Sphingomyelinase and ceramide activate mitogen-activated protein kinase in myeloid HL-60 cells

Enhancement of Sphingomyelinase-Induced Endothelial Nitric Oxide Synthase-Mediated Vasorelaxation in a Murine Model of Type 2 Diabetes

Sphingolipids are important biological mediators both in health and disease. We investigated the vascular effects of enhanced sphingomyelinase (SMase) activity in a mouse model of type 2 diabetes mellitus (T2DM) to gain an understanding of the signaling pathways involved. Myography was used to measure changes in the tone of the thoracic aorta after administration of 0.2 U/mL neutral SMase in the presence or absence of the thromboxane prostanoid (TP) receptor antagonist SQ 29,548 and the nitric oxide synthase (NOS) inhibitor L-NAME. In precontracted aortic segments of non-diabetic mice, SMase induced transient contraction and subsequent weak relaxation, whereas vessels of diabetic (Lepr^db/Lepr^db, referred to as db/db) mice showed marked relaxation. In the presence of the TP receptor antagonist, SMase induced enhanced relaxation in both groups, which was 3-fold stronger in the vessels of db/db mice as compared to controls and could not be abolished by ceramidase or sphingosine-kinase inhibitors. Co-administration of the NOS inhibitor L-NAME abolished vasorelaxation in both groups. Our results indicate dual vasoactive effects of SMase: TP-mediated vasoconstriction and NO-mediated vasorelaxation. Surprisingly, in spite of the general endothelial dysfunction in T2DM, the endothelial NOS-mediated vasorelaxant effect of SMase was markedly enhanced.

PD-1-induced T cell exhaustion is controlled by a Drp1-dependent mechanism

Programmed cell death‐1 (PD‐1) signaling downregulates the T‐cell response, promoting an exhausted state in tumor‐infiltrating T cells, through mostly unveiled molecular mechanisms. Dynamin‐related protein‐1 (Drp1)‐dependent mitochondrial fission plays a crucial role in sustaining T‐cell motility, proliferation, survival, and glycolytic engagement. Interestingly, such processes are exactly those inhibited by PD‐1 in tumor‐infiltrating T cells. Here, we show that PD‐1^pos CD8⁺ T cells infiltrating an MC38 (murine adenocarcinoma)‐derived murine tumor mass have a downregulated Drp1 activity and more elongated mitochondria compared with PD‐1^neg counterparts. Also, PD‐1^pos lymphocytic elements infiltrating a human colon cancer rarely express active Drp1. Mechanistically, PD‐1 signaling directly prevents mitochondrial fragmentation following T‐cell stimulation by downregulating Drp1 phosphorylation on Ser616, via regulation of the ERK1/2 and mTOR pathways. In addition, downregulation of Drp1 activity in tumor‐infiltrating PD‐1^pos CD8⁺ T cells seems to be a mechanism exploited by PD‐1 signaling to reduce motility and proliferation of these cells. Overall, our data indicate that the modulation of Drp1 activity in tumor‐infiltrating T cells may become a valuable target to ameliorate the anticancer immune response in future immunotherapy approaches.

Regulation of SREBPs by Sphingomyelin in Adipocytes via a Caveolin and Ras-ERK-MAPK-CREB Signaling Pathway

Sterol response element binding protein (SREBP) is a key transcription factor in insulin and glucose metabolism. We previously demonstrated that elevated levels of membrane sphingomyelin (SM) were related to peroxisome proliferator–activated receptor-γ (PPARγ), which is a known target gene of SREBP-1 in adipocytes. However, the role of SM in SREBP expression in adipocytes remains unknown. In human abdominal adipose tissue from obese women with various concentrations of fasting plasma insulin, SREBP-1 proteins decreased in parallel with increases in membrane SM levels. An inverse correlation was found between the membrane SM content and the levels of SREBP-1c/ERK/Ras/PPARγ/CREB proteins. For the first time, we demonstrate the effects of SM and its signaling pathway in 3T3-F442A adipocytes. These cells were enriched or unenriched with SM in a range of concentrations similar to those observed in obese subjects by adding exogenous natural SMs (having different acyl chain lengths) or by inhibiting neutral sphingomyelinase. SM accumulated in caveolae of the plasma membrane within 24 h and then in the intracellular space. SM enrichment decreased SREBP-1 through the inhibition of extracellular signal-regulated protein kinase (ERK) but not JNK or p38 mitogen-activated protein kinase (MAPK). Ras/Raf-1/MEK1/2 and KSR proteins, which are upstream mediators of ERK, were down-regulated, whereas SREBP-2/caveolin and cholesterol were up-regulated. In SM-unmodulated adipocytes treated with DL-1-Phenyl-2-Palmitoylamino-3-morpholino-1-propanol (PPMP), where the ceramide level increased, the expression levels of SREBPs and ERK were modulated in an opposite direction relative to the SM-enriched cells. SM inhibited the insulin-induced expression of SREBP-1. Rosiglitazone, which is an anti-diabetic agent and potent activator of PPARγ, reversed the effects of SM on SREBP-1, PPARγ and CREB. Taken together, these findings provide novel insights indicating that excess membrane SM might be critical for regulating SREBPs in adipocytes via a MAPK-dependent pathway.

Inhibition of host extracellular signal-regulated kinase (ERK) activation decreases new world alphavirus multiplication in infected cells

New World alphaviruses belonging to the family Togaviridae are classified as emerging infectious agents and Category B select agents. Our study is focused on the role of the host extracellular signal-regulated kinase (ERK) in the infectious process of New World alphaviruses. Infection of human cells by Venezuelan equine encephalitis virus (VEEV) results in the activation of the ERK-signaling cascade. Inhibition of ERK1/2 by the small molecule inhibitor Ag-126 results in inhibition of viral multiplication. Ag-126-mediated inhibition of VEEV was due to potential effects on early and late stages of the infectious process. While expression of viral proteins was down-regulated in Ag-126 treated cells, we did not observe any influence of Ag-126 on the nuclear distribution of capsid. Finally, Ag-126 exerted a broad-spectrum inhibitory effect on New World alphavirus multiplication, thus indicating that the host kinase, ERK, is a broad-spectrum candidate for development of novel therapeutics against New World alphaviruses.

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VEEV infection activated multiple components of the ERK signaling cascade.

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Inhibition of ERK activation using Ag-126 inhibited VEEV multiplication.

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Activation of ERK by Ceramide C6 increased infectious titers of TC-83.

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Ag-126 inhibited virulent strains of all New World alphaviruses.

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Ag-126 treatment increased percent survival of infected cells.

Pro- and anti-apoptotic roles for IGF-I in TNF-alpha-induced apoptosis: a MAP kinase mediated mechanism

OBJECTIVE

The concept of skeletal muscle homeostasis--often viewed as the net balance between two separate processes, namely protein degradation and protein synthesis--are not occurring independently of each other, but are finely co-ordinated by a web of intricate signalling networks.

MATERIALS AND METHODS

Using rodent muscle cell lines we have investigated TNF-alpha/IGF-I interactions, in an attempt to mimic and understand mechanisms underlying the wasting process.

RESULTS AND CONCLUSION

When myoblast cells are incubated with TNF-alpha (10 ng ml(- 1)) maximal damage ( approximately 21% +/- 0.7 myoblast death, p < 0.05) was induced. Co-incubation of TNF-alpha (10 ng ml(- 1)) with IGF-I resulted in cell survival ( approximately 50% reduction in myoblast death, p < 0.05), however, myotube formation was not evident. In contrast, a novel role of IGF-I has been identified whereby co-incubation of muscle cells with IGF-I (1.5 ng ml(- 1)) and a non-apoptotic dose of TNF-alpha (1.25 ng ml(- 1); sufficient to block differentiation) unexpectedly were shown not to rescue a block on differentiation but to facilitate significant myoblast death (p < 0.05). Interestingly, pre-administration of PD98059, a MAPK signal-blocking agent followed by co-incubation of 1.25 ng ml(- 1) TNF-alpha and 1.5 ng ml(- 1) IGF-I, reduced death to baseline levels (p < 0.05). We show for the first time that IGF-I can be apoptotic in the absence of TNF-alpha-induced cell death.

