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Blood, Vol. 91 No. 8 (April 15), 1998:
pp. 3059-3065
Adaptive Response of Iron Absorption to Anemia, Increased
Erythropoiesis, Iron Deficiency, and Iron Loading in
 2-Microglobulin Knockout Mice
By
Manuela Santos,
Hans Clevers,
Maria de Sousa, and
J.J.M. Marx
From the Departments of Immunology and Internal Medicine and
Eijkman-Winkler Institute, University Hospital Utrecht, Utrecht, The
Netherlands; and Molecular Pathology and Immunology, Abel Salazar
Institute for the Biomedical Sciences, Porto, Portugal.
 |
ABSTRACT |
Recently, a novel gene of the major histocompatibility complex (MHC)
class I family, HFE (HLA-H), has been found to be mutated in a large
proportion of hereditary hemochromatosis (HH) patients. Further support
for a causative role of HFE in this disease comes from the observation
that  2-microglobulin knockout (2m /) mice, that
fail to express MHC class I products, develop iron overload. We have
now used this animal model of HH to examine the capacity to adapt iron
absorption in response to altered iron metabolism in the absence of
2m-dependent molecule(s). Mucosal uptake, mucosal transfer and
retention of iron were measured in control and 2m/
mice with altered iron metabolism. Mucosal uptake of Fe(III), but not
of Fe(II), by the mutant mice was significantly higher when compared
with B6 control mice. Mucosal transfer in the 2m/
mice was higher, independent of the iron form tested. No significant differences were found in iron absorption between control and 2m/ mice when anemia was induced either by
repetitive bleeding or by hemolysis through phenylhydrazine treatment.
However, iron absorption in mice made anemic by dietary deprivation of
iron was significantly higher in the mutant mice. Furthermore, the 2m/ mice manifested an impaired capacity to
downmodulate iron absorption when dietary or parenterally iron-loaded.
The expression of the defect in iron absorption in the
2m/ mice is quantitative, with iron absorption
being excessively high for the size of body iron stores. The higher
iron absorption capacity in the 2m/ mice may
involve the initial step of ferric mucosal uptake and the subsequent
step of mucosal transfer of iron to the plasma.
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INTRODUCTION |
IRON HOMEOSTASIS is maintained primarily
by controlling intestinal absorption.1 Several factors are
known to affect iron absorption, including body iron stores
(store-regulator2) and the rate of erythropoiesis
(erythroid-regulator3,4). At present, it is not known
whether these two main regulators of iron absorption operate through
the same mechanism(s) in the intestinal mucosa, although there is some
evidence that they may act independently. Inborn abnormalities of the
iron regulators occur, as for example in the iron overload syndrome
hereditary hemochromatosis (HH). HH is an autosomal recessive disease,
characterized by a defect in regulation of iron absorption, an increase
of transferrin saturation, and progressive iron deposition
predominantly in parenchymal cells of several organs.5
Recently, a novel gene of the major histocompatibility complex (MHC)
class I family, HFE (HLA-H), has been found to be mutated in a large
proportion of HH patients.6 Previously, we characterized
iron metabolism in 2-microglobulin knockout (2m/) mice that fail to express MHC class
I products.7,8 Transferrin saturation in the
2m/ mice is abnormally high and pathologic
iron depositions occur predominantly in liver parenchymal
cells.8,9 We have now used this animal model of HH to
examine the role of 2m-dependent molecules in the regulation of iron
absorption. Iron absorption is a multistep process, consisting of the
initial mucosal iron uptake from the lumen of the gut and the
subsequent transfer of iron into the plasma. In mice, reduction of
Fe(III) to Fe(II) is a prerequisite for iron uptake by the
intestine.10 Recently, a persistent increase in the
reduction and uptake of iron by the intestine in HH patients has been
reported,11 whereas other studies indicate that the mucosal
transfer of iron is also increased.12 Our previous studies
on the absorption of a reduced form of iron [Fe(II)] in
2m/ mice showed that the mutant mice fail
to limit the transfer of iron from mucosal cells into the plasma,
whereas the mucosal uptake is similar to normal B6 mice.8
Thus, the reducing step in this study was not investigated. In view of
this, we examined iron uptake, transfer, and retention in
2m/ and B6 control mice with altered iron
metabolism to determine (1) the absorption of both Fe(III) and Fe(II);
(2) the organ distribution of the newly absorbed, radiolabeled iron;
(3) whether iron absorption in 2m/ mice is
differently affected by erythroid demand versus altered iron stores;
and (4) the step of iron absorption at which the defect is located.
