Targeting double-strand breaks to replicating DNA identifies a subpathway of DSB repair that is defective in ataxia-telangiectasia cells.

The critical cellular defect(s) and basis for cell killing by ionizing radiation in ataxia-telangiectasia (A-T) are unknown. We use the topoisomerase I inhibitor camptothecin (CPT), which kills mainly S-phase cells and induces DSBs predominantly in replication forks, to show that A-T cells are defective in the repair of this particular subclass of DSBs. CPT-treated A-T cells reaching G2 have abnormally high levels of chromatid exchanges (viewed as prematurely condensed G2 chromosomes); aberrations in normal cells are mostly chromatid breaks. Transfectants of A-T cells with the wild-type ATM cDNA are corrected for CPT sensitivity, chromatid aberrations, and the DSB repair defect. These data suggest that in normal cells ATM, the A-T protein, probably recognizes DSBs in active replicons and targets the repair machinery to the breaks; in addition, the ATM protein is involved in the suppression of low-fidelity, adventitious rejoining between replication-associated DSBs. The loss of ATM functions therefore leads to genome destabilization, sensitivity to DSB-inducing agents and to the cancer-promoting illegitimate exchange events that follow.     

Crocidolite asbestos inhibits pentose phosphate oxidative pathway and glucose 6-phosphate dehydrogenase activity in human lung epithelial cells

The cytotoxicity of asbestos has been related to its ability to increase the production of reactive oxygen species (ROS), via the iron-catalyzed reduction of oxygen and/or the activation of NADPH oxidase. The pentose phosphate pathway (PPP) is generally activated by the cell exposure to oxidant molecules. Contrary to our expectations, asbestos (crocidolite) fibers caused a dose- and time-dependent inhibition of PPP and decreased its activation by an oxidative stress in human lung epithelial cells A549. In parallel, the intracellular activity of the PPP rate-limiting enzyme, glucose 6-phosphate dehydrogenase (G6PD), was significantly diminished by crocidolite exposure. This inhibition was selective, as the activity of other PPP and glycolysis enzymes was not modified, and was not attributable to a decreased expression of G6PD. On the opposite, the incubation with glass fibers MMVF10 did not modify PPP and G6PD activity. PPP and G6PD inhibition did not correlate with the increased nitric oxide (NO) production elicited by crocidolite in A549 cells. Experiments with the purified enzyme suggest that crocidolite inhibits G6PD by directly interacting with the protein. We propose here a new mechanism of asbestos-evoked oxidative stress, wherein fibers increase the intracellular ROS levels also by inhibiting the main antioxidant pathway of the cell.     

Genome Instability in Ataxia Telangiectasia (A-T) Families: Camptothecin-Induced Damage to Replicating DNA Discriminates between Obligate A-T Heterozygotes,A-T Homozygotes and Controls

Previously we used the topoisomerase I inhibitor camptothecin (CPT), which kills mainly S-phase cells primarily by inducing double strand breaks (DSBs) in replication forks, to show that ataxia telangiectasia (A-T) fibroblasts are defective in the repair of this particular subclass of DSBs. CPT treated A-T cells reaching G2 have abnormally high levels of chromatid exchanges, viewed as prematurely condensed G2 chromosomes (G2 PCC), compared with normal cells where aberrations are mostly chromatid breaks. Here we show that A-T lymphoblastoid cells established from individuals with different mutations in the ATM gene also exhibit increased levels of chromosomal exchanges in response to CPT, indicating that the replication-associated DSBs are misrepaired in all these cells. From family studies we show that the presence of a single mutated allele in obligate A-T heterozygotes leads to intermediate levels of chromosomal exchanges in CPT-treated lymphoblastoid cells, thus providing a functional and sensitive assay to identify these individuals.     

