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New Insights into Erythropoiesis: The Roles of Folate, Vitamin B12, and Iron


Abstract and Figures

Erythropoiesis is the process in which new erythrocytes are produced. These new erythrocytes replace the oldest erythrocytes (normally about one percent) that are phagocytosed and destroyed each day. Folate, vitamin B12, and iron have crucial roles in erythropoiesis. Erythroblasts require folate and vitamin B12 for proliferation during their differentiation. Deficiency of folate or vitamin B12 inhibits purine and thymidylate syntheses, impairs DNA synthesis, and causes erythroblast apoptosis, resulting in anemia from ineffective erythropoiesis. Erythroblasts require large amounts of iron for hemoglobin synthesis. Large amounts of iron are recycled daily with hemoglobin breakdown from destroyed old erythrocytes. Many recently identified proteins are involved in absorption, storage, and cellular export of nonheme iron and in erythroblast uptake and utilization of iron. Erythroblast heme levels regulate uptake of iron and globin synthesis such that iron deficiency causes anemia by retarded production rates with smaller, less hemoglobinized erythrocytes.
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(Some corrections may occur before final publication online and in print)
Annu. Rev. Nutr. 2004. 24:105–31
doi: 10.1146/annurev.nutr.24.012003.132306
of Folate, Vitamin B
, and Iron
Mark J. Koury
Department of Medicine, Vanderbilt University School of Medicine and
Veterans Affairs Tennessee Valley Healthcare System, Nashville, Tennessee 37232;
Prem Ponka
Departments of Physiology and Medicine, Lady Davis Institute for Medical Research of
the Jewish General Hospital, McGill University, Montreal, Quebec, H3T 1E2, Canada;
Key Words apoptosis, heme, erythrocytes, iron deficiency anemia, megaloblastic
Abstract Erythropoiesis is the process in which new erythrocytes are produced.
These new erythrocytes replace the oldest erythrocytes (normally about one percent)
that are phagocytosed and destroyed each day. Folate, vitamin B
, and iron have
crucial roles in erythropoiesis. Erythroblasts require folate and vitamin B
for prolif-
eration during their differentiation. Deficiency of folate or vitamin B
inhibits purine
and thymidylate syntheses, impairs DNA synthesis, and causes erythroblast apopto-
sis, resulting in anemia from ineffective erythropoiesis. Erythroblasts require large
amounts of iron for hemoglobin synthesis. Large amounts of iron are recycled daily
with hemoglobin breakdown from destroyed old erythrocytes. Many recently identi-
fied proteins are involved in absorption, storage, and cellular export of nonheme iron
and in erythroblast uptake and utilization of iron. Erythroblast heme levels regulate
uptake of iron and globin synthesis such that iron deficiency causes anemia by retarded
production rates with smaller, less hemoglobinized erythrocytes.
*The US Government has the right to retain a nonexclusive, royalty-free license in and to
any copyright covering this paper.
ABBREVIATIONS ALA-S2/eALA-S, erythroid-specific 5-aminolevulinic-acid syn-
thase; BFU-E, burst-forming unit-erythroid; CFU-E, colony-forming unit-erythroid;
Dcytb, duodenal cytochrome b; eIF-2, eukaryotic initiation factor 2; EPO, erythropoi-
etin; FBP, folate-binding protein; GI, gastrointestinal; HO-1, heme oxygenase 1; HRI,
heme-regulated inhibitor; IRE, iron-responsive element; Ireg1/MTP1, ferroportin 1; IRP,
iron regulatory protein; LIP, labile iron pool; Nramp/DMT1, divalent metal transporter 1;
NTBI, nontransferrin-bound iron; RFC, reduced folate carrier; THF, tetrahydrofolate;
UTR, untranslated region.
First published online as a Review in Advance on March 10, 2004
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INTRODUCTION .....................................................106
IN ERYTHROPOIESIS ..............111
PREREQUISITE FOR ERYTHROPOIESIS ...............................117
BY DEVELOPING ERYTHROID CELLS ................................121
CELLS .............................................................121
HEMOGLOBIN SYNTHESIS ..........................................122
Erythropoiesis is the process by which the hematopoietic tissue of the bone mar-
row produces red blood cells (erythrocytes). The mean lifespan of a normal human
erythrocyte is about 120 days. Erythrocytes are involved in transporting carbon
dioxide and nitric oxide, but their principal function is to deliver oxygen from the
lungs to the other tissues of the body. The amount of oxygen delivered to the tissues
is a function of the number of circulating erythrocytes. In normal adults, approx-
imately 200 billion of the oldest erythrocytes (about 1% of the total number) are
replaced every day by an equal number of newly formed erythrocytes. In situations
in which the erythrocytes are abnormally lost from the circulation by bleeding or by
increased destruction (hemolysis), the rate of new erythrocyte production can ex-
ceed one trillion per day. Thus, erythropoiesis is a dynamic process that can respond
promptly to the need for more oxygen delivery. Among the numerous requirements
for active erythropoiesis are adequate supplies of three nutrientsfolate, cobal-
amin (vitamin B
), and iron. Deciency of each of these three nutrients can lead to
decreased erythrocyte production and subsequently to decreased numbers of cir-
culating erythrocytes (anemia). Advances in erythropoiesis research have helped
to explain the roles of these nutrients in the production of erythrocytes and how
their respective deciency states cause anemia. Recently reported ndings related
to the development of nutrition-related anemias that will be reviewed here include:
(a) the uptake and intracellular effects of folate, vitamin B
, and iron; (b) the in-
duction of programmed death (apoptosis) of erythroid progenitor cells in folate or
vitamin B
deciency; and (c) the cellular mechanisms in iron-decient erythrob-
lasts that avoid apoptosis, but nonetheless decrease erythrocyte production.
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Figure 1 Stages of erythroid differentiation. Specic stages of erythroid differenti-
ation beginning with the burst-forming unit-erythroid (BFU-E) and ending with the
mature erythrocyte (RBC) are shown. Multiple cell divisions occur between stages
prior to the polychromatophilic erythroblast (polyEB) stage after which the cells do
not divide. Periods of high cellular proliferation, erythropoietin (EPO) dependence, and
hemoglobin synthesis are demarcated. Other abbreviations: CFU-E, colony-forming
unit-erythroid; ProEB, proerythroblasts; BasoEB, basophilic erythroblast; OrthoEB,
orthochromatic erythroblast; and Retic, reticulocyte.
All types of blood cells have a nite life span and must be constantly replaced
by new cells formed in the hematopoietic tissue. Erythropoiesis is a continuous
process of proliferation and differentiation beginning with the hematopoietic stem
cell and ending with the erythrocyte (56, 60) (Figure 1). Hematopoietic stem cells
are rare, less than one in ten thousand nucleated cells of the bone marrow, and they
can self-renew or differentiate into all of the cells in the blood and the immune
system. Their commitment to differentiation and the subsequent commitment of
their progeny to the erythroid lineage appear to be stochastic events, but may be
related to the prevalence and association of specic DNA transcription factors
(18). The earliest stage of progenitor cell differentiation that is committed to the
erythroid lineage is the burst-forming unit-erythroid (BFU-E; Figure 1). Human
BFU-Es are dened by their ability to form large burstsof erythroblast colonies
or one very large colony of erythroblasts, after two to three weeks in semisolid
tissue culture. Erythroid bursts can contain more than one thousand erythroblasts
and, thus, a single BFU-E and its progeny can have ten or more rounds of cell
division before reaching the terminal postmitotic stages of differentiation. The
next dened stage is the colony-forming unit-erythroid (CFU-E; Figure 1). Human
CFU-Es require one week to form single colonies of up to 64 erythroblasts in
tissue culture. Thus, CFU-E and their progeny have six or fewer rounds of cell
division. The erythropoietic stages subsequent to CFU-E are dened by their light
microscopic appearance in stained preparations. Cellular proliferation is not shown
in Figure 1, but the percentage of cells in active cell cycle is greatest in the CFU-
E and proerythroblast stages, and cell division ceases at the polychromatophilic
Erythropoietin (EPO) is the principal regulator of erythropoiesis (56, 63). EPO
is a glycoprotein hormone produced by a subset of peritubular, interstitial cells
in the renal cortex (61, 64). A few of these cells produce EPO under normal
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circumstances. In response to decreased oxygen delivery as occurs with anemia,
the number of these interstitial cells that produce EPO increases exponentially
(62). In the bone marrow, EPO acts upon erythroid progenitors in the stages from
CFU-E to the earliest of basophilic erythroblasts. This period of EPO dependence
precedes and does not overlap the period of hemoglobin synthesis (Figure 1).
As shown in Figure 1, progenitor cells in these stages are dependent upon EPO to
prevent apoptosis (55, 119), but they display widely varying degrees of dependence
(54). To survive this period of dependence, most erythroid progenitor cells require
greater EPO concentrations than those concentrations normally found in the blood.
Thus, the normal daily production rate of 200 billion erythrocytes requires the
survival of only a minority of the maximal number of EPO-dependent erythroid
progenitor cells.
Anemia occurs when the number of circulating erythrocytes is decreased. If
the anemia is due to transient blood loss or hemolysis and the kidneys and bone
marrow are normal, the erythropoietic system corrects the anemia. Specically,
the decreased erythrocytes in anemia reduce oxygen delivery, and the kidneys
respond by increasing EPO production. The increased EPO results in the survival
of more erythroid progenitor cells in the EPO-dependent stages and subsequently
increased erythrocyte production. The increased erythrocytes in the circulation
deliver more oxygen, lowering the elevated EPO levels, and ultimately returning
the erythrocyte production rate and the number of circulating erythrocytes to their
normal, steady-state levels prior to the onset of anemia. In many anemias, however,
the kidneys are not normal, resulting in deciency of EPO, or the hematopoietic
tissue of the bone marrow is not normal, resulting in an inability to respond to
the EPO. Among the numerous causes of an inability to respond to erythropoietic
demand are deciencies of folate, vitamin B
, and iron. The majority of nutrition-
related anemias can be attributed to deciency of one of these nutrients (48).
Folate and vitamin B
are both required for the extensive DNA synthesis that
accompanies the production of hundreds of billions of new erythrocytes each day.
