Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:6.5.1.2 (DNA ligase)
2,749 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The activity of a DNA repair enzyme, O6-alkylguanine-DNA alkyltransferase (AGT), was studied in gastric mucosa of 15 Macaca fascicularis monkeys before and during chronic oral exposure to the ethylating carcinogen N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) in order to investigate possible causes of inter-individual differences in susceptibility to its gastrocarcinogenic effect. A wide range of AGT activity (307-1903 fmol/mg protein, mean 695) was found before treatment and it decreased during the first year of exposure (means 627, 479 and 452 fmol/mg protein respectively at 6, 12 and 18 months after the beginning of the experiment). The carcinogenesis study is under way and to date four monkeys with low initial AGT level in gastric mucosa died of gastric cancer. The relevance of AGT level measurement for prediction of individual susceptibility to ENNG is discussed.
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PMID:Individual levels of activity of O6-alkylguanine-DNA alkyltransferase in monkey gastric mucosa during chronic exposure to a gastrocarcinogen N-ethyl-N'-nitro-N-nitrosoguanidine. 827 34

In Escherichia coli, the repair of 3-methyladenine (3MeA) DNA lesions by DNA glycosylases prevents alkylation induced cell death. We described previously the isolation of a human 3MeA DNA glycosylase (AAG) cDNA that maps to chromosome 16 and hybridizes to specific genomic DNA fragments from a number of mammals, including mouse. As a first step in the generation of a 3MeA DNA glycosylase deficient mouse by homologous replacement in embryonic stem cells, we have cloned the mouse 3MeA DNA glycosylase cDNA. The cloned 1095 base pair cDNA contains a complete 333 amino acid open reading frame that predicts a 36.5 kDa protein and hybridizes to a 1.5 kb mRNA transcript. Mouse 3MeA DNA glycosylase (Aag) transcript levels vary by up to 21 fold among tissues, being highest in the testes and lowest in the heart. The Aag cDNA encodes a glycosylase able to release 3MeA, 7-methylguanine (7MeG) and 3-methylguanine (3MeG) from alkylated DNA. The expression of Aag in E. coli provides substantial resistance against killing by methylating agents, but, unlike its E. coli counterparts, the Aag glycosylase fails to protect against killing by ethylating and propylating agents. A 232 amino acid stretch of the predicted mouse protein shares extensive amino acid identity with rat (93%) and human (83%) 3MeA DNA glycosylases and we observe that all three mammalian glycosylases have a bipartite nuclear localization signal. The Aag gene maps to mouse chromosome 11, suggesting a segment of conserved synteny between mouse chromosome 11 and human chromosome 16, which bears the human 3MeA DNA glycosylase gene. Cloning the mouse 3MeA DNA glycosylase cDNA is a step toward understanding the role of this DNA repair enzyme in mammals.
Carcinogenesis 1993 Feb
PMID:Cloning and characterization of a mouse 3-methyladenine/7-methyl-guanine/3-methylguanine DNA glycosylase cDNA whose gene maps to chromosome 11. 843 58

Formation of single strand breaks in nuclear DNA induced by hepatocarcinogens aflatoxin B1 and N-nitrosodimethylamine was observed to be more pronounced in rats maintained on a riboflavin-deficient diet compared to that on a normal diet. This increased damage was reversed on riboflavin supplementation. The induction of repair enzymes poly(ADP-ribose) polymerase, DNA polymerase beta and DNA ligase was significantly higher in riboflavin-deficient rats following DNA damage caused by the administration of carcinogens. Riboflavin supplementation brought down the induction to the levels found in rats maintained on normal diet. Since damage to DNA and its altered repair may relate to carcinogenesis, modulation of these parameters by riboflavin suggests a potential chemopreventive role of this vitamin.
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PMID:Modulation of carcinogen-induced DNA damage and repair enzyme activity by dietary riboflavin. 855 99

Suppressed expression of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT), characterized as the Mer- phenotype, occurs only in malignant or transformed cell lines. To investigate the relationship between the transformation process and loss of MGMT expression, we derived 20 cloned lines of IMR90 normal fibroblasts transfected with the plasmid pSV3neo expressing the SV40 large-T antigen. Of the five lines that were grown until crisis phase, four emerged as continuously proliferating immortal lines. Of these, only one retained MGMT, the other three having become Mer-. In every case the loss of MGMT coincided with the final phase of immortalization following crisis. Because these were cloned cell lines it is clear that the phenotypic change to Mer- is not merely due to selection of a Mer- cell from the initial population, but must involve a cellular change in MGMT regulation. It is not clear if increased mutation rate associated with loss of MGMT results in increased frequency of an immortalization event or if an immortalization event, such as telomere disruption, results in MGMT suppression. In addition, we have shown that, consistent with previous observations, both hypermethylation in promoter sequences and hypomethylation of downstream sequences in the body of the gene were closely associated with loss of MGMT expression. These studies also illustrate the utility of these new cloned cell lines for characterizing molecular events associated with transformation and immortalization.
Carcinogenesis 1996 Feb
PMID:Changes in O6-methylguanine-DNA methyltransferase expression during immortalization of cloned human fibroblasts. 862 42

