NADPH Oxidase-Mediated Activation of Neutral Sphingomyelinase Is Responsible for Diesel Particulate Extract-Induced Keratinocyte Apoptosis.

Human epidermis is positioned at the interface with the external environment, protecting our bodies against external challenges, including air pollutants. Emerging evidence suggests that diesel particulate extract (DPE), a major component of air pollution, leads to impairment of diverse cellular functions in keratinocytes (KC). In this study, we investigated the cellular mechanism underlying DPE-induced KC apoptosis. We first addressed cell death occurring in KC exposed to DPE, paralleled by increased activation of NADPH oxidases (NOXs) and subsequent ROS generation. Blockade of NOX activation with a specific inhibitor attenuated the expected DPE-induced KC apoptosis. In contrast, pre-treatment with a specific inhibitor of reactive oxygen species (ROS) generation did not reverse DPE/NOX-mediated increase in KC apoptosis. We next noted that NOX-mediated KC apoptosis is mainly attributable to neutral sphingomyelinase (SMase)-mediated stimulation of ceramides, which is a well-known pro-apoptotic lipid. Moreover, we found that inhibition of NOX activation significantly attenuated DPE-mediated increase in the ratio of ceramide to its key metabolite sphingosine-1-phosphate (S1P), an important determinant of cell fate. Together, these results suggest that activation of neutral SMase serves as a key downstream signal for the DPE/NOX activation-mediated alteration in ceramide and S1P productions, and subsequent KC apoptosis.

Dietary inulin decreases circulating ceramides by suppressing neutral sphingomyelinase expression and activity in mice.

Elevated circulating levels of ceramides (Cers) are associated with increased risk of cardiometabolic diseases, and Cers may play a causative role in metabolic dysfunction that precedes cardiac events, such as mortality as a result of coronary artery disease. Although the mechanisms involved are likely complex, these associations suggest that lowering circulating Cer levels could be protective against cardiovascular diseases. Conversely, dietary fibers, such as inulin, have been reported to promote cardiovascular and metabolic health. However, the mechanisms involved in these protective processes also are not well understood. We studied the effects of inulin on lipid metabolism with a model of atherosclerosis in LDL receptor-deficient mice using lipidomics and transcriptomics. Plasma and tissues were collected at 10 days and/or 12 weeks after feeding mice an atherogenic diet supplemented with inulin or cellulose (control). Compared with controls, inulin-fed mice displayed a decreased C16:0/C24:0 plasma Cer ratio and lower levels of circulating Cers associated with VLDL and LDL. Liver transcriptomic analysis revealed that Smpd3, a gene that encodes neutral SMase (NSMase), was downregulated by 2-fold in inulin-fed mice. Hepatic NSMase activity was 3-fold lower in inulin-fed mice than in controls. Furthermore, liver redox status and compositions of phosphatidylserine and FFA species, the major factors that determine NSMase activity, were also modified by inulin. Taken together, these results showed that, in mice, inulin can decrease plasma Cer levels through reductions in NSMase expression and activity, suggesting a mechanism by which fiber could reduce cardiometabolic disease risk.

Genome-Wide DNA Methylation Profiles of Phlegm-Dampness Constitution.

BACKGROUND/AIMS: Metabolic diseases are leading health concerns in today's global society. In traditional Chinese medicine (TCM), one body type studied is the phlegm-dampness constitution (PC), which predisposes individuals to complex metabolic disorders. Genomic studies have revealed the potential metabolic disorders and the molecular features of PC. The role of epigenetics in the regulation of PC, however, is unknown. METHODS: We analyzed a genome-wide DNA methylation in 12 volunteers using Illumina Infinium Human Methylation450 BeadChip on peripheral blood mononuclear cells (PBMCs). Eight volunteers had PC and 4 had balanced constitutions. RESULTS: Methylation data indicated a genome-scale hyper-methylation pattern in PC. We located 288 differentially methylated probes (DMPs). A total of 256 genes were mapped, and some of these were metabolic-related. SQSTM1, DLGAP2 and DAB1 indicated diabetes mellitus; HOXC4 and SMPD3, obesity; and GRWD1 and ATP10A, insulin resistance. According to Ingenuity Pathway Analysis (IPA), differentially methylated genes were abundant in multiple metabolic pathways. CONCLUSION: Our results suggest the potential risk for metabolic disorders in individuals with PC. We also explain the clinical characteristics of PC with DNA methylation features.

Acid Sphingomyelinase Inhibition Prevents Development of Sepsis Sequelae in the Murine Liver.

