Mitochondria and ceramide: intertwined roles in regulation of apoptosis

5,7,3'-Trihydroxy-3,4'-dimethoxyflavone inhibits the tubulin polymerization and activates the sphingomyelin pathway

Flavonoids are polyphenolic compounds which display a vast array of biological activities and are among the most promising anti-cancer agents. The derivative of quercetin, 5,7,3'-trihydroxy-3,4'-dimethoxyflavone (THDF), is a natural flavonoid that inhibits cell proliferation and induces apoptosis in human leukemia cells. Here we show that THDF induces cell-cycle arrest in the M phase and inhibits tubulin polymerization. This was associated with the accumulation of cyclin B1 and p21(Cip1) , changes in the phosphorylation status of cyclin B1, Cdk1, Cdc25C, and MPM-2, and activation of the acidic sphingomyelinase (ASMase). Moreover, desipramine attenuated THDF-mediated cell death, indicating a crucial role of ASMase in the mechanism of cell death. In vivo studies on the athymic nude mouse xenograft model also confirmed that THDF inhibits growth of human leukemia cells and suggest that this compound may have therapeutic value.

Targeting of survivin by nanoliposomal ceramide induces complete remission in a rat model of NK-LGL leukemia

The natural killer (NK) type of aggressive large granular lymphocytic (LGL) leukemia is a fatal illness that pursues a rapid clinical course. There are no effective therapies for this illness, and pathogenetic mechanisms remain undefined. Here we report that the survivin was highly expressed in both aggressive and chronic leukemic NK cells but not in normal NK cells. In vitro treatment of human and rat NK-LGL leukemia cells with cell-permeable, short-chain C₆-ceramide (C₆) in nanoliposomal formulation led to caspase-dependent apoptosis and diminished survivin protein expression, in a time- and dose-dependent manner. Importantly, systemic intravenous delivery of nanoliposomal ceramide induced complete remission in the syngeneic Fischer F344 rat model of aggressive NK-LGL leukemia. Therapeutic efficacy was associated with decreased expression of survivin in vivo. These data suggest that in vivo targeting of survivin through delivery of nanoliposomal C₆-ceramide may be a promising therapeutic approach for a fatal leukemia.

Characteristics of the rat cardiac sphingolipid pool in two mitochondrial subpopulations

Mitochondrial sphingolipids play a diverse role in normal cardiac function and diseases, yet a precise quantification of cardiac mitochondrial sphingolipids has never been performed. Therefore, rat heart interfibrillary mitochondria (IFM) and subsarcolemmal mitochondria (SSM) were isolated, lipids extracted, and sphingolipids quantified by LC-tandem mass spectrometry. Results showed that sphingomyelin (approximately 10,000 pmol/mg protein) was the predominant sphingolipid regardless of mitochondrial subpopulation, and measurable amounts of ceramide (approximately 70 pmol/mg protein) sphingosine, and sphinganine were also found in IFM and SSM. Both mitochondrial populations contained similar quantities of sphingolipids except for ceramide which was much higher in SSM. Analysis of sphingolipid isoforms revealed ten different sphingomyelins and six ceramides that differed from 16- to 24-carbon units in their acyl side chains. Sub-fractionation experiments further showed that sphingolipids are a constituent part of the inner mitochondrial membrane. Furthermore, inner membrane ceramide levels were 32% lower versus whole mitochondria (45 pmol/mg protein). Three ceramide isotypes (C20-, C22-, and C24-ceramide) accounted for the lower amounts. The concentrations of the ceramides present in the inner membranes of SSM and IFM differed greatly. Overall, mitochondrial sphingolipid content reflected levels seen in cardiac tissue, but the specific ceramide distribution distinguished IFM and SSM from each other.

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.

Identification and characterization of murine mitochondria-associated neutral sphingomyelinase (MA-nSMase), the mammalian sphingomyelin phosphodiesterase 5

Sphingolipids play important roles in regulating cellular responses. Although mitochondria contain sphingolipids, direct regulation of their levels in mitochondria or mitochondria-associated membranes is mostly unclear. Neutral SMase (N-SMase) isoforms, which catalyze hydrolysis of sphingomyelin (SM) to ceramide and phosphocholine, have been found in the mitochondria of yeast and zebrafish, yet their existence in mammalian mitochondria remains unknown. Here, we have identified and cloned a cDNA based on nSMase homologous sequences. This cDNA encodes a novel protein of 483 amino acids that displays significant homology to nSMase2 and possesses the same catalytic core residues as members of the extended N-SMase family. A transiently expressed V5-tagged protein co-localized with both mitochondria and endoplasmic reticulum markers in MCF-7 and HEK293 cells; accordingly, the enzyme is referred to as mitochondria-associated nSMase (MA-nSMase). MA-nSMase was highly expressed in testis, pancreas, epididymis, and brain. MA-nSMase had an absolute requirement for cations such as Mg(2+) and Mn(2+) and activation by the anionic phospholipids, especially phosphatidylserine and the mitochondrial cardiolipin. Importantly, overexpression of MA-nSMase in HEK293 cells significantly increased in vitro N-SMase activity and also modulated the levels of SM and ceramide, indicating that the identified cDNA encodes a functional SMase. Thus, these studies identify and characterize, for the first time, a mammalian MA-nSMase. The characterization of MA-nSMase described here will contribute to our understanding of pathways regulated by sphingolipid metabolites, particularly with reference to the mitochondria and associated organelles.

Sphingolipids and gangliosides of the nervous system in membrane function and dysfunction

Simple sphingolipids such as ceramide and sphingomyelin (SM) as well as more complex glycosphingolipids play very important roles in cell function under physiological conditions and during disease development and progression. Sphingolipids are particularly abundant in the nervous system. Due to their amphiphilic nature they localize to cellular membranes and many of their roles in health and disease result from membrane reorganization and from lipid interaction with proteins within cellular membranes. In this review we discuss some of the functions of sphingolipids in processes that entail cellular membranes and their role in neurodegenerative diseases, with an emphasis on SM, ceramide and gangliosides.

Lipid oversupply, selective insulin resistance, and lipotoxicity: molecular mechanisms

The accumulation of fat in tissues not suited for lipid storage has deleterious consequences on organ function, leading to cellular damage that underlies diabetes, heart disease, and hypertension. To combat these lipotoxic events, several therapeutics improve insulin sensitivity and/or ameliorate features of metabolic disease by limiting the inappropriate deposition of fat in peripheral tissues (i.e. thiazolidinediones, metformin, and statins). Recent advances in genomics and lipidomics have accelerated progress towards understanding the pathogenic events associated with the excessive production, underutilization, or inefficient storage of fat. Herein we review studies applying pharmacological or genetic strategies to manipulate the expression or activity of enzymes controlling lipid deposition, in order to gain a clearer understanding of the molecular mechanisms by which fatty acids contribute to metabolic disease.

