Zip14 (Slc39a14) mediates non-transferrin-bound iron uptake into cells

Abstract

Zip14 is a member of the SLC39A zinc transporter family, which is involved in zinc uptake by cells. Up-regulation of Zip14 by IL-6 appears to contribute to the hepatic zinc accumulation and hypozincemia of inflammation. At least three members of the SLC39A family transport other trace elements, such as iron and manganese, in addition to zinc. We analyzed the capability of Zip14 to mediate non-transferrin-bound iron (NTBI) uptake by overexpressing mouse Zip14 in HEK 293H cells and Sf9 insect cells. Zip14 was found to localize to the plasma membrane, and its overexpression increased the uptake of both 65 Zn and 59 Fe. Addition of bathophenanthroline sulfonate, a cell-impermeant ferrous iron chelator, inhibited Zip14-mediated iron uptake from ferric citrate, suggesting that iron is taken up by HEK cells as Fe 2؉ . Iron uptake by HEK and Sf9 cells expressing Zip14 was inhibited by zinc. Suppression of endogenous Zip14 expression by using Zip14 siRNA reduced the uptake of both iron and zinc by AML12 mouse hepatocytes. Zip14 siRNA treatment also decreased metallothionein mRNA levels, suggesting that compensatory mechanisms were not sufficient to restore intracellular zinc. Collectively, these results indicate that Zip14 can mediate the uptake of zinc and NTBI into cells and that it may play a role in zinc and iron metabolism in hepatocytes, where this transporter is abundantly expressed. Because NTBI is commonly found in plasma of patients with hemochromatosis and transfusional iron overload, Zip14-mediated NTBI uptake may contribute to the hepatic iron loading that characterizes these diseases.hemochromatosis ͉ iron transport ͉ liver ͉ zinc transport ͉ inflammation

The role of endocytic pathways in cellular uptake of plasma non-transferrin iron

“…29,30 Despite these limitations, attempts have been made to assess NTBI transport by using NTBI-simulating complexes of radiolabeled iron in protein free settings and by artificial inclusion of reductants in order to render the iron transportable by various voltage activatable Ca channels [25][26][27] or by a putative Zn transporter. 23,24 The pathophysiological significance of such approaches is still a subject of debate, and likewise the effects of Ca-channel blockers on ironassociated cardiac damage. 27 Considering the complex nature of plasma NTBI, we choice to use native NTBI containing sera from hypertransfused thalassemia major patients in conjunction with two major strategies for tracing iron ingress into cell compartments.…”

“…3,22 The pathological accumulation of iron in multiple tissues observed in iron overload conditions is thought to be caused by excessive influx of iron into plasma, as well as unbalanced erythrophagocytosis by reticuloendothelial cells in spleen and liver that lead to persistently high circulating NTBI levels which are responsible for the systemic iron dispersal. The hypothesis that tissue iron overload is caused by NTBI is largely based on the assumption that NTBI is randomly transported into cells by unregulated mechanisms, presumably via non-specific divalent cation transporters 23,24 or calcium channels, [25][26][27] subsequent to extracellular reduction of iron(III) to iron(II) by a cell-surface iron reductase such as Dcytb. Chronic exposure of cells in vitro to artificial iron complexes that presumably mimic NTBI (usually ferric citrate) [10][11][12]28,29 has been shown to generate cellular iron overload as indicated by increased ferritin levels, ROS generation, protein and DNA oxidation, and other indicators of cell damage.…”

Cardiac ferroportin regulates cellular iron homeostasis and is important for cardiac function

“…This contrast points to possible differences between human and murine cardiomyocytes in iron entry, export, storage, or detoxification pathways. In the context of iron overload, the entry of NTBI into cardiomyocytes is recognized as a contributor to cardiac iron loading and subsequent dysfunction (18)(19)(20). Therefore, interesting questions are whether known NTBI transporters differ in human and mouse cardiomyocytes and whether such differences account for the divergent cardiac phenotypes.…”

“…These effects occur against a background of otherwise normal systemic iron homeostasis. Iron enters cardiomyocytes primarily as transferrin (Tf), through transferrin receptor 1 (TfR1) but also can enter as nonTf-bound iron (NTBI) through L-type calcium channels, T-type calcium channels, the divalent metal transporter DMT1, and the zinc transporter Zip14 (18)(19)(20). In the cell, excess iron is stored and detoxified in a protein complex called "ferritin" (21).…”

Hepatic ZIP14-mediated zinc transport is required for adaptation to endoplasmic reticulum stress

“…2D and 5D) supports the notion that GRP94 is a critical factor that supports recovery of ER homeostasis. Although ZIP14 can contribute to uptake of manganese and non-transferrinbound iron under certain circumstances (49,50), the hepatic concentration of these metals after TM administration was comparable between WT and Zip14 KO mice (Fig. S1 E and F).…”