N-acetylsphingosine (C2-ceramide) inhibited neutrophil superoxide formation and calcium influx

Unwrapping the mechanisms of ceramide and fatty acid-initiated signals leading to immune-inflammatory responses in obesity

Obesity is considered a global epidemic developed in part as a consequence of the overconsumption of high fat diets. One of the main negative outcomes of obesity is the development of low-grade chronic systemic inflammation, induced by dysregulated immune responses, which can lead to multiple obesity-related diseases. Ceramides are a group of bioactive lipids known to be elevated in obesity and obesity-associated conditions, including cardiovascular disease and type II diabetes. Ceramides may be key players in promoting an obesity-induced inflammatory environment due to their ability to activate key pathways such as Toll-like receptor 4 (TLR4) and NLR pyrin domain containing receptor 3 (Nlrp3), while studies have shown that inhibition of ceramide synthesis gives rise to an anti-inflammatory environment. N-3 polyunsaturated fatty acids (n-3 PUFA) have been of interest due to their anti-inflammatory actions and shown to have beneficial effects in obesity-related diseases. This review will highlight the impact of ceramides in promoting an obesity-induced inflammatory microenvironment and discuss how n-3 PUFA could potentially counteract these responses and have a regulatory effect promoting immune homeostasis.

Sphinganine-Analog Mycotoxins (SAMs): Chemical Structures, Bioactivities, and Genetic Controls

Sphinganine-analog mycotoxins (SAMs) including fumonisins and A. alternata f. sp. Lycopersici (AAL) toxins are a group of related mycotoxins produced by plant pathogenic fungi in the Fusarium genus and in Alternaria alternata f. sp. Lycopersici, respectively. SAMs have shown diverse cytotoxicity and phytotoxicity, causing adverse impacts on plants, animals, and humans, and are a destructive force to crop production worldwide. This review summarizes the structural diversity of SAMs and encapsulates the relationships between their structures and biological activities. The toxicity of SAMs on plants and animals is mainly attributed to their inhibitory activity against the ceramide biosynthesis enzyme, influencing the sphingolipid metabolism and causing programmed cell death. We also reviewed the detoxification methods against SAMs and how plants develop resistance to SAMs. Genetic and evolutionary analyses revealed that the FUM (fumonisins biosynthetic) gene cluster was responsible for fumonisin biosynthesis in Fusarium spp. Sequence comparisons among species within the genus Fusarium suggested that mutations and multiple horizontal gene transfers involving the FUM gene cluster were responsible for the interspecific difference in fumonisin synthesis. We finish by describing methods for monitoring and quantifying SAMs in food and agricultural products.

The role of dihydrosphingolipids in disease

Dihydrosphingolipids refer to sphingolipids early in the biosynthetic pathway that do not contain a C4-trans-double bond in the sphingoid backbone: 3-ketosphinganine (3-ketoSph), dihydrosphingosine (dhSph), dihydrosphingosine-1-phosphate (dhS1P) and dihydroceramide (dhCer). Recent advances in research related to sphingolipid biochemistry have shed light on the importance of sphingolipids in terms of cellular signalling in health and disease. However, dihydrosphingolipids have received less attention and research is lacking especially in terms of their molecular mechanisms of action. This is despite studies implicating them in the pathophysiology of disease, for example dhCer in predicting type 2 diabetes in obese individuals, dhS1P in cardiovascular diseases and dhSph in hepato-renal toxicity. This review gives a comprehensive summary of research in the last 10-15 years on the dihydrosphingolipids, 3-ketoSph, dhSph, dhS1P and dhCer, and their relevant roles in different diseases. It also highlights gaps in research that could be of future interest.

Rotenone induces neurotoxicity through Rac1-dependent activation of NADPH oxidase in SHSY-5Y cells

Neurodegenerative diseases are attributed to impairment of the ubiquitin-proteasome system (UPS). Oxidative stress has been considered a contributing factor in the pathology of impaired UPS by promoting protein misfolding and subsequent protein aggregate formation. Increasing evidence suggests that NADPH oxidase is a likely source of excessive oxidative stress in neurodegenerative disorders. However, the mechanism of activation and its role in impaired UPS is not understood. We show that activation of NADPH oxidase in a neuroblastoma cell line (SHSY-5Y) resulted in increased oxidative and nitrosative stress, elevated cytosolic calcium, ER-stress, impaired UPS, and apoptosis. Rac1 inhibition mitigated the oxidative/nitrosative stress, prevented calcium-dependent ER-stress, and partially rescued UPS function. These findings demonstrate that Rac1 and NADPH oxidase play an important role in rotenone neurotoxicity.

Palmitate induces transcriptional regulation of BACE1 and presenilin by STAT3 in neurons mediated by astrocytes

Deregulation of calcium has been implicated in neurodegenerative diseases, including Alzheimer's disease (AD). Previously, we showed that saturated free-fatty acid, palmitate, causes AD-like changes in primary cortical neurons mediated by astrocytes. However, the molecular mechanisms by which conditioned medium from astrocytes cultured in palmitate induce AD-like changes in neurons are unknown. This study demonstrates that this condition medium from astrocytes elevates calcium level in the neurons, which subsequently increases calpain activity, a calcium-dependent protease, leading to enhance p25/Cdk5 activity and phosphorylation and activation of the STAT3 (signal transducer and activator of transcription) transcription factor. Inhibiting calpain or Cdk5 significantly reduces the upregulation in nuclear level of pSTAT3, which we found to transcriptionally regulate both BACE1 and presenilin-1, the latter is a catalytic subunit of γ-secretase. Decreasing pSTAT3 levels reduced the mRNA levels of both BACE1 and presenilin-1 to near control levels. These data demonstrate a signal pathway leading to the activation of STAT3, and the generation of the amyloid peptide. Thus, our results suggest that STAT3 is an important potential therapeutic target of AD pathogenesis.

Effects of sphingosine and sphingosine analogues on the free radical production by stimulated neutrophils: ESR and chemiluminescence studies

Sphingolipids inhibit the activation of the neutrophil (PMN) NADPH oxidase by protein kinase C pathway. By electron spin resonance spectroscopy (ESR) and chemiluminescence (CL), we studied the effects of sphingosine (SPN) and ceramide analogues on phorbol 12-myristate 13-acetate (PMA, 5 × 10^-7M) stimulated PMN (6 × 10⁶ cells). By ESR with spin trapping (100 mM DMPO: 5,5-dimethyl-1-pyrroline-Noxide), we showed that SPN (5 to 8 × 10^-6M), C₂-ceramide (N-acetyl SPN) and C₆-ceramide (N-hexanoyl SPN) at the final concentration of 2 × 10^-5 and 2 × 10^-4M inhibit the production of free radicals by stimulated PMN. The ESR spectrum of stimulated PMN was that of DMPO-superoxide anion spin adduct. Inhibition by 5 × 10^-6M SPN was equivalent to that of 30 U/ml SOD. SPN (5 to 8 × 10^-6M) has no effect on in vitro systems generating superoxide anion (xanthine 50 mM/xanthine oxidase 110 mU/ml) or hydroxyl radical (Fenton reaction: 88 mM H₂O₂, 0.01 mM Fe²⁺ and 0.01 mM EDTA). SPN and N-acetyl SPN also inhibited the CL of PMA stimulated PMN in a dose dependent manner (from 2 × 10^-6 to 10^-5M), but N-hexanoyl SPN was less active (from 2 × 10^-5 to 2 × 10^-4M). These effects were compared with those of known PMN inhibitors, superoxide dismutase, catalase and azide. SPN was a better inhibitor compared with these agents. The complete inhibition by SPN of ESR signal and CL of stimulated PMN confirms that this compound or one of its metabolites act at the level of NADPH-oxidase, the key enzyme responsible for production of oxygen-derived free radicals.