Targeting the conversion of ceramide to sphingosine 1-phosphate as a novel strategy for cancer therapy

Sphingolipids not only function as structural components of cell membranes but also act as signaling molecules to regulate fundamental cellular responses, such as cell death and differentiation, proliferation and certain types of inflammation. Particularly the cellular balance between ceramide and sphingosine 1-phosphate seems to be crucial for a cell's decision to either undergo apoptosis or proliferate, two events which are implicated in tumor development and growth. Whereas ceramide possesses proapoptotic capacity in many cell types, sphingosine 1-phosphate acts as a counterplayer able to induce cell proliferation and protect cells from undergoing apoptosis. Therefore, tipping the balance in favour of ceramide production, i.e. by inhibiting ceramidase or sphingosine kinase activities has potential to support its proapoptotic action and hence represents a promising rational approach to effective cancer therapy. This review highlights most recent data on the regulation of cellular sphingolipid formation and their potential implication in tumor development, and provides perspectives for their use as targets in molecular intervention therapy.

Protein Kinase C-induced Activation of a Ceramide/Protein Phosphatase 1 Pathway Leading to Dephosphorylation of p38 MAPK

Recently we showed that, in human breast cancer cells, activation of protein kinase C by 4beta-phorbol 12-myristate 13-acetate (PMA) produced ceramide formed from the salvage pathway (Becker, K. P., Kitatani, K., Idkowiak-Baldys, J., Bielawski, J., and Hannun, Y. A. (2005) J. Biol. Chem. 280, 2606-2612). In this study, we investigated intracellular signaling events mediated by this novel activated pathway of ceramide generation. PMA treatment resulted in transient activation of mitogen-activated protein kinases (ERK1/2, JNK1/2, and p38) followed by dephosphorylation/inactivation. Interestingly, fumonisin B1 (FB1), an inhibitor of the salvage pathway, attenuated loss of phosphorylation of p38, suggesting a role for ceramide in p38 dephosphorylation. This was confirmed by knock-down of longevity-assurance homologue 5, which partially suppressed the formation of C(16)-ceramide induced by PMA and increased the phosphorylation of p38. These results demonstrate a role for the salvage pathway in feedback inhibition of p38. To determine which protein phosphatases act in this pathway, specific knock-down of serine/threonine protein phosphatases was performed, and it was observed that knock-down of protein phosphatase 1 (PP1) catalytic subunits significantly increased p38 phosphorylation, suggesting activation of PP1 results in an inhibitory effect on p38. Moreover, PMA recruited PP1 catalytic subunits to mitochondria, and this was significantly suppressed by FB1. In addition, phospho-p38 resided in PMA-stimulated mitochondria. Upon PMA treatment, a mitochondria-enriched/purified fraction exhibited significant increases in C(16)-ceramide, a major ceramide specie, which was suppressed by FB1. Taken together, these data suggest that accumulation of C(16)-ceramide in mitochondria formed from the protein kinase C-dependent salvage pathway results at least in part from the action of longevity-assurance homologue 5, and the generated ceramide modulates the p38 cascade via PP1.

Sphingolipids and cell signaling: involvement in apoptosis and atherogenesis

This review considers various functional aspects of cell sphingolipids (sphingomyelin, ceramides) and lysosphingolipids (sphingosine-1-phosphate (S1P) and sphingosine phosphorylcholine). Good evidence now exists that they are actively involved in numerous cell-signaling processes. The enzymes responsible for formation and interconversion of cell sphingolipids (sphingomyelinases, ceramidase, sphingosine kinase, S1P-lyase) exhibit high sensitivity to various stimulating factors. This determines the content of individual cell sphingolipids and therefore the mode of cell response. Special attention is paid to preferential localization of sphingolipids in the rigid plasma membrane domains (rafts) coupled to many signal proteins. The suggestion is discussed that ceramide signaling may be based on the modification of fine molecular interactions in lipid rafts, resulting in its clusterization inducing the signal transduction. The review also highlights involvement of sphingolipids in cell proliferation, apoptosis, and in processes implicated to atherosclerosis.

Penta-Acetyl Geniposide Induce Apoptosis in C6 Glioma Cells by Modulating the Activation of Neutral Sphingomyelinase-Induced p75 Nerve Growth Factor Receptor and Protein Kinase Cδ Pathway

In our previous studies, we demonstrated the apoptotic cascades protein kinase C (PKC) delta/c-Jun NH2-terminal kinase (JNK)/Fas/caspases induced by penta-acetyl geniposide [(Ac)5GP]. However, the upstream signals mediating PKCdelta activation have not yet been clarified. Ceramide, mainly generated from the degradation of sphingomyelin, was hypothesized upstream above PKCdelta in (Ac)5GP-transduced apoptosis. Furthermore, nerve growth factor (NGF)/p75 is supposed to be involved because(Ac)5GP-induced apoptosis was demonstrated previously in glioma cells. In the present study, (Ac)5GP was shown to activate neutral sphingomyelinase (N-SMase) immediately, with its maximum at 15 min. The NGF and p75 enhanced by (Ac)5GP was inhibited when added with GW4869, the N-SMase inhibitor, indicating NGF/p75 as the downstream signals of N-SMase/ceramide. To investigate whether N-SMase is involved in (Ac)5GP-transduced apoptotic pathway, cells were treated with (Ac)5GP added with or without GW4869. It showed that N-SMase inhibition blocked FasL expression and caspase 3 activation. Likewise, p75 antagonist peptide attenuated the FasL/caspase 3 expression. The PKCdelta translocation induced by (Ac)5GP was also eliminated by GW4869 and p75 antagonist peptide. To further confirm whether N-SMase activation plays an important role in (Ac)5GP-induced apoptosis, cells were analyzed the apoptotic rate by 4', 6-diamidino-2-phenylindole (DAPI) staining. (Ac)5GP-induced apoptosis was reduced 40 and 80% by 10 and 20 microM GW4869, respectively. It indicated that N-SMase activation is pivotal in (Ac)5GP-mediated apoptosis. In conclusion, SMase and NGF/p75 are suggested to mediate upstream above PKCdelta, thus transducing FasL/caspase cascades in (Ac)5GP-induced apoptosis.

Reactive oxygen species production by mitochondria in endothelial cells exposed to reoxygenation after hypoxia and glucose depletion is mediated by ceramide

In endothelium, reoxygenation after hypoxia (H/R) has been shown to induce production of reactive oxygen species (ROS) by complex III of the mitochondrial respiratory chain. The purpose of the present study was to test the involvement of ceramide in this phenomenon. Human umbilical vein endothelial cells underwent 2 h of hypoxia (Po2, ∼20 mmHg) without glucose and 1 h of reoxygenation (Po2, ∼120 mmHg) with glucose. ROS production was measured by the fluorescent marker 2′,7′-dichlorodihydrofluorescein diacetate, and cell death by propidium iodide. We showed that 1) after 1 h of reoxygenation, fluorescence had risen and that ROS production was inhibited by desipramine, an inhibitor of sphingomyelinase, an enzyme responsible for ceramide production (126 ± 7% vs. 48 ± 12%, P < 0.05); 2) administration of ceramide ( N-acetylsphingosine) per se (i.e., in the absence of H/R) induced ROS production (65 ± 3%), which was inhibited by complex III inhibitor: antimycin A (24 ± 3%, P < 0.0001), or stigmatellin (31 ± 2%, P < 0.0001); 3) hypoxia/reoxygenation-induced ROS production was not affected by either ceramide-activated protein kinase inhibitor dimethyl aminopurine or mitochondrial permeability transition inhibitor cyclosporin A but was significantly inhibited by the antiapoptotic protein Bcl-2 (82 ± 8%, P < 0.05); 4) ceramide-induced ROS production was also inhibited by Bcl-2 (41 ± 4%, P < 0.0001). These results demonstrate that in endothelial cells submitted to hypoxia and glucose depletion followed by reoxygenation with glucose, the pathway implicated in mitochondrial complex III ROS production is ceramide dependent and is decreased by the antiapoptotic protein Bcl-2.

