Mice lacking mitochondrial ferritin are more sensitive to doxorubicin-mediated cardiotoxicity

Mitochondrial ferritin (FtMt) is a recently identified ferritin type that accumulates specifically in the mitochondria [ 1 ]. Human FtMt, which is encoded by an intronless gene, is synthesized as a precursor with a long N-terminal targeting sequence that is cleaved in the mature protein. The mature FtMt has a functional ferroxidase center and forms stable ferritin shells that readily accumulate iron [ 2 ]. Its 3D structure is analogous to that of the H-ferritin [ 3 ]. The FtMt transcript does not contain a functional iron responsive element (IRE) sequence, and thus, FtMt expression is not controlled by intracellular iron levels. In humans, FtMt was found to be expressed in the testis [ 4 ], neurons [ 5 ], and in the erythroblasts of subjects with genetic or acquired sideroblastic anemia [ 6 ], where it is responsible of the mitochondrial iron deposits of the sideroblasts. Since human FtMt is not detectable in cultured cells, most of the data on its function have been obtained by analyzing the phenotype of overexpressing cells. These studies showed that, by virtue of its ferroxidase activity, FtMt actively sequesters iron inside the mitochondria at the expense of cytosolic iron [ 7 ]. It also reduces iron-mediated oxidative damage of mitochondria [ 8 ] and rescues some defects caused by frataxin deficiency in yeast [ 9 ] and HeLa cells [ 10 ]. More recently, it has been shown that overexpression of FtMt in K562 erythroid cells reduced Jak/STAT signaling and increased apoptosis [ 11 ]. In sideroblasts, erythroid progenitors FtMt expression occurred at the early stage of cell differentiation and was accompanied by reduced iron availability and increased apoptosis [ 12 , 13 ]. Moreover, FtMt expressing cells transplanted in nude mice grew more slowly than their control counterpart [ 14 ]. Altogether, FtMt seems to protect mitochondria against iron-dependent oxidative damage and also modifies cellular iron distribution by attracting iron from the cytosol to mitochondria [ 15 ]. Specific antibodies have been raised against mouse FtMt that were used to study its distribution in mouse organs. It was found that FtMt is strongly expressed in the testis, particularly in the spermatocytes, and also in the heart, kidney, Purkinje cells, and some neurons, and generally in cells with high respiratory activity that actively use iron enzymes and produce reactive oxygen species (ROS) [ 16 ]. In line with the results obtained in the cell lines, FtMt was also found to decrease the sensitivity of mitochondria to oxidative damage in SLA mice [ 17 ] and to protect neuronal cells from oxidative damage [ 18 – 20 ]. The role of FtMt in neurodegenerative diseases has been recently reviewed [ 21 ]. However, the recently described FtMt-deficient mice in the C57BL/6J strain did not show any evident phenotype and produced siderocytes/sideroblasts similar to the control FtMt +/+ even under conditions of vitamin B6 deficiency [ 22 ]. It is unlikely that the lack of phenotype in FtMt deficiency is due to redundancy of the function, since FtMt is the only known mitochondrial iron storage protein. Although FtMt appears dispensable under physiological conditions, it may protect against damages under specific conditions. FtMt is highly expressed in the heart; thus, we considered that it may protect its mitochondria against oxidative damage, in particular from the injury induced by doxorubicin (Dox), a well-characterized anthracycline whose extensive use for the cure of a variety of tumors is hampered by a recognized cardiotoxicity [ 23 – 25 ]. Anthracyclines, including Dox, possess a high affinity for cardiolipin, a negatively charged phospholipid of the inner mitochondrial membrane [ 26 ], and hence, they are retained at high concentrations in the mitochondrial compartment. Confocal microscopy experiments showed that in H9c2 cardiomyocytes, Dox localized to mitochondrial sites of redox cycling and ROