Ceramide-induced killing of normal and malignant human lymphocytes is by a non-apoptotic mechanism

C6-ceramide nanoliposomes target the Warburg effect in chronic lymphocytic leukemia

Ceramide is a sphingolipid metabolite that induces cancer cell death. When C6-ceramide is encapsulated in a nanoliposome bilayer formulation, cell death is selectively induced in tumor models. However, the mechanism underlying this selectivity is unknown. As most tumors exhibit a preferential switch to glycolysis, as described in the "Warburg effect", we hypothesize that ceramide nanoliposomes selectively target this glycolytic pathway in cancer. We utilize chronic lymphocytic leukemia (CLL) as a cancer model, which has an increased dependency on glycolysis. In CLL cells, we demonstrate that C6-ceramide nanoliposomes, but not control nanoliposomes, induce caspase 3/7-independent necrotic cell death. Nanoliposomal ceramide inhibits both the RNA and protein expression of GAPDH, an enzyme in the glycolytic pathway, which is overexpressed in CLL. To confirm that ceramide targets GAPDH, we demonstrate that downregulation of GAPDH potentiates the decrease in ATP after ceramide treatment and exogenous pyruvate treatment as well as GAPDH overexpression partially rescues ceramide-induced necrosis. Finally, an in vivo murine model of CLL shows that nanoliposomal C6-ceramide treatment elicits tumor regression, concomitant with GAPDH downregulation. We conclude that selective inhibition of the glycolytic pathway in CLL cells with nanoliposomal C6-ceramide could potentially be an effective therapy for leukemia by targeting the Warburg effect.

Accelerated growth of skin carcinoma following fludarabine therapy for chronic lymphocytic leukemia

We present four patients with chronic lymphocytic leukemia treated with fludarabine, who developed aggressive skin cancer after years of quiescence, a short time after institution of treatment. Their leukemias responded well to therapy with fludarabine with initial treatment as well as relapse. Three patients had recurrence with basal cell carcinomas with multiple, rapidly growing tumors and one had recurrence of both basal and squamous cancers and eventually died of metastatic squamous cell carcinoma. Fludarabine induces prolonged period of lymphopenia, affecting especially the T cell population, which is crucial in the defense against skin cancers. There appears to be a direct association between fludarabine and the flare up of skin cancers in these patients, possibly analogous to the increased incidence of these malignancies in patients on chronic cyclosporine immunosuppression.

The pivotal role of c-Jun NH2-terminal kinase-mediated Beclin 1 expression during anticancer agents-induced autophagy in cancer cells

The c-Jun NH2-terminal kinase (JNK) pathway represents one subgroup of MAP kinases that are activated primarily by cytokines and exposure to environmental stress. Autophagy is a protein-degradation system characterized by the formation of double-membrane vacuoles termed autophagosomes. Autophagy-related gene beclin 1 plays a key role in autophagosome formation. However, the relationships between activation of JNK pathway, autophagy induction and Beclin 1 expression remain elusive. In this study, we used human cancer cell lines CNE2 and Hep3B to investigate the role of JNK-mediated Beclin 1 expression in ceramide-induced autophagic cell death. Ceramide-treated cells exhibited the characteristics of autophagy (that is, acidic vesicular organelle formation and the LC3-II generation). JNK was activated in these two cell lines exposed to ceramide and the phosphorylation of c-Jun also increased. In the meantime, we found that ceramide upregulated Beclin 1 expression in cancer cells. The upregulation of Beclin 1 expression could be blocked by SP600125 (a specific inhibitor of JNK) or a small interfering RNA (siRNA) directed against JNK1/2 or c-Jun. Chromatin immunoprecipitation and luciferase reporter analysis revealed that c-Jun was involved in the regulation of beclin 1 transcription in response to ceramide treatment. In addition, inhibition of JNK activity by SP600125 could inhibit autophagy induction by ceramide. Furthermore, Beclin 1 knockdown by siRNA also inhibited ceramide-mediated autophagic cell death. JNK-mediated Beclin 1 expression was also observed in topotecan-induced autophagy. These data suggest that activation of JNK pathway can mediate Beclin 1 expression, which plays a key role in autophagic cell death in cancer cells.

