Chromium(VI) Enhances (±)-anti-7β,8α-Dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene-Induced Cytotoxicity and Mutagenicity in Mammalian Cells through Its Inhibitory Effect on Nucleotide Excision Repair

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Abstract
Chromium(VI) [Cr(VI)], a ubiquitous environmental contaminant, is a well-known carcinogen to both humans and experimental animals, although it is a weak mutagen by itself. Occupational exposure to Cr(VI) is strongly associated with a high incidence of lung cancer, but the underlying mechanisms remain unclear. Tobacco smoking is the major cause of lung cancer, and polycyclic aromatic hydrocarbons (PAHs) in tobacco smoke are the major etiological agents. Since humans are frequently exposed to both Cr(VI) and PAHs, it is possible that Cr(VI) and PAHs have a synergistic effect on mutagenecity and cytotoxicity that contributes to the high incidence of lung cancer associated with exposure to both agents. In this study, we tested this possibility by determining the effect of Cr(VI) exposure on (±)-anti-7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE, an active metabolite of PAHs) induced cytotoxicity, mutagenicity, and DNA adduct formation in Chinese hamster ovary (CHO) cells. Using the adenine phosphoribosyltransferase (APRT+) → APRT- forward mutation assay, we found that while Cr(VI) alone induced low mutation frequency, it greatly enhanced BPDE-induced mutations in nucleotide excision repair (NER)-proficient CHO cells. Cr(VI) exposure also greatly enhanced BPDE-induced killing in NER-proficient cells. It is known that the cytotoxicity and mutagenicity of BPDE are mainly caused by the formation of DNA adduct, which are removed by NER. To test the possibility that the enhancement of cytotoxicity and mutagenicity by Cr(VI) is caused by the inhibition of NER, NER-deficient cells were used, and the enhancement effects of Cr(VI) were not observed in those cells. We further found that while Cr(VI) exposure does not change the total BPDE−DNA adduct formation, it significantly inhibited the repair of BPDE−DNA adducts from genomic DNA in NER-proficient cells. Using a host cell reactivation assay, we found that the repair of BPDE−DNA adduct in a luciferase reporter gene is greatly inhibited after Cr(VI) exposure in NER-proficient cells while not in NER-deficient cells. Together these results clearly demonstrate that Cr(VI) exposure can greatly enhance the mutagenicity and cytotoxicity of PAHs by inhibiting the cellular NER pathway, and this may constitute an important mechanism for Cr(VI)-induced human carcinogenesis.