Isolation of full-length ATM cDNA and correction of the ataxia-telangiectasia cellular phenotype

Chemical screen identifies shikonin as a broad DNA damage response inhibitor that enhances chemotherapy through inhibiting ATM and ATR

DNA damage response (DDR) is a highly conserved genome surveillance mechanism that preserves cell viability in the presence of chemotherapeutic drugs. Hence, small molecules that inhibit DDR are expected to enhance the anti-cancer effect of chemotherapy. Through a recent chemical library screen, we identified shikonin as an inhibitor that strongly suppressed DDR activated by various chemotherapeutic drugs in cancer cell lines derived from different origins. Mechanistically, shikonin inhibited the activation of ataxia telangiectasia mutated (ATM), and to a lesser degree ATM and RAD3-related (ATR), two master upstream regulators of the DDR signal, through inducing degradation of ATM and ATR-interacting protein (ATRIP), an obligate associating protein of ATR, respectively. As a result of DDR inhibition, shikonin enhanced the anti-cancer effect of chemotherapeutic drugs in both cell cultures and in mouse models. While degradation of ATRIP is proteasome dependent, that of ATM depends on caspase- and lysosome-, but not proteasome. Overexpression of ATM significantly mitigated DDR inhibition and cell death induced by shikonin and chemotherapeutic drugs. These novel findings reveal shikonin as a pan DDR inhibitor and identify ATM as a primary factor in determining the chemo sensitizing effect of shikonin. Our data may facilitate the development of shikonin and its derivatives as potential chemotherapy sensitizers through inducing ATM degradation.

A Review of Breast Cancer Risk Factors in Adolescents and Young Adults

Simple Summary

Cancer diagnosed in patients between the ages of 15 and 39 deserves special consideration. Diagnoses within this cohort of adolescents and young adults include childhood cancers which present at an older age than expected, or an early presentation of cancers that are typically observed in older adults, such as breast cancer. Cancers within this age group are associated with worse disease-free and overall survival rates, and the incidence of these cases are rising. Knowing an individual’s susceptibility to disease can change their clinical management and allow for the risk-testing of relatives. This review discusses the risk factors that contribute to breast cancer in this unique cohort of patients, including inherited genetic risk factors, as well as environmental and lifestyle factors. We also describe risk models that allow clinicians to quantify a patient’s lifetime risk of developing disease.

Abstract

Cancer in adolescents and young adults (AYAs) deserves special consideration for several reasons. AYA cancers encompass paediatric malignancies that present at an older age than expected, or early-onset of cancers that are typically observed in adults. However, disease diagnosed in the AYA population is distinct to those same cancers which are diagnosed in a paediatric or older adult setting. Worse disease-free and overall survival outcomes are observed in the AYA setting, and the incidence of AYA cancers is increasing. Knowledge of an individual’s underlying cancer predisposition can influence their clinical care and may facilitate early tumour surveillance strategies and cascade testing of at-risk relatives. This information can further influence reproductive decision making. In this review we discuss the risk factors contributing to AYA breast cancer, such as heritable predisposition, environmental, and lifestyle factors. We also describe a number of risk models which incorporate genetic factors that aid clinicians in quantifying an individual’s lifetime risk of disease.

In vitro dexamethasone treatment does not induce alternative ATM transcripts in cells from Ataxia-Telangiectasia patients

Short term treatment with low doses of glucocorticoid analogues has been shown to ameliorate neurological symptoms in Ataxia–Telangiectasia (A–T), a rare autosomal recessive multisystem disease that mainly affects the cerebellum, immune system, and lungs. Molecular mechanisms underlying this clinical observation are unclear. We aimed at evaluating the effect of dexamethasone on the induction of alternative ATM transcripts (ATMdexa1). We showed that dexamethasone cannot induce an alternative ATM transcript in control and A–T lymphoblasts and primary fibroblasts, or in an ATM-knockout HeLa cell line. We also demonstrated that some of the reported readouts associated with ATMdexa1 are due to cellular artifacts and the direct induction of γH2AX by dexamethasone via DNA-PK. Finally, we suggest caution in interpreting dexamethasone effects in vitro for the results to be translated into a rational use of the drug in A–T patients.

Diversity of ATM gene variants: a population-based genome data analysis for precision medicine

BACKGROUND

Ataxia-telangiectasia (AT) is a rare autosomal recessive disorder that causes deficiency or dysfunction of the ataxia-telangiectasia mutated (ATM) protein. Not only AT patients, but also certain ATM heterozygous mutation carriers show a significantly reduced life expectancy due to cancer and ischemic heart disease; in particular, female carriers having particular alleles have an increased risk of breast cancer. The frequency of such risk heterozygotes at a population level remains to be fully determined, and evidence-based preventive medical guidelines have not yet been established.

METHODS

Using the 3.5KJPNv2 allele frequency panel of Japanese Multi Omics Reference Panel v201902, which shows single-nucleotide variant (SNV) and insertion/deletion (INDEL) allele frequencies from 3552 Japanese healthy individuals, we investigated the diversity of ATM gene variants.

RESULTS

We detected 2845 (2370 SNV and 475 INDEL) variants in the ATM gene, including 1338 (1160 SNV and 178 INDEL) novel variants. Also, we found a stop-gained SNV (NC_000008.11:g.108115650G > A (p.Trp266*)) and a disruptive-inframe-deletion (NC_000008.11:g. 108181014AAGAAAAGTATGGATGATCAAG/A (p.Ala1945_Phe1952delinsVal) and two frameshift INDELs (NC_000008.11:g.108119714CAA/C (p.Glu376fs) and NC_000008.11:g.108203577CTTATA/C (p.Ile2629fs)), which would be novel variants predicted to lead to loss of ATM functionality.

CONCLUSION

The combination of population-based biobanking and human genomics provided a novel insight of diversity of ATM gene variants at a population level. For the advancement of precision medicine, such approach will be useful to predict novel pathogenic/likely pathogenic variants in the ATM gene and to establish preventive medical guidelines for certain ATM heterozygotes pertaining to their risk of particular diseases.

ATM directs DNA damage responses and proteostasis via genetically separable pathways

The protein kinase ATM is a master regulator of the DNA damage response but also responds directly to oxidative stress. Loss of ATM causes ataxia telangiectasia, a neurodegenerative disorder with pleiotropic symptoms that include cerebellar dysfunction, cancer, diabetes, and premature aging. We genetically separated the activation of ATM by DNA damage from that by oxidative stress using separation-of-function mutations. We found that deficient activation of ATM by the Mre11-Rad50-Nbs1 complex and DNA double-strand breaks resulted in loss of cell viability, checkpoint activation, and DNA end resection in response to DNA damage. In contrast, loss of oxidative activation of ATM had minimal effects on DNA damage-related outcomes but blocked ATM-mediated initiation of checkpoint responses after oxidative stress and resulted in deficiencies in mitochondrial function and autophagy. In addition, expression of a variant ATM incapable of activation by oxidative stress resulted in widespread protein aggregation. These results indicate a direct relationship between the mechanism of ATM activation and its effects on cellular metabolism and DNA damage responses in human cells and implicate ATM in the control of protein homeostasis.