Sphingolipids and the sphingosine kinase inhibitor, SKI II, induce BCL-2-independent apoptosis in human prostatic adenocarcinoma cells

BACKGROUND

Elevated BCL-2 is one mechanism of therapeutic resistance in prostate cancer (PC), and new approaches are needed to overcome such resistance.

METHODS

We evaluated the effects of BCL-2 over-expression in human prostatic adenocarcinoma cells on their susceptibility to sphingolipids (SLs) and to the sphingosine kinase (SpK) inhibitor, SKI II.

RESULTS

In survival assays, no significant differences were observed in the responses to sphingosine or ceramide among parental PC-3 cells lacking detectable BCL-2 and BCL-2 over-expressing PC-3 transfectants; similarly, the responses to dimethyl-sphingosine (DMSP) of parental LNCaP cells and a BCL-2 over-expressing LNCaP transfectant were equivalent. SKI II induced protracted, BCL-2-independent survival loss in both PC-3 and LNCaP parental/transfectant pairs; in contrast, DMSP induced rapid cell shrinkage, caspase activation and caspase-dependent DNA fragmentation. DMSP-induced DNA fragmentation and loss of mitochondrial membrane potential were equivalent in BCL-2 transfectants and parental PC-3 cells and were not associated with BCL-2 downregulation. DMSP-mediated cytotoxicity was not associated with the enhanced production of reactive oxygen intermediates. SL analyses of parental and transfectant PC-3 cells did not reveal increased levels of sphingosine-1-phosphate in the BCL-2 transfectants; further, there only a modest early shift, corresponding to apoptotic onset, in pro- versus anti-apoptotic SLs in response to DMSP treatment.

CONCLUSIONS

Thus, in contrast to the inhibitory effects of BCL-2 on apoptosis induced by various agents in tumor cells, SKI II and selected pro-apoptotic SLs appear atypical in their independence from such inhibition, and may have merits as new candidates for treatment of AI PC.

Anticancer activity of a ceramide analog containing a disulfide linkage

The effects of exogenous short-chain ceramide (1) on the arrest of growth of cancer cells in vitro and induction of apoptosis have been well documented. In the present study, an analog of 1 with a disulfide linkage, N-(4',5'-dithiaheptanoyl)-D-erythro-ceramide (2), was synthesized and found to be significantly more antiproliferative and cytotoxic than 1 in BT549, A549, and DU145 cancer cells. The activity was correlated with a reduction in cellular glutathione (GSH) level. Ceramide analogs with a tri- and tetra-sulfide moiety were also prepared, but they did not deplete cellular GSH levels and did not possess antiproliferative or cytotoxic properties.

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.

Beyond apoptosis: nonapoptotic cell death in physiology and disease

Apoptosis is a morphologically defined form of programmed cell death (PCD) that is mediated by the activation of members of the caspase family. Analysis of death-receptor signaling in lymphocytes has revealed that caspase-dependent signaling pathways are also linked to cell death by nonapoptotic mechanisms, indicating that apoptosis is not the only form of PCD. Under physiological and pathological conditions, cells demonstrate a high degree of flexibility in cell-death responses, as is reflected in the existence of a variety of mechanisms, including necrosis-like PCD, autophagy (or type II PCD), and accidental necrosis. In this review, we discuss recent data suggesting that canonical apoptotic pathways, including death-receptor signaling, control caspase-dependent and -independent cell-death pathways.Key words: apoptosis, necrosis, nonapoptotic programmed cell death, death receptors, ceramides.

P-glycoprotein is implicated in the inhibition of ceramide-induced apoptosis in TF-1 acute myeloid leukemia cells by modulation of the glucosylceramide synthase pathway

OBJECTIVE

Ceramide, an intermediate of apoptosis induction in response to chemotherapy, can be detoxified by glycosylation at the cytoplasmic surface of the Golgi membrane. P-glycoprotein (p-gp) might augment ceramide glycosylation by translocating glucosylceramide (GC) across the Golgi membrane. We aimed to show that glucosylceramide synthase (GCS) activity is linked to p-gp expression and resistance to ceramide-induced apoptosis in acute myeloid leukemia (AML).