An Imperfect G2M Checkpoint Contributes to Chromosome Instability Following Irradiation of S and G2 Phase Cells

DNA double strand break (DSB) repair and checkpoint control represent two major mechanisms that function to reduce chromosomal instability following ionising irradiation (IR). Ataxia telangiectasia (A-T) cells have long been known to have defective checkpoint responses. Recent studies have shown that they also have a DSB repair defect following IR raising the issue of how ATM’s repair and checkpoint functions interplay to maintain chromosomal stability. A-T and Artemis cells manifest an identical and epistatic repair defect throughout the cell cycle demonstrating that ATM’s major repair defect following IR represents Artemis-dependent end-processing. Artemis cells show efficient G2/M checkpoint induction and a prolonged arrest relative to normal cells. Following irradiation of G2 cells, this checkpoint is dependent on ATM and A-T cells fail to show checkpoint arrest. In contrast, cells irradiated during S phase initiate a G2/M checkpoint which is independent of ATM and, significantly, both Artemis and A-T cells show a prolonged arrest at the G2/M checkpoint likely reflecting their repair defect. Strikingly, the G2/M checkpoint is released before the completion of repair when approximately 10-20 DSBs remain both for S phase and G2 phase irradiated cells. This defined sensitivity level of the G2/M checkpoint explains the prolonged arrest in repair-deficient relative to normal cells and provides a conceptual framework for the co-operative phenotype between checkpoint and repair functions in maintaining chromosomal stability.     

Neuroprotection by glucose-6-phosphate dehydrogenase and the pentose phosphate pathway

Glucose-6-phosphate dehydrogenase (G6PD), the rate limiting enzyme that channels glucose catabolism from glycolysis into the pentose phosphate pathway (PPP), is vital for the production of reduced nicotinamide adenine dinucleotide phosphate (NADPH) in cells. NADPH is in turn a substrate for glutathione reductase, which reduces oxidized glutathione disulfide to sulfhydryl glutathione. Best known for inherited deficiencies underlying acute hemolytic anemia due to elevated oxidative stress by food or medication, G6PD, and PPP activation have been associated with neuroprotection. Recent works have now provided more definitive evidence for G6PD's protective role in ischemic brain injury and strengthened its links to neurodegeneration. In Drosophila models, improved proteostasis and lifespan extension result from an increased PPP flux due to G6PD induction, which is phenocopied by transgenic overexpression of G6PD in neurons. Moderate transgenic expression of G6PD was also shown to improve healthspan in mouse. Here, the deciphered and implicated roles of G6PD and PPP in protection against brain injury, neurodegenerative diseases, and in healthspan/lifespan extensions are discussed together with an important caveat, namely NADPH oxidase (NOX) activity and the oxidative stress generated by the latter. Activation of G6PD with selective inhibition of NOX activity could be a viable neuroprotective strategy for brain injury, disease, and aging.     

Requirement of the MRN complex for ATM activation by DNA damage

The ATM protein kinase is a primary activator of the cellular response to DNA double-strand breaks (DSBs). In response to DSBs, ATM is activated and phosphorylates key players in various branches of the DNA damage response network. ATM deficiency causes the genetic disorder ataxia-telangiectasia (A-T), characterized by cerebellar degeneration, immunodeficiency, radiation sensitivity, chromosomal instability and cancer predisposition. The MRN complex, whose core contains the Mre11, Rad50 and Nbs1 proteins, is involved in the initial processing of DSBs. Hypomorphic mutations in the NBS1 and MRE11 genes lead to two other genomic instability disorders: the Nijmegen breakage syndrome (NBS) and A-T like disease (A-TLD), respectively. The order in which ATM and MRN act in the early phase of the DSB response is unclear. Here we show that functional MRN is required for ATM activation, and consequently for timely activation of ATM-mediated pathways. Collectively, these and previous results assign to components of the MRN complex roles upstream and downstream of ATM in the DNA damage response pathway and explain the clinical resemblance between A-T and A-TLD.     

Nuclear ataxia-telangiectasia mutated (ATM) mediates the cellular response to DNA double strand breaks in human neuron-like cells

The protein kinase ATM (ataxia-telangiectasia mutated) activates the cellular response to double strand breaks (DSBs), a highly cytotoxic DNA lesion. ATM is activated by DSBs and in turn phosphorylates key players in numerous damage response pathways. ATM is missing or inactivated in the autosomal recessive disorder ataxia-telangiectasia (A-T), which is characterized by neuronal degeneration, immunodeficiency, genomic instability, radiation sensitivity, and cancer predisposition. The predominant symptom of A-T is a progressive loss of movement coordination due to ongoing degeneration of the cerebellar cortex and peripheral neuropathy. A major deficiency in understanding A-T is the lack of information on the role of ATM in neurons. It is unclear whether the ATM-mediated DSB response operates in these cells similarly to proliferating cells. Furthermore, ATM was reported to be cytoplasmic in neurons and suggested to function in these cells in capacities other than the DNA damage response. Recently we obtained genetic molecular evidence that the neuronal degeneration in A-T does result from defective DNA damage response. We therefore undertook to investigate this response in a model system of human neuron-like cells (NLCs) obtained by neuronal differentiation in culture. ATM was largely nuclear in NLCs, and their ATM-mediated responses to DSBs were similar to those of proliferating cells. Knocking down ATM did not interfere with neuronal differentiation but abolished ATM-mediated damage responses in NLCs. We concluded that nuclear ATM mediates the DSB response in NLCs similarly to in proliferating cells. Attempts to understand the neurodegeneration in A-T should be directed to investigating the DSB response in the nervous system.     

