA (ILK-H) reduced ILK protein level, mRNA level and kinase activity. ILK knockdown by ILK-H suppressed the basal and TGFbeta1-stimulated invasion and migration. Further, ILK-H reduced the basal and TGFbeta1-stimulated secretion of uPA, and increased the secretion of its inhibitor (PAI-1). Conversely, ILK-H did not affect TGFbeta1-stimulated secretion of MMP2 and its cell-associated activator MT1-MMP. Additionally, TGFbeta1 activated Smad2 phosphorylation, and this was not affected by ILK knockdown. Earlier reports indicate that Smad2 activation increased the expression of MMP2 and MT1-MMP. Thus, TGFbeta1 may act through ILK-independent and Smad2-dependent signaling in regulating MMP2 and MT1-MMP in SKOV3 cells. Collectively, this study suggests that ILK serves as a key mediator in TGFbeta1 regulation of uPA/PAI-1 system critical for the invasiveness of human ovarian cancer cells. And ILK is a potential target for cancer therapy.     

Roles of sphingosine-1-phosphate (S1P) receptors in malignant behavior of glioma cells. Differential effects of S1P2 on cell migration and invasiveness

Sphingosine-1-phosphate (S1P) is a bioactive lipid that signals through a family of five G-protein-coupled receptors, termed S1P(1-5). S1P stimulates growth and invasiveness of glioma cells, and high expression levels of the enzyme that forms S1P, sphingosine kinase-1, correlate with short survival of glioma patients. In this study we examined the mechanism of S1P stimulation of glioma cell proliferation and invasion by either overexpressing or knocking down, by RNA interference, S1P receptor expression in glioma cell lines. S1P(1), S1P(2) and S1P(3) all contribute positively to S1P-stimulated glioma cell proliferation, with S1P(1) being the major contributor. Stimulation of glioma cell proliferation by these receptors correlated with activation of ERK MAP kinase. S1P(5) blocks glioma cell proliferation, and inhibits ERK activation. S1P(1) and S1P(3) enhance glioma cell migration and invasion. S1P(2) inhibits migration through Rho activation, Rho kinase signaling and stress fiber formation, but unexpectedly, enhances glioma cell invasiveness by stimulating cell adhesion. S1P(2) also potently enhances expression of the matricellular protein CCN1/Cyr61, which has been implicated in tumor cell adhesion, and invasion as well as tumor angiogenesis. A neutralizing antibody to CCN1 blocked S1P(2)-stimulated glioma invasion. Thus, while S1P(2) decreases glioma cell motility, it may enhance invasion through induction of proteins that modulate glioma cell interaction with the extracellular matrix.     

Calcium influx and signaling in yeast stimulated by intracellular sphingosine 1-phosphate accumulation

In mammalian cells, intracellular sphingosine 1-phosphate (S1P) can stimulate calcium release from intracellular organelles, resulting in the activation of downstream signaling pathways. The budding yeast Saccharomyces cerevisiae expresses enzymes that can synthesize and degrade S1P and related molecules, but their possible role in calcium signaling has not yet been tested. Here we examine the effects of S1P accumulation on calcium signaling using a variety of yeast mutants. Treatment of yeast cells with exogenous sphingosine stimulated Ca(2+) accumulation through two distinct pathways. The first pathway required the Cch1p and Mid1p subunits of a Ca(2+) influx channel, depended upon the function of sphingosine kinases (Lcb4p and Lcb5p), and was inhibited by the functions of S1P lyase (Dpl1p) and the S1P phosphatase (Lcb3p). The biologically inactive stereoisomer of sphingosine did not activate this Ca(2+) influx pathway, suggesting that the active S1P isomer specifically stimulates a calcium-signaling mechanism in yeast. The second Ca(2+) influx pathway stimulated by the addition of sphingosine was not stereospecific, was not dependent on the sphingosine kinases, occurred only at higher doses of added sphingosine, and therefore was likely to be nonspecific. Mutants lacking both S1P lyase and phosphatase (dpl1 lcb3 double mutants) exhibited constitutively high Ca(2+) accumulation and signaling in the absence of added sphingosine, and these effects were dependent on the sphingosine kinases. These results show that endogenous S1P-related molecules can also trigger Ca(2+) accumulation and signaling. Several stimuli previously shown to evoke calcium signaling in wild-type cells were examined in lcb4 lcb5 double mutants. All of the stimuli produced calcium signals independent of sphingosine kinase activity, suggesting that phosphorylated sphingoid bases might serve as messengers of calcium signaling in yeast during an unknown cellular response.     