All proliferating cells require iron, but the iron requirements of erythroid cells in
the late basophilic erythroblast through reticulocyte stages, when hemoglobin is
synthesized and accumulates (Figure 1), are much greater than all other cell types.
Folate, an essential nutrient found in the tissues of plants and animals, consists
of a pteridine (2-amino-4-hydroxy-pteridine) ring attached to para-aminobenzoate
with a polyglutamyl tail. The reduced tetrahydrofolate (THF) form acts as a co-
factor in multiple biochemical reactions by donating or accepting one-carbon
units (4, 108). Folate is present in both plant and animal tissues, most commonly
in the form of 5-methyl-THF. Reduced folates are absorbed in the jejunum af-
ter enzymatic cleavage to the monoglutamic form (40, 42). The absorbed folate
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enters the blood and circulates in the body as 5-methyl-THF monoglutamate. Once
folate is transported from the blood into a cell, it is retained there through the ac-
tion of folylpolyglutamate synthetase that converts the folate to the polyglutamyl
form (108). Folate is transported into cells by several mechanisms, but the endo-
cytotic mechanism involving two specic glucosylphosphatidylinositol-anchored,
cell-surface folate-binding proteins (FPBs) and the bidirectional membrane trans-
porter termed the reduced folate carrier (RFC) are the best characterized (72).
Both of these transport mechanisms have been examined in erythropoietic cells.
In vitro studies with antibodies to the FBPs showed that FBPs are expressed on
early stage hematopoietic cells, but they do not transport signicant amounts of fo-
late (98). Similar in vitro studies show some morphological changes in the progeny
of BFU-E and CFU-E when antibodies to FBPs are added to the culture medium,
but surprisingly the growth of these erythroid progenitors are enhanced by the
antibodies (5, 6). Mice that are rendered null for one of the FBPs by homologous
recombination have embryonic lethality due to neural defects, but no hematopoi-
etic defect has been described (83). Mice rendered null for the RFC also have
embryonic lethality, but the defect appears to involve the hematopoietic system
(123). The prenatal mice can be rescued by loading the mothers with high doses
of folic acid, but the pups die from hematopoietic failure a few weeks after birth
(123), indicating that the RFC is necessary for folate transport in erythroid cells.
Clinical folate deciency in the developed countries has been associated with
those who have poor nutrition, such as the elderly or those with alcoholism. To
reduce the incidence of neural tube defects that develop during the rst trimester of
pregnancy, grain products in the United States have been fortied with folic acid for
the last seven years. Considering serum folate concentrations of less than 3 ng/ml
as folate deciency, one study found a reduction in folate-decient individuals
among a middle-aged/older population from 22% prior to fortication to 1.7%
after fortication (52). Despite this dramatic decrease in the incidence of folate
deciency in the general population, others remain at increased risk for developing
deciency due to specic medical conditions. Among the individuals at increased
risk are those with intestinal malabsorption; general malnutrition; high erythrocyte
turnover rates such as in chronic hemolytic anemias; anticonvulsant medications
that interfere with folate absorption or utilization such as phenytoin; and antibiotic
medications with antifolate actions such as trimethoprim/sulfamethoxazole.
Vitamin B
(cobalamin), an essential nutrient consisting of a tetrapyrrole (cor-
rin) ring containing cobalt that is attached to 5,6-dimethylbenzimidazolyl ribonu-
cleotide, is produced in microorganisms and is found in animal tissues. Vitamin
is a coenzyme in two biochemical reactions in humans. One of these re-
actions is the transfer of a methyl group from 5-methyl-THF to homocysteine
via methylcobalamin, thereby regenerating methionine (4, 108) (Figure 2). This
reaction represents the link between folate and vitamin B
coenzymes and ap-
pears to account for the requirement of both vitamins in normal erythropoiesis
(106, 114). The absorption of vitamin B
is a relatively complex process (4, 106,
107). Protein-bound vitamin B
in food is released by stomach acid and binds to
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Figure 2 DNA synthesis pathways that require folate or vitamin B
Abbreviations: THF, tetrahydrofolate; 5,10-CH
-THF, methylenetetrahydrofolate; 10-
CHO-THF, formyltetrahydrofolate; 5-CH
-THF, methyltetrahydrofolate; DHF, dihy-
drofolate; DNA-CH
, methylated DNA; dUMP, deoxyuridylate; dTMP, thymidylate;
dATP, deoxyadenosine triphosphate; dGTP, deoxyguanosine triphosphate; and dTTP,
thymidine triphosphate.
specic vitamin B
binding glycoproteins termed haptocorrins that are present in
the secretions of the salivary glands and stomach. In the duodenum, the haptocor-
rins are digested, and the vitamin B
binds to intrinsic factor, another glycoprotein
secreted by the stomach. The vitamin B
intrinsic factor complex subsequently
binds to specic receptors in the terminal ileum. These receptors consist of cubilin,
which binds the vitamin B
intrinsic factor complex, and megalin, an associated
membrane transport protein (78). After endocytosis in the ileal epithelium, the
vitamin B
is freed from the intrinsic factor and binds to transcobalamin II (apo-
transcobalamin) that is produced by the microvascular endothelium of ileal villi
(95). The vitamin B
transcobalamin II complex, termed holotranscobalamin II,
enters the blood where it is the functional carrier of vitamin B
to the other cells of
the body. Holotranscobalamin II binds to specic homodimerized receptor proteins
that are displayed on the surface of many different types of cells (107). Although
this receptor has not been examined directly in hematopoetic tissue, its mecha-
nism of transport in the other cells is via endocytosis with subsequent intracellular
release of vitamin B
from its complex with transcobalamin II. The incidence
of vitamin B
deciency increases signicantly with age such that up to 15% of
older individuals are decient in developed countries (8, 110). Most often this age-
related deciency appears to be due to atrophic gastritis and the resultant inability
to dissociate vitamin B
from the proteins to which it is bound in food (19). Other
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causes include the autoimmune gastropathy termed pernicious anemia, in which
both intrinsic factor and gastric acid are not produced; intestinal malabsorption
involving the terminal ileum; previous surgery that removed the stomach or ileum;
medications that interfere with gastric acid secretion such as H
-histamine receptor
blockers or proton pump inhibitors; and a strict vegan diet.
The importance of adequate folate and vitamin B
in erythropoiesis is demon-
strated by megaloblastic anemia, the clinical disease that can occur with deciency
of either vitamin. Megaloblastic anemia affects all hematopoietic lineages, but it is
most prominent in the erythroid lineage. Megaloblastic anemia is characterized by
pancytopenia with macrocytic erythrocytes, hypersegmented neutrophilic granu-
locytes, and reticulocytopenia. The bone marrow has increased numbers of large
immature-appearing erythroblasts and myeloblasts (i.e., megaloblasts) that are un-
dergoing increased rates of premature death as shown by elevated serum bilirubin,
lactate dehydrogenase, myeloperoxidase, and by increased iron turnover. This in-
creased death of hematopoietic cells prior to their maturation is termed ineffective
hematopoiesis. Studies in patients with anemia due to folate or vitamin B
ciency have shown that impaired DNA synthesis and its sequelae are key elements
in the increased hematopoietic cell death that characterizes these anemias. The pe-
riod of high proliferation rates during erythropoiesis (Figure 1) makes the erythroid
progenitor cells more susceptible than other types of cells to the impaired DNA
synthesis in folate or vitamin B
deciency. Erythroblasts from patients with fo-
late or vitamin B
deciency anemia had no active incorporation of
into DNA despite total DNA content between 2N and 4N that characterizes cells in
DNA synthesis (i.e., in S-phase of the cell cycle) (75, 115, 121). Flow cytometry
of bone marrow cells from patients with folate or vitamin deciency had increased
percentages of cells in S-phase compared to controls (50). When rates of DNA syn-
thesis were examined directly in mitogen-stimulated blood lymphocytes of patients
with folate- or vitamin B
deciency anemia, they were decreased (117), but a
similar study using bone marrow cells did not show a decreased rate (13). Impaired
DNA synthesis would be expected to result in chromosomal breakage and pos-
sibly nuclear damage. Previous studies have shown that chromosomal breakage
is markedly increased in the bone marrow cells of patients with folate or vita-
min B
deciency anemia (44, 75). Also, erythrocyte micronuclei (Howell-Jolly
bodies), a marker of genetic damage when they are increased in splenectomized
patients, are most increased in those patients who have folate or vitamin B
deciency (70).
One-carbon units are required in three biochemical pathways involved in the
synthesis of DNA. These pathways are shown in Figure 2. They are (a) two steps
in the de novo synthesis of purines in which 10-formyl-THF provides two carbons
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of the purine ring structure; (b) the reaction catalyzed by thymidylate synthase in
which 5,10-methylene-THF provides the methylene group and reducing equiva-
lents for the methylation of deoxyuridylate to form thymidylate; and (c) the re-
action catalyzed by DNA methyltransferase in which 5-methyl-THF provides the
methyl group (indirectly through remethylation of homocysteine to form methio-
nine and subsequently S-adenosylmethionine) for the methylation of cytosines
in DNA. As mentioned above, the methylcobalamin form of vitamin B
is the
coenzyme involved in the transfer of the methyl group from 5-methyl-THF to ho-
mocysteine, thereby regenerating methionine and THF (Figure 2c). With vitamin
deciency, not only does inhibited methionine regeneration lead to decreased
S-adenosylmethionine and increased homocysteine and S-adenosylhomocysteine,
but 5-methyl-THF accumulates intracellularly, while other forms of THF, specif-
ically the 10-formyl-THF required for purine synthesis and the 5,10-methylene-
THF required for thymidylate synthesis, decrease (108). This predicted trapping
of intracellular folate in the 5-methyl-THF form (46) resulting in intracellular de-
ciencies of other forms of folate including those required for de novo synthesis of
deoxynucleotides was demonstrated in the bone marrow cells of rats made func-
tionally vitamin B
decient by nitrous oxide exposure (49). In folate or vitamin
deciency, the de novo synthesis of deoxynucleotides is decreased, resulting in
impaired synthesis and repair of DNA, and ultimately, in cell death. Erythropoiesis
under these deciency conditions is termed ineffective because the erythroid cells
are present in the hematopoietic tissue, but most of them cannot mature to the
late stages of differentiation before undergoing apoptosis. The decreased numbers
of erythroid cells surviving until the postmitotic, terminal stages in ineffective
erythropoiesis leads to anemia.