Ceramide regulates gemcitabine-induced senescence and apoptosis in human pancreatic cancer cell lines

Bioactive sphingolipids are potent intracellular signaling molecules having profound effects on cell death, growth, and differentiation. Pharmacologic manipulation of sphingolipid levels could have a significant effect on the induction of apoptosis by anticancer agents, and thus, improve treatment efficacy. We observed that gemcitabine cannot completely kill AsPc1 and Panc1 human pancreatic cancer cells in culture; even at high concentrations of gemcitabine, 30% to 40% of the cells remain viable. By adding sphingomyelin to the culture medium, gemcitabine-induced cell death increased synergistically to >90%. Panc1 cells that survived high concentrations of gemcitabine had an increase in beta-galactosidase activity, a marker of senescence. The inclusion of sphingomyelin with gemcitabine reduced beta-galactosidase activity, as compared with cells treated with gemcitabine alone. Expression of p21(waf1/cip1) in both cell lines exposed to sphingomyelin, gemcitabine, and gemcitabine + sphingomyelin varied relative to the untreated group. C(8)-ceramide induced both cell death and senescence in a dose-dependent manner. These results indicate that gemcitabine induces senescence in pancreatic cancer cells and that sphingomyelin-enhanced chemosensitivity is achieved through reducing the induction of senescence by redirecting the cell to enter the apoptotic pathway. Ceramide levels seem to be critical to this decision, with cell cycle progression being uninhibited at low ceramide levels, senescence induced at moderate levels, and apoptosis initiated at high levels. Our results provide further evidence that targeting the sphingolipid metabolism is a means of enhancing the efficacy of chemotherapeutic agents.

The solute carrier 44A1 is a mitochondrial protein and mediates choline transport

Choline oxidation to betaine takes place in the mitochondria; however, a protein regulating mitochondrial choline transport was never identified. The purpose of this study was to analyze subcellular localization of the solute carrier 44A1 (SLC44A1), a plasma membrane choline transporter sensitive to inhibition by hemicholinium-3. We generated N- and C-terminal-SLC44A1-specific antibodies and analyzed localization of endogenous and overexpressed SLC44A1 in C2C12 mouse muscle cells, MCF7 human breast cancer cells, and mouse tissues using confocal microscopy, differential centrifugation, and Western blotting. We further performed choline uptake competition studies on isolated mitochondria using the specific inhibitor hemicholinium-3 and SLC44A1 antibodies, and analyzed mitochondria of FL83B hepatocytes after the targeted knock-down of SLC44A1 using siRNA technology. In addition, we analyzed SLC44A1 expression during choline deficiency. Localization studies revealed plasma membrane, cytosolic, microsomal, and mitochondrial localization of endogenous and His-tagged SLC44A1. Uptake studies in isolated mitochondria show an accumulation of (3)H-choline, which is strongly inhibited by hemicholinium-3 (60%), by an excess of unlabeled choline (97%), and by both SLC44A1 antibodies. SLC44A1 mRNA and protein expression were down-regulated during choline deficiency. These data clearly establish SLC44A1 as an important mediator of choline transport across both the plasma membrane and the mitochondrial membrane.

Metabonomics research of diabetic nephropathy and type 2 diabetes mellitus based on UPLC-oaTOF-MS system

Ultra performance liquid chromatography (UPLC) coupled with orthogonal acceleration time-of-flight (oaTOF) mass spectrometry has showed great potential in diabetes research. In this paper, a UPLC-oaTOF-MS system was employed to distinguish the global serum profiles of 8 diabetic nephropathy (DN) patients, 33 type 2 diabetes mellitus (T2DM) patients and 25 healthy volunteers, and tried to find potential biomarkers. The UPLC system produced information-rich chromatograms with typical measured peak widths of 4 s, generating peak capacities of 225 in 15 min. Furthermore, principal component analysis (PCA) was used for group differentiation and marker selection. As shown in the scores plot, the distinct clustering between the patients and controls was observed, and DN and T2DM patients were also separated into two individual groups. Several compounds were tentatively identified based on accurate mass, isotopic pattern and MS/MS information. In addition, significant changes in the serum level of leucine, dihydrosphingosine and phytosphingosine were noted, indicating the perturbations of amino acid metabolism and phospholipid metabolism in diabetic diseases, which having implications in clinical diagnosis and treatment.

Role of JNK1-dependent Bcl-2 phosphorylation in ceramide-induced macroautophagy

Macroautophagy is a vacuolar lysosomal catabolic pathway that is stimulated during periods of nutrient starvation to preserve cell integrity. Ceramide is a bioactive sphingolipid associated with a large range of cell processes. Here we show that short-chain ceramides (C(2)-ceramide and C(6)-ceramide) and stimulation of the de novo ceramide synthesis by tamoxifen induce the dissociation of the complex formed between the autophagy protein Beclin 1 and the anti-apoptotic protein Bcl-2. This dissociation is required for macroautophagy to be induced either in response to ceramide or to starvation. Three potential phosphorylation sites, Thr(69), Ser(70), and Ser(87), located in the non-structural N-terminal loop of Bcl-2, play major roles in the dissociation of Bcl-2 from Beclin 1. We further show that activation of c-Jun N-terminal protein kinase 1 by ceramide is required both to phosphorylate Bcl-2 and to stimulate macroautophagy. These findings reveal a new aspect of sphingolipid signaling in up-regulating a major cell process involved in cell adaptation to stress.

