Differential expression of genes related to HFE and iron status in mouse duodenal epithelium.

Iron absorption, distribution, use, and storage are thought to be tightly regulated since altered iron stores may lead to cellular damage and disease. HFE, the hereditary hemochromatosis gene product, is expressed in the crypts of the duodenum, but the molecular mechanism by which it contributes to the inhibition of iron absorption is still unknown. In this study we aimed to identify transcriptional profiles in the duodenal epithelium of Hfe(-/-) mice. We used dedicated microarrays to compare gene expression among the duodenum of Hfe(-/-) mice, induced iron overload mice, and control mice. We found 151 differentially expressed genes and unknown sequences between Hfe(-/-) mice and normal littermates. Gene profiling revealed a gene subset more specific for Hfe inactivation. The functional annotation of upregulated genes highlighted that mucus production and cell maintenance may account for the influence of Hfe on epithelium integrity and luminal iron uptake.

Over-expression of wild-type and mutant HFE in a human melanocytic cell line reveals an intracellular bridge between MHC class I pathway and transferrin iron uptake.

Hereditary hemochromatosis (HH) is a frequent recessive disorder of iron metabolism characterised by systemic iron overload. In Northern Europe, more than 90% of HH patients are homozygous for a mis-sense mutation (C282Y) in the HFE1 gene product. The HFE protein is the heavy chain of a MHC class I-related molecule and associates with beta2 microglobulin and the transferrin receptor. Its precise roles in iron metabolism and in the pathophysiology of HH are still unclear. In order to identify the cellular processing of HFE, an important step towards the understanding of the function of the protein, we stably over-expressed the wild type and mutated forms fused to the Green Fluorescent Protein in a melanocytic MHC class I expressing cell line, the Mel Juso cell line. In wild type and mutant clones, the fusion proteins were not detected at the cell surface but only in the cytoplasm. Their sub-cellular localisation was determined by co-labelling of cells with organite-specific antibodies and confocal microscopy. HFE-GFP followed initially HLA class I intracellular processing but co-localised with transferrin in early endosomes without recycling at the cell surface. The C282Y-GFP fusion protein followed a different folding pathway to exit endoplasmic reticulum. Over-expression of the wild-type protein lead to a decrease in diferric transferrin uptake. Our model will be of use in the elucidation of the functional interaction between intracellular HFE and iron transporters transferrin/transferrin receptor complexes and Slc11A2 (also named N-Ramp2 or DMT1) in different endosomal compartments.

Iron overload in mice expressing HFE exclusively in the intestinal villi provides evidence that HFE regulates a functional cross-talk between crypt and villi enterocytes.

Hereditary hemochromatosis (HH), a common autosomal recessive disorder due to a mutation in HFE, which encodes an atypical MHC class I glycoprotein, is characterized by excessive absorption of dietary iron. Little is known however of the apparently complex pathophysiology of HFE involvement in the process of iron influx. Here, in order to tackle the issue in vivo, we decided to target HFE expression exclusively to the relevant tissue, intestinal epithelium. This was achieved by putting HFE under transcriptional control of the rat fatty acid binding protein (Fabpi) promoter. Quite unexpectedly, Fabpi-HFE mice had significantly elevated serum transferrin saturation levels in comparison to those of normal littermates. By a careful, layer by layer analysis of transgene expression along the crypt-villus axis, we were able to affirm that the ectopic expression of transgenic HFE in the differentiated villi enterocytes was responsible for ferric hyperabsorption, a phenomenon exacerbated in the absence of endogenous HFE expression, which we assessed by crossing the transgene onto an HFE(-/-) (knockout) background. This forced dichotomy between the absence of HFE in the crypt and expression in the villi provides experimental support that HFE functions as a "gatekeeper," regulating the cross-talk between the crypt and villi enterocytes and thereby modulating the avidity of mature enterocytes for dietary iron.