Sphingosine 1-Phosphate Produced by Sphingosine Kinase 2 Intrinsically Controls Platelet Aggregation In Vitro and In Vivo

RATIONALE

Platelets are known to play a crucial role in hemostasis. Sphingosine kinases (Sphk) 1 and 2 catalyze the conversion of sphingosine to the bioactive metabolite sphingosine 1-phosphate (S1P). Although platelets are able to secrete S1P on activation, little is known about a potential intrinsic effect of S1P on platelet function.

OBJECTIVE

To investigate the role of Sphk1- and Sphk2-derived S1P in the regulation of platelet function.

METHODS AND RESULTS

We found a 100-fold reduction in intracellular S1P levels in platelets derived from Sphk2(-/-) mutants compared with Sphk1(-/-) or wild-type mice, as analyzed by mass spectrometry. Sphk2(-/-) platelets also failed to secrete S1P on stimulation. Blood from Sphk2-deficient mice showed decreased aggregation after protease-activated receptor 4-peptide and adenosine diphosphate stimulation in vitro, as assessed by whole blood impedance aggregometry. We revealed that S1P controls platelet aggregation via the sphingosine 1-phosphate receptor 1 through modulation of protease-activated receptor 4-peptide and adenosine diphosphate-induced platelet activation. Finally, we show by intravital microscopy that defective platelet aggregation in Sphk2-deficient mice translates into reduced arterial thrombus stability in vivo.

CONCLUSIONS

We demonstrate that Sphk2 is the major Sphk isoform responsible for the generation of S1P in platelets and plays a pivotal intrinsic role in the control of platelet activation. Correspondingly, Sphk2-deficient mice are protected from arterial thrombosis after vascular injury, but have normal bleeding times. Targeting this pathway could therefore present a new therapeutic strategy to prevent thrombosis.

Partial deficiency of sphingosine-1-phosphate lyase confers protection in experimental autoimmune encephalomyelitis

Background

Sphingosine-1-phosphate (S1P) regulates the egress of T cells from lymphoid organs; levels of S1P in the tissues are controlled by S1P lyase (Sgpl1). Hence, Sgpl1 offers a target to block T cell-dependent inflammatory processes. However, the involvement of Sgpl1 in models of disease has not been fully elucidated yet, since Sgpl1 KO mice have a short life-span.

Methodology

We generated inducible Sgpl1 KO mice featuring partial reduction of Sgpl1 activity and analyzed them with respect to sphingolipid levels, T-cell distribution, and response in models of inflammation.

Principal Findings

The partially Sgpl1 deficient mice are viable but feature profound reduction of peripheral T cells, similar to the constitutive KO mice. While thymic T cell development in these mice appears normal, mature T cells are retained in thymus and lymph nodes, leading to reduced T cell numbers in spleen and blood, with a skewing towards increased proportions of memory T cells and T regulatory cells. The therapeutic relevance of Sgpl1 is demonstrated by the fact that the inducible KO mice are protected in experimental autoimmune encephalomyelitis (EAE). T cell immigration into the CNS was found to be profoundly reduced. Since S1P levels in the brain of the animals are unchanged, we conclude that protection in EAE is due to the peripheral effect on T cells, leading to reduced CNS immigration, rather than on local effects in the CNS.

Significance

The data suggest Sgpl1 as a novel therapeutic target for the treatment of multiple sclerosis.

Ovalbumin-induced plasma interleukin-4 levels are reduced in ceramide kinase-deficient DO11.10 RAG1-/- mice

Ceramide kinase (CERK) produces the bioactive lipid ceramide-1-phosphate (C1P) and is a key regulator of ceramide and dihydroceramide levels. It is likely that CERK and C1P play a role in inflammatory processes but the cells involved and the mechanisms used remain to be clarified. In particular, the impact of CERK on T-cell biology has not been studied so far. Here, we used Cerk-/- mice backcrossed with DO11.10/RAG1-/- mice to probe the effect of CERK ablation on T-cell activation. Levels of interleukin (IL)-2, IL-4, IL-5, IL-13, of tumor necrosis factor (TNF)-alpha, and of interferon (INF)-gamma were recorded following ovalbumin challenge in vivo and using ovalbumin-treated splenocytes ex- vivo. Absence of CERK led to a significant decrease in the production of IL-4, thus suggesting that CERK may polarize T cells towards the TH2 cell subtype. However, the importance of CERK to TH2 cell biology will have to be investigated further because in a model of asthma, which is TH2-cell driven, Cerk-/- mice responded like wild-type animals.

Sphingosine kinase 1 is essential for proteinase-activated receptor-1 signalling in epithelial and endothelial cells

There is accumulating evidence that activation of sphingosine kinase 1 (SPHK1) is an important element in intracellular signalling cascades initiated by stimulation of multiple receptors, including certain growth factor, cytokine, and also G-protein coupled receptors. We here report that stimulation of the lung epithelial cell line A549 by thrombin leads to transient increase of SPHK1 activity and elevation of intracellular sphingosine-1-phosphate (S1P); abrogation of this stimulation by SPHK1-specific siRNA, pharmacological inhibition, or expression of a dominant-negative SPHK1 mutant blocks the response to thrombin, as measured by secretion of MCP-1, IL-6, IL-8, and PGE(2). Using selective stimulation of proteinase-activated receptors (PARs) a specific involvement of SPHK1 in the PAR-1 induced responses in A549 cell, including activation of NFkappaB, was evident, while PAR-2 and PAR-4 responses were independent of SPHK1. Moreover, PAR-1 or thrombin-induced cytokine production and adhesion factor expression of human umbilical vein endothelial cells was also seen to depend on SPHK1. Using dermal microvascular endothelial cells from SPHK1-deficient mice, we showed that absence of the enzyme abrogates MCP-1 production induced in these cells upon treatment with thrombin or PAR-1 activating peptide. We propose SPHK1 inhibition as a novel way to block PAR-1 mediated signalling, which could be useful in treatment of a number of diseases, in particular in atherosclerosis.

Neutropenia with impaired immune response to Streptococcus pneumoniae in ceramide kinase-deficient mice

In mammals, ceramide kinase (CerK)-mediated phosphorylation of ceramide is the only known pathway to ceramide-1-phosphate (C1P), a recently identified signaling sphingolipid metabolite. To help delineate the roles of CerK and C1P, we knocked out the gene of CerK in BALB/c mice by homologous recombination. All in vitro as well as cell-based assays indicated that CerK activity is completely abolished in Cerk-/- mice. Labeling with radioactive orthophosphate showed a profound reduction in the levels of de novo C1P formed in Cerk-/- macrophages. Consistently, mass spectrometry analysis revealed a major contribution of CerK to the formation of C16-C1P. However, the significant residual C1P levels in Cerk-/- animals indicate that alternative routes to C1P exist. Furthermore, serum levels of proapoptotic ceramide in these animals were significantly increased while levels of dihydroceramide as the biosynthetic precursor were reduced. Previous literature pointed to a role of CerK or C1P in innate immune cell function. Using a variety of mechanistic and disease models, as well as primary cells, we found that macrophage- and mast cell-dependent readouts are barely affected in the absence of CerK. However, the number of neutrophils was strikingly reduced in blood and spleen of Cerk-/- animals. When tested in a model of fulminant pneumonia, Cerk-/- animals developed a more severe disease, lending support to a defect in neutrophil homeostasis following CerK ablation. These results identify ceramide kinase as a key regulator of C1P, dihydroceramide and ceramide levels, with important implications for neutrophil homeostasis and innate immunity regulation.

