Regulation of Mitochondrial Iron Import through Differential Turnover of Mitoferrin 1 and Mitoferrin 2

Abstract

Mitoferrin 1 and mitoferrin 2 are homologous members of the mitochondrial solute carrier family. Mitoferrin 1 is required for mitochondrial iron delivery in developing erythrocytes. Here we show that mitoferrin 1 and mitoferrin 2 contribute to mitochondrial iron delivery in a variety of cells. Reductions in mitoferrin 1 and/or mitoferrin 2 levels by RNA interference result in decreased mitochondrial iron accumulation, heme synthesis, and iron-sulfur cluster synthesis. The ectopic expression of mitoferrin 1 in nonerythroid cells silenced for mitoferrin 2 or the expression of mitoferrin 2 in cells silenced for mitoferrin 1 restored heme synthesis to "baseline" levels. The ectopic expression of mitoferrin 2, however, did not support hemoglobinization in erythroid cells deficient in mitoferrin 1. Mitoferrin 2 could not restore heme synthesis in developing erythroid cells because of an inability of the protein to accumulate in mitochondria. The half-life of mitoferrin 1 was increased in developing erythroid cells, while the half-life of mitoferrin 2 did not change. These results suggest that mitochondrial iron accumulation is tightly regulated and that controlling mitoferrin levels within the mitochondrial membrane provides a mechanism to regulate mitochondrial iron levels.Iron is a required element for all eukaryotes, but iron can be toxic at high concentrations. Consequently, the cellular acquisition of iron is highly regulated, as is the concentration of free iron in biological fluids. The regulation of iron concentration is extended to cellular organelles that either store or utilize iron. Mitochondria utilize iron for the synthesis of heme and ironsulfur (Fe-S) clusters. These prosthetic groups are used within the mitochondria and are exported for use by cytosolic and nuclear proteins. The mechanisms that regulate mitochondrial iron levels are not known, although it is clear that mitochondrial iron levels must be regulated. For example, the loss of function mutations in genes that encode enzymes required for Fe-S cluster synthesis or the Atm1 transporter that exports Fe-S clusters, results in excessive mitochondrial iron accumulation in yeast and humans (for a review, see reference 11).The mechanisms that regulate mitochondrial iron pools are not well defined. Mitochondrial iron pools might be regulated at the level of import. Mitoferrin 1 (Mfrn1) has been shown to be required for mitochondrial iron import in developing erythroid cells. A mutation in zebrafish Mfrn1 (frascati) or the deletion of mouse Mfrn1 leads to defects in hemoglobinization due to a deficit in mitochondrial iron uptake (17). The phenotype of frascati zebrafish is restricted to developing red blood cells; other cell types showed no evidence of a mitochondrial iron phenotype. Mfrn1 has a paralogue, Mfrn2, and both genes have homologues MRS3 and MRS4 in Saccharomyces cerevisiae. Yeast with deletions of MRS3 and MRS4 grows poorly under low iron conditions due to impaired mitochondrial iron acquisition (5, 10, 13, 23). In yeast, the expression of...

Mitoferrin-2-dependent Mitochondrial Iron Uptake Sensitizes Human Head and Neck Squamous Carcinoma Cells to Photodynamic Therapy

“…Mfrn1 is highly expressed in erythroid cells, whereas nonerythroid tissues express Mfrn2 (20). There is no sequence homology between MCU and Mfrn2 (15,16).…”

“…is an iron transporter localized in the mitochondrial inner membrane that transports iron from the cytosol into mitochondria (20,34). Because bafilomycin-induced toxicity during PDT is likely related to mitochondrial iron uptake, we assessed Mfrn2 expression in head and neck cancer cells.…”

The role of lysosomes in iron metabolism and recycling

“…However, the details of how iron is targeted to these organelles or how heme is exported to the cytosol remain unclear. The recently detected proteins mitoferrin 1 and mitoferrin 2 might act as iron-transporters over the inner mitochondrial membranes (Paradkar et al, 2009, Shaw et al, 2006, although they cannot be the only transporters involved because their deletion is not lethal. Since mitochondria are the main cellular producers of superoxide and hydrogen peroxide, one has to anticipate that mitochondrial iron is predominantly kept in a non-redox-active form since otherwise fulminant production of hydroxyl radicals would result.…”

ABCs of erythroid mitochondrial iron uptake

“…In collaboration with Kaplan's group, they noted that Mfrn1 protein was stabilized as erythroid precursors underwent differentiation, consistent with a key role in mitochondrial iron acquisition (6). Now, they show that Mfrn1 stabilization is accomplished through interaction with another protein that is highly restricted to erythroid mitochondria, Abcb10 (also known as Abc-me) (2).…”