C16 ceramide is crucial for triacylglycerol-induced apoptosis in macrophages

Triacylglycerol (TG) accumulation caused by adipose triglyceride lipase (ATGL) deficiency or very low-density lipoprotein (VLDL) loading of wild-type (Wt) macrophages results in mitochondrial-mediated apoptosis. This phenotype is correlated to depletion of Ca(2+) from the endoplasmic reticulum (ER), an event known to induce the unfolded protein response (UPR). Here, we show that ER stress in TG-rich macrophages activates the UPR, resulting in increased abundance of the chaperone GRP78/BiP, the induction of pancreatic ER kinase-like ER kinase, phosphorylation and activation of eukaryotic translation initiation factor 2A, the translocation of activating transcription factor (ATF)4 and ATF6 to the nucleus and the induction of the cell death executor CCAAT/enhancer-binding protein homologous protein. C16:0 ceramide concentrations were increased in Atgl-/- and VLDL-loaded Wt macrophages. Overexpression of ceramide synthases was sufficient to induce mitochondrial apoptosis in Wt macrophages. In accordance, inhibition of ceramide synthases in Atgl-/- macrophages by fumonisin B1 (FB1) resulted in specific inhibition of C16:0 ceramide, whereas intracellular TG concentrations remained high. Although the UPR was still activated in Atgl-/- macrophages, FB1 treatment rescued Atgl-/- macrophages from mitochondrial dysfunction and programmed cell death. We conclude that C16:0 ceramide elicits apoptosis in Atgl-/- macrophages by activation of the mitochondrial apoptosis pathway.

An obligate role for membrane-associated neutral sphingomyelinase activity in orienting chemotactic migration of human neutrophils

For polymorphonuclear neutrophils (PMNs) to orient migration to chemotactic gradients, weak external asymmetries must be amplified into larger internal signaling gradients. Lipid mediators, associated with the plasma membrane and within the cell, participate in generating these gradients. This study examined the role in PMN chemotaxis of neutral sphingomyelinase (N-SMase), a plasma membrane-associated enzyme that converts sphingomyelin to ceramide. A noncompetitive N-SMase inhibitor, GW4869 (5 mM, 5 minutes), did not inhibit PMN motility (as percentage of motile cells, or mean cell velocity), but it abrogated any orientation of movement toward the source of the chemotaxin, formylmethionylleucylphenylanaline (FMLP) (net displacement along the gradient axis in micrometers, or as percentage of total migration distance). This defect could be completely reversed by treatment with lignoceric ceramide (5 μg/ml, 15 minutes). Immunolocalization studies demonstrated that N-SMase (1) distributes preferentially toward the leading edge of some elongated cells, (2) is associated with the plasma membrane, (3) is more than 99.5% localized to the cytofacial aspect of the plasma membrane, (4) is excluded from pseudopodial extensions, and (5) increases rapidly in response to FMLP. Morphologically, the inhibition of N-SMase limited cellular spreading and the extension of sheet-like pseudopods. Elongated PMNs demonstrated a polarized distribution of GTPases, with Rac 1/2 accumulated at, and RhoA excluded from, the front of the cell. This polarity was negated by N-SMase inhibition and restored by lignoceric ceramide. We conclude that N-SMase at the cytofacial plasma membrane is an essential element for the proper orientation of PMNs in FMLP gradients, at least in part by polarizing the distribution of Rac 1/2 and RhoA GTPases.

Inhibition of corneal inflammation by liposomal delivery of short-chain, C-6 ceramide

Ceramide is recognized as an antiproliferative and proapoptotic sphingolipid metabolite; however, the role of ceramide in inflammation is not well understood. To determine the role of C6-ceramide in regulating inflammatory responses, human corneal epithelial cells were treated with C6-ceramide in 80 nm diameter nanoliposome bilayer formulation (Lip-C6) prior to stimulation with UV-killed Staphylococcus aureus. Lip-C6 (5 muM) inhibited the phosphorylation of proinflammatory and proapoptotic MAP kinases JNK and p38 and production of neutrophil chemotactic cytokines CXCL1, CXCL5, and CXCL8. Lip-C6 also blocked CXC chemokine production by human and murine neutrophils. To determine the effect of Lip-C6 in vivo, a murine model of corneal inflammation was used in which LPS or S. aureus added to the abraded corneal surface induces neutrophil infiltration to the corneal stroma, resulting in increased corneal haze. Mice were treated topically with 2 nMoles (811 ng) Lip-C6 or with control liposomes prior to, or following, LPS or S. aureus stimulation. We found that corneal inflammation was significantly inhibited by Lip-C6 but not control liposomes given prior to, or following, activation by LPS or S. aureus. Furthermore, Lip-C6 did not induce apoptosis of corneal epithelial cells in vitro or in vivo, nor did it inhibit corneal wound healing. Together, these findings demonstrate a novel, anti-inflammatory, nontoxic, therapeutic role for liposomally delivered short-chain ceramide.

Mechanisms by which fatty acids regulate leucocyte function

Fatty acids (FAs) have been shown to alter leucocyte function and thus to modulate inflammatory and immune responses. In this review, the effects of FAs on several aspects of lymphocyte, neutrophil and macrophage function are discussed. The mechanisms by which FAs modulate the production of lipid mediators, activity of intracellular signalling pathways, activity of lipid-raft-associated proteins, binding to TLRs (Toll-like receptors), control of gene expression, activation of transcription factors, induction of cell death and production of reactive oxygen and nitrogen species are described in this review. The rationale for the use of specific FAs to treat patients with impaired immune function is explained. Substantial improvement in the therapeutic usage of FAs or FA derivatives may be possible based on an improvement in the understanding of the precise molecular mechanisms of action with respect to the different leucocyte types and outcome with respect to the inflammatory responses.

C2-ceramide induces vasodilation in phenylephrine-induced pre-contracted rat thoracic aorta: role of RhoA/Rho-kinase and intracellular Ca2+ concentration

It is known that ceramide may play an important regulatory role in vascular tone although its effect on vascular tone and the mechanisms involved are controversial. The present study was designed to investigate the effects of ceramide and its key initial regulators, TNF-alpha and neutral sphingomyelinase (SMase), on vascular tone of isolated rat thoracic aortic rings and elucidate the mechanisms involved in the changes in vascular tone induced by ceramide. Contractile responses and Fura-2 Ca2+ signals were measured in rat thoracic aortic rings or strips. 10(-5) M C2-ceramide, 0.1 U/ml neutral sphingomyelinase (SMase), and 5x10(-7) g/ml TNF-alpha had no effect on resting tone in rat thoracic aortic rings. However, in phenylephrine-induced pre-contracted rings, treatment with ceramide, SMase, and TNF-alpha evoked a gradual but sustained vasodilation. Vasodilation effect in response to 10(-5) M C2-ceramide was not significantly changed by the absence or presence of endothelium, a cyclooxygenase pathway inhibitor (10(-6) M indomethacin), or PKC inhibitors (10(-5) M H-7 & 5x10(-7) M calphostin-C). Pretreatment with 1 microM Y-27632, a RhoA/Rho-kinase inhibitor, significantly inhibited the phenylephrine-induced contraction itself as well as the C2-ceramide-induced vasodilation. Pre-treatment with 10(-5) M C2-ceramide had no effect on phasic rise in [Ca2+]i and tension evoked by stimulation with 10(-8) M phenylephrine, but post-treatment of C2-ceramide significantly reduced the phenylephrine-induced secondary tonic [Ca2+]i and tension plateau. Our results indicate that C2-ceramide induces vasodilation in phenylephrine-induced pre-contracted rat thoracic aorta. Furthermore, inhibition of phenylephrine-induced activation of RhoA/Rho-kinase pathway as well as phenylephrine-induced elevations in [Ca2+]i are clearly a key factors in C2-ceramide-induced vasodilation.

Ceramides induce programmed cell death in Arabidopsis cells in a calcium-dependent manner

While the role of C2-ceramide in the induction of programmed cell death (PCD) in animal systems has been well documented, little is known of its role in plant cells. Here we show that C2-ceramide induces PCD in Arabidopsis suspension cultures, which is preceded by the generation of a calcium transient and an increase in reactive oxygen species (ROS). Inhibition of the calcium transient prevented cell death, whereas inhibition of ROS had no effect on cell survival. These observations suggest that calcium signalling plays a role in ceramide-induced PCD but is independent of the generation of ROS.

