Neutral sphingomyelinase 2 (nSMase2) is a phosphoprotein regulated by calcineurin (PP2B)

Inhibition of neutral sphingomyelinase 2 impairs HIV-1 envelope formation and substantially delays or eliminates viral rebound

Although HIV-1 Gag is known to drive viral assembly and budding, the precise mechanisms by which the lipid composition of the plasma membrane is remodeled during assembly are incompletely understood. Here, we provide evidence that the sphingomyelin hydrolase neutral sphingomyelinase 2 (nSMase2) interacts with HIV-1 Gag and through the hydrolysis of sphingomyelin creates ceramide that is necessary for proper formation of the viral envelope and viral maturation. Inhibition or depletion of nSMase2 resulted in the production of noninfectious HIV-1 virions with incomplete Gag lattices lacking condensed conical cores. Inhibition of nSMase2 in HIV-1-infected humanized mouse models with a potent and selective inhibitor of nSMase2 termed PDDC [phenyl(R)-(1-(3-(3,4-dimethoxyphenyl)-2, 6-dimethylimidazo[1,2-b]pyridazin-8-yl) pyrrolidin-3-yl)-carbamate] produced a linear reduction in levels of HIV-1 in plasma. If undetectable plasma levels of HIV-1 were achieved with PDDC treatment, viral rebound did not occur for up to 4 wk when PDDC was discontinued. In vivo and tissue culture results suggest that PDDC selectively kills cells with actively replicating HIV-1. Collectively, this work demonstrates that nSMase2 is a critical regulator of HIV-1 replication and suggests that nSMase2 could be an important therapeutic target with the potential to kill HIV-1-infected cells.

Lipid peroxidation and sphingolipid alterations in the cerebral cortex and hypothalamus of rats fed a high-protein diet

OBJECTIVES

High-protein diets (HPDs) are widely accepted to enhance satiety and energy expenditure and thus have become a popular strategy to lose weight and facilitate muscle protein synthesis. However, long-term high-protein consumption could be linked with metabolic and clinical problems such as renal and liver dysfunctions. This study verified the effects of 8-wk high-protein ingestion on lipid peroxidation and sphingolipid metabolism in the plasma, cerebral cortex, and hypothalamus in rats.

METHODS

Immunoenzymatic and spectrophotometric methods were applied to assess oxidation-reduction (redox) biomarkers and neutral sphingomyelinase activity, whereas gas-liquid chromatography and high-performance liquid chromatography were used to examine sphingolipid levels.

RESULTS

The vast majority of HPD-related alterations was restricted to the hypothalamus. Specifically, an increased rate of lipid peroxidation (increased lipid hydroperoxides, 8-isoprostanes, and thiobarbituric acid reactive substances) associated with ceramide accumulation via the activation of de novo synthesis (decreased sphinganine), salvage pathway (decreased sphingosine), and sphingomyelin hydrolysis (decreased sphingomyelin and increased neutral sphingomyelinase activity) was noted.

CONCLUSIONS

This study showed that HPD substantially affected hypothalamic metabolic pathways, which potentially alter cerebral output signals to the peripheral tissues.

Adult alcohol drinking and emotional tone are mediated by neutral sphingomyelinase during development in males

Alcohol use, abuse, and addiction, and resulting health hazards are highly sex-dependent with unknown mechanisms. Previously, strong links between the SMPD3 gene and its coded protein neutral sphingomyelinase 2 (NSM) and alcohol abuse, emotional behavior, and bone defects were discovered and multiple mechanisms were identified for females. Here we report strong sex-dimorphisms for central, but not for peripheral mechanisms of NSM action in mouse models. Reduced NSM activity resulted in enhanced alcohol consumption in males, but delayed conditioned rewarding effects. It enhanced the acute dopamine response to alcohol, but decreased monoaminergic systems adaptations to chronic alcohol. Reduced NSM activity increased depression- and anxiety-like behavior, but was not involved in alcohol use for the self-management of the emotional state. Constitutively reduced NSM activity impaired structural development in the brain and enhanced lipidomic sensitivity to chronic alcohol. While the central effects were mostly opposite to NSM function in females, similar roles in bone-mediated osteocalcin release and its effects on alcohol drinking and emotional behavior were observed. These findings support the view that the NSM and multiple downstream mechanism may be a source of the sex-differences in alcohol use and emotional behavior.

Mechanisms underlying Nrf2 nuclear translocation by non-lethal levels of hydrogen peroxide: p38 MAPK-dependent neutral sphingomyelinase2 membrane trafficking and ceramide/PKCζ/CK2 signaling

Hydrogen peroxide is an aerobic metabolite playing a central role in redox signaling and oxidative stress. H₂O₂ could activate redox sensitive transcription factors, such as Nrf2, AP-1 and NF-κB by different manners. In some cells, treatment with non-lethal levels of H₂O₂ induces rapid activation of Nrf2, which upregulates expression of a set of genes involved in glutathione (GSH) synthesis and defenses against oxidative damage. It depends on two steps, the rapid translational activation of Nrf2 and facilitation of Nrf2 nuclear translocation. We review the molecular mechanisms by which H₂O₂ induces nuclear translocation of Nrf2 in cultured cells by highlighting the role of neutral sphingomyelinase 2 (nSMase2), a GSH sensor. H₂O₂ enters cells through aquaporin channels in the plasma membrane and is rapidly reduced to H₂O by GSH peroxidases to consume cellular GSH, resulting in nSMase2 activation to generate ceramide. H₂O₂ also activates p38 MAP kinase, which enhances transfer of nSMase2 from perinuclear regions to plasma membrane lipid rafts to accelerate ceramide generation. Low levels of ceramide activate PKCζ, which then activates casein kinase 2 (CK2). These protein kinases are able to phosphorylate Nrf2 to stabilize and activate it. Notably, Nrf2 also binds to caveolin-1 (Cav1), which protects Nrf2 from Keap1-mediated degradation and limits Nrf2 nuclear translocation. We propose that Cav1serves as a signaling hub for the control of H₂O₂-mediated phosphorylation of Nrf2 by kinases, which results in release of Nrf2 from Cav1 to facilitate nuclear translocation. In summary, H₂O₂ induces GSH depletion which is recovered by Nrf2 activation dependent on p38/nSMase2/ceramide signaling.

Sphingomyelinase Disables Inactivation in Endogenous PIEZO1 Channels

Endogenous PIEZO1 channels of native endothelium lack the hallmark inactivation often seen when these channels are overexpressed in cell lines. Because prior work showed that the force of shear stress activates sphingomyelinase in endothelium, we considered if sphingomyelinase is relevant to endogenous PIEZO1. Patch clamping was used to quantify PIEZO1-mediated signals in freshly isolated murine endothelium exposed to the mechanical forces caused by shear stress and membrane stretch. Neutral sphingomyelinase inhibitors and genetic disruption of sphingomyelin phosphodiesterase 3 (SMPD3) cause PIEZO1 to switch to profoundly inactivating behavior. Ceramide (a key product of SMPD3) rescues non-inactivating channel behavior. Its co-product, phosphoryl choline, has no effect. In contrast to ceramide, sphingomyelin (the SMPD3 substrate) does not affect inactivation but alters channel force sensitivity. The data suggest that sphingomyelinase activity, ceramide, and sphingomyelin are determinants of native PIEZO gating that enable sustained activity.

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SMPD3 sphingomyelinase enables long-lasting PIEZO1 activity in response to force

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Ceramide, a key lipid product of SMPD3, promotes long-lasting activity

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Sphingomyelin, the SMPD3 substrate, does not affect the duration of activity

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Sphingomyelin alters PIEZO1 force sensitivity

Neutral sphingomyelinase-2 and cardiometabolic diseases

Sphingolipids, in particular ceramides, play vital role in pathophysiological processes linked to metabolic syndrome, with implications in the development of insulin resistance, pancreatic ß-cell dysfunction, type 2 diabetes, atherosclerosis, inflammation, nonalcoholic steatohepatitis, and cancer. Ceramides are produced by the hydrolysis of sphingomyelin, catalyzed by different sphingomyelinases, including neutral sphingomyelinase 2 (nSMase2), whose dysregulation appears to underlie many of the inflammation-related pathologies. In this review, we discuss the current knowledge on the biochemistry of nSMase2 and ceramide production and its regulation by inflammatory cytokines, with particular reference to cardiometabolic diseases. nSMase2 contribution to pathogenic processes appears to involve cyclical feed-forward interaction with proinflammatory cytokines, such as TNF-α and IL-1ß, which activate nSMase2 and the production of ceramides, that in turn triggers the synthesis and release of inflammatory cytokines. We elaborate these pathogenic interactions at the molecular level and discuss the potential therapeutic benefits of inhibiting nSMase2 against inflammation-driven cardiometabolic diseases.

