Protective roles for ATM in cellular response to oxidative stress

Mouse models for induced genetic instability at endogenous loci

Exposure to environmental factors and genetic predisposition of an individual may lead individually or in combination to various genetic diseases including cancer. These diseases may be a consequence of genetic instability resulting in large-scale genomic rearrangements, such as DNA deletions, duplications, and translocations. This review focuses on mouse assays detecting genetic instability at endogenous loci. The frequency of DNA deletions by homologous recombination at the pink-eyed unstable (p(un)) locus is elevated in mice with mutations in ATM, Trp53, Gadd45, and WRN genes and after exposure to carcinogens. Other quantitative in vivo assays detecting loss of heterozygosity events, such as the mammalian spot assay, Dlb-1 mouse and Aprt mouse assays, are also reviewed. These in vivo test systems may predict hazardous effects of an environmental agent and/or genetic predisposition to cancer.

Effect ofAtmDisruption on Spontaneously Arising and Radiation-Induced Deletion Mutations in Mouse Liver

Deletion mutations were efficiently recovered in mouse liver after total-body irradiation with X rays by using a transgenic mouse "gpt-delta" system that harbored a lambda EG10 shuttle vector with the red and gam genes for Spi- (sensitive to P2 lysogen interference) selection. We incorporated this system into homozygous Atm-knockout mice as a model of the radiosensitive hereditary disease ataxia telangiectasia (AT). Lambda phages recovered from the livers of X-irradiated mice with the Atm+/+ genotype showed a dose-dependent increase in the Spi- mutant frequency up to sixfold at 50 Gy over the unirradiated control of 2.8x10(-6). The livers from Atm-/- mice yielded a virtually identical dose-response curve for X rays with a background fraction of 2.4x10(-6). Structural analyses revealed no significant difference in the proportion of -1 frameshifts and larger deletions between Atm+/+ and Atm-/- mice, although larger deletions prevailed in X-ray-induced Spi- mutants irrespective of Atm status. While a possible defect in DNA repair after irradiation has been strongly indicated in the literature for nondividing cultured cells in vitro from AT patients, the Atm disruption does not significantly affect radiation mutagenesis in the stationary mouse liver in vivo.

Oxidative stress in ataxia telangiectasia

Ataxia telangiectasia is one of a group of recessive hereditary genomic instability disorders and is characterized by progressive neurodegeneration, immunodeficiency and cancer susceptibility. Heterozygotes for the mutated gene are more susceptible to cancer and to ischaemic heart disease. The affected gene, ATM (ataxia telangiectasia mutated), has been cloned and codes for a protein kinase (ATM), which orchestrates the cellular response to DNA double-strand breaks after ionising radiation. An underlying feature of ataxia telangiectasia is oxidative stress and there is chronic activation of stress response pathways in tissues showing pathology such as the cerebellum, but not in the cerebrum or liver. ATM has also been shown to be activated by insulin and to have a wider role in signal transduction and cell growth. Many, but not all, aspects of the phenotype can be attributed to a defective DNA damage response. The oxidative stress may result directly from accumulated DNA damage in affected tissues or ATM may have an additional role in sensing/modulating redox homeostasis. The basis for the observed tissue specificity of the oxidative damage in ataxia telangiectasia is not clear.

Fanconi anaemia proteins: major roles in cell protection against oxidative damage

Fanconi anaemia (FA) is a cancer-prone genetic disorder that is characterised by cytogenetic instability and redox abnormalities. Although rare subtypes of FA (B, D1 and D2) have been implicated in DNA repair through links with BRCA1 and BRCA2, such a role has yet to be demonstrated for gene products of the common subtypes. Instead, these products have been strongly implicated in xenobiotic metabolism and redox homeostasis through interactions of FANCC with cytochrome P-450 reductase and with glutathione S-transferase, and of FANCG with cytochrome P-450 2E1, as well as redox-dependent signalling through an interaction between FANCA and Akt kinase. We hypothesise that FA proteins act directly (via FANCC and FANCG) and indirectly (via FANCA, BRCA2 and FANCD2) with the machinery of cellular defence to modulate oxidative stress. The latter interactions may co-ordinate the link between the response to DNA damage and oxidative stress parameters (3, 6-12).

