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PubMed2001

Effect of ultraviolet light, methyl methanesulfonate and ionizing radiation on the genotoxic response and apoptosis of mouse fibroblasts lacking c-Fos, p53 or both.

c-Fos and p53 are DNA damage-inducible proteins that are involved in gene regulation, cell cycle checkpoint control and cell proliferation following exposure to genotoxic agents. To investigate comparatively the role of c-Fos and p53 in the maintenance of genomic stability and the induction of apoptosis, we generated mouse fibroblast cell lines from knockout mice deficient for either c-fos (fos -/-) or p53 (p53-/-) or for both gene products (fosp53-/-). The sensitivity of these established cell lines was compared with the corresponding wild-type cells as to the cytotoxic, clastogenic and apoptosis-inducing effects of ultraviolet (UV-C) light and methyl methanesulfonate (MMS). Additionally, we analysed the frequency of apoptosis of the cell lines after treatment with ionizing radiation (IR). We observed c-fos-/-, p53-/- and fosp53-/- cells to be more sensitive than wild-type cells with respect to cell death, as measured in a cytotoxicity (MTT) assay. Regarding apoptosis, all deficient cell lines displayed hypersensitivity to UV-C light, MMS and IR. With chromosomal aberrations as the endpoint, the sensitivity of the double-knockout cells was between wild-type and single-knockouts. The results indicate that both c-Fos and p53 play an important role in protecting fibroblasts against a broad range of genotoxic agents. The results also show that, in fibroblasts, apoptosis induced by UV-C light, MMS and IR does not require p53 and that, in this cell type, p53 rather protects against DNA damage-induced apoptotic cell death.

Animals↗
PubMed2004

A novel mechanism of chemoprotection by sulforaphane: inhibition of histone deacetylase.

Sulforaphane (SFN), a compound found at high levels in broccoli and broccoli sprouts, is a potent inducer of phase 2 detoxification enzymes and inhibits tumorigenesis in animal models. SFN also has a marked effect on cell cycle checkpoint controls and cell survival and/or apoptosis in various cancer cells, through mechanisms that are poorly understood. We tested the hypothesis that SFN acts as an inhibitor of histone deacetylase (HDAC). In human embryonic kidney 293 cells, SFN dose-dependently increased the activity of a beta-catenin-responsive reporter (TOPflash), without altering beta-catenin or HDAC protein levels. Cytoplasmic and nuclear extracts from these cells had diminished HDAC activity, and both global and localized histone acetylation was increased, compared with untreated controls. Studies with SFN and with media from SFN-treated cells indicated that the parent compound was not responsible for the inhibition of HDAC, and this was confirmed using an inhibitor of glutathione S-transferase, which blocked the first step in the metabolism of SFN, via the mercapturic acid pathway. Whereas SFN and its glutathione conjugate (SFN-GSH) had little or no effect, the two major metabolites SFN-cysteine and SFN-N-acetylcysteine were effective HDAC inhibitors in vitro. Finally, several of these findings were recapitulated in HCT116 human colorectal cancer cells: SFN dose-dependently increased TOPflash reporter activity and inhibited HDAC activity, there was an increase in acetylated histones and in p21(Cip1/Waf1), and chromatin immunoprecipitation assays revealed an increase in acetylated histones bound to the P21 promoter. Collectively, these findings suggest that SFN may be effective as a tumor-suppressing agent and as a chemotherapeutic agent, alone or in combination with other HDAC inhibitors currently undergoing clinical trials.

Acetylation↗
PubMed1999

Modulation of apoptosis signaling pathways and cell cycle regulation.

