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1 Departments of Cellular and Structural Biology, The University of Texas Health Science Center, San Antonio, Texas, USA
* To whom correspondence should be addressed. E-mail: ghio.andy{at}epa.gov.
Ferroportin 1 (FPN1; aka MTP1, IREG1 and SLC40A1), which was originally identified as a basolateral iron transporter crucial for nutritional iron absorption in the intestine, is expressed in airway epithelia and upregulated when these cells are exposed to iron. Using immunofluoresence labeling and confocal microscopic imaging techniques, we demonstrate that in human and rodent lungs, FPN1 localizes subcellularly to the apical but not basolateral membrane of the airway epithelial cells. The role of airway epithelial cells in iron mobilization in the lung was studied using an in vitro model of the polarized airway epithelium. Normal human bronchial epithelial cells, grown on membrane supports until differentiated, were exposed to iron and the efficiency and direction of iron transportation were studied. We found that these cells can efficiently take up iron across the apical but not basolateral surface in a concentration-dependent manner. Most of the iron taken up by the cells is then released into the medium within 8 hours in the form of less reactive protein-bound complexes including ferritin and transferrin. Interestingly, iron release also occurred across the apical but not basolateral membrane. Our findings indicate that FPN1, depending on its subcellular location, could have distinct functions in iron homeostasis in different cells and tissues. While it is responsible for exporting nutrient iron from enterocytes to the circulation in the intestine, it likely plays a key role in iron detoxification in airway epithelial cells in the lung.
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