Cell specific differences in O6-methylguanine-DNA methyltransferase activity and removal of O6-methylguanine in rat pulmonary cells

Publisher Website
Google Scholar
Abstract
Previous studies have demonstrated that cell specificity exists for the alkylation of DNA from lung cells following treatment of rats with the tobacco specific carcinogen 4-( N -methyl- N -nitrosamino)-1-(3-pyridyl)-1-butanone (NNK). The concentration of the promutagenic adduct O6 -methylguanine (O 6 MG) was found to be greatest in Clara cells followed by macrophages, type II cells and alveolar small cells. The purpose of this study was to measure the activity of the repair protein O6 -methylguanine-DNA methyltransferase ( O6 MGMT) and to determine whether differences exist for the removal of O6 MG among pulmonary cell types. Constitutive activity of O6 MGMT was 2-fold greater in macrophages and type II cells than alveolar small cells and Clara cells. Treatment for 4 days with NNK (10 mg/kg/day) had no effect on O 6 MGMT activity in macrophages, but decreased activity in alveolar small cells and type II cells by 57 and 84%, respectively. O 6 MGMT activity was reduced to below limits of detection in Clara cells following treatment with NNK. The effect of NNK on O6 MGMT activity was consistent with rates of removal of O6 MG in macrophages and Clara cells. The loss of O6 MG from DNA of macrophages followed first order kinetics ( t½ = 48 h) while very little loss of this adduct was observed in Clara cells over an 8 day period following cessation of carcinogen treatment. Even though O6 MGMT activity was reduced in alveolar small cells and type II cells, ∼90% of the O 6 MG bound to DNA in these cell types was removed within 8 days after treatment was discontinued. The loss of O 6 MG from pulmonary cells appears to result largely from the removal of this adduct by O 6 MGMT since rates of cell turnover were very low (0.5–1.5%/day) in the lung and were not affected by treatment with NNK. This study indicates that the activity of O 6 MGMT and the rate of resynthesis of this repair enzyme differ considerably among pulmonary cells following the methylation of DNA. The high concentration of O 6 MG in Clara cells and the low rate of repair of this promutagenic adduct may be critical factors in the potent pulmonary carcinogenicity induced by the tobacco specific carcinogen NNK.

This publication has 24 references indexed in Scilit: