Magnesium-dependent sphingomyelinase

Nipping disease in the bud: nSMase2 inhibitors as therapeutics in extracellular vesicle-mediated diseases

Extracellular vesicles (EVs) are indispensable mediators of intercellular communication, but they can also assume a nefarious role by ferrying pathological cargo that contributes to neurological, oncological, inflammatory, and infectious diseases. The canonical pathway for generating EVs involves the endosomal sorting complexes required for transport (ESCRT) machinery, but an alternative pathway is induced by the enrichment of lipid membrane ceramides generated by neutral sphingomyelinase 2 (nSMase2). Inhibition of nSMase2 has become an attractive therapeutic strategy for inhibiting EV biogenesis, and a growing number of small-molecule nSMase2 inhibitors have shown promising therapeutic activity in preclinical disease models. This review outlines the function of EVs, their potential role in disease, the discovery and efficacy of nSMase2 inhibitors, and the path to translate these findings into therapeutics.

Sphingolipids in Alzheimer's disease, how can we target them?

Altered levels of sphingolipids and their metabolites in the brain, and the related downstream effects on neuronal homeostasis and the immune system, provide a framework for understanding mechanisms in neurodegenerative disorders and for developing new intervention strategies. In this review we will discuss: the metabolites of sphingolipids that function as second messengers; and functional aberrations of the pathway resulting in Alzheimer's disease (AD) pathophysiology. Focusing on the central product of the sphingolipid pathway ceramide, we describ approaches to pharmacologically decrease ceramide levels in the brain and we argue on how the sphingolipid pathway may represent a new framework for developing novel intervention strategies in AD. We also highlight the possible use of clinical and non-clinical drugs to modulate the sphingolipid pathway and sphingolipid-related biological cascades.

Enriched Expression of Neutral Sphingomyelinase 2 in the Striatum is Essential for Regulation of Lipid Raft Content and Motor Coordination

Sphingomyelinases are a family of enzymes that hydrolyze sphingomyelin to generate phosphocholine and ceramide. The brain distribution and function of neutral sphingomyelinase 2 (nSMase2) were elucidated in this study. nSMase2 mRNA expression was greatest in the striatum, followed by the prefrontal cortex, hippocampus, cerebellum, thalamus, brainstem, and olfactory bulb. The striatum had the highest level of nSMase2 protein expression, followed by the prefrontal cortex, thalamus, hippocampus, brainstem, and cerebellum. Dense immunolabeling was observed in the striatum, including the caudate-putamen, while moderately dense staining was found in the olfactory bulb and cerebral neocortex. Electron microscopy of the caudate-putamen showed nSMase2 immunoreaction product was present in small diameter dendrites or dendritic spines, that formed asymmetrical synapses with unlabeled axon terminals containing small round vesicles; and characteristics of glutamatergic axons. Lipidomic analysis of the striatum showed increase in long chain sphingomyelins, SM36:1 and SM38:1 after inhibition of nSMase activity. Quantitative proteomic analysis of striatal lipid raft fraction showed many proteins were downregulated by more than 2-fold after inhibition or antisense knockdown of nSMase; consistent with the notion that nSMase2 activity is important for aggregation or clustering of proteins in lipid rafts. Inhibition or antisense knockdown of nSMase2 in the caudate-putamen resulted in motor deficits in the rotarod and narrow beam tests; as well as decreased acoustic startle and improved prepulse inhibition of the startle reflex. Together, results indicate an important function of nSMase2 in the striatum.

Ceramide and neurodegeneration: susceptibility of neurons and oligodendrocytes to cell damage and death

Neurodegenerative disorders are marked by extensive neuronal apoptosis and gliosis. Although several apoptosis-inducing agents have been described, understanding of the regulatory mechanisms underlying modes of cell death is incomplete. A major breakthrough in delineation of the mechanism of cell death came from elucidation of the sphingomyelin (SM)-ceramide pathway that has received worldwide attention in recent years. The SM pathway induces apoptosis, differentiation, proliferation, and growth arrest depending upon cell and receptor types, and on downstream targets. Sphingomyelin, a plasma membrane constituent, is abundant in mammalian nervous system, and ceramide, its primary catabolic product released by activation of either neutral or acidic sphingomyelinase, serves as a potential lipid second messenger or mediator molecule modulating diverse cellular signaling pathways. Neutral sphingomyelinase (NSMase) is a key enzyme in the regulated activation of the SM cycle and is particularly sensitive to oxidative stress. In a context of increasing clarification of the mechanisms of neurodegeneration, we thought that it would be useful to review details of recent findings that we and others have made concerning different pro-apoptotic neurotoxins including proinflammatory cytokines, hypoxia-induced SM hydrolysis and ceramide production that induce cell death in human primary neurons and primary oligodendrocytes: redox sensitive events. What has and is emerging is a vista of therapeutically important ceramide regulation affecting a variety of different neurodegenerative and neuroinflammatory disorders.

Effect of sphingomyelinase-mediated generation of ceramide on aggregation of low-density lipoprotein

This study addresses the response-to-retention hypothesis, which states that the subendothelial retention of atherogenic lipoproteins is the necessary and sufficient condition for the initiation of atherosclerosis. Here we focus on the relationship between the generation of ceramide in the low-density lipoprotein (LDL) phospholipid monolayer and the resulting aggregation of LDL particles. This study provides the first measurement of neutral, Mg (2+)-dependent Sphingomyelinase (Smase)-mediated ceramide formation from LDL-sphingomyelin and does so for a range of enzyme concentrations (0-0.22 units Smase/mL). The kinetics of ceramide generation was measured using a fluorescence assay for the above enzyme concentrations with a fixed substrate concentration (0.33 mg LDL/mL). The kinetics of LDL aggregate formation was measured by dynamic light scattering (DLS, method of cumulants) for identical enzyme concentrations. Ceramide concentration profiles were fit with a modification of the Michaelis-Menten model ( k a = 1.11 x 10 (-1) microM (-1) min (-1), k -a = 6.54 x 10 (2) microM (-1) min (-1), k 1 = 3.33 x 10 (1) microM (-1) min (-1), k -1 = 1.41 x 10 (-2) min (-1), k cat = 8.05 x 10 (1) min (-1), K M = 2.418 microM, k deact = 4.66 x 10 (-2) microM (-1) min (-1)) that accounts for the effects of enzyme attachment to the LDL monolayer and for deactivation of Smase due to product inhibition. LDL aggregation is described by a mass action model as explained in previous studies. A key result of this work is the finding that LDL aggregate size depends directly on ceramide concentration and is independent of enzyme concentration. This study demonstrates how principles of colloid science are relevant to important biomedical problems.