Targeting ceramide metabolism with a potent and specific ceramide kinase inhibitor

Ceramide kinase (CerK) produces the bioactive lipid ceramide-1-phosphate (C1P) and appears as a key enzyme for controlling ceramide levels. In this study, we discovered and characterized adamantane-1-carboxylic acid (2-benzoylamino-benzothiazol-6-yl)amide (NVP-231), a potent, specific, and reversible CerK inhibitor that competitively inhibits binding of ceramide to CerK. NVP-231 is active in the low nanomolar range on purified as well as cellular CerK and abrogates phosphorylation of ceramide, resulting in decreased endogenous C1P levels. When combined with another ceramide metabolizing inhibitor, such as tamoxifen, NVP-231 synergistically increased ceramide levels and reduced cell growth. Therefore, NVP-231 represents a novel and promising compound for controlling ceramide metabolism that may provide insight into CerK physiological function.

FTY720 rescue therapy in the dark agouti rat model of experimental autoimmune encephalomyelitis: expression of central nervous system genes and reversal of blood-brain-barrier damage

FTY720 (fingolimod) is an oral sphingosine-1 phosphate (S1P) receptor modulator in phase III development for the treatment of multiple sclerosis. To further investigate its mode of action, we analyzed gene expression in the central nervous system (CNS) during experimental autoimmune encephalomyelitis (EAE). FTY720 downregulated inflammatory genes in addition to vascular adhesion molecules. It decreased the matrix metalloproteinase gene MMP-9 and increased its counterregulator--tissue inhibitor of metalloproteinase, TIMP-1--resulting in a proteolytic balance that favors preservation of blood-brain-barrier (BBB) integrity. Furthermore, FTY720 reduced S1P lyase that increases the S1P concentration in the brain, in line with a marked reversal of neurological deficits and raising the possibility for enhanced triggering of S1P receptors on resident brain cells. This is accompanied by an increase in S1P(1) and S1P(5) in contrast with the attenuation of S1P(3) and S1P(4). Late-stage rescue therapy with FTY720, even up to 1 month after EAE onset, reversed BBB leakiness and reduced demyelination, along with normalization of neurologic function. Our results indicate rapid blockade of ongoing disease processes by FTY720, and structural restoration of the CNS parenchyma, which is likely caused by the inhibition of autoimmune T cell infiltration and direct modulation of microvascular and/or glial cells.

Sphingolipid metabolizing enzymes as novel therapeutic targets

Pharmacological interference with sphingolipid metabolizing enzymes promises to provide novel ways to modulate cellular pathways relevant in multiple diseases. In this review, we focus on two sphingolipid signaling molecules, sphingosine-1-phosphate (S1P) and ceramide, as they are involved in cell fate decisions (survival vs. apoptosis) and in a wide range of pathophysiological processes. For S1P, we will discuss sphingosine kinases and S1P lyase as the enzymes which are crucial for its production and degradation, respectively, emphasizing the potential therapeutic usefulness of inhibitors of these enzymes. For ceramide, we will concentrate on acid sphingomyelinase, and critically review the substantial literature which implicates this enzyme as a worthwhile target for pharmacological inhibitors. It will become clear that the task to validate these enzymes as drug targets is not finished and many questions regarding the therapeutic usefulness of their inhibitors remain unanswered. Still this approach holds promise for a number of totally new therapies, and, on the way, detailed insight into sphingolipid signaling pathways can be gained.

FTY720-phosphate is dephosphorylated by lipid phosphate phosphatase 3

FTY720 is a novel immunomodulatory drug efficacious in the treatment of multiple sclerosis. The drug is converted in vivo to the monophosphate, FTY720-P, by sphingosine kinase 2. This conversion is incomplete, suggesting opposing actions of kinase and phosphatase activities. To address which of the known lipid phosphatases might dephosphorylate FTY720-P, we overexpressed the broad specificity lipid phosphatases LPP1-3, and the specific S1P phosphatases (SPP1 and 2) in HEK293 cells, and performed in vitro assays using lysates of transfected cells. Among LPPs, only LPP3 was able to dephosphorylate FTY720-P; among SPPs, only SPP1 showed activity against FTY720-P. On intact cells, LPP3 acted as an ecto-phosphatase or FTY720-P, thus representing the major phosphatase involved in the equilibrium between FTY720 and FTY720-P observed in vivo.

FTY720, an immunomodulatory sphingolipid mimetic: translation of a novel mechanism into clinical benefit in multiple sclerosis

FTY720 (fingolimod; 2-amino-2[2-(4-octylphenyl)ethyl]-1,3-propanediol, Novartis) is the prototype of a new generation of immunomodulators. The drug is the result of extensive chemical derivatisation based on the natural product myriocin, isolated from the ascomycete Isaria sinclairii. FTY720 bears structural similarity to sphingosine, a naturally occurring sphingolipid. As with sphingosine, FTY720 is effectively phosphorylated by sphingosine kinases in vivo and the phosphorylated drug targets G-protein-coupled receptors for sphingosine-1-phosphate (S1P). Gene deletion and reverse pharmacology studies have shown that FTY720 acts at S1P1 receptors on lymphocytes and the endothelium, thereby inhibiting the egress of T- and B cells from secondary lymphoid organs into the blood and their recirculation to inflamed tissues. Animal studies suggest that this novel mechanism translates into effective treatments for several autoimmune diseases and a recently completed Phase II clinical trial highlighted FTY720 as a potential therapy for relapsing-remitting multiple sclerosis.

Sphingosine kinase inhibitor suppresses a Th1 polarization via the inhibition of immunostimulatory activity in murine bone marrow-derived dendritic cells

Sphingosine kinase (Sphk) has been shown to be activated by growth factor and survival factors, and one of its products, sphingosine-1-phosphate, plays an important role in the regulation of various cellular responses. However, the effect of Sphk on the maturation and immunostimulatory function of dendritic cells (DCs) still remains largely unknown. In this study, we examined whether sphingosine kinase inhibitor (SKI) can influence co-stimulatory molecules (CD40, CD80, CD86 and MHC class II) and cytokine production (IL-12 and IL-10) in murine bone marrow-derived DCs. SKI significantly inhibited co-stimulatory molecules in DCs. SKI suppressed IL-12 production by DCs and IFN-gamma production by T cells. In addition, SKI-inhibited LPS induced the translocation of nuclear factor-kappaB, whereas it did not affect the degradation of IL-1 receptor-associated kinase-1 by LPS. These novel findings provide new insight into the immunopharmacological role of SKI in terms of its effects on DCs. These findings open a possibility for further understanding of the immunopharmacological functions of SKI, as well as therapeutic adjuvants for the treatment of DC-related acute and chronic diseases.

