Lysophospholipid receptor-dependent and -independent calcium signaling

Cholestasis-Associated Pruritus and Its Pruritogens

Pruritus is a debilitating symptom of various cholestatic disorders, including primary biliary cholangitis (PBC), primary sclerosing cholangitis (PSC) and inherited progressive familial intrahepatic cholestasis (PFIC). The molecular mechanisms leading to cholestasis-associated pruritus are still unresolved and the involved pruritogens are indecisive. As a consequence of pruritus, patients suffer from sleep deprivation, loss of daytime concentration, auto-mutilation and sometimes even suicidal ideations. Current guideline-approved therapy of cholestasis-associated pruritus includes stepwise administration of several medications, which may alleviate complaints in some, but not all affected patients. Therefore, also experimental therapeutic approaches are required to improve patients' quality of life. This article reviews the current state of research on pruritogens and their receptors, and shortly discusses the most recent experimental therapies.

Increased S1P induces S1PR2 internalization to blunt the sensitivity of colorectal cancer to 5-fluorouracil via promoting intracellular uracil generation

Sphingosine-1-phosphate (S1P), the backbone of most sphingolipids, activating S1P receptors (S1PRs) and the downstream G protein signaling has been implicated in chemoresistance. In this study we investigated the role of S1PR2 internalization in 5-fluorouracil (5-FU) resistance in human colorectal cancer (CRC). Clinical data of randomly selected 60 CRC specimens showed the correlation between S1PR2 internalization and increased intracellular uracil (P < 0.001). Then we explored the regulatory mechanisms in CRC model of villin-S1PR2-/- mice and CRC cell lines. We showed that co-administration of S1P promoted S1PR2 internalization from plasma membrane (PM) to endoplasmic reticulum (ER), thus blunted 5-FU efficacy against colorectal tumors in WT mice, compared to that in S1PR2-/- mice. In HCT116 and HT-29 cells, application of S1P (10 μM) empowered S1PR2 to internalize from PM to ER, thus inducing 5-FU resistance, whereas the specific S1PR2 inhibitor JTE-013 (10 μM) effectively inhibited S1P-induced S1PR2 internalization. Using Mag-Fluo-AM-labeling [Ca2+]ER and LC-ESI-MS/MS, we revealed that internalized S1PR2 triggered elevating [Ca2+]ER levels to activate PERK-eLF2α-ATF4 signaling in HCT116 cells. The activated ATF4 upregulated RNASET2-mediated uracil generation, which impaired exogenous 5-FU uptake to blunt 5-FU therapy. Overall, this study reveals a previously unrecognized mechanism of 5-FU resistance resulted from S1PR2 internalization-upregulated uracil generation in colorectal cancer, and provides the novel insight into the significance of S1PR2 localization in predicting the benefit of CRC patients from 5-FU-based chemotherapy.

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.

Metabolic alterations in the erythrocyte during blood-stage development of the malaria parasite

Background

Human blood cells (erythrocytes) serve as hosts for the malaria parasite Plasmodium falciparum during its 48-h intraerythrocytic developmental cycle (IDC). Established in vitro protocols allow for the study of host–parasite interactions during this phase and, in particular, high-resolution metabolomics can provide a window into host–parasite interactions that support parasite development.

Methods

Uninfected and parasite-infected erythrocyte cultures were maintained at 2% haematocrit for the duration of the IDC, while parasitaemia was maintained at 7% in the infected cultures. The parasite-infected cultures were synchronized to obtain stage-dependent information of parasite development during the IDC. Samples were collected in quadruplicate at six time points from the uninfected and parasite-infected cultures and global metabolomics was used to analyse cell fractions of these cultures.

Results

In uninfected and parasite-infected cultures during the IDC, 501 intracellular metabolites, including 223 lipid metabolites, were successfully quantified. Of these, 19 distinct metabolites were present only in the parasite-infected culture, 10 of which increased to twofold in abundance during the IDC. This work quantified approximately five times the metabolites measured in previous studies of similar research scope, which allowed for more detailed analyses. Enrichment in lipid metabolism pathways exhibited a time-dependent association with different classes of lipids during the IDC. Specifically, enrichment occurred in sphingolipids at the earlier stages, and subsequently in lysophospholipid and phospholipid metabolites at the intermediate and end stages of the IDC, respectively. In addition, there was an accumulation of 18-, 20-, and 22-carbon polyunsaturated fatty acids, which produce eicosanoids and promote gametocytogenesis in infected erythrocyte cultures.

Conclusions

The current study revealed a number of heretofore unidentified metabolic components of the host–parasite system, which the parasite may exploit in a time-dependent manner to grow over the course of its development in the blood stage. Notably, the analyses identified components, such as precursors of immunomodulatory molecules, stage-dependent lipid dynamics, and metabolites, unique to parasite-infected cultures. These conclusions are reinforced by the metabolic alterations that were characterized during the IDC, which were in close agreement with those known from previous studies of blood-stage infection.

Effects of lysophospholipid on growth performance, carcass yield, intestinal development, and bone quality in broilers

A study was conducted to evaluate the effects of supplementing different levels of lysophospholipid (LPL) to normal or reduced energy diets on growth performance, carcass yield, intestinal morphology, and skeletal development in broilers. A total of 960 one-day-old Cobb 500 male birds were allocated using a 2 × 4 factorial arrangement with 2 energy levels (NE: normal and RE: 100 kcal/kg metabolizable energy reduction) and 4 LPL supplement levels (0, 0.025, 0.050, and 0.075%). Three diet phases were fed throughout the trial: starter (days 0 to 7), grower (days 8 to 21), and finisher (days 22 to 42) phases. Body weight (BW), feed intake (FI), and feed conversion ratio were calculated at the end of each phase. At day 7 and 21, duodenum and jejunum samples were collected for intestinal morphology and claudin-3 expression analyses, and tibia were sampled for bone quality analyses. At day 42, 4 birds per replicate were selected to measure carcass yield. The results showed low metabolizable energy diets impaired bird's growth performance, intestine development, and bone quality. The 0.075% LPL supplement in NE improved BW, BW gain, and FI in the finisher and overall period compared with no LPL supplement in NE (P < 0.05). In RE, the 0.025% LPL supplement significantly improved growth performance compared to the other treatments in RE (P < 0.05). The interactions on processing parameters were detected with LPL supplement in NE diets; 0.025, 0.05, and 0.075% LPL supplements significantly increased pectoral major percentages compared to the one without LPL supplement in NE (P < 0.05). The 0.075% LPL supplement increased dressing percentage (cold carcass weight/live BW) compared with the others (P < 0.05). The intestine morphology results showed LPL had positive effects on intestine development mainly during the early age (day 7) and claudin-3 expression at both day 7 and 21. Furthermore, LPL supplement significantly increased the total Ca and P deposition and positively affected the bone structure development. In summary, dietary LPL supplementation promoted growth performance, carcass yield, intestinal development, intestinal health, and bone quality.

cAMP guided his way: a life for G protein-mediated signal transduction and molecular pharmacology-tribute to Karl H. Jakobs

Karl H. Jakobs, former editor-in-chief of Naunyn-Schmiedeberg's Archives of Pharmacology and renowned molecular pharmacologist, passed away in April 2018. In this article, his scientific achievements regarding G protein-mediated signal transduction and regulation of canonical pathways are summarized. Particularly, the discovery of inhibitory G proteins for adenylyl cyclase, methods for the analysis of receptor-G protein interactions, GTP supply by nucleoside diphosphate kinases, mechanisms in phospholipase C and phospholipase D activity regulation, as well as the development of the concept of sphingosine-1-phosphate as extra- and intracellular messenger will presented. His seminal scientific and methodological contributions are put in a general and timely perspective to display and honor his outstanding input to the current knowledge in molecular pharmacology.