Characterization of neutral sphingomyelinase activity and isoform expression in rodent skeletal muscle mitochondria

Skeletal muscle is composed of fiber types that differ in mitochondrial content, antioxidant capacity, and susceptibility to apoptosis. Ceramides have been linked to oxidative stress-mediated apoptotic intracellular signalling and the enzyme neutral sphingomyelinase (nSMase) is, in part, responsible for generating these ceramides through the hydrolysis of sphingomyelin. Despite the role of ceramides in mediating apoptosis, there is a gap in the literature regarding nSMase in skeletal muscle mitochondria. This study aimed to characterize total nSMase activity and individual isoform expression in isolated subsarcolemmal (SS) mitochondria from soleus, diaphragm, plantaris, and extensor digitorum longus (EDL). Total nSMase activity did not differ between muscle types. nSMase2 content was detectable in all muscles and higher in EDL, soleus, and plantaris compared to diaphragm whereas nSMase3 was undetectable in all muscles. Finally, total nSMase activity positively correlated to nSMase2 protein content in soleus but not the other muscles. These findings suggest that nSMase associated with SS mitochondria may play a role in intracellular signalling processes involving ceramides in skeletal muscle and nSMase2 may be the key isoform, specifically in slow twitch muscle like soleus. Further studies are needed to fully elucidate the specific contribution of nSMase, along with the role of the various isoforms and mitochondrial subpopulation in generating mitochondrial ceramides in skeletal muscle, and its potential effects on mediating apoptosis.

Protein Binding and Stability of Drug Candidates: The Achilles' Heel in In Vitro Potency Assays

In the present scenario of drug discovery, several screening filters ensure a rigorous nomination of clinical candidates. One of these screens is the determination of IC₅₀, the concentration of drug at half-maximal inhibitory concentration, also known as a potency assay. However, various nuances pertaining to the design, execution, and interpretation of in vitro potency results suggest a sizeable opportunity for the generation of erroneous data. The focus areas of this article include: (1) examining the requirement for the addition of serum albumin in in vitro potency assays, (2) problems encountered with cell lysates, and (3) drug candidate stability concerns during in vitro potency assays/high-throughput screening. Based on this assessment, the interpretation of the data generated using cell-based systems (i.e., lysates with or without the addition of fetal bovine serum) should be carried out with caution for in vitro potency testing, and the inclusion of a correction factor for non-specific protein binding should be considered. The addition of serum albumin to a cell-free system should be restricted to drugs having high protein binding (≥ 90%). Additionally, stability assessment of analytes should be considered to avoid dubious in vitro potency outcomes due to degraded material or active metabolite(s).

Sphingomyelinases and Liver Diseases

Sphingolipids (SLs) are critical components of membrane bilayers that play a crucial role in their physico-chemical properties. Ceramide is the prototype and most studied SL due to its role as a second messenger in the regulation of multiple signaling pathways and cellular processes. Ceramide is a heterogeneous lipid entity determined by the length of the fatty acyl chain linked to its carbon backbone sphingosine, which can be generated either by de novo synthesis from serine and palmitoyl-CoA in the endoplasmic reticulum or via sphingomyelin (SM) hydrolysis by sphingomyelinases (SMases). Unlike de novo synthesis, SMase-induced SM hydrolysis represents a rapid and transient mechanism of ceramide generation in specific intracellular sites that accounts for the diverse biological effects of ceramide. Several SMases have been described at the molecular level, which exhibit different pH requirements for activity: neutral, acid or alkaline. Among the SMases, the neutral (NSMase) and acid (ASMase) are the best characterized for their contribution to signaling pathways and role in diverse pathologies, including liver diseases. As part of a Special Issue (Phospholipases: From Structure to Biological Function), the present invited review summarizes the physiological functions of NSMase and ASMase and their role in chronic and metabolic liver diseases, of which the most relevant is nonalcoholic steatohepatitis and its progression to hepatocellular carcinoma, due to the association with the obesity and type 2 diabetes epidemic. A better understanding of the regulation and role of SMases in liver pathology may offer the opportunity for novel treatments of liver diseases.

Immuno-Surgical Management of Pancreatic Cancer with Analysis of Cancer Exosomes

Exosomes (EXs), a type of extracellular vesicles secreted from various cells and especially cancer cells, mesenchymal cells, macrophages and other cells in the tumor microenvironment (TME), are involved in biologically malignant behaviors of cancers. Recent studies have revealed that EXs contain microRNAs on their inside and express proteins and glycolipids on their outsides, every component of which plays a role in the transmission of genetic and/or epigenetic information in cell-to-cell communications. It is also known that miRNAs are involved in the signal transduction. Thus, EXs may be useful for monitoring the TME of tumor tissues and the invasion and metastasis, processes that are associated with patient survival. Because several solid tumors secrete immune checkpoint proteins, including programmed cell death-ligand 1, the EX-mediated mechanisms are suggested to be potent targets for monitoring patients. Therefore, a companion therapeutic approach against cancer metastasis to distant organs is proposed when surgical removal of the primary tumor is performed. However, EXs and immune checkpoint mechanisms in pancreatic cancer are not fully understood, we provide an update on the recent advances in this field and evidence that EXs will be useful for maximizing patient benefit in precision medicine.

Sphingomyelin phosphodiesterase 3 methylation and silencing in oral squamous cell carcinoma results in increased migration and invasion and altered stress response

Neutral sphingomyelinase 2 (nSMase2), the product of the sphingomyelin phosphodiesterase 3 (SMPD3) gene, catalyzes the hydrolysis of sphingomyelin to ceramide. Ceramide acts on various signaling pathways to influence cell proliferation, survival, and stress response. Altered levels of sphingolipids and ceramides have been reported in various cancer types, including oral squamous cell carcinoma (OSCC). OSCC patients exhibit a poor 5-year survival rate of 50%, a figure that has remained stagnant for decades. To overcome this requires a better understanding of the molecular events driving this disease. The molecular analysis of the oral cavity reported here has identified the SMPD3 promoter region as a site of frequent hypermethylation and downregulation in pre-malignant and malignant tissues as compared with healthy control tissues. While lentivirus-induced overexpression of SMPD3 in cell models of oral dysplasia and OSCC did not significantly alter proliferation, it did decrease migration and invasion and increased resistance to the epidermal growth factor receptor (EGFR) inhibitor erlotinib. These results suggest that SMPD3 downregulation is a common event in OSCC progression and may promote the spread of tumor cells.

Synaptic vesicle generation from activity-dependent bulk endosomes requires a dephosphorylation-dependent dynamin-syndapin interaction

Activity‐dependent bulk endocytosis generates synaptic vesicles (SVs) during intense neuronal activity via a two‐step process. First, bulk endosomes are formed direct from the plasma membrane from which SVs are then generated. SV generation from bulk endosomes requires the efflux of previously accumulated calcium and activation of the protein phosphatase calcineurin. However, it is still unknown how calcineurin mediates SV generation. We addressed this question using a series of acute interventions that decoupled the generation of SVs from bulk endosomes in rat primary neuronal culture. This was achieved by either disruption of protein–protein interactions via delivery of competitive peptides, or inhibition of enzyme activity by known inhibitors. SV generation was monitored using either a morphological horseradish peroxidase assay or an optical assay that monitors the replenishment of the reserve SV pool. We found that SV generation was inhibited by, (i) peptides that disrupt calcineurin interactions, (ii) an inhibitor of dynamin I GTPase activity and (iii) peptides that disrupt the phosphorylation‐dependent dynamin I–syndapin I interaction. Peptides that disrupted syndapin I interactions with eps15 homology domain‐containing proteins had no effect. This revealed that (i) calcineurin must be localized at bulk endosomes to mediate its effect, (ii) dynamin I GTPase activity is essential for SV fission and (iii) the calcineurin‐dependent interaction between dynamin I and syndapin I is essential for SV generation. We therefore propose that a calcineurin‐dependent dephosphorylation cascade that requires both dynamin I GTPase and syndapin I lipid‐deforming activity is essential for SV generation from bulk endosomes.

Role of Neutral Sphingomyelinase-2 (NSM 2) in the Control of T Cell Plasma Membrane Lipid Composition and Cholesterol Homeostasis

The activity of neutral sphingomyelinase-2 (NSM2) to catalyze the conversion of sphingomyelin (SM) to ceramide and phosphocholine at the cytosolic leaflet of plasma membrane (PM) is important in T cell receptor (TCR) signaling. We recently identified PKCζ as a major NSM2 downstream effector which regulates microtubular polarization. It remained, however, unclear to what extent NSM2 activity affected overall composition of PM lipids and downstream effector lipids in antigen stimulated T cells. Here, we provide a detailed lipidomics analyses on PM fractions isolated from TCR stimulated wild type and NSM2 deficient (ΔNSM) Jurkat T cells. This revealed that in addition to that of sphingolipids, NSM2 depletion also affected concentrations of many other lipids. In particular, NSM2 ablation resulted in increase of lyso-phosphatidylcholine (LPC) and lyso-phosphatidylethanolamine (LPE) which both govern PM biophysical properties. Crucially, TCR dependent upregulation of the important T cell signaling lipid diacylglycerol (DAG), which is fundamental for activation of conventional and novel PKCs, was abolished in ΔNSM cells. Moreover, NSM2 activity was found to play an important role in PM cholesterol transport to the endoplasmic reticulum (ER) and production of cholesteryl esters (CE) there. Most importantly, CE accumulation was essential to sustain human T cell proliferation. Accordingly, inhibition of CE generating enzymes, the cholesterol acetyltransferases ACAT1/SOAT1 and ACAT2/SOAT2, impaired TCR driven expansion of both CD4⁺ and CD8⁺ T cells. In summary, our study reveals an important role of NSM2 in regulating T cell functions by its multiple effects on PM lipids and cholesterol homeostasis.

