jkk-1 and mek-1 regulate body movement coordination and response to heavy metals through jnk-1 in Caenorhabditis elegans.

Although in vitro evidence suggests two c-Jun N-terminal kinase (JNK) kinases, MKK4 and MKK7, transactivate JNK, in vivo confirmation is incomplete. In fact, JNK deficiency may differ from the composite deficiency of MKK4 and MKK7 in Drosophila and mice. Recently, the Caenorhabditis elegans homolog of human JNK, jnk-1, and two MKK-7s, mek-1 and jkk-1, were cloned. Here we characterize jnk-1, which encodes two isoforms JNK-1 alpha and JNK-1 beta. A null allele, jnk-1(gk7), yielded worms with defective body movement coordination and modest mechanosensory deficits. Similarly to jkk-1 mutants, elimination of GABAergic signals suppressed the jnk-1(gk7) locomotion defect. Like mek-1 nulls, jnk-1(gk7) showed copper and cadmium hypersensitivity. Conditional expression of JNK-1 isoforms rescued these defects, suggesting that they are not due to developmental errors. While jkk-1 or mek-1 inactivation mimicked jnk-1(gk7) locomotion and heavy metal stress defects, respectively, mkk-4 inactivation did not, but rather yielded defective egg laying. Our results delineate at least two different JNK pathways through jkk-1 and mek-1 in C.elegans, and define interaction between MKK7, but not MKK4, and JNK.

Oocyte apoptosis is suppressed by disruption of the acid sphingomyelinase gene or by sphingosine-1-phosphate therapy.

The time at which ovarian failure (menopause) occurs in females is determined by the size of the oocyte reserve provided at birth, as well as by the rate at which this endowment is depleted throughout post-natal life. Here we show that disruption of the gene for acid sphingomyelinase in female mice suppressed the normal apoptotic deletion of fetal oocytes, leading to neonatal ovarian hyperplasia. Ex vivo, oocytes lacking the gene for acid sphingomyelinase or wild-type oocytes treated with sphingosine-1-phosphate resisted developmental apoptosis and apoptosis induced by anti-cancer therapy, confirming cell autonomy of the death defect. Moreover, radiation-induced oocyte loss in adult wild-type female mice, the event that drives premature ovarian failure and infertility in female cancer patients, was completely prevented by in vivo therapy with sphingosine-1-phosphate. Thus, the sphingomyelin pathway regulates developmental death of oocytes, and sphingosine-1-phosphate provides a new approach to preserve ovarian function in vivo.

Role of acidic sphingomyelinase in Fas/CD95-mediated cell death.

Engagement of the Fas receptor has been reported to induce ceramide generation via activation of acidic sphingomyelinase (aSMase). However, the role of aSMase in Fas-mediated cell death is controversial. Using genetically engineered mice deficient in the aSMase gene (aSMase(-/-)), we found that thymocytes, concanavalin A-activated T cells, and lipopolysaccharide-activated B cells derived from both aSMase(-/-) and aSMase(+/+) mice were equally sensitive to Fas-mediated cell death, triggered by either anti-Fas antibody or Fas ligand in vitro. Similarly, activation-induced apoptosis of T lymphocytes was unaffected by the status of aSMase, and aSMase(-/-) mice failed to show immunological symptoms seen in animals with defects in Fas function. In vivo, intravenous injection of 3 microg/25 g mouse body weight of anti-Fas Jo2 antibody into aSMase(-/-) mice failed to affect hepatocyte apoptosis or mortality, whereas massive hepatocyte apoptosis and animal death occurred in wild type littermates. Animals heterozygous for aSMase deficiency were also significantly protected. Susceptibility of aSMase(-/-) mice to anti-Fas antibody was demonstrated with higher antibody doses (>/=4 microg/25 g mouse). These data indicate a role for aSMase in Fas-mediated cell death in some but not all tissues.

Radiation-induced apoptosis of endothelial cells in the murine central nervous system: protection by fibroblast growth factor and sphingomyelinase deficiency.