Autotaxin in Pathophysiology and Pulmonary Fibrosis

Lysophospholipid signaling is emerging as a druggable regulator of pathophysiological responses, and especially fibrosis, exemplified by the relative ongoing clinical trials in idiopathic pulmonary fibrosis (IPF) patients. In this review, we focus on ectonucleotide pyrophosphatase-phosphodiesterase 2 (ENPP2), or as more widely known Autotaxin (ATX), a secreted lysophospholipase D (lysoPLD) largely responsible for extracellular lysophosphatidic acid (LPA) production. In turn, LPA is a bioactive phospholipid autacoid, forming locally upon increased ATX levels and acting also locally through its receptors, likely guided by ATX's structural conformation and cell surface associations. Increased ATX activity levels have been detected in many inflammatory and fibroproliferative conditions, while genetic and pharmacologic studies have confirmed a pleiotropic participation of ATX/LPA in different processes and disorders. In pulmonary fibrosis, ATX levels rise in the broncheoalveolar fluid (BALF) and stimulate LPA production. LPA engagement of its receptors activate multiple G-protein mediated signal transduction pathways leading to different responses from pulmonary cells including the production of pro-inflammatory signals from stressed epithelial cells, the modulation of endothelial physiology, the activation of TGF signaling and the stimulation of fibroblast accumulation. Genetic or pharmacologic targeting of the ATX/LPA axis attenuated disease development in animal models, thus providing the proof of principle for therapeutic interventions.

The novel sphingosine-1-phosphate receptors antagonist AD2900 affects lymphocyte activation and inhibits T-cell entry into the lymph nodes

Sphingolipid derivatives play key roles in immune cell migration and function. Synthetic sphingolipid analogues are used as therapeutics to intervene various inflammatory and malignant conditions. We hypothesize that different analogs have different effects on immune cells and therefore can be used as treatment for specific diseases. This study examines the properties of the novel synthetic sphingolipid analog, AD2900, and its effects on immune cell activation and lymphocyte localization in homeostasis. AD2900 is an antagonist for all sphingosine-1-phosphate (S1P) receptors. It demonstrates a significant inhibitory effect on the proliferation of activated human peripheral blood mononuclear cells, which is dependent on cAMP reduction and calcium signal transduction but not on phospholipase C activation. AD2900 causes a significant but reversible downregulation of S1P1 expression on the cell surface. AD2900 administration to C57BL/6J mice leads to the accumulation of T cells in the blood and spleen and in turn reduces T-cell number in the lymph nodes. Moreover, AD2900 treatment shows significant effects on the localization of T-cell subpopulations. These results demonstrate the key roles of S1P in T-cell trafficking in a steady state and suggest a potential clinical application for AD2900. Notably, this sphingolipid analog does not cause a severe lymphopenia. The clinical effect of AD2900 in hemato-oncologic diseases and immune-related diseases needs further investigation.

Autocrine lysophosphatidic acid signaling activates β-catenin and promotes lung allograft fibrosis

Tissue fibrosis is the primary cause of long-term graft failure after organ transplantation. In lung allografts, progressive terminal airway fibrosis leads to an irreversible decline in lung function termed bronchiolitis obliterans syndrome (BOS). Here, we have identified an autocrine pathway linking nuclear factor of activated T cells 2 (NFAT1), autotaxin (ATX), lysophosphatidic acid (LPA), and β-catenin that contributes to progression of fibrosis in lung allografts. Mesenchymal cells (MCs) derived from fibrotic lung allografts (BOS MCs) demonstrated constitutive nuclear β-catenin expression that was dependent on autocrine ATX secretion and LPA signaling. We found that NFAT1 upstream of ATX regulated expression of ATX as well as β-catenin. Silencing NFAT1 in BOS MCs suppressed ATX expression, and sustained overexpression of NFAT1 increased ATX expression and activity in non-fibrotic MCs. LPA signaling induced NFAT1 nuclear translocation, suggesting that autocrine LPA synthesis promotes NFAT1 transcriptional activation and ATX secretion in a positive feedback loop. In an in vivo mouse orthotopic lung transplant model of BOS, antagonism of the LPA receptor (LPA1) or ATX inhibition decreased allograft fibrosis and was associated with lower active β-catenin and dephosphorylated NFAT1 expression. Lung allografts from β-catenin reporter mice demonstrated reduced β-catenin transcriptional activation in the presence of LPA1 antagonist, confirming an in vivo role for LPA signaling in β-catenin activation.

Sphingosylphosphorylcholine inhibits macrophage adhesion to vascular smooth muscle cells

Inflammation in de-endothelialised arteries contributes to the development of cardiovascular diseases. The process that initiates this inflammatory response is the adhesion of monocytes/macrophages to exposed vascular smooth muscle cells, typically stimulated by cytokines such as tumour necrosis factor-α (TNF). The aim of this study was to determine the effect of the sphingolipid sphingosylphosphorylcholine (SPC) on the interaction of monocytes/macrophages with vascular smooth muscle cells. Rat aortic smooth muscle cells and rat bone marrow-derived macrophages were co-cultured using an in vitro assay following incubation with sphingolipids to assess inter-cellular adhesion. We reveal that SPC inhibits the TNF-induced adhesion of macrophages to smooth muscle cells. This anti-adhesive effect was the result of SPC-induced changes to the smooth muscle cells (but not the macrophages) and was mediated, at least partly, via the sphingosine 1-phosphate receptor subtype 2. Lipid raft domains were also required. Although SPC did not alter expression or membrane distribution of the adhesion proteins intercellular adhesion molecule-1 and vascular cellular adhesion protein-1 in smooth muscle cells, SPC preincubation inhibited the TNF-induced increase in inducible nitric oxide synthase (NOS2) resulting in a subsequent decrease in nitric oxide production. Inhibiting NOS2 activation in smooth muscle cells led to a decrease in the adhesion of macrophages to smooth muscle cells. This study has therefore delineated a novel pathway which can inhibit the interaction between macrophages and vascular smooth muscle cells via SPC-induced repression of NOS2 expression. This mechanism could represent a potential drug target in vascular disease.

Inhibition of Sphingosine Kinase 1 Ameliorates Angiotensin II-Induced Hypertension and Inhibits Transmembrane Calcium Entry via Store-Operated Calcium Channel

Angiotensin II (AngII) plays a critical role in the regulation of vascular tone and blood pressure mainly via regulation of Ca(2+) mobilization. Several reports have implicated sphingosine kinase 1 (SK1)/sphingosine 1-phosphate (S1P) in the mobilization of intracellular Ca(2+) through a yet-undefined mechanism. Here we demonstrate that AngII-induces biphasic calcium entry in vascular smooth muscle cells, consisting of an immediate peak due to inositol tris-phosphate-dependent release of intracellular calcium, followed by a sustained transmembrane Ca(2+) influx through store-operated calcium channels (SOCs). Inhibition of SK1 attenuates the second phase of transmembrane Ca(2+) influx, suggesting a role for SK1 in AngII-dependent activation of SOC. Intracellular S1P triggers SOC-dependent Ca(2+) influx independent of S1P receptors, whereas external application of S1P stimulated S1P receptor-dependent Ca(2+) influx that is insensitive to inhibitors of SOCs, suggesting that the SK1/S1P axis regulates store-operated calcium entry via intracellular rather than extracellular actions. Genetic deletion of SK1 significantly inhibits both the acute hypertensive response to AngII in anaesthetized SK1 knockout mice and the sustained hypertensive response to continuous infusion of AngII in conscious animals. Collectively these data implicate SK1 as the missing link that connects the angiotensin AT1A receptor to transmembrane Ca(2+) influx and identify SOCs as a potential intracellular target for SK1.