ATM gene regulates oxygen-glucose deprivation-induced nuclear factor-kappaB DNA-binding activity and downstream apoptotic cascade in mouse cerebrovascular endothelial cells

BACKGROUND AND PURPOSE

Cells lacking the ATM (ataxia telangectasia mutated) gene are hypersensitive to DNA damage caused by a variety of insults. ATM may regulate oxidative stress-induced signaling cascades involving nuclear factor-kappaB (NF-kappaB), a transcription factor that is upstream of a wide variety of stress-responsive genes. We investigated the potential interaction of ATM and NF-kappaB after oxygen-glucose deprivation (OGD) in cerebral endothelial cells (CECs).

METHODS

Primary cultures of mouse CECs were subjected to OGD in the absence or presence of ATM antisense oligonucleotides or the NF-kappaB inhibitor SN50. ATM expression was determined with the use of reverse transcription-polymerase chain reaction and Western blot, and NF-kappaB activity was assessed by electrophoretic mobility shift assay. Cells were assessed for mitochondrial DNA damage with the use of long polymerase chain reaction and were assessed for caspase-3 and caspase-8 activity with the use of fluorogenic substrates. Cell death was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide and LDH release.

RESULTS

OGD stimulated ATM gene expression at the mRNA and protein level in CECs as early as 1 hour after OGD initiation. ATM gene knockdown with the use of an antisense oligonucleotide suppressed OGD-induced ATM protein expression, which was accompanied by an attenuation of NF-kappaB activation and the subsequent expression of downstream genes, including the antiapoptotic gene c-IAP2. ATM knockdown also accentuated OGD-induced mitochondrial DNA damage and the activation of caspase-3 and caspase-8, leading to enhanced CEC death. The specific NF-kappaB inhibitor SN50 mimicked the effects of ATM knockdown.

CONCLUSIONS

We conclude that ATM may play a cytoprotective role in OGD-induced CEC death via a NF-kappaB-dependent signaling pathway.

Biochemical identification of a neutral sphingomyelinase 1 (NSM1)-like enzyme as the major NSM activity in the DT40 B-cell line: absence of a role in the apoptotic response to endoplasmic reticulum stress

DT40 cells have approx. 10-fold higher Mg2+-dependent neutral sphingomyelinase (NSM) activity in comparison with other B-cell lines and contain very low acidic sphingomyelinase activity. Purification of this activity from DT40 cell membranes suggested the presence of one major NSM isoform. Although complete purification of this isoform could not be achieved, partially purified fractions were examined further with regard to the known characteristics of previously partially purified NSMs and the two cloned enzymes exhibiting in vitro NSM activity (NSM1 and NSM2). For a direct comparative study, highly purified brain preparations, purified NSM1 protein and Bacillus cereus enzyme were used. Analysis of the enzymic properties of the partially purified DT40 NSM, such as cation dependence, substrate specificity, redox regulation and stimulation by phosphatidylserine, together with the localization of this enzyme to the endoplasmic reticulum (ER), suggested that this NSM from DT40 cells corresponds to NSM1. Further studies aimed to correlate presence of the high levels of this NSM1-like activity in DT40 cells with the ability of these cells to accumulate ceramide and undergo apoptosis. When DT40 cells were stimulated to apoptose by a variety of agents, including the ER stress, an increase in endogenous ceramide levels was observed. However, these responses were not enhanced compared with another B-cell line (Nalm-6), characterized by low sphingomyelinase activity. In addition, DT40 cells were not more susceptible to ceramide accumulation and apoptosis when exposed to the ER stress compared with other apoptotic agents. Inhibition of de novo synthesis of ceramide partially inhibited its accumulation, indicating that the ceramide production in DT40 cells could be complex and, under some conditions, could involve both sphingomyelin hydrolysis and ceramide synthesis.