ATM-dependent phosphorylation of MRE11 controls extent of resection during homology directed repair by signalling through Exonuclease 1

The MRE11/RAD50/NBS1 (MRN) complex plays a central role as a sensor of DNA double strand breaks (DSB) and is responsible for the efficient activation of ataxia-telangiectasia mutated (ATM) kinase. Once activated ATM in turn phosphorylates RAD50 and NBS1, important for cell cycle control, DNA repair and cell survival. We report here that MRE11 is also phosphorylated by ATM at S676 and S678 in response to agents that induce DNA DSB, is dependent on the presence of NBS1, and does not affect the association of members of the complex or ATM activation. A phosphosite mutant (MRE11S676AS678A) cell line showed decreased cell survival and increased chromosomal aberrations after radiation exposure indicating a defect in DNA repair. Use of GFP-based DNA repair reporter substrates in MRE11S676AS678A cells revealed a defect in homology directed repair (HDR) but single strand annealing was not affected. More detailed investigation revealed that MRE11S676AS678A cells resected DNA ends to a greater extent at sites undergoing HDR. Furthermore, while ATM-dependent phosphorylation of Kap1 and SMC1 was normal in MRE11S676AS678A cells, there was no phosphorylation of Exonuclease 1 consistent with the defect in HDR. These results describe a novel role for ATM-dependent phosphorylation of MRE11 in limiting the extent of resection mediated through Exonuclease 1.

Ataxia-telangiectasia-mutated protein kinase levels stratify patients with pancreatic adenocarcinoma into prognostic subgroups with loss being a strong indicator of poor survival

OBJECTIVES

Recently, aberrations in the gene encoding for ataxia-telangiectasia-mutated (ATM) protein kinase have been reported for pancreatic ductal adenocarcinomas (PDAC). These findings argue that ATM deficiency may play a role during carcinogenesis. Therefore, in this study, we investigated the clinical relevance of ATM expression and ATM activation in PDAC.

METHODS

Both ATM expression and nuclear phosphoSer1981-ATM levels were assessed by immunohistochemistry in a cohort of 133 PDAC and correlated with clinicopathological parameters.

RESULTS

We found stratification in prognostic subgroups. Complete loss of Ser1981-ATM was indicative of the worst prognosis (median survival, 10.8 vs 14.3 months [low expression] vs 31.1 months [high expression], P < 0.001). Similarly, analysis of ATM expression demonstrated absent expression levels of ATM to be associated with dismal prognosis (median survival, 9.6 months), whereas expression of ATM in general was associated with increased survival (17.7 months, P = 0.001).

CONCLUSIONS

Our analysis shows that both ATM expression and activated ATM are prognostic markers in PDAC with respect to standard clinicopathological parameters. These results suggest that ATM should be further explored as prognostic as well as predictive factor with respect to conventional chemotherapies and for putative synthetic lethal approaches.

DNA double-strand breaks activate ATM independent of mitochondrial dysfunction in A549 cells

Excessive nuclear or mitochondrial DNA damage can lead to mitochondrial dysfunction, decreased energy production, and increased generation of reactive oxygen species (ROS). Although numerous cell signaling pathways are activated when cells are injured, the ataxia telangiectasia mutant (ATM) protein has emerged as a major regulator of the response to both mitochondrial dysfunction and nuclear DNA double-strand breaks (DSBs). Because mitochondrial dysfunction is often a response to excessive DNA damage, it has been difficult to determine whether nuclear and/or mitochondrial DNA DSBs activate ATM independent of mitochondrial dysfunction. In this study, mitochondrial and nuclear DNA DSBs were generated in the A549 human lung adenocarcinoma cell line by infecting with retroviruses expressing the restriction endonuclease PstI fused to a mitochondrial targeting sequence (MTS) or nuclear localization sequence (NLS) and a hemagglutinin antigen epitope tag (HA). Expression of MTS-PstI-HA or NLS-PstI-HA activated the DNA damage response defined by phosphorylation of ATM, the tumor suppressor protein p53 (TP53), KRAB-associated protein (KAP)-1, and structural maintenance of chromosomes (SMC)-1. Phosphorylated ATM and SMC1 were detected in nuclear fractions, whereas phosphorylated TP53 and KAP1 were detected in both mitochondrial and nuclear fractions. PstI also enhanced expression of the cyclin-dependent kinase inhibitor p21 and inhibited cell growth. This response to DNA damage occurred in the absence of detectable mitochondrial dysfunction and excess production of ROS. These findings reveal that DNA DSBs are sufficient to activate ATM independent of mitochondrial dysfunction and suggest that the activated form of ATM and some of its substrates are restricted to the nuclear compartment, regardless of the site of DNA damage.

A-TWinnipeg: Pathogenesis of rare ATM missense mutation c.6200C>A with decreased protein expression and downstream signaling, early-onset dystonia, cancer, and life-threatening radiotoxicity

We studied 10 Mennonite patients who carry the c.6200C>A missense mutation (p.A2067D) in the ATM gene, all of whom exhibited a phenotypic variant of ataxia-telangiectasia (A-T) that is characterized by early-onset dystonia and late-onset mild ataxia, as previously described. This report provides the pathogenetic evidence for this mutation on cellular functions. Several patients have developed cancer and subsequently experienced life-threatening adverse reactions to radiation (radiotoxicity) and/or chemotherapy. As the c.6200C>A mutation is, thus far, unique to the Mennonite population and is always associated with the same haplotype or haplovariant, it was important to rule out any possible confounding DNA variant on the same haplotype. Lymphoblastoid cells derived from Mennonite patients expressed small amounts of ATM protein, which had no autophosphorylation activity at ATM Ser1981, and trace-to-absent transphosphorylation of downstream ATM targets. A-T lymphoblastoid cells stably transfected with ATM cDNA which had been mutated for c.6200C>A did not show a detectable amount of ATM protein. The same stable cell line with mutated ATM cDNA also showed a trace-to-absent transphosphorylation of downstream ATM targets SMC1pSer966 and KAP1pSer824. From these results, we conclude that c.6200A is the disease-causing ATM mutation on this haplotype. The presence of at least trace amounts of ATM kinase activity on some immunoblots may account for the late-onset, mild ataxia of these patients. The cause of the dystonia remains unclear. Because this dystonia-ataxia phenotype is often encountered in the Mennonite population in association with cancer and adverse reactions to chemotherapy, an early diagnosis is important.

Smg1 haploinsufficiency predisposes to tumor formation and inflammation

SMG1 is a member of the phosphoinositide kinase-like kinase family of proteins that includes ATM, ATR, and DNA-PK, proteins with known roles in DNA damage and cellular stress responses. SMG1 has a well-characterized role in nonsense-mediated decay as well as suggested roles in the DNA damage response, resistance to oxidative stress, regulation of hypoxic responses, and apoptosis. To understand the roles of SMG1 further, we generated a Genetrap Smg1 mouse model. Smg1 homozygous KO mice were early embryonic lethal, but Smg1 heterozygous mice showed a predisposition to a range of cancers, particularly lung and hematopoietic malignancies, as well as development of chronic inflammation. These mice did not display deficiencies in known roles of SMG1, including nonsense-mediated decay. However, they showed elevated basal tissue and serum cytokine levels, indicating low-level inflammation before the development of tumors. Smg1 heterozygous mice also showed evidence of oxidative damage in tissues. These data suggest that the inflammation observed in Smg1 haploinsufficiency contributes to susceptibility to cancer and that Smg1-deficient animals represent a model of inflammation-enhanced cancer development.

Comprehensive profiling of radiosensitive human cell lines with DNA damage response assays identifies the neutral comet assay as a potential surrogate for clonogenic survival

In an effort to explore the possible causes of human radiosensitivity and identify more rapid assays for cellular radiosensitivity, we interrogated a set of assays that evaluate cellular functions involved in recognition and repair of DNA double-strand breaks: (1) neutral comet assay, (2) radiation-induced γ-H2AX focus formation, (3) the temporal kinetics of structural maintenance of chromosomes 1 phosphorylation, (4) intra-S-phase checkpoint integrity, and (5) mitochondrial respiration. We characterized a unique panel of 19 "radiosensitive" human lymphoblastoid cell lines from individuals with undiagnosed diseases suggestive of a DNA repair disorder. Radiosensitivity was defined by reduced cellular survival using a clonogenic survival assay. Each assay identified cell lines with defects in DNA damage response functions. The highest concordance rate observed, 89% (17/19), was between an abnormal neutral comet assay and reduced survival by the colony survival assay. Our data also suggested that the neutral comet assay would be a more rapid surrogate for analyzing DNA repair/processing disorders.