Involvement of neutral ceramidase in ceramide metabolism at the plasma membrane and in extracellular milieu

Neutral ceramidase is a type II integral membrane protein, which is occasionally secreted into the extracellular milieu after the processing of its N-terminal anchor. We found that when overexpressed in CHOP cells, neutral ceramidase hydrolyzed cell surface ceramide, which increased in amount after the treatment of cells with bacterial sphingomyelinase, leading to an increase in the cellular level of sphingosine and sphingosine 1-phosphate. On the other hand, knockdown of the endogenous enzyme by siRNA decreased the cellular level of both sphingolipid metabolites. The treatment of cells with bovine serum albumin significantly reduced the cellular level of sphingosine, but not sphingosine 1-phosphate, generated by overexpression of the enzyme. The cellular level of sphingosine 1-phosphate increased with overexpression of the cytosolic sphingosine kinase. These results suggest that sphingosine 1-phosphate is mainly produced inside of the cell after the incorporation of sphingosine generated on the plasma membranes. The enzyme also seems to participate in the hydrolysis of serum-derived ceramide in the vascular system. Significant amounts of sphingosine as well as sphingosine 1-phosphate were generated in the cell-free conditioned medium of ceramidase transfectants, compared with mock transfectants. No increase in these metabolites was observed if serum or bacterial sphingomyelinase was omitted from the conditioned medium, suggesting that the major source of ceramide is the serum-derived sphingomyelin. A sphingosine 1-phosphate receptor, S1P(1), was internalized much faster by the treatment of S1P(1)-overexpressing cells with conditioned medium of ceramidase transfectants than that of mock transfectants. Collectively, these results clearly indicate that the enzyme is involved in the metabolism of ceramide at the plasma membrane and in the extracellular milieu, which could regulate sphingosine 1-phosphate-mediated signaling through the generation of sphingosine.

Antineoplastic agents. 545. Isolation and structure of turbostatins 1-4 from the Asian marine mollusk Turbo stenogyrus

The cancer cell line bioassay-guided separation of an extract from the marine mollusk Turbo stenogyrus led to the isolation of four new cerebrosides designated turbostatins 1-4 (1-4). The structure of each glycolipid was determined by interpreting results of a series of HR-APCI-MS and NMR (1D and 2D) spectral analyses. All four turbostatins exhibited significant (GI50 0.15-2.6 microg/mL) cancer cell growth inhibition against the murine P388 lymphocytic leukemia and a panel of human cancer cell lines.

Acid sphingomyelinase in macrophage apoptosis

PURPOSE OF REVIEW

This article presents an overview of the recent progress in understanding metabolic and functional interrelationships of biologically active sphingolipids related to the sphingomyelin signal transduction pathway in relation to the regulation of apoptosis in macrophages.

RECENT FINDINGS

Ceramide generation is an essential, early step in apoptosis in numerous systems. There are several mechanisms for ceramide generation, including activation of plasma membrane, lysosomal, nuclear, and mitochondrial sphingomyelinases, and induction of de-novo synthesis of ceramide. Some of the proapoptotic actions of ceramide are to facilitate assembly of death receptor complexes in the plasma membrane, to prevent the activation of protein kinase B/Akt, and to promote the activation of caspase 3. Failure of macrophages in developing atherosclerotic plaques to undergo apoptosis is a possible contributor to plaque expansion. At low concentrations, oxidized LDL has been shown to prevent apoptosis induced by growth factor withdrawal in cultured bone marrow-derived macrophages, in part by inhibiting sphingomyelinase and preventing generation of ceramide. At high concentrations, however, oxidized LDL can induce apoptosis or necrosis of macrophages.

SUMMARY

Sphingolipid signal transduction pathways play an important role in the regulation of growth and survival pathways in macrophages. These are directly relevant to the pathogenesis of a variety of chronic inflammatory disorders, including atherosclerosis.

Acute cutaneous barrier disruption activates epidermal p44/42 and p38 mitogen-activated protein kinases in human and hairless guinea pig skin

Acute cutaneous barrier disruption of the skin elicits various homeostatic repair responses in the epidermis. Although several candidates for the signaling mechanisms that induce these responses have been reported, e.g. the calcium and ion concentration, peroxisome proliferator-activated receptor-alpha, and TNF-alpha signaling mediated by sphingomyelinases, the exact nature of the signals remains undertermined. Therefore, assuming that an important group of serine/threonine-signaling kinases, mitogen- and SAPK/JNK, might link the barrier disruption to the subsequent homeostatic responses, the activation of three MAPKs in hairless guinea pig or in human skin after barrier disruption was investigated. The epidermal barrier was insulated with tape stripping or organic solvents, and Western blotting, and immune complex kinase assay. In the skin of hairless guinea pigs, p44/42 MAPK and p38 MAPK, but nor SAPK/JNK, were continued to be activated for at least 180 min. The activation of p44/42 which positively correlated with the number of tape strippings, whereas K+ sucrose solution suppressed its activation. The activation of p44/42 MAPK was also induced by treatment of the skin with organic solvents. In similar fashion, p44/42 and p38 MAPKs were found to be activated in human skin after tape stripping. These results for strongly suggest that the activation of p44/42 and p38 MAPKs links the stimuli of barrier disruption to the subsequent homeostatic responses to repair the barrier defect.

Inhibition of erythrocyte phosphatidylserine exposure by urea and Cl-

Osmotic shock by addition of sucrose to the medium stimulates erythrocyte sphingomyelinase with subsequent ceramide formation and triggers Ca(2+) entry through stimulation of cation channels. Both ceramide and Ca(2+) activate an erythrocyte scramblase, leading to breakdown of phosphatidylserine asymmetry, a typical feature of apoptosis. Because erythrocytes are regularly exposed to osmotic shock during passage of kidney medulla, the present study explored the influence of NaCl and urea on erythrocyte phosphatidylserine exposure as determined by annexin binding. The percentage of annexin-binding erythrocytes increased from <5 to 80 +/- 4% (n = 4) upon addition of 650 mM sucrose, an effect paralleled by activation of the cation channel and stimulation of ceramide formation. The number of annexin-binding erythrocytes increased only to 18% after addition of 325 mM NaCl and was not increased by addition of 650 mM urea. According to whole cell patch-clamp experiments, the cation conductance was activated by replacement of extracellular Cl(-) with gluconate at isotonic conditions or by addition of hypertonic sucrose or urea. Although stimulating the cation conductance, urea abrogated the annexin binding and concomitant increase of ceramide levels induced by osmotic cell shrinkage. In vitro sphingomyelinase assays demonstrated a direct inhibitory effect of urea on sphingomyelinase activity. Urea did not significantly interfere with annexin binding after addition of ceramide. In conclusion, both Cl(-) and urea blunt erythrocyte phosphatidylserine exposure after osmotic shock. Whereas Cl(-) is effective through inhibition of the cation conductance, urea exerts its effect through inhibition of sphingomyelinase, thus blunting formation of ceramide.

