Iron metabolism

Regulation of intracellular iron levels in iron-acceptor and iron-donor cells

In recent years many new genes and proteins were identified with crucial functions in iron metabolism. This gave an explosion of our knowledge and understanding of iron related disorders. Mutations have been found that are responsible for disturbances in iron transport, leading to either iron overload or iron deficiency. For experts in the field, these new findings clarify the sky and open new routes for exploring hitherto hidden fields of research. For the physician, however, iron metabolism may become even more complicated. In this review, we have tried to assemble all new iron related genes into the context of pathophysiology. Important results from animal experiments, mainly derived from knockout mouse models, are included in this review as they often explain the phenotype of human disease.

The outcome of Leishmania major experimental infection in BALB/c mice can be modulated by exogenously delivered iron

We previously established that Leishmania promastigotes express a transferrin receptor and that iron chelators inhibit promastigote growth in vitro. Thus, we were interested in modulating the vertebrate host iron pool and to monitor whether such changes will affect the outcome of L. major infection in BALB / c mice, inoculated in the footpad with 106 stationary phase promastigotes. Treatment of mice with desferrioxamine resulted in a slight delay of the development of cutaneous lesions. In contrast and unexpectedly, systemic iron delivery, at early time points of parasite delivery, significantly limited footpad pathology. Accordingly, parasite loads at the site of parasite delivery, the draining lymph node, liver and spleen were significantly reduced in iron-loaded mice. Importantly, the "protective" effect of iron delivery correlated with the presence, at the site of inoculation, of lower levels of IL-4 and IL-10 transcripts while both IFN-gamma and inducible nitric oxide synthase transcripts were at higher levels. The presence of more type 1 cytokine transcripts was further supported by the increased levels of IgG2a in their sera. These data strongly suggest that susceptibility to L. major as assessed in the footpad model is modifiable by interventions that alter the iron status of the host at early time points of parasite delivery.

Regulation of mammalian iron homeostasis

Iron homeostasis is regulated with respect to uptake, storage and utilization. Newer work is presented that defines proteins responsible for iron transport, sequestration and sensing, and that addresses their regulation at the cellular and organismal levels by ambient iron concentrations, demand for erythropoiesis, body iron burden, and redox stimuli.

Crystal structure of the hereditary haemochromatosis protein HFE complexed with transferrin receptor

HFE is related to major histocompatibility complex (MHC) class I proteins and is mutated in the iron-overload disease hereditary haemochromatosis. HFE binds to the transferrin receptor (TfR), a receptor by which cells acquire iron-loaded transferrin. The 2.8 A crystal structure of a complex between the extracellular portions of HFE and TfR shows two HFE molecules which grasp each side of a twofold symmetric TfR dimer. On a cell membrane containing both proteins, HFE would 'lie down' parallel to the membrane, such that the HFE helices that delineate the counterpart of the MHC peptide-binding groove make extensive contacts with helices in the TfR dimerization domain. The structures of TfR alone and complexed with HFE differ in their domain arrangement and dimer interfaces, providing a mechanism for communicating binding events between TfR chains. The HFE-TfR complex suggests a binding site for transferrin on TfR and sheds light upon the function of HFE in regulating iron homeostasis.

The hemochromatosis protein HFE competes with transferrin for binding to the transferrin receptor

HFE is a class I major histocompatibility complex (MHC)-related protein that is mutated in patients with the iron overload disease hereditary hemochromatosis. HFE binds to transferrin receptor (TfR), the receptor used by cells to obtain iron in the form of diferric transferrin (Fe-Tf). Previous studies demonstrated that HFE and Fe-Tf can bind simultaneously to TfR to form a ternary complex, and that membrane-bound or soluble HFE binding to cell surface TfR results in a reduction in the affinity of TfR for Fe-Tf. We studied the inhibition by soluble HFE of the interaction between soluble TfR and Fe-Tf using radioactivity-based and biosensor-based assays. The results demonstrate that HFE inhibits the TfR:Fe-Tf interaction by binding at or near the Fe-Tf binding site on TfR, and that the Fe-Tf:TfR:HFE ternary complex consists of one Fe-Tf and one HFE bound to a TfR homodimer.