Apoptosis can be described as multiple pathways converging from numerous different initiating events and insults, such as anticancer agents. These pathways converge on a common irreversible execution phase in which proteases and nucleases digest the doomed cell. Counteracting the signals to die are a variety of pathways that enhance cell survival and that may become constitutively active as a result of oncogenic transformation. Studies of apoptosis have identified many cellular factors that play a role in the decision as to whether a cell lives or dies. These factors include the p53 tumor suppressor, the Bcl-2 family of proteins, and a variety of intracellular signal transduction pathways, all of which may provide novel therapeutic targets. It also is possible to take advantage of the defect in cell cycle regulation that occurs in cells with mutant p53; such cells are susceptible to agents that inhibit DNA damage-induced cell cycle checkpoints at S and G2 phase. Cell cycle perturbation occurs following treatment with all anticancer drugs and a knowledge of the kinetics of such events should facilitate design of synergistic rather than antagonistic schedules. These concepts have been developed in cell culture models and it is essential to determine whether the mechanisms defined also occur in patients receiving therapy. Accordingly, tumors need to undergo serial biopsies during therapy and be analyzed for perturbation in cell cycle or apoptosis-regulating proteins. The results of such studies should facilitate the rational design of chemotherapy combinations.

Animals↗
PubMed1996

Expression of HPV16 E6 or E7 increases integration of foreign DNA.

In most invasive cervical carcinomas, high-risk human papillomavirus (HPV) DNA is integrated into the host genome, while in pre-invasive cervical lesions the viral genome is typically maintained exclusively as an episome. In contrast, integration of low-risk HPV DNA is rare, as is the association of low-risk HPVs with carcinomas. High-risk HPV integration is associated with a selective growth advantage of affected cells, and hence, integration is likely to be an important genetic alteration contributing to cervical tumor progression. Expression of high-risk, but not low-risk, HPV E6 or E7 proteins disrupts the p53-dependent G1 arrest that cells normally display in response to DNA damage. Absence of this cell cycle checkpoint may predispose cells containing high-risk HPVs to genetic instability and to the accumulation of the genetic alterations that appear to be required for HPV-associated cervical tumor progression. We hypothesized that integration of high-risk HPV DNA into the host cell genome may be facilitated by E6- and/or E7-mediated disruption of the normal DNA damage response pathway. To test this hypothesis, we assessed the integration frequency of a reporter plasmid (pHyGal) in RKO cells expressing individual E6 or E7 genes of either high-risk (HPV16) or low-risk (HPV6, HPV11) type viruses. Cells expressing HPV16 E6 or HPV16 E7 exhibited a significantly increased frequency of pHyGal integration in comparison to RKO control cells or cells expressing low-risk HPV E6 or E7. Thus, expression of high-risk, but not low-risk, E6 and E7 proteins increases the frequency of foreign DNA integration into the host genome. These findings suggest that at least some of the difference in oncogenic potential observed between high-risk and low-risk HPV types may be determined by the increased ability of high-risk HPVs to integrate into host DNA.

Cinnamates↗
PubMed2000

Differential toxicities of anticancer agents among DNA repair and checkpoint mutants of Saccharomyces cerevisiae.

Most cytotoxic anticancer agents damage DNA directly, interfere with DNA metabolism or chromosome segregation, and are particularly toxic in dividing cells. Although a considerable amount of information on the mechanisms of action of these agents is available, the molecular bases for selective tumor cell killing by chemotherapy are largely unknown. Many genetic alterations found in sporadic and hereditary cancers affect functions in DNA repair and cell cycle control and result in sensitivity to DNA damaging agents. We have therefore set out to determine the effects of these cancer mutations on sensitivity or resistance to various chemotherapeutic agents. Because most of the affected genes are well conserved among eukaryotes, we have carried out a comprehensive analysis of a panel of isogenic yeast strains, each defective in a particular DNA repair or cell cycle checkpoint function, for sensitivity to the Food and Drug Administration-approved cytotoxic anticancer agents. Widely different toxicity profiles were observed for 23 agents and X-rays, indicating that the type of DNA repair and cell cycle checkpoint mutations in individual tumors could strongly influence the outcome of a particular chemotherapeutic regimen.