The ATM protein kinase and cellular redox signaling: beyond the DNA damage response

The ataxia-telangiectasia mutated (ATM) protein kinase is best known for its role in the DNA damage response, but recent findings suggest that it also functions as a redox sensor that controls the levels of reactive oxygen species in human cells. Here, we review evidence supporting the conclusion that ATM can be directly activated by oxidation, as well as various observations from ATM-deficient patients and mouse models that point to the importance of ATM in oxidative stress responses. We also discuss the roles of this kinase in regulating mitochondrial function and metabolic control through its action on tumor suppressor p53, AMP-activated protein kinase (AMPK), mammalian target of rapamycin (mTOR) and hypoxia-inducible factor 1 (HIF1), and how the regulation of these enzymes may be affected in ATM-deficient patients and in cancer cells.

Autophosphorylation and ATM activation: additional sites add to the complexity

The recognition and signaling of DNA double strand breaks involves the participation of multiple proteins, including the protein kinase ATM (mutated in ataxia-telangiectasia). ATM kinase is activated in the vicinity of the break and is recruited to the break site by the Mre11-Rad50-Nbs1 complex, where it is fully activated. In human cells, the activation process involves autophosphorylation on three sites (Ser(367), Ser(1893), and Ser(1981)) and acetylation on Lys(3016). We now describe the identification of a new ATM phosphorylation site, Thr(P)(1885) and an additional autophosphorylation site, Ser(P)(2996), that is highly DNA damage-inducible. We also confirm that human and murine ATM share five identical phosphorylation sites. We targeted the ATM phosphorylation sites, Ser(367) and Ser(2996), for further study by generating phosphospecific antibodies against these sites and demonstrated that phosphorylation of both was rapidly induced by radiation. These phosphorylations were abolished by a specific inhibitor of ATM and were dependent on ATM and the Mre11-Rad50-Nbs1 complex. As found for Ser(P)(1981), ATM phosphorylated at Ser(367) and Ser(2996) localized to sites of DNA damage induced by radiation, but ATM recruitment was not dependent on phosphorylation at these sites. Phosphorylation at Ser(367) and Ser(2996) was functionally important because mutant forms of ATM were defective in correcting the S phase checkpoint defect and restoring radioresistance in ataxia-telangiectasia cells. These data provide further support for the importance of autophosphorylation in the activation and function of ATM in vivo.

Efficacy of citicoline as an acute stroke treatment

Citicoline (cytidine-5'-diphosphocholine or CDP-choline) is a precursor essential for the synthesis of phosphatidylcholine, one of the cell membrane components that is degraded during cerebral ischemia to free fatty acids and free radicals. Animal studies suggest that citicoline may protect cell membranes by accelerating resynthesis of phospholipids and suppressing the release of free fatty acids, stabilizing cell membranes, and reducing free radical generation. Numerous experimental stroke studies with citicoline have shown improved outcome and reduced infarct size in both ischemic and hemorrhagic stroke models. Citicoline has been studied worldwide in both ischemic and hemorrhagic clinical stroke with excellent safety and possibly efficacy found in several trials. A meta-analysis of four randomized US clinical citicoline trials concluded that treatment with oral citicoline within the first 24 h after a moderate to severe stroke is safe and increases the probability of complete recovery at 3 months. Citicoline clinical efficacy trials are now continuing outside of the US in both ischemic and hemorrhagic stroke. A citicoline supplement is now available from several sources on the internet.

Ionizing radiation-dependent and independent phosphorylation of the 32-kDa subunit of replication protein A during mitosis

The human single-stranded DNA-binding protein, replication protein A (RPA), is regulated by the N-terminal phosphorylation of its 32-kDa subunit, RPA2. RPA2 is hyperphosphorylated in response to various DNA-damaging agents and also phosphorylated in a cell-cycle-dependent manner during S- and M-phase, primarily at two CDK consensus sites, S23 and S29. Here we generated two monoclonal phospho-specific antibodies directed against these CDK sites. These phospho-specific RPA2-(P)-S23 and RPA2-(P)-S29 antibodies recognized mitotically phosphorylated RPA2 with high specificity. In addition, the RPA2-(P)-S23 antibody recognized the S-phase-specific phosphorylation of RPA2, suggesting that during S-phase only S23 is phosphorylated, whereas during M-phase both CDK sites, S23 and S29, are phosphorylated. Immunofluorescence microscopy revealed that the mitotic phosphorylation of RPA2 starts at the onset of mitosis, and dephosphorylation occurs during late cytokinesis. In mitotic cells treated with ionizing radiation (IR), we observed a rapid hyperphosphorylation of RPA2 in addition to its mitotic phosphorylation at S23 and S29, associated with a significant change in the subcellular localization of RPA. Our data also indicate that the RPA2 hyperphosphorylation in response to IR is facilitated by the activity of both ATM and DNA-PK, and is associated with activation of the Chk2 pathway.

Functional and computational assessment of missense variants in the ataxia-telangiectasia mutated (ATM) gene: mutations with increased cancer risk

The functional consequences of missense variants are often difficult to predict. This becomes especially relevant when DNA sequence changes are used to determine a diagnosis or prognosis. To analyze the consequences of 12 missense variants in patients with mild forms of ataxia-telangiectasia (A-T), we employed site-directed mutagenesis of ataxia-telangiectasia mutated (ATM) cDNA followed by stable transfections into a single A-T cell line to isolate the effects of each allele on the cellular phenotype. After induction of the transfected cells with CdCl2, we monitored for successful ATM transcription and subsequently assessed: 1) intracellular ATM protein levels; 2) ionizing radiation (IR)-induced ATM kinase activity; and 3) cellular radiosensitivity. We then calculated SIFT and PolyPhen scores for the missense changes. Nine variants produced little or no correction of the A-T cellular phenotype and were interpreted to be ATM mutations; SIFT/PolyPhen scores supported this. Three variants corrected the cellular phenotype, suggesting that they represented benign variants or polymorphisms. SIFT and PolyPhen scores supported the functional analyses for one of these variants (c.1709T>C); the other two were predicted to be "not tolerated" (c.6188G>A and c.6325T>G) and were classified as "operationally neutral." Genotype/phenotype relationships were compared: three deleterious missense variants were associated with an increased risk of cancer (c.6679C>T, c.7271T>G, and c.8494C>T). In situ mutagenesis represents an effective experimental approach for distinguishing deleterious missense mutations from benign or operationally neutral missense variants.

Etoposide induces ATM-dependent mitochondrial biogenesis through AMPK activation

Background

DNA damage such as double-stranded DNA breaks (DSBs) has been reported to stimulate mitochondrial biogenesis. However, the underlying mechanism is poorly understood. The major player in response to DSBs is ATM (ataxia telangiectasia mutated). Upon sensing DSBs, ATM is activated through autophosphorylation and phosphorylates a number of substrates for DNA repair, cell cycle regulation and apoptosis. ATM has been reported to phosphorylate the α subunit of AMP-activated protein kinase (AMPK), which senses AMP/ATP ratio in cells, and can be activated by upstream kinases. Here we provide evidence for a novel role of ATM in mitochondrial biogenesis through AMPK activation in response to etoposide-induced DNA damage.

