The ATM protein kinase: regulating the cellular response to genotoxic stress, and more

Direct activation of ATM by resveratrol under oxidizing conditions

Resveratrol has been widely reported to reduce cancer progression in model systems and to selectively induce cell death in transformed cell lines. Many enzymes have been reported to respond to resveratrol in mammalian cells, including the Ataxia-Telangiectasia Mutated (ATM) protein kinase that acts in DNA damage recognition, signaling, and repair. Here we investigate the responses of ATM to resveratrol exposure in normal and transformed human cell lines and find that ATM autophosphorylation and substrate phosphorylation is stimulated by resveratrol in a manner that is promoted by reactive oxygen species (ROS). We observe direct stimulatory effects of resveratrol on purified ATM in vitro and find that the catalytic efficiency of the kinase on a model substrate is increased by resveratrol. In the purified system we also observe a requirement for oxidation, as the effect of resveratrol on ATM signaling is substantially reduced by agents that prevent disulfide bond formation in ATM. These results demonstrate that resveratrol effects on ATM are direct, and suggest a mechanism by which the oxidizing environment of transformed cells promotes ATM activity and blocks cell proliferation.

Pot1a prevents telomere dysfunction and ATM-dependent neuronal loss

Genome stability is essential for neural development and the prevention of neurological disease. Here we determined how DNA damage signaling from dysfunctional telomeres affects neurogenesis. We found that telomere uncapping by Pot1a inactivation resulted in an Atm-dependent loss of cerebellar interneurons and granule neuron precursors in the mouse nervous system. The activation of Atm by Pot1a loss occurred in an Atr-dependent manner, revealing an Atr to Atm signaling axis in the nervous system after telomere dysfunction. In contrast to telomere lesions, Brca2 inactivation in neural progenitors also led to ablation of cerebellar interneurons, but this did not require Atm. These data reveal that neural cell loss after DNA damage selectively engages Atm signaling, highlighting how specific DNA lesions can dictate neuropathology arising in human neurodegenerative syndromes.

Generation and repair of AID-initiated DNA lesions in B lymphocytes

Activation-induced deaminase (AID) initiates the secondary antibody diversification process in B lymphocytes. In mammalian B cells, this process includes somatic hypermutation (SHM) and class switch recombination (CSR), both of which require AID. AID induces U:G mismatch lesions in DNA that are subsequently converted into point mutations or DNA double stranded breaks during SHM/CSR. In a physiological context, AID targets immunoglobulin (Ig) loci to mediate SHM/CSR. However, recent studies reveal genome-wide access of AID to numerous non-Ig loci. Thus, AID poses a threat to the genome of B cells if AID-initiated DNA lesions cannot be properly repaired. In this review, we focus on the molecular mechanisms that regulate the specificity of AID targeting and the repair pathways responsible for processing AID-initiated DNA lesions.

Aberrant topoisomerase-1 DNA lesions are pathogenic in neurodegenerative genome instability syndromes

DNA damage is considered to be a prime factor in several spinocerebellar neurodegenerative diseases; however, the DNA lesions underpinning disease etiology are unknown. We observed the endogenous accumulation of pathogenic topoisomerase-1 (Top1)-DNA cleavage complexes (Top1ccs) in murine models of ataxia telangiectasia and spinocerebellar ataxia with axonal neuropathy 1. We found that the defective DNA damage response factors in these two diseases cooperatively modulated Top1cc turnover in a non-epistatic and ATM kinase-independent manner. Furthermore, coincident neural inactivation of ATM and DNA single-strand break repair factors, including tyrosyl-DNA phosphodiesterase-1 or XRCC1, resulted in increased Top1cc formation and excessive DNA damage and neurodevelopmental defects. Notably, direct Top1 poisoning to elevate Top1cc levels phenocopied the neuropathology of the mouse models described above. Our results identify a critical endogenous pathogenic lesion associated with neurodegenerative syndromes arising from DNA repair deficiency, indicating that genome integrity is important for preventing disease in the nervous system.

Congenital microcephaly

The underlying etiologies of genetic congenital microcephaly are complex and multifactorial. Recently, with the exponential growth in the identification and characterization of novel genetic causes of congenital microcephaly, there has been a consolidation and emergence of certain themes concerning underlying pathomechanisms. These include abnormal mitotic microtubule spindle structure, numerical and structural abnormalities of the centrosome, altered cilia function, impaired DNA repair, DNA Damage Response signaling and DNA replication, along with attenuated cell cycle checkpoint proficiency. Many of these processes are highly interconnected. Interestingly, a defect in a gene whose encoded protein has a canonical function in one of these processes can often have multiple impacts at the cellular level involving several of these pathways. Here, we overview the key pathomechanistic themes underlying profound congenital microcephaly, and emphasize their interconnected nature.

Interactions Between Ataxia Telangiectasia Mutated Kinase Inhibition, Poly(ADP-ribose) Polymerase-1 Inhibition and BRCA1 Status in Breast Cancer Cells

BACKGROUND

Cells harboring BRCA1/BRCA2 mutations are hypersensitive to inhibition of poly(ADP-ribose) polymerase-1 (PARP-1). We recently showed that interference with PARP-1 activity by NU1025 is strongly cytotoxic for BRCA1-positive BT-20 cells but not BRCA1-deficient SKBr-3 cells. These unexpected observations prompted speculation that other PARP-1 inhibitor(s) may be more cytotoxic towards SKBr-3 cells. In addition, interference with the DNA damage signaling pathway via (for instance) Ataxia telangiectasia mutated (ATM) kinase inhibition may induce synthetic lethality in DNA repair-deficient breast cancer cells and pharmacological interference with ATM activity may sensitize breast cancer cells to PARP-1 inactivation.

METHODS

We determined drug cytotoxicity in human MCF-7 and SKBr-3 breast cancer cells using the CellTiterGLO Luminescent cell viability assay and a Tecan multi-label, multitask plate counter to measure generated luminescence. Changes in cell cycle progression were monitored by flow cytometric measurement of DNA content in cells stained with propidium iodide.

RESULTS

Unlike NU1025, AZD2461, a new PARP-1 inhibitor, markedly reduced the numbers of living MCF-7 and SKBr-3 cells. ATM kinase inhibition (CP466722) was also cytotoxic for both MCF-7 and SKBr-3 cells. Furthermore, AZD2461 enhanced the cytotoxicity of CP466722 in both cell lines by inducing apoptosis, and concurrent inhibition of ATM and PARP-1 reduced cell proliferation more strongly than either single treatment.

CONCLUSIONS

Our data show that inhibition of PARP-1 by AZD2461 is synthetically lethal for NU1025-resistant MCF-7 and SKBr-3 breast cancer cells. They also indicate that DNA damage signaling is essential for survival of both SKBr-3 and MCF-7 cells, especially after inactivation of PARP-1.