Regulation of G6PD acetylation by SIRT2 and KAT9 modulates NADPH homeostasis and cell survival during oxidative stress

Glucose‐6‐phosphate dehydrogenase (G6PD) is a key enzyme in the pentose phosphate pathway (PPP) and plays an essential role in the oxidative stress response by producing NADPH, the main intracellular reductant. G6PD deficiency is the most common human enzyme defect, affecting more than 400 million people worldwide. Here, we show that G6PD is negatively regulated by acetylation on lysine 403 (K403), an evolutionarily conserved residue. The K403 acetylated G6PD is incapable of forming active dimers and displays a complete loss of activity. Knockdown of G6PD sensitizes cells to oxidative stress, and re‐expression of wild‐type G6PD, but not the K403 acetylation mimetic mutant, rescues cells from oxidative injury. Moreover, we show that cells sense extracellular oxidative stimuli to decrease G6PD acetylation in a SIRT2‐dependent manner. The SIRT2‐mediated deacetylation and activation of G6PD stimulates PPP to supply cytosolic NADPH to counteract oxidative damage and protect mouse erythrocytes. We also identified KAT9/ELP3 as a potential acetyltransferase of G6PD. Our study uncovers a previously unknown mechanism by which acetylation negatively regulates G6PD activity to maintain cellular NADPH homeostasis during oxidative stress.     

A critical role of glucose-6-phosphate dehydrogenase in TAp73-mediated cell proliferation

The pentose phosphate pathway (PPP) provides ribose and NADPH that support biosynthesis and antioxidant defense. Our recent findings suggest that the p53-related protein TAp73 enhances the PPP flux. TAp73 stimulates the expression of glucose-6-phophate dehydrogenase (G6PD), the rate-limiting enzymes of the PPP. Through this regulation, TAp73 promotes the accumulation of macromolecules and increases cellular capability to withstand oxidative stresses. TAp73 also regulates other metabolic enzymes, and the relative importance of these targets in TAp73-mediated cell growth is not well understood. Here we show that, like in other cell lines, TAp73 is required for supporting proliferation and maintaining the expression of G6PD in the human lung cancer H1299 cells. Restoration of G6PD expression almost fully rescues the defects in cell growth caused by TAp73 knockdown, suggesting that G6PD is the major proliferative target of TAp73 in these cells. G6PD expression is elevated in various tumors, correlating with the upregulation of TAp73. These results indicate that TAp73 may function as an oncogene, and that G6PD is likely a focal point of regulation in oncogenic growth.     

MEC1-dependent redistribution of the Sir3 silencing protein from telomeres to DNA double-strand breaks.

The yeast Sir2/3/4p complex is found in abundance at telomeres, where it participates in the formation of silent heterochromatin and telomere maintenance. Here, we show that Sir3p is released from telomeres in response to DNA double-strand breaks (DSBs), binds to DSBs, and mediates their repair, independent of cell mating type. Sir3p relocalization is S phase specific and, importantly, requires the DNA damage checkpoint genes MEC1 and RAD9. MEC1 is a homolog of ATM, mutations in which cause ataxia telangiectasia (A-T), a disease characterized by various neurologic and immunologic abnormalities, a predisposition for cancer, and a cellular defect in repair of DSBs. This novel mode by which preformed DNA repair machinery is mobilized by DNA damage sensors may have implications for human diseases resulting from defective DSB repair.     