Cellular response to oxidative stress: signaling for suicide and survival

Reactive oxygen species (ROS), whether produced endogenously as a consequence of normal cell functions or derived from external sources, pose a constant threat to cells living in an aerobic environment as they can result in severe damage to DNA, protein, and lipids. The importance of oxidative damage to the pathogenesis of many diseases as well as to degenerative processes of aging has becoming increasingly apparent over the past few years. Cells contain a number of antioxidant defenses to minimize fluctuations in ROS, but ROS generation often exceeds the cell's antioxidant capacity, resulting in a condition termed oxidative stress. Host survival depends upon the ability of cells and tissues to adapt to or resist the stress, and repair or remove damaged molecules or cells. Numerous stress response mechanisms have evolved for these purposes, and they are rapidly activated in response to oxidative insults. Some of the pathways are preferentially linked to enhanced survival, while others are more frequently associated with cell death. Still others have been implicated in both extremes depending on the particular circumstances. In this review, we discuss the various signaling pathways known to be activated in response to oxidative stress in mammalian cells, the mechanisms leading to their activation, and their roles in influencing cell survival. These pathways constitute important avenues for therapeutic interventions aimed at limiting oxidative damage or attenuating its sequelae.

Ubiquitination capabilities in response to neocarzinostatin and H(2)O(2) stress in cell lines from patients with ataxia-telangiectasia

The human genetic disorder ataxia-telangiectasia (A-T) is due to lack of functional ATM, a protein kinase which is involved in cellular responses to DNA double strand breaks (DSBs) and possibly other oxidative stresses, as well as in regulation of several fundamental cellular functions. Studies regarding responses in A-T cells to the induction of DSBs utilize ionizing radiation or radiomimetic chemicals, such as neocarzinostatin (NCS), which induce DNA DSBs. This critical DNA lesion activates many defense systems, such as the cell cycle checkpoints. The cell cycle is also regulated through a timed and coordinated degradation of regulatory proteins via the ubiquitin pathway. Our recent studies indicate that the ubiquitin pathway is influenced by the cellular redox status and that it is the major cellular pathway for removal of oxidized proteins. Accordingly, we hypothesized that the absence of a functional ATM protein might involve perturbations to the ubiquitin pathway as well. We show here that upon treatment with NCS, there was a transient 50-70% increase in endogenous ubiquitin conjugates in A-T and wt lymphoblastoid cells. Ubiquitin conjugation capabilities per se and levels of substrates for conjugation were also similarly enhanced in wt and A-T cells upon NCS treatment. We also compared the ubiquitination response in A-T and wt cells using H(2)O(2) as the stress, in view of preexisting evidence of the effects of H(2)O(2) on ubiquitination capabilities in other types of cells. As with NCS treatment, there was an approximately 45% increase in endogenous ubiquitin conjugates by 2-4 h after exposure to H(2)O(2). Both cell types showed a rapid 50-150% increase in de novo formed 125I-ubiquitin conjugates. As compared with wt cells, unexposed A-T cells had higher endogenous levels of conjugates and enhanced conjugation capability. However, A-T cells mounted a more muted ubiquitination response to the stress. The enhanced ubiquitin conjugation in unstressed A-T cells and attenuated ability of these cells to respond to stress are consistent with the A-T cells being under oxidative stress and with their having an 'aged' phenotype. The indication that ubiquitin conjugate levels and ubiquitin conjugation capabilities are enhanced upon oxidative stress without significant changes in GSSG/GSH ratios indicates that assays of ubiquitination provide a sensitive measure of cellular stress. The data also add support to the impression that potentiated ubiquitination response to mild oxidative stress is a generalizable phenomenon.

Elevated oxidative stress in patients with ataxia telangiectasia

Ataxia telangiectasia (AT) is a pleiotropic genetic disorder characterized by progressive neurodegeneration, especially of cerebellar Purkinje cells, immunodeficiency, increased incidence of cancer, and premature aging. The disease is caused by functional inactivation of the ATM (AT-mutated) gene product, which is thought to act as a sensor of reactive oxygen species and oxidative damage of cellular macromolecules and DNA. The compound phenotype of AT might thus be linked to a continuous state of oxidative stress leading to an increase of programmed cell death (apoptosis). To assess this hypothesis, we analyzed lipid peroxidation products and the oxidative stress associated DNA base damage 8-hydroxy-2-deoxyguanosine in patients with AT. Oxidative damage to lipids and DNA was found to be markedly increased in AT patients. These results indicate that ATM might play an important role in the maintenance of cell homeostasis in response to oxidative damage. In this context, a better control of levels of reactive oxygen species could be a rational foundation of therapeutic intervention to help alleviate some of the symptoms associated with AT.

