Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.1.1.37 (DNA methyltransferase)
 4,983 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

DNA in mammalian cells is enzymatically methylated at the 5-position of cytosine via S-adenosylmethionine and DNA methyltransferase. Several chemical carcinogens have been shown to inhibit this reaction, altering DNA methylation. We have been studying the mechanism by which carcinogens alter the methylation of DNA in order to better understand the cellular regulation of DNA methylase activity and to understand the role, if any, of DNA methylation in the carcinogenic process. We have utilized an in vitro assay for DNA methylase isolated from purified rat-liver nuclei. Ethionine, a liver carcinogen, given to rats 17 hr after partial hepatectomy inhibited the incorporation of [methyl-3H]-methionine into 5-methylcytosine residues of DNA. DNA isolated from these ethionine-treated rats was able to accept methyl groups from S-adenosylmethionine 8 times more than control DNA. It was further demonstrated that S-adenosylethionine competitively inhibited the DNA methylase resulting in hypomethylated DNA. N-Methyl-N-nitro-N-nitrosoguanidine reacted with the DNA methylase at the sulfhydryl sites inactivating the enzyme. Methylnitrosourea did not react directly with the methylase enzyme, but when reacted with DNA, the DNA methylase activity was inhibited by the carcinogen alkylated DNA. Sodium selenite also inhibited the enzyme non-competitively with a Ki of 6.7 microM. 5-Azacytidine prevented the 2 to 3 fold increase in DNA methylase seen 2 days following partial hepatectomy. All of these data with various carcinogens, altering DNA methylation by different mechanisms, support the hypothesis that DNA methylation plays a role in the initiation of carcinogenesis.
 ...
PMID:Studies on DNA methyltransferase and alteration of the enzyme activity by chemical carcinogens. 243 29

We have selected two N-methyl-N-nitrosourea (MNU)-resistant derivatives of the SV40-transformed, alkyltransferase-deficient (Mex-) human fibroblast cell line MRC5V1. Both derivatives remain Mex-. They are cross-resistant to methylmethanesulphonate (MMS) and 6-thioguanine (6TG) but not 2,6-diaminopurine. They show increased sensitivity to the bifunctional chloroethylating agent mitozolomide (MTZ). We have transfected MRC5V1 and one of our MNU-resistant lines with the bacterial O6-methylguanine (O6-MeG)-DNA methyltransferase (ada) gene. Transfectants of MRC5V1 are significantly more resistant to MNU but exhibit only a small increase in resistance to MMS and MTZ. Transfectants of the MNU-resistant derivative exhibit only a small additional increase in resistance to MNU, no further increase in resistance to MMS and a large increase in resistance to MTZ. The pattern of resistance to cytotoxic agents of these transfectants suggests that a second mechanism of resistance to MNU, independent of alkyltransferase expression, is operating in our resistant lines. This mechanism apparently enables the cells to tolerate O6-MeG and 6TG, but not chloroethyl adducts in their DNA.
Carcinogenesis 1989 May
PMID:Properties of N-methyl-N-nitrosourea-resistant, Mex- derivatives of an SV40-immortalized human fibroblast cell line. 253 16

Deficiency in DNA repair has been linked to aging, mutagenesis, carcinogenesis and several types of primary neuronal degeneration. O6-Methylguanine-DNA methyltransferase is a key enzyme in the repair of DNA alkylation damage that removes a methyl group from the O6 position of methylguanine. This study was carried out to determine whether there were any changes in the activity of this enzyme in lymphocytes of patients with Alzheimer's disease (AD) as compared to lymphocytes of age-matched non-demented elderly. The transferase activity in lymphocytes from 19 elderly patients with AD (mean 87.7 fmole/100 micrograms protein +/- SD 44.7) was not statistically different from that in 19 age/sex-matched controls (mean 91.3 fmole/100 micrograms protein +/- SD 40.0). There was no significant trend with age in transferase activity and the activity levels in the elderly subjects studied were the same as those reported previously for younger individuals by this laboratory. It is concluded that a reduction in O6-methylguanine-DNA methyltransferase activity is unlikely to be involved in the etiology or the pathogenesis of AD.
 ...
PMID:O6-methylguanine-DNA methyltransferase in lymphocytes of the elderly with and without Alzheimer's disease. 261 77

