Activated neutrophils inhibit nucleotide excision repair in human pulmonary epithelial cells: role of myeloperoxidase Free

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Chronic inflammation is considered to be a risk factor for the development of epithelial cell-derived malignancies. However, the underlying processes have not yet been fully defined at the biochemical and molecular level. In the lung, polymorphonuclear neutrophils are thought to affect carcinogenesis by accelerating the metabolism of inhaled chemical carcinogens, thereby, causing enhanced formation of pro-mutagenic DNA adducts. In this study, we hypothesized that neutrophils interfere with the removal of such DNA adducts by inhibiting nucleotide excision repair (NER) in epithelial target cells. Human alveolar epithelial cells (A549) were co-incubated with phorbol-ester-activated neutrophils and phenotypical assessment of NER capacity in the A549 cells was performed, using a modified comet-assay. Activated neutrophils caused a significant reduction of NER in the co-cultured epithelial cells. This inhibitory action of neutrophils was abrogated upon addition of the myeloperoxidase (MPO) inhibitor 4-ABAH. In parallel experiments, the MPO product hypochlorous acid (HOCl) caused a comparable acute dose-dependent inhibition of NER. These observations were supported by showing that HOCl caused a delayed removal of DNA adducts in benzo[a]pyrene-diolepoxide (BPDE)-exposed A549 cells. NER inhibition by HOCl was persistent for up to 24 hours and was independent of cytotoxicity or loss of cellular ATP. Further studies indicate that the acute HOCl-induced inhibition of NER can only partly be explained by direct oxidative modification of repair proteins. In an attempt to explain the more persistent effects of HOCl, we analysed the expression of relevant NER genes (ERCC-1, XPA, XPC, XPF and XPG) and found that HOCl significantly reduced XPC gene expression. In conclusion, our data indicate that neutrophils are potent inhibitors of nucleotide excision repair through MPO-catalysed formation of HOCl. These observations may provide a further biological explanation for the association between inflammation and lung cancer risk.