Intracellular S1P generation is essential for S1P-induced motility of human lung endothelial cells: role of sphingosine kinase 1 and S1P lyase. PLoS One. 2011;6:e16571. [PMID: 21304987 PMCID: PMC3031585 DOI: 10.1371/journal.pone.0016571]

Nucleic acid-based nanotherapeutics for treating sepsis and associated organ injuries

In recent years, gene therapy has been made possible with the success of nucleic acid drugs against sepsis and its related organ dysfunction. Therapeutics based on nucleic acids such as small interfering RNAs (siRNAs), microRNAs (miRNAs), messenger RNAs (mRNAs), and plasmid DNAs (pDNAs) guarantee to treat previously undruggable diseases. The advantage of nucleic acid-based therapy against sepsis lies in the development of nanocarriers, achieving targeted and controlled gene delivery for improved efficacy with minimal adverse effects. Entrapment into nanocarriers also ameliorates the poor cellular uptake of naked nucleic acids. In this study, we discuss the current state of the art in nanoparticles for nucleic acid delivery to treat hyperinflammation and apoptosis associated with sepsis. The optimized design of the nanoparticles through physicochemical property modification and ligand conjugation can target specific organs-such as lung, heart, kidney, and liver-to mitigate multiple sepsis-associated organ injuries. This review highlights the nanomaterials designed for fabricating the anti-sepsis nanosystems, their physicochemical characterization, the mechanisms of nucleic acid-based therapy in working against sepsis, and the potential for promoting the therapeutic efficiency of the nucleic acids. The current investigations associated with nanoparticulate nucleic acid application in sepsis management are summarized in this paper. Noteworthily, the potential application of nanotherapeutic nucleic acids allows for a novel strategy to treat sepsis. Further clinical studies are required to confirm the findings in cell- and animal-based experiments. The capability of large-scale production and reproducibility of nanoparticle products are also critical for commercialization. It is expected that numerous anti-sepsis possibilities will be investigated for nucleic acid-based nanotherapeutics in the future.

Sphingosine 1-phosphate signaling during infection and immunity

Sphingolipids are essential components of all eukaryotic membranes. The bioactive sphingolipid molecule, Sphingosine 1-Phosphate (S1P), regulates various important biological functions. This review aims to provide a comprehensive overview of the role of S1P signaling pathway in various immune cell functions under different pathophysiological conditions including bacterial and viral infections, autoimmune disorders, inflammation, and cancer. We covered the aspects of S1P pathways in NOD/TLR pathways, bacterial and viral infections, autoimmune disorders, and tumor immunology. This implies that targeting S1P signaling can be used as a strategy to block these pathologies. Our current understanding of targeting various components of S1P signaling for therapeutic purposes and the present status of S1P pathway inhibitors or modulators in disease conditions where the host immune system plays a pivotal role is the primary focus of this review.

The Role of Sphingolipids in Regulating Vascular Permeability in Idiopathic Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a disease that causes scarring and fibrotic transformation of the lung parenchyma, resulting in the progressive loss of respiratory function and, often, death. Current treatments that target profibrotic factors can slow the rate of progression but are unable to ultimately stop it. In the past decade, many studies have shown that increased vascular permeability may be both a predictive and perpetuating factor in fibrogenesis. Consequently, there is a search for therapeutic targets to try and modulate vascular permeability in fibrotic lungs. One such class of targets that show great promise is sphingolipids. Sphingolipids are common in cell membranes and are increasingly recognized as critical to many cell signaling pathways, including those that affect the integrity of the vascular endothelial barrier. In this focused review we look at sphingolipids, particularly the sphingosine-1-phosphate (S1P) axis and its effects on vascular permeability, and how those effects may affect the pathogenesis of IPF. We further examine existing S1P modulators and their potential efficacy as therapeutics for IPF.

Silencing IQGAP1 alleviates hepatic fibrogenesis via blocking bone marrow mesenchymal stromal cell recruitment to fibrotic liver

IQ motif-containing guanosine triphosphatase (GTPase)-activating protein 1 (IQGAP1) is a cytosolic scaffolding protein involved in cell migration. Our previous studies suggest sphingosine 1-phosphate (S1P) triggers bone marrow (BM) mesenchymal stromal cells (BMSCs) to damaged liver, thereby promoting liver fibrosis. However, the role of IQGAP1 in S1P-induced BMSC migration and liver fibrogenesis remains unclear. Chimeric mice of BM cell labeled by EGFP were used to build methionine-choline-deficient and high-fat (MCDHF)-diet-induced mouse liver fibrosis. IQGAP1 small interfering RNA (siRNA) was utilized to silence IQGAP1 in vivo. IQGAP1 expression is significantly elevated in MCDHF-diet-induced mouse fibrotic livers. Positive correlations are presented between IQGAP1 and fibrosis hallmarks expressions in human and mouse fibrotic livers. In vitro, depressing IQGAP1 expression blocks S1P-induced motility and cytoskeleton remodeling of BMSCs. S1P facilitates IQGAP1 aggregating to plasma membrane via S1P receptor 3 (S1PR₃) and Cdc42/Rac1. In addition, IQGAP1 binds to Cdc42/Rac1, regulating S1P-induced activation of Cdc42/Rac1 and mediating BMSC migration in concert. In vivo, silencing IQGAP1 reduces the recruitment of BMSCs to impaired liver and effectively alleviates liver fibrosis induced by MCDHF diet. Together, silencing IQGAP1 relieves liver fibrosis by blocking BMSC migration, providing an effective therapeutic strategy for liver fibrosis.

Therapeutic CFTR Correction Normalizes Systemic and Lung-Specific S1P Level Alterations Associated with Heart Failure

Heart failure (HF) is among the main causes of death worldwide. Alterations of sphingosine-1-phosphate (S1P) signaling have been linked to HF as well as to target organ damage that is often associated with HF. S1P's availability is controlled by the cystic fibrosis transmembrane regulator (CFTR), which acts as a critical bottleneck for intracellular S1P degradation. HF induces CFTR downregulation in cells, tissues and organs, including the lung. Whether CFTR alterations during HF also affect systemic and tissue-specific S1P concentrations has not been investigated. Here, we set out to study the relationship between S1P and CFTR expression in the HF lung. Mice with HF, induced by myocardial infarction, were treated with the CFTR corrector compound C18 starting ten weeks post-myocardial infarction for two consecutive weeks. CFTR expression, S1P concentrations, and immune cell frequencies were determined in vehicle- and C18-treated HF mice and sham controls using Western blotting, flow cytometry, mass spectrometry, and qPCR. HF led to decreased pulmonary CFTR expression, which was accompanied by elevated S1P concentrations and a pro-inflammatory state in the lungs. Systemically, HF associated with higher S1P plasma levels compared to sham-operated controls and presented with higher S1P receptor 1-positive immune cells in the spleen. CFTR correction with C18 attenuated the HF-associated alterations in pulmonary CFTR expression and, hence, led to lower pulmonary S1P levels, which was accompanied by reduced lung inflammation. Collectively, these data suggest an important role for the CFTR-S1P axis in HF-mediated systemic and pulmonary inflammation.

Regulation of Rac1 Activation in Choroidal Endothelial Cells: Insights into Mechanisms in Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is one of the leading causes of blindness worldwide. Vision loss from the neovascular form is associated with the invasion of choroidal endothelial cells into the neural retina to form vision-threatening macular neovascularization (MNV). Anti-angiogenic agents are the current standard of care but are effective in only ~50% of AMD cases. The molecular mechanisms involved in invasive MNV point to the importance of regulating signaling pathways that lead to pathologic biologic outcomes. In studies testing the effects of AMD-related stresses, activation of the Rho GTPase, Rac1, was found to be important for the choroidal endothelial cell invasion into the neural retina. However, current approaches to prevent Rac1 activation are inefficient and less effective. We summarize active Rac1-mediated mechanisms that regulate choroidal endothelial cell migration. Specifically, we discuss our work regarding the role of a multidomain protein, IQ motif containing GTPase activating protein 1 (IQGAP1), in sustaining pathologic Rac1 activation and a mechanism by which active Rap1, a Ras-like GTPase, may prevent active Rac1-mediated choroidal endothelial cell migration.