Calcium-independent phospholipase A2 (iPLA2 beta)-mediated ceramide generation plays a key role in the cross-talk between the endoplasmic reticulum (ER) and mitochondria during ER stress-induced insulin-secreting cell apoptosis

Endoplasmic reticulum (ER) stress induces INS-1 cell apoptosis by a pathway involving Ca(2+)-independent phospholipase A(2) (iPLA(2)beta)-mediated ceramide generation, but the mechanism by which iPLA(2)beta and ceramides contribute to apoptosis is not well understood. We report here that both caspase-12 and caspase-3 are activated in INS-1 cells following induction of ER stress with thapsigargin, but only caspase-3 cleavage is amplified in iPLA(2)beta overexpressing INS-1 cells (OE), relative to empty vector-transfected cells, and is suppressed by iPLA(2)beta inhibition. ER stress also led to the release of cytochrome c and Smac and, unexpectedly, their accumulation in the cytosol is amplified in OE cells. These findings raise the likelihood that iPLA(2)beta participates in ER stress-induced apoptosis by activating the intrinsic apoptotic pathway. Consistent with this possibility, we find that ER stress promotes iPLA(2)beta accumulation in the mitochondria, opening of mitochondrial permeability transition pore, and loss in mitochondrial membrane potential (Delta Psi) in INS-1 cells and that these changes are amplified in OE cells. ER stress also led to greater ceramide generation in ER and mitochondria fractions of OE cells. Exposure to ceramide alone induces loss in Delta Psi and apoptosis and these are suppressed by forskolin. ER stress-induced mitochondrial dysfunction and apoptosis are also inhibited by forskolin, as well as by inactivation of iPLA(2)beta or NSMase, suggesting that iPLA(2)beta-mediated generation of ceramides via sphingomyelin hydrolysis during ER stress affect the mitochondria. In support, inhibition of iPLA(2)beta or NSMase prevents cytochrome c release. Collectively, our findings indicate that the iPLA(2)beta-ceramide axis plays a critical role in activating the mitochondrial apoptotic pathway in insulin-secreting cells during ER stress.

Regulation of ceramide-induced neuronal death: cell metabolism meets neurodegeneration

The present review explores the role of ceramides in neuronal apoptosis, as well as the recent discovery of the signaling pathways involved in this process placing particular emphasis on the correlation between cellular metabolism and neuronal death. Endogenous levels of ceramides are increased following various pro-apoptotic stimuli which have been identified as potential causes of chronic and acute neurodegenerative diseases. Ceramides induce changes in multiple enzymes and cell signaling components. The early inhibition of the neuronal survival pathway regulated by phosphatidil-inositol-3-kinase/protein kinase B or AKT mediated by ceramide may be a relevant early event in the decision of neuronal survival/death. It may perturb several molecular and metabolic functions. In particular it might decrease glycolysis through rapid modulation of hexokinase activity. This would in turn generate limited amounts of mitochondrial substrates leading to mitochondrial dysfunction and neuronal apoptosis. Subtle and early metabolic alterations caused by inhibition of the PI3K/AKT pathway mediated by ceramide may potentially work with genes associated with neurodegenerative diseases such as Parkinson's and Alzheimer's disease. Together they may be determinant steps in downstream events leading to neuronal apoptosis. Therefore, reinforcement of the PI3K/AKT pathway could constitute an important neuroprotective strategy.

Cytotoxicity and apoptosis enhancement in brain tumor cells upon coadministration of paclitaxel and ceramide in nanoemulsion formulations

The objective of this study was to examine augmentation of therapeutic activity in human glioblastoma cells with combination of paclitaxel (PTX) and the apoptotic signaling molecule, C(6)-ceramide (CER), when administered in novel oil-in-water nanoemulsions. The nanoemulsions were formulated with pine-nut oil, which has high concentrations of essential polyunsaturated fatty acid (PUFA). Drug-containing nanoemulsions were characterized for particle size, surface charge, and the particle morphology was examined with transmission electron microscopy (TEM). Epi-fluorescent microscopy was used to analyze nanoemulsion-encapsulated rhodamine-labeled PTX and NBD-labeled CER uptake and distribution in U-118 human glioblastoma cells. Cell viability was assessed with the MTS (formazan) assay, while apoptotic activity of PTX and CER was evaluated with caspase-3/7 activation and flow cytometry. Nanoemulsion formulations with the oil droplet size of approximately 200 nm in diameter were prepared with PTX, CER, and combination of the two agents. When administered to U-118 cells, significant enhancement in cytotoxicity was observed with combination of PTX and CER as compared to administration of individual agents. The increase in cytotoxicity correlated with enhancement in apoptotic activity in cells treated with combination of PTX and CER. The results of these studies show that oil-in-water nanoemulsions can be designed with combination therapy for enhancement of cytotoxic effect in brain tumor cells. In addition, PTX and CER can be used together to augment therapeutic activity, especially in aggressive tumor models such as glioblastoma.

Organelle-targeted nanocarriers: specific delivery of liposomal ceramide to mitochondria enhances its cytotoxicity in vitro and in vivo

To further increase the therapeutic activity of drugs known to act on intracellular target sites, in vivo drug delivery approaches must actively mediate the specific delivery of drug molecules to the subcellular site of action. We show here that surface modification of nanocarriers with mitochondriotropic triphenylphosphonium cations facilitates the efficient subcellular delivery of a model drug to mitochondria of mammalian cells and improves its activity in vitro and in vivo.

Sphingolipids, insulin resistance, and metabolic disease: new insights from in vivo manipulation of sphingolipid metabolism

Obesity and dyslipidemia are risk factors for metabolic disorders including diabetes and cardiovascular disease. Sphingolipids such as ceramide and glucosylceramides, while being a relatively minor component of the lipid milieu in most tissues, may be among the most pathogenic lipids in the onset of the sequelae associated with excess adiposity. Circulating factors associated with obesity (e.g., saturated fatty acids, inflammatory cytokines) selectively induce enzymes that promote sphingolipid synthesis, and lipidomic profiling reveals relationships between tissue sphingolipid levels and certain metabolic diseases. Moreover, studies in cultured cells and isolated tissues implicate sphingolipids in certain cellular events associated with diabetes and cardiovascular disease, including insulin resistance, pancreatic beta-cell failure, cardiomyopathy, and vascular dysfunction. However, definitive evidence that sphingolipids contribute to insulin resistance, diabetes, and atherosclerosis has come only recently, as researchers have found that pharmacological inhibition or genetic ablation of enzymes controlling sphingolipid synthesis in rodents ameliorates each of these conditions. Herein we will review the role of ceramide and other sphingolipid metabolites in insulin resistance, beta-cell failure, cardiomyopathy, and vascular dysfunction, focusing on these in vivo studies that identify enzymes controlling sphingolipid metabolism as therapeutic targets for combating metabolic disease.

