Iron regulatory protein as an endogenous sensor of iron in rat intestinal mucosa. Possible implications for the regulation of iron absorption.

Abstract

Duodenal enterocytes adjust intestinal iron absorption to the body's state of iron repletion. Here we tested how iron supply from the blood modulates the RNA-binding activity of iron regulatory proteins (IRP-1 and IRP-2) in immature duodenal rat enterocytes, and whether the modulation is compatible with the hypothesis that IRPs, in turn, may regulate the expression of iron transport proteins in maturating enterocytes during migration to the villus tips. Tissue uptake of parenterally applied 59Fe along the duodenal crypt-villus axis was compared to local IRP-1 and IRP-2 activity and to duodenal 59Fe transport capacity 12 h, 48 h, and 72 h after intravenous iron administration to iron-deficient rats. IRP-1 and IRP-2 activity was significantly increased in iron-deficiency. 59Fe administrated from the blood side was almost exclusively taken up by crypt enterocytes. Accordingly, the activity of IRP-1 decreased at this site 12 h after parenteral iron administration, but remained high at the villus tips. After 48 h the bulk of 59Fe containing enterocytes had migrated to the villus tips. Correspondingly, IRP-1 activity was decreased at duodenal villus tips after 48 h. IRP-2 activity also tended to decrease, though the change was statistically not significant. IRP-2 activity remained significantly higher at duodenal villus tips than in crypts, even after 72 h. Intestinal iron absorption capacity decreased with the same delay as IRP-1 activity after intravenous iron administration. In the ileum 59Fe uptake from the blood and IRP activity showed no significant difference between crypt and villus region. Luminal administration of iron decreased duodenal IRP-1 and IRP-2 activity at tips and crypts within 2 h. Thus, recently absorbed iron becomes available to cytosolic IRP during its passage through the enterocyte. Our results are compatible with a role of IRPs in gearing the expression of intestinal iron transporters in the duodenal brushborder to the body's state of iron repletion.