Exploration of liver cancer genomes

Liver cancer is the third leading cause of cancer-related death worldwide. Advances in sequencing technologies have enabled the examination of liver cancer genomes at high resolution; somatic mutations, structural alterations, HBV integration, RNA editing and retrotransposon changes have been comprehensively identified. Furthermore, integrated analyses of trans-omics data (genome, transcriptome and methylome data) have identified multiple critical genes and pathways implicated in hepatocarcinogenesis. These analyses have uncovered potential therapeutic targets, including growth factor signalling, WNT signalling, the NFE2L2-mediated oxidative pathway and chromatin modifying factors, and paved the way for new molecular classifications for clinical application. The aetiological factors associated with liver cancer are well understood; however, their effects on the accumulation of somatic changes and the influence of ethnic variation in risk factors still remain unknown. The international collaborations of cancer genome sequencing projects are expected to contribute to an improved understanding of risk evaluation, diagnosis and therapy for this cancer.

The Role of Translational Regulation in Survival after Radiation Damage; an Opportunity for Proteomics Analysis

In this review, we will summarize the data from different model systems that illustrate the need for proteome-wide analyses of the biological consequences of ionizing radiation (IR). IR remains one of three main therapy choices for oncology, the others being surgery and chemotherapy. Understanding how cells and tissues respond to IR is essential for improving therapeutic regimes against cancer. Numerous studies demonstrating the changes in the transcriptome following exposure to IR, in diverse systems, can be found in the scientific literature. However, the limitation of our knowledge is illustrated by the fact that the number of transcripts that change after IR exposure is approximately an order of magnitude lower than the number of transcripts that re-localize to or from ribosomes under similar conditions. Furthermore, changes in the post-translational modifications of proteins (phosphorylation, acetylation as well as degradation) are profoundly important for the cellular response to IR. These considerations make proteomics a highly suitable tool for mechanistic studies of the effect of IR. Strikingly such studies remain outnumbered by those utilizing proteomics for diagnostic purposes such as the identification of biomarkers for the outcome of radiation therapy. Here we will discuss the role of the ribosome and translational regulation in the survival and preservation of cells and tissues after exposure to ionizing radiation. In doing so we hope to provide a strong incentive for the study of proteome-wide changes following IR exposure.

A fine-scale dissection of the DNA double-strand break repair machinery and its implications for breast cancer therapy

DNA-damage response machinery is crucial to maintain the genomic integrity of cells, by enabling effective repair of even highly lethal lesions such as DNA double-strand breaks (DSBs). Defects in specific genes acquired through mutations, copy-number alterations or epigenetic changes can alter the balance of these pathways, triggering cancerous potential in cells. Selective killing of cancer cells by sensitizing them to further DNA damage, especially by induction of DSBs, therefore requires careful modulation of DSB-repair pathways. Here, we review the latest knowledge on the two DSB-repair pathways, homologous recombination and non-homologous end joining in human, describing in detail the functions of their components and the key mechanisms contributing to the repair. Such an in-depth characterization of these pathways enables a more mechanistic understanding of how cells respond to therapies, and suggests molecules and processes that can be explored as potential therapeutic targets. One such avenue that has shown immense promise is via the exploitation of synthetic lethal relationships, for which the BRCA1-PARP1 relationship is particularly notable. Here, we describe how this relationship functions and the manner in which cancer cells acquire therapy resistance by restoring their DSB repair potential.

DNA replication stress in CHK1-depleted tumour cells triggers premature (S-phase) mitosis through inappropriate activation of Aurora kinase B

The disruption of DNA replication in cells triggers checkpoint responses that slow-down S-phase progression and protect replication fork integrity. These checkpoints are also determinants of cell fate and can help maintain cell viability or trigger cell death pathways. CHK1 has a pivotal role in such S-phase responses. It helps maintain fork integrity during replication stress and protects cells from several catastrophic fates including premature mitosis, premature chromosome condensation and apoptosis. Here we investigated the role of CHK1 in protecting cancer cells from premature mitosis and apoptosis. We show that premature mitosis (characterized by the induction of histone H3 phosphorylation, aberrant chromatin condensation, and persistent RPA foci in arrested S-phase cells) is induced in p53-deficient tumour cells depleted of CHK1 when DNA synthesis is disrupted. These events are accompanied by an activation of Aurora kinase B in S-phase cells that is essential for histone H3 Ser10 phosphorylation. Histone H3 phosphorylation precedes the induction of apoptosis in p53-/- tumour cell lines but does not appear to be required for this fate as an Aurora kinase inhibitor suppresses phosphorylation of both Aurora B and histone H3 but has little effect on cell death. In contrast, only a small fraction of p53+/+ tumour cells shows this premature mitotic response, although they undergo a more rapid and robust apoptotic response. Taken together, our results suggest a novel role for CHK1 in the control of Aurora B activation during DNA replication stress and support the idea that premature mitosis is a distinct cell fate triggered by the disruption of DNA replication when CHK1 function is suppressed.

ATM increases activation-induced cytidine deaminase activity at downstream S regions during class-switch recombination

Activation-induced cytidine deaminase (AID) initiates Ab class-switch recombination (CSR) in activated B cells resulting in exchanging the IgH C region and improved Ab effector function. During CSR, AID instigates DNA double-strand break (DSB) formation in switch (S) regions located upstream of C region genes. DSBs are necessary for CSR, but improper regulation of DSBs can lead to chromosomal translocations that can result in B cell lymphoma. The protein kinase ataxia telangiectasia mutated (ATM) is an important proximal regulator of the DNA damage response (DDR), and translocations involving S regions are increased in its absence. ATM phosphorylates H2AX, which recruits other DNA damage response (DDR) proteins, including mediator of DNA damage checkpoint 1 (Mdc1) and p53 binding protein 1 (53BP1), to sites of DNA damage. As these DDR proteins all function to promote repair and recombination of DSBs during CSR, we examined whether mouse splenic B cells deficient in these proteins would show alterations in S region DSBs when undergoing CSR. We find that in atm(-/-) cells Sμ DSBs are increased, whereas DSBs in downstream Sγ regions are decreased. We also find that mutations in the unrearranged Sγ3 segment are reduced in atm(-/-) cells. Our data suggest that ATM increases AID targeting and activity at downstream acceptor S regions during CSR and that in atm(-/-) cells Sμ DSBs accumulate as they lack a recombination partner.