The results show that, in 2m/ mice,
mucosal uptake and retention of iron delivered as Fe(III) is higher
than in control mice, while no differences in mucosal uptake are
detectable from a test dose containing Fe(II). Mucosal transfer when
testing Fe(III) or Fe(II) is similar and significantly increased.
Upregulation of iron absorption in response to dietary iron deficiency
is excessively high in 2m/ mice, but
increased to the same extent as control mice when erythroid demand
increases. Furthermore, 2m/ mice have a
limited capacity to downregulate iron absorption in response to
increased iron stores. Taken together, these results show that the
expression of the defect in iron absorption in the 2m/ mice is quantitative, with iron
absorption being excessively high for the size of body iron stores.
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MATERIALS AND METHODS |
Animals and Treatments
C57BL/6 (B6) female mice aged 6 to 8 weeks were purchased from the IFFA
Credo (Brussels, Belgium), and used as controls. The 2m/ mice13 were purchased from
Jackson ImmunoResearch Laboratories (West Grove, PA) and further bred
in our animal facility. The mutant mice had been back-crossed 11 times
onto the C57BL/6. Control animals received a commercial diet (RMH-B;
Hope Farms, Woerden, The Netherlands). Dietary iron deficiency was
induced by placing mice on a purified diet with low iron
content14 and demineralized water for a period of 2 weeks.
Dietary iron-loading was obtained by placing mice on an iron-enriched
diet containing 2.5% (wt/wt) carbonyl iron (Sigma Immunochemicals, St
Louis, MO) for 2 weeks. Before iron absorption testing, all animals
received the same control diet for 2 days to avoid any direct influence
of the diet on intestinal iron absorption. To induce parenteral
iron-loading, 5 mg of iron-dextran (Sigma Immunochemicals) was injected
subcutaneously and the mice were left for 2 weeks to allow for iron
redistribution. Hemolytic anemia was induced through phenylhydrazine
(PHZ; Sigma Immunochemicals) by subcutaneous injection of 60 mg/kg of
body weight on 2 consecutive days, and iron absorption was measured at
day 4. To induce anemia through phlebotomy, a total of approximately 1 mL of blood was extracted over a period of 2 weeks by retro-orbital puncture.
For all animal experiments, written consent was obtained from the local
Animal Experiments Committee of Utrecht University (Utrecht, The
Netherlands).
Hematologic Measurements
Heparinized blood was obtained by orbital puncture under
diethylether anesthesia. Red blood cell count (RBC), hemoglobin
(Hb), hematocrit (HCT), and mean corpuscular volume (MCV) were
determined using a Cell-Dyn 1600 counter (Sequoia-Turner Corp, Mountain
View, CA).
Measurement of Liver Iron Concentration
Liver samples were weighed wet and then dried overnight at 106°C
and weighed again. The dried samples were ashed in an oven at 500°C
for 17 hours and then fully solubilized in 6 mol/L HCl; the final
solution was adjusted with demineralized water to a final HCl
concentration of 1.2 mol/L. Iron concentration of the samples was
determined by flame atomic absorption spectrometry (Varian SpectrAA 250 Plus; Varian, Mulgrave, Victoria, Australia).