Sphingomyelin Breakdown in T Cells: Role of Membrane Compartmentalization in T Cell Signaling and Interference by a Pathogen

Sphingolipids are major components of cellular membranes, and at steady-state level, their metabolic fluxes are tightly controlled. On challenge by external signals, they undergo rapid turnover, which substantially affects the biophysical properties of membrane lipid and protein compartments and, consequently, signaling and morphodynamics. In T cells, external cues translate into formation of membrane microdomains where proximal signaling platforms essential for metabolic reprograming and cytoskeletal reorganization are organized. This review will focus on sphingomyelinases, which mediate sphingomyelin breakdown and ensuing ceramide release that have been implicated in T-cell viability and function. Acting at the sphingomyelin pool at the extrafacial or cytosolic leaflet of cellular membranes, acid and neutral sphingomyelinases organize ceramide-enriched membrane microdomains that regulate T-cell homeostatic activity and, upon stimulation, compartmentalize receptors, membrane proximal signaling complexes, and cytoskeletal dynamics as essential for initiating T-cell motility and interaction with endothelia and antigen-presenting cells. Prominent examples to be discussed in this review include death receptor family members, integrins, CD3, and CD28 and their associated signalosomes. Progress made with regard to experimental tools has greatly aided our understanding of the role of bioactive sphingolipids in T-cell biology at a molecular level and of targets explored by a model pathogen (measles virus) to specifically interfere with their physiological activity.

The neutral sphingomyelinase 2 in T cell receptor signaling and polarity

By hydrolyzing its substrate sphingomyelin at the cytosolic leaflet of cellular membranes, the neutral sphingomyelinase 2 (NSM2) generates microdomains which serve as docking sites for signaling proteins and thereby, functions to regulate signal relay. This has been particularly studied in cellular stress responses while the regulatory role of this enzyme in the immune cell compartment has only recently emerged. In T cells, phenotypic polarization by co-ordinated cytoskeletal remodeling is central to motility and interaction with endothelial or antigen-presenting cells during tissue recruitment or immune synapse formation, respectively. This review highlights studies adressing the role of NSM2 in T cell polarity in which the enzyme plays a major role in regulating cytoskeletal dynamics.

The juxtamembrane linker in neutral sphingomyelinase-2 functions as an intramolecular allosteric switch that activates the enzyme

Neutral sphingomyelinase 2 (nSMase2) produces the bioactive lipid ceramide and has important roles in neurodegeneration, cancer, and exosome formation. Although nSMase2 has low basal activity, it is fully activated by phosphatidylserine (PS). Previous work showed that interdomain interactions within nSMase2 are needed for PS activation. Here, we use multiple approaches, including small angle X-ray scattering, hydrogen-deuterium exchange-MS, circular dichroism and thermal shift assays, and membrane yeast two-hybrid assays, to define the mechanism mediating this interdomain interactions within nSMase2. In contrast to what we previously assumed, we demonstrate that PS binding at the N-terminal and juxtamembrane regions of nSMase2 rather acts as a conformational switch leading to interdomain interactions that are critical to enzyme activation. Our work assigns a unique function for a class of linkers of lipid-activated, membrane-associated proteins. It indicates that the linker actively participates in the activation mechanism via intramolecular interactions, unlike the canonical linkers that typically aid protein dimerization or localization.

Mechanism of Rapid Nuclear Factor-E2-Related Factor 2 (Nrf2) Activation via Membrane-Associated Estrogen Receptors: Roles of NADPH Oxidase 1, Neutral Sphingomyelinase 2 and Epidermal Growth Factor Receptor (EGFR)

Membrane-associated estrogen receptors (ER)-α36 and G protein-coupled estrogen receptor (GPER) play important roles in the estrogen’s rapid non-genomic actions including stimulation of cell proliferation. Estrogen via these receptors induces rapid activation of transcription factor nuclear factor-E2-related factor 2 (Nrf2), a master regulator of detoxification and antioxidant systems, playing a key role in the metabolic reprogramming to support cell proliferation. This review highlights the possible mechanism underlying rapid Nrf2 activation via membrane-associated estrogen receptors by estrogen and phytoestrogens. Stimulation of ER-α36-GPER signaling complex rapidly induces Src-mediated transactivation of epidermal growth factor receptor (EGFR) leading to a kinase-mediated signaling cascade. We propose a novel hypothesis that ER-α36-GPER signaling initially induces rapid and temporal activation of NADPH oxidase 1 to generate superoxide, which subsequently activates redox-sensitive neutral sphingomyelinase 2 generating the lipid signaling mediator ceramide. Generation of ceramide is required for Ras activation and ceramide-protein kinase C ζ-casein kinase 2 (CK2) signaling. Notably, CK2 enhances chaperone activity of the Cdc37-Hsp90 complex supporting activation of various signaling kinases including Src, Raf and Akt (protein kinase B). Activation of Nrf2 may be induced by cooperation of two signaling pathways, (i) Nrf2 stabilization by direct phosphorylation by CK2 and (ii) EGFR-Ras-PI 3 kinase (PI3K)-Akt axis which inhibits glycogen synthase kinase 3β leading to enhanced nuclear transport and stability of Nrf2.

Neutral Sphingomyelinase Inhibition Alleviates LPS-Induced Microglia Activation and Neuroinflammation after Experimental Traumatic Brain Injury

Neuroinflammation is one of the key secondary injury mechanisms triggered by traumatic brain injury (TBI). Microglial activation, a hallmark of brain neuroinflammation, plays a critical role in regulating immune responses after TBI and contributes to progressive neurodegeneration and neurologic deficits following brain trauma. Here we evaluated the role of neutral sphingomyelinase (nSMase) in microglial activation by examining the effects of the nSMase inhibitors altenusin and GW4869 in vitro (using BV2 microglia cells and primary microglia), as well as in a controlled cortical injury (CCI) model in adult male C57BL/6 mice. Pretreatment of altenusin or GW4869 prior to lipopolysaccharide (LPS) stimulation for 4 or 24 hours, significantly downregulated gene expression of the pro-inflammatory mediators TNF-α, IL-1β, IL-6, iNOS, and CCL2 in microglia and reduced the release of nitric oxide and TNF-α These nSMase inhibitors also attenuated the release of microparticles and phosphorylation of p38 MAPK and ERK1/2. In addition, altenusin pretreatment also reduced the gene expression of multiple inflammatory markers associated with microglial activation after experimental TBI, including TNF-α, IL-1β, IL-6, iNOS, CCL2, CD68, NOX2, and p22phox Overall, our data demonstrate that nSMase inhibitors attenuate multiple inflammatory pathways associated with microglial activation in vitro and after experimental TBI. Thus, nSMase inhibitors may represent promising therapeutics agents targeting neuroinflammation.

Contribution of neutral sphingomyelinases to in vitro virulence of Entamoeba histolytica

Amoebiasis is a worldwide health problem caused by the pathogen Entamoeba histolytica. Several virulence factors have been implicated in host invasion, immune evasion, and tissue damage. There are still new factors that remain to be elucidated and characterized. In this work, we obtained amoebic transfectants overexpressing three of the neutral sphingomyelinase enzymes encoded in the E. histolytica genome. The EhnSM3 overexpression induced an increase in hemolytic and cytotoxic activities, besides an increase in gene expression of amoebapore A, B, and C. Meanwhile the EhnSM1 and EhnSM2 overexpression caused an increase in cytopathic activity. In all the neutral sphingomyelinases overexpressing strains, the gene expression levels for cysteine proteinase 5, adhesin 112 and, heavy and light Gal/GalNAc lectin subunits were not affected. We propose that the increase of cytotoxic and lytic effect of EhnSM3 overexpressed strain can be related to the sum of the effect of EhnSM3 plus amoebapores, in a process cell contact-dependent or as mediator by inducing the gene expression of amoebapores enabling a link between EhnSM3 with the virulence phenotype in E. histolytica. Our results suggest a differential role for neutral sphingomyelinases in E. histolytica virulence.