Antimetabolites, Antineoplastic↗
PubMed2000

The human checkpoint protein hRad17 interacts with the PCNA-like proteins hRad1, hHus1, and hRad9.

DNA damage activates cell cycle checkpoints that prevent progression through the cell cycle. In yeast, the DNA damage checkpoint response is regulated by a series of genes that have mammalian homologs, including rad1, rad9, hus1, and rad17. On the basis of sequence homology, yeast and human Rad1, Rad9, and Hus1 protein homologs are predicted to structurally resemble the sliding clamp PCNA. Likewise, Rad17 homologs have extensive homology with replication factor C (RFC) subunits (p36, p37, p38, p40, and p140), which form a clamp loader for PCNA. These observations predict that Rad1, Hus1, and Rad9 might interact with Rad17 as a clamp-clamp loader pair during the DNA damage response. In this report, we demonstrate that endogenous human Rad17 (hRad17) interacts with the PCNA-related checkpoint proteins hRad1, hRad9, and hHus1. Mutational analysis of hRad1 and hRad17 demonstrates that this interaction has properties similar to the interaction between RFC and PCNA, a well characterized clamp-clamp loader pair. Moreover, we show that DNA damage affects the association of hRad17 with the clamp-like checkpoint proteins. Collectively, these data provide the first experimental evidence that hRad17 interacts with the PCNA-like proteins hRad1, hHus1, and hRad9 in manner similar to the interaction between RFC and PCNA.

Cell Cycle↗
PubMed2005

Gadd45a, a p53- and BRCA1-regulated stress protein, in cellular response to DNA damage.

Mammalian cells exhibit complex, but intricate cellular responses to genotoxic stress, including cell cycle checkpoints, DNA repair and apoptosis. Inactivation of these important biological events may result in genomic instability and cell transformation, as well as alterations of therapeutic sensitivity. Gadd45a, a p53- and BRCA1-regulated stress-inducible gene, has been characterized as one of the important players that participate in cellular response to a variety of DNA damage agents. Interestingly, the signaling machinery that regulates Gadd45a induction by genotoxic stress involves both p53-dependent and -independent pathways; the later may employ BRCA1-related or MAP kinase-mediated signals. Gadd45a protein has been reported to interact with multiple important cellular proteins, including Cdc2 protein kinase, proliferating cell nuclear antigen (PCNA), p21Waf1/Cip1 protein, core histone protein and MTK/MEKK4, an up-stream activator of the JNK/SAPK pathway, indicating that Gadd45a may play important roles in the control of cell cycle checkpoint, DNA repair process, and signaling transduction. The importance of Gadd45a in maintaining genomic integrity is well manifested by the demonstration that disruption of endogenous Gadd45a in mice results in genomic instability and increased carcinogenesis. Therefore, Gadd45a appears to be an important component in the cellular defense network that is required for maintenance of genomic stability.

Animals↗
PubMed2001

Bisimidazoacridones induce a potent cytostatic effect in colon tumor cells that sensitizes them to killing by UCN-01.