Ceramide induces p38 MAPK and JNK activation through a mechanism involving a thioredoxin-interacting protein-mediated pathway

Ceramide, a tumor-suppressor lipid, is generated by sphingomyelin hydrolysis or by de novo synthesis when cells are activated by various stress stimuli as well as when cancer cells are subjected to genotoxic chemotherapy. Ceramide may modulate apoptotic signaling pathways; however, its transcription-dependent effects remain unclear. Our data showed that actinomycin D partially inhibited ceramide-induced apoptosis. Using microarray analysis, we found that ceramide up-regulated a tumor suppressor gene called thioredoxin-interacting protein (Txnip). Similarly, the chemotherapeutic agent etoposide induced Txnip expression en route to apoptosis, which was blocked by inhibitors of ceramide production. Txnip colocalized with thioredoxin and reduced its activity, which caused dissociation of thioredoxin from apoptosis signal-regulating kinase 1 (ASK1). Cells expressing ASK1 siRNA were more resistant to ceramide-induced apoptosis. Ceramide caused ASK1-regulated p38 mitogen-activated protein kinase (MAPK) and JNK activation, as well as activation of the endoplasmic reticulum (ER) stress cascade, and pharmacologic or siRNA-mediated inhibition of p38 MAPK or JNK partially reduced ceramide-induced mitochondria-mediated apoptosis. Furthermore, ceramide-induced ASK1, p38, and JNK phosphorylation and cell apoptosis were inhibited by Txnip siRNA transfection. Taken together, we show that ceramide exhibits a mechanism of transcriptional regulation involving up-regulation of Txnip expression, also induced by etoposide, which results in ASK1 activation, ER stress, and p38 and JNK phosphorylation, all leading to apoptosis.

Ceramide-induced formation of ROS and ATP depletion trigger necrosis in lymphoid cells

In lymphocytes, Fas activation leads to both apoptosis and necrosis, whereby the latter form of cell death is linked to delayed production of endogenous ceramide and is mimicked by exogenous administration of long- and short-chain ceramides. Here molecular events associated with noncanonical necrotic cell death downstream of ceramide were investigated in A20 B lymphoma and Jurkat T cells. Cell-permeable, C6-ceramide (C6), but not dihydro-C6-ceramide (DH-C6), induced necrosis in a time- and dose-dependent fashion. Rapid formation of reactive oxygen species (ROS) within 30 min of C6 addition detected by a dihydrorhodamine fluorescence assay, as well as by electron spin resonance, was accompanied by loss of mitochondrial membrane potential. The presence of N-acetylcysteine or ROS scavengers like Tiron, but not Trolox, attenuated ceramide-induced necrosis. Alternatively, adenovirus-mediated expression of catalase in A20 cells also attenuated cell necrosis but not apoptosis. Necrotic cell death observed following C6 exposure was associated with a pronounced decrease in ATP levels and Tiron significantly delayed ATP depletion in both A20 and Jurkat cells. Thus, apoptotic and necrotic death induced by ceramide in lymphocytes occurs via distinct mechanisms. Furthermore, ceramide-induced necrotic cell death is linked here to loss of mitochondrial membrane potential, production of ROS, and intracellular ATP depletion.

The flavonoid quercetin induces apoptosis and inhibits migration through a MAPK-dependent mechanism in osteoblasts

The present study was undertaken to evaluate effects of quercetin, a major dietary flavonoid occurring in foods of plant origin, on cell viability and migration of osteoblastic cells. Quercetin inhibited cell viability, which was largely attributed to apoptosis, in a dose-and time-dependent manner in osteoblastic cells. Similar cytotoxicity of quercetin was observed in adipose tissue-derived stromal cells. Quercetin exerted a protective effect against H(2)O(2)-induced cell death, whereas it increased TNF-alpha-induced cell death. Western blot analysis showed that quercetin induced activation of ERK and p38, but not JNK. Quercetin-induced cell death was prevented by the ERK inhibitor PD98059, but not by inhibitors of p38 and JNK. Quercetin increased Bax expression and caused depolarization of mitochondrial membrane potential, which were inhibited by PD98059. Quercetin induced caspase-3 activation, and the quercetininduced cell death was prevented by caspase inhibitors. Quercetin inhibited cell migration, and its effect was prevented by inhibitors of ERK and p38. Taken together, these findings suggest that quercetin induces apoptosis through a mitochondria-dependent mechanism involving ERK activation and inhibits migration through activation of ERK and p38 pathways. Quercetin may exert both protective and deleterious effects in bone repair.

Ceramide synthesis in the endoplasmic reticulum can permeabilize mitochondria to proapoptotic proteins

Increased mitochondrial ceramide levels are associated with the initiation of apoptosis. There is evidence that ceramide is causal. Thus, the conversion of the precursor, dihydroceramide, to ceramide by the enzyme dihydroceramide desaturase may be important in preparing the cell for apoptosis. Ceramide can initiate apoptosis by permeabilizing the mitochondrial outer membrane to apoptosis-inducing proteins. However, the mitochondrion's ability to produce ceramide may be limited by its proteome. Here, we show that ceramide synthesized in isolated mammalian endoplasmic reticulum (ER) vesicles from either C8-dihydroceramide or sphingosine to produce long-chain ceramide can transfer to isolated mitochondria. The rate of transfer is consistent with a simple collision model. The transfer of the long-chain ceramide is faster than expected for an uncatalyzed process. Sufficient ceramide is transferred to permeabilize the outer membrane to cytochrome c and adenylate kinase. The mitochondria-associated membranes, ER-like membranes that are tightly associated with isolated mitochondria, can produce enough ceramide to permeabilize the outer membrane transiently. Thus, this ceramide exchange obviates the need for a complete ceramide de novo pathway in mitochondria to increase ceramide levels to the critical value required for functional changes, such as ceramide channel self-assembly followed by protein release.

Ceramide induces mitochondrial abnormalities in insulin-secreting INS-1 cells: potential mechanisms underlying ceramide-mediated metabolic dysfunction of the beta cell

C2-ceramide, a cell permeable analogue of ceramide [CER] markedly reduced mitochondrial membrane potential [MMP] in insulin-secreting INS cells, which was followed by a significant accumulation of cytochrome c [Cyt c] into the cytosolic compartment. In a manner akin to CER, exposure of these cells to interleukin-1beta [IL-1beta] also resulted in reduction in MMP and cytosolic accumulation of Cyt c. Further, long-term exposure of these cells to either CER [but not its inactive analogue] or IL-1beta caused a marked reduction in their metabolic viability. However, unlike IL-1beta, which increased nitric oxide [NO] release, CER-treatment of INS cells had no effects of CER on NO release were demonstrable. Together, these findings suggest that CER-induced mitochondrial effects may not be mediated via iNOS gene expression and NO production. CER also activated an okadaic acid -sensitive protein phosphatase [CAPP] in the purified mitochondrial fraction, suggesting that CAPP might represent one of the target proteins for CER in the beta cell mitochondria. Together, our findings suggest direct detrimental effects of CER on mitochondrial function in beta cells leading to their dysfunction and demise via apoptosis. Moreover, our findings provide evidence for a potential difference in the mechanisms underlying CER- and IL-1beta-induced mitochondrial defects and apoptotic demise of the effete beta cell.

