Transferrin receptor activity and localisation in the rat duodenum.

It is not known how the efficiency of intestinal iron absorption is regulated. One hypothesis suggests that an interaction between the transferrin receptor (TfR) and the haemochromatosis protein (HFE) regulates the level of iron loading in crypt cells. The hypothesis goes on to suggest that this determines the amount of transport protein, expressed in villus enterocytes, that is involved in iron absorption. Mice with haploinsufficiency for TfR are iron deficient and this is thought to be caused by reduced iron absorption. This suggests that TfR may play a role in the regulation and/or mechanism of iron absorption. We investigated TfR function and distribution by measuring iron uptake from plasma transferrin and by immunohistochemistry. The uptake of transferrin-bound iron (Tf-Fe2) into mucosal cells subsequently separated along the crypt-villus axis was compared to the presence of TfR, determined by immunohistochemistry using frozen and wax sections. Frozen sections showed TfR staining in crypt and villus epithelial cells. In wax sections TfR was only identified in a supranuclear region commencing in enterocytes at the crypt-villus junction and attaining greatest levels at the villus tip. This indicates that the processing of wax tissue exposes a TfR epitope that otherwise remains undetectable when studied in frozen sections. This appearance in paraffin sections was inversely related to the uptake of Tf-Fe2. Supranuclear TfR was not associated with lysosomes, since there was no difference in the uptake of normal Tf-Fe2 and that of the non-digestible cellobiose Tf-Fe2, and Western blot analysis revealed similar amounts of TfR in crypt and villus cells. Also the uptake of Tf-Fe2 increased linearly with time, albeit less in villus than crypt cells, suggesting that maturation of an efflux system in villus cells is not responsible for this difference. We hypothesise that TfR in the supranuclear region of villus enterocytes may play a role in iron absorption.