Glycine N-methyltransferase tumor susceptibility gene in the benzo(a)pyrene-detoxification pathway

Glycine N-methyltransferase (GNMT) affects genetic stability by (a) regulating the ratio of S-adenosylmethionine to S-adenosylhomocystine and (b) binding to folate. 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 benzo(a)pyrene (BaP) detoxification pathway. Results from an indirect immunofluorescent antibody assay showed that GNMT was expressed in cell cytoplasm before BaP treatment and translocated to cell nuclei after BaP treatment. Compared with cells transfected with the vector plasmid, the number 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. According to an aryl hydrocarbon hydroxylase enzyme activity assay, GNMT inhibited BaP-induced cytochrome P450 1A1 enzyme activity. Automated BaP docking using a Lamarckian genetic algorithm with GNMT X-ray crystallography revealed a BaP 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 binds readily with other DNA methyltransferases, including HhaI, HaeIII, PvuII methyltransferases and human DNA methyltransferase 2. We therefore hypothesized that BaP-DNA methyltransferase and BaP-GNMT interactions may contribute to carcinogenesis.