Methodology/Principal Findings

Three pairs of human ATM+ and ATM- cells were employed. Cells treated with etoposide exhibited an ATM-dependent increase in mitochondrial mass as measured by 10-N-Nonyl-Acridine Orange and MitoTracker Green FM staining, as well as an increase in mitochondrial DNA content. In addition, the expression of several known mitochondrial biogenesis regulators such as the major mitochondrial transcription factor NRF-1, PGC-1α and TFAM was also elevated in response to etoposide treatment as monitored by RT-PCR. Three pieces of evidence suggest that etoposide-induced mitochondrial biogenesis is due to ATM-dependent activation of AMPK. First, etoposide induced ATM-dependent phosphorylation of AMPK α subunit at Thr172, indicative of AMPK activation. Second, inhibition of AMPK blocked etoposide-induced mitochondrial biogenesis. Third, activation of AMPK by AICAR (an AMP analogue) stimulated mitochondrial biogenesis in an ATM-dependent manner, suggesting that ATM may be an upstream kinase of AMPK in the mitochondrial biogenesis pathway.

Conclusions/Significance

These results suggest that activation of ATM by etoposide can lead to mitochondrial biogenesis through AMPK activation. We propose that ATM-dependent mitochondrial biogenesis may play a role in DNA damage response and ROS regulation, and that defect in ATM-dependent mitochondrial biogenesis could contribute to the manifestations of A-T disease.

Senataxin, defective in ataxia oculomotor apraxia type 2, is involved in the defense against oxidative DNA damage

Adefective response to DNA damage is observed in several human autosomal recessive ataxias with oculomotor apraxia, including ataxia-telangiectasia. We report that senataxin, defective in ataxia oculomotor apraxia (AOA) type 2, is a nuclear protein involved in the DNA damage response. AOA2 cells are sensitive to H₂O₂, camptothecin, and mitomycin C, but not to ionizing radiation, and sensitivity was rescued with full-length SETX cDNA. AOA2 cells exhibited constitutive oxidative DNA damage and enhanced chromosomal instability in response to H₂O₂. Rejoining of H₂O₂-induced DNA double-strand breaks (DSBs) was significantly reduced in AOA2 cells compared to controls, and there was no evidence for a defect in DNA single-strand break repair. This defect in DSB repair was corrected by full-length SETX cDNA. These results provide evidence that an additional member of the autosomal recessive AOA is also characterized by a defective response to DNA damage, which may contribute to the neurodegeneration seen in this syndrome.

Ataxia Telangiectasia Mutated Down-regulates Phospho-Extracellular Signal-Regulated Kinase 1/2 via Activation of MKP-1 in Response to Radiation

Ataxia telangiectasia mutated (ATM) kinase plays a crucial role in the cellular response to DNA damage and in radiation resistance. Although much effort has focused on the relationship between ATM and other nuclear signal transducers, little is known about interactions between ATM and mitogenic signaling pathways. In this study, we show a novel relationship between ATM kinase and extracellular signal-regulated kinase 1/2 (ERK1/2), a key mitogenic stimulator. Activation of ATM by radiation down-regulates phospho-ERK1/2 and its downstream signaling via increased expression of mitogen-activated protein kinase phosphatase MKP-1 in both cell culture and tumor models. This dephosphorylation of ERK1/2 is independent of epidermal growth factor receptor (EGFR) activity and is associated with radioresistance. These findings show a new function for ATM in the control of mitogenic pathways affecting cell signaling and emphasize the key role of ATM in coordinating the cellular response to DNA damage.

Involvement of novel autophosphorylation sites in ATM activation

ATM kinase plays a central role in signaling DNA double-strand breaks to cell cycle checkpoints and to the DNA repair machinery. Although the exact mechanism of ATM activation remains unknown, efficient activation requires the Mre11 complex, autophosphorylation on S1981 and the involvement of protein phosphatases and acetylases. We report here the identification of several additional phosphorylation sites on ATM in response to DNA damage, including autophosphorylation on pS367 and pS1893. ATM autophosphorylates all these sites in vitro in response to DNA damage. Antibodies against phosphoserine 1893 revealed rapid and persistent phosphorylation at this site after in vivo activation of ATM kinase by ionizing radiation, paralleling that observed for S1981 phosphorylation. Phosphorylation was dependent on functional ATM and on the Mre11 complex. All three autophosphorylation sites are physiologically important parts of the DNA damage response, as phosphorylation site mutants (S367A, S1893A and S1981A) were each defective in ATM signaling in vivo and each failed to correct radiosensitivity, genome instability and cell cycle checkpoint defects in ataxia-telangiectasia cells. We conclude that there are at least three functionally important radiation-induced autophosphorylation events in ATM.

Expression of human ATM cDNA in Atm-deficient mouse brain mediated by HSV-1 amplicon vector

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder characterized by neurodegeneration, immunodeficiency, cancer predisposition, genome instability, and radiation sensitivity. Herpes simplex virus type 1 (HSV-1) amplicon vectors provide a means to deliver large genes to the nervous system efficiently and safely. We have generated an amplicon vector, carrying human FLAG-tagged A-T mutated (ATM), as well as an enhanced green fluorescent protein (EGFP) marker gene. Due to the lack of effective and reliable antibodies for ATM and FLAG appropriate for immunohistochemistry in mouse tissue sections, expression of the human FLAG-tagged ATM was confirmed in the mouse cerebellum at the RNA level by reverse transcription followed by quantitative PCR, and by radioactive in situ hybridization. In addition, we were able to immunoprecipitate the full-length human ATM protein from the cerebella of Atm -/- mice post-infection. This vector has been injected into the cerebella of Atm -/- mice with gene delivery to thousands of cells, including Purkinje cells, based on the EGFP marker gene. The expression of human FLAG-tagged ATM has been demonstrated in the cerebella of Atm-/- mice at the transcription and translational level three days post-infection. To our knowledge, this is the first report of vector-mediated delivery of the human ATM cDNA to an Atm -/- mouse. These vectors provide the groundwork to develop gene therapy approaches for A-T patients.

Identification of Domains of Ataxia-telangiectasia Mutated Required for Nuclear Localization and Chromatin Association

Ataxia-telangiectasia mutated (ATM) is essential for rapid induction of cellular responses to DNA double strand breaks (DSBs). In this study, we mapped a nuclear localization signal (NLS), 385KRKK388, within the amino terminus of ATM and demonstrate its recognition by the conventional nuclear import receptor, the importin alpha1/beta1 heterodimer. Although mutation of this NLS resulted in green fluorescent protein (GFP) x ATM(NLSm) localizing predominantly within the cytoplasm, small amounts of nuclear GFP x ATM(NLSm) were still sufficient to elicit a DNA damage response. Insertion of an heterologous nuclear export signal between GFP and ATM(NLSm) resulted in complete cytoplasmic localization of ATM, concomitantly reducing the level of substrate phosphorylation and increasing radiosensitivity, which indicates a functional requirement for ATM nuclear localization. Interestingly, the carboxyl-terminal half of ATM, containing the kinase domain, which localizes to the cytoplasm, could not autophosphorylate itself or phosphorylate substrates, nor could it correct radiosensitivity in response to DSBs even when targeted to the nucleus by insertion of an exogenous NLS, demonstrating that the ATM amino terminus is required for optimal ATM function. Moreover, we have shown that the recruitment/retention of ATM at DSBs requires its kinase activity because a kinase-dead mutant of GFP x ATM failed to form damage-induced foci. Using deletion mutation analysis we mapped a domain in ATM (amino acids 5-224) required for its association with chromatin, which may target ATM to sites of DNA damage. Combined, these data indicate that the amino terminus of ATM is crucial not only for nuclear localization but also for chromatin association, thereby facilitating the kinase activity of ATM in vivo.

