Modulation of cell signalling by ceramides

Impact of mitochondrial beta-oxidation in fatty acid-mediated inhibition of glioma cell proliferation

Tetradecylthioacetic acid (TTA), which cannot be beta-oxidized, exerts growth-limiting properties in glioma cells. In order to investigate the importance of modulated lipid metabolism and alterations in mitochondrial properties in this cell death process, we incubated glioma cells both with TTA and the oxidizable fatty acid palmitic acid (PA), in the presence of L-carnitine and the carnitine palmitoyltransferase inhibitors etomoxir and aminocarnitine. L-carnitine partly abolished the PA-mediated growth reduction of glioma cells, whereas etomoxir and aminocarnitine enhanced the antiproliferative effect of PA. The production of acid-soluble products increased and the incorporation of PA into glycerolipids decreased after L-carnitine supplementation. L-carnitine was found to enhance the antiproliferative effect of TTA, but did not affect the incorporation of TTA into glycerolipids, or ceramide. PDMP, sphingosine 1-phosphate, desipramine, fumonisin B(1), and L-cycloserine were able not to rescue the glioma cells from PA and TTA-induced growth inhibition, suggesting that increased ceramide production is not important in the growth reduction. TTA-mediated growth inhibition was accompanied with an increased uptake of PA and increased incorporation of PA into triacylglycerol (TG). Our data suggest that mitochondrial functions are involved in fatty acid-mediated growth inhibition. Whether there is a causal relationship between TG accumulation and the apoptotic process remains to be determined.

A lysophosphatidic acid analogue is revealed as a potent inhibitor of phosphatidylcholine synthesis, inducing apoptosis

A previous study demonstrated that cross-desensitization experiments performed with the lysophosphatidic acid (LPA) analogues (R)- and (S)-N-palmitoyl-norleucinol 1-phosphate (PNPAs) inhibited LPA-induced platelet aggregation without any stereospecificity. Here we report opposite biological effects of the two enantiomers on mitogenesis of IMR-90 fibroblasts in relation to their respective metabolism. (R)PNPA was proliferative, while (S)PNPA induced apoptosis by specifically inhibiting phosphatidylcholine biosynthesis at the last step of the CDP-choline pathway controlled by cholinephosphotransferase. This effect was not direct but required dephosphorylation of PNPAs by ecto-lipid phosphate phosphatase before cellular uptake of the generated N-palmitoyl-norleucinols (PNOHs). Inhibition of cholinephosphotransferase by the derivative (S)PNOH was confirmed by an in vitro assay. (S)PNPA proapoptotic effects led us to clarify the mechanism linking cholinephosphotransferase inhibition to apoptosis. Three proapoptotic responses were observed: the activation of caspase-3, the production of ceramides from newly synthesized pools (as demonstrated by the inhibitor Fumonisin B1) and finally the activation of stress-activated protein kinase, p38 and c-Jun N-terminal kinases 1/2, as a result of ceramide increase. Thus our data demonstrate that synthetic analogues of LPA might display stereospecific effects leading to apoptosis independently of classical LPA-activated pathways.

Sphingomyelinases: enzymology and membrane activity

This paper reviews our present knowledge of sphingomyelinases as enzymes, and as enzymes acting on a membrane constituent lipid, sphingomyelin. Six types of sphingomyelinases are considered, namely acidic, secretory, Mg(2+)-dependent neutral, Mg(2+)-independent neutral, alkaline, and bacterial enzymes with both phospholipase C and sphingomyelinase activity. Sphingomyelinase assay methods and specific inhibitors are reviewed. Kinetic and mechanistic studies are summarized, a kinetic model and a general-base catalytic mechanism are proposed. Sphingomyelinase-membrane interactions are considered from the point of view of the influence of lipids on the enzyme activity. Moreover, effects of sphingomyelinase activity on membrane architecture (increased membrane permeability, membrane aggregation and fusion) are described. Finally, a number of open questions on the above topics are enunciated.

Substantial sulfatide deficiency and ceramide elevation in very early Alzheimer's disease: potential role in disease pathogenesis

In addition to pathology in the gray matter, there are also abnormalities in the white matter in Alzheimer's disease (AD). Sulfatide species are a class of myelin-specific sphingolipids and are involved in certain diseases of the central nervous system. To assess whether sulfatide content in gray and white matter in human subjects is associated with both the presence of Alzheimer's disease (AD) pathology as well as the stage of dementia, we analyzed the sulfatide content of brain tissue lipid extracts by electrospray ionization mass spectrometry from 22 subjects whose cognitive status at time of death varied from no dementia to very severe dementia. All subjects with dementia had AD pathology. The results demonstrate that: (i) sulfatides were depleted up to 93% in gray matter and up to 58% in white matter from all examined brain regions from AD subjects with very mild dementia, whereas all other major classes of lipid (except plasmalogen) in these subjects were not altered in comparison to those from age-matched subjects with no dementia; (ii) there was no apparent deficiency in the biosynthesis of sulfatides in very mild AD subjects as characterized by the examination of galactocerebroside sulfotransferase activities in post-mortem brain tissues; (iii) the content of ceramides (a class of potential degradation products of sulfatides) was elevated more than three-fold in white matter and peaked at the stage of very mild dementia. The findings demonstrate that a marked decrease in sulfatides is associated with AD pathology even in subjects with very mild dementia and that these changes may be linked with early events in the pathological process of AD.