Normal neutrophil functions in sphingosine kinase type 1 and 2 knockout mice

Sphingosine kinase (SPHK) has been implicated as an important element in neutrophil responses to diverse stimulatory agents. To get more insight into the role of the type 1 and 2 isoforms of SPHK in neutrophil functions, we made use of the respective SPHK knockout mice. Neutrophils isolated from the bone marrow of these mice showed normal increase of intracellular Ca(2+) when stimulated in vitro by fMLP, platelet-activating factor, the anaphylatoxin C5a, or ATP, and normal migration towards fMLP and C5a. Also, recruitment of neutrophils into the peritoneum towards the chemokines KC and MIP-2 or to LPS, and into the peripheral blood after fMLP injection was similar in SPHK knockout strains and wild-type animals. An in vivo model of bacterial lung infection revealed an accelerated progression of disease in SPHK2 (but not SPHK1) knockout mice as compared to wild-type controls. However, effector functions of SPHK-deficient neutrophils, such as superoxide production, beta-glucuronidase release and their capacity to kill bacteria were unchanged as compared to wild-type cells. To conclude, the data derived from SPHK knockout mice do not support the hypothesis that any of the two lipid kinases plays a crucial role in signalling downstream of various neutrophil stimuli; SPHKs appear not to be essential for neutrophil recruitment and effector functions.

A critical beta6-beta7 loop in the pleckstrin homology domain of ceramide kinase

CerK (ceramide kinase) produces ceramide 1-phosphate, a sphingophospholipid with recognized signalling properties. It localizes to the Golgi complex and fractionates essentially between detergent-soluble and -insoluble fractions; however, the determinants are unknown. Here, we made a detailed mutagenesis study of the N-terminal PH domain (pleckstrin homology domain) of CerK, based on modelling, and identified key positively charged amino acid residues within an unusual motif in the loop interconnecting beta-strands 6 and 7. These residues are critical for CerK membrane association and polyphosphoinositide binding and activity. Their mutagenesis results in increased thermolability, sensitivity to proteolysis, reduced apparent molecular mass as well as propensity of the recombinant mutant protein to aggregate, indicating that this loop impacts the overall conformation of the CerK protein. This is in contrast with most PH domains whose function strongly relies on charges located in the beta1-beta2 loop.

Effects of ceramide-1-phosphate on cultured cells: dependence on dodecane in the vehicle

Ceramide-1-phosphate (C1P), the product of ceramide kinase, is a sphingophospholipid with recently recognized signaling properties. In particular, it was reported to be mitogenic and capable of direct stimulation of cytosolic phospholipase A(2alpha). Much of the present knowledge has relied on the use of C1P of various acyl chain lengths, together with diverse protocols to deliver it to cultured cells. A mixture of ethanol (or methanol) with dodecane, as the vehicle, has become popular. However, the contribution of this solvent to the observed effects of C1P has not been documented. Here, we show that addition of C1P in ethanol-dodecane to culture medium leads to irreversible cytotoxic effects. These culminate in mitochondrial swelling, vacuole formation, and cell death. Not only the toxicity of C1P, but also its ability to trigger prostaglandin E2 release, is fully dependent upon addition of a premade C1P-dodecane mixture. Furthermore, we show that these effects are not restricted to C1P. They result from the capacity of dodecane to interact with phospholipids; hence, they go undetected with a vehicle control. This study should raise awareness about the use of dodecane for phospholipid delivery and, in turn, help in unraveling C1P signaling, which is still poorly understood.

n-Butyrate inhibits Jun NH(2)-terminal kinase activation and cytokine transcription in mast cells

Mast cells are well known to contribute to type I allergic conditions but only recently have been brought in association with chronic relapsing/remitting autoimmune diseases such as celiac disease and ulcerative colitis. Since the bacterial metabolite n-butyrate is considered to counteract intestinal inflammation we investigated the effects of this short chain fatty acid on mast cell activation. Using RNAse protection assays and reporter gene technology we show that n-butyrate downregulates TNF-alpha transcription. This correlates with an impaired activation of the Jun NH(2)-terminal kinase (JNK) but not other MAP kinases such as ERK and p38 that are largely unaffected by n-butyrate. As a consequence, we observed a decreased nuclear activity of AP-1 and NF-AT transcription factors. These results indicate that n-butyrate inhibits critical inflammatory mediators in mast cells by relatively selectively targeting the JNK signalling.

Sphingosine 1-phosphate phosphatase 2 is induced during inflammatory responses

Sphingosine 1-phosphate (S1P) levels in cells and, consequently, its bioactivity as a signalling molecule are controlled by the action of enzymes responsible for its synthesis and degradation. In the present report, we examined alterations in expression patterns of enzymes involved in S1P-metabolism (sphingosine kinases including their splice variants, sphingosine 1-phosphate phosphatases, and sphingosine 1-phosphate lyase) under certain inflammatory conditions. We found that sphingosine kinase type 1 (SPHK1) mRNA could be triggered in a cell type-specific manner; individual SPHK1 splice variants were induced with similar kinetics. Remarkably, expression and activity of S1P phosphatase 2 (SPP2) was found to be highly upregulated by inflammatory stimuli in a variety of cells (e.g., neutrophils, endothelial cells). Bandshift analysis using oligonucleotides spanning predicted NFkappaB sites within the SPP2 promoter and silencing of NFkappaB/RelA via RelA-directed siRNA demonstrated that SPP2 is an NFkappaB-dependent gene. Silencing of SPP2 expression in endothelial cells, in turn, led to a marked reduction of TNF-alpha-induced IL-1beta mRNA and protein and to a partial reduction of induced IL-8, suggesting a pro-inflammatory role of SPP2. Notably, up-regulation of SPP2 was detected in samples of lesional skin of patients with psoriasis, an inflammatory skin disease. This study provides detailed insights into the regulation of SPP2 gene expression and suggests that SPP2 might be a novel player in pro-inflammatory signalling.

Pulmonary and vascular pharmacology of sphingosine 1-phosphate

Dysregulation of vasomotor tone, endothelial barrier function and immune cell trafficking are central to the pathology of many lung diseases, including acute lung injury, adult respiratory distress syndrome, chronic obstructive pulmonary disease and asthma. There is increasing evidence that the serum sphingolipid sphingosine 1-phosphate and its G-protein-coupled receptors are pivotal not only in the regulation of lymphocyte migration, but also in the maintenance of vascular homeostasis and the preservation of permeability barriers that separate discrete compartments in the lung.

NBD-labeled derivatives of the immunomodulatory drug FTY720 as tools for metabolism and mode of action studies

Fluorescently labeled chiral analogs of the immunomodulatory drug FTY720 and its corresponding phosphates with variable aliphatic spacers between the aromatic ring and the NBD label have been synthesized. Determining the influence of the spacer on the in vitro phosphorylation rate by SPHK1 and 2 resulted in the identification of NBD-(R)-AAL 1c,d which are phosphorylated with an efficiency comparable to that of the unlabeled FTY720 analog (R)-AAL. Furthermore, the NBD-(R)-AAL phosphates 10c,d were proven to be a functional S1P receptor agonist.