Ceramide inhibits L-type calcium channel currents in GH3 cells

In this study, we investigated the effect of ceramide on the L-type Ca2+ channel (L-channel) in GH3 cells. We found that C6-ceramide, but not C6-dihydroceramide, the inactive analogue, had an inhibitory effect on BayK 8644-stimulated GH release. Using patch clamp analysis, C6- and C2-ceramide, but not C6-dihydroceramide, were found to inhibit the L-channel current. Increasing intracellular ceramide level with sphingomyelinase also inhibited the L-channel current. The inhibitory effect of ceramide on the L-channel current was attenuated by calphostin C, a myristolated pseudosubstrate protein kinase C (PKC) inhibitor, and lavendustin A, a tyrosine kinase inhibitor. Combined treatment with lavendustin A and the myristolated PKC inhibitor blocked the effect of ceramide on the L-channel current. These results indicate that ceramide, a lipid messenger of the sphingomyelin pathway, is an important regulator of the L-channel in GH3 cells and both tyrosine kinase and PKC are involved in this effect of ceramide.

Ceramide induces caspase-dependent and -independent apoptosis in A-431 cells

We investigated the ceramide-induced apoptosis and potential mechanism in A-431 cells. Ceramide treatment causes the round up and the death of A-431 cells that is associated with p38 activation and can be observed in 10 h. Short-time ceramide treatment-induced cell death is not associated with the typical apoptotic phenotypes, such as the translocation of phosphatidylserine (PS) from inner layer to outer layer of the plasma membrane, loss of mitochondrial membrane potential, DNA fragmentation, caspase activation, and PARP or PKC-delta degradation. SB202190, a specific inhibitor of p38 mitogen-activated protein (MAP) kinase, but not caspase inhibitor, blocks the cell death induced by short-time ceramide treatment (within 12 h). Whereas neither inhibition of p38 MAP kinase nor inhibition of caspases blocks cell death induced by prolonged ceramide treatment. Moreover, incubation of cells with ceramide for a long time (over 12 h) results in the reduction of proportion of S phase accompanied with typical apoptotic cell death phenotypes that are different from the cell death induced by short-time ceramide treatment. Our data demonstrated that ceramide-induced apoptotic cell death involves both caspase-dependent and caspase-independent signaling pathways. The caspase-independent cell death that occurred in relatively early stage of ceramide treatment is mediated via p38 MAP kinase, which can progress into a stage that is associated with changes of cell cycle events and involves both caspase-dependent and -independent mechanisms.

Differential binding of ceramide to MEKK1 in glomerular endothelial and mesangial cells

Previously, we have shown that ceramide is able to directly bind to and activate c-Raf and to trigger the downstream classical mitogen-activated protein kinase (MAPK/ERK) cascade in glomerular mesangial cells [Proc. Natl. Acad. Sci. USA 93 (1996) 6959]. In this study, we show that ceramide acts differently in glomerular endothelial cells in that treatment of endothelial cells with exogenous ceramide leads to a potent activation of the stress-activated protein kinase (SAPK/JNK) cascade but not to an activation of the classical ERK cascade. A similar effect was observed with the inflammatory cytokines TNFalpha and IL-1beta, which activate a sphingomyelinase and thereby increase intracellular ceramide levels. The activation of JNKs as shown by c-Jun phosphorylation assays was paralleled by increased phosphorylation of the two JNK isoforms, p45 and p54. In addition, also the activator of JNKs, SEK1, was found to be increasingly phosphorylated by exogenous ceramide as well as by TNFalpha. In contrast, dihydroceramide had no effect on JNK or SEK1 phosphorylation. To see whether ceramide directly binds to MEKK1, which is the c-Raf analog in the SAPK cascade, a radioiodinated photoaffinity labeling analogue of ceramide, (N-[3-[[[2-(125I)iodo-4-[3-(trifluoromethyl)-3H-diazirin-3-yl]benzyl]oxy]-carbonyl] propanoyl]-D-erythro-sphingosine) ([125I]TID-ceramide) was used. Stimulation of endothelial cells with this [125I]TID-ceramide for 5 min followed by a short photolysis defined MEKK1 as a direct target of ceramide. With the same method, protein kinase C-alpha (PKC-alpha) was identified as a ceramide target. In contrast, no binding to c-Raf or the MEKK1 activator p65-PAK could be detected. A direct binding of ceramide to MEKK1 was also confirmed by affinity chromatography using a ceramide-coupled sepharose column. Furthermore, the ceramide-activated SAPK/JNK cascade is clearly involved in the mechanism of apoptosis, since in the presence of a JNK inhibitor, ceramide-induced DNA fragmentation is significantly reduced. In summary, we have shown that ceramide potently activates the SAPK cascade but not the ERK cascade in endothelial cells, which contrasts to mesangial cells where ceramide activates the ERK pathway and has only a minor effect on the SAPK cascade. Regarding the direct target of ceramide binding and action in endothelial cells, we identified MEKK1 as a further member of the growing family of ceramide-activated protein kinases.

Clostridium perfringens alpha-toxin activates the sphingomyelin metabolism system in sheep erythrocytes

Clostridium perfringens alpha-toxin induces hemolysis of rabbit erythrocytes through the activation of glycerophospholipid metabolism. Sheep erythrocytes contain large amounts of sphingomyelin (SM) but not phosphatidylcholine. We investigated the relationship between the toxin-induced hemolysis and SM metabolic system in sheep erythrocytes. Alpha-toxin simultaneously induced hemolysis and a reduction in the levels of SM and formation of ceramide and sphingosine 1-phosphate (S1P). N-Oleoylethanolamine, a ceramidase inhibitor, inhibited the toxin-induced hemolysis and caused ceramide to accumulate in the toxin-treated cells. Furthermore, dl-threo-dihydrosphingosine and B-5354c, isolated from a novel marine bacterium, both sphingosine kinase inhibitors, blocked the toxin-induced hemolysis and production of S1P and caused sphingosine to accumulate. These observations suggest that the toxin-induced activation of the SM metabolic system is closely related to hemolysis. S1P potentiated the toxin-induced hemolysis of saponin-permeabilized erythrocytes but had no effect on that of intact cells. Preincubation of lysated sheep erythrocytes with pertussis toxin blocked the alpha-toxin-induced formation of ceramide from SM. In addition, incubation of C. botulinum C3 exoenzyme-treated lysates of sheep erythrocytes with alpha-toxin caused an accumulation of sphingosine and inhibition of the formation of S1P. These observations suggest that the alpha-toxin-induced hemolysis of sheep erythrocytes is dependent on the activation of the SM metabolic system through GTP-binding proteins, especially the formation of S1P.

Mitochondrial cytochrome c release mediates ceramide-induced activator protein 2 activation and gene expression in keratinocytes

The intracellular signaling pathway(s) through which second messenger ceramides induce gene expression in human cells has not yet been characterized. In the present study, ceramide-induced expression of intercellular adhesion molecule-1 (ICAM-1), which requires activation of transcription factor activator protein 2 (AP-2), was found to be mediated through a mitochondrial pathway. Inhibitors of mitochondrial electron transport chain (e.g. rotenone, thenoyltrifluoroacetone, and antimycin A) reduced ceramide-induced ICAM-1 expression. Stimulation of human keratinocytes with cell-permeant ceramides at concentrations that did not induce apoptosis (no activation of caspases 3, 8, and 9 and no nucleosomal fragmentation) but caused AP-2 activation and ICAM-1 induction released cytochrome c (cyt c) from mitochondria into the cytoplasm of cells. This cyt c release was an indispensable prerequisite for effective ceramide signaling, because its inhibition by modulating the mitochondrial megachannel with bonkrekic acid or carboxyatractyloside prevented ceramide-induced AP-2 activation and ICAM-1 expression. Analysis of the interaction between cyt c and AP-2 revealed that cyt c oxidized AP-2 and that this redox regulation greatly enhanced the DNA binding capacity of AP-2. Mitochondria thus have a previously unrecognized function in signaling ceramide-induced transcription factor activation and gene regulation.

