Ironing out Ferroportin

Abstract

Maintaining physiologic iron concentrations in tissues is critical for metabolism and host defense. Iron absorption in the duodenum, recycling of iron from senescent erythrocytes, and iron mobilization from storage in macrophages and hepatocytes constitute the major iron flows into plasma for distribution to tissues, predominantly for erythropoiesis. All iron transfer to plasma occurs through the iron exporter ferroportin. The concentration of functional membrane-associated ferroportin is controlled by its ligand, the iron-regulatory hormone hepcidin, and fine-tuned by regulatory mechanisms serving iron homeostasis, oxygen utilization, host defense, and erythropoiesis. Fundamental questions about the structure and biology of ferroportin remain to be answered.

Figure 1. Systemic flows of elemental iron

The release of iron into blood plasma is mediated by the iron transporter ferroportin which also serves as the control point for the regulation of iron flows. In plasma, iron is carried by transferrin (Tf-Fe). Red blood cells (RBCs) contain the majority of the body iron, and iron fluxes in humans are dominated by the utilization of iron by erythroblasts in the marrow, and recycling of iron from the hemoglobin of senescent erythrocytes by erythrophagocytosing macrophages (iMΦ=iron-recycling macrophages, mostly in the spleen). Other cells in the body utilize smaller amounts of iron, and this too is recycled by macrophages after cells die. The liver is the major storage organ for iron from which iron can be mobilized in times of need, and where it is deposited when there is a surplus of iron in the body. Duodenal absorption of iron is the sole external source of iron, and compensates for the losses of iron from the body which are normally relatively small. In pregnant women, iron is transferred to fetal blood through ferroportin in the placenta.

Figure 2. Ferroportin regulation

Cellular iron efflux is proportional to the concentration of ferroportin molecules on the membrane, and multiple levels of ferroportin regulation exist in different cell types. Panel A: In iron-recycling macrophages, following erythrophagocytosis both heme and iron increase ferroportin production. Heme regulates ferroportin transcriptionally via the Bach1/Nrf2 complex (heme causes degradation of repressor Bach1, resulting in the stimulation of Fpn transcription by Nrf2). Iron regulates ferroportin translationally through modulating the interaction of IRPs with IREs in the 5′ UTR of ferroportin (iron prevents IRPs from binding to Fpn mRNA, allowing translation to proceed). During infections with intracellular pathogens, increased NO production leads to the activation of Nrf2 and increased ferroportin transcription. Inflammation can also suppress ferroportin transcription through as yet unknown mechanisms. Ferroportin is post-translationally regulated by hepcidin-mediated endocytosis and proteolysis. Panel B: In duodenal enterocytes, hypoxia induces ferroportin transcription via HIF2alpha. Retention of iron in enterocytes during iron deficiency is avoided by the presence of ferroportin mRNAs that lack 5′IRE. Hepcidin controls iron efflux from enterocytes posttranslationallly by inducing the endocytosis and proteolysis of ferroportin. Panel C: Integration of diverse local and systemic influences on cellular iron efflux through ferroportin regulation at the transcriptional, translational and posttranslational level.

Figure 3. Alternating access model of iron transport by ferroportin

Ferroportin is a member of the major facilitator superfamily of transporters which share a structural feature known as the MFS fold: 12 transmembrane segments (TM) organized into two 6-helix halves, connected by a large cytoplasmic loop between TM-6 and -7. Transport across the membrane likely occurs via the alternate-access (rocker-switch) mechanism whereby the two halves of the molecule cycle through inward-facing, occluded and outward-facing conformations to facilitate substrate transport. We hypothesize that i) ferrous iron binds to ferroportin in the open-in conformation, ferroportin flips to the open-out conformation and releases ferrous iron to be oxidized by a ferroxidase (Ferrox) and delivered to transferrin (Tf) in the ferric form; and ii) that hepcidin binds to the open out conformation inducing a conformational change that results in ubiquitination of the connecting loop and endocytosis of ferroportin.