PURPOSE: To determine the ability of WMC26, a prototypic bisimidazoacridone (BIA), to induce apoptosis in sensitive colon adenocarcinoma cells and to advance the hypothesis that cancer cells that are growth-arrested by WMC26 are predisposed to undergo apoptotic death by abrogators of cell cycle checkpoints. METHODS: The antiproliferative activity of WMC26 was examined in detail by a 4-day MTT assay, cell counting, BrdU incorporation and a two-color LIVE/DEAD assay. To detect apoptosis a number of established techniques were used, including gel electrophoresis, flow cytometry, and confocal laser microscopy of treated cells. The activity of senescence-associated beta-galactosidase in treated cells was also analyzed. RESULTS: WMC26, at physiological concentrations, induced complete and longlasting growth arrest of HCT116 cells in culture but did not trigger cell death. The growth-arrested cells (blocked at G1 and G2/M cell cycle checkpoints) did not synthesize DNA but were metabolically active and had intact plasma membranes. Although they resembled the senescence-like phenotype reported to be induced by treatment with some antitumor agents, the cells did not express senescence-associated beta-galactosidase, an indicator of the senescence-like state. Treatment of WMC26 growth-arrested cells with 1 microM UCN-01, an abrogator of the G2/M checkpoint, caused a very rapid (1 h) change in morphology and cell death within 72 h. CONCLUSIONS: BIAs do not induce apoptosis in sensitive colon tumor cells. They are highly cytostatic but only marginally toxic to the cells even at concentrations 100-fold higher than those sufficient for complete growth arrest. In this respect WMC26 differs from some other DNA-interacting antitumor agents that produce cell growth arrest at low concentrations but are toxic at higher doses. The complete growth arrest induced by WMC26 in colon cancer cells sensitized them to apoptotic death induced by UCN-01. This finding suggests that a combination of WMC26 and cyclin-dependent kinase inhibitors may be an attractive treatment method for colon cancer that utilizes the highly tumor-selective activity of WMC26.

Adenocarcinoma↗
PubMed2002

BCR/ABL regulates response to DNA damage: the role in resistance to genotoxic treatment and in genomic instability.

BCR/ABL regulates cell proliferation, apoptosis, differentiation and adhesion. In addition, BCR/ABL can induce resistance to cytostatic drugs and irradiation by modulation of DNA repair mechanisms, cell cycle checkpoints and Bcl-2 protein family members. Upon DNA damage BCR/ABL not only enhances reparation of DNA lesions (e.g. homologous recombination repair), but also prolongs activation of cell cycle checkpoints (e.g. G2/M) providing more time for repair of otherwise lethal lesions. Moreover, by modification of anti-apoptotic members of the Bcl-2 family (e.g. upregulation of Bcl-x(L)) BCR/ABL provides a cytoplasmic 'umbrella' protecting mitochondria from the 'rain' of apoptotic signals coming from the damaged DNA in the nucleus, thus preventing release of cytochrome c and activation of caspases. The unrepaired and/or aberrantly repaired (but not lethal) DNA lesions resulting from spontaneous and/or drug-induced damage can accumulate in BCR/ABL-transformed cells leading to genomic instability and malignant progression of the disease. Inhibition of BCR/ABL kinase activity by STI571 (Gleevec, imatinib mesylate) reverses drug resistance and, in combination with standard chemotherapeutics can exert strong anti-leukemia effect.

Antineoplastic Agents↗
PubMed2001

Cell cycle in the fucus zygote parallels a somatic cell cycle but displays a unique translational regulation of cyclin-dependent kinases.

In eukaryotic cells, the basic machinery of cell cycle control is highly conserved. In particular, many cellular events during cell cycle progression are controlled by cyclin-dependent kinases (CDKs). The cell cycle in animal early embryos, however, differs substantially from that of somatic cells or yeasts. For example, cell cycle checkpoints that ensure that the sequence of cell cycle events is correct have been described in somatic cells and yeasts but are largely absent in embryonic cells. Furthermore, the regulation of CDKs is substantially different in the embryonic and somatic cells. In this study, we address the nature of the first cell cycle in the brown alga Fucus, which is evolutionarily distant from the model systems classically used for cell cycle studies in embryos. This cycle consists of well-defined G1, S, G2, and M phases. The purine derivative olomoucine inhibited CDKs activity in vivo and in vitro and induced different cell cycle arrests, including at the G1/S transition, suggesting that, as in somatic cells, CDKs tightly control cell cycle progression. The cell cycle of Fucus zygotes presented the other main features of a somatic cell cycle, such as a functional spindle assembly checkpoint that targets CDKs and the regulation of the early synthesis of two PSTAIRE CDKs, p32 and p34, and the associated histone H1 kinase activity as well as the regulation of CDKs by tyrosine phosphorylation. Surprisingly, the synthesis after fertilization of p32 and p34 was translationally regulated, a regulation not described previously for CDKs. Finally, our results suggest that the activation of mitotic CDKs relies on an autocatalytic amplification mechanism.

