Mutations in the neutral sphingomyelinase gene SMPD3 implicate the ceramide pathway in human leukemias

Soluble ligands for NK cell receptors promote evasion of chronic lymphocytic leukemia cells from NK cell anti-tumor activity

Natural killer (NK) cells are a major component of the anti-tumor immune response. NK cell dysfunctions have been reported in various hematologic malignancies, including chronic lymphocytic leukemia (CLL). Here we investigated the role of tumor cell-released soluble and exosomal ligands for NK cell receptors that modulate NK cell activity. Soluble CLL plasma factors suppressed NK cell cytotoxicity and down-regulated the surface receptors CD16 and CD56 on NK cells of healthy donors. The inhibition of NK cell cytotoxicity was attributed to the soluble ligand BAG6/BAT3 that engages the activating receptor NKp30 expressed on NK cells. Soluble BAG6 was detectable in the plasma of CLL patients, with the highest levels at the advanced disease stages. In contrast, NK cells were activated when BAG6 was presented on the surface of exosomes. The latter form was induced in non-CLL cells by cellular stress via an nSmase2-dependent pathway. Such cells were eliminated by lymphocytes in a xenograft tumor model in vivo. Here, exosomal BAG6 was essential for tumor cell killing because BAG6-deficient cells evaded immune detection. Taken together, the findings show that the dysregulated balance of exosomal vs soluble BAG6 expression may cause immune evasion of CLL cells.

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.

Ceramide transfer protein and cancer

Sphingolipids are important structural components of membranes, and play an equally important role in basic cellular processes as second messengers. Recently, sphingolipids are receiving increasing attention in cancer research. Ceramide is the central molecule that regulates sphingolipid metabolism forming the basic structural backbone of sphingolipids and the precursor of all complex sphingolipids. It is been proposed to be an important regulator of tumor cell death following exposure to stress stimuli. The increase or decrease of ceramide levels leading to change in sensitivity of cancer cells to stress stimuli provides support for a central role of ceramide signaling in cell death. In this review, we have focused on ceramide transfer protein (CERT) as a major regulator of ceramide flux in the cell.

Sphingolipid metabolism cooperates with BAK and BAX to promote the mitochondrial pathway of apoptosis

Mitochondria are functionally and physically associated with heterotypic membranes, yet little is known about how these interactions impact mitochondrial outer-membrane permeabilization (MOMP) and apoptosis. We observed that dissociation of heterotypic membranes from mitochondria inhibited BAK/BAX-dependent cytochrome c (cyto c) release. Biochemical purification of neutral sphingomyelinases that correlated with MOMP sensitization suggested that sphingolipid metabolism coordinates BAK/BAX activation. Using purified lipids and enzymes, sensitivity to MOMP was achieved by in vitro reconstitution of the sphingolipid metabolic pathway. Sphingolipid metabolism inhibitors blocked MOMP from heavy membrane preparations but failed to influence MOMP in the presence of sphingolipid-reconstituted, purified mitochondria. Furthermore, the sphingolipid products, sphingosine-1-PO(4) and hexadecenal, cooperated specifically with BAK and BAX, respectively. Sphingolipid metabolism was also required for cellular responses to apoptosis. Our studies suggest that BAK/BAX activation and apoptosis are coordinated through BH3-only proteins and a specific lipid milieu that is maintained by heterotypic membrane-mitochondrial interactions.

Drug targeting of sphingolipid metabolism: sphingomyelinases and ceramidases

Sphingolipids represent a class of diverse bioactive lipid molecules that are increasingly appreciated as key modulators of diverse physiologic and pathophysiologic processes that include cell growth, cell death, autophagy, angiogenesis, and stress and inflammatory responses. Sphingomyelinases and ceramidases are key enzymes of sphingolipid metabolism that regulate the formation and degradation of ceramide, one of the most intensely studied classes of sphingolipids. Improved understanding of these enzymes that control not only the levels of ceramide but also the complex interconversion of sphingolipid metabolites has provided the foundation for the functional analysis of the roles of sphingolipids. Our current understanding of the roles of various sphingolipids in the regulation of different cellular processes has come from loss-of-function/gain-of-function studies utilizing genetic deletion/downregulation/overexpression of enzymes of sphingolipid metabolism (e.g. knockout animals, RNA interference) and from the use of pharmacologic inhibitors of these same enzymes. While genetic approaches to evaluate the functional roles of sphingolipid enzymes have been instrumental in advancing the field, the use of pharmacologic inhibitors has been equally important in identifying new roles for sphingolipids in important cellular processes.The latter also promises the development of novel therapeutic targets with implications for cancer therapy, inflammation, diabetes, and neurodegeneration. In this review, we focus on the status and use of pharmacologic compounds that inhibit sphingomyelinases and ceramidases, and we will review the history, current uses and future directions for various small molecule inhibitors, and will highlight studies in which inhibitors of sphingolipid metabolizing enzymes have been used to effectively treat models of human disease.