Gastrointestinal Iron Absorption
Measurement of iron absorption was performed as previously
described.15 Briefly, in a series of experiments,
59Fe(III) citrate was added to Fe(II) as ferrous sulphate
solution with a 20-fold molar excess of L-ascorbic acid to reduce the
Fe(III). 51CrCl3 was added as a nonabsorbable
indicator. The total amount of Fe(II) per test dose was 5 µg per
mouse and had a final volume of 0.3 mL. When absorption of Fe(III) was
tested, ferric-citrate (Sigma Immunochemicals) was added to obtain a
total of 5 µg per mouse, with a 20-fold molar excess of sodium
citrate dihydrate (Sigma Immunochemicals) to maintain mononuclear
ferric-citrate complexes and to prevent precipitation. Each mouse
received approximately 50 kBq of 59Fe and 200 kBq of
51Cr.
The test dose was orally applied with the use of an olive-tipped
oroesophageal needle. Total body radioactivity was measured with a
whole-body gamma counter (Automatic Scanner DS4/4S; Tracelab Ltd,
Weybridge, Surrey, UK) with separate detection windows for 59Fe and 51Cr peaks. The values were corrected
for radioisotope decay, contribution of 59Fe to the
51Cr peak, and day-to-day fluctuations of the scanner with
the use of a radium source. Mucosal uptake of iron (MU) was calculated from the activity of 59Fe and 51Cr administered
(measured immediately after test dose administration and considered as
100%) and the activity of 59Fe (F1) and 51Cr
(C1) found within the body 22 hours later, using the following equation: MU = 100 × (F1  C1)/(100 C1)%. F1 and
C1 were expressed as the percentage of the amount of 59Fe
and 51Cr administered, respectively. 59Fe
retention (IR) was determined by whole-body counting 4 days after the
administration of the test dose. The mucosal transfer fraction of iron
(MT) was determined as the ratio IR/MU.
Tissue Distribution of Gastrointestinally Absorbed Iron
To determine the tissue distribution of 59Fe after
gastrointestinal administration of Fe(II), animals were killed at day
6, and whole organs were then removed and washed in physiologic salt. The radioactivity of whole organs was measured in a gamma counter (Packard Instruments, Downers Grove, IL).
Statistical Analysis
Results are presented as the mean ± SD or, when indicated, as the
mean ± SEM. The Student's t-test was used for comparison between the control and knockout mouse groups. The level of
significance was pre-set at P < .05.
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RESULTS |
Iron Absorption of Fe(II) Versus Fe(III)
Reduction of Fe(III) to Fe(II) seems to be a prerequisite for iron
uptake by the intestine in both mice and humans.10,11 This
enzymatic step is bypassed if iron reaches the microvillous membrane as
Fe(II), eg, when excess ascorbic acid or orange juice is added to the
radioactive iron test dose.12,16 We therefore compared
Fe(III) and Fe(II) iron absorption steps in control B6 and
2m/ mice. As shown in
Table 1, mucosal uptake of Fe(III) was
significantly lower than Fe(II) in control mice, but not in the
2m/ mice. Mucosal transfer of iron,
representing transport across the basolateral membrane into the plasma,
was higher in 2m/ mice than in control
mice, regardless of the iron oxidation state. Accordingly, ultimate
iron retention was significantly higher in the
2m/ mice compared with control mice when
iron was administered in the ferric form.
Tissue Distribution of Gastrointestinally Absorbed Iron
We next examined organ distribution of newly absorbed 59Fe
administered as Fe(II). We chose to test iron absorption of ferrous iron to achieve similar values of iron retention in both mice strains
and therefore compare tissue iron distribution resulting from a similar
amount of radioactive iron. As shown by the radioactivity detected in
different organs, higher amounts of iron were deposited in the liver of
2m/ mice, with somewhat lesser amounts
being found in spleen and blood (Fig 1A).
Iron retention, here defined as the percentage of 59Fe
found in the body 6 days after the administration of an oral radioactive test dose, was plotted against the recovered radioactivity from liver, spleen, and blood (Fig 1B). As expected, iron retention from the ferrous test dose was not significantly different between the
two mouse strains. The differences reside in the quantity distributed
in the liver.
