Duodenal cytochrome b (Dcytb; encoded by the Cybrd1 gene), a ferric reductase expressed at the brush border of duodenal enterocytes,1 has been proposed to reduce dietary ferric iron, thereby facilitating its transport into the mucosal cells by the ferrous iron transporter divalent metal transporter 1. To define the role of Dcytb in intestinal iron absorption, Gunshin et al2 have taken the important step of generating a Dcytb knockout mouse. From their analysis of the hepatic iron levels of these animals, they concluded that Dcytb is not necessary for dietary iron absorption in mice. However, this conclusion should be interpreted with caution, as no direct measurements of iron absorption were made and the reliance on liver iron levels does not provide unequivocal evidence for or against an absorption defect. The authors show that Cybrd1-/- mice maintained on standard rodent chow have hepatic iron levels similar to wild-type mice, and at face value this would suggest that Dcytb plays no role in iron absorption under normal conditions. However, the diet used for these studies (Prolab RMH 3000 LabDiet; PMI Richmond, Richmond, IN) contains a large amount of iron (380 mg/kg), including added ferrous iron, and thus both wild-type and Cybrd1-/- mice would likely absorb comparable quantities of iron as the ferric iron reduction step is bypassed. Thus the results obtained are not unexpected. To adequately define the role of Dcytb, direct iron absorption studies should be carried out or the mice should be maintained on a diet containing ferric iron only. The former approach is preferable.
In the second part of the study, animals were maintained on an iron-deficient diet to determine whether Dcytb plays a role in absorption when iron is limiting. Liver iron content was measured and found to be similar in both Cybrd1-/- mice and controls. As iron absorption was not measured, this experiment gives no information about the role of Dcytb in absorption and simply shows that liver iron stores in both strains of mice decrease when the amount of iron in the diet is low. To demonstrate this point, we present comparable data from the sla mouse. These animals carry a deletion in the Heph gene (which encodes the ferroxidase hephaestin)3 and have defective basolateral export of iron from intestinal enterocytes.4 When sla mice and control animals are placed on an iron-deficient diet for 6 weeks there is no difference in their liver iron content (Figure 1), similar to the results presented for the Cybrd1-/- mouse. However, when intestinal iron absorption was measured in these animals we found that sla mice had a significantly lower absorption than control mice (Figure 1). This clearly shows that liver iron levels cannot be used as a surrogate marker of iron absorption under these conditions. We believe that the question of whether Dcytb plays a role in iron absorption remains unresolved. Only direct measurements of iron absorption using radioactive ferric iron in knockout and control animals, maintained on both standard and iron-deficient diets, will answer this question definitively.
Supported in part by grants from the National Health and Medical Research Council of Australia; the National Institute of Diabetes, Digestive Diseases, and Kidney (DK57648); and the Human Frontier Science Program (RGY0328/2001-M).
References
- 1.McKie AT, Barrow D, Latunde-Dada GO, et al. An iron-regulated ferric reductase associated with the absorption of dietary iron. Science. 2001;291: 1755-1759. [DOI] [PubMed] [Google Scholar]
- 2.Gunshin H, Starr CN, DiRenzo C, et al. Cybrd1 (duodenal cytochrome b) is not necessary for dietary iron absorption in mice. Blood. 2005;106: 2879-2883. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Vulpe CD, Kuo YM, Murphy TL, et al. Hephaestin, a ceruloplasmin homologue implicated in intestinal iron transport, is defective in the sla mouse. Nat Genet. 1999;21: 195-199. [DOI] [PubMed] [Google Scholar]
- 4.Pinkerton PH, Bannerman RM. Hereditary defect in iron absorption in mice. Nature. 1967;216: 482-483. [DOI] [PubMed] [Google Scholar]
- 5.Frazer DM, Wilkins SJ, Becker EM, et al. Hepcidin expression inversely correlates with the expression of duodenal iron transporters and iron absorption in rats. Gastroenterology. 2002;123: 835-844. [DOI] [PubMed] [Google Scholar]
- 6.Torrence JD, Bothwell TH. A simple technique for measuring storage iron concentrations in formalinised liver samples. S Afr J Med Sci. 1968;33: 9-11. [PubMed] [Google Scholar]