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.

Identification of novel anionic phospholipid binding domains in neutral sphingomyelinase 2 with selective binding preference

Sphingolipids such as ceramide are recognized as vital regulators of many biological processes. Neutral sphingomyelinase 2 (nSMase2) is one of the key enzymes regulating ceramide production. It was previously shown that the enzymatic activity of nSMase2 was dependent on anionic phospholipids (APLs). In this study, the structural requirements for APL-selective binding of nSMase2 were determined and characterized. Using lipid-protein overlay assays, nSMase2 interacted specifically and directly with several APLs, including phosphatidylserine and phosphatidic acid. Lipid-protein binding studies of deletion mutants identified two discrete APL binding domains in the N terminus of nSMase2. Further, mutagenesis experiments pinpointed the core sequences and major cationic amino acids in the domains that are necessary for the cooperative activation of nSMase2 by APLs. The first domain included the first amino-terminal hydrophobic segment and Arg-33, which were essential for nSMase2 to interact with APLs. The second binding domain was comprised of the second hydrophobic segment and Arg-92 and Arg-93. Moreover, mutation of one or both domains decreased APL binding and APL-dependent catalytic activity of nSMase2. Further, mutation of both domains in nSMase2 reduced its plasma membrane localization. Finally, these binding domains are also important for the capability of nSMase2 to rescue the defects of yeast lacking the nSMase homologue, ISC1. In conclusion, these data have identified the APL binding domains of nSMase2 for the first time. The analysis of interactions between nSMase2 and APLs will contribute to our understanding of signaling pathways mediated by sphingolipid metabolites.

Acid sphingomyelinase in macrophage biology

Macrophages play a central role in innate immune responses, in disposal of cholesterol, and in tissue homeostasis and remodeling. To perform these vital functions macrophages display high endosomal/lysosomal activities. Recent studies have highlighted that acid sphingomyelinase (ASMase), which generates ceramide from sphingomyelin, is involved in modulation of membrane structures and signal transduction in addition to its metabolic role in the lysosome. In this review, we bring together studies on ASMase, its different forms and locations that are necessary for the macrophage to accomplish its diverse functions. We also address the importance of ASMase to several disease processes that are mediated by activated macrophages.

Neutral sphingomyelinase-2 mediates growth arrest by retinoic acid through modulation of ribosomal S6 kinase

All-trans-retinoic acid (ATRA) induces growth arrest of many cell types. Previous studies have reported that ATRA can modulate cellular sphingolipids, but the role of sphingolipids in the ATRA response is not clear. Using MCF-7 cells as a model system, we show that ATRA stimulates an increase in ceramide levels followed by G(0)/G(1) growth arrest. Notably, induction of nSMase2 was the primary effect of ATRA on the sphingolipid network and was both time- and dose-dependent. Importantly, pretreatment with nSMase2 siRNA significantly inhibited ATRA effects on ceramide levels and growth arrest. In contrast, nSMase2 overexpression was sufficient to increase ceramide levels and induce G(0)/G(1) growth arrest of asynchronous MCF-7 cells. Surprisingly, neither ATRA stimulation nor nSMase2 overexpression had significant effects on classical cell cycle regulators such as p21/WAF1 or retinoblastoma. In contrast, ATRA suppressed phosphorylation of ribosomal S6 kinase (S6K) and its downstream targets S6 and eIF4B. Importantly, these effects were significantly inhibited by nSMase2 siRNA. Reciprocally, nSMase2 overexpression was sufficient to suppress S6K phosphorylation and signaling. Notably, neither ATRA effects nor nSMase2 effects on S6K phosphorylation required the ceramide-activated protein phosphatase PP2A, previously identified as important for S6K regulation. Finally, nSMase2 overexpression was sufficient to decrease translation as measured by methionine incorporation and analysis of polyribosome profiles. Taken together, these results implicate nSMase2 as a major component of ATRA-induced growth arrest of MCF-7 cells and identify S6K as a novel downstream target of nSMase2.

