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

Mammalian cells regulate iron levels tightly through the activity of iron-r egulatory proteins (IRPs) that bind to RNA motifs called iron-responsive el ements (IREs), When cells become iron-depleted, IRPs bind to IREs present i n the mRNAs of ferritin and the transferrin receptor, resulting in diminish ed translation of the ferritin mRNA and increased translation of the transf errin receptor mRNA. Likewise, intestinal epithelial cells regulate iron ab sorption by a process that also depends on the intracellular levels of iron . Although intestinal epithelial cells have an active IRE/IRP system, it ha s not been proven that this system is involved in the regulation of iron ab sorption in these cells. In this study, we characterized the effect of over expression of the ferritin IRE on iron absorption by Caco-2 cells, a model of intestinal epithelial cells. Cells overexpressing ferritin IRE had incre ased levels of ferritin, whereas the levels of the transferrin receptor wer e decreased. Iron absorption in IRE-transfected cells was deregulated: iron uptake from the apical medium was increased, but the capacity to retain th is newly incorporated iron diminished. Cells overexpressing IRE were not ab le to control iron absorption as a function of intracellular iron, because both iron-deficient cells as well as iron-loaded cells absorbed similarly h igh levels of iron. The labile iron pool of IRE-transfected cell was extrem ely low. Likewise, the reduction of the labile iron pool in control cells r esulted in cells having increased iron absorption. These results indicate t hat cells overexpressing IRE do not regulate iron absorption, an effect ass ociated with decreased levels of the regulatory iron pool.