Down-regulation of ATM protein sensitizes human prostate cancer cells to radiation-induced apoptosis

Treatment with the protein kinase C activator 12-O-tetradecanoylphorbol 12-acetate (TPA) enables radiation-resistant LNCaP human prostate cancer cells to undergo radiation-induced apoptosis, mediated via activation of the enzyme ceramide synthase (CS) and de novo synthesis of the sphingolipid ceramide (Garzotto, M., Haimovitz-Friedman, A., Liao, W. C., White-Jones, M., Huryk, R., Heston, D. W. W., Cardon-Cardo, C., Kolesnick, R., and Fuks, Z. (1999) Cancer Res. 59, 5194-5201). Here, we show that TPA functions to decrease the cellular level of the ATM (ataxia telangiectasia mutated) protein, known to repress CS activation (Liao, W.-C., Haimovitz-Friedman, A., Persaud, R., McLoughlin, M., Ehleiter, D., Zhang, N., Gatei, M., Lavin, M., Kolesnick, R., and Fuks, Z. (1999) J. Biol. Chem. 274, 17908-17917). Gel shift analysis in LNCaP and CWR22-Rv1 cells demonstrated a significant reduction in DNA binding of the Sp1 transcription factor to the ATM promoter, and quantitative reverse transcription-PCR showed a 50% reduction of ATM mRNA between 8 and 16 h of TPA treatment, indicating that TPA inhibits ATM transcription. Furthermore, treatment of LNCaP, CWR22-Rv1, PC-3, and DU-145 human prostate cells with antisense-ATM oligonucleotides, which markedly reduced cellular ATM levels, significantly enhanced radiation-induced CS activation and apoptosis, leading to apoptosis at doses as a low as 1 gray. These data suggest that the CS pathway initiates a generic mode of radiation-induced apoptosis in human prostate cancer cells, regulated by a suppressive function of ATM, and that ATM might represent a potential target for pharmacologic inactivation with potential clinical applications in human prostate cancer.

IGF-1 phosphorylates AMPK-alpha subunit in ATM-dependent and LKB1-independent manner

Serine/threonine protein kinase AMP-activated protein kinase (AMPK) is a key metabolic stress-responsive factor that promotes the adaptation of cells to their microenvironment. Elevated concentrations of intracellular AMP, caused by metabolic stress, are known to activate AMPK by phosphorylation of the catalytic subunit. Recently, the tumor suppressor serine/threonine protein kinase LKB1 was identified as an upstream kinases, AMPKKs. In the current study, we found that stimulation with growth factors also caused AMPK-alpha subunit phosphorylation. Interestingly, even an LKB1-nonexpressing cancer cell line, HeLa, exhibited growth factor-stimulated AMPK-alpha subunit phosphorylation, suggesting the presence of an LKB1-independent pathway for AMPK-alpha subunit phosphorylation. In the human pancreatic cancer cell line PANC-1, AMPK-alpha subunit phosphorylation promoted by IGF-1 was suppressed by antisense ataxia telangiectasia mutated (ATM) expression. We found that IGF-1 also induced AMPK-alpha subunit phosphorylation in the human normal fibroblast TIG103 cell line, but failed to do so in a human fibroblast AT2-KY cell line lacking ATM. Immunoprecipitates of ATM collected from IGF-1-stimulated cells also caused the phosphorylation of the AMPK-alpha subunit in vitro. IGF-1-stimulated ATM phosphorylation at both threonine and tyrosine residues, and our results demonstrated that the phosphorylation of tyrosine in the ATM molecule is important for AMPK-alpha subunit phosphorylation during IGF-1 signaling. These results suggest that IGF-1 induces AMPK-alpha subunit phosphorylation via an ATM-dependent and LKB1-independent pathway.

A CpG-rich bidirectional promoter induces the T-cell death-associated gene 51 and downregulates an inversely oriented transcript during early T-cell activation

The human T-cell death-associated gene 51 (TDAG51) is upregulated upon lymphocyte stimulation and in the context of ER stress. Moreover, TDAG51 plays a role in programmed cell death and tumorigenesis. We performed an extensive TDAG51 promoter analysis and found a strong CpG-rich bidirectional promoter within the first 582 nucleotides of the TDAG51 reference DNA complementary to RNA (cDNA). Upon stimulation of primary human T cells, this promoter modulated the downregulation of a newly detected head-to-head oriented transcript. Mapping of the transcription start points revealed that the 5' regions of the TDAG51 mRNA and of the newly identified transcript did not overlap in T cells. Thus, the TDAG51 locus shows an operon-like organization of two head-to-head oriented transcripts that are inversely regulated in T lymphocytes by a CpG-rich bidirectional promoter.

Autophosphorylation of ataxia-telangiectasia mutated is regulated by protein phosphatase 2A

Ionizing radiation induces autophosphorylation of the ataxia-telangiectasia mutated (ATM) protein kinase on serine 1981; however, the precise mechanisms that regulate ATM activation are not fully understood. Here, we show that the protein phosphatase inhibitor okadaic acid (OA) induces autophosphorylation of ATM on serine 1981 in unirradiated cells at concentrations that inhibit protein phosphatase 2A-like activity in vitro. OA did not induce gamma-H2AX foci, suggesting that it induces ATM autophosphorylation by inactivation of a protein phosphatase rather than by inducing DNA double-strand breaks. In support of this, we show that ATM interacts with the scaffolding (A) subunit of protein phosphatase 2A (PP2A), that the scaffolding and catalytic (C) subunits of PP2A interact with ATM in undamaged cells and that immunoprecipitates of ATM from undamaged cells contain PP2A-like protein phosphatase activity. Moreover, we show that IR induces phosphorylation-dependent dissociation of PP2A from ATM and loss of the associated protein phosphatase activity. We propose that PP2A plays an important role in the regulation of ATM autophosphorylation and activity in vivo.

The impact of carboxy nitroxide antioxidants on irradiated ataxia telangiectasia cells

Three water-soluble carboxy nitroxide antioxidants, 5-carboxy-1,1,3,3-tetramethylisoindolin-2-yloxyl, 4-carboxy-2,2,6,6-tetramethylpiperidin-1-yloxyl, and 3-carboxy-2,2,5,5-tetramethylpyrrolidin-1-yloxyl, show significant impact on the postirradiation survival rates of ataxia telangiectasia (A-T) cells compared to normal cells, an assay which represents a model for understanding the impact of ROS damage on the A-T phenotype. The effects of these antioxidants are much more significant than those of vitamin E or Trolox (a water-soluble vitamin E analog), studied using the same cell survival model.

ATM protein purified from vaccinia virus expression system: DNA binding requirements for kinase activation

The ataxia-telangiectasia mutated (ATM) gene product plays a role in responding to double stand DNA breaks. Some biochemical studies of ATM function have been hampered by lack of an efficient expression system and abundant purified ATM protein. We report the construction of a vaccinia virus expressing ATM, vWR-ATM, which was used to produce large amounts of functional FLAG-tagged ATM protein (FLAG-ATM) in HeLa cells. Kinase activity of the purified FLAG-ATM was dependent on manganese and inhibited with wortmannin. Using the FLAG-ATM recombinant protein, GST-p53 serine 15 phosphorylation increased in the presence of damaged DNA. PHAS-1 phosphorylation was found to be DNA independent. Purified FLAG-ATM was recovered in the autophosphorylated form, as demonstrated by phosphorylation of ATM serine 1981. As shown by atomic force microscopy, FLAG-ATM bound to linear DNA both at broken ends and in mid-strands. Vaccinia virus is the most efficient ATM expression system described to date.