Arachidonic acid release by cPLA2 may be causally related to NO-induced apoptosis in vascular smooth muscle cells

Apoptosis has been shown to occur in vascular smooth muscle cells during the development of atherosclerosis. In order to investigate the possible role of arachidonic acid during apoptosis in vascular smooth muscle, we induced apoptosis in cultured rat aortal smooth muscle cells (SMCs) by treatment with either UV (ultraviolet) radiation, tumor necrosis factor-alpha (TNF-alpha) or NO donor drugs (sodium nitroprusside, or S-nitroso-N-acetyl-D-penicillamine, SNAP). Apoptosis was detected by either DNA fragmentation analysis or by TUNEL analysis. UV radiation, TNF-alpha and NO were observed to stimulate apoptosis in the cells as well as to stimulate arachidonate release from the cells. NO also increased levels of cPLA2 in the cells, which is an enzyme that is frequently activated in cells that release arachidonate. These agents stimulated arachidonate release somewhat earlier than they stimulated apoptosis in the cells. The inhibition of cPLA2 by arachidonyl trifluoromethyl ketone (AACOCF3) also led to the inhibition of arachidonate release from the cells as well as the inhibition of nitroprusside stimulated apoptosis. Arachidonic acid itself could induce apoptosis in the cultured cells. These observations provide evidence that arachidonate may be involved in apoptosis in vascular smooth muscle.

Characterization and direct quantitation of ceramide molecular species from lipid extracts of biological samples by electrospray ionization tandem mass spectrometry

A rapid, simple, and reliable method has been developed for the characterization and quantitation of ceramide molecular species directly from chloroform extracts of biological samples by electrospray ionization tandem mass spectrometry (ESI/MS/MS). By exploiting the differential fragmentation patterns of deprotonated ceramide ions, individual 2-hydroxy and nonhydroxy ceramide molecular species were readily identified by ESI/MS/MS with the neutral loss of fragments of mass 256.2 and 327.3 which correspond to sphingosine derivatives. The ions generated from the neutral loss of 256.2 (i.e., [M - H - 256.2](-)) are unique for ceramides with N-acyl sphingosine with the 18-carbon homolog. However, the sensitivity for nonhydroxy ceramides in ESI/MS/MS with the neutral loss of 256.2 is approximately threefold higher than that for 2-hydroxy ceramides. The ions resulting from the neutral loss of 327.3 (i.e., [M - H - 327.3](-)) are specific for 2-hydroxy ceramides. Additionally, all ceramides including both 2-hydroxy and nonhydroxy forms can be confirmed and accurately quantitated by ESI/MS/MS with the neutral loss of 240.2 after correction for (13)C isotope factors. This methodology demonstrated a 1000-fold linear dynamic range and a detection limit at the subfemtomole range and was applied to directly quantitate ceramide molecular species in chloroform extracts of biological samples including brain tissues and cell cultures.

Ceramides and sphingomyelins in skeletal muscles of the rat: content and composition. Effect of prolonged exercise

The sphingomyelin-signaling pathway has been described in many tissues. Ceramide is the main second messenger in this pathway. Ceramide has also been shown to be present in skeletal muscles; however, there are few data on the regulation of the content of ceramide in muscle tissue. Moreover, there are no data on the content of particular ceramides or their composition in the muscles. The aim of the present study was to examine the content and composition of fatty acids (FA) in ceramide and sphingomyelin moieties and the activity of neutral Mg(2+)-dependent sphingomyelinase in different skeletal muscle types of the rat at rest and after exhausting exercise of moderate intensity. The experiments were carried out on male Wistar rats, divided into two groups: 1) control and 2) exercised until exhaustion on a treadmill. Soleus and red and white gastrocnemius muscles were taken. Ceramide and sphingomyelin were separated by TLC. The content of individual FA in the two compounds was determined by gas-liquid chromatography. Twelve different ceramides and sphingomyelins were identified and quantified in each muscle type according to the FA residues. Saturated FA consisted of 68, 67, and 66% of total ceramide-FA and 75, 77, and 78% of total sphingomyelin-FA in soleus and red and white gastrocnemius, respectively. The total content of ceramide- and sphingomyelin-FA and the activity of sphingomyelinase were highest in the soleus and lowest in the white gastrocnemius. Exercise resulted in a reduction in the total content of ceramide- and sphingomyelin-FA in each muscle. This was accounted for mostly by a reduction in content in the amount of saturated FA. Exercise reduced the activity of neutral Mg(2+)-dependent sphingomyelinase in the soleus and red gastrocnemius and did not affect its activity in the white gastrocnemius. We conclude that the sphingomyelin-signaling pathway is present in skeletal muscles and that it is influenced by prolonged exercise.

