Tranferrins receptor association and endosomal localization of soluble HFE are not sufficient for regulation of cellular iron homeostasis

Iron uptake and storage are tightly regulated to guarantee sufficient iron for essential cellular processes and to prevent the production of damaging free radicals. A non-classical class I MHC molecule, the hemochromatosis factor HFE, has been shown to regulate iron metabolism, potentially via its direct interaction with the transferrin receptor (TfR). In this study, we demonstrate that a soluble beta(2)microglobulin-HFE monochain (sHFE) folds with beta(2)microglobulin (beta(2)m) and associates with the TfR, indicating that the transmembrane and cytoplasmic domains are not necessary for assembly and trafficking through the ER-Golgi network. We also demonstrate human TfR-specific uptake and accumulation of extracellular sHFE by treated cells. The sHFE localized to the endosomal compartment albeit we observed variation in the time taken for endosomal trafficking between different cell types. The sHFE monochain was effective in reducing Tf uptake into cells, however this did not correlate to any changes in TfR or ferritin synthesis, in contrast to the HFE-induced increase and decrease of TfR and ferritin, respectively. These findings of incongruent sHFE activity suggest that either variation in affinity binding of sHFE to TfR prevents efficient modulation of iron-regulated proteins or that HFE has multiple functions some of which may be independent of TfR but dependent on interactions within the endosomal compartment for effective modulation of iron metabolism. (C) 2004 Wiley-Liss, Inc.