Asymmetrical macromolecular complex formation of lysophosphatidic acid receptor 2 (LPA2) mediates gradient sensing in fibroblasts

Chemotactic migration of fibroblasts toward growth factors relies on their capacity to sense minute extracellular gradients and respond to spatially confined receptor-mediated signals. Currently, mechanisms underlying the gradient sensing of fibroblasts remain poorly understood. Using single-particle tracking methodology, we determined that a lysophosphatidic acid (LPA) gradient induces a spatiotemporally restricted decrease in the mobility of LPA receptor 2 (LPA2) on chemotactic fibroblasts. The onset of decreased LPA2 mobility correlates to the spatial recruitment and coupling to LPA2-interacting proteins that anchor the complex to the cytoskeleton. These localized PDZ motif-mediated macromolecular complexes of LPA2 trigger a Ca(2+) puff gradient that governs gradient sensing and directional migration in response to LPA. Disruption of the PDZ motif-mediated assembly of the macromolecular complex of LPA2 disorganizes the gradient of Ca(2+) puffs, disrupts gradient sensing, and reduces the directional migration of fibroblasts toward LPA. Our findings illustrate that the asymmetric macromolecular complex formation of chemoattractant receptors mediates gradient sensing and provides a new mechanistic basis for models to describe gradient sensing of fibroblasts.

PPARγ agonists upregulate sphingosine 1-phosphate (S1P) receptor 1 expression, which in turn reduces S1P-induced [Ca(2+)]i increases in renal mesangial cells

We previously identified peroxisome proliferator-activated receptor gamma (PPARγ) agonists (thiazolidinediones, TZDs) as modulators of the sphingolipid metabolism in renal mesangial cells. TZDs upregulated sphingosine kinase 1 (SK-1) and increased the formation of intracellular sphingosine 1-phosphate (S1P), which in turn reduced the expression of pro-fibrotic connective tissue growth factor. Since S1P also acts as extracellular ligand at specific S1P receptors (S1PR, S1P1-5), we investigated here the effect of TZDs on S1PR expression in mesangial cells and evaluated the functional consequences by measuring S1P-induced increases in intracellular free Ca(2+) concentration ([Ca(2+)]i). Treatment with two different TZDs, troglitazone and rosiglitazone, enhanced S1P1 mRNA and protein expression in rat mesangial cells, whereas S1P2-5 expression levels were not altered. Upregulation of S1P1 mRNA upon TZD treatment was also detected in human mesangial cells and mouse glomeruli. PPARγ antagonism and promoter studies revealed that the TZD-dependent S1P1 mRNA induction involved a functional PPAR response element in the S1P1 promoter. Pharmacological approaches disclosed that S1P-induced [Ca(2+)]i increases in rat mesangial cells were predominantly mediated by S1P2 and S1P3. Interestingly, the transcriptional upregulation of S1P1 by TZDs resulted in a reduction of S1P-induced [Ca(2+)]i increases, which was reversed by the S1P1/3 antagonist VPC-23019, the protein kinase C (PKC) inhibitor PKC-412, and by S1P1 siRNA. These data suggest that PPARγ-dependent upregulation of S1P1 leads to an inhibition of S1P-induced Ca(2+) signaling in a PKC-dependent manner. Overall, these results reveal that TZDs not only modulate intracellular S1P levels but also regulate S1PR signaling by increasing S1P1 expression in mesangial cells.

Sphingosine-1-phosphate phosphatase 1 regulates keratinocyte differentiation and epidermal homeostasis

Sphingosine 1-phosphate (S1P) is a bioactive lipid whose levels are tightly regulated by its synthesis and degradation. Intracellularly, S1P is dephosphorylated by the actions of two S1P-specific phosphatases, sphingosine-1-phosphate phosphatases 1 and 2. To identify the physiological functions of S1P phosphatase 1, we have studied mice with its gene, Sgpp1, deleted. Sgpp1(-/-) mice appeared normal at birth, but during the 1st week of life they exhibited stunted growth and suffered desquamation, with most dying before weaning. Both Sgpp1(-/-) pups and surviving adults exhibited multiple epidermal abnormalities. Interestingly, the epidermal permeability barrier developed normally during embryogenesis in Sgpp1(-/-) mice. Keratinocytes isolated from the skin of Sgpp1(-/-) pups had increased intracellular S1P levels and displayed a gene expression profile that indicated overexpression of genes associated with keratinocyte differentiation. The results reveal S1P metabolism as a regulator of keratinocyte differentiation and epidermal homeostasis.

Sphingosine kinase and sphingosine 1-phosphate in the heart: a decade of progress

Activation of sphingosine kinase/sphingosine 1-phosphate (SK/S1P)-mediated signaling has emerged as a critical cardioprotective pathway in response to acute ischemia/reperfusion injury. S1P is released in both ischemic pre- and post-conditioning. Application of exogenous S1P to cultured cardiac myocytes subjected to hypoxia or treatment of isolated hearts either before ischemia or at the onset of reperfusion exerts prosurvival effects. Synthetic congeners of S1P such as FTY720 mimic these responses. Gene targeted mice null for the SK1 isoform whose hearts are subjected to ischemia/reperfusion injury exhibit increased infarct size and respond poorly either to ischemic pre- or postconditioning. Measurements of cardiac SK activity and S1P parallel these observations. Experiments in SK2 knockout mice have revealed that this isoform is necessary for survival in the heart. High density lipoprotein (HDL) is a major carrier of S1P, and studies of hearts in which selected S1P receptors have been inhibited implicate the S1P cargo of HDL in cardioprotection. Inhibition of S1P lyase, an endogenous enzyme that degrades S1P, also leads to cardioprotection. These observations have considerable relevance for future therapeutic approaches to acute and chronic myocardial injury. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Sphingosine kinase 1 is overexpressed and promotes proliferation in human thyroid cancer

Sphingosine kinase 1 (SphK1), an oncogenic kinase, has been previously found to be elevated in various types of human cancer and play a role in tumor development and progression. Nevertheless, the biological and clinical significance of SphK1 in thyroid cancer is largely unknown. Here, we demonstrate that the expression of SphK1 is generally up-regulated in thyroid cancer and that its expression level is correlated with the degree of thyroid malignancy. Silencing SphK1 by specific RNA interference is able to suppress the proliferation of thyroid cancer cells, and SphK1 expression level is strongly associated with the expression of proliferation cell nuclear antigen in thyroid cancer tissues. Of particular note is that depletion of SphK1 results in dephosphorylation of protein kinase B and glycogen synthase kinase-3β and subsequent inactivation of β-catenin-T-cell factor/lymphoid enhancing factor transcriptional activity. Hence, taken together, our study has identified SphK1 as a proproliferative oncogenic kinase, an Akt/glycogen synthase kinase-3β/β-catenin activator, and probably a biomarker for thyroid cancer as well.