Parallel profiling of the transcriptome, cistrome, and epigenome in the cellular response to ionizing radiation

The DNA damage response (DDR) is a vast signaling network that is robustly activated by DNA double-strand breaks, the critical lesion induced by ionizing radiation (IR). Although much of this response operates at the protein level, a critical component of the network sustains many DDR branches by modulating the cellular transcriptome. Using deep sequencing, we delineated three layers in the transcriptional response to IR in human breast cancer cells: changes in the expression of genes encoding proteins or long noncoding RNAs, alterations in genomic binding by key transcription factors, and dynamics of epigenetic markers of active promoters and enhancers. We identified protein-coding and previously unidentified noncoding genes that were responsive to IR, and demonstrated that IR-induced transcriptional dynamics was mediated largely by the transcription factors p53 and nuclear factor κB (NF-κB) and was primarily dependent on the kinase ataxia-telangiectasia mutated (ATM). The resultant data set provides a rich resource for understanding a basic, underlying component of a critical cellular stress response.

CCT128930 induces cell cycle arrest, DNA damage, and autophagy independent of Akt inhibition

PI3K/Akt/mTOR pathway plays an important role in tumor progression and anti-cancer drug resistance. The aim of the present study is to determine the antitumor effect of CCT128930, a novel small molecule inhibitor of Akt, in the HepG2 hepatoma cancer cells. Our results showed that at low concentrations, CCT128930 increased, but not inhibited, the phosphorylation of Akt in HepG2 and A549 cells. CCT128930 inhibited cell proliferation by inducing cell cycle arrest in G1 phase through downregulation of cyclinD1 and Cdc25A, and upregulation of p21, p27 and p53. A higher dose (20 μM) of CCT128930 triggered cell apoptosis with activation of caspase-3, caspase-9, and PARP. Treatment with CCT128930 increased phosphorylation of ERK and JNK in HepG2 cells. CCT128930 activated DNA damage response of HepG2 cell characterized by phosphorylation of H2AX, ATM (ataxia-telangiectasia mutated), Chk1 and Chk2. Upon exposure to CCT128930 at a higher concentration, HepG2 cells exhibited autophagy was accompanied by an increase the levels of LC3-II and Beclin-1. Blocking autophagy using chloroquine magnified CCT128930-induced apoptotic cell death and the phosphorylation of H2AX. The results in this study have advanced our current understandings of the anti-cancer mechanisms of CCT128930 in cancer cells.

Immune deficiency in Ataxia-Telangiectasia: a longitudinal study of 44 patients

Ataxia-Telangiectasia (A-T) is a genetic condition leading to neurological defects and immune deficiency. The nature of the immune deficiency is highly variable, and in some cases causes significant morbidity and mortality due to recurrent sinopulmonary infections. Although the neurological defects in A-T are progressive, the natural history of the immune deficiency in A-T has not been evaluated formally. In this study we analyse the clinical history and immunological data in 44 patients with A-T who attended the National Ataxia-Telangiectasia clinic in Nottingham between 2001 and 2011. Using patient medical records and Nottingham University Hospitals (NUH) National Health Service Trust medical IT systems, data regarding clinical history, use of immunoglobulin replacement therapy, total immunoglobulin levels, specific antibody levels and lymphocyte subset counts were obtained. T cell receptor spectratyping results in some patients were already available and, where possible, repeat blood samples were collected for analysis. This study shows that subtle quantitative changes in certain immunological parameters such as lymphocyte subset counts may occur in patients with A-T over time. However, in general, for the majority of patients the severity of immune deficiency (both clinically and in terms of immunological blood markers) does not seem to deteriorate significantly with time. This finding serves to inform the long-term management of this cohort of patients because, if recurrent respiratory tract infections present later in life, then other contributory factors (e.g. cough/swallowing difficulties, underlying lung disease) should be investigated aggressively. Our findings also offer some form of reassurance for parents of children with A-T, which is otherwise a progressively severely debilitating condition.

Live longer on MARS: a yeast paradigm of mitochondrial adaptive ROS signaling in aging

Adaptive responses to stress, including hormesis, have been implicated in longevity, but their mechanisms and outcomes are not fully understood. Here, I briefly summarize a longevity mechanism elucidated in the budding yeast chronological lifespan model by which Mitochondrial Adaptive ROS Signaling (MARS) promotes beneficial epigenetic and metabolic remodeling. The potential relevance of MARS to the human disease Ataxia-Telangiectasia and as a potential anti-aging target is discussed.

Chemotherapeutic compounds targeting the DNA double-strand break repair pathways: the good, the bad, and the promising

The repair of DNA double-strand breaks (DSBs) is a critical cellular mechanism that exists to ensure genomic stability. DNA DSBs are the most deleterious type of insult to a cell's genetic material and can lead to genomic instability, apoptosis, or senescence. Incorrectly repaired DNA DSBs have the potential to produce chromosomal translocations and genomic instability, potentially leading to cancer. The prevalence of DNA DSBs in cancer due to unregulated growth and errors in repair opens up a potential therapeutic window in the treatment of cancers. The cellular response to DNA DSBs is comprised of two pathways to ensure DNA breaks are repaired: homologous recombination and non-homologous end joining. Identifying chemotherapeutic compounds targeting proteins involved in these DNA repair pathways has shown promise as a cancer therapy for patients, either as a monotherapy or in combination with genotoxic drugs. From the beginning, there have been a number of chemotherapeutic compounds that have yielded successful responses in the clinic, a number that have failed (CGK-733 and iniparib), and a number of promising targets for future studies identified. This review looks in detail at how the cell responds to these DNA DSBs and investigates the chemotherapeutic avenues that have been and are currently being explored to target this repair process.

Nitric oxide induces ataxia telangiectasia mutated (ATM) protein-dependent γH2AX protein formation in pancreatic β cells

In this study, the effects of cytokines on the activation of the DNA double strand break repair factors histone H2AX (H2AX) and ataxia telangiectasia mutated (ATM) were examined in pancreatic β cells. We show that cytokines stimulate H2AX phosphorylation (γH2AX formation) in rat islets and insulinoma cells in a nitric oxide- and ATM-dependent manner. In contrast to the well documented role of ATM in DNA repair, ATM does not appear to participate in the repair of nitric oxide-induced DNA damage. Instead, nitric oxide-induced γH2AX formation correlates temporally with the onset of irreversible DNA damage and the induction of apoptosis. Furthermore, inhibition of ATM attenuates cytokine-induced caspase activation. These findings show that the formation of DNA double strand breaks correlates with ATM activation, irreversible DNA damage, and ATM-dependent induction of apoptosis in cytokine-treated β cells.

