Introduction
Iron is a paradoxical element, essential for living organsms but also potentially
toxic. Indeed, iron has the ability to readily accept and donate electrons, interconverting
from soluble ferrous form (Fe2+) to the insoluble ferric form (Fe3+). This capacity
allows iron to play a major role in oxygen transport (as the central part of hemoglobin)
but also in electron transfer, nitrogen fixation or DNA synthesis, all essential reactions
for living organisms. Indeed, iron deficiency is the main cause of anemia [1] as well
as a cause of fatigue [2,3] and decreased effort capacity [4,5]. However, despite
a high frequency of anemia among critically ill patients, with 60 to 66% being anemic
at intensive care unit (ICU) admission [6,7], to date little is known about iron deficiency
and iron metabolism in critically ill patients [8]. The interaction between inflammation
and iron metabolism interferes with the usual iron metabolism variables and renders
this metabolism difficult to investigate [9,10].
The recent discovery of hepcidin (the master regulator of iron metabolism) has shed
new light on the regulation of iron homeostasis and has helped our understanding of
complex clinical situations, such as those observed in critically ill patients, where
several regulatory circuits interfere with iron metabolism [11]. The purpose of this
article is to review iron metabolism and anemia in critically ill patients as well
as the role of hepcidin, and to discuss the indications for iron supplementation in
these patients.
Iron metabolism overview and the role of hepcidin
Although iron is essential for life, it may also be toxic because of its capacity
to react with oxygen and to promote the production of free radicals. This duality
is found in human pathology: Iron deficiency (because of poor iron intake, abnormal
blood losses etc...) presents with anemia and fatigue; whereas iron overload (mainly
in hereditary hemochromatosis and following repeated blood transfusions) induces multiple
organ dysfunctions (including liver fibrosis, cirrhosis, cardiomyopathy, diabetes...).
This explains why iron homeostasis must be finely tuned to avoid both deficiency and
excess.
Iron turnover in the organism occurs almost in a closed circuit (Figure 1). Indeed,
global iron turnover through losses (because of bleeding or cell desquamation) and
dietary uptake (by duodenal cells) is only 1 to 2 mg per day, compared to approximately
3 to 4 g of iron contained in the organism. In fact, most of the iron available for
erythropoiesis comes from the catabolism of senescent red blood cells (RBCs) by macrophages
in the reticulo-endothelial system (called eythrophagocytosis). As shown in Figure
1, more than two-thirds of the body's iron content is incorporated into hemoglobin,
either in bone marrow erythroid progenitors or in circulating RBCs. Once aged, these
RBCs are internalized and hemoglobin is degraded in tissue macrophages. Iron is then
transferred to the macrophage cytosol and either released into the blood flow or stored
in ferritin molecules. In the plasma, transferrin binds newly released iron to allow
its mobilization from storage sites (mainly the spleen and to a lesser extent the
liver) to utilization sites (mainly the bone marrow). Erythropoiesis requires about
25 to 30 mg of iron daily. It has to be stressed that the amount of iron present in
the plasma at any time is small (about 3 mg) compared to the daily amount of iron
needed for erythropoiesis. Iron metabolism is therefore finely tuned, with hepcidin
being central to its regulation [12].
Figure 1
Distribution of iron in the body. Erythrocytes contain almost two thirds of all body
iron. Any blood loss may thus lead to direct iron loss. Serum iron, representing less
than 1/103 of the total iron content, is very limited at any time compared to the
daily amount of iron needed for erythropoiesis. Hepatocytes and tissue macrophages
are the main sites of iron storage. Iron is absorbed by intestinal cells through the
duodenal metal transporter (DMT-1 apical transporter) and exported into the blood
circulation via ferroportin.
Hepcidin is a small 25 amino acid peptide mainly produced by the liver. It is produced
as an 84 amino acid pre-pro-peptide. Pro-hepcidin has been shown to be biologically
inactive. Hepcidin acts by binding to ferroportin, which is the sole known iron exporter
[13]. The binding of hepcidin to ferroportin induces its internalization and degradation
in the cytosol, which prevents the release of intracellular iron [13]. Ferroportin
is mainly expressed in macrophages and duodenal cells, allowing, respectively, iron
recycling (after eythrophagocytosis) and iron absorption from the digestive lumen
(after internalization of iron through natural resistance-associated macrophage protein
[nRAMP]/duodenal metal transporter [DMT1]). Induction of hepcidin synthesis may thus
lead to iron-restricted erythropoiesis (by inhibiting the release of iron from macrophages
to the bone marrow) and to dietary iron deficiency (by inhibiting the uptake of iron
from the digestive duodenal cells). Hepcidin acts as a 'hyposideremic' hormone, aimed
at inhibiting iron absorption and reducing the level of iron in the blood.