Melo E, B. C. T. Coimbra L, Lapa MS, Acencio MMP, Freitas-Filho O, Capelozzi VL, Teixeira LR, Fernandes CJCS, Jatene FB, Laurindo FRM, Terra-Filho M. Shear stress-exposed pulmonary artery endothelial cells fail to upregulate HSP70 in chronic thromboembolic pulmonary hypertension

The pathophysiological mechanisms underlying chronic thromboembolic pulmonary hypertension (CTEPH) are still unclear. Endothelial cell (EC) remodeling is believed to contribute to this pulmonary disease triggered by thrombus and hemodynamic forces disbalance. Recently, we showed that HSP70 levels decrease by proatherogenic shear stress. Molecular chaperones play a major role in proteostasis in neurological, cancer and inflammatory/ infectious diseases. To shed light on microvascular responses in CTEPH, we characterized the expression of molecular chaperones and annexin A2, a component of the fibrinolytic system. There is no animal model that reproduces microvascular changes in CTEPH, and this fact led us to isolated endothelial cells from patients with CTEPH undergoing pulmonary endarterectomy (PEA). We exposed CTEPH-EC and control human pulmonary endothelial cells (HPAEC) to high- (15 dynes/cm2) or low- (5 dynes/cm2) shear stress. After high-magnitude shear stress HPAEC upregulated heat shock protein 70kDa (HSP70) and the HSP ER paralogs 78 and 94kDa glucose-regulated protein (GRP78 and 94), whereas CTEPH-ECs failed to exhibit this response. At static conditions, both HSP70 and HSP90 families in CTEPH-EC are decreased. Importantly, immunohistochemistry analysis showed that HSP70 expression was downregulated in vivo, and annexin A2 was upregulated. Interestingly, wound healing and angiogenesis assays revealed that HSP70 inhibition with VER-155008 further impaired CTEPH-EC migratory responses. These results implicate HSP70 as a novel master regulator of endothelial dysfunction in type 4 PH. Overall, we first show that global failure of HSP upregulation is a hallmark of CTEPH pathogenesis and propose HSP70 as a potential biomarker of this condition.

Unbalanced Sphingolipid Metabolism and Its Implications for the Pathogenesis of Psoriasis

Sphingolipids (SLs), which have structural and biological responsibilities in the human epidermis, are importantly involved in the maintenance of the skin barrier and regulate cellular processes, such as the proliferation, differentiation and apoptosis of keratinocytes (KCs). As many dermatologic diseases, including psoriasis (PsO), intricately characterized by perturbations in these cellular processes, are associated with altered composition and unbalanced metabolism of epidermal SLs, more education to precisely determine the role of SLs, especially in the pathogenesis of skin disorders, is needed. PsO is caused by a complex interplay between skin barrier disruption, immune dysregulation, host genetics and environmental triggers. The contribution of particular cellular compartments and organelles in SL metabolism, a process related to dysfunction of lysosomes in PsO, seems to have a significant impact on lysosomal signalling linked to a modulation of the immune-mediated inflammation accompanying this dermatosis and is not fully understood. It is also worth noting that a prominent skin disorder, such as PsO, has diminished levels of the main epidermal SL ceramide (Cer), reflecting altered SL metabolism, that may contribute not only to pathogenesis but also to disease severity and/or progression. This review provides a brief synopsis of the implications of SLs in PsO, aims to elucidate the roles of these molecules in complex cellular processes deregulated in diseased skin tissue and highlights the need for increased research in the field. The significance of SLs as structural and signalling molecules and their actions in inflammation, in which these components are factors responsible for vascular endothelium abnormalities in the development of PsO, are discussed.

Sphingosine kinase 1-associated autophagy differs between neurons and astrocytes

Autophagy is a degradative pathway for removing aggregated proteins, damaged organelles, and parasites. Evidence indicates that autophagic pathways differ between cell types. In neurons, autophagy plays a homeostatic role, compared to a survival mechanism employed by starving non-neuronal cells. We investigated if sphingosine kinase 1 (SK1)-associated autophagy differs between two symbiotic brain cell types—neurons and astrocytes. SK1 synthesizes sphingosine-1-phosphate, which regulates autophagy in non-neuronal cells and in neurons. We found that benzoxazine autophagy inducers upregulate SK1 and neuroprotective autophagy in neurons, but not in astrocytes. Starvation enhances SK1-associated autophagy in astrocytes, but not in neurons. In astrocytes, SK1 is cytoprotective and promotes the degradation of an autophagy substrate, mutant huntingtin, the protein that causes Huntington’s disease. Overexpressed SK1 is unexpectedly toxic to neurons, and its toxicity localizes to the neuronal soma, demonstrating an intricate relationship between the localization of SK1’s activity and neurotoxicity. Our results underscore the importance of cell type-specific autophagic differences in any efforts to target autophagy therapeutically.

Sphingolipids in Ventilator Induced Lung Injury: Role of Sphingosine-1-Phosphate Lyase

Mechanical ventilation (MV) performed in respiratory failure patients to maintain lung function leads to ventilator-induced lung injury (VILI). This study investigates the role of sphingolipids and sphingolipid metabolizing enzymes in VILI using a rodent model of VILI and alveolar epithelial cells subjected to cyclic stretch (CS). MV (0 PEEP (Positive End Expiratory Pressure), 30 mL/kg, 4 h) in mice enhanced sphingosine-1-phosphate lyase (S1PL) expression, and ceramide levels, and decreased S1P levels in lung tissue, thereby leading to lung inflammation, injury and apoptosis. Accumulation of S1P in cells is a balance between its synthesis catalyzed by sphingosine kinase (SphK) 1 and 2 and catabolism mediated by S1P phosphatases and S1PL. Thus, the role of S1PL and SphK1 in VILI was investigated using Sgpl1+/- and Sphk1-/- mice. Partial genetic deletion of Sgpl1 protected mice against VILI, whereas deletion of SphK1 accentuated VILI in mice. Alveolar epithelial MLE-12 cells subjected to pathophysiological 18% cyclic stretch (CS) exhibited increased S1PL protein expression and dysregulation of sphingoid bases levels as compared to physiological 5% CS. Pre-treatment of MLE-12 cells with S1PL inhibitor, 4-deoxypyridoxine, attenuated 18% CS-induced barrier dysfunction, minimized cell apoptosis and cytokine secretion. These results suggest that inhibition of S1PL that increases S1P levels may offer protection against VILI.

Defective Sphingosine-1-phosphate metabolism is a druggable target in Huntington's disease

Huntington’s disease is characterized by a complex and heterogeneous pathogenic profile. Studies have shown that disturbance in lipid homeostasis may represent a critical determinant in the progression of several neurodegenerative disorders. The recognition of perturbed lipid metabolism is only recently becoming evident in HD. In order to provide more insight into the nature of such a perturbation and into the effect its modulation may have in HD pathology, we investigated the metabolism of Sphingosine-1-phosphate (S1P), one of the most important bioactive lipids, in both animal models and patient samples. Here, we demonstrated that S1P metabolism is significantly disrupted in HD even at early stage of the disease and importantly, we revealed that such a dysfunction represents a common denominator among multiple disease models ranging from cells to humans through mouse models. Interestingly, the in vitro anti-apoptotic and the pro-survival actions seen after modulation of S1P-metabolizing enzymes allows this axis to emerge as a new druggable target and unfolds its promising therapeutic potential for the development of more effective and targeted interventions against this incurable condition.