Functional expression of ATM gene carried by HSV amplicon vector in vitro and in vivo

Ataxia-telangiectasia (AT) is a human autosomal recessive disease with a pleiotropic phenotype characterized by cerebellar degeneration, immunodeficiency, premature aging, cancer predisposition, and radiation sensitivity. The gene mutated in AT, ATM (for AT-mutated), had been cloned and found to have ionizing radiation and oxidative stress-inducible kinase activity. No treatment can stop the progression of the disease. In this study, the complete open-reading frame of ATM cDNA was cloned into a Herpes simplex virus type-1 (HSV-1) amplicon vector (pTO-ATM), and the transduction of cultured AT cells was demonstrated by immunohistochemistry and Western blot analysis. Functional gene expression was evaluated by cell colony-forming assays following exposure to oxidative stress. The survival of AT cells with ATM gene transduction was about 100% higher compared to nontransduced cells after t-butyl hydroperoxide treatments. Next, the normal ATM gene expression in different regions of the rat brain was studied. Immunohistochemistry staining demonstrated weak endogenous ATM protein expression in neurons of the caudate-putamen, with significantly higher levels of expression detected in neurons in other brain regions. Exogenous ATM gene expression from pTO-ATM after viral transduction in the caudate-putamen of the adult rat was examined. At 3 days after injection of the pTO-ATM viral vector, abundant positive ATM staining of the neurons was found at the injection sites, in comparison to the controls. These data demonstrate that the relatively large ATM cDNA can be transduced and expressed in vitro and in vivo from an HSV amplicon viral vector. These data provide initial evidence that the replacement of the ATM gene into the cells of AT patients might be possible some day.

HSV-1 amplicon vector-mediated expression of ATM cDNA and correction of the ataxia-telangiectasia cellular phenotype

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder characterized by neurodegeneration, immunodeficiency, cancer predisposition, genome instability, and radiation sensitivity. Previous research has shown that it is possible to correct the hereditary deficiency A-T by DNA transfection in cell culture, but the large size of the ATM cDNA (9 kb) limits the use of many vector types for gene replacement. HSV-1 amplicon vectors provide a means to deliver large genes to cells efficiently and without toxicity. In this study, the FLAG-tagged cDNA for human ATM was inserted into an HSV-1 amplicon under control of the CMV promoter (designated as HGC-ATM). FLAG-ATM expression was confirmed in 293T/17 cells and human A-T fibroblasts (GM9607) after transduction, by immunoprecipitation, Western analysis, and immunocytochemistry. Functional recovery was assessed by two independent assays. First, in vitro kinase assay showed that vector-derived ATM in GM9607 cells could successfully phosphorylate wt p53 using recombinant GST-p53(1-101). Second, in A-T cells infected with the HGC-ATM vector, the extent of accumulation in G2/M phase at 24 h postirradiation was similar to that observed in cells with wild-type endogenous ATM and lower than that observed in A-T cells infected with a control vector. Thus, these vectors provide a tool to test the feasibility of HSV-amplicons as gene therapy vectors for A-T.

Identification of a novel protein kinase mediating Akt survival signaling to the ATM protein

We identified a novel human AMP-activated protein kinase (AMPK) family member, designated ARK5, encoding 661 amino acids with an estimated molecular mass of 74 kDa. The putative amino acid sequence reveals 47, 45.8, 42.4, and 55% homology to AMPK-alpha1, AMPK-alpha2, MELK, and SNARK, respectively, suggesting that it is a new member of the AMPK family. It has a putative Akt phosphorylation motif at amino acids 595-600, and Ser(600) was found to be phosphorylated by active Akt resulting in the activation of kinase activity toward the SAMS peptide, a consensus AMPK substrate. During nutrient starvation, ARK5 supported the survival of cells in an Akt-dependent manner. In addition, we also demonstrated that ARK5, when activated by Akt, phosphorylated the ATM protein that is mutated in the human genetic disorder ataxia-telangiectasia and also induced the phosphorylation of p53. On the basis of our current findings, we propose that a novel AMPK family member, ARK5, is the tumor cell survival factor activated by Akt and acts as an ATM kinase under the conditions of nutrient starvation.

Upregulation of FasL and apoptosis in thymic lymphomas in Atm knock-in mice

Several members of the phosphatidylinositol 3-kinase family play key roles in recognising and responding to damage in DNA, induced by a variety of chemicals and other agents. One of these, ATM, the product of the gene mutated in the human genetic disorder ataxia-telangiectasia (A-T), recognises double strand breaks in DNA caused by ionizing radiation and radiomimetic chemicals. In order to study DNA damage recognition and the abnormalities of genome instability and cancer predisposition that occur in A-T patients, we generated a mouse model expressing a mutant form of Atm corresponding to a common human mutation. In this model, a 9 nucleotide in-frame deletion was introduced into the Atm gene and has been designated Atm-Delta SRI. These animals had a longer lifespan than Atm gene disrupted mice (Atm(-/-)) and they developed less thymic lymphomas. A characteristic of the lymphomas appearing in Atm-Delta SRI mice was an increased rate of apoptosis compared to the corresponding tumours in Atm(-/-) mice. Increased expression of FasL in these tumours may account for the higher levels of apoptosis. These results demonstrate that expression of mutant Atm in mice gives rise to phenotypic differences compared to Atm(-/-) mice and has implications for heterogeneity described in the human syndrome.

ATM induction insufficiency in a radiosensitive breast-cancer patient

The ataxia telangiectasia (A-T) gene (ATM) is a dominant breast cancer gene with tumour suppressor activity. ATM also regulates cellular sensitivity to ionising radiation (IR) presumably through its role as a facilitator of DNA repair. In normal cells and tissues the ATM protein is rapidly induced by IR to threshold/maximum levels. The kinase function of the ATM protein is also rapidly activated in response to IR. The fact that women carriers of ATM mutations can have an increased risk of developing breast cancer and that many sporadic breast tumours have reduced levels of the ATM protein broadens the scope of ATM's tumour suppressor within the breast. This report describes the downregulation of ATM protein levels in a radiosensitive breast cancer patient. Postinduction ATM levels were up to tenfold lower in the patient's fresh tissues compared to normal controls. These results might indicate a much broader role for ATM anomalies in breast cancer aetiology.

Mice heterozygous for mutation in Atm, the gene involved in ataxia-telangiectasia, have heightened susceptibility to cancer

Ataxia-telangiectasia is characterized by radiosensitivity, genome instability and predisposition to cancer. Heterozygous carriers of ATM, the gene defective in ataxia-telangiectasia, have a higher than normal risk of developing breast and other cancers. We demonstrate here that Atm 'knock-in' (Atm-Delta SRI) heterozygous mice harboring an in-frame deletion corresponding to the human 7636del9 mutation show an increased susceptibility to developing tumors. In contrast, no tumors are observed in Atm knockout (Atm(+/-)) heterozygous mice. In parallel, we report the appearance of tumors in 6 humans from 12 families who are heterozygous for the 7636del9 mutation. Expression of ATM cDNA containing the 7636del9 mutation had a dominant-negative effect in control cells, inhibiting radiation-induced ATM kinase activity in vivo and in vitro. This reduces the survival of these cells after radiation exposure and enhances the level of radiation-induced chromosomal aberrations. These results show for the first time that mouse carriers of a mutated Atm that are capable of expressing Atm have a higher risk of cancer. This finding provides further support for cancer predisposition in human ataxia-telangiectasia carriers.

