Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P16104 (H2AX)
 3,930 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

During the evolution of cancer, the incipient tumour experiences 'oncogenic stress', which evokes a counter-response to eliminate such hazardous cells. However, the nature of this stress remains elusive, as does the inducible anti-cancer barrier that elicits growth arrest or cell death. Here we show that in clinical specimens from different stages of human tumours of the urinary bladder, breast, lung and colon, the early precursor lesions (but not normal tissues) commonly express markers of an activated DNA damage response. These include phosphorylated kinases ATM and Chk2, and phosphorylated histone H2AX and p53. Similar checkpoint responses were induced in cultured cells upon expression of different oncogenes that deregulate DNA replication. Together with genetic analyses, including a genome-wide assessment of allelic imbalances, our data indicate that early in tumorigenesis (before genomic instability and malignant conversion), human cells activate an ATR/ATM-regulated DNA damage response network that delays or prevents cancer. Mutations compromising this checkpoint, including defects in the ATM-Chk2-p53 pathway, might allow cell proliferation, survival, increased genomic instability and tumour progression.
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PMID:DNA damage response as a candidate anti-cancer barrier in early human tumorigenesis. 1582 43

DNA damage checkpoint genes, such as p53, are frequently mutated in human cancer, but the selective pressure for their inactivation remains elusive. We analysed a panel of human lung hyperplasias, all of which retained wild-type p53 genes and had no signs of gross chromosomal instability, and found signs of a DNA damage response, including histone H2AX and Chk2 phosphorylation, p53 accumulation, focal staining of p53 binding protein 1 (53BP1) and apoptosis. Progression to carcinoma was associated with p53 or 53BP1 inactivation and decreased apoptosis. A DNA damage response was also observed in dysplastic nevi and in human skin xenografts, in which hyperplasia was induced by overexpression of growth factors. Both lung and experimentally-induced skin hyperplasias showed allelic imbalance at loci that are prone to DNA double-strand break formation when DNA replication is compromised (common fragile sites). We propose that, from its earliest stages, cancer development is associated with DNA replication stress, which leads to DNA double-strand breaks, genomic instability and selective pressure for p53 mutations.
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PMID:Activation of the DNA damage checkpoint and genomic instability in human precancerous lesions. 1582 43

Human checkpoint kinase 1 (Chk1) is an essential kinase required to preserve genome stability. Here, we show that Chk1 inhibition by two distinct drugs, UCN-01 and CEP-3891, or by Chk1 small interfering RNA (siRNA) leads to phosphorylation of ATR targets. Chk1-inhibition triggered rapid, pan-nuclear phosphorylation of histone H2AX, p53, Smc1, replication protein A, and Chk1 itself in human S-phase cells. These phosphorylations were inhibited by ATR siRNA and caffeine, but they occurred independently of ATM. Chk1 inhibition also caused an increased initiation of DNA replication, which was accompanied by increased amounts of nonextractable RPA protein, formation of single-stranded DNA, and induction of DNA strand breaks. Moreover, these responses were prevented by siRNA-mediated downregulation of Cdk2 or the replication initiation protein Cdc45, or by addition of the CDK inhibitor roscovitine. We propose that Chk1 is required during normal S phase to avoid aberrantly increased initiation of DNA replication, thereby protecting against DNA breakage. These results may help explain why Chk1 is an essential kinase and should be taken into account when drugs to inhibit this kinase are considered for use in cancer treatment.
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PMID:Inhibition of human Chk1 causes increased initiation of DNA replication, phosphorylation of ATR targets, and DNA breakage. 1583 61

Chromium(VI) is a toxic and carcinogenic metal that causes the formation of DNA phosphate-based adducts. Cr-DNA adducts are genotoxic in human cells, although they do not block replication in vitro. Here, we report that induction of cytotoxicity in Cr(VI)-treated human colon cells and mouse embryonic fibroblasts requires the presence of all major mismatch repair (MMR) proteins. Cr-DNA adducts lost their ability to block replication of Cr-modified plasmids in human colon cells lacking MLH1 protein. The presence of functional mismatch repair caused induction of p53-independent apoptosis associated with activation of caspases 2 and 7. Processing of Cr-DNA damage by mismatch repair resulted in the extensive formation of gamma-H2AX foci in G(2) phase, indicating generation of double-stranded breaks as secondary toxic lesions. Induction of gamma-H2AX foci was observed at 6 to 12 h postexposure, which was followed by activation of apoptosis in the absence of significant G(2) arrest. Our results demonstrate that mismatch repair system triggers toxic responses to Cr-DNA backbone modifications through stress mechanisms that are significantly different from those for other forms of DNA damage. Selection for Cr(VI) resistant, MMR-deficient cells may explain the very high frequency of lung cancers with microsatellite instability among chromate workers.
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PMID:Mismatch repair proteins are activators of toxic responses to chromium-DNA damage. 1583 65