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| Fig 1.
Distribution of gastointestinally absorbed
59Fe. (A) Control B6 ( ) and 2m/
( ) mice received a radioactive test dose solution of ferrous iron.
At day 6, the mice were exsanguinated and their organs were dissected.
The amount of radioactivity in selected organs was measured in a gamma
counter. The data are presented as the mean specific activity (counts
per minute per gram of wet weight of tissue) ± SD (n = 12; combined data from 2 experiments). Mean values in livers and
spleens from 2m/ mice were significantly different
from those of B6 mice (Student's t-test): P < .05. (B) Radioactivity recovered from livers, spleens, and blood versus iron
retention in control B6 ( ) and 2m/ ( ) mice
from the same experiments. Iron retention was determined at day 6 in a
whole-body counter. Individual values for each mouse are shown.
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Iron Absorption in Mice With Altered Iron Metabolism
Effect of modifying the erythroid regulator.
In the next set of experiments, we examined the capacity of
2m/ to regulate iron absorption in
response to increased erythropoiesis demand in the presence of
unaltered or decreased iron stores. For this purpose, two models were
used to stimulate the erythroid regulator. In one group, the mice were
phlebotomized, and in another group, hemolytic anemia was induced by
PHZ treatment. Thus, we could evaluate iron absorption changes in mice
with increased erythropoiesis demand in two different situations: with
depleted iron stores (phlebotomy treatment) and with unchanged iron
stores (PHZ treatment). Hematological values measured at day 0, ie,
when the test dose was administered, and at day 4, ie, when ultimate iron retention was measured, are given in
Table 2. PHZ treatment resulted in a more
severe reduction of RBCs in B6 (44%) when compared with
2m/ mice (23%). MCV was significantly
increased at day 4 in both mice strains, reflecting a young cell
population as a result of increased erythropoiesis. Less pronounced
changes were observed after phlebotomy treatment in respect to RBC
counts, Hb, and MCV. Recovery of RBC and Hb values was observed in both
mouse strains at day 4 (Table 2). These results show that both
treatments were effective in increasing erythropoiesis demand at the
time the radiolabeled iron was administrated.
Liver iron concentration in untreated 2m/
mice was significantly higher than in untreated B6 mice (P < .001; Table 3). PHZ-treated B6 mice showed
a significant increase in liver iron concentration (P < .0001). In contrast, PHZ-treated 2m/ mice
significantly decreased liver iron concentration (P < .01; Table 3). This indicates that
2m/ mice are able to rapidly mobilize the
excess liver iron in response to increased demand for erythropoiesis.
Indeed, when a higher dose of PHZ was used (120 mg/kg body weight), all
B6 control mice died, whereas all 2m/ mice
survived (data not shown). After repetitive bleeding, no statistically
significant changes in liver iron concentration were observed between
treated and untreated mouse groups (Table 3).
As shown in Fig 2, mucosal uptake, mucosal
transfer, and retention of Fe(III) was again significantly higher in
untreated 2m/ mice than in untreated B6
mice. Phlebotomy treatment induced an increase in mucosal uptake,
mucosal transfer, and retention of iron in B6 mice that was not
significantly different from the mutant mice. Interestingly, PHZ
treatment, which does not alter the size of total body iron stores,
induced an even greater increase in mucosal iron uptake and retention,
which was similar in both mice strains (Fig 2). This finding supports
the notion that the erythroid regulator surpasses the store regulator
in acute hemolysis in a way that is independent of 2m-associated
molecule(s).
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| Fig 2.