METHODS

Apoptosis and cell-cycle analysis were measured using propidium iodide staining and flow cytometry. Fluorescent microscopy assessed p-gp expression in, and rhodamine 123 uptake by, the Golgi. P-gp interaction with GC was assessed by modulation of rhodamine accumulation. The GCS activity assay was based upon the transfer of UDP-(3)H-glucose to C8-ceramide to form radiolabeled GC, by rate-limiting cell-derived GCS. TLC and fluorimetry were used to measure the metabolites of fluorescent ceramide. Cell viability was measured using 7-amino-actinomycin D staining and flow cytometry with an internal standard for cell enumeration.

RESULTS

P-gp(+) cell lines (KG1a, TF-1) were resistant to C8-ceramide-induced apoptosis compared to p-gp(-) cell lines (HL-60, U937). P-gp inhibitors GF120918 and cyclosporin A enhanced ceramide-induced apoptosis in the p-gp expressing cells. P-gp expression was identified in the Golgi of these cells. Pgp's efflux function in TF-1 but not KG1a cells was inhibited by glucosylceramide. In the presence of p-gp inhibitors, R123 accumulation in the Golgi of TF-1 cells was lost, and GCS activity and lactosylceramide formation were downregulated. Intact cells were necessary for the involvement of p-gp in the regulation of GCS activity.

CONCLUSION

Our data suggests that ceramide induces apoptosis in AML cells and that p-gp confers resistance to ceramide-induced apoptosis, with modulation of the ceramide-glucosylceramide pathway making a marked contribution to this resistance in TF-1 cells.

Pivotal role of the cell death factor BNIP3 in ceramide-induced autophagic cell death in malignant glioma cells

The sphingolipid ceramide has been recognized as an important second messenger implicated in regulating diverse signaling pathways especially for apoptosis. Very little is known, however, about the molecular mechanisms underlying nonapoptotic cell death induced by ceramide. In the present study, we first demonstrate that ceramide induces nonapoptotic cell death in malignant glioma cells. The cell death was accompanied by several specific features characteristic of autophagy: presence of numerous autophagic vacuoles in the cytoplasm, development of the acidic vesicular organelles, autophagosome membrane association of microtubule-associated protein light chain 3 (LC3), and a marked increase in expression levels of two forms of LC3 protein (LC3-I and LC3-II). We additionally demonstrate that ceramide decreases mitochondrial membrane potential and activates the transcription of death-inducing mitochondrial protein, BNIP3, resulting in increased expression levels of its mRNA and protein in malignant glioma cells. Moreover, tumor cells transfected with BNIP3 gene undergo autophagy in the absence of ceramide. These results suggest that ceramide induces autophagic cell death in malignant glioma cells via activation of BNIP3. This study adds a new concept to characterize the pathways by which ceramide acts to induce nonapoptotic autophagic cell death in malignant gliomas.

The histone deacetylase inhibitor MS-275 interacts synergistically with fludarabine to induce apoptosis in human leukemia cells

