Regulation of DNA repair in response to oxidative stress in human cells
The research presented in this thesis examined the regulation of DNA repair pathways in response to agents shown to induce oxidative stress and oxidative damage to cellular DNA in human dermal fibroblasts. Non-cytotoxic doses of hydrogen peroxide (H2O2), UVB irradiation, hypochlorite (H0C1) and vitamin C were established. Measurement of base excision repair (BER) and nucleotide excision repair (NER) factors using a ribonuclease protection assay showed little modulation of gene transcript levels in response to treatment with H2O2, UVB, H0C1 or vitamin C. Significant changes in protein expression of the BER factors 8-oxoguanine glycosylase (hOGGl) and human apurinic/apyrimidinic endonuclease (hAPE) and of the NER factors human homologue of Rad23 (hHR23B) and Xeroderma pigmentosum protein A (XPA) in response to oxidative stress were not observed. Likewise, significant changes in endonuclease nicking activity (measured as the excision activity at a known site of oxidative DNA damage) were not observed. Studies in MCF-7, MDA-MB-468, MDA-MB-436, MDA-MB-231, T47-D, ZR-75.1 and HBL-100 human breast cancer cell lines revealed significantly elevated endonuclease nicking activity in the MDA-MB-436 cell line compared to human dermal fibroblasts. hi contrast to the other breast cancer cell lines under investigation, MDA-MB-436 cells were found to have a relatively low gene expression of the nucleotide pool sanitiser, human MutT homologue (hMTHl). The other breast cancer cell lines possessed increased gene expression of hMTHl relative to dermal fibroblasts, which may be as a response to increased oxidative stress generated as a consequence of the carcinogenesis process. In conclusion, regulation of DNA repair pathways in response to oxidative stress may be an important consequence in the carcinogenesis process. hMTHl expression and endonuclease nicking activity may provide useful tools for investigations into carcinogenesis in the future.