Because hepcidin plays this central role in iron metabolism regulation, its synthesis
is finely regulated (Figure 2) [11,12]. Hepcidin synthesis is induced by iron overload
and inflammation, whereas iron deficiency, hypoxia and erythroid expansion repress
its synthesis. The molecular mechanisms implicated in these complex regulations are
not fully understood (see [12] for review), but the induction of hepcidin synthesis
by inflammation has been shown to be interleukin (IL)-6 dependent [14]. This interaction
between hepcidin and inflammation makes hepcidin the principal agent responsible for
the iron-restricted erythropoiesis observed during chronic diseases, ultimately leading
to the 'anemia of chronic disease' (or anemia of inflammation) [15,16]. On the other
hand, hepcidin synthesis is repressed by both iron deficiency and stimulation of erythropoiesis
[11,12]. Although the precise mechanisms involved in the repression of hepcidin are
not fully understood, it appears that matriptase 2, a membrane-bound serine protease
expressed in hepatocytes, seems to play a key role in repressing hepcidin synthesis
in iron deficiency conditions [17]. Repression of hepcidin by erythropoiesis stimulation
is even less well understood, but seems to involve bone marrow erythropoietic activity
rather than erythropoietin itself [18,19]. Hypoxia-inducible factor (HIF) or CCAAT
enhancer binding protein-alpha pathways have also been proposed [12]. In human pathology,
little is known. Growth differentiation factor 15, a member of the transforming growth
factor (TGF)-β family produced by late erythroblasts, has been found in high levels
in patients with beta-thalassemia syndromes and has been shown to repress hepcidin
synthesis [20]. These two opposite stimuli are found in the anemia of critically ill
patients, as discussed below.
Figure 2
Regulation of iron metabolism in anemia of the critically ill patient. Two opposite
stimuli regulate hepcidin, which is the master regulator of iron metabolism. Hepcidin
binds to ferroportin, inducing its internalization and destruction, thus avoiding
iron export. Inflammation induces hepcidin synthesis, while iron deficiency, blood
spoliation and erythropoiesis stimulation repress it. A low hepcidin level is required
to allow iron export and its utilization for erythropoiesis. Apo-Tf: apotransferrin;
Tf-Fe: transferrin bound iron.
Implication of iron metabolism in the anemia of the critically Ill: hepcidin as a
diagnostic tool?
Anemia is not only very frequent among critically ill patients, it is also associated
with increased transfusion rates and worse outcomes (increased length of stay, increased
mortality) [6,7]. However, recent recommendations have led to a decrease in transfusion
triggers [21]. Nowadays, anemia is present at ICU discharge in at least 75% of all
patients when considering their last measured hemoglobin levels [22]. Furthermore,
anemia may also be prolonged after discharge, with a median time to recovery of 11
weeks and more than half of the patients still anemic 6 months after ICU discharge
[23]. There is, therefore, need for a better understanding of the mechanisms of anemia
in the critically ill and an evaluation of therapeutic options.
The two main contributing factors for anemia in the critically ill are inflammation
and iron deficiency, which have opposite effects on iron metabolism (see above). Until
recently, inflammation, rather than iron deficiency, was considered to play the major
role. Indeed, the iron profile of critically ill patients constantly shows hallmarks
of anemia of inflammation. However, this topic has not been considered a matter of
great interest in the past, with few studies undertaken [9]. Inflammation is frequent
in critical illness, whatever the underlying pathology. The anemia of critically ill
patients is indeed similar to the anemia of inflammation, with blunted erythropoietic
response and activation of RBC destruction by macrophages [15,24]. Low serum iron
and high ferritin levels constitute the typical iron profile of critically ill patients
and are indicative of an inflammatory iron profile [25,26].
Because ferritin synthesis is induced by inflammation (through IL-1) independently
of the level of iron stores, elevated ferritin levels are no longer indicative of
iron stores in the context of inflammation [10]. Thus, despite an iron profile that
mimics iron overload (with high ferritin levels), iron deficiency may exist in these
critically ill patients.
