IRON ABSORPTION: THE EFFECT OF AN IRON-DEFICIENT DIET

[Advances in iron metabolism: a transition state]

PURPOSE

Advances towards the understanding of gene regulation and protein function recently discovered through iron metabolism disorders are the subject of this review.

CURRENT KNOWLEDGE AND KEY POINTS

Within a few years the discovery of genes that determine heritable defects of cellular iron uptake or regulation in mice as in humans have provided new insights for investigation into iron metabolism pathways.

FUTURE PROSPECTS AND PROJECTS

It is still unclear how connections are made between new proteins in iron uptake, trafficking and regulation of iron homeostasis. Gene expression studies using microarrays technology in different iron conditions should help to explore iron homeostasis further.

Co-localization of the mammalian hemochromatosis gene product (HFE) and a newly identified transferrin receptor (TfR2) in intestinal tissue and cells

Mutations in the HFE gene and a newly identified second transferrin receptor gene, TfR2, cause hemochromatosis. The cognate proteins, HFE and TfR2, are therefore of key importance in human iron homeostasis. HFE is expressed in small intestinal crypt cells where transferrin-iron entry may determine subsequent iron absorption by mature enterocytes, but the physiological function of TfR2 is unknown. Using specific peptide antisera, we examined the duodenal localization of HFE and TfR2 in humans and mice, with and without HFE deficiency, by confocal microscopy. We also investigated potential interactions of these proteins in human intestinal cells in situ. Duodenal expression of HFE and TfR2 (but not TfR1) in wild-type mice and humans was restricted to crypt cells, in which they co-localized. HFE deficiency disrupted this interaction, altering the cellular distribution of TfR2 in human crypts. In human Caco-2 cells, HFE and TfR2 co-localized to a distinct CD63-negative vesicular compartment showing marked signal enhancement on exposure to iron-saturated transferrin ligand, indicating that HFE preferentially interacts with TfR2 in a specialized early endosomal transport pathway for transferrin-iron. This interaction occurs specifically in small intestinal crypt cells that differentiate to become iron-absorbing enterocytes. Our immunohistochemical findings provide evidence for a novel mechanism for the regulation of iron balance in mammals.

Hepcidin expression inversely correlates with the expression of duodenal iron transporters and iron absorption in rats

BACKGROUND & AIMS

Hepcidin is an antimicrobial peptide thought to be involved in the regulation of intestinal iron absorption. To further investigate its role in this process, we examined hepatic and duodenal gene expression in rats after the switch from a control diet to an iron-deficient diet.

METHODS

Adult rats on an iron-replete diet were switched to an iron-deficient diet and the expression of iron homeostasis molecules in duodenal and liver tissue was studied over 14 days. Intestinal iron absorption was determined at these same time-points by measuring the retention of an oral dose of (59)Fe.

RESULTS

Iron absorption increased 2.7-fold within 6 days of switching to an iron-deficient diet and was accompanied by an increase in the duodenal expression of Dcytb, divalent metal transporter 1, and Ireg1. These changes precisely correlated with decreases in hepatic hepcidin expression and transferrin saturation. No change in iron stores or hematologic parameters was detected.

CONCLUSIONS

This study showed a close relationship between the expression of hepcidin, duodenal iron transporters, and iron absorption. Both hepcidin expression and iron absorption can be regulated before iron stores and erythropoiesis are affected, and transferrin saturation may signal such changes.

Iron absorption in rats increased by yeast glucan

The effects of brewer's yeast cell walls and two of its components, glucan and mannan, on the absorption of 59Fe by anemic rats were investigated. After administration of the label, the percentage of 59Fe taken up into the blood of group given glucan was generally similar to that of a group given yeast cell walls, both values were higher than in controls. The incorporation of 59Fe into the small intestines was higher in the group given glucan than in the controls or a group given a glucan-mannan mixture. Glucan is the main substance in yeast cell walls that increases iron absorption.

Molecular aspects of iron absorption and HFE expression

Hereditary hemochromatosis, a disease of iron overload, occurs in about 1 in 200-400 Caucasians. The gene mutated in this disorder is termed HFE. The product of this gene, HFE protein, is homologous to major histocompatibility complex class I proteins, but HFE does not present peptides to T cells. Based on recent structural, biochemical, and cell biological studies, transferrin receptor (TfR) is a ligand for HFE. This association directly links HFE protein to the TfR-mediated regulation of iron homeostasis. Although evidence is accumulating that binding of HFE to TfR is critical for the effects of HFE, the final pieces in the HFE puzzle have not been established. This review focuses on recent advances in HFE research and presents a hypothetical model of HFE function.

Regulation of intracellular iron levels in iron-acceptor and iron-donor cells

In recent years many new genes and proteins were identified with crucial functions in iron metabolism. This gave an explosion of our knowledge and understanding of iron related disorders. Mutations have been found that are responsible for disturbances in iron transport, leading to either iron overload or iron deficiency. For experts in the field, these new findings clarify the sky and open new routes for exploring hitherto hidden fields of research. For the physician, however, iron metabolism may become even more complicated. In this review, we have tried to assemble all new iron related genes into the context of pathophysiology. Important results from animal experiments, mainly derived from knockout mouse models, are included in this review as they often explain the phenotype of human disease.

