atm and p53 cooperate in apoptosis and suppression of tumorigenesis, but not in resistance to acute radiation toxicity

DNA damage response in brain tumors: A Society for Neuro-Oncology consensus review on mechanisms and translational efforts in neuro-oncology

DNA damage response (DDR) mechanisms are critical to maintenance of overall genomic stability, and their dysfunction can contribute to oncogenesis. Significant advances in our understanding of DDR pathways have raised the possibility of developing therapies that exploit these processes. In this expert-driven consensus review, we examine mechanisms of response to DNA damage, progress in development of DDR inhibitors in IDH-wild-type glioblastoma and IDH-mutant gliomas, and other important considerations such as biomarker development, preclinical models, combination therapies, mechanisms of resistance and clinical trial design considerations.

Aberrant ATM signaling and homology-directed DNA repair as a vulnerability of p53-mutant GBM to AZD1390-mediated radiosensitization

ATM is a key mediator of radiation response, and pharmacological inhibition of ATM is a rational strategy to radiosensitize tumors. AZD1390 is a brain-penetrant ATM inhibitor and a potent radiosensitizer. This study evaluated the spectrum of radiosensitizing effects and the impact of TP53 mutation status in a panel of IDH1 wild-type (WT) glioblastoma (GBM) patient-derived xenografts (PDXs). AZD1390 suppressed radiation-induced ATM signaling, abrogated G0-G1 arrest, and promoted a proapoptotic response specifically in p53-mutant GBM in vitro. In a preclinical trial using 10 orthotopic GBM models, AZD1390/RT afforded benefit in a cohort of TP53-mutant tumors but not in TP53-WT PDXs. In mechanistic studies, increased endogenous DNA damage and constitutive ATM signaling were observed in TP53-mutant, but not in TP53-WT, PDXs. In plasmid-based reporter assays, GBM43 (TP53-mutant) showed elevated DNA repair capacity compared with that in GBM14 (p53-WT), whereas treatment with AZD1390 specifically suppressed homologous recombination (HR) efficiency, in part, by stalling RAD51 unloading. Furthermore, overexpression of a dominant-negative TP53 (p53DD) construct resulted in enhanced basal ATM signaling, HR activity, and AZD1390-mediated radiosensitization in GBM14. Analyzing RNA-seq data from TCGA showed up-regulation of HR pathway genes in TP53-mutant human GBM. Together, our results imply that increased basal ATM signaling and enhanced dependence on HR represent a unique susceptibility of TP53-mutant cells to ATM inhibitor-mediated radiosensitization.

In vivo RNA-seq and ChIP-seq analyses show an obligatory role for the C terminus of p53 in conferring tissue-specific radiation sensitivity

Thymus and spleen, in contrast to liver, are radiosensitive tissues in which p53-dependent apoptosis is triggered after whole-body radiation in vivo. Combined RNA sequencing (RNA-seq) and chromatin immunoprecipitation sequencing (ChIP-seq) analyses of radiation-treated mouse organs identifies both shared and tissue-specific p53 transcriptional responses. As expected, the p53 targets shared among thymus and spleen are enriched in apoptotic targets. The inability to upregulate these genes in the liver is not due to reduced gene occupancy. Use of an engineered mouse model shows that deletion of the C terminus of p53 can confer radiation-induced expression of p53 apoptotic targets in the liver with concomitant increased cell death. Global RNA-seq analysis reveals that an additional role of the C terminus is also needed for transcriptional activation of liver-specific p53 targets. It is hypothesized that both suppression of apoptotic gene expression combined with enhanced activation of liver-specific targets confers tissue-specific radio-resistance.

Histological analysis of sleep and circadian brain circuitry in cranial radiation-induced hypersomnolence (C-RIH) mouse model

Disrupted sleep, including daytime hypersomnolence, is a core symptom reported by primary brain tumor patients and often manifests after radiotherapy. The biological mechanisms driving the onset of sleep disturbances after cranial radiation remains unclear but may result from treatment-induced injury to neural circuits controlling sleep behavior, both circadian and homeostatic. Here, we develop a mouse model of cranial radiation-induced hypersomnolence which recapitulates the human experience. Additionally, we used the model to explore the impact of radiation on the brain. We demonstrated that the DNA damage response following radiation varies across the brain, with homeostatic sleep and cognitive regions expressing higher levels of γH2AX, a marker of DNA damage, than the circadian suprachiasmatic nucleus (SCN). These findings were supported by in vitro studies comparing radiation effects in SCN and cortical astrocytes. Moreover, in our mouse model, MRI identified structural effects in cognitive and homeostatic sleep regions two-months post-treatment. While the findings are preliminary, they suggest that homeostatic sleep and cognitive circuits are vulnerable to radiation and these findings may be relevant to optimizing treatment plans for patients.

Targeting the ATM Kinase to Enhance the Efficacy of Radiotherapy and Outcomes for Cancer Patients

Targeting the DNA damage response represents a promising approach to improve the efficacy of radiation therapy. One appealing target for this approach is the serine/threonine kinase ataxia telangiectasia mutated (ATM), which is activated by DNA double strand breaks to orchestrate the cellular response to ionizing radiation. Small-molecule inhibitors targeting ATM have entered clinical trials testing their safety in combination with radiation therapy or in combination with other DNA damaging agents. Here, we review biochemical, genetic, and cellular functional studies of ATM, phenotypes associated with germline and somatic cancer mutations in ATM in humans, and experiments in genetically engineered mouse models that support a rationale for investigating ATM inhibitors as radiosensitizers for cancer therapy. These data identify important synthetic lethal relationships, which suggest that ATM inhibitors may be particularly effective in tumors with defects in other nodes of the DNA damage response. The potential for ATM inhibition to improve immunotherapy responses in preclinical models represents another emerging area of research. We summarize ongoing clinical trials of ATM inhibitors with radiotherapy. We also discuss critical ongoing areas of investigation that include discovery of biomarkers that predict for radiosensitization by ATM inhibitors and identification of effective combinations of ATM inhibitors, radiation therapy, other DNA damage response-directed therapies, and/or immunotherapies.

iTRAQ-based proteomic analysis reveals potential osteogenesis-promoted role of ATM in strontium-incorporated titanium implant

The present study aims to reveal the osteogenic roles played by DNA damage response biomarkers through implementing isobaric tags for relative and absolute quantitation (iTRAQ) technique. First, sandblasted large-grit double acid-etched (SLA) titanium implant and strontium-incorporated (SLA-Sr) titanium implant were used for inserting in the tibiae of rats. iTRAQ technique was used to detect protein expression changes and identify differentially expressed proteins (DEPs). In total, 19,343 peptides and 4280 proteins were screened out. Among them, 91 and 138 DEPs were identified in the SLA-Sr group after implantation for 3 and 7 days, respectively. Ataxia-telangiectasia mutated (ATM) protein up-regulated on the 3rd day showed a trend of further up-regulation on the 7th day. Moreover, functional enrichment analyses were also conducted to explore the biological function of DEPs during the initial stage of osseointegration in vivo, which revealed that the biological functions of the DEPs on the 7th day were mainly related to "mismatch repair" and "mitotic G1 DNA damage checkpoint." Analysis of the Reactome signaling pathway showed that ATM was associated with TP53's regulation and activation. Finally, DNA damage repair related genes were selected for validation at mRNA and protein expression levels. Real-time reverse transcription-polymerase chain reaction and immunohistochemistry validation results demonstrated that mRNA expression level of ATM was higher in SLA-Sr group. In conclusion, SLA-Sr titanium implant could initiate DNA damage repair by activating expression levels of ATM. This study was striving to reveal new faces of better osseointegration and shedding light on the biological function and underlying mechanisms of this important procedure.