Iron absorption in mice with altered iron metabolism. (A)
Mucosal uptake and (B) mucosal transfer of iron. (C) Iron retention. B6
() and 2m/ () mice were divided into 6 groups (n = 6 per group) and treated as described in Materials and
Methods. Hemolysis was obtained by phenylhydrazine injection of 60 mg/kg body weight. Phlebotomy resulted in 1 mL blood loss from each
mouse. The iron-deficient diet contained 6 mg Fe/Kg and the
iron-enriched diet contained 2.5% wt/wt carbonyl iron. All other
groups were maintained on a standard diet containing 164 mg Fe/kg.
Parenteral loading was obtained by injecting 5 mg of iron-dextran.
Mucosal uptake was measured at day 1 after the administration of a test
dose of ferric iron, and iron retention was measured at day 4. Mucosal
transfer of iron was calculated from the ratio IR/MU. The data are
presented as the mean ± SEM. ($) Student's t-test for
comparison of 2m/ mice with B6 mice, same
treatment (P < .05). (#) Student's t-test for
comparison between untreated (control) and treated mice (P < .05).
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Effect of modifying the store regulator.
To manipulate the store regulator, animals were fed an iron-deficient
or an iron-supplemented diet. A third group was parenterally iron-loaded, by injection of iron-dextran. Thus, iron absorption was
evaluated in mice with insufficient erythropoiesis due to limited
dietary iron and in mice with increased iron stores predominantly in
parenchymal cells (dietary iron) versus reticulo-endothelial (RE) cells
(parenteral iron-dextran). This distinction in cellular iron stores is
important, because in HH patients excess iron is deposited in
parenchymal cells, whereas little iron is found in RE cells until the
later stages of the disease.17,18 Because intestinal iron
absorption is inversely related to RE iron stores, an iron-handling
defect of RE cells has been suggested to be responsible for both excess
iron deposition in parenchymal cells and lack of feedback regulation of
duodenal iron uptake in HH.19
Feeding of an iron-deficient diet to growing animals resulted in a
lower liver iron concentration in B6 mice compared with the
B6-untreated group (P < .0001), but remarkably not in
2m/ mice (Table 3). A modest but
significant decrease of Hb values at day 0 (Table 2) and increased
mucosal uptake and transfer of iron, resulting in an increased iron
retention (Fig 2), was observed in both mouse strains placed on an
iron-deficient diet. Paradoxically, despite the fact that
2m/ mice have increased amounts of iron
stores (Table 3), these alterations in iron absorption steps were
significantly more pronounced in these mice (Fig 2). However, taking
into account the differences already seen in untreated groups, it
should be noted that both mouse strains responded with a twofold to
threefold increase in iron retention.
Iron-dextran treatment resulted in a more severe iron overload of the
liver when compared with a 2-week iron-enriched regime (Table 3).
Iron-dextran complexes are recovered from the plasma through
phagocytosis by cells of the RE system, leading to the appearance of
heavy iron-granules in Kupffer cells in both mouse strains (data not
shown). No significant changes in mucosal uptake of iron were observed
in these groups when compared with untreated groups (Fig 2A). Thus, the
decrease observed in iron retention is a consequence of downmodulation
of the mucosal transfer step (Fig 2B and C). Importantly, mucosal
transfer and the ultimate iron retention in iron-dextran treated
2m/ mice was significantly higher than in
the corresponding B6 mouse group, approaching the levels observed in
nontreated B6 mice.
As previously reported,8 dietary iron loading resulted in
visible iron deposits in Kupffer and parenchymal cells of B6 mice and
predominantly in parenchymal cells of 2m/
mice (data not shown). Compared with parenteral iron loading, the major
difference in iron absorption values after dietary iron loading was an
additional decrease in mucosal uptake observed in B6 mice, but not in
2m/ mice (Fig 2A). Again, mucosal transfer
and the ultimate iron retention, although slightly decreased in dietary
iron-loaded 2m/ mice, was at the level
seen in untreated B6 mice (Fig 2B and C). These results clearly show
that 2m/ mice retain some ability to
regulate iron absorption in response to altered iron stores. However,
the ultimate iron retention is invariably higher than in the
corresponding B6 mouse group (Fig 3).