Ceramide 1-phosphate enhances calcium entry through voltage-operated calcium channels by a protein kinase C-dependent mechanism in GH4C1 rat pituitary cells

Sphingomyelin derivatives modulate a multitude of cellular processes, including the regulation of [Ca2+]i (the intracellular free calcium concentration). Previous studies have shown that these metabolites often inhibit calcium entry through VOCCs (voltage-operated calcium channels). In the present study, we show that, in pituitary GH4C1 cells, C1P (C2-ceramide 1-phosphate) enhances calcium entry in a dose-dependent manner. The phospholipase C inhibitor U73122 attenuated the response. C1P invoked a small, but significant, increase in the formation of inositol phosphates. Pre-treatment of the cells with pertussis toxin was without an effect on the C1P-evoked increase in [Ca2+]i. The effect of C1P was critically dependent on extracellular calcium, since no increase in [Ca2+]i was observed when cells in a calcium-free buffer were stimulated with C1P. Furthermore, if the cells were retreated with 300 nM of the VOCC inhibitor nimodipine, the effect of C1P was almost totally abolished. In addition, ceramide C8-1-phosphate evoked an increase in [Ca2+]i, but the onset of the response was slow compared with that of C1P. In cells treated with 1 mM thapsigargin for 15 min, C1P still evoked an increase in [Ca2+]i. In patch-clamp experiments in the whole-cell mode, C1P enhanced calcium entry through the VOCCs compared with vehicle-treated cells. Dialysis of the cells with C1P did not enhance the calcium current. On-cell patch-clamp experiments showed an enhanced probability of the VOCCs being open (P(open)) in the presence of C1P. Inhibition of PKC (protein kinase C) with GF109203X and down-regulation of PKC with PMA attenuated the C1P-evoked increase in [Ca2+]i. Furthermore, down-regulation of PKC abolished the effect of C1P on P(open). This is the first report showing that a sphingomyelin derivative enhances calcium entry through VOCCs.

Regulation of polymorphonuclear leukocyte degranulation and oxidant production by ceramide through inhibition of phospholipase D

Exogenous C(2)-ceramide has been shown to inhibit polymorphonuclear leukocyte (PMN) phagocytosis through inhibition of phospholipase D (PLD) and downstream events, including activation of extracellular signal-regulated kinases 1 and 2, leading to the hyphothesis that the sphingomyelinase pathway is involved in termination of phagocytosis. Here it is postulated that increased PLD activity generating phosphatidic acid and diacylglycerol (DAG) is essential for superoxide release and degranulation and that ceramide, previously shown to be generated during PMN activation, inhibits PLD activation, thereby leading to inhibition of PMN function. When PMNs were primed with granulocyte colony-stimulating factor (G-CSF) and then activated with N-formyl-methionyl-leucyl-phenylalanine (FMLP), C(2)-ceramide (10 microM) completely inhibited release of superoxide, lactoferrin, and gelatinase; the DAG analog sn-1,2-didecanoylglycerol (DiC10) (10 microM) restored oxidase activation and degranulation in the ceramide-treated cells. Similarly, C(2)-ceramide inhibited oxidase activity and degranulation of PMNs treated with cytochalasin B followed by FMLP, and DiC10 restored function. In contrast, C(2)-ceramide did not inhibit phosphorylation of p47phox or p38 mitogen-activated protein kinase, or translocation of p47phox, PLD-containing organelles, adenosine diphosphate-ribosylation factor 1, RhoA, protein kinase C (PKC)-beta or PKC-alpha to the plasma membrane in G-CSF or cytochalasin B-treated, FMLP-activated PMNs. PLD activity increased by 3-fold in G-CSF-primed PMNs stimulated by FMLP and by 30-fold in cytochalasin B-treated PMNs stimulated by FMLP. Both PLD activities were completely inhibited by 10 microM C(2)-ceramide. In conclusion, superoxide, gelatinase, and lactoferrin release require activation of the PLD pathway in primed PMNs and cytochalasin B-treated PMNs. Ceramide may affect protein interactions with PLD in the plasma membrane, thereby attenuating PMN activation.

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.

Thapsigargin-induced calcium entry and apoptotic death of neutrophils are blocked by activation of protein kinase C

Intracellular free calcium ([Ca(2+)](i)) concentration, free oxygen radical (FOR) production, DNA breakdown, and plasma membrane phosphorylation were studied in human neutrophils activated with thapsigargin and phorbol myrisate acetate (PMA). Thapsigargin produced a rapid and sustained rise of [Ca(2+)](i), activated the endonuclease, and caused the breakdown of the neutrophil's DNA with a half-time close to 6 h. The protein kinase C activator PMA failed to inhibit the initial rise of [Ca(2+)](i), but inhibited the second phase of thapsigargin-induced calcium transient and completely blocked the activation of the endonuclease induced by thapsigargin. Thapsigargin induced a minor and delayed production of FOR, whereas PMA caused an abrupt and sustained FOR production that was enhanced by thapsigargin. Two plasma membrane proteins close to 50 and 64 kD were phosphorylated in PMA-activated neutrophils. These results suggest that the nonphosphorylated form of the membrane protein permits basal and thapsigargin-induced calcium entry. Phosphorylation by PMA of plasma membrane protein inhibits calcium uptake in both resting and thapsigargin-activated neutrophils and contributes to the block of the activation of the apoptotic endonuclease.

Elevated ceramide is downstream of altered calcium homeostasis in low serum-induced apoptosis

Two immortalized cell lines, sup (+) and sup (-), derived from mutagenized Syrian hamster embryo cells, were used to study the relationship and temporal order between calcium and ceramide signals during apoptosis. The early preneoplastic cells, termed sup (+), suppress tumorigenicity when hybridized with tumor cells, whereas later-stage sup (-) cells do not. In reduced serum conditions, sup (+) cells cease proliferating and undergo apoptosis; in contrast, sup (-) cells continue slow growth and undergo necrosis. In sup (+) cells, decreased endoplasmic reticulum (ER) calcium occurs 4 h after low serum treatment and precedes apoptosis. Significant elevations in ceramide are observed 16 h after reduced serum treatment of sup (+) cells but are not found in sup (-) cells. Inhibiting ER calcium depletion in low serum-treated sup (+) cells by treating with high levels of calcium prevents both ceramide generation and apoptosis. Conversely, inducing ER calcium depletion in sup (-) cells by treating with low serum plus thapsigargin results in elevated ceramide levels and apoptosis. Furthermore, C(6)-ceramide treatment induced apoptosis of sup (-) cells in low serum, a condition that does not normally cause apoptosis. C(6)-ceramide treatment did not induce apoptosis in either sup (+) or sup (-) cells in 10% serum but did cause G(2)/M arrest. These studies show that ceramide production is downstream of ER calcium release.

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.

Surfactant modulates calcium response of neutrophils to physiologic stimulation via cell membrane depolarization

Pulmonary surfactant (PS) reduces inflammation in the lung by poorly understood mechanisms. We have observed that surfactant-associated proteins (SAP) insert monovalent cation channels in artificial membranes. Neutrophils are primary mediators of acute pulmonary inflammation, and their functions are activated by increases in cytosolic ionized calcium concentration ([Ca2+]) and by changes in membrane potential. We hypothesize that PS inserts SAP-dependent cation channels in neutrophils, causing membrane depolarization, altered [Ca2+] response, and depressed activation. Human neutrophils were isolated, exposed to PS+SAP (1% Survanta), PS-SAP (1% Exosurf), or buffer, and washed before activating with selected stimulants. PS+SAP reduced phorbol ester- and formyl peptide-stimulated adherence and aggregation by 38% (p < 0.05) and 54% (p < 0.02), respectively. PS+SAP also inhibited the formyl peptide-induced [Ca2+] response of neutrophils (p < 0.01), but only in the presence of external Ca2+. Further characterization of this inhibition demonstrated that PS+SAP blocked formyl peptide-induced influx of both Ca2+ and Mn2+, and that this inhibition was present during activation by other neutrophil stimulants (IL-8, immune complexes). Prior depolarization of neutrophils with gramicidin-D similarly inhibited the [Ca2+] response of neutrophils to formyl peptide, and analysis of neutrophil membrane potential by 3,3'-dipentyloxaearbocyanine iodide (diOC5(3)) fluorescence revealed that PS+SAP induced rapid neutrophil depolarization. In contrast, PS-SAP exhibited little effect on neutrophil function, [Ca2+], or membrane potential. We conclude that PS+SAP decreases neutrophil adherence and aggregation responses, blocks Ca2+ influx after physiologic stimulation, and decreases membrane potential. We speculate that these effects are caused by membrane depolarization via SAP-dependent cation channel insertion, and that all of these effects contribute to the antiinflammatory properties of PS+SAP.