Ceramide accelerates dephosphorylation of extracellular signal-regulated kinase 1/2 to decrease prostaglandin D(2) production in RBL-2H3 cells

In the present study, the effect of ceramide on antigen-stimulated phosphorylation of extracellular signal-regulated kinase (ERK) in the mechanism responsible for regulating production of prostaglandin (PG) D(2) was investigated in the mast cell line, RBL-2H3 cells. Cell-permeable C(6)-ceramide (N-hexanoylsphingosine) suppressed antigen-stimulated phosphorylation of ERK1/2 and p38 mitogen-activated protein kinase. Ceramide also inhibited production of PGD(2) and an increase in the activity of cytosolic phospholipase A(2) (cPLA(2)), whereas it did not influence the tyrosine phosphorylation of major cellular proteins in response to antigen. The ceramide-induced inhibition of ERK1/2 phosphorylation and of cPLA(2) activation was suppressed by orthovanadate, a tyrosine phosphatase inhibitor, but not by okadaic acid, a serine/threonine phosphatase inhibitor. Addition of ceramide to the lysate prepared from antigen-stimulated cells reduced the phosphorylated ERK1/2, and orthovanadate effectively prevented the reduction. These results suggest that ceramide accelerates the dephosphorylation of phosphorylated ERK1/2 via activation of a protein tyrosine phosphatase, thus preventing activation of cPLA(2) and production of PGD(2).

Determination of ceramides and diglycerides by the diglyceride kinase assay

This review will discuss the utilization of the diglyceride (DG) kinase assay as an analytical method that allows the simultaneous quantitation of DG and ceramide from cell and tissue samples. This enzymatic approach is sensitive, quantitative, and linear over a broad range (20 pmol to 20-25 nmol) for both classes of lipids. It is also practical in that it can be applied to crude lipid extracts and used to process many samples (up to 100) concurrently. However, it has become apparent that this assay has not been conducted optimally, primarily as a result of lack of adherence to its basic principles. The principles illustrated here are also useful to all enzymatic quantitative methods.

5-Aminosalicylate stimulates phospholipase D activity in macrophages

5-Aminosalicylate, which is considered to be the active moiety of sulfasalazine, is one of the most widely used agents for treatment of inflammatory bowel disease. However, its mechanism of action is unclear. In this report, we provide evidence that the phospholipase D pathway is a target for this drug in macrophages. Activation of phospholipase D leads to the generation of important second messengers such as phosphatidic acid, lysophosphatidic acid and diacylglycerol, all of which can regulate cellular responses involved in inflammation. Murine peritoneal macrophages were labeled with [(3)H]myristate, incubated with various drugs, agonists, or inhibitors, and phospholipase D activity was assayed. 5-Aminosalicylate or sulfasalazine stimulated phospholipase D in a time- and concentration-dependent manner. Chelation of extracellular Ca(2+) inhibited phospholipase D activation by either of these drugs whereas pretreatment of macrophages with the tyrosine kinase inhibitor genistein had no effect. Downregulation of protein kinase C by prolonged incubation with phorbol ester completely blocked the activation of phospholipase D. Pertussis toxin decreased the activation of phospholipase D. The levels of inositol 1,4,5-trisphosphate increased by 260% after treatment of macrophages with 5-aminosalicylate. A phosphoinositide-specific phospholipase C inhibitor U73122 blocked phospholipase D activation completely. Interestingly, long-term preincubation of the macrophages with a relatively low concentration of 5-aminosalicylate that did not stimulate phospholipase D activity by itself, potentiated the effect of phorbol ester-induced activation of phospholipase D. Taken together, these results show that 5-aminosalicylate activates phospholipase D via a pathway involving inositol 1,4,5-trisphosphate generation, calcium fluxes, and Gi/Go. Although the mechanisms by which phospholipase D activation by 5-aminosalicylate or sulfasalazine might attenuate inflammatory responses in the intestine remain to be defined, these results highlight a novel potential mechanism of action for these drugs.

1H- and (31)P-MR spectroscopy of primary and recurrent human brain tumors in vitro: malignancy-characteristic profiles of water soluble and lipophilic spectral components

In vitro NMR spectroscopy was performed on specimen of human brain tumors. From all patients, tissue samples of primary tumors and their first recurrences were examined. (31)P- and (1)H-spectra were recorded from samples of meningioma, astrocytoma and glioblastoma. A double extraction procedure of the tissue samples permitted acquisition of information from the membrane fraction and from the cytosolic fraction. (31)P-spectra were used to analyze the lipophilic fraction (phospholipids of the membrane) of the tissue extracts, while the (1)H-spectra reflected information on the metabolic alterations of the hydrophilic, cytosolic fraction of the tissue. The tumor types showed distinctive spectral patterns in both the (31)P- and the (1)H-spectra. Based on the total detectable (31)P signal, the level of phosphatidylcholine was about 34% lower in primary astrocytomas than in primary glioblastomas (p = 0.0003), whereas the level of sphingomyelin was about 45% lower in primary glioblastomas than in primary astrocytomas (p = 0.0061). A similar tendency of these phospholipids was observed when comparing primary and recurrent astrocytoma samples from the same individuals [+15% (p = 0.0103) and -23% (p = 0.0314) change, respectively]. (1)H-spectra of gliomas were characterized by an increase of the ratios of alanine, glycine and choline over creatine as a function of the degree of malignancy. In agreement with findings in the (31)P-spectra, the (1)H-spectra of recurrent astrocytomas showed metabolic profiles of increased malignancy in comparison to their primary occurrence. Since gliomas tend to increase in malignancy upon recurrence, this may reflect evolving tumor metabolism. (1)H-spectra of meningiomas showed the highest ratio of alanine over creatine accompanied by a near absence of myo-inositol. Phospholipid profiles of meningiomas showed higher fractional contents of phosphatidylcholine along with lower phosphatidylserine compared to astrocytomas, while higher phosphatidylethanolamine and sphingomyelin fractional contents distinguished meningiomas from glioblastomas. The extraction method being used in this study combined with high-resolution (1)H- and (31)P-MRS provides a wide range of biochemical information, which enables differentiation not only between tumor types but also between primary and recurrent gliomas, reflecting an evolving tumor metabolism.

