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.
Intestinal iron absorption and liver iron content in sla and C57BL/6J mice on an iron-deficient diet. Animals were maintained on an iron-deficient diet for 6 weeks from weaning, and iron absorption was determined by whole-body counting following the oral administration of radioactive iron as previously described.5 Absorption (▪) is presented as the percentage of the initial iron dose retained by the animals 5 days after dosing. Hepatic iron measurements (□) were performed on liver tissue as previously described.6 Data represent the mean ± SEM of 3 animals. Statistical differences were assessed using Student t test. P = .001 for iron absorption. P = .86 for liver iron content.
Intestinal iron absorption and liver iron content in sla and C57BL/6J mice on an iron-deficient diet. Animals were maintained on an iron-deficient diet for 6 weeks from weaning, and iron absorption was determined by whole-body counting following the oral administration of radioactive iron as previously described.5 Absorption (▪) is presented as the percentage of the initial iron dose retained by the animals 5 days after dosing. Hepatic iron measurements (□) were performed on liver tissue as previously described.6 Data represent the mean ± SEM of 3 animals. Statistical differences were assessed using Student t test. P = .001 for iron absorption. P = .86 for liver iron content.
Cybrd1 is not essential in mice
We appreciate the interest that Frazer and colleagues have shown in our work. However, in contrast to their interpretation, the purpose of our initial paper1 was not to define the possible role of Cybrd1 (duodenal cytochrome b [Dcytb]) in intestinal iron absorption but rather to determine whether it is essential for the procurement of iron for utilization and storage in vivo. Our results clearly showed that it is not essential in129S6/SvEvTac mice, whether they were fed a standard lab diet or an iron-deficient diet.
Frazer and his coworkers are correct that our standard chow contains ferrous iron, possibly eliminating the need for an enzymatic ferric reductase. However, as we reported,1 animals lacking Cybrd1 maintained iron stores comparable to wild-type mice after 8 weeks on an iron-deficient diet. In unpublished studies, we continued Cybrd1-/- mice on the iron-deficient diet for 6 months. Even under these conditions, their tissue iron stores and hematologic parameters were indistinguishable from wild type. Clearly, Cybrd1 is not essential for viability, for erythroid iron assimilation, or for maintenance of liver iron stores. While our data do not rule out a defect in intestinal iron absorption in Cybrd1-/- mice, they argue against a major role for Cybrd1 in vivo. For comparison, mice lacking the iron transporter Slc11a2 (divalent metal transporter 1 [DMT1]) in the intestine show a very severe reduction in liver iron stores and profound anemia that is undoubtedly due to a failure of intestinal iron absorption.2
We have not yet attempted to address whether Cybrd1 facilitates intestinal iron absorption. Other ferric reductases have been identified recently,3 raising the possibility that there may be functional redundancy in dietary iron reduction. Future experiments should answer these questions.
Correspondence: Nancy C. Andrews, Children's Hospital Boston, 300 Longwood Ave, Boston, MA 02115; e-mail: nandrews@enders.tch.harvard.edu
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).
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