Mitochondrial superoxide production and nuclear factor erythroid 2-related factor 2 activation in p75 neurotrophin receptor-induced motor neuron apoptosis

Nerve growth factor (NGF) can induce apoptosis by signaling through the p75 neurotrophin receptor (p75(NTR)) in several nerve cell populations. Cultured embryonic motor neurons expressing p75(NTR) are not vulnerable to NGF unless they are exposed to an exogenous flux of nitric oxide (*NO). In the present study, we show that p75(NTR)-mediated apoptosis in motor neurons involved neutral sphingomyelinase activation, increased mitochondrial superoxide production, and cytochrome c release to the cytosol. The mitochondria-targeted antioxidants mitoQ and mitoCP prevented neuronal loss, further evidencing the role of mitochondria in NGF-induced apoptosis. In motor neurons overexpressing the amyotrophic lateral sclerosis (ALS)-linked superoxide dismutase 1(G93A) (SOD1(G93A)) mutation, NGF induced apoptosis even in the absence of an external source of *NO. The increased susceptibility of SOD1(G93A) motor neurons to NGF was associated to decreased nuclear factor erythroid 2-related factor 2 (Nrf2) expression and downregulation of the enzymes involved in glutathione biosynthesis. In agreement, depletion of glutathione in nontransgenic motor neurons reproduced the effect of SOD1(G93A) expression, increasing their sensitivity to NGF. In contrast, rising antioxidant defenses by Nrf2 activation prevented NGF-induced apoptosis. Together, our data indicate that p75(NTR)-mediated motor neuron apoptosis involves ceramide-dependent increased mitochondrial superoxide production. This apoptotic pathway is facilitated by the expression of ALS-linked SOD1 mutations and critically modulated by Nrf2 activity.

Insulin-like growth factor-1-dependent maintenance of neuronal metabolism through the phosphatidylinositol 3-kinase-Akt pathway is inhibited by C2-ceramide in CAD cells

Ceramide is a lipid second-messenger generated in response to stimuli associated with neurodegeneration that induces apoptosis, a mechanism underlying neuronal death in Parkinson's disease. We tested the hypothesis that insulin-like growth factor-1 (IGF-1) could mediate a metabolic response in CAD cells, a dopaminergic cell line of mesencephalic origin that differentiate into a neuronal-like phenotype upon serum removal, extend processes resembling neurites, synthesize abundant dopamine and noradrenaline and express the catecholaminergic biosynthetic enzymes tyrosine hydroxylase and dopamine beta-hydroxylase, and that this process was phosphatidylinositol 3-kinase (PI 3-K)-Akt-dependent and could be inhibited by C(2)-ceramide. The metabolic response was evaluated as real-time changes in extracellular acidification rate (ECAR) using microphysiometry. The IGF-1-induced ECAR response was associated with increased glycolysis, determined by increased NAD(P)H reduction, elevated hexokinase activity and Akt phosphorylation. C(2)-ceramide inhibited all these changes in a dose-dependent manner, and was specific, as it was not induced by the inactive C(2)-ceramide analogue C(2)-dihydroceramide. Inhibition of the upstream kinase, PI 3-K, also inhibited Akt phosphorylation and the metabolic response to IGF-1, similar to C(2)-ceramide. Decreased mitochondrial membrane potential occurred after loss of Akt phosphorylation. These results show that IGF-1 can rapidly modulate neuronal metabolism through PI 3-K-Akt and that early metabolic inhibition induced by C(2)-ceramide involves blockade of the PI 3-K-Akt pathway, and may compromise the first step of glycolysis. This may represent a new early event in the C(2)-ceramide-induced cell death pathway that could coordinate subsequent changes in mitochondria and commitment of neurons to apoptosis.

A novel role for protein kinase Cdelta-mediated phosphorylation of acid sphingomyelinase in UV light-induced mitochondrial injury

Multiple studies have addressed the mechanisms by which ultraviolet (UV) light induces cell death, and a few have focused on stress mediators such as acid sphingomyelinase (ASMase) or protein kinase Cdelta (PKCdelta). Based on a recent study that identified a novel mechanism of activation of ASMase through phosphorylation, the current study was undertaken to determine the upstream mechanisms regulating ASMase in response to UV and to investigate the role of ASMase and its phosphorylation at S508 as an integral event during UV light-induced cell death. Exposure of MCF-7 breast cancer cells to UV light type C (UVC) transiently activated ASMase with maximal activity detected at 10 min postirradiation. A significant increase in C16-ceramide was detected concomitant with a decrease in C16-sphingomyelin. In marked contrast, cells overexpressing the ASMase(S508A) mutant, which could not be phosphorylated, had no change in either ASMase activity or ceramide levels post-UV radiation. Loss of PKCdelta by RNA interference or its inhibition by rottlerin blocked ASMase phosphorylation and membrane targeting, thus implicating PKCdelta upstream of ASMase activation by UV light. Further investigations revealed that UV radiation altered mitochondrial morphology from elongated tubules to fragmented perinuclear organelles, consistent with the onset of the apoptotic cascade. Importantly, cells overexpressing ASMase(S508A) were protected (>50%) from UV light-induced mitochondrial fragmentation. Mechanistically, the results showed that ASMase(S508A) cells had 50% less active Bax than ASMase(WT) cells. These molecular differences culminated in resistance of ASMase(S508) cells to UVC-induced cell death (25%) as compared to ASMase(WT) cells (46%). Taken together, this study provides key molecular insights into activation of ASMase in response to UV light, the role of PKCdelta in this activation, and the role of ASMase in mediating apoptotic responses.

Ceramide: From Embryos to Tumors

Ceramides are ubiquitous lipids that have important functions integral to apoptotic signaling. Several therapeutic agents currently exist that induce ceramide-dependent apoptosis in cancerous cells, and a number of enzymes involved in ceramide metabolism are beginning to be recognized as potential targets for cancer therapy. Recent research shows that evasion of ceramide-dependent apoptosis is essential at the earliest stages of embryonic development and is an important mechanism of multidrug resistance. Although ceramide-based strategies for treating cancer are promising, current data about ceramide-resistant tumors require further understanding of the role of ceramide in apoptosis.