Chemical and biological basis for development of novel radioprotective drugs for cancer therapy

Ionizing radiation (IR) causes chemical changes in biological systems through direct interaction with the macromolecules or by causing radiolysis of water. This property of IR is harnessed in the clinic for radiotherapy in almost 50% of cancers patients. Despite the advent of stereotactic radiotherapy instruments and other advancements in shielding techniques, the inadvertent deposition of radiation dose in the surrounding normal tissue can cause late effects of radiation injury in normal tissues. Radioprotectors, which are chemical or biological agents, can reduce or mitigate these toxic side-effects of radiotherapy in cancer patients and also during radiation accidents. The desired characteristics of an ideal radioprotector include low chemical toxicity, high risk to benefit ratio and specific protection of normal cells against the harmful effects of radiation without compromising the cytotoxic effects of IR on cancer cells. Since reactive oxygen species (ROS) are the major contributors of IR mediated toxicity, plethora of studies have highlighted the potential role of antioxidants to protect against IR induced damage. However, owing to the lack of any clinically approved radioprotector against whole body radiation, researchers have shifted the focus toward finding alternate targets that could be exploited for the development of novel agents. The present review provides a comprehensive insight in to the different strategies, encompassing prime molecular targets, which have been employed to develop radiation protectors/countermeasures. It is anticipated that understanding such factors will lead to the development of novel strategies for increasing the outcome of radiotherapy by minimizing normal tissue toxicity.

FATC Domain Deletion Compromises ATM Protein Stability, Blocks Lymphocyte Development, and Promotes Lymphomagenesis

Ataxia-telangiectasia mutated (ATM) kinase is a master regulator of the DNA damage response, and loss of ATM leads to primary immunodeficiency and greatly increased risk for lymphoid malignancies. The FATC domain is conserved in phosphatidylinositol-3-kinase-related protein kinases (PIKKs). Truncation mutation in the FATC domain (R3047X) selectively compromised reactive oxygen species-induced ATM activation in cell-free assays. In this article, we show that in mouse models, knock-in ATM-R3057X mutation (Atm⁠ ^RX ⁠, corresponding to R3047X in human ATM) severely compromises ATM protein stability and causes T cell developmental defects, B cell Ig class-switch recombination defects, and infertility resembling ATM-null. The residual ATM-R3057X protein retains minimal yet functional measurable DNA damage-induced checkpoint activation and significantly delays lymphomagenesis in Atm⁠ ^RX/RX ⁠ mice compared with Atm⁠ ^-/- ⁠. Together, these results support a physiological role of the FATC domain in ATM protein stability and show that the presence of minimal residual ATM-R3057X protein can prevent growth retardation and delay tumorigenesis without restoring lymphocyte development and fertility.

Tumor genotype dictates radiosensitization after Atm deletion in primary brainstem glioma models

Diffuse intrinsic pontine glioma (DIPG) kills more children than any other type of brain tumor. Despite clinical trials testing many chemotherapeutic agents, palliative radiotherapy remains the standard treatment. Here, we utilized Cre/loxP technology to show that deleting Ataxia telangiectasia mutated (Atm) in primary mouse models of DIPG can enhance tumor radiosensitivity. Genetic deletion of Atm improved survival of mice with p53-deficient but not p53 wild-type gliomas after radiotherapy. Similar to patients with DIPG, mice with p53 wild-type tumors had improved survival after radiotherapy independent of Atm deletion. Primary p53 wild-type tumor cell lines induced proapoptotic genes after radiation and repressed the NRF2 target, NAD(P)H quinone dehydrogenase 1 (Nqo1). Tumors lacking p53 and Ink4a/Arf expressed the highest level of Nqo1 and were most resistant to radiation, but deletion of Atm enhanced the radiation response. These results suggest that tumor genotype may determine whether inhibition of ATM during radiotherapy will be an effective clinical approach to treat DIPGs.

Selenium nanoparticles act as an intestinal p53 inhibitor mitigating chemotherapy-induced diarrhea in mice

Selenium, at high-dose levels approaching its toxicity, protects tissues from dose-limiting toxicities of many cancer chemotherapeutics without compromising their therapeutic effects on tumors, there by allowing the delivery of higher chemotherapeutic doses to achieve increased cure rate. In this regard, selenium nanoparticles (SeNPs), which show the lowest toxicity among extensively investigated selenium compounds including methylselenocysteine and selenomethionine, are more promising for application. The key issue remains to be resolved is whether low-toxicity SeNPs possess a selective protective mechanism. p53 or p53-regulated thrombospondin-1 has each been confirmed to be an appropriate target for therapeutic suppression to reduce side effects of anticancer therapy. The present study demonstrated that SeNPs transiently suppressed the expression of many intestinal p53-associated genes in healthy mice. SeNPs did not interfere with tumor-suppressive effect of nedaplatin, a cisplatin analogue; however, effectively reduced nedaplatin-evoked diarrhea. Nedaplatin-induced diarrhea was associated with activation of intestinal p53 and high expression of intestinal thrombospondin-1. The preventive effect of SeNPs on nedaplatin-induced diarrhea was correlated with a powerful concomitant suppression of p53 and thrombospondin-1. Moreover, the high-dose SeNPs used in the present study did not suppress growth nor caused liver and kidney injuries as well as alterations of hematological parameters in healthy mice. Overall, the present study reveals that chemotherapeutic selectivity conferred by SeNPs involves a dual suppression of two well-documented targets, the p53 and thrombospondin-1, providing mechanistic and pharmacologic insights on low-toxicity SeNPs as a potential chemoprotectant for mitigating chemotherapy-induced diarrhea.

Regulation of response to radiotherapy by β-arrestin1 in Non-small cell lung cancer

β-arrestin1 serves as scaffold proteins participating in multiple signaling pathways. However, there were few researches focusing on the impact of β-arrestin1 on DNA damage response (DDR). Non-small cell lung cancer cell (NSCLC) lines were transfected with β-arrestin1 plasmids or siRNA and received radiation treatment. MTT and colony formation assay were performed to assess the proliferation and viability of tumor cells. Flow cytometry was used to evaluate the impact of β-arrestin1 on radiation-induced apoptosis. Western blotting was applied to detect protein expression in apoptosis, DDR, ERK and NF-kB pathways. We used qRT-PCR to test ATR, H2AX, β-arrestin1 mRNA level in cancer tissues compared with para-carcinoma tissues. Co-IP was performed to evaluate the interaction between β-arrestin1 and ATR or H2AX. Comet assay was used to detect DNA damage. β-arrestin1 mRNA level co-related with ATR and H2AX levels in cancer tissues, and β-arrestin1 bound to ATR and H2AX directly or indirectly. Overexpression of β-arrestin1 enhanced the DNA damage response pathway activation and increase DNA damage and apoptosis. Interestingly, suppression of β-arrestin1 inhibited cell proliferation and attenuated ERK and NF-kB pathways activation induced by radiation. Overexpression of β-arrestin1 enhances DDR pathway activation induced by radiation, as well as downstream apoptosis, and depletion of β-arrestin1 inhibits DDR pathway. Meanwhile β-arrestin1 regulates cell proliferation by suppression of ERK and NF-kB pathways. Manipulation of β-arrestin1 status modulates radiosensitivity for NSCLC.