Targeting sphingosine-1-phosphate signaling in lung diseases

Sphingosine-1-phosphate (S1P), a simple, bioactive sphingolipid metabolite, plays a key role, both intracellularly and extracellularly, in various cellular processes such as proliferation, survival, migration, inflammation, angiogenesis, and endothelial barrier integrity. The cellular S1P level is low and is tightly regulated by its synthesis and degradation. Sphingosine Kinases (SphKs) 1 and 2, catalyze the ATP-dependent phosphorylation of sphingosine to S1P, while the degradation is mediated by the reversible dephosphorylation catalyzed by the S1P phosphatases and lipid phosphate phosphatases and the irreversible degradation to hexadecenal and ethanolamine phosphate by sphingosine-1-phosphate lyase (S1PL). As a ligand for specific G-protein-coupled receptors, S1P1-5, which are differentially expressed in different cell types, S1P generates downstream signals that play crucial role in developmental and disease related pathologies. In addition to acting extracellularly on receptors located on the plasma membrane, S1P can also act intracellularly, independently of S1P1-5, affecting calcium homeostasis and cell proliferation. The SphKs /S1P /S1PL metabolic pathway is implicated in numerous human pathologies including respiratory disorders, thereby raising the possibility that manipulating intracellular S1P levels could offer therapeutic potential in ameliorating lung diseases. This review focuses on the prospects of targeting S1P signaling and S1P metabolizing enzymes using small molecule inhibitors, receptor agonists, and antagonists in the treatment of lung diseases.

Role of Sphingosine Kinase 1 and S1P Transporter Spns2 in HGF-mediated Lamellipodia Formation in Lung Endothelium

Hepatocyte growth factor (HGF) signaling via c-Met is known to promote endothelial cell motility and angiogenesis. We have previously reported that HGF stimulates lamellipodia formation and motility of human lung microvascular endothelial cells (HLMVECs) via PI3K/Akt signal transduction and reactive oxygen species generation. Here, we report a role for HGF-induced intracellular sphingosine-1-phosphate (S1P) generation catalyzed by sphingosine kinase 1 (SphK1), S1P transporter, spinster homolog 2 (Spns2), and S1P receptor, S1P₁, in lamellipodia formation and perhaps motility of HLMVECs. HGF stimulated SphK1 phosphorylation and enhanced intracellular S1P levels in HLMVECs, which was blocked by inhibition of SphK1. HGF enhanced co-localization of SphK1/p-SphK1 with actin/cortactin in lamellipodia and down-regulation or inhibition of SphK1 attenuated HGF-induced lamellipodia formation in HLMVECs. In addition, down-regulation of Spns2 also suppressed HGF-induced lamellipodia formation, suggesting a key role for inside-out S1P signaling. The HGF-mediated phosphorylation of SphK1 and its localization in lamellipodia was dependent on c-Met and ERK1/2 signaling, but not the PI3K/Akt pathway; however, blocking PI3K/Akt signaling attenuated HGF-mediated phosphorylation of Spns2. Down-regulation of S1P₁, but not S1P₂ or S1P₃, with specific siRNA attenuated HGF-induced lamellipodia formation. Further, HGF enhanced association of Spns2 with S1P₁ that was blocked by inhibiting SphK1 activity with PF-543. Moreover, HGF-induced migration of HLMVECs was attenuated by down-regulation of Spns2_. Taken together, these results suggest that HGF/c-Met-mediated lamellipodia formation, and perhaps motility is dependent on intracellular generation of S1P via activation and localization of SphK1 to cell periphery and Spns2-mediated extracellular transportation of S1P and its inside-out signaling via S1P₁.

Hyperoxia-induced p47phox activation and ROS generation is mediated through S1P transporter Spns2, and S1P/S1P1&2 signaling axis in lung endothelium

Hyperoxia-induced lung injury adversely affects ICU patients and neonates on ventilator assisted breathing. The underlying culprit appears to be reactive oxygen species (ROS)-induced lung damage. The major contributor of hyperoxia-induced ROS is activation of the multiprotein enzyme complex NADPH oxidase. Sphingosine-1-phosphate (S1P) signaling is known to be involved in hyperoxia-mediated ROS generation; however, the mechanism(s) of S1P-induced NADPH oxidase activation is unclear. Here, we investigated various steps in the S1P signaling pathway mediating ROS production in response to hyperoxia in lung endothelium. Of the two closely related sphingosine kinases (SphKs)1 and 2, which synthesize S1P from sphingosine, only Sphk1(-/-) mice conferred protection against hyperoxia-induced lung injury. S1P is metabolized predominantly by S1P lyase and partial deletion of Sgpl1 (Sgpl1(+/-)) in mice accentuated lung injury. Hyperoxia stimulated S1P accumulation in human lung microvascular endothelial cells (HLMVECs), and downregulation of S1P transporter spinster homolog 2 (Spns2) or S1P receptors S1P1&2, but not S1P3, using specific siRNA attenuated hyperoxia-induced p47(phox) translocation to cell periphery and ROS generation in HLMVECs. These results suggest a role for Spns2 and S1P1&2 in hyperoxia-mediated ROS generation. In addition, p47(phox) (phox:phagocyte oxidase) activation and ROS generation was also reduced by PF543, a specific SphK1 inhibitor in HLMVECs. Our data indicate a novel role for Spns2 and S1P1&2 in the activation of p47(phox) and production of ROS involved in hyperoxia-mediated lung injury in neonatal and adult mice.

S1P prophylaxis mitigates acute hypobaric hypoxia-induced molecular, biochemical, and metabolic disturbances: A preclinical report

Sphingosine-1-phosphate (S1P) is emerging to have hypoxic preconditioning potential in various preclinical studies. The study aims to evaluate the preclinical preconditioning efficacy of exogenously administered S1P against acute hypobaric hypoxia (HH)-induced pathological disturbances. Male Sprague Dawley rats (200 ± 20 g) were preconditioned with 1, 10, and 100 μg/kg body weight (b.w.) S1P (i.v.) for three consecutive days. On the third day, S1P preconditioned animals, along with hypoxia control animals, were exposed to HH equivalent to 7,620 m (280 mm Hg) for 6 h. Postexposure status of cardiac energy production, circulatory vasoactive mediators, pulmonary and cerebral oxidative damage, and inflammation were assessed. HH exposure led to cardiac energy deficit indicated by low ATP levels and pronounced AMPK activation levels, raised circulatory levels of brain natriuretic peptide and endothelin-1 with respect to total nitrate (NOx), redox imbalance, inflammation, and alterations in NOx levels in the pulmonary and cerebral tissues. These pathological precursors have been routinely reported to be coincident with high-altitude diseases. Preconditioning with S1P, especially 1 µg/kg b.w. dose, was seen to reverse the manifestation of these pathological disturbances. The protective efficacy could be attributed, at least in part, to enhanced activity of cardioprotective protein kinase C and activation of small GTPase Rac1, which led to further induction of hypoxia-adaptive molecular mediators: hypoxia-inducible factor (HIF)-1α and Hsp70. This is a first such report, to the best of our knowledge, elucidating the mechanism of exogenous S1P-mediated HIF-1α/Hsp70 induction. Conclusively, systemic preconditioning with 1 μg/kg b.w. S1P in rats protects against acute HH-induced pathological disturbances. © 2016 IUBMB Life 68(5):365-375, 2016.

Molecular mechanisms in lipopolysaccharide-induced pulmonary endothelial barrier dysfunction

The confluent pulmonary endothelium plays an important role as a semi-permeable barrier between the vascular space of blood vessels and the underlying tissues, and it contributes to the maintenance of circulatory fluid homeostasis. Pulmonary endothelial barrier dysfunction is a pivotal early step in the development of a variety of high mortality diseases, such as acute lung injury (ALI). Endothelium barrier dysfunction in response to inflammatory or infectious mediators, including lipopolysaccharide (LPS), is accompanied by invertible cell deformation and interendothelial gap formation. However, specific pharmacological therapies aiming at ameliorating pulmonary endothelial barrier function in patients are still lacking. A full understanding of the fundamental mechanisms that are involved in the regulation of pulmonary endothelial permeability is essential for the development of barrier protective therapeutic strategies. Therefore, this review summarizes several important molecular mechanisms involved in LPS-induced changes in pulmonary endothelial barrier function. As for barrier-disruption, the activation of myosin light chain kinase (MLCK), RhoA and tyrosine kinases; increase of calcium influx; and apoptosis of the endothelium lead to an elevation of lung endothelial permeability. Additionally, the activation of Rac1, Cdc42, protease activated receptor 1 (PAR1) and adenosine receptors (ARs), as well as the increase of cyclic AMP and sphingosine-1-phosphate (S1P) content, protect against LPS-induced lung endothelial barrier dysfunction. Furthermore, current regulatory factors and strategies against the development of LPS-induced lung endothelial hyper-permeability are discussed.

