The uncoupling of autophagy and zinc homeostasis in airway epithelial cells as a fundamental contributor to COPD

Abstract

The proper regulation of zinc (Zn) trafficking proteins and the cellular distribution of Zn are critical for the maintenance of autophagic processes. However, there have been no studies that have examined Zn dyshomeostasis and the disease-related modulation of autophagy observed in the airways afflicted with chronic obstructive pulmonary disease (COPD). We hypothesized that dysregulated autophagy in airway epithelial cells (AECs) is related to Zn dysregulation in cigarette smoke (CS)-induced COPD. We applied a human ex vivo air-liquid interface model, a murine model of smoke exposure, and human lung tissues and investigated Zn, ZIP1, and ZIP2 Zn-influx proteins, autophagy [microtubule-associated 1A/1B-light chain-3 (LC3), Beclin-1], autophagic flux (Sequestosome), apoptosis [Bcl2; X-linked inhibitor of apoptosis (XIAP), poly (ADP)-ribose polymerase (PARP)], and inflammation [thymic stromal lymphopoietin (TSLP), regulated on activation, normal T cell expressed and secreted (RANTES), and IL-1β]. Lung tissues from CS-exposed mice exhibit reduced free-Zn in AECs, with elevated ZIP1 and diminished ZIP2 expression. Interestingly, increased LC3 colocalized with ZIP1, suggesting an autophagic requirement for free-Zn to support its catabolic function. In human AECs, autophagy was initiated but was unable to efficiently degrade cellular debris, as evidenced by stable Beclin-1 and increased LC3-II, but with a concomitant elevation in Sequestosome. Autophagic dysfunction due to CS exposure coupled with Zn depletion also induced apoptosis, with the reduction of antiapoptotic and antiautophagic proteins Bcl2 and XIAP and PARP cleavage. This was accompanied by an increase in RANTES and TSLP, an activator of adaptive immunity. We conclude that the uncoupling of Zn trafficking and autophagy in AECs constitutes a fundamental disease-related mechanism for COPD pathogenesis and could provide a new therapeutic target.

Keywords: COPD; apoptosis; autophagy; epithelium; inflammation; zinc.