Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UMLS:C0004135 (ATM)
 13,001 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Diploid human fibroblast strains were treated for 10 min with inhibitors of type I and type II DNA topoisomerases, and after removal of the inhibitors, the rate of initiation of DNA synthesis at replicon origins was determined. By alkaline elution chromatography, 4'-(9-acridinylamino)methanesulfon-m-anisidide (amsacrine), an inhibitor of DNA topoisomerase II, was shown to produce DNA strand breaks. These strand breaks are thought to reflect drug-induced stabilization of topoisomerase-DNA cleavable complexes. Removal of the drug led to a rapid resealing of the strand breaks by dissociation of the complexes. Velocity sedimentation analysis was used to quantify the effects of amsacrine treatment on DNA replication. It was demonstrated that transient exposure to low concentrations of amsacrine inhibited replicon initiation but did not substantially affect DNA chainelongation within operating replicons. Maximal inhibition of replicon initiation occurred 20 to 30 min after drug treatment, and the initiation rate recovered 30 to 90 min later. Ataxia telangiectasia cells displayed normal levels of amsacrine-induced DNA strand breaks during stabilization of cleavable complexes but failed to downregulate replicon initiation after exposure to the topoisomerase inhibitor. Thus, inhibition of replicon initiation in response to DNA damage appears to be an active process which requires a gene product which is defective or missing in ataxia telangiectasia cells. In normal human fibroblasts, the inhibition of DNA topoisomerase I by camptothecin produced reversible DNA strand breaks. Transient exposure to this drug also inhibited replicon initiation. These results suggest that the cellular response pathway which downregulates replicon initiation following genotoxic damage may respond to perturbations of chromatin structure which accompany stabilization of topoisomerase-DNA cleavable complexes.
 ...
PMID:Inhibition of replicon initiation in human cells following stabilization of topoisomerase-DNA cleavable complexes. 164 93

Ataxia telangiectasia (AT) cells are known to be hypersensitive to ionizing radiations and to drugs such as bleomycin and epipodophyllotoxin VP16, a topoisomerase II poison. Both of these produce DNA double-strand breaks even if through different mechanisms. In this work we analyzed the sensitivity to bleomycin and to epipodophyllotoxin of AT cells after transfection with pR plasmid. This plasmid, interacting with bacterial SOS repair pathways, expresses itself in mammalian cells conferring cell resistance to the SOS inducers UV and 4NQO and cell sensitivity to different drugs such as bleomycin. This effect is presumably due to the interaction of pR products with double-strand breaks. Our findings indicate that pR plasmid, in both AT lines tested (AT5BIVA fibroblasts and ATL6 lymphoblasts), expresses itself (increasing UV protection) and amplifies the already enhanced AT cell sensitivity to both bleomycin and VP16.
 ...
PMID:Human cells (normal and ataxia telangiectasia) transfected with pR plasmid are hypersensitive to DNA strand-breaking agents. 171 24

A number of characteristics in the human genetic disorder ataxia-telangiectasia are compatible with an alteration to chromatin structure or the recognition of that structure by an enzyme or DNA binding protein. We describe here reduce activity of DNA topoisomerase type II in a number of Epstein Barr Virus-transformed ataxia-telangiectasia lymphoblastoid cell lines. Enzyme activity was reduced 10-fold or greater in 4 out of 5 cell lines compared to controls. In the remaining cell line approximately a 2-3 fold reduction was evident in partially purified extracts. DNA topoisomerase type I activity was found to be the same as controls in all the cell lines. Northern blot analysis revealed that the same level of DNA topoisomerase II mRNA was expressed in ataxia-telangiectasia and control cell lines. The size and amount of the enzyme did not differ appreciably from that observed in control cells. The reduced activity of DNA topoisomerase II in ataxia-telangiectasis cells might be explained by amino acid substitutions, small deletions in DNA or by a defect in post-translational modification in these cells.
 ...
PMID:Defective DNA topoisomerase II in ataxia-telangiectasia cells. 196 59