Insulin-like growth factors mediate heterotrimeric G protein-dependent ERK1/2 activation by transactivating sphingosine 1-phosphate receptors

Although several studies have shown that a subset of insulin-like growth factor (IGF) signals require the activation of heterotrimeric G proteins, the molecular mechanisms underlying IGF-stimulated G protein signaling remain poorly understood. Here, we have studied the mechanism by which endogenous IGF receptors activate the ERK1/2 mitogen-activated protein kinase cascade in HEK293 cells. In these cells, treatment with pertussis toxin and expression of a Galpha(q/11)-(305-359) peptide that inhibits G(q/11) signaling additively inhibited IGF-stimulated ERK1/2 activation, indicating that the signal was almost completely G protein-dependent. Treatment with IGF-1 or IGF-2 promoted translocation of green fluorescent protein (GFP)-tagged sphingosine kinase (SK) 1 from the cytosol to the plasma membrane, increased endogenous SK activity within 30 s of stimulation, and caused a statistically significant increase in intracellular and extracellular sphingosine 1-phosphate (S1P) concentration. Using a GFP-tagged S1P1 receptor as a biological sensor for the generation of physiologically relevant S1P levels, we found that IGF-1 and IGF-2 induced GFP-S1P receptor internalization and that the effect was blocked by pretreatment with the SK inhibitor, dimethylsphingosine. Treating cells with dimethylsphingosine, silencing SK1 expression by RNA interference, and blocking endogenous S1P receptors with the competitive antagonist VPC23019 all significantly inhibited IGF-stimulated ERK1/2 activation, suggesting that IGFs elicit G protein-dependent ERK1/2 activation by stimulating SK1-dependent transactivation of S1P receptors. Given the ubiquity of SK and S1P receptor expression, S1P receptor transactivation may represent a general mechanism for G protein-dependent signaling by non-G protein-coupled receptors.     

Roles of sphingosine 1-phosphate on tumorigenesis

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid with a variety of biological activities.It is generated from the conversion of ceramide to sphingosine by ceramidase and the subsequent conversion of sphingosine to S1P,which is catalyzed by sphingosine kinases.Through increasing its intracellular levels by sphingolipid metabolism and binding to its cell surface receptors,S1P regulates several physiological and pathological processes,including cell proliferation,migration,angiogenesis and autophagy.These processes are responsible for tumor growth,metastasis and invasion and promote tumor survival.Since ceramide and S1P have distinct functions in regulating in cell fate decision,the balance between the ceramide/sphingosine/S1P rheostat becomes a potent therapeutic target for cancer cells.Herein,we summarize our current understanding of S1P signaling on tumorigenesis and its potential as a target for cancer therapy.     

Activin receptor-like kinase (ALK)1 is an antagonistic mediator of lateral TGFbeta/ALK5 signaling

Transforming growth factor-beta (TGFbeta) regulates the activation state of the endothelium via two opposing type I receptor/Smad pathways. Activin receptor-like kinase-1 (ALK1) induces Smad1/5 phosphorylation, leading to an increase in endothelial cell proliferation and migration, while ALK5 promotes Smad2/3 activation and inhibits both processes. Here, we report that ALK5 is important for TGFbeta/ALK1 signaling; endothelial cells lacking ALK5 are deficient in TGFbeta/ALK1-induced responses. More specifically, we show that ALK5 mediates a TGFbeta-dependent recruitment of ALK1 into a TGFbeta receptor complex and that the ALK5 kinase activity is required for optimal ALK1 activation. TGFbeta type II receptor is also required for ALK1 activation by TGFbeta. Interestingly, ALK1 not only induces a biological response opposite to that of ALK5 but also directly antagonizes ALK5/Smad signaling.     