ATM: genome stability, neuronal development, and cancer cross paths

One of the cornerstones of the web of signaling pathways governing cellular life and differentiation is the DNA damage response. It spans a complex network of pathways, ranging from DNA repair to modulation of numerous processes in the cell. DNA double-strand breaks (DSBs), which are formed as a result of genotoxic stress or normal recombinational processes, are extremely lethal lesions that rapidly mobilize this intricate defense system. The master controller that pilots cellular responses to DSBs is the ATM protein kinase, which turns on this network by phosphorylating key players in its various branches. ATM is the protein product of the gene mutated in the human genetic disorder ataxia-telangiectasia (A-T), which is characterized by neuronal degeneration, immunodeficiency, sterility, genomic instability, cancer predisposition, and radiation sensitivity. The clinical and cellular phenotype of A-T attests to the numerous roles of ATM, on the one hand, and to the link between the DNA damage response and developmental processes on the other hand. Recent studies of this protein and its effectors, combined with a thorough investigation of animal models of A-T, have led to new insights into the mode of action of this master controller of the DNA damage response. The evidence that ATM is involved in signaling pathways other than those related to damage response, particularly ones relating to cellular growth and differentiation, reinforces the multifaceted nature of this protein, in which genome stability, developmental processes, and cancer cross paths.

Vesnarinone causes oxidative damage by inhibiting catalase function through ceramide action in myeloid cell apoptosis

Vesnarinone is an effective inotropic agent for treating congestive heart failure, but its clinical usage is restricted because of the severe side effect of agranulocytosis. In myeloid HL-60 cells, vesnarinone increased the intracellular content of a proapoptotic lipid mediator, ceramide, in a time- and dose-dependent manner. Vesnarinone-induced apoptosis was significantly enhanced by simultaneous treatment with a cell-permeable N-acetyl sphingosine (C2-ceramide). Treatment with neither vesnarinone, C2-ceramide, nor simultaneously with vesnarinone and C2-ceramide caused a marked increase of reactive oxygen intermediates (ROI) generation measured by the 2',7'-dichlorofluorescin method. However, oxidative damage judged by the production of lipid peroxidates and the nitroblue tetrazolium-reducing ability were enhanced more significantly by simultaneous treatment with vesnarinone and C2-ceramide than by vesnarinone alone. Moreover, vesnarinone inhibited catalase function both at the protein and activity level, and this inhibition was synergistically enhanced by C2-ceramide, and vesnarinone-induced oxidative damage and apoptosis were significantly suppressed by treatment of HL-60 cells with purified catalase. C2-ceramide enhanced vesnarinone-induced inhibition of the ROI-scavenging enzyme catalase at the levels of protein and activity in HL-60 cells; in contrast, however, vesnarinone did not induce ceramide generation, oxidative damage, or catalase depletion in HL-60/ves cells, where vesnarinone could not induce apoptosis. Taken together, the results suggest that vesnarinone induces myeloid cell apoptosis by increasing oxidative damage via ceramide-induced inhibition of catalase function.

