Co-trafficking of HFE, a Nonclassical Major Histocompatibility Complex Class I Protein, with the Transferrin Receptor Implies a Role in Intracellular Iron Regulation

Cindy N Gross,Alivelu Irrinki,John N Feder,Caroline A Enns

doi:10.1074/jbc.273.34.22068

Abstract

The mechanism by which a novel major histocompatibility complex class I protein, HFE, regulates iron uptake into the body is not known. HFE is the product of the gene that is mutated in >80% of hereditary hemochromatosis patients. It was recently found to coprecipitate with the transferrin receptor (Feder, J. N., Penny, D. M., Irrinki, A., Lee, V. K., Lebron, J. A., Watson, N., Tsuchihashi, Z., Sigal, E., Bjorkman, P. J., and Schatzman, R. C. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 1472-1477; Parkkila, S., Waheed, A., Britton, R. S., Bacon, B. R., Zhou, X. Y., Tomatsu, S., Fleming, R.E. , and Sly, W. S. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 13198-13202) and to decrease the affinity of transferrin for the transferrin receptor (Feder et al.). In this study, HeLa cells were transfected with HFE under the control of the tetracycline-repressible promoter. We demonstrate that HFE and the transferrin receptor are capable of associating with each other within 30 min of their synthesis with pulse-chase experiments. HFE and the transferrin receptor co-immunoprecipitate throughout the biosynthetic pathway. Excess HFE is rapidly degraded, whereas the HFE-transferrin receptor complex is stable. Immunofluorescence experiments indicate that they also endocytose into transferrin-positive compartments. Combined, these results suggest a role for the transferrin receptor in HFE trafficking. Cells expressing HFE have modestly increased levels of transferrin receptor and drastically reduced levels of ferritin. These results implicate HFE further in the modulation of iron levels in the cell.

Highlights

Hereditary hemochromatosis is an autosomal recessive disease more common in people of Northern European descent than cystic fibrosis, phenylketonuria, and muscular dystrophy combined [1]
We investigated whether Asn-linked glycosylation was necessary for the association of FLAG epitopetagged HFE (fHFE) with transferrin receptor (TfR). tetϩ and tetϪ fWTHFE/tTA HeLa cells were treated with tunicamycin to inhibit Asn-linked glycosylation
To continue to define the relationship among iron, Tf, TfR, and HFE, we have investigated the biosynthesis and trafficking of fHFE in relation to TfR and have shown that HFE may regulate intracellular iron levels since its expression greatly decreases the levels of ferritin and modestly increases the levels of TfR

Summary

Introduction

Hereditary hemochromatosis is an autosomal recessive disease more common in people of Northern European descent than cystic fibrosis, phenylketonuria, and muscular dystrophy combined [1]. The results of this study demonstrate that cells expressing HFE have decreased ferritin levels and increased TfR number, implying low intracellular iron levels.

Results

Conclusion