Tobacco-Related Exposure Upregulates Circ_0035266 to Exacerbate Inflammatory Responses in Human Bronchial Epithelial Cells

One of the most carcinogenic chemicals found in cigarette tobacco smoke is 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), which has been confirmed to be associated with the etiology of diverse cancers. Lipopolysaccharide (LPS), another biologically active component of cigarette smoke, is a risk factor which enhances NNK-induced lung tumorigenesis due to chronic lung inflammation. Although inflammatory responses play critical roles in the initiation of many tumors, our knowledge about the mechanisms of NNK+LPS on inflammation is currently limited. Here, we investigated the inflammatory effects of NNK+LPS in human bronchial epithelial cells (BEAS-2B) and explored the underlying mechanisms mediated by circular RNAs (circRNAs). We identified a novel circRNA, circ_0035266, which was strongly upregulated in NNK+LPS-induced BEAS-2B cells and enhanced the inflammatory responses to NNK+LPS by regulating the secretion of pro-inflammatory cytokines interleukin (IL)-6 and IL-8. Specifically, circ_0035266 knockdown alleviated NNK+LPS-induced inflammatory responses, whereas overexpression of circ_0035266 had the opposite effect. Moreover, dual-luciferase reporter and fluorescence in situ hybridization (FISH) assays verified that circ_0035266 bound to miR-181d-5p directly in the cytoplasm. qRT-PCR, dual-luciferase reporter assays, and Western blot analyses showed that DDX3X (DDX3) was the downstream target of miR-181d-5p and that DDX3X expression levels were modulated by circ_0035266. These results suggested that circ_0035266 served as a competitive endogenous RNA for miR-181d-5p to regulate DDX3X expression, which is involved in the modulation of NNK+LPS-induced inflammatory responses in BEAS-2B cells.