Because humans are unable to excrete excess iron, iron balance is physiologically maintained by the control of iron absorption in the proximal portion of the duodenum. Iron overload develops if regulation of iron balance is bypassed by parenteral injections of iron or transfusion. Normally, only about 1 to 1.5 mg of iron of the 10 to 20 mg in the adult diet is absorbed to balance obligatory losses. Both nonheme iron and heme iron enter through the microvillous brush border at the apical (luminal) surface of the intestinal enterocytes (Fig. 1). Nonheme dietary iron is predominantly ferric (Fe3+) and, before absorption, is converted to ferrous (Fe2+) iron either by the reducing action of ascorbate or by the action of brush border ferrireductases, such as membrane-associated duodenal cytochrome B (DCYTB) and likely others. The ferrous iron is then absorbed through DMT1, the same ferrous iron transporter that provides an exit for iron from the endosome (see earlier). To transport ferrous iron, DMT1 requires an inward proton gradient that is generated by the mucosa-acidifying sodium/proton exchangers NHE2 and NHE3.
Fig1. ABSORPTION OF DIETARY IRON BY THE DUODENAL ENTEROCYTE. In the gastrointestinal lumen, dietary iron is presented to the enterocyte as heme or nonheme iron. Heme iron uptake is not well characterized, and the specific membrane transporter remains uncertain. After absorption, heme oxygenase 1 (HO-1) releases iron from heme into a common cytosolic pool. Nonheme dietary iron is predominantly ferric (Fe3+) and, before absorption, is converted to ferrous (Fe2+) iron either by the reducing action of other dietary constituents or by the action of brush border ferrireductases, such as membrane-associated duodenal cytochrome B (DCYTB) and likely others. Ferrous iron is then transported across the apical membrane by the divalent metal transporter 1 (DMT1) into the common cytosolic iron pool. Iron is delivered to ferroportin by a chaperone (likely poly(rC)-binding protein 2 [PCB2]), exported through ferroportin, regulated by hepcidin, with hephaestin or circulating ceruloplasmin acting as ferrioxidases that convert Fe2+ to Fe3+ before loading it onto transferrin (Tf). Cytosolic iron in excess of systemic needs may be carried to ferritin by the cytosolic iron chaperone poly(rC)-binding protein 1 (PCBP1), retained, and then lost when the enterocyte is shed. In addition to regulation by hepcidin, enterocyte iron absorption is modulated by hypoxia-inducible factor 2α (HIF-2α) and the iron-regulatory proteins (IRP1 and IRP2). See text for details. Fe2Tf, Diferric transferrin. (Modified with permission from Anderson GJ, Frazer DM, McLaren GD. Iron absorption and metabolism. Curr Opin Gastroenterol. 2009;25:129.)
The exact means by which heme iron is absorbed are still uncertain, but when inside the enterocyte, inducible heme oxygenase 1 releases the iron from protoporphyrin, apparently into a common pathway with absorbed dietary nonheme iron. In the enterocyte cytosol, the iron can be (1) retained for cellular requirements or stored in cytosolic ferritin and then lost when the enterocyte is exfoliated or (2) exported through ferroportin on the enterocyte basolateral membrane. Iron export through ferroportin requires oxidation by membrane-bound hephaestin or circulating ceruloplasmin to the ferric form for binding by plasma transferrin. Control of duodenal iron uptake is intricate,30 depending on both systemic factors (hepcidin control of ferroportin) and local modulation of iron absorption through transcriptional (HIF-2α) and posttranscriptional (by the iron-regulatory protein/ iron-responsive element system) mechanisms. Expression of FPN1B, which lacks the iron-responsive element in its 5′ untranslated region, allows enterocytes to bypass iron-regulatory protein repression of ferroportin iron export even when cells throughout the body are iron-deficient.