Role of programmed cell death in development

Programmed cell death (PCD) is an integral part of both animal and plant development. In animals, model systems such as Caenorhabditis elegans, Drosophila melanogaster, and mice have shown a general cell death profile of induction, caspase mediation, cell death, and phagocytosis. Tremendous strides have been made in cell death research in animals in the past decade. The ordering of the C. elegans genes Ced-3, 4 and 9, identification of caspase-activated DNase that degrades nuclear DNA during PCD, identification of signal transduction modules involving caspases as well as the caspase-independent pathway, and the involvement of mitochondria are some of the findings of immense value in understanding animal PCDs. Similarly, the caspase inactivation mechanisms of infecting viruses to stall host cell death give a new dimension to the viral infection process. However, plant cell death profiles provide an entirely different scenario. The presence of a cell wall that cannot be phagocytosed, absence of the hallmarks of animal PCDs such as DNA laddering, formation of apoptotic bodies, a cell-death-specific nuclease, a biochemical machinery of killer enzymes such as caspases all point to novel ways of cell elimination. Large gaps in our understanding of plant cell death have prompted speculative inferences and comparisons with animal cell death mechanisms. This paper deals with both animals and plants for a holistic view on cell death in eukaryotes.

Seasonal changes in thermotropic behavior of phosphatidylcholine and phosphatidylethanolamine in different organs of the ascidian Halocynthia aurantium

Differential scanning calorimetry and polarising microscopy were used to investigate the crystal-liquid crystal-isotropic melt phase transitions of phosphatidylcholine (PC), and phosphatidylethanolamine (PE), isolated from muscles, gill pouches, gonads and digestive glands of Halocynthia aurantium, collected in summer and winter. We also analyzed the fatty chain composition of these phospholipids. In summer, the crystalline to liquid crystalline phase transitions of PC and PE from different organs were more co-operative than in winter. Their peak maximum temperatures were close and temperature ranges overlapped for summer samples. Peak maximum temperatures of winter samples decreased sharply, by 18-27 degrees C for PC and by 10-44 degrees C for PE, respectively, depending on the organ. Total heat changes of transitions also decreased. Thermograms were completely located at temperatures below -1.7 degrees C (minimal temperature of seawater in winter). In contrast to summer samples, peak maximum temperatures for PC and PE in winter differed significantly, (by 14-30 degrees C depending on organ), while the temperature ranges of their transitions still showed considerable overlap. Simultaneously, the temperature ranges of the liquid crystalline to isotropic phase transitions decreased. The main reason for changes in thermotropic behavior of phospholipids seems to be the decrease of saturated/unsaturated ratios. The existence of stable and thermoadaptative labile phospholipid pools in the membrane structure is proposed. The relationship of these transitions to low- and high-temperature adaptation is discussed.

Induction of MRP1 and gamma-glutamylcysteine synthetase gene expression by interleukin 1beta is mediated by nitric oxide-related signalings in human colorectal cancer cells

Treatment of human colorectal cancer cells HT29 with interleukin 1beta (IL-1beta) induces expression of the multidrug resistance protein (MRP1) gene encoding the ATP-dependent glutathione S-conjugate export (GS-X) pump and the gamma-glutamylcysteine synthetase (gamma-GCSh) gene encoding heavy (catalytic) subunit of gamma-glutamylcysteine synthetase, the rate-limiting enzyme for the biosynthesis of glutathione (GSH). The induction can be suppressed by N(G)-methyl-L-arginine, a specific inhibitor of nitric oxide synthase (NOS). These results suggest that IL-1beta-mediated MRP1 and gamma-GCSh induction involve nitric oxide (NO) -related signaling. Further supports to the involvement of NO in the induction of MRP1 and gamma-GCSh expression are made by the following observations. (i) Expression of MRP1 and gamma-GCSh genes were induced by treating the cells with NO donors, i.e., S-nitro-N-acetyl-D,L-penicillamide (SNAP) and S-nitroso-L-glutathione, in a concentration-dependent manner. (ii) Ectopic expression of inducible NOS (iNOS) activity by transfecting expressible recombinant iNOS cDNA encoding functional iNOS but not the nonfunctional version resulted in elevated expression of MRP1 and gamma-GCSh. We also demonstrated that HT-29 cells treated with either 1L-1beta or SNAP induced ceramide production, and addition of C2 or C6 ceramides into cultured HT-29 cells resulted in induction of gamma-GCSh but not MRP1 expression. Collectively, our results demonstrate that induction of MRP1 and gamma-GCSh by IL-1beta is regulated, at least in part, by an NO-related signaling, and induction of gamma-GCSh is by NO-related ceramide signaling.