An in vivo murine model (10) and its in vitro extension (57) of folate-decient
erythropoiesis have provided some new insights into the cellular events that lead to
erythroid cell apoptosis in folate deciency. In this model, mice are fed an amino
acidbased, folate-free diet that induces a pancytopenia with all of the charac-
teristics of the human hematopoietic disease that results from folate or vitamin
deciency (10). To study the cellular events of folate-decient erythropoiesis
in a puried population of developmentally synchronized cells, mice are fed the
folate-free diet before and during the acute erythroblastosis phase of Friend virus
disease. This virus induces a proliferation of erythroid cells that accumulate at
the proerythroblast stage of differentiation, and when combined with the folate-
free diet yield a population of folate-decient proerythroblasts. When cultured
with EPO under folate-sufcient conditions almost all of these proerythroblasts
differentiate into reticulocytes, but when cultured with EPO under folate-decient
conditions, most of these proerythroblasts undergo apoptosis before differentiating
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into reticulocytes (57). Proerythroblasts freshly isolated from folate-decient mice
have decreases in all forms of folate (58). During their differentiation in vitro, the
erythroblasts cultured under folate-decient conditions accumulate in S-phase of
the cell cycle. The majority of these cells undergo apoptosis in S-phase (59). The
folate-decient erythroid cells can be saved from their apoptotic fate if they are
supplied in vitrowith sufcient amounts of both thymidine and a purine that can
be salvaged to provide the necessary deoxynucleotides that permit DNA synthesis
(59). Hypoxanthine, inosine, adenosine, and deoxyadenosine are effective for this
purine salvage, but guanosine and deoxyguanosine are not. The medium supple-
mentation required in vitro for the survival and completion of erythroid differenti-
ation is 60 µmol/L for the purine and 20 µmol/L for thymidine, indicating that the
defects in DNA replication and repair that lead to apoptosis in folate-decient ery-
throid cells are due to impaired de novo synthesis of primarily purines (Figure 2a)
and secondarily thymidylate (Figure 2b). The methylation of cytosines in the DNA
of folate-decient murine erythroblasts is the same as in control erythroblasts (DJ
Park and MJ Koury, unpublished data). Similarly, bone marrow cells of patients
with vitamin B
deciency anemia and bone marrow cells of controls had similar
percentages of methylated cytosines in their respective DNAs (97). These results
in mice and humans suggest that inhibition of DNA methylation by folate or vita-
min B
deciency (Figure 2c) does not play a role in the anemias resulting from
deciency of these vitamins.
The mechanism by which the DNA damage in folate or vitamin B
leads to apoptosis in hematopoietic cells has not been established. The inhibited
conversion of deoxyuridylate to thymidylate has been associated with increased
uracil misincorporation into DNA due to the utilization by DNA polymerase of de-
oxyuridine triphosphate in lieu of thymidine triphosphate (11, 12, 116). However,
one study in vitamin B
decient patients (97), and another in folate-decient
patients (99), did not nd this increased incorporation of uracil in DNA of blood
cells. Uracils misincorporated close to one another on opposite DNA strands have
been proposed as a source of double-stranded DNA breakage in eukaryotic cells
(38) and lead to double-stranded DNA cleavage in an experimental prokaryotic
system (28). The DNA of the folate-decient erythroblasts in the in vitro murine
model have only a two- to threefold increased proportion of misincorporated uracil
compared to controls (58), suggesting that uracil misincorporation may not be a
signicant source of DNA strand breakage leading to apoptosis. These two- to
threefold increases in uracil misincorporation in folate-decient erythroblasts are
similar to the changes seen in lymphocytes of folate-decient rats that have ev-
idence of DNA damage (31), but less than found in patients with megaloblastic
anemia (116). The rescue of folate-decient erythroblasts by exogenous purines
and thymidine suggests that insufcient deoxynucleotide triphosphates may be
the cause of DNA damage and apoptosis. Murine granulocyte progenitors treated
with the antifolate methotrexate are similarly rescued by exogenous purines and
thymidine (84). Although one study of bone marrow from patients with mega-
loblastic anemia found increases in all deoxynucleotides (51), others have shown
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specic deoxynucleotide depletions in splenic cells of folate-decient rats (53),
human lymphocytes cultured under folate-decient conditions (111), and cell lines
treated with antifolates (9, 122). Murine erythroblasts accumulate increased p53
protein when they are cultured under folate-decient conditions (58), suggesting
that p53 expression is an indicator of DNA damage in folate-decient erythrob-
lasts. However, when erythroblasts from p53-null mice are cultured under folate-
decient conditions, they have similar rates of apoptosis as do their p53 wild-type
littermates, which indicates that p53 is not necessary for the apoptosis that results
from folate deciency-induced DNA damage (59).
The murine in vivo and in vitro systems have also provided insights into the
morphological changes of ineffective erythropoiesis that occur in folate or vitamin
deciency. When mice are made folate-decient by being fed the folate-free
diet, they develop a macrocytic anemia with decreased reticulocytes (10, 58). As the
folate-deciency anemia progresses, the bone marrow hematopoietic cells of the
mice, including the erythroid cells, have decreased numbers of total nucleated cells,
increased size of the individual cells, and increased numbers of cells undergoing
apoptosis. While the absolute numbers of reticulocytes are decreased in folate-
decient mice, the absolute numbers of CFU-Es are increased in their bone marrow
and spleen compared to controls (10). This result indicates that the folate-decient
mice, like their human counterparts, have increased EPO levels in response to the
anemia, with a resultant increased survival of erythroid cells in the CFU-E and
other early stages of the EPO-dependent period. However, most of these increased
CFU-Es do not survive during the subsequent stages of erythropoiesis, but rather
they succumb to apoptosis, most often while in S-phase of the cell cycle. Erythroid
cells that are in the CFU-E stage or in S-phase during the post-CFU-E stages of
differentiation are larger and more immature appearing than the normal erythroid
cells which accumulate in the G
phase during the terminal stages of erythroid
differentiation. Together, the shift to earlier stages of erythroid differentiation and
the accumulation of cells in S-phase contribute to the increased size and immature
appearance of erythroid cells in the bone marrow that characterize megaloblastic
anemias (58).
Iron is an essential element that is a component of heme-containing proteins
(i.e., hemoglobin, myoglobin, and cytochromes) and innumerable nonheme iron-
containing proteins with vital functions in many metabolic processes of all cells.
However, at pH 7.4 and physiological oxygen tension, the relatively soluble fer-
rous ion is readily oxidized to the ferric ion, which forms virtually insoluble ferric
hydroxides. Moreover, unless bound to specic ligands, iron plays a key role in
the formation of harmful oxygen radicals, which ultimately cause peroxidative
damage to vital cell structures. Because of this virtual insolubility and potential
toxicity, specialized mechanisms and molecules for the acquisition, transport, and
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storage of iron in a soluble, nontoxic form have evolved to meet cellular and or-
ganismal iron requirements. Moreover, organisms are equipped with sophisticated
mechanisms that prevent the expansion of a catalytically active intracellular iron
pool, while maintaining sufcient concentrations for metabolic use (2, 90, 100,
Cellular iron acquisition and its proper intracellular targeting into functional
iron proteins depend on an array of other proteins. Traditional proteins involved
in iron metabolism include transferrin, transferrin receptor, and ferritin, but re-
cent research has identied a number of novel genes whose products emerge as
important players in iron metabolism (Table 1).
Iron represents 55 and 45 mg per kilogram of body weight in adult men and
women, respectively. Normally, about 60% to 70% of total body iron is present in
hemoglobin in circulating erythrocytes. In vertebrates, iron is transported within
the body between sites of absorption, storage, and utilization by the plasma glyco-
protein, transferrin, which binds ferric iron very tightly but reversibly. The daily
turnover of transferrin iron is roughly 30 mg and, normally, about 80% of this
iron is transported to the bone marrow for hemoglobin synthesis in developing
erythroid cells. Senescent erythrocytes are phagocytosed by macrophages of the
reticuloendothelial system where the heme moiety is split from hemoglobin and
catabolized enzymatically via heme oxygenase-1 (HO-1) (71). Iron, which is lib-
erated from its connement within the protoporphyrin ring inside macrophages, is
returned almost quantitatively to the circulation. The remaining 5 mg of the daily
plasma iron turnover is exchanged with nonerythroid tissues, namely, the liver.
About 1 mg of dietary iron is absorbed daily, and the total organismal iron balance
is maintained by a daily loss of 1 mg via nonspecic mechanisms (mostly cell
desquamation) (100).
Several important features of organismal iron metabolism must be mentioned.
First, iron turnover is virtually an internal event in the body, and most of the iron
turning over is used for the synthesis of hemoglobin in erythroid cells. Second,
at least some nonerythroid cells can acquire nontransferrin-bound iron (NTBI),
and this process likely operates in vivo only in severely iron-overloaded patients
who have NTBI in their plasma. However, hemoglobin synthesis is stringently
dependent on transferrin as the source of iron for erythroid cells. Third, although
iron absorption is required for efcient erythrocyte formation on a long-term ba-
sis, quantitatively the most important source of iron for day-to-day erythropoiesis
is macrophages that recycle hemoglobin iron. Fourth, erythrocytes contain about
45,000-fold more heme iron (20 mmol/L) than nonheme iron (440 nmol/L) (100).
The fact that all iron for hemoglobin synthesis comes from transferrin and that
this delivery system operates so efciently, leaving mature erythrocytes with neg-
ligible amounts of nonheme iron, suggests that the iron transport machinery in
erythroid cells is an integral part of the heme biosynthesis pathway. It seems rea-
sonable to propose that the evolutionary forces that led to the development of highly
hemoglobinized erythrocytes also dramatically affected numerous aspects of iron
metabolism in developing erythroid cells, making them unique in this regard.