Human T-Cell Development and Thymic Egress: An Infectious Disease Perspective

Emigration of mature naïve CD4 SP T cells from the human thymus to the periphery is not fully understood, although elucidation of the mechanisms that govern egress of T cells is crucial to understanding both basic immunology and the immune response in diseases such as HIV infection. Recent work has brought to light the requirement for sphingosine-1-phosphate (S1P) and its receptors in a variety of fields including mature naïve T-cell egress from the thymus of mice. We are examining the expression and function of this novel requisite T-cell egress receptor within the human thymus, characterizing changes observed in the expression and function of this receptor in infectious diseases. To perform this work, we use a variety of humanized murine models reviewed in this article. Future work in the field of T-cell egress, especially as it pertains to S1P receptors, should advance the fields of basic T-cell immunology and immunopathology and open new avenues for exploration into novel therapeutics.

The sphingosine kinase 1/sphingosine-1-phosphate pathway in pulmonary arterial hypertension

RATIONALE

Sphingosine kinases (SphKs) 1 and 2 regulate the synthesis of the bioactive sphingolipid sphingosine-1-phosphate (S1P), an important lipid mediator that promotes cell proliferation, migration, and angiogenesis.

OBJECTIVES

We aimed to examine whether SphKs and their product, S1P, play a role in the development of pulmonary arterial hypertension (PAH).

METHODS

SphK1(-/-), SphK2(-/-), and S1P lyase heterozygous (Sgpl1(+/-)) mice, a pharmacologic SphK inhibitor (SKI2), and a S1P receptor 2 (S1PR2) antagonist (JTE013) were used in rodent models of hypoxia-mediated pulmonary hypertension (HPH). S1P levels in lung tissues from patients with PAH and pulmonary arteries (PAs) from rodent models of HPH were measured.

MEASUREMENTS AND MAIN RESULTS

mRNA and protein levels of SphK1, but not SphK2, were significantly increased in the lungs and isolated PA smooth muscle cells (PASMCs) from patients with PAH, and in lungs of experimental rodent models of HPH. S1P levels were increased in lungs of patients with PAH and PAs from rodent models of HPH. Unlike SphK2(-/-) mice, SphK1(-/-) mice were protected against HPH, whereas Sgpl1(+/-) mice were more susceptible to HPH. Pharmacologic SphK1 and S1PR2 inhibition prevented the development of HPH in rodent models of HPH. Overexpression of SphK1 and stimulation with S1P potentially via ligation of S1PR2 promoted PASMC proliferation in vitro, whereas SphK1 deficiency inhibited PASMC proliferation.

CONCLUSIONS

The SphK1/S1P axis is a novel pathway in PAH that promotes PASMC proliferation, a major contributor to pulmonary vascular remodeling. Our results suggest that this pathway is a potential therapeutic target in PAH.

Sphingosine 1-phosphate lyase inhibition by 2-acetyl-4-(tetrahydroxybutyl)imidazole (THI) under conditions of vitamin B6 deficiency

Caramel food colorant 2-acetyl-4-(tetrahydroxybutyl)imidazole (THI) causes lymphopenia in animals through sphingosine 1-phosphate lyase (SPL) inhibition. However, this mechanism of action is partly still controversial because THI did not inhibit SPL in vitro either in cell-free or in cell-based systems. It is thought that the in vitro experimental conditions which have been used so far were not suitable for the evaluation of SPL inhibition, especially in case of cell-based experiments. We speculated that the key factor might be the coenzyme pyridoxal 5'-phosphate (PLP), an active form of vitamin B6 (VB6), because media used in cell-based assays usually contain an excess amount of VB6 which leads to the activation of SPL. By the use of VB6-deficient culture medium, we could regulate apo- (without PLP) and holo- (with PLP) SPL enzyme in cultured cells, resulting in the successful detection of SPL inhibition by THI. Although the observed inhibitory effect was not as strong as that of 4-deoxypyridoxine (a VB6 analog SPL inhibitor), these findings may be useful for further understanding the mechanism of action of THI.

Critical role of Spns2, a sphingosine-1-phosphate transporter, in lung cancer cell survival and migration

The sphingosine-1-phosphate (S1P) transporter Spns2 regulates myocardial precursor migration in zebrafish and lymphocyte trafficking in mice. However, its function in cancer has not been investigated. We show here that ectopic Spns2 expression induced apoptosis and its knockdown enhanced cell migration in non-small cell lung cancer (NSCLC) cells. Metabolically, Spns2 expression increased the extracellular S1P level while its knockdown the intracellular. Pharmacological inhibition of S1P synthesis abolished the augmented cell migration mediated by Spns2 knockdown, indicating that intracellular S1P plays a key role in this process. Cell signaling studies indicated that Spns2 expression impaired GSK-3β and Stat3 mediated pro-survival pathways. Conversely, these pathways were activated by Spns2 knockdown, which explains the increased cell migration since they are also crucial for migration. Alterations of Spns2 were found to affect several enzymes involved in S1P metabolism, including sphingosine kinases, S1P phosphatases, and S1P lyase 1. Genetically, Spns2 mRNA level was found to be reduced in advanced lung cancer (LC) patients as quantified by using a small scale qPCR array. These data show for the first time that Spns2 plays key roles in regulating the cellular functions in NSCLC cells, and that its down-regulation is a potential risk factor for LC.

Cardiovascular effects of fingolimod: A review article

Multiple sclerosis (MS) is a chronic auto-immune disease. Most therapeutic strategies for treatment of this disease direct immune modulation and control of inflammatory processes. First-line therapeutic agents showed moderate efficacy and frequent side-effects with moderate efficacy in trials. Their parental administration and limited long-term adherence restrict their efficacy compared with second-line therapies. Fingolimod as a second-line therapeutic agent has been shown to reduce annualized relapse rate, risk of disability progression and inflammatory activity of relapsing MS. Safety and efficacy FTY720: Safety and efficacy issues are the main metrics for judgment of drug efficacy. In this article, we focus on cardiovascular effects of FTY720 treatment. Effect of FTY720 on rate and rhythm, impact of FTY720 on endothelial cells, its atheroprotective effects, its effects on cardiac transplantation outcomes, vascular complications of FTY720, effects of FTY720 on endocrine functions and interaction of FTY720 with cardioactive agents are explained in this review article.

Sphingolipids and antimicrobial peptides: function and roles in atopic dermatitis

Inflammatory skin diseases such as atopic dermatitis (AD) and rosacea were complicated by barrier abrogation and deficiency in innate immunity. The first defender of epidermal innate immune response is the antimicrobial peptides (AMPs) that exhibit a broad-spectrum antimicrobial activity against multiple pathogens, including Gram-positive and Gram-negative bacteria, viruses, and fungi. The deficiency of these AMPs in the skin of AD fails to protect our body against virulent pathogen infections. In contrast to AD where there is a suppression of AMPs, rosacea is characterized by overexpression of cathelicidin antimicrobial peptide (CAMP), the products of which result in chronic epidermal inflammation. In this regard, AMP generation that is controlled by a key ceramide metabolite S1P-dependent mechanism could be considered as alternate therapeutic approaches to treat these skin disorders, i.e., Increased S1P levels strongly stimulated the CAMP expression which elevated the antimicrobial activity against multiple pathogens resulting the improved AD patient skin.

