Ferroportin‐mediated cellular iron efflux requires extracellular calcium

Abstract

Ferroportin—the only known cellular iron exporter—is responsible for iron efflux from enterocytes and macrophages to the blood plasma. This transporter, under the control of hepcidin, serves as a key point of regulation in human iron homeostasis. The thermodynamic driving force for ferroportin‐mediated iron efflux is not known at present. To test the hypothesis that ferroportin is an iron/calcium antiporter, we expressed human ferroportin in RNA‐injected Xenopus oocytes and examined its activity by using radiotracer assays as described (Mitchell et al, Am J Physiol Cell Physiol 306: C450–C459, 2014). Ferroportin expression stimulated 55Fe efflux in the presence of 2 mM extracellular Ca2+ [mean (SD) rate constant, k = 3.5 (1.8) × 10−3 min−1; P < 0.001] but not in its absence (0 mM Ca2+, 1 mM EGTA) [k = 0.45 (0.60) × 10−3 min−1], the latter no different from control. Mg2+ could not substitute for Ca2+. Whereas extracellular Ca2+ stimulated ferroportin‐mediated 55Fe efflux (with half‐maximal [Ca], K0.5 ∼ 0.6 mM), microinjection of iron did not stimulate the uptake of 100 μM 45Ca2+ in oocytes expressing ferroportin (P = 0.7). We conclude that calcium is a required cofactor—but not a transported substrate—of ferroportin. Our study suggests that ferroportin‐mediated iron efflux may be limited in conditions of hypocalcemia. DK080047, DK082717