PF, Brogan JR, Kurimasa A, Chen DJ, Bedford JS, Chen BPC. Differential radiosensitivity phenotypes of DNA-PKcs mutations affecting NHEJ and HRR systems following irradiation with gamma-rays or very low fluences of alpha particles

We have examined cell-cycle dependence of chromosomal aberration induction and cell killing after high or low dose-rate γ irradiation in cells bearing DNA-PKcs mutations in the S2056 cluster, the T2609 cluster, or the kinase domain. We also compared sister chromatid exchanges (SCE) production by very low fluences of α-particles in DNA-PKcs mutant cells, and in homologous recombination repair (HRR) mutant cells including Rad51C, Rad51D, and Fancg/xrcc9. Generally, chromosomal aberrations and cell killing by γ-rays were similarly affected by mutations in DNA-PKcs, and these mutant cells were more sensitive in G1 than in S/G2 phase. In G1-irradiated DNA-PKcs mutant cells, both chromosome- and chromatid-type breaks and exchanges were in excess than wild-type cells. For cells irradiated in late S/G2 phase, mutant cells showed very high yields of chromatid breaks compared to wild-type cells. Few exchanges were seen in DNA-PKcs-null, Ku80-null, or DNA-PKcs kinase dead mutants, but exchanges in excess were detected in the S2506 or T2609 cluster mutants. SCE induction by very low doses of α-particles is resulted from bystander effects in cells not traversed by α-particles. SCE seen in wild-type cells was completely abolished in Rad51C- or Rad51D-deficient cells, but near normal in Fancg/xrcc9 cells. In marked contrast, very high levels of SCEs were observed in DNA-PKcs-null, DNA-PKcs kinase-dead and Ku80-null mutants. SCE induction was also abolished in T2609 cluster mutant cells, but was only slightly reduced in the S2056 cluster mutant cells. Since both non-homologous end-joining (NHEJ) and HRR systems utilize initial DNA lesions as a substrate, these results suggest the possibility of a competitive interference phenomenon operating between NHEJ and at least the Rad51C/D components of HRR; the level of interaction between damaged DNA and a particular DNA-PK component may determine the level of interaction of such DNA with a relevant HRR component.

Tug of war between survival and death: exploring ATM function in cancer

Ataxia-telangiectasia mutated (ATM) kinase is a one of the main guardian of genome stability and plays a central role in the DNA damage response (DDR). The deregulation of these pathways is strongly linked to cancer initiation and progression as well as to the development of therapeutic approaches. These observations, along with reports that identify ATM loss of function as an event that may promote tumor initiation and progression, point to ATM as a bona fide tumor suppressor. The identification of ATM as a positive modulator of several signalling networks that sustain tumorigenesis, including oxidative stress, hypoxia, receptor tyrosine kinase and AKT serine-threonine kinase activation, raise the question of whether ATM function in cancer may be more complex. This review aims to give a complete overview on the work of several labs that links ATM to the control of the balance between cell survival, proliferation and death in cancer.

Cytolethal distending toxin (CDT) is a radiomimetic agent and induces persistent levels of DNA double-strand breaks in human fibroblasts

Cytolethal distending toxin (CDT) is a unique genotoxin produced by several pathogenic bacteria. The tripartite protein toxin is internalized into mammalian cells via endocytosis followed by retrograde transport to the ER. Upon translocation into the nucleus, CDT catalyzes the formation of DNA double-strand breaks (DSBs) due to its intrinsic endonuclease activity. In the present study, we compared the DNA damage response (DDR) in human fibroblasts triggered by recombinant CDT to that of ionizing radiation (IR), a well-known DSB inducer. Furthermore, we dissected the pathways involved in the detection and repair of CDT-induced DNA lesions. qRT-PCR array-based mRNA and western blot analyses showed a partial overlap in the DDR pattern elicited by CDT and IR, with strong activation of both the ATM-Chk2 and the ATR-Chk1 axis. In line with its in vitro DNase I-like activity on plasmid DNA, neutral and alkaline Comet assay revealed predominant induction of DSBs in CDT-treated fibroblasts, whereas irradiation of cells generated higher amounts of SSBs and alkali-labile sites. Using confocal microscopy, the dynamics of the DSB surrogate marker γ-H2AX was monitored after pulse treatment with CDT or IR. In contrast to the fast induction and disappearance of γ-H2AX-foci observed in irradiated cells, the number of γ-H2AX-foci induced by CDT were formed with a delay and persisted. 53BP1 foci were also generated following CDT treatment and co-localized with γ-H2AX foci. We further demonstrated that ATM-deficient cells are very sensitive to CDT-induced DNA damage as reflected by increased cell death rates with concomitant cleavage of caspase-3 and PARP-1. Finally, we provided novel evidence that both homologous recombination (HR) and non-homologous end joining (NHEJ) protect against CDT-elicited DSBs. In conclusion, the findings suggest that CDT functions as a radiomimetic agent and, therefore, is an attractive tool for selectively inducing persistent levels of DSBs and unveiling the associated cellular responses.

R-loops in proliferating cells but not in the brain: implications for AOA2 and other autosomal recessive ataxias

Disruption of the Setx gene, defective in ataxia oculomotor apraxia type 2 (AOA2) leads to the accumulation of DNA/RNA hybrids (R-loops), failure of meiotic recombination and infertility in mice. We report here the presence of R-loops in the testes from other autosomal recessive ataxia mouse models, which correlate with fertility in these disorders. R-loops were coincident in cells showing high basal levels of DNA double strand breaks and in those cells undergoing apoptosis. Depletion of Setx led to high basal levels of R-loops and these were enhanced further by DNA damage both in vitro and in vivo in tissues with proliferating cells. There was no evidence for accumulation of R-loops in the brains of mice where Setx, Atm, Tdp1 or Aptx genes were disrupted. These data provide further evidence for genome destabilization as a consequence of disrupted transcription in the presence of DNA double strand breaks arising during DNA replication or recombination. They also suggest that R-loop accumulation does not contribute to the neurodegenerative phenotype in these autosomal recessive ataxias.

Novel PI3K and mTOR Inhibitor NVP-BEZ235 Radiosensitizes Breast Cancer Cell Lines under Normoxic and Hypoxic Conditions

In the present study, we assessed, if the novel dual phosphatidylinositol 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) inhibitor NVP-BEZ235 radiosensitizes triple negative (TN) MDA-MB-231 and estrogen receptor (ER) positive MCF-7 cells to ionizing radiation under various oxygen conditions, simulating different microenvironments as occurring in the majority of breast cancers (BCs). Irradiation (IR) of BC cells cultivated in hypoxic conditions revealed increased radioresistance compared to normoxic controls. Treatment with NVP-BEZ235 completely circumvented this hypoxia-induced effects and radiosensitized normoxic, reoxygenated, and hypoxic cells to similar extents. Furthermore, NVP-BEZ235 treatment suppressed HIF-1α expression and PI3K/mTOR signaling, induced autophagy, and caused protracted DNA damage repair in both cell lines in all tested oxygen conditions. Moreover, after incubation with NVP-BEZ235, MCF-7 cells revealed depletion of phospho-AKT and considerable signs of apoptosis, which were significantly enhanced by radiation. Our findings clearly demonstrate that NVP-BEZ235 has a clinical relevant potential as a radiosensitizer in BC treatment.

