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

Abstract

Mammalian cells regulate iron levels tightly through the activity of iron-regulatory proteins (IRPs) that bind to RNA motifs called iron-responsive elements (IREs).When cells become iron-depleted, IRPs bind to IREs present in the mRNAs of ferritin and the transferrin receptor, resulting in diminished translation of the ferritin mRNA and increased translation of the transferrin receptor mRNA. Likewise, intestinal epithelial cells regulate iron absorption by a process that also depends on the intracellular levels of iron. Although intestinal epithelial cells have an active IRE/IRP system, it has not been proven that this system is involved in the regulation of iron absorption in these cells. In this study, we characterized the effect of overexpression of the ferritin IRE on iron absorption by Caco-2 cells, a model of intestinal epithelial cells. Cells overexpressing ferritin IRE had increased levels of ferritin, whereas the levels of the transferrin receptor were decreased. Iron absorption in IRE-transfected cells was deregulated: iron uptake from the apical medium was increased, but the capacity to retain this newly incorporated iron diminished. Cells overexpressing IRE were not able to control iron absorption as a function of intracellular iron, because both iron-deficient cells as well as iron-loaded cells absorbed similarly high levels of iron. The labile iron pool of IREtransfected cell was extremely low. Likewise, the reduction of the labile iron pool in control cells resulted in cells having increased iron absorption. These results indicate that cells overexpressing IRE do not regulate iron absorption, an effect associated with decreased levels of the regulatory iron pool.

Ferrous bisglycinate increased iron transportation through DMT1 and PepT1 in pig intestinal epithelial cells compared with ferrous sulphate

“…Besides DMT1, TfR and ferritin also play important roles in iron absorption in the intestine. DMT1 has two functional isomers, one of which contains a single iron reaction element (IRE) in 3′-UTR and is bound by IRP (iron-regulatory proteins) in vitro; the other isomer lacks 3′-UTR (Gárate and Núñez, 2000). Iron absorption is dependent on iron concentrations in the epithelial cells of the intestine (Ludwiczek et al, 2004).…”

“…It has been recognized that a high concentration of iron could reduce the combination of IRP/IRE, thus reducing the stability of DMT1 mRNA and further, affecting iron absorption. When iron is depleted in cells, IRPs inhibit ferritin expression and induce TfR expression (Gárate and Núñez, 2000). In our study, mRNA expression of TfR was significantly higher in the ferrous bisglycinate groups than in the ferrous sulphate ones when the treatment concentrations were 0.5 and 1 mmol · l -1 , and treatment time was 24 h, implying that the iron concentration within IPEC cells was higher in the ferrous bisglycinate groups, which could be a result of more iron being transported from outside into IPEC cells via DMT1.…”

Increased IRP1 and IRP2 RNA binding activity accompanies a reduction of the labile iron pool in HFE‐expressing cells*

“…The transfer of iron across the duodenal epithelium is inversely proportional to the relative iron repletion of an organism (reviewed in Bothwell et al, 1995). Increasing evidence points to the involvement of IRP machinery, or at the very least the labile iron pool, in the regulation of dietary iron uptake and transfer across the enterocyte (Schumann et al, 1999;Garate and Nunez, 2000). The speci®c effects of IRPs on enterocyte proteins of iron transport and homeostasis are just beginning to be investigated (Wardrop and Richardson, 1999;Abboud and Haile, 2000;Donovan et al, 2000;McKie et al, 2000).…”

Ferritin Heavy Chain–Mediated Iron Homeostasis and Subsequent Increased Reactive Oxygen Species Production Are Essential for Epithelial-Mesenchymal Transition

“…10). Repression of FHC leads to iron release from the ferritin complex and an increase in the cellular LIP (11,12), whereas overexpression of FHC can decrease LIP (13,14). LIP is a pool of chelatable and redox active iron (10).…”