Mechanisms of HFE-induced regulation of iron homeostasis: Insights from the W81A HFE mutation

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Abstract
The mechanisms by which the hereditary hemochromatosis protein, HFE, decreases transferrin-mediated iron uptake were examined. Coimmunoprecipitation studies using solubilized cell extracts demonstrated that transferrin (Tf) competed with HFE for binding to the transferrin receptor (TfR) similar to previous in vitro studies using soluble truncated forms of HFE and the TfR. At concentrations of Tf approaching those found in the blood, no differences in Tf binding to cells were detected, which is consistent with the lower binding constant of HFE for TfR versus Tf. However, cells expressing HFE still showed a decrease in Tf-mediated iron uptake at concentrations of Tf sufficient to dissociate HFE from the TfR. These results indicate that the association of HFE with TfR is not essential for its ability to lower intracellular iron stores. To test the effect of HFE on lowering intracellular iron levels independently of its association with TfR, a mutated HFE (fW81AHFE) that shows greatly reduced affinity for the TfR was transfected into tetracycline-controlled transactivator HeLa cells. HeLa cells expressing fW81AHFE behaved in a similar manner to cells expressing wild-type HFE with respect to decreased intracellular iron levels measured by iron regulatory protein gel-shift assays and ferritin levels. The results indicate that HFE can lower intracellular iron levels independently of its interaction with the TfR.