Identification of Mg2+ -dependent neutral sphingomyelinase 1 as a mediator of heat stress-induced ceramide generation and apoptosis

Neutral sphingomyelinases (SMases) are involved in the induction of ceramide-mediated proapoptotic signaling under heat stress conditions. Although ceramide is an important mediator of apoptosis, the neutral SMase that is activated under heat stress has not been identified. In this study, we cloned an Mg(2+)-dependent neutral SMase from a zebrafish embryonic cell cDNA library using an Escherichia coli expression-cloning vector. Screening of the clones using an SMase activity assay with C(6)-7-nitro-2-1,3-benzoxadiazol-4-yl-sphingomyelin as the substrate resulted in the isolation of one neutral SMase cDNA clone. This cDNA encoded a polypeptide of 420 amino acids (putative molecular weight: 46,900) containing two predicted transmembrane domains in its C-terminal region. The cloned neutral SMase 1 acted as a mediator of stress-induced apoptosis. Bacterially expressed recombinant neutral SMase 1 hydrolyzed [choline-methyl-(14)C]sphingomyelin optimally at pH 7.5 in the presence of an Mg(2+) ion. In zebrafish embryonic cells, the endogenous SMase enzyme was localized in the microsomal fraction. In FLAG-tagged SMase-overexpressing cells, neutral SMase 1 colocalized with a Golgi marker in a cytochemical analysis. Inactivation of the enzyme by an antisense phosphorothioate oligonucleotide repressed the induction of ceramide generation, caspase-3 activation, and apoptotic cell death by heat stress. Thus, neutral SMase 1 participates in an inducible ceramide-mediating, proapoptotic signaling pathway that operates in heat-induced apoptosis in zebrafish embryonic cells.

The unexpected role of acid sphingomyelinase in cell death and the pathophysiology of common diseases

Acid sphingomyelinase (ASM; E.C. 3.1.4.12) is best known for its involvement in the lysosomal storage disorder Niemann-Pick disease (NPD). Through studies that began by investigating this rare disease, recent findings have uncovered the important role of this enzyme in the initiation of ceramide-mediated signal transduction. This unique function involves translocation of the enzyme from intracellular compartments to the outer leaflet of the cell membrane, where hydrolysis of sphingomyelin into ceramide initiates membrane reorganization and facilitates the formation and coalescence of lipid microdomains. These microdomains are sites of protein-protein interactions that lead to downstream signaling, and perturbation of microdomain formation influences the pathophysiology of many common diseases. The initial observations implicating ASM in this process have come from studies using cells from patients with NPD or from ASM knockout (ASMKO) mice, where the genetic deficiency of this enzymatic activity has been shown to protect these cells and animals from stress-induced and developmental apoptosis. This review will discuss the complex biology of this enzyme in the context of these new findings and its recently reported importance in common human diseases, including cancer, sepsis, cardiovascular, pulmonary, liver, and neurological diseases as well as the potential for using ASM (or ASM inhibitors) as therapeutic agents.

Ceramide as a TLR4 agonist; a putative signalling intermediate between sphingolipid receptors for microbial ligands and TLR4

Mucosal Toll-like receptors (TLRs) respond to pathogens, but remain inert to the indigenous flora, suggesting that the TLRs can receive pathogen-specific signals. For example, TLR4 signalling is activated in CD14-negative epithelial cells by P-fimbriated, uropathogenic Escherichia coli, but not by lipopolysaccharide. The fimbriae use glycosphingolipids as recognition receptors and there is release of ceramide, which is the membrane-anchoring domain of the receptors. In this study, ceramide was identified as a TLR4 agonist and as a putative signalling intermediate between the glycosphingolipid recognition receptors and TLR4. Exogenous ceramide activated a TLR4-dependent epithelial cell response, as shown by exposing stably transfected TLR4-positive or -negative human embryonal kidney cells to C2 and C6 ceramide. A similar, TLR4-dependent response occurred after deliberate release of endogenous long-chained ceramide with sphingomyelinase. Microbial ligands with glycosphingolipid specificity (P fimbriae or the B subunit of Shiga toxin) were shown to increase the levels of ceramide and to trigger a TLR4-dependent response in epithelial cells. The results show that ceramide activates TLR4 signalling and suggest that this mechanism might allow pathogens to elicit mucosal TLR4 responses by perturbing sphingolipid receptors for virulence ligands like P fimbriae.

Sphingolipids in apoptosis, survival and regeneration in the nervous system

Simple sphingolipids such as ceramide, sphingosine and sphingosine 1-phosphate are key regulators of diverse cellular functions. Their roles in the nervous system are supported by extensive evidence derived primarily from studies in cultured cells. More recently animal studies and studies with human samples have revealed the importance of ceramide and its metabolites in the development and progression of neurodegenerative disorders. The roles of sphingolipids in neurons and glial cells are complex, cell dependent, and many times contradictory. In this review I will summarize the effects elicited by ceramide and ceramide metabolites in cells of the nervous system, in particular those effects related to cell survival and death, emphasizing the molecular mechanisms involved. I also discuss recent evidence for the implication of sphingolipids in the development and progression of certain dementias.

TRAIL and Ceramide

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a clinically useful cytokine. TRAIL induces apoptosis in a wide variety of transformed cells, but does not cause toxicity to most normal cells. Recent studies show that death receptors (DR4 and DR5), decoy receptors (DcR1 and DcR2), and death inhibitors (FLIP, FAP-1, and IAP) are responsible for the differential sensitivity to TRAIL of normal and tumor cells. Several researchers have also shown that genotoxic agents, such as chemotherapeutic agents and ionizing radiation, enhance TRAIL-induced cytotoxicity by increasing DR5 gene expression or decreasing the intracellular level of FLIP, an antiapoptotic protein. Previous studies have shown that ceramide helps to regulate a cell's response to various forms of stress. Stress-induced alterations in the intracellular concentration of ceramide occur through the activation of a variety of enzymes that synthesize or catabolize ceramide. Increases in intracellular ceramide levels modulate apoptosis by acting through key proteases, phosphatases, and kinases. This review discusses the interaction between TRAIL and ceramide signaling pathways in regulating apoptotic death.

Reduction in alkaline sphingomyelinase in colorectal tumorigenesis is not related to the APC gene mutation

BACKGROUND AND AIMS

The sphingomyelin pathway is an important intracellular mechanism in regulating cell growth. The first step in this pathway is catalysed by sphingomyelinases. Alkaline sphingomyelinase is specifically located in the intestinal tract. Markedly reduced alkaline sphingomyelinase activities have been found in sporadic colorectal tumours and in familial adenomatous polyposis adenomas. Since the adenomatous polyposis coli (APC) gene is mutated in about 80% of sporadic colorectal tumors, and familial adenomatous polyposis is the consequence of a germline mutation of the same gene, we examined whether low alkaline sphingomyelinase activity is linked to APC gene mutations.

PATIENTS AND METHODS

Both germline and sporadic adenomatous polyposis coli gene mutations were studied. Alkaline, neutral, and acid sphingomyelinase activities were measured in the intestinal mucosa and content of multiple intestinal neoplasia mice, a murine model of familial adenomatous polyposis and compared to control mice. Alkaline sphingomyelinase activity was also measured in 11 human rectal tumors with APC gene mutation and compared with 9 control tumors without mutation.

