A small molecule redistributes iron in ferroportin-deficient mice and patient-derived primary macrophages

Abstract

Deficiencies of the transmembrane iron-transporting protein ferroportin (FPN1) cause the iron misdistribution that underlies ferroportin disease, anemia of inflammation, and several other human diseases and conditions. A small molecule natural product, hinokitiol, was recently shown to serve as a surrogate transmembrane iron transporter that can restore hemoglobinization in zebrafish deficient in other iron transporting proteins and can increase gut iron absorption in FPN1-deficient flatiron mice. However, whether hinokitiol can restore normal iron physiology in FPN1-deficient animals or primary cells from patients and the mechanisms underlying such targeted activities remain unknown. Here, we show that hinokitiol redistributes iron from the liver to red blood cells in flatiron mice, thereby increasing hemoglobin and hematocrit. Mechanistic studies confirm that hinokitiol functions as a surrogate transmembrane iron transporter to release iron trapped within liver macrophages, that hinokitiol-Fe complexes transfer iron to transferrin, and that the resulting transferrin-Fe complexes drive red blood cell maturation in a transferrin-receptor–dependent manner. We also show in FPN1-deficient primary macrophages derived from patients with ferroportin disease that hinokitiol moves labile iron from inside to outside cells and decreases intracellular ferritin levels. The mobilization of nonlabile iron is accompanied by reductions in intracellular ferritin, consistent with the activation of regulated ferritin proteolysis. These findings collectively provide foundational support for the translation of small molecule iron transporters into therapies for human diseases caused by iron misdistribution.

A Rationally Designed Complex Replenishes the Transferrin Iron Pool Directly and with High Specificity

“…Recent studies demonstrated that the natural product hinokitiol acts as a cell-permeant Fe chelator and facilitates Fe redistribution across pathophysiologic gradients in rodents. 25,26 An aqueous formulation of iron pyrophosphate citrate (FPC), which directly replenishes the transferrin-Fe pool upon intravenous infusion, has an FDA indication for use in CKD patients receiving hemodialysis. 27,28 However, care must be taken to ensure that serum FPC concentrations do not exceed transferrin's capacity to bind Fe as exposure to toxic labile Fe occurs above this threshold.…”

Another tool in the toolkit to manage iron overload

“…Ekaputri et al. ( 5 ) discuss, in PNAS, the biological activity and potential therapeutic use of hinokitiol, a small, plant-derived molecule used in traditional Asian medicine that also has the potential to mobilize iron in the setting of iron overload.…”

“…The studies of hinokitiol described by Ekaputri et al. ( 5 ) indicate this small molecule works in a fundamentally different way.…”

“…In the studies by Ekaputri et al. ( 5 ), the authors use a mouse model of ferroportin deficiency in which the animals exhibit macrophage iron overload, low levels of iron in serum, and mild anemia ( 5 ). Treatment of these animals with hinokitiol results in the mobilization of iron from the liver and spleen with concomitant increases in serum iron and improvements in anemia ( Fig.…”

“…Because the hinokitiol iron mobilized from ferroportin-deficient cells was rapidly exchanged and converted to transferrin iron in the blood, hinokitiol did not facilitate iron excretion from the ferroportin-deficient mice ( 5 ). It did, however, facilitate the redistribution of iron from intracellular stores to the blood where the iron could then be presented to the erythron.…”