Tumor necrosis factor-alpha-mediated signal transduction in human neutrophils: involvement of sphingomyelin metabolites in the priming effect of TNF-alpha on the fMLP-stimulated superoxide production

We investigated the mechanism underlying the priming effect of TNF-alpha on fMLP-stimulated superoxide production in human neutrophils. TNF-alpha enhanced fMLP-stimulated superoxide production in a concentration-dependent manner. TNF-alpha also induced sphingomyelin (SM) hydrolysis and increased the formation of its metabolite, sphingosine-1-phosphate (SP-1-P). The treatment of neutrophils with sphingomyelinase also resulted in a similar priming effect. C2 ceramide produced a concentration-dependent inhibition of fMLP-stimulated superoxide production within the concentration range of 1-30 microM. Sphingosine had a dual effect on fMLP-stimulated superoxide generation, exhibiting a priming effect at lower concentrations (0.2-1 microM), but an inhibitory effect at higher concentrations (1-30 microM). SP-1-P (1-30 microM), showed a concentration-dependent enhancement of fMLP stimulated superoxide production. Furthermore, after treating neutrophils with DL-threo-dihydro-sphingosine, a competitive inhibitor of sphingosine kinase, TNF-alpha produced a similar dual effect as observed with sphingosine. These results strongly suggest that SM hydrolysis plays a key role in the intracellular signal transduction mediating the TNF-alpha-mediated priming effect.

Actin cytoskeletal function is spared, but apoptosis is increased, in WAS patient hematopoietic cells

Mutations in the Wiskott-Aldrich syndrome protein (WASP) have been hypothesized to cause defective actin cytoskeletal function. This resultant dysfunction of the actin cytoskeleton has been implicated in the pathogenesis of Wiskott-Aldrich syndrome (WAS). In contrast, it was found that stimulated actin polymerization is kinetically normal in the hematopoietic lineages affected in WAS. It was also found that the actin cytoskeleton in WAS platelets is capable of producing the hallmark cytoarchitectural features associated with activation. Further analysis revealed accelerated cell death in WAS lymphocytes as evidenced by increased caspase-3 activity. This increased activity resulted in accelerated apoptosis of these cells. CD95 expression was also increased in these cells, suggesting an up-regulation in the FAS pathway in WAS lymphocytes. Additionally, inhibition of actin polymerization in lymphocytes using cytochalasin B did not accelerate apoptosis in these cells. This suggests that the accelerated apoptosis observed in WAS lymphocytes was not secondary to an underlying defect in actin polymerization caused by mutation of the WAS gene. These data indicate that WASP does not play a universal role in signaling actin polymerization, but does play a role in delaying cell death. Therefore, the principal consequence of mutations in theWAS gene is to accelerate lymphocyte apoptosis, potentially through up-regulation of the FAS-mediated cell death pathway. This accelerated apoptosis may ultimately give rise to the clinical manifestations observed in WAS.

Advances in the signal transduction of ceramide and related sphingolipids

Recently, the sphingolipid metabolites ceramide, sphingosine, ceramide 1-P, and sphingosine 1-P have been implicated as second messengers involved in many different cellular functions. Publications on this topic are appearing at a rapidly increasing rate and new developments in this field are also appearing rapidly. It is thus important to summarize the results obtained from many different laboratories and from different fields of research to obtain a clearer picture of the importance of sphingolipid metabolites. This article reviews the studies from the last few years and includes the effects of a variety of extracellular agents on sphingolipid signal transduction pathways in different tissues and cells and on the mechanisms of regulation. Sphingomyelin exists in a number of functionally distinct pools and is composed of distinct molecular species. Sphingomyelin metabolites may be formed by many different pathways. For example, the generation of ceramide from sphingomyelin can be catalyzed by at least five different sphingomyelinases. A large variety of stimuli can induce the generation of ceramide, leading to activation or inhibition of various cellular events such as proliferation, differentiation, apoptosis, and inflammation. The effect of ceramide on these physiological processes is due to its many different downstream targets. It can activate ceramide-activated protein kinases and ceramide-activated protein phosphatases. It also activates or inhibits PKCs, PLD, PLA2, PC-PLC, nitric oxide synthase, and the ERK and SAPK/JNK signaling cascades. Ceramide activates or inhibits transcription factors, modulates calcium homeostasis and interacts with the retinoblastoma protein to regulate cell cycle progression. Most of the work in this field has involved the study of ceramide effects, but the roles of the other three sphingomyelin metabolites is now attracting much attention. The complex interactions between signaling components and ceramide and the controls regulating these interactions are now being identified and are presented in this review.

Human milk effects on neutrophil calcium metabolism: blockade of calcium influx after agonist stimulation

Neutrophils are the predominant cellular mediators of acute inflammation, and human milk suppresses multiple neutrophil functions. We sought to determine whether these effects were mediated through disruption of normal intracellular Ca2+ homeostasis. Exposure of human neutrophils to human milk, followed by washing, resulted in altered Ca2+ transient responses to formyl-peptide stimulation in which the peak cytosolic free Ca2+ concentration ([free Ca]) was the same as in unexposed cells, but the postpeak decline in [free Ca] was more rapid. This effect was observed after human milk exposures as brief as 10 s, persisted for up to 4 h after human milk removal, and was concentration dependent. On the basis of experiments examining Ca2+-free conditions followed by Ca2+ supplementation, and experiments examining spontaneous and stimulated manganese and barium influx into neutrophils, the human milk effect was due to blockade of Ca2+ influx. Decreased Ca2+ transient responses to other physiologic stimuli (IL-8, opsonized Staphylococcus aureus, and immune complexes) were observed after human milk exposures. Rat intestinal epithelial cells and HL-60 cells failed to show these effects, suggesting a selective effect on mature inflammatory cells. Characterization of the Ca2+-blocking activity showed it was heat and acid stable in human milk with a molecular mass between 30-100 kD. Commercial human milk lactoferrin exhibited Ca2+ influx blockade activity, but recombinant human lactoferrin showed none. Separation of the activity by heparin affinity chromatography showed that it was distinct from lactoferrin. Human milk-induced blockade of Ca2+ influx provides a potential mechanism for broad suppression of neutrophil functions that may contribute to the antiinflammatory properties of human milk.

Phosphatidic acid is a potent and selective inhibitor of protein phosphatase 1 and an inhibitor of ceramide-mediated responses

In the present study, we report that phosphatidic acid (PA) functions as a novel, potent, and selective inhibitor of protein phosphatase 1 (PP1). The catalytic subunit of PP1alpha was inhibited by PA dose-dependently in a noncompetitive manner with a K(i) value of 80 nM. The inhibition by PA was specific to PP1 as PA failed to inhibit protein phosphatase 2A (PP2A) or PP2B. Furthermore, PA was the most effective and potent inhibitor of PP1 compared with other phospholipids. Because we recently showed that ceramides activated PP1, we next examined the effects of PA on ceramide stimulation of PP1. PA inhibited both basal and ceramide-stimulated PP1 activities, and ceramide showed potent and stereoselective activation of PP1 in the presence of PA. Next, the effects of PA on ceramide-induced responses were examined. Molt-4 cells took up PA dose- and time-dependently such that by 1 and 3 h, uptake of PA was 0.37 and 0. 65% of total PA added, respectively. PA at 30 microM and calyculin A at 10 nM (an inhibitor of PP1 and PP2A at low concentrations), but not okadaic acid at 10 nM (a PP2A inhibitor at low concentrations) prevented poly(ADP-ribose) polymerase proteolysis induced by C(6)-ceramide. Moreover, the combination of PA with okadaic acid prevented retinoblastoma gene product dephosphorylation induced by C(6)-ceramide. These data suggest that PA functions as a specific regulator of PP1 and may reverse or counteract those effects of ceramide that are mediated by PP1, such as apoptosis and retinoblastoma gene product dephosphorylation.