RESULTS

Alkaline, neutral, and acid sphingomyelinase activities were present in the small intestine and colon in both mice types with no differences in hydrolytic capacity or distribution pattern. In sporadic rectal tumors similar alkaline sphingomyelinase activities were identified in tumors with somatic APC gene mutations as in samples without mutations. In the tumors without detectable APC mutations beta-catenin was analyzed, but no mutation was detected.

CONCLUSION

Alkaline sphingomyelinase is not directly linked to adenomatous polyposis coli gene mutations.

Development of intestinal alkaline sphingomyelinase in rat fetus and newborn rat

Sphingomyelin metabolism is a novel signal transduction pathway related to cell differentiation, proliferation, and apoptosis. Alkaline sphingomyelinase (alk-SMase) is specifically present in the intestinal tract of many species. The enzyme is important in digestion of dietary sphingomyelin. Milk is the only exogenous source of sphingomyelin for an infant, and digestion of milk sphingomyelin may be important for development of intestinal mucosa. It is unknown whether alk-SMase is present before birth and whether it changes after birth and during the suckling period. We studied activities, expression, and distribution of alk-SMase in rat fetus and newborn. The changes of acid and neutral SMase as well as alkaline phosphatase were analyzed for comparison. Little activity of alk-SMase was identified up to gestation day 20, but increased 10 times during the following 2 d. After birth, the activity continued to increase during the following 4 wk. Western blot using IgY antibody against rat alk-SMase failed to identify the enzyme at gestation day 20 but clearly showed the protein at day 22. The distribution pattern of the enzyme along the intestinal tract in fetus was largely the same as in adult animals, but became more pronounced after birth. Short-term weaning had no effect on alk-SMase activity. The activities of acid and neutral SMase were high at gestation day 20 and decreased significantly before birth. The changes of alk-SMase also differed from those of alkaline phosphatase, another brush border enzyme. Thus, we conclude that alk-SMase is rapidly expressed during the last days of gestation and that the newborn rat acquires the ability to digest milk sphingomyelin early in life.

Sphingomyelin hydrolysis during apoptosis

Sphingolipid breakdown products are now being recognized as important players in apoptosis. Ceramide, which is considered to serve as second messenger, is mainly generated by hydrolysis of the membrane sphingophospholipid sphingomyelin (SM) through the action of a sphingomyelinase (SMase). However, little is known about the localization and regulation of this phenomenon. Here, we summarize the current knowledge on the function of SM hydrolysis in apoptosis signaling. In particular, the present review focuses on the role of neutral sphingomyelinase (N-SMase) in the generation of the proapoptotic ceramide. This enzyme is regulated by several mechanisms, including the tumor necrosis factor (TNF) receptor-associated protein FAN (for factor associated with N-SMase activation) and oxidative stress. These observations place SMase activation and SM hydrolysis as early events in the apoptosis signaling cascade.

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.

Chronic colitis is associated with a reduction of mucosal alkaline sphingomyelinase activity

BACKGROUND AND AIMS

The hydrolysis of sphingomyelin (SM) generates key molecules regulating cell growth. Animal cancer studies support an inhibitory role for this pathway in the malignant transformation of the colonic mucosa. The activity of a specific intestinal alkaline sphingomyelinase (SMase), which hydrolyzes SM, is reduced in colorectal tumors. In this study we measured alkaline SMase activity in patients with longstanding colitis and assessed if a reduction can be used as a marker in surveillance of high risk patients.

METHODS

Alkaline SMase activity was measured in 139 colonic biopsies from 34 patients with longstanding, extensive colitis and from 11 controls. Fifteen patients had earlier diagnosis of dysplasia or DNA aneuploidy. Alkaline SMase activity was related to histologic dysplasia and DNA aneuploidy assessed by flow cytometry, patient age, and duration of disease.

RESULTS

Alkaline SMase activity was significantly lower in the patient group with and without dysplasia compared with controls (p = 0.006). In biopsies, an association was not found between alkaline SMase activity, dysplasia, or DNA ploidy. However, alkaline SMase activity decreased with age both in patients and controls (p = 0.008).

CONCLUSIONS

Reduction of alkaline SMase activity seen in colorectal cancer and adenomas is also present in patients with chronic colitis. It is not complementary to dysplasia or DNA-aneuploidy in the identification of high risk patients. The age-associated decrease of alkaline SMase activity seems to be a general phenomenon indicating premature senescence of the mucosa in longstanding colitis.

Neutral sphingomyelinase 1 deficiency in the mouse causes no lipid storage disease

Sphingomyelin is a major lipid in the bilayer of subcellular membranes of eukaryotic cells. Different sphingomyelinases catalyze the initial step in the catabolism of sphingomyelin, the hydrolysis to phosphocholine and ceramide. Sphingomyelinases have been postulated to generate ceramide as a lipophilic second messenger in intracellular signaling pathways involved in cell proliferation, differentiation, or apoptosis. To elucidate the function of the first cloned Mg(2+)-dependent, neutral sphingomyelinase (nSMase 1) in sphingomyelin catabolism and its potential role in signaling processes in a genetic and molecular approach, we have generated an nSMase 1-null mutant mouse line by gene targeting. The nSMase 1-deficient mice show an inconspicuous phenotype and no accumulation or changed metabolism of sphingomyelin or other lipids, despite grossly reduced nSMase activity in all organs except brain. We also addressed the recent proposal that nSMase 1 possesses lysophospholipase C activity. The unaltered metabolism of lysophosphatidylcholine or lyso-platelet-activating factor excludes the proposed role of nSMase 1 as a lysophospholipase C.

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

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

Permeability barrier disorder in Niemann-Pick disease: sphingomyelin-ceramide processing required for normal barrier homeostasis

Prior studies have established the requirement for enzymatic hydrolysis of glucosylceramides to ceramide for epidermal barrier homeostasis. In this study, we asked whether sphingomyelin-derived ceramide, resulting from acid-sphingomyelinase activity, is also required for normal barrier function. We showed first, that a subset of Niemann-Pick patients with severe acid-sphingomyelinase deficiency (i.e., <2% residual activity) demonstrate abnormal permeability barrier homeostasis, i.e., delayed recovery kinetics following acute barrier disruption by cellophane tape-stripping. To obtain further mechanistic insights into the potential requirement for sphingomyelin-to-ceramide processing for the barrier, we next studied the role of acid-sphingomyelinase in hairless mouse skin. Murine epidermis contains abundant acid-sphingomyelinase activity (optimal pH 5.1-5.6). Two hours following acute barrier disruption by tape-stripping, acid-sphingomyelinase activity increases 1. 44-fold (p<0.008 versus vehicle-treated controls), an increase that is blocked by a single topical application of the acid-sphingomyelinase inhibitor, palmitoyldihydrosphingosine. Furthermore, both palmitoyldihydrosphingosine and desipramine, a chemically and mechanically unrelated acid-sphingomyelinase inhibitor, significantly delay barrier recovery both 2 and 4 h after acute barrier abrogation. Inhibitor application also causes both an increase in sphingomyelin content, and a reduction of normal extracellular lamellar membrane structures, in the stratum corneum. Both of the inhibitor-induced delays in barrier recovery can be overridden by co-applications of topical ceramide, demonstrating that an alteration of the ceramide-sphingomyelin ratio, rather than sphingomyelin accumulation, is likely responsible for the barrier abnormalities that occur with acid-sphingomyelinase deficiency. These studies demonstrate an important role for enzymatic processing of sphingomyelin-to-ceramide by acid-sphingomyelinase as a mechanism for generating a portion of the stratum corneum ceramides for permeability barrier homeostasis in mammalian skin.

