Lack of ceramide generation in TF-1 human myeloid leukemic cells resistant to ionizing radiation

Emerging Roles of Ceramides in Breast Cancer Biology and Therapy

One of the classic hallmarks of cancer is the imbalance between elevated cell proliferation and reduced cell death. Ceramide, a bioactive sphingolipid that can regulate this balance, has long been implicated in cancer. While the effects of ceramide on cell death and therapeutic efficacy are well established, emerging evidence indicates that ceramide turnover to downstream sphingolipids, such as sphingomyelin, hexosylceramides, sphingosine-1-phosphate, and ceramide-1-phosphate, is equally important in driving pro-tumorigenic phenotypes, such as proliferation, survival, migration, stemness, and therapy resistance. The complex and dynamic sphingolipid network has been extensively studied in several cancers, including breast cancer, to find key sphingolipidomic alterations that can be exploited to develop new therapeutic strategies to improve patient outcomes. Here, we review how the current literature shapes our understanding of how ceramide synthesis and turnover are altered in breast cancer and how these changes offer potential strategies to improve breast cancer therapy.

Ceramide Kinase Inhibition Drives Ferroptosis and Sensitivity to Cisplatin in Mutant KRAS Lung Cancer by Dysregulating VDAC-Mediated Mitochondria Function

Ceramide kinase (CERK) is the mammalian lipid kinase from which the bioactive sphingolipid, ceramide-1-phosphate (C1P), is derived. CERK has been implicated in several promalignant phenotypes with little known as to mechanistic underpinnings. In this study, the mechanism of how CERK inhibition decreases cell survival in mutant (Mut) KRAS non-small cell lung cancer (NSCLC), a major lung cancer subtype, was revealed. Specifically, NSCLC cells possessing a KRAS mutation were more responsive to inhibition, downregulation, and genetic ablation of CERK compared with those with wild-type (WT) KRAS regarding a reduction in cell survival. Inhibition of CERK induced ferroptosis in Mut KRAS NSCLC cells, which required elevating VDAC-regulated mitochondria membrane potential (MMP) and the generation of cellular reactive oxygen species (ROS). Importantly, through modulation of VDAC, CERK inhibition synergized with the first-line NSCLC treatment, cisplatin, in reducing cell survival and in vivo tumor growth. Further mechanistic studies indicated that CERK inhibition affected MMP and cell survival by limiting AKT activation and translocation to mitochondria, and thus, blocking VDAC phosphorylation and tubulin recruitment.

IMPLICATIONS

Our findings depict how CERK inhibition may serve as a new key point in combination therapeutic strategy for NSCLC, specifically precision therapeutics targeting NSCLC possessing a KRAS mutation.

γ-Tocotrienol induces apoptosis in pancreatic cancer cells by upregulation of ceramide synthesis and modulation of sphingolipid transport

Background

Ceramide synthesis and metabolism is a promising target in cancer drug development. γ-tocotrienol (GT3), a member of the vitamin E family, orchestrates multiple effects that ensure the induction of apoptosis in both, wild-type and RAS-mutated pancreatic cancer cells. Here, we investigated whether these effects involve changes in ceramide synthesis and transport.

Methods

The effects of GT3 on the synthesis of ceramide via the de novo pathway, and the hydrolysis of sphingomyelin were analyzed by the expression levels of the enzymes serine palmitoyl transferase, ceramide synthase-6, and dihydroceramide desaturase, and acid sphingomyelinase in wild-type RAS BxPC3, and RAS-mutated MIA PaCa-2 and Panc 1 pancreatic cancer cells. Quantitative changes in ceramides, dihydroceramides, and sphingomyelin at the cell membrane were detected by LCMS. Modulation of ceramide transport by GT3 was studied by immunochemistry of CERT and ARV-1, and the subsequent effects at the cell membrane was analyzed via immunofluorescence of ceramide, caveolin, and DR5.

Results

GT3 favors the upregulation of ceramide by stimulating synthesis at the ER and the plasma membrane. Additionally, the conversion of newly synthesized ceramide to sphingomyelin and glucosylceramide at the Golgi is prevented by the inhibition of CERT. Modulation ARV1 and previously observed inhibition of the HMG-CoA pathway, contribute to changes in membrane structure and signaling functions, allows the clustering of DR5, effectively initiating apoptosis.

Conclusions

Our results suggest that GT3 targets ceramide synthesis and transport, and that the upregulation of ceramide and modulation of transporters CERT and ARV1 are important contributors to the apoptotic properties demonstrated by GT3 in pancreatic cancer cells.

The emerging role of FTY720 (Fingolimod) in cancer treatment

FTY720 (Fingolimod) is a clinically approved immunomodulating therapy for multiple sclerosis that sequesters T-cells to lymph nodes through functional antagonism of sphingosine-1-phosphate 1 receptor. FTY720 also demonstrates a proven efficacy in multiple in vitro and in vivo cancer models, suggesting a potential therapeutic role in cancer patients. A potential anticancer mechanism of FTY720 is through the inhibition of sphingosine kinase 1, a proto-oncogene with in vitro and clinical cancer association. In addition, FTY720's anticancer properties may be attributable to actions on several other molecular targets. This study focuses on reviewing the emerging evidence regarding the anticancer properties and molecular targets of FTY720. While the clinical transition of FTY720 is currently limited by its immune suppression effects, studies aiming at FTY720 delivery and release together with identifying its key synergetic combinations and relevant patient subsets may lead to its rapid introduction into the clinic.

Exploring the link between ceramide and ionizing radiation

The aim of radiotherapy is to eradicate cancer cells with ionizing radiation; tumor cell death following irradiation can be induced by several signaling pathways, most of which are triggered as a consequence of DNA damage, the primary and major relevant cell response to radiation. Several lines of evidence demonstrated that ceramide, a crucial sensor and/or effector of different signalling pathways promoting cell cycle arrest, death and differentiation, is directly involved in the molecular mechanisms underlying cellular response to irradiation. Most of the studies strongly support a direct relationship between ceramide accumulation and radiation-induced cell death, mainly apoptosis; for this reason, defining the contribution of the multiple metabolic pathways leading to ceramide formation and the causes of its dysregulated metabolism represent the main goal in order to elucidate the ceramide-mediated signaling in radiotherapy. In this review, we summarize the current knowledge concerning the different routes leading to ceramide accumulation in radiation-induced cell response with particular regard to the role of the enzymes involved in both ceramide neogenesis and catabolism. Emphasis is placed on sphingolipid breakdown as mechanism of ceramide generation activated following cell irradiation; the functional relevance of this pathway, and the role of glycosphingolipid glycohydrolases as direct targets of ionizing radiation are also discussed. These new findings add a further attractive point of investigation to better define the complex interplay between sphingolipid metabolism and radiation therapy.

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.