Injury to the central nervous system (CNS) by ionizing radiation may be a consequence of damage to the vascular endothelium. Recent studies showed that radiation-induced apoptosis of endothelial cells in vitro and in the lung in vivo is mediated by the lipid second messenger ceramide via activation of acid sphingomyelinase (ASM). This apoptotic response to radiation can be inhibited by basic fibroblast growth factor or by genetic mutation of ASM. In the CNS, single-dose radiation has been shown to result in a 15% loss of endothelial cells within 24 h, but whether or not this loss is associated with apoptosis remains unknown. In the present studies, dose- and time-dependent induction of apoptosis was observed in the C57BL/6 mouse CNS. Apoptosis was quantified by terminal deoxynucleotidyl transferase-mediated nick end labeling, and specific endothelial apoptosis was determined by histochemical double labeling with terminal deoxynucleotidyl transferase-mediated nick end labeling and Lycopersicon esculentum lectin. Beginning at 4 h after single-dose radiation, apoptosis was ongoing for 24 h and peaked at 12 h at an incidence of 0.7-1.4% of the total cells in spinal cord sections. Up to 20% of the apoptotic cells were endothelial. This effect was also seen in multiple regions of the brain (medulla, pons, and hippocampus). A significant reduction of radiation-induced apoptosis was observed after i.v. basic fibroblast growth factor treatment (0.45-4.5 microg/mouse). Identical results were noted in C3H/HeJ mice. Furthermore, irradiated ASM knockout mice displayed as much as a 70% reduction in endothelial apoptosis. This study demonstrates that ionizing radiation induces early endothelial cell apoptosis throughout the CNS. These data are consistent with recent evidence linking radiation-induced stress with ceramide and suggest approaches to modify the apoptotic response in control of radiation toxicity in the CNS.

Sphingolipid signaling in gonadal development and function.

Sphingolipid second messengers, such as ceramide and sphingosine-1-phosphate, signal proliferation, differentiation and death in mammalian cells. The object of this article is to highlight the potential impact of this new information on the study of female and male gonadal development and function. Since the generation of competent gametes by both sexes is precisely regulated by maturational (meiotic) and apoptotic (quality-control) checkpoints, it is proposed that lipid signaling molecules serve as important contributors to the regulation of gametogenesis. The function of sphingolipid molecules in mediating stress- or damage-induced apoptosis in the germ line, an event most-likely associated with impaired gonadal function and infertility, is also discussed. Collectively, these areas represent exciting research directions that may ultimately lead to the development of new therapeutics to coordinate and control fertility in males and females.

Signal transduction of stress via ceramide.

The sphingomyelin (SM) pathway is a ubiquitous, evolutionarily conserved signalling system analogous to conventional systems such as the cAMP and phosphoinositide pathways. Ceramide, which serves as second messenger in this pathway, is generated from SM by the action of a neutral or acidic SMase, or by de novo synthesis co-ordinated through the enzyme ceramide synthase. A number of direct targets for ceramide action have now been identified, including ceramide-activated protein kinase, ceramide-activated protein phosphatase and protein kinase Czeta, which couple the SM pathway to well defined intracellular signalling cascades. The SM pathway induces differentiation, proliferation or growth arrest, depending on the cell type. Very often, however, the outcome of signalling through this pathway is apoptosis. Mammalian systems respond to diverse stresses with ceramide generation, and recent studies show that yeast manifest a form of this response. Thus ceramide signalling is an older stress response system than the caspase/apoptotic death pathway, and hence these two pathways must have become linked later in evolution. Signalling of the stress response through ceramide appears to play a role in the development of human diseases, including ischaemia/reperfusion injury, insulin resistance and diabetes, atherogenesis, septic shock and ovarian failure. Further, ceramide signalling mediates the therapeutic effects of chemotherapy and radiation in some cells. An understanding of the mechanisms by which ceramide regulates physiological and pathological events in specific cells may provide new targets for pharmacological intervention.

Ceramide generation by the Reaper protein is not blocked by the caspase inhibitor, p35.