Short-chain 3-ketoceramides, strong apoptosis inducers against human leukemia HL-60 cells

Ceramides act as a second messenger of the apoptotic signaling process. The allylic alcohol portion comprising the C-3, C-4, and C-5 carbons is essential for this function. The suggestion has been made that this alcohol moiety is oxidized in mitochondria to a carbonyl moiety, with the generation of reactive oxygen species. However, there is no established precedent for the apoptotic performance of 3-ketoceramides thus presumed. In this work, we have synthesized three different types of short-chain 3-ketoceramides, that is, (2S,4E)-2-acetylamino-3-oxo-4-octadecen-1-ol (A), (2S,4E,6E)-2-acetylamino-3-oxo-4,6-octadecadien-1-ol (B), and (2S,4E)-2-acetylamino-1-methoxy-3-oxo-4-octadecene (C), and demonstrated that these 3-ketoceramides are capable of inducing effective apoptosis in human leukemia HL-60 cells. In particular, the two monoenoic compounds, A and C, are far more powerful than the corresponding alcoholic analogue, N-acetyl-D-erythro-sphingosine. Observations of DNA fragmentation, caspase-3 activation, and cytochrome c release from mitochondria provide substantiated evidence for mitochondrial apoptosis and the effects of exogenous glutathione on these phenomena are also discussed.

Diabetes alters sphingolipid metabolism in the retina: a potential mechanism of cell death in diabetic retinopathy

Dysregulated sphingolipid metabolism causes neuronal cell death and is associated with insulin resistance and diseases. Thus, we hypothesized that diabetes-induced changes in retinal sphingolipid metabolism may contribute to neuronal pathologies in diabetic retinopathy. ESI-MS/MS was used to measure ceramide content and ceramide metabolites in whole retinas after 2, 4, and 8 weeks of streptozotocin-induced diabetes. After 4 and 8 weeks of diabetes, a approximately 30% decrease in total ceramide content was observed, concomitant with a significant approximately 30% increase in glucosylceramide levels in fed diabetic rats compared with their age-matched controls. Acute insulin therapy as well as a short-term lowering of glucose via fasting did not affect the increase in glucosylceramide composition. To assess the putative biological consequences of the increase in glucosylceramide composition, R28 retinal neurons were treated with glucosylceramide synthase inhibitors. Inhibiting glycosphingolipid metabolism increased insulin sensitivity in retinal neurons. Glycosphingolipid inhibitors augmented insulin-stimulated p70 S6kinase activity in the presence of inhibitory concentrations of high glucose or glucosamine. Inhibition of glycosphingolipid synthesis also suppressed glucosamine- and interleukin-1beta-induced death. Consistent with these inhibitor studies, pharmacological accumulation of glycosphingolipids increased activation of the endoplasmic reticulum stress response, a putative modulator of insulin resistance and neuronal apoptosis. It is speculated that an increase in glucosylceramide, and possibly higher-order glycosphingolipids, could contribute to the pathogenesis of diabetic retinopathy by contributing to local insulin resistance, resulting in neuronal cell death. Thus, dysfunctional glycosphingolipid metabolism may contribute to metabolic stress in diabetes, and therapeutic strategies to restore normal sphingolipid metabolism may be a viable approach for treatment of diabetic retinopathy.

Sphingolipids in apoptosis, survival and regeneration in the nervous system

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

Tailoring structure-function and targeting properties of ceramides by site-specific cationization

In the course of our studies on compartment-specific lipid-mediated cell regulation, we identified an intimate connection between ceramides (Cers) and the mitochondria-dependent death-signaling pathways. Here, we report on a new class of cationic Cer mimics, dubbed ceramidoids, designed to act as organelle-targeted sphingolipids (SPLs), based on conjugates of Cer and dihydroceramide (dhCer) with pyridinium salts (CCPS and dhCCPS, respectively). Ceramidoids having the pyridinium salt unit (PSU) placed internally (alpha and gamma- CCPS) or as a tether (omega-CCPS) in the N-acyl moiety were prepared by N-acylation of sphingoid bases with different omega-bromo acids or pyridine carboxylic acid chlorides following capping with respective pyridines or alkyl bromides. Consistent with their design, these analogs, showed a significantly improved solubility in water, well-resolved NMR spectra in D(2)O, broadly modified hydrophobicity, fast cellular uptake, and higher anticancer activities in cells in comparison to uncharged counterparts. Structure-activity relationship (SAR) studies in MCF-7 breast carcinoma cells revealed that the location of the PSU and its overall chain length affected markedly the cytotoxic effects of these ceramidoids. All omega-CCPSs were more potent (IC(50/48 h): 0.6-8.0 microM) than their alpha/gamma-CCPS (IC(50/48 h): 8-20 microM) or D-erythro-C6-Cer (IC(50/48 h): 15 microM) analogs. omega-DhCCPSs were also moderately potent (IC(50/48 h): 2.5-12.5 microM). Long-chain omega-dhCCPSs were rapidly and efficiently oxidized in cells to the corresponding omega-CCPSs, as established by LC-MS analysis. CCPS analogs also induced acute changes in the levels and composition of endogenous Cers (upregulation of C16-, C14-, and C18-Cers, and downregulation of C24:0- and C24:1-Cers). These novel ceramidoids illustrate the feasibility of compartment-targeted lipids, and they should be useful in cell-based studies as well as potential novel therapeutics.

Sphingolipid targets in cancer therapy

Considerable progress has been made recently in our understanding of the role of ceramide in the induction of apoptotic cell death. Ceramide is produced by cancer cells in response to exposure to radiation and most chemotherapeutics and is an intracellular second messenger that activates enzymes, leading to apoptosis. Because of its central role in apoptosis, pharmacologic manipulation of intracellular ceramide levels should result in attenuation or enhancement of drug resistance. This may be achieved through direct application of sphingolipids or by the inhibition/activation of the enzymes that either produce or use ceramide. In addition, attention should be given to the subcellular location of ceramide generation, because this has been shown to affect the biological activity of sphingolipids. This review summarizes the sphingolipid biosynthetic pathway, as it relates to the identification of important targets for drug discovery, and the development of novel agents capable of enhancing chemotherapy.

Sphingosine increases the permeability of model and cell membranes

Sphingosine, at 5-15 mol % total lipids, remarkably increases the permeability to aqueous solutes of liposomal and erythrocyte ghost membranes. The increased permeability cannot be interpreted in terms of leakage occurring at the early stages of a putative membrane solubilization by sphingosine, nor is it due to a sphingosine-induced generation of nonlamellar structures, or flip-flop lipid movement. Instead, sphingosine stabilizes (rigidifies) gel domains in membranes, raising their melting temperatures and increasing the transition cooperativity. Structural defects originating during the lateral phase separation of the "more rigid" and "less rigid" domains are likely sites for the leakage of aqueous solutes to the extravesicular medium. The presence of coexisting domains in the plasma membrane makes it a target for sphingosine permeabilization. The sphingosine-induced increase in rigidity and breakdown of the plasma membrane permeability barrier could be responsible for some of the physiological effects of sphingosine.