Ceramides and skin function

Ceramides are the major lipid constituent of lamellar sheets present in the intercellular spaces of the stratum corneum. These lamellar sheets are thought to provide the barrier property of the epidermis. It is generally accepted that the intercellular lipid domain is composed of approximately equimolar concentrations of free fatty acids, cholesterol, and ceramides. Ceramides are a structurally heterogeneous and complex group of sphingolipids containing derivatives of sphingosine bases in amide linkage with a variety of fatty acids. Differences in chain length, type and extent of hydroxylation, saturation etc. are responsible for the heterogeneity of the epidermal sphingolipids. It is well known that ceramides play an essential role in structuring and maintaining the water permeability barrier function of the skin. In conjunction with the other stratum corneum lipids, they form ordered structures. An essential factor is the physical state of the lipid chains in the nonpolar regions of the bilayers. The stratum corneum intercellular lipid lamellae, the aliphatic chains in the ceramides and the fatty acids are mostly straight long-chain saturated compounds with a high melting point and a small polar head group. This means that at physiological temperatures, the lipid chains are mostly in a solid crystalline or gel state, which exhibits low lateral diffusional properties and is less permeable than the state of liquid crystalline membranes, which are present at higher temperatures. The link between skin disorders and changes in barrier lipid composition, especially in ceramides, is difficult to prove because of the many variables involved. However, most skin disorders that have a diminished barrier function present a decrease in total ceramide content with some differences in the ceramide pattern. Formulations containing lipids identical to those in skin and, in particular, some ceramide supplementation could improve disturbed skin conditions. Incomplete lipid mixtures yield abnormal lamellar body contents, and disorder intercellular lamellae, whereas complete lipid mixtures result in normal lamellar bodies and intercellular bilayers. The utilization of physiological lipids according to these parameters have potential as new forms of topical therapy for dermatoses. An alternative strategy to improving barrier function by topical application of the various mature lipid species is to enhance the natural lipid-synthetic capability of the epidermis through the topical delivery of lipid precursors.

Transient mechanoactivation of neutral sphingomyelinase in caveolae to generate ceramide

The vascular endothelium acutely autoregulates blood flow in vivo in part through unknown mechanosensing mechanisms. Here, we report the discovery of a new acute mechanotransduction pathway. Hemodynamic stressors from increased vascular flow and pressure in situ rapidly and transiently induce the activity of neutral sphingomyelinase but not that acid sphingomyelinase in a time- and flow rate-dependent manner, followed by the generation of ceramides. This acute mechanoactivation occurs directly at the luminal endothelial cell surface primarily in caveolae enriched in sphingomyelin and neutral sphingomyelinase, but not acid sphingomyelinase. Scyphostatin, which specifically blocks neutral but not acid sphingomyelinase, inhibits mechano-induced neutral sphingomyelinase activity as well as downstream activation of extracellular signal-regulated kinase 1 and 2 (ERK1 and ERK2) by increased flow in situ. We postulate a novel physiological function for neutral sphingomyelinase as a new mechanosensor initiating the ERK cascade and possibly other mechanotransduction pathways.

Growth hormone-induced diacylglycerol and ceramide formation via Galpha i3 and Gbeta gamma in GH4 pituitary cells. Potentiation by dopamine-D2 receptor activation

Growth hormone (GH) secretion is regulated by indirect negative feedback mechanisms. To address whether GH has direct actions on pituitary cells, lipid signaling in GH(4)ZR(7) somatomammotroph cells was examined. GH (EC(50) = 5 nm) stimulated diacylglycerol (DAG) and ceramide formation in parallel by over 10-fold within 15 min and persisting for >3 h. GH-induced DAG/ceramide formation was blocked by pertussis toxin (PTX) implicating G(i)/G(o) proteins and was potentiated 1.5-fold by activation of G(i)/G(o)-coupled dopamine-D2S receptors, which had no effect alone. Following PTX pretreatment, only PTX-resistant Galpha(i)3, not Galpha(o) or Galpha(i)2, rescued GH-induced DAG/ceramide signaling. GH-induced DAG/ceramide formation was also blocked in cells expressing Gbetagamma blocker GRK-ct. In GH(4)ZR(7) cells, GH induced phosphorylation of JAK2 and STAT5, which was blocked by PTX and mimicked by ceramide analogue C2-ceramide or sphingomyelinase treatment to increase endogenous ceramide. We conclude that in GH(4) pituitary cells, GH induces formation of DAG/ceramide via a novel Galpha(i)3/Gbetagamma-dependent pathway. This novel pathway suggests a mechanism for autocrine feedback regulation by GH of pituitary function.

Lipoarabinomannan-induced cell signaling involves ceramide and mitogen-activated protein kinase

Lipoarabinomannan (LAM) is a major cell wall-associated lipoglycan, produced in large amounts (15 mg/g of bacteria) in different species of mycobacteria. Our laboratory has previously reported that LAM from Mycobacterium smegmatis exerts its cytotoxic activity via inhibition of protein kinase C, a key signaling molecule inside the mononuclear cells (S. Ghosh, S. Pal, S. Das, S. K. Dasgupta, and S. Majumdar, FEMS Immunol. Med. Microbiol. 21:181-188, 1998). In this study we report that LAM from Mycobacterium tuberculosis induces a signal transduction pathway in favor of survivability of the host cells via the generation of ceramide, a novel second messenger. The endogenous ceramide level in mononuclear cells was found to be enhanced during LAM treatment. The effects of LAM on protein tyrosine phosphorylation in human peripheral blood mononuclear cells were examined. LAM enhanced the tyrosine phosphorylation of p42 mitogen-activated protein kinase and phosphoinositol 3-kinase (PI3 kinase) and dephosphorylation of stress-activated protein kinase. LAM-induced phosphorylation of p42 (extracellular signal-regulated kinase 2) was further enhanced by wortmannin, a PI3 kinase inhibitor. To examine whether these effects are due to elevation of endogenous ceramide, we exposed the cells to cell-permeative C(2)-ceramide exogenously and studied the activities of different protein kinases. Fluorescence-activated cell sorter analysis and morphological studies showed that LAM induces cell survival. Therefore, these results suggest the ability of LAM to induce ceramide in the altered signaling pathway and help in cell survival.

Regulation of nociceptin/orphanin FQ gene expression in astrocytes by ceramide

Ceramide is generated in response to inflammatory mediators and oxidative stress. Since these stimuli dramatically increase nociceptin/orphanin FQ (N/OFQ) gene expression in astrocytes we evaluated the regulation of N/OFQ by ceramide and the signaling mechanisms involved. We found that ceramide induced N/OFQ mRNA levels 22-fold after 24 h. In astrocytes ceramide stimulated the JNK, p38 and ERK MAP kinase pathways and also led to the activation of the transcription factors CREB and NFkappaB. Using specific inhibitors of signaling pathways we determined that N/OFQ gene induction is mediated by ERK and p38 MAP kinases. ERK activation may be induced by protein kinase C and it leads to CREB phosphorylation. The NFkappaB pathway also appears to be crucial for ceramide-induced N/OFQ gene expression.

