Journal
AMERICAN JOURNAL OF PHYSIOLOGY-CELL PHYSIOLOGY
Volume 306, Issue 5, Pages C450-C459Publisher
AMER PHYSIOLOGICAL SOC
DOI: 10.1152/ajpcell.00348.2013
Keywords
cobalt transport; enterocytes; iron overload; iron transport; macrophages; membrane transport; trace metal nutrition; Xenopus laevis oocyte; zinc transport
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Funding
- Public Health Service Grants from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) [R01 DK080047, R01 DK082717, P30 DK078392]
- University of Cincinnati
- U.S. Department of Energy (DoE), Nuclear Physics Isotope Program
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Iron homeostasis is achieved by regulating the intestinal absorption of the metal and its recycling by macrophages. Iron export from enterocytes or macrophages to blood plasma is thought to be mediated by ferroportin under the control of hepcidin. Although ferroportin was identified over a decade ago, little is understood about how it works. We expressed in Xenopus oocytes a human ferroportin-enhanced green fluorescent protein fusion protein and observed using confocal microscopy its exclusive plasma-membrane localization. As a first step in its characterization, we established an assay to detect functional expression of ferroportin by microinjecting oocytes with Fe-55 and measuring efflux. Ferroportin expression increased the first-order rate constants describing Fe-55 efflux up to 300-fold over control. Ferroportin-mediated Fe-55 efflux was saturable, temperature-dependent (activation energy, E-a approximate to 17 kcal/mol), maximal at extracellular pH approximate to 7.5, and inactivated at extracellular pH < 6.0. We estimated that ferroportin reacts with iron at its intracellular aspect with apparent affinity constant < 10(-7) M. Ferroportin expression also stimulated efflux of Zn-65 and Co-57 but not of Cu-64, Cd-109, or Mn-54. Hepcidin treatment of oocytes inhibited efflux of Fe-55, Zn-65, and Co-57. Whereas hepcidin administration in mice resulted in a marked hypoferremia within 4 h, we observed no effect on serum zinc levels in those same animals. We conclude that ferroportin is an iron-preferring cellular metal-efflux transporter with a narrow substrate profile that includes cobalt and zinc. Whereas hepcidin strongly regulated serum iron levels in the mouse, we found no evidence that ferroportin plays an important role in zinc homeostasis.
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