Influence of Chromatin Structure and Radical Scavengers on Yields of Radiation-Induced 8-oxo-dG and DNA Strand Breaks in Cellular Model Systems

Abstract

Radiation-induced formation of 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dG) and DNA strand breaks was studied in cultured cells with normal or modified chromatin structure. Human fibroblasts were irradiated as cellular monolayers (intact cells), nuclear monolayers (permeabilized cells with intact chromatin structure), and nucleoid monolayers (permeabilized and salt-treated cells with histone-free DNA). 8-oxo-dG was assayed with reverse-phase HPLC coupled to an electrochemical detector and strand breaks with the alkali unwinding assay. Depletion of low-molecular-weight nuclear components increased the radiation-induced formation of 8-oxo-dG fivefold compared to twofold for the formation of strand breaks. Removal of both low-molecular-weight components and histones increased the yield of 8-oxo-dG 46-fold and the yield of strand breaks 43-fold. Removal of only the histones thus leads to a two times greater increase in the yield of strand breaks compared to 8-oxo-dG. Addition of radical scavengers to nuclear and nucleoid monolayers provided a significantly better protection against the formation of 8-oxo-dG relative to the formation of strand breaks. These results suggest that in intact cells, 8-oxo-dG is preferentially formed in histone-free structures of chromatin, indicating a larger role for the indirect effect of radiation in the formation of 8-oxo-dG than in the formation of strand breaks.