SIRT5 promotes IDH2 desuccinylation and G6PD deglutarylation to enhance cellular antioxidant defense

Excess in mitochondrial reactive oxygen species (ROS) is considered as a major cause of cellular oxidative stress. NADPH, the main intracellular reductant, has a key role in keeping glutathione in its reduced form GSH, which scavenges ROS and thus protects the cell from oxidative damage. Here, we report that SIRT5 desuccinylates and deglutarylates isocitrate dehydrogenase 2 (IDH2) and glucose‐6‐phosphate dehydrogenase (G6PD), respectively, and thus activates both NADPH‐producing enzymes. Moreover, we show that knockdown or knockout of SIRT5 leads to high levels of cellular ROS. SIRT5 inactivation leads to the inhibition of IDH2 and G6PD, thereby decreasing NADPH production, lowering GSH, impairing the ability to scavenge ROS, and increasing cellular susceptibility to oxidative stress. Our study uncovers a SIRT5‐dependent mechanism that regulates cellular NADPH homeostasis and redox potential by promoting IDH2 desuccinylation and G6PD deglutarylation.     

Glycolytic cancer cells lacking 6-phosphogluconate dehydrogenase metabolize glucose to induce senescence

We show that knockdown of 6-phosphogluconate dehydrogenase (6PGD) of the pentose phosphate pathway (PPP) inhibits growth of lung cancer cells by senescence induction. This inhibition is not due to a defect in the oxidative PPP per se. NADPH and ribose phosphate production are normal in 6PGD knockdown cells and shutdown of PPP by knockdown of glucose-6-phosphate dehydrogenase (G6PD) has little effect on cell growth. Moreover, 6PGD knockdown cells can proliferate when the PPP is bypassed by using fructose instead of glucose in medium. Significantly, G6PD knockdown rescues proliferation of cells lacking 6PGD, suggesting an accumulation of growth inhibitory glucose metabolics in cells lacking 6PGD. Therefore, 6PGD inhibition may provide a novel strategy to treat glycolyic tumors such as lung cancer.     

ATM and Artemis promote homologous recombination of radiation‐induced DNA double‐strand breaks in G2

Homologous recombination (HR) and non‐homologous end joining (NHEJ) represent distinct pathways for repairing DNA double‐strand breaks (DSBs). Previous work implicated Artemis and ATM in an NHEJ‐dependent process, which repairs a defined subset of radiation‐induced DSBs in G1‐phase. Here, we show that in G2, as in G1, NHEJ represents the major DSB‐repair pathway whereas HR is only essential for repair of ～15% of X‐ or γ‐ray‐induced DSBs. In addition to requiring the known HR proteins, Brca2, Rad51 and Rad54, repair of radiation‐induced DSBs by HR in G2 also involves Artemis and ATM suggesting that they promote NHEJ during G1 but HR during G2. The dependency for ATM for repair is relieved by depleting KAP‐1, providing evidence that HR in G2 repairs heterochromatin‐associated DSBs. Although not core HR proteins, ATM and Artemis are required for efficient formation of single‐stranded DNA and Rad51 foci at radiation‐induced DSBs in G2 with Artemis function requiring its endonuclease activity. We suggest that Artemis endonuclease removes lesions or secondary structures, which inhibit end resection and preclude the completion of HR or NHEJ.     

Peroxynitrite protects neurons against nitric oxide-mediated apoptosis. A key role for glucose-6-phosphate dehydrogenase activity in neuroprotection

Peroxynitrite is thought to be a nitric oxide-derived neurotoxic effector molecule involved in the disruption of key energy-related metabolic targets. To assess the consequences of such interference in cellular glucose metabolism and viability, we studied the possible modulatory role played by peroxynitrite in glucose oxidation in neurons and astrocytes in primary culture. Here, we report that peroxynitrite triggered rapid stimulation of pentose phosphate pathway (PPP) activity and the accumulation of NADPH, an essential cofactor for glutathione regeneration. In contrast to peroxynitrite, nitric oxide elicited NADPH depletion, glutathione oxidation, and apoptotic cell death in neurons, but not in astrocytes. These events were noticeably counteracted by pretreatment of neurons with peroxynitrite. In an attempt to elucidate the mechanism responsible for this PPP stimulation and neuroprotection, we found evidence consistent with both exogenous and endogenous peroxynitrite-mediated activation of glucose-6-phosphate dehydrogenase (G6PD), an enzyme that catalyzes the first rate-limiting step in the PPP. Moreover, functional overexpression of the G6PD gene in stably transformed PC12 cells induced NADPH accumulation and offered remarkable resistance against nitric oxide-mediated apoptosis, whereas G6PD gene-targeted antisense inhibition depleted NADPH levels and exacerbated cellular vulnerability. In light of these results, we suggest that G6PD activation represents a novel role for peroxynitrite in neuroprotection against nitric oxide-mediated apoptosis.     