Ginsenoside compound K inhibits angiogenesis via regulation of sphingosine kinase-1 in human umbilical vein endothelial cells

Ginsenoside compound K (CK) is a metabolite of the protopanaxadiol-type saponins of Panax ginseng C.A. Meyer (Araliaceae), has long been used to treat against the development of cancer, inflammation, allergies, and diabetes. This study examined the anti-angiogenic properties of CK against sphingosine 1-phosphate (S1P)-induced cell migration via regulation of sphingosine kinase 1 (SPHK1) in human umbilical vein endothelial cells (HUVEC). Studies on S1P-induced cell migration, expression of SPHK1 and MMPs and analysis of sphingolipid metabolites by LC-MS/MS were examined after the treatment of CK (2.5, 5, 10 μg/mL) in HUVEC. S1P produced by SPHK1 is also involved in cell growth, migration, and protection of apoptosis; therefore, we sought to investigate whether ginsenosides are able to regulate SPHK1. For this purpose, we developed an inhibitory assay of SPHK1 activity and an analytical method for detection of S1P and other sphingolipid metabolites in HUVEC. Ginsenoside CK inhibited 100 nM S1P-induced cell migrations in a dose-dependent manner. Among tested ginsenosides, CK exclusively inhibited S1P production, SPHK1 activity and SPHK1 expression in HUVEC, whereas expression of the pro-apoptotic sphingolipids, sphingosine and ceramide, was increased in response to CK. The major subspecies of the increased ceramide was C24:0-ceramide. CK also disrupted the sphingolipid rheostat, which ultimately influences cell fate, and dose-dependently inhibited HUVEC migration by reducing expression of metalloproteinases (MMPs). Ginsenoside CK acts as a unique HUVEC migration inhibitor by regulating MMP expression, as well as the activity of SPHK1 and its related sphingolipid metabolites.

Autotaxin and lysophosphatidic acid signalling in lung pathophysiology

Autotaxin (ATX or ENPP2) is a secreted glycoprotein widely present in biological fluids. ATX primarily functions as a plasma lysophospholipase D and is largely responsible for the bulk of lysophosphatidic acid (LPA) production in the plasma and at inflamed and/or malignant sites. LPA is a phospholipid mediator produced in various conditions both in cells and in biological fluids, and it evokes growth-factor-like responses, including cell growth, survival, differentiation and motility, in almost all cell types. The large variety of LPA effector functions is attributed to at least six G-protein coupled LPA receptors (LPARs) with overlapping specificities and widespread distribution. Increased ATX/LPA/LPAR levels have been detected in a large variety of cancers and transformed cell lines, as well as in non-malignant inflamed tissues, suggesting a possible involvement of ATX in chronic inflammatory disorders and cancer. In this review, we focus exclusively on the role of the ATX/LPA axis in pulmonary pathophysiology, analysing the effects of ATX/LPA on pulmonary cells and leukocytes in vitro and in the context of pulmonary pathophysiological situations in vivo and in human diseases.

The activation of RhoC in vascular endothelial cells is required for the S1P receptor type 2-induced inhibition of angiogenesis

Sphingosine-1-phosphate (S1P) is a multifunctional phospholipid inducing a variety of cellular responses in endothelial cells (EC). S1P responses are mediated by five G protein coupled receptors of which three types (S1P1R-S1P3R) have been described to be of importance in vascular endothelial cells (EC). Whereas the S1P1R regulates endothelial barrier function by coupling to Gαi and the monomeric GTPase Rac1, the signaling pathways involved in the S1P-induced regulation of angiogenesis are ill defined. We therefore studied the sprouting of human umbilical vein EC (HUVEC) in vitro and analyzed the activation of the RhoGTPases RhoA and RhoC. Physiological relevant concentrations of S1P (100-300nM) induce a moderate activation of RhoA and RhoC. Inhibition or siRNA-mediated depletion of the S1P2R preferentially decreased the activation of RhoC. Both manipulations caused an increase of sprouting in a spheroid based in vitro sprouting assay. Interestingly, a similar increase in sprouting was detected after effective siRNA-mediated knockdown of RhoC. In contrast, the depletion of RhoA had no influence on sprouting. Furthermore, suppression of the activity of G proteins of the Gα12/13 subfamily by adenoviral overexpression of the regulator of G protein signaling domain of LSC as well as siRNA-mediated knockdown of the Rho specific guanine nucleotide exchange factor leukemia associated RhoGEF (LARG) inhibited the S1P-induced activation of RhoC and concomitantly increased sprouting of HUVEC with similar efficacy. We conclude that the angiogenic sprouting of EC is suppressed via the S1P2R subtype. Thus, the increase in basal sprouting can be attributed to blocking of the inhibitory action of autocrine S1P stimulating the S1P2R. This inhibitory pathway involves the activation of RhoC via Gα12/13 and LARG, while the simultaneously occurring activation of RhoA is apparently dispensable here.

Sphingosine-1-phosphate, FTY720, and sphingosine-1-phosphate receptors in the pathobiology of acute lung injury

Acute lung injury (ALI) attributable to sepsis or mechanical ventilation and subacute lung injury because of ionizing radiation (RILI) share profound increases in vascular permeability as a key element and a common pathway driving increased morbidity and mortality. Unfortunately, despite advances in the understanding of lung pathophysiology, specific therapies do not yet exist for the treatment of ALI or RILI, or for the alleviation of unremitting pulmonary leakage, which serves as a defining feature of the illness. A critical need exists for new mechanistic insights that can lead to novel strategies, biomarkers, and therapies to reduce lung injury. Sphingosine 1-phosphate (S1P) is a naturally occurring bioactive sphingolipid that acts extracellularly via its G protein-coupled S1P1-5 as well as intracellularly on various targets. S1P-mediated cellular responses are regulated by the synthesis of S1P, catalyzed by sphingosine kinases 1 and 2, and by the degradation of S1P mediated by lipid phosphate phosphatases, S1P phosphatases, and S1P lyase. We and others have demonstrated that S1P is a potent angiogenic factor that enhances lung endothelial cell integrity and an inhibitor of vascular permeability and alveolar flooding in preclinical animal models of ALI. In addition to S1P, S1P analogues such as 2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol (FTY720), FTY720 phosphate, and FTY720 phosphonates offer therapeutic potential in murine models of lung injury. This translational review summarizes the roles of S1P, S1P analogues, S1P-metabolizing enzymes, and S1P receptors in the pathophysiology of lung injury, with particular emphasis on the development of potential novel biomarkers and S1P-based therapies for ALI and RILI.

Therapeutic potential of targeting SK1 in human cancers

Sphingosine kinase 1 (SK1) is a lipid enzyme with oncogenic properties that converts the proapoptotic lipids ceramide and sphingosine into the antiapoptotic lipid sphingosine-1-phosphate and activates the signal transduction pathways that lead to cell proliferation, migration, the activation of the inflammatory response, and the impairment of apoptosis. There is compelling evidence that SK1 activation contributes to cancer progression leading to increased oncogenic transformation, tumor growth, resistance to therapies, tumor neovascularization, and metastatic spread. High levels of SK1 expression or activity have been associated with a poor prognosis in several human cancers. Recent studies using cancer cell and mouse models demonstrate a significant potential for SK1-targeting therapies to synergize with the effects of chemotherapy and radiotherapy; however, until recently the absence of clinically applicable SK1 inhibitors has limited the translation of these findings into patients. With the recent discovery of SK1 inhibiting properties of a clinically approved drug FTY720 (Fingolimod), SK1 has gained significant attention from both clinicians and the pharmaceutical industry and it is hoped that trials of newly developed SK1 inhibitors may follow soon. This review provides an overview of the SK1 signaling, its relevance to cancer progression, and the potential clinical significance of targeting SK1 for improved local or systemic control of human cancers.