Ceramide in stress response

Evidence has consistently indicated that activation of sphingomyelinases and/or ceramide synthases and the resulting accumulation of ceramide mediate cellular responses to stressors such as lipopolysaccharide, interleukin 1beta, tumor necrosis factor alpha, serum deprivation, irradiation and various antitumor treatments. Recent studies had identified the genes encoding most of the enzymes responsible for the generation of ceramide and ongoing research is aimed at characterizing their individual functions in cellular response to stress. This chapter discusses the seminal and more recent discoveries in regards to the pathways responsible for the accumulation of ceramide during stress and the mechanisms by which ceramide affects cell functions. The former group includes the roles of neutral sphingomyelinase 2, serine palmitoyltransferase, ceramide synthases, as well as the secretory and endosomal/lysosomal forms of acid sphingomyelinase. The latter summarizes the mechanisms by which ceramide activate its direct targets, PKCzeta, PP2A and cathepsin D. The ability of ceramide to affect membrane organization is discussed in the light of its relevance to cell signaling. Emerging evidence to support the previously assumed notion that ceramide acts in a strictly structure-specific manner are also included. These findings are described in the context of several physiological and pathophysiological conditions, namely septic shock, obesity-induced insulin resistance, aging and apoptosis of tumor cells in response to radiation and chemotherapy.

Remodeling of sphingolipids by plasma membrane associated enzymes

The sphingolipid plasma membrane content and pattern is the result of several processes, among which the main, in term of quantity, are: neo-biosynthesis in endoplasmic reticulum and Golgi apparatus, membrane turnover with final catabolism in lysosomes and membrane shedding. In addition to this, past and recent data suggest that the head group of sphingolipids can be opportunely modified at the plasma membrane level, probably inside specific membrane lipid domains, by the action of enzymes involved in the sphingolipids metabolism, working directly at the cell surface. The number of membrane enzymes, hydrolases and transferases, acting on membrane sphingolipids is growing very rapidly. In this report we describe some properties of these enzymes.

Targeting sphingolipid metabolism in head and neck cancer: rational therapeutic potentials

IMPORTANCE OF THE FIELD

Ceramide accumulation has been shown to be a conserved mechanism of apoptosis initiation in normal physiological processes as well as in response to cancer treatments. Therefore, it is unsurprising that many cancers develop aberrations of sphingolipid metabolism that prevent the accumulation of ceramide, whether by reduction of ceramide generation or by enhanced ceramide catabolism, particularly dangerous when catabolism leads to generation of pro-tumor sphingosine-1-phosphate and ceramide-1-phosphate. Numerous studies have now implicated dysregulation of sphingolipid metabolism in head and neck cancers.

AREAS COVERED IN THIS REVIEW

This review highlights the importance of sphingolipid metabolism and brings sphingolipid metabolism to the forefront in the investigation of novel therapies for head and neck cancer. It reviews sphingolipid-centric therapies under investigation in preclinical and clinical trials of cancers of the head and neck.

WHAT THE READER WILL GAIN

The roles of sphingolipids and sphingolipid metabolism in cancer are reviewed and the reader will be brought up to date with discoveries in the field of sphingolipid metabolism in head and neck cancer.

TAKE HOME MESSAGE

As treatments for head and neck cancers are currently limited, the potential of targeting sphingolipid metabolism should be taken into consideration as we seek novel ways to combat this group of tumors.

Mammalian neutral sphingomyelinases: regulation and roles in cell signaling responses

Ceramide, a bioactive lipid, has been extensively studied and identified as an essential bioactive molecule in mediating cellular signaling pathways. Sphingomyelinase (SMase), (EC 3.1.4.12) catalyzes the cleavage of the phosphodiester bond in sphingomyelin (SM) to form ceramide and phosphocholine. In mammals, three Mg(2+)-dependent neutral SMases termed nSMase1, nSMase2 and nSMase3 have been identified. Among the three enzymes, nSMase2 is the most studied and has been implicated in multiple physiological responses including cell growth arrest, apoptosis, development and inflammation. In this review, we summarize recent findings for the cloned nSMases and discuss the insights for their roles in regulation ceramide metabolism and cellular signaling pathway.