The molecular mechanisms of maladaptive response in liver tissue with respect to the acute and post-acute phase of sepsis are not yet fully understood. Long-term sepsis survivors might develop hepatocellular/hepatobiliary injury and fibrosis. Here, we demonstrate that acid sphingomyelinase, an important regulator of hepatocyte apoptosis and hepatic stellate cell (HSC) activation, is linked to the promotion of liver dysfunction in the acute phase of sepsis as well as to fibrogenesis in the long-term. In both phases, we observed a beneficial effect of partial genetic sphingomyelinase deficiency in heterozygous animals (smpd1+/-) on oxidative stress levels, hepatobiliary function, macrophage infiltration and on HSC activation. Strikingly, similar to heterozygote expression of SMPD1, either preventative (p-smpd1+/+) or therapeutic (t-smpd1+/+) pharmacological treatment strategies with desipramine - a functional inhibitor of acid sphingomyelinase (FIASMA) - significantly improved liver function and survival. The inhibition of sphingomyelinase exhibited a protective effect on liver function in the acute-phase, and the reduction of HSC activation diminished development of sepsis-associated liver fibrosis in the post-acute phase of sepsis. In summary, targeting sphingomyelinase with FDA-approved drugs is a novel promising strategy to overcome sepsis-induced liver dysfunction.

Neutral sphingomyelinase 2 modulates cytotoxic effects of protopanaxadiol on different human cancer cells.

BACKGROUND: Some of ginsenosides, root extracts from Panax ginseng, exert cytotoxicity against cancer cells through disruption of membrane subdomains called lipid rafts. Protopanaxadiol (PPD) exhibits the highest cytotoxic effect among 8 ginsenosides which we evaluated for anti-cancer activity. We investigated if PPD disrupts lipid rafts in its cytotoxic effects and also the possible mechanisms. METHODS: Eight ginsenosides were evaluated using different cancer cells and cell viability assays. The potent ginsenoside, PPD was investigated for its roles in lipid raft disruption and downstream pathways to apoptosis of cancer cells. Anti-cancer effects of PPD was also investigated in vivo using mouse xenograft model. RESULTS: PPD consistently exerts its potent cytotoxicity in 2 cell survival assays using 5 different cancer cell lines. PPD disrupts lipid rafts in different ways from methyl-ß-cyclodextrin (MßCD) depleting cholesterol out of the subdomains, since lipid raft proteins were differentially modulated by the saponin. During disruption of lipid rafts, PPD activated neutral sphingomyelinase 2 (nSMase 2) hydrolyzing membrane sphingomyelins into pro-apoptotic intracellular ceramides. Furthermore, PPD demonstrated its anti-cancer activities against K562 tumor cells in mouse xenograft model, confirming its potential as an adjunct or chemotherapeutic agent by itself in vivo. CONCLUSIONS: This study demonstrates that neutral sphingomyelinase 2 is responsible for the cytotoxicity of PPD through production of apoptotic ceramides from membrane sphingomyelins. Thus neutral sphingomyelinase 2 and its relevant mechanisms may potentially be employed in cancer chemotherapies.

The unconventional role of acid sphingomyelinase in regulation of retinal microangiopathy in diabetic human and animal models.

OBJECTIVE: Acid sphingomyelinase (ASM) is an important early responder in inflammatory cytokine signaling. The role of ASM in retinal vascular inflammation and vessel loss associated with diabetic retinopathy is not known and represents the goal of this study. RESEARCH DESIGN AND METHODS: Protein and gene expression profiles were determined by quantitative RT-PCR and Western blot. ASM activity was determined using Amplex Red sphingomyelinase assay. Caveolar lipid composition was analyzed by nano-electrospray ionization tandem mass spectrometry. Streptozotocin-induced diabetes and retinal ischemia-reperfusion models were used in in vivo studies. RESULTS: We identify endothelial caveolae-associated ASM as an essential component in mediating inflammation and vascular pathology in in vivo and in vitro models of diabetic retinopathy. Human retinal endothelial cells (HREC), in contrast with glial and epithelial cells, express the plasma membrane form of ASM that overlaps with caveolin-1. Treatment of HREC with docosahexaenoic acid (DHA) specifically reduces expression of the caveolae-associated ASM, prevents a tumor necrosis factor-α-induced increase in the ceramide-to-sphingomyelin ratio in the caveolae, and inhibits cytokine-induced inflammatory signaling. ASM is expressed in both vascular and neuroretina; however, only vascular ASM is specifically increased in the retinas of animal models at the vasodegenerative phase of diabetic retinopathy. The absence of ASM in ASM(-/-) mice or inhibition of ASM activity by DHA prevents acellular capillary formation. CONCLUSIONS: This is the first study demonstrating activation of ASM in the retinal vasculature of diabetic retinopathy animal models. Inhibition of ASM could be further explored as a potential therapeutic strategy in treating diabetic retinopathy.

