Down-regulation of the DNA-repair endonuclease 8-oxo-guanine DNA glycosylase 1 (hOGG1) by sodium dichromate in cultured human A549 lung carcinoma cells

Abstract

Hexavalent chromium is a genotoxic human pulmonary carcinogen that elevates DNA oxidation, apparently through the generation of reactive DNA-damaging intermediates including CrV, CrIV and reactive oxygen species. We tested the hypothesis that elevation of DNA oxidation may also be through inhibition of the expression of the repair glycosylase for 8-oxo deoxyguanine (hOGG1) in cultured A549 human lung epithelial cells. Treatment with sodium dichromate (0-100 μM, 16 h) resulted in a concentration-dependent decrease in the levels of OGG1 mRNA as measured by both RT-PCR and RNase protection assay. Sodium dichromate at 25 μM and above gave a marked reduction of OGG1 mRNA expression which was not seen at 1 μM and below. No effect on the expression of the apurinic endonuclease hAPE or the house-keeping gene GAPDH was observed at any of the concentrations of sodium dichromate investigated. Treatment of cells with the pro-oxidant H2O2 (0-200 μM) for 16 h had no detectable effect on the levels of OGG1 mRNA or protein expression suggesting that the effect of sodium dichromate is not mediated by H2O2. Western blotting demonstrated that sodium dichromate (100 μM; 16 h and >25 μM; 28 h) markedly reduced levels of OGG1 protein in nuclear cell extracts. Additionally, treatment of cells with sodium dichromate (>25 μM, 28 h) resulted in a concentration-dependent decrease in the ability of nuclear extracts to nick a synthetic oligonucleotide containing 8-oxo deoxyguanine (8-oxo dG). We conclude that the elevation of 8-oxo dG levels observed in A549 cells treated with sodium dichromate may be, at least in part, due to a reduced capacity to repair endogenous and hexavalent chromium-induced 8-oxo dG.