Expression of the double-stranded RNA-dependent kinase PKR influences osteosarcoma attachment independent growth, migration, and invasion

Osteosarcoma (OS) is a rare, insidious tumor of mesenchymal origin that most often affects children, adolescents, and young adults. While the primary tumor can be controlled with chemotherapy and surgery, it is the lung metastases that are eventually fatal. Multiple studies into the initial drivers of OS development have been undertaken, but few of these have examined innate immune/inflammatory signaling. A central figure in inflammatory signaling is the innate immune/stress-activated kinase double-stranded RNA-dependent protein kinase (PKR). To characterize the role of PKR in OS, U2OS, and SaOS-2 osteosarcoma cell lines were stably transfected with wild-type or dominant-negative (DN) PKR. Overexpression of PKR enhanced colony formation in soft agar (U2OS and SaOS-2), enhanced cellular migration (U2OS), and invasive migration (SaOS-2). In contrast, overexpression of DN-PKR inhibited attachment-independent growth, migration and/or invasion. These data demonstrate a role for inflammatory signaling in OS formation and migration/invasion and suggest the status of PKR expression/activation may have prognostic value.

AIMP3 Deletion Induces Acute Radiation Syndrome-like Phenotype in Mice

Genomes are mostly protected from constant DNA-damaging threats, either internal or external, which ultimately sustain the organism. Herein, we report that AIMP3, a previously demonstrated tumour suppressor, plays an essential role in maintaining genome integrity in adult mice. Upon induction of the temporal systemic deletion of AIMP3 by tamoxifen in adult mice, the animals developed an acute radiation syndrome-like phenotype, typified by scleroderma, hypotrophy of haematopoietic cells and organs, and intestinal failure. Induction of γH2AX, an early marker of DNA double-strand breaks, was observed in the spleen, intestine, and the highly replicating embryonic cortex. In addition, sub-lethal irradiation of AIMP3 mKO mice dramatically affected organ damage and survival. Using isolated MEFs from conditional KO mice or AIMP3 knockdown cells, we confirmed the presence of spontaneously occurring DNA double-strand breaks by COMET assay and γH2AX induction. Furthermore, γH2AX removal was delayed, and homologous DNA repair activity was significantly reduced. Reduction of RPA foci formation and subsequent Rad51 foci formation probably underlie the significant reduction in homologous recombination activity in the absence of AIMP3. Together, our data demonstrate that AIMP3 plays a role in genome stability through the DNA repair process.

Sp1 phosphorylation by ATM downregulates BER and promotes cell elimination in response to persistent DNA damage

ATM (ataxia-telangiectasia mutated) is a central molecule for DNA quality control. Its activation by DNA damage promotes cell-cycle delay, which facilitates DNA repair prior to replication. On the other hand, persistent DNA damage has been implicated in ATM-dependent cell death via apoptosis; however, the mechanisms underlying this process remain elusive. Here we find that, in response to persistent DNA strand breaks, ATM phosphorylates transcription factor Sp1 and initiates its degradation. We show that Sp1 controls expression of the key base excision repair gene XRCC1, essential for DNA strand break repair. Therefore, degradation of Sp1 leads to a vicious cycle that involves suppression of DNA repair and further aggravation of the load of DNA damage. This activates transcription of pro-apoptotic genes and renders cells susceptible to elimination via both apoptosis and natural killer cells. These findings constitute a previously unrecognized 'gatekeeper' function of ATM as a detector of cells with persistent DNA damage.

Synergy between Prkdc and Trp53 regulates stem cell proliferation and GI-ARS after irradiation

Ionizing radiation (IR) is one of the most widely used treatments for cancer. However, acute damage to the gastrointestinal tract or gastrointestinal acute radiation syndrome (GI-ARS) is a major dose-limiting side effect, and the mechanisms that underlie this remain unclear. Here we use mouse models to explore the relative roles of DNA repair, apoptosis, and cell cycle arrest in radiation response. IR induces DNA double strand breaks and DNA-PK mutant Prkdc^scid/scid mice are sensitive to GI-ARS due to an inability to repair these breaks. IR also activates the tumor suppressor p53 to trigger apoptotic cell death within intestinal crypt cells and p53 deficient mice are resistant to apoptosis. To determine if DNA-PK and p53 interact to govern radiosensitivity, we compared the response of single and compound mutant mice to 8 Gy IR. Compound mutant Prkdc^scid/scid/Trp53^-/-mice died earliest due to severe GI-ARS. While both Prkdc^scid/scid and Prkdc^scid/scid/Trp53^-/-mutant mice had higher levels of IR-induced DNA damage, particularly within the stem cell compartment of the intestinal crypt, in Prkdc^scid/scid/Trp53^-/-mice these damaged cells abnormally progressed through the cell cycle resulting in mitotic cell death. This led to a loss of Paneth cells and a failure to regenerate the differentiated epithelial cells required for intestinal function. IR-induced apoptosis did not correlate with radiosensitivity. Overall, these data reveal that DNA repair, mediated by DNA-PK, and cell cycle arrest, mediated by p53, cooperate to protect the stem cell niche after DNA damage, suggesting combination approaches to modulate both pathways may be beneficial to reduce GI-ARS. As many cancers harbor p53 mutations, this also suggests targeting DNA-PK may be effective to enhance sensitivity of p53 mutant tumors to radiation.

Pharmacologic ATM but not ATR kinase inhibition abrogates p21-dependent G1 arrest and promotes gastrointestinal syndrome after total body irradiation

We show that ATM kinase inhibition using AZ31 prior to 9 or 9.25 Gy total body irradiation (TBI) reduced median time to moribund in mice to 8 days. ATR kinase inhibition using AZD6738 prior to TBI did not reduce median time to moribund. The striking finding associated with ATM inhibition prior to TBI was increased crypt loss within the intestine epithelium. ATM inhibition reduced upregulation of p21, an inhibitor of cyclin-dependent kinases, and blocked G1 arrest after TBI thereby increasing the number of S phase cells in crypts in wild-type but not Cdkn1a(p21^CIP/WAF1)−/− mice. In contrast, ATR inhibition increased upregulation of p21 after TBI. Thus, ATM activity is essential for p21-dependent arrest while ATR inhibition may potentiate arrest in crypt cells after TBI. Nevertheless, ATM inhibition reduced median time to moribund in Cdkn1a(p21^CIP/WAF1)−/− mice after TBI. ATM inhibition also increased cell death in crypts at 4 h in Cdkn1a(p21^CIP/WAF1)−/−, earlier than at 24 h in wild-type mice after TBI. In contrast, ATR inhibition decreased cell death in crypts in Cdkn1a(p21^CIP/WAF1)−/− mice at 4 h after TBI. We conclude that ATM activity is essential for p21-dependent and p21-independent mechanisms that radioprotect intestinal crypts and that ATM inhibition promotes GI syndrome after TBI.

The role of gene mutations and gene products in intestinal tissue reactions from ionising radiation

The response of the intestine to (low linear-energy-transfer) ionising radiation is reviewed regarding the cellular basis to the reactions, the regenerative processes which restore the tissue, and external agents which aid its recovery. In the steady-state, it is generally considered that the crypt cell lineages in both small and large intestine are maintained by a small number of stem cells, but there are differences for example in the composition of their niche residence and in the numbers of transit cell generations. Various cell surface markers are now available to indentify particular lineage cell types. Radiation doses up to 1Gy cause apoptotic stem-cell death in particular locations, at higher doses to >6Gy Lgr5⁺ stem cells are required for normal intestinal recovery, and at >8Gy some crypts are sterilised and the probability of animal death from intestinal injury increases with higher doses. Mutations in repair genes, tumour suppressor genes, and survival genes cause various degrees of stem cell and clonogenic cell radiosensitisation. Recent evidence is suggesting much plasticity in the crypt cell lineage, potentially contributing to flexibility in the hierarchical lineage, clonogen number variations and the sensitisation differences. Knockout mice for many different genes have been used to detect their role in both steady state and in irradiated conditions, expected to lead to further insight to the damage and restorative processes. Many different external agents have been used to ameliorate intestinal reactions, including prostaglandins, interleukins, angiogenic and epithelial growth factors, other cytokines, and intraluminal factors.

Inactivation of the ATMIN/ATM pathway protects against glioblastoma formation

Glioblastoma multiforme (GBM) is the most aggressive human primary brain cancer. Using a Trp53-deficient mouse model of GBM, we show that genetic inactivation of the Atm cofactor Atmin, which is dispensable for embryonic and adult neural development, strongly suppresses GBM formation. Mechanistically, expression of several GBM-associated genes, including Pdgfra, was normalized by Atmin deletion in the Trp53-null background. Pharmacological ATM inhibition also reduced Pdgfra expression, and reduced the proliferation of Trp53-deficient primary glioma cells from murine and human tumors, while normal neural stem cells were unaffected. Analysis of GBM datasets showed that PDGFRA expression is also significantly increased in human TP53-mutant compared with TP53-wild-type tumors. Moreover, combined treatment with ATM and PDGFRA inhibitors efficiently killed TP53-mutant primary human GBM cells, but not untransformed neural stem cells. These results reveal a new requirement for ATMIN-dependent ATM signaling in TP53-deficient GBM, indicating a pro-tumorigenic role for ATM in the context of these tumors.

A novel mouse model for ataxia-telangiectasia with a N-terminal mutation displays a behavioral defect and a low incidence of lymphoma but no increased oxidative burden

Ataxia-telangiectasia (A-T) is a rare multi-system disorder caused by mutations in the ATM gene. Significant heterogeneity exists in the underlying genetic mutations and clinical phenotypes. A number of mouse models have been generated that harbor mutations in the distal region of the gene, and a recent study suggests the presence of residual ATM protein in the brain of one such model. These mice recapitulate many of the characteristics of A-T seen in humans, with the notable exception of neurodegeneration. In order to study how an N-terminal mutation affects the disease phenotype, we generated an inducible Atm mutant mouse model (Atm(tm1Mmpl/tm1Mmpl), referred to as A-T [M]) predicted to express only the first 62 amino acids of Atm. Cells derived from A-T [M] mutant mice exhibited reduced cellular proliferation and an altered DNA damage response, but surprisingly, showed no evidence of an oxidative imbalance. Examination of the A-T [M] animals revealed an altered immunophenotype consistent with A-T. In contrast to mice harboring C-terminal Atm mutations that disproportionately develop thymic lymphomas, A-T [M] mice developed lymphoma at a similar rate as human A-T patients. Morphological analyses of A-T [M] cerebella revealed no substantial cellular defects, similar to other models of A-T, although mice display behavioral defects consistent with cerebellar dysfunction. Overall, these results suggest that loss of Atm is not necessarily associated with an oxidized phenotype as has been previously proposed and that loss of ATM protein is not sufficient to induce cerebellar degeneration in mice.

MicroRNA-21 is involved in X-ray irradiation resistance in K562 leukaemia cells

BACKGROUND

Many studies have demonstrated that microRNA-21 (miR-21) acts as an oncogene in the tumourigenesis of a variety of tumours and may be involved in the chemotherapeutic drug resistance of tumour cells. In this study, we utilized the leukaemia cell line K562 as an in vitro cell model to investigate whether miR-21 is involved in X-ray irradiation resistance.

METHODS

Retroviral transduction and antisense oligonucleotide transfection were used to overexpress or knock down miR-21 expression, respectively. An MTT assay was used to measure cell viability, and western blotting was performed to detect the expression of the miR-21 target gene, PTEN (phosphatase and tensin homologue), and its downstream signalling components, phosphatidylinositol 3-kinase (PI3K), and AKT.

RESULTS

The overexpression of miR-21 decreased the protein expression levels of PTEN, increased the phosphorylation level of AKT, and enhanced the X-ray irradiation resistance in K562 cells. In contrast, the knockdown of miR-21 increased the PTEN protein expression, reduced the phosphorylation levels of the AKT, and increased the sensitivity of K562 cells to X-ray irradiation. The overexpression of PTEN or the knockdown of AKT also increased the sensitivity of K562 cells to X-ray irradiation.

CONCLUSION

By regulating the expression of its target gene PTEN, which subsequently affects the PI3K/AKT signalling pathway, miR-21 exerts its regulatory role on the radiation sensitivity of K562 cells. These results may help to provide the basis for microRNA-based targeted therapies to overcome radiation resistance in tumour cells.

Atm deletion with dual recombinase technology preferentially radiosensitizes tumor endothelium

Cells isolated from patients with ataxia telangiectasia are exquisitely sensitive to ionizing radiation. Kinase inhibitors of ATM, the gene mutated in ataxia telangiectasia, can sensitize tumor cells to radiation therapy, but concern that inhibiting ATM in normal tissues will also increase normal tissue toxicity from radiation has limited their clinical application. Endothelial cell damage can contribute to the development of long-term side effects after radiation therapy, but the role of endothelial cell death in tumor response to radiation therapy remains controversial. Here, we developed dual recombinase technology using both FlpO and Cre recombinases to generate primary sarcomas in mice with endothelial cell-specific deletion of Atm to determine whether loss of Atm in endothelial cells sensitizes tumors and normal tissues to radiation. Although deletion of Atm in proliferating tumor endothelial cells enhanced the response of sarcomas to radiation, Atm deletion in quiescent endothelial cells of the heart did not sensitize mice to radiation-induced myocardial necrosis. Blocking cell cycle progression reversed the effect of Atm loss on tumor endothelial cell radiosensitivity. These results indicate that endothelial cells must progress through the cell cycle in order to be radiosensitized by Atm deletion.

ATM signalling and cancer

ATM, the protein kinase mutated in the rare human disease ataxia telangiectasia (A-T), has been the focus of intense scrutiny over the past two decades. Initially this was because of the unusual radiosensitive phenotype of cells from A-T patients, and latterly because investigating ATM signalling has yielded valuable insights into the DNA damage response, redox signalling and cancer. With the recent explosion in genomic data, ATM alterations have been revealed both in the germline as a predisposing factor for cancer and as somatic changes in tumours themselves. Here we review these findings, as well as advances in the understanding of ATM signalling mechanisms in cancer and ATM inhibition as a strategy for cancer treatment.

Role of DNA damage in cardiovascular disease

Patients with some progeroid syndromes, such as Werner syndrome, exhibit atherosclerotic cardiovascular disease (CVD) at a young age as a manifestation of premature aging. Recent studies have revealed that most progeroid syndromes are caused by genetic defects in specific molecules involved in the DNA damage response, a cornerstone of genome stability. Ionizing radiation is one of the most potent genotoxic stimuli and causes various kinds of DNA damage. Further, there is increasing evidence that therapeutic radiation treatments can cause cardiovascular complications. Here, we describe the DNA damage and subsequent response, review recent advances in the understanding of the molecular basis of progeroid syndromes (especially those syndromes that involve CVD), review the pathological and epidemiological analysis of radiation-induced CVD, and discuss the possible role of DNA damage and the DNA damage response in the pathogenesis of atherosclerotic CVD.

Leukemia inhibitory factor promotes nasopharyngeal carcinoma progression and radioresistance

Radioresistance of EBV-associated nasopharyngeal carcinoma (NPC) is associated with poor prognosis for patients with this form of cancer. Here, we found that NPC patients had increased serum levels of leukemia inhibitory factor (LIF) and that higher LIF levels correlated with local tumor recurrence. Furthermore, in vitro studies with NPC cells and in vivo xenograft mouse studies demonstrated that LIF critically contributes to NPC tumor growth and radioresistance. Using these model systems, we found that LIF treatment activated the mTORC1/p70S6K signaling pathway, enhanced tumor growth, inhibited DNA damage responses, and enhanced radioresistance. Treatment with either soluble LIF receptor (sLIFR), a LIF antagonist, or the mTOR inhibitor rapamycin reversed LIF-mediated effects, resulting in growth arrest and increased sensitivity to γ irradiation. Immunohistochemical (IHC) analyses of human NPC biopsies revealed that LIF and LIFR were overexpressed in tumor cells and that LIF expression correlated with the presence of the activated p-p70S6K. Finally, we found that the EBV-encoded protein latent membrane protein 1 (LMP1) enhances LIF production. Together, our findings indicate that LIF promotes NPC tumorigenesis and suggest that serum LIF levels may predict local recurrence and radiosensitivity in NPC patients.

Recent progress in mouse models for tumor suppressor genes and its implications in human cancer

Gain-of-function mutations in oncogenes and loss-of-function mutations in tumor suppressor genes (TSG) lead to cancer. In most human cancers, these mutations occur in somatic tissues. However, hereditary forms of cancer exist for which individuals are heterozygous for a germline mutation in a TSG locus at birth. The second allele is frequently inactivated by gene deletion, point mutation, or promoter methylation in classical TSGs that meet Knudson's two-hit hypothesis. Conversely, the second allele remains as wild-type, even in tumors in which the gene is haplo-insufficient for tumor suppression. This article highlights the importance of PTEN, APC, and other tumor suppressors for counteracting aberrant PI3K, β-catenin, and other oncogenic signaling pathways. We discuss the use of gene-engineered mouse models (GEMM) of human cancer focusing on Pten and Apc knockout mice that recapitulate key genetic events involved in initiation and progression of human neoplasia. Finally, the therapeutic potential of targeting these tumor suppressor and oncogene signaling networks is discussed.

Apoptosis and survival

The term apoptosis first appeared in the biomedical literature in 1972, to delineate a structurally distinctive mode of cell death responsible for cell loss within living tissues. The cardinal morphological features are cell shrinkage, accompanied by transient but violent bubbling and blebbing from the surface, and culminating in separation of the cell into a cluster of membrane-bounded bodies. Changes in several cell surface molecules also ensure that, in tissues, apoptotic cells are immediately recognised and phagocytosed by their neighbours. However, it is important to note that apoptosis is only one form of cell death and the particular death pathway that is the most important determinant for cancer therapy is not necessarily that which has the fastest kinetics, as is the bias in many laboratories, but rather that which displays the most sensitive dose-response relationship.

Pharmacological inhibition of ATM by KU55933 stimulates ATM transcription

Ataxia-telangiectasia mutated (ATM) kinase is a component of a signalling mechanism that determines the process of decision-making in response to DNA damage and involves the participation of multiple proteins. ATM is activated by DNA double-strand breaks (DSBs) through the Mre11–Rad50–Nbs1 (MRN) DNA repair complex, and orchestrates signalling cascades that initiate the DNA damage response. Cells lacking ATM are hypersensitive to insults, particularly genotoxic stress, induced through radiation or radiomimetic drugs. Here, we investigate the degree of ATM activation during time-dependent treatment with genotoxic agents and the effects of ATM on phospho-induction and localization of its downstream substrates. Additionally, we have demonstrated a new cell-cycle-independent mechanism of ATM gene regulation following ATM kinase inhibition with KU5593. Inhibition of ATM activity causes induction of ATM protein followed by oscillation and this mechanism is governed at the transcriptional level. Furthermore, this autoregulatory induction of ATM is also accompanied by a transient upregulation of p53, pATR and E2F1 levels. Since ATM inhibition is believed to sensitize cancer cells to genotoxic agents, this novel insight into the mechanism of ATM regulation might be useful for designing more precise strategies for modulation of ATM activity in cancer therapy.

Protein kinases and transcription factors activation in response to UV-radiation of skin: implications for carcinogenesis

Solar ultraviolet (UV) radiation is an important environmental factor that leads to immune suppression, inflammation, photoaging, and skin carcinogenesis. Here, we reviewed the specific signal transduction pathways and transcription factors involved in the cellular response to UV-irradiation. Increasing experimental data supporting a role for p38, MAPK, JNK, ERK1/2, and ATM kinases in the response network to UV exposure is discussed. We also reviewed the participation of NF-κB, AP-1, and NRF2 transcription factors in the control of gene expression after UV-irradiation. In addition, we discussed the promising chemotherapeutic intervention of transcription factors signaling by natural compounds. Finally, we focused on the review of data emerging from the use of DNA microarray technology to determine changes in global gene expression in keratinocytes and melanocytes in response to UV treatment. Efforts to obtain a comprehensive portrait of the transcriptional events regulating photodamage of intact human epidermis after UV exposure reveals the existence of novel factors participating in UV-induced cell death. Progress in understanding the multitude of mechanisms induced by UV-irradiation could lead to the potential use of protein kinases and novel proteins as specific targets for the prevention and control of skin cancer.

A small molecule binding to the coactivator CREB-binding protein blocks apoptosis in cardiomyocytes

As a master transcription factor in cellular responses to external stress, tumor suppressor p53 is tightly regulated. Excessive p53 activity during myocardial ischemia causes irreversible cellular injury and cardiomyocyte death. p53 activation is dependent on lysine acetylation by the lysine acetyltransferase and transcriptional coactivator CREB-binding protein (CBP) and on acetylation-directed CBP recruitment for p53 target gene expression. Here, we report a small molecule ischemin, developed with a structure-guided approach to inhibit the acetyl-lysine binding activity of the bromodomain of CBP. We show that ischemin alters post-translational modifications on p53 and histones, inhibits p53 interaction with CBP and transcriptional activity in cells, and prevents apoptosis in ischemic cardiomyocytes. Our study suggests small molecule modulation of acetylation-mediated interactions in gene transcription as a new approach to therapeutic interventions of human disorders such as myocardial ischemia.