Acid sphingomyelinase contributes to evodiamine-induced apoptosis in human gastric cancer SGC-7901 cells.

Evodiamine-induced apoptosis has been shown to have anticancer activity by eradication of some carcinoma cell lines. This study was designed to evaluate the effects of evodiamine on the viability of human gastric cancer SGC-7901 cells and to define the cell death pathway. Flow cytometry detection showed that 1.5 µM evodiamine significantly induced SGC-7901 cell apoptosis in a time-dependent manner. This apoptosis was partially inhibited by the pancaspase inhibitor carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoro-methylketone, which suggests that evodiamine-induced apoptosis in SGC-7901 cells is partially caspase independent. Further, the total content of sphingomyelin was decreased and expression of acid sphingomyelinase (aSMase) and neutral SMase genes in the SGC-7901cells was upregulated. Protein expression of aSMase, which was exposed to evodiamine, was shown to be increased by western blot analysis and could have been responsible for inducing caspase-independent apoptosis. Our results indicate that evodiamine stimulates upregulation of aSMase expression and hydrolysis of sphingomyelin into ceramide, which might be one of the mechanisms by which apoptosis occurs in SGC-7901 cells.

Acid sphingomyelinase deficiency protects from cisplatin-induced gastrointestinal damage.

Cisplatin is one of the most effectively used chemotherapeutic agents for cancer treatment. However, in humans, important cytotoxic side effects are observed including dose-limiting renal damage and profound gastrointestinal symptomatology. The toxic responses to cisplatin in mice are similar to those in human patients. Here, we evaluated whether the acid sphingomyelinase (Asm) mediates at least some of the toxic in vivo effects of cisplatin. To this end, we determined the toxic effects of a single intraperitoneal dose of cisplatin (27 mg/kg) in wild type (Asm(+/+)) and Asm-deficient mice (Asm(-/-)). Tissue injury and apoptosis were determined histologically on hematoxylin-eosin and TUNEL (terminal deoxynucleotidyl transferase (TdT)-mediated nick end labeling) stainings 3, 12, 36 and 72 h after treatment. Our results revealed severe toxicity of cisplatin in Asm(+/+) mice with increased numbers of apoptotic cells in the thymus and small intestine. In marked contrast, Asm(-/-) mice were resistant to cisplatin and no apoptosis was observed in these organs after treatment. Moreover, cisplatin treatment primarily triggered apoptosis of endothelial cells in microvessels of intestine and thymus, an effect that was absent in mice lacking Asm. The data thus suggest that at least some toxic effects of cisplatin are mediated by the Asm in vivo resulting in early death of endothelial cells and consecutive organ damage.

Dietary sphingomyelin inhibits colonic tumorigenesis with an up-regulation of alkaline sphingomyelinase expression in ICR mice.

BACKGROUND: Sphingomyelin (SM) hydrolysis generates biologically active products regulating cell growth, differentiation and apoptosis. Dietary SM has been found to inhibit colonic tumorigenesis. Alkaline sphingomyelinase (alk-SMase) is the key enzyme responsible for sphingomyelin digestion in the gut. Whether or not dietary sphingomyelin affects alk-SMase expression was examined in a colon cancer animal model. MATERIALS AND METHODS: Imprinting control region (ICR) mice were injected with 1,2-dimethylhydrazine (DMH) and then fed a diet with or without SM (0.5 g/kg in diet) for 22 weeks. The colonic tumorigenesis and alk-SMase activity were determined and alk-SMase expression was examined by Western blot and PCR. RESULTS: Dietary SM inhibited the tumorigenesis and increased the alk-SMase activity in the colon by 65%. The increased activity was associated with increased enzyme protein and mRNA expression. No changes of acid and neutral sphingomyelinase activities were found. CONCLUSION: Long-term supplementation with dietary sphingomyelin up-regulates colonic alk-SMase expression, which may contribute to the inhibitory effects of sphingomyelin against colonic carcinogenesis.

Thalidomide-induced antiangiogenic action is mediated by ceramide through depletion of VEGF receptors, and is antagonized by sphingosine-1-phosphate.