Regulators of Ca(2+) signaling in mast cells: potential targets for treatment of mast cell-related diseases?

A calcium signal is essential for degranulation, generation of eicosanoids and optimal production of cytokines in mast cells in response to antigen and other stimulants. The signal is initiated by phospholipase C-mediated production of inositol1,4,5-trisphosphate resulting in release of stored Ca(2+) from the endoplasmic reticulum (ER) and Golgi. Depletion of these stores activates influx of extracellular Ca(2+), usually referred to as store-operated calcium entry (SOCE), through the interaction of the Ca(2+)-sensor, stromal interacting molecule-1 (STIM1 ), in ER with Orai1(CRACM1) and transient receptor potential canonical (TRPC) channel proteins in the plasma membrane (PM). This interaction is enabled by microtubular-directed reorganization of ER to form ER/PM contact points or "punctae" in which STIM1 and channel proteins colocalize. The ensuing influx of Ca(2+) replenishes Ca(2+) stores and sustains elevated levels of cytosolic Ca(2+) ions-the obligatory signal for mast-cell activation. In addition, the signal can acquire spatial and dynamic characteristics (e.g., calcium puffs, waves, oscillations) that encode signals for specific functional outputs. This is achieved by coordinated regulation of Ca(2+) fluxes through ATP-dependent Ca(2+)-pumps and ion exchangers in mitochondria, ER and PM. As discussed in this chapter, studies in mast cells revealed much about the mechanisms described above but little about allergic and autoimmune diseases although studies in other types of cells have exposed genetic defects that lead to aberrant calcium signaling in immune diseases. Pharmacologic agents that inhibit or activate the regulatory components of calcium signaling in mast cells are also discussed along with the prospects for development of novel SOCE inhibitors that may prove beneficial in the treatment inflammatory mast-cell related diseases.

The molecular mechanism of cholestatic pruritus

Pruritus is a frequent symptom in patients with cholestatic liver diseases. Pruritus can be excruciating and, in rare cases, become a primary indication for liver transplantation. The molecular mechanism of itch signal transduction is largely unclear. It was our hypothesis that compounds which accumulate in the circulation during cholestasis act as direct or indirect pruritogens by affecting signaling in itch fibers. To test this, we screened plasma samples of a large group of patients with various cholestatic conditions for their capacity to activate neuroblastoma cells. Quite strikingly, we found that samples from itchy cholestatic patients caused a significantly higher activation than samples from non-itchy cholestatic patients and healthy controls. Purification revealed lysophosphatidic acid (LPA) as the active compound. LPA is a very potent signaling lipid that can activate cells through various LPA receptors. Subsequently, we could demonstrate that cholestatic patients with pruritus have highly elevated levels of serum autotaxin (ATX), the enzyme that converts lysophosphatidylcholine into LPA. This is a striking finding as ATX has never been connected to itch perception thus far. We have also shown that LPA, when injected intradermally, causes itching in mice. On the basis of our results, we hypothesize that during cholestasis, expression of ATX is induced and gives rise to increased local formation of LPA near unmyelinated nerve endings of itch fibers. LPA then activates these neurons through one of the LPA receptors, which in turn potentiates action potentials along itch fibers.

Quantitative determination of lysophosphatidic acids (LPAs) in human saliva and gingival crevicular fluid (GCF) by LC-MS/MS

Lysophosphatidic acid (LPA) is a phospholipid mediator that plays multiple cellular functions by acting through G protein-coupled LPA receptors. LPAs are known to be key mediators in inflammation, and several lines of evidence suggest a role for LPAs in inflammatory periodontal diseases. A simple and sensitive liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) method has been developed and validated to quantify LPA species (LPA 18:0, LPA 16:0, LPA 18:1 and LPA 20:4) in human saliva and gingival crevicular fluid (GCF). LPA 17:0 was used as an internal standard and the LPA species were extracted from saliva by liquid-liquid extraction using butanol. Chromatography was performed using a Macherey-Nagel NUCLEODUR® C8 Gravity Column (125 mm × 2.0 mm ID) with a mixture of methanol/water: 75/25 (v/v) containing 0.5% formic acid and 5 mM ammonium formate (mobile phase A) and methanol/water: 99/0.5 (v/v) containing 0.5% formic acid and 5mM ammonium formate (mobile phase B) at a flow rate of 0.5 mL/min. LPAs were detected by a linear ion trap-triple quadrupole mass spectrometer with a total run time of 8.5 min. The limit of quantification (LOQ) in saliva was 1 ng/mL for all LPA species and the method was validated over the range of 1-200 ng/mL. The method was validated in GCF over the ranges of 10-500 ng/mL for LPA 18:0 and LPA 16:0, and 5-500 ng/mL for LPA 18:1 and LPA 20:4. This sensitive LC-MS/MS assay was successfully applied to obtain quantitative data of individual LPA levels from control subjects and patients with various periodontal diseases. All four LPA species were consistently elevated in samples obtained from periodontal diseases, which supports a role of LPAs in the pathogenesis of periodontal diseases.

Mediators of pruritus during cholestasis

PURPOSE OF REVIEW

Pruritus is a frequent symptom in patients with cholestatic liver diseases. Itching may be excruciating, may seriously impair quality of life and even induce suicidal ideation in the most severe cases.

RECENT FINDINGS

The molecular mechanism of itch signal transduction in cholestasis is largely unclear. It may be caused or potentiated by compounds that accumulate in the circulation during cholestasis, which either directly or indirectly affect signalling in itch fibres. In the past, bile salts and endogenous opioids have been proposed but never been proven to be key factors in itch perception during cholestasis. We have performed a screen for compounds in plasma from patients with various cholestatic conditions for their capacity to activate neuronal cell lines. In these sera, we could identify a potent neuronal activator as lysophosphatidic acid (LPA). LPA is a very potent signalling phospholipid that can activate cells through various LPA receptors. Quite strikingly, samples from itchy cholestatic patients contained higher amounts of LPA. These increased levels of LPA turned out to be caused by elevated levels of serum autotaxin, the enzyme that converts lysophosphatidylcholine into LPA. This is a striking finding, as autotaxin has never been connected to itch perception thus far. We have also shown that LPA, when injected intradermally, caused scratching behaviour in mice.

SUMMARY

On the basis of our results, we hypothesize that during cholestasis expression of autotaxin is induced, which gives rise to increased local formation of LPA near unmyelinated nerve endings of itch fibres. LPA activates these neurons through one of the LPA receptors, which in turn potentiates action potentials along itch fibres leading to the perception of pruritus.