Differential expression of transferrin receptor in duodenal mucosa in iron overload. Evidence for a site-specific defect in genetic hemochromatosis

In genetic hemochromatosis, metabolic studies have demonstrated inappropriately increased iron absorption by cells of the duodenal mucosa. It is not clear whether this reflects an intrinsic abnormality of iron homeostasis at this site or is a consequence of a more generalized defect in cellular iron metabolism particularly involving the liver. We have previously used the expression of iron-related proteins as markers of iron homeostasis and have demonstrated normal regulation of the transferrin receptor and ferritin in the liver in this condition. In the present study we used immunohistochemical techniques to study transferrin-receptor expression in the gastrointestinal epithelium in normal subjects and patients with iron overload. In untreated genetic hemochromatosis and normal subjects, villus epithelial cells expressed receptor in the basolateral, subnuclear region. In contrast, in patients with secondary iron overload, receptor staining was absent in villus epithelial cells. The cells in the duodenal crypts showed intense staining for the transferrin receptor in all subjects investigated, a finding consistent with the known behavior of this receptor in proliferating cells. Given that body iron stores in both types of iron overload were comparable, these findings indicating a failure of down-regulation of the villus enterocyte transferrin receptor in genetic hemochromatosis may reflect the presence of a regulatory defect associated with the inability to control iron absorption in this condition.

The influence of previous iron intake on the estimation of bioavailability of Fe from a test meal given to rats

Iron retention and its subsequent distribution from 3 g 59Fe extrinsically-labelled wholewheat four in rats given a low (8 micrograms Fe/g)- or high (566 micrograms Fe/g)-Fe diet during the previous 3 d was measured (Expt 1). The mean (with SE) proportion of Fe retained from wholewheat flour was 0.46 (0.04) in the group given the low-Fe diet and 0.15 (0.01) in the group given the high-Fe diet (P less than 0.001). There was no difference in distribution of absorbed 59Fe in the tissues examined. The procedure was repeated in rats given diets containing a range of Fe concentrations, groups 1-6 respectively: 8, 77, 136, 334, 566, 1270 micrograms Fe/g (Expt 2). The mean (with SE) proportions of Fe retained in groups 1-6 respectively were 0.60 (0.02), 0.34 (0.02), 0.30 (0.02), 0.20 (0.20), 0.17 (0.02), 0.09 (0.01). Regression analysis showed that Fe retention was a function of the logarithm of the Fe concentration of the diet consumed before the test meal (R -0.997, P less than 0.0001) where Fe retained (microgram) = 95 -28 log10 Fe concentration of diet. Rats were given a low-, medium- or high-Fe diet (8, 136 or 1270 micrograms Fe/g respectively) for 1 or 2 d instead of 3 d before measuring Fe retention from 3 g wholewheat flour (Expt 3). The mean (with SE) proportions of Fe retained in rats given the low-, medium- or high-Fe diets for 1 d were 0.45 (0.02), 0.25 (0.02) and 0.13 (0.01) (P less than 0.001) and for 2 d 0.47 (0.03), 0.31 (0.03) and 0.18 (0.02) (P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution and metabolism of iron in muscles of iron-deficient rats

Iron-deficiency anemia leads directly to both reduced hemoglobin levels and work performance in humans and experimental animals. In an attempt to observe a direct link between work performance and insufficient iron at the cellular level, we produced severe iron deficiency in female weanling Sprague-Dawley rats following five weeks on a low-iron diet. Deficient rats were compared with normal animals to observe major changes in hematological parameters, body weight, and growth of certain organs and tissues. The overall growth of iron-deficient animals was approximately 50% of normal. The ratio of organ weight: body weight increased in heart, liver, spleen, kidney, brain, and soleus muscle in response to iron deficiency. Further, mitochondria from heart and red muscle retained their iron more effectively under the stress of iron deficiency than mitochondria from liver and spleen.Metabolism of iron in normal and depleted tissue was measured using tracer amounts of(59)Fe administered orally. As expected, there was greater uptake of tracer iron by iron-deficient animals. The major organ of iron accumulation was the spleen, but significant amounts of isotope were also localized in heart and brain. In all muscle tissue examined the(59)Fe preferentially entered the mitochondria. Enhanced mitochondrial uptake of iron prior to any detectable change in the hemoglobin level in experimental animals may be indicative of nonhemoglobin related biochemical changes and/or decrements in work capacity.

Lead and iron absorption from rat small intestine: the effect of dietary Fe deficiency

1. When lead is administered in drinking-water iron-deficient rats retain more Pb than Fe-replete rats (Six & Goyer, 1972; Klauder & Petering, 1975). In the present study the relationship between the absorption of Pb and Fe was investigated. 2. Adult male rats were transfered to a milk-based diet fed with or without supplementary Fe (180 mg Fe/kg as ferrous sulphate). After 7--9 d the absorption of duodenally-administered 203Pb and 59Fe was measured as uptake of radioactivity from the gastrointestinal tract after 90 min. 59Fe absorption was increased in rats given the unsupplemented diet for 7 d and was further increased in rats kept on the diet for up to 7 weeks. 203Pb absorption was not consistently increased by either short- or long-term Fe deprivation. 3. Much of the 203Pb in homogenates of the upper small intestine was bound to soluble protein of which up to 85% was dialysable. In contrast little 59Fe was dialysable. Only a small proportion of the soluble musosal Pb was associated with ferritin during gel-filtration chromatography although 203Pb precipitated together with carrier rat-liver ferritin with an antibody to rat-liver ferritin. 4. There appeared to be no direct relationship between the transfer of Fe and Pb across the small intestine of the adult rat.

Inhibition of Protein Synthesis: A Mechanism for the Production of Impaired Iron Absorption

Treatment of rats with cycloheximide results in a defect in intestinal iron transport. A similar defect occurs after the parenteral administration of iron. Under both conditions there is impaired uptake of iron into the mucosal cells as well as defective transfer from the intestinal mucosa. It is suggested that the interference in iron transport may be due to a deficiency of an unidenti-fild carrier Substance.