Effects of phospholipids on sphingomyelin hydrolysis induced by intestinal alkaline sphingomyelinase: an in vitro study

Digestion of dietary sphingomyelin (SM) is catalyzed by intestinal alkaline sphingomyelinase (SMase) and may have important implications in colonic tumorigenesis. Previous studies demonstrated that the digestion and absorption of dietary SM was slow and incomplete and that the colon was exposed to SM and its hydrolytic products including ceramide. In the present work, we studied the influences of glycerophospholipids and hydrolytic products of phosphatidylcholine (PC; i.e., lyso-PC, fatty acid, diacylglycerol, and phosphorylcholine) on SM hydrolysis induced by purified rat intestinal alkaline SMase in the presence of 10 mM taurocholate. It was found that various phospholipids including PC, phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidylethanolamine (PE), and phosphatidic acid (PA) inhibit alkaline SMase activity in a dose-dependent manner, with the degree of inhibition being in the order PA > PS > PI > PC > PE. Similar inhibition was also seen in a buffer of pH 7.4, which is close to the physiologic pH in the middle of the small intestine. When the effects of hydrolytic products of PC were studied, lyso-PC, oleic acid, and 1,2-dioleoyl glycerol also inhibited alkaline SMase activity, whereas phosphorylcholine enhanced SMase activity. However, in the absence of bile salt, acid phospholipids including PA, PS, and PI mildly stimulated alkaline SMase activity whereas PC and PE had no effect. It is concluded that in the presence of bile salts, glycerophospholipids and their hydrolytic products inhibit intestinal alkaline SMase activity. This may contribute to the slow rate of SM digestion in the upper small intestine.

Characterization and subcellular localization of murine and human magnesium-dependent neutral sphingomyelinase

Sphingomyelinases (SMases) catalyze the hydrolysis of sphingomyelin, an essential lipid constituent of the plasma membrane, lysosomal membranes, endoplasmic reticulum, and the Golgi membrane stacks of mammalian cells. In this study, we report the biochemical and functional characterization and subcellular localization of magnesium-dependent nSMase1 from overexpressing human embryonic kidney (HEK293) cells. Site-directed mutagenesis of conserved residues probably involved in the enzymatic sphingomyelin cleavage as well as the removal of one or both putative transmembrane domains lead to the complete loss of enzymatic activity of human nSMase1 expressed in HEK293 cells. Polyclonal antibodies raised against recombinant mammalian nSMase1 immunoprecipitated and inactivated the enzyme in membrane extracts of overexpressing HEK293 cells and different murine tissues. Cell fractionation combined with immunoprecipitation studies localized the nSMase1 protein predominantly in the microsomal fraction. The enzyme colocalized with marker proteins of the endoplasmic reticulum and the Golgi apparatus in immunocytochemistry. Anti-nSMase1 antibodies did not affect the nSMase activity in the plasma membrane fraction and membrane extracts from murine brain. Our study leads to the conclusion that nSMase1 is one of at least two mammalian neutral sphingomyelinases with different subcellular localization, tissue specificity, and enzymatic properties.

Alkaline sphingomyelinases and ceramidases of the gastrointestinal tract

In addition to the acid and neutral sphingomyelinases (SMase) that occur in most tissues, distinct alkaline sphingomyelinases occur in the mucosa of the gastrointestinal tract and human bile. These enzymes exhibit characteristic properties with regard to bile-salt dependence, protease resistance, and longitudinal distribution in the gut. Alkaline SMase has now been partially purified from human bile and from rat small intestine. It is thought to have a role in sphingomyelin (SM) digestion but may also be important for the generation of antiproliferative sphingolipid messengers in the gut. It occurs throughout the whole length of the intestine and also in the colon. It is decreased in colon cancer tissue compared to surrounding mucosa and is extremely low in colon mucosa from patients with familial adenomatous polyposis (FAP). This chapter reviews the properties and potential physiological and pathophysiological significance of alkaline SMase. It also briefly summarizes the knowledge about sphingolipid digestion and about the ceramidases of the gut.

Neutral sphingomyelinase: past, present and future

Sphingomyelin and its metabolic products are now known to have second messenger functions in a variety of cellular signaling pathways. At the epicenter of the sphingomyelin--cell signaling pathway is a family of phospholipases called sphingomyelinases. These enzymes cleave sphingomyelin to produce ceramide and phosphocholine. Ceramide in turn serves as a lipid second messenger that induces a variety of cell regulatory phenomenon such as programmed cell death (apoptosis), cell differentiation, cell proliferation, and sterol homeostasis. Neutral sphingomyelinase (N-SMase) is a Mg2+ sensitive enzyme that can be activated by a host of physiologically relevant and structurally diverse molecules like tumor necrosis factor-alpha (TNF-alpha), oxidized human low density lipoproteins (Ox-LDL), and several growth factors. Large amounts of ceramide accumulate in human fatty streaks and plaques along with Ox-LDL, growth factors, and proinflammatory cytokines in human atherosclerosis. A further role of ceramide and N-SMase in atherosclerosis was uncovered by the finding that Ox-LDL and TNF-alpha stimulated N-SMase activity. In turn, ceramide and/or a homolog serves as an important stress signaling molecule in signal transduction, which leads to apoptosis. Interestingly, an antibody against N-SMase can abrogate Ox-LDL and TNF-alpha induced apoptosis, and therefore may be useful for additional studies of apoptosis in experimental animals. Overexpression of recombinant human N-SMase in human aortic smooth muscle cells markedly stimulate apoptosis, presumably via the multioligomerization of the 'death domain'. Since plaque stability is an integral aspect of atherosclerosis management, activation of N-SMase and subsequent apoptosis may be vital events in the onset of plaque rupture, stroke and heart failure. In contrast to these observations in human hepatocytes, TNF-alpha mediated N-SMase activation did not induce apoptosis. Rather it stimulated the maturation of sterol regulatory element (SRE) binding protein (SREBP-1). Moreover, a cell permeable ceramide was found to reconstitute the phenomenon above in a sterol-independent fashion. These findings provide alternate avenues for therapy of patients with hypercholesterolemia and atherosclerosis. The findings reported here suggests that N-SMase plays important cell regulatory roles and provide an exciting opportunity to further these findings to understand the pathophysiology of human disease states.