Identification and characterization of an oocyte factor required for porcine nuclear reprogramming

Nuclear reprogramming of somatic cells can be induced by oocyte factors. Despite numerous attempts, the factors responsible for successful nuclear reprogramming remain elusive. In the present study, we found that porcine oocytes with the first polar body collected at 42 h of in vitro maturation had a stronger ability to support early development of cloned embryos than porcine oocytes with the first polar body collected at 33 h of in vitro maturation. To explore the key reprogramming factors responsible for the difference, we compared proteome signatures of the two groups of oocytes. 18 differentially expressed proteins between these two groups of oocytes were discovered by mass spectrometry (MS). Among these proteins, we especially focused on vimentin (VIM). A certain amount of VIM protein was stored in oocytes and accumulated during oocyte maturation, and maternal VIM was specifically incorporated into transferred somatic nuclei during nuclear reprogramming. When maternal VIM function was inhibited by anti-VIM antibody, the rate of cloned embryos developing to blastocysts was significantly lower than that of IgG antibody-injected embryos and non-injected embryos (12.24 versus 22.57 and 21.10%; p < 0.05), but the development of in vitro fertilization and parthenogenetic activation embryos was not affected. Furthermore, we found that DNA double strand breaks dramatically increased and that the p53 pathway was activated in cloned embryos when VIM function was inhibited. This study demonstrates that maternal VIM, as a genomic protector, is crucial for nuclear reprogramming in porcine cloned embryos.

DNA repair diseases: What do they tell us about cancer and aging?

The discovery of DNA repair defects in human syndromes, initially in xeroderma pigmentosum (XP) but later in many others, led to striking observations on the association of molecular defects and patients' clinical phenotypes. For example, patients with syndromes resulting from defective nucleotide excision repair (NER) or translesion synthesis (TLS) present high levels of skin cancer in areas exposed to sunlight. However, some defects in NER also lead to more severe symptoms, such as developmental and neurological impairment and signs of premature aging. Skin cancer in XP patients is clearly associated with increased mutagenesis and genomic instability, reflecting the defective repair of DNA lesions. By analogy, more severe symptoms observed in NER-defective patients have also been associated with defective repair, likely involving cell death after transcription blockage of damaged templates. Endogenously induced DNA lesions, particularly through oxidative stress, have been identified as responsible for these severe pathologies. However, this association is not that clear and alternative explanations have been proposed. Despite high levels of exposure to intense sunlight, patients from tropical countries receive little attention or care, which likely also reflects the lack of understanding of how DNA damage causes cancer and premature aging.

Kaposi's sarcoma-associated herpesvirus induces the ATM and H2AX DNA damage response early during de novo infection of primary endothelial cells, which play roles in latency establishment

The DNA damage response (DDR) that evolved to repair host cell DNA damage also recognizes viral DNA entering the nucleus during infections. Here, we investigated the modulation of DDR signaling during de novo infection of primary endothelial cells by Kaposi's sarcoma-associated herpesvirus (KSHV). Phosphorylation of representative DDR-associated proteins, such as ataxia telangiectasia mutated (ATM) and H2AX, was induced as early as 30 min (0.5 h) postinfection and persisted during in vitro KSHV latency. Phosphorylated H2AX (γH2AX) colocalized at 30 min (0.5 h) with the KSHV genome entering the nuclei. Total H2AX protein levels also increased, and the increase was attributed to a decrease in degradative H2AX Lys48-linked polyubiquitination with a concomitant increase in Lys63-linked polyubiquitination that was shown to increase protein stability. ATM and H2AX phosphorylation and γH2AX nuclear foci were also induced by UV-inactivated KSHV, which ceased at later times of infection. Inhibition of ATM kinase activity by KU-55933 and H2AX knockdown by small interfering RNA significantly reduced the expression of the KSHV latency-associated nuclear antigen 1 (LANA-1; ORF73) and LANA-1 nuclear puncta. Knockdown of H2AX also resulted in a >80% reduction in the nuclear KSHV DNA copy numbers. Similar results were also observed in ATM-negative cells, although comparable levels of viral DNA entered ATM-negative and ATM-positive cell nuclei. In contrast, knockdown of CHK1 and CHK2 did not affect ORF73 expression. Collectively, these results demonstrate that KSHV induces ATM and H2AX, a selective arm of the DDR, for the establishment and maintenance of its latency during de novo infection of primary endothelial cells.

IMPORTANCE

Eukaryotic cells mount a DNA damage response (DDR) to sense and repair different types of cellular DNA damage. In addition, DDR also recognizes exogenous genetic material, such as the viral DNA genome entering the nucleus during infections. The present study was undertaken to determine whether de novo Kaposi's sarcoma-associated herpesvirus (KSHV) infection modulates DDR. Our results demonstrate that early during de novo infection of primary endothelial cells, KSHV induces a selective arm of DDR signaling, such as the ATM kinase and its downstream target, H2AX, which are essential for KSHV's latent gene expression and the establishment of latency. These studies suggest that targeting ATM and H2AX could serve as an attractive strategy to block the establishment of KSHV latent infection and the associated malignancies.

The Rad50 hook domain regulates DNA damage signaling and tumorigenesis

The Mre11 complex (Mre11, Rad50, and Nbs1) is a central component of the DNA damage response (DDR), governing both double-strand break repair and DDR signaling. Rad50 contains a highly conserved Zn(2+)-dependent homodimerization interface, the Rad50 hook domain. Mutations that inactivate the hook domain produce a null phenotype. In this study, we analyzed mutants with reduced hook domain function in an effort to stratify hook-dependent Mre11 complex functions. One of these alleles, Rad50(46), conferred reduced Zn(2+) affinity and dimerization efficiency. Homozygous Rad50(46/46) mutations were lethal in mice. However, in the presence of wild-type Rad50, Rad50(46) exerted a dominant gain-of-function phenotype associated with chronic DDR signaling. At the organismal level, Rad50(+/46) exhibited hydrocephalus, liver tumorigenesis, and defects in primitive hematopoietic and gametogenic cells. These outcomes were dependent on ATM, as all phenotypes were mitigated in Rad50(+/46) Atm(+/-) mice. These data reveal that the murine Rad50 hook domain strongly influences Mre11 complex-dependent DDR signaling, tissue homeostasis, and tumorigenesis.

