Phosphorylation of p65 is required for zinc oxide nanoparticle-induced interleukin 8 expression in human bronchial epithelial cells

Abstract

Background: Exposure to zinc oxide (ZnO) in environmental and occupational settings causes acute pulmonary responses through the induction of proinflammatory mediators such as interleukin-8 (IL-8). Objective: We investigated the effect of ZnO nanoparticles on IL-8 expression and the underlying mechanisms in human bronchial epithelial cells. Methods: We determined IL-8 mRNA and protein expression in primary human bronchial epithelial cells and the BEAS-2B human bronchial epithelial cell line using reverse-transcriptase polymerase chain reaction and the enzyme-linked immunosorbent assay, respectively. Transcriptional activity of IL-8 promoter and nuclear factor kappa B (NFκB) in ZnO-treated BEAS-2B cells was measured using transient gene transfection of the luciferase reporter construct with or without p65 constructs. Phosphorylation and degradation of IκBα, an inhibitor of NF-κB, and hosphorylation of p65 were detected using immunoblotting. Binding of p65 to the IL-8 promoter was examined using the chromatin immunoprecipitation assay. Results: ZnO exposure (2-8 μg/mL) increased IL-8 mRNA and protein expression. Inhibition of transcription with actinomycin D blocked ZnO-induced IL-8 expression, which was consistent with the observation that ZnO exposure increased IL-8 promoter reporter activity. Further study demonstrated that the κB-binding site in the IL-8 promoter was required for ZnO-induced IL-8 transcriptional activation. ZnO stimulation modestly elevated IκBαhorylation and degradation. Moreover, ZnO exposure also increased the binding of p65 to the IL-8 promoter and p65 phosphorylation at serines 276 and 536. Overexpression of p65 constructs mutated at serines 276 or 536 significantly reduced ZnO-induced increase in IL-8 promoter reporter activity. Conclusion: p65 phosphorylation and IkBα phosphorylation and degradation are the primary mechanisms involved in ZnO nanoparticle-induced IL-8 expression in human bronchial epithelial cells.