Sphingomyelin-degrading pathways in human cells role in cell signalling

The ubiquitous sphingophospholipid sphingomyelin (SM) can be hydrolysed in human cells to ceramide by different sphingomyelinases (SMases). These enzymes exert a dual role, enabling not only the turnover of membrane SM and the degradation of exogenous (lipoprotein) SM, but also the signal-induced generation of the lipid second messenger ceramide. This review focuses on the function(s) of the different SMases in living cells. While both lysosomal and non-lysosomal pathways that ensure SM hydrolysis in intact cells can be distinguished, the precise contribution of each of these SM-cleaving enzymes to the production of ceramide as a signalling molecule remains to be clarified.

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.

Familial adenomatous polyposis is associated with a marked decrease in alkaline sphingomyelinase activity: a key factor to the unrestrained cell proliferation?

The hydrolysis of sphingomyelin generates key molecules regulating cell growth and inducing apoptosis. Data from animal cancer models support an inhibitory role for this pathway in the malignant transformation of the colonic mucosa. In the intestinal tract, a sphingomyelinase with an optimum alkaline pH has been identified. We recently found that the activity of alkaline sphingomyelinase is significantly decreased in colorectal adenocarcinomas, indicating a potential anticarcinogenic role of this enzyme. To further examine whether the reduction of sphingomyelinase is present already in the premalignant state of neoplastic transformation, we measured sphingomyelinase activities in patients with familial adenomatous polyposis (FAP) and in sporadic colorectal tubulovillous adenomas. Tissue samples were taken from adenomas and surrounding macroscopically normal mucosa from 11 FAP patients operated with ileorectal anastomosis, from three FAP patients with intact colon, from 13 patients with sporadic colorectal adenomas and from 12 controls. Activities of acid, neutral and alkaline sphingomyelinase were measured together with alkaline phosphatase. In FAP adenoma tissue, alkaline sphingomyelinase activity was reduced by 90% compared to controls (P < 0.0001), acid sphingomyelinase by 66% (P < 0.01) and neutral sphingomyelinase by 54% (P < 0.05). Similar reductions were found in the surrounding mucosa. In sporadic adenoma tissue, only alkaline sphingomyelinase was reduced significantly, by 57% (P < 0.05). Alkaline phosphatase was not changed in FAP adenomas, but decreased in the sporadic adenomas. We conclude that the markedly reduced levels of alkaline sphingomyelinase activities in FAP adenomas and in the surrounding mucosa may be a pathogenic factor that can lead to unrestrained cell proliferation and neoplastic transformation.

Ursodeoxycholic acid increases the activities of alkaline sphingomyelinase and caspase-3 in the rat colon

BACKGROUND

Ursodeoxycholic acid (UDCA) has been found to inhibit the development of colon carcinoma induced by chemical carcinogens with unidentified mechanisms. Sphingomyelin metabolism has emerged as a novel signal transduction pathway closely related to cell proliferation and apoptosis. We recently found that alkaline sphingomyelinase (SMase) activity was decreased in human colon cancer. The present study is to investigate whether UDCA has effect on the levels of SMase and whether the activity of caspase-3, a key regulatory protease in apoptosis that can be activated by sphingomyelin breakdown products, is also influenced by UDCA.

METHODS

Rats were fed UDCA in amounts ranging from 37.5 to 300 mg/kg/day for 10 days by gavage. The colonic mucosa was scraped, homogenized, and sonicated. The activities of acid, neutral and alkaline SMases, and caspase-3 were determined.

RESULTS

UDCA dose-dependently increased alkaline SMase activity in colonic mucosa and faeces, slightly increased acid SMase activity in the mucosa, and had no effect on neutral SMase. UDCA also dose-dependently increased caspase-3 activity in the colonic mucosa, and the increase correlated significantly with the changes in alkaline but not that in acid or neutral SMase activity.

CONCLUSIONS

UDCA increases alkaline sphingomyelinase and caspase-3 activities, which might be a mechanism involved in its anticarcinogenic effect on colon cancer development.

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.

Purification and characterization of a membrane bound neutral pH optimum magnesium-dependent and phosphatidylserine-stimulated sphingomyelinase from rat brain

Sphingomyelin hydrolysis and ceramide generation catalyzed by sphingomyelinases (SMase) are key components of the signaling pathways in cytokine- and stress-induced cellular responses. In this study, we report the partial purification and characterization of the membrane bound, neutral pH optimal, and magnesium-dependent SMase (N-SMase) from rat brain. Proteins from Triton X-100 extract of brain membrane were purified sequentially with DEAE-Sephacel, heparin-Sepharose, ceramic hydroxyapatite, Mono Q, phenyl-Superose, and Superose 12 column chromatography. After eight purification steps, the specific activity of the enzyme increased by 3030-fold over the brain homogenate. The enzyme hydrolyzed sphingomyelin but not phosphatidylcholine and its activity was dependent upon magnesium with an optimal pH of 7.5 and a native pI of 5.2. Delipidation of the enzyme through chromatographic purification or by extraction with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid followed by gel filtration revealed that the enzyme became increasingly dependent on phosphatidylserine (PS). Up to 20-fold stimulation was observed with PS whereas other lipids examined were either ineffective or only mildly stimulatory. The Km of the enzyme for substrate sphingomyelin (3.4 mol %) was not affected by PS. The highly purified enzyme was inhibited by glutathione with a >95% inhibition observed with 3 mM glutathione and with a Hill number calculated at approximately 8. The significance of these results to the regulation of N-SMase is discussed.

Digestion of ceramide by human milk bile salt-stimulated lipase

BACKGROUND

There is a renewed interest in metabolism of sphingolipids because of their role in signal transduction. Sphingomyelin is the dominating phospholipid in human milk but its metabolism and possible function in the gastrointestinal tract of breast fed infants is unknown. We explored whether bile salt-stimulated milk lipase has a role in sphingolipid metabolism.

METHODS

In vitro assays of sphingomyelinase and ceramidase activities, using radiolabeled substrates, human milk samples and purified native and recombinant variants of bile salt-stimulated milk lipase with or without known activators or inhibitors.

RESULTS

Human whey and purified lipase catalysed hydrolysis of palmitoyl-labeled ceramide with the highest rate around pH 8.5-9.0. 1 mg of lipase hydrolysed 0.7 micromol ceramide in one hour at pH 8.5 in presence of 4 mM bile salt. The activity of whey was inhibited by antibodies towards human bile salt-stimulated milk lipase, indicating that this lipase accounted for virtually all ceramidase activity in the milk. In contrast, bile salt-stimulated milk lipase showed no activity against sphingomyelin. However we give evidence of a separate, hitherto unknown, acid sphingomyelinase in human milk. Under the used in vitro conditions this sphingomyelinase could account for hydrolysis of half of milk sphingomyelin in one hour.