We previously demonstrated that the nitroxide antioxidant tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) increased latency to tumorigenesis and doubled (100%) the lifespan of Atm-deficient mice, a mouse model of ataxia telangiectasia, which displays accelerated oxidative damage and stress. Tempol treatment of cancer-prone p53-deficient mice resulted in a small but significant (25%) increase in lifespan by prolonging latency to tumorigenesis, demonstrating that existing oxidative stress and damage are not necessary for the chemopreventative effects of tempol. However, the relatively small effect on latency in p53-deficient mice and the finding that tempol-mediated resistance to oxidative insult was p53-dependent suggested a more direct role of p53 in the chemopreventative effects of tempol. Surprisingly, tempol treatment specifically increased serine 18 phosphorylation of p53 (but not gamma-H2AX) and p21 expression in primary thymocytes in vitro in a p53-dependent fashion. Inhibition of phosphoinositide 3-kinase (PI3K) family members suggested that SMG-1 was responsible for the tempol-mediated enhancement of p53 serine 18 phosphorylation. These data suggest that the chemopreventative effect of tempol is not solely due to the reduction of oxidative stress and damage but may also be related to redox-mediated signaling functions that include p53 pathway activation.
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PMID:Cancer chemoprevention by the antioxidant tempol acts partially via the p53 tumor suppressor. 1588 86

Norethindrone is a commonly used drug for contraception and hormone replacement therapy, whose carcinogenic potential is still controversial. We applied a novel and particularly sensitive method to screen for DNA damage with special attention to double-strand breaks (DSBs) and identified norethindrone to be likely genotoxic and therefore potentially mutagenic: a p53-reporter assay served as a first, high-throughput screening method and was followed by the immunofluorescent detection of phosphorylated H2AX as a sensitive assay for the presence of DSBs. Norethindrone at concentrations of 2-100 microg/ml activated p53 and phosphorylated H2AX specifically and in a dose-dependent manner. No p53 activation or H2AX phosphorylation was detected using a panel of structurally/functionally related drugs. The overall amount of DNA damage induced by norethindrone was low as compared with etoposide and ionizing radiation. Consistently, norethindrone treatment did not cause a cell cycle arrest. DSBs were not detected with the neutral comet assay, a less sensitive method for DSB assessment than H2AX phosphorylation. Our findings in the p53-reporter and gamma-H2AX assays could not be ascribed to common DSB-causing artifacts in standard genotoxicity screening, including drug precipitation, high cytotoxicity levels and increased apoptosis. Therefore, our study suggests that norethindrone induces DSBs in our experimental setting, both complementing and adding a new aspect to the existing literature on the genotoxic potential of norethindrone. As the effective concentrations of norethindrone used in our assays were approximately 100- to 1000-fold higher than therapeutical doses, the significance of these findings with regard to human exposure still remains to be determined.
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PMID:Novel genotoxicity assays identify norethindrone to activate p53 and phosphorylate H2AX. 1590 98

Renal medullary cells normally are exposed to extraordinarily high interstitial NaCl concentration as part of the urinary concentrating mechanism, yet they survive and function. Acute elevation of NaCl to a moderate level causes transient cell cycle arrest in culture. Higher levels of NaCl, within the range found in the inner medulla, cause apoptosis. Recently, it was surprising to discover that even moderately high levels of NaCl cause DNA double-strand breaks. The DNA breaks persist in cultured cells that are proliferating rapidly after adaptation to high NaCl, and DNA breaks normally are present in the renal inner medulla in vivo. High NaCl inhibits repair of broken DNA both in culture and in vivo, but the DNA is rapidly repaired if the level of NaCl is reduced. The inhibition of DNA repair is associated with suppressed activity of some DNA damage-response proteins like Mre11, Chk1, and H2AX but not that of others, like GADD45, p53, ataxia telangiectasia-mutated kinase (ATM), and Ku86. In this review, we consider possible mechanisms by which the renal cells escape the known dangerous consequences of persistent DNA damage. Furthermore, we consider that the persistent DNA damage may be a sensor of hypertonicity that activates ATM kinase to provide a signal that contributes to protective osmotic regulation.
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PMID:DNA damage and osmotic regulation in the kidney. 1595 78

