D-erythro-N,N-dimethylsphingosine inhibits bFGF-induced proliferation of cerebral, aortic and coronary smooth muscle cells

Sphingosine-1-Phosphate Metabolism and Signaling in Kidney Diseases

In the past few decades, sphingolipids and sphingolipid metabolites have gained attention because of their essential role in the pathogenesis and progression of kidney diseases. Studies in models of experimental and clinical nephropathies have described accumulation of sphingolipids and sphingolipid metabolites, and it has become clear that the intracellular sphingolipid composition of renal cells is an important determinant of renal function. Proper function of the glomerular filtration barrier depends heavily on the integrity of lipid rafts, which include sphingolipids as key components. In addition to contributing to the structural integrity of membranes, sphingolipid metabolites, such as sphingosine-1-phosphate (S1P), play important roles as second messengers regulating biologic processes, such as cell growth, differentiation, migration, and apoptosis. This review will focus on the role of S1P in renal cells and how aberrant extracellular and intracellular S1P signaling contributes to the pathogenesis and progression of kidney diseases.

β-arrestin 1 transfection induced cell death in high grade glioma in vitro

The best known functions of β-arrestins (β-arr) are to regulate G protein-coupled receptors (GPCR) signaling through receptor desensitization and internalization. Many reports also suggest that β-arrs play important role in immune regulation and inflammatory responses, under physiological and pathological conditions. Recent studies have shown that β-arr 1 silencing halts proliferation and increases temozolomide (TMZ) response in glioblastoma (GBM) cells. The focus of this paper is to analyze the role of β-arr 1 overexpression in the 18 high grade glioma (HGG) cell line in terms of viability and their response to TMZ treatment. For this reason, the cell line was transfected with β-arr 1 and the effect was analyzed after 24 h, 48 h and 72 h in terms of proliferation and treatment response. We observed that β-arr 1 overexpression induced a time and dose dependant inhibition in the HGG cells. Unexpectedly, β-arr transfection resulted in a very mild increase in TMZ toxicity after 24 h, becoming non-statistically significant at 72 h. In conclusion, we showed that β-arr 1 overexpression inhibits cell proliferation in the 18 cell line but only has a very modest effect on treatment response with the alkylating agent TMZ.

Sphingolipids in neutrophil function and inflammatory responses: Mechanisms and implications for intestinal immunity and inflammation in ulcerative colitis

Bioactive sphingolipids are regulators of immune cell function and play critical roles in inflammatory conditions including ulcerative colitis. As one of the major forms of inflammatory bowel disease, ulcerative colitis pathophysiology is characterized by an aberrant intestinal inflammatory response that persists causing chronic inflammation and tissue injury. Innate immune cells play an integral role in normal intestinal homeostasis but their dysregulation is thought to contribute to the pathogenesis of ulcerative colitis. In particular, neutrophils are key effector cells and are first line defenders against invading pathogens. While the activity of neutrophils in the intestinal mucosa is required for homeostasis, regulatory mechanisms are equally important to prevent unnecessary activation. In ulcerative colitis, unregulated neutrophil inflammatory mechanisms promote tissue injury and loss of homeostasis. Aberrant neutrophil function represents an early checkpoint in the detrimental cycle of chronic intestinal inflammation; thus, dissecting the mechanisms by which these cells are regulated both before and during disease is essential for understanding the pathogenesis of ulcerative colitis. We present an analysis of the role of sphingolipids in the regulation of neutrophil function and the implication of this relationship in ulcerative colitis.

Pharmacological relevance and potential of sphingosine 1-phosphate in the vascular system

Sphingosine-1-phosphate (S1P) was identified as a crucial molecule for regulating immune responses, inflammatory processes as well as influencing the cardiovascular system. S1P mediates differentiation, proliferation and migration during vascular development and homoeostasis. S1P is a naturally occurring lipid metabolite and is present in human blood in nanomolar concentrations. S1P is not only involved in physiological but also in pathophysiological processes. Therefore, this complex signalling system is potentially interesting for pharmacological intervention. Modulation of the system might influence inflammatory, angiogenic or vasoregulatory processes. S1P activates G-protein coupled receptors, namely S1P(1-5) , whereas only S1P(1-3) is present in vascular cells. S1P can also act as an intracellular signalling molecule. This review highlights the pharmacological potential of S1P signalling in the vascular system by giving an overview of S1P-mediated processes in endothelial cells (ECs) and vascular smooth muscle cells (VSMCs). After a short summary of S1P metabolism and signalling pathways, the role of S1P in EC and VSMC proliferation and migration, the cause of relaxation and constriction of arterial blood vessels, the protective functions on endothelial apoptosis, as well as the regulatory function in leukocyte adhesion and inflammatory responses are summarized. This is followed by a detailed description of currently known pharmacological agonists and antagonists as new tools for mediating S1P signalling in the vasculature. The variety of effects influenced by S1P provides plenty of therapeutic targets currently under investigation for potential pharmacological intervention.