IgE-dependent activation of sphingosine kinases 1 and 2 and secretion of sphingosine 1-phosphate requires Fyn kinase and contributes to mast cell responses

Engagement of the high affinity receptor for IgE (FcepsilonRI) on mast cells results in the production and secretion of sphingosine 1-phosphate (S1P), a lipid metabolite present in the lungs of allergen-challenged asthmatics. Herein we report that two isoforms of sphingosine kinase (SphK1 and SphK2) are expressed and activated upon FcepsilonRI engagement of bone marrow-derived mast cells (BMMC). Fyn kinase is required for FcepsilonRI coupling to SphK1 and -2 and for subsequent S1P production. Normal activation of SphK1 and -2 was restored by expression of wild type Fyn but only partly with a kinase-defective Fyn, indicating that induction of SphK1 and SphK2 depended on both catalytic and noncatalytic properties of Fyn. Downstream of Fyn, the requirements for SphK1 activation differed from that of SphK2. Whereas SphK1 was considerably dependent on the adapter Grb2-associated binder 2 and phosphatidylinositol 3-OH kinase, SphK2 showed minimal dependence on these molecules. Fyn-deficient BMMC were defective in chemotaxis and, as previously reported, in degranulation. These functional responses were partly reconstituted by the addition of exogenous S1P to FcepsilonRI-stimulated cells. Taken together with our previous study, which demonstrated delayed SphK activation in Lyn-deficient BMMC, we propose a cooperative role between Fyn and Lyn kinases in the activation of SphKs, which contributes to mast cell responses.

Sphingosine kinase type 2 is essential for lymphopenia induced by the immunomodulatory drug FTY720

FTY720, a potent immunomodulatory drug in phase 2/3 clinical trials, induces rapid and reversible sequestration of lymphocytes into secondary lymphoid organs, thereby preventing their migration to sites of inflammation. As prerequisite for its function, phosphorylation of FTY720 to yield a potent agonist of the sphingosine-1-phosphate receptor S1P(1) is required in vivo, catalyzed by an as-yet-unknown kinase. Here, we report on the generation of sphingosine kinase 2 (SPHK2) knockout mice and demonstrate that this enzyme is essential for FTY720 phosphate formation in vivo. Consequently, administration of FTY720 does not induce lymphopenia in SPHK2-deficient mice. After direct dosage of FTY720 phosphate, lymphopenia is only transient in this strain, indicating that SPHK2 is constantly required to maintain FTY720 phosphate levels in vivo.

The role of sphingosine and ceramide kinases in inflammatory responses

The 1-phosphates of sphingosine and ceramide (S1P and C1P) have emerged as key representatives of a new group of lipid signalling molecules. S1P is known to act both as an extracellular mediator and as an intracellular 'second messenger,' while C1P currently is only known for its intracellular actions. Therefore, sphingosine and ceramide kinases, the enzymes involved in the generation of these lipid mediators, are now in the spotlight. This review summarizes current information on structure, localization, substrate specificity, activation, and binding partners of these kinases, and then focuses on discoveries in relation to immune cell regulation and inflammation, addressing in particular mast cell activation and degranulation, IL-12 signalling, prostaglandin biosynthesis, monocyte activation, and neutrophil priming.

Characterization of a ceramide kinase-like protein

Ceramide is a key player governing cell fate, and its conversion to ceramide-1-phosphate by ceramide kinase (CERK) is emerging as an important mean to regulate apoptosis and inflammatory processes. We identified a new ceramide kinase homolog, designated CERK-like protein (CERKL) and we compared it to the known CERK. Real time-PCR analysis of human tissues revealed a restricted pattern of expression for CERKL mRNA. Surprisingly, various ceramides, known substrates for CERK, were not phosphorylated by CERKL in vitro. Upon 32P(i)-pulse labeling of COS-1 cells transiently expressing CERKL, or incubation with NBD-C6-ceramide, ceramide-1-phosphate was not detected. After recombinant expression in COS-1 cells, CERKL was partially recovered in the soluble fraction, as a phosphorylated protein. Live cell imaging indicated localization of GFP-tagged CERKL to many cell compartments, including specific association with nucleoli. Two splice variants of CERKL did not localize to nucleoli nor did a CERKL variant with a point mutation in the putative ATP binding site. We also studied a naturally occurring CERKL mutant (R257X), recently linked to the pathology of retinitis pigmentosa. It accumulated in the nucleus but was not associated with nucleoli. Treatment with the calcium ionophore A23187 led to clearing of CERKL from nucleoli, but had no effect on the R257X CERKL mutant. Collectively, although kinase activity of CERKL remains to be proven, these findings suggest a functional link between CERKL and its nucleolar localization. Furthermore, we propose that the cause for retinitis pigmentosa in patients bearing the CERKL R257X mutation might be the accumulation of a truncated CERKL protein in the nucleus.

Basal and induced sphingosine kinase 1 activity in A549 carcinoma cells: function in cell survival and IL-1beta and TNF-alpha induced production of inflammatory mediators

Sphingosine-1-phosphate, a lipid mediator produced by sphingosine kinases, regulates diverse cellular processes, ranging from cell growth and survival to effector functions, such as proinflammatory mediator synthesis. Using human A549 epithelial lung carcinoma cells as a model system, we observed transient upregulation of sphingosine kinase type 1 (SPHK1) enzyme activity upon stimulation with both TNF-alpha or IL-1beta. This transient activation of SPHK1 was found to be required for cytokine-induced COX-2 transcription and PGE2 production, since not only specific siRNA (abolishing both basal and induced SPHK1 enzyme activity), but also a dominant-negative SPHK1 mutant (suppressing induced SPHK1 activity only) both reduced COX-2 and PGE2. Furthermore, TNF-alpha- or IL-1beta-induced transcription of selected cytokines, chemokines, and adhesion molecules (IL-6, RANTES, MCP-1, and VCAM-1) was found to require SPHK1 activation. Suppression of SPHK1 activation led to reduction of cytokine-induced IkappaBalpha phosphorylation and consequently diminished NFkappaB activity due to reduced nuclear translocation of RelA (p65), explaining the dependence of inflammatory mediator production on SPHK1 activation. Inhibition of basal SPHK1 activity by N,N-dimethylsphingosine or by downregulation of its expression using siRNA induced spontaneous apoptosis in A549 cells, an effect that can be explained through interference with constitutive NFkappaB activity in this cell type. In contrast, expression of the dominant-negative mutant did not induce apoptosis. Taken together, these findings demonstrate a role of SPHK1 activation in proinflammatory signalling and of SPHK1 basal activity in survival of A549 lung carcinoma cells.

Ceramide kinase targeting and activity determined by its N-terminal pleckstrin homology domain

The N-terminus of ceramide kinase (CERK) is thought to be myristoylated and to contain a pleckstrin homology (PH) domain. We found that deletion of this region (DeltaPH-CERK) ablates activity. This is not due to prevention of N-terminal myristoylation since a G2A CERK mutant, which cannot be myristoylated, was active. CERK was able to bind liposomes, as well as the isolated unmyristoylated PH domain; DeltaPH-CERK was not. Upon analysis of EGFP-tagged proteins, CERK was found associated with the Golgi complex. Osmotic cell swelling induced translocation of CERK to the plasma membrane and formation of large vesicles displaying bound CERK. None of these features occurred with DeltaPH-CERK, which remained disseminated throughout the cytoplasm. These findings show that the PH domain of CERK is essential for localization, translocation, and activity of this lipid kinase.

