Role of HFE in Hepatic Iron Overload: Novel Findings Demonstrate Direct Effects of Mutant C282Y on Hepcidin and Calreticulin mRNA Expression.

Studies done in non-hepatic cell lines, focusing on the interaction between HFE with TFR1 and β-2M proved insufficient to explain the discrepancies found in the clinical penetrance of hemochromatosis in subjects carrying the C282Y mutation. Our first goal was to investigate the role of HFE wild type (wt) and mutant proteins (C282Y and H63D) in a human hepatic cell line, focusing on the cellular localization and interaction of HFE with the expression of other iron related proteins. HFE mutant C282Y was found to be retained in the endoplasmic reticulum (ER). Thus, in addition, we investigated the effect of HFE wt and mutant proteins on Calreticulin, which is a chaperon protein that responds to ER stress and has a protective effect on oxidative damage in some cell lines. Here we report setting up a stable transfection of wt- and mutant-HFE in a hepatic cell line (HepG2) and examine the intracellular distribution of wt- and HFE mutants, their effect on iron intake independently of TFR1 and on the expression of other iron and ER stress response genes, namely Hepcidin and Calreticulin. In addition, we validated some of the novel effects of HFE on Calreticulin using peripheral blood mononuclear cells from HFE patients. The localization of the HFE variants was analyzed using KDEL and Golgin-97 as ER and the Golgi complex markers, respectively. HFE C282Y shows a high degree of overlap with the ER markers, confirming a retention of this variant in this organelle. Over-expression of the HFE wt impaired the intake of ⁵⁵Fe relatively to transfected control cells (P<0.008) independently of TFR1, as demonstrated by RNAi silencing. Hamp RNA expression was decreased in cells over expressing C282Y in comparison to HFE wt cells (P<0.011). Finally over-expression of HFE wt decreases Calreticulin mRNA, whereas the C282Y had an opposite effect, compared to the control cell line. A similar result was observed in peripheral blood mononuclear cells (PMBC) of C282Y homozygous HFE patients, compared to wild type blood donors (P<0.006). Interestingly, this data suggest that synthesis of the HFE mutant C282Y triggers a protective effect on oxidative damage mediated by Calreticulin. In fact, HepG2 cells over-expressing C282Y showed lower levels of ROS than HFE wt (P<0.004). This observation might contribute to explain some of the discrepancies seen in the clinical penetrance of the disease in C282Y carrying subjects. The direct effect of the mutant HFE C282Y on mRNA expression of hepcidin also demonstrated here for the first time corroborates and provides a molecular basis for earlier reports of low hepcidin levels in HH patients and in Hfe-KO mice.