Ceramide kinase promotes tumor cell survival and mammary tumor recurrence

Recurrent breast cancer is typically an incurable disease and, as such, is disproportionately responsible for deaths from this disease. Recurrent breast cancers arise from the pool of disseminated tumor cells (DTC) that survive adjuvant or neoadjuvant therapy, and patients with detectable DTCs following therapy are at substantially increased risk for recurrence. Consequently, the identification of pathways that contribute to the survival of breast cancer cells following therapy could aid in the development of more effective therapies that decrease the burden of residual disease and thereby reduce the risk of breast cancer recurrence. We now report that ceramide kinase (Cerk) is required for mammary tumor recurrence following HER2/neu pathway inhibition and is spontaneously upregulated during tumor recurrence in multiple genetically engineered mouse models for breast cancer. We find that Cerk is rapidly upregulated in tumor cells following HER2/neu downregulation or treatment with Adriamycin and that Cerk is required for tumor cell survival following HER2/neu downregulation. Consistent with our observations in mouse models, analysis of gene expression profiles from more than 2,200 patients revealed that elevated CERK expression is associated with an increased risk of recurrence in women with breast cancer. In addition, although CERK expression is associated with aggressive subtypes of breast cancer, including those that are estrogen receptor-negative, HER2(+), basal-like, or high grade, its association with poor clinical outcome is independent of these clinicopathologic variables. Together, our findings identify a functional role for Cerk in breast cancer recurrence and suggest the clinical utility of agents targeted against this prosurvival pathway.

Radiation-induced acid ceramidase confers prostate cancer resistance and tumor relapse

Escape of prostate cancer (PCa) cells from ionizing radiation-induced (IR-induced) killing leads to disease progression and cancer relapse. The influence of sphingolipids, such as ceramide and its metabolite sphingosine 1-phosphate, on signal transduction pathways under cell stress is important to survival adaptation responses. In this study, we demonstrate that ceramide-deacylating enzyme acid ceramidase (AC) was preferentially upregulated in irradiated PCa cells. Radiation-induced AC gene transactivation by activator protein 1 (AP-1) binding on the proximal promoter was sensitive to inhibition of de novo ceramide biosynthesis, as demonstrated by promoter reporter and ChIP-qPCR analyses. Our data indicate that a protective feedback mechanism mitigates the apoptotic effect of IR-induced ceramide generation. We found that deregulation of c-Jun induced marked radiosensitization in vivo and in vitro, which was rescued by ectopic AC overexpression. AC overexpression in PCa clonogens that survived a fractionated 80-Gy IR course was associated with increased radioresistance and proliferation, suggesting a role for AC in radiotherapy failure and relapse. Immunohistochemical analysis of human PCa tissues revealed higher levels of AC after radiotherapy failure than those in therapy-naive PCa, prostatic intraepithelial neoplasia, or benign tissues. Addition of an AC inhibitor to an animal model of xenograft irradiation produced radiosensitization and prevention of relapse. These data indicate that AC is a potentially tractable target for adjuvant radiotherapy.

Therapeutic potential of targeting SK1 in human cancers

Sphingosine kinase 1 (SK1) is a lipid enzyme with oncogenic properties that converts the proapoptotic lipids ceramide and sphingosine into the antiapoptotic lipid sphingosine-1-phosphate and activates the signal transduction pathways that lead to cell proliferation, migration, the activation of the inflammatory response, and the impairment of apoptosis. There is compelling evidence that SK1 activation contributes to cancer progression leading to increased oncogenic transformation, tumor growth, resistance to therapies, tumor neovascularization, and metastatic spread. High levels of SK1 expression or activity have been associated with a poor prognosis in several human cancers. Recent studies using cancer cell and mouse models demonstrate a significant potential for SK1-targeting therapies to synergize with the effects of chemotherapy and radiotherapy; however, until recently the absence of clinically applicable SK1 inhibitors has limited the translation of these findings into patients. With the recent discovery of SK1 inhibiting properties of a clinically approved drug FTY720 (Fingolimod), SK1 has gained significant attention from both clinicians and the pharmaceutical industry and it is hoped that trials of newly developed SK1 inhibitors may follow soon. This review provides an overview of the SK1 signaling, its relevance to cancer progression, and the potential clinical significance of targeting SK1 for improved local or systemic control of human cancers.

Interdiction of sphingolipid metabolism to improve standard cancer therapies

Non-surgical therapies for human malignancies must negotiate complex cell signaling pathways to impede cancer cell growth, ideally promoting death of cancer cells while sparing healthy tissue. For most of the past half century, medical approaches for treating cancer have relied primarily on cytotoxic chemotherapeutics that interfere with DNA replication and cell division, susceptibilities of rapidly dividing cancer cells. As a consequence, these therapies exert considerable cell stress, promoting the generation of ceramide through de novo synthesis and recycling of complex glycosphingolipids and sphingomyelin into apoptotic ceramide. Radiotherapy of cancer exerts similar geno- and cytotoxic cell stresses, and generation of ceramide following ionizing radiation therapy is a well-described feature of radiation-induced cell death. Emerging evidence now describes sphingolipids as mediators of death in response to newer targeted therapies, cementing ceramide generation as a common mechanism of cell death in response to cancer therapy. Many studies have now shown that dysregulation of ceramide accumulation-whether by reduced generation or accelerated metabolism-is a common mechanism of resistance to standard cancer therapies. The aims of this chapter will be to discuss described mechanisms of cancer resistance to therapy related to dysregulation of sphingolipid metabolism and to explore clinical and preclinical approaches to interdict sphingolipid metabolism to improve outcomes of standard cancer therapies.

Ceramide function in the brain: when a slight tilt is enough

Ceramide, the precursor of all complex sphingolipids, is a potent signaling molecule that mediates key events of cellular pathophysiology. In the nervous system, the sphingolipid metabolism has an important impact. Neurons are polarized cells and their normal functions, such as neuronal connectivity and synaptic transmission, rely on selective trafficking of molecules across plasma membrane. Sphingolipids are abundant on neural cellular membranes and represent potent regulators of brain homeostasis. Ceramide intracellular levels are fine-tuned and alteration of the sphingolipid-ceramide profile contributes to the development of age-related, neurological and neuroinflammatory diseases. The purpose of this review is to guide the reader towards a better understanding of the sphingolipid-ceramide pathway system. First, ceramide biology is presented including structure, physical properties and metabolism. Second, we describe the function of ceramide as a lipid second messenger in cell physiology. Finally, we highlight the relevance of sphingolipids and ceramide in the progression of different neurodegenerative diseases.

Anti-proliferative effects of the novel ceramide analog (S)-2-(benzylideneamino)-3-hydroxy-N-tetrade-cylpropanamide in chemoresistant cancer

The ceramide-sphingosine-1-phosphate rheostat is a promising therapeutic target. Here, the novel ceramide analog (S)-2-(benzylideneamino)-3-hydroxy-N-tetrade-cylpropanamide is shown to block proliferation and enhance the efficacy of the clinical chemotherapeutics, etoposide and doxorubicin. These results demonstrate the therapeutic potential of this compound in treating both endocrine resistant and chemoresistant breast cancer.

