Aldehyde-mediated protein degradation is responsible for the inhibition of nucleotide excision repair by cigarette sidestream smoke

We recently reported that cigarette sidestream smoke (CSS) can delay nucleotide excision repair (NER), which was due to the inhibition of repair protein accumulation to DNA damage sites. However, the mechanisms how the protein recruitment was inhibited remains unclear. We hypothesized that aldehydes in CSS could be a candidate taking a role for the inhibition, and tested our hypothesis by removing aldehydes from CSS using cigarette-filters. The NER inhibition potency of CSS or filtered CSS (F-CSS) was compared using human keratinocyte cell line, HaCaT. Cigarette-filters were able to reduce total aldehydes in CSS by half. Pretreating cells with CSS and F-CSS enhanced UVB-induced cell death, with the effect of CSS weakened by filtration. CSS strongly inhibited the repair of UVB-induced DNA damage, pyrimidine(6-4)pyrimidone photoproducts (6-4PPs), where the recruitments of repair molecules, TFIIH and XPG, were slowed down. F-CSS showed similar inhibition of NER and accumulation of related proteins, but the effect was weaker than CSS. Semicarbazide (SEM), an aldehyde-trapping agent, alleviated the NER delay induced by both CSS and F-CSS, further confirming that aldehydes in CSS were the main cause for the inhibition of NER and that the different amounts of aldehydes in CSS and F-CSS were responsible for the different inhibition efficiency. Furthermore, TFIIH level was decreased by treatment with CSS and restored in the presence of proteasome inhibitor, indicating that the degradation of NER proteins might be the cause of the inhibition of NER-protein recruitment. These results supported our hypothesis that aldehydes in CSS are the main contributor for the NER inhibition via protein degradation, and reconfirmed that exposure to CSS without filtration could be a severe threat to human health.