The involvement of cell cycle checkpoint-mutations in the mutagenesis induced in Drosophila by a longer wavelength light band of solar UV

Solar ultraviolet radiation is considered to be injurious rather than necessary for most organisms living on the earth. It is reported that the risk of skin cancer in humans increases by the depletion of the ozone layer. We have examined the genotoxicity of solar ultraviolet, especially the longer wavelength light, using Drosophila. Recently, we have demonstrated that light of wavelength up to 340 nm is mutagenic on Drosophila larvae. Using an excision repair-deficient Drosophila strain (mus201), we have obtained results suggesting that the lesion caused in larvae by the 320 nm-light irradiation may be similar to the damage induced by irradiation at 310 nm, and that light of 330 and 340 nm may induce damage different from that induced by 310 and 320 nm-light. To examine the difference in DNA damage induced by light of a particular wavelength, we performed monochromatic irradiation on larvae of two Drosophila strains; one excision repair-deficient (mei-9) and another postreplication repair-deficient (mei-41). 310 and 320 nm-light was more mutagenic in the mei-9 strain than in mei-41, whereas 330 and 340 nm-light was more mutagenic in mei-41 than in mei-9. It is demonstrated that the mei-41 gene is a homologue of the human atm gene which is responsible for a cell cycle checkpoint. This result suggests that 310-320 nm-light induces DNA damage that is subject to nucleotide excision repair (NER) and that 330-360 nm-light causes damage to be recognized by the cell cycle checkpoint but it is not repairable by NER.

Immunohistochemical study of p53-associated proteins in rat brain following lithium-pilocarpine status epilepticus

Activation of the p53-stress response pathway has been implicated in excitotoxic neuronal cell death. Recent studies have demonstrated an age-dependent induction of both p53 mRNA and protein in the rat brain following lithium-pilocarpine-mediated status epilepticus (LPSE). We investigated whether other proteins that have been shown to participate in the p53 cascade are induced by LPSE. We used immunohistochemistry to examine the expression of Mdm2, Bax, CD95/Fas/APO-1, ATM, Ref-1 and ubiquitin. A significant increase in nuclear Mdm2 immunoreactivity, which colocalized with p53, was observed in cells within hippocampal pyramidal cell layers, dentate gyrus, piriform cortex, amygdala and thalamus. Dual immunofluorescence microscopy revealed a reduction in free ubiquitin expression in cells with p53 and Mdm2 accumulation. Increased immunoreactivity for CD95/Fas/APO-1 and Bax was also detected in the same p53-positive cells. Moreover, expression of Ref-1 and ATM, which are involved in the response to oxidative stress-induced DNA damage and regulation of p53 function, were increased. Colocalization of Ref-1 and p53 suggests that Ref-1 might activate p53 function in LPSE-induced neurodegeneration. In contrast, ATM immunoreactivity was predominantly cytoplasmic suggesting that ATM may not directly modulate p53 activity in injured neurons. These results extend our previous observations with regard to activation and stabilization of p53 in injured central nervous system neurons. The data indicate that p53 induction following LPSE may activate downstream pro-apoptotic genes leading to neurodegeneration.

Requirement of ATM in UVA-induced signaling and apoptosis

Solar UVA, but not UVC, reaches the earth's surface and therefore is an important etiological factor for the induction of human skin cancer. ATM kinase is an important regulator of cell survival and cell cycle checkpoints. Here, we observe that UVA, unlike UVC, triggers ATM kinase activity, and the activation may occur through reactive oxygen species produced after irradiation of cells with UVA. We also show that ATM activation is involved in the apoptotic response to UVA but not UVC. Furthermore, we provide evidence that ATM-dependent p53 and c-Jun N-terminal kinase (JNK) pathways are linked to UVA-induced apoptosis. On the other hand, UVC-induced apoptosis occurs through ATR-dependent p53 phosphorylation as well as the JNK pathway. Therefore, these results suggest that ATM, like p53, is involved in the UVA-induced apoptosis to suppress carcinogenesis.

ATM, a central controller of cellular responses to DNA damage

Mutations in the ATM gene lead to the genetic disorder ataxia-telangiectasia. ATM encodes a protein kinase that is mainly distributed in the nucleus of proliferating cells. Recent studies reveal that ATM regulates multiple cell cycle checkpoints by phosphorylating different targets at different stages of the cell cycle. ATM also functions in the regulation of DNA repair and apoptosis, suggesting that it is a central regulator of responses to DNA double-strand breaks.