Interactions between the novel benzamide histone deacetylase (HDAC) inhibitor MS-275 and fludarabine were examined in lymphoid and myeloid human leukemia cells in relation to mitochondrial injury, signal transduction events, and apoptosis. Prior exposure of Jurkat lymphoblastic leukemia cells to a marginally toxic concentration of MS-275 (e.g., 500 nM) for 24 h sharply increased mitochondrial injury, caspase activation, and apoptosis in response to a minimally toxic concentration of fludarabine (500 nM), resulting in highly synergistic antileukemic interactions and loss of clonogenic survival. Simultaneous exposure to MS-275 and fludarabine also led to synergistic effects, but these were not as pronounced as observed with sequential treatment. Similar interactions were noted in the case of (a) other human leukemia cell lines (e.g., U937, CCRF-CEM); (b) other HDAC inhibitors (e.g., sodium butyrate); and (c) other nucleoside analogues (e.g., 1-beta-D-arabinofuranosylcytosine, gemcitabine). Potentiation of fludarabine lethality by MS-275 was associated with acetylation of histones H3 and H4, down-regulation of the antiapoptotic proteins XIAP and Mcl-1, enhanced cytosolic release of proapoptotic mitochondrial proteins (e.g., cytochrome c, Smac/DIABLO, and apoptosis-inducing factor), and caspase activation. It was also accompanied by the caspase-dependent down-regulation of p27(KIP1), cyclins A, E, and D(1), and cleavage and diminished phosphorylation of retinoblastoma protein. However, increased lethality of the combination was not associated with enhanced fludarabine triphosphate formation or DNA incorporation and occurred despite a slight reduction in the S-phase fraction. Prior exposure to MS-275 attenuated fludarabine-mediated activation of MEK1/2, extracellular signal-regulated kinase, and Akt, and enhanced c-Jun NH(2)-terminal kinase phosphorylation; furthermore, inducible expression of constitutively active MEK1/2 or Akt significantly diminished MS-275/fludarabine-induced lethality. Combined exposure of cells to MS-275 and fludarabine was associated with a significant increase in generation of reactive oxygen species; moreover, both the increase in reactive oxygen species and apoptosis were largely attenuated by coadministration of the free radical scavenger L-N-acetylcysteine. Finally, prior administration of MS-275 markedly potentiated fludarabine-mediated generation of the proapoptotic lipid second messenger ceramide. Taken together, these findings indicate that the HDAC inhibitor MS-275 induces multiple perturbations in signal transduction, survival, and cell cycle regulatory pathways that lower the threshold for fludarabine-mediated mitochondrial injury and apoptosis in human leukemia cells. They also provide insights into possible mechanisms by which novel, clinically relevant HDAC inhibitors might be used to enhance the antileukemic activity of established nucleoside analogues such as fludarabine.

Synergistic antiproliferative and apoptotic effects induced by epidermal growth factor receptor and protein kinase a inhibitors in human prostatic cancer cell lines

Our results revealed that the blockade of epidermal growth factor receptor (EGFR) tyrosine kinase and protein kinase A (PKA) signaling pathways by specific inhibitors (PD153035 and Rp-cAMPs) leads to a synergistic inhibition of EGF- and serum-stimulated growth of human prostatic cancer cells (LNCaP, DU145 and PC3) concomitant with an arrest in the G1 phase of cellular cycle. Of particular interest, the combination of PD153035 and Rp-cAMPs also caused a more substantial apoptotic/necrotic death of these prostatic cancer cells as compared to drugs alone. Moreover, we observed that the inhibition of acidic sphingomyelinase and caspase cascades results in a marked reduction of DNA fragmentation and apoptotic death induced by PD153035, alone or in combination with Rp-cAMPs, in EGF stimulated PC3 cells. This suggests that these agents might mediate their cytotoxic effects at least in part via the ceramide generation and activation of caspase signaling pathways. N-oleoylethanolamine (OE), an inhibitor of acidic ceramidase, consistently potentiated the apoptotic effects of PD153035 in all the prostatic cancer cell lines tested. Additionally, the cellular ceramide content estimated for PC3 cells was increased after treatment with PD153035, alone or in combination, at a lower dose with OE and Rp-cAMPs. The synergistic apoptotic effect of PD153035 plus Rp-cAMPs induced in PC3 was also accompanied by a significant rate of mitochondrial membrane depolarization and release of cytochrome c into cytosol as compared to drugs alone. Combined, the results indicated that the simultaneous inhibition of EGFR and PKA signaling cascades might lead to a more massive apoptotic death of metastatic prostatic cancer cells by increasing ceramide accumulation and activating of caspase cascade of a mitochondrial dependent manner.

New advances on structural and biological functions of ceramide in apoptotic/necrotic cell death and cancer

Recent data on the cellular ceramide functions and its involvement in the apoptotic/necrotic cell death as well as its anticarcinogenic properties are presented. The emphasis is on the connections between the ceramide and caspase signaling pathways during the apoptotic cell death process. Notably, the experimental strategies and pharmacological tools used for establishment of the role of ceramide in triggering cell death are described. Moreover, the importance of a compartmentation of endogenous ceramide within the plasma membrane microdomains, lysosomes and mitochondria is discussed. Information on the deregulated functions of ceramide and caspase signaling pathways in several metastatic cancer types is also presented.