Animals↗
PubMed2006

Differential processing of antitumour-active and antitumour-inactive trans platinum compounds by SKOV-3 ovarian cancer cells.

In order to compare the mechanistic properties of the antitumour-active trans platinum complex trans-[PtCl(2){Z-HN=C(OMe)Me}(NH(3))] (trans-Z) and of the antitumour-inactive isomer of cisplatin trans-[PtCl(2)(NH(3))(2)] (trans-DDP), the differential processing of the two compounds by SKOV-3 ovarian cancer cells has been investigated. trans-Z and trans-DDP enter cells with the same efficacy, but trans-Z shows a two-fold higher affinity for cellular DNA. The treatment with trans-DDP IC(50) determines an initial and transient cytostatic effect, paralleled by a moderate increase of apoptosis and by sequential and reversible arrests in S and G(2)/M phases of cell-cycle. In contrast, trans-Z IC(50) determines an initial cytotoxic effect, a more persistent and marked increase of apoptosis, and a more marked and prolonged arrest in S and G(2)/M phases of the cell-cycle. Treatment-induced gene expression modifications indicate that phenotypic effects of trans-DDP are driven by an initial and transient up-regulation of some genes related to cell-cycle checkpoint and arrest networks, whereas the more dramatic phenotypic effects of trans-Z are driven by a persistent up-regulation of more numerous genes involved in cell-cycle checkpoint and arrest networks, and in genome stability and DNA repair. Therefore, molecular and cellular events have been identified which are produced by trans-Z but not by trans-DDP, and which likely represent the mechanistic basis of antitumour activity of trans-Z in the SKOV-3 system.

Animals↗
PubMed2000

Role for ATM in DNA damage-induced phosphorylation of BRCA1.

The human genetic disorder ataxia-telangiectasia is characterized by immunodeficiency, progressive cerebellar ataxia, radiosensitivity, cell cycle checkpoint defects, and cancer predisposition. The gene product [ataxia-telangiectasia mutation (ATM)] mutated in this syndrome is a component of the DNA damage detection pathway. Loss of ATM function in human and mouse cells causes defects in DNA repair and cell cycle checkpoint control and, not surprisingly, humans and mice with compromised ATM function are prone to cancers. An excess of breast cancer in the relatives of ataxia-telangiectasia patients has also been reported by epidemiological studies. Predisposition to breast and ovarian cancers is also observed in women with germline mutations in BRCA1, a tumor suppressor gene. BRCA1 is a nuclear protein with a cell cycle-regulated expression pattern and is hyperphosphorylated in response to DNA-damaging agents. Here we show that rapid ionizing radiation-induced in vivo phosphorylation of BRCA1 requires the presence of functional ATM protein. Furthermore, we show that ATM interacts with BRCA1, and this association is enhanced by radiation. We also demonstrate that BRCA1 is a substrate of ATM kinase in vitro and in vivo. Using phospho-specific antibodies against serines 1387, 1423, and 1457 of BRCA1, we demonstrate radiation-induced, ATM-dependent phosphorylation of BRCA1 at these sites. These findings show that BRCA1 is regulated by an ATM-dependent mechanism as a part of the cellular response to DNA damage. This interaction between ATM and BRCA1 argues in favor of the involvement of particular aspects of ATM function in breast cancer predisposition.

Ataxia Telangiectasia↗
PubMed1998

Splitting the ATM: distinct repair and checkpoint defects in ataxia-telangiectasia.