Abnormal expression of the nuclear-associated enzyme DNA topoisomerase II (topoisomerase II) has been implicated in the in vitro phenotype of radiation hypersensitive ataxia-telangiectasia (A-T) cells and in modifying sensitivity of eukaryotic cells to topoisomerase II-inhibitor drugs [e.g., the DNA intercalator amsacrine (mAMSA)]. To study such relationships, various SV40- and Epstein-Barr Virus-transformed human cell lines derived from normal, A-T, or UV-sensitive xeroderma pigmentosum donors have been assayed for their sensitivity to mAMSA together with direct and indirect measurements of topoisomerase II expression. We report on the identification of an SV40-transformed A-T fibroblast cell line with abnormally high levels of topoisomerase II in nuclear protein extracts as determined by immunoblotting, measurement of kinetoplast DNA decatenation activity, and mAMSA-dependent DNA-protein cross-linking activity in a filter binding assay. Using a flow cytometric assay for the analysis of reactivity of nuclei with a polyclonal antitopoisomerase II antibody, overproduction was found to occur in all phases of the cell cycle. High levels of topoisomerase II were associated with hypersensitivity (5-10-fold) to mAMSA-induced cell cycle delay, cell kill, and DNA strand breakage (assayed under protein-denaturing conditions). Xeroderma pigmentosum (group A) cells demonstrated normal responses to mAMSA. The results provide evidence that cellular potential for the generation of topoisomerase II-dependent DNA damage is a major factor in governing the sensitivity to mAMSA. Furthermore, underexpression of topoisomerase II does not appear to be a primary factor in describing the in vitro A-T phenotype. The findings also relate to how changes in chromatin structure and function may either reflect or dictate the expression of topoisomerase II in human cells.
 ...
PMID:Cellular consequences of overproduction of DNA topoisomerase II in an ataxia-telangiectasia cell line. 253 42

Ataxia telangiectasia (AT) cell lines are characterised by their hypersensitivity to ionizing radiation and bleomycin, and their failure to inhibit DNA synthesis after DNA damage. A recent report [Singh et al. (1988) Nucl. Acids Res. 16, 3919-3929] indicated that a reduction in topoisomerase II (topo II) activity was a feature of AT lymphoblast cell lines. We have studied the possible role of DNA topoisomerases in determining the phenotype of an AT fibroblast cell line. AT5BIVA cells are sensitive to the topo II inhibitors etoposide (VP16) and amsacrine (m-AMSA), compared to normal human fibroblasts (MRC5-V1 and VA13). AT5BIVA cells express a 3-fold higher level of topo II protein than MRC5-V1 cells, and 6-fold higher than VA13. This is reflected in elevated topo II activity in AT5BIVA cells. Untransformed AT5BI cells also show elevated topo II activity compared to untransformed normal cells. The extent of overproduction of topo II in AT5BIVA cells is comparable with that seen in a mutant Chinese hamster cell line, ADR-1, which is similarly hypersensitive to both bleomycin and topo II inhibitors. However, ADR-1 cells show neither hypersensitivity to ionizing radiation nor abnormal inhibition of DNA synthesis following DNA damage. Topo II overproduction per se does not appear sufficient to generate an "AT-like" phenotype. AT5BIVA cells express a reduced level of topoisomerase I (topo I) and are hypersensitive to the topo I inhibitor, camptothecin. ADR-1 cells express a normal level of topo I, indicating that a reduction in the level of topo I is not the inevitable consequence of an elevation in topo II.
 ...
PMID:Overproduction of topoisomerase II in an ataxia telangiectasia fibroblast cell line: comparison with a topoisomerase II-overproducing hamster cell mutant. 253 56

We have utilized DNA transfer and recombinant DNA techniques to probe DNA double-strand break repair in the human ionizing radiation-sensitive genetic syndrome ataxia-telangiectasia (A-T). Using restriction enzyme-generated double-strand breaks in the coding sequence of a selectable gene we have detected a significantly greater frequency of mis-repair of such breaks in a permanent A-T cell line compared with cell lines of normal radiosensitivity. This mis-repair in A-T can plausibly explain many of the clinical features of the disease but was insufficiently detailed to address the broad problem of DNA repair mechanisms relevant to ionizing radiation-induced damage. To extend these observations of DNA double-strand break mis-repair we have now applied this type of repair assay to novel, de novo induced mammalian X-ray-sensitive cell lines and to appropriate Escherichia coli mutants. In both cellular systems we have now found some equivalence to the A-T repair defect. In particular, studies on one E. coli mutant have provided evidence suggesting an involvement of a topoisomerase activity in DNA double-strand break mis-repair, which may be relevant to the biochemical defect in A-T.
 ...
PMID:Molecular studies on the nature of the repair defect in ataxia-telangiectasia and their implications for cellular radiobiology. 282 Oct 21