Cigarette smoke extracts promote vascular smooth muscle cell proliferation and enhances contractile responses in the vasculature and airway

Cigarette smoke exposure is a strong risk factor for cardiovascular and respiratory diseases. However, the knowledge about how cigarette smoke induces damage to vasculature and airway is limited. The present study was designed to examine the effects of cigarette smoke particles extracted by heptane (heptane-soluble smoke particles, HSP), by water (water-soluble smoke particles, WSP) and by DMSO (DMSO-soluble smoke particles, DSP), which represent lipophilic, hydrophilic and ambiphoteric constituents from the cigarette smoke, respectively. Human aortic smooth muscle cell (HASMC) proliferation was assessed in cell culture. Rat resistance artery and airway contractile responses to serotonin, U46619, phenylephrine, noradrenaline, acetylcholine, des-Arg⁹-bradykinin, bradykinin, sarafotoxin 6c and endothelin-1 were monitored by a sensitive myograph system. Immunocytochemistry and cell-based phosphoELISA assay were used to demonstrate activation of extracellular signal-regulated kinases 1/2 (ERK1/2). For the first time, our results demonstrate that although all the three extracts promote HASMC proliferation, the HSP and DSP effects occur earlier. HSP and DSP, but not WSP, increase the contractile responses to sarafotoxin 6c, U46619 or bradykinin in rat mesenteric artery and/or in bronchi. ERK1/2 is activated by HSP and DSP in HASMCs and inhibition of ERK1/2 abrogated the smoke extracts-induced HASMC proliferation, while blockage of nicotinic receptors had no effects, suggesting that the toxic effects of the smoke extracts occur via activation of intracellular ERK1/2 signalling, but not nicotinic receptors.

The sphingosine kinase inhibitor N,N-dimethylsphingosine inhibits neointimal hyperplasia

BACKGROUND AND PURPOSE

Sphingosine-1-phosphate and its receptors may be involved in vascular smooth muscle cell (VSMC) proliferation following vascular injury. Here, we evaluate the effect of d-erythro-N,N-dimethylsphingosine (DMS), a sphingosine kinase (SK) inhibitor, on VSMC proliferation, apoptosis and neointimal formation.

EXPERIMENTAL APPROACH

Growth responses in vitro to fetal calf serum (FCS) were measured by [(3)H]-thymidine incorporation and extracellular signal-regulated kinase-1/2 (ERK-1/2) activation in quiescent primary cultures of porcine VSMC in the presence and absence of various concentrations of the SK inhibitor DMS. In vivo treatment with DMS was delivered with a local endoluminal catheter, following balloon injury of coronary arteries. The artery intimal formation was investigated by angiography, myography and histomorphometry.

KEY RESULTS

In vitro experiments indicated that DMS induced a dose-dependent reduction in [(3)H]-thymidine incorporation and ERK-1/2 activation via a protein kinase C (PKC) independent mechanism with an IC(50) value of 12 +/- 6 and 15 +/- 10 microM respectively. DMS also reduced Akt signalling. Four weeks following in vivo delivery of DMS, complete functional endothelial regeneration was observed in all treatment groups, with significant reduction of intimal formation (vehicle 23.7 +/- 4.6% vs. DMS infusion 8.92 +/- 2.9%, P < 0.05).

CONCLUSIONS AND IMPLICATIONS

Taken together, these results demonstrate that local administration of the SK inhibitor, DMS, reduced neointimal formation, and this effect could involve inhibition of ERK-1/2 and Akt signalling, and modulation of smooth muscle growth.