Thalidomide, which is clinically recognized as an efficient therapeutic agent for multiple myeloma, has been thought to exert antiangiogenic action through an unknown mechanism. We here show a novel mechanism of thalidomide-induced antiangiogenesis in zebrafish embryos. Thalidomide induces the defect of major blood vessels, which is demonstrated by their morphologic loss and confirmed by the depletion of vascular endothelial growth factor (VEGF) receptors such as neuropilin-1 and Flk-1. Transient increase of ceramide content through activation of neutral sphingomyelinase (nSMase) precedes thalidomide-induced vascular defect in the embryos. Synthetic cell permeable ceramide, N-acetylsphingosine (C2-ceramide) inhibits embryonic angiogenesis as well as thalidomide. The blockade of ceramide generation by antisense morpholino oligonucleotides for nSMase prevents thalidomide-induced ceramide generation and vascular defect. In contrast to ceramide, sphingosine-1-phosphate (S1P) inhibits nSMase-dependent ceramide generation and restores thalidomide-induced embryonic vascular defect with an increase of expression of VEGF receptors. In human umbilical vein endothelial cells (HUVECs), thalidomide-induced inhibition of cell growth, generation of ceramide through nSMase, and depletion of VEGF receptors are restored to the control levels by pretreatment with S1P. These results suggest that thalidomide-induced antiangiogenic action is regulated by the balance between ceramide and S1P signal.

Interleukin-2-induced survival of natural killer (NK) cells involving phosphatidylinositol-3 kinase-dependent reduction of ceramide through acid sphingomyelinase, sphingomyelin synthase, and glucosylceramide synthase.

Interleukin 2 (IL-2) rescued human natural killer (NK) KHYG-1 cells from apoptosis along with a reduction of ceramide. Conversely, an increase of ceramide inhibited IL-2-rescued survival. IL-2 deprivation-induced activation of acid sphingomyelinase (SMase) and inhibition of glucosylceramide synthase (GCS) and sphingomyelin synthase (SMS) were normalized by IL-2 supplementation. A phosphatidyl inositol-3 (PI-3) kinase inhibitor, LY294002, inhibited IL-2-rescued survival, but a mitogen-activated protein kinase inhibitor, PD98059, and an inhibitor of Janus tyrosine kinase/signal transducer and activator of transcription pathway, AG490, did not. LY294002 inhibited IL-2-induced reduction of ceramide through activation of acid SMase and inhibition of GCS and SMS, suggesting the positive involvement of PI-3 kinase in ceramide reduction through enzymatic regulation. Indeed, a constitutively active PI-3 kinase enhanced growth rate and ceramide reduction through inhibition of acid SMase and activation of GCS and SMS. Further, LY294002 inhibited IL-2-induced changes of transcriptional level as well as mRNA and protein levels in acid SMase and GCS but did not affect the stability of the mRNAs. These results suggest that PI-3 kinase-dependent reduction of ceramide through regulation of acid SMase, GCS, and SMS plays a role in IL-2-rescued survival of NK cells.

Identification of one exon deletion of intestinal alkaline sphingomyelinase in colon cancer HT-29 cells and a differentiation-related expression of the wild-type enzyme in Caco-2 cells.

Sphingomyelin (SM) metabolism in the gut has been implicated in colonic tumorigenesis. Intestinal alkaline sphingomyelinase (alk-SMase) hydrolyses SM in the intestinal content and at the brush border. The enzyme activity is decreased in the tissues of human colorectal tumours. This study examines whether site or chain-mutation of alk-SMase occurs in colon cancer HT-29 cells and Caco-2 cells. Total RNA was isolated and the cDNA of alk-SMase was amplified by RT-PCR. The size of the cDNA from HT-29 cells was smaller than that of the wild-type cDNA. DNA sequencing identified a deletion of exon 4 in alk-SMase cDNA in HT-29 cells. No mutation in genomic alk-SMase DNA from exon 3 to 5 was identified. The exon 4 deletion was caused by a shift of RNA splice site in chromosome 17q25. In Caco-2 cells, no mutation of alk-SMase cDNA was identified. Transient expression in COS-7 cells showed that the enzyme from the cDNA in HT-29 cells had little alk-SMase activity whereas that in Caco-2 cells was as active as the wild-type alk-SMase. The deleted region included residue His353, which is predicted to form a substrate-binding site of alk-SMase. H353A substitution resulted in a protein with no alk-SMase activity. In monolayer cultured Caco-2 cells and HT-29 cells the alk-SMase activities were low. However, to culture the cells under polarizing conditions increased alk-SMase activity and reduced SM level in Caco-2 cells. The alk-SMase activity varied in parallel with alkaline phosphatase activity. In conclusion, we identified an inactive deletion in alk-SMase in HT-29 cells, and a differentiation-related expression of the enzyme in Caco-2 cells. The results provide a molecular mechanism related to previous findings of reduced alk-SMase activity in human colon cancers.