Structure of human nSMase2 reveals an interdomain allosteric activation mechanism for ceramide generation

Neutral sphingomyelinase 2 (nSMase2, product of the SMPD3 gene) is a key enzyme for ceramide generation that is involved in regulating cellular stress responses and exosome-mediated intercellular communication. nSMase2 is activated by diverse stimuli, including the anionic phospholipid phosphatidylserine. Phosphatidylserine binds to an integral-membrane N-terminal domain (NTD); however, how the NTD activates the C-terminal catalytic domain is unclear. Here, we identify the complete catalytic domain of nSMase2, which was misannotated because of a large insertion. We find the soluble catalytic domain interacts directly with the membrane-associated NTD, which serves as both a membrane anchor and an allosteric activator. The juxtamembrane region, which links the NTD and the catalytic domain, is necessary and sufficient for activation. Furthermore, we provide a mechanistic basis for this phenomenon using the crystal structure of the human nSMase2 catalytic domain determined at 1.85-Å resolution. The structure reveals a DNase-I-type fold with a hydrophobic track leading to the active site that is blocked by an evolutionarily conserved motif which we term the "DK switch." Structural analysis of nSMase2 and the extended N-SMase family shows that the DK switch can adopt different conformations to reposition a universally conserved Asp (D) residue involved in catalysis. Mutation of this Asp residue in nSMase2 disrupts catalysis, allosteric activation, stimulation by phosphatidylserine, and pharmacological inhibition by the lipid-competitive inhibitor GW4869. Taken together, these results demonstrate that the DK switch regulates ceramide generation by nSMase2 and is governed by an allosteric interdomain interaction at the membrane interface.

ATRA transcriptionally induces nSMase2 through CBP/p300-mediated histone acetylation

Neutral sphingomyelinase-2 (nSMase2) is a key ceramide-producing enzyme in cellular stress responses. While many posttranslational regulators of nSMase2 are known, emerging evidence suggests a more protracted regulation of nSMase2 at the transcriptional level. Previously, we reported that nSMase2 is induced by all-trans retinoic acid (ATRA) in MCF7 cells and implicated nSMase2 in ATRA-induced growth arrest. Here, we further investigated how ATRA regulates nSMase2. We find that ATRA regulates nSMase2 transcriptionally through the retinoic acid receptor-α, but this is independent of previously identified transcriptional regulators of nSMase2 (Sp1, Sp3, Runx2) and is not through increased promoter activity. Epigenetically, the nSMase2 gene is not repressively methylated in MCF7 cells. However, inhibition of histone deacetylases (HDACs) with trichostatin A (TSA) induced nSMase2 comparably to ATRA; furthermore, combined ATRA and TSA treatment was not additive, suggesting ATRA regulates nSMase2 through direct modulation of histone acetylation. Confirming this, the histone acetyltransferases CREB-binding protein and p300 were required for ATRA induction of nSMase2. Finally, use of class-specific HDAC inhibitors suggested that HDAC4 and/or HDAC5 are negative regulators of nSMase2 expression. Collectively, these results identify a novel pathway of nSMase2 regulation and suggest that physiological or pharmacological modulation of histone acetylation can directly affect nSMase2 levels.

The neutral sphingomyelinase-2 is involved in angiogenic signaling triggered by oxidized LDL

Capillaries of the external part of the normal arterial wall constitute the vasa vasorum network. In atherosclerotic lesions, neovascularization occurs in areas of intimal hyperplasia where it may promote plaque expansion, and intraplaque hemorrhage. Oxidized LDL that are present in atherosclerotic areas activate various angiogenic signaling pathways, including reactive oxygen species and the sphingosine kinase/sphingosine-1-phosphate pathway. We aimed to investigate whether oxidized LDL-induced angiogenesis requires neutral sphingomyelinase-2 activation and the neutral sphingomyelinase-2/sphingosine kinase-1 pathway. The role of neutral sphingomyelinase-2 in angiogenic signaling was investigated in Human Microvascular Endothelial Cells (HMEC-1) forming capillary tube on Matrigel and in vivo in the Matrigel plug assay in C57BL/6 mice and in the chicken chorioallantoic membrane model. Low concentration of human oxidized LDL elicits HMEC-1 capillary tube formation and neutral sphingomyelinase-2 activation, which were blocked by neutral sphingomyelinase-2 inhibitors, GW4869 and specific siRNA. This angiogenic effect was mimicked by low concentration of C6-Ceramide and was inhibited by sphingosine kinase-1 inhibitors. Upstream of neutral sphingomyelinase-2, oxidized LDL-induced activation required LOX-1, reactive oxygen species generation by NADPH oxidase and p38-MAPK activation. Inhibition of sphingosine kinase-1 blocked the angiogenic response and triggered HMEC-1 apoptosis. Low concentration of oxidized LDL was angiogenic in vivo, both in the Matrigel plug assay in mice and in the chorioallantoic membrane model, and was blocked by GW4869. In conclusion, low oxLDL concentration triggers sprouting angiogenesis that involves ROS-induced activation of the neutral sphingomyelinase-2/sphingosine kinase-1 pathway, and is effectively inhibited by GW4869.

The hyaluronic acid inhibitor 4-methylumbelliferone is an NSMase2 activator-role of Ceramide in MU anti-tumor activity

Increased synthesis of hyaluronic acid (HA) is often associated with increased metastatic potential and invasivity of tumor cells. 4-Methylumbelliferone (MU) is an inhibitor of HA synthesis, and has been studied as a potential anti-tumor drug to inhibit the growth of primary tumors and distant metastasis of tumor cells. Although several studies reported that the anticancer effects of MU are mediated by inhibition of HA signaling, the mechanism still needs to be clarified. In a previous study we demonstrated the regulation of HA synthesis by ceramide, and now show how MU activated neutral sphingomyelinase2 (NSMase2) generates ceramides and mediates MU induced inhibition of HA synthesis, cell migration and invasion, and apoptosis of tumor cells. Using a HA enriched mouse oligodendroglioma cell line G26-24 we found that MU elevated the activity of NSMase2 and increased ceramide levels, which in turn increased phosphatase PP2A activity. Further, the activated PP2A reduced phosphorylation of Akt, decreased activities of HA synthase2 (HAS2) and calpains, and inhibited both the synthesis of HA, and the migration and invasion of G26-24 tumor cells. In addition, MU mediated ceramide stimulated activation of p53 and caspase-3, reduced SIRT1 expression and decreased G26-24 viability. The mechanism of the MU anticancer therefore initially involves NSMase2/ceramide/PP2A/AKT/HAS2/caspase-3/p53/SIRT1 and the calpain signaling pathway, suggesting that ceramides play a key role in the ability of a tumor to become aggressively metastatic and grow.

P53-dependent upregulation of neutral sphingomyelinase-2: role in doxorubicin-induced growth arrest

Neutral sphingomyelinase-2 (nSMase2) is a ceramide-generating enzyme that has been implicated in growth arrest, apoptosis and exosome secretion. Although previous studies have reported transcriptional upregulation of nSMase2 in response to daunorubicin, through Sp1 and Sp3 transcription factors, the role of the DNA damage pathway in regulating nSMase2 remains unclear. In this study, we show that doxorubicin induces a dose-dependent induction of nSMase2 mRNA and protein with concomitant increases in nSMase activity and ceramide levels. Upregulation of nSMase2 was dependent on ATR, Chk1 and p53, thus placing it downstream of the DNA damage pathway. Moreover, overexpression of p53 was sufficient to transcriptionally induce nSMase2, without the need for DNA damage. DNA-binding mutants as well as acetylation mutants of p53 were unable to induce nSMase2, suggesting a role of nSMase2 in growth arrest. Moreover, knockdown of nSMase2 prevented doxorubicin-induced growth arrest. Finally, p53-induced nSMase2 upregulation appears to occur via a novel transcription start site upstream of exon 3. These results identify nSMase2 as a novel p53 target gene, regulated by the DNA damage pathway to induce cell growth arrest.

Src regulates cigarette smoke-induced ceramide generation via neutral sphingomyelinase 2 in the airway epithelium

We previously demonstrated that the neutral sphingomyelinase (nSMase) 2 is the sole sphingomyelinase activated during cigarette smoke (CS)-induced oxidative stress of human airway epithelial cells, leading to ceramide generation and subsequent apoptosis of affected cells. Since then, we reported that nSMase2 is a phosphoprotein, the degree of enzymatic activity and stability of which are dictated by its degree of phosphorylation. Simultaneously, the non-receptor tyrosine kinase and proto-oncogene Src has increasingly become a target of interest in both smoking-related lung injury, such as chronic obstructive pulmonary disease, and lung cancer. Within this context, we tested and now present Src as a regulator of ceramide generation via modulation of nSMase2 phosphorylation and activity during CS-induced oxidative stress. Specifically, we provide evidence that Src activity is necessary for both CS-induced ceramide accumulation in vivo (129/Sv mice) and in vitro (human airway epithelial cells) and for nSMase2 activity during CS-induced oxidative stress. Moreover, because nSMase2 is exclusively phosphorylated on serines, we show that this occurs through Src-dependent activation of the serine/threonine kinase p38 mitogen-activated protein kinase during oxidative stress. Finally, we provide evidence that Src and p38 mitogen-activated protein kinase activities are critical for regulating nSMase2 phosphorylation. This study provides insights into a molecular target involved in smoking-related lung injury, represented here as nSMase2, and its modulation by the oncogene Src.