The p53 tumor suppressor protein regulates hematopoietic stem cell fate

The p53 tumor suppressor protein is a key transcription factor that regulates several signaling pathways involved in the cell's response to stress. Through stress-induced activation, p53 accumulates and triggers the expression of target genes that protect the genetic integrity of all cells including hematopoietic stem cells (HSCs). These protective mechanisms include cell-cycle arrest, DNA repair, induction of apoptosis, or initiation of senescence. In addition to its function under stress conditions, p53 has important functions during steady-state hematopoiesis, regulating HSC quiescence and self-renewal. In addition, it appears that p53 levels affect HSC competition for the hematopoietic niche, with the less p53 activated HSCs preferentially surviving. The specific genes and precise mechanisms underlying p53's effects on normal HSCs are slowly being clarified. p53 also plays an important role in leukemia stem cell (LSC) behavior, with p53 loss affecting drug resistance and disease progression. Pharmacologic activation of p53 function could overcome the adverse impact of p53 inactivation in LSCs. Thus, understanding the p53 regulatory mechanisms active in HSCs and LSCs may promote the development of new therapeutic strategies that could eliminate the population of largely quiescent LSCs.

Absence of Wip1 partially rescues Atm deficiency phenotypes in mice

Wild-type p53-induced phosphatase 1 (WIP1) is a serine/threonine phosphatase that dephosphorylates proteins in the ataxia telangiectasia mutated (ATM)-initiated DNA damage response pathway. WIP1 may have a homeostatic role in ATM signaling by returning the cell to a normal pre-stress state following completion of DNA repair. To better understand the effects of WIP1 on ATM signaling, we crossed Atm-deficient mice to Wip1-deficient mice and characterized phenotypes of the double knockout progeny. We hypothesized that the absence of Wip1 might rescue Atm deficiency phenotypes. Atm null mice, like ATM-deficient humans with the inherited syndrome ataxia telangiectasia, exhibit radiation sensitivity, fertility defects, and are T-cell lymphoma prone. Most double knockout mice were largely protected from lymphoma development and had a greatly extended lifespan compared with Atm null mice. Double knockout mice had increased p53 and H2AX phosphorylation and p21 expression compared with their Atm null counterparts, indicating enhanced p53 and DNA damage responses. Additionally, double knockout splenocytes displayed reduced chromosomal instability compared with Atm null mice. Finally, doubly null mice were partially rescued from gametogenesis defects observed in Atm null mice. These results indicate that inhibition of WIP1 may represent a useful strategy for cancer treatment in general and A-T patients in particular.

Uncoupling p53 functions in radiation-induced intestinal damage via PUMA and p21

The role of p53 in tissue protection is not well understood. Loss of p53 blocks apoptosis in the intestinal crypts following irradiation but paradoxically accelerates gastrointestinal (GI) damage and death. PUMA and p21 are the major mediators of p53-dependent apoptosis and cell-cycle checkpoints, respectively. To better understand these two arms of p53 response in radiation-induced GI damage, we compared animal survival, as well as apoptosis, proliferation, cell-cycle progression, DNA damage, and regeneration in the crypts of WT, p53 knockout (KO), PUMA KO, p21 KO, and p21/PUMA double KO (DKO) mice in a whole body irradiation model. Deficiency in p53 or p21 led to shortened survival but accelerated crypt regeneration associated with massive nonapoptotic cell death. Nonapoptotic cell death is characterized by aberrant cell-cycle progression, persistent DNA damage, rampant replication stress, and genome instability. PUMA deficiency alone enhanced survival and crypt regeneration by blocking apoptosis but failed to rescue delayed nonapoptotic crypt death or shortened survival in p21 KO mice. These studies help to better understand p53 functions in tissue injury and regeneration and to potentially improve strategies to protect or mitigate intestinal damage induced by radiation.

Identification of dAven, a Drosophila melanogaster ortholog of the cell cycle regulator Aven

Aven is a regulator of the DNA-damage response and G2/M cell cycle progression. Overexpression of Aven is associated with poor prognosis in patients with childhood acute lymphoblastic leukemia and acute myeloid leukemia, and altered intracellular Aven distribution is associated with infiltrating ductal carcinoma and papillary carcinoma breast cancer subtypes. Although Aven orthologs have been identified in most vertebrate species, no Aven gene has been reported in invertebrates. Here, we describe a Drosophila melanogaster open reading frame (ORF) that shares sequence and functional similarities with vertebrate Aven genes. The protein encoded by this ORF, which we named dAven, contains several domains that are highly conserved among Aven proteins of fish, amphibian, bird and mammalian origins. In flies, knockdown of dAven by RNA interference (RNAi) resulted in lethality when its expression was reduced either ubiquitously or in fat cells using Gal4 drivers. Animals undergoing moderate dAven knockdown in the fat body had smaller fat cells displaying condensed chromosomes and increased levels of the mitotic marker phosphorylated histone H3 (PHH3), suggesting that dAven was required for normal cell cycle progression in this tissue. Remarkably, expression of dAven in Xenopus egg extracts resulted in G2/M arrest that was comparable to that caused by human Aven. Taken together, these results suggest that, like its vertebrate counterparts, dAven plays a role in cell cycle regulation. Drosophila could be an excellent model for studying the function of Aven and identifying cellular factors that influence its activity, revealing information that may be relevant to human disease.

Transcriptome characterization uncovers the molecular response of hematopoietic cells to ionizing radiation

Ionizing radiation causes rapid and acute suppression of hematopoietic cells that manifests as the hematopoietic syndrome. However, the roles of molecules and regulatory pathways induced in vivo by irradiation of different hematopoietic cells have not been completely elaborated. Using a strategy that combined different microarray bioinformatics tools, we identified gene networks that might be involved in the early response of hematopoietic cells radiation response in vivo. The grouping of similar time-ordered gene expression profiles using quality threshold clustering enabled the successful identification of common binding sites for 56 transcription factors that may be involved in the regulation of the early radiation response. We also identified novel genes that are responsive to the transformation-related protein 53; all of these genes were biologically validated in p53-transgenic null mice. Extension of the analysis to purified bone marrow cells including highly purified long-term hematopoietic stem cells, combined with functional classification, provided evidence of gene expression modifications that were largely unknown in this primitive population. Our methodology proved particularly useful for analyzing the transcriptional regulation of the complex ionizing radiation response of hematopoietic cells. Our data may help to elucidate the molecular mechanisms involved in tissue radiosensitivity and to identify potential targets for improving treatment in radiation emergencies.