Opposite effects of dihydrosphingosine 1-phosphate and sphingosine 1-phosphate on transforming growth factor-beta/Smad signaling are mediated through the PTEN/PPM1A-dependent pathway

Transforming growth factor-beta (TGF-beta) is an important regulator of physiological connective tissue biosynthesis and plays a central role in pathological tissue fibrosis. Previous studies have established that a biologically active lipid mediator, sphingosine 1-phosphate (S1P), mimics some of the profibrotic functions of TGF-beta through cross-activation of Smad signaling. Here we report that another product of sphingosine kinase, dihydrosphingosine 1-phosphate (dhS1P), has an opposite role in the regulation of TGF-beta signaling. In contrast to S1P, dhS1P inhibits TGF-beta-induced Smad2/3 phosphorylation and up-regulation of collagen synthesis. The effects of dhS1P require a lipid phosphatase, PTEN, a key modulator of cell growth and survival. dhS1P stimulates phosphorylation of the C-terminal domain of PTEN and its subsequent translocation into the nucleus. We demonstrate a novel function of nuclear PTEN as a co-factor of the Smad2/3 phosphatase, PPM1A. Complex formation of PTEN with PPM1A does not require the lipid phosphatase activity but depends on phosphorylation of the serine/threonine residues located in the C-terminal domain of PTEN. Upon complex formation with PTEN, PPM1A is protected from degradation induced by the TGF-beta signaling. Consequently, overexpression of PTEN abrogates TGF-beta-induced Smad2/3 phosphorylation. This study establishes a novel role for nuclear PTEN in the stabilization of PPM1A. PTEN-mediated cross-talk between the sphingolipid and TGF-beta signaling pathways may play an important role in physiological and pathological TGF-beta signaling.     

Sphingosine kinase type 2 activation by ERK-mediated phosphorylation

Sphingosine 1-phosphate (S1P), a potent lipid mediator, is a ligand for a family of five G protein-coupled receptors (S1P(1-5)) that have been shown to regulate a variety of biological responses important for cancer progression. The cellular level of S1P is low and tightly regulated in a spatio-temporal manner through its synthesis catalyzed by two sphingosine kinases, denoted SphK1 and SphK2. Many stimuli activate and translocate SphK1 to the plasma membrane by mechanisms that are dependent on its phosphorylation. Much less is known about activation of SphK2. Here we demonstrate that epidermal growth factor (EGF) as well as the protein kinase C activator, phorbol ester, induce rapid phosphorylation of hSphK2 which was markedly reduced by inhibition of MEK1/ERK pathway. Down-regulation of ERK1 blocked EGF-induced phosphorylation of SphK2. Recombinant ERK1 phosphorylated hSphK2 in vitro and increased its enzymatic activity. ERK1 also was found to be in a complex with hSphK2 in vivo. Site-directed mutagenesis indicated that hSphK2 is phosphorylated on Ser-351 and Thr-578 by ERK1 and that phosphorylation of these residues is important for EGF-stimulated migration of MDA-MB-453 cells. These studies provide the first clues to the mechanism of agonist-mediated SphK2 activation and enhance understanding of the regulation of SphK2 activity by phosphorylation and its role in movement of human breast cancer cells toward EGF.     