Inhibition of DNA double-strand break repair by the dual PI3K/mTOR inhibitor NVP-BEZ235 as a strategy for radiosensitization of glioblastoma

PURPOSE

Inhibitors of the DNA damage response (DDR) have great potential for radiosensitization of numerous cancers, including glioblastomas, which are extremely radio- and chemoresistant brain tumors. Currently, there are no DNA double-strand break (DSB) repair inhibitors that have been successful in treating glioblastoma. Our laboratory previously demonstrated that the dual phosphoinositide 3-kinase/mTOR inhibitor NVP-BEZ235 can potently inhibit the two central DDR kinases, DNA-dependent protein kinase catalytic subunit (DNA-PKcs) and ataxia-telangiectasia mutated (ATM), in vitro. Here, we tested whether NVP-BEZ235 could also inhibit ATM and DNA-PKcs in tumors in vivo and assessed its potential as a radio- and chemosensitizer in preclinical mouse glioblastoma models.

EXPERIMENTAL DESIGN

The radiosensitizing effect of NVP-BEZ235 was tested by following tumor growth in subcutaneous and orthotopic glioblastoma models. Tumors were generated using the radioresistant U87-vIII glioma cell line and GBM9 neurospheres in nude mice. These tumors were then treated with ionizing radiation and/or NVP-BEZ235 and analyzed for DNA-PKcs and ATM activation, DSB repair inhibition, and attenuation of growth.

RESULTS

NVP-BEZ235 potently inhibited both DNA-PKcs and ATM kinases and attenuated the repair of ionizing radiation-induced DNA damage in tumors. This resulted in striking tumor radiosensitization, which extended the survival of brain tumor-bearing mice. Notably, tumors displayed a higher DSB-load when compared with normal brain tissue. NVP-BEZ235 also sensitized a subset of subcutaneous tumors to temozolomide, a drug routinely used concurrently with ionizing radiation for the treatment of glioblastoma.

CONCLUSIONS

These results demonstrate that it may be possible to significantly improve glioblastoma therapy by combining ionizing radiation with potent and bioavailable DNA repair inhibitors such as NVP-BEZ235.

Impact of genomic damage and ageing on stem cell function

Impairment of stem cell function contributes to the progressive deterioration of tissue maintenance and repair with ageing. Evidence is mounting that age-dependent accumulation of DNA damage in both stem cells and cells that comprise the stem cell microenvironment are partly responsible for stem cell dysfunction with ageing. Here, we review the impact of the various types of DNA damage that accumulate with ageing on stem cell functionality, as well as the development of cancer. We discuss DNA-damage-induced cell intrinsic and extrinsic alterations that influence these processes, and review recent advances in understanding systemic adjustments to DNA damage and how they affect stem cells.

ATM specifically mediates repair of double-strand breaks with blocked DNA ends

Ataxia telangiectasia is caused by mutations in ATM and represents a paradigm for cancer predisposition and neurodegenerative syndromes linked to deficiencies in the DNA-damage response. The role of ATM as a key regulator of signalling following DNA double-strand breaks (DSBs) has been dissected in extraordinary detail, but the impact of this process on DSB repair still remains controversial. Here we develop novel genetic and molecular tools to modify the structure of DSB ends and demonstrate that ATM is indeed required for efficient and accurate DSB repair, preventing cell death and genome instability, but exclusively when the ends are irreversibly blocked. We therefore identify the nature of ATM involvement in DSB repair, presenting blocked DNA ends as a possible pathogenic trigger of ataxia telangiectasia and related disorders.

ATM kinase promotes both caspase-8 and caspase-9 activation during TNF-α-induced apoptosis of HeLa cells

In this study, we show that atraxia telangiectasia mutated kinase (ATM) activity is generally upregulated by different apoptotic stimuli, i.e. TNF-α, TRAIL, paclitaxel, or UV. Apoptotic progression is markedly attenuated by siATM-RNA through down regulation of caspase-8 and caspase-9 in parallel with decreases in FLIP-S (short form of cellular FLICE inhibitory protein) protein levels and Bid cleavage. In addition, ATM activity is upregulated through t-Cdc6 while caspase-8 and caspase-9 activities increase. Taken together, we suggest that ATM regulates caspase-8 activation by influencing levels of FLIP-S, ATM kinase activity is upregulated by t-Cdc6, and increased ATM activity plays an essential role in the amplification of apoptosis in TNF-α-stimulated HeLa cells.

The role of oxidative DNA damage in radiation induced bystander effect

Ionizing radiation (IR) has been described as a double-edged sword, since it is used for diagnostic and therapeutic medical applications, and at the same time it is a well known human mutagen and carcinogen, causing wide-ranging chromosomal aberrations. It is nowadays accepted that the detrimental effects of IR are not restricted only in the irradiated cells, but also to non-irradiated bystander or even distant cells manifesting various biological effects. This review presents the role of oxidative stress in the induction of bystander effects referring to the types of the implicated oxidative DNA lesions, the contributing intercellular and intracellular stress mediators, the way they are transmitted from irradiated to bystander cells and finally, the complex role of the bystander effect in the therapeutic efficacy of radiation treatment of cancer.

ATM controls proper mitotic spindle structure

The recessive ataxia-telangiectasia (A-T) syndrome is characterized by cerebellar degeneration, immunodeficiency, cancer susceptibility, premature aging, and insulin-resistant diabetes and is caused by loss of function of the ATM kinase, a member of the phosphoinositide 3-kinase-like protein kinases (PIKKs) family. ATM plays a crucial role in the DNA damage response (DDR); however, the complexity of A-T features suggests that ATM may regulate other cellular functions. Here we show that ATM affects proper bipolar mitotic spindle structure independently of DNA damage. In addition, we find that in mitosis ATM forms a complex with the poly(ADP)ribose (PAR) polymerase Tankyrase (TNKS) 1, the spindle pole protein NuMA1, and breast cancer susceptibility protein BRCA1, another crucial DDR player. Our evidence indicates that the complex is required for efficient poly(ADP)ribosylation of NuMA1. We find further that a mutant NuMA1 version, non-phosphorylatable at potential ATM-dependent phosphorylation sites, is poorly PARylated and induces loss of spindle bipolarity. Our findings may help to explain crucial A-T features and provide further mechanistic rationale for TNKS inhibition in cancer therapy.

DNA-PKcs activates the Chk2-Brca1 pathway during mitosis to ensure chromosomal stability

The catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) is known to have a critical role in DNA double-strand break repair. We have previously reported that DNA-PKcs is activated when cells enter mitosis and functions in mitotic spindle assembly and chromosome segregation. Here we report that DNA-PKcs is the upstream regulator of the Chk2-Brca1 pathway, which impacts microtubule dynamics, kinetochore attachment and chromosomal segregation in mitosis. Downstream from Chk2, Brca1 promotes monoubiquitination of γ-tubulin to inhibit microtubule nucleation and growth. We found that DNA-PKcs is essential for mitotic Chk2 phosphorylation at Thr68. As in Chk2- and Brca1-deficient cells, loss of DNA-PKcs resulted in chromosome misalignment and lagging during anaphase owing to elevation in microtubule dynamics. Importantly, these mitotic aberrations in DNA-PKcs-defective cells were alleviated by the overexpression of phosphomimetic Chk2 or Brca1 mutant proteins but not their wild-type counterparts. Taken together, these results demonstrate that DNA-PKcs regulates mitotic spindle organization and chromosomal instability via the Chk2-Brca1 signaling pathway.