Further characterization of mammalian ceramide kinase: substrate delivery and (stereo)specificity, tissue distribution, and subcellular localization studies

Recombinant human ceramide kinase (HsCERK) was analyzed with regard to dependence on divalent cations and to substrate delivery, spectrum, specificity, and stereoselectivity. Depending on the chain length of the ceramide, either albumin for short-chain ceramide or a mixed micellar form (octylglucoside/cardiolipin) for long-chain ceramide was preferred for the substrate delivery, the former resulting in higher activities. Bacterially expressed HsCERK was highly dependent on Mg2+ ions, much less on Ca2+ ions. A clear preference for the d-erythro isomer was seen. Various N-acylated amino alcohols were no substrate, but N-hexanoyl-1-O-hexadecyl-2-desoxy-2-amino-sn-glycerol and N-tetradecanoyl-2S-amino-1-butanol were phosphorylated, suggesting that the secondary hydroxy group is not required for recognition. The properties of HsCERK, expressed in CHO cells, were similar to those of the bacterially expressed protein, including the Mg2+ dependence. In mouse, the highest activities were found in testis and cerebellum, and upon subcellular fractionation the activity was recovered mainly in the microsomal fraction. This fits with the plasma membrane localization in CHO cells, which was mediated by the N-terminal putative pleckstrin domain. No evidence for phosphorylation of ceramide by the recently described multiple lipid kinase was found. The latter kinase is localized in the mitochondria, but no firm conclusions with regard to its substrate could be drawn.

An Efficient Synthesis of d-erythro- and d-threo-Sphingosine from d-Glucose: Olefin Cross-Metathesis Approach

[reaction: see text] The D-erythro- and D-threo-sphingosine were synthesized via E-selective olefin cross-metathesis using a D-glucose-derived building block and long-chain terminal alkene.

ATP-induced apoptosis of thymocytes is mediated by activation of P2 X 7 receptor and involves de novo ceramide synthesis and mitochondria

Thymocytes were reported to undergo apoptosis in the presence of extracellular ATP through the activation of the purinergic receptors P2 X 1R, P2 X 7R or both. We investigated the identity of the P2 X R and the signaling pathways involved in ATP-mediated apoptosis. Apoptosis elicited by ATP was prevented by inhibition of P2 X 7R, or in thymocytes bearing a mutated P2 X 7R, and reproduced with a P2 X 7R agonist, but not with a P2 X 1R agonist. Stimulation of thymocytes with either ATP or a P2 X 7R agonist was found to stimulate a late de novo ceramide synthesis and mitochondrial alterations. Inhibition of either processes attenuated apoptosis. Interestingly, stimulation with either ATP or a P2 X 1R agonist induced an early ceramide accumulation and a weak caspases-3/7 activation that did not lead to apoptosis. In conclusion, de novo ceramide generation and mitochondrial alterations, both resulting from P2 X 7R activation, were implicated in ATP-induced thymocyte apoptosis.

Ceramide induces neuronal apoptosis through mitogen-activated protein kinases and causes release of multiple mitochondrial proteins

Ceramide accumulates in neurons during various disorders associated with acute or chronic neurodegeneration. In these studies, we investigated the mechanisms of ceramide-induced apoptosis in primary cortical neurons using exogenous C(2) ceramide as well as inducing endogenous ceramide accumulation using inhibitors of glucosylceramide synthetase. Ceramide induced the translocation of certain, but not all, pro-apoptotic mitochondrial proteins: cytochrome c, Omi, SMAC, and AIF were released from the mitochondria, whereas Endonuclease G was not. Ceramide also selectively altered the phosphorylation state of members of the MAPK superfamily, causing dephosphorylation of ERK1/2 and hyperphosphorylation of p38 MAP kinases, but not affecting the phosphorylation of JNK or ERK5. Inhibitors of the p38 MAP kinase pathway (SB-202190 or SB-203580) and an inhibitor of the ERK1/2 pathway (U0126) reduced ceramide-induced neuronal death. These p38 and ERK1/2 inhibitors appear to block ceramide-activated apoptotic signaling upstream of the mitochondria, as they attenuated mitochondrial release of cytochrome c, Omi, AIF, and SMAC, as well as reducing ceramide-induced caspase-3 activation.

A mitochondrial pool of sphingomyelin is involved in TNFalpha-induced Bax translocation to mitochondria

We recently showed that targeting bSMase (bacterial sphingomyelinase) specifically to mitochondria caused accumulation of ceramide in mitochondria, and induced cytochrome c release and cell death [Birbes, El Bawab, Hannun and Obeid (2001) FASEB J., 15, 2669-2679]. In the present study, we investigated the role of this mitochondrial pool of ceramide in response to a receptor-mediated event, namely TNFalpha (tumour necrosis factor alpha), and the involvement of this mitochondrial pool of ceramide in Bax translocation to mitochondria, an event that precedes cytochrome c release. Treatment of MCF7 cells with TNFalpha caused an increase in ceramide levels in the mitochondrial fraction which accompanied Bax translocation to mitochondria. Targeting bSMase to mitochondria specifically resulted in Bax translocation to mitochondria, suggesting that the mitochondrial ceramide pool is involved in Bax translocation. Moreover, in a reconstituted cell-free system, treatment of isolated mitochondria with bSMase enhanced Bax association with mitochondrial membranes. Collectively, these results suggest that the generation of ceramide in mitochondria in response to TNFalpha is sufficient to induce Bax translocation to mitochondria and subsequent cytochrome c release and cell death.

Role of mitogen-activated protein kinases in hydrogen peroxide-induced cell death in osteoblastic cells

Oxidative stress is known to induce cell death in a wide variety of cell types, apparently by modulating intracellular signaling pathways. However, the underlying mechanism by which oxidants induce cell death remains unclear. The present study was undertaken to determine the role of the mitogen-activated protein kinase subfamilies in hydrogen peroxide (H2O2)-induced cell death of osteoblastic cells. H2O2 resulted in a time- and dose-dependent cell death, which was, in part, attributed to apoptosis. H2O2-induced cell death was prevented by iron chelator, hydroxyl radical scavengers. But H2O2-induced cell death was not affected by 3-aminobenzamide, an inhibitor of poly (ADP-ribose) polymerase activation. H2O2 treatment caused a transient activation of extracellular signal-regulated kinase (ERK), followed by sustained activation. Cell death induced by H2O2 was prevented by PD98059, an inhibitor of ERK upstream kinase MEK1/2. But H2O2 induced a transient activation of p38 and c-Jun N-terminal kinase (JNK) without sustained activation and inhibitors of these kinses were not effective in preventing the cell death. H2O2 increased Bax expression and produced hyperpolarization of mitochondrial membrane potential and its effect was prevented by PD98059. The ERK activation and cell death induced by H2O2 were not dependent on the phosphorylation of epidermal growth factor receptor. Taken together, these findings suggest that the ERK signaling pathway plays an active role in mediating H2O2-induced apoptosis of osteoblasts and functions upstream of mitochondria-dependent pathway to initiate the apoptotic signal.