Suppression of glucosylceramide synthase restores p53-dependent apoptosis in mutant p53 cancer cells

Tumor suppressor p53 plays an essential role in protecting cells from malignant transformation by inducing cell-cycle arrest and apoptosis. Mutant p53 that is detected in more than 50% of cases of cancers loses its role in suppression of tumors but gains in oncogenic function. Strategies to convert mutant p53 into wild-type p53 have been suggested for cancer prevention and treatment, but they face a variety of challenges. Here, we report an alternative approach that involves suppression of glucosylceramide synthase (GCS), an enzyme that glycosylates ceramide and blunts its proapoptotic activity in cancer cells. Human ovarian cancer cells expressing mutant p53 displayed resistance to apoptosis induced by DNA damage. We found that GCS silencing sensitized these mutant p53 cells to doxorubicin but did not affect the sensitivity of cells with wild-type p53. GCS silencing increased the levels of phosphorylated p53 and p53-responsive genes, including p21(Waf1/Cip1), Bax, and Puma, consistent with a redirection of the mutant p53 cells to apoptosis. Reactivated p53-dependent apoptosis was similarly verified in p53-mutant tumors where GCS was silenced. Inhibition of ceramide synthase with fumonisin B1 prevented p53 reactivation induced by GCS silencing, whereas addition of exogenous C6-ceramide reactivated p53 function in p53-mutant cells. Our findings indicate that restoring active ceramide to cells can resuscitate wild-type p53 function in p53-mutant cells, offering preclinical support for a novel type of mechanism-based therapy in the many human cancers harboring p53 mutations.

Apoptotic sphingolipid ceramide in cancer therapy

Apoptosis, also called programmed cell death, is physiologically and pathologically involved in cellular homeostasis. Escape of apoptotic signaling is a critical strategy commonly used for cancer tumorigenesis. Ceramide, a derivative of sphingolipid breakdown products, acts as second messenger for multiple extracellular stimuli including growth factors, chemical agents, and environmental stresses, such as hypoxia, and heat stress as well as irradiation. Also, ceramide acts as tumor-suppressor lipid because a variety of stress stimuli cause apoptosis by increasing intracellular ceramide to initiate apoptotic signaling. Defects on ceramide generation and sphingolipid metabolism are developed for cancer cell survival and cancer therapy resistance. Alternatively, targeting ceramide metabolism to correct these defects might provide opportunities to overcome cancer therapy resistance.

Cancer treatment strategies targeting sphingolipid metabolism

Ceramide and sphingosine-1-phosphate are related sphingolipid metabolites that can be generated through a de novo biosynthetic route or derived from the recycling of membrane sphingomyelin. Both these lipids regulate cellular responses to stress, with generally opposing effects. Sphingosine-1-phosphate functions as a growth and survival factor, acting as a ligand for a family of G protein-coupled receptors, whereas ceramide activates intrinsic and extrinsic apoptotic pathways through receptor-independent mechanisms. A growing body of evidence has implicated ceramide, sphingosine-1-phosphate and the genes involved in their synthesis, catabolism and signaling in various aspects of oncogenesis, cancer progression and drug- and radiation resistance. This may be explained in part by the finding that both lipids impinge upon the PI3K/ AKT pathway, which represses apoptosis and autophagy. In addition, sphingolipids influence cell cycle progression, telomerase function, cell migration and stem cell biology. Considering the central role of ceramide in mediating physiological as well as pharmacologically stimulated apoptosis, ceramide can be considered a tumor-suppressor lipid. In contrast, sphingosine-1-phosphate can be considered a tumor-promoting lipid, and the enzyme responsible for its synthesis functions as an oncogene. Not surprisingly, genetic mutations that result in reduced ceramide generation, increased sphingosine-1-phosphate synthesis or which reduce steady state ceramide levels and increase sphingosine-1-phosphate levels have been identified as mechanisms of tumor progression and drug resistance in cancer cells. Pharmacological tools for modulating sphingolipid pathways are being developed and represent novel therapeutic strategies for the treatment of cancer.

Acid ceramidase upregulation in prostate cancer cells confers resistance to radiation: AC inhibition, a potential radiosensitizer

Radiation resistance in a subset of prostate tumors remains a challenge to prostate cancer radiotherapy. The current study on the effects of radiation on prostate cancer cells reveals that radiation programs an unpredicted resistance mechanism by upregulating acid ceramidase (AC). Irradiated cells demonstrated limited changes of ceramide levels while elevating levels of sphingosine and sphingosine-1-phosphate. By genetically downregulating AC with small interfering RNA (siRNA), we observed radiosensitization of cells using clonogenic and cytotoxicity assays. Conversely, AC overexpression further decreased sensitivity to radiation. We also observed that radiation-induced AC upregulation was sufficient to create cross-resistance to chemotherapy as demonstrated by decreased sensitivity to Taxol and C(6) ceramide compared to controls. Lower levels of caspase 3/7 activity were detected in cells pretreated with radiation, also indicating increased resistance. Finally, utilization of the small molecule AC inhibitor, LCL385, sensitized PPC-1 cells to radiation and significantly decreased tumor xenograft growth. These data suggest a new mechanism of cancer cell resistance to radiation, through upregulation of AC that is, in part, mediated by application of the therapy itself. An improved understanding of radiotherapy and the application of combination therapy achieved in this study offer new opportunities for the modulation of radiation effects in the treatment of cancer.

Downregulating sphingosine kinase-1 for cancer therapy

BACKGROUND

The sphingolipids ceramide and sphingosine 1-phosphate (S1P) are key regulators of cell death and proliferation. The subtle balance between their intracellular levels is governed mainly by sphingosine kinase-1, which produces the pro-survival S1P. Sphingosine kinase-1 is an oncogene; is overexpressed in many tumors; protects cancer cells from apoptosis in vitro and in vivo; and its activity is decreased by anticancer therapies. Hence, sphingosine kinase-1 appears to be a target of interest for therapeutic manipulation.

OBJECTIVE

This review considers recent developments regarding the involvement of sphingosine kinase-1 as a therapeutic target for cancer, and describes the pharmacological tools currently available.

RESULTS/CONCLUSION

The studies described provide strong evidence that strategies to kill cancer cells via sphingosine kinase-1 inhibition are valid and could have a favorable therapeutic index.