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TABLE 1 Some proteins involved in iron metabolism
Protein Function Result of deficiency Reference
Transferrin (Tf) Fe
-carrier in plasma Severe Fe deciency anemia; (88, 90)
generalized Fe overload
Tf receptor Membrane receptor for diferric-Tf Embryonic lethality (67, 91)
Ferritin (H and L) Cellular Fe storage H: embryonic lethality (7, 32, 90)
IRP (1 and 2) Fe sensors; bind to IREs IRP2: brain Fe overload (2, 15, 65, 76, 104)
DMT1/DCT1/Nramp2 Membrane transporter for Fe
Hypochromic microcytic anemia (16,17, 34, 41)
Duodenal cytochrome b Ferric reductase (provides Fe
Unknown (73)
(Dcytb) for DMT1 in duodenum)
Ferroportin 1/Ireg1/MTP1 Fe export from cells Hemochromatosis type 4 (1, 29, 74, 79, 88)
Ceruloplasmin (Cp) Regulation of Fe export from cells Hypochromic microcytic anemia (45)
Hephaestin Regulation of Fe export from enterocytes Hypochromic microcytic anemia (3, 112)
(membrane-bound Cp homolog)
ALA-S2/eALA-S First enzyme of heme synthesis; X-linked sideroblastic anemia (33, 85)
5-aminolevulinic-acid synthase
Ferrochelatase Last enzyme of heme synthesis; Fe
Erythropoietic protoporphyria (24, 85)
insertion into protoporphyrin IX
Mitochondrial ferritin Mitochondrial Fe storage (?) Unknown; high expression in (20, 30)
Heme oxygenase-1(HO-1) Recycling of hemoglobin Fe Severe anemia and inammation (71, 88, 93, 94, 120)
Hepcidin Plasma peptide which appears to Fe overload; overexpression of hepcidin (35, 8082)
inhibit Fe absorption causes severe Fe deciency anemia
Abbreviations: ALA-S2/eALA-S, erythroid-specic 5-aminolevulinic-acid synthase; DCT, divalent cation transporter; DMT, divalent metal transporter; IRE, iron-responsive element;
IRP, iron regulatory protein.
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Iron deciency is the most prevalent cause of anemia, affecting more than half a
billion people worldwide. The anemia of iron deciency is caused by a decreased
supply of iron for heme synthesis and, consequently, hemoglobin formation in
developing erythroid cells. Decreased hemoglobinization leads to the production
of erythrocytes that are smaller than normal (microcytic) and contain reduced
amounts of hemoglobin (hypochromic). Blood loss is the most common cause of
iron deciency. One milliliter of blood contains about 0.5 mg of iron and, hence,
a steady blood loss of as little as 3 to 4 mL per day (1.5 to 2 mg of iron) can
result in a negative iron balance. In men and postmenopausal women, unexplained
iron deciency is nearly always due to occult bleeding from the gastrointestinal
(GI) tract. Sources of GI bleeding include hemorrhoids, hiatus hernia, peptic ul-
ceration, diverticulosis, tumors of the stomach and colon, adenomatous polyps,
colitis, esophageal varices, and ingestion of salicylates, steroids, and nonsteroidal
anti-inammatory agents. Worldwide, the leading cause of GI blood loss is hook-
worm infection (87). In premenopausal women, menstrual blood loss is the most
common cause of iron deciency. The average menstrual blood loss in normal
healthy women is about 40 mL, and women who lose 80 mL or more become iron-
decient. Increased iron requirements during periods of rapid growth, diminished
iron absorption, or both may also cause iron deciency.
In the anemia of chronic disease, iron-decient erythropoiesis results from a
defect in the recycling of hemoglobin iron in the reticuloendothelial system (109).
In patients with anemia of chronic inammation, there appears to be a defect in
the release of iron from macrophages that is probably caused by cytokine-induced
ferritin synthesis. As a result, iron is plentiful in macrophages, but this iron is not
available to erythroid precursors.
There are specialized mammalian cells that must export iron. Absorption of di-
etary iron for transfer to transferrin in plasma requires iron efux across the ba-
solateral surface of the intestinal epithelia. A second major site of iron release
is from macrophages where senescent or damaged red cells are degraded to ex-
port the metal from hemoglobin and provide it for binding to transferrin. Iron
release from these donor cells to plasma transferrin is poorly understood, but
a number of recent studies have provided new clues in this important area of
iron metabolism. A likely candidate for iron export from cells is ferroportin 1
(29), also known as Ireg1 (74) or MTP1 (1), with the ferroxidase activity of hep-
haestin (112) and ceruloplasmin (45) facilitating the movement of iron across
the membranes of enterocytes and macrophages, respectively. Ceruloplasmin and
hephaestin exhibit a high degree of homology; both proteins contain several cop-
per atoms that are necessary for their ferroxidase (i.e., oxidation of Fe
to Fe
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Figure 3 Scheme of possible iron (Fe) pathways in reticuloendothelial macrophages
involved in the recycling of hemoglobin iron. Following phagocytosis of senescent red
blood cells (RBCs), the erythrocyte membrane is lysed and heme is transported to the
endoplasmic reticulum (E.R.) to be degraded by heme oxygenase-1 (HO-1). Most of
the iron derived from hemoglobin catabolism is promptly returned to the circulation,
likely being transported across the plasma membrane by ferroportin 1. In Kupffer cells,
ferroportin 1 (MTP1, 1reg1) is present not only at the plasma membrane but is also
present in the cytoplasm (1). (Reprinted from Reference 89 and used with permission.)
At the end of an erythrocytes life, it is phagocytosed by cells of the reticuloen-
dothelial system and iron is liberated from its connement within the protopor-
phyrin ring by HO-1. These cells have an enormous capacity to purge themselves
of iron, which is likely exported via ferroportin 1 (Figure 3). However, the mecha-
nism involved in the regulation of macrophage iron output is unknown (47). It has
recently been proposed (35, 37) that the plasma peptide hepcidin may be involved
in the regulation of iron release from macrophages, but direct evidence to support
this hypothesis is missing.
Normally, the body iron content in humans is maintained within narrow limits
by the regulation of intestinal iron absorption (77). Both heme and elemental
iron are absorbed through the brush border of the upper small intestine. Heme
iron is more readily available for absorption but usually constitutes only a small
fraction of dietary iron. Heme (derived from hemoglobin or myoglobin) is taken
up intact, probably via specic high-afnity heme-binding sites in the mucosal
brush border (39, 118) (Figure 4). After entering the intestinal epithelial cells, iron
is enzymatically released from heme by HO-1.
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Figure 4 Iron transport across the intestinal epithelium. Iron (Fe) must cross two
membranes to be transferred across the absorptive epithelium. The apical transporter
has been identied as Nramp2/divalent metal transporter 1 (DMT1). It acts in concert
with duodenal cytochrome b (Dcytb) that reduces ferric iron. The basolateral trans-
porter, ferroportin 1, requires ferroxidase activity of hephaestin (ceruloplasmin-like
molecule) for the transfer of iron to the plasma. Hephaestin is depicted here at the
basolateral surface of the cell, but it is not known whether it functions at this location.
Heme iron is taken up by a separate process that is not well characterized. Excess iron
within enterocytes is stored as ferritin. (Reprinted from Reference 89 and used with
Elemental Fe
is virtually insoluble at neutral pH and, therefore, the availability
of dietary iron for intestinal absorption depends on the composition of intestinal
secretions as well as ligands and reducing agents present in the diet. Ascorbic acid is
the most powerful promoter of nonheme iron absorption, which is also enhanced by
the organic acids (e.g., citric acid and amino acids). On the other hand, compounds
that form insoluble complexes with iron (e.g., phosphates, phytates, and tannin)
prevent absorption. Similarly, conditions in which there is a failure of gastric acid
secretion (e.g., atrophic gastritis) may signicantly reduce the availability of iron
for absorption.
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The process of inorganic iron absorption is not fully understood, but a com-
pelling candidate for an iron transporter has recently been identied. Nramp2/
divalent metal transporter 1 (DMT1), which is involved in iron transport across
the endosomal membrane (see below), is also a principal transporter of iron in
the intestine (41, 101). Nramp2/DMT1 transports only the ferrous (reduced) form
of iron, and this explains why reducing agents enhance iron absorption. More-
over, the duodenal brush border contains a ferric reductase, duodenal cytochrome
b (Dcytb) (73), which plays a role in the formation of Fe
prior to its transport
into the enterocyte. The chemical nature of iron in the labile intermediate pool
in enterocytes is unknown, but recently a novel protein necessary for iron egress
from enterocytes was identied. This protein, ferroportin 1 (1, 29, 74), is identical
to the Fe
exporter involved in iron egress from macrophages (Figure 3). The
ferroxidase activity of hephaestin (3, 112), a membrane-bound ceruloplasmin (45)
homologue, also plays an important role in iron export from intestinal epithelial
cells to the circulation. Hephaestin is not an iron transporter itself but likely inter-
acts with the ferroportin 1 to facilitate the movement of iron across the membrane
(Figure 4). Hephaestin is mutated in sex-linked anemia (sla/sla) mice that take up
iron from the intestinal lumen into the epithelial cells normally, but the subsequent
exit of iron into the circulation is diminished (112). It is of interest that during the
process of absorption, iron undergoes at least two changes in its oxidation status:
reduction at the brush border and oxidation at the basolateral membrane.
Physiologically, the major factors affecting iron absorption are the amount of
body iron stores and the rate of erythropoiesis (77). The uptake of iron by mucosal
cells is inversely proportional to total body iron content but seems to be independent
of changes in plasma iron or transferrin concentration. The 3
UTR of mRNA for
Nramp2/DMT1 expressed in intestinal cells contains the iron-responsive element
(IRE) (16, 101); hence, based on the IRE/iron regulatory protein (IRP) paradigm
(see below), diminished Fe levels would be expected to increase Nramp2/DMT1
expression and vice versa. It is unclear how increased erythropoietic activity
(increased plasma iron turnover?) enhances iron absorption. Hypoxia can di-
rectly stimulate iron absorption, independent of changes in erythroid activity.