Coronin 1B regulates S1P-induced human lung endothelial cell chemotaxis: role of PLD2, protein kinase C and Rac1 signal transduction

Coronins are a highly conserved family of actin binding proteins that regulate actin-dependent processes such as cell motility and endocytosis. We found that treatment of human pulmonary artery endothelial cells (HPAECs) with the bioactive lipid, sphingosine-1-phosphate (S1P) rapidly stimulates coronin 1B translocation to lamellipodia at the cell leading edge, which is required for S1P-induced chemotaxis. Further, S1P-induced chemotaxis of HPAECs was attenuated by pretreatment with small interfering RNA (siRNA) targeting coronin 1B (∼36%), PLD2 (∼45%) or Rac1 (∼50%) compared to scrambled siRNA controls. Down regulation PLD2 expression by siRNA also attenuated S1P-induced coronin 1B translocation to the leading edge of the cell periphery while PLD1 silencing had no effect. Also, S1P-induced coronin 1B redistribution to cell periphery and chemotaxis was attenuated by inhibition of Rac1 and over-expression of dominant negative PKC δ, ε and ζ isoforms in HPAECs. These results demonstrate that S1P activation of PLD2, PKC and Rac1 is part of the signaling cascade that regulates coronin 1B translocation to the cell periphery and the ensuing cell chemotaxis.

Targeting sphingosine kinase 1 attenuates bleomycin-induced pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic and progressive interstitial lung disease, wherein transforming growth factor β (TGF-β) and sphingosine-1-phosphate (S1P) contribute to the pathogenesis of fibrosis. However, the in vivo contribution of sphingosine kinase (SphK) in fibrotic processes has not been documented. Microarray analysis of blood mononuclear cells from patients with IPF and SphK1- or SphK2-knockdown mice and SphK inhibitor were used to assess the role of SphKs in fibrogenesis. The expression of SphK1/2 negatively correlated with lung function and survival in patients with IPF. Also, the expression of SphK1 was increased in lung tissues from patients with IPF and bleomycin-challenged mice. Knockdown of SphK1, but not SphK2, increased survival and resistance to pulmonary fibrosis in bleomycin-challenged mice. Administration of SphK inhibitor reduced bleomycin-induced mortality and pulmonary fibrosis in mice. Knockdown of SphK1 or treatment with SphK inhibitor attenuated S1P generation and TGF-β secretion in a bleomycin-induced lung fibrosis mouse model that was accompanied by reduced phosphorylation of Smad2 and MAPKs in lung tissue. In vitro, bleomycin-induced expression of SphK1 in lung fibroblast was found to be TGF-β dependent. Taken together, these data indicate that SphK1 plays a critical role in the pathology of lung fibrosis and is a novel therapeutic target.

Regulation of the sphingosine kinase/sphingosine 1-phosphate pathway

Sphingolipids have emerged as pleiotropic signaling molecules with roles in numerous cellular and biological functions. Defining the regulatory mechanisms governing sphingolipid metabolism is crucial in order to develop a complete understanding of the biological functions of sphingolipid metabolites. The sphingosine kinase/ sphingosine 1-phosphate pathway was originally thought to function in the irreversible breakdown of sphingoid bases; however, in the last few decades it has materialized as an extremely important signaling pathway involved in a plethora of cellular events contributing to both normal and pathophysiological events. Recognition of the SK/S1P pathway as a second messaging system has aided in the identification of many mechanisms of its regulation; however, a cohesive, global understanding of the regulatory mechanisms controlling the SK/S1P pathway is lacking. In this chapter, the role of the SK/S1P pathway as a second messenger is discussed, and its role in mediating TNF-α- and EGF-induced biologies is examined. This work provides a comprehensive look into the roles and regulation of the sphingosine kinase/ sphingosine 1-phosphate pathway and highlights the potential of the pathway as a therapeutic target.

Sphingolipids in lung endothelial biology and regulation of vascular integrity

Of the multiple and diverse homeostatic events that involve the lung vascular endothelium, participation in preserving vascular integrity and therefore organ function is paramount. We were the first to show that the lipid growth factor and angiogenic factor, sphingosine-1-phosphate, is a critical agonist involved in regulation of human lung vascular barrier function (Garcia et al. J Clin Invest, 2011). Utilizing both in vitro models and preclinical murine, rat, and canine models of acute and chronic inflammatory lung injury, we have shown that S1Ps, as well as multiple S1P analogues such as FTY720 and ftysiponate, serve as protective agents limiting the disruption of the vascular EC monolayer in the pulmonary microcirculation and attenuate parenchymal accumulation of inflammatory cells and high protein containing extravasated fluid, thereby reducing interstitial and alveolar edema. The vasculo-protective mechanism of these therapeutic effects occurs via ligation of specific G-protein-coupled receptors and an intricate interplay of S1P with other factors (such as MAPKS, ROCKs, Rho, Rac1) with rearrangement of the endothelial cytoskeleton to form strong cortical actin rings in the cell periphery and enhanced cell-to-cell and cell-to-matrix tethering dynamics. This cascade leads to reinforcement of focal adhesions and paracellular junctional complexes via cadherin, paxillin, catenins, and zona occludens. S1P through its interaction with Rac and Rho influences the cytoskeletal rearrangement indicated in the later stages of angiogenesis as a stabilizing force, preventing excessive vascular permeability. These properties translate into a therapeutic potential for acute and chronic inflammatory lung injuries. S1P has potential for providing a paradigm shift in the approach to disruption of critical endothelial gatekeeper function, loss of lung vascular integrity, and increased vascular permeability, defining features of acute lung injury (ALI), and may prove to exhibit an intrinsically protective role in the pulmonary vasculature ameliorating agonist- or sepsis-induced pulmonary injury and vascular leakage.

Cellular assay for the characterization of sphingosine-1-phosphate lyase inhibitors

Sphingosine-1-phosphate (S1P) lyase represents a target for therapeutic intervention in immune regulation. Inhibitors of the lyase can be identified by established biochemical assays, but a cellular test system for such inhibitors has not been described so far. We found that silencing or inhibition of S1P lyase with short interfering RNA (siRNA) or active site-directed inhibitors in cultured mammalian cells does not cause a relevant increase of S1P in the cells as measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). However, the addition of sphingosine to cultures of cell lines or primary cells provides a source of intracellular S1P that is susceptible to degradation by the lyase and, hence, increases on inhibition or silencing of the enzyme. The assay was optimized with respect to sphingosine concentration, incubation time, and cell density and was established for routine use with HEK293 cells. The assay was found to be suitable for the testing of novel active site-directed S1P lyase inhibitors, providing important information on their relative potency in intact cells.

Sphingosine kinase 1 is required for mesothelioma cell proliferation: role of histone acetylation

Background

Malignant pleural mesothelioma (MPM) is a devastating disease with an overall poor prognosis. Despite the recent advances in targeted molecular therapies, there is a clear and urgent need for the identification of novel mesothelioma targets for the development of highly efficacious therapeutics.

Methodology/Principal Findings

In this study, we report that the expression of Sphingosine Kinase 1 (SphK1) protein was preferentially elevated in MPM tumor tissues (49 epithelioid and 13 sarcomatoid) compared to normal tissue (n = 13). In addition, we also observed significantly elevated levels of SphK1 and SphK2 mRNA and SphK1 protein expression in MPM cell lines such as H2691, H513 and H2461 compared to the non-malignant mesothelial Met5 cells. The underlying mechanism appears to be mediated by SphK1 induced upregulation of select gene transcription programs such as that of CBP/p300 and PCAF, two histone acetyl transferases (HAT), and the down regulation of cell cycle dependent kinase inhibitor genes such as p27Kip1 and p21Cip1. In addition, using immunoprecipitates of anti-acetylated histone antibody from SphK inhibitor, SphK-I2 treated Met5A and H2691 cell lysates, we also showed activation of other cell proliferation related genes, such as Top2A (DNA replication), AKB (chromosome remodeling and mitotic spindle formation), and suppression of p21 CIP1 and p27KIP1. The CDK2, HAT1 and MYST2 were, however, unaffected in the above study. Using SphK inhibitor and specific siRNA targeting either SphK1 or SphK2, we also unequivocally established that SphK1, but not SphK2, promotes H2691 mesothelioma cell proliferation. Using a multi-walled carbon nanotubes induced peritoneal mesothelioma mouse model, we showed that the SphK1−/− null mice exhibited significantly less inflammation and granulamatous nodules compared to their wild type counterparts.