JunB and Bcl-2 overexpression results in protection against cell death of nigral neurons following axotomy

Transection of the medial forebrain bundle is a well established approach to investigate neuronal cell body response in the derived neuronal populations of the substantia nigra pars compacta (SNC). This model of central axotomy leads in mouse within 50 days post transection to degeneration of up to 70% of the affected SNC neurons. A central component of the axotomy induced alterations leading to neuronal degeneration is the rapid induction, lasting expression and activation of the c-Jun transcription factor. However, the role of c-Jun in the process of neuronal degeneration is not fully understood. Since null mutations of c-Jun cause embryonic lethality, this study was designed to investigate the impact of two c-Jun modulating proteins on neuronal survival after axotomy in transgenic mice: JunB, a Jun family member affecting c-Jun expression, and Bcl-2, an antiapoptotic protooncogene interacting among others with the c-Jun N-terminal kinases. In JunB as well as in Bcl-2 transgenic mice the long term survival rate of transected SNC neurons was remarkably increased when compared to wildtype controls. These effects were obviously achieved by cellular modulations directly following axotomy: Whereas JunB overexpression attenuated c-Jun induction and simultaneously led to a higher phosphorylation rate of c-Jun in SNC neurons, Bcl-2 overexpression did not influence c-Jun expression, but resulted in a reduced phosphorylation state of c-Jun in transected SNC neurons. We therefore conclude that the early phosphorylation rate of c-Jun might play an important role for the long term fate of transected neurons.

Ceramide promotes the death of human cervical tumor cells in the absence of biochemical and morphological markers of apoptosis

C8-ceramide, a synthetic cell-permeable analog of endogenous ceramides, interfered with cell proliferation, and was cytotoxic to papilloma virus-containing human cervix carcinoma cells, CALO, INBL, and HeLa, that match two clinical stages of tumor progression. C8-ceramide (3 microM) markedly reduced the tumor cell number after 48 h of treatment, an effect that endured even after the removal of C8-ceramide. The carcinoma cells showed morphologic changes, characteristic of necrosis and released lactate dehydrogenase (LDH). A biologically inactive analog C8-dihydro-ceramide had no effect on cell viability in any of the cell lines tested. Seventy-two hours after C8-ceramide treatment none of the biochemical and morphological markers characteristic of apoptosis: (a) nuclear chromatin condensation, (b) DNA fragmentation, (c) proteolysis of the caspase-3 substrate poly-(ADP-ribose)-polymerase (PARP), and (d) appearance of phosphatidylserine on the external cell membrane, were observed. C8-ceramide had no effect on human cervix fibroblasts and induced a mild reduction (30%) in the proliferation of normal human cervix epithelia and HeLa cells (IV-B metastatic stage). The cytotoxicity of C8-ceramide was restricted to CALO (early II-B) and INBL (IV-A non-metastatic) carcinoma cells. The possible application of ceramide in the treatment of early stages of cervical cancer is discussed.

Adenovirus infection of primary malignant lymphoid cells

Adenovirus infection represents a cellular stress that induces host cell pro-apoptotic responses. To overcome this barrier to productive infection, viral polypeptides modulate a variety of host cell pathways. The interface of these early viral gene products with key cellular regulatory proteins has provided considerable information concerning basic cellular mechanisms operative in cell cycle regulation, transcriptional control and apoptosis. The overlap of these mechanisms with those impacted during oncogenesis provides the opportunity to use adenoviruses and adenovirus mutants to characterize the state of key regulatory pathways in specific malignant cells. For example, adenoviruses mediate cytotoxicity after infection of chronic lymphocytic leukemia (CLL) cells, mantle cell lymphoma (MCL) cells and multiple myeloma cell lines. Specific adenovirus mutants demonstrate enhanced cytotoxicity and, in many cases, apoptosis is not the primary mechanism of cell death. Analysis of these infections with respect to both the features of the primary malignant cell and the mechanisms of adenovirus-mediated cytotoxicity holds the prospect of providing novel insights into the status of key regulatory pathways in individual patient malignant cells. These studies also hold the prospect of supporting the development of specific attenuated adenoviruses as therapeutic agents with selective cytotoxicity for specific primary lymphoid malignancies.