Sphingosine kinase regulates oxidized low density lipoprotein-mediated calcium oscillations and macrophage survival

We recently reported that oxidized LDL (oxLDL) induces an oscillatory increase in intracellular calcium ([Ca(2+)](i)) levels in macrophages. Furthermore, we have shown that these [Ca(2+)](i) oscillations mediate oxLDL's ability to inhibit macrophage apoptosis in response to growth factor deprivation. However, the signal transduction pathways by which oxLDL induces [Ca(2+)](i) oscillations have not been elucidated. In this study, we show that these oscillations are mediated in part by intracellular mechanisms, as depleting extracellular Ca(2+) did not completely abolish the effect. Inhibiting sarco-endoplasmic reticulum ATPase (SERCA) completely blocked [Ca(2+)](i) oscillations, suggesting a role for Ca(2+) reuptake by the ER. The addition of oxLDL resulted in an almost immediate activation of sphingosine kinase (SK), which can increase sphingosine-1-phosphate (S1P) levels by phosphorylating sphingosine. Moreover, S1P was shown to be as effective as oxLDL in blocking macrophage apoptosis and producing [Ca(2+)](i) oscillations. This suggests that the mechanism in which oxLDL generates [Ca(2+)](i) oscillations may be 1) activation of SK, 2) SK-mediated increase in S1P levels, 3) S1P-mediated Ca(2+) release from intracellular stores, and 4) SERCA-mediated Ca(2+) reuptake back into the ER.

The Localization and Activity of Sphingosine Kinase 1 Are Coordinately Regulated with Actin Cytoskeletal Dynamics in Macrophages

The physiologic and pathologic functions of sphingosine kinase (SK) require translocation to specific membrane compartments. We tested the hypothesis that interactions with actin filaments regulate the localization of SK1 to membrane surfaces, including the plasma membrane and phagosome. Macrophage activation is accompanied by a marked increase in association of SK1 with actin filaments. Catalytically-inactive (CI)- and phosphorylation-defective (PD)-SK1 mutants exhibited reductions in plasma membrane translocation, colocalization with cortical actin filaments, membrane ruffling, and lamellipodia formation, compared with wild-type (WT)-SK1. However, translocation of CI- and PD-SK1 to phagosomes were equivalent to WT-SK1. SK1 exhibited constitutive- and stimulus-enhanced association with actin filaments and F-actin-enriched membrane fractions in both intact macrophages and a novel in vitro assay. In contrast, SK1 bound G-actin only under stimulated conditions. Actin inhibitors disrupted SK1 localization and modulated its activity. Conversely, reduction of SK1 levels or activity via RNA interference or specific chemical inhibition resulted in dysregulation of actin filaments. Thus, the localization and activity of SK1 are coordinately regulated with actin dynamics during macrophage activation.

Activation of sphingosine kinase-1 mediates inhibition of vascular smooth muscle cell apoptosis by hyperglycemia

Vascular smooth muscle cell (VSMC) apoptosis plays an essential role in vascular development and atherosclerosis. Hyperglycemia inhibits VSMC apoptosis, which may contribute to the development of diabetic vasculopathy. In the present study, we analyzed the mechanism of high-glucose-induced anti-apoptotic effect in cultured human aortic smooth muscle cells (HASMCs). Compared with normoglycemia, exposure of HASMCs to hyperglycemia but not mannitol significantly increased sphingosine kinase 1 (SK1) activity but not SK2 activity. This increase was inhibited by protein kinase C (PKC) inhibitor GF109203X, the antioxidant N-acetylcysteine, and the reduced form of glutathione. The mechanism of SK1 activation by high glucose involves plasma membrane translocation. In addition, hyperglycemia markedly inhibited serum withdrawal-induced apoptosis in HASMCs. Importantly, inhibition of SK1 by either a competitive inhibitor N',N'-dimethylsphingosine or expression of dominant-negative mutant of SK1(G82D) or specific small interference RNA knockdown substantially attenuated hyperglycemia-induced anti-apoptotic effect and anti-apoptotic protein Bcl-2 expression in HASMCs. Moreover, SK1-mediated anti-apoptotic effect requires the intracellular effects of sphingosine-1-phosphate. We conclude that hyperglycemia stimulates SK1 activity via PKC- and oxidative stress-dependent pathways, leading to decreased apoptosis in HASMCs. Taken together, these observations have important implications for understanding the roles of the SK1 signaling pathway in the pathogenesis of diabetic vasculopathy.