Early activation of sphingosine kinase in mast cells and recruitment to FcepsilonRI are mediated by its interaction with Lyn kinase

Sphingosine kinase has been recognized as an essential signaling molecule that mediates the intracellular conversion of sphingosine to sphingosine-1-phosphate. In mast cells, induction of sphingosine kinase and generation of sphingosine-1-phosphate have been linked to the initial rise in Ca(2+), released from internal stores, and to degranulation. These events either precede or are concomitant with the activation of phospholipase C-gamma and the generation of inositol trisphosphate. Here we show that sphingosine kinase type 1 (SPHK1) interacts directly with the tyrosine kinase Lyn and that this interaction leads to the recruitment of this lipid kinase to the high-affinity receptor for immunoglobulin E (FcepsilonRI). The interaction of SPHK1 with Lyn caused enhanced lipid and tyrosine kinase activity. After FcepsilonRI triggering, enhanced sphingosine kinase activity was associated with FcepsilonRI in sphingolipid-enriched rafts of mast cells. Bone marrow-derived mast cells from Lyn(-/)(-) mice, compared to syngeneic wild-type cells, were defective in the initial induction of SPHK1 activity, and the defect was overcome by retroviral Lyn expression. These findings position the activation of SPHK1 as an FcepsilonRI proximal event.

Fluorescence-labeled sphingosines as substrates of sphingosine kinases 1 and 2

Fluorescently labeled d-erythro-sphingosines have been successfully synthesized in 9 and 12 steps starting from commercially available Garner aldehyde. Determining the influence of the nature, position and linkage of the label on the in vitro phosphorylation rate by sphingosine kinases 1 and 2 resulted in the identification of a pyrene- and a NBD-labeled sphingosine which are both phosphorylated with efficiency comparable to the natural substrate.

Activation of mast cells by incorporation of cholesterol into rafts

IgE plus antigen-stimulated mast cells degranulate, synthesize leukotrienes and secrete cytokines. According to the coalescence model this process is initiated in specific membrane compartments termed rafts. There, enhanced levels of glycosphingolipids and cholesterol stabilize the interaction of FcepsilonRI and Lyn, and thus facilitate the first steps of signal transduction. Enforced changes in raft architecture by cholesterol deprivation and exogenous application of glycosphingolipids influence these early events by loss of tyrosine kinase activity or receptor-independent signal initiation respectively. Here we show that exogenously added cholesterol accumulates in rafts and activates mast cells. An investigation of the signaling events reveals that in contrast to IgE plus antigen-mediated stimulation, cholesterol triggers p38 mitogen-activated protein kinase and preferentially induces expression of FosB. Consequently, a comparative large-scale microarray analysis demonstrates that a number of IgE plus antigen-induced immediate early genes (peak expression at 30 min after induction) are repressed by cholesterol. These changes further translate into altered expression levels and time kinetics of a number of early genes (peak expression at 2 h after stimulation). As the most prominent example for cholesterol-dependent genes, we identified PAI1 (plasminogen activator inhibitor 1), a protein regarded as a risk factor for atherosclerosis.

Phosphorylation of the immunomodulatory drug FTY720 by sphingosine kinases

The immunomodulatory drug FTY720 is phosphorylated in vivo, and the resulting FTY720 phosphate as a ligand for sphingosine-1-phosphate receptors is responsible for the unique biological effects of the compound. So far, phosphorylation of FTY720 by murine sphingosine kinase (SPHK) 1a had been documented. We found that, while FTY720 is also phosphorylated by human SPHK1, the human type 2 isoform phosphorylates the drug 30-fold more efficiently, because of a lower Km of FTY720 for SPHK2. Similarly, murine SPHK2 was more efficient than SPHK1a. Among splice variants of the human SPHKs, an N-terminally extended SPHK2 isoform was even more active than SPHK2 itself. Further SPHK superfamily members, namely ceramide kinase and a "SPHK-like" protein, failed to phosphorylate sphingosine and FTY720. Thus, only SPHK1 and 2 appear to be capable of phosphorylating FTY720. Using selective assay conditions, SPHK1 and 2 activities in murine tissues were measured. While activity of SPHK2 toward sphingosine was generally lower than of SPHK1, FTY720 phosphorylation was higher under conditions favoring SPHK2. In human endothelial cells, while activity of SPHK1 toward sphingosine was 2-fold higher than of SPHK2, FTY720 phosphorylation was 7-fold faster under SPHK2 assay conditions. Finally, FTY720 was poorly phosphorylated in human blood as compared with rodent blood, in line with the low activity of SPHK1 and in particular of SPHK2 in human blood. To conclude, both SPHK1 and 2 are capable of phosphorylating FTY720, but SPHK2 is quantitatively more important than SPHK1.

Prevention of CD40-triggered dendritic cell maturation and induction of T-cell hyporeactivity by targeting of Janus kinase 3

Pharmacological targeting of Janus kinase 3 (JAK3) has been employed successfully to control allograft rejection and graft-vs.-host disease (GVHD). Recent evidence suggests that in addition to its involvement in common-gamma chain (cgamma) signaling of cytokine receptors, JAK3 is also engaged in the CD40 signaling pathway of peripheral blood monocytes. In this study, we assessed the consequences of JAK3 inhibition during CD40-induced maturation of myeloid dendritic cells (DCs), and tested the impact thereof on the induction of T-cell alloreactivity. Dendritic cells triggering through CD40 induced JAK3 activity, the expression of costimulatory molecules, production of IL-12, and potent allogeneic stimulatory capacity. In contrast, JAK3 inhibition with the rationally designed JAK3 inhibitor WHI-P-154 prevented these effects arresting the DCs at an immature level. Interestingly, DCs exposed to the JAK3-inhibitor during CD40-ligation induced a state of hyporeactivity in alloreactive T cells that was reversible upon exogenous IL-2 supplementation to secondary cultures. These results suggest that immunosuppressive therapies targeting the tyrosine kinase JAK3 may also affect the function of myeloid cells. This property of JAK3 inhibitors therefore represents a further level of interference, which together with the well-established suppression of cgamma signaling could be responsible for their clinical efficacy.

Suppression of early T-cell-receptor-triggered cellular activation by the Janus kinase 3 inhibitor WHI-P-154

BACKGROUND

Therapeutic targeting of Janus kinase 3 (JAK3) has received particular attention, because it is associated with the common gamma signaling of cytokine receptors and thus vitally influences T-cell growth and survival. Recent evidence, however, indicates a critical role for JAK3 in signaling linked to the T-cell antigen receptor.

METHODS

In this study we investigated whether targeting JAK3 with a rationally designed inhibitor affects early T-cell activation events. T cells were stimulated by CD3 and CD28 cross-linking, and interleukin (IL)-2 production, activation marker expression, increase of free intracellular Ca2+ concentration, activation of the extracellular-related kinase, and nuclear translocation of transcription factors were evaluated.