Neutral sphingomyelinase-3 is a DNA damage and nongenotoxic stress-regulated gene that is deregulated in human malignancies

In this study, we report the characterization of a novel genotoxic and nongenotoxic stress-regulated gene that we had previously named as SKNY. Our results indicate that SKNY encodes the recently identified neutral sphingomyelinase-3 (nSMase3; hereafter SKNY is referred to as nSMase3). Examination of nSMase3 subcellular distribution reveals nSMase3 to localize to the endoplasmic reticulum (ER), and deletion of a COOH-terminal region containing its putative transmembrane domain and ER targeting signal partly alters its compartmentalization to the ER. Treatment with genotoxic Adriamycin and nongenotoxic tumor necrosis factor-alpha up-regulates endogenous nSMase3 expression, albeit with different kinetics. Tumor necrosis factor-alpha up-regulates nSMase3 expression within 2 h that lasts beyond 24 h and declines to control levels by 36 h. Adriamycin up-regulation of nSMase3 is transient, occurs within 30 min, and declines to control levels by 120 min. Prolonged treatment with Adriamycin by 24 h and beyond, however, causes a down-regulation in nSMase3 expression. Activation of wild-type p53 also down-regulates nSMase3 expression, suggesting that DNA damage-mediated nSMase3 down-regulation seems to occur partly through the tumor suppressor p53. Overexpression of exogenous nSMase3 sensitizes cells to Adriamycin-induced cell killing, a finding consistent with the proposed proapoptotic role of nSMase enzymes and nSMase-generated ceramide. We further investigated nSMase3 expression in various human malignancies and found its expression to be deregulated in several types of primary tumors when compared with their matching normal tissues. Collectively, our results have identified nSMase3 to be an important molecule that is linked to tumorigenesis and cellular stress response.

G2/M checkpoint stringency is a key parameter in the sensitivity of AML cells to genotoxic stress

Acute myeloid leukemia (AML) cells exposed to genotoxic agents arrest their cell cycle at the G2/M checkpoint and are inherently chemoresistant. To understand the mechanism of this chemoresistance, we compared the ability of immature CD34+ versus mature CD34- AML cell lines (KG1a and U937, respectively) to recover from a DNA damage-induced cell cycle checkpoint in G2. Here, we report that KG1a cells have a more stringent G2/M checkpoint response than U937 cells. We show that in both cell types, the CDC25B phosphatase participates in the G2/M checkpoint recovery and that its expression is upregulated. Furthermore, we show that CHK1 inhibition by UCN-01 in immature KG1a cells allows checkpoint exit and induces sensitivity to genotoxic agents. Similarly, UCN-01 treatment potentializes genotoxic-induced inhibition of colony formation efficiency of primary leukemic cells from AML patients. Altogether, our results demonstrate that checkpoint stringency varies during the maturation process and indicate that targeting checkpoint mechanisms might represent an attractive therapeutic opportunity for chemoresistant immature AML cells.

Sphingosine kinase-1--a potential therapeutic target in cancer

Sphingolipid metabolites play critical functions in the regulation of a number of fundamental biological processes including cancer. Whereas ceramide and sphingosine mediate and trigger apoptosis or cell growth arrest, sphingosine 1-phosphate promotes proliferation and cell survival. The delicate equilibrium between the intracellular levels of each of these sphingolipids is controlled by the enzymes that either produce or degrade these metabolites. Sphingosine kinase-1 is a crucial regulator of this two-pan balance, because it produces the prosurvival sphingosine 1-phosphate, and reduces the content of both ceramide and sphingosine, the proapoptotic sphingolipids. Sphingosine kinase-1 controls the levels of sphingolipids having opposite effects on cell survival/death, its gene was found to be of oncogenic nature, its mRNA is overexpressed in many solid tumors, its overexpression protects cells from apoptosis and its activity is decreased during anticancer treatments. Therefore, sphingosine kinase-1 appears to be a target of interest for therapeutic manipulation via its pharmacological inhibition. Strategies to kill tumor cells by increasing their ceramide and/or sphingosine content while blocking sphingosine 1-phosphate generation should have a favorable therapeutic index.

Penta-acetyl geniposide induce apoptosis in C6 glioma cells by modulating the activation of neutral sphingomyelinase-induced p75 nerve growth factor receptor and protein kinase Cdelta pathway

In our previous studies, we demonstrated the apoptotic cascades protein kinase C (PKC) delta/c-Jun NH2-terminal kinase (JNK)/Fas/caspases induced by penta-acetyl geniposide [(Ac)5GP]. However, the upstream signals mediating PKCdelta activation have not yet been clarified. Ceramide, mainly generated from the degradation of sphingomyelin, was hypothesized upstream above PKCdelta in (Ac)5GP-transduced apoptosis. Furthermore, nerve growth factor (NGF)/p75 is supposed to be involved because(Ac)5GP-induced apoptosis was demonstrated previously in glioma cells. In the present study, (Ac)5GP was shown to activate neutral sphingomyelinase (N-SMase) immediately, with its maximum at 15 min. The NGF and p75 enhanced by (Ac)5GP was inhibited when added with GW4869, the N-SMase inhibitor, indicating NGF/p75 as the downstream signals of N-SMase/ceramide. To investigate whether N-SMase is involved in (Ac)5GP-transduced apoptotic pathway, cells were treated with (Ac)5GP added with or without GW4869. It showed that N-SMase inhibition blocked FasL expression and caspase 3 activation. Likewise, p75 antagonist peptide attenuated the FasL/caspase 3 expression. The PKCdelta translocation induced by (Ac)5GP was also eliminated by GW4869 and p75 antagonist peptide. To further confirm whether N-SMase activation plays an important role in (Ac)5GP-induced apoptosis, cells were analyzed the apoptotic rate by 4', 6-diamidino-2-phenylindole (DAPI) staining. (Ac)5GP-induced apoptosis was reduced 40 and 80% by 10 and 20 microM GW4869, respectively. It indicated that N-SMase activation is pivotal in (Ac)5GP-mediated apoptosis. In conclusion, SMase and NGF/p75 are suggested to mediate upstream above PKCdelta, thus transducing FasL/caspase cascades in (Ac)5GP-induced apoptosis.

Overcoming MDR-associated chemoresistance in HL-60 acute myeloid leukemia cells by targeting sphingosine kinase-1

We examined the involvement of sphingosine kinase-1, a critical regulator of the sphingolipid balance, in susceptibility to antineoplastic agents of either sensitive or multidrug-resistant acute myeloid leukemia cells. Contrary to parental HL-60 cells, doxorubicin and etoposide failed to trigger apoptosis in chemoresistant HL-60/Doxo and HL-60NP16 cells overexpressing MRP1 and MDR1, respectively. Chemosensitive HL-60 cells displayed sphingosine kinase-1 inhibition coupled with ceramide generation. In contrast, chemoresistant HL-60/ Doxo and HL-60/VP16 had sustained sphingosine kinase-1 activity and did not produce ceramide during treatment. Enforced expression of sphingosine kinase-1 in chemosensitive HL-60 cells resulted in marked inhibition of apoptosis that was mediated by blockade of mitochondrial cytochrome c efflux hence suggesting a control of apoptosis at the pre-mitochondrial level. Incubation with cell-permeable ceramide of chemoresistant cells led to a sphingosine kinase-1 inhibition and apoptosis both prevented by sphingosine kinase-1 over-expression. Furthermore, F-12509a, a new sphingosine kinase inhibitor, led to ceramide accumulation, decrease in sphingosine 1-phosphate content and caused apoptosis equally in chemosensitive and chemoresistant cell lines that is inhibited by adding sphingosine 1-phosphate or overexpressing sphingosine kinase-1. F-12509a induced classical apoptosis hallmarks namely nuclear fragmentation, caspase-3 cleavage as well as downregulation of antiapoptotic XIAP, and release of cytochrome c and SMAC/Diablo.