Sphingosine kinase 1 expression is regulated by signaling through PI3K, AKT2, and mTOR in human coronary artery smooth muscle cells

Sphingosine kinase 1 (SphK1) is a lipid kinase implicated in mitogenic signaling pathways in vascular smooth muscle cells. We demonstrate that human coronary artery smooth muscle (HCASM) cells require SphK1 for growth and that SphK1 mRNA and protein levels are elevated in PDGF stimulated HCASM cells. To determine the mechanism of PDGF-induced SphK1 expression, we used pharmacological inhibitors of the PI3K/AKT/mTOR signaling pathway. Wortmannin, SH-5, and rapamycin significantly blocked PDGF-stimulated induction of SphK1 mRNA and protein expression, indicating a regulatory role of the PI3K/AKT/mTOR pathway in SphK1 expression. To determine which isoform of AKT regulates SphK1 mRNA and protein levels, siRNAs specific for AKT1, AKT2, and AKT3 were used. We show that AKT2 siRNA significantly blocked PDGF-stimulated increases in SphK1 mRNA and protein expression levels as well as SphK1 enzymatic activity levels. In contrast, AKT1 or AKT3 siRNA did not have an effect. Together, these results demonstrate that the PI3K/AKT/mTOR signaling pathway is involved in regulation of SphK1, with AKT2 playing a key role in PDGF-induced SphK1 expression in HCASM cells.

Regulation of receptor tyrosine kinase signaling by GRKs and beta-arrestins

Receptor tyrosine kinases (RTKs) are a unique family of cell surface receptors, each containing a common intracellular domain that has tyrosine kinase activity. However, RTKs share many signaling molecules with another unique family of cell surface receptors, the seven-transmembrane receptors (7TMRs), and these receptor families can activate similar signaling cascades. In this review of RTK signaling, we describe the role of cross talk between RTKs and 7TMRs, focusing specifically on the role played in this process by beta-arrestins and by G proteins.

Sphingosine kinase-dependent activation of endothelial nitric oxide synthase by angiotensin II

OBJECTIVE

In addition to their role in programmed cell death, cell survival, and cell growth, sphingolipid metabolites such as ceramide, sphingosine, and sphingosine-1-phosphate have vasoactive properties. Besides their occurrence in blood, they can also be formed locally in the vascular wall itself in response to external stimuli. This study was performed to investigate whether vasoactive compounds modulate sphingolipid metabolism in the vascular wall and how this might contribute to the vascular responses.

METHODS AND RESULTS

In isolated rat carotid arteries, the contractile responses to angiotensin II are enhanced by the sphingosine kinase inhibitor dimethylsphingosine. Endothelium removal or NO synthase inhibition by N(omega)-nitro-L-arginine results in a similar enhancement. Angiotensin II concentration-dependently induces NO production in an endothelial cell line, which can be diminished by dimethylsphingosine. Using immunoblotting and intracellular calcium measurements, we demonstrate that this sphingosine kinase-dependent endothelial NO synthase activation is mediated via both phosphatidylinositol 3-kinase/Akt and calcium-dependent pathways.

CONCLUSIONS

Angiotensin II induces a sphingosine kinase-dependent activation of endothelial NO synthase, which partially counteracts the contractile responses in isolated artery preparations. This pathway may be of importance under pathological circumstances with reduced NO bioavailability. Moreover, a disturbed sphingolipid metabolism in the vascular wall may lead to reduced NO bioavailability and endothelial dysfunction.

Sphingosine kinase: biochemical and cellular regulation and role in disease

Sphingolipids have emerged as molecules whose metabolism is regulated leading to generation of bioactive products including ceramide, sphingosine, and sphingosine-1-phosphate. The balance between cellular levels of these bioactive products is increasingly recognized to be critical to cell regulation; whereby, ceramide and sphingosine cause apoptosis and growth arrest phenotypes, and sphingosine-1-phosphate mediates proliferative and angiogenic responses. Sphingosine kinase is a key enzyme in modulating the levels of these lipids and is emerging as an important and regulated enzyme. This review is geared at mechanisms of regulation of sphingosine kinase and the coming to light of its role in disease.

Sphingosine-1-phosphate is released by cerebellar astrocytes in response to bFGF and induces astrocyte proliferation through Gi-protein-coupled receptors

