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

---

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

The human DNA repair enzyme, methylguanine-DNA methyltransferase (MGMT, M(r) 21,000), which protects cells against the mutagenic effect of alkylating carcinogens, was found to be localized in the cell nucleus (except the nucleolus) by immunofluorescence staining using polyclonal and monoclonal antibodies. The supporting experiments came from differential staining of the MGMT-deficient (mer-) and -proficient (mer+) cells, Western blotting analysis, and specific antibody depletion studies with the immobilized fusion protein, GSTMGMT-glutathione-Sepharose. Its localization in the nucleus agrees with its biological function and possibly explains the ineffective protection of mammalian cells (mer-) transfected with the Escherichia coli MGMT genes from bifunctional alkylating agents.
...
PMID:Intracellular localization of human DNA repair enzyme methylguanine-DNA methyltransferase by antibodies and its importance. 138 61

Regulation of the expression of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT) has been investigated in a number of human lymphoblastoid cell lines. In a number of Mex- cell lines that do not express methyltransferase activity, CpG sequences in the mgmt gene were hypomethylated with respect to methyltransferase-expressing Mex+ lines. In the cell line GM1953(S), in which the mgmt gene is coregulated with the thymidine kinase and galactokinase genes, reexpression of all three activities was experimentally induced. In this case, the mgmt gene in the nonexpressing cells was found to be hypermethylated and underwent a demethylation at CpG sequences that was coincident with the reappearance of the mgmt mRNA and the three enzyme activities. The simultaneous silencing of three activities in these cells was correlated with an increase in DNA 5-methylcytosine that was widespread throughout the genome. The data indicate that MGMT expression can be controlled epigenetically in human lymphoid cell lines, although the relationship between cytosine methylation and MGMT expression is complex. Furthermore, the rapid alterations in methylation in GM1953(S) cells indicate the existence of signals that can induce widespread and abrupt alterations in cytosine methylation in human cells in culture.
...
PMID:Epigenetic silencing of the DNA repair enzyme O6-methylguanine-DNA methyltransferase in Mex- human cells. 139 30

The level of the DNA repair enzyme O6-alkylguanine-DNA alkyltransferase (MGMT) was examined in benign and malignant skin tumors induced with different initiating and promoting agents and from both SENCAR and Sensitive SENCAR Inbred (SSIN) mice. The MGMT levels in the tumors were approximately one-half the level observed in normal surrounding epidermis and in keratinocytes from untreated controls. In addition, a carcinoma-producing cell line, VT 17DT, derived from papillomas in SENCAR mice had no detectable MGMT activity (Mer- phenotype), whereas in the non-tumor forming line, 3PC, MGMT activity was comparable to that in papillomas. The comparatively low level of MGMT in papillomas may contribute to their ease of conversion to squamous cell carcinomas by N-ethyl-N-nitrosourea or n-methyl-N'-nitro-N-nitrosoguanidine. MGMT activity was also determined in the epidermis of non-exposed mice of various stocks and strains. Epidermal MGMT activity was similar to levels in the corresponding livers and was, in general, parallel with stock/strain susceptibility to tumor formation. This is the first report that examined MGMT activity in skin tumors and normal keratinocytes in the mice of several stocks and strains.
...
PMID:O6-alkylguanine-DNA alkyltransferase activity in epidermal tumor and normal epidermal cells of mice of various stocks and strains. 163 96

When animals are treated with carcinogenic agents that alkylate O6-guanine residues, the incidence of tumors in specific tissues often relates inversely to the level of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT) present in the tissue. Similarly, the hypersensitivity to anticancer chloroethylnitrosoureas of some human tumor cell lines is believed to result from their deficiency in MGMT. We have undertaken a comprehensive investigation of MGMT expression in a panel of nine characterized human glioma cell lines. Methyltransferase activity determined by incubating protein extracts of these glioma lines with [3H]methylated DNA ranged from undetectable in six lines (the Mer- phenotype) to greater than 0.8 pmol/mg in two lines (U-373 MG and D-392 MG). MGMT protein was undetectable in Western blots of the Mer- cell extracts probed with specific anti-MGMT monoclonal antibodies. Consistent with these results, steady-state levels of MGMT mRNA, determined by Northern blot analysis, were detectable only in the three Mer+ glioma lines (U-373 MG, D-392 MG, D-263 MG). Southern analysis of EcoRI-digested DNA probed with MGMT cDNA revealed no amplification, rearrangement or deletions of the MGMT gene in any of the glioma cell lines. This is the first report that examines MGMT expression at the biochemical, molecular and genetic levels in a particular tumor type. These studies suggest that transcriptional regulation is the basis of the Mer- phenotype in these malignant human glioma cell lines, since no gross structural or quantitative abnormalities of the MGMT gene were seen in the phenotypically Mer- lines.
...
PMID:Expression of O6-methylguanine-DNA methyltransferase in malignant human glioma cell lines. 189 34