Ataxia-telangiectasia (A-T) is an autosomal recessive human disorder that, because of its multisystem nature, is of interest to scientists and clinicians from many disciplines. A-T patients have defects in the neurological and immune systems, telangiectasia in the eyes and face, and are, in addition, cancer-prone and radiation-sensitive. A-T cell lines have a range of diverse phenotypes including sensitivity to ionizing radiation and defects in cell-cycle checkpoint control. The ATM protein is a member of the PI 3-kinase-like superfamily, and it has been widely accepted that A-T cells represent mammalian cell-cycle checkpoint mutants and that the radiation sensitivity is a consequence of this defect. However, several lines of evidence suggest that A-T cells have distinct repair and checkpoint defects. A-T cells therefore appear to harbour dual checkpoint/repair defects. Here, we review the evidence supporting this contention and consider its implications for an analysis of the A-T phenotype.

Animals↗
PubMed2006

CtIP, a multivalent adaptor connecting transcriptional regulation, checkpoint control and tumor suppression.

CtIP was originally characterized as an interacting protein for the transcriptional corepressor CtBP. In addition to transcriptional regulation, CtIP plays an important role in the DNA damage response and cell cycle checkpoint control. Recent evidence points to a viral factor-like S phase promoting activity of CtIP in counteracting Rb repression to facilitate E2F-directed transactivation during the G(1)/S transition. Genetic studies in mice revealed the haploid insufficiency of CtIP in suppressing tumorigenesis associated with shortened life span. Thus, CtIP emerges as a multivalent adaptor that connects cell cycle checkpoint control, transcriptional regulation and tumor suppression.

Animals↗
PubMed2000

Chk1 is an essential kinase that is regulated by Atr and required for the G(2)/M DNA damage checkpoint.

Chk1, an evolutionarily conserved protein kinase, has been implicated in cell cycle checkpoint control in lower eukaryotes. By gene disruption, we show that CHK1 deficiency results in a severe proliferation defect and death in embryonic stem (ES) cells, and peri-implantation embryonic lethality in mice. Through analysis of a conditional CHK1-deficient cell line, we demonstrate that ES cells lacking Chk1 have a defective G(2)/M DNA damage checkpoint in response to gamma-irradiation (IR). CHK1 heterozygosity modestly enhances the tumorigenesis phenotype of WNT-1 transgenic mice. We show that in human cells, Chk1 is phosphorylated on serine 345 (S345) in response to UV, IR, and hydroxyurea (HU). Overexpression of wild-type Atr enhances, whereas overexpression of the kinase-defective mutant Atr inhibits S345 phosphorylation of Chk1 induced by UV treatment. Taken together, these data indicate that Chk1 plays an essential role in the mammalian DNA damage checkpoint, embryonic development, and tumor suppression, and that Atr regulates Chk1.

Alleles↗
PubMed2003

Human T-cell leukemia virus-I tax oncoprotein functionally targets a subnuclear complex involved in cellular DNA damage-response.

The virally encoded oncoprotein Tax has been implicated in HTLV-1-mediated cellular transformation. The exact mechanism by which this protein contributes to the oncogenic process is not known. However, it has been hypothesized that Tax induces genomic instability via repression of cellular DNA repair. We examined the effect of de novo Tax expression upon the cell cycle, because appropriate activation of cell cycle checkpoints is essential to a robust damage-repair response. Upon induction of tax expression, Jurkat T-cells displayed a pronounced accumulation in G2/M that was reversible by caffeine. We examined the G2-specific checkpoint signaling response in these cells and found activation of the ATM/chk2-mediated pathway, whereas the ATR/chk1-mediated response was unaffected. Immunoprecipitation with anti-chk2 antibody results in co-precipitation of Tax demonstrating a direct interaction of Tax with a chk2-containing complex. We also show that Tax targets a discrete nuclear site and co-localizes with chk2 and not chk1. This nuclear site, previously identified as Tax Speckled Structures (TSS), also contains the early damage response factor 53BP1. The recruitment of 53BP1 to TSS is dependent upon ATM signaling and requires expression of Tax. Specifically, Tax expression induces redistribution of diffuse nuclear 53BP1 to the TSS foci. Taken together these data suggest that the TSS describe a unique nuclear site involved in DNA damage recognition, repair response, and cell cycle checkpoint activation. We suggest that association of Tax with this multifunctional subnuclear site results in disruption of a subset of the site-specific activities and contributes to cellular genomic instability.