RESULTS

We found that JAK3 inhibitor treatment dramatically impaired T-cell-receptor (TCR)-induced IL-2 production, surface activation marker expression (CD69, CD154), and homotypic T-cell aggregation. Accordingly, mRNA production of IL-2, interferon-gamma, and IL-10 was profoundly inhibited. Molecular analysis revealed that TCR-triggered phosphorylation of phospholipase C-gamma1, increase in cytoplasmic Ca2+ concentration, and activation of extracellular-related kinase were markedly reduced by the JAK3 inhibitor, resulting in substantially decreased DNA binding of nuclear factor of activated T cells and alkaline phosphatase-1 and subsequent IL-2 promoter activation. Remarkably, on TCR-independent stimulation, IL-2 production, CD69 expression, and blast formation were completely insensitive to JAK3 inhibitor treatment.

CONCLUSION

These data indicate that pharmacologic targeting of JAK3 uncouples early TCR-triggered signaling from essential downstream events, which may have important implications for the use of such compounds in T-cell-mediated disorders such as allograft rejection or graft-versus-host disease.

Protein kinase A regulates GATA-3-dependent activation of IL-5 gene expression in Th2 cells

Treatment of Th cells with compounds that elevate cAMP levels augments Th2-type lymphokine expression, in particular the synthesis of IL-5. Using primary murine CD4(+) T lymphocytes, we show in this study that inhibition of protein kinase A (PKA) activity in Th2 effector cells impairs IL-5 synthesis, whereas the expression of PKA catalytic subunit alpha enhances IL-5 synthesis in Th0 cells. In addition, we observed by coexpression of PKA catalytic subunit and GATA-3 in Th1 cells that the stimulatory effect of PKA is dependent on GATA-3 activity. These data demonstrate that activation of PKA in Th effector cells induces the IL-5 gene expression in a GATA-3-dependent manner.

The Th2 cell cytokines IL-4 and IL-13 regulate found in inflammatory zone 1/resistin-like molecule alpha gene expression by a STAT6 and CCAAT/enhancer-binding protein-dependent mechanism

The onset of allergic inflammation in the lung is driven by a complex genetic program. This study shows that found in inflammatory zone (FIZZ)1 and FIZZ2, but not FIZZ3, gene expression was up-regulated 6 h after Ag challenge in a mouse model of acute pulmonary inflammation. Induction of both genes was abolished in allergen-challenged STAT6-deficient mice. FIZZ1, but not FIZZ2, mRNA was up-regulated upon incubation of the myeloid cell line BMnot with IL-4. The promoter region of FIZZ1 contains functional binding sites for STAT6 and C/EBP. FIZZ1 promoter reporter gene constructs responded to IL-4 and IL-13 stimulation in transiently transfected cells. Point mutations in the STAT6 or the C/EBP site led to loss of cytokine responsiveness indicating that IL-4-mediated induction of murine FIZZ1 is orchestrated by the coordinate action of STAT6 and C/EBP. It is concluded that the expression of the genes encoding FIZZ1 and FIZZ2, but not FIZZ3, is induced in allergen-challenged lungs in a STAT6-dependent fashion. STAT6 directly regulates IL-4- and IL-13-triggered induction of FIZZ1 expression at the transcriptional level by cooperation with C/EBP. Induction of FIZZ2 gene expression most likely occurs independent of a direct effect by these cytokines and may be due to indirect STAT6-driven mechanisms.

Novel mode of interference with nuclear factor of activated T-cells regulation in T-cells by the bacterial metabolite n-butyrate

The transcription factor nuclear factor of activated T-cells (NF-AT) plays an essential role in the activation of many early immune response genes. A dynamic equilibrium between calcineurin and cellular kinases controls its phosphorylation and thus regulates its activity by determining its subcellular localization. Here, we demonstrate that T-cell activation in the presence of the bacterial metabolite n-butyrate, which leads to inhibition of interleukin-2 transcription, is characterized by the maintenance of the activity of counter-regulatory kinases glycogen synthase kinase 3 and protein kinase A as well as persistence of intracellular cAMP levels, whereas calcium response and mitogen-activated protein kinase activation were indistinguishable from cells stimulated in the absence of n-butyrate. Nuclear binding of NF-AT was decreased but other transcription factors implicated in interleukin-2 expression such as AP1 and nuclear factor kappaB were unaffected. The effect on NF-AT binding appeared to be the result of increased nuclear export because the export inhibitor leptomycin B completely restored nuclear binding of NF-AT. We, therefore, provide first evidence for interference with NF-AT regulation alternative to the currently understood inhibition of nuclear import. This mechanism might represent a bacterial strategy to subvert host defense, which could be of particular clinical importance in the gastrointestinal tract where high amounts of n-butyrate are physiologically present.

The immune modulator FTY720 targets sphingosine 1-phosphate receptors

Immunosuppressant drugs such as cyclosporin have allowed widespread organ transplantation, but their utility remains limited by toxicities, and they are ineffective in chronic management of autoimmune diseases such as multiple sclerosis. In contrast, the immune modulating drug FTY720 is efficacious in a variety of transplant and autoimmune models without inducing a generalized immunosuppressed state and is effective in human kidney transplantation. FTY720 elicits a lymphopenia resulting from a reversible redistribution of lymphocytes from circulation to secondary lymphoid tissues by unknown mechanisms. Using FTY720 and several analogs, we show now that FTY720 is phosphorylated by sphingosine kinase; the phosphorylated compound is a potent agonist at four sphingosine 1-phosphate receptors and represents the therapeutic principle in a rodent model of multiple sclerosis. Our results suggest that FTY720, after phosphorylation, acts through sphingosine 1-phosphate signaling pathways to modulate chemotactic responses and lymphocyte trafficking.

The synthesis and biological characterization of a ceramide library

A facile synthesis of a combinatorial ceramide library and their activities in the NF-kappaB pathway and in apoptosis induction/prevention were demonstrated. A novel NF-kappaB activating molecule was discovered among ceramide containing beta-galactose, and the structural requirements of ceramides for apoptosis induction was elucidated.

Sphingolipids and the regulation of the immune response

Over the last decade evidence has accumulated that sphingolipids are important and specific signalling molecules for cell-to-cell communication (mediator function) as well as for intracellular signalling processes (second messenger function). In addition, glycosylated sphingolipids are essential building blocks of rafts thereby participating in the initiation of receptor mediated signalling events. In immunology, processes such as T cell apoptosis, Th1 versus Th2 T cell differentiation, phagocytosis, and allergic excitability are either influenced or directly regulated by this class of lipids. Models such as the 'dual function concept' (differentiation of structural components versus signalling molecules) and the 'rheostat concept' (the balance of two or more sphingolipids is essential for the biological function) describe the multiple properties of these signalling molecules.