The effect of different vitamin A status on events following DNA damage by hepatocarcinogens was investigated in rats. Formation of single-strand breaks in nuclear DNA induced by aflatoxin B1 and N-nitrosodimethylamine was observed to be more pronounced after vitamin A-deficiency. This enhanced damage was reversed upon vitamin A supplementation. Subsequent to DNA damage, the induction of repair enzymes poly(ADP-ribose) polymerase, DNA polymerase beta and DNA ligase was found to be significantly higher in vitamin A-deficient rats. Vitamin A supplementation brought down the induction to the levels found in rats maintained on normal diet. Vitamin A thus may control carcinogenesis by manipulating molecular events at the initiation stage.
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PMID:Effect of different vitamin A status on carcinogen-induced DNA damage and repair enzymes in rats. 872 17

Repair of alkylated bases in DNA is performed by O6-methylguanine-DNA methyltransferase (MGMT) and a set of enzymes of the base excision repair pathway involving N-methylpurine-DNA glycosylase (MPG), apurinic endonuclease (APE), DNA polymerase beta (Pol beta) and DNA ligase. The level of expression of these enzymes may exert a profound effect on resistance of cells towards alkylating drugs. We have comparatively analyzed the expression of MGMT and the different base excision repair genes in rat hepatoma cells (line H4IIE) after exposure to alkylating agents, X-rays and the glucocorticoid hormone dexamethasone. Furthermore, the effect of these agents on the activity of the cloned human MGMT promoter was assayed. Exposure of cells to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) or ionizing radiation increased MGMT mRNA levels up to 4.5-fold. Under the same conditions of treatment, exerting only a weak toxic effect, MPG and DNA ligase I mRNA levels were not enhanced, whereas the amounts of APE and Pol beta mRNA transiently increased by approximately 2-fold after X-ray and MNNG treatment, respectively. Dexamethasone induced both MGMT, APE and Pol beta mRNA and the induction paralleled the increase in mRNA of the glucocorticoid-dependent gene tyrosine aminotransferase. The observed increase in MGMT mRNA was due to promoter activation, which was shown in transient transfection assays with MGMT promoter-CAT reporter constructs in H4IIE cells. In these assays, the human MGMT promoter was found to be induced by methylating agents (MNNG and methyl methanesulfonate), ionizing radiation and dexamethasone. Weak induction of the promoter was observed after UV irradiation. Treatment with the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate was ineffective in promoter activation. The transfected MGMT promoter was not inducible by mutagens in HeLa S3 cells, which do not respond with induction of the endogenous MGMT gene. This is the first report showing hormone induction of a DNA repair gene (MGMT). The induction of MGMT and other genes encoding enzymes involved in DNA alkylation damage repair may be relevant in cancer therapy by causing resistance of tumor cells to alkylating drugs.
Carcinogenesis 1996 Nov
PMID:Induction of the alkyltransferase (MGMT) gene by DNA damaging agents and the glucocorticoid dexamethasone and comparison with the response of base excision repair genes. 896 45

Nitric oxide-induced modifications of DNA occur either by directly altering DNA chemically through reactive nitrogen oxide species (RNOS) or indirectly by inhibiting various repair processes. DNA ligases are enzymes which rejoin single-strand breaks and are critical for DNA integrity during processes such as gene transcription and repair. The eukaryotic and T4 DNA ligases are active in the presence of ATP and act in two steps: the formation of protein-AMP intermediates, then the ligation of DNA breaks. When T4 DNA ligase was exposed to the NO generator DEA/NO (Et2N[NO(NO)]Na), a concentration- and time-dependent inhibition of these two steps, adenylylation of the protein and ligation of the substrate, was observed. This inhibition was abated by the presence of cysteine, suggesting that RNOS, rather than NO, mediated the inhibition of the ligase activity. As mammalian and T4 DNA ligases act by the same mechanism, the inhibition of DNA ligase may explain the increase in single-strand breaks reported for cells exposed to NO and provides a mechanism to increase DNA lesions without direct chemical modification of DNA by NO or RNOS.
Carcinogenesis 1996 Nov
PMID:Nitric oxide inhibits DNA ligase activity: potential mechanisms for NO-mediated DNA damage. 896 69