Sphingolipid players in the leukemia arena

Sphingolipids function as bioactive mediators of different cellular processes, mostly proliferation, survival, differentiation and apoptosis, besides being structural components of cellular membranes. Involvement of sphingolipid metabolism in cancerogenesis was demonstrated in solid tumors as well as in hematological malignancies. Herein, we describe the main biological and clinical aspects of leukemias and summarize data regarding sphingolipids as mediators of apoptosis triggered in response to anti-leukemic agents and synthetic analogs as inducers of cell death as well. We also report the contribution of molecules that modulate sphingolipid metabolism to development of encouraging strategies for leukemia treatment. Finally we address how deregulation of sphingolipid metabolism is associated to occurrence of therapy resistance both in vitro and in vivo. Sphingolipids can be considered promising therapeutic tools alone or in combination with other compounds, as well as valid targets in the attempt to eradicate leukemia and overcome drug resistance.

Sphingolipids as modulators of cancer cell death: potential therapeutic targets

Through modifications in the fine membrane structure, cell-cell or cell-matrix interactions, and/or modulation of intracellular signaling pathways, sphingolipids can affect the tumorigenic potential of numerous cell types. Whereas ceramide and its metabolites have been described as regulators of cell growth and apoptosis, these lipids as well as other sphingolipid molecules can modulate the ability of malignant cells to grow and resist anticancer treatments, and their susceptibility to non-apoptotic cell deaths. This review summarizes our current knowledge on the properties of sphingolipids in the regulation of cancer cell death and tumor development. It also provides an update on the potential perspectives of manipulating sphingolipid metabolism and using sphingolipid analogues in anticancer therapy.

Apoptosis induced by intracellular ceramide accumulation in MDA-MB-435 breast carcinoma cells is dependent on the generation of reactive oxygen species

Strategies to promote intracellular ceramide accumulation in cancer cells may have therapeutic utility because ceramide is an important second messenger during apoptosis. Exposure to cell-permeable C(6) ceramide or tricyclodecan-9-yl-xanthate (an inducer of de novo ceramide synthesis and an inhibitor of sphingomyelin synthase) caused MDA-MB-435 human breast carcinoma cells to die by apoptosis. Concomitant treatment with the ceramidase inhibitor D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol (MAPP) or the glucosylceramide synthase inhibitor 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP) potentiated the cytotoxic effect of C(6) ceramide, indicating that C(6) ceramide-mediated cytotoxicity was antagonized by the action of ceramidases and glucosylceramide synthase. Interestingly, treatment with PPMP alone, but not MAPP alone, also induced apoptosis in MDA-MB-435 cells, suggesting that conversion to glucosylceramide rather than catabolism by ceramidases prevented endogenous ceramide from reaching cytotoxic levels. C(6) ceramide-induced apoptosis in MDA-MB-435 cells was associated with the generation of reactive oxygen species, and was inhibited by the antioxidants N-acetylcysteine and glutathione. Although mitochondrial membrane integrity was disrupted in C(6) ceramide-treated MDA-MB-435 cells, apoptosis was not mediated by caspases because there was no protective effect by the pan-caspase inhibitor z-VAD-fmk. Collectively, these findings indicate that strategies to enhance intracellular ceramide accumulation in malignant cells might offer a novel approach to the treatment of breast cancer.

Sphingosine kinase-1 as a chemotherapy sensor in prostate adenocarcinoma cell and mouse models

Systemic chemotherapy was considered of modest efficacy in prostate cancer until the recent introduction of taxanes. We took advantage of the known differential effect of camptothecin and docetaxel on human PC-3 and LNCaP prostate cancer cells to determine their effect on sphingosine kinase-1 (SphK1) activity and subsequent ceramide/sphingosine 1-phosphate (S1P) balance in relation with cell survival. In vitro, docetaxel and camptothecin induced strong inhibition of SphK1 and elevation of the ceramide/S1P ratio only in cell lines sensitive to these drugs. SphK1 overexpression in both cell lines impaired the efficacy of chemotherapy by decreasing the ceramide/S1P ratio. Alternatively, silencing SphK1 by RNA interference or pharmacologic inhibition induced apoptosis coupled with ceramide elevation and loss of S1P. The differential effect of both chemotherapeutics was confirmed in an orthotopic PC-3/green fluorescent protein model established in nude mice. Docetaxel induced a stronger SphK1 inhibition and ceramide/S1P ratio elevation than camptothecin. This was accompanied by a smaller tumor volume and the reduced occurrence and number of metastases. SphK1-overexpressing PC-3 cells implanted in animals developed remarkably larger tumors and resistance to docetaxel treatment. These results provide the first in vivo demonstration of SphK1 as a sensor of chemotherapy.

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.

Amyloid-beta peptide induces oligodendrocyte death by activating the neutral sphingomyelinase-ceramide pathway

Amyloid-beta peptide (Abeta) accumulation in senile plaques, a pathological hallmark of Alzheimer's disease (AD), has been implicated in neuronal degeneration. We have recently demonstrated that Abeta induced oligodendrocyte (OLG) apoptosis, suggesting a role in white matter pathology in AD. Here, we explore the molecular mechanisms involved in Abeta-induced OLG death, examining the potential role of ceramide, a known apoptogenic mediator. Both Abeta and ceramide induced OLG death. In addition, Abeta activated neutral sphingomyelinase (nSMase), but not acidic sphingomyelinase, resulting in increased ceramide generation. Blocking ceramide degradation with N-oleoyl-ethanolamine exacerbated Abeta cytotoxicity; and addition of bacterial sphingomyelinase (mimicking cellular nSMase activity) induced OLG death. Furthermore, nSMase inhibition by 3-O-methyl-sphingomyelin or by gene knockdown using antisense oligonucleotides attenuated Abeta-induced OLG death. Glutathione (GSH) precursors inhibited Abeta activation of nSMase and prevented OLG death, whereas GSH depletors increased nSMase activity and Abeta-induced death. These results suggest that Abeta induces OLG death by activating the nSMase-ceramide cascade via an oxidative mechanism.

