Mitochondrial ferritin limits oxidative damage regulating mitochondrial iron availability: hypothesis for a protective role in Friedreich ataxia

Mitochondrial ferritin (FtMt) is a nuclear-encoded iron-sequestering protein that specifically localizes in mitochondria. In mice it is highly expressed in cells characterized by high-energy consumption, while is undetectable in iron storage tissues like liver and spleen. FtMt expression in mammalian cells was shown to cause a shift of iron from cytosol to mitochondria, and in yeast it rescued the defects associated with frataxin deficiency. To study the role of FtMt in oxidative damage, we analyzed the effect of its expression in HeLa cells after incubation with H2O2 and Antimycin A, and after a long-term growth in glucose-free media that enhances mitochondrial respiratory activity. FtMt reduced the level of reactive oxygen species (ROS), increased the level of adenosine 5'triphosphate and the activity of mitochondrial Fe-S enzymes, and had a positive effect on cell viability. Furthermore, FtMt expression reduces the size of cytosolic and mitochondrial labile iron pools. In cells grown in glucose-free media, FtMt level was reduced owing to faster degradation rate, however it still protected the activity of mitochondrial Fe-S enzymes without affecting the cytosolic iron status. In addition, FtMt expression in fibroblasts from Friedreich ataxia (FRDA) patients prevented the formation of ROS and partially rescued the impaired activity of mitochondrial Fe-S enzymes, caused by frataxin deficiency. These results indicate that the primary function of FtMt involves the control of ROS formation through the regulation of mitochondrial iron availability. They are consistent with the expression pattern of FtMt observed in mouse tissues, suggesting a FtMt protective role in cells characterized by defective iron homeostasis and respiration, such as in FRDA.