Functional expression in Xenopus oocytes reveals that human ferroportin is an iron exporter shared with zinc

Abstract

Iron homeostasis is achieved by regulating the intestinal absorption of the metal and its recycling by macrophages. Export of iron from enterocytes or macrophages to the blood is thought to be mediated by ferroportin (Fpn) under the control of hepcidin. Some mutations in Fpn result in macrophage iron loading whereas others, as a result of insensitivity to hepcidin, cause systemic iron overload. Whereas Fpn was identified a decade ago, there remains a paucity of information about how it works. We expressed GFP‐tagged human Fpn in RNA‐injected Xenopus oocytes and observed using confocal microscopy exclusive plasma‐membrane localization. As a first step in its characterization, we established an assay to detect functional expression of Fpn by microinjecting oocytes with 55Fe and measuring the 55Fe efflux over 30 min. Expression of Fpn increased the first‐order rate constants describing 55Fe efflux between 4‐fold and 100‐fold over control. Fpn‐mediated 55Fe efflux was maximal when the injectate contained nitrilotriacetic acid (NTA), and when we provided apotransferrin, bathophenanthroline disulfonate (BPS) and NTA in the incubation medium, whereas inclusion of ceruloplasmin had only a modest effect. Fpn‐mediated 55Fe was accelerated in pH 8.2 medium compared with pH 7.2 or pH 6.2 (P < 0.001). Fpn expression also stimulated the efflux of 65Zn (P < 0.001) but not of 54Mn, 64Cu or 109Cd (P > 0.11). We conclude that Fpn mediates cellular iron efflux and that Zn2+ counts among its narrow substrate profile.