Overcoming resistance to gamma-rays in squamous carcinoma cells by poly-drug elevation of ceramide levels

Recent strategies to sensitize radioresistant tumours are based on combining gamma-irradiation with inducers of apoptosis. We report that the combination of three inhibitors of sphingolipid metabolism, DL-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol.HCl(DL-PDMP)+imipramine +/- D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol (D-MAPP), with 10-Gy irradiation triggers both mitotic and apoptotic killing in radioresistant SQ20B squamous carcinoma cells. In these cells, apoptosis is defective due to a lack of ceramide generation upstream, which cannot be explained by sphingomyelinase (neutral and acidic) deficiency or rapid derivation to the sphingolipid pathway. We present evidence of a functional transduction death pathway when ceramide generation is restored, which involves the mitochondrial-mediated pathway coupled to alterations in redox status and to executive caspases activation. The poly-drug treatment restored apoptosis to levels similar to those observed in radiosensitive SCC61 squamous carcinoma cells. Simultaneous exposure to gamma-irradiation and poly-drug treatment acted synergistically in SQ20B cells to produce a marked increase in both mitochondrial dysfunction and caspase cleavage, which led to a 7.8-fold increase in apoptosis within 48 h, relative to irradiated cells. Moreover, the results suggest that the ceramide released by irradiation or poly-drug treatment converges upon common cellular targets. Modulation of endogenous ceramide levels by inhibitors of sphingolipid metabolism may represent a new cellular target for the sensitization of radioresistant tumours to gamma-ray therapy.

Liposomal delivery enhances short-chain ceramide-induced apoptosis of breast cancer cells

It is therapeutically desirable to effectively deliver ceramide, an antimitogenic and proapoptotic lipid second messenger, to transformed cell types. However, the targeted delivery of cell-permeable ceramide analogs, including C6-ceramide, to cells may be impeded by the hydrophobicity of these bioactive lipids, resulting in reduced efficacy. The objective of this study is to develop and optimize liposomal vehicles to augment ceramide delivery to a breast adenocarcinoma cell line. We designed conventional, cationic, and pegylated drug release vesicles to efficaciously deliver ceramide to MDA-MB-231 breast adenocarcinoma cells. In vitro pharmacokinetic analysis demonstrated that liposomal ceramide delivery resulted in significantly greater accumulation of ceramide in MDA-MB-231 cells. Ceramide-formulated liposomes significantly inhibited MDA-MB-231 cell proliferation as compared with nonliposomal administration of ceramide. Ceramide-induced apoptosis correlated with the pharmacokinetic profile and the diminished proliferation in this highly aggressive, metastatic cell line. Liposomal ceramide formulations inhibited phosphorylated Akt levels and stimulated caspase-3/7 activity more effectively than nonliposomal ceramide, events consistent with apoptosis. Together, these results indicate that bioactive ceramide analogs can be incorporated into conventional, cationic, or pegylated liposomal vehicles for improved drug delivery and release.

Radiation and ceramide-induced apoptosis

Ceramide is a sphingolipid that acts as a second messenger in ubiquitous, evolutionarily conserved, signaling systems. Emerging data suggest that radiation acts directly on the plasma membrane of several cell types, activating acid sphingomyelinase, which generates ceramide by enzymatic hydrolysis of sphingomyelin. Ceramide then acts as a second messenger in initiating an apoptotic response via the mitochondrial system. Radiation-induced DNA damage can also initiate ceramide generation by activation of mitochondrial ceramide synthase and de novo synthesis of ceramide. In some cells and tissues, BAX is activated downstream of ceramide, regulating commitment to the apoptotic process via release of mitochondrial cytochrome c. Genetic and pharmacologic studies in vivo showed that radiation targets the acid sphingomyelinase apoptotic system of microvascular endothelial cells in the lungs, intestines and brain, as well as in oocytes, to initiate the pathogenesis of tissue damage. Regulated ceramide metabolism may produce metabolites, such as sphingosine 1-phosphate, shown to signal antiapoptosis, thus controlling the intensity of the apoptotic response and constituting a mechanism for radiation sensitivity or resistance. An improved understanding of this signaling system may offer new opportunities for the modulation of radiation effects in the treatment of cancer.

Increasing endogenous ceramide using inhibitors of sphingolipid metabolism maximizes ionizing radiation-induced mitochondrial injury and apoptotic cell killing

To enhance the killing effects of ionizing radiation, we amplified the endogenous ceramide signal in Jurkat cell cultures using 3 different inhibitors of sphingolipid metabolism: DL-PDMP, D-MAPP and imipramine. Of the various possible drug combinations, only DL-PDMP (20 microM) + imipramine (20 microM) and DL-PDMP (20 microM) + imipramine (20 microM) + D-MAPP (5 microM) induced a major increase in ceramide levels, reaching 240% and 340% of control values, respectively, after incubation for 48 hr. With these models, we demonstrate that endogenously formed ceramide triggers time- and concentration-dependent apoptosis through induction of mitochondrial injury and activation of the caspase pathway. Cellular dysfunction includes alterations to the cellular redox potential, as assessed by the generation of ROS and total glutathione depletion, and a drop in Delta Psi(m). A parallel elevation of mitochondrial ceramide levels was also observed. The combination of DL-PDMP + imipramine +/- D-MAPP with 10 Gy irradiation produced cumulative effects leading to apoptosis via mitochondrial collapse and activation of the caspase cascade. The association efficiency was confirmed in normal and acid sphingomyelinase-deficient lymphoid cell lines. Taken together, these results suggest that increasing endogenous ceramide levels may potentially be very valuable when combined with ionizing radiation in tumor therapy.

Kit signaling inhibits the sphingomyelin-ceramide pathway through PLCγ1: implication in stem cell factor radioprotective effect

Previous studies demonstrated that Kit activation confers radioprotection. However, the mechanism by which Kit signaling interferes with cellular response to ionizing radiation (IR) has not been firmly established. Based on the role of the sphingomyelin (SM) cycle apoptotic pathway in IR-induced apoptosis, we hypothesized that one of the Kit signaling components might inhibit IR-induced ceramide production or ceramide-induced apoptosis. Results show that, in both Ba/F3 and 32D murine cell lines transfected with wild-type c-kit, stem cell factor (SCF) stimulation resulted in a significant reduction of IR-induced apoptosis and cytotoxicity, whereas DNA repair remained unaffected. Moreover, SCF stimulation inhibited IR-induced neutral sphingomyelinase (N-SMase) stimulation and ceramide production. The SCF inhibitory effect on SM cycle was not influenced by wortmannin, a phosphoinositide-3 kinase (PI3K) inhibitor. The SCF protective effect was maintained in 32D-KitYF719 cells in which the PI3K/Akt signaling pathway is abolished due to mutation in Kit docking site for PI3K. In contrast, phospholipase C γ (PLCγ) inhibition by U73122 totally restored IR-induced N-SMase stimulation, ceramide production, and apoptosis in Kit-activated cells. Moreover, SCF did not protect 32D-KitYF728 cells (lacking a functional docking site for PLCγ1), from IR-induced SM cycle. Finally, SCF-induced radioprotection of human CD34+ bone marrow cells was also inhibited by U73122. Altogether, these results suggest that SCF radioprotection is due to PLCγ1-dependent negative regulation of IR-induced N-SMase stimulation. Beyond the scope of Kit-expressing cells, it suggests that PLCγ1 status could greatly influence the post-DNA damage cellular response to IR, and perhaps, to other genotoxic agents.