Functional link between BLM defective in Bloom's syndrome and the ataxia-telangiectasia-mutated protein, ATM

Chromosome aberrations, genomic instability, and cancer predisposition are hallmarks of a number of syndromes in which the defective genes recognize and/or repair DNA damage or are involved in some aspect of DNA processing. We report here direct interaction between BLM, mutated in Bloom's Syndrome (BS), and ATM, mutated is ataxia-telangiectasia, and we have mapped the sites of interaction. Full-length BLM cDNA corrected sister chromatid exchange (SCE) and radiosensitivity in BS cells. Mitotic phosphorylation of BLM was partially dependent on ATM, and phosphorylation sites on BLM were identified. A phosphospecific antibody against one of these sites (Thr-99) revealed radiation-induced phosphorylation, which was defective in ataxia-telangiectasia cells. Stable cell lines expressing phosphorylation site mutants failed to correct radiosensitivity in BS cells but corrected SCE. These mutants also sensitized normal control cells to radiation and increased radiation-induced chromosome aberrations but did not cause SCE numbers to increase. These data suggest that ATM and BLM function together in recognizing abnormal DNA structures by direct interaction and that these phosphorylation sites in BLM are important for radiosensitivity status but not for SCE frequency.

Missense mutations but not allelic variants alter the function of ATM by dominant interference in patients with breast cancer

The human genetic disorder ataxia-telangiectasia (A-T) is characterized by hypersensitivity to ionizing radiation and an elevated risk of malignancy. Epidemiological data support an increased risk for breast and other cancers in A-T heterozygotes. However, screening breast cancer cases for truncating mutations in the ATM (A-T mutated) gene has failed largely to reveal an increased incidence in these patients. It has been hypothesized that ATM missense mutations are implicated in breast cancer, and there is some evidence to support this. The presence of a large variety of rare missense variants in addition to common polymorphisms in ATM makes it difficult to establish such a relationship by association studies. To investigate the functional significance of these changes we have introduced missense substitutions, identified in either A-T or breast cancer patients, into ATM cDNA before establishing stable cell lines to determine their effect on ATM function. Pathogenic missense mutations and neutral missense variants were distinguished initially by their capacity to correct the radiosensitive phenotype in A-T cells. Furthermore missense mutations abolished the radiation-induced kinase activity of ATM in normal control cells, caused chromosome instability, and reduced cell viability in irradiated control cells, whereas neutral variants failed to do so. Mutant ATM was expressed at the same level as endogenous protein, and interference with normal ATM function seemed to be by multimerization. This approach represents a means of identifying genuine ATM mutations and addressing the significance of missense changes in the ATM gene in a variety of cancers including breast cancer.

ATM phosphorylates histone H2AX in response to DNA double-strand breaks

A very early step in the response of mammalian cells to DNA double-strand breaks is the phosphorylation of histone H2AX at serine 139 at the sites of DNA damage. Although the phosphatidylinositol 3-kinases, DNA-PK (DNA-dependent protein kinase), ATM (ataxia telangiectasia mutated), and ATR (ATM and Rad3-related), have all been implicated in H2AX phosphorylation, the specific kinase involved has not yet been identified. To definitively identify the specific kinase(s) that phosphorylates H2AX in vivo, we have utilized DNA-PKcs-/- and Atm-/- cell lines and mouse embryonic fibroblasts. We find that H2AX phosphorylation and nuclear focus formation are normal in DNA-PKcs-/- cells and severely compromised in Atm-/- cells. We also find that ATM can phosphorylate H2AX in vitro and that ectopic expression of ATM in Atm-/- fibroblasts restores H2AX phosphorylation in vivo. The minimal H2AX phosphorylation in Atm-/- fibroblasts can be abolished by low concentrations of wortmannin suggesting that DNA-PK, rather than ATR, is responsible for low levels of H2AX phosphorylation in the absence of ATM. Our results clearly establish ATM as the major kinase involved in the phosphorylation of H2AX and suggest that ATM is one of the earliest kinases to be activated in the cellular response to double-strand breaks.

ATM, a central controller of cellular responses to DNA damage

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

ATM as a target for novel radiosensitizers

DNA damage checkpoints are complex signal transduction pathways that are critical for normal cellular recovery following potentially lethal genotoxic insults. The ataxia-telangiectasia mutated (ATM) protein kinase is a critical component in these pathways and integrates the cellular response to damage by phosphorylating key proteins involved in cell cycle regulation and DNA repair. Lack of normal ATM function in the inherited ataxia-telangiectasia (A-T) syndrome results in a pleiotropic clinical syndrome characterized by a marked increased risk of cancer and profound hypersensitivity to ionizing radiation. Cells derived from patients with A-T share some of these attributes with genomic instability, loss of normal cell cycle arrest pathways, defects in DNA repair and increased radiation sensitivity. The radiosensitivity of A-T cells suggests that pharmacological inhibitors of the ATM kinase should be effective radiosensitizing agents. In fact, caffeine inhibits ATM kinase activity at concentrations that result in an A-T-like phenotype with loss of cell cycle checkpoints and hypersensitivity to ionizing radiation. Although the clinical use of caffeine as a radiosensitizer is limited by potentially lethal systemic toxicities, more potent methyl xanthines may selectively inhibit the ATM pathway at clinically achievable levels. Interestingly, caffeine and other methyl xanthines preferentially radiosensitize cells that lack normal p53 function. Because p53 is commonly inactivated in epithelial malignancies, this suggests that small molecule inhibitors of ATM might selectively sensitize the majority of tumors to the lethal effects of ionizing radiation while sparing normal tissues.

UV-induced hyperphosphorylation of replication protein a depends on DNA replication and expression of ATM protein

Exposure to DNA-damaging agents triggers signal transduction pathways that are thought to play a role in maintenance of genomic stability. A key protein in the cellular processes of nucleotide excision repair, DNA recombination, and DNA double-strand break repair is the single-stranded DNA binding protein, RPA. We showed previously that the p34 subunit of RPA becomes hyperphosphorylated as a delayed response (4-8 h) to UV radiation (10-30 J/m(2)). Here we show that UV-induced RPA-p34 hyperphosphorylation depends on expression of ATM, the product of the gene mutated in the human genetic disorder ataxia telangiectasia (A-T). UV-induced RPA-p34 hyperphosphorylation was not observed in A-T cells, but this response was restored by ATM expression. Furthermore, purified ATM kinase phosphorylates the p34 subunit of RPA complex in vitro at many of the same sites that are phosphorylated in vivo after UV radiation. Induction of this DNA damage response was also dependent on DNA replication; inhibition of DNA replication by aphidicolin prevented induction of RPA-p34 hyperphosphorylation by UV radiation. We postulate that this pathway is triggered by the accumulation of aberrant DNA replication intermediates, resulting from DNA replication fork blockage by UV photoproducts. Further, we suggest that RPA-p34 is hyperphosphorylated as a participant in the recombinational postreplication repair of these replication products. Successful resolution of these replication intermediates reduces the accumulation of chromosomal aberrations that would otherwise occur as a consequence of UV radiation.

Activation of p53 transcriptional activity requires ATM's kinase domain and multiple N-terminal serine residues of p53

The ATM protein kinase regulates the cell's response to DNA damage by regulating cell cycle checkpoints and DNA repair. ATM phosphorylates several proteins involved in the DNA-damage response, including p53. We have examined the mechanism by which ATM regulates p53's transcriptional activity. Here, we demonstrate that reintroduction of ATM into AT cells restores the activation of p53 by the radio-mimetic agent bleomycin. Further, p53 activation is lost when a kinase inactive ATM is used, or if the N-terminal of ATM is deleted. In addition, AT cells stably expressing ATM showed decreased sensitivity to Ionizing Radiation-induced cell killing, whereas cells expressing kinase inactive ATM or N-terminally deleted ATM were indistinguishable from AT cells. Finally, single point-mutations of serines 15, 20, 33 or 37 did not individually block the ATM-dependent activation of p53 transcriptional activity by bleomycin. However, double mutations of either serines 15 and 20 or serines 33 and 37 blocked the ability of ATM to activate p53. Our results indicate that the N-terminal of ATM and ATM's kinase activity are required for activation of p53's transcriptional activity and restoration of normal sensitivity to DNA damage. In addition, activation of p53 by ATM requires multiple serine residues in p53's transactivation domain.

