Regulation of Transferrin-Mediated Iron Uptake by HFE, The Protein Defective in Hereditary Hemochromatosis

The protein defective in hereditary hemochromatosis, called HFE, is similar to MHC class I-type proteins and associates with β2-microglobulin (β2M). Its association with β2Mwas previously shown to be necessary for its stability, normal intracellular processing, and cell surface expression in transfected COS cells. Here we use stably transfected Chinese hamster ovary cell lines expressing both HFE and β2Mor HFE alone to study the effects of β2Mon the stability and maturation of the HFE protein and on the role of HFE in transferrin receptor 1 (TfR1)-mediated iron uptake. In agreement with prior studies on other cell lines, we found that overexpression of HFE, without overexpressing β2M, resulted in a decrease in TfR1-dependent iron uptake and in lower iron levels in the cells, as evidenced by ferritin and TfR1 levels measured at steady state. However, overexpression of both HFE and β2Mhad the reverse effect and resulted in an increase in TfR1-dependent iron uptake and increased iron levels in the cells. The HFE-β2Mcomplex did not affect the affinity of TfR1 for transferrin or the internalization rate of transferrin-bound TfR1. Instead, HFE-β2Menhanced the rate of recycling of TfR1 and resulted in an increase in the steady-state level of TfR1 at the cell surface of stably transfected cells. We propose that Chinese hamster ovary cells provide a model to explain the effect of the HFE-β2Mcomplex in duodenal crypt cells, where the HFE-β2Mcomplex appears to facilitate the uptake of transferrin-bound iron to sense the level of body iron stores. Impairment of this process in duodenal crypt cells leads them to be iron poor and to signal the differentiating enterocytes to take up iron excessively after they mature into villus cells in the duodenum of hereditary hemochromatosis patients.