Furthermore, no significant differences were found between iron
absorption in dietary and parenterally iron-loaded
2m/ mice (Fig 2). Thus, despite increased
iron deposition in Kupffer cells, achieved by iron-dextran injections
in 2m/, no further feedback regulation of
the levels of iron absorption was observed. This is in agreement with
previous observations that reconstitution of
2m/ mice with normal hematopoietic cells
redistributes the iron from parenchymal to Kupffer cells, but does not
correct the mucosal defect.8
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| Fig 3.
Adaptive response of iron absorption in B6 and
2m/ mice to dietary manipulations. Animals
received a radioactive test dose solution of ferric iron and iron
retention was measured at day 4. Data represent mean values for iron
retention in mice fed an iron-enriched (), iron-deficient (), or
standard diet ( ).
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DISCUSSION |
It was expected that further elucidation of the mechanism(s) of iron
absorption would be possible as soon as the gene responsible for HH, an
HLA-A-linked disease of iron metabolism, could be identified. Recently, a new candidate gene for this disease has been
found.6 The new gene encodes an MHC class I-like protein,
and two mutations were found in more than 85% of HH patients. One of
the mutations, present in 83% of the HH patients, results in a
cysteine to tyrosine substitution at amino acid 282 (C282Y) and is
thought to interfere with the binding of the HLA-H heavy chain to
2m.20 Further support for a causative role of HLA-H in
this disease comes from the previous observation that
2m/ mice develop iron
overload.7-9 These mutant mice are unable to express MHC
class I products, including the mouse homolog of HLA-H (HFE) gene,
recently identified as MR2.21
Iron Absorption of Fe(II) Versus Fe(III)
Reduction of Fe(III) to Fe(II) has been shown to be a prerequisite for
iron uptake by the intestine both in mice and
humans.10,11,22,23 A significant increase in the duodenal
mucosal Fe(III) reducing activity in HH patients compared with normal
controls has been reported recently.11 Mucosal uptake of
ferric iron by the 2m/ mice was
significantly higher when compared with B6 mice, whereas no differences
were detected in mucosal uptake of ferrous iron. Our results comparing
ferric iron absorption in B6 and 2m/ mice
are compatible with a higher reducing capacity of the intestinal mucosa
in mice lacking functional 2m and 2m-dependent molecules. In
addition, a higher mucosal transfer capacity independent of the iron
form administrated has been confirmed.
Tissue Distribution of Gastrointestinally Absorbed Iron
Six days after the oral administration of radiolabeled iron, increased
amounts of 59Fe were found in the livers of
2m/ mice compared with B6 mice. Taking
into account that 2m/ mice have a higher
transferrin saturation and reduced amounts of apo-transferrin in
plasma,8 these results indicate that gastrointestinally
absorbed iron is released from mucosal cells independently of the
availability of transferrin in plasma and is deposited in the liver.
This is in agreement with previous findings in hypotransferrinemic
mice, characterized by a heritable reduction in circulating
transferrin, leading to anemia but also to increased storage of iron in
the liver.24 In these mice, the gastrointestinally absorbed
iron not bound to transferrin is deposited in the liver, suggesting the
existence of an uptake system for non-transferrin-bound
iron.25
Iron Absorption in Mice With Altered Iron Metabolism
Iron homeostasis is maintained essentially by controlling intestinal
iron absorption.1 Several factors have been identified that
affect iron absorption, including body iron stores,2 the rate of erythropoiesis,3 hypoxia,26 and
inflammation.27
We have used our animal model of HH to examine the capacity to adapt
iron absorption in response to altered iron metabolism in the absence
of 2m-dependent molecule(s). We wanted to differentiate between the
erythroid regulator and the store regulator. Therefore, we stimulated
differentially these iron regulators. The results showed that, as far
as the erythroid regulator is concerned, no major differences in iron
absorption between B6 control mice and 2m/
mice were found. These treatments more severely affected B6 mice than
2m/ mice, probably due to the increased
iron stores already present in the 2m/
mice. Both steps of iron absorption, ie, mucosal uptake and mucosal transfer, seem to increase in response to PHZ and to phlebotomy treatment. This supports the notion that the erythroid regulator surpasses the store regulator in acute hemolysis in a way that is
independent of 2m-associated molecule(s).