The excision repair of u.v. damage has been supposed to involve an initial action of DNA topoisomerase II, since some pre-incision step is sensitive to novobiocin, a topoisomerase II inhibitor. But novobiocin also affects mitochondrial structure and ATP metabolism, and this may account for its apparent inhibition of energy-dependent excision repair. We have investigated the effects of etoposide, another inhibitor of topoisomerase II, on u.v.-irradiated human cells: it is a more specific agent with no immediate side-effects on mitochondria. But etoposide is without effect on cellular excision repair, at the pre-incision stage or at the later stages of either DNA resynthesis or strand break ligation; nor does it potentiate cell killing after u.v. irradiation. The chromosome decondensation that accompanies incomplete excision repair in mitotic cells is likewise not greatly affected by etoposide. Therefore, if topoisomerase II is involved in excision repair or its regulation, it acts through a process that in whole cells is insensitive to etoposide. In ataxia telangiectasia cells, which are known to be hypersensitive to etoposide, the mitochondrial activities are not abnormally affected.
 ...
PMID:Action of etoposide (VP-16-123) on human cells: no evidence for topoisomerase II involvement in excision repair of u.v.-induced DNA damage, nor for mitochondrial hypersensitivity in ataxia telangiectasia. 282 76

DNA topoisomerase type I and II activities were determined by serial dilution in nuclear extracts from control and ataxia-telangiectasia lymphoblastoid cells. Topoisomerase I activity, assayed by relaxation of supercoiled plasmid DNA, was found to be approximately the same in both cell types. In order to remove interference from topoisomerase I, the activity of topoisomerase II was measured by the unknotting of knotted P4 phage DNA in the presence of ATP. The activity of topoisomerase II was markedly reduced in two ataxia-telangiectasia cell lines, AT2ABR and AT8ABR, compared to controls. This reduction in activity was detected with increasing concentration of protein and in time course experiments at a single protein concentration. A third cell line, AT3ABR, did not have a detectably lower activity of topoisomerase II when assayed under these conditions. The difference in topoisomerase II activity in the ataxia-telangiectasia cell lines examined may reflect to some extent the heterogeneity observed in this syndrome.
 ...
PMID:A defect in DNA topoisomerase II activity in ataxia-telangiectasia cells. 282

Considerable evidence supports a defect at the level of chromatin structure or recognition of that structure in cells from patients with the human genetic disorder ataxia-telangiectasia. Accordingly, we have investigated the activities of enzymes that alter the topology of DNA in Epstein Barr Virus-transformed lymphoblastoid cells from patients with this syndrome. Reduced activity of DNA topoisomerase II, determined by unknotting of P4 phage DNA, was observed in partially purified extracts from 5 ataxia-telangiectasia cell lines. The levels of enzyme activity was reduced substantially in 4 of these cell lines and to a lesser extent in the other cell line compared to controls. DNA topoisomerase I, assayed by relaxation of supercoiled DNA, was found to be present at comparable levels in both cell types. Reduced activity of topoisomerase II in ataxia-telangiectasia is compatible with the molecular, cellular and clinical changes described in this syndrome.
 ...
PMID:Reduced DNA topoisomerase II activity in ataxia-telangiectasia cells. 283 4

The cytotoxic and cell kinetic effects of the epipodophyllotoxin 4,6-demethylepipodophyllotoxin-9-(4,6-O-ethylidene-beta-D-glucopyr anoside) (VP-16) in cultured mammalian cells are thought to relate to the induction of DNA damage, specifically DNA strand interruptions. In an effort to explore this relationship in human cells we have identified a VP-16-hypersensitive human cell system, namely an SV40-transformed fibroblast line (AT5BIVA) originally derived from an ataxia telangiectasia (AT) patient. Evidence is presented that enhanced VP-16 sensitivity may be a consistent in vitro feature at AT derived cells. However, the intrinsic sensitivity (DNA strand breaks per lethal hit quantitated by nucleoid sedimentation) was the same for AT5BIVA and a corresponding normal control, indicating that the AT cell line accumulated more drug-induced DNA damage during short-term VP-16 exposures. It is suggested that AT cells may have abnormal topoisomerase II activity. The cell cycle responses of normal and AT cells to VP-16 exposure were complex, with the generation of parasynchronous S phase populations and the accumulation of cells in G2. Differences in cell killing or DNA strand breakage between normal and AT cells could only be correlated with the magnitude and kinetics of the G2 retention phenomenon. In short, there are several similarities in the action of ionizing radiation and VP-16. We suggest that the sensitivity of cellular DNA to VP-16-induced DNA damage and the kinetics of the G2 delay may be useful parameters for predicting the survival probability of drug-treated human tumor populations.
 ...
PMID:Predominant role for DNA damage in etoposide-induced cytotoxicity and cell cycle perturbation in human SV40-transformed fibroblasts. 301 31


1 2 3 4 5 6 7 8 9 10 Next >>