Breaking the Barrier: Using a Novel In Vitro Organotypic Airway Trans-Epithelial Exposure Model to Study the Depth of Inhaled Chemical Toxicity
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The toxicity of inhaled xenobiotics is often attributed to oxidative stress and inflammation; however, the specific underlying cellular and molecular mechanisms remain to be fully understood. Bridging this gap will require the use of robust and broadly-applicable in vitro organotypic models to provide insight into the role that the different cell types within the airway microenvironment play in exposure effects and susceptibility. We hypothesized that exposure of an intact epithelial barrier to an inhaled toxicant would cause oxidative stress and the induction of a pro-inflammatory response in underlying airway fibroblasts, a cell type crucial to maintaining tissue homeostasis in the airway. To test this hypothesis, we constructed a “trans-epithelial” exposure model (TEEM) to recapitulate an in vivo exposure of the airway microenvironment by incorporating an intact human bronchial epithelial cell (HBEC) layer to separate human lung fibroblasts (HLF) from the exposure material while also allowing assessment of the direct and trans-epithelial effects of toxicant exposure in the HBEC and HLF, respectively. Time course (2-24 hours) analysis of exposure outcomes following exposure of the TEEM to the model toxicant diesel exhaust particulates (DEP) demonstrated that the kinetics and magnitude of oxidative stress-responsive gene expression is similar between the two cell types; however, peak pro-inflammatory gene induction in HLF was delayed, but more prolonged in HLF relative to HBEC. Genes involved in glutathione homeostasis and hydrogen peroxide (H2O2) signaling (NQO1, TRX1, PTGS2 and GCLM1) were also alternatively regulated in response to DEP exposure, and their induction was attenuated by pre-treatment with the antioxidants N-acetyl-cysteine (NAC) and ascorbic acid. Further, pre-treatment of HLF with antioxidants led to an attenuation of both oxidative stress-responsive and pro-inflammatory gene expression in DEP-exposed HBEC. The findings presented here demonstrate that while receiving no direct exposures, HLF are both a target and a mediator of the effects of inhaled chemical exposures. Further, this readily-accessible in vitro organotypic model of the human airway microenvironment allows for the identification of xenobiotic exposure effects beyond the epithelium, as well as the investigation of their underlying cellular and molecular mechanisms.