Transcriptional downregulation of ATM by EGF is defective in ataxia-telangiectasia cells expressing mutant protein

There is evidence that ATM plays a wider role in intracellular signalling in addition to DNA damage recognition and cell cycle control. In this report we show that activation of the EGF receptor is defective in ataxia-telangiectasia (A-T) cells and that sustained stimulation of cells with EGF downregulates ATM protein in control cells but not in A-T cells expressing mutant protein. Concomitant with the downregulation of ATM, DNA-binding activity of the transcription factor Sp1 decreased in controls after EGF treatment but increased from a lower basal level in A-T cells to that in untreated control cells. Mutation in two Sp1 consensus sequences in the ATM promoter reduced markedly the capacity of the promoter to support luciferase activity in a reporter assay. Overexpression of anti-sense ATM cDNA in control cells decreased the basal level of Sp1, which in turn was increased by subsequent treatment of cells with EGF, similar to that observed in A-T cells. On the other hand full-length ATM cDNA increased the basal level of Sp1 binding in A-T cells, and in response to EGF Sp1 binding decreased, confirming that this is an ATM-dependent process. Contrary to that observed in control cells there was no radiation-induced change in ATM protein in EGF-treated A-T cells and likewise no alteration in Sp1 binding activity. The results demonstrate that EGF-induced downregulation of ATM (mutant) protein in A-T cells is defective and this appears to be due to less efficient EGFR activation and abnormal Sp1 regulation.

ATM protein expression correlates with radioresistance in primary glioblastoma cells in culture

PURPOSE

Glioblastoma multiforme (GBM) is one of the malignancies most resistant to radiation therapy. In contrast, cells derived from individuals with ataxia telangiectasia (AT), possessing mutations in the ATM gene, demonstrate increased sensitivity to ionizing radiation. Using a collection of glioma specimens adapted to tissue culture and several established GBM cell lines, we investigated the relationship between ATM protein expression and radiosensitivity. The three aims of our study were to: (1) quantify ATM protein levels in cultured glioma cells; (2) measure the correlation between ATM protein levels and radiation sensitivity; and (3) examine the dependence of ATM on p53 status.

METHODS AND MATERIALS

Glioma specimens were collected, catalogued, and adapted to grow in culture. Levels of ATM, p53, and p21 proteins were determined by Western blot. Radiation sensitivities were determined by clonogenic assays. p53 mutation status was determined by DNA sequencing. Correlations were identified by linear regression analysis.

RESULTS

ATM protein levels were variable in the primary gliomas. Glioma cell lines demonstrated significantly lower levels of ATM protein. Clonogenic assays of cell strains and cell lines yielded survival fractions (SF2s) consistent with the radioresistant behavior of GBM tumors in vivo. Regression analysis revealed a high correlation between ATM protein levels and SF2 for primary glioma cell strains, but not for established GBM cell lines. p53 status failed to predict radiosensitivity.

CONCLUSION

We have demonstrated that while our collection of low passage cell cultures depends on ATM for their resistance to IR, established cell lines may acquire adaptive characteristics which downplay the role of the ATM gene product in vitro. Therefore, attenuating ATM gene expression may be a successful strategy in the treatment of GBM tumors.

Epidermal growth factor sensitizes cells to ionizing radiation by down-regulating protein mutated in ataxia-telangiectasia

Epidermal growth factor (EGF) has been reported to either sensitize or protect cells against ionizing radiation. We report here that EGF increases radiosensitivity in both human fibroblasts and lymphoblasts and down-regulates both ATM (mutated in ataxia-telangiectasia (A-T)) and the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs). No further radiosensitization was observed in A-T cells after pretreatment with EGF. The down-regulation of ATM occurs at the transcriptional level. Concomitant with the down-regulation of ATM, the DNA binding activity of the transcription factor Sp1 decreased. A causal relationship was established between these observations by demonstrating that up-regulation of Sp1 DNA binding activity by granulocyte/macrophage colony-stimulating factor rapidly reversed the EGF-induced decrease in ATM protein and restored radiosensitivity to normal levels. Failure to radiosensitize EGF-treated cells to the same extent as observed for A-T cells can be explained by induction of ATM protein and kinase activity with time post-irradiation. Although ionizing radiation damage to DNA rapidly activates ATM kinase and cell cycle checkpoints, we have provided evidence for the first time that alteration in the amount of ATM protein occurs in response to both EGF and radiation exposure. Taken together these data support complex control of ATM function that has important repercussions for targeting ATM to improve radiotherapeutic benefit.

Radiation-induced chromatid breaks and deficient DNA repair in cancer predisposition

Deficient repair of DNA double-strand breaks, resulting in an abnormally high frequency of chromatid breaks after G(2) exposure of cells to radiation, appears to be associated with cancer predisposition. Unrepaired DNA strand breaks contribute to genomic instability. Unrepaired chromatid breaks representing DNA strand breaks can result in chromosome deletions, translocations and gene amplifications seen in human cancers. This cytogenetic response of cells to radiation may be useful as a marker of cancer susceptibility and in identifying individuals at risk of developing cancer in cancer families.

Transactivation-deficient p73alpha (p73Deltaexon2) inhibits apoptosis and competes with p53

p73 has recently been identified as a structural and functional homolog of the tumor suppressor protein p53. Overexpression of p53 activates transcription of p53 effector genes, causes growth inhibition and induced apoptosis. We describe here the effects of a tumor-derived truncated transcript of p73alpha (p73Deltaexon2) on p53 function and on cell death. This transcript, which lacks the acidic N-terminus corresponding to the transactivation domain of p53, was initially detected in a neuroblastoma cell line. Overexpression of p73Deltaexon2 partially protects lymphoblastoid cells against apoptosis induced by anti-Fas antibody or cisplatin. By cotransfecting p73Deltaexon2 with wild-type p53 in the p53 null line Saos 2, we found that this truncated transcript reduces the ability of wild-type p53 to promote apoptosis. This anti-apoptotic effect was also observed when p73Deltaexon2 was co-transfected with full-length p73 (p73alpha). This was further substantiated by suppression of p53 transactivation of the effector gene p21/Waf1 in p73Deltaexon2 transfected cells and by inhibition of expression of a reporter gene under the control of the p53 promoter. Thus, this truncated form of p73 can act as a dominant-negative agent towards transactivation by p53 and p73alpha, highlighting the potential implications of these findings for p53 signaling pathway. Furthermore, we demonstrate the existence of a p73Deltaexon2 transcript in a very significant proportion (46%) of breast cancer cell lines. However, a large spectrum of normal and malignant tissues need to be surveyed to determine whether this transdominant p73 variant occurs in a tumor-specific manner.

Genetic evidence of a role for ATM in functional interaction between human T-cell leukemia virus type 1 Tax and p53

Recent evidence from several investigators suggest that the human T-cell leukemia virus type 1 Tax oncoprotein represses the transcriptional activity of the tumor suppressor protein, p53. An examination of published findings reveals serious controversy as to the mechanism(s) utilized by Tax to inhibit p53 activity and whether the same mechanism is used by Tax in adherent and suspension cells. Here, we have investigated Tax-p53 interaction simultaneously in adherent epithelial (HeLa and Saos) and suspension T-lymphocyte (Jurkat) cells. Our results indicate that Tax activity through the CREB/CREB-binding protein (CBP), but not NF-kappaB, pathway is needed to repress the transcriptional activity of p53 in all tested cell lines. However, we did find that while CBP binding by Tax is necessary, it is not sufficient for inhibiting p53 function. Based on knockout cell studies, we correlated a strong genetic requirement for the ATM, but not protein kinase-dependent DNA, protein in conferring a Tax-p53-repressive phenotype.