Autologous plasma activates Akt/protein kinase B and enhances basal survival and resistance to DNA damage-induced apoptosis in B-chronic lymphocytic leukaemia cells

We have studied the actions of autologous plasma on both basal and DNA damage-induced apoptosis in B-chronic lymphocytic leukaemia (B-CLL) cells. Apoptosis was quantified using morphological criteria and Western blot analysis for the apoptosis-specific p85 fragment of poly(ADP ribose) polymerase. Cell viability was estimated using the methyl thiazol tetrazolium bromide dye reduction assay. Plasma cultures showed lower rates of basal apoptosis as well as a decreased cytotoxic response to chlorambucil and gamma-radiation compared with cultures in fetal calf serum. Experiments using neutralizing antibodies suggested that the protective actions of plasma could not be accounted for by interleukin 4, the interferons alpha or gamma or stromal cell-derived factor 1, each of which have been shown to protect B-CLL cells from apoptosis in vitro. Plasma addition to B-CLL cells resulted in rapid activation of the Akt protein kinase, a key signalling enzyme that has been implicated in anti-apoptotic signalling. LY294002, an inhibitor of phosphatidylinositol 3'-kinase, blocked Akt activation by plasma. To the best of our knowledge, this is the first report to show that factors present in plasma promote basal survival of B-CLL cells and resistance to cytotoxic drugs via stimulation of the Akt cytoprotective-signalling pathway. Pharmacological blockade of this pathway may have potential in the development of novel therapeutic strategies for B-CLL treatment.

Targeting ceramide metabolism--a strategy for overcoming drug resistance

Inherent or acquired drug resistance, which frequently characterizes cancer cells, is caused by multiple mechanisms, including dysfunctional metabolism of the lipid second messenger ceramide. Ceramide, the basic structural unit of the sphingolipids, plays a role in activating cell death signals initiated by cytokines, chemotherapeutic agents, and ionizing radiation. Recent discoveries about the metabolism of ceramide suggest that this agent may have an important influence on the effectiveness of various cancer therapeutics. In particular, the cytotoxic effect of chemotherapy is decreased when generation of ceramide is impaired but is increased when the degradation of ceramide is blocked. Herein, we review the mechanisms of resistance to chemotherapeutic agents in terms of ceramide metabolism.

Cell cycle arrest of hematopoietic cell lines after treatment with ceramide is commonly associated with retinoblastoma activation

BACKGROUND

Leukaemia cells differ from their normal counterparts in that their ability to properly regulate survival, proliferation, differentiation, and apoptosis is aberrant. Understanding the molecular mechanisms controlling cell proliferation and developing therapeutic strategies to correct nonfunctional regulatory mechanisms are emerging areas of medical research. Ceramide, a metabolite of membrane sphingomyelin hydrolysis, has recently emerged as a key regulator of cellular proliferation, differentiation, and apoptosis in leukaemia cells.

METHODS

Leukaemia cell lines were treated with a biologically active analogue of ceramide, C(2)-ceramide. Cell cycle status was assessed flow cytometrically using propidium iodide. Induction of apoptosis was confirmed by annexin V staining of externalised phosphatidylserine and retinoblastoma activation was determined by Western blotting.

RESULTS

C(2)-ceramide induced activation of retinoblastoma tumour suppressor protein, G(0)/G(1) cell cycle arrest, or apoptosis in leukaemia cell lines. In addition, these effects differed depending upon cell type, thus confirming the pleiotropic nature of the ceramide signalling pathway. Most cells studied responded to exogenous C(2)-ceramide by entering growth arrest, evidently resulting from activation of retinoblastoma protein, and by displaying some degree of apoptosis.

CONCLUSIONS

Taken together, these findings suggest that signalling via ceramide has novel therapeutic applications for treatment of leukaemia.

