Mechanisms of iron mediated regulation of the duodenal iron transporters divalent metal transporter 1 and ferroportin 1.

Intestinal iron absorption is regulated by the body's demands for iron. Identification of divalent metal transporter 1 (DMT1) and ferroportin 1 (FPN1) has improved our understanding of iron transport across the intestinal epithelium. Although DMT1 and FPN1 mRNA bear an iron responsive element (IRE) within its untranslated regions which should cause susceptibility to iron mediated posttranscriptional regulation the latter has not been shown so far. The effects of iron perturbations on DMT1 and FPN1 expression were investigated in CaCo2 cells and in primary tissue cultures of human duodenal biopsies by means of Northern Blot, Western Blot, RNA-bandshift and Nuclear Run off analysis. Both DMT1 and FPN1 mRNA levels were increased upon treatment of CaCo2 cells with desferrioxamine, whereas iron treatment resulted in the opposite effect. These changes were paralleled by the respective alterations in DMT1 and FPN1 protein expression. Although desferrioxamine treatment increased the binding affinity of iron regulatory protein-1 to DMT1- and FPN1-IRE, the mRNA half life of DMT1 mRNA remained unchanged. Nuclear run-off analysis then demonstrated that the effects of iron and desferrioxamine on DMT1 and FPN1 mRNA expression are rather due to modulation of transcription of these genes. Our results demonstrate that iron unidirectionally regulates the expression of the two ferrous ion transporters DMT1 and FPN1 by affecting their transcription. This provides evidence for a negative feed-back loop between intracellular iron availability and transmembrane iron transport.