Acceleration by ceramide of calcium-dependent translocation of phospholipase A2 from cytosol to membranes in platelets

The effect of ceramide on Ca2+-dependent translocation of cytosolic phospholipase A2 (cPLA2) to membranes was studied. Pretreatment of platelets with sphingomyelinase or C6-ceramide (N-hexanoylsphingosine) led to apparent enhancement of Ca2+-ionophore A23187-stimulated arachidonic acid release but did not affect the cytosolic phospholipase A2 (cPLA2) activity. Under these conditions, the cPLA2 proteins in membranes increased significantly, compared with those by A23187 alone. Sphingomyelinase and C6-ceramide, but not C6-dihydroceramide, a control analog of C6-ceramide, also facilitated the Ca2+-dependent increase in the cPLA2 protein, as well as the activity, in membranes induced by addition of Ca2+ into platelet lysate. Protein kinase Calpha, which possesses a Ca2+-dependent lipid binding domain, was increased in membranes in a Ca2+-dependent manner, but the increase was not accelerated by sphingomyelinase or C6-ceramide. These findings suggest that ceramide in membranes potentiates Ca2+-dependent cPLA2 translocation from cytosol to membranes, probably through modification of membrane phospholipid organization.

Glycerophospholipids in brain: their metabolism, incorporation into membranes, functions, and involvement in neurological disorders

Neural membranes contain several classes of glycerophospholipids which turnover at different rates with respect to their structure and localization in different cells and membranes. The glycerophospholipid composition of neural membranes greatly alters their functional efficacy. The length of glycerophospholipid acyl chain and the degree of saturation are important determinants of many membrane characteristics including the formation of lateral domains that are rich in polyunsaturated fatty acids. Receptor-mediated degradation of glycerophospholipids by phospholipases A(l), A(2), C, and D results in generation of second messengers such as arachidonic acid, eicosanoids, platelet activating factor and diacylglycerol. Thus, neural membrane phospholipids are a reservoir for second messengers. They are also involved in apoptosis, modulation of activities of transporters, and membrane-bound enzymes. Marked alterations in neural membrane glycerophospholipid composition have been reported to occur in neurological disorders. These alterations result in changes in membrane fluidity and permeability. These processes along with the accumulation of lipid peroxides and compromised energy metabolism may be responsible for the neurodegeneration observed in neurological disorders.

Oxalate-induced ceramide accumulation in Madin-Darby canine kidney and LLC-PK1 cells

BACKGROUND

Oxalate exposure produces oxidant stress in renal epithelial cells leading to death of some cells and adaptation of others. The pathways involved in these diverse actions remain unclear, but appear to involve activation of phospholipase A2 (PLA2) and redistribution of membrane phospholipids. The present studies examined the possibility that oxalate actions may also involve increased accumulation of ceramide, a lipid-signaling molecule implicated in a variety of pathways, including those leading to apoptotic cell death.

METHODS

Ceramide accumulation was examined in renal epithelial cells from pig kidney (LLC-PK1 cells) and from dog kidney [Madin-Darby canine kidney (MDCK cells)] using the diacylglycerol kinase assay. Sphingomyelin degradation was assessed by monitoring the disappearance of 3H-sphingomyelin from cells that had been prelabeled with [3H]-choline. The effects of oxalate were compared with those of other oxidants (peroxide, xanthine/xanthine oxidase), other organic acids (formate and citrate), and a known activator of sphingomyelinase in these cells [tumor necrosis factor-alpha (TNF-alpha)]. Separate studies determined whether oxalate-induced accumulation of ceramide could be blocked by pretreatment with antioxidants [Mn (III) tetrakis (1-methyl-4-pyridyl) porphyrin (Mn TMPyP, a superoxide dismutase mimetic) or N-acetylcysteine (NAC; an antioxidant)], with an inhibitor of ceramide synthase [fumonisin B1 (FB1)] or with an inhibitor of PLA2 [arachidonyl trifluoromethylketone (AACOCF3)].

RESULTS

Oxalate exposure produced a significant time- and concentration-dependent increase in cellular ceramide. A reciprocal decrease in 3H-sphingomyelin was observed under these conditions. Increases in cellular ceramide levels were also observed after treatment with other oxidants (hydrogen peroxide, and xanthine/xanthine oxidase), activators of sphingomyelinase (TNF-alpha), exogenous sphingomyelinase, or arachidonic acid. Formate produced similar (albeit smaller) effects, and citrate did not. The oxidant-induced increases in ceramide were attenuated by pretreatment with NAC (a glutathione precursor) and MnTMPyP (a superoxide dismutase mimetic), suggesting a role for cellular redox states. The oxalate-induced increase in ceramide was also attenuated by pretreatment with AACOCF3, suggesting a role for PLA2. Pretreatment with FB1 produced a small but statistically insignificant attenuation of the response to oxalate.