Feeding an iron-deficient diet also increases both mucosal uptake and
mucosal transfer of iron, with the opposite occurring when feeding an
iron-enriched diet. The changes in mucosal uptake observed in these
models could partially be caused by the altered iron content of the
diet, although the mice were fed with normal diet 2 days before testing
to normalize the intestinal lumen iron content. By comparison,
injection of iron-dextran, while resulting in a higher iron overload of
the liver, does not induce significant changes in the initial step of
mucosal uptake. Rather, the resulting lower iron retention seems to
reflect a lower transfer of iron from the mucosal cell into the blood.
This means that these two processes, ie, mucosal uptake and mucosal
transfer of iron, can be separately regulated. Further support for this
view appears from studies performed in other genetically defective mice
strains.28,29 One of these is sex-linked anemia (sla),
in which the defect is expressed as reduced transfer of iron from
mucosal cells to the blood.28,30 The second abnormality
reported in mice is hereditary microcytic anemia
(mk).29 In contrast to sla, mk mice
display a markedly reduced uptake of iron across the microvillous
membrane but no reduction in the subsequent transfer across the
basolateral membrane. Recently, the mk has been identified as
Nramp2,31 identical to a divalent cation-transporter that
transports Fe(II).32 The data from sla and
mk mice suggest that at least two independent iron-transporting
complexes are involved in the handling of iron within mucosal cells.
The present results indicate that in 2m/
mice both steps of mucosal uptake and mucosal transfer are
quantitatively affected. Indeed, the results concerning the store
regulator clearly show that in 2m/ mice,
although being able to respond by increasing or decreasing iron
absorption, the ultimate iron retention is invariably higher than seen
in B6 mice (Fig 3). Importantly, in HH patients, iron absorption is
also regulated to some extent. In fact, iron absorption is found to be
within the normal range in many patients at presentation. With
reduction of iron stores by phlebotomy, absorption increased to high
levels and only became suppressed as iron stores
reaccumulated.33 Thus, it can be said that HH is associated
with a quantitative defect of intestinal function, in which net
absorption of iron appears to be "inappropriate for the size of the
storage pool of iron."33-36
Because of the similarities in iron metabolism seen in HH and
2m/ mice,8 this suggests that
heterodimers composed of  -heavy chain and 2m (HFE in
humans6 and MR2 in mice21) rather than -heavy chain or 2m subunit per se may influence iron absorption. A possible interpretation could be that HFE/MR2 product may negatively interfere with the expression of Fe(II)-transporter(s) (eg, Nramp2) and/or with the expression of ferrireductase complexes
presumably present at or near the microvillous membrane.
In conclusion, this study shows that the defect in the regulation of
iron absorption in 2m/ mice is
quantitative rather than qualitative, sharing similarities with what
has been observed in HH patients. The expression of the defect may
involve the initial step of ferric mucosal uptake and the subsequent
step of mucosal transfer of iron to the plasma.
| |
FOOTNOTES |
Submitted August 4, 1997;
accepted December 3, 1997.
Supported by a grant from Junta Nacional de Investigação
Científica e Tecnológica-PRAXIS XXI (BD/2866/94).
Address reprint requests to Manuela Santos, Department of
Immunology, University Hospital Utrecht, Room F03.821, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. section
1734 solely to indicate this fact.
| |
ACKNOWLEDGMENT |
The authors are grateful to Toon Hesp, Jan Smits, and Else Dorrestein
for taking care of the animals; to Drs K. Wienk, F. Arosa, and G. Porto
for valuable discussions; and to Dr N. Barker for reviewing the
manuscript.
| |
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Abstract
Introduction
Abstract