Regulation of ryanodine receptors by sphingosylphosphorylcholine: involvement of both calmodulin-dependent and -independent mechanisms

Sphingosylphosphorylcholine (SPC), a lipid mediator with putative second messenger functions, has been reported to regulate ryanodine receptors (RyRs), Ca2+ channels of the sarco/endoplasmic reticulum. RyRs are also regulated by the ubiquitous Ca2+ sensor calmodulin (CaM), and we have previously shown that SPC disrupts the complex of CaM and the peptide corresponding to the CaM-binding domain of the skeletal muscle Ca2+ release channel (RyR1). Here we report that SPC also displaces Ca2+-bound CaM from the intact RyR1, which we hypothesized might lead to channel activation by relieving the negative feedback Ca2+CaM exerts on the channel. We could not demonstrate such channel activation as we have found that SPC has a direct, CaM-independent inhibitory effect on channel activity, confirmed by both single channel measurements and [3H]ryanodine binding assays. In the presence of Ca2+CaM, however, the addition of SPC did not reduce [3H]ryanodine binding, which we could explain by assuming that the direct inhibitory action of the sphingolipid was negated by the simultaneous displacement of inhibitory Ca2+CaM. Additional experiments revealed that RyRs are unlikely to be responsible for SPC-elicited Ca2+ release from brain microsomes, and that SPC does not exert detergent-like effects on sarcoplasmic reticulum vesicles. We conclude that regulation of RyRs by SPC involves both CaM-dependent and -independent mechanisms, thus, the sphingolipid might play a physiological role in RyR regulation, but channel activation previously attributed to SPC is unlikely.

Diversity of lysophosphatidic acid receptor-mediated intracellular calcium signaling in early cortical neurogenesis

Lysophosphatidic acid (LPA) is a membrane-derived lysophospholipid that can induce pleomorphic effects in neural progenitor cells (NPCs) from the cerebral cortex, including alterations in ionic conductance. LPA-induced, calcium-mediated conductance changes have been reported; however, the underlying molecular mechanisms have not been determined. We show here that activation of specific cognate receptors accounts for nearly all intracellular calcium responses evoked by LPA in acutely cultured nestin-positive NPCs from the developing mouse cerebral cortex. Fast-onset changes in intracellular calcium levels required release from thapsigargin-sensitive stores by a pertussis toxin-insensitive mechanism. The influx of extracellular calcium through Cd(2+)/Ni(2+)-insensitive influx pathways, approximately one-half of which were Gd(3+) sensitive, contributed to the temporal diversity of responses. Quantitative reverse transcription-PCR revealed the presence of all five known LPA receptors in primary NPCs, with prominent expression of LPA(1), LPA(2), and LPA(4). Combined genetic and pharmacological studies indicated that NPC responses were mediated by LPA(1) (approximately 30% of the cells), LPA(2) (approximately 30%), a combination of receptors on single cells (approximately 30%), and non-LPA(1,2,3) pathways (approximately 10%). LPA responsivity was significantly reduced in more differentiated TuJ1(+) cells within cultures. Calcium transients in a large proportion of LPA-responsive NPCs were also initiated by the closely related signaling lipid S1P (sphingosine-1-phosphate). These data demonstrate for the first time the involvement of LPA receptors in mediating surprisingly diverse NPC calcium responses involving multiple receptor subtypes that function within a single cell. Compared with other known factors, lysophospholipids represent the major activator of calcium signaling identified within NPCs at this early stage in corticogenesis.

ATP-independent glucose stimulation of sphingosine kinase in rat pancreatic islets

Sphingosine kinase (SPHK) catalyzes sphingosine 1-phosphate production, promoting cell survival and reducing apoptosis in isolated rat pancreatic islets. Glucose, the primary islet beta-cell growth factor and insulin secretagogue, increased islet SPHK activity by 3- to 5-fold following acute (1 h) or prolonged (7 days) stimulation. Prolonged stimulation of islets with glucose induced SPHK1a and SPHK2 mRNA levels; there were no changes in SPHK protein expression. To isolate the metabolic effects of glucose on SPHK activation, islets were stimulated with glucose analogs or metabolites. 2-deoxy-D-glucose (2-DG), an analog phosphorylated by glucokinase but not an effective energy source, activated SPHK similarly to glucose. In contrast, 3-o-methylglucose (3-oMeG), which is transported but neither phosphorylated nor metabolized, did not increase islet SPHK activity. Glyceraldehyde and alpha-ketoisocaproic acid (KIC), metabolites that stimulate glycolysis and the citric acid cycle, respectively, did not activate islet SPHK. Moreover, inorganic phosphate blocked glucose-induced SPHK activation. A role for SPHK activity in beta-cell growth was confirmed when small interfering (si)SPHK2 RNA transfection reduced rat insulinoma INS-1e cell SPHK levels and activity and cell growth. Glucose induced an early and sustained increase in islet SPHK activity that was dependent on glucose phosphorylation, but independent of ATP generation or new protein biosynthesis. Glucose-supported beta-cell growth appears to be in part mediated by SPHK activity.

Sphingosine-1-phosphate receptors: zooming in on ligand-induced intracellular trafficking and its functional implications

Regulatory processes including receptor phosphorylation and intracellular trafficking, also referred to as receptor internalization, are important processes to terminate G protein-coupled receptor (GPCR) signaling. Compelling evidence now indicates that internalization of a receptor is not necessarily the endpoint of signaling, but can also be the beginning of the activation of intracellular signaling pathways. Sphingosine-1-phosphate (S1P) receptors, which are activated by the endogenous phospholipid S1P, belong to the family of GPCRs. Interestingly, there is evidence indicating differential intracellular trafficking of one of the S1P receptor subtypes, the S1P1 receptor, upon agonist activation by either S1P or the synthetic agonist FTY720-P. Moreover, the differential effect of FTY720-P on S1P1 receptor regulation has been suggested to be the mechanism of action of this drug, which is now in Phase III clinical trials for the treatment of multiple sclerosis. It is thus of importance to get a good insight into the regulation of S1P receptors. This review therefore gives a detailed overview about the current state of knowledge on S1P receptor internalization and its functional implications, including some data on nuclear signaling of S1P receptors.

Sphingolipids and the orchestration of endothelium-derived vasoactive factors: when endothelial function demands greasing

Vasomotor tone is regulated by a complex interplay of a variety of extrinsic neurohumoral and intrinsic factors. It is the endothelium that has a major influence on smooth muscle cell tone via the release of intrinsic vasoactive factors and is therefore an important regulator of vasomotor tone. Sphingolipids are an emerging class of lipid mediators with important physiological properties. In the last two decades it has not only become increasingly clear that sphingolipid signaling plays a pivotal role in immune function, but also its role in the vascular system is now becoming more recognized. In this mini-review we will highlight the possible cross-talk between sphingolipids and intrinsic vasoactive factors released by the endothelium. Via this cross-talk sphingolipids can orchestrate vasomotor tone and may therefore also be involved in the pathophysiology of disease states associated with endothelial dysfunction.

Ca2+ signaling induced by sphingosine 1-phosphate and lysophosphatidic acid in mouse B cells

Lysophospholipids (LPLs) such as lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are chemotactic for lymphocytes, and increases of in cytosolic [Ca(2+)] signal the regulation of lymphocyte activation and migration. Here, the authors investigated the effects of LPA and S1P on [Ca(2+)](c) in mouse B cell lines (WEHI-231 and Bal-17) and primary B cells isolated from mouse spleen and bone marrow, and focused on the modulation of store-operated Ca(2+) entry (SOCE) by LPLs. In Bal-17 (a mature B cell line) both LPA and S1P induced a transient [Ca(2+)](c) increase via a phospholipase C pathway. In addition, pretreatment with LPLs was found to augment thapsigargin-induced SOCE in Bal-17 cells. However, in WEHI-231 (an immature B cell line) LPLs had no significant effect on [Ca(2+)](c) or SOCE. Furthermore, in freshly isolated splenic B cells (SBCs) and bone marrow B cells (BMBCs), LPLs induced only a small increase in [Ca(2+)](c). Interestingly, however, pretreatment with LPLs markedly increased SOCE in primary B cells, and this augmentation was more prominent in BMBCs than SBCs. The unidirectional influx of Ca(2+) was measured using Ba(2+) as a surrogate ion. Similarly, Ba(2+) influx was also found to be markedly increased by LPLs in SBCs and BMBCs. Summarizing, LPLs were found to strongly augment SOCE-mediated Ca(2+)-signaling in mouse B cells. However, unlike the mature Bal-17 cell line, PLC-dependent Ca(2+) release was insignificant in primary B cells and inWEHI-231.