C2-ceramide attenuates phenylephrine-induced vasoconstriction and elevation in [Ca2+]i in rat aortic smooth muscle

In the present study, we examined the effects of cell-permeable C2-ceramide on contraction of aortic smooth muscle and intracellular free Ca2+ ([Ca2+]i). C2-ceramide (10(-7) to 10(-4) M) alone did not elicit any significant changes in either basal tension or resting levels of [Ca2+]i in rat aortic smooth muscle. However, C2-ceramide (10(-7) to 10(-4) M) attenuated phenylephrine-induced contractions in isolated rat aortic rings in a concentration-related manner, and inhibited elevations in [Ca2+]i in cultured rat aortic smooth muscle cells induced by phenylephrine. C2-ceramide-induced relaxation was found to be only slightly endothelium-dependent. However, nitric oxide inhibitors (L-NNA, L-NMMA), an inhibitor of prostanoid synthesis (indomethacin), an inhibitor of opiate actions, and several inhibitors of the pharmacologic actions of various vasoactive amines all failed to interfere with the vasorelaxant responses of C2-ceramide. Three different inhibitors of protein kinase C, when used in a wide concentration range, also failed to interfere with the ceramide-induced relaxations. Our results suggest that the sphingomyelin-signaling pathway may play an important regulatory role in arterial wall tone.

Tumor necrosis factor-alpha, sphingomyelinase, and ceramide inhibit store-operated calcium entry in thyroid FRTL-5 cells

Tumor necrosis factor alpha (TNF-alpha) is a potent inhibitor of proliferation in several cell types, including thyroid FRTL-5 cells. As intracellular free calcium ([Ca2+]i) is a major signal in activating proliferation, we investigated the effect of TNF-alpha on calcium fluxes in FRTL-5 cells. TNF-alpha per se did not modulate resting [Ca2+]i. However, preincubation (10 min) of the cells with 1-100 ng/ml TNF-alpha decreased the thapsigargin (Tg)-evoked store-operated calcium entry in a concentration-dependent manner. TNF-alpha did not inhibit the mobilization of sequestered calcium. To investigate whether the effect of TNF-alpha on calcium entry was mediated via the sphingomyelinase pathway, the cells were pretreated with sphingomyelinase (SMase) prior to stimulation with Tg. SMase inhibited the Tg-evoked calcium entry in a concentration-dependent manner. Furthermore, an inhibition of calcium entry was obtained after preincubation of the cells with the membrane-permeable C2-ceramide and C6-ceramide analogues. The inactive ceramides dihydro-C2 and dihydro-C6 showed only marginal effects. Neither SMase, C2-ceramide, nor C6-ceramide affected the release of sequestered calcium. C2- and C6-ceramide also decreased the ATP-evoked calcium entry, without affecting the release of sequestered calcium. The effect of TNF-alpha and SMase was inhibited by the kinase inhibitor staurosporin and by the protein kinase C (PKC) inhibitor calphostin C but not by down-regulation of PKC. However, we were unable to measure a significant activation of PKC using TNF-alpha or C6-ceramide. The effect of TNF-alpha was not mediated via activation of either c-Jun N-terminal kinase or p38 kinase. We were unable to detect an increase in the ceramide (or sphingosine) content of the cells after stimulation with TNF-alpha for up to 30 min. Thus, one mechanism of action of TNF-alpha, SMase, and ceramide on thyroid FRTL-5 cells is to inhibit calcium entry.

Caspases Mediate Tumor Necrosis Factor-–Induced Neutrophil Apoptosis and Downregulation of Reactive Oxygen Production

Tumor necrosis factor- (TNF-) exerts two separate effects on neutrophils, stimulating effector functions while simultaneously inducing apoptosis. We examined here the involvement of caspases in neutrophil apoptosis and the effect of TNF-–induced apoptosis on reactive oxygen production. Immunoblotting and affinity labeling showed activation of caspase-8, caspase-3, and a caspase with a large subunit of 18 kD (T18) in TNF-–treated neutrophils. Active caspase-6 and -7 were not detectable in this cell type. Caspase-8 activated caspase-3 and T18 in neutrophil cytoplasmic extracts. zVAD-fmk blocked neutrophil apoptosis, in parallel with the inhibition of caspase activation. TNF-–induced caspase activation was accompanied by a decrease in the ability of neutrophils to release superoxide anion. Conversely, TNF- treatment in the presence of zVAD-fmk caused a prolonged augmentation of superoxide release. Granulocyte-macrophage colony-stimulating factor inhibited TNF-–induced caspase activation and apoptosis, while reversing the diminution in superoxide release. These observations not only suggest that a caspase cascade mediates apoptotic events and downregulates oxygen radical production in TNF-–treated neutrophils, but also raise the possibility that suppression of caspase activation with enhanced proinflammatory actions of TNF- may underlie the pathogenesis of inflammatory diseases.

Caspases Mediate Tumor Necrosis Factor-–Induced Neutrophil Apoptosis and Downregulation of Reactive Oxygen Production

Tumor necrosis factor- (TNF-) exerts two separate effects on neutrophils, stimulating effector functions while simultaneously inducing apoptosis. We examined here the involvement of caspases in neutrophil apoptosis and the effect of TNF-–induced apoptosis on reactive oxygen production. Immunoblotting and affinity labeling showed activation of caspase-8, caspase-3, and a caspase with a large subunit of 18 kD (T18) in TNF-–treated neutrophils. Active caspase-6 and -7 were not detectable in this cell type. Caspase-8 activated caspase-3 and T18 in neutrophil cytoplasmic extracts. zVAD-fmk blocked neutrophil apoptosis, in parallel with the inhibition of caspase activation. TNF-–induced caspase activation was accompanied by a decrease in the ability of neutrophils to release superoxide anion. Conversely, TNF- treatment in the presence of zVAD-fmk caused a prolonged augmentation of superoxide release. Granulocyte-macrophage colony-stimulating factor inhibited TNF-–induced caspase activation and apoptosis, while reversing the diminution in superoxide release. These observations not only suggest that a caspase cascade mediates apoptotic events and downregulates oxygen radical production in TNF-–treated neutrophils, but also raise the possibility that suppression of caspase activation with enhanced proinflammatory actions of TNF- may underlie the pathogenesis of inflammatory diseases.

The formation of ceramide-1-phosphate during neutrophil phagocytosis and its role in liposome fusion

Ceramide, a product of agonist-stimulated sphingomyelinase activation, is known to be generated during the phagocytosis of antibody-coated erythrocytes by polymorphonuclear leukocytes. Agonist-stimulated formation of ceramide-1-phosphate is now shown to occur in 32PO4-labeled neutrophils. Ceramide-1-phosphate is formed by a calcium-dependent ceramide kinase, found predominately in the neutrophil plasma membrane. The neutrophil kinase is specific for ceramide because, in contrast to the bacterial diglyceride kinase, ceramide is not phosphorylated under conditions specific for diglyceride phosphorylation. Conversely, 1,2-diacylglycerol does not serve as substrate for the neutrophil ceramide kinase. Ceramide kinase activation occurs in a time-dependent fashion, reaching peak activity 10 min after formyl peptide stimulation and challenge with antibody-coated erythrocytes. The lipid kinase activity is optimal at pH 6.8. Because the formation of the phagolysosome is a critical event in phagocytosis, the effect of ceramide-1-phosphate in promoting the fusion of liposomes was determined. Both the addition of increasing concentrations of sphingomyelinase D and ceramide-1-phosphate promoted liposomal fusion. In summary, ceramide-1-phosphate is formed during phagocytosis through activation of ceramide kinase. Ceramide-1-phosphate may promote phagolysosome formation.

Protein phosphatase inhibitors exert specific and nonspecific effects on calcium influx in thapsigargin-treated human neutrophils

C2-ceramide but not inhibitors of phosphatase types 1 and 2A (okadaic acid, calyculin A, tautomycin) blocked store-regulated Ca2+ entry induced in human neutrophils by thapsigargin. This contrasts with previous results showing that both types of compounds inhibit Ca2+ influx in fmet-leu-phe-treated cells. In present studies, phosphatase inhibitors increased the rate of secondary Ca2+ influx in a temperature-dependent manner. Their mechanism of action appeared to be independent of phosphatase inhibition since the inactive congeners, norokadaone and tetraacetyl okadaic acid, also potentiated Ca2+ influx at similar concentrations. When Ca2+ stores were predischarged by thapsigargin, okadaic acid but not norokadaone acted synergistically with fMLP to inhibit subsequent Ca2+ entry. Results suggest that blockade of Ca2+ influx in neutrophils is mediated by a phosphorylation reaction that is prolonged by phosphatase inhibitors. The requisite phosphorylation occurs in fMLP-activated cells but may be absent in cells incubated with thapsigargin.