Indeed, daily blood losses are far from negligible, either through repeated blood
sampling [6,27], surgical site bleeding, other invasive procedures (drainage, catheter
placement, renal replacement therapy...) or occult bleeding [26]. The median blood
loss for anemic critically ill patients has been estimated to be as high as 128 ml
per day [26]. This may represent a median iron loss as high as 64 mg per day. As daily
iron intake is less than 20 fold iron losses, iron deficiency could easily appear
in critically ill patients.
Iron deficiency may thus coexist with inflammation. In addition, iron deficiency is
not infrequent in the general population [28], and also in the elderly [3,29] or patients
suffering from heart failure [30]. The frequency of iron deficiency on ICU admission
may thus be around 35% [31,32]. However, the diagnosis of iron deficiency is difficult
in the context of inflammation because the usual indicators of iron deficiency are
no longer valid [9,10]. Because inflammation induces ferritin synthesis, serum ferritin
levels are no longer indicative of iron stores. New biological markers are thus required
for the diagnosis of iron deficiency in the context of inflammation (Figure 3) [10].
Below are the main biological markers that can be used:
Figure 3
Biological variable of iron metabolism.
• Percentage of hypochromic RBCs. These hypochromic RBCs result from iron-restricted
erythropoiesis. Schematically, a value of > 10% hypochromic erythrocytes (normal <
2.5%) is indicative of iron-restricted erythropoiesis over the past 3 months (this
being the RBC lifespan).
• Reticulocyte hemoglobin content. Reticulocyte hemoglobin content below 28 pg is
also indicative of iron-restricted erythropoiesis over the past 2 to 3 days (this
being the lifespan of reticulocytes). Recently, a low reticulocyte hemoglobin content
on admission was shown to be associated with higher transfusion rates in critically
ill patients [32].
• Erythrocyte zinc protoporphyrin (ZPP). During erythropoiesis, Fe is normally incorporated
into protoporphyrin IX to form heme. In iron deficiency, zinc is substituted for iron,
leading to the formation of ZPP. Increased erythrocyte ZPP is thus indicative of iron
deficiency.
• Soluble transferrin receptor (sTfR). Transferrin receptors allow the internalization
of iron into erythroid progenitor. Their synthesis is increased as bone marrow erythropoietic
activity increases. When iron supply is insufficient, a truncated form of transferrin
receptor appears in the serum. sTfR is thus indicative of iron-deficiency anemia.
This marker is widely proposed, however there is no gold standard for its measurement.
• sTfR/log ferritin ratio (called the ferritin index). This is proposed as a marker
to differentiate between anemia of inflammation and the combined situation of iron
deficiency and anemia of inflammation, taking into account the "uncovered need for
iron" on the one hand and the "iron stores" on the other [15].
Complex algorithms combining all these variables have been proposed for the diagnosis
of iron deficiency in the presence of inflammation [10,15]; however, none are clinically
validated and the cut-off values for each variable are unknown. Moreover, all but
sTfR cannot be used after recent blood transfusion.
Being central to iron metabolism, hepcidin may be a marker of iron deficiency, even
in the presence of inflammation. Indeed, using animal models, we and others have demonstrated
that hepcidin can be repressed despite inflammation [33-35] and that this repression
is associated with spleen iron mobilization [34]. These observations reinforce the
concept that iron deficiency may coexist with anemia of inflammation [15]. Measurement
of hepcidin concentrations may thus be helpful for the diagnosis of iron deficiency
in the context of inflammation. Additionally, many hepcidin assays have been recently
developed [36]. Most studies evaluating the use of hepcidin concentrations to diagnose
iron deficiency during inflammation have used ELISA-based values showing virtually
undetectable levels [35] or normal values [37,38] of hepcidin despite inflammation
(supposed to increase hepcidin synthesis). Measurement of hepcidin concentrations
could be accurate in the diagnosis of iron deficiency in critically ill anemic patients
using a cut-off value of less than 130 ng/l [38].
Is there a place for iron supplementation or treatment in critically ill patients?
Because iron deficiency may coexist with inflammation in critically ill patients [9,10,32,38]
and because iron may be mobilized from spleen stores in the presence of inflammation
[34,35], one could propose that iron be given to critically ill patients.
