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1 Medicine, The Davis Heart and Lung Research Institute, Columbus, Ohio, United States; Pharmacy, The Ohio State University, Columbus, Ohio, United States
2 Medicine, The Davis Heart and Lung Research Institute, Columbus, Ohio, United States
* To whom correspondence should be addressed. E-mail: daren.knoell{at}osumc.edu.
Apoptosis plays a causative role in acute lung injury in part due to epithelial cell loss. We recently reported that zinc protects the lung epithelium during inflammatory stress whereas depletion of intracellular zinc enhances extrinsic apoptosis. In this investigation, we evaluated the relationship between zinc, caspase-3, and cell-to-cell contact via proteins that form the adherens junction complex. Cell-adhesion proteins are directly responsible for formation of the mechanical barrier of the lung epithelium. We hypothesized that exposure to inflammatory cytokines in conjunction with zinc deprivation, would induce caspase-3 leading to degradation of junction proteins, loss of cell-to-cell contact, and compromised barrier function. Primary human upper airway and Type I/II alveolar epithelial cultures were obtained from multiple donors and exposed to inflammatory stimuli that provoke extrinsic apoptosis, in addition to depletion of intracellular zinc. We observed that zinc deprivation combined with tumor necrosis factor alpha (TNF), interferon gamma (IFN), and Fas receptor ligation accelerates caspase-3 activation, proteolysis of E-cadherin and -catenin, and cellular apoptosis, leading to increased paracellular leak across monolayers of both upper airway and alveolar lung epithelial cultures. Zinc supplementation inhibited apoptosis and paracellular leak whereas, caspase inhibition was less effective. We conclude that zinc is a vital factor in the lung epithelium that protects against death receptor-mediated apoptosis and barrier dysfunction. Further, our findings suggest that although caspase-3 inhibition reduces lung epithelial apoptosis, it does not prevent mechanical dysfunction. These findings facilitate future studies aimed at developing therapeutic strategies to prevent acute lung injury.
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