From genetics to cellular physiology. Focus on "Regulation of transferrin-induced endocytosis by wild-type and C282Y-mutant HFE in transfected HeLa cells".

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 ironloaded 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 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: [email protected]). Am J Physiol Cell Physiol 282: C971–C972, 2002; 10.1152/ajpcell.00009.2002.