Because blood transfusion is not an option to fully correct the anemia in critically
ill patients [6,21], the use of alternatives such as erythropoiesis-stimulating agents
or iron has been suggested. Erythropoiesis-stimulating agents have already been studied
in the critically ill. They have not been shown to be useful [39] and are beyond the
scope of this review. In addition, iron deficiency may concern up to 40% of critically
ill patients [10,31,32,38]. Iron may thus be needed not only for erythropoiesis but
also to correct all the disorders associated with iron deficiency, having been shown
to improve functional capacity in women [40] and in cardiac patients [41]. However,
iron is also a toxic compound with the ability to induce oxidative stress or to promote
bacterial growth and may thus not be suitable in the ICU context. Indeed, free iron
may induce oxidative stress through the Fenton reaction. Large amounts of iron, exceeding
the transferrin iron-binding capacity, may thus be toxic by inducing the release of
free iron and causing oxidative stress. This probably explains the increased mortality
associated with large amounts of iron administration (around the DL50) observed in
an animal model of peritonitis [42]. However, no increase in oxidative stress has
been demonstrated in human practice [43]. There is also a link between iron and infection,
with iron being needed for bacterial growth. The decrease in serum iron concentration
may be a defense mechanism against bacterial proliferation. However, bacteria have
developed mechanisms for iron acquisition including the release of siderophores. The
respective affinity for iron between transferrin and siderophores is probably what
matters [44]. In clinical studies, this link between iron and infection has essentially
been supported by experimental data on microorganisms and retrospective studies in
hemodialysis patients showing an association between hyperfer-ritinemia and the likelihood
of infection. However, available observational studies in postoperative or critically
ill patients show no association between intravenous iron administration and risk
of infection [45]. Furthermore, iron deficiency is associated with impaired immunity
[46] and may, therefore, be responsible for increased susceptibility to infection
[32] as well as being associated with increased length of stay in the ICU [31].
Iron may thus be suggested to correct iron deficiency, even in the presence of inflammation,
similar to its proposed use in the treatment of patients with cancer-induced anemia
[15,47]. Iron may be given using either intravenous or enteral routes. For the latter,
ferrous iron is used. Iron absorption requires a mildly acidic medium (i.e., without
concomitant use of proton pump inhibitors) and ascorbic acid. However, absorption
may be reduced by inflammation because of the decrease in ferroportin levels induced
by hepcidin, or because of frequent gastrointestinal adverse effects. The intravenous
route allows administration of much higher doses with few adverse effects (with the
notable exception of anaphylactic shock following iron dextran injections) and no
difficulty of absorption. A recent meta-analysis showed that non-dextran iron was
superior to enteral iron for the correction of anemia, with few adverse effects [48].
However, the only available study of intravenous iron showed no beneficial effect
on erythropoiesis when used without erythropoiesis-stimulating agents [25]. The only
study of iron deficiency treatment in critically ill patients is the study by Pieracci
et al., which showed a reduced transfusion rate in patients with baseline iron deficiency
treated with enteral iron supplementation (ferrous sulfate 325 mg three times daily)
[49]. In this study, oral iron supplementation was not associated with an increased
risk of infection.
Iron may therefore be proposed either to correct iron deficiency and/or to enhance
the response to erythropoiesis-stimulating agents in critically ill patients, but
further studies are needed to rule out the potential risks of iron treatment (i.e.,
oxidative stress induction, increased risk of infection) and to define the best route
of administration. In Figure 4, we propose an algorithm for iron deficiency diagnosis
and treatment. We believe that iron should be given to critically ill patients only
in cases of iron deficiency, at best defined according to a low hepcidin level. The
dose of iron needed may be assessed using the following formula:
Figure 4
Algorithm for diagnosis and treatment of iron deficiency (proposal not yet supported
by clinical trial evidence). ESA: erythropoiesis-stimulating agents; CRP: C-reactive
protein; sTfR: soluble transferrin receptor.
iron deficit
=
body weight
(
kg
)
×
(
target Hb
−
actual Hb
)
×
2.4.
Because elevated iron concentrations induce the synthesis of hepcidin, which in turn
may reduce iron availability, the total dose of iron should be given using fractionated
injections. Further clinical studies are needed to validate these propositions.
Conclusion
The discovery of hepcidin sheds new light on our knowledge of iron metabolism and
may enable easier recognition of iron deficiency in the presence of inflammation in
critically ill patients. This opens new areas of research exploring the role of iron
treatment for these patients.
Competing interests
The authors declare that they have no competing interests.
List of abbreviations used
RBC: red blood cell; sTfR: soluble transferring receptor; ZPP: zinc protoporphyrin.