Characterization of CDw92 as a member of the choline transporter-like protein family regulated specifically on dendritic cells

CDw92 is a 70-kDa surface protein broadly expressed on leukocytes and endothelial cells. In this manuscript, we present the molecular cloning of the CDw92 molecule by using a highly efficient retroviral expression cloning system. Sequence analysis of the CDw92 cDNA revealed a length of 2679 bp. The 1959-bp open reading frame encodes a protein of 652 amino acids. Computational analysis of the CDw92 protein sequence indicates 10 transmembrane domains, three potential N-linked glycosylation sites, and an amino acid stretch in the C-terminal region that is related to the immunoreceptor tyrosine-based inhibitory motif. Comparison of the sequence of the CDw92 clone presented in this study with various database entries show that it is a C-terminal variant of human choline transporter-like protein 1, a member of a recently identified family of multitransmembrane surface proteins. Furthermore, we found that CDw92 is stably expressed on monocytes, PBLs, and endothelial cells, as we did not yet find modulation of expression by various stimuli on these cells. In contrast to this factor-independent expression of CDw92, we detected a specific regulation of CDw92 on monocyte-derived dendritic cells (Mo-DCs). Maturation of Mo-DCs by ionomycin or calcium ionophore resulted in down-regulation of CDw92 and incubation of these cells with IL-10 in a specific re-expression. Moreover, targeting of CDw92 on LPS-treated Mo-DCs by CDw92 mAb VIM15b augmented the LPS-induced IL-10 production 2.8-fold. Together, these data suggest a crucial role of the CDw92 protein in the biology and regulation of the function of leukocytes in particular DCs.

Beyond a structural component: sphingolipids in immunology

Two major classes of lipids participating in signaling cascades in immune cells are known today. One comprises glycerol-based lipids with diacylglycerol as its most prominent member that mediates the activation of classical and novel protein kinase C molecules. The second group contains the sphingolipids, with the best-investigated representatives being sphingosine, sphingosine-1-phosphate, and ceramide. In the last years the latter two molecules have especially received considerable attention for their modulatory capacity in the course of an apoptotic response. Today it is clear that sphingolipids are ubiquitously distributed in all eukaryotic cells, especially in cellular membranes, where they were previously thought to fulfil an exclusively structural role. Recent findings, however, have demonstrated functions beyond this. Sphingolipid specific G-protein coupled receptors were identified and their role as intracellular second messengers has been further elucidated. In addition, glycosphingolipids, in particular, are enriched in certain membrane compartments, known as detergent resistant membranes. These serve as entry sites for several receptor-mediated signaling events by stabilizing receptor/kinase interactions, suggesting an involvement in the initiation of signaling cascades. Altogether, these findings have led to new insights into both the role of these lipids in signaling as well as the underlying pathology of several diseases with imbalances in the sphingolipid metabolism. The development of these disorders has mainly been attributed to the toxic potential of lysosphingolipids up to now. In addition, attempts have been made to develop compounds and drugs containing the sphingolipid backbone for influencing diseases associated with unwanted cell activation (e.g, cancer, inflammatory processes). These novel findings and developments are reviewed in the following.

Sphingolipids: second messengers, mediators and raft constituents in signaling

Recently, evidence has accumulated to show that sphingolipids exert an important function in signaling. These lipids serve as intracellular second messengers and as extracellular mediators. Furthermore, glycosylated sphingolipids are essential components of membrane rafts, which serve as platforms for the initiation of signaling cascades. Here, Eva Prieschl and Thomas Baumruker summarize current findings in leukocytes illustrating these different facets.

Major histocompatibility complex class II- fetal skin dendritic cells are potent accessory cells of polyclonal T-cell responses

Whereas dendritic cells (DC) and Langerhans cells (LC) isolated from organs of adult individuals express surface major histocompatibility complex (MHC) class II antigens, DC lines generated from fetal murine skin, while capable of activating naive, allogeneic CD8+ T cells in a MHC class I-restricted fashion, do not exhibit anti-MHC class II surface reactivity and fail to stimulate the proliferation of naive, allogeneic CD4+ T cells. To test whether the CD45+ MHC class I+ CD80+ DC line 80/1 expresses incompetent, or fails to transcribe, MHC class II molecules, we performed biochemical and molecular studies using Western blot and polymerase chain reaction analysis. We found that 80/1 DC express MHC class II molecules neither at the protein nor at the transcriptional level. Ultrastructural examination of these cells revealed the presence of a LC-like morphology with indented nuclei, active cytoplasm, intermediate filaments and dendritic processes. In contrast to adult LC, no LC-specific cytoplasmic organelles (Birbeck granules) were present. Functionally, 80/1 DC in the presence, but not in the absence, of concanavalin A and anti-T-cell receptor monoclonal antibodies stimulated a vigorous proliferative response of naive CD4+ and CD8+ T cells. Furthermore, we found that the anti-CD3-induced stimulation of naive CD4+ and CD8+ T cells was critically dependent on the expression of FcgammaR on 80/1 DC and that the requirement for co-stimulation depends on the intensity of T-cell receptor signalling.

The role of sphingosine kinase in the signaling initiated at the high-affinity receptor for IgE (FcepsilonRI) in mast cells

Over the last few years, sphingolipids have emerged as an additional class of lipids participating in signaling events in various cell types. The best-investigated examples so far are ceramide and sphingosine-1-phosphate. Ceramide-activated protein kinase and sphingosine kinase are two enzymes which respond to and generate these mediators. In particular, sphingosine kinase, its substrate sphingosine and the product sphingosine-1-phosphate have recently been implicated in the signaling cascades initiated at the FcepsilonRI of mast cells. High intracellular levels of sphingosine seem to serve as an 'intracellular' inhibitor which is 'deactivated' by the action of sphingosine kinase, due to the conversion to sphingosine-1-phosphate. One mode of action of the inhibitory process in this cell type is prevention of the activation of the mitogen-activated protein (MAP) kinase pathway. Sphingosine-1-phosphate itself, the product of this enzymatic reaction, is believed to lead to Ca(2+) mobilization and to stimulate the MAP kinase pathway. The existence and function of this second messenger explains the 'IP3 gap' described in mast cells after FcepsilonRI activation. Therefore, a picture emerges whereby the balance of these two lipid molecules seems to be decisive for the activation of mast cells by IgE plus antigen, with sphingosine kinase acting as a permissive switch for stimulation.

Glycosphingolipid-induced relocation of Lyn and Syk into detergent-resistant membranes results in mast cell activation

Sphingosine, sphingosine-1-phosphate, and the more complex sphingolipid ceramide exert strong immunomodulatory effects on a variety of leukocytes. However, little is known regarding such a potential of glycosphingolipids, a class of sugar derivatives of sphingosine. Here we demonstrate that galactosylsphingosine, one of the smallest representatives of this group, accumulates in the detergent-resistant membranes resulting in the relocation of the tyrosine kinases Lyn and Syk into this compartment. The result of this is an enhanced tyrosine phosphorylation and kinase activity leading to priming and activation of mast cells by conveying a weak yet significant activation of the mitogen-activated protein kinase pathway(s). In comparison to IgE/Ag triggering, galactosylsphingosine stimulates the mitogen-activated protein kinase pathway more rapidly and favors c-Jun NH2-terminal kinase 1 activation over extracellular signal-regulatory kinase 1 and 2. At the transcription factor level, this "ultratransient signaling event" results in an activation of JunD as the predominant AP-1 component. In this respect, the effects of galactosylsphingosine are clearly distinct from the signaling elicited by other sphingolipids without the sugar moiety, such as sphingosine-1-phosphate.

