Regulatory mechanisms of haptoglobin on particulate matter-induced epithelial-to-mesenchymal transition in bronchial epithelial cells.

Background: It has been proposed that repeated exposure of bronchial epithelial cells to atmospheric particulate matter (PM) could disrupt airway epithelial integrity and lead to epithelial-to-mesenchymal transition (EMT) and ultimately airway remodeling. The molecular mechanisms underlying PM-related bronchial epithelial EMT have not yet been elucidated. The aim of this research is to clarify the molecular mechanism of EMT upon PM exposure.

Methods: Using an in vivo mouse model of PM-induced airway inflammation and an in vitro model of PM-stimulated bronchial epithelial cells, we clarified the role of haptoglobin (HP) in PM-induced bronchial epithelial EMT. The expression of HP in lung tissues was evaluated by immunohistochemistry (IHC). Western blotting (WB) and immunofluorescence staining were used to analyze EMT-related protein expression and the relevant signaling pathways in the changes in lung tissues and bronchial epithelial cells upon PM exposure. HP small interfering RNA (siRNA) was used to implement the interference of endogenous HP.

Results: In vivo experiments showed elevated HP expression in the bronchial epithelium upon PM exposure. IHC and WB showed that E-cadherin expression was decreased, and vimentin expression was increased in bronchial epithelial cells. Moreover, WB results showed that the phosphorylation levels of signal transducer and activator of transcription 3 (STAT3) and extracellular regulated protein kinases (ERK) were elevated in the lung parenchymal tissue of mice. The results of in vitro molecular mechanism experiments showed that compared with those of the control group, the phosphorylation levels of STAT3 and ERK in the PM group increased progressively with higher concentrations of PM and longer stimulation durations. BEAS-2B cells were pretreated with stattic (STAT3 inhibitor) and/or U0126 (ERK inhibitor), and it was found that either stattic or U0126 inhibited PM-induced reduction of E-cadherin expression and elevation of vimentin expression, and the inhibitory effect was most significant when both inhibitors were pretreated simultaneously. Through transfection of BEAS-2B cells with HP siRNA, WB results showed that HP siRNA partially reversed the PM-induced reduction in E-cadherin expression and elevation of vimentin expression, in addition to the reduction in the phosphorylation levels of the STAT3 and ERK.

Conclusions: HP is an important mediator of PM-induced EMT in bronchial epithelial cells and promotes PM-induced EMT in bronchial epithelial cells through activation of the STAT3 signaling pathway and the ERK signaling pathway. Inhibition of HP expression attenuates PM exposure-induced EMT in bronchial epithelial cells.