Administration of hepatocarcinogens aflatoxin B1 and N-nitrosodimethylamine to rats caused single-strand breaks in nuclear DNA. Inclusion in the diet of rutin, a naturally occurring phenolic flavonoid glycoside, significantly reduced the appearance of such breaks. The protection against DNA damage was found to be reduction in the induction of repair enzymes polymerase, DNA polymerase beta and DNA ligase. Even associated with poly(ADP-ribose) a marginal dose of rutin was effective in this regard. Since DNA damage and inefficient repair are expected to initiate the process of carcinogenesis, modulation by rutin of these parameters emphasizes the protective role of this flavonoid against carcinogenesis induced by chemical carcinogens.
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PMID:Protective effect of rutin, a flavonol glycoside, on the carcinogen-induced DNA damage and repair enzymes in rats. 902 Sep 19

Poor folate status may be important in the aetiology of several epithelial cell malignancies including cancer of the uterine cervix. Folic acid is essential in the synthesis of purine nucleotides and the pyrimidine nucleoside thymidine and it is probable that imbalances in these DNA precursors negatively effect DNA stability and may ultimately lead to malignant transformation. The development of a modified 'comet assay' using the bacterial DNA repair enzyme uracil DNA glycosylase, to detect misincorporated uracil in human DNA is reported here. The effect of perturbing folic acid and deoxyuridine levels on uracil misincorporation in normal human lymphocytes and cultured human tumour cells was investigated using this assay. HeLa cells and peripheral human lymphocytes incubated as agarose-embedded nucleoids, with 1 unit of uracil DNA glycosylase per microg of DNA, contained low levels of uracil in their DNA. Both HeLa cells and stimulated human lymphocytes cultured in folate-deficient medium were growth arrested. Incubating human lymphocytes in folate-deficient medium significantly increased the level of uracil detected compared with control cells. HeLa cells showed an increase in non-specific DNA damage (strand breaks). Deoxyuridine (100 microM) significantly increased the level of uracil detected in the DNA of both folate-deficient and control HeLa cells. It appears that this modified comet assay specifically detects misincorporated uracil in single human cells. It should, therefore, prove valuable in determining the role of folic acid status in DNA instability and cancer.
Carcinogenesis 1997 Sep
PMID:Uracil misincorporation in human DNA detected using single cell gel electrophoresis. 932 65

Uracil can arise in DNA by misincorporation of dUTP into nascent DNA and/or by cytosine deamination in established DNA. Based on recent findings, both pathways appear to be promoted in the methyl-deficient model of hepatocarcinogenesis. A chronic increase in the ratio dUTP:dTTP with folate/methyl deficiency can result in a futile cycle of excision and reiterative uracil misincorporation leading to premutagenic apyrimidinic (AP) sites, DNA strand breaks, DNA fragmentation and apoptotic cell death. The progressive accumulation of unmethylated cytosines with chronic methyl deficiency will increase the potential for cytosine deamination to uracil and further stress uracil mismatch repair mechanisms. Uracil is removed by a highly specific uracil-DNA glycosylase (UDG) leaving an AP site that is subsequently repaired by sequential action of AP endonuclease, 5'-phosphodiesterase, a DNA polymerase and DNA ligase. Since the DNA polymerases cannot distinguish between dUTP and dTTP, an increase in dUTP:dTTP ratio will promote uracil misincorporation during both DNA replication and repair synthesis. The misincorporation of uracil for thymine (5-methyluracil) may constitute a genetically significant form of DNA hypomethylation distinct from cytosine hypomethylation. In the present study a significant increase in the level of uracil in liver DNA as early as 3 weeks after initiation of folate/methyl deficiency was accompanied by parallel increases in DNA strand breaks, AP sites and increased levels of AP endonuclease mRNA. In addition, uracil was also detected within the p53 gene sequence using UDG PCR techniques. Increased levels of uracil in DNA implies that the capacity for uracil base excision repair is exceeded with chronic folate/methyl deficiency. It is possible that enzyme-induced extrahelical bases, AP sites and DNA strand breaks interact to negatively affect the stability of the DNA helix and stress the structural limits of permissible uracil base excision repair activity. Thus substitution of uracil for thymine induces repair-related premutagenic lesions and a novel form of DNA hypomethylation that may relate to tumor promotion in the methyl-deficient model of hepatocarcinogenesis.
Carcinogenesis 1997 Nov
PMID:Presence and consequence of uracil in preneoplastic DNA from folate/methyl-deficient rats. 939 4


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