The activity of a new nitrosourea (TCNU) based on the endogenous amino acid taurine was assessed in three human lung cancer xenografts growing in immunodeficient mice. Moderate activity (specific growth delays of 0.63 and 1.13 compared with controls) was seen in two non-small cell tumours after a single oral administration of 20 mg-1kg. This dose was curative in a small cell xenograft. By using high performance liquid chromatography it was possible to detect parent drug in the tumours as well as the plasma and tissues after oral administration of TCNU. Drug sensitivity was correlated inversely with the amount of the DNA repair enzyme 0(6)-methylguanine-DNA methyltransferase assayed from extracts of the tumour cells but not with the levels of parent drug within the tumour. This compound appears to have unique pharmacokinetic properties compared with other chloroethylnitrosoureas.
...
PMID:Activity of a new nitrosourea (TCNU) in human lung cancer xenografts. 339 Mar 69

Adducts of O6-alkylguanine in DNA that are induced by cytotoxic, carcinogenic or mutagenic alkylating agents can be removed by the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT). Human tumor cell lines that do not express this enzyme (Mer-) are hypersensitive to the effects of such alkylating agents, although the molecular basis of MGMT gene suppression is not yet understood. Previous studies suggested that Mer- cells deficient in this enzyme lack neither the gene nor the trans-acting factors necessary for normal transcription. Methylation of CpG dinucleotides is an attractive mechanism to account for suppression of the MGMT gene; however, there have been reports of both direct and inverse correlations between methylation and MGMT expression. We previously demonstrated an inverse correlation between methylation at a single SmaI site in the human MGMT promoter and gene expression. To substantiate this observation, we examined additional CpGs in the promoters of three Mer+ and three Mer- cell lines, using rare methylation-sensitive restriction sites, and then sought to identify the region where methylation correlated with gene expression. Six CpGs in the region from -245 bp to +225 bp (relative to the transcription start site) were completely unmethylated in all Mer+ cells, whereas in Mer- cells were at least partially methylated. The methylation status of CpGs further upstream did not correlate with MGMT expression. We conclude, therefore, that the association between CpG methylation and suppressed MGMT gene activity extends to sites other than SmaI but is limited to a core region of the promoter.
...
PMID:Localization of methylation sites in the human O6-methylguanine-DNA methyltransferase promoter: correlation with gene suppression. 778 59

O6-Alkylguanine derivatives are well known as chemical modulators of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT). Depletion of the enzyme by these derivatives leads to increase sensitivity of tumor cells to chloroethylnitrosoureas. We tested the effect of O6-methylguanine, O6-benzylguanine, O6-(p-methylbenzyl)guanine, O6-(p-chlorobenzyl)guanine, O6-(p-methoxybenzyl)guanine, O6-methylhypoxanthine and O6-benzylhypoxanthine on the sensitivity of tumor cell lines to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3- nitrosourea hydrochloride (ACNU) using a colorimetric cytotoxicity assay. The sensitivity of MGMT-proficient tumor cells including HeLA S3, C6-1, C6-2/ACNU, U-138 MG and U-373 MG cells was greatly enhanced by 2 hr pretreatment of 10-100 microM O6-benzylguanine, O6-(p-methylbenzyl)guanine and O6-(p-chlorobenzyl)guanine, but not by O6-methylguanine or O6-methylhypoxanthine. O6-(p-methylbenzyl)guanine moderately sensitized the 2 cell lines, HeLa S3 and C6-1, tested in our study to ACNU cytotoxicity. O6-Benzylhypoxanthine at the high concentration (100 microM) sensitized, to some extent, 3 MGMT-proficient cell lines. Lesser degrees of enhancement by the O6-benzylguanine derivatives were noted in MGMT-deficient tumor cells. Biological effects of O6-alkylguanine derivatives on enhancing ACNU cytotoxicity of tumor cells suggest that the exocyclic 2-amino and O6-benzyl groups in O6-benzylguanine skeleton are both essential for the inhibition of MGMT activity.
...
PMID:Enhancing effect of O6-alkylguanine derivatives on chloroethylnitrosourea cytotoxicity toward tumor cells. 807 57