Interestingly, the gene for Nramp2/DMT1 seems to contain regulatory elements
that can be responsible for its increased transcription under hypoxic conditions
Recent research, based on genetic studies, revealed that hepcidin probably plays,
either directly or indirectly, an important role in iron metabolism. In its presumed
active form, hepcidin is a 22- or 25-amino acid peptide that has intrinsic antimicro-
bial activity (82). Mice that are unable to express hepcidin develop iron overload
associated with decreased iron levels in macrophages (80), whereas animals that
overexpress hepcidin develop lethal iron deciency anemia (81). Hence, it has been
suggested (35, 37) that hepcidin may be a putative signaling molecule mediating
communication between the sites of iron storage (hepatocytes and macrophages)
and iron release from duodenal enterocytes or macrophages. However, thus far no
study has demonstrated that circulating hepcidin itself plays a direct role in iron
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With some notable exceptions (e.g., enterocytes), physiologically, virtually all the
cells in the organism, including erythroid precursors, take up iron from transferrin.
Delivery of iron to cells occurs following the binding of transferrin to transferrin
receptors on the cell membrane (91, 100). The transferrin receptor complexes are
then internalized by endocytosis, and iron is released from transferrin by a process
involving endosomal acidication. Identifying the mechanism of iron transport
across the endosomal membrane was elusive, but a compelling candidate for an
endosomal iron transporter has been identied (34, 41). The transporter, Nramp2
(also known as DMT1 or DCT1, divalent cation transporter 1), is encoded by
a gene that belongs to the Nramp (natural resistance-associated macrophage
protein) family of genes (21). Interestingly, Nramp2 generates two alternatively
spliced mRNAs that differ at their 3
untranslated regions (UTRs) by the presence
or absence of the IRE and that encode two proteins with distinct carboxy termini
(16, 17). Isoform II (derived from non-IRE-containing mRNA; for the denition
of IRE see below) has been identied as the major Nramp2 protein isoform that is
expressed in the developing erythroid cells (17). Also, Nramp2 was not found to be
a limiting factor in erythroid cell iron acquisition via the physiological, transferrin-
dependent, pathway. Because the substrate for Nramp2/DMT1 is Fe
, reduction
of Fe
must occur in endosomes, but little is known about this process. A cDNA
encoding a plasma membrane di-heme protein present in mouse duodenal cells
was found to exhibit ferric reductase activity (73). This protein (Dcytb) belongs to
the cytochrome b561 family of plasma membrane reductases, and it would seem
important to examine whether this or a similar b-type cytochrome is involved in
reduction within endosomes. Following its escape from endosomes, iron is
transported to intracellular sites of use and/or storage in ferritin, but this aspect
of iron metabolism, including the nature of the elusive intermediary pool of iron
and its cellular trafcking, remains enigmatic. Only in erythroid cells does some
evidence exist for specic targeting of iron toward mitochondria, the sites of heme
production by ferrochelatase, the enzyme that inserts Fe
into protoporphyrin
IX. This targeting is demonstrated in hemoglobin-synthesizing cells, where iron
acquired from transferrin continues to ow into mitochondria, even when the
synthesis of protoporphyrin IX is markedly suppressed (85). Moreover, inhibition
of endosome motility decreases the rate of
Fe incorporation into heme from
labeled endosomes, suggesting that in erythroid cells a transient mitochondrion-
endosome interaction may be involved in iron translocation to ferrochelatase (92).
In general, cells are equipped with a remarkable regulatory system that tightly con-
trols iron levels in the labile iron pool (LIP), that is, iron-in-transit among various
intracellular compartments. Sensitive control mechanisms exist that monitor iron
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levels in the LIP and prevent its expansion, while still making the metal available
for iron-dependent proteins and enzymes. In general, enlargement of the LIP leads
to a stimulation of ferritin synthesis and to a decrease in the expression of trans-
ferrin receptors; the opposite scenario develops when this pool is depleted of iron.
Pivotal players in this regulation are IRP1 and IRP2, which senseiron levels in
the LIP.
Iron-dependent regulation of both ferritin and transferrin receptors occurs post-
transcriptionally and is mediated by virtually identical IREs. IREs present in the
UTRs of mRNAs, as in ferritin and erythroid-specic 5-aminolevulinic-acid
synthase (ALA-S2, the rst enzyme of heme biosynthesis), mediate inhibition
of translation of the respective mRNAs in iron-deprived cells. Similar IREs are
also present in the 3
UTR of the transferrin receptor mRNA. These IREs confer
differential stability to transferrin receptor mRNAs as a function of cellular iron
levels. The IREs are nucleotide sequences that are recognized by specic cytosolic
RNA-binding proteins known as IRP1 and IRP2. An IRE-binding form of each
IRP accumulates in iron-depleted cells, but the mechanism of accumulation differs.
When cellular iron is low, IRP1 is in a form that can bind to IREs, and IRP2 (which
has constitutive RNA-binding activity) is stable. Binding of IRPs to IREs found in
the 5
end of mRNA (ferritin, erythroid-specic ALA-S2) inhibits translation of
these transcripts, whereas binding to IREs in the 3
UTR of the transferrin receptor
mRNA (and probably also in the intestinal form of mRNA for Nramp2/DMT1)
stabilizes the transcripts. Hence, iron deciency promotes cellular iron acquisi-
tion and possibly intestinal iron absorption while it decreases levels of the cellular
iron-storing protein, ferritin. On the other hand, the expansion of the LIP inacti-
vates IRP1 and leads to a degradation of IRP2, resulting in efcient translation of
ferritin mRNA (and ALA-S2 mRNA in erythroid cells) and rapid degradation of
transferrin receptor mRNA (2, 15, 76, 91, 100, 104).
Some cells and tissues with specic requirements for iron evolved mechanisms
that can override the IRE/IRP-dependent control of transferrin receptor formation.
Erythroid cells, which are the most avid consumers of iron in organisms, use
primarily a transcriptional mechanism to maintain very high transferrin receptor
levels (68, 85). Moreover, erythroid cells are equipped with an important regulatory
mechanism that coordinates protoporphyrin IX formation with iron supply (85).
Because the 5
UTR of mRNA for erythroid-specic ALA-S2 contains the IRE,
the formation of ALA-S2 (the rate-limiting enzyme of porphyrin biosynthesis)
and, consequently, protoporphyrin depends on the availability of iron.
Although three different and totally distinct pathways are involved in hemoglobin
synthesis, virtually no intermediates, i.e., globin chains, porphyrin intermedi-
ates, or iron, accumulate in the developing erythroblasts and reticulocytes. This
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regulation is achieved, at least in part, by a series of negative and positive feedback
mechanisms in which both iron and heme play important roles. First and foremost,
the supply of iron via the transferrin-receptor pathway limits, and thus controls,
heme synthesis rate in erythroid precursors. Moreover, in erythroid cells uncom-
mitted heme inhibits cellular iron acquisition and, consequently, heme synthesis.
Furthermore, availability of heme is essential for the synthesis of globin at both the
transcriptional and, more importantly, the translational levels (85, 86). Numerous
reports indicate that heme stimulates globin gene transcription and is probably in-
volved in promoting some other aspects of erythroid differentiation (105). Hemin
treatment of erythroid precursors leads to rapid accumulation of globin mRNA,
whereas heme deciency leads to a decrease in globin mRNA levels (27, 36, 102,
103, 105). These effects can probably be explained by heme-mediated upregulation
of the erythroid transcription factor NF-E2 binding activity (105).
It has long been known that the translation of globin in intact reticulocytes and
their lysates is dependent on the availability of heme (14, 23, 69, 124). Heme de-
ciency inhibits protein synthesis through activation of heme-regulated inhibitor
(HRI). HRI is a cyclic adenosine monophosphate (AMP)-independent protein ki-
nase that specically phosphorylates the α-subunit of eukaryotic initiation factor
2 (eIF-2). Recent research has revealed that autophosphorylation of threonine 485
is essential for the phosphorylation and activation of HRI and is required for the
acquisition of the eIF-2α kinase activity (96). During translation initiation, eIF-
2-GTP, associated with Met-tRNA
, binds to 40 S subunit and participates in
the recognition of the initiation codon. After translation initiation, eIF-2-GTP is
hydrolyzed to eIF-2-GDP. Because eIF-2 has a much greater afnity for GDP, a
guanine nucleotide exchange factor, eIF-2B, is required to recycle eIF-2 to the
GTP-bound form. Phosphorylation of eIF-α at serine 51 blocks the activity of
eIF-2B, reducing the level of eIF-2-GTP. Heme binding to HRI inhibits the phos-
phorylation of eIF-2α by HRI, resulting in an efcient translation of globin and
probably other proteins in erythroid cells (23, 113). Association of heme with
HRI inhibits the enzyme by promoting the formation of disulde bonds, perhaps
between two HRI subunits (23). Disulde bond formation reverses the inhibition
of protein synthesis seen during heme deciency. Interestingly, HRI contains two
sequences that are similar to the heme regulatory motif found in numerous other
proteins whose functions are regulated by heme. Importantly, the HRI homodimer
has two distinct types of heme-binding sites (22). Binding of heme to the rst site
is stable (i.e., HRI is a hemoprotein), whereas binding of heme to the second site
is responsible for the rapid down-regulation of HRI activity (22). The mRNA for
HRI is present in uninduced murine erythroleukemia cells and is increased after
the induction of erythroid differentiation. This accumulation of HRI mRNA in dif-
ferentiating murine erythroleukemia cells is dependent upon the presence of heme
because an inhibitor of heme synthesis markedly reduces HRI mRNA accumula-
tion (26); hence, HRI plays an important physiological role in the translation of
globin and probably other proteins synthesized in erythroid cells. This conclusion
is further supported by the nding that expression of dominant-negative mutants
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of HRI in murine erythroleukemia cells increased hemoglobin production in these
cells upon DMSO induction of erythroid differentiation (25).
Mice rendered null for the HRI gene (HRI
) appear to be normal, fertile,
and without gross abnormalities of hematologic parameters (43). However, in
erythroid cells from iron-decient HRI
mice, a marked increase in both α-
and β-globin synthesis led to accumulated globins that were devoid of heme and
aggregated within the erythrocytes and their precursors. This resulted in a hy-
perchromic normocytic anemia with decreased erythrocyte counts, compensatory
erythroid hyperplasia, and accelerated apoptosis in bone marrow and spleen (i.e.,
ineffective erythropoiesis). These important results established the physiological
role of HRI in balancing the synthesis of α- and β-globins with the availability of
heme in developing erythroid cells. Moreover, these results have demonstrated that
the translational regulation of HRI in iron deciency is essential for the survival
of erythroid precursors (43).
