Ceramide does not act as a general second messenger for ultraviolet-induced apoptosis

Sphingolipids and mitochondrial apoptosis

The sphingolipid family of lipids modulate several cellular processes, including proliferation, cell cycle regulation, inflammatory signaling pathways, and cell death. Several members of the sphingolipid pathway have opposing functions and thus imbalances in sphingolipid metabolism result in deregulated cellular processes, which cause or contribute to diseases and disorders in humans. A key cellular process regulated by sphingolipids is apoptosis, or programmed cell death. Sphingolipids play an important role in both extrinsic and intrinsic apoptotic pathways depending on the stimuli, cell type and cellular response to the stress. During mitochondrial-mediated apoptosis, multiple pathways converge on mitochondria and induce mitochondrial outer membrane permeabilization (MOMP). MOMP results in the release of intermembrane space proteins such as cytochrome c and Apaf1 into the cytosol where they activate the caspases and DNases that execute cell death. The precise molecular components of the pore(s) responsible for MOMP are unknown, but sphingolipids are thought to play a role. Here, we review evidence for a role of sphingolipids in the induction of mitochondrial-mediated apoptosis with a focus on potential underlying molecular mechanisms by which altered sphingolipid metabolism indirectly or directly induce MOMP. Data available on these mechanisms is reviewed, and the focus and limitations of previous and current studies are discussed to present important unanswered questions and potential future directions.

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.

Effects of ceramide inhibition on radiation-induced apoptosis in human leukemia MOLT-4 cells

In the present study, using inhibitors of ceramide synthase (fumonisin B1), ketosphinganine synthetase (L-cycloserine), acid sphingomyelinase (D609 and desipramine) and neutral sphingomyelinase (GW4869), the role of ceramide in X-ray-induced apoptosis was investigated in MOLT-4 cells. The diacylglycerol kinase (DGK) assay showed that the intracellular concentration of ceramide increased time-dependently after X irradiation of cells, and this radiation-induced accumulation of ceramide did not occur prior to the appearance of apoptotic cells. Treatment with D609 significantly inhibited radiation-induced apoptosis, but did not inhibit the increase of intracellular ceramide. Treatment with desipramine or GW4869 prevented neither radiation-induced apoptosis nor the induced increase of ceramide. On the other hand, fumonisin B1 and L-cycloserine had no effect on the radiation-induced induction of apoptosis, in spite of significant inhibition of the radiation-induced ceramide. From these results, it was suggested that the increase of the intracellular concentration of ceramide was not essential for radiation-induced apoptosis in MOLT-4 cells.

Mitochondrial ceramide increases in UV-irradiated HeLa cells and is mainly derived from hydrolysis of sphingomyelin

Sphingolipids are important signaling molecules in many biologic processes, but little is known about their organelle-specific roles. Using HeLa cells, we investigated the effects of UV and etoposide-induced apoptosis on the contents of sphingomyelin (SM) and ceramide in subcellular compartments. UV irradiation of HeLa cells increased the levels of SM in all subcellular fractions, but the change was most dramatic in mitochondria. Using diacylglycerol kinase assays to quantify ceramide, we found that the levels of ceramide in mitochondria increased as early as 2 h after UV irradiation and remained elevated at 6 h. The increase in mitochondrial SM and ceramide was inhibited by D609, an inhibitor of sphingomyelinase and SM synthase. The inhibition of sphingolipid production correlated with protection of the mitochondrial transmembrane potential and prevention of cytochrome c release following UV irradiation. In contrast, myriocin, an inhibitor of the de novo ceramide synthesis pathway, only partially suppressed the production of ceramides in mitochondria and cannot suppress UV-induced apoptosis. Fumonicin B1, an inhibitor of ceramide synthase, can only prevent mitochondrial ceramide synthesis and UV-induced apoptosis in a small degree. These results indicate that mitochondrial ceramide production in UV-irradiated HeLa cells is not mediated by the de novo synthesis pathway, but mainly through SM hydrolysis.

Ceramide: second messenger or modulator of membrane structure and dynamics?

The physiological role of ceramide formation in response to cell stimulation remains controversial. Here, we emphasize that ceramide is not a priori an apoptotic signalling molecule. Recent work points out that the conversion of sphingomyelin into ceramide can play a membrane structural (physical) role, with consequences for membrane microdomain function, membrane vesiculation, fusion/fission and vesicular trafficking. These processes contribute to cellular signalling. At the Golgi, ceramide takes part in a metabolic flux towards sphingomyelin, diacylglycerol and glycosphingolipids, which drives lipid raft formation and vesicular transport towards the plasma membrane. At the cell surface, receptor clustering in lipid rafts and the formation of endosomes can be facilitated by transient ceramide formation. Also, signalling towards mitochondria may involve glycosphingolipid-containing vesicles. Ceramide may affect the permeability of the mitochondrial outer membrane and the release of cytochrome c. In the effector phase of apoptosis, the breakdown of plasma membrane sphingomyelin to ceramide is a consequence of lipid scrambling, and may regulate apoptotic body formation. Thus ceramide formation serves many different functions at distinct locations in the cell. Given the limited capacity for spontaneous intracellular diffusion or membrane flip-flop of natural ceramide species, the topology and membrane sidedness of ceramide generation are crucial determinants of its impact on cell biology.

The adenovirus e3 promoter is sensitive to activation signals in human T cells

The group C adenoviruses typically cause acute respiratory disease in young children. In addition, a persistent phase of infection has been observed in which virus may be shed for years without producing overt pathology. Our laboratory recently reported that group C adenovirus DNA can be found in tonsil and adenoid T lymphocytes from the majority of pediatric donors (C. T. Garnett, D. Erdman, W. Xu, and L. R. Gooding, J. Virol. 76:10608-10616, 2002). This finding suggests that immune evasion strategies of human adenoviruses may be directed, in part, toward protection of persistently or latently infected T lymphocytes. Many of the adenoviral gene products implicated in prevention of immune destruction of virus-infected cells are encoded within the E3 transcription unit. In this study, the E3 promoter was evaluated for sensitivity to T-cell activation signals by using a promoter reporter plasmid. Indeed, this promoter is extremely sensitive to T-cell activation, with phorbol myristate acetate (PMA) plus ionomycin increasing E3-directed transcription 100-fold. By comparison, in the same cells E1A expression leads to a 5.5-fold increase in transcription from the E3 promoter. In contrast to induction by E1A, activation by PMA plus ionomycin requires the two E3 NF-kappaB binding sites. Interestingly, expression of E1A inhibits induction of the E3 promoter in response to T-cell activation while increasing E3 promoter activity in unactivated cells. Collectively, these data suggest that the E3 promoter may have evolved the capacity to respond to T-cell activation in the absence of E1A expression and may act to upregulate antiapoptotic gene expression in order to promote survival of persistently infected T lymphocytes.