Approx. 20% of human tumor cell lines (termed Mer-) are deficient in the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT; E.C.2.1.1.63). Such cells possess the MGMT gene and promoter sequences but have virtually no mRNA or protein. Cytosine methylation of gene sequences has been proposed as a mechanism by which MGMT could be suppressed in Mer- cells; however, the experimental evidence does not uniformly support this idea. We therefore investigated the effect of in vitro methylation of the MGMT promoter in a reporter gene construct transfected into cultured human cells. DNA methylation by HpaII or HhaI methylases suppressed the activity of the promoter, although the effect was not absolute. The occurrence of partial intracellular demethylation of promoter sequences may account for the incomplete inhibition of transcription. A model that attempts to reconcile the opposing views on the role of cytosine methylation in MGMT gene expression is presented.
...
PMID:In vitro methylation of the human O6-methylguanine-DNA methyltransferase promoter reduces transcription. 811 Aug 28

Activation of the Ha-ras oncogene in N-methyl-N-nitrosourea (MNU)-induced rat mammary tumors has been well documented. Such Ha-ras activation is thought to be brought about by direct action of carcinogens resulting in a G-->A transition at the second nucleotide of codon 12. However, a DNA repair enzyme, O6-methylguanine-DNA methyltransferase (MGMT), can specifically remove methyl groups from O6-methylguanine, which is a major mutagenic and carcinogenic DNA lesion leading to the G-->A transition. In this study, we compared the amount of MGMT mRNA in MNU-induced rat mammary tumors with and without such Ha-ras activation. A single injection of MNU into 82 female Sprague-Dawley rats induced 80 mammary carcinomas. RNase protection analysis and subsequent sequencing revealed that 42 of 65 randomly selected tumors contained Ha-ras oncogenes activated by the G-->A transition. The amount of MGMT mRNA was then measured by means of reverse transcriptase-mediated polymerase chain reaction (RT-PCR) amplification and Southern hybridization. No obvious difference in the level of MGMT mRNA was detected between the two tumor groups. In addition, in the course of our experiment, five of 42 tumors classified as containing activated Ha-ras oncogenes proved to contain low percentages of tumor cells with the Ha-ras activation. These results suggest that Ha-ras activation in MNU-induced rat mammary tumors may not necessarily be influenced by differences in MGMT activity. They also raise the possibility that activation of other oncogenes and/or inactivation of unidentified tumor suppressor gene(s) may be involved in development of a certain proportion of tumors with activated Ha-ras oncogenes, as is suspected in the case of tumors without Ha-ras activation.
...
PMID:Comparison of O6-methylguanine-DNA methyltransferase mRNA levels in Ha-ras mutated and non-mutated rat mammary tumors induced by N-methyl-N-nitrosourea. 811 29

Chloroethylnitrosoureas (CENUs) alkylate DNA at specific sites and inhibit DNA replication in tumor cells. O6-Alkylguanine moieties resulting from alkylation of guanine bases are thought to be one of most lethal adducts in living cells. Effectiveness of CENUs is known to relate well with an enzymic activity of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT), which recognizes and removes O6-alkylguanine. To improve therapeutic results of CENUs, we have measured MGMT activity of human brain tumors and studied the relationship between MGMT activity and clinical responsiveness to I-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU). Thirty-seven patients with brain tumors were entered into the study. The neoplasms included gliomas, non-glial tumors, and brain metastases. The MGMT activity of gliomas was significantly lower than that of non-glial tumors and brain metastases. No significant difference in the enzyme activity was noted between low- and high-grade gliomas. Out of the 22 gliomas 5 tumors indicated a value below 60 fmol/mg, suggestive of a methyl excision repair minus (Mer-) tumor. Two out of 3 evaluable patients with a Mer- tumor responded well to post-operative ACNU adjuvant chemotherapy. Our results suggest that brain tumors include a certain percentage of Mer- phenotype tumors, and that CENUs such as ACNU should be applied selectively on tumors with a low MGMT activity in order to increase the therapeutic effectiveness.
...
PMID:Influence of O6-methylguanine-DNA methyltransferase activity on chloroethylnitrosourea chemotherapy in brain tumors. 839 42


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

---