In conclusion, in erythroid cells iron is not only the substrate for the synthesis
of hemoglobin but also participates in its regulation. Moreover, the iron proto-
porphyrin complex appears to enhance globin gene transcription, is essential for
globin translation, and supplies the prosthetic group for hemoglobin assembly.
Erythropoiesis during normal conditions, folate deciency, and iron deciency is
shown in Figure 5 (see color insert). Erythropoiesis during vitamin B
is similar to that shown for folate deciency. In normal erythropoiesis, a minor-
ity of the EPO-dependent cells survives the EPO-dependent period, giving rise
to basophilic erythroblasts that divide and mature into orthochromatic erythrob-
lasts and reticulocytes. Because the periods of EPO dependence and hemoglobin
synthesis do not overlap, apoptosis of progenitors during normal erythropoiesis
does not increase serum bilirubin. During folate-decient erythropoiesis, increased
apoptosis due to DNA damage extends into the post-EPO-dependent stages, where
hemoglobin synthesis has begun but has not yet reached high levels. This apopto-
sis of cells that have begun hemoglobin synthesis causes slightly increased serum
bilirubin. Those folate-decient erythroblasts surviving to the late stages produce
fewer but larger reticulocytes, leading to macrocytic anemia. The anemia induces
EPO production, which in turn increases the survival of cells in the EPO-dependent
stages compared to normal erythropoiesis. However, the expansion of these EPO-
dependent populations is relatively incomplete due to increased apoptosis from
the folate deciency. In iron deciency, decreased synthesis of heme results in
the decreased protein translation, especially of globins, through the enhanced
action of HRI. This decreased protein translation in the iron-decient erythroid
cells results in retarded reticulocyte production and smaller, less hemoglobinized
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reticulocytes, leading to microcytic anemia. The resultant anemia induces EPO
production, which decreases the apoptosis in the EPO-dependent cells relative
to normal erythropoiesis. This increased survival in the EPO-dependent stages,
however, does not result in increased reticulocyte production due to the inhibitory
effect of HRI during the subsequent hemoglobin synthesis stages.
MJK is supported by a Merit Review Award from the Department of Veterans
Affairs. PP thanks the Canadian Institutes of Health Research for support. The
authors thank Alex Sheftel, Conrad Wagner, and Maurice Bondurant for their
helpful discussions and suggestions, and Sandy Fraiberg and Michael Forbes for
excellent editorial assistance.
The Annual Review of Nutrition is online at
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Figure 5 Models of erythropoiesis. Period of erythroid differentiation shown extends
from the actively dividing CFU-E through the postmitotic Ortho-EB and Retics, which
are shown with their enucleated nuclei. Irregular nuclear fragments in the Poly-EB and
earlier stages represent apoptotic cells. The arrows between the stages represent rela-
tive rates of progression between stages. Colors are as stained with 3,3’-dimethoxy-
benzidine and hematoxylin. Orange represents accumulated hemoglobin.
Abbreviations: Baso-EB, basophilic erythroblast; CFU-E, colony-forming unit-ery-
throid; Ortho-EB, Orthochromatic erythroblast; Poly-EB, polychromatophilic ery-
throblast; Pro-EB, proerythroblast; Retic, reticulocyte. Reprinted with modifications
from Koury MJ, Horne DW, Brown ZA, Pietenpol JA, Blount BC, Ames BN, Hard R,
and Koury ST. 1997. Apoptosis of late-stage erythroblasts in megaloblastic anemia:
association with DNA damage and macrocyte production. Blood 89:4617–23.
Copyright American Society of Hematology, used with permission.
Koury.qxd 12/24/2003 1:39 PM Page 1
... Iron is bound, transported, and delivered around the body by the transferrin glycoprotein 8 , while the main intracellular iron storage, ferritin, provides a long-term reserve of iron for the formation of hemoglobin and other heme proteins [9][10][11] . Serum iron, serum ferritin, transferrin saturation percentage (TSP), and the total iron-binding capacity (TIBC) of transferrin are biochemical measurements that are commonly used together to assess an individual's iron status 12 . ...
... b: 1 Anemias (iron deficiency and other anemias), 2 Platelet indices (platelet count, platelet crit, platelet dist. width), 3 White blood cell counts (lymphocytes, leukocytes, monocytes), 4 Seated height, 5 Water mass, 6 Fat-free mass, 7 Phlebitis and thrombophlebitis, 8 Non-alcoholic liver disease, 9 Gamma-glutamyl Transferase, 10 Direct bilirubin, 11 Urate, 12 Creatinine. c: 1 Mean platelet volume, 2 Platelet crit, 3 Platelet dist. ...
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Iron is essential for many biological processes, but iron levels must be tightly regulated to avoid harmful effects of both iron deficiency and overload. Here, we perform genome-wide association studies on four iron-related biomarkers (serum iron, serum ferritin, transferrin saturation, total iron-binding capacity) in the Trøndelag Health Study (HUNT), the Michigan Genomics Initiative (MGI), and the SardiNIA study, followed by their meta-analysis with publicly available summary statistics, analyzing up to 257,953 individuals. We identify 123 genetic loci associated with iron traits. Among 19 novel protein-altering variants, we observe a rare missense variant (rs367731784) in HUNT, which suggests a role for DNAJC13 in transferrin recycling. We further validate recently published results using genetic risk scores for each biomarker in HUNT (6% variance in serum iron explained) and present linear and non-linear Mendelian randomization analyses of the traits on all-cause mortality. We find evidence of a harmful effect of increased serum iron and transferrin saturation in linear analyses that estimate population-averaged effects. However, there was weak evidence of a protective effect of increasing serum iron at the very low end of its distribution. Our findings contribute to our understanding of the genes affecting iron status and its consequences on human health.
... The prevalence of dyslipidemia in Taiwan has been increasing because of changes in lifestyle and eating habits. According to the findings of the Nutrition and Health Survey in Taiwan (NAHSIT) 2016-2019, the prevalence of hyperlipidemia, defined as triglycerides (TG) ≥ 2.26 mmol/L, total cholesterol (TC) ≥ 6.21 mmol/L, or antihyperlipidemic medication in women aged [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39], and 40-49 years, were 5.1%, 8.0%, and 14.0%, ...
... respectively, among adult women in six ethnic groups in the US [34], suggesting that an animal diet may play a substantial role in the supply of iron and vitamin B 12 . Iron, vitamin B 12 , and folate are predominant nutrients for erythropoiesis, and a lack of these nutrients could consequently lead to the development of anemia [35]. ...
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Anemia and dyslipidemia often occurs in premenopausal women. This study investigated the association between dietary patterns and anemia among dyslipidemic women in Taiwan. This study recruited 22,631 dyslipidemic women aged 20–45 years between 2001 and 2015. The dietary assessment was collected by a validated food frequency questionnaire. The biochemical data including blood lipids, red blood cells, hemoglobin, hematocrit, and C-reactive protein (CRP) were retrieved from the database. Women with a combined high plant diet (HP) and low animal diet (LA) were associated with a lower prevalence of obesity (11.7%), central obesity (16.0%), high total cholesterol (16.4%), and high low-density lipoprotein cholesterol (11.9%), and had lower hemoglobin (12.9 ± 1.4 g/dL), hematocrit (38.8 ± 3.6%), and CRP levels (20.6 ± 31.4 nmol/L). The low plant diet (LP) + high animal diet (HA) pattern was negatively associated with moderate to severe anemia (OR: 0.76, 95% CI: 0.64–0.92, p = 0.004) compared to the low plant diet (LP) + low animal diet (LA) pattern. However, the HP + LA pattern was positively correlated with moderate to severe anemia (OR: 1.22, 95% CI: 1.04–1.43, p = 0.015). In conclusion, a low plant and high animal diet plays a role in preventing anemia development among dyslipidemic women.
... Anaemia in the reproductive age of women is defined as the haemoglobin level of less than 11 g per decilitre [7]. The deficiency of micronutrients like iron, zinc, vitamin B12, vitamin A, and folic acid is linked to inadequate nutrient consumption and is the primary predictor of anaemia [8][9][10][11][12][13]. In addition, large-scale studies have found that low socioeconomic status and lack of education are major determinants of anaemia in women [14][15][16]. ...
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Background Anaemia among women is a public health problem with associated adverse outcomes for mother and child. This study investigates the determinants of women’s anaemia in two Bengals; West Bengal (a province of India) and Bangladesh. These two spaces are inhabitated by Bengali speaking population since historic past. The study argues that open defecation, contraceptive method use and food consumption patterns are playing crucial role in explaining anaemia. Methods Using non-pregnant women belonging to different religious groups, we analyzed a total of 21,032 women aged 15–49 from the nationally representative cross-sectional surveys, i.e., Bangladesh Demographic Health Survey (BDHS-VI, 2011) and National Family Health Survey (NFHS round 4, 2015–16). We performed spatial, bivariate and logistic regression analyses to unfold the important risk factors of anaemia in two Bengals. Results The prevalence of anaemia was 64% in West Bengal and 41% in Bangladesh. The significant risk factors explaining anaemia were use of sterilization, vegetarian diet and open defecation. Further, women who used groundwater (tube well or well) for drinking suffered more from anaemia. Also, younger women, poor, less educated and having more children were highly likely to be anaemic. The study also indicates that those who frequently consumed non-vegetarian items and fruits in West Bengal and experienced household food security in Bangladesh were less prone to be anaemic. Hindus of West Bengal, followed by Muslims of that state and then Hindus of Bangladesh were at the higher risk of anaemia compared to Muslims of Bangladesh, indicating the stronger role of space over religion in addressing anaemia. Unlike West Bengal, Bangladesh observed distinct regional differences in women's anaemia. Conclusions Propagating the choices of contraception mainly Pill/ injection/IUDs and making the availability of iron rich food along with a favourable community environment in terms of safe drinking water and improved sanitation besides better education and economic condition can help to tackle anaemia in limited-resource areas.