Neutral sphingomyelinase-2 is a redox sensitive enzyme: role of catalytic cysteine residues in regulation of enzymatic activity through changes in oligomeric state

Neutral sphingomyelinase-2 (nSMase-2) is the major sphingomyelinase activated in response to pro-inflammatory cytokines and during oxidative stress. It is a membrane-bound 655 amino acid protein containing 22 cysteine residues. In this study, we expressed recombinant mouse nSMase-2 protein in Escherichia coli, and investigated whether nSMase-2 is a redox sensitive enzyme. Our results demonstrate that nSMase-2 exists as both monomers and multimers that are associated with high and low enzymatic activity respectively. Mutational analysis of nSMase-2 identified within its C-terminal catalytic domain several oxidant-sensitive cysteine residues that were shown to be involved in enzyme oligomerization. Changing Cys(617) to Ser for example is a gain-of-function mutation associated with a decreased propensity for oligomerization. Alternatively, nSMase-2 expression in a bacterial strain that lacks endogenous thioredoxin, Rosetta-gami2, results in increased oligomer formation and lower enzyme activity. Phenotypic rescue was accomplished by treating nSMase-2 lysates with recombinant human thioredoxin. This indicates that nSMase-2 may be a novel substrate for thioredoxin. FRET analysis confirmed the presence of nSMase-2 multimers in mammalian HEK cells and their localization to the plasma membrane. In conclusion, our results identify nSMase-2 as a redox-sensitive enzyme, whose basal activity is influenced by thioredoxin-mediated changes in its oligomeric state.

Lung injury and lung cancer caused by cigarette smoke-induced oxidative stress: Molecular mechanisms and therapeutic opportunities involving the ceramide-generating machinery and epidermal growth factor receptor

Chronic obstructive pulmonary disease (COPD) and lung cancer are frequently caused by tobacco smoking. However, these diseases present opposite phenotypes involving redox signaling at the cellular level. While COPD is characterized by excessive airway epithelial cell death and lung injury, lung cancer is caused by uncontrolled epithelial cell proliferation. Notably, epidemiological studies have demonstrated that lung cancer incidence is significantly higher in patients who have preexisting emphysema/lung injury. However, the molecular link and common cell signaling events underlying lung injury diseases and lung cancer are poorly understood. This review focuses on studies of molecular mechanism(s) underlying smoking-related lung injury (COPD) and lung cancer. Specifically, the role of the ceramide-generating machinery during cigarette smoke-induced oxidative stress leading to both apoptosis and proliferation of lung epithelial cells is emphasized. Over recent years, it has been established that ceramide is a sphingolipid playing a major role in lung epithelia structure/function leading to lung injury in chronic pulmonary diseases. However, new and unexpected findings draw attention to its potential role in lung development, cell proliferation, and tumorigenesis. To address this dichotomy in detail, evidence is presented regarding several protein targets, including Src, p38 mitogen-activated protein kinase, and neutral sphingomyelinase 2, the major sphingomyelinase that controls ceramide generation during oxidative stress. Furthermore, their roles are presented not only in apoptosis and lung injury but also in enhancing cell proliferation, lung cancer development, and resistance to epidermal growth factor receptor-targeted therapy for treating lung cancer.

Roles and regulation of neutral sphingomyelinase-2 in cellular and pathological processes

Our understanding of the functions of ceramide signaling has advanced tremendously over the past decade. In this review, we focus on the roles and regulation of neutral sphingomyelinase 2 (nSMase2), an enzyme that generates the bioactive lipid ceramide through the hydrolysis of the membrane lipid sphingomyelin. A large body of work has now implicated nSMase2 in a diverse set of cellular functions, physiological processes, and disease pathologies. We discuss different aspects of this enzyme's regulation from transcriptional, post-translational, and biochemical. Furthermore, we highlight nSMase2 involvement in cellular processes including inflammatory signaling, exosome generation, cell growth, and apoptosis, which in turn play important roles in pathologies such as cancer metastasis, Alzheimer's disease, and other organ systems disorders. Lastly, we examine avenues where targeted nSMase2-inhibition may be clinically beneficial in disease scenarios.

Oxidation-induced conformational changes in calcineurin determined by covalent labeling and tandem mass spectrometry

The Ca²⁺/calmodulin activated phosphatase, calcineurin, is inactivated by H₂O₂ or superoxide-induced oxidation, both in vivo and in vitro. However, the potential for global and/or local conformation changes occurring within calcineurin as a function of oxidative modification, that may play a role in the inactivation process, has not been examined. Here, the susceptibility of calcineurin methionine residues toward H₂O₂-induced oxidation were determined using a multienzyme digestion strategy coupled with capillary HPLC–electrospray ionization mass spectrometry and tandem mass spectrometry analysis. Then, regions within the protein complex that underwent significant conformational perturbation upon oxidative modification were identified by monitoring changes in the modification rates of accessible lysine residues between native and oxidized forms of calcineurin, using an amine-specific covalent labeling reagent, S,S′-dimethylthiobutanoylhydroxysuccinimide ester (DMBNHS), and tandem mass spectrometry. Importantly, methionine residues found to be highly susceptible toward oxidation, and the lysine residues exhibiting large increases in accessibility upon oxidation, were all located in calcineurin functional domains involved in Ca²⁺/CaM binding regulated calcineurin stimulation. These findings therefore provide initial support for the novel mechanistic hypothesis that oxidation-induced global and/or local conformational changes within calcineurin contribute to inactivation via (i) impairing the interaction between calcineurin A and calcineurin B, (ii) altering the low-affinity Ca²⁺ binding site in calcineurin B, (iii) inhibiting calmodulin binding to calcineurin A, and/or (iv) by altering the affinity between the calcineurin A autoinhibitory domain and the catalytic center.

Trk receptors need neutral sphingomyelinase activity to promote cell viability

Neurotrophins are a group of secreted polypeptides, which comprises Nerve Growth Factor (NGF) and Brain-Derived Neurotrophic Factor (BDNF). Each neurotrophin can bind specifically to a tyrosine kinase Trk receptor (TrkA, TrkB or TrkC), while all of the neurotrophins can bind, with similar affinity, to the p75 neurotrophin receptor (p75(NTR)). Experiments on cell viability promotion by BDNF in granule neurons or by NGF in PC12 cells show that neurotrophin-exerted cell viability is neutral sphingomyelinase (nSMase)-dependent, since GW4869 or siRNA knockdown abrogates the protective effects, as well as neurotrophin-induced Akt phosphorylation. Finally, the assessment of nSMase activity promotion drives to the conclusion that neurotrophins can promote cell viability through Trk receptors in a manner depending on basal nSMase but not through SMase activity enhancement.

Early activation of nSMase2/ceramide pathway in astrocytes is involved in ischemia-associated neuronal damage via inflammation in rat hippocampi

BACKGROUND

Ceramide accumulation is considered a contributing factor to neuronal dysfunction and damage. However, the underlying mechanisms that occur following ischemic insult are still unclear.

METHODS

In the present study, we established cerebral ischemia models using four-vessel occlusion and oxygen-glucose deprivation methods. The hippocampus neural cells were subjected to immunohistochemistry and immunofluorescence staining for ceramide and neutral sphingomyelinase 2 (nSMase2) levels; immunoprecipitation and immunoblot analysis for nSMase2, receptor for activated C kinase 1 (RACK1), embryonic ectoderm development (EED), p38 mitogen-activated protein kinase (p38MAPK) and phosphorylated p38MAPK expression; SMase assay for nSMase and acid sphingomyelinase (aSMase) activity; real-time reverse transcription polymerase chain reaction for cytokine expression; and Nissl, microtubule-associated protein 2 and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling staining.

RESULTS

We found considerable production of ceramide in astrocytes, but not in neurons, during early cerebral ischemia. This was accompanied by the induction of nSMase (but not aSMase) activity in the rat hippocampi. The inhibition of nSMase2 activity effectively reduced ceramide accumulation in astrocytes and alleviated neuronal damage to some extent. Meanwhile, the expression levels of proinflammatory cytokines, including tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β) and IL-6, were found to be upregulated, which may have played an import role in neuronal damage mediated by the nSMase2/ceramide pathway. Although enhanced binding of nSMase2 with RACK1 and EED were also observed after cerebral ischemia, nSMase2 activity was not blocked by the TNF-α receptor inhibitor through RACK1/EED signaling. p38MAPK, but not protein kinase Cζ or protein phosphatase 2B, was able to induce nSMase2 activation after ischemia. p38MAPK can be induced by A2B adenosine receptor (A2BAR) activity.