Dissociation of calmodulin-target peptide complexes by the lipid mediator sphingosylphosphorylcholine: implications in calcium signaling

Previously we have identified the lipid mediator sphingosylphosphorylcholine (SPC) as the first potentially endogenous inhibitor of the ubiquitous Ca2+ sensor calmodulin (CaM) (Kovacs, E., and Liliom, K. (2008) Biochem. J. 410, 427-437). Here we give mechanistic insight into CaM inhibition by SPC, based on fluorescence stopped-flow studies with the model CaM-binding domain melittin. We demonstrate that both the peptide and SPC micelles bind to CaM in a rapid and reversible manner with comparable affinities. Furthermore, we present kinetic evidence that both species compete for the same target site on CaM, and thus SPC can be considered as a competitive inhibitor of CaM-target peptide interactions. We also show that SPC disrupts the complex of CaM and the CaM-binding domain of ryanodine receptor type 1, inositol 1,4,5-trisphosphate receptor type 1, and the plasma membrane Ca2+ pump. By interfering with these interactions, thus inhibiting the negative feedback that CaM has on Ca2+ signaling, we hypothesize that SPC could lead to Ca2+ mobilization in vivo. Hence, we suggest that the action of the sphingolipid on CaM might explain the previously recognized phenomenon that SPC liberates Ca2+ from intracellular stores. Moreover, we demonstrate that unlike traditional synthetic CaM inhibitors, SPC disrupts the complex between not only the Ca2+-saturated but also the apo form of the protein and the target peptide, suggesting a completely novel regulation for target proteins that constitutively bind CaM, such as ryanodine receptors.

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.

Regulation of Ca2+ signaling with particular focus on mast cells

Calcium signals mediate diverse cellular functions in immunological cells. Early studies with mast cells, then a preeminent model for studying Ca2+-dependent exocytosis, revealed several basic features of calcium signaling in non-electrically excitable cells. Subsequent studies in these and other cells further defined the basic processes such as inositol 1,4,5-trisphosphate-mediated release of Ca2+ from Ca2+ stores in the endoplasmic reticulum (ER); coupling of ER store depletion to influx of external Ca2+ through a calcium-release activated calcium (CRAC) channel now attributed to the interaction of the ER Ca2+ sensor, stromal interacting molecule-1 (STIM1), with a unique Ca2+-channel protein, Orai1/CRACM1, and subsequent uptake of excess Ca2+ into ER and mitochondria through ATP-dependent Ca2+ pumps. In addition, transient receptor potential channels and ion exchangers also contribute to the generation of calcium signals that may be global or have dynamic (e.g., waves and oscillations) and spatial resolution for specific functional readouts. This review discusses past and recent developments in this field of research, the pharmacologic agents that have assisted in these endeavors, and the mast cell as an exemplar for sorting out how calcium signals may regulate multiple outputs in a single cell.

Transforming growth factor-beta2 upregulates sphingosine kinase-1 activity, which in turn attenuates the fibrotic response to TGF-beta2 by impeding CTGF expression

Transforming growth factor-beta2 (TGF-beta2) stimulates the expression of pro-fibrotic connective tissue growth factor (CTGF) during the course of renal disease. Because sphingosine kinase-1 (SK-1) activity is also upregulated by TGF-beta, we studied its effect on CTGF expression and on the development of renal fibrosis. When TGF-beta2 was added to an immortalized human podocyte cell line we found that it activated the promoter of SK-1, resulting in upregulation of its mRNA and protein expression. Further, depletion of SK-1 by small interfering RNA or its pharmacological inhibition led to accelerated CTGF expression in the podocytes. Over-expression of SK-1 reduced CTGF induction, an effect mediated by intracellular sphingosine-1-phosphate. In vivo, SK-1 expression was also increased in the podocytes of kidney sections of patients with diabetic nephropathy when compared to normal sections of kidney obtained from patients with renal cancer. Similarly, in a mouse model of streptozotocin-induced diabetic nephropathy, SK-1 and CTGF were upregulated in podocytes. In SK-1 deficient mice, exacerbation of disease was detected by increased albuminuria and CTGF expression when compared to wild-type mice. Thus, SK-1 activity has a protective role in the fibrotic process and its deletion or inhibition aggravates fibrotic disease.

Sphingosine kinase and sphingosine 1-phosphate in cardioprotection

Activation of sphingosine kinase/sphingosine 1-phosphate-mediated signaling has emerged as a critical cardioprotective pathway in response to acute ischemia/reperfusion injury. Application of exogenous sphingosine 1-phosphate (S1P) in cultured cardiac myocytes subjected to hypoxia or treatment of isolated hearts either before ischemia or at the onset of reperfusion (pharmacologic preconditioning or postconditioning) exerts prosurvival effects. Synthetic congeners of S1P mimic these responses. Gene-targeted mice null for the sphingosine kinase 1 isoform whose hearts are subjected to ischemia/reperfusion injury exhibit increased infarct size and respond poorly either to ischemic preconditioning or to ischemic postconditioning. Measurements of cardiac sphingosine kinase activity and S1P parallel these observations. High-density lipoprotein is a major carrier of S1P, and studies of hearts in which selected S1P receptors have been deleted implicate the S1P cargo of high-density lipoprotein in cardioprotection. These observations have considerable relevance for future therapeutic approaches to acute and chronic myocardial injury.

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

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

The sphingosine 1-phosphate receptor 5 and sphingosine kinases 1 and 2 are localised in centrosomes: possible role in regulating cell division

We show here that the endogenous sphingosine 1-phosphate 5 receptor (S1P(5), a G protein coupled receptor (GPCR) whose natural ligand is sphingosine 1-phosphate (S1P)) and sphingosine kinases 1 and 2 (SK1 and SK2), which catalyse formation of S1P, are co-localised in the centrosome of mammalian cells, where they may participate in regulating mitosis. The centrosome is a site for active GTP-GDP cycling involving the G-protein, G(i) and tubulin, which are required for spindle pole organization and force generation during cell division. Therefore, the presence of S1P(5) (which normally functions as a plasma membrane guanine nucleotide exchange factor, GEF) and sphingosine kinases in the centrosome might suggest that S1P(5) may function as a ligand activated GEF in regulating G-protein-dependent spindle formation and mitosis. The addition of S1P to cells inhibits trafficking of S1P(5) to the centrosome, suggesting a dynamic shuttling endocytic mechanism controlled by ligand occupancy of cell surface receptor. We therefore propose that the centrosomal S1P(5) receptor might function as an intracellular target of S1P linked to regulation of mitosis.

