Novel cellular defenses against iron and oxidation: Ferritin and autophagocytosis preserve lysosomal stability in airway epithelium

Abstract

Adsorbed to a variety of particles, iron may be carried to the lungs by inhalation thereby contributing to a number of inflammatory lung disorders. Redox-active iron is a potent catalyst of oxidative processes, but intracellularly it is bound primarily to ferritin in a non-reactive form and probably is catalytically active largely within the lysosomal compartment. Damage to the membranes of these organelles causes the release to the cytosol of a host of powerful hydrolytic enzymes, inducing apoptotic or necrotic cell death. The results of this study, using cultured BEAS-2B cells, which are adenovirus transformed human bronchial epithelial cells, and A549 cells, which have characteristics similar to type II alveolar epithelial cells, suggest that the varying abilities of different types of lung cells to resist oxidative stress may be due to differences in intralysosomal iron chelation. Cellular ferritin and iron were assayed by ELISA and atomic absorption, while plasma and lysosomal membrane stability were evaluated by the acridine orange uptake and trypan blue dye exclusion tests, respectively. Normally, and also after exposure to an iron complex, A549 cells contained significantly more ferritin (2.26 ± 0.60 versus 0.63 ± 0.33 ng/μg protein, P <0.001) and less iron (0.96 ± 0.14 versus 1.48 ± 0.21 ng/μg protein, P <0.05) than did BEAS-2B cells. Probably as a consequence, iron-exposed A549 cells displayed more stable lysosomes (P <0.05) and better survival (P <0.05) following oxidative stress. Following starvation-induced autophagocytosis, which also enhances resistance to oxidant stress, the A549 cells showed a significant reduction in ferritin, and the BEAS-2B cells did not. These results suggest that intralysosomal ferritin enhances lysosomal stability by iron-chelation, preventing Fenton-type chemistry. This notion was further supported by the finding that endocytosis of apoferritin, added to the medium, stabilized lysosomes (P <0.001 versus P <0.01) and increased survival (P <0.01 versus P <0.05) of iron-loaded A549 and BEAS-2B cells. Assuming that primary cell lines of the alveolar and bronchial epithelium behave in a similar manner as these respiratory cell lines, intrabronchial instillation of apoferritin-containing liposomes may in the future be a treatment for iron-dependent airway inflammatory processes.