CONCLUSIONS

These results indicate that the inhibition of ceramide production in astrocytes by targeting A2BAR/p38MAPK/nSMase2 signaling may represent a viable approach for attenuating inflammatory responses and neuronal damage after cerebral ischemia.

A signaling cascade mediated by ceramide, src and PDGFRβ coordinates the activation of the redox-sensitive neutral sphingomyelinase-2 and sphingosine kinase-1

Stress-inducing agents, including oxidative stress, generate the sphingolipid mediators ceramide (Cer) and sphingosine-1-phosphate (S1P) that are involved in stress-induced cellular responses. The two redox-sensitive neutral sphingomyelinase-2 (nSMase2) and sphingosine kinase-1 (SK1) participate in transducing stress signaling to ceramide and S1P, respectively; however, whether these key enzymes are coordinately regulated is not known. We investigated whether a signaling link coordinates nSMase2 and SK1 activation by H2O2. In mesenchymal cells, H2O2 elicits a dose-dependent biphasic effect, mitogenic at low concentration (5μM), and anti-proliferative and toxic at high concentration (100μM). Low H2O2 concentration triggered activation of nSMase2 and SK1 through a nSMase2/Cer-dependent signaling pathway that acted upstream of activation of SK1. Further results implicated src and the trans-activation of PDGFRβ, as supported by the blocking effect of specific siRNAs, pharmacological inhibitors, and genetically deficient cells for nSMase2, src and SK1. The H2O2-induced src/PDGFRβ/SK1 signaling cascade was impaired in nSMase2-deficient fro/fro cells and was rescued by exogenous C2Cer that activated src/PDGFRβ/SK1. Thus, the results define a nSMase2/SK1 signaling pathway implicated in the mitogenic response to low oxidative stress. On the other hand, high oxidative stress induced inhibition of SK1. The results also showed that the toxicity of high H2O2 concentration was comparable in control and nSMase2-deficient cells. Taken together the results identify a tightly coordinated nSMase2/SK1 pathway that mediates the mitogenic effects of H2O2 and may sense the degree of oxidative stress.

Sphingolipid metabolism and neutral sphingomyelinases

Sphingolipids are an important class of lipid molecules that play fundamental roles in our cells and body. Beyond a structural role, it is now clearly established that sphingolipids serve as bioactive signaling molecules to regulate diverse processes including inflammatory signaling, cell death, proliferation, and pain sensing. Sphingolipid metabolites have been implicated in the onset and progression of various diseases including cancer, lung disease, diabetes, and lysosomal storage disorders. Here we review sphingolipid metabolism to introduce basic concepts as well as emerging complexities in sphingolipid function gained from modern technological advances and detailed cell and animal studies. Furthermore, we discuss the family of neutral sphingomyelinases (N-SMases), which generate ceramide through the hydrolysis of sphingomyelin and are key enzymes in sphingolipid metabolism. Four mammalian N-SMase enzymes have now been identified. Most prominent is nSMase2 with established roles in bone mineralization, exosome formation, and cellular stress responses. Function for the other N-SMases has been more enigmatic and is an area of active investigation. The known properties and potential role(s) of each enzyme are discussed to help guide future studies.

Lung cancer and lung injury: the dual role of ceramide

Sphingolipids play key roles in cancer, yet our current understanding of sphingolipid function in lung cancer is limited to a few key players. The best characterized of these are sphingosine-1-phosphate and ceramide which are described for their opposing roles in cell fate. However, because sphingolipids as a whole are readily interconverted by a complex enzymatic machinery, no single sphingolipid appears to have exactly one role. Instead, the roles of specific sphingolipids appear to be context specific as demonstrated by findings that ceramide-1-phosphate has both proliferative and apoptotic effects depending on its concentration. Therefore, we present herein several years of research on ceramide, a sphingolipid linked to apoptotic signaling, that is emerging in cancer research for its potential roles in proliferation and cell-to-cell communication via exosomes.Ceramide is a well-studied sphingolipid in both normal and pathological conditions ranging from skin development to lung cancer. Interestingly, several groups have previously reported its increased levels in emphysema patients who are smokers, a patient subpopulation greatly susceptible to lung cancer. However, the molecular mechanisms through which cigarette smoke (CS) and ceramide accumulation lead to lung cancer, non-small cell lung cancer (NSCLC) specifically, are unknown.Interestingly, recent studies clearly establish that two signaling pathways are activated during CS exposure in the lung airway. One centers on the activation of neutral sphingomyelinase2 (nSMase2), an enzyme that hydrolyzes sphingomyelin to ceramide. The other pathway focuses on the oncogenic EGF receptor (EGFR), which becomes aberrantly activated but not degraded, leading to prolonged proliferative signaling. Recent studies show that these two signaling pathways may actually converge and integrate. Specifically, Goldkorn et al. demonstrated that during CS exposure, EGFR is favorably co-localized in ceramide-enriched regions of the plasma membrane, proposing that nSMase2/ceramide plays a role in the aberrant EGFR activation, leading to augmented tumorigenic signaling. Moreover, new findings indicate that CS exposure may induce resistance to the tyrosine kinase inhibitors (TKIs), used for treatment of NSCLC, merely through posttranslational molecular alterations. Furthermore, structural anomalies of the CS-activated EGFR appear to be supported by the excess ceramide produced by the CS-activated nSMase2 in the plasma membrane of lung epithelial cells.We present in this chapter the progression of the sphingolipid field in lung cancer using ceramide as an example. However, many crucial questions remain to be answered regarding the role of sphingolipids in lung cancer because of the glut of promising observations.

Knockout of SOD1 alters murine hepatic glycolysis, gluconeogenesis, and lipogenesis

We previously observed a stronger effect of knockout of Cu,Zn-superoxide dismutase (SOD1) than that of Se-dependent glutathione peroxidase 1 (GPX1) on murine body weight and glucose homeostasis. Two experiments were conducted to determine how hepatic lipid profiles and key metabolic regulators were correlated with this difference. SOD1(-/-) and GPX1(-/-) mice and their respective wild-type (WT) littermates (n=6 or 7/group, male) were fed a Se-adequate Torula yeast-sucrose diet and killed at 6 months of age to collect liver samples. In Experiment 1, fasted SOD1(-/-) mice displayed pyruvate intolerance and a 61% decrease (P<0.05) in liver glycogen compared with their WT littermates. The former had lower (P<0.05) activities of phosphoenolpyruvate carboxykinase, total protein phosphatase, and protein phosphatase 2A, but a higher (P<0.05) activity of glucokinase in the liver than the latter. In contrast, hepatic concentrations of total cholesterol, triglycerides, and nonesterified fatty acids were increased by 11 to 100% (P<0.05) in the SOD1(-/-) mice. Meanwhile, these mice had elevated (P<0.05) hepatic protein levels of sterol-regulatory element binding proteins 1 and 2, p53 MAPK, total and phosphorylated AMP-activated protein kinase α1 protein, protein tyrosine phosphatase 1B, and protein phosphatase 2B. In Experiment 2, GPX1(-/-) mice and their WT littermates were compared, but showed no difference in any of the measures. In conclusion, knockout of SOD1, but not GPX1, led to a decreased liver glycogen storage synchronized with pyruvate intolerance and elevated hepatic lipid profiles in adult mice. This striking comparison was possibly due to unique impacts of these two knockouts on intracellular tone of H(2)O(2) and key regulators of liver gluconeogenesis, glycolysis, and lipogenesis.

Contrary effects of sphingosine-1-phosphate on expression of α-smooth muscle actin in transforming growth factor β1-stimulated lung fibroblasts

Transforming growth factor-β1 (TGFβ1) plays a pivotal role in fibrosis in various organs including the lung. Following pulmonary injury, TGFβ1 stimulates conversion of fibroblasts to myofibroblasts that are mainly characterized by up-regulation of α-smooth muscle actin (αSMA) expression, and the resulting excess production of extracellular matrix proteins causes fibrosis with loss of alveolar function. The present study was undertaken to define the role of the sphingosine-1-phosphate (S1P) pathway in TGFβ1-induced expression of αSMA in human fetal lung fibroblasts, HFL1 cells. Analysis of mRNA revealed the existence of S1P(1), S1P(2), and S1P(3) receptor mRNAs. Treatment with TGFβ1 increased sphingosine kinase (SphK) activity and S1P(3) receptor mRNA at 24h after stimulation, and pharmacological data showed the involvement of sphingomyelinase, SphK, and S1P(3) receptor in the TGFβ1-induced up-regulation of αSMA with and without serum. Treatment with pertussis toxin and S1P(1) receptor antagonist W146 enhanced αSMA expression by TGFβ1/serum, and S1P decreased and increased αSMA levels with and without serum, respectively. TGFβ1 increased cyclooxygenase-2 expression in a manner dependent on serum and the sphingomyelinase/SphK pathway, and the response was decreased by pertussis toxin. Prostaglandin E(2), formed by TGFβ1/serum stimulation, decreased the TGFβ1-induced expression of αSMA via EP prostanoid receptor. These data suggest that S1P formed by TGFβ1 stimulation has diverse effects on the expression of αSMA, inhibition via the S1P(1) receptor-mediated and serum-dependent expression of cyclooxygenase-2 and the resulting formation of prostaglandin E(2), and stimulation via the S1P(3) receptor in a serum-independent manner.