Systemic delivery of liposomal short-chain ceramide limits solid tumor growth in murine models of breast adenocarcinoma

In vitro tumor cell culture models have illuminated the potential therapeutic utility of elevating the intracellular concentration of the antimitogenic and proapoptotic sphingolipid, ceramide. However, although cell-permeable, short-chain ceramide is an effective apoptotic agent in vitro, its use as an in vivo, systemically delivered therapeutic is limited by its inherent lipid hydrophobicity and physicochemical properties. Here, we report that the systemic i.v. delivery of C6-ceramide (C6) in a pegylated liposomal formulation significantly limited the growth of solid tumors in a syngeneic BALB/c mouse tumor model of breast adenocarcinoma. Over a 3-week treatment period, a well-tolerated dose of 36 mg/kg liposomal-C6 elicited a >6-fold reduction in tumor size compared with empty ghost liposomes. Histologic analyses of solid tumors from liposomal-C6-treated mice showed a marked increase in the presence of apoptotic cells, with a coincident decrease in cellular proliferation and in the development of a microvessel network. Liposomal-C6 accumulated within caveolae and mitochondria, suggesting putative mechanisms by which ceramide induces selective cancer cell cytotoxicity. A pharmacokinetic analysis of systemic liposomal-C6 delivery showed that the pegylated liposomal formulation follows first-order kinetics in the blood and achieves a steady-state concentration in tumor tissue. Confirming the therapeutic utility of i.v. liposomal-C6 administration, we also shown diminution of solid tumor growth in a human xenograft model of breast cancer. Together, these results indicate that bioactive ceramide analogues can be incorporated into pegylated liposomal vehicles for improved solubility, drug delivery, and antineoplastic efficacy.

Mitochondrial ceramide and the induction of apoptosis

In most cell types, a key event in apoptosis is the release of proapoptotic intermembrane space proteins from mitochondria to the cytoplasm. In general, it is the release of these intermembrane space proteins that is responsible for the activation of caspases and DNases that are responsible for the execution of apoptosis. The mechanism for the increased permeability of the mitochondrial outer membrane during the induction phase of apoptosis is currently unknown and highly debated. This review will focus on one such proposed mechanism, namely, the formation of ceramide channels in the mitochondrial outer membrane. Ceramides are known to play a major regulatory role in apoptosis by inducing the release of proapoptotic proteins from the mitochondria. As mitochondria are known to contain the enzymes responsible for the synthesis and hydrolysis of ceramide, there exists a mechanism for regulating the level of ceramide in mitochondria. In addition, mitochondrial ceramide levels have been shown to be elevated prior to the induction phase of apoptosis. Ceramide has been shown to form large protein permeable channels in planar phospholipid and mitochondrial outer membranes. Thus, ceramide channels are good candidates for the pathway with which proapoptotic proteins are released from mitochondria during the induction phase of apoptosis.

Down-regulation of ATM protein sensitizes human prostate cancer cells to radiation-induced apoptosis

Treatment with the protein kinase C activator 12-O-tetradecanoylphorbol 12-acetate (TPA) enables radiation-resistant LNCaP human prostate cancer cells to undergo radiation-induced apoptosis, mediated via activation of the enzyme ceramide synthase (CS) and de novo synthesis of the sphingolipid ceramide (Garzotto, M., Haimovitz-Friedman, A., Liao, W. C., White-Jones, M., Huryk, R., Heston, D. W. W., Cardon-Cardo, C., Kolesnick, R., and Fuks, Z. (1999) Cancer Res. 59, 5194-5201). Here, we show that TPA functions to decrease the cellular level of the ATM (ataxia telangiectasia mutated) protein, known to repress CS activation (Liao, W.-C., Haimovitz-Friedman, A., Persaud, R., McLoughlin, M., Ehleiter, D., Zhang, N., Gatei, M., Lavin, M., Kolesnick, R., and Fuks, Z. (1999) J. Biol. Chem. 274, 17908-17917). Gel shift analysis in LNCaP and CWR22-Rv1 cells demonstrated a significant reduction in DNA binding of the Sp1 transcription factor to the ATM promoter, and quantitative reverse transcription-PCR showed a 50% reduction of ATM mRNA between 8 and 16 h of TPA treatment, indicating that TPA inhibits ATM transcription. Furthermore, treatment of LNCaP, CWR22-Rv1, PC-3, and DU-145 human prostate cells with antisense-ATM oligonucleotides, which markedly reduced cellular ATM levels, significantly enhanced radiation-induced CS activation and apoptosis, leading to apoptosis at doses as a low as 1 gray. These data suggest that the CS pathway initiates a generic mode of radiation-induced apoptosis in human prostate cancer cells, regulated by a suppressive function of ATM, and that ATM might represent a potential target for pharmacologic inactivation with potential clinical applications in human prostate cancer.

Sphingosine Kinase 1 (SK1) Is Recruited to Nascent Phagosomes in Human Macrophages: Inhibition of SK1 Translocation by Mycobacterium tuberculosis

Mycobacterium tuberculosis (M.tb) is a leading cause of global infectious mortality. The pathogenesis of tuberculosis involves inhibition of phagosome maturation, leading to survival of M.tb within human macrophages. A key determinant is M.tb-induced inhibition of macrophage sphingosine kinase (SK) activity, which normally induces Ca2+ signaling and phagosome maturation. Our objective was to determine the spatial localization of SK during phagocytosis and its inhibition by M.tb. Stimulation of SK activity by killed M.tb, live Staphylococcus aureus, or latex beads was associated with translocation of cytosolic SK1 to the phagosome membrane. In contrast, SK1 did not associate with phagosomes containing live M.tb. To characterize the mechanism of phagosomal translocation, live cell confocal microscopy was used to compare the localization of wild-type SK1, catalytically inactive SK1G82D, and a phosphorylation-defective mutant that does not undergo plasma membrane translocation (SK1S225A). The magnitude and kinetics of translocation of SK1G82D and SK1S225A to latex bead phagosomes were indistinguishable from those of wild-type SK1, indicating that novel determinants regulate the association of SK1 with nascent phagosomes. These data are consistent with a model in which M.tb inhibits both the activation and phagosomal translocation of SK1 to block the localized Ca2+ transients required for phagosome maturation.