Alkylation at the O6 position of guanine leading to miscoding during DNA replication has been shown to correlate with mutagenesis both in bacteria and mammalian cells. The widely used Chinese hamster ovary cells (CHO) are unable to remove O6-methylguanine (O6-meG) due to the absence of O6-meG DNA methyltransferase (MT) activity. Recently Ding et al. [Mol. Cell. Biol. (1985) 5, 3293-3296] transfected CHO cells with human liver DNA obtaining a line provided with a function for the repair of O6-meG. We confirmed the presence of MT activity in this particular clone (14,300 molecules/cell). We used this MT-proficient cell line as compared with the original MT-deficient CHO cell line to analyse the relevance of repair of this lesion on cell killing, ouabain resistance (ouar) mutations and sister chromatid exchanges (SCEs) induced by methylating agents. MT-proficient cells were more resistant than MT-deficient ones to the cytotoxic and mutagenic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and N-methyl-N-nitrosourea (MNU). Furthermore a lower number of MNNG-induced SCEs were found in MT-proficient CHO than in MT-deficient cells. Similar ouar mutation frequencies were recorded in the two cell lines after 4-nitroquinoline-1-oxide (4NQO) treatment showing that the differences in cytotoxicity and mutagenesis are restricted to treatment with alkylating agents.
Carcinogenesis 1987 Oct
PMID:Cytotoxicity, mutations and SCEs induced by methylating agents are reduced in CHO cells expressing an active mammalian O6-methylguanine-DNA methyltransferase gene. 311 13

To clarify the involvement of O6-methylguanine (O6-MeG) in mutagenesis, we have been trying to isolate Mer+ cells from a HeLa S3 Mer- cell line, and to compare the mutation frequencies between the cell lines. We previously isolated the N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-resistant cells, MR10-1, from HeLa S3 Mer- cells. However, the MR10-1 cells still had only a little O6-MeG-DNA methyltransferase (MT) activity. In the present study, we have isolated two 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU)-resistant cells, ACr41 and ACr42, from the MR10-1 cells. The two ACr cells had increased MT activities. The ACr cells were also significantly more resistant to 1-(2-chloroethyl)-1-nitrosourea and slightly more resistant to MNNG than the MR10-1 cells. When the mutation frequencies were tested at the hypoxanthine-guanine phosphoribosyl transferase and ouabain loci in these cell lines, the two ACr cells were more resistant to the mutagenic effect of MNNG than the MR10-1 cells. These results show the linkage between the resistance to the cytotoxic effect of nitrosourea compounds and MT activity, and strongly support the hypothesis that O6-MeG is the main pre-mutagenic lesion induced by MNNG.
Carcinogenesis 1988 Jun
PMID:Increased O6-methylguanine-DNA methyltransferase activity and reduced mutability in 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2- chloroethyl)-3-nitrosourea-resistant HeLa S3 cells. 316 31

We have isolated an isogenic O6-methylguanine (O6-MeG)-DNA methyltransferase-defective mutant from a HeLa cell line. This mutant exhibits excess DNA strand breaks and considerable cytotoxicity after N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) treatment. The increased frequency of strand breaks after MNNG treatment was not abolished by DNA synthesis inhibitors. We propose that the presence of unrepaired O6-MeG lesions leads to excess strand breaks and these, in turn, are mainly responsible for the cytotoxicity.
Carcinogenesis 1988 Oct
PMID:O6-methylguanine-DNA methyltransferase-defective human cell mutant: O6-methylguanine, DNA strand breaks and cytotoxicity. 316 54

The effect of nutritional zinc-deficiency on the activities of O6-alkylguanine:DNA methyltransferase (AGT) in 9 rat tissues including liver, lung, kidney, spleen, brain, esophagus, forestomach, gastric-stomach and small intestine has been examined. Individual tissue extracts prepared from zinc-deficient and pair-fed, zinc-sufficient rats were incubated with N-[3H]methylnitrosourea-methylated calf thymus DNA for 1 h. The activities of AGT in these tissues were measured by two methods: (a) the transfer of the methyl group from O6-methylguanine in substrate DNA to AGT protein, and (b) the determination of the ratio of O6-methylguanine:7-methylguanine remaining in substrate DNA following incubation. AGT activities (expressed as fmol protein methylated/h per mg protein) were significantly reduced in the esophagus, spleen and lungs of zinc-deficient rats as compared to those in their corresponding zinc-sufficient counterparts. The ratio of O6-methylguanine:7-methylguanine was also reduced in the esophagus of the zinc-deficient rat. These results were consistent with our earlier findings that dietary zinc-deficiency enhances nitrosamine-induced esophageal carcinogenesis in rats.
 ...
PMID:Effect of nutritional zinc-deficiency on O6-alkylguanine-DNA-methyl-transferase activities in rat tissues. 319 74