Effects of C2-ceramide on the Malme-3M melanoma cell line

Ceramide is implicated in the regulation of various signaling pathways leading to proliferation, differentiation or apoptotic cell death, but there have been few investigations about the effects of ceramide on the cell growth and the melanogenesis of melanocytes. In the present study, we investigated the effects of cell-permeable ceramide on Malme-3M human melanoma cell line. MTT proliferation assay showed that C2-ceramide inhibited the growth of Malme-3M cells in a dose-dependent manner. Cell cycle analysis confirmed the inhibition of DNA synthesis by a reduction in the S phase and an increase in the G0/G1 phase. Flow cytometric analysis for apoptotic cells and morphological observations indicated that the antiproliferative effect of C2-ceramide was not due to apoptosis. We next investigated the effects of C2-ceramide on the pigmentation of Malme-3M melanoma cells. The results showed that C2-ceramide induced only a slight decrease of tyrosinase activity and melanin synthesis. To investigate the ceramide signaling pathway, we studied the influence of C2-ceramide on extracellular signal-regulated kinase (ERK) and Akt activation by Western blot. We demonstrated that the amount of phosphorylated Akt was decreased by C2-ceramide, whereas ERK was activated transiently. Because of a well-known involvement of ceramide in apoptosis, we further investigated the level of caspase-3 and HSP70 after treatment of C2-ceramide. We found that the caspase-3 was not activated and the expression of HSP70 increased moderately. In conclusion, C2-ceramide inhibited the cell growth of Malme-3M cells without the induction of apoptosis. We suggest that increased HSP70 may be related to the resistance against apoptosis.

TNF receptor subtype signalling: differences and cellular consequences

Tumour necrosis factor-alpha (TNF alpha) is a multifunctional cytokine belonging to a family of ligands with an associated family of receptor proteins. The pleiotropic actions of TNF range from proliferative responses such as cell growth and differentiation, to inflammatory effects and the mediation of immune responses, to destructive cellular outcomes such as apoptotic and necrotic cell death mechanisms. Activated TNF receptors mediate the association of distinct adaptor proteins that regulate a variety of signalling processes including kinase or phosphatase activation, lipase stimulation, and protease induction. Moreover, the cytokine regulates the activities of transcription factors, heterotrimeric or monomeric G-proteins and calcium ion homeostasis in order to orchestrate its cellular functions. This review addresses the structural basis of TNF signalling, the pathways employed with their cellular consequences, and focuses on the specific role played by each of the two TNF receptor isotypes, TNFR1 and TNFR2.

Ceramide-induced cell death in primary neuronal cultures: upregulation of ceramide levels during neuronal apoptosis

Ceramide is a sphingolipid that has been implicated both in apoptosis and protection from cell death. We show that in both rat cerebellar granule cells and cortical neuronal cultures application of C(2)-ceramide causes cell death in a dose- and time-dependent manner. Similar effects were observed with the exogenous application of bacterial sphingomyelinase, which hydrolyzes sphingomyelin located on the outer leaflet of the plasma membrane and leads to endogenous ceramide accumulation. Furthermore, endogenous ceramide levels were increased during apoptosis induced by nutrient deprivation or etoposide treatment. These findings suggest that upregulation of ceramide levels, which may be generated through activation of sphingomyelinase, contributes to neuronal apoptosis.

Ceramide generation in situ alters leukocyte cytoskeletal organization and beta 2-integrin function and causes complete degranulation

Ceramide levels increase in activated polymorphonuclear neutrophils, and here we show that endogenous ceramide induced degranulation and superoxide generation and increased surface beta(2)-integrin expression. Ceramide accumulation reveals a bifurcation in integrin function, as it abolished agonist-induced adhesion to planar surfaces, yet had little effect on homotypic aggregation. We increased cellular ceramide content by treating polymorphonuclear neutrophils with sphingomyelinase C and controlled for loss of sphingomyelin by pretreatment with sphingomyelinase D to generate ceramide phosphate, which is not a substrate for sphingomyelinase C. Pretreatment with the latter enzyme blocked all the effects of sphingomyelinase C. Ceramide generation caused a Ca(2+) flux and complete degranulation of both primary and secondary granules and increased surface beta(2)-integrin expression. These integrins were in a nonfunctional state, and subsequent activation with platelet-activating factor or formyl-methionyl-leucyl-phenylalanine induced beta(2)-integrin-dependent homotypic aggregation. However, these cells were completely unable to adhere to surfaces via beta(2)-integrins. This was not due to a defect in the integrins themselves because the active conformation could be achieved by cation switching. Rather, ceramide affected cytoskeletal organization and inside-out signaling, leading to affinity maturation. Cytochalasin D induced the same disparity between aggregation and surface adhesion. We conclude that ceramide affects F-actin rearrangement, leading to massive degranulation, and reveals differences in beta(2)-integrin-mediated adhesive events.

Sphingolipid mediators in cardiovascular cell biology and pathology

Sphingolipids have emerged as a new class of lipid mediators. In response to various extracellular stimuli, sphingolipid turnover can be stimulated in vascular cells and cardiac myocytes. Subsequent generation of sphingolipid molecules such as ceramide, sphingosine, and sphingosine-1-phosphate, is followed by regulation of ion fluxes and activation of various signaling pathways leading to smooth muscle cell proliferation, endothelial cell differentiation or apoptotic cell death, cell contraction, retraction, or migration. The importance of sphingolipids in cardiovascular signaling is illustrated by recent observations implicating them in physiological processes such as vasculogenesis as well as in frequent pathological conditions, including atherosclerosis and its complications.

Ceramide inhibits the outward potassium current in rat pinealocytes

In the present study, we investigated the effect of ceramide on the outward K(+) current in rat pinealocytes using whole cell and single channel recordings. Three components of the whole cell outward K(+) current were separated, an iberiotoxin (IBTX)-sensitive K(+) current (I(KCa)), a transient A current (I(A)) and a delayed rectifier current (I(K)). C6-ceramide reduced all three components of the outward K(+) current. C6-ceramide (30 microM) caused a 53% inhibition of I(KCa) [a component that is generated by the IBTX-sensitive K(+) channel (BK channel)], a 27% inhibition of I(A) and a 17% inhibition of I(K). Additional studies showed that the BK channel was not inhibited by dihydroC6-ceramide, the inactive analog of C6-ceramide, but mimicked by sphingomyelinase which increased intracellular ceramide. The ceramide inhibition of the BK channel was only partly dependent on its inhibition of the L-type Ca(2+) channel. Studies using specific kinase inhibitors showed that calphostin C (a protein kinase C inhibitor) and to a lesser degree lavendustin A (a tyrosine kinase inhibitor) were effective in reducing the ceramide inhibition of I(KCa). Taken together, our results show that, in rat pinealocytes, ceramide reduces the outward K(+) current predominantly by inhibiting I(KCa). Moreover, protein kinase C appears to be the main kinase involved in the ceramide inhibition of I(KCa).