Cloning and characterization of the mammalian brain-specific, Mg2+-dependent neutral sphingomyelinase.

The enzymatic breakdown of sphingomyelin by sphingomyelinases is considered the major source of the second messenger ceramide. Studies on the contribution of the various described acidic and neutral sphingomyelinases to the signaling pool of ceramide have been hampered by the lack of molecular data on the neutral sphingomyelinases (nSMases). We recently identified a mammalian nSMase, an integral membrane protein with remote similarity to bacterial sphingomyelinases. However, its ubiquitous expression pattern is in contrast to previous findings that sphingomyelinase activity is found mainly in brain tissues. By using an improved database search method, combined with phylogenetic analysis, we identified a second mammalian nSMase (nSMase2) with predominant expression in the brain. The sphingomyelinase activity of nSMase2 has a neutral pH optimum, depends on Mg(2+) ions, and is activated by unsaturated fatty acids and phosphatidylserine. Immunofluorescence reveals a neuron-specific punctate perinuclear staining, which colocalizes with a Golgi marker in a number of cell lines. The likely identity of nSMase2 with cca1, a rat protein involved in contact inhibition of 3Y1 fibroblasts, suggests a role for this enzyme in cell cycle arrest. Both mammalian nSMases are members of a superfamily of Mg(2+)-dependent phosphohydrolases, which also contains nucleases, inositol phosphatases, and bacterial toxins.

Molecular cloning, characterization, and expression of a novel human neutral sphingomyelinase.

Neutral sphingomyelinase (N-SMase) has emerged as an important cell membrane-associated enzyme that participates in several signal transduction and cell regulatory phenomena. Using expression cloning, we have identified a 3.7-kilobase pair cDNA transcript for N-SMase whose open reading frame predicts a 397-amino acid polypeptide. Transfection of COS-7 cells with cDNA for N-SMase resulted in a marked increase in N-SMase activity. Recombinant N-SMase (r-N-SMase) had the following physical-chemical properties. Mg(2+) activated and Cu(2+) and glutathione inhibited the activity of r-N-SMase. In contrast, dithiothreitol did not alter the activity of the enzyme. Of several phospholipids examined, sphingomyelin was the preferred substrate for r-N-SMase. The apparent molecular mass of r-N-SMase derived from COS-7 cells was approximately 90 kDa, similar to the native neutral sphingomyelinase prepared from human urine. However, upon expression in Escherichia coli, the apparent molecular mass of the recombinant enzyme was approximately 45 kDa. We speculate that this apparent difference in recombinant enzymes derived from COS-7 and E. coli cells may be due to extensive post-transcriptional changes. r-N-SMase has numerous post-transcriptional modification sites such as phosphorylation sites via protein kinase C, casein kinase II, tyrosine kinase, and cAMP- and cGMP-dependent protein kinases as well as sites for glycosylation and myristoylation. Amino acid sequence alignment studies revealed that r-N-SMase has some similarity to acid sphingomyelinase and significant homology to the death domains of tumor necrosis factor-alpha receptor-1 and Fas/Apo-I. We believe that the molecular cloning and characterization of N-SMase cDNA will accelerate the process to define its role as a key regulator in apoptosis, lipid and lipoprotein metabolism, and other cell regulatory pathways.

Identification of gene sequences overexpressed in senescent and Werner syndrome human fibroblasts.

The phenotype of replicative senescence is a dominant trait in human diploid fibroblasts (HDF). Therefore, we have sought to identify overexpressed and/or newly expressed causal genes by constructing and screening a subtracted cDNA library derived from polyA+RNA of prematurely senescent Werner syndrome (WS) HDF. We have identified 15 cDNA clones that are overexpressed in senescent and WS HDF. Among them are six known sequences coding for: acid sphingomyelinase, fibronectin, SPARC, nm23-metastasis suppressor protein, and two translation factors, eIF-2 beta and EF-1 alpha. Among the 10 unknown clones are: S1-5, which encodes a secreted protein containing EGF-like domains and paradoxically stimulates DNA synthesis of young HDF in an autocrine and paracrine manner, S1-3, which encodes a protein containing "zinc finger" domains, suggesting nucleic acid binding properties; S1-15, which shows sequence similarities to human alpha 2-chimerin; and S2-6, which represents a new member of the LIM family of proteins. The other five clones do not have any significant homology to known sequences. Steady-state mRNA levels of all gene sequences thus far studied are elevated in both WS and senescent normal HDF when compared to young HDF, which suggests that senescent and WS HDF enter a final common pathway where multiple gene overexpression may generate diverse antiproliferative mechanisms and pathogenic sequelae.