CONCLUSIONS

Oxalate exposure produces a marked accumulation of ceramide in renal epithelial cells by a process that is redox sensitive and mediated in part by activation of PLA2. Since cellular sphingomyelin decreased as ceramide increased, it seems likely that oxalate actions are mediated, at least in part, by an increase in sphingomyelinase activity, although alterations in ceramide synthase are also possible. Further study is required to define the steps involved in oxalate actions and to determine the extent to which ceramide signaling mediates oxalate actions.

Dimyristoylphosphatidylcholine/C16:0-ceramide binary liposomes studied by differential scanning calorimetry and wide- and small-angle x-ray scattering

Ceramide has recently been established as a central messenger in the signaling cascades controlling cell behavior. Physicochemical studies have revealed a strong tendency of this lipid toward phase separation in mixtures with phosphatidylcholines. The thermal phase behavior and structure of fully hydrated binary membranes composed of dimyristoylphosphatidylcholine (DMPC) and N-palmitoyl-ceramide (C16:0-ceramide, up to a mole fraction X(cer) = 0.35) were resolved in further detail by high-sensitivity differential scanning calorimetry (DSC) and x-ray diffraction. Both methods reveal very strong hysteresis in the thermal phase behavior of ceramide-containing membranes. A partial phase diagram was constructed based on results from a combination of these two methods. DSC heating scans show that with increased X(cer) the pretransition temperature T(p) first increases, whereafter at X(cer) > 0.06 it can no longer be resolved. The main transition enthalpy DeltaH remains practically unaltered while its width increases significantly, and the upper phase boundary temperature of the mixture shifts to approximately 63 degrees C at X(cer) = 0.30. Upon cooling, profound phase separation is evident, and for all of the studied compositions there is an endotherm in the region close to the T(m) for DMPC. At X(cer) >/= 0.03 a second endotherm is evident at higher temperatures, starting at 32.1 degrees C and reaching 54.6 degrees C at X(cer) = 0.30. X-ray small-angle reflection heating scans reveal a lamellar phase within the temperature range of 15-60 degrees C, regardless of composition. The pretransition is observed up to X(cer) < 0.18, together with an increase in T(p). In the gel phase the lamellar repeat distance d increases from approximately 61 A at X(cer) = 0. 03, to 67 A at X(cer) = 0.35. In the fluid phase increasing X(cer) from 0.06 to 0.35 augments d from 61 A to 64 A. An L(beta')/L(alpha) (ripple/fluid) phase coexistence region is observed at high temperatures (from 31 to 56.5 degrees C) when X(cer) > 0.03. With cooling from temperatures above 50 degrees C we observe a slow increase in d as the coexistence region is entered. A sudden solidification into a metastable, modulated gel phase with high d values is observed for all compositions at approximately 24 degrees C. The anomalous swelling for up to X(cer) = 0.30 in the transition region is interpreted as an indication of bilayer softening and thermally reduced bending rigidity.

Stimulation of phospholipase D activity by oxidized LDL in mouse peritoneal macrophages

Oxidation of LDL is an important factor in the development of atherosclerosis. However, the mechanisms by which oxidized LDL exerts its atherogenic actions are poorly understood. In the present work, we show that oxidized LDL stimulates phospholipase D (PLD) activity in mouse peritoneal macrophages and that this effect increases with the degree of LDL oxidation. Oxidative modification of LDL results in the production of lipid peroxides and the conversion of phosphatidylcholine to lysophosphatidylcholine. Although we found that lysophosphatidylcholine alone activates PLD, the stimulation of this enzyme activity by oxidized LDL is independent of lysophosphatidylcholine formation. Also, 7-ketocholesterol, the major oxysterol in oxidized LDL, failed to stimulate PLD activity. To determine the mechanism(s) whereby oxidized LDL activates PLD, the possible involvements of protein kinase C and tyrosine phosphorylation were investigated. Pretreatment of macrophages with the protein kinase C inhibitor Ro-32-0432 or downregulation of protein kinase C activity by prolonged incubation with 100 nmol/L 4beta-phorbol 12-myristate 13-acetate did not alter the stimulatory effect of oxidized LDL on PLD activation. However, oxidized LDL stimulated tyrosine phosphorylation of several macrophage proteins, and preincubation of the macrophages with genistein, a tyrosine kinase inhibitor, blocked the activation of PLD by oxidized LDL. In addition, pretreatment with orthovanadate, which inhibits tyrosine phosphatases, enhanced basal and oxidized LDL-stimulated PLD activity. Pretreatment of macrophages with pertussis toxin decreased the stimulatory effect of oxidized LDL, indicating that GTP-binding proteins may also be involved in the activation of PLD by oxidized LDL. We also found that the platelet-activating factor receptor antagonists WEB 2086 and L-659,989 inhibit the oxidized LDL stimulation of PLD, suggesting a role for platelet-activating factor receptor in this process. The stimulation of the PLD pathway by oxidized LDL may be of importance in atherogenesis, because PLD activation leads to generation of important second messengers such as phosphatidate, lysophosphatidate, and diacylglycerol, which are known to regulate many cellular functions.