Acrolein, a reactive metabolite of cyclophosphamide, may be responsible for bladder cancer induced by cyclophosphamide. DNA methylase was isolated from the liver and urothelium of rats by high salt extraction of purified nuclei. Acrolein at 10 microM inhibited liver and bladder DNA methylase activity by 30-50%. Kinetic studies with the liver enzyme showed a competitive type of inhibition with a Ki of 6.7 microM. Both dithiothreitol and glutathione afforded protection to the enzyme when added to the assay. At near equimolar concentrations of glutathione to acrolein, the methylase retained 80-90% activity. An increase in DNA had no effect on the inhibition by acrolein, whereas increased amounts of protein protected against acrolein inhibition, suggesting that acrolein reacted with the DNA methylase protein. On the other hand, DNA that had been reacted with acrolein was unable to serve as a substrate for DNA methylase. As the DNA adducts increased the methylation of the DNA decreased. Thus, acrolein has the ability to react with DNA and the DNA methylase protein, either of which results in inhibition of DNA methylation.
Carcinogenesis 1988 Mar
PMID:Inhibition of DNA methylase activity by acrolein. 334 85

O6-Methylguanine-DNA methyltransferase (MT)-proficient Chinese hamster ovary cells were grown in the presence of low, gradually increasing levels of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) with the aim of selecting MNNG-resistant cell lines. Six resistant clones with two levels of resistance were isolated. A 3-fold increase in survival was observed in clones 13, 14 and 15 and a greater than 10-fold increase in clones A, B and C. Cross resistance to N-methyl-N-nitrosourea but not to mitomycin C was observed. By comparison with the parental MT-proficient cells, MT activity was doubled in two resistant clones (13 and B) irrespective of their resistance levels. DNA glycosylase activity responsible for the removal of 7-methylguanine and 3-methyladenine showed similar levels in resistant clones 13 and B, in the MT-proficient cells and in the original MT-deficient cells. Alkylation-induced DNA damage, as measured by alkaline elution at the same MNNG dose, was higher in clones 13 and B than in the parental cells. The induction of sister chromatid exchanges by MNNG was inversely related to the resistance levels, thus paralleling the induction of cytotoxicity. These results suggest the existence in Chinese hamster ovary cells of at least two independent functions which control resistance to methylating agents, one possibly being the capacity to repair O6-methylguanine.
Carcinogenesis 1988 Jul
PMID:Isolation of clones displaying enhanced resistance to methylating agents in O6-methylguanine-DNA methyltransferase-proficient CHO cells. 338 40

O4-Alkylthymine-DNA adducts have been implicated as causative lesions in chemical mutagenesis and carcinogenesis. To directly assess the mutagenic potential of these adducts in vivo, we have designed an enzymatic technique for introducing nucleotide analogues at predetermined sites of biologically active DNA. Escherichia coli DNA polymerase I was used in vitro to incorporate a single O4-methylthymine residue at the 3' terminus of an oligonucleotide primer opposite the adenine residue of the amber codon in bacteriophage phi X174 am3 DNA. After further extension of the primer with unmodified nucleotides, the partial-duplex product was transfected into E. coli spheroplasts. Replication of the site-specifically methylated DNA in E. coli deficient in O4-methylthymine-DNA methyltransferase (ada-) yielded 10-fold more mutant progeny phage than replication of nonmethylated DNA; no increase in mutation frequency was observed after replication in repair-proficient (ada+) E. coli. The DNA from 20 independently isolated mutant plaques all contained A.T----G.C transitions at the original site of O4-methylthymine incorporation. These data demonstrate that O4-methylthymine induces base-substitution mutations in E. coli and suggest that this adduct may be involved in mutagenesis by N-nitroso methylating agents. This enzymatic technique for site-specific mutagenesis provides an alternative to the chemical synthesis of oligonucleotides containing altered bases.
 ...
PMID:Mutagenic potential of O4-methylthymine in vivo determined by an enzymatic approach to site-specific mutagenesis. 346 67


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