Aminoflavone (5-amino-2,3-fluorophenyl)-6,8-difluoro-7-methyl-4H-1-benzopyran-4-one) (NSC 686288) is a candidate for possible advancement to phase I clinical trial. Aminoflavone has a unique activity profile in the NCI 60 cell lines (COMPARE analysis; http://www.dtp.nci.nih.gov/docs/dtp_search.html), and exhibits potent cellular and animal antitumor activity. To elucidate the mechanism of action of aminoflavone, we studied DNA damage in MCF-7 cells. Aminoflavone induced DNA-protein cross-links (DPC) and DNA single-strand breaks (SSB). Aminoflavone induced high levels of DPC and much lower level of SSB than camptothecin, which induces equal levels of DPC and SSB due to the trapping topoisomerase I-DNA complexes. Accordingly, neither topoisomerase I nor topoisomerase II were detectable in the aminoflavone-induced DPC. Aminoflavone also induced dose- and time-dependent histone H2AX phosphorylation (gamma-H2AX). Gamma-H2AX foci occurred with DPC formation, and like DPC, persisted after aminoflavone removal. Aphidicolin prevented gamma-H2AX formation, suggesting that gamma-H2AX foci correspond to replication-associated DNA double-strand breaks. Accordingly, no gamma-H2AX foci were found in proliferating cell nuclear antigen-negative or in mitotic cells. Bromodeoxyuridine incorporation and fluorescence-activated cell sorting analyses showed DNA synthesis inhibition uniformly throughout the S phase after exposure to aminoflavone. Aminoflavone also induced RPA2 and p53 phosphorylation, and induced p21(Waf1/Cip1) and MDM2, demonstrating S-phase checkpoint activation. These studies suggest that aminoflavone produces replication-dependent DNA lesions and S-phase checkpoint activation following DPC formation. Gamma-H2AX may be a useful clinical marker for monitoring the efficacy of aminoflavone in tumor therapies.
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PMID:DNA-protein cross-links and replication-dependent histone H2AX phosphorylation induced by aminoflavone (NSC 686288), a novel anticancer agent active against human breast cancer cells. 1595 81

Telomere attrition and other forms of telomere damage can activate the ATM kinase pathway. What generates the DNA damage signal at mammalian chromosome ends or at other double-strand breaks is not known. Telomere dysfunction is often accompanied by disappearance of the 3' telomeric overhang, raising the possibility that DNA degradation could generate the structure that signals. Here we address these issues by studying telomere structure after conditional deletion of mouse TRF2, the protective factor at telomeres. Upon removal of TRF2 from TRF2(F/-) p53-/- mouse embryo fibroblasts, a telomere damage response is observed at most chromosome ends. As expected, the telomeres lose the 3' overhang and are processed by the non-homologous end-joining pathway. Non-homologous end joining of telomeres was abrogated in DNA ligase IV-deficient (Lig4-/-) cells. Unexpectedly, the telomeres of TRF2-/- Lig4-/- p53-/- cells persisted in a free state without undergoing detectable DNA degradation. Notably, the telomeres retained their 3' overhangs, but they were recognized as sites of DNA damage, accumulating the DNA damage response factors 53BP1 and gamma-H2AX, and activating the ATM kinase. Thus, activation of the ATM kinase pathway at chromosome ends does not require overhang degradation or other overt DNA processing.
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PMID:DNA processing is not required for ATM-mediated telomere damage response after TRF2 deletion. 1596 70

Hypomorphic mutations which lead to decreased function of the NBS1 gene are responsible for Nijmegen breakage syndrome, a rare autosomal recessive hereditary disorder that imparts an increased predisposition to development of malignancy. The NBS1 protein is a component of the MRE11/RAD50/NBS1 complex that plays a critical role in cellular responses to DNA damage and the maintenance of chromosomal integrity. Using small interfering RNA transfection, we have knocked down NBS1 protein levels and analyzed relevant phenotypes in two closely related human lymphoblastoid cell lines with different p53 status, namely wild-type TK6 and mutated WTK1. Both TK6 and WTK1 cells showed an increased level of ionizing radiation-induced mutation at the TK and HPRT loci, impaired phosphorylation of H2AX (gamma-H2AX), and impaired activation of the cell cycle checkpoint regulating kinase, Chk2. In TK6 cells, ionizing radiation-induced accumulation of p53/p21 and apoptosis were reduced. There was a differential response to ionizing radiation-induced cell killing between TK6 and WTK1 cells after NBS1 knockdown; TK6 cells were more resistant to killing, whereas WTK1 cells were more sensitive. NBS1 deficiency also resulted in a significant increase in telomere association that was independent of radiation exposure and p53 status. Our results provide the first experimental evidence that NBS1 deficiency in human cells leads to hypermutability and telomere associations, phenotypes that may contribute to the cancer predisposition seen among patients with this disease.
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PMID:NBS1 knockdown by small interfering RNA increases ionizing radiation mutagenesis and telomere association in human cells. 1599 26


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