Regulation of tissue- and stimulus-specific cell fate decisions by p53 in vivo

The tumour suppressor p53 pathway is often inactivated by multiple mechanisms in the genesis of human cancers. Aberrant cellular proliferation, DNA damage, hypoxia, and ribosomal stress cause activation of the p53 tumour suppressor with multiple possible consequences to the cell: cell death, cell cycle arrest, or senescence. These mechanisms ultimately ensure that the cell does not replicate, and are thus potent tumour suppressor mechanisms. An important question that has eluded the field is how p53 makes these cell fate decisions. This review summarizes the current status of knowledge regarding p53-mediated stress and tissue-dependent cell fate decisions in mouse models and human tumours.

Oncogene-induced senescence: the bright and dark side of the response

In late 1990s, it was shown that activated oncogenes are able to induce senescence. Since then large leaps in understanding this phenomenon have been achieved. There is substantial evidence supporting oncogene-induced senescence (OIS) as a potent antitumor barrier in vivo. Multiple pathways participating in cell cycle regulation, DNA damage signaling, immune response, and bioenergetics regulate the process. Despite its beneficial effects the senescent cell is thought to promote carcinogenesis and age-related disease in a nonautonomous manner. Here, we highlight the works dealing with all these aspects and discuss the studies proposing therapeutic exploitation of OIS.

Chk1 suppressed cell death

The role of Chk1 in the cellular response to DNA replication stress is well established. However recent work indicates a novel role for Chk1 in the suppression of apoptosis following the disruption of DNA replication or DNA damage. This review will consider these findings in the context of known pathways of Chk1 signalling and potential applications of therapies that target Chk1.

Convergent transcription through a long CAG tract destabilizes repeats and induces apoptosis

Short repetitive sequences are common in the human genome, and many fall within transcription units. We have previously shown that transcription through CAG repeat tracts destabilizes them in a way that depends on transcription-coupled nucleotide excision repair and mismatch repair. Recent observations that antisense transcription accompanies sense transcription in many human genes led us to test the effects of antisense transcription on triplet repeat instability in human cells. Here, we report that simultaneous sense and antisense transcription (convergent transcription) initiated from two inducible promoters flanking a CAG95 tract in a nonessential gene enhances repeat instability synergistically, arrests the cell cycle, and causes massive cell death via apoptosis. Using chemical inhibitors and small interfering RNA (siRNA) knockdowns, we identified the ATR (ataxia-telangiectasia mutated [ATM] and Rad3 related) signaling pathway as a key mediator of this cellular response. RNA polymerase II, replication protein A (RPA), and components of the ATR signaling pathway accumulate at convergently transcribed repeat tracts, accompanied by phosphorylation of ATR, CHK1, and p53. Cell death depends on simultaneous sense and antisense transcription and is proportional to their relative levels, it requires the presence of the repeat tract, and it occurs in both proliferating and nonproliferating cells. Convergent transcription through a CAG repeat represents a novel mechanism for triggering a cellular stress response, one that is initiated by events at a single locus in the genome and resembles the response to DNA damage.

Aven blocks DNA damage-induced apoptosis by stabilising Bcl-xL

Induction of apoptosis by DNA-damaging agents involves the activation of mitochondrial apoptotic pathway. Aven has been identified as an antiapoptotic protein and has been shown to activate ATM in response to DNA damage. In this study, we demonstrated that enforced expression of Aven blocks UV-irradiation-, SN-38- or cisplatin-induced apoptosis upstream of mitochondria by stabilising Bcl-xL protein levels in breast cancer cells. Aven silencing by RNA interference markedly enhanced apoptotic response following treatment with DNA-damaging agents. Aven is complexed with Bcl-xL in untreated breast cancer cells and treatment with DNA-damaging agents led to decreased Aven/Bcl-xL interaction. Importantly, Bcl-xL was necessary for the prosurvival activity of Aven and depletion of Bcl-xL abrogated Aven-mediated protection against DNA damage-induced apoptosis. Analysis of breast cancer tissue microarrays revealed decreased Aven nuclear expression in breast cancer tissues compared with non-neoplastic breast tissues. In particular, we detected reduced nuclear expression of Aven in infiltrating ductal carcinoma and papillary carcinoma breast cancer subtypes compared with non-neoplastic breast tissues and infiltrating lobular breast cancer tissues. Our results suggest that Aven is an important mediator in DNA damage-induced apoptotic signalling in breast cancer cells and its nuclear expression is altered in breast cancer tissues, which may contribute to genomic instability in breast cancer tumours.

Disrupting circadian homeostasis of sympathetic signaling promotes tumor development in mice

Background

Cell proliferation in all rapidly renewing mammalian tissues follows a circadian rhythm that is often disrupted in advanced-stage tumors. Epidemiologic studies have revealed a clear link between disruption of circadian rhythms and cancer development in humans. Mice lacking the circadian genes Period1 and 2 (Per) or Cryptochrome1 and 2 (Cry) are deficient in cell cycle regulation and Per2 mutant mice are cancer-prone. However, it remains unclear how circadian rhythm in cell proliferation is generated in vivo and why disruption of circadian rhythm may lead to tumorigenesis.

Methodology/Principal Findings

Mice lacking Per1 and 2, Cry1 and 2, or one copy of Bmal1, all show increased spontaneous and radiation-induced tumor development. The neoplastic growth of Per-mutant somatic cells is not controlled cell-autonomously but is dependent upon extracellular mitogenic signals. Among the circadian output pathways, the rhythmic sympathetic signaling plays a key role in the central-peripheral timing mechanism that simultaneously activates the cell cycle clock via AP1-controlled Myc induction and p53 via peripheral clock-controlled ATM activation. Jet-lag promptly desynchronizes the central clock-SNS-peripheral clock axis, abolishes the peripheral clock-dependent ATM activation, and activates myc oncogenic potential, leading to tumor development in the same organ systems in wild-type and circadian gene-mutant mice.

Conclusions/Significance

Tumor suppression in vivo is a clock-controlled physiological function. The central circadian clock paces extracellular mitogenic signals that drive peripheral clock-controlled expression of key cell cycle and tumor suppressor genes to generate a circadian rhythm in cell proliferation. Frequent disruption of circadian rhythm is an important tumor promoting factor.

LIN28B confers radio-resistance through the posttranscriptional control of KRAS

To screen the differentially expressed microRNAs related to radio-resistance, we compared the microRNA profiles of lung cancer cells with different responses to ionizing radiation (IR). Of 328 microRNAs in microarray, 27 microRNAs were differentially expressed in NCI-H460 (H460) and NCI-H1299 (H1299) cells. Among them, let-7g was down-regulated in radio-resistant H1299 cells, and the level of let-7g was higher in radio-sensitive cells like Caski, H460, and ME180 in qRT-PCR analysis than in radio-resistant cells like A549, H1299, DLD1, and HeLa. Over-expression of let-7g in H1299 cells could suppress the translation of KRAS, and increase the sensitivity to IR. When we knockdown the expression of LIN28B, an upstream regulator of let-7g, the level of mature let-7g was increased in H1299 cells and the sensitivity to IR was also enhanced in LIN28B knockdown cells. From these data, we suggest that LIN28B plays an important role in radiation responses of lung cancer cells through inhibiting let-7g processing and increasing translation of KRAS.

