Transcript Iron overload Iron deficiency

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Body iron content – 3-4g
◦ Hb, iron containing proteins, bound to Tf, storage
(ferritin, haemosiderin).
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Iron homeostasis is regulated strictly at level
of intestinal absorption.
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Haem diet – very readily absorbed via haem
carrier protein 1 (apical bruish border
membrane of duodenal enterocytes) i.e.
higher bioavailability.
Remainder of dietary iron poorly absorbed
(10%).
◦ Ascorbic acid enhances absorption of non-animal
sources of iron; tannates inhibit absorption.
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Fe2+ better absorbed cf. Fe3+.
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Fe3+ freed from food binding sites in
stomach, binds to mucin, travels to
duodenum and small bowel.
◦ Haem iron - carrier protein (endocytosis).
◦ Fe3+ - attachment to an integrin.
◦ Fe2+ - intestinal transporter DMT1.
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Iron then enters cytosol, binds to cytosolic
low molecular weight iron carriers and
proteins e.g. Mobilferrin (shuttles iron with
help of ATP) to basolateral membrane
Export from basolateral membrane via
duodenal iron exporter.
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Upon release into circulation, re-oxidised to
Fe3+, loaded onto transferrin.
◦ Site of influence of HFE gene product, +/caeruloplasmin (known ferroxidase).
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Iron absorption regulated by many stimuli –
◦ Iron stores.
◦ Degree of erythropoiesis (increased with increased
erythropoiesis, reticulocytosis).
◦ Ineffective erythropoiesis.
◦ Mobilferrin – mechanism of loss in iron replete
state.
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Transferrin and TfR.
Ferritin.
Iron responsive element-binding protein (IRE-BP)
aka iron regulatory protein/factor (IRP/IRF).
HFE.
Divalent metal transporter (DMT1, Nramp2,
DCT1,Slc11a) – duodenal iron transporter.
Ferroportin and hephaestin, iron export proteins.
Hepcidin.
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Encoded on long arm of chromosome 3.
Half life 8 days.
Hepatic synthesis.
Complete lack incompatible with life
(hypotransferrinaemia).
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Also on long arm of chromosome 3.
homodimeric transmembrane protein.
◦ Found in most cells. Most dense on erythroid
precursors, hepatocytes, placental cells.
◦ Restricted expression: both TfR1 and TfR2 present
at high levels in hepatocytes, epithelial cells of
small intestine including duodenal crypt cells.
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Each TfR binds 2 diferric Tf molecules.
Uptake by clustering on clathrin coated pits,
then endocytosed.
Iron off-loaded in acidified vacuoles,
apotransferrin-TfR complex recycled to cell
surface, apo-Tf then released back into
circulation.
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Cellular storage protein for iron.
L and H chains (chromosome 19, 11).
Synthesis controlled at 2 levels –
◦ DNA transcription via its promotor.
◦ mRNA translation via interactions with iron
regulatory proteins.
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Acute phase reactant.
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Ferritin in erythroid precursors may be of special
importance in haem synthesis especially at
beginning of Hb accumulation, when Tf-TfR
pathway still in sufficient.
When ferritin accumulates, it aggregates,
proteolyzed by lysosomal enzymes, , then
converted to iron-rich, poorly characterised
haemosiderin, which releases iron slowly.
M-ferritin – present in mitochondria. Expression
correlated with tissues that have high
mitochondrial number, rather than those
involved in iron storage.
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Sensing iron-regulatory proteins modulate
synthesis of TfR, ferritin, DMT1.
◦ IRP1 and IRP2 – cytosolic RNA binding proteins.
Bind to iron-responsive elements located in 5’ or 3’
untranslated regions of specific mRNAs encoding
ferritin, TfR, DMT1 and (in erythroid cells) eALAS.
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Binding of IRPs to IREs at 5’ end of transcrips
of e.g. Ferritin, eALAS – decreases rate of
synthesis; binding to 3’ end of transcripts
e.g. TfR or DMT1, mRNA half life prolonged,
increased synthesis.
IRE-IRP complex senses state of iron balance
– conformational change.
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End result – in iron overload, increased
ferritin (for adequate storage), decreased TfR
(minimise further iron entry into cell), and
vice versa in iron deficiency.
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Expression in GIT limited to cells in deep
crypts in proximity to site of iron absorption.
HFE protein associated with TfR, acts to
modulate uptake of Tf-bound iron into crypt
cells.
Along with hepcidin, acts as iron sensor.
Hereditary haemochromatosis with HFE gene
mutation - inability to bind beta 2microglobulin, impaired cellular trafficking,
reduced incorporation into the cell
membrane, reduced association with TfR1.
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Divalent metal transporter protein – iron
transporter (also Pb, Zn, Cu).
Widely expressed, esp. in proximal
duodenum.
Isoform containing iron responsive element
(Nramp2 isoform I) specifically upregulated in
iron deficiency, greatest expression at brush
border of apical pole of enterocytes in apical
2/3 of villi.
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Increased body iron stores – enhanced uptake
of iron from circulation into crypt cells.
Increasing intracellular iron into crypt cells,
differentiating enterocytes migrating up to
villus tip downregulate iron transporter
DMT1, reducing absorption of dietary iron
from gut.
Inverse relationship between ferritin levels in
serum, and DMT1 levels in duodenal cells.
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Transporting iron from basolateral membrane
of enterocytes to circulation; from
macrophage (from effete RBCs) into
circulation for formation of new Hb.
◦ Ferroportin.
◦ Hephaestin.
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Ferroportin-1 in basal portion of placental
syncytiotrophoblasts, basolateral surface of
duodenal enterocytes, macrophages,
hepatocytes.
Upregulated by amount of available iron,
downregulated through interaction with
hepcidin.