CONCLUSIONS

Human milk bile salt-stimulated milk lipase hydrolyses ceramide and may thus have a role in sphingomyelin digestion, but only after initial hydrolysis to ceramide and phosphorylcholine. Part of the latter could be carried out in the stomach by the acid milk sphingomyelinase now described. We speculate that these two milk enzymes may be of importance for optimal use of human milk sphingolipids.

Sphingomyelinase activity in human platelets

The sphingolipid metabolites, ceramide, sphingosine, and sphingosine-1-phosphate, may be involved in several signalling pathways and may regulate cell functions such as cell growth, secretion, differentiation, and apoptosis. During activation of human platelets by thrombin, sphingosine-1-phosphate is released from platelets and can potentiate their aggregation. Thrombin also causes an increase in platelet sphingosine levels. Since these molecules can be derived from sphingomyelin, we have determined whether platelets possess sphingomyelinase and whether this enzyme is regulated during platelet function. Using radioactive sphingomyelin as substrate, we assayed sphingomyelinase activity over the range of pH 4 to 10 and observed optimal activity at pH 5.0-5.5. Little activity was found at neutral or alkaline pH, and the presence of Mg++, Ca++, Zn++, or EDTA in the reaction mixture had little effect on the pH profile. Activation of platelets by thrombin or ADP had no effect on sphingomyelinase activity, but thrombin caused secretion of the acid-sphingomyelinase activity into the media. Thus, human platelets contain an acid-sphingomyelinase which is secreted during thrombin-induced platelet activation.

Myelin contains neutral sphingomyelinase activity that is stimulated by tumor necrosis factor-alpha

Purified myelin from mouse brain was found to contain two forms of neutral sphingomyelinase, one Mg2+ dependent and the other Mg2+ independent. The former had a pH optimum of 7.5 and Km of 0.35 mM, whereas the corresponding values for the latter were pH 8.0 and Km 3.03 mM. Specific activity of the Mg(2+)-dependent enzyme showed a rostral-caudal gradient, ranging from 75 nmol/mg protein/hr in myelin from cerebral hemispheres to 21 nmol/mg protein/hr in myelin from spinal cord. Relative specific activity was approximately 20% that of brain stem or cerebral hemisphere homogenate. Treatment of myelin with taurocholate or high salt concentration did not significantly reduce activity of the Mg(2+)-dependent enzyme. The activity of that enzyme did not change with time or in the presence or absence of protease inhibitors; by contrast, that of Mg(2+)-independent enzyme decreased sharply in the absence of protease inhibitors but rose in their presence. To test for the effect of tumor necrosis factor-alpha (TNF alpha) on myelin sphingomyelinase, mouse brain myelin was labeled in vivo by intracerebral injection of [3H]acetate into 18-20-day-old mice. After 40 hr, brain stems were removed, minced, and treated with TNF alpha in Krebs-Ringer solution, after which myelin was immediately isolated. Separation and counting of individual lipids revealed TNF alpha treatment to cause increased labeling of myelin ceramide and cholesterol ester with concomitant decrease in myelin sphingomyelin. Western blotting of myelin proteins using antibodies to the two TNF alpha receptors as probes revealed the presence of the p75 receptor. Implications of these findings in relation to possible mechanisms of autoimmune demyelination are discussed.

Induction of apoptosis and potentiation of TNF- and Fas-mediated apoptosis in U937 cells by the xanthogenate compound D609

Apoptosis induced by ligation of either the tumor necrosis factor (TNF) p55 receptor or the Fas/APO-1/CD95 receptor has been suggested to require ceramide as a signaling molecule. Ceramide is formed as a result of sphingomyelinase (SMase) activation in the sphingomyelin cycle, and ligation of TNF and Fas receptors has been reported to stimulate SMase activity. We have studied the effects of D609, a xanthogenate compound with antitumoral properties, on TNF- or Fas-induced apoptosis of monocytic U937 cells. First, the effects of D609 on SMase activity were assessed using in vitro assays for neutral and acidic SMase, and the results suggested that D609 caused a modest stimulation of the activity of both SMases in U937 cells. Exposure of U937 cells for 6 h to TNF or anti-Fas mAb induced apoptosis in 40-45% of the cells, as measured by fluorescent staining of nuclear chromatin. Cotreatment with D609 potentiated TNF- as well as Fas-mediated apoptosis up to 70 and 90%, respectively. Furthermore, in incubations with D609 alone, 60% of the cells became apoptotic within 16 h. Since D609 has been reported to inhibit protein kinase C (PKC) activity, the effect of phorbol 12-myristate 13-acetate (PMA) on D609-induced apoptosis was investigated. PMA was found to inhibit D609-induced apoptosis in U937 cells as well as cell death induced by TNF and anti-Fas mAb. Thus, PKC inactivation may play an important role in the regulation of apoptosis in U937 cells. In summary, the present results show that D609 stimulates SMase activity, potentiates TNF- and Fas-induced apoptosis, and induces apoptosis on its own in U937 cells.

Sphingoid bioregulators in the differentiation of cells of neural origin

The involvement of ceramide in the differentiation of two neuroblastoma cell lines, Neuro2a and SH-SY5Y, and cerebellar granule cells in primary culture was investigated. The following results were obtained: (a) the cellular content of ceramide markedly increased with induced differentiation of Neuro2a cells (inducers: RA, FCS deprivation), SH-SY5Y cells (inducers: RA, PMA), and spontaneous differentiation of cerebellar granule cells; (b) all the investigated cells in the differentiated form displayed a higher ability to produce ceramide from exogenously administered [3H]Sph-SM and expressed a higher content of neutral sphingomyelinase and, in the case of cerebellar granule cells, also of acidic sphingomyelinase; (c) inhibition of ceramide biosynthesis by Fumonisin B1 blocked the process of differentiation in Neuro2a and cerebellar granule cells; and (d) treatments capable of enhancing ceramide level (administration of sphingosine or C2-Ceramide) induced differentiation in both Neuro2a and SH-SY5Y cells. The data obtained support the notion that ceramide plays a general biomodulatory role in neural cell differentiation.