Ceramide in apoptosis: a revisited role

The sphingolipid ceramide has recently emerged as a new transducer or modulator of apoptotic cell death. This function, however, has recently been challenged. Here, in the light of recent observations, the role of ceramide in apoptosis signaling is discussed.

Elevation of glucosylceramide in multidrug-resistant cancer cells and accumulation in cytoplasmic droplets

Multidrug-resistant (MDR) cancer cells have been shown to have an accumulation of glucosylceramide (GlcCer). In this study, we aim at localizing, at subcellular level, where these lipids accumulate. Neutral lipids and phospholipid containing organelles have been identified using confocal fluorescence microscopy and microspectrofluorometry by monitoring the emission of the fluorescent probe Nile-red. Data from confocal fluorescence microscopy analysis shows accumulation of neutral lipids in cytoplasmic droplets of MDR human carcinoma MCF7R cells. Microspectrofluorometric measurements show an increase of the gold-yellow emission intensity in MCF7R cells, corresponding to neutral lipids. Similar observations were made in human MDR vincristine-HL60 and doxorubicin-KB selected cells. Total cellular glucosylceramide (GlcCer) measurements using [(3)H]-palmitic acid and thin layer chromatography show a significant increase of GlcCer in MCF7R cells. Moreover, MCF7R cells treated with fluorescent GlcCer-bodipy exhibit an accumulation of this lipid in cytoplasmic droplets. Treatment of MCF7R cells with 1-phenyl-2-palmitoylamino-3-morpholino-1-propanolol (PPMP), a potent inhibitor of GlcCer synthase, attenuates the Nile-red fluorescence emission emanating from these structures and reverses MDR. Moreover, Golgi compartments stained with fluorescent PPMP-bodipy, show an increase in the Golgi compartments density. Treatment of MCF7R cells with cyclosporine A (CSA), tamoxifen (TMX) and 3'-azido-3'deoxythymidine (AZT) leads to the same effect observed in the presence of PPMP. Treatment of MCF7 and MCF7R with the beta-glucosidase inhibitor conduritol beta-epoxide (CBE) significantly increases resistance to daunorubicin only in MCF7R cells. These data demonstrate also that: (i) CSA, an inhibitor of MDR, has an additional target in addition to P-glycoprotein; and (ii) TMX (used in breast cancer treatment and prevention) and AZT (used in the treatment of HIV) could have side effects by disturbing lipid metabolism and inhibiting many cellular functions required in normal cells.

Kit signaling and negative regulation of daunorubicin-induced apoptosis: role of phospholipase Cgamma

Previous studies have demonstrated that activation of Kit by stem cell factor (SCF), its natural ligand, or by gain-of-function point mutation in its intracellular domain, confers significant protection against apoptosis induced by growth factor deprivation or radiation. However, the effects of Kit activation on the cellular response to anti-tumor agents have not been so extensively documented. This study shows that daunorubicin-induced apoptosis and cytotoxicity were reduced in the murine Ba/F3 cells transfected with Kit (Ba/F3-Kit) in the presence of SCF and in Ba/F3 cells transfected with a constitutively active Kit variant (Ba/F3-KitDelta27), compared to either parental Ba/F3 (Ba/F3-wt) or unstimulated Ba/F3-Kit cells. In Ba/F3-wt and in Ba/F3-Kit cells, daunorubicin stimulated within 8-15 min neutral sphingomyelinase and ceramide production but not in SCF-stimulated Ba/F3-Kit or in Ba/F3-KitDelta27 whereas all Ba/F3 cells were equally sensitive to exogenous cell-permeant C6-ceramide. In Ba/F3-Kit, SCF-induced Kit activation resulted in a rapid phospholipase Cgamma (PLCgamma) tyrosine phosphorylation followed by diacylglycerol release and protein kinase C (PKC) stimulation. U-73122, a PLCgamma inhibitor, not only blocked diacylglycerol production and PKC stimulation but also restored daunorubicin-induced sphingomyelinase stimulation, ceramide production, and apoptosis. These results suggest that Kit activation results in PLCgamma-mediated PKC-dependent sphingomyelinase inhibition which contributes to drug resistance in Kit-related malignancies.

Natural ceramide reverses Fas resistance of acid sphingomyelinase(-/-) hepatocytes

The role of the second messenger ceramide in Fas-mediated death requires clarification. To address this issue, we generated hepatocytes from paired acid sphingomyelinase (ASMase; asmase)(+/+) and asmase(-/-) mice. asmase(-/-) hepatocytes, derived from 8-week-old mice, manifested normal sphingomyelin content and normal morphological, biochemical, and biologic features. Nonetheless, ASMase-deficient hepatocytes did not display rapid ceramide elevation or apoptosis in response to Jo2 anti-Fas antibody. asmase(-/-) hepatocytes were not inherently resistant to apoptosis because staurosporine, which did not induce early ceramide elevation, stimulated a normal apoptotic response. The addition of low nanomolar quantities of natural C16-ceramide, which by itself did not induce apoptosis, completely restored the apoptotic response to anti-Fas in asmase(-/-) hepatocytes. Other sphingolipids did not replace natural ceramide and restore Fas sensitivity. Overcoming resistance to Fas in asmase(-/-) hepatocytes by natural ceramide is evidence that it is the lack of ceramide and not ASMase which determines the apoptotic phenotype. The ability of natural ceramide to rescue the phenotype without reversing the genotype provides evidence that ceramide is obligate for Fas induction of apoptosis in hepatocytes.

Lysosomal sphingomyelinase is not solicited for apoptosis signaling

Stress-induced activation of an acidic sphingomyelinase leading to generation of ceramide, an important lipid mediator, has been associated with apoptosis; however, the implication of this hydrolase has been questioned. The present study aimed at re-evaluating the role of this lysosomal enzyme in apoptosis initiated by different apoptotic inducers. The sensitivity of a series of acid sphingomyelinase-deficient cell lines derived from Niemann-Pick disease patients to stress-induced apoptosis was investigated. We have now shown that stress stimuli, such as anthracyclines, ionizing radiation, and Fas ligation trigger similar apoptotic hallmarks in normal and acid sphingomyelinase-deficient cell lines. Retrovirus-mediated gene correction of enzyme deficiency in Niemann-Pick cells does not modify response to apoptosis. Ceramide production is comparable in normal and Niemann-Pick cells, and increased activity of neutral sphingomyelinase is observed. Thus, our findings cast serious doubts that lysosomal sphingomyelinase activation is responsible for stress-induced apoptosis of cultured cells.