... These increases in erythrocyte size and hemoglobin content in the patients randomized to vadadustat compared with those randomized to darbepoetin are most consistent with improved iron delivery to erythroblasts, where increased intracellular iron enhances heme synthesis that, in turn, increases hemoglobin accumulation and cell size.57 Erythroblasts of vadadustat-treated patients should contain increased heme due to HIF increasing transcription of the erythroidspecific ALAS-2 gene 58 and iron-regulatory protein increasing translation of ALAS-2 mRNAs.59 Increased erythroblast heme, in turn, induces enhanced heme-regulated inhibitor-controlled synthesis of globins and other proteins, 57 leading to production of larger erythrocytes containing more hemoglobin (i.e., increased mean MCV and MCH) as observed in patients randomized to vadadustat compared with those randomized to darbepoetin. ...
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Patients with chronic kidney disease (CKD) develop anemia largely because of inappropriately low erythropoietin (EPO) production and insufficient iron available to erythroid precursors. In four phase 3, randomized, open‐label, clinical trials in dialysis‐dependent and non–dialysis‐dependent patients with CKD and anemia, the hypoxia‐inducible factor prolyl hydroxylase inhibitor, vadadustat, was noninferior to the erythropoiesis‐stimulating agent, darbepoetin alfa, in increasing and maintaining target hemoglobin concentrations. In these trials, vadadustat increased the concentrations of serum EPO, the numbers of circulating erythrocytes, and the numbers of circulating reticulocytes. Achieved hemoglobin concentrations were similar in patients treated with either vadadustat or darbepoetin alfa, but compared with patients receiving darbepoetin alfa, those receiving vadadustat had erythrocytes with increased mean corpuscular volume and mean corpuscular hemoglobin, while the red cell distribution width was decreased. Increased serum transferrin concentrations, as measured by total iron‐binding capacity, combined with stable serum iron concentrations, resulted in decreased transferrin saturation in patients randomized to vadadustat compared with patients randomized to darbepoetin alfa. The decreases in transferrin saturation were associated with relatively greater declines in serum hepcidin and ferritin in patients receiving vadadustat compared with those receiving darbepoetin alfa. These results for serum transferrin saturation, hepcidin, ferritin, and erythrocyte indices were consistent with improved iron availability in the patients receiving vadadustat. Thus, overall, vadadustat had beneficial effects on three aspects of erythropoiesis in patients with anemia associated with CKD: increased endogenous EPO production, improved iron availability to erythroid cells, and increased reticulocytes in the circulation.
... Folate and vitamin B12 are required in the synthesis of SAM, the sole methyl donor in numerous methylation reactions involving proteins, phospholipids, and biogenic amines. In the case of folate or vitamin B deficiency, the S-adenosylmethionine synthesis may become severely impaired, which might be the cause of many neuropsychiatries (28,(44)(45)(46). Reduced synthesis of S-adenosylmethionine could be due to a hypomethylation state, leading in turn to impaired synthesis of proteins and neurotransmitters necessary for the brain structural integrity, and OCD was further induced (47). ...
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Background Obsessive–compulsive disorder (OCD) a complex neuropsychiatric disorder, is characterized by irresistible obsessive thinking and compulsive behavior. Folate is a member of water-soluble vitamins in the human body and sustains many normal daily activities (e.g., exercise, sleep, and memory). Homocysteine, a sulfur-containing non-essential amino acid, has been investigated in numerous psychiatric disorders (e.g., OCD). Vitamin B12 is a type of complex organic compound with cobalt contained. Moreover, vitamin B12 and folate deficiency and high levels of homocysteine were found to have an effect on brain functions and also lead to non-specific psychiatric symptoms. Objectives This study aimed to confirm the epidemiological evidence of OCD and investigate whether vitamin B12, folate, and homocysteine have an effect on the etiology of OCD. Methods A systematic search was conducted on eight databases (i.e., PubMed, Embase, Web of Science, the Cochrane Library, China Biology Medicine disc, China National Knowledge Infrastructure, Wanfang Database, China Science and Technology Journal Database), and the retrieval time was up to March 2021. The available articles involving patients with OCD with/without abnormal serum levels of vitamin B12, folate, and homocysteine were comprehensively reviewed and analyzed. Results A total of 5 studies involving 309 patients were included in this meta-analysis, including 172 cases in the experimental group and 137 in the control group. The content of folate in the OCD group was not significantly different from that in the control group (SMD = −0.089, 95%CI −0.755 to 0.577, p = 0.794). And serum homocysteine was significantly higher in the patients with OCD (SMD = 1.132, 95%CI 0.486 to 1.778, p = 0.001). Vitamin B12 was significantly lower in patients with OCD (SMD = −0.583, 95%CI −0.938 to −0.229, p = 0.001). Conclusions This meta-analysis shows serum high levels of homocysteine, low levels of vitamin B12, and normal folate level are closely correlated with OCD. However, high-quality case-control studies should be further conducted to explore the correlation between serum levels of vitamin B12, folate, homocysteine, and OCD. Systematic Review Registration ; PROSPERO (Number CRD#42021262161 ).
... For example, vitamin D, other than regulating bone metabolism, also plays extra-skeletal functions, including regulation of the immune system response [5], as well as antioxidant properties of different molecules, such as vitamin E, vitamin C, and beta-carotene, are able to protect biological systems from various stressors. Furthermore, vitamins B9 and B12 are involved in homocysteine metabolism and also play a pivotal role in the process of erythropoiesis [6], and vitamin C, other than being a powerful antioxidant, is essential for iron absorption [7]. ...
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Numerous approaches demonstrate how nutritional intake can be sufficient to ensure the necessary supply of vitamins. However, it is evident that not all vitamins are contained in all foods, so it is necessary either to combine different food groups or to use a vitamin supplement to be well-fed. During pregnancy, deficiencies are often exacerbated due to increased energy and nutritional demands, causing adverse outcomes in mother and child. Micronutrient supplementation could lead to optimal pregnancy outcomes being essential for proper metabolic activities that are involved in tissue growth and functioning in the developing fetus. In order to establish adequate vitamin supplementation, various conditions should be considered, such as metabolism, nutrition and genetic elements. This review accurately evaluated vitamin requirements and possible toxic effects during pregnancy. Much attention was given to investigate the mechanisms of cell response and risk assessment of practical applications to improve quality of life. Importantly, genetic studies suggest that common allelic variants and polymorphisms may play an important role in vitamin metabolism during pregnancy. Changes in gene expression of different proteins involved in micronutrients’ metabolism may influence the physiological needs of the pregnant woman.
... Vitamin levels recorded in this study are relatively comparable with those in other Arthrospira species [83][84]. In spirulina, vitamins and coenzymes synthesize haemoglobin and act upon hematopoietic oxidative processes in the bone marrow [85]. GSH is a powerful antioxidant that has been commercialized and is widely used in many fields. ...
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Increasing the consumption of natural substances has increased the demand for biological sources such as Spirulina platensis. The study quantitatively investigates the antioxidant potential and phytonutrient contents in aqueous and ethanol extracts of spirulina. The spirulina was collected from a local farm near Pondicherry and mass cultured in our research laboratory. The spirulina biomass was evaluated for antioxidant potential viz. catalase, SOD, GPx, Vitamin C, Vitamin E, and reduced GSH; phytonutrients contents like total phenol, flavonoid, tannin, carbohydrates, and proteins in both aqueous and ethanolic extracts of spirulina. Significant enzymatic antioxidant activity was observed for ethanolic extract. However, aqueous extracts were higher for catalase, SOD, and GPx activity. The same trend was observed for non-enzymatic activities. Total phenol, flavonoid, and tannin content were observed and high in aqueous extract. However, protein and carbohydrate content were higher in ethanolic extract. We observed a significant change in antioxidant activity and phytonutrient content in ethanolic extract than in aqueous extracts. The strong antioxidant property and higher phytonutrient contents of spirulina can play a vital role in the dietary supplement and combating malnutrition.
... Hence, the primary treatment of anaemia focuses on the use of an iron salt either alone or in a combination of Folic Acid and/or Cyanocobalamin. It should be noted, however, that effective treatment depends on the identification of the actual cause and accordingly the treatment 10 . ...
Malnutrition is a condition in which there is an imbalance in the proportion of essential nutrients that the body requires to stay healthy. The most prevalent form of malnutrition is micronutrient deficiency and of this iron deficiency anaemia is the most common type, affecting over 2 billion people worldwide. Micronutrients are a special class of vital ingredients that are required in minuscule quantities to perform a wide range of physiological functions. The current therapy for the treatment of Iron deficiency anaemia has many drawbacks like gastrointestinal side effects, erratic absorption of iron, etc. To overcome the above side effects, we tried to develop and evaluate a site-specific sustained-release combined drug delivery system of Iron and Folic acid. Iron as Ferrous ions is preferentially absorbed in the jejunum. To facilitate the absorption of iron, the gastro retentive multiunit particulate system (GRMUPs) was developed comprising Ferrous Ascorbate is a source of iron. Colon targeted tablet was developed for Folic acid to localize and maximize the absorption of Folic acid in the colon. The objective was to avoid competitive absorption of two micro-nutrients in the jejunum. A factorial design was employed to optimize the formulation of GRMUPs effervescent system containing Ferrous Ascorbate. The GRMUPs were evaluated for floating lag time, floating time, and drug release profile. Folic acid core tablets were coated with colon targeted film and were evaluated for site-specific release in the simulated colonic fluid. Further, both the systems were filled in a capsule to provide once-a-day administration.
Shellfish, in particular bivalves, are an often-overlooked source of vitamin B12 (B12) in the human diet although they have significantly higher tissue levels of B12 than other animal meat or fish sources, including all vertebrates. However, the origins and key metabolic processes involving B12 in bivalves remain largely unknown. In this study, we examined the distribution of B12 in tissues of several adult Australian bivalve species and assessed hypotheses concerning their B12 utilisation and principal uptake, specifically whether it is derived from diet or gut microbiome. Pacific oysters, Crassostrea gigas, and Goolwa cockles, Plebidonax deltoides (‘pipis’), are both high in B12 (28.0–49.4 μg/100 g total per individual). Vitamin B12 tissue distribution, particularly in oysters, varied significantly, with higher amounts in the adductor muscle (44.0–96.7 μg/100 g), and other tissues, such as gonads, were relatively low (12.7–35.9 μg/100 g). In comparison, concentrations of B12 in the adductor muscle and roe of Southern Australian scallops, Pecten fumatus, were appreciably lower (3.4–10.8 μg/100 g). We also demonstrated that microalgal feed commonly grown in aquaculture can be supplemented directly with B12, resulting in an enriched feed. However, the B12-enriched diet did not transfer to a significant increase in oyster larval B12 concentrations, contradicting our theory that vitamin uptake through feed was a primary B12 source. Vitamin B12 concentrations across oyster larval life stages showed a significant decrease post metamorphosis, which indicates a higher utilisation of B12 during this life event. Our findings also provide insight into B12 uptake and tissue distribution in bivalve species, which can aid the aquaculture industry in promotion of bivalves as a valuable source of dietary B12 for human consumers, while also suggesting ways to optimise vitamin supplementation in bivalve hatchery production.
