Effect of oxidative DNA damage in promoter elements on transcription factor binding

Reactive oxygen species produced by endogenous metabolic activity and exposure to a multitude of exogenous agents impact cells in a variety of ways. The DNA base damage 8-oxodeoxyguanosine (8-oxodG) is a prominent indicator of oxidative stress and has been well-characterized as a premutagenic lesion in mammalian cells and putative initiator of the carcinogenic process. Commensurate with the recent interest in epigenetic pathways of cancer causation we investigated how 8-oxodG alters the interaction between cis elements located on gene promoters and sequence-specific DNA binding proteins associated with these promoters. Consensus binding sequences for the transcription factors AP-1, NF-kappaB and Sp1 were modified site-specifically at guanine residues and electrophoretic mobility shift assays were performed to assess DNA-protein interactions. Our results indicate that whereas a single 8-oxodG was sufficient to inhibit transcription factor binding to AP-1 and Sp1 sequences it had no effect on binding to NF-kappaB, regardless of its position. We conclude from these data that minor alterations in base composition at a crucial position within some, but not all, promoter elements have the ability to disrupt transcription factor binding. The lack of inhibition by damaged NF-kappaB sequences suggests that DNA-protein contact sites may not be as determinative for stable p50 binding to this promoter as other, as yet undefined, structural parameters.