The Reaper (Rpr) gene encodes a 65-amino acid protein that induces apoptosis in Drosophila by an unknown mechanism. A previous study reported that Rpr expression induced generation of the lipid second messenger ceramide and through use of the peptide caspase inhibitor N-benzyloxycarbonyl-VAD-fluoromethylketone(zVAD.fmk ) ordered ceramide generation downstream of caspases in SL2 cells (Pronk, G. J. , Ramer, K., Amiri, P., and Williams, L. T. (1996) Science 271, 808-810). The present study re-evaluates these events in SL2 cells transfected with cDNA for Rpr, with or without the baculovirus caspase inhibitor p35, under the control of the metallothionein promoter. Following copper addition, Rpr protein was detected at 1.5 h and maximal at 2.5 h. Ceramide generation and caspase activation occurred nearly simultaneously, each detectable at 2-2.5 h and maximal at 6 h. Ceramide levels increased from a base line of 5 pmol/nmol lipid phosphorus to a maximum of 10 pmol/nmol lipid phosphorus. Identical increases in ceramide were detected using the enzymatic 1,2-diacylglycerol kinase assay or the non-enzymatic o-phthalaldehyde derivatization high pressure liquid chromatography assay. In contrast, diacylglycerol levels were not increased by Rpr expression. Apoptosis, first detected at 4 h, was maximal at 16 h. Co-expression of p35 did not affect Rpr-induced ceramide generation, whereas caspase activation and apoptosis were abolished. In contrast, zVAD.fmk inhibited ceramide generation and apoptosis. These data show that Rpr-induced ceramide generation is upstream or independent of p35-inhibitable caspases and demonstrate differences in the actions of peptide and p35 caspase inhibitors.

Regulation of ceramide production and apoptosis.

Ceramide is a sphingosine-based lipid signaling molecule that regulates cellular differentiation, proliferation, and apoptosis. The emerging picture suggests that coupling of ceramide to specific signaling cascades is both stimulus and cell-type specific. Ceramide action is determined within the context of other stimuli and by the subcellular topology of its production. Here, we discuss the pathways of ceramide generation and the interaction of ceramide with caspases and other apoptotic signaling cascades.

Ceramide signaling in apoptosis.

The sphingomyelin pathway is a ubiquitous, evolutionarily conserved signaling system initiated by hydrolysis of the plasma membrane phospholipid sphingomyelin to generate ceramide. Ceramide acts as a second messenger in activating the apoptotic cascade. Diverse cytokine receptors and environmental stresses utilize ceramide to signal apoptosis. In several cell systems ceramide links to the stress-activated protein kinase (SAPK)/c-jun kinase (JNK) cascade to signal apoptosis. The engagement of the sphingomyelin pathway in signaling apoptosis is tightly regulated by anti-apoptotic control mechanisms, and the balance between pro- and anti-apoptotic systems determines the magnitude of the apoptotic response in vitro and in vivo. This review describes the known elements and molecular ordering of ceramide-mediated apoptosis and the anti-apoptotic mechanisms that regulate its expression. Understanding of pro- and anti-apoptotic signaling involved in ceramide-mediated apoptosis and the modes of their co-ordinated function may yield opportunities for pharmacological interventions with potential for clinical applications.

Differential inhibition of radiation-induced apoptosis.

The most common mechanism by which radiation kills cells is the induction of DNA double-strand breaks that results in the loss of cell proliferation. Even though apoptosis is increasingly identified in experimental systems in vitro and in vivo, it is still generally regarded as a rare mode of radiation-induced cell kill with minor relevance for the clinical effects of radiation. This review will focus on pro- and antiapoptotic signaling that affects the apoptotic outcome in irradiated mammalian cells. In particular, we will concentrate on the sphingomyelin/ceramide signal transduction pathway which is involved in initiation of stress-induced apoptosis in a variety of normal and neoplastic cells. We will also discuss the crosstalk between the sphingomyelin/ceramide pathway and the protein kinase C pathway which constitutes an antiapoptotic pathway, and the potential for pharmacological modulation to increase the fraction of apoptotic cells undergoing apoptosis after radiation exposure.

Fas-mediated apoptosis and sphingomyelinase signal transduction: the role of ceramide as a second messenger for apoptosis.

