Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

VI. Iron Metabolism in Cells
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induced by interleukin-6 (IL-6), IL-10, and glucocorticoids
( Graversen et al ., 2002 ). The heme-hemopexin complexes
undergo endocytosis after binding to CD91, also called the
low-density lipoprotein receptor-related protein (LRP) or
α 2 -macroglobulin receptor. Once inside the cell, the complexes
are transported to lysosomes for degradation, and
receptors are recycled to the cell surface ( Hvidberg et al .,
2005 ).
C. Iron Metabolism in Hepatocytes
The liver is essential for normal iron homeostasis in the
body. It regulates iron movement into and around the body
through hepcidin synthesis, accounts for about 40% of the
body iron stores, and is the site of synthesis of apotransferrin,
ceruloplasmin, haptoglobin, and hemopexin plasma
proteins ( Anderson and Frazer, 2005 ).
Hepatocytes synthesize hepcidin in an endocrine manner
to regulate iron metabolism in the body. Hepcidin is a small
cationic peptide (25 amino acids in humans) that forms from
a larger prohepcidin peptide (84 amino acids) ( Park et al .,
2001 ). Hepcidin has been cloned, expressed, and sequenced
in several species, including dogs ( Fry et al ., 2004 ; Verga
Falzacappa and Muckenthaler, 2005 ). Because of its small
size, hepcidin is rapidly cleared by the kidney and is measurable
in urine. Using immunohistochemistry techniques,
prohepcidin is localized to organelles of the secretory pathway,
particularly the Golgi apparatus, of hepatocytes suggesting
that molecules may accumulate before receiving
a signal for secretion ( Wallace et al ., 2005 ). Prohepcidin
circulates in plasma, but its plasma concentration does not
correlate well with urinary hepcidin concentrations or other
iron parameters ( Kemna et al ., 2005 ). The site, nature, and
potential regulation of the conversion of prohepcidin to hepcidin
remain to be identified.
A number of systemic stimuli, including body iron
stores, anemia, hypoxia, degree of erythropoiesis, and
inflammation have been reported to modulate hepcidin
levels; however, the pathways involved in hepcidin production
remain to be elucidated ( Anderson and Frazer, 2005 ;
Steele et al ., 2005 ). Anemia, hypoxia, and erythropoietin
themselves do not seem to be of primary importance ( Pak
et al ., 2006 ; Vokurka et al ., 2006 ). Hepcidin production is
modulated by body iron requirements, which are largely
influenced by the magnitude of erythropoiesis present
( Wilkins et al ., 2006 ). A signal arising from erythropoietic
activity in bone marrow is proposed to regulate hepcidin
production, but its nature is unknown at the time of
this manuscript preparation ( Pak et al ., 2006 ). The content
of diferric transferrin in plasma may be a key indicator of
body iron requirement ( Wilkins et al ., 2006 ). Many questions
remain concerning how hepcidin expression is modulated
in response to body iron requirements; however,
three molecules (hemochromatosis [HFE] protein, TfR2,
and hemojuvelin) have been identified that are involved in
the regulation of this pathway(s). Defects in each of these
molecules in humans or mice have resulted in inappropriately
low hepcidin levels and increased iron absorption
leading to hemochromatosis. It is hypothesized that hepcidin
is regulated by HFE and TfR2 on hepatocyte surfaces
in response to plasma transferrin saturation. TfR2 has a
25-fold lower affinity for diferric transferrin than does
TfR1 and would presumably become active when diferric
transferrin concentrations are high ( Anderson and Frazer,
2005 ). The pathway involving plasma diferric transferrin
concentration may not be the only way hepcidin production
is regulated. Hemojuvelin may modulate this pathway
or affect hepcidin synthesis independently ( Steele et al .,
2005 ). Studies indicate that hepcidin synthesis is stimulated
by certain bone morphogenetic proteins (BMPs)
that bind to hemojuvelin, regulating hepcidin expression
through a BMP signal transduction pathway ( Truksa et
al ., 2006 ). In addition, hepcidin expression increases in
response to inflammation, and this increase precedes any
alteration in transferrin saturation ( Steele et al ., 2005 ).
Most of the iron stored within the liver is found in
hepatocytes stored as ferritin. Hepatocytes secrete a glycosylated
form of ferritin into plasma ( Ghosh et al ., 2004 ).
Ferritin in human plasma is primarily composed of L subunits,
but H subunits predominate in ferritin in dog plasma
( Watanabe et al ., 2000 ). Plasma ferritin concentrations
generally correlate with body iron stores; however, ferritin
is an acute phase protein that increases in plasma during
inflammation ( Torti and Torti, 2002 ).
Hepatocyte membranes contain ferritin receptors.
However, serum ferritin is present in low concentrations and
contains little iron, so it is unlikely that hepatocytes take up
significant amounts of iron from plasma by this mechanism
under normal conditions ( Anderson and Frazer, 2005 ; Ponka
et al ., 1998 ). Like other cells, hepatocytes utilize TfR1
to transport iron into the cells. NTBI is thought to be chelated
by small organic acids, such as citrate, but some may
be bound loosely to proteins such as albumin ( Anderson and
Frazer, 2005 ). The liver rapidly clears NTBI. In experimental
studies using mice, the half-time of clearance of NTBI
in plasma was 30 sec, compared to a half-time clearance
for transferrin-bound iron of 50 min (Craven et al ., 1987 ).
Like macrophages and enterocytes, hepatocytes utilize
ferroportin to export iron to plasma. Ferroportin expression
is lower in hepatocytes than in enterocytes and
macrophages, which may help explain why these cells preferentially
accumulate iron in most iron-overload conditions
(Rivera et al ., 2005 ).
As discussed earlier, transferrin (apotransferrin with
bound Fe 3 ions) is critical for normal iron transport to cells.
Liver synthesis and release of apotransferrin to plasma is
increased in most species in response to iron deficiency (but
not generally in dogs) and decreased in response to inflammation
(negative acute phase protein). Transferrin deficient