Induction of p53 renders ATM-deficient mice refractory to hepatocarcinogenesis

BACKGROUND & AIMS

p53 Mutations are very common in human hepatocellular carcinoma, and induction of hepatic p53 expression causes lysis of implanted hepatoblastoma cells in a chimeric mouse. Ataxia Telangiectasia Mutated (ATM) kinase senses DNA strand breaks and induces p53. Our aims were to establish whether ATM deficiency alters the carcinogenic response of hepatocytes to diethylnitrosamine (DEN).

METHODS

Male ATM-deficient (ATM(-/-)), heterozygote (ATM(+/-)), and wild-type (WT) mice were injected with DEN at age 15 days, and animals were killed up to 12 months to assess p53, cell cycle, apoptosis, and liver tumor development.

RESULTS

Whereas >80% of WT and ATM(+/-) mice developed hepatocellular carcinoma (HCC), at 9-12 months, ATM(-/-) mice remained refractory to DEN-induced HCC up to 15 months. At 6 and 9 months, and compared with WT mice, p53 and p19(ARF) expression were greatly enhanced in ATM(-/-) liver associated with up-regulation of ATR and Chk1; cleaved caspase-3 immunohistochemistry and caspase-3 activity were also significantly increased. Whereas livers of DEN-treated ATM(-/-) mice showed markers of senescence (beta-galactosidase, Cxcl-1), up-regulation of telomerase occurred concurrently. The possibility that such balanced senescence could result in immortalization was demonstrated in hepatocytes prepared at 9 months from DEN-treated ATM(-/-) liver.

CONCLUSIONS

Hepatocarcinogenesis is abrogated in ATM-deficient mice in association with induction of ATR, Chk1, p53, and p19(ARF). Resultant cell cycle arrest and apoptosis of DNA-damaged cells are possible mechanisms that underlie this unique "refractoriness" to malignant transformation in DEN-initiated ATM(-/-) hepatocytes. The findings also show that prolonged up-regulation of p53 associated with some features of senescence does not inevitably cause organ failure.

Atm-deficient mice exhibit increased sensitivity to dextran sulfate sodium-induced colitis characterized by elevated DNA damage and persistent immune activation

The role of ataxia telangiectasia mutated (ATM), a DNA double-strand break recognition and response protein, in inflammation and inflammatory diseases is unclear. We have previously shown that high levels of systemic DNA damage are induced by intestinal inflammation in wild-type mice. To determine the effect of Atm deficiency in inflammation, we induced experimental colitis in Atm(-/-), Atm(+/-), and wild-type mice via dextran sulfate sodium (DSS) administration. Atm(-/-) mice had higher disease activity indices and rates of mortality compared with heterozygous and wild-type mice. Systemic DNA damage and immune response were characterized in peripheral blood throughout and after three cycles of treatment. Atm(-/-) mice showed increased sensitivity to levels of DNA strand breaks in peripheral leukocytes, as well as micronucleus formation in erythroblasts, compared with heterozygous and wild-type mice, especially during remission periods and after the end of treatment. Markers of reactive oxygen and nitrogen species-mediated damage, including 8-oxoguanine and nitrotyrosine, were present both in the distal colon and in peripheral leukocytes, with Atm(-/-) mice manifesting more 8-oxoguanine formation than wild-type mice. Atm(-/-) mice showed greater upregulation of inflammatory cytokines and significantly higher percentages of activated CD69+ and CD44+ T cells in the peripheral blood throughout treatment. ATM, therefore, may be a critical immunoregulatory factor dampening the deleterious effects of chronic DSS-induced inflammation, necessary for systemic genomic stability and homeostasis of the gut epithelial barrier.

Deletion of Puma protects hematopoietic stem cells and confers long-term survival in response to high-dose gamma-irradiation

Molecular paradigms underlying the death of hematopoietic stem cells (HSCs) induced by ionizing radiation are poorly defined. We have examined the role of Puma (p53 up-regulated mediator of apoptosis) in apoptosis of HSCs after radiation injury. In the absence of Puma, HSCs were highly resistant to gamma-radiation in a cell autonomous manner. As a result, Puma-null mice or the wild-type mice reconstituted with Puma-null bone marrow cells were strikingly able to survive for a long term after high-dose gamma-radiation that normally would pose 100% lethality on wild-type animals. Interestingly, there was no increase of malignancy in the exposed animals. Such profound beneficial effects of Puma deficiency were likely associated with better maintained quiescence and more efficient DNA repair in the stem cells. This study demonstrates that Puma is a unique mediator in radiation-induced death of HSCs. Puma may be a potential target for developing an effective treatment aimed to protect HSCs from lethal radiation.

Genetically engineered mouse models in cancer research

Mouse models of human cancer have played a vital role in understanding tumorigenesis and answering experimental questions that other systems cannot address. Advances continue to be made that allow better understanding of the mechanisms of tumor development, and therefore the identification of better therapeutic and diagnostic strategies. We review major advances that have been made in modeling cancer in the mouse and specific areas of research that have been explored with mouse models. For example, although there are differences between mice and humans, new models are able to more accurately model sporadic human cancers by specifically controlling timing and location of mutations, even within single cells. As hypotheses are developed in human and cell culture systems, engineered mice provide the most tractable and accurate test of their validity in vivo. For example, largely through the use of these models, the microenvironment has been established to play a critical role in tumorigenesis, since tumor development and the interaction with surrounding stroma can be studied as both evolve. These mouse models have specifically fueled our understanding of cancer initiation, immune system roles, tumor angiogenesis, invasion, and metastasis, and the relevance of molecular diversity observed among human cancers. Currently, these models are being designed to facilitate in vivo imaging to track both primary and metastatic tumor development from much earlier stages than previously possible. Finally, the approaches developed in this field to achieve basic understanding are emerging as effective tools to guide much needed development of treatment strategies, diagnostic strategies, and patient stratification strategies in clinical research.

Changes in transcriptome after in vivo exposure to ionising radiation reveal a highly specialised liver response

PURPOSE

To identify transcriptional gene-networks involved in the early in vivo response of liver cells to radiation exposure and improve our understanding of the molecular processes responsible for tissue radiosensitivity.

MATERIALS AND METHODS

Transcriptome variations of liver RNA samples were measured 3 hours post-irradiation using microarray technology. The results were confirmed and extended using real-time polymerase-chain-reaction (RT-PCR).

RESULTS

We identified quantitative changes in the expression of 126 genes, most of which were observed for the first time. We show that some modifications, such as the upregulation of the cyclin-dependent kinase inhibitor 1A (Cdkn1A) gene, persisted for at least two months after the initial exposure. Other genes regulated by the transformation-related protein 53 (Trp53/p53) such as Bcl2-associated X protein (Bax) or etoposide-induced-2.4 (Ei24/PIG8) were not upregulated. Grouping differentially expressed genes into functional categories revealed that the primary response of liver cells to radiation exposure was the enhancement of oxidoreductase activity and inhibition of cell proliferation, involving cell cycle progression and apoptosis-related genes.

CONCLUSIONS

The data provides evidence of gene expression modifications associated with the hepatic response to radiation exposure. One of the main differences observed with radiation-sensitive tissues such as the spleen was cell proliferation. The comparison of our data with transcriptome modifications in different biological models enabled the identification of networks of genes that might be co-regulated. Overall, our expression data revealed genes and cellular pathways that might help to improve our understanding of the molecular basis underlying tissue radiosensitivity and to identify possible targets for novel therapeutic strategies.