Department of Pharmaceutical Sciences, School of Pharmacy, University of Southern California, Los Angeles, California 90089-9121
HEREDITARY HEMOCHROMATOSIS (HH) is a
common inherited disorder of people of Northern European descent,
affecting some 1 in 400 people. HH is a disorder of iron metabolism
characterized by iron overload in many organs, particularly in the
liver, pancreas, heart, and pituitary, leading to multiorgan
dysfunction and premature death. Positional cloning of the gene for HH
resulted in the identification of a relatively widely expressed gene,
named HFE, that was curiously homologous in its predicted amino acid
sequence and structure to major histocompatibility complex (MHC) class
I molecules (1). Thus, from the genetic data, the role of
HFE in iron metabolism could not be immediately deduced.
However, since the initial cloning of HFE, progress in the
characterization of its role in iron metabolism at the cellular and
molecular level has been impressively rapid. By using physiological systems and heterologous expression systems, several groups have begun
to characterize the role of HFE in cellular iron uptake (2-9), with the latest contribution by Schwake et
al., the current article in focus (Ref. 10,
see p. C973 in this issue). The collective data
from these groups have dramatically changed the way in which we think
about cellular iron uptake mediated by the transferrin receptor (TfR),
the key cell surface receptor that mediates cellular iron uptake by
binding to extracellular iron-loaded transferrin and endocytosing it.
Such data include showing in immunoprecipitation studies that wild-type
HFE is physically associated with the TfR (2-4, 6,
9). Wild-type HFE was also found to colocalize with the TfR at
the plasma membrane and in intracellular compartments, presumably
endocytic organelles (3, 7). An apparent functional consequence of the association of wild-type HFE with TfR is that the
affinity of cell surface TfR for transferrin is lowered (2, 4, 5,
9). All of these data suggest that wild-type HFE cotraffics with
the TfR, and this association may regulate cellular iron uptake into
cells by altering the affinity of the TfR for transferrin either at the
cell surface or in endocytic compartments. Further supportive evidence
for a role of HFE in the regulation of TfR-mediated iron uptake is that
the most common mutation found in HFE, the missense C282Y mutation
occurring in over 80% of HH patients, nearly eliminates the capacity
of HFE to interact with the TfR (2), and the mutated HFE
protein is retained in the endoplasmic reticulum/Golgi apparatus
(7). In addition, expression of the mutated C282Y HFE did
not alter the affinity of cell surface TfR for transferrin
(2). Thus, in cells expressing mutated C282Y HFE,
transferrin (iron) uptake would be predicted to be higher, which would
provide a plausible mechanism for iron overload in cells and organs of
HH patients.
In the study in focus here, Schwake et al. (10) have
applied a powerful biophysical approach, cell-attached and whole cell capacitance measurements, to characterize the role of HFE in
transferrin internalization. They were able to measure
transferrin-dependent endocytotic events in a heterologous cell system
in which the expression level of HFE and the mutated C282Y HFE could be
regulated by a tetracycline-sensitive promoter. Their data suggest an
alternate, although not necessarily mutually exclusive, function for
HFE: a negative regulator of TfR internalization from the cell surface. This study makes several important, novel contributions, not only with
respect to characterization of HFE-regulated cellular iron uptake but
also to cell physiology in general. First, this study provides
surprising evidence suggesting that transferrin stimulates endocytotic
events, presumably by stimulation of the internalization of the TfR, a
receptor traditionally considered to be a constitutively endocytosing
receptor (that is, one whose endocytosis is not dependent on the
presence of ligand). Thus the TfR, perhaps by virtue of its interaction
with HFE, may actually be a tightly regulated endocytotic receptor for
iron uptake. Second, as the role of HFE in the regulation of TfR
internalization has been controversial, the characterization by
capacitance measurements of the role of HFE as a negative regulator of
TfR internalization is significant, particularly with respect to
understanding the overall mechanism by which HFE regulates iron
metabolism. However, because there are conflicting data in this area
(4, 8, 9), it will be an important one to resolve in the
future. Third, Schwake et al. have been able to show that the C282Y
mutation significantly attenuates HFE's negative regulatory function
in transferrin-stimulated endocytosis, providing an alternative or
additional mechanism to account for increased iron uptake in cells and
organs of HH patients. Fourth, they demonstrate that capacitance
measurements are sensitive enough to characterize the kinetics of
ligand-receptor endocytotic events and may therefore be applicable to
the study of a wide variety of endocytosing receptors and be an
approach of significant interest to cell biologists studying
receptor-mediated endocytosis.
Intriguing questions remain with respect to HFE and its role in iron
metabolism. By virtue of its homology to MHC class I molecules, which
bind to
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2-microglobulin, HFE predictably has also been
shown to bind to
2-microglobulin (2, 4, 6, 9). Thus the TfR may exist in a ternary complex with
2-microglobulin and HFE. Although the contribution of
2-microglobulin in iron metabolism has yet to be fully
characterized, it plays at least an indirect role because the binding
of
2-microglobulin to HFE regulates HFE trafficking
along the biosynthetic pathway (7). In addition, the HFE
C282Y mutation abrogates this interaction, resulting in the retention
of mutated HFE in the early biosynthetic pathway (7).
Clearly, the regulation of the dynamic interactions among HFE,
2-microglobulin, and the TfR represents an important future area of investigation in the cellular physiology of iron metabolism, perhaps employing additional novel approaches. More importantly, hints of bench-to-bedside benefits of the research into
HFE come from the report that soluble truncated fragments of HFE
complexed to
2-microglobulin inhibit transferrin binding to the TfR (2); perhaps such soluble fragments or
derivatives thereof could be used therapeutically to alleviate iron
overload in HH patients. In conclusion, there remains continued promise for success in an area that has progressed rapidly from genetics to
cellular and molecular physiology, using novel approaches such as that
used in the article in focus.
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ACKNOWLEDGEMENTS |
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This essay was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant DK-51588.
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FOOTNOTES |
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Address for reprint requests and other correspondence: C. T. Okamoto, Dept. of Pharmaceutical Sciences, School of Pharmacy, Univ. of Southern California, 1985 Zonal Ave., Los Angeles, CA 90089-9121 (E-mail: cokamoto{at}hsc.usc.edu).
10.1152/ajpcell.00009.2002
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