Carrier Proteins↗
PubMed1999

G1 checkpoint protein and p53 abnormalities occur in most invasive transitional cell carcinomas of the urinary bladder.

The G1 cell cycle checkpoint regulates entry into S phase for normal cells. Components of the G1 checkpoint, including retinoblastoma (Rb) protein, cyclin D1 and p16INK4a, are commonly altered in human malignancies, abrogating cell cycle control. Using immunohistochemistry, we examined 79 invasive transitional cell carcinomas of the urinary bladder treated by cystectomy, for loss of Rb or p16INK4a protein and for cyclin D1 overexpression. As p53 is also involved in cell cycle control, its expression was studied as well. Rb protein loss occurred in 23/79 cases (29%); it was inversely correlated with loss of p16INK4a, which occurred in 15/79 cases (19%). One biphenotypic case, with Rb+p16- and Rb-p16+ areas, was identified as well. Cyclin D1 was overexpressed in 21/79 carcinomas (27%), all of which retained Rb protein. Fifty of 79 tumours (63%) showed aberrant accumulation of p53 protein; p53 staining did not correlate with Rb, p16INK4a, or cyclin D1 status. Overall, 70% of bladder carcinomas showed abnormalities in one or more of the intrinsic proteins of the G1 checkpoint (Rb, p16INK4a and cyclin D1). Only 15% of all bladder carcinomas (12/79) showed a normal phenotype for all four proteins. In a multivariate survival analysis, cyclin D1 overexpression was linked to less aggressive disease and relatively favourable outcome. In our series, Rb, p16INK4a and p53 status did not reach statistical significance as prognostic factors. In conclusion, G1 restriction point defects can be identified in the majority of bladder carcinomas. Our findings support the hypothesis that cyclin D1 and p16INK4a can cooperate to dysregulate the cell cycle, but that loss of Rb protein abolishes the G1 checkpoint completely, removing any selective advantage for cells that alter additional cell cycle proteins.

Aged↗
PubMed2001

The Ataxia telangiectasia gene product is required for oxidative stress-induced G1 and G2 checkpoint function in human fibroblasts.

Ataxia telangiectasia (AT) is an autosomal recessive disorder characterized by neuronal degeneration accompanied by ataxia, telangiectasias, acute cancer predisposition, and sensitivity to ionizing radiation (IR). Cells from individuals with AT show unusual sensitivity to IR, severely attenuated cell cycle checkpoint functions, and poor p53 induction in response to IR compared with normal human fibroblasts (NHFs). The gene mutated in AT (ATM) has been cloned, and its product, pATM, has IR-inducible kinase activity. The AT phenotype has been suggested to be a consequence, at least in part, of an inability to respond appropriately to oxidative damage. To test this hypothesis, we examined the ability of NHFs and AT dermal fibroblasts to respond to t-butyl hydroperoxide and IR treatment. AT fibroblasts exhibit, in comparison to NHFs, increased sensitivity to the toxicity of t-butyl hydroperoxide, as measured by colony-forming efficiency assays. Unlike NHFs, AT fibroblasts fail to show G(1) and G(2) phase checkpoint functions or to induce p53 in response to t-butyl hydroperoxide. Treatment of NHFs with t-butyl hydroperoxide activates pATM-associated kinase activity. Our results indicate that pATM is involved in responding to certain aspects of oxidative damage and in signaling this information to downstream effectors of the cell cycle checkpoint functions. Our data further suggest that some of the pathologies seen in AT could arise as a consequence of an inability to respond normally to oxidative damage.

Adolescent↗