C2-ceramide attenuates prostaglandin F2alpha-induced vasoconstriction and elevation of [Ca2+]i in canine cerebral vascular smooth muscle

Sphingolipids have emerged as important components of signal transduction pathways involved in a variety of cellular processes. In the present study, we examined the effects of C2-ceramide, a cell-permeable sphingolipid, on contraction of canine cerebral vascular smooth muscle and intracellular free Ca2+ ([Ca2+]i). C2-ceramide (10(-8)-10(-4) M) alone did not elicit any significant changes in either basal tension or resting levels of [Ca2+]i in canine cerebrovascular muscle. However, C2-ceramide (10(-7)-10(-4) M) attenuated prostaglandin F2alpha (PGF2alpha)-induced contractions in isolated canine cerebrovascular smooth muscle rings. C2-ceramide (10(-5) M) inhibited the secondary phasic rise of [Ca2+]i evoked by PGF2alpha in cultured canine cerebral vascular smooth muscle cells, resulting in decreases in the elevation in [Ca2+]i. NO inhibitors (L-NNA, L-NMMA), an inhibitor of prostanoid synthesis (indomethacin), an inhibitor of opiate actions and several inhibitors of the pharmacologic actions of various vasoactive amines all failed to interfere with the vasorelaxant response of C2-ceramide. Our results suggest that the sphingomyelin signaling pathway may play an important regulatory role in cerebral arterial wall tone.

Signal transduction of stress via ceramide

The sphingomyelin (SM) pathway is a ubiquitous, evolutionarily conserved signalling system analogous to conventional systems such as the cAMP and phosphoinositide pathways. Ceramide, which serves as second messenger in this pathway, is generated from SM by the action of a neutral or acidic SMase, or by de novo synthesis co-ordinated through the enzyme ceramide synthase. A number of direct targets for ceramide action have now been identified, including ceramide-activated protein kinase, ceramide-activated protein phosphatase and protein kinase Czeta, which couple the SM pathway to well defined intracellular signalling cascades. The SM pathway induces differentiation, proliferation or growth arrest, depending on the cell type. Very often, however, the outcome of signalling through this pathway is apoptosis. Mammalian systems respond to diverse stresses with ceramide generation, and recent studies show that yeast manifest a form of this response. Thus ceramide signalling is an older stress response system than the caspase/apoptotic death pathway, and hence these two pathways must have become linked later in evolution. Signalling of the stress response through ceramide appears to play a role in the development of human diseases, including ischaemia/reperfusion injury, insulin resistance and diabetes, atherogenesis, septic shock and ovarian failure. Further, ceramide signalling mediates the therapeutic effects of chemotherapy and radiation in some cells. An understanding of the mechanisms by which ceramide regulates physiological and pathological events in specific cells may provide new targets for pharmacological intervention.

Products of Sphingolipid Catabolism Block Activation of the p21-Activated Protein Kinases in Neutrophils

Neutrophils stimulated with the chemoatttractant FMLP are known to exhibit a rapid and transient activation of two p21-activated protein kinases (Paks) with molecular masses of approximately 63 and 69 kDa. Paks can be detected by their ability to undergo renaturation and catalyze the phosphorylation of a peptide substrate that corresponds to amino acid residues 297 to 331 of the 47-kDa subunit of the nicotinamide-adenine dinucleotide phosphate-oxidase complex (p47-phox) fixed within a gel. In this study, we demonstrate that N-acetylsphingosine (C2-ceramide) and a variety of sphingoid bases (e.g., d-erythrosphingosine) block activation of the 63- and 69-kDa Paks in neutrophils. The concentrations of these lipids that were effective in blocking Pak activation were similar to those that inhibit a variety of neutrophil responses. Activation of the 63- and 69-kDa Paks was also markedly reduced in neutrophils treated with sphingomyelinase before stimulation. Moreover, we report that addition of C2-ceramide or d-erythrosphingosine to neutrophils after stimulation with FMLP markedly enhances the rate of Pak inactivation. These effects were not mimicked by arachidonate, which is a potent disorganizing agent of neutrophil membranes. These data support and extend the proposal that sphingoid bases may establish a set point in neutrophils for positive stimuli.

Calcium release-activated calcium current (ICRAC) is a direct target for sphingosine

Whole cell patch-clamp recordings were made to study the regulation of the store-operated calcium release-activated calcium current (ICRAC) by metabolites involved in the sphingomyelin pathway in RBL-2H3 cells. Sphingosine, a regulator of cell growth, inhibits ICRAC completely within 200 s and independently from conversion to either sphingosine 1-phosphate or ceramide. Structural analogs of sphingosine, including N,N-dimethylsphingosine, DL-threo-dihydrosphingosine, and N-acetylsphingosine (C2-ceramide) also block ICRAC. This effect is always accompanied by an elevation of whole cell membrane capacitance. These sphingolipids appear, therefore, to accumulate in the plasma membrane and directly block ICRAC channels. Sphingosylphosphorylcholine also increases capacitance but does not inhibit ICRAC, demonstrating structural specificity and that the elevation of capacitance is necessary but not sufficient for block. Nerve growth factor, which is known to break down sphingomyelin, inhibits ICRAC, and this inhibition can be antagonized by reducing sphingosine production with L-cycloserine, suggesting that ICRAC is a physiologically relevant and direct target of sphingosine. We propose that sphingosine directly blocks ICRAC, suggesting that the sphingomyelin pathway is involved in ICRAC regulation.

Purification and characterization of ceramide-activated protein phosphatases

Ceramide has emerged as a potential regulator of diverse cellular functions, and a few direct targets have been identified for its action including protein kinases and phosphatases. In this study, we have purified the predominant ceramide-activated protein phosphatase (CAPP) from rat brain. Utilizing a novel chromatographic approach, CAPP was purified to near homogeneity using hydrophobic interaction chromatography on phenyl Sepharose followed by anion-exchange chromatography on MonoQ. The purified protein was composed of three major bands on SDS-polyacrylamide gel electrophoresis which comigrated with the three subunits of heterotrimeric PP2A. Immunologic studies further identified CAPP to be composed predominantly of heterotrimeric AB'C and ABalphaC as well as heterodimeric PP2A (AC), where C is the catalytic subunit, and A and B are regulatory subunits. These results were also supported by the coelution of CAPP with trimeric and dimeric PP2A on size-exclusion chromatography. These studies provide a convenient and efficient method for the isolation of trimeric and dimeric PP2A, and they allow the biochemical investigation of CAPP.

Activation of a Plasma Membrane–Associated Neutral Sphingomyelinase and Concomitant Ceramide Accumulation During IgG-Dependent Phagocytosis in Human Polymorphonuclear Leukocytes

The sphingomyelin cycle, which plays an important role in regulation of cell growth, differentiation, and apoptosis, involves the formation of ceramide by the action of a membrane-associated, Mg2+-dependent, neutral sphingomyelinase and/or a lysosomal acid sphingomyelinase. In human polymorphonuclear leukocytes (PMNs), ceramide production correlates with and plays a role in the regulation of functional responses such as oxidant release and Fcγ receptor-mediated phagocytosis. To increase our understanding of the sphingomyelin cycle in human PMNs, the cellular location of neutral and acid sphingomyelinases was investigated in resting, formylmethionylleucylphenylalanine (FMLP)-activated, and FMLP-activated PMNs engaged in phagocytosis. In resting PMNs, a Mg2+-dependent, neutral sphingomyelinase was the predominant activity and was localized to the plasma membrane fractions along with the majority of ceramide. Upon FMLP-activation, there was a 1.9-fold increase in this neutral, Mg2+-dependent sphingomyelinase activity, which increased to 2.7-fold subsequent to phagocytosis of IgG opsonized targets. This increase in sphingomyelinase activity was restricted to the plasma membrane fractions, which were also the site of increased ceramide levels. Phospholipase D (PLD) activity, which is a target of ceramide action and is required for phagocytosis, was also found primarily in the plasma membrane fractions of FMLP-activated and phagocytosing PMNs. Our findings indicate that in human PMNs engaged in phagocytosis, the sphingomyelin cycle is restricted to the plasma membrane where intracellular targets of ceramide action, such as PLD, are localized.