In this article, we review the role of sphingomyelinases and ceramide in the Fas-mediated apoptosis signal transduction cascade. Several stimuli, including ligation of Fas, have been shown to enhance either neutral and/or acidic sphingomyelinase activity and increase ceramide content in intact cells or cell membrane preparations. Ceramide seems to have different functions, including induction of apoptosis, growth arrest, and/or differentiation, depending on cell type or location of sphingomyelin hydrolysis within the cell. Several putative targets for ceramide activity, including a kinase and a phosphatase, have also been identified. While ceramide and acidic sphingomyelinase activity appear to be involved in apoptotic signalling for Fas and other members of the tumour necrosis factor receptor family, it is clear that other signals and mechanisms are necessary for Fas-mediated apoptosis.

Requirement for ceramide-initiated SAPK/JNK signalling in stress-induced apoptosis.

The induction of programmed cell death, or apoptosis, involves activation of a signalling system, many elements of which remain unknown. The sphingomyelin pathway, initiated by hydrolysis of the phospholipid sphingomyelin in the cell membrane to generate the second messenger ceramide, is thought to mediate apoptosis in response to tumour-necrosis factor (TNF)-alpha, to Fas ligand and to X-rays. It is not known whether it plays a role in the stimulation of other forms of stress-induced apoptosis. Given that environmental stresses also stimulate a stress-activated protein kinase (SAPK/JNK), the sphingomyelin and SAPK/JNK signalling systems may be coordinated in induction of apoptosis. Here we report that ceramide initiates apoptosis through the SAPK cascade and provide evidence for a signalling mechanism that integrates cytokine- and stress-activated apoptosis.

Cell-free reconstitution of Fas-, UV radiation- and ceramide-induced apoptosis.

Cell-free systems are valuable tools for the dissection of complex cellular processes. Here we show that cytoplasmic extracts from cells exposed to anti-Fas antibody or UV radiation contain an activity capable of reproducing morphological changes typical of apoptosis in nuclei added to these extracts, as well as internucleosomal cleavage of DNA and proteolysis of a protein known to be cleaved during the apoptosis of intact cells. Extracts from control cell populations were inactive in this respect. These effects were partly blocked by the addition of purified Bcl-2 protein or a competitive inhibitor peptide of interleukin-1 beta-converting enzyme to the extracts. Furthermore, apoptotic activity was induced in cytoplasmic extracts from untreated cells by the addition of ceramide, a lipid second messenger implicated recently in apoptosis signaling. These extracts should prove highly useful in the dissection of molecular events that occur during apoptosis.

Inhibition of ceramide-induced apoptosis by Bcl-2.

Ceramide, a long chain sphingolipid that is generated intracellularly upon hydrolysis of membrane-associated sphingomyelin, has recently been implicated as a second messenger-like molecule that is produced distal to ligation of the tumour necrosis factor receptor type 1 (TNFR1), as well as the related Fas (CD95/Apo-1) molecule. It is well established that ligation of TNFR1 or Fas leads to apoptosis in most cases. Furthermore, it has also recently been demonstrated that exposure to cell-permeable synthetic ceramides can result in apoptosis in many cases. These and other observations have led to the hypothesis that accumulation of intracellular ceramide may be a common element of several pathways that result in apoptosis. Here we show that exposure to synthetic ceramides triggers apoptosis in the human T lymphoblastoid cell lines, CEM and Jurkat, and that overexpression of the apoptosis-repressor protein, Bcl-2, renders these cells resistant to the apoptosis-inducing effects of ceramide, as well as to several other stimuli. Since exposure to ceramides can result in either cell proliferation, differentiation, cycle arrest, or death, the level of Bcl-2 expression in a cell may be an important factor in determining the outcome of signals that result in intracellular generation of this sphingolipid.

Does endotoxin stimulate cells by mimicking ceramide?

Recent studies indicate that tumor necrosis factor (TNF) and interleukin 1 (IL-1) stimulate cells via the intracellular messenger ceramide. Bacterial endotoxin (lipopolysaccharide; LPS) shows strong structural and functional resemblance to ceramide. Here, Samuel Wright and Richard Kolesnick review data suggesting that LPS may provoke cellular responses by mimicking the action of ceramide.