Molecular consequences of human mast cell activation following immunoglobulin E-high-affinity immunoglobulin E receptor (IgE-FcepsilonRI) interaction

The cross-linking by immunoglobulin E of its high-affinity receptor, FcepsilonRI, on mast cells initiates a complex series of biochemical events leading to degranulation and the synthesis and secretion of eicosanoids and cytokines through the action of transcription factors, such as nuclear factor-kappaB. The initial activation involves the phosphorylation of FcepsilonRI beta- and gamma-subunits through the actions of the tyrosine kinases lyn and syk. For the purposes of description, the subsequent events may be grouped in three cascades characterized by the key proteins involved. First, the phospholipase C-inositol phosphate cascade activates protein kinase C and is largely responsible for calcium mobilization and influx. Second, activation of Ras and Raf via mitogen-activated protein kinase causes the production of arachidonic acid metabolites. Third, the generation of sphingosine and sphingosine-1-phosphate occurs through activation of sphingomyelinase. While the early signaling events tend to be specific for the cited cascades, there is an increasing overlap of activated proteins with the downstream propagation of the signal. It is the balanced interaction between these proteins that culminates in degranulation, synthesis, and release of eicosanoids and cytokines.

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.

Characterization of acidic and neutral sphingomyelinase activities in crude extracts of HL-60 cells

The enzymatic activities of acidic and neutral sphingomyelinases (aSMase and nSMase) in crude extracts of HL-60 cells prepared by short ultrasonic irradiation (sonicates) were characterized. It was found that although both have similar Km and Vmax (approximately 0.2 mM and approximately 3.5 nmol/mg per h, respectively), the two activities differ in many other aspects, including the following: (1) the aSMase activity has higher stability at 37 degrees C; (2) the aSMase is much less sensitive to Triton X-100 ( > 5 mM), compared with < or = 0.4 mM for the nSMase; (3) the nSMase, but not the aSMase, can discriminate between the natural bovine sphingomyelin substrate and the fluorescent substrate lissamine rhodamine dodecanoyl sphingosyl phosphocholine, suggesting that nSMase has higher substrate specificity. TNFalpha, which upon incubation with the HL-60 cells induces cellular SM hydrolysis, does not affect Km or Vmax of the nSMase in HL-60 sonicates. This suggests that TNFalpha may operate through translocation of either the enzyme or the substrate, thereby enhancing substrate availability and rate of hydrolysis, and not through enzyme activation. The relevance of these studies to the sphingomyelin cycle is discussed.

Dietary factors in human colorectal cancer

Colorectal cancer is a significant cause of mortality in Western societies. The progression of the disease from normal colonic epithelium to the acquisition of the malignant phenotype is accompanied by numerous genetic and epigenetic alterations. Compelling experimental and epidemiological evidence indicates that diet and nutrition are key factors in the modulation of colorectal cancer. A salient case in point is the recent observation that a dietary regimen based on a Western-style diet provokes in the rodent colon the appearance of preneoplastic lesions in the absence of any genotoxic insult. This review mainly describes dietary factors that inhibit the development and progression of colorectal cancer. Much is unknown about the precise mechanisms of action of chemically disparate nutrients and how they interfere with the development and progression of this disease. Current knowledge about this important issue is summarized. We believe that continuing scrutiny and precise assessment of the benefits (and potential risks) of nutrients in the treatment and prevention of colorectal cancer will prove significant to controlling this devastating disease.

The platelet-activating factor receptor antagonist L-659,989 inhibits phospholipase D activity

The platelet-activating factor (PAF) receptor antagonist L-659,989 [(+/-)-trans-2-(3-methoxy-5-methylsulfonyl-4-propoxyphenyl)-5-(3,4, 5-trimethoxyphenyl)tetrahydrofuran)] has been reported to be a specific inhibitor of the PAF receptor and as such, it is widely used for assessment of PAF receptor mediated biological effects. We report here that L-659,989 may not be as specific as previously reported because it is also a potent inhibitor of phospholipase D activity. At concentrations of 30 micrograms/ml, L-659,989 inhibited basal and agonist-stimulated phospholipase D activity by about 55% and 70-100% respectively, through a mechanism that may involve the generation of intracellular ceramides. Another PAF receptor antagonist, WEB-2086, did not affect phospholipase D activity at concentrations up to 50 micrograms/ml. Either of these inhibitors when present at 20 micrograms/ml are reported to fully block the effects of PAF. Furthermore, L-659,989 directly inhibited the activity of bacterial PLD in vitro. These results indicate that caution is required in the interpretation of results derived from the use of L-659,989.

Nitric oxide in septic shock

Septic shock is a major cause of death following trauma and is a persistent problem in surgical patients throughout the world. It is characterised by hypotension and vascular collapse, with a failure of the major organs within the body. The role of excessive nitric oxide (NO) production, following the cytokine-dependent induction of the inducible nitric oxide synthase (iNOS), in the development of septic shock is discussed. Emphasis is placed upon the signal-transduction process by which iNOS is induced and the role of NO in cellular energy dysfunction and the abnormal function of the cardiovascular system and liver during septic shock.