Regulation of MAP kinase by calcium-sensing receptor in bovine parathyroid and CaR-transfected HEK293 cells

Regulation of the extracellular signal-regulated kinase 1 and 2 (ERK1/2) pathway by the extracellular calcium (Ca2+o)-sensing receptor (CaR) was investigated in bovine parathyroid and CaR-transfected human embryonic kidney (HEKCaR) cells. Elevating Ca2+o or adding the selective CaR activator NPS R-467 elicited rapid, dose-dependent phosphorylation of ERK1/2. These phosphorylations were attenuated by pretreatment with pertussis toxin (PTX) or by treatment with the phosphotyrosine kinase (PTK) inhibitors genistein and herbimycin, the phosphatidylinositol-specific phospholipase C (PI-PLC) inhibitor U-73122, or the protein kinase C (PKC) inhibitor GF109203X and were enhanced by the PKC activator phorbol 12-myristate 13-acetate. Combined treatment with PTX and inhibitors of both PKC and PTK nearly abolished high Ca2+o-evoked ERK1/2 activation in HEKCaR cells, demonstrating CaR-mediated coupling via both Gq and G(i). High Ca2+o increased serine phosphorylation of the 85-kDa cytosolic phospholipase A2 (cPLA2) in both parathyroid and HEKCaR cells. The selective mitogen-activated protein kinase (MAPK) inhibitor PD98059 abolished high-Ca2+o)-induced ERK1/2 activation and reduced cPLA2 phosphorylation in both cell types, documenting MAPK's role in cPLA2 activation. Thus our data suggest that the CaR activates MAPK through PKC, presumably through Gq/11-mediated activation of PI-PLC, as well as through G(i)- and PTK-dependent pathway(s) in bovine parathyroid and HEKCaR cells and indicate the importance of MAPK in cPLA2 activation.

Involvement of Fas receptor and not tumor necrosis factor-alpha receptor in ultraviolet-induced activation of acid sphingomyelinase

Fas receptor and tumor necrosis factor receptor-1 (TNFR1) mediate the activation of acid sphingomyelinase (ASMase), which catalyzes the hydrolysis of sphingomyelin to ceramide. Ceramide acts as a second messenger in mediating cell growth, differentiation, stress response, and apoptosis. Ultraviolet (UV) irradiation induces Fas receptor and TNFR1 aggregation. However, the roles of Fas receptor and TNFR1 in mediating UV-induced ASMase activation have not been explored. In this report, we demonstrate that Fas receptor, not TNFR1, mediated UV-induced activation of ASMase. Our data indicate that ASMase activity was not induced with UV irradiation but by TNFalpha in MCF-7 cells that expressed low levels of Fas receptor. In contrast, ASMase was activated by UV irradiation or TNFalpha treatment in Fas stably transfected MCF-7 cells. Immunofluorescence staining of TNFR1 on MCF-7 cells showed that TNFR1 was aggregated after treatment with UV irradiation or TNFalpha. However, UV-induced aggregation of TNFR1 did not lead to induction of ASMase activity. These results suggest that Fas receptor aggregation is solely responsible for UV-induced activation of ASMase. Further, with the use of BJAB and dominant-negative Fas-associated death domain-containing protein (FADD) stably transfected BJAB cells, we demonstrated that dominant-negative FADD partly inhibited UV-induced ASMase activation. Our results suggest that FADD is involved in UV-induced and Fas-mediated signaling pathways for activation of ASMase. Mol. Carcinog. 30:47-55, 2001.

Regulation of FOS by different compartmental stresses induced by low levels of ionizing radiation

We irradiated different cellular compartments and measured changes in expression of the FOS gene at the mRNA and protein levels. [(3)H]Thymidine and tritiated water were used to irradiate the nucleus and the whole cell, respectively. (125)I-Concanavalin A binding was used to irradiate the cell membrane differentially. Changes in FOS mRNA and protein levels were measured using semi-quantitative RT-PCR and SDS-PAGE Western blotting, respectively. Irradiation of the nucleus or the whole cell at a dose rate of 0.075 Gy/h caused no change in the level of FOS mRNA expression, but modestly (1.5-fold) induced FOS protein after 0.5 h. Irradiation of the nucleus at a dose rate of 0.43 Gy/h induced FOS mRNA by 1.5-fold after 0.5 h, but there was no significant effect after whole-cell irradiation. FOS protein was transiently induced 2.5-fold above control levels 0.5 h after a 0. 43-Gy/h exposure of the nucleus or the whole cell. Irradiation of the cell membrane at a dose rate of 1.8 Gy/h for up to 2 h caused no change in the levels of expression of FOS mRNA or protein, but a dose rate of 6.8 Gy/h transiently increased the level of FOS mRNA 3-fold after 0.5 h. These data demonstrate the complexity of the cellular response to radiation-induced damage at low doses. The lack of quantitative agreement between the transcript and protein levels for FOS suggests a role for post-transcriptional regulation.

Sphingomyelin modulates capacitation of human sperm in vitro

Ejaculated mammalian sperm must mature (capacitate) before they can undergo acrosomal exocytosis and fertilize an egg. Loss of sperm sterols is an early step in capacitation. Because sphingomyelin slows cholesterol efflux from other cells, the role of sphingomyelin in capacitation was tested. Human sperm were exposed to sphingomyelinase and then incubated for as long as 24 h. The ability of sperm to acrosome-react in response to progesterone was tested to measure capacitation. Sphingomyelinase-treated sperm became responsive to progesterone approximately 10 h earlier than control sperm. Sphingomyelinase also increased spontaneous acrosomal exocytosis. The effects of sphingomyelinase were accompanied by accelerated losses of the inhibitory sterols, cholesterol and desmosterol. To test whether sphingomyelinase-generated ceramide might promote capacitation, sperm were incubated for 8 h with the cell-permeable ceramide N:-hexanoylsphingosine (25 microM) or with solvent. Ceramide increased the incidence of progesterone-responsive sperm and, at later times, spontaneously reacted sperm. N:-Hexanoylsphinganine, an inactive control ceramide, had no effect. These results suggest that sphingomyelin in the sperm influences the rate of capacitation by slowing the loss of sterols, and that exogenous sphingomyelinase accelerates capacitation by speeding the loss of sterols and by generating ceramide.

Role of ceramide in neuronal cell death and differentiation

Ceramide is a sphingolipid metabolite that has been implicated in cellular apoptosis and differentiation. It has been shown to induce apoptosis in various mammalian cell lines and, more recently, has been implicated in neuronal apoptosis. Although the mechanisms of ceramide-induced cell death have not been fully elucidated, they appear to involve a number of signal transduction pathways, including proline-directed kinases, phosphatases, phospholipases, transcription factors, and caspases. Interestingly, ceramide also appears to promote survival and differentiation in certain neuronal systems, when applied at lower concentrations and/or at different developmental stages. Together, studies to date indicate an important multipotential role for this lipid in cell death and differentiation.

The putative benzene metabolite 2,3, 5-tris(glutathion-S-yl)hydroquinone depletes glutathione, stimulates sphingomyelin turnover, and induces apoptosis in HL-60 cells

In this study, we show that 2,3,5-tris(glutathion-S-yl)hydroquinone (TGHQ), a putative metabolite of benzene, induces apoptosis in human promyelocytic leukemia (HL-60) cells. Prior to the onset of apoptosis, TGHQ depletes intracellular glutathione (GSH) in a reactive oxygen species (ROS)-independent manner. Neutral, Mg(2+)-dependent sphingomyelinases, which are normally inhibited by GSH, are subsequently activated, as evidenced by increases in intracellular ceramide and depletion of sphingomyelin. As ceramide levels rise, effector caspase (DEVDase) activity steadily increases. Interestingly, while catalase has no effect on TGHQ-mediated depletion of GSH, this hydrogen peroxide (H(2)O(2)) scavenger does inhibit DEVDase activity and apoptosis, provided the enzyme is added to HL-60 cells before an increase in ceramide can be observed. Since ceramide analogues inhibit the mitochondrial respiratory chain, these data imply that ceramide-mediated generation of H(2)O(2) is necessary for the activation of effector caspases-3 and/or -7, and apoptosis. In summary, these studies indicate that TGHQ, and perhaps many quinol-based toxicants and chemotherapeutics, may induce apoptosis in hematopoietic cells by depleting GSH and inducing the proapoptotic ceramide-signaling pathway.