Hypersensitivity to radiation-induced non-apoptotic and apoptotic death in cell lines from patients with the ICF chromosome instability syndrome

Immunodeficiency, centromeric region instability, and facial anomalies (ICF), a rare recessive chromosome instability syndrome, involves the loss of DNA methyltransferase 3B activity and the consequent hypomethylation of a small portion of the genome. We demonstrate for the first time that ICF cells are strongly hypersensitive to a genotoxic agent, namely, ionizing radiation. However, unlike cell lines from patients with ataxia telangiectasia or Nijmegen breakage syndrome, chromosome instability syndromes also associated with unusual sensitivity to ionizing radiation, ICF cells did not show any deficiencies in their cell cycle checkpoints. ICF lymphoblastoid cell lines demonstrated increased apoptosis, long-term cell cycle arrest, and loss of viability in clonogenicity assays after irradiation compared to analogous normal cell lines. Also, the ICF cell lines were subject to high frequencies of rapid non-apoptotic cell death upon irradiation but not to abnormally high levels of radiation-induced, cytogenetically detectable chromosome abnormalities. ICF-associated undermethylation of some regulatory gene(s) might lead to an exaggerated response to radiation-induced breaks in DNA yielding increased rates of cell death and irreversible cell cycle arrest. As a defense against their frequent spontaneous breaks in chromosomes 1 and 16, ICF patients may be abnormally prone to chromosome break-induced apoptosis, non-apoptotic cell death, and permanent cell cycle arrest so as to minimize the number of cycling cells with spontaneous rearrangements. A similarly increased cell death and cycle-arrest response to chromosome breaks due to cancer-linked DNA hypomethylation might occur during carcinogenesis.

Ceramide induces cell death in the human prostatic carcinoma cell lines PC3 and DU145 but does not seem to be involved in Fas-mediated apoptosis

Treatment of cells with synthetic C2-ceramide has been reported to induce apoptosis in several cell systems, and endogenously formed ceramide has been proposed to act as a second messenger, activating signaling pathways which contribute to the execution of apoptotic cell death after Fas ligation or tumor necrosis factor receptor-1 ligation. In this study, we examined the effect of exogenously administered C2-ceramide on the human prostatic carcinoma cell lines PC3 (Fas-sensitive) and DU145 (Fas-resistant). In both cell lines, C2-ceramide induced cell death in a dose-dependent manner, whereas a structural analog, C2-dihydroceramide, did not. The pan-caspase inhibitor zVAD-fmk did not prevent C2-ceramide-induced cell death but did prevent C2-ceramide-induced DNA fragmentation, indicating that apoptotic and non-apoptotic mechanisms are involved in C2-ceramide-induced death. Interestingly, cycloheximide prevented C2-ceramide-induced DNA fragmentation, indicating that ceramide-induced apoptosis in PC3 and DU145 requires new protein synthesis. In addition, because cycloheximide converts Fas-resistant DU145 to Fas-sensitive as assessed by DNA fragmentation, ceramide does not seem to play a major role in the Fas-mediated pathway in this cell line. We also determined the levels of endogenous sphingomyelin after Fas ligation in PC3. No decrease of sphingomyelin levels could be detected after Fas activation. We conclude that sphingomyelinase-generated ceramide does not play a role in Fas-mediated apoptosis in PC3, and that there are fundamental differences in the mechanisms of cell death induced by C2-ceramide and Fas ligation.

Killing of T lymphocytes by synthetic ceramide is by a nonapoptotic mechanism and is abrogated following mitogenic activation

Ceramide induces apoptosis in leukemia cell lines and has been proposed as a potential therapeutic agent in malignancies refractory to conventional treatment. Here we show that synthetic N-acetyl-d-erythro-sphingosine (C2 ceramide) kills normal human T lymphocytes by a caspase-independent nonapoptotic mechanism. By contrast, T cells were induced to caspase-dependent apoptosis by okadaic acid. Furthermore, C2 ceramide treatment of the Jurkat leukemia cell line induced killing by apoptosis. Activation of T lymphocytes by phytohemagglutinin abrogated killing by C2 ceramide. The data here suggest that ceramide triggers caspase-dependent apoptosis in leukemia cells lines, but activates caspase-independent nonapoptotic killing of resting T lymphocytes which is abrogated following mitogenic activation.