Signalling sphingomyelinases: which, where, how and why?

A major lipid signalling pathway in mammalian cells implicates the activation of sphingomyelinase (SMase), which upon cell stimulation hydrolyses the ubiquitous sphingophospholipid sphingomyelin to ceramide. This review summarizes our current knowledge on the nature and regulation of signalling SMase(s). Because of the controversy on the identity of this(these) phospholipase(s), the roles of various SMases in cell signalling are discussed. Special attention is also given to the subcellular site of action of signalling SMases and to the cellular factors that positively or negatively control their activity. These regulating agents include lipids (arachidonic acid, diacylglycerol and ceramide), kinases, proteases, glutathione and other proteins.

Involvement of lipid mediators on cytokine signaling and induction of secretory phospholipase A2 in immortalized astrocytes (DITNC)

Our previous studies demonstrated the ability of proinflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha) and interleukin 1beta (IL-1beta), to stimulate NFkappaB/DNA binding and synthesis of secretory phospholipase A2 (sPLA2) in immortalized astrocytes (DITNC). In this study, we examined possible involvement of lipid mediators in the cytokine action. Using [14C]serine to label sphingomyelin and ceramide in these cells, subsequent exposure of cells to cytokines did not result in alteration of sphingomyelin/ceramide ratio. Furthermore, neither exogenous sphingomyelinase nor cell-permeable ceramides could stimulate NFkappaB/DNA binding. On the other hand, C-2 ceramide (0.3 microM) as well as other lipid mediators, such as lysophosphatidylcholine and arachidonic acid, were able to elicit a small increase in sPLA2 and potentiate the induction of sPLA2 by TNF-alpha. When DITNC cells were prelabeled with [32P]Pi, an increase in labeled phosphatidic acid (PA) was observed on treatment of cells with IL-1beta (200 U/mL). However, despite the ability of phorbol myristate acetate (PMA) to stimulate phospholipase D (PLD) and synthesis of phosphatidylethanol (PEt) in these cells, PLD activity was not affected by IL-1beta. With the [32P]labeled cells, however, PA-phosphohydrolase inhibitors, such as chlorpromazine and propranolol, could elicit large increases in labeled PA, indicating active PA metabolism in these cells. Cytokines also caused an increase in levels of diacylglycerol (DG) in these cells, although the source of this lipid pool is presently not understood. Taken together, these results provide evidence for the participation of PA and DG in cytokine signaling activity. Furthermore, although cytokines did not cause the release of ceramide, lipid mediators, such as lysophospholipids, and AA could modulate cytokine-mediated induction of sPLA2 in astrocytes.

Lipid second messengers derived from glycerolipids and sphingolipids, and their role in smooth muscle function

The processes that link activation of an external receptor to the internal mechanisms that elicit a physiological response have been the subject of extensive investigation. It has been established that rather than just being an inert barrier to protect the cell from environmental damage, there are populations of phospholipids located within the plasma membrane that act as a reservoir for signalling molecules and when a receptor binds its appropriate activating ligand a chain of events is initiated which leads to the breakdown of these lipids and the release of second messengers. Such processes are rapid enough for physiological responses to be effected. The purpose of this review is to examine the profile of lipid second messengers derived from glycerophospholipids and sphingolipids. In the former class are included phosphoinositide and phosphatidylcholine and the latter includes sphingomyelin. Hydrolysis of such parent compounds is mediated by phospholipases and the profile of metabolites appears to be agonist specific and modulated by a number of mechanisms including heterotrimeric G-protein subunits, small G-proteins, alterations in intracellular calcium concentration, protein kinase C and tyrosine kinases. The recent interest in sphingolipids, particularly in vascular smooth muscle cells, has been provoked by the observation that ceramide and sphingoid base formation is observed in response to vasoconstrictor hormones.

The effects of dietary lipids on gene expression and apoptosis

The beneficial effects of dietary FO with respect to autoimmune disease, CVD and some types of cancer are well established. Studies conducted over the last 10-15 years have established the potent effects of FO on gene expression in the previously mentioned diseases. The effects of dietary FO appear to be selective in nature, with the expression of individual genes simultaneously being increased, decreased or completely unaffected. In order to elucidate the molecular mechanism(s) involved, recent studies have focused on analysing the effects of the long-chain polyunsaturated n-3 fatty acids EPA and DHA which are highly enriched in FO and thought to be the primary mediators of its biological activity. Indeed, it has been found that EPA and DHA appear to both directly and indirectly modulate gene expression in vivo, depending on the gene examined. The direct effects of EPA and DHA are most probably mediated by their ability to bind to positive and/or negative regulatory transcription factors, while the indirect effects appear to be mediated through alterations in the generation of intracellular lipid second messengers (e.g. diacylglycerol and ceramide). Future studies need to be focused on further elucidation of the inter- and intracellular signalling events mediated by dietary n-3 fatty acids. Understanding the molecular mechanism(s) modified by dietary FO will ultimately lead to improved dietary strategies to aid in the prevention of autoimmune disease, CVD and/or certain types of cancer.