Sphingosine kinase regulation and cardioprotection

Activation of sphingosine kinase/sphingosine-1-phosphate (SK/S1P)-mediated signalling has been recognized as critical for cardioprotection in response to acute ischaemia/reperfusion injury. Incubation of S1P with cultured cardiac myocytes subjected to hypoxia or treatment of isolated hearts either before ischaemia or at the onset of reperfusion (pharmacologic pre- or postconditioning) results in reduced myocyte injury. Synthetic agonists active at S1P receptors mimic these responses. Gene-targeted mice null for the SK1 isoform whose hearts are subjected to ischaemia/reperfusion injury exhibit increased infarct size and respond poorly either to ischaemic pre- or postconditioning. Measurements of cardiac SK activity and S1P parallel these observations. Ischaemic postconditioning combined with sphingosine and S1P rescues the heart from prolonged ischaemia. These observations may have considerable relevance for future therapeutic approaches to acute and chronic myocardial injury.

Volume-sensitive NADPH oxidase activity and taurine efflux in NIH3T3 mouse fibroblasts

Reactive oxygen species (ROS) are produced in NIH3T3 fibroblasts during hypotonic stress, and H(2)O(2) potentiates the concomitant release of the organic osmolyte taurine (Lambert IH. J Membr Biol 192: 19-32, 2003). The increase in ROS production [5-(and-6)-carboxy-2', 7'-dichlorodihydrofluorescein diacetate fluorescence] is detectable after a reduction in the extracellular osmolarity from 335 mosM (isotonic) to 300 mosM and reaches a maximal value after a reduction to 260 mosM. The swelling-induced ROS production is reduced by the flavoprotein inhibitor diphenylene iodonium chloride (25 microM) but is unaffected by the nitric oxide synthase inhibitor N omega-nitro-l-arginine methyl ester, indicating that the volume-sensitive ROS production is NADPH oxidase dependent. NIH3T3 cells express the NADPH oxidase components: p22 phox, a NOX4 isotype; p47 phox; and p67 phox (real-time PCR). Exposure to the Ca2+-mobilizing agonist ATP (10 microM) potentiates the release of taurine but has no effect on ROS production under hypotonic conditions. On the other hand, addition of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA, 100 nM) or the lipid messenger lysophosphatidic acid (LPA, 10 nM) potentiates the swelling-induced taurine release as well as the ROS production. Overexpression of Rac1 or p47 phox or p47 phox knockdown [small interfering (si)RNA] had no effect on the swelling-induced ROS production or taurine release. NOX4 knockdown (siRNA) impairs the increase in the ROS production and the concomitant taurine release following osmotic exposure. It is suggested that a NOX4 isotype plus p22 phox account for the swelling-induced increase in the ROS production in NIH3T3 cells and that the oxidase activity is potentiated by PKC and LPA but not by Ca2+.

K6PC-5, a sphingosine kinase activator, induces anti-aging effects in intrinsically aged skin through intracellular Ca2+ signaling

BACKGROUND

Sphingosine-1-phosphate (S1P), a bioactive sphingolipid metabolite, regulates multiple cellular responses such as Ca(2+) signaling, growth, survival, and differentiation. Because sphingosine kinase (SK) is the enzyme directly responsible for the production of S1P, many factors have been identified that regulate its activity and subsequent S1P levels. To date, there are no reports to demonstrate a chemically induced, direct activation of SK.

OBJECTIVE

Here we have studied the effects of K6PC-5 as a newly synthesized SK activator on fibroblast proliferation in both human fibroblasts and aged mouse skin. To demonstrate that K6PC-5 has S1P-mediated action mechanism in fibroblasts, we have measured SK-dependent intracellular Ca(2+) signaling.

METHODS

Fibroblasts were cultured primarily from human foreskin and were used to study the effect of K6PC-5 and S1P on intracellular Ca(2+) signaling and fibroblast proliferation. Changes in intracellular Ca(2+) were detected by fluorescence with fura-2/AM. To study skin anti-aging effects of K6PC-5, we used intrinsically aged hairless mice (56 weeks old).

RESULTS

K6PC-5 promoted fibroblast proliferation and procollagen production in human fibroblasts significantly. K6PC-5 induced intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations in human fibroblasts. Both dimethylsphingosine and dihydroxysphingosine, SK inhibitors, and the transfection of SK1-siRNA blocked the K6PC-5-induced increases in [Ca(2+)](i), an effect independent of the classical PLC/IP(3)-mediated pathway. The K6PC-5-induced [Ca(2+)](i) oscillations were dependent on thapsigargin-sensitive Ca(2+) stores and Ca(2+) entry. Topical application of K6PC-5 for 2 weeks to intrinsically aged hairless mice enhanced fibroblast proliferation, collagen production, and eventually increased dermal thickness (10%). K6PC-5 also promoted specific epidermal differentiation marker proteins, including involucrin, loricrin, filaggrin, and keratin 5, without any alterations on epidermal barrier function.

CONCLUSION

These results suggest that K6PC-5 acts to regulate fibroblast proliferation through intracellular S1P production, and can further promote keratinocyte differentiation. We anticipate that the regulation of S1P levels may represent a novel approach for the treatment of skin disorders, including skin aging.

Anti-apoptotic effect of sphingosine-1-phosphate and platelet-derived growth factor in human embryonic stem cells

Human embryonic stem (hES) cells hold great promise for use in regenerative medicine. However, technologies first need to be established to maintain hES cells efficiently in vitro. Understanding the signaling networks involved in hES cell maintenance will prove to be essential to the development of such culture systems. Previously, we described a serum-free medium capable of supporting prolonged hES cell maintenance using sphingosine-1-phosphate (S1P) and platelet-derived growth factor (PDGF). Here, we describe an anti-apoptotic effect of S1P and PDGF in hES cells and demonstrate a direct effect of S1P in preventing hES cell apoptosis. Western blot analysis shows that S1P stimulates the phosphorylation of the mitogen-activated protein (MAP) kinases Erk1/2 but not of Akt, whereas PDGF stimulates both Erk1/2 and Akt phosphorylation. Moreover, our study suggests that the Erk1/2 and PI3K/Akt signaling pathways act independently of each other. Furthermore, neither S1P nor PDGF modify intracellular calcium concentration ([Ca(2+)]( i )) and Smad2 phosphorylation. Using pharmacological inhibitors of Erk1/2 and PI3K, our results demonstrate a critical role of the Erk1/2 and PI3K/Akt signaling pathways in mediating the anti-apoptotic effect of S1P and PDGF on hES cells. However, inhibition of the mammalian target of rapamycin (mTOR), a common downstream effector of Erk1/2 and PI3K/Akt, has no effect on hES cell apoptosis.

Unraveling the complexities of sphingosine-1-phosphate function: the mast cell model

Sphingosine-1-phosphate (S1P) is a lipid mediator involved in diverse biological processes, from vascular and neural development to the regulation of lymphocyte trafficking. Many of its functions are regulated by five widely expressed S1P G-protein-coupled receptors (S1P(1-5)). S1P is produced mostly intracellularly, thus, much of its potential as an autocrine and paracrine mediator depends on how, when, and where it is generated or secreted out of the cells. However, S1P can also have intracellular activity independent of its receptors, adding to the complexity of S1P function. The mast cell, a major effector cell during an allergic response, has proven instrumental towards understanding the complex regulation and function of S1P. Antigen (Ag) engagement of the IgE receptor in mast cells stimulates sphingosine kinases, which generate S1P and are involved in the activation of calcium fluxes critical for mast cell responses. In addition, mast cells secrete considerable amounts of S1P upon activation, thus affecting the surrounding tissues and recruiting inflammatory cells. Export of S1P is also involved in the autocrine transactivation of S1P receptors present in mast cells. The in vivo response of mast cells, however, is not strictly dependent on their ability to generate S1P, but they are also affected by changes in S1P in the environment previous to Ag challenge. This review will discuss the recent advances towards understanding the intricacies of S1P generation, secretion and regulation in mast cells. In addition, how S1P receptors are activated and their involvement in mast cell functions will also be covered, including new insights on the role of S1P in the mast cell-mediated allergic response of systemic anaphylaxis.