Cell autonomous apoptosis defects in acid sphingomyelinase knockout fibroblasts

A body of evidence suggests that stress-induced sphingomyelin hydrolysis to the second messenger ceramide initiates apoptosis in some cells. Although studies using lymphoblasts from Niemann-Pick disease patients or acid sphingomyelinase (ASMase)-deficient mice have provided genetic support for this hypothesis, these models have not been universally accepted as definitive. Here, we show that mouse embryonic fibroblasts (MEFs) prepared from asmase mice manifest cell autonomous defects in apoptosis in response to several stresses. In particular, asmase(-/-) MEFs failed to generate ceramide and were totally resistant to radiation-induced apoptosis but remained sensitive to staurosporine, which did not induce ceramide. asmase(-/-) MEFs were also partially resistant to tumor necrosis factor alpha/ actinomycin D and serum withdrawal. Thus, resistance to apoptosis in asmase(-/-) MEFs was not global but rather stress type specific. Most importantly, the sensitivity to stress could be restored in the asmase(-/-) MEFs by administration of natural ceramide. Overcoming apoptosis resistance by natural ceramide is evidence that it is the lack of ceramide, not ASMase, that determines apoptosis sensitivity. The ability to rescue the apoptotic phenotype without reversing the genotype by the product of the enzymatic deficiency provides proof that ceramide is obligate for apoptosis induction in response to some stresses.

Activation of a nuclear sphingomyelinase in radiation-induced apoptosis

The subcellular origin of ceramide signaling in ionizing radiation-triggered apoptosis was investigated using two previously described subclones of the autonomous erythro-myeloblastic cell line TF-1, radio-resistant and -sensitive TF-1-34 and TF-1-33, respectively. We show in nuclei-free lysates and cytoplasts that both cell lines failed to generate ceramide in response to ionizing radiation. Moreover, whereas cytoplasts did respond to anti-Fas stimulation through phosphatidylserine externalization, no effect was observed with ionizing radiation. Only in highly purified nuclei preparations did we observe ceramide generation, neutral sphingomyelinase activation, and apoptotic features (PARP cleavage, nuclear fragmentation, DNA laddering) in TF-1-33, but not in TF-1-34 cells. These observations suggest that nuclear sphingomyelinase and ceramide formation may contribute to ionizing radiation-triggered apoptosis.

Growth arrest and cell death in the breast tumor cell in response to ionizing radiation and chemotherapeutic agents which induce DNA damage

Breast tumor cells are relatively refractory to apoptosis in response to modalities which induce DNA damage such as ionizing radiation and the topoisomerase II inhibitor, adriamycin. Various factors which may modulate the apoptotic response to DNA damage include the p53 status of the cell, levels and activity of the Bax and Bcl-2 families of proteins, activation of NF-kappa B, relative levels of insulin like growth factor and insulin-like growth factor binding proteins, activation of MAP kinases and PI3/Akt kinases, (the absence of) ceramide generation and the CD95 (APO1/Fas) signaling pathway. Prolonged growth arrest associated with replicative senescence may represent an alternative and reciprocal response to DNA-damage induced apoptosis that is p53 and/or p21waf1/cip1 dependent while delayed apoptosis may occur in p53 mutant breast tumor cells which fail to maintain the growth-arrested state. Clearly, the absence of an immediate apoptotic response to DNA damage does not eliminate other avenues leading to cell death and loss of self-renewal capacity in the breast tumor cell. Nevertheless, prolonged growth arrest (even if ultimately succeeded by apoptotic or necrotic cell death) could provide an opportunity for subpopulations of breast tumor cells to recover proliferative capacity and to develop resistance to subsequent clinical intervention.

Ceramide mediates radiation-induced death of endothelium

The sphingomyelin (SM) pathway is an ubiquitous, evolutionarily conserved signaling system, analogous to conventional systems such as the cAMP and phosphoinositide pathways. Ceramide is generated from SM by the action of a neutral or acid SMase, or by de novo synthesis coordinated through the enzyme ceramide synthase. Once generated, ceramide may serve as a second messenger in signaling responses to physiologic or environmental stimuli, or may be converted to a variety of structural or effector molecules. In the radiation response, ceramide serves as a second messenger in initiating apoptosis, while some of its metabolites block apoptosis. In certain cells, such as endothelial, lymphoid and haematopoietic cells, ceramide mediates apoptosis while in others ceramide may serve only as a co-signal for or play no role in the death response. Regulated ceramide metabolism may determine the balance between pro- and anti-apoptotic signals, and hence, the intensity of the apoptotic response, thus constituting a mechanism of radiation sensitivity or resistance. This paradigm may offer new opportunities for modulation of the radiation effects in the treatment of cancer. Chemical modifiers of ceramide metabolism may be useful to enhance the therapeutic effects or reduce the toxicity of radiation treatment.

Stress-induced apoptosis is not mediated by endolysosomal ceramide

A major lipid-signaling pathway in mammalian cells implicates the generation of ceramide from the ubiquitous sphingolipid sphingomyelin (SM). Hydrolysis of SM by a sphingomyelinase present in acidic compartments has been reported to mediate, via the production of ceramide, the apoptotic cell death triggered by stress-inducing agents. In the present study, we investigated whether the ceramide formed within or accumulated in lysosomes indeed triggers apoptosis. A series of observations strongly suggests that ceramide involved in stress-induced apoptosis is not endolysosomal: 1) Although short-chain ceramides induced apoptosis, loading cells with natural ceramide through receptor-mediated endocytosis did not result in cell death. 2) Neither TNF-alpha nor anti-CD95 induced the degradation to ceramide of a natural SM that had been first introduced selectively into acidic compartments. 3) Stimulation of SV40-transformed fibroblasts by TNF-alpha or CD40 ligand resulted in apoptosis equally well in cells derived from control individuals and from patients affected with Farber disease, having a genetic defect of acid ceramidase activity leading to lysosomal accumulation of ceramide. Also, induction of apoptosis using anti-CD95 (Fas) or anti-CD40 antibodies, TNF-alpha, daunorubicin, and ionizing radiation was similar in control and Farber disease lymphoid cells. In all cases, apoptosis was preceded by a comparable increase of intracellular ceramide levels. 4) Retroviral-mediated gene transfer and overexpression of acid ceramidase in Farber fibroblasts, which led to complete metabolic correction of the ceramide catabolic defect, did not affect the cell response to TNF-alpha and CD40 ligand.

Molecular mechanisms of ionizing radiation-induced apoptosis

Ionizing radiation activates not only signalling pathways in the nucleus as a result of DNA damage, but also signalling pathways initiated at the level of the plasma membrane. Proteins involved in DNA damage recognition include poly(ADP ribose) polymerase (PARP), DNA-dependent protein kinase, p53 and ataxia- telangiectasia mutated (ATM). Many of these proteins are inactivated by caspases during the execution phase of apoptosis. Signalling pathways outside the nucleus involve tyrosine kinases such as stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK), protein kinase C, ceramide and reactive oxygen species. Recent evidence shows that tumour cells resistant to ionizing radiation-induced apoptosis have defective ceramide signalling. How these signalling pathways converge to activate the caspases is presently unknown, although in some cell types a role for calpain has been suggested.