Glycine N-Methyltransferase Tumor Susceptibility Gene in the Matabolic Pathways of Benzo(a)pyrene and Alfatoxin B1

• Main Authors: Shih-Yin Chen, 陳世殷
• Other Authors: Yi-Ming Chen
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/28766771346946756398

博士 === 國立陽明大學 === 公共衛生研究所 === 93 === Glycine N-methyltransferase (GNMT) affects genetic stability by regulating the ratio of S-adenosylmethionine to S-adenosylhomocystine and binding to folate. Previously, we reported that the expression level of GNMT was diminished in human hepatocellular carcinoma (HCC）. In addition, human GNMT genotypes had high rate of loss of heterozygosity in the HCC tissues. Based on the identification of GNMT as a 4 S polyaromatic hydrocarbon-binding protein, we used liver cancer cell lines that expressed GNMT either transiently or stably in cDNA transfections to analyze the role of GNMT in the metabolic pathways of benzo(a)pyrene (BaP) and aflatoxin B1(AFB1). Results from an indirect immunofluorescent antibody assay showed that GNMT was expressed in cell cytoplasm before BaP and AFB1 treatment and translocated to cell nuclei after BaP and AFB1 treatment. Compared with cells transfected with the vector plasmid, the level of BaP-7,8-diol 9,10-epoxide-DNA adducts that formed in GNMT-expressing cells was significantly reduced. Furthermore, the dose-dependent inhibition of BaP-7,8-diol 9,10-epoxide-DNA adduct formation by GNMT was observed in HepG2 cells infected with different multiplicities of infection of recombinant adenoviruses carrying GNMT cDNA. Competitive ELISA showed that the presence of GNMT can reduce 8,9-dihyro-8-(2,6-diamino-4-oxo-3,4-dihydropyrimid-5-yl-formamido)-9- hydroxy-AFB1-DNA adduct formation. In addition, a cytotoxicity assay was used to determine the effect of GNMT on the cells treated with BaP and AFB1 and the results showed that the expression of GNMT could decrease the BaP and AFB1-induced cytotoxicity. According to an aryl hydrocarbon hydroxylase enzyme activity assay, GNMT inhibited BaP-induced cytochrome P450 1A1 enzyme activity. Automated BaP and AFB1 docking using a Lamarckian genetic algorithm with GNMT X-ray crystallography revealed the BaP and AFB1 preference for the S-adenosylmethionine-binding domain of the dimeric form of GNMT, a novel finding of a cellular defense against potentially damaging exposures. In addition to GNMT, results from docking experiments showed that BaP and AFB1 binds readily with other DNA methyltransferases, including HhaI methyltransferases and human DNA methyltransferase 2. We therefore hypothesized that BaP or AFB1-DNA methyltransferase and BaP or AFB1-GNMT interactions may contribute to carcinogenesis.