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Mutation in mice with sex-linked anaemia –
enterocytes are iron loaded, but efflux
through basolateral membrane inhibited.
Homology to caeruloplasmin.
Link between iron deficiency and copper
deficiency – administration of copper
facilitates egress of iron from tissue(s) into
circulation.
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SFT-mediated transport has properties
defined for Tf-independent iron uptake,
transporting iron across lipid bilayer. Process
dependent on Cu.
Has ferrireductase activity.
Cytosolic localisation in recycling endosomes,
stimulates Tf bound iron assimilation.
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25 aa peptide hormone.
Chromosome 19.
Synthesized by hepatocyes. Intrinsic
antimicrobial activity.
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Binds ferroportin, complex internalised and
degraded.
Resultant decrease in efflux of iron from cells
to plasma
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Iron – stimulated with increased iron levels
Inflammation, infection (and endotoxin)
Hypoxia - downregulated
Erythropoiesis – downregulated in anaemia,
oxidative stress, ineffective erythropoiesis.
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BMP - members of TGF-b superfamily which
regulate cell proliferation, differentiation,
apoptosis.
Targets BMP receptors type I and II,
resulting in phosphorylation of cytoplasmic
R-Smads.
R-Smads associate with Smad4, translocate
to nucleus, activates transcription of target
genes (in this case hepcidin).
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BMP2, 4, 5, 6, 7, 9 increase hepcidin
expression in hepatic cells.
◦ Individual members of BMP family interact with
different combinations of type I and II receptors.
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BMP’s effect on cellular response also
modulated by BMP coreceptors.
◦ Hemojuvelin (HJV) - iron-specific, stimulates
BMP2/4 pathway.
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Member of family of repulsive guidance
molecules (RGMs) - coreceptors of BMP
receptors.
Chromosome 1.
Disruptive mutations cause juvenile
haemochromatosis.
2 forms -
◦ GPI linked membrane form - stimulates BMP
signalling and hepcidin expression.
◦ Soluble HJV (sHJV) - antagonist of BMP signalling.
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Production stimulated by increased plasma
iron and tissue stores.
Negative feedback - hepcidin decreases
release of iron into plasma (from
macrophages and enterocytes).
Fe-Tf increases hepcidin mRNA production
(dose dependent relationship).
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HFE interacts with TfR1, but dissociates when
Fe-Tf binds to TfR1.
◦ Amount of free HFE proportional to Tf-Fe.
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TfR2 – Tf-Fe stabilises TfR2 protein in dose
dependent fashion.
◦ Fe-Tf binding increases fraction of TfR2 localizing to
recycling endosomes, decreases fraction of TfR2
localizing to late endosomes where it is targeted for
degradation.
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TfR2 competes with TfR1 for binding to HFE.
◦ HFE-TfR2 may regulate hepcidin expression by
promoting HJV/BMP signalling, impacting upon
hepcidin expression.
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Hepcidin decreased in iron-deficiency
anaemia, hereditary anaemias with ineffective
erythropoiesis, and mouse models of anaemia
from bleeding and haemolysis. Response not
seen when erythropoiesis suppressed.
◦ Allows greater availability of iron for erythropoiesis.
◦ Degree of anaemia by itself doesn’t seem as
important.
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Nature of erythropoietic regulator of hepcidin
is unknown – proteins secreted by developing
erythrocytes?
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Mechanism particularly important in ironloading anaemias.
◦ Urinary hepcidin very low in untransfused
patients with thalassaemia intermedia, despite
high serum and tissue iron levels.
◦ Very high erythropoietic activity overrides
hepcidin regulation by iron.
◦ Severe hepcidin suppression leads to increased
iron absorption and development of lethal iron
overload.
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Member of TGF-b superfamily, mediates
hepcidin suppression in thalassaemia.
Secreted during erythroblast maturation.
Suppresses hepcidin mRNA production in
primary human hepatocytes.
Uncertain whether GDF15 plays role in
pathogenesis other than that of ineffective
erythropoiesis.
◦ Levels much lower in sera of sickle cell anaemia,
MDS.
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Physiological relevance uncertain.
Hypoxia-inducing factor (HIF) is the main
mediator of oxygen-regulated gene
expression.
◦ VHL deficiency results in VHL protein deficiency,
hence stabilisation of HIF. Resultant decrease in
hepcidin levels.
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IL-6 a prominent inducer of hepcidin, through
STAT-3 dependent transcriptional mechanism.
◦ Other cytokines may also induce hepcidin independent
of IL-6.
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Macrophage also express hepcidin in response
to micobial stimulation.
◦ Hepcidin may function in autocrine manner to degrade
macrophage ferroportin, causing local retention of iron
in macrophages.
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Inflammatory stimuli acting through TNFa
suppresses HJV mRNA, thus perhaps
preventing iron-regulatory pathway from
suppressing hepcidin during hypoferraemia of
inflammation.
Iron overload
Iron deficiency
Hypotransferrinaemia - recessive
HFE gene mutation
TfR2 gene mutation – recessive
Ferroportin mutation – autosomal
dominant
Hepcidin mutations
Hemojuvelin mutations
H ferritin mutation - dominant
TMPRSS6 mutation - IRIDA
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IDA unresponsive to oral iron
supplementation, partially responsive to
parenteral iron administration.
Likely autosomal recessive.
?22q12-13 – encodes type II transmembrane
serine protease (matriptase-2), primarily
expressed in liver.
Defect in iron uptake.
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Elevated urinary hepcidin cf. normal iron
deficiency.
◦ ?reason for failure to absorb iron despite iron
deficiency.
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Still unclear how mutations lead to
ainappropriately elevated hepcidin.
◦ Negative regulator of hepcidin transcription in
mouse models.