Identification of an alkaline sphingomyelinase activity in human bile

The hydrolysis of sphingomyelin has been found to generate important signals regulating cell proliferation, differentiation and apoptosis. However, the enzymes responsible for digestion of dietary sphingomyelin have not been well documented. This study demonstrates the occurrence of a sphingomyelinase (SMase) in both human hepatic bile and gallbladder bile. The enzyme was equally found in both bacteria negative and positive bile samples and in samples obtained from patients with or without gallbladder diseases. A bacteria-free gallbladder bile was used for characterization. It was found that bile SMase hydrolyzed sphingomyelin to phosphorylcholine and ceramide with negligible activity against either phosphatidylcholine or p-nitrophenyl phosphate. The enzyme preferred an alkaline condition and the optimal pH was 9. The activity of this alkaline SMase was bile salt dependent and was fully activated by 4-6 mM bile salts. Triton X-100, the non-ionic detergent did not activate bile SMase. Ca2+ and Mg2+ ions had no significant effect at optimal bile salt concentration. The molecular mass of this enzyme was about 85 kDa as measured by Sephadex G200 gel chromatography. In conclusion, we demonstrated a SMase in bile which differs markedly from the known acid and neutral SMase. Its potential important roles in sphingomyelin digestion and gallbladder diseases require further investigation.

Role of the ceramide-signaling pathway in cytokine responses to P-fimbriated Escherichia coli

Escherichia coli express fimbriae-associated adhesins through which they attach to mucosal cells and activate a cytokine response. The receptors for E. coli P fimbriae are the globoseries of glycosphingolipids; Gal alpha 1-->4Gal beta-containing oligosaccharides bound to ceramide in the outer leaflet of the lipid bilayer. The receptors for type 1 fimbriae are mannosylated glycoproteins rather than glycolipids. This study tested the hypothesis that P-fimbriated E. coli elicit a cytokine response through the release of ceramide in the receptor-bearing cell. We used the A498 human kidney cell line, which expressed functional receptors for P and type 1 fimbriae and secreted higher levels of interleukin (IL)-6 when exposed to the fimbriated strains than to isogenic nonfimbriated controls. P-fimbriated E. coli caused the release of ceramide and increased the phosphorylation of ceramide to ceramide 1-phosphate. The IL-6 response to P-fimbriated E. coli was reduced by inhibitors of serine/threonine kinases but not by other protein kinase inhibitors. In contrast, ceramide levels were not influenced by type 1-fimbriated E. coli, and the IL-6 response was insensitive to the serine/threonine kinase inhibitors. These results demonstrate that the ceramide-signaling pathway is activated by P-fimbriated E. coli, and that the receptor specificity of the P fimbriae influences this process. We propose that this activation pathway contributes to the cytokine induction by P-fimbriated E. coli in epithelial cells.

A mediator role of ceramide in the regulation of neuroblastoma Neuro2a cell differentiation

Current studies indicate that ceramide is involved in the regulation of important cell functions, namely cell growth, differentiation, and apoptosis. In the present study, the possible role of ceramide in the differentiation of neuroblastoma Neuro2a cells was investigated. The following results were obtained. (a) Ceramide content of Neuro2a cells, induced to differentiate by retinoic acid (RA) treatment rapidly increased after addition of RA, was maintained at high levels in RA-differentiated cells and returned to the starting levels with removal of RA and reversal of differentiation; under the same conditions, the sphingosine content remained unchanged. (b) After a short pulse with [3H]sphingomyelin or [3H]sphingosine or L-[3H]serine, the metabolic formation of ceramide was markedly higher and more rapid in RA-differentiated than undifferentiated cells. (c) Inhibitors of ceramide biosynthesis (Fumonisin B1, beta-chloroalanine and L-cycloserine) diminished the extent of the differentiating effect of RA and concomitantly Cer content decreased. (d) The activity of neutral sphingomyelinase increased after addition of RA, maintained high levels in RA-differentiated cells, and returned to the initial levels with removal of RA. (e) Experimental conditions that cause an elevation of ceramide content (treatment with sphingosine or ceramide or C2-ceramide or bacterial sphingomyelinase) inhibited cell proliferation and stimulated neurite outgrowth; dihydro-analogues of sphingosine, ceramide, and C2-ceramide had no effect on differentiation. (f) treatment with Fumonisin B1 completely inhibited sphingosine-induced differentiation. These data suggest a specific bioregulatory function of ceramide in the control of Neuro2a cell growth and differentiation and pose the general hypothesis of a mediator role of ceramide in the differentiation of cells of neural origin.

Alkaline sphingomyelinase activity in rat gastrointestinal tract: distribution and characteristics

Previous studies indicated that there was an alkaline sphingomyelinase (SMase) activity in small intestine, but its properties have not been studied in detail. In the present work, we studied the distribution of this enzyme activity in rat gastrointestinal tract and characterized it in intestinal mucosal homogenates. Little alkaline SMase activity was detected in the stomach and the duodenum. The activity in both mucosa and intestinal content increased in the small intestine and reached the maximum at the distal jejunum, then declined in the ileum and slightly increased again in the colon. The activity distribution pattern differed markedly from those of acid SMase and alkaline phosphatase. Little alkaline SMase activity could be found in bile, liver and pancreas before or after treatment with trypsin. The optimum pH of the alkaline SMase was 9. It specifically hydrolyzed sphingomyelin (SM), not phosphatidylcholine, to ceramide and phosphocholine. The alkaline SMase was bile salt dependent and was optionally activated by 3 mM bile salts. Triton X-100 could not mimic the effect of bile salt, rather dose-dependently inhibited the enzyme activity. Ca2+, Mg2+ did not change the alkaline SMase activity in the presence of bile salts, and reduced the activity in the absence of bile salt. Trypsin inactivated acid SMase in pancreas, liver and duodenum but had no influence on intestinal alkaline SMase activity. In conclusion, the intestinal alkaline SMase has a specific distribution pattern and the characters of it differ in several respects from the known acid and neutral SMases.

Degradation of fluorescent and radiolabelled sphingomyelins in intact cells by a non-lysosomal pathway

The aim of the present study was to investigate the role of the entitled neutral, sphingomyelinase in the non-lysosomal pathway of sphingomyelin degradation by intact cells (Spence et al. (1983) J. Biol. Chem. 258, 8595-8600; Levade et al. (1991) J. Biol. Chem. 266, 13519-13529). The uptake and degradation of sphingomyelin by intact living cells was studied using cell lines exhibiting a wide range of activity levels of acid, lysosomal and neutral sphingomyelinases as determined in vitro on cell homogenates by their respective standard assays. For this purpose, neuroblastoma, skin fibroblasts, lymphoid and leukemic cell lines, some of them derived from patients with Niemann-Pick disease (deficient in the acid, lysosomal sphingomyelinase) were incubated with radioactive, [oleoyl-3H]sphingomyelin or fluorescent, pyrene-sulfonylaminoundecanoyl-sphingomyelin. Either compound was taken up by a pathway which was not receptor-mediated and hydrolyzed by all intact cells, including those derived from Niemann-Pick disease patients. Moreover, their degradation by the intact cells was not inhibited by treatment with chloroquine, indicating hydrolysis by a non-lysosomal sphingomyelinase. The intracellular sphingomyelin degradation rates showed no correlation with the activity of the 'classical' neutral sphingomyelinase as determined in vitro. In particular, fibroblasts derived from Niemann-Pick patients lacking the lysosomal sphingomyelinase, and having no detectable in vitro activity of the 'classical' neutral sphingomyelinase, were able to degrade the exogenously supplied sphingomyelins. Indeed, in vitro these cells were shown to exhibit neutral, magnesium- and dithiothreitol-dependent sphingomyelinase activities, that might contribute to the non-lysosomal pathway for sphingomyelin degradation to ceramide in intact cells.

