Regulation of Fe absorption by cultured intestinal epithelia (Caco-2) cell monolayers with varied Fe status

Abstract

Body Fe homeostasis is maintained through the regulation of Fe absorption by the intestinal epithelia. Working under the hypothesis that the intracellular concentration of Fe is instrumental in the control of its transepithelial flux, we investigated in vitro which steps in Fe absorption are regulated by cellular Fe content. For that study, Caco-2 cells containing different concentrations of intracellular 55Fe were grown in porous filters, and the apical-to-cell-to- basolateral flux of 59Fe was then determined. We found that 1) at low (up to 0.1 mM) intracellular Fe content the apical-to-basal Fe transport was primarily regulated by a decrease in apical Fe uptake (first stage of regulation), 2) at higher levels of intracellular Fe (0.1-1 mM) the transepithelial Fe flux was regulated by intracellular factors that sequester most of the Fe taken up at the apical surface (second stage of regulation), and 3) a fraction of the apical-to-basolateral Fe flux was not regulated by the intracellular concentration of Fe. Ferritin synthesis preceded the onset of the second stage of regulation, suggesting a causal relationship between intracellular Fe levels, ferritin levels, and regulation of Fe absorption.

DMT1, a physiologically relevant apical Cu1+transporter of intestinal cells

“…Interestingly, equimolar concentrations of Fe 2ϩ , in the absence of ascorbate, afforded a considerable reduction of Cu(II)-histidine, as observed by the (1), we evaluated copper transport in cells with varied levels of iron. As previously reported (30), 59 Fe uptake in Caco-2 cells inversely correlated with intracellular iron concentration (Fig. 4A).…”

“…They express high levels of GLUT5, a glucose transporter found in the brush border of fetal and adult small intestine (20). Moreover, they express transferrin receptors in the basolateral membrane and the DMT1 transporter in the apical membrane, and they have an active IRE/IRP (iron-responsive element/iron regulatory protein) system that regulates apical iron uptake and transepithelial iron transport as a function of intracellular iron levels (2,16,30).…”

Overexpression of the Ferritin Iron-responsive Element Decreases the Labile Iron Pool and Abolishes the Regulation of Iron Absorption by Intestinal Epithelial (Caco-2) Cells

“…Equilibrium Loading of Caco-2 Cells with 55 Fe-Cells with known concentrations of intracellular iron were obtained as described (3 treated and seeded at a density of 1 ϫ 10 5 cells/flask and cultured as above. After a second trypsin treatment, the cells were plated on 1-cm 2 polycarbonate inserts (Transwell, COSTAR, Cambridge, MA) and were cultured in media as before for 12-14 days, with change of media every 3 or 4 days.…”

“…Intestinal epithelial cells respond to a fall in body iron stores by increasing the absorption of dietary iron, so the extent of iron transport through the intestinal epithelium is inversely related to the content of body iron stores (1)(2)(3). Knowledge of the molecular mechanisms involved in the regulation of transepithelial iron transport through the intestine remains elusive, in part because of the heterogeneity in age and iron content of intestinal cells.…”

“…In particular, Caco-2 cells have been described as an excellent in vitro model of human enterocytes (4,5). Caco-2 cells grown on bicameral inserts exhibit both apical iron uptake (3,4) and transferrin-mediated basolateral iron uptake (6). Caco-2 cells reduce Fe 3ϩ to Fe 2ϩ in the apical medium, and this reduction correlates with increased iron uptake (7,8).…”