Iron supply determines apical/basolateral membrane distribution of intestinal iron transporters DMT1 and ferroportin 1

Abstract

Núñez MT, Tapia V, Rojas A, Aguirre P, Gómez F, Nualart F. Iron supply determines apical/basolateral membrane distribution of intestinal iron transporters DMT1 and ferroportin 1. Am J Physiol Cell Physiol 298: C477-C485, 2010. First published December 9, 2009; doi:10.1152/ajpcell.00168.2009.-Intestinal iron absorption comprises the coordinated activity of the influx transporter divalent metal transporter 1 (DMT1) and the efflux transporter ferroportin (FPN). In this work, we studied the movement of DMT1 and FPN between cellular compartments as a function of iron supply. In rat duodenum, iron gavage resulted in the relocation of DMT1 to basal domains and the internalization of basolateral FPN. Considerable FPN was also found in apical domains. In Caco-2 cells, the apical-to-basal movement of cyan fluorescent protein-tagged DMT1 was complete 90 min after the addition of iron. Steady-state membrane localization studies in Caco-2 cells revealed that iron status determined the apical/ basolateral membrane distribution of DMT1 and FPN. In agreement with the membrane distribution of the transporters, 55 Fe flux experiments revealed inward and outward iron fluxes at both membrane domains. Antisense oligonucleotides targeted to DMT1 or FPN inhibited basolateral iron uptake and apical iron efflux, respectively, indicating the participation of DMT1 and FPN in these fluxes. The fluxes were regulated by the iron supply; increased iron reduced apical uptake and basal efflux and increased basal uptake and apical efflux. These findings suggest a novel mechanism of regulation of intestinal iron absorption based on inward and outward fluxes at both membrane domains, and repositioning of DMT1 and FPN between membrane and intracellular compartments as a function of iron supply. This mechanism should be complementary to those based in the transcriptional or translational regulation of iron transport proteins.intestinal iron absorption; divalent metal transporter 1; mucosal block IN THE ABSENCE OF A CONTROLLED excretion mechanism, iron levels in the body are regulated mainly by its passage through the duodenum epithelia. Traditionally, intestinal iron absorption is divided into three sequential steps: the uptake of iron from the intestinal lumen; an intracellular phase, in which iron binds to cytosolic components; and a transfer step, in which iron passes from the cells to the blood plasma. The uptake of iron from the lumen of the intestine is mediated by the Fe 2ϩ -H ϩ cotransporter divalent metal transporter 1 (DMT1) (19).Once inside the enterocyte, iron integrates into a cytosolic pool of weakly bound iron called the labile iron pool (LIP) (16,25). The nature of the LIP-binding counterpart is unknown, but it has been ascribed to diverse low-molecular-weight substances such as phosphate, nucleotides, hydroxyl, amino, and sulfydryl groups (23,36). From the LIP, iron distributes into ferritin and other iron-requiring proteins (15, 24). Iron exit from the enterocyte is mediated by the efflux transporter ferroportin (FPN), the only m...

Human iron transporters

“…Also yet to be well defined is the role of DMT1 (here shown with a ?) in possible basolateral uptake of iron noted in basolateral membrane vesicles [24] and recently supported [37]. Núñez et al also propose that Fpn can allow iron to exit back into the lumen of the GI tract (also shown here with a ?).…”

“…Critical experiments are still needed to learn if these explanations for DMT1 movement are reliable. The findings that departure of DMT1 from the apical membrane correlates with decreased apical flux in Caco-2 cells and that antisense to DMT1 inhibits this flux [37] both support the mucosal block as an explanation, but participation of DMT1 in transcytosis is not excluded by such data. Iron exit from enterocytes depends on the transporter Fpn plus oxidation of Fe 2?…”

“…Transcytosis is definitely the stage about which the least is known. Although there is evidence that DMT1 moves with the iron [32], Núñez' group [37] recently reminded us that such data can also account for a loss of iron uptake after an iron challenge, traditionally called the mucosal block [54]. Earlier, there were similar observations in Caco2 cells supporting this interpretation [46].…”

“…. We have already covered regulation of the export process [14] but call attention here to how repositioning of Fpn may help to accomplish this regulation even without turnover of the Fpn in the proteosome [37]. These cells recycle the iron to the marrow for reutilization in erythropoiesis.…”

DMT1 (IRE) expression in intestinal and erythroid cells is regulated by peripheral benzodiazepine receptor-associated protein 7

“…The exact mechanisms by which iron is transported by DMT1 are obscure. We, and others (22,30), have observed that in intestinal epithelial cells with iron feeding DMT1 is internalized from the BBM into vesicles within the subapical cytosolic space. It is not known whether the internalization of DMT1 is required for iron transport and/or is part of the iron uptake regulatory mechanism, although transcytosis of vesicles derived from the apical BBM and vesicles derived from the basolateral surface has been described.…”