Ceramide in apoptosis signaling: relationship with oxidative stress

Ceramide is one of the major sphingosine-based lipid second messengers that is generated in response to various extracellular agents. However, while widespread attention has focused on ceramide as a second messenger involved in the induction of apoptosis, important issues with regard to the mechanisms of ceramide formation and mode of action remain to be addressed. Several lines of evidence suggest that ceramide and oxidative stress are intimately related in cell death induction. This review focuses on the putative relationships between oxidative stress and sphingolipid metabolism in the apoptotic process and discusses the potential mechanisms that connect and regulate the two phenomena.

Ionizing radiation-induced apoptosis in ataxia-telangiectasia fibroblasts. Roles of caspase-9 and cellular inhibitor of apoptosis protein-1

Ionizing radiation (IR) has been shown to induce apoptosis to a greater extent in a fibroblast cell line AT5BIVA derived from an individual with ataxia-telangiectasia (AT) than in control fibroblasts. However, the signaling pathway that underlies IR-induced apoptosis in AT cells has remained unknown. The mechanism of apoptosis in response to gamma-irradiation has now been examined in three AT fibroblast lines (AT3BIVA, AT4BIVA, and AT5BIVA) derived from different individuals with AT. The apoptotic indexes of these cell lines at 72 h after irradiation were 12, 31, and 35%, respectively, compared with a value of 2.3% for control fibroblasts. Immunoblot analysis and fluorometric assays revealed that the extents of IR-induced activation of caspase-3 and caspase-9 were markedly greater in AT4BIVA and AT5BIVA cells than in AT3BIVA and control cells. Furthermore, the basal abundance of the apoptotic inhibitor, a cellular inhibitor of apoptosis proteins (c-IAP-1), was markedly reduced in AT4BIVA and AT5BIVA cells compared with that in AT3BIVA and control cells. The overexpression of either caspase-9 mutant forms or recombinant c-IAP-1 in AT5BIVA cells inhibited the IR-induced activation of caspases-3 and 9 and reduced the apoptotic index of the irradiated cells. These results indicate that the extent of IR-induced apoptosis in different AT cell lines is inversely related to the abundance of c-IAP-1 and directly related to the extent of activation of caspases-3 and 9.

Elevated Cu/Zn-SOD exacerbates radiation sensitivity and hematopoietic abnormalities of Atm-deficient mice

Patients with the genetic disorder ataxia-telangiectasia (A-T) display a pleiotropic phenotype that includes neurodegeneration, immunodeficiency, cancer predisposition and hypersensitivity to ionizing radiation. The gene responsible is ATM, and ATM:-knockout mice recapitulate most features of A-T. In order to study the involvement of oxidative stress in the A-T phenotype, we examined mice deficient for Atm and overexpressing human Cu/Zn superoxide dismutase (SOD1). We report that elevated levels of SOD1 exacerbate specific features of the murine Atm- deficient phenotype, including abnormalities in hematopoiesis and radiosensitivity. The data are consistent with the possibility that oxidative stress contributes to some of the clinical features associated with the A-T phenotype.

Efficient DNA base excision repair in ataxia telangiectasia cells

Ataxia telangiectasia (A-T) cells are sensitive to a broad range of free-radical-producing and alkylating agents. Damage caused by such agents is in part repaired by base excision [base excision repair (BER)]. Two BER pathways have been demonstrated in mammalian cells: a single-nucleotide-insertion pathway and a long-patch pathway involving resynthesis of 2-10 nucleotides. Although early studies failed to detect DNA-repair defects in A-T cells exposed to ionizing radiation and radiomimetic agents, more recent experiments performed in non-dividing A-T cells and the demonstrated interaction of the A-T-mutated protein (ATM) with the BRCA1 gene product suggest that a DNA-repair defect may underlie, at least in part, the radiation sensitivity in A-T cells. We have analysed BER of a single abasic site or a single uracil in two A-T families, using an in vitro BER system. In both families, the mutation involved was homozygous and completely inactivated the ATM protein. No difference was observed between affected individuals and heterozygous or homozygous wild-type relatives in their capacity to perform DNA repair by either one-nucleotide insertion or the long-patch pathway. Hence, the putative DNA-repair defect in A-T cells, if any, does not involve BER.