DNA damage response genes and the development of cancer metastasis

DNA damage response genes play vital roles in the maintenance of a healthy genome. Defects in cell cycle checkpoint and DNA repair genes, especially mutation or aberrant downregulation, are associated with a wide spectrum of human disease, including a predisposition to the development of neurodegenerative conditions and cancer. On the other hand, upregulation of DNA damage response and repair genes can also cause cancer, as well as increase resistance of cancer cells to DNA damaging therapy. In recent years, it has become evident that many of the genes involved in DNA damage repair have additional roles in tumorigenesis, most prominently by acting as transcriptional (co-)factors. Although defects in these genes are causally connected to tumor initiation, their role in tumor progression is more controversial and it seems to depend on tumor type. In some tumors like melanoma, cell cycle checkpoint/DNA repair gene upregulation is associated with tumor metastasis, whereas in a number of other cancers the opposite has been observed. Several genes that participate in the DNA damage response, such as RAD9, PARP1, BRCA1, ATM and TP53 have been associated with metastasis by a number of in vitro biochemical and cellular assays, by examining human tumor specimens by immunohistochemistry or by DNA genome-wide gene expression profiling. Many of these genes act as transcriptional effectors to regulate other genes implicated in the pathogenesis of cancer. Furthermore, they are aberrantly expressed in numerous human tumors and are causally related to tumorigenesis. However, whether the DNA damage repair function of these genes is required to promote metastasis or another activity is responsible (e.g., transcription control) has not been determined. Importantly, despite some compelling in vitro evidence, investigations are still needed to demonstrate the role of cell cycle checkpoint and DNA repair genes in regulating metastatic phenotypes in vivo.

Natural compounds with anti-ageing activity

Ageing is a complex molecular process driven by diverse molecular pathways and biochemical events that are promoted by both environmental and genetic factors. Specifically, ageing is defined as a time-dependent decline of functional capacity and stress resistance, associated with increased chance of morbidity and mortality. These effects relate to age-related gradual accumulation of stressors that result in increasingly damaged biomolecules which eventually compromise cellular homeostasis. Nevertheless, the findings that genetic or diet interventions can increase lifespan in evolutionarily diverse organisms indicate that mortality can be postponed. Natural compounds represent an extraordinary inventory of high diversity structural scaffolds that can offer promising candidate chemical entities in the major healthcare challenge of increasing health span and/or delaying ageing. Herein, those natural compounds (either pure forms or extracts) that have been found to delay cellular senescence or in vivo ageing will be critically reviewed and summarized according to affected cellular signalling pathways. Moreover, the chemical structures of the identified natural compounds along with the profile of extracts related to their bioactive components will be presented and discussed. Finally, novel potential molecular targets for screening natural compounds for anti-ageing activity, as well as the idea that anti-ageing interventions represent a systemic approach that is also effective against age-related diseases will be discussed.

Mammalian Fe-S cluster biogenesis and its implication in disease

Iron-sulfur (Fe-S) clusters are inorganic cofactors that are ubiquitous and essential. Due to their chemical versatility, Fe-S clusters are implicated in a wide range of protein functions including mitochondrial respiration and DNA repair. Composed of iron and sulfur, they are sensible to oxygen and their biogenesis requires a highly conserved protein machinery that facilitates assembly of the cluster as well as its insertion into apoproteins. Mitochondria are the central cellular compartment for Fe-S cluster biogenesis in eukaryotic cells and the importance of proper function of this biogenesis for life is highlighted by a constantly increasing number of human genetic diseases that are associated with dysfunction of this Fe-S cluster biogenesis pathway. Although these disorders are rare and appear dissimilar, common aspects are found among them. This review will give an overview on what is known on mammalian Fe-S cluster biogenesis today, by putting it into the context of what is known from studies from lower model organisms, and focuses on the associated diseases, by drawing attention to the respective mutations. Finally, it outlines the importance of adequate cellular and murine models to uncover not only each protein function, but to resolve their role and requirement throughout the mammalian organism.

Predictability, efficacy and safety of radiosensitization of glioblastoma-initiating cells by the ATM inhibitor KU-60019

We have previously shown that pharmacological inhibition of ataxia telangiectasia mutated (ATM) protein sensitizes glioblastoma-initiating cells (GICs) to ionizing radiation (IR). Herein, we report the experimental conditions to overcome GIC radioresistance in vitro using the specific ATM inhibitor KU-60019, two major determinants of the tumor response to this drug and the absence of toxicity of this treatment in vitro and in vivo. Repeated treatments with KU-60019 followed by IR substantially delayed GIC proliferation in vitro and even eradicated radioresistant cells, whereas GIC treated with vehicle plus radiation recovered early and expanded. The tumor response to the drug occurred under a cutoff level of expression of TP53 and over a cutoff level of expression of phosphatidylinositol 3-kinase (PI3K). No increased clastogenicity or point mutagenicity was induced by KU-60019 plus radiation when compared to vehicle plus radiation. No significant histological changes to the brain or other organs were observed after prolonged infusion into the brain of KU-60019 at millimolar concentrations. Taken together, these findings suggest that GIC-driven tumors with low expression of TP53 and high expression of PI3K might be effectively and safely radiosensitized by KU-60019.

A majority of human melanoma cell lines exhibits an S phase-specific defect in excision of UV-induced DNA photoproducts

It is well established that efficient removal of highly-promutagenic UV-induced dipyrimidine photoproducts via nucleotide excision repair (NER) is required for protection against sunlight-associated malignant melanoma. Nonetheless, the extent to which reduced NER capacity might contribute to individual melanoma susceptibility in the general population remains unclear. Here we show that among a panel of 14 human melanoma strains, 11 exhibit significant inhibition of DNA photoproduct removal during S phase relative to G0/G1 or G2/M. Evidence is presented that this cell cycle-specific NER defect correlates with enhanced apoptosis and reduced clonogenic survival following UV irradiation. In addition, melanoma strains deficient in S phase-specific DNA photoproduct removal manifest significantly lower levels of phosphorylated histone H2AX at 1 h post-UV, suggesting diminished activation of ataxia telangiectasia and Rad 3-related (ATR) kinase, i.e., a primary orchestrator of the cellular response to UV-induced DNA replication stress. Consistently, in the case of DNA photoproduct excision-proficient melanoma cells, siRNA-mediated depletion of ATR (but not of its immediate downstream effector kinase Chk1) engenders deficient NER specifically during S. On the other hand simultaneous siRNA-mediated depletion of ataxia telangiectasia mutated kinase (ATM) and DNA-dependent protein kinase catalytic subunit (DNA-PKcs) exerts no significant effect on either phosphorylation of H2AX at 1 h post-UV or the efficiency of DNA photoproduct removal. Our data suggest that defective NER exclusively during S phase, possibly associated with decreased ATR signaling, may constitute an heretofore unrecognized determinant in melanoma pathogenesis.