Formation of endogenous free sphingoid bases in cells induced by changing medium conditions

Sphingoid bases are precursors and breakdown products of sphingolipids and may function as second messengers. Here we have tested the hypothesis that sphingoid bases are produced in cells in response to external stimuli. Using a high-performance liquid chromatography system, the pattern and the amounts of free sphingoid bases in various cell types (i.e., NIH-3T3, A431, NG108-15) were determined. The predominant sphingoid base in these mammalian cells was identified as C-18 sphingosine, followed by C-18 sphinganine (dihydrosphingosine). In all cells examined, the levels of endogenous sphingoid bases can be rapidly elevated by replacing cell-conditioned medium with Hepes-buffered saline or with fresh medium, causing a dramatic increase (up to 9.5-fold) in sphingosine levels within 60 min; sphinganine levels were raised to a lesser extent (up to 4.5-fold). Addition of ammonium ions inhibited the generation of sphingoid bases. These results suggest that the machinery for metabolizing sphingoid bases can be stimulated rapidly, although the exact nature of the stimulus remains obscure. Nevertheless, the ability to control sphingosine formation in cells by changing medium conditions provides a powerful tool for investigations of the physiological roles of endogenous sphingosine.

Characterization of the neutral pH-optimum sphingomyelinase from rat brain: inhibition by copper II and ganglioside GM3

A neutral pH-optimum sphingomyelinase (N-SMase), solubilized from rat brain membranes, was characterized with respect to metal and membrane lipid effects. Chromatofocusing chromatography, which separates proteins according to pI, showed two N-SMase activities. One eluted at pH 4.7 and the other required 0.4 M NaCl before elution. Kinetically, the two preparations appeared similar. The N-SMase eluting at pH 4.7 was most extensively studied here. Of the phospholipids studied, only phosphatidylserine showed any influence on N-SMase and that was to increase its activity by as much as 50%. Neither serine nor phosphatidic acid had any effect. Of the cations tested, none was able to replace Mg2+ as a required activator. However, it was found that several metals were inhibitory, with Cu2+ being most effective (IC50 = 5 microM). Gangliosides, particularly the monosialoganglioside, GM3 (IC50 approximately 50 microM), inhibited N-SMase. Other glycolipids showed little effect on activity, even the immediate precursor to GM3 - lactosylceramide. The ganglioside sugar, N-acetylneuraminic acid, also had no effect on N-SMase activity. None of these inhibitors affected the acidic pH-optimum sphingomyelinase. Other sphingolipid compounds such as ceramide - the enzymatic product - and sphingosylphosphorylcholine (lysosphingomyelin) showed no capacity to inhibit N-Smase, implying that the enzyme may have a selective substrate-binding site.

Ceramide: a new lipid "second messenger"?

1-Alpha, 25-dihydroxyvitamin D3, tumor necrosis factor alpha, and gamma-interferon induce sphingomyelin turnover to ceramide in HL-60 cells. The time frame of the turnover is consistent with ceramide formation being a mediator of growth and differentiation; furthermore, differentiation can be induced by addition of exogenous ceramides alone. Hence, the action of the biologically active form of vitamin D and other nutritionally relevant factors may be mediated by lipid second messenger pathways involving sphingolipids.

Intracellular transport and metabolism of sphingomyelin

SM is unique among the phospholipids because it is restricted to the lumenal aspect of organelles involved in the secretory and endocytic pathways. Given the intracellular sites of SM biosynthesis and hydrolysis, and the interconnections between these sites by vesicle-mediated transport pathways, the basic mechanism for maintaining the intracellular distribution of SM seems clear. It remains to be determined how SM metabolism and transport are coordinated to maintain the SM content of each organelle. For example, the size of the SM pool at the cell surface is maintained by regulation of at least five processes: transport of newly synthesized SM from the Golgi apparatus, plasma membrane lipid recycling, local SM synthesis, local SM hydrolysis, and SM transport from the cell surface to lysosomes. Although SM cannot undergo spontaneous transbilayer movement, SM metabolism generates both DAG, Cer and (indirectly) SPhB which can rapidly 'flip-flop', and thus gain access to the cytoplasmic leaflet of a membrane. It is of particular interest that these lipid species may be involved in the regulation of PK-C, suggesting that SM metabolism could play a role in signal transduction. However, physiological effects of endogenous Cer and SPhB remain elusive, even though the pharmacological effect of SPhB on PK-C is well established. Aside from the direct generation of second messengers, stimulation of SM hydrolysis has also been shown to induce cholesterol movement from the cell surface to intracellular membranes. It is not known whether this reflects the possibility that cholesterol may act as a second messenger. Alternatively, this phenomenon suggests that SM metabolism may cause rapid changes in the physical properties of the cell surface. For example, erythrocytes extensively treated with exogenously-added SMase will undergo endovesiculation It is tempting to speculate that any involvement of SM in the regulation of intracellular processes requires a combination of both the generation of biochemical second messengers and the alteration of membrane biophysical properties that can result from SM metabolism.

Human placental sphingomyelinase. Purification to homogeneity, antigenic properties and partial amino-acid sequences of the enzyme

Sphingomyelinase from human placenta was purified to homogeneity in five steps: concanavalin A Sepharose, butyl agarose. Blue Sepharose, sphingosylphosphocholine Sepharose chromatography and FPLC-Mono Q. This lysosomal enzyme has a pH optimum around pH 5.0-6.0. It is a glycoprotein with an approximate molecular mass of 70 kDa which is reduced to 60 kDa by enzymatic deglycosylation. Monospecific antibodies against sphingomyelinase were isolated using sphingomyelinase covalently linked to Sepharose as affinity matrix. These antibodies effectively inhibit the sphingomyelinase activity. Peptides were released from sphingomyelinase by cyanogen bromide or proteolytically by trypsin, proteinase V8 and Lys C for gas phase sequencing. Amino-acid sequences are reported which proved to be the prerequisite for antibody and oligonucleotide screening of the respective human placenta cDNA libraries for the determination of the complete amino acid sequence of human lysosomal sphingomyelinase. In situ hybridisation with a labelled antisense RNA synthesized in vitro using cloned sphingomyelinase-specific cDNA as template, which encodes the peptide sequences described here, revealed the strong expression of sphingomyelinase in human placental villi and normal fibroblasts. Fibroblasts of a Niemann-Pick patient, however, were free of mRNA expressing the sphingomyelinase described here.