Conclusions/Significance

The lipid kinase SphK1 plays a positive and essential role in the growth and development of malignant mesothelioma and is therefore a likely therapeutic target.

FTY720 protects cardiac microvessels of diabetes: a critical role of S1P1/3 in diabetic heart disease

Background: Diabetes is associated with an increased risk of cardiac microvascular disease. The mechanisms by which this damage occurs are unknown. However, research suggests that signaling through the sphingosine-1-phosphates receptor 1 and 3 (S1P1/3) by FTY720, a sphiongolipid drug that is structually similar to SIP, may play a role in the treatment on cardiac microvascular dysfunction in diabetes. We hypothesized that FTY720 might exert the cardioprotective effects of S1P1 and S1P3 viaprotein kinase C-beta (PKCβ II) signaling pathway.

Methodology/Principal Findings: Transthoracic echocardiography was performed to detect the change of cardiac function. Scanning and transmission electron microscope with lanthanum tracer were used to determine microvascular ultrastructure and permeability in vivo. Apoptosis was detected by TUNEL and CD31 dual labeling in paraffin-embedded sections. Laser capture miscrodissection was used to assess cardiac micovascular endothelial cells (CMECs) in vivo. RT-PCR and Western blot analysis were used to determine the mRNA levels and protein expression of S1P1, S1P3, and PKCβ II. In the diabetic rats vs. controls, cardiac capillaries showed significantly higher density; CD31 positive endothelial cells were significantly reduced; the apoptosis index of cardiac endothlial cells was significantly higher. And FTY720 could increase the expressional level of S1P1 and boost S1P3 trasnslocation from membrane to nuclear, then ameliorate cardiac microvascular barrier impairment and pathologic angiogenesis induced by diabetes. In addition, overexpression of PKCβ II significantly decreased the protective effect of FTY720.

Conclusions: Our study represents that the deregulation of S1P1 and S1P3 is an important signalresponsible for cardiac microvascular dysfunction in diabetes. FTY720 might be competent to serve as a potential therapeutic approach for diabetic heart disease through ameliorating cardiac microvascular barrier impairment and pathologic angiogenesis, which might be partly dependent on PKCβII-mediated signaling pathway.

Sphingosine kinase 1 deficiency exacerbates LPS-induced neuroinflammation

The pathogenesis of inflammation in the central nervous system (CNS), which contributes to numerous neurodegenerative diseases and results in encephalopathy and neuroinflammation, is poorly understood. Sphingolipid metabolism plays a crucial role in maintaining cellular processes in the CNS, and thus mediates the various pathological consequences of inflammation. For a better understanding of the role of sphingosine kinase activation during neuroinflammation, we developed a bacterial lipopolysaccharide (LPS)-induced brain injury model. The onset of the inflammatory response was observed beginning 4 hours after intracerebral injection of LPS into the lateral ventricles of the brain. A comparison of established neuroinflammatory parameters such as white matter rarefactions, development of cytotoxic edema, astrogliosis, loss of oligodendrocytes, and major cytokines levels in wild type and knockout mice suggested that the neuroinflammatory response in SphK1-/- mice was significantly upregulated. At 6 hours after intracerebroventricular injection of LPS in SphK1-/- mice, the immunoreactivity of the microglia markers and astrocyte marker glial fibrillary acidic protein (GFAP) were significantly increased, while the oligodendrocyte marker O4 was decreased compared to WT mice. Furthermore, western blotting data showed increased levels of GFAP. These results suggest that SphK1 activation is involved in the regulation of LPS induced brain injury. RESEARCH HIGHLIGHTS: • Lipopolysaccharide (LPS) intracerebral injection induces severe neuroinflammation. • Sphingosine kinase 1 deletion worsens the effect of the LPS. • Overexpression of SphK1 might be a potential new treatment approach to neuroinflammation.

IQGAP1 is necessary for pulmonary vascular barrier protection in murine acute lung injury and pneumonia

We recently reported that integrin α(v)β(3) is necessary for vascular barrier protection in mouse models of acute lung injury and peritonitis. Here, we used mass spectrometric sequencing of integrin complexes to isolate the novel β(3)-integrin binding partner IQGAP1. Like integrin β(3), IQGAP1 localized to the endothelial cell-cell junction after sphingosine-1-phosphate (S1P) treatment, and IQGAP1 knockdown prevented cortical actin formation and barrier enhancement in response to S1P. Furthermore, knockdown of IQGAP1 prevented localization of integrin α(v)β(3) to the cell-cell junction. Similar to β(3)-null animals, IQGAP1-null mice had increased pulmonary vascular leak compared with wild-type controls 3 days after intratracheal LPS. In an Escherichia coli pneumonia model, IQGAP1 knockout mice had increased lung weights, lung water, and lung extravascular plasma equivalents of (125)I-labeled albumin compared with wild-type controls. Taken together, these experiments indicate that IQGAP1 is necessary for S1P-mediated vascular barrier protection during acute lung injury and is required for junctional localization of the barrier-protective integrin α(v)β(3).

Involvement of ceramide in cell death responses in the pulmonary circulation

Ceramides are signaling sphingolipids involved in cellular homeostasis but also in pathological processes such as unwanted apoptosis, growth arrest, oxidative stress, or senescence. Several enzymatic pathways are responsible for the synthesis of ceramides, which can be activated in response to exogenous stimuli such as cytokines, radiation, or oxidative stress. Endothelial cells are particularly rich in acid sphingomyelinases, which can be rapidly activated to produce ceramides, both intracellular and at the plasma membrane. In addition, neutral sphingomyelinases, the de novo pathway and the ceramide recycling pathway, may generate excessive ceramides involved in endothelial cell responses. When up-regulated, ceramides trigger signaling pathways that culminate in endothelial cell death, which in murine lungs has been linked to the development of emphysema-like disease. Furthermore, ceramides may be released paracellularly where they are believed to exert paracrine activities. Such effects, along with ceramides released by inflammatory mediators, may contribute to lung inflammation and pulmonary edema, because ceramide-challenged pulmonary endothelial cells exhibit decreased barrier function, independent of apoptosis. Reestablishing the sphingolipid homeostasis, either by modulating ceramide synthesis or by opposing its biological effects through augmentation of the prosurvival sphingosine-1 phosphate, may alleviate acute or chronic pulmonary conditions characterized by vascular endothelial cell death or dysfunction.

Novel methods for the quantification of (2E)-hexadecenal by liquid chromatography with detection by either ESI QTOF tandem mass spectrometry or fluorescence measurement

Sphingosine-1-phosphate lyase (SPL) is the only known enzyme that irreversibly cleaves sphingosine-1-phosphate (S1P) into phosphoethanolamine and (2E)-hexadecenal during the final step of sphingolipid catabolism. Because S1P is involved in a wide range of physiological and diseased processes, determining the activity of the degrading enzyme is of great interest. Therefore, we developed two procedures based on liquid chromatography (LC) for analysing (2E)-hexadecenal, which is one of the two S1P degradation products. After separation, two different quantification methods were performed, tandem mass spectrometry (MS) and fluorescence detection. However, (2E)-hexadecenal as a long-chain aldehyde is not ionisable by electrospray ionisation (ESI) for MS quantification and has an insufficient number of corresponding double bonds for fluorescence detection. Therefore, we investigated 2-diphenylacetyl-1,3-indandione-1-hydrazone (DAIH) as a derivatisation reagent. DAIH transforms the aldehyde into an ionisable and fluorescent analogue for quantitative analysis. Our conditions were optimised to obtain the outstanding limit of detection (LOD) of 1 fmol per sample (30 μL) for LC-MS/MS and 0.75 pmol per sample (200 μL) for LC determination with fluorescence detection. We developed an extraction procedure to separate and concentrate (2E)-hexadecenal from biological samples for these measurements. To confirm our new methods, we analysed the (2E)-hexadecenal level of different cell lines and human plasma for the first time ever. Furthermore, we treated HT-29 cells with different concentrations of 4-deoxypyridoxine (DOP), which competitively inhibits pyridoxal-5-phosphate (P5P), an essential cofactor for SPL activity, and observed a significant decrease in (2E)-hexadecenal relative to the untreated cells.