Hyperosmolarity-induced lipid droplet formation depends on ceramide production by neutral sphingomyelinase 2

Hyperosmolarity (HO) imposes a remarkable stress on membranes, especially in tissues in direct contact with the external environment. Our efforts were focused on revealing stress-induced lipid changes that precede the inflammatory cytokine response in human corneal epithelial cells exposed to increasing osmolarity. We used a lipidomic analysis that detected significant and systematic changes in the lipid profile, highly correlated with sodium concentrations in the medium. Ceramides and triglycerides (TGs) were the most-responsive lipid classes, with gradual increases of up to 2- and 3-fold, respectively, when compared with control. The source of ceramide proved to be sphingomyelin hydrolysis, and neutral sphingomyelinase 2 (NSM2) activity showed a 2-fold increase 1 h after HO stress, whereas transcription increased 3-fold. Both TG accumulation and IL-8 secretion were shown to be dependent on ceramide production by specific knock-down of NSM2. In HCE cells, diglyceride acyltransferase 1 was responsible for the TG synthesis, but the enzyme activity had no effect on cytokine secretion. Hence, NSM2 plays a key role in the cellular response to hyperosmolar stress, and its activity regulates both cytokine secretion and lipid droplet formation.

Neutral sphingomyelinase 2 activity and protein stability are modulated by phosphorylation of five conserved serines

We previously presented that the neutral sphingomyelinase 2 (nSMase2) is the only SMase activated in human airway epithelial (HAE) cells following exposure to oxidative stress (ox-stress), yielding ceramide accumulation and thereby inducing apoptosis. Furthermore, we reported that nSMase2 is a phospho-protein in which the level of phosphorylation controls nSMase2 activation induced by ox-stress. Here we identify five specific serines that are phosphorylated in nSMase2 and demonstrate that their phosphorylation controls the nSMase2 activity upon ox-stress exposure in an interdependent manner. Furthermore, we show that the nSMase2 protein stability and thus its level of expression is also post-translationally regulated by these five serine phosphorylation sites. This study provides initial structure/function insights regarding nSMase2 phosphorylation sites and offers some new links for future studies aiming to fully elucidate nSMase2 regulatory machinery.

Sphingolipid metabolism, oxidant signaling, and contractile function of skeletal muscle

SIGNIFICANCE

Sphingolipids are a class of bioactive lipids that regulate diverse cell functions. Ceramide, sphingosine, and sphingosine-1-phosphate accumulate in tissues such as liver, brain, and lung under conditions of cellular stress, including oxidative stress. The activity of some sphingolipid metabolizing enzymes, chiefly the sphingomyelinases, is stimulated during inflammation and in response to oxidative stress. Ceramide, the sphingomyelinase product, as well as the ceramide metabolite, sphingosine-1-phosphate, can induce the generation of more reactive oxygen species, propagating further inflammation.

RECENT ADVANCES

This review article summarizes information on sphingolipid biochemistry and signaling pertinent to skeletal muscle and describes the potential influence of sphingolipids on contractile function.

CRITICAL ISSUES

It encompasses topics related to (1) the pathways for complex sphingolipid biosynthesis and degradation, emphasizing sphingolipid regulation in various muscle fiber types and subcellular compartments; (2) the emerging evidence that implicates ceramide, sphingosine, and sphingosine-1-phosphate as regulators of muscle oxidant activity, and (3) sphingolipid effects on contractile function and fatigue.

FUTURE DIRECTIONS

We propose that prolonged inflammatory conditions alter ceramide, sphingosine, and sphingosine-1-phosphate levels in skeletal muscle and that these changes promote the weakness, premature fatigue, and cachexia that plague individuals with heart failure, cancer, diabetes, and other chronic inflammatory diseases.

TLR4 protein contributes to cigarette smoke-induced matrix metalloproteinase-1 (MMP-1) expression in chronic obstructive pulmonary disease

Cigarette smoke is the major risk factor associated with the development of chronic obstructive pulmonary disease and alters expression of proteolytic enzymes that contribute to disease pathology. Previously, we reported that smoke exposure leads to the induction of matrix metalloproteinase-1 (MMP-1) through the activation of ERK1/2, which is critical to the development of emphysema. To date, the upstream signaling pathway by which cigarette smoke induces MMP-1 expression has been undefined. This study demonstrates that cigarette smoke mediates MMP-1 expression via activation of the TLR4 signaling cascade. In vitro cell culture studies demonstrated that cigarette smoke-induced MMP-1 was regulated by TLR4 via MyD88/IRAK1. Blockade of TLR4 or inhibition of IRAK1 prevented cigarette smoke induction of MMP-1. Mice exposed to acute levels of cigarette smoke exhibited increased TLR4 expression. To further confirm the in vivo relevance of this signaling pathway, rabbits exposed to acute cigarette smoke were found to have elevated TLR4 signaling and subsequent MMP-1 expression. Additionally, lungs from smokers exhibited elevated TLR4 and MMP-1 levels. Therefore, our data indicate that TLR4 signaling, through MyD88 and IRAK1, plays a predominant role in MMP-1 induction by cigarette smoke. The identification of the TLR4 pathway as a regulator of smoke-induced protease production presents a series of novel targets for future therapy in chronic obstructive pulmonary disease.

EGF receptor exposed to oxidative stress acquires abnormal phosphorylation and aberrant activated conformation that impairs canonical dimerization

Crystallographic studies have offered understanding of how receptor tyrosine kinases from the ErbB family are regulated by their growth factor ligands. A conformational change of the EGFR (ErbB1) was shown to occur upon ligand binding, where a solely ligand-mediated mode of dimerization/activation was documented. However, this dogma of dimerization/activation was revolutionized by the discovery of constitutively active ligand-independent EGFR mutants. In addition, other ligand-independent activation mechanisms may occur. We have shown that oxidative stress (ox-stress), induced by hydrogen peroxide or cigarette smoke, activates EGFR differently than its ligand, EGF, thereby inducing aberrant phosphorylation and impaired trafficking and degradation of EGFR. Here we demonstrate that ox-stress activation of EGFR is ligand-independent, does not induce "classical" receptor dimerization and is not inhibited by the tyrosine kinase inhibitor AG1478. Thus, an unprecedented, apparently activated, state is found for EGFR under ox-stress. Furthermore, this activation mechanism is temperature-dependent, suggesting the simultaneous involvement of membrane structure. We propose that ceramide increase under ox-stress disrupts cholesterol-enriched rafts leading to EGFR re-localization into the rigid, ceramide-enriched rafts. This increase in ceramide also supports EGFR aberrant trafficking to a peri-nuclear region. Therefore, the EGFR unprecedented and activated conformation could be sustained by simultaneous alterations in membrane structure under ox-stress.

Genome-wide screening for genes associated with FK506 sensitivity in fission yeast

We have been studying calcineurin signal transduction pathway in fission yeast Schizosaccharomyces pombe (S. pombe) by developing a genetic screen for mutants that show hypersensitivity to the immunosuppressive calcineurin inhibitor FK506 (tacrolimus). In the present study, to identify nonessential genes that are functionally related to the calcineurin signaling pathway, we performed a genome-wide screen of 3004 haploid deletion strains and confirmed 72 deletion strains to be FK506 sensitive. These 72 genes are classified into nine functional groups to include membrane trafficking (16 genes), signal transduction (10 genes), ubiquitination (8 genes), chromatin remodeling (6 genes), cytokinesis (4 genes), ribosomal protein (3 genes), RNA binding protein (3 genes), and a variety of other known functions (17 genes) or still unknown functions (5 genes) in the biological system. In our previous screening of FK506-sensitive mutants we isolated several membrane-trafficking mutants showing defective cell wall integrity. Here, we further examined the vacuolar fusion, the v-SNARE synaptobrevin Syb1 localization, and the sensitivity to the β-glucan synthase inhibitor micafungin in these 72 FK506-sensitive strains. Results showed that 25 deletion strains exhibited abnormal vacuole fusion, 19 deletion strains exhibited Syb1 mislocalization, and 14 deletion strains exhibited both abnormal vacuole fusion and Syb1 mislocalization, while 42 deletion strains showed both normal vacuole fusion and Syb1 localization. Likewise, 16 deletion strains showed sensitivity to micafungin. Altogether, our present study indicates that calcineurin mediates a plethora of physiological processes in fission yeast, and that calcineurin is extensively involved in cross-talk between signaling pathways.