Nutraceuticals are food or parts of food that contain bioactives with physiological as well as medicinal effects. They can help maintain human health and are useful in preventing numerous acute and chronic diseases. Various food products such as prebiotics, probiotics, dietary fibers, fatty acids (polyunsaturated), antioxidants, spices, herbs, nutrients and dietary supplements can be considered nutraceuticals. Byproducts from food and agricultural sources are also valuable source of nutraceuticals e.g. leaves, peels, stems, seed flours. Nutraceuticals possess both nutritional as well as therapeutic value. Nutraceuticals are helpful for the prevention of various diseases such as overweight, CVD, cancer, osteoporosis, arthritis, increased sugar and cholesterol levels, etc. This chapter provides a preamble for the book about nutraceuticals, the various types, sources and importance.
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Divalent metal transporter 1 (DMT1) is the major transferrin-independent iron uptake system at the apical pole of intestinal cells, but it may also transport iron across the membrane of acidified endosomes in peripheral tissues. Iron transport and expression of the 2 isoforms of DMT1 was studied in erythroid cells that consume large quantities of iron for biosynthesis of hemoglobin. In mk/mk mice that express a loss-of-function mutant variant of DMT1, reticulocytes have a decreased cellular iron uptake and iron incorporation into heme. Interestingly, iron release from transferrin inside the endosome is normal in mk/mk reticulocytes, suggesting a subsequent defect in Fe++ transport across the endosomal membrane. Studies by immunoblotting using membrane fractions from peripheral blood or spleen from normal mice where reticulocytosis was induced by erythropoietin (EPO) or phenylhydrazine (PHZ) treatment suggest that DMT1 is coexpressed with transferrin receptor (TfR) in erythroid cells. Coexpression of DMT1 and TfR in reticulocytes was also detected by double immunofluorescence and confocal microscopy. Experiments with isoform-specific anti-DMT1 antiserum strongly suggest that it is the non–iron-response element containing isoform II of DMT1 that is predominantly expressed by the erythroid cells. As opposed to wild-type reticulocytes, mk/mk reticulocytes express little if any DMT1, despite robust expression of TfR, suggesting a possible effect of the mutation on stability and targeting of DMT1 isoform II in these cells. Together, these results provide further evidence that DMT1 plays a central role in iron acquisition via the transferrin cycle in erythroid cells.
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Folate deficiency causes massive incorporation of uracil into human DNA (4 million per cell) and chromosome breaks. The likely mechanism is the deficient methylation of dUMP to dTMP and subsequent incorporation of uracil into DNA by DNA polymerase. During repair of uracil in DNA, transient nicks are formed; two opposing nicks could lead to chromosome breaks. Both high DNA uracil levels and elevated micronucleus frequency (a measure of chromosome breaks) are reversed by folate administration. A significant proportion of the U.S. population has low folate levels, in the range associated with elevated uracil misincorporation and chromosome breaks. Such breaks could contribute to the increased risk of cancer and cognitive defects associated with folate deficiency in humans.
Direct marrow cytogenetic preparations were examined in 14 cases of anemia associated with deficiency in vitamin B12 and/or folate. Seven cases showed distinct alterations in chromosome structure consisting of increased chromosome breakage, incomplete chromosome contraction and centromere spreading. These abnormalities were not present after vitamin replacement and were generally less distinct in cases with major medical complications or with only mild megaloblastic changes. No distinct aberrations in chromosome number or karyotype were found. The cytogenetic changes observed seem compatible with current concepts of megaloblastic cell division and with the role of folate and vitamin B12 in DNA metabolism.
Vitamin B-12 deficiency is present in up to 15% of the elderly population as documented by elevated methylmalonic acid with or without elevated total homocysteine concentrations in combination with low or low-normal vitamin B-12 concentrations. Clinical signs and symptoms of vitamin B-12 deficiency are insensitive in elderly subjects and comorbidity in these subjects makes responses to therapy difficult to interpret. Many elderly subjects with hyperhomocysteinemia have undiagnosed vitamin B-12 deficiency with elevated serum methylmalonic acid concentrations. Therefore, such elderly subjects should not receive folic acid supplementation before their vitamin B-12 status is diagnosed. Oral vitamin B-12 supplementation may be effective in lowering serum methylmalonic acid values in the elderly. However, the dose of vitamin B-12 in most common multivitamin preparations is too low for this purpose. Research efforts should be directed toward determining practical methods for diagnosing and treating vitamin B-12 deficiency in the millions of elderly subjects with undiagnosed deficiency.
Stressed mammalian cells up-regulate heme oxygenase 1 (Hmox1; EC, which catabolizes heme to biliverdin, carbon monoxide, and free iron. To assess the potential role of Hmox1 in cellular antioxidant defense, we analyzed the responses of cells from mice lacking functional Hmox1 to oxidative challenges. Cultured Hmox1(-/-) embryonic fibroblasts demonstrated high oxygen free radical production when exposed to hemin, hydrogen peroxide, paraquat, or cadmium chloride, and they were hypersensitive to cytotoxicity caused by hemin and hydrogen peroxide. Furthermore, young adult Hmox1(-/-) mice were vulnerable to mortality and hepatic necrosis when challenged with endotoxin. Our in vitro and in vivo results provide genetic evidence that up-regulation of Hmox1 serves as an adaptive mechanism to protect cells from oxidative damage during stress.
This study was designed to identify the cellular component of the intestinal villus where transcobalamin II (TCII) is synthesized, because this protein provides an essential function in the intestinal absorption of vitamin B-12 (cobalamin, Cbl). When a segment of proximal or distal small intestine of the guinea pig is cultured in medium containing [Co-57]Cbl, TCII[Co-57]Cbl appears within 15 min. Northern blot analysis of RNA from both proximal and distal small intestine identified the TCII transcript. In situ hybridization of the distal ileum with S-35-labeled TCII antisense transcript localized grains predominantly in crypts and in the lower third and central core of the villi. Grains were also evident at the base of the enterocytes in close apposition with the vascular network, whereas few grains appeared in the apical region of the columnar cells. This study provides evidence that TCII is constitutively expressed in the intestinal villi where vascular endothelium is abundant. In the distal ileum, where the intrinsic factor (IF) receptor is expressed, after uptake of IF-Cbl and the subsequent binding of free Cbl to TCII synthesized in the villi, the TCII-Cbl complex enters the microcirculation and passes into the portal blood.
The anemia of chronic disease traditionally is defined as a hypoproliferative anemia of no apparent cause that occurs in association with an inflammatory, infectious, or neoplastic disorder, and resolves when the underlying disorder is corrected. Disordered iron metabolism as manifested by a low serum iron, decreased serum transferrin, decreased transferrin saturation, increased serum ferritin, increased reticuloendothelial iron stores, increased erythrocyte-free protoporphyrin, and reduced iron absorption, is a characteristic feature of the anemia of chronic disease and has been thought to be a major factor contributing to the syndrome. A mild shortening of red cell life span also occurs. However, we now know that impaired erythropoietin production and impaired responsiveness of erythroid progenitor cells to this hormone are also important abnormalities contributing to the anemia of chronic disease, and appear to be due to the effects of inflammatory cytokines. Increased intracellular iron may also have a role in the inhibition of erythropoietin production, since the oxygen sensor is a hemoprotein. While the role of inflammatory cytokines in the pathogenesis of anemia of chronic disease appears unequivocal, it has become apparent that disordered iron metabolism, while characteristic of this form of anemia, may not be central to its pathogenesis. It is undisputed that iron absorption is reduced, and that iron administered intravenously is rapidly sequestered in the reticuloendothelial system; however, iron delivery to the bone marrow is not impaired, and erythroid iron utilization is not markedly depressed in anemia of chronic disease. Importantly, recombinant erythropoietin therapy can correct the anemia of chronic disease, but it cannot correct the anemia due to iron deficiency. This refutes the concept that the lack of available iron is central to the pathogenesis of the syndrome. Indeed, it is highly likely that abnormalities such as reduced iron absorption and decreased erythroblast transferrin-receptor expression largely result from decreased erythropoietin production and inhibition of its activity by inflammatory cytokines.
The folates are made up of a pterdine ring attached to a p-aminobenzoate and a polyglutamyl chain. The active form is tetrahydrofolate which can have C1 units enzymically attached. These C1 units (as a formyl group) are passed on to enzymes in the purine pathway that insert the C−2 and C−8 into the purine ring. A methylene group (−CH2−) attached to tetrahydrofolate is used to convert the uracil-type pyrimidine base found in RNA into the thymine base found in DNA. A further folate cofactor, i.e. 5-methyltetrahydrofolate, is involved in the remethylation of the homocysteine produced in the methylation cycle back to methionine. After activation to S-adenosylmethionine this acts as a methyl donor for the dozens of different methyltransferases present in all cells. Folate deficiency results in reduction of purine and pyrimidine biosynthesis and consequently DNA biosynthesis and cell division. This process is most easily seen in a reduction of erythrocytes causing anaemia. Reduction in the methylation cycle has multiple effects less easy to identify. One such effect is certainly on the nerve cells, because interruption of the methylation cycle causing neuropathy can also happen in vitamin B12 deficiency due to reduced activity of the vitamin B12-dependent enzyme methionine synthase (EC In vitamin B12 deficiency, blocking of the methylation cycle causes the folate cofactors in the cell to become trapped as 5-methyltetrahydrofolate. This process in turn produces a pseudo folate deficiency in such cells, preventing cell division and giving rise to an anaemia identical to that seen in folate deficiency.