Induction of tissue factor production but not the upregulation of adhesion molecule expression by ceramide in human vascular endothelial cells

Binding of tumor necrosis factor-alpha (TNF-alpha) to p60 TNF-alpha receptor induces the activation of sphingomyelinase to generate ceramide, which in turn activates certain protein kinases and phosphatases, resulting in various TNF-alpha-mediated biological effects. We have investigated the role for the sphingomyelin/ceramide pathway in the TNF-alpha-induced upregulation of adhesion molecule expression and tissue factor production of human endothelial cells. TNF-alpha stimulated human umbilical vascular endothelial cells (HUVECs) to upregulate the expression of vascular cell adhesion molecule-1 (VCAM-1), intercellular adhesion molecule-1 (ICAM-1) and HLA class I molecules in addition to the induction of procoagulant tissue factor production. C2-ceramide, a highly cell-permeable ceramide analog, was able to stimulate HUVECs to produce tissue factor activity as well as TNF-alpha. However, C2-ceramide did not stimulate HUVECs to upregulate the expression of VCAM-1, ICAM-1 and HLA class I molecules. These results suggest that there exist both the ceramide-dependent and -independent pathways in TNF-alpha signal transduction system in human vascular endothelial cells.

Sphingomyelinase and ceramide analogs induce contraction and rises in [Ca(2+)](i) in canine cerebral vascular muscle

Studies were designed to investigate effects of neutral sphingomyelinase (N-SMase) and ceramide analogs as well as phosphorylcholine on vascular tone and Ca(2+) mobilization in isolated canine cerebral arterial smooth muscle. N-SMase (0.001-0.1 U/ml) provoked a gradual but sustained vasoconstriction of arterial rings in a concentration-related manner that was endothelium independent. Incubation of denuded arterial rings in Ca(2+)-free medium or pretreatment with verapamil in extracellular Ca(2+) resulted in a reduction of the N-SMase-evoked constriction. Exposure of arterial rings to 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid (BAPTA)-AM did not, however, result in a reduction of N-SMase-induced constriction. Both staurosporine and bisindolymaleimide I attenuated N-SMase-induced contractions to 66% and 72% of control, respectively. N-SMase caused gradual and sustained rises in intracellular Ca(2+) concentration ([Ca(2+)](i)) in primary cultured cerebral vascular smooth muscle cells. Pretreatment of these cultured cells with nimodipine and verapamil caused a steady decline in N-SMase-induced rises in [Ca(2+)](i). Exposure of the cells to Ca(2+)-free solution reversed the [Ca(2+)](i)-induced rise triggered by N-SMase to the resting baseline. Both C(8) and C(16) ceramide (10(-9)-10(-6) M), but not phosphorylcholine, constricted denuded canine arterial rings in a concentration-related manner and elevated [Ca(2+)](i). Our results suggest that the sphingomyelin-signaling pathway, via a probable release of ceramide molecules, may play an important role in regulation of cerebral arterial wall tone.

Sphingomyelin metabolites in vascular cell signaling and atherogenesis

The atherosclerotic lesion most probably develops through a number of cellular events which implicate all vascular cell types and include synthesis of extracellular proteins, cell proliferation, differentiation and death. Sphingolipids and sphingolipid metabolizing enzymes may play important roles in atherogenesis, not only because of lipoprotein alterations but also by mediating a number of cellular events which are believed to be crucial in the development of the vascular lesions such as proliferation or cell death. Exogenous sphingolipids may mediate various biological effects such as apoptosis, mitogenesis or differentiation depending on the cell type. Moreover, several molecules present in the atherogenic lesion, such as oxidized LDL, growth factors or cytokines, which activate intracellular signaling pathways leading to vascular cell modifications, can stimulate sphingomyelin hydrolysis and generation of ceramide (and other metabolites as sphingosine-1-phosphate). Here we review the potential implication of the sphingomyelin/ceramide cycle in vascular cell signaling related to atherosclerosis, and more generally the role of sphingolipids in the events observed during the atherosclerotic process as cell differentiation, migration, adhesion, retraction, proliferation and death.

The role of ceramide in the modulation of intracellular Ca2+ levels by interleukin 1 beta in rat cortical synaptosomes

We recently demonstrated that the pro-inflammatory cytokine, interleukin 1beta (IL-1beta) elevates intracellular free Ca2+ levels ([Ca2+]i) in rat cortical synaptosomes in a manner involving activation of the IL-1 receptor and stimulation of p42 mitogen-activated protein (MAP) kinase. We now report that the effects of IL-1beta on [Ca2+]i are mimicked by the sphingolipid metabolite ceramide. In cortical synaptosomes ceramide elevates [Ca2+]i in a p42 MAP kinase-dependent manner, and we conclude that the effects of IL-1beta on Ca2+ homeostasis involve ceramide as an upstream component of the p42 MAP kinase pathway.

Purification and characterization of neutral sphingomyelinase from Helicobacter pylori

Phospholipase activities of human gastric bacterium, Helicobacter pylori, are regarded as the pathogenic factors owing to their actions on epithelial cell membranes. In this study, we purified and characterized neutral sphingomyelinase (N-SMase) from the superficial components of H. pylori strains for the first time. N-SMase was purified 2083-fold with an overall recovery of 37%. The purification steps included acid glycine extraction, ammonium sulfate precipitation, CM-Sepharose, Mono-Q, and Sephadex G-75 column chromatography. Approximate molecular mass for the native N-SMase was around 32 kDa. When N-omega-trinitrophenylaminolauryl sphingomyelin (TNPAL-SM) was used as a substrate, the purified enzyme exhibited a K(m) of 6.7 microM and a V(max) of 15.6 nmol of TNPAL-sphingosine/h/mg of protein at 37 degrees C in 50 mM phosphate-buffered saline, pH 7.4. N-SMase reaches optimal activity at pH 7.4 and has a pI of 7.15. The enzyme activity is magnesium dependent and specifically hydrolyzed sphingomyelin and phosphatidylethanolamine. The enzyme also exhibits hemolytic activity on human erythrocytes. According to Western blot analysis, a rabbit antiserum against purified N-SMase from H. pylori cross-reacted with SMase from Bacillus cereus. Sera from individuals with H. pylori infection but not uninfected ones recognizing the purified N-SMase indicated that it was produced in vivo. In enzyme-linked immunosorbent assays, the purified N-SMase used as an antigen was as effective as crude protein antigens in detecting human antibodies to H. pylori.

Radiation-induced expression of functional Fas ligand in EBV-positive human nasopharyngeal carcinoma cells

Ionizing radiation remains a major therapeutic tool against human cancers, especially epithelial tumors, which account for the majority of human malignancies. Although Fas and Fas-L are essential determinants of apoptosis, few data support their role in the cytotoxic effect of ionizing radiation. Epstein-Barr-virus (EBV)-positive nasopharyngeal carcinoma (NPC) were chosen to address this question owing to their known sensitivity to ionizing radiation and their constitutive expression of the Fas-receptor. We here report that, in xenografted NPC cells, Fas-L expression, which was very low in basal conditions, was dramatically increased by tumor irradiation. Both the Fas receptor and the Fas ligand were found to be functional in this model, and a high proportion of irradiated NPC cells underwent apoptosis following tumor irradiation. Induction of Fas-L expression and apoptosis were observed for doses as low as 2 Gy. These data show an increase in Fas-L expression upon irradiation exposure, and strongly suggest that, in some epithelial malignancies, Fas-mediated apoptosis can play a major role in the anti-tumor effect of ionizing radiation, in the range of doses used for therapeutic applications.