Stress-induced sphingolipid signaling: role of type-2 neutral sphingomyelinase in murine cell apoptosis and proliferation

Background

Sphingomyelin hydrolysis in response to stress-inducing agents, and subsequent ceramide generation, are implicated in various cellular responses, including apoptosis, inflammation and proliferation, depending on the nature of the different acidic or neutral sphingomyelinases. This study was carried out to investigate whether the neutral Mg²⁺-dependent neutral sphingomyelinase-2 (nSMase2) plays a role in the cellular signaling evoked by TNFalpha and oxidized LDLs, two stress-inducing agents, which are mitogenic at low concentrations and proapoptotic at higher concentrations.

Methodology and Principal Findings

For this purpose, we used nSMase2-deficient cells from homozygous fro/fro (fragilitas ossium) mice and nSMase2-deficient cells reconstituted with a V5-tagged nSMase2. We report that the genetic defect of nSMase2 (in fibroblasts from fro/fro mice) does not alter the TNFalpha and oxidized LDLs-mediated apoptotic response. Likewise, the hepatic toxicity of TNFalpha is similar in wild type and fro mice, thus is independent of nSMase2 activation. In contrast, the mitogenic response elicited by low concentrations of TNFalpha and oxidized LDLs (but not fetal calf serum) requires nSMase2 activation.

Conclusion and Significance

nSMase2 activation is not involved in apoptosis mediated by TNFalpha and oxidized LDLs in murine fibroblasts, and in the hepatotoxicity of TNFalpha in mice, but is required for the mitogenic response to stress-inducing agents.

Acid ceramidase upregulation in prostate cancer: role in tumor development and implications for therapy

Bioactive sphingolipids, such as ceramide, sphingosine and sphingosine-1-phosphate are known bio-effector molecules which play important roles in various aspects of cancer biology including cell proliferation, growth arrest, apoptosis, metastasis, senescence and inflammation. Therefore, enzymes involved in ceramide metabolism are gaining recognition as being critical regulators of cancer cell growth and/or survival. We previously observed that the ceramide metabolizing enzyme, acid ceramidase (AC) is upregulated in tumor tissues. Studies have now concluded that this creates a dysfunctional ceramide pathway, which is responsible for tumor progression and resistance to chemotherapy and radiation. This suggests that development of small-molecule drugs that inhibit AC enzyme activity is a promising approach for improving standard cancer therapy and patient's clinical outcomes.

Sphingomyelin metabolism at the plasma membrane: implications for bioactive sphingolipids

The plasma membrane (PM) is a major resource for production of bioactive lipids and contains a large proportion of the cellular sphingomyelin (SM) content. Consequently, the regulation of SM levels at the PM by enzymes such as sphingomyelinase (SMase) and SM synthase 2 (SMS2) can have profound effects - both on biophysical properties of the membrane, but also on cellular signaling. Over the past 20 years, there has been considerable research into the physiological and cellular functions associated with regulation of SM levels, notably with regards to the production of ceramide. In this review, we will summarize this research with particular focus on the SMases and SMS2. We will outline what biological functions are associated with SM metabolism/production at the PM, and discuss what we believe are major challenges that need to be addressed in future studies.

Upregulation of smpd3 via BMP2 stimulation and Runx2

Deletion of smpd3 induces osteogenesis and dentinogenesis imperfecta in mice. smpd3 is highly elevated in the parietal bones of developing mouse calvaria, but not in sutural mesenchymes. Here, we examine the mechanism of smpd3 regulation, which involves BMP2 stimulation of Runx2. smpd3 mRNA expression increased in response to BMP2 treatment and Runx2 transfection in C2C12 cells. The Runx2-responsive element (RRE) encoded within the -562 to -557 region is important for activation of the smpd3 promoter by Runx2. Electrophoretic mobility shift assays revealed that Runx2 binds strongly to the -355 to -350 RRE and less strongly to the -562 to -557 site. Thus, the smpd3 promoter is activated by BMP2 and is directly regulated by the Runx2 transcription factor. This novel description of smpd3 regulation will aid further studies of bone development and osteogenesis.