Subtype-specific residues involved in ligand activation of the endothelial differentiation gene family lysophosphatidic acid receptors

Lysophosphatidic acid (LPA) is a ligand for three endothelial differentiation gene family G protein-coupled receptors, LPA(1-3). We performed computational modeling-guided mutagenesis of conserved residues in transmembrane domains 3, 4, 5, and 7 of LPA(1-3) predicted to interact with the glycerophosphate motif of LPA C18:1. The mutants were expressed in RH7777 cells, and the efficacy (E(max)) and potency (EC(50)) of LPA-elicited Ca(2+) transients were measured. Mutation to alanine of R3.28 universally decreased both the efficacy and potency in LPA(1-3) and eliminated strong ionic interactions in the modeled LPA complexes. The alanine mutation at Q3.29 decreased modeled interactions and activation in LPA(1) and LPA(2) more than in LPA(3). The mutation W4.64A had no effect on activation and modeled LPA interaction of LPA(1) and LPA(2) but reduced the activation and modeled interactions of LPA(3). The R5.38A mutant of LPA(2) and R5.38N mutant of LPA(3) showed diminished activation by LPA; however, in LPA(1) the D5.38A mutation did not, and mutation to arginine enhanced receptor activation. In LPA(2), K7.36A decreased the potency of LPA; in LPA(1) this same mutation increased the E(max). In LPA(3), R7.36A had almost no effect on receptor activation; however, the mutation K7.35A increased the EC(50) in response to LPA 10-fold. In LPA(1-3), the mutation Q3.29E caused a modest increase in EC(50) in response to LPA but caused the LPA receptors to become more responsive to sphingosine 1-phosphate (S1P). Surprisingly micromolar concentrations of S1P activated the wild type LPA(2) and LPA(3) receptors, indicating that S1P may function as a weak agonist of endothelial differentiation gene family LPA receptors.

Low concentrations of sphingosylphosphorylcholine enhance pulmonary artery vasoreactivity: the role of protein kinase C delta and Ca2+ entry

Sphingosylphosphorylcholine (SPC) is a powerful vasoconstrictor, but in vitro its EC(50) is approximately 100-fold more than plasma concentrations. We examined whether subcontractile concentrations of SPC (100 nmol/L of SPC, and independent of the endothelium, 2-aminoethoxydiphenylborane-sensitive Ca(2+) entry, and Rho kinase. It was abolished by the phospholipase C inhibitor U73122, the broad spectrum protein kinase C (PKC) inhibitor Ro31-8220, and the PKC delta inhibitor rottlerin, but not by Gö6976, which is ineffective against PKC delta. The potentiation could be attributed to enhancement of Ca(2+) entry. SPC also potentiated the responses to prostaglandin F(2 alpha) and U436619, which activate a 2-aminoethoxydiphenylborane sensitive nonselective cation channel in intrapulmonary arteries. In this case, potentiation was partially inhibited by diltiazem but abolished by 2-aminoethoxydiphenylborane, Ro31-8220, and rottlerin. SPC (1 micromol/L) caused translocation of PKC delta to the perinuclear region and cytoskeleton of cultured intrapulmonary artery smooth muscle cells. We present the novel finding that low, subcontractile concentrations of SPC potentiate Ca(2+) entry in intrapulmonary arteries through both voltage-dependent and independent pathways via a receptor-dependent mechanism involving PKC delta. This has implications for the physiological role of SPC, especially in cardiovascular disease, where SPC is reported to be elevated.

Sphingolipid metabolites selectively elicit increases in nuclear calcium concentration in cell suspension cultures and in isolated nuclei of tobacco

Sphingolipids are known to interfere with calcium-based signalling pathways. Here we report that these compounds modulate nuclear calcium signalling in tobacco BY-2 cells. Nuclear protein kinase activity phosphorylated endogenous sphingoid long-chain bases (LCBs), suggesting that LCBs are actively metabolized in the nucleus of tobacco BY-2 cells. The Delta4-unsaturated LCB D-erythro-sphingosine and the saturated LCB D-ribo-phytosphingosine elicited increases in free calcium in the nucleus in a dose-dependent and structure-related manner. However, neither sphingosine-1-phosphate nor C2-ceramide was able to stimulate nuclear calcium changes. N-,N-Dimethyl-D-erythro-sphingosine, a structural analogue of D-erythro-sphingosine, was the most efficient LCB so far tested in eliciting nuclear calcium changes both in intact tobacco BY-2 cells and in isolated nuclei. TRP channel inhibitors prevent the effect of DMS, suggesting that LCBs may activate TRP-like channels located on the inner nuclear membrane Collectively, the obtained data show that nuclei respond to LCBs on their own independently of the cytosolic compartment.

Lysophospholipid receptors in cell signaling

There is increasing evidence that different phospholipids are involved in regulation of various cell processes and cell-cell interactions. Lysophospholipids (lysophosphatidic acid, lysophosphatidylcholine) and a number of lysosphingolipids play particular roles in these regulations. Their effects are mediated by specific G-protein-coupled receptors. G-Protein coupled signal transduction to the cell nucleus involving a chain of intracellular protein kinases induces the main effects in cells--growth, proliferation, survival, or apoptosis. This review summarizes recent data on various groups of lysophospholipid receptors and their cell signal transduction pathways.

Sphingosine‐1‐phosphate and the immunosuppressant, FTY720‐phosphate, regulate detrusor muscle tone

Overactive bladder syndrome (OBS) results from disturbances of bladder function. Bladder smooth muscle (detrusor) exhibits spontaneous rhythmic activity (tone) independent of neurogenic control, which is enhanced in patients with OBS. We have now uncovered a prominent role for the bioactive sphingolipid metabolite, sphingosine-1-phosphate (S1P), in regulating rabbit detrusor smooth muscle tone and contraction. S1P-induced contraction of detrusor muscle was dependent on stretch and intracellular calcium. Although detrusor expresses the S1P receptors S1P1 and S1P2, only S1P2 appeared to be involved in S1P-induced contraction, since SEW2871 (S1P1 agonist) and dihydro-S1P (potent agonist for all S1P receptors except S1P2) were poor contractile agents. In agreement, the S1P2 antagonist JTE013 inhibited S1P-induced contraction. The fast, transient muscle contraction (phasic) mediated by S1P was dependent on phospholipase C (PLC) whereas the slower, sustained contraction (tonic) was not. Surprisingly, the immunosuppressant FTY720-phosphate, an agonist for all S1P receptors except S1P2, had distinct contractile properties and also induced slow, sustained contraction. Thus, FTY720-phosphate and/or S1P may regulate calcium channels in an S1P receptor-independent manner. Collectively, our results demonstrate that S1P may regulate detrusor smooth muscle tone and suggest that dysregulation of complex S1P signaling might contribute to OBS.