Sphingosine-1-phosphate signaling and its role in disease

The bioactive sphingolipid metabolite sphingosine-1-phosphate (S1P) is now recognized as a critical regulator of many physiological and pathophysiological processes, including cancer, atherosclerosis, diabetes and osteoporosis. S1P is produced in cells by two sphingosine kinase isoenzymes, SphK1 and SphK2. Many cells secrete S1P, which can then act in an autocrine or paracrine manner. Most of the known actions of S1P are mediated by a family of five specific G protein-coupled receptors. More recently, it was shown that S1P also has important intracellular targets involved in inflammation, cancer and Alzheimer's disease. This suggests that S1P actions are much more complex than previously thought, with important ramifications for development of therapeutics. This review highlights recent advances in our understanding of the mechanisms of action of S1P and its roles in disease.

Therapeutic potential of targeting sphingosine kinase 1 in prostate cancer

Sphingosine kinase 1 (SK1) is a lipid enzyme with oncogenic properties that converts the proapoptotic lipid sphingosine into the antiapoptotic lipid sphingosine-1-phosphate, which activates the signal transduction pathways that lead to cell proliferation, migration, activation of the inflammatory response and impairment of apoptosis. Compelling evidence suggests that SK1 activation contributes to cancer progression leading to increased oncogenic transformation, tumor growth, resistance to therapies, tumor neovascularization and metastatic spread. High levels of SK1 expression or activity have been associated with poor prognosis in several cancers, including those of the prostate. Recent studies using prostate cancer cell and mouse models demonstrate a significant potential for SK1-targeting therapies to synergize with the effects of docetaxel chemotherapy and radiotherapy. However, until recently the absence of clinically applicable SK1 inhibitors has limited the translation of these findings into patients. With the recent discovery that clinically approved drug fingolimod has SK1-inhibiting properties, SK1 has gained significant attention from both clinicians and the pharmaceutical industry and it is hoped that trials of newly developed SK1 inhibitors might follow soon.

Ablation of sphingosine kinase-2 inhibits tumor cell proliferation and migration

Sphingosine kinases (SK) regulate the balance between proapoptotic ceramides and mitogenic sphingosine-1-phosphate (S1P); however, the functions of the two isoenzymes (SK1 and SK2) in tumor cells are not well defined. Therefore, RNA interference was used to assess the individual roles of SK1 and SK2 in tumor cell sphingolipid metabolism, proliferation, and migration/invasion. Treatment of A498, Caki-1, or MDA-MB-231 cells with siRNAs specific for SK1 or SK2 effectively suppressed the expression of the target mRNA and protein. Ablation of SK1 did not affect mRNA or protein levels of SK2 and reduced intracellular levels of S1P while elevating ceramide levels. In contrast, ablation of SK2 elevated mRNA, protein, and activity levels of SK1 and increased cellular S1P levels. Interestingly, cell proliferation and migration/invasion were suppressed more by SK2-selective ablation than by SK1-selective ablation, showing that the increased S1P does not rescue these phenotypes. Similarly, exogenous S1P did not rescue the cells from the antiproliferative or antimigratory effects of the siRNAs. Consistent with these results, differential effects of SK1- and SK2-selective siRNAs on signaling proteins, including p53, p21, ERK1, ERK2, FAK, and VCAM1, indicate that SK1 and SK2 have only partially overlapping functions in tumor cells. Overall, these data indicate that loss of SK2 has stronger anticancer effects than does suppression of SK1. Consequently, selective inhibitors of SK2 may provide optimal targeting of this pathway in cancer chemotherapy.

Sphingosine kinase 1 and sphingosine 1-phosphate receptor 3 are functionally upregulated on astrocytes under pro-inflammatory conditions

BACKGROUND

Reactive astrocytes are implicated in the development and maintenance of neuroinflammation in the demyelinating disease multiple sclerosis (MS). The sphingosine kinase 1 (SphK1)/sphingosine1-phosphate (S1P) receptor signaling pathway is involved in modulation of the inflammatory response in many cell types, but the role of S1P receptor subtype 3 (S1P(3)) signaling and SphK1 in activated rat astrocytes has not been defined.

METHODOLOGY/PRINCIPAL FINDINGS

Using immunohistochemistry we observed the upregulation of S1P(3) and SphK1 expression on reactive astrocytes and SphK1 on macrophages in MS lesions. Increased mRNA and protein expression of S1P(3) and SphK1, as measured by qPCR and Western blotting respectively, was observed after treatment of rat primary astrocyte cultures with the pro-inflammatory stimulus lipopolysaccharide (LPS). Activation of SphK by LPS stimulation was confirmed by SphK activity assay and was blocked by the use of the SphK inhibitor SKI (2-(p-hydroxyanilino)-4-(p-chlorphenyl) thiazole. Treatment of astrocytes with a selective S1P(3) agonist led to increased phosphorylation of extracellular signal-regulated kinase (ERK)-1/2), which was further elevated with a LPS pre-challenge, suggesting that S1P(3) upregulation can lead to increased functionality. Moreover, astrocyte migration in a scratch assay was induced by S1P and LPS and this LPS-induced migration was sensitive to inhibition of SphK1, and independent of cell proliferation. In addition, S1P induced secretion of the potentially neuroprotective chemokine CXCL1, which was increased when astrocytes were pre-challenged with LPS. A more prominent role of S1P(3) signaling compared to S1P(1) signaling was demonstrated by the use of selective S1P(3) or S1P(1) agonists.

CONCLUSION/SIGNIFICANCE

In summary, our data demonstrate that the SphK1/S1P(3) signaling axis is upregulated when astrocytes are activated by LPS. This signaling pathway appears to play a role in the establishment and maintenance of astrocyte activation. Upregulation of the pathway in MS may be detrimental, e.g. through enhancing astrogliosis, or beneficial through increased remyelination via CXCL1.

Photolysis of caged sphingosine-1-phosphate induces barrier enhancement and intracellular activation of lung endothelial cell signaling pathways

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that mediates cellular functions by ligation via G protein-coupled S1P receptors. In addition to its extracellular action, S1P also has intracellular effects; however, the signaling pathways modulated by intracellular S1P remain poorly defined. We have previously demonstrated a novel pathway of intracellular S1P generation in human lung endothelial cells (ECs). In the present study, we examined the role of intracellular S1P generated by photolysis of caged S1P on EC barrier regulation and signal transduction. Intracellular S1P released from caged S1P caused mobilization of intracellular calcium, induced activation of MAPKs, redistributed cortactin, vascular endothelial cadherin, and β-catenin to cell periphery, and tightened endothelial barrier in human pulmonary artery ECs. Treatment of cells with pertussis toxin (PTx) had no effect on caged S1P-mediated effects on Ca(2+) mobilization, reorganization of cytoskeleton, cell adherens junction proteins, and barrier enhancement; however, extracellular S1P effects were significantly attenuated by PTx. Additionally, intracellular S1P also activated small GTPase Rac1 and its effector Ras GTPase-activating-like protein IQGAP1, suggesting involvement of these proteins in the S1P-mediated changes in cell-to-cell adhesion contacts. Downregulation of sphingosine kinase 1 (SphK1), but not SphK2, with siRNA or inhibition of SphK activity with an inhibitor 2-(p-hydroxyanilino)-4-(p-chlorophenyl) thiazole (CII) attenuated exogenously administrated S1P-induced EC permeability. Furthermore, S1P1 receptor inhibitor SB649164 abolished exogenous S1P-induced transendothelial resistance changes but had no effect on intracellular S1P generated by photolysis of caged S1P. These results provide evidence that intracellular S1P modulates signal transduction in lung ECs via signaling pathway(s) independent of S1P receptors.