Sphingosine kinase and sphingosine-1-phosphate in liver pathobiology

Over 20?years ago, sphingosine-1-phosphate (S1P) was discovered to be a bioactive signaling molecule. Subsequent studies later identified two related kinases, sphingosine kinase 1 and 2, which are responsible for the phosphorylation of sphingosine to S1P. Many stimuli increase sphingosine kinase activity and S1P production and secretion. Outside the cell, S1P can bind to and activate five S1P-specific G protein-coupled receptors (S1PR1–5) to regulate many important cellular and physiological processes in an autocrine or paracrine manner. S1P is found in high concentrations in the blood where it functions to control vascular integrity and trafficking of lymphocytes. Obesity increases blood S1P levels in humans and mice. With the world wide increase in obesity linked to consumption of high-fat, high-sugar diets, S1P is emerging as an accomplice in liver pathobiology, including acute liver failure, metabolic syndrome, control of blood lipid and glucose homeostasis, nonalcoholic fatty liver disease, and liver fibrosis. Here, we review recent research on the importance of sphingosine kinases, S1P, and S1PRs in liver pathobiology, with a focus on exciting insights for new therapeutic modalities that target S1P signaling axes for a variety of liver diseases.     

A Novel Aminothiazole KY-05009 with Potential to Inhibit Traf2- and Nck-Interacting Kinase (TNIK) Attenuates TGF-β1-Mediated Epithelial-to-Mesenchymal Transition in Human Lung Adenocarcinoma A549 Cells

Transforming growth factor (TGF)-β triggers the epithelial-to-mesenchymal transition (EMT) of cancer cells via well-orchestrated crosstalk between Smad and non-Smad signaling pathways, including Wnt/β-catenin. Since EMT-induced motility and invasion play a critical role in cancer metastasis, EMT-related molecules are emerging as novel targets of anti-cancer therapies. Traf2- and Nck-interacting kinase (TNIK) has recently been considered as a first-in-class anti-cancer target molecule to regulate Wnt signaling pathway, but pharmacologic inhibition of its EMT activity has not yet been studied. Here, using 5-(4-methylbenzamido)-2-(phenylamino)thiazole-4-carboxamide (KY-05009) with TNIK-inhibitory activity, its efficacy to inhibit EMT in cancer cells was validated. The molecular docking/binding study revealed the binding of KY-05009 in the hinge region of TNIK, and the inhibitory activity of KY-05009 against TNIK was confirmed by an ATP competition assay (K _i, 100 nM). In A549 cells, KY-05009 significantly and strongly inhibited the TGF-β-activated EMT through the attenuation of Smad and non-Smad signaling pathways, including the Wnt, NF-κB, FAK-Src-paxillin-related focal adhesion, and MAP kinases (ERK and JNK) signaling pathways. Continuing efforts to identify and validate potential therapeutic targets associated with EMT, such as TNIK, provide new and improved therapies for treating and/or preventing EMT-based disorders, such as cancer metastasis and fibrosis.     

Endoglin null endothelial cells proliferate faster and are more responsive to transforming growth factor beta1 with higher affinity receptors and an activated Alk1 pathway

Endoglin is an accessory receptor for transforming growth factor beta (TGFbeta) in endothelial cells, essential for vascular development. Its pivotal role in angiogenesis is underscored in Endoglin null (Eng-/-) murine embryos, which die at mid-gestation (E10.5) from impaired yolk sac vessel formation. Moreover, mutations in endoglin and the endothelial-specific TGFbeta type I receptor, ALK1, are linked to hereditary hemorrhagic telangiectasia. To determine the role of endoglin in TGFbeta pathways, we derived murine endothelial cell lines from Eng+/+ and Eng-/- embryos (E9.0). Whereas Eng+/+ cells were only partially growth inhibited by TGFbeta, Eng-/- cells displayed a potent anti-proliferative response. TGFbeta-dependent Smad2 phosphorylation and Smad2/3 translocation were unchanged in the Eng-/- cells. In contrast, TGFbeta treatment led to a more rapid activation of the Smad1/5 pathway in Eng null cells that was apparent at lower TGFbeta concentrations. Enhanced activity of the Smad1 pathway in Eng-/- cells was reflected in higher expression of ALK1-dependent genes such as Id1, Smad6, and Smad7. Analysis of cell surface receptors revealed that the TGFbeta type I receptor, ALK5, which is required for ALK1 function, was increased in Eng-/- cells. TGFbeta receptor complexes were less numerous but displayed a higher binding affinity. These results suggest that endoglin modulates TGFbeta signaling in endothelial cells by regulating surface TGFbeta receptors and suppressing Smad1 activation. Thus an altered balance in TGFbeta receptors and downstream Smad pathways may underlie defects in vascular development and homeostasis.     