The essential function of the MRN complex in the resolution of endogenous replication intermediates

The MRN complex (Mre11/Rad50/Nbs1) is important in double-strand break (DSB) recognition, end resection, replication fork stabilization, and ATM and ATR activation. Complete deletion of MRN is incompatible with cell and organism life, presumably due to replication-born DSBs; however, the underlying mechanism remains unknown. We devised a noninvasive high-content assay, termed high-content microscopy-assisted cell-cycle phenotyping (hiMAC), to investigate the fate of cells lacking Nbs1. Surprisingly, deletion of Nbs1 does not kill cells during replication. The primary lesions in Nbs1-deleted cells are replication intermediates that result from defective resolution rather than fork destabilization. These lesions are converted to DSBs in the subsequent G2 phase, which subsequently activate Chk1, delay G2 progression, and lead to chromosome instability. Nbs1-deleted cells establish a DSB equilibrium that permits cell cycling but activates p53, causing G1 and G2 arrest, and cell death. Thus, we identify a physiological role of Nbs1 in the resolution of stalled replication forks.

Recognition of tumors by the innate immune system and natural killer cells

In recent years, roles of the immune system in immune surveillance of cancer have been explored using a variety of approaches. The roles of the adaptive immune system have been a major emphasis, but increasing evidence supports a role for innate immune effector cells such as natural killer (NK) cells in tumor surveillance. Here, we discuss some of the evidence for roles in tumor surveillance of innate immune cells. In particular, we focus on NK cells and other immune cells that express germline-encoded receptors, often labeled NK receptors. The impact of these receptors and the cells that express them on tumor suppression is summarized. We discuss in detail some of the pathways and events in tumor cells that induce or upregulate cell-surface expression of the ligands for these receptors, and the logic of how those pathways serve to identify malignant, or potentially malignant cells. How tumors often evade tumor suppression mediated by innate killer cells is another major subject of the review. We end with a discussion on some of the implications of the various findings with respect to possible therapeutic approaches.

DPP-4 inhibitors repress foam cell formation by inhibiting scavenger receptors through protein kinase C pathway

Studies show that dipeptidyl peptidase-4 (DPP-4) inhibitors may have an anti-atherosclerotic effect. Since foam cells are key components of atherosclerotic plaque, we studied the effect of DPP-4 inhibitors on foam cell formation. Foam cell formation was studied by treatment of THP-1 macrophages with oxidized low-density lipoprotein in the absence or presence of DPP-4 inhibitors (sitagliptin and NVPDPP728). The expression of scavenger receptors SRA, CD36 and LOX-1 was measured, and their role in foam cell formation in the presence of DPP-4 inhibitors was examined. In additional studies, role of protein kinase C and A in the effect of DPP-4 inhibitors was examined. Foam cell formation was markedly reduced by both DPP-4 inhibitors, as was the expression of CD36 and LOX-1 (CD36 ≫ LOX-1), but not SRA. Simultaneously, there was a reduction in phosphorylated PKC, but not PKA, content. Recovery of phosphorylated PKC following treatment of cells negated the effect of DPP-4 inhibitors on foam cell formation. Further, overexpression of CD36 or LOX-1 blocked the effect of DPP-4 inhibitors on foam cell formation. DPP-4 inhibitors repress foam cell formation through the inhibition of SRs CD36 and LOX-1, most likely via the inhibition of PKC activity. This study provides novel insights into the mechanism of inhibition of atherosclerosis by DPP-4 inhibitors.

Interplay between the cell cycle and double-strand break response in mammalian cells

The cell cycle is intimately associated with the ability of cells to sense and respond to and repair DNA damage. Understanding how cell cycle progression, particularly DNA replication and cell division, are regulated and how DNA damage can affect these processes has been the subject of intense research. Recent evidence suggests that the repair of DNA damage is regulated by the cell cycle, and that cell cycle factors are closely associated with repair factors and participate in cellular decisions regarding how to respond to and repair damage. Precise regulation of cell cycle progression in the presence of DNA damage is essential to maintain genomic stability and avoid the accumulation of chromosomal aberrations that can promote tumor formation. In this review, we discuss the current understanding of how mammalian cells induce cell cycle checkpoints in response to DNA double-strand breaks. In addition, we discuss how cell cycle factors modulate DNA repair pathways to facilitate proper repair of DNA lesions.

Tcrδ translocations that delete the Bcl11b haploinsufficient tumor suppressor gene promote atm-deficient T cell acute lymphoblastic leukemia

ATM is the master regulator of the cellular response to DNA double strand breaks (DSBs). Deficiency of ATM predisposes humans and mice to αβ T lymphoid cancers with clonal translocations between the T cell receptor (TCR) α/δ locus and a 450 kb region of synteny on human chromosome 14 and mouse chromosome 12. While these translocations target and activate the TCL1 oncogene at 14q32 to cause T cell pro-lymphocytic leukemia (T-PLL), the TCRα/δ;14q32 translocations in ATM-deficient T cell acute lymphoblastic leukemia (T-ALL) have not been characterized and their role in cancer pathogenesis remains unknown. The corresponding lesion in Atm-deficient mouse T-ALLs is a chromosome t(12;14) translocation with Tcrδ genes fused to sequences on chromosome 12; although these translocations do not activate Tcl1, they delete the Bcl11b haploinsufficient tumor suppressor gene. To assess whether Tcrδ translocations that inactivate one copy of Bcl11b promote transformation of Atm-deficient cells, we analyzed Atm(-/-) mice with mono-allelic Bcl11b deletion initiating in thymocytes concomitant with Tcrδ recombination. Inactivation of one Bcl11b copy had no effect on the predisposition of Atm(-/-) mice to clonal T-ALLs. Yet, none of these T-ALLs had a clonal chromosome t(12;14) translocation that deleted Bcl11b indicating that Tcrδ translocations that inactivate a copy of Bcl11b promote transformation of Atm-deficient thymocytes. Our data demonstrate that antigen receptor locus translocations can cause cancer by deleting a tumor suppressor gene. We discuss the implications of these findings for the etiology and therapy of T-ALLs associated with ATM deficiency and TCRα/δ translocations targeting the 14q32 cytogenetic region.