The influence of heterochromatin on DNA double strand break repair: Getting the strong, silent type to relax

DNA non-homologous end-joining (NHEJ) and homologous recombination (HR) represent the major DNA double strand break (DSB) pathways in mammalian cells, whilst ataxia telangiectasia mutated (ATM) lies at the core of the DSB signalling response. ATM signalling plays a major role in modifying chromatin structure in the vicinity of the DSB and increasing evidence suggests that this function influences the DSB rejoining process. DSBs have long been known to be repaired with two (or more) component kinetics. The majority (～85%) of DSBs are repaired with fast kinetics in a predominantly ATM-independent manner. In contrast, ～15% of radiation-induced DSBs are repaired with markedly slower kinetics via a process that requires ATM and those mediator proteins, such as MDC1 or 53BP1, that accumulate at ionising radiation induced foci (IRIF). DSBs repaired with slow kinetics predominantly localise to the periphery of genomic heterochromatin (HC). Indeed, there is mounting evidence that chromatin complexity and not damage complexity confers slow DSB repair kinetics. ATM's role in HC-DSB repair involves the direct phosphorylation of KAP-1, a key HC formation factor. KAP-1 phosphorylation (pKAP-1) arises in both a pan-nuclear and a focal manner after radiation and ATM-dependent pKAP-1 is essential for DSB repair within HC regions. Mediator proteins such as 53BP1, which are also essential for HC-DSB repair, are expendable for pan-nuclear pKAP-1 whilst being essential for pKAP-1 formation at IRIF. Data suggests that the essential function of the mediator proteins is to promote the retention of activated ATM at DSBs, concentrating the phosphorylation of KAP-1 at HC DSBs. DSBs arising in G2 phase are also repaired with fast and slow kinetics but, in contrast to G0/G1 where they all DSBs are repaired by NHEJ, the slow component of DSB repair in G2 phase represents an HR process involving the Artemis endonuclease. Results suggest that whilst NHEJ repairs the majority of DSBs in G2 phase, Artemis-dependent HR uniquely repairs HC DSBs. Collectively, these recent studies highlight not only how chromatin complexity influences the factors required for DSB repair but also the pathway choice.     

SIRT2 controls the pentose phosphate switch

The most common enzyme defect in humans is glucose‐6‐phosphate dehydrogenase (G6PD) deficiency, which affects more than 400 million people. G6PD shunts glucose into the pentose phosphate pathway (PPP) to generate nucleotides and reducing potential in the form of NADPH. In this issue, Wang et al ( 2014 ) show that G6PD activity is post‐translationally regulated by SIRT2, a cytoplasmic NAD⁺‐dependent deacetylase, thereby linking NAD⁺ levels to DNA repair and oxidative defences, and identifying potential new approaches to treating this common genetic disease.     

Roles of nibrin and AtM/ATR kinases on the G2 checkpoint under endogenous or radio-induced DNA damage

Checkpoint response to DNA damage involves the activation of DNA repair and G2 lengthening subpathways. The roles of nibrin (NBS1) and the ATM/ATR kinases in the G2 DNA damage checkpoint, evoked by endogenous and radio-induced DNA damage, were analyzed in control, A-T and NBS lymphoblast cell lines. Short-term responses to G2 treatments were evaluated by recording changes in the yield of chromosomal aberrations in the ensuing mitosis, due to G2 checkpoint adaptation, and also in the duration of G2 itself. The role of ATM/ATR in the G2 checkpoint pathway repairing chromosomal aberrations was unveiled by caffeine inhibition of both kinases in G2. In the control cell lines, nibrin and ATM cooperated to provide optimum G2 repair for endogenous DNA damage. In the A-T cells, ATR kinase substituted successfully for ATM, even though no G2 lengthening occurred. X-ray irradiation (0.4 Gy) in G2 increased chromosomal aberrations and lengthened G2, in both mutant and control cells. However, the repair of radio-induced DNA damage took place only in the controls. It was associated with nibrin-ATM interaction, and ATR did not substitute for ATM. The absence of nibrin prevented the repair of both endogenous and radio-induced DNA damage in the NBS cells and partially affected the induction of G2 lengthening.