Activation of a Plasma Membrane–Associated Neutral Sphingomyelinase and Concomitant Ceramide Accumulation During IgG-Dependent Phagocytosis in Human Polymorphonuclear Leukocytes

The sphingomyelin cycle, which plays an important role in regulation of cell growth, differentiation, and apoptosis, involves the formation of ceramide by the action of a membrane-associated, Mg2+-dependent, neutral sphingomyelinase and/or a lysosomal acid sphingomyelinase. In human polymorphonuclear leukocytes (PMNs), ceramide production correlates with and plays a role in the regulation of functional responses such as oxidant release and Fcγ receptor-mediated phagocytosis. To increase our understanding of the sphingomyelin cycle in human PMNs, the cellular location of neutral and acid sphingomyelinases was investigated in resting, formylmethionylleucylphenylalanine (FMLP)-activated, and FMLP-activated PMNs engaged in phagocytosis. In resting PMNs, a Mg2+-dependent, neutral sphingomyelinase was the predominant activity and was localized to the plasma membrane fractions along with the majority of ceramide. Upon FMLP-activation, there was a 1.9-fold increase in this neutral, Mg2+-dependent sphingomyelinase activity, which increased to 2.7-fold subsequent to phagocytosis of IgG opsonized targets. This increase in sphingomyelinase activity was restricted to the plasma membrane fractions, which were also the site of increased ceramide levels. Phospholipase D (PLD) activity, which is a target of ceramide action and is required for phagocytosis, was also found primarily in the plasma membrane fractions of FMLP-activated and phagocytosing PMNs. Our findings indicate that in human PMNs engaged in phagocytosis, the sphingomyelin cycle is restricted to the plasma membrane where intracellular targets of ceramide action, such as PLD, are localized.

Altered sphingomyelinase and ceramide expression in the setting of ischemic and nephrotoxic acute renal failure

Diverse physical and chemical stimuli can activate sphingomyelinases (SMases), resulting in sphingomyelin (SM) hydrolysis with ceramide release. Since ceramide can profoundly impact a host of homeostatic mechanisms, the concept of a "SM (or SMase) signaling pathway" has emerged. We recently documented that ceramide levels fall abruptly during renal ischemia, and then rebound to twice normal values during early reperfusion (30 to 90 min) Therefore, the present study assessed whether these ceramide changes are paralleled, and hence potentially mediated, by comparable changes in SMase activity. Mice were subjected to 45 minutes of renal ischemia +/- 30 minutes, 90 minutes, or 24 hours of reperfusion. Renal cortices (or isolated proximal tubules) were then assayed for SMase activity (acidic, neutral forms). To characterize whether early post-ischemic ceramide increments are a relatively persistent event, ceramide was assayed following a 24-hour reperfusion period. Finally, to assess whether the observed perturbations were unique to post-ischemic injury, SMase and ceramide were quantified in the setting of glycerol-induced myohemoglobinuria and anti-glomerular basement membrane (alpha GBM) antibody-induced acute renal failure (ARF). Ischemia induced abrupt declines (approximately 50%) in both acidic and neutral SMase activities, and these persisted in an unremitting fashion throughout 24 hours of reperfusion. Nevertheless, increased ceramide expression (2x normal) resulted. Myohemoglobinuria also suppressed acidic/neutral SMases, and again, "paradoxical" ceramide increments were observed. Finally, alpha GBM nephritis increased ceramide levels, but in this instance, a correlate was increased SMase activity. These results suggest that: (1) ceramide is an acute renal "stress rectant" increasing in response to diverse renal insults; (2) this response may occur independently of the classic SM pathway, since the ceramide increments can seemingly be dissociated from increased SMase activity; and (3) given the well documented impact of ceramide and the SM(ase) pathway on apoptosis, cell proliferation, differentiation, and tissue inflammation, the present results have potentially broad ranging implications for the induction and evolution of diverse forms of ARF.

Signaling mechanisms of elevated neutrophil O2- generation after burn injury

A full skin thickness burn injury was produced in anesthetized rats by exposing 25% of total body surface area to 98 degrees C water for 10 s. Sham (exposed to 37 degrees C water) and burn rats were killed 1, 3, 7, or 10 days later. The role of Ca2+ signaling and Ca(2+)-related protein kinase C (PKC) activation in neutrophil O2- generation was ascertained by evaluating the effect of treatment of the rats with the Ca2+ entry blocker, diltiazem. There was an overt enhancement of O2- generation by polymorphonuclear leukocytes from burn rats on days 1, 3, and 7 postburn, with the peak release occurring on day 3 postburn. O2- generation comparable to the sham was noted on day 10 after the burn. O2- releases on days 1, 3, and 7 postburn were accompanied by marked elevation of Cai2+ and PKC responses. Like the O2- release, intracellular Ca2+ concentration ([Ca2+]i) response on day 10 after burn was suppressed to levels found in the sham group. The treatment of burn rats with diltiazem prevented the upregulation of both [Ca2+]i and PKC responses as well as O2- generation in neutrophils in rats on days 1, 3, and 7 after the burn. Because previous studies have shown that increases in [Ca2+]i precede O2- generation and degranulation, our results suggest that neutrophil O2- release enhancement in the early stages after burn injury (e.g., days 1-7 postburn) results from an overactivation of the Cai2+ and PKC signaling pathways. The heightened O2- generation during the early burn injury phase might play a role in tissue damage in one or more of host organs.

Programmed cell death in plant disease: the purpose and promise of cellular suicide

The interaction of pathogens with plants leads to a disruption in cellular homeostasis, often leading to cell death, in both compatible and incompatible relationships. The mechanistic basis of this cellular disruption and consequent death is complex and poorly characterized, but it is established that host responses to pathogens are dependent on gene expression, involve signal transduction, and require energy. Recent data suggest that in animals, a genetically regulated, signal transduction-dependent programmed cell death process, commonly referred to as apoptosis, is conserved over a wide range of phyla. The basic function of apoptosis is to direct the selective elimination of certain cells during development, but it also is a master template that is involved in host responses to many pathogens. Programmed cell death in plants, while widely observed, has not been studied extensively at either the biochemical or genetic level. Current data suggest that activation or suppression of programmed cell death may underlie diseases in plants as it does in animals. This review describes some of the fundamental characteristics of apoptosis in animals and points to a number of connections to programmed cell death in plants that may lead to both a better understanding of disease processes and novel strategies for engineering disease resistance in plants.

Activation of neutral sphingomyelinase in human neutrophils by polyunsaturated fatty acids

Although unesterified polyunsaturated fatty acids (PUFA) have been shown to elicit marked changes in neutrophil function, the associated signal transduction processes require clarification. In this study we examined the effect of PUFA on the sphingomyelin (SM)-signalling cycle in human neutrophils. Treatment of neutrophils with eicosatetraenoic acid [arachidonic acid, 20:4(n-6)] caused a decrease in the mass of cellular SM and an increase in the level of ceramide. 20:4(n-6)-stimulated neutral sphingomyelinase (SMase) activity of the leucocytes in a time- and concentration-dependent manner. Other unsaturated fatty acids, docosahexaenoic [22:6(n-3)], eicosapentaenoic [20:5(n-3)], octadecenoic [oleic, 18:1(n-9)] and octadecadienoic [linoleic, 18:2(n-6)] acids also had the capacity to activate neutral SMase; however, certain 20:4(n-6) derivatives ¿20:4(n-6) methyl ester [20:4(n-6)ME], 15-hydroperoxyeicosatetraenoic (15-HPETE) and 15-hydroxyeicosatetraenoic (15-HETE) acids¿, very-long-chain PUFA ¿tetracosatetraenoic [24:4(n-6)] and octacosatetraenoic [28:4(n-6)] acids¿ and saturated fatty acids [octadecanoic (stearic, 18:0) and eicosanoic (arachidic, 20:0) acids] had no significant effect. Activation of neutral SMase by 20:4(n-6) appeared to involve metabolism via 20:4(n-6)CoA (arachidonoyl CoA) and was not dependent on prostaglandin and leukotriene synthesis. All of the fatty acids and derivatives tested failed to activate acidic SMase of neutrophils. Ceramide was found to inhibit 20:4(n-6)-induced superoxide generation by the cells. It is envisaged that the PUFA-induced ceramide production in neutrophils plays a role in the regulation of biological responses.