A novel anti-inflammatory drug, SDZ ASM 981, for the treatment of skin diseases: in vitro pharmacology

SDZ ASM 981, a novel ascomycin macrolactam derivative, has high anti-inflammatory activity in animal models of allergic contact dermatitis and shows clinical efficacy in atopic dermatitis, allergic contact dermatitis and psoriasis, after topical application. Here we report on the in vitro activities of this promising new drug. SDZ ASM 981 inhibits the proliferation of human T cells after antigen-specific or non-specific stimulation. It downregulates the production of Th1 [interleukin (IL)-2, interferon-gamma] and Th2 (IL-4, IL-10) type cytokines after antigen-specific stimulation of a human T-helper cell clone isolated from the skin of an atopic dermatitis patient. SDZ ASM 981 inhibits the phorbol myristate acetate/phytohaemagglutinin-stimulated transcription of a reporter gene coupled to the human IL-2 promoter in the human T-cell line Jurkat and the IgE/antigen-mediated transcription of a reporter gene coupled to the human tumour necrosis factor (TNF)-alpha promoter in the murine mast-cell line CPII. It does not, however, affect the human TNF-alpha promoter controlled transcription of a reporter gene in a murine dendritic cell line (DC18 RGA) after stimulation via the FcgammaRIII receptor. SDZ ASM 981 also prevents the release of preformed pro-inflammatory mediators from mast cells, as shown in the murine cell line CPII after stimulation with IgE/antigen. In summary, these results demonstrate that SDZ ASM 981 is a specific inhibitor of the production of pro-inflammatory cytokines from T cells and mast cells in vitro.

The balance between sphingosine and sphingosine-1-phosphate is decisive for mast cell activation after Fc epsilon receptor I triggering

Over the last few years, sphingolipids have been identified as potent second messenger molecules modulating cell growth and activation. A newly emerging facet to this class of lipids suggests a picture where the balance between two counterregulatory lipids (as shown in the particular example of ceramide and sphingosine-1-phosphate in T lymphocyte apoptosis) determines the cell fate by setting the stage for various protein signaling cascades. Here, we provide a further example of such a decisive balance composed of the two lipids sphingosine and sphingosine-1-phosphate that determines the allergic responsiveness of mast cells. High intracellular concentrations of sphingosine act as a potent inhibitor of the immunoglobulin (Ig)E plus antigen-mediated leukotriene synthesis and cytokine production by preventing activation of the mitogen-activated protein kinase pathway. In contrast, high intracellular levels of sphingosine-1-phosphate, also secreted by allergically stimulated mast cells, activate the mitogen-activated protein kinase pathway, resulting in hexosaminidase and leukotriene release, or in combination with ionomycin, give cytokine production. Equivalent high concentrations of sphingosine-1-phosphate are dominant over sphingosine as they counteract its inhibitory potential. Therefore, it might be inferred that sphingosine-kinase is pivotal to the activation of signaling cascades initiated at the Fc epsilon receptor I by modulating the balance of the counterregulatory lipids.

TNF-alpha and IL-5 gene induction in IgE plus antigen-stimulated mast cells require common and distinct signaling pathways

BACKGROUND

Mast cells produce a variety of cytokines and chemokines in a timely and tightly controlled fashion if stimulated via the FcepsilonRI. Evidence is accumulating that the transcriptional induction of the corresponding genes and the release of these mediators are dependent on common and mediator-specific components of the signal transduction and transcription factor machinery.

METHODS

We addressed this issue by comparing the effects of mitogen activated protein (MAP) kinase pathway inhibitors and protein kinase C (PKC) inhibitors on the induction of TNF-alpha and IL-5 after IgE plus antigen (Ag) stimulation in CPII mouse mast cells using Western blot analyses and transient transfections of reporter gene plasmids.

RESULTS

TNF-alpha shows a strict dependence on the MAP kinase pathway, while IL-5 is either activated by PMA-dependent PKCs or along the MAP kinase pathway. In addition, both mediators are sensitive to PKCmu inhibition, suggesting involvement of this atypical, non-PMA dependent PKC in the overall induction process.

CONCLUSION

While the two cytokines were recently shown to be regulated by a member of the nuclear factor of activated T-cells (NF-AT) transcription factor family, activator protein 1 (AP1) was identified as a cofactor at the TNF-alpha promoter while a GATA family member comprised the cofactor at the IL-5 promoter. This suggests that the differences in requirement for signal transduction cascades are the result of a different usage of NF-AT cofactors for transcription of each cytokine in mast cells.

Nrf1 in a complex with fosB, c-jun, junD and ATF2 forms the AP1 component at the TNF alpha promoter in stimulated mast cells

Cap 'n' collar-basic leucine zipper (CNC-bZIP) proteins are widely implicated in developmental processes throughout different species. Evidence is accumulating that some of them are also participating in induced gene expression in the adult. Here we show that the three CNC-bZIP members NF-E2, Nrf1 and Nrf2 are constitutively expressed in the murine mast cell line CPII and that they form transcription factor complexes with several AP1 binding proteins. Upon induction, complexes are observed at the 2 x NF-E2 consensus binding site and the extended kappa3/AP1(+) site of the TNFalpha promoter. The interaction of Nrf1 with c -jun, junD, fosB and ATF2 in mast cells is in contrast to the recently reported binding of Nrf1 alone at the kappa3/AP1(-) site in dendritic cells. We speculated that this may be the result of the expression of isoforms of Nrf1 in mast cells. Using a PCR cloning strategy, we have isolated six novel splice variants of this transcription factor. Some of them have deleted the translational stop codon, resulting in an Nrf1 protein with an altered leucine zipper region. Expression of this altered binding/interaction domain interferes with TNFalpha induction, indicating an interaction of this splice variant with the active AP1/NF-AT complex at this promoter.

CD99 Engagement on Human Peripheral Blood T Cells Results in TCR/CD3-Dependent Cellular Activation and Allows for Th1-Restricted Cytokine Production

We have assessed the functional effect of CD99 engagement on resting human peripheral blood (PB) T cells. CD99, as detected by the mAb 3B2/TA8, is constitutively expressed on all PB T cells and becomes further up-regulated upon cellular activation. In this study we demonstrate that cross-linking of the CD99 molecule with the agonistic mAb 3B2/TA8 cooperates with suboptimal TCR/CD3 signals, but not with phorbol ester, ionomycin, or CD28 mAb stimulation, to induce proliferation of resting PB T cells. Comparable stimulatory effects were observed with the CD99 mAb 12E7. Characterization of the signaling pathways involved revealed that CD99 engagement leads to the elevation of intracellular Ca2+, which is dependent on the cell surface expression of the TCR/CD3 complex. No CD99 mAb-induced calcium mobilization was observed on TCR/CD3-modulated or TCR/CD3-negative T cells. To examine the impact of CD99 stimulation on subsequent cytokine production by T cells, we cross-linked CD99 molecules in the presence of a suboptimal TCR/CD3 trigger followed by determination of intracellular cytokine levels. Significantly, T cell lines as well as Th1 and Th0 clones synthesized TNF-α and IFN-γ after this treatment. In contrast, Th2 clones were unable to produce IL-4 or IFN-γ when stimulated in a similar fashion. We conclude that CD99 is a receptor that mediates TCR/CD3-dependent activation of resting PB T cells and specifically induces Th1-type cytokine production in polyclonally activated T cell lines, Th1 and Th0 clones.