Calcineurin selectively docks with the dynamin Ixb splice variant to regulate activity-dependent bulk endocytosis

Depolarization of nerve terminals stimulates rapid dephosphorylation of two isoforms of dynamin I (dynI), mediated by the calcium-dependent phosphatase calcineurin (CaN). Dephosphorylation at the major phosphorylation sites Ser-774/778 promotes a dynI-syndapin I interaction for a specific mode of synaptic vesicle endocytosis called activity-dependent bulk endocytosis (ADBE). DynI has two main splice variants at its extreme C terminus, long or short (dynIxa and dynIxb) varying only by 20 (xa) or 7 (xb) residues. Recombinant GST fusion proteins of dynIxa and dynIxb proline-rich domains (PRDs) were used to pull down interacting proteins from rat brain nerve terminals. Both bound equally to syndapin, but dynIxb PRD exclusively bound to the catalytic subunit of CaNA, which recruited CaNB. Binding of CaN was increased in the presence of calcium and was accompanied by further recruitment of calmodulin. Point mutations showed that the entire C terminus of dynIxb is a CaN docking site related to a conserved CaN docking motif (PXIXI(T/S)). This sequence is unique to dynIxb among all other dynamin variants or genes. Peptide mimetics of the dynIxb tail blocked CaN binding in vitro and selectively inhibited depolarization-evoked dynI dephosphorylation in nerve terminals but not of other dephosphins. Therefore, docking to dynIxb is required for the regulation of both dynI splice variants, yet it does not regulate the phosphorylation cycle of other dephosphins. The peptide blocked ADBE, but not clathrin-mediated endocytosis of synaptic vesicles. Our results indicate that Ca(2+) influx regulates assembly of a fully active CaN-calmodulin complex selectively on the tail of dynIxb and that the complex is recruited to sites of ADBE in nerve terminals.

Diet-induced elevations in serum cholesterol are associated with alterations in hippocampal lipid metabolism and increased oxidative stress

The structure and function of the hippocampus, a brain region critical for learning and memory, is impaired by obesity and hyperlipidemia. Peripheral cholesterol and sphingolipids increase progressively with aging and are associated with a range of age-related diseases. However, the mechanisms linking peripheral cholesterol metabolism to hippocampal neuroplasticity remain poorly understood. To determine whether diets that elevate serum cholesterol influence lipid metabolism in the hippocampus, we maintained rats on a diet with high amounts of saturated fat and simple sugars for 3 months and then analyzed hippocampal lipid species using tandem mass spectrometry. The high fat diet was associated with increased serum and liver cholesterol and triglyceride levels, and also promoted cholesterol accumulation in the hippocampus. Increases in hippocampal cholesterol were associated with elevated galactosyl ceramide and sphingomyelin. To determine whether changes in lipid composition exerted biological effects, we measured levels of the lipid peroxidation products 4-hydroxynonenal-lysine and 4-hydroxynonenal-histidine; both were increased locally in the hippocampus, indicative of cell membrane-associated oxidative stress. Taken together, these observations support the existence of a potentially pathogenic relationship between dietary fat intake, peripheral cholesterol and triglyceride levels, brain cell sphingolipid metabolism, and oxidative stress.

Protein phosphatase 2A and neutral sphingomyelinase 2 regulate IRAK-1 protein ubiquitination and degradation in response to interleukin-1beta

The IL-1β signaling cascade is initiated by the phosphorylation of IL-1β receptor-associated kinase-1 (IRAK-1), followed by its ubiquitination and degradation. This paper investigates the regulation of IRAK-1 degradation in primary hepatocytes and in HEK cells overexpressing the IL-1β receptor. We provide evidence that protein phosphatase 2A (PP2A) is a negative regulator of the phosphorylation, Lys(48)-linked ubiquitination, and degradation of IRAK-1. PP2A catalytic activity increased within 30 min of stimulation with IL-1β. siRNA against PP2A catalytic subunit (PP2Ac) or treatment with pharmacological inhibitor, okadaic acid, enhanced IRAK-1 Lys(48)-linked ubiquitination and degradation. Direct interaction between PP2Ac and IRAK-1 was observed, suggesting that IRAK-1 might be a PP2A substrate. The mechanisms of PP2A activation by IL-1β involved neutral sphingomyelinase-2 (NSMase-2) and an accumulation of ceramide. Overexpression of NSMase-2 delayed IRAK-1 degradation in a PP2A-dependent manner, whereas NSMase-2 silencing had the opposite effect. The addition of sphingomyelinase, ceramide, or a proteasome inhibitor all led to retention of IRAK-1 at the cell membrane and to increased JNK phosphorylation. This study suggests that NSMase-2- and PP2A-dependent regulation of IRAK-1 degradation is a novel mechanism to fine tune the magnitude of IL-1β response.

The neutral sphingomyelinase family: identifying biochemical connections

Neutral sphingomyelinases (N-SMases) are considered to be key mediators of stress-induced ceramide production. The extended family of N-SMases is a subset of the DNaseI superfamily and comprises members from bacteria, yeast and mammals. In recent years, the identification and cloning of mammalian N-SMase family members has led to significant advances in understanding their physiological roles and regulation. However, there is still limited information on their regulation at the biochemical and molecular level. In this review, we summarize current knowledge about the biochemical regulation of the eukaryotic N-SMases and identify the major areas where knowledge is lacking. In recent years, research into the roles and regulation of N-SMases has moved in great strides with the cloning and characterization of multiple N-SMase isoforms and the development of knockout mice. However, as researchers continue to move forward in understanding the physiological functions of these various N-SMase isoforms, it has become exceedingly important to define howthese isoforms are regulated at the biochemical and molecular level. This is crucial for the development of future tools to study N-SMase signaling such as, for example, phospho-specific antibodies designating activation states. This is also an important part of identifying novel roles of N-SMases in physiological and pathological states. Finally, only by obtaining a more complete understanding of the workings of these enzymes at the molecular level, will investigators be able to design appropriate compounds that can target and inhibit their activity both efficiently and specifically. Certainly, the last of these is crucial when considering the potential of N-SMases as therapeutic targets. With this in mind, we sincerely hope that the next decade of research will even surpass the last ten years in advancing our understanding of the eukaryotic N-SMase family.

Lung injury and cancer: Mechanistic insights into ceramide and EGFR signaling under cigarette smoke

Cigarette smoke has been connected to an array of chronic lung diseases and is a major source of morbidity and mortality. Active smoking is responsible for approximately 90% of lung cancer cases. In addition, cigarette smoke is associated with other chronic pulmonary diseases such as pulmonary edema, chronic bronchitis, and pulmonary emphysema, the last two also termed chronic obstructive pulmonary disease (COPD). Lung cancer and COPD are developed very frequently in chronic cigarette smokers. It has been known for some time that lung cancer incidence increases in patients with COPD. Even the existence of some low-grade emphysema without noticeable airflow obstruction is associated with significantly elevated risk of lung cancer. These recent clinical insights demand new thinking and exploration of novel mechanistic studies to fully understand these observations. Lung injury and repair involve cell death and hyperplasia of airway epithelial cells and infiltration of inflammatory cells. All of these occur simultaneously. The mechanisms of cell death and hyperplasia in the lung constitute two sides of the coin of lung injury and repair. However, most molecular studies in airway epithelial cells center on the mechanism(s) of either cell growth and proliferation or cell death and the ceramide-generating machinery that drives aberrant induction of apoptotic cell death. Very few address both sides of the coin as an outcome of cigarette smoke exposure, which is the focus of this review.

Neutral sphingomyelinase 2: a novel target in cigarette smoke-induced apoptosis and lung injury

Chronic obstructive pulmonary disease (COPD) is caused by exposure to cigarette smoke (CS). One mechanism of CS-induced lung injury is aberrant generation of ceramide, which leads to elevated apoptosis of epithelial and endothelial cells in the alveolar spaces. Recently, we discovered that CS-induced ceramide generation and apoptosis in pulmonary cells is governed by neutral sphingomyelinase (nSMase) 2. In the current experiments, we expanded our studies to investigate whether nSMase2 governs ceramide generation and apoptosis in vivo using rodent and human models of CS-induced lung injury. We found that exposure of mice or rats to CS leads to colocalizing elevations of ceramide levels and terminal deoxynucleotidyl transferase mediated X-dUTP nick end labeling-positive cells in lung tissues. These increases are nSMase2 dependent, and are abrogated by treatment with N-acetyl cysteine or anti-nSMase2 small interfering RNA (siRNA). We further showed that mice that are heterozygous for nSMase2 demonstrate significant decrease in ceramide generation after CS exposure, whereas acidic sphingomyelinase (aSMase) knockout mice maintain wild-type ceramide levels, confirming our previous findings (in human airway epithelial cells) that only nSMase2, and not aSMase, is activated by CS exposure. Lastly, we found that lung tissues from patients with emphysema (smokers) display significantly higher levels of nSMase2 expression compared with lung tissues from healthy control subjects. Taken together, these data establish the central in vivo role of nSMase2 in ceramide generation, aberrant apoptosis, and lung injury under CS exposure, underscoring its promise as a novel target for the prevention of CS-induced airspace destruction.