Knockdown of copper-transporting ATPase 1 (Atp7a) impairs iron flux in fully-differentiated rat (IEC-6) and human (Caco-2) intestinal epithelial cells

Abstract

Intestinal iron absorption is highly regulated since no mechanism for iron excretion exists. We previously demonstrated that expression of an intestinal copper transporter (Atp7a) increases in parallel with genes encoding iron transporters in the rat duodenal epithelium during iron deprivation (Am. J. Physiol.: Gastrointest. Liver Physiol., 2005, 288, G964–G971). This led us to postulate that Atp7a may influence intestinal iron flux. Therefore, to test the hypothesis that Atp7a is required for optimal iron transport, we silenced Atp7a in rat IEC-6 and human Caco-2 cells. Iron transport was subsequently quantified in fully-differentiated cells plated on collagen-coated, transwell inserts. Interestingly, ⁵⁹Fe uptake and efflux were impaired in both cell lines by Atp7a silencing. Concurrent changes in the expression of key iron transport-related genes were also noted in IEC-6 cells. Expression of Dmt1 (the iron importer), Dcytb (an apical membrane ferrireductase) and Fpn1 (the iron exporter) was decreased in Atp7a knockdown (KD) cells. Paradoxically, cell-surface ferrireductase activity increased (>5-fold) in Atp7a KD cells despite decreased Dcytb mRNA expression. Moreover, increased expression (>10-fold) of hephaestin (an iron oxidase involved in iron efflux) was associated with increased ferroxidase activity in KD cells. Increases in ferrireductase and ferroxidase activity may be compensatory responses to increase iron flux. In summary, in these reductionist models of the mammalian intestinal epithelium, Atp7a KD altered expression of iron transporters and impaired iron flux. Since Atp7a is a copper transporter, it is a logical supposition that perturbations in intracellular copper homeostasis underlie the noted biologic changes in these cell lines.