The mitogenic role of sphingosine-1-phosphate (S1P) and its involvement in basic fibroblast growth factor (bFGF)-induced proliferation were examined in primary cultures of cerebellar astrocytes. Exposure to bFGF resulted in a rapid increase of extracellular S1P formation, bFGF inducing astrocytes to release S1P, but not sphingosine kinase, in the extracellular milieu. The SK inhibitor N,N-dimethylsphingosine inhibited S1P release as well as bFGF-induced growth stimulation. S1P application in quiescent astrocytes caused a dose-dependent increase in DNA synthesis. This gliotrophic effect was induced by a brief exposure to low nanomolar S1P, mimicked by the S1P receptor agonist dihydro-S1P, and inhibited by pertussis toxin (PTX), an inactivator of G(i)/G(o)-proteins. S1P also induced activation of extracellular signal-regulated kinase that was inhibited again by PTX. Moreover, the S1P lyase inhibitor 4-deoxypyridoxine induced the cellular accumulation of S1P but did not affect DNA synthesis. These results support the view that S1P exerted a mitogenic effect on cerebellar astrocytes extracellularly, most likely through cell surface S1P receptors. In agreement, mRNAs for S1P1, S1P2, and S1P3 receptors are expressed in cerebellar astrocytes (Anelli et al., 2005. J Neurochem 92:1204-1215). Ceramide, a negative regulator of astrocyte proliferation and down-regulated by bFGF (Riboni et al., 2002. Cerebellum 1:129-135), efficiently inhibited S1P-induced proliferation. The S1P action appears to be part of an autocrine/paracrine cascade stimulated by bFGF and, together with ceramide down-regulation, essential for astrocytes to respond to bFGF. The results suggest that S1P and bFGF/S1P may play an important role in physiopathological glial proliferation, such as brain development, reactive gliosis and brain tumor formation.

Inflammation and Atherosclerosis

PURPOSE

The aim of this article is to discuss the role of inflammation in atherosclerosis.

SUMMARY

An initial chemical, mechanical or immunological insult induces endothelial dysfunction. This triggers a cascade of inflammatory reactions, in which monocytes, macrophages, T lymphocytes and vascular smooth muscle cells participate. Leukocyte adhesion molecules, cytokines, growth factors and metalloproteinases participate in all stages of atherogenesis. Almost all of the traditional risk factors for atherosclerosis are associated with and participate in the inflammatory process. Many infectious agents, mainly Chlamydia pneumoniae, have been proposed as potential triggers of the cascade. The immune system has been implicated in plaque formation, through the activation of cellular and humoral immunity against innate or microbial heat shock protein 60. Methods of detection of systemic or local plaque inflammation have been developed and research is being conducted on the potential use of anti-inflammatory and antibiotic drugs in atherosclerosis.

Sphingosine-1-phosphate signaling promotes critical migratory events in vasculogenesis

Here we have investigated the role of sphingosine-1-phosphate (S1P) signaling in the process of vasculogenesis in the mouse embryo. At stages preceding the formation of blood vessels (7.5-8 dpc) in the embryo proper, yolk sac, and allantois, the S1P receptor S1P(2) is expressed in conjunction with S1P(1) and/or S1P(3). Additionally, sphingosine kinase-2 (SK2), an enzyme that catalyzes the formation of S1P, is expressed in these tissues throughout periods of vasculogenesis. Using the cultured mouse allantois explant model of blood vessel formation, we found that vasculogenesis was dependent on S1P signaling. We showed that S1P could replace the ability of serum to promote vasculogenesis in cultured allantois explants. Instead of small poorly reticulated clusters of rounded endothelial cells that formed under serum-free conditions, S1P promoted the formation of elongated endothelial cells that arranged into expansive branched networks of capillary-like vessels. These effects could not be reproduced by vascular endothelial growth factor or basic fibroblast growth factor administration. The ability of S1P to promote blood vessel formation was not due to effects on cell survival or on changes in numbers of endothelial cells (Flk1(+)/PECAM(+)), angioblasts (Flk1(+)/PECAM(-)), or undifferentiated mesodermal cells (Flk1(-)/PECAM(-)). The S1P effect on blood vessel formation was attributed to it promoting migratory activities of angioblasts and early endothelial cells required for the expansion of vascular networks. Together, our findings suggest that migratory events critical to the de novo formation of blood vessels are under the influence of S1P, possibly synthesized via the action of SK2, with signaling mediated by S1P receptors that include S1P(1), S1P(2), and S1P(3).

The role of G-protein coupled receptors and associated proteins in receptor tyrosine kinase signal transduction

It is well established that stimulation of G-protein coupled receptors (GPCRs) can activate signalling from receptor tyrosine kinases by a process termed transactivation. Indeed, in recent years, it has become apparent that transactivation is a general phenomenon that has been demonstrated for many unrelated GPCRs and receptor tyrosine kinases. In this case the GPCR/G-protein participation is up-stream of the receptor tyrosine kinase. Substantial research has addressed these findings but meanwhile another mechanism of cross talk has been slowly emerging. For over a decade, a growing body of evidence has demonstrated that numerous growth factors use G-proteins and attendant signalling molecules such as beta-arrestins that participate down-stream of the receptor tyrosine kinase to signal to effectors, such as p42/p44 MAPK. This review highlights this novel mechanism of cross talk between receptor tyrosine kinases and GPCRs, which is distinct from growth factor receptor transactivation by GPCRs.