Ceramide inhibits nitric oxide production in alveolar macrophages of endotoxin and ethanol plus endotoxin-treated rats

The effect of ceramide on nitric oxide (NO) formation was studied in rat alveolar macrophages (AMs). Rats were infused with ethanol (EtOH) for 3 h, or the EtOH infusion was combined with i.v. injection of endotoxin (ET) 90 min into the infusion. Controls were infused with saline. Alveolar macrophages were obtained by bronchoalveolar lavage and were cultured for 20 h in the presence and absence of ET, interferon-gamma (IFN), C6 ceramide (N-hexanoylsphingosine), and C2 dihydroceramide. Nitric oxide formation was assessed by measurement of nitrites in the medium. C6 ceramide significantly suppressed NO formation in response to in vitro addition of ET, but not IFN. C2 dihydroceramide caused no inhibition. The ceramide effect appears to be not only stimulus specific, but also specific to AMs, since NO formation by Kupffer cells and liver infiltrated neutrophils was not affected. The results suggest involvement of the sphingomyelin cycle in ET-stimulated NO formation in rat AMs.

Direct effect of ceramide on the mitochondrial electron transport chain leads to generation of reactive oxygen species. Role of mitochondrial glutathione

Ceramide is a sphingolipid that is generated in the signaling of inflammatory cytokines such as tumor necrosis factor (TNF), which exerts many functional roles depending on the cell type where it is produced. Since TNF cytotoxicity is mediated by overproduction of reactive oxygen species from mitochondria, we have examined the role of ceramide in generation of oxidative stress in isolated rat liver mitochondria. The present studies demonstrate that addition of N-acetylsphingosine (C2-ceramide) to mitochondria led to an increase of fluorescence of dihydrorhodamine 123 or dichlorofluorescein-stained mitochondria, indicating formation of hydrogen peroxide. Such effect was significant at 0.25 microM and maximal at 1-5 microM C2, decreasing at greater concentrations. This inductive effect of ceramide was mimicked by N-hexanoylsphingosine at the same concentration range, whereas the immediate precursor of C2, C2-dihydroceramide increased hydrogen peroxide at 1-5 microM. Sphingosine generated hydrogen peroxide at concentrations >/=10 microM, whereas diacylglycerol failed to increase hydrogen peroxide. The increase in hydrogen peroxide induced by C2 was not triggered by mitochondrial permeability transition as C2 did not induce mitochondrial swelling. Blocking electron transport chain at complex I and II prevented the increase in hydrogen peroxide induced by C2; however, interruption of electron flow at complex III by antimycin A potentiated the inductive effect of C2. Depletion of matrix GSH prior to exposure to ceramide resulted in a potentiated increase (2-fold) of hydrogen peroxide generation, leading to lipid peroxidation and loss of activity of respiratory chain complex IV compared with GSH-repleted mitochondria. Mitochondria isolated from TNF-treated cells showed an increase (2-3-fold) in the amount of ceramide compared with mitochondria from untreated cells. These results suggest that mitochondria are a target of ceramide produced in the signaling of TNF whose effect on mitochondrial electron transport chain leads to overproduction of hydrogen peroxide and consequently this phenomena may account for the generation of reactive oxygen species during TNF cytotoxicity.

C2-ceramide and C6-ceramide inhibited priming for enhanced release of superoxide in monocytes, but had no effect on the killing of leukaemic cells by monocytes

Ceramide acts as an intracellular second messenger in cellular signal transduction. We examined the effects of two cell-permeable ceramides, C2-ceramide and C6-ceramide, on human monocyte functions. After monocytes were primed with lipopolysaccharide (LPS) or interferon-gamma (IFN-gamma) for 18 hr in suspension culture, they produced a high amount of superoxide (O2-) when triggered by phorbol myristate acetate. C2- or C6-ceramide inhibited O2- release from monocytes primed with LPS (1 ng/ml) or IFN-gamma (100 U/ml), but did not affect unprimed monocytes. An analogue, C2-dihydroceramide, was inactive. C2-ceramide was most effective at 6 microM, and C6-ceramide at 60 microM. C2- or C6-ceramide at these concentrations was not toxic for monocytes, as assessed by trypan blue exclusion and by the 3-[4, 5-dimethylthiazol-2-y1]-2,5 diphenyl tetrazolium bromide (MTT) assay which measures the ability of live cells to produce formazan. C2-ceramide (20 microM) had no effect on the killing of leukaemic cells (HL-60 and K562 cells) by monocytes treated with IFN-gamma, LPS, or both for 18 hr, with killing assessed by an 111 Indium-releasing assay. C2-ceramide (20 microM) induced secretion of low amounts of tumour necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1 beta) from the monocytes. But C2-ceramide did not alter the higher secretion of TNF-alpha or IL-1 beta from monocytes treated with IFN-gamma or LPS. Thus the cell-permeable ceramides acted like antagonists of LPS, rather than analogues of LPS, as has been proposed. The results here showed that the signal transduction pathway for O2- release by monocytes differed from that for the cytolysis of leukaemic cells, and confirmed that oxygen radicals are not involved in cytolysis.

Ceramide Inhibits IgG-Dependent Phagocytosis in Human Polymorphonuclear Leukocytes

Ceramide is a product of agonist-induced sphingolipid metabolism in several cell types, including polymorphonuclear leukocytes (PMNs). In adherent PMNs, the kinetics of ceramide production correspond with the termination of fMLP-stimulated H2O2 release. Furthermore, short chain ceramides inhibit fMLP-mediated H2O2 release in adherent PMNs. In the present study, we investigated the effects of short chain ceramides and sphingoid bases on phagocytosis of IgG-opsonized erythrocytes (EIgG) by suspended PMNs activated with fMLP. N-Acetylsphingosine, N-acetylphytosphingosine, phytosphingosine, sphingosine, and dihydrosphingosine, but not N-acetyldihydrosphingosine, inhibited phagocytosis of EIgG. In contrast, these same lipids did not inhibit fMLP-mediated chemotaxis. Endogenous ceramide levels increased within the first few minutes of phagocytosis, with a significant (P < .05) accumulation by 30 minutes, the time by which phagocytosis was terminated. Neutral sphingomyelinase activity paralleled the increase in ceramide, consistent with the generation of ceramide by the hydrolysis of sphingomyelin. The N-acetyl-conjugated sphingols (C2 ceramides) blocked phosphatidylethanol formation indicating that phospholipase D (PLD) is an intracellular target of ceramide action. These data suggest that ceramides, generated through activation of the sphingomyelin cycle, act as negative regulators of FcγR-mediated phagocytosis.

Functions of ceramide in coordinating cellular responses to stress

Sphingolipid metabolites participate in key events of signal transduction and cell regulation. In the sphingomyelin cycle, a number of extracellular agents and insults (such as tumor necrosis factor, Fas ligands, and chemotherapeutic agents) cause the activation of sphingomyelinases, which act on membrane sphingomyelin and release ceramide. Multiple experimental approaches suggest an important role for ceramide in regulating such diverse responses as cell cycle arrest, apoptosis, and cell senescence. In vitro, ceramide activates a serine-threonine protein phosphatase, and in cells it regulates protein phosphorylation as well as multiple downstream targets [such as interleukin converting enzyme (ICE)-like proteases, stress-activated protein kinases, and the retinoblastoma gene product] that mediate its distinct cellular effects. This spectrum of inducers of ceramide accumulation and the nature of ceramide-mediated responses suggest that ceramide is a key component of intracellular stress response pathways.

Sphingosine-induced inhibition of capacitative calcium influx in CFPAC-1 cells

Sphingosine (10 microM) induced mobilization of intracellular Ca2+ stores in the pancreatic duct adenocarcinoma cell line CFPAC-1. The effect was specific for sphingosine, since the sphingosine analog C2-ceramide had no effect. Sphingosine did not cause Ca2+ entry from extracellular medium, as also shown by following Mn2+ quenching of Fura-2 fluorescence. Furthermore, sphingosine, similarly to the mitochondrial inhibitors rotenone and oligomycin, strongly inhibited the rate of Mn2+ entry triggered by both thapsigargin- and agonist-induced depletion of intracellular stores. The uptake of rhodamine 123, a lipophilic cation which estimates mitochondrial energy level, was reduced by sphingosine to an extent similar to that observed in the presence of mitochondrial inhibitors. It is suggested that impairment of mitochondrial function might be responsible for inhibition of capacitative Ca2+ entry caused by sphingosine.