Angiotensin II increases CTGF expression via MAPKs/TGF-β1/TRAF6 pathway in atrial fibroblasts

The activation of transforming growth factor-β1(TGF-β1)/Smad signaling pathway and increased expression of connective tissue growth factor (CTGF) induced by angiotensin II (AngII) have been proposed as a mechanism for atrial fibrosis. However, whether TGFβ1/non-Smad signaling pathways involved in AngII-induced fibrogenetic factor expression remained unknown. Recently tumor necrosis factor receptor associated factor 6 (TRAF6)/TGFβ-associated kinase 1 (TAK1) has been shown to be crucial for the activation of TGF-β1/non-Smad signaling pathways. In the present study, we explored the role of TGF-β1/TRAF6 pathway in AngII-induced CTGF expression in cultured adult atrial fibroblasts. AngII (1 μM) provoked the activation of P38 mitogen activated protein kinase (P38 MAPK), extracellular signal-regulated kinase 1/2(ERK1/2) and c-Jun NH(2)-terminal kinase (JNK). AngII (1 μM) also promoted TGFβ1, TRAF6, CTGF expression and TAK1 phosphorylation, which were suppressed by angiotensin type I receptor antagonist (Losartan) as well as p38 MAPK inhibitor (SB202190), ERK1/2 inhibitor (PD98059) and JNK inhibitor (SP600125). Meanwhile, both TGFβ1 antibody and TRAF6 siRNA decreased the stimulatory effect of AngII on TRAF6, CTGF expression and TAK1 phosphorylation, which also attenuated AngII-induced atrial fibroblasts proliferation. In summary, the MAPKs/TGFβ1/TRAF6 pathway is an important signaling pathway in AngII-induced CTGF expression, and inhibition of TRAF6 may therefore represent a new target for reversing Ang II-induced atrial fibrosis.     

Protein tyrosine phosphatase 1B regulates TGF beta 1-induced Smad2 activation through PI3 kinase-dependent pathway

Insulin is known to modulate transforming growth factor-beta (TGFbeta) signaling. In this report, by using the IN Cell Analyzer 1000, the fluorescence cell imaging instrument, we demonstrated that protein tyrosine phosphatase 1B (PTP1B) could regulate TGFbeta1-induced Smad2 activation in a PI3 kinase-dependent manner. By using the CHO cells stably expressing EGFP-Smad2, we showed that TGFbeta1 effectively stimulated Smad2 nuclear translocation in CHO cells. When pretreated with insulin, TGFbeta1-induced Smad2 nuclear entry was dramatically decreased. Furthermore, both the PI3K inhibitor LY294002 and the dominant negative AKT (DN-AKT) abolished the inhibitory effects of insulin, demonstrating that the inhibition of Smad2 activation by insulin was PI3K/AKT dependent. Since PTP1B negatively modulates insulin signaling, we further addressed the effects of PTP1B on insulin-mediated inhibition of Smad2 activation. Our data showed that overexpression of PTP1B effectively attenuated insulin-induced inhibition of Smad2 stimulation. Moreover, the PTP1B inhibitor, 3-(3,5-dibromo-4-hydroxy-benzoyl)-2-ethyl-benzofuran-6-sulfonicacid-(4-(thiazol-2-ylsulfamyl)-phenyl)-amide (Compound-2), recovered insulin inhibition of Smad2 activation. In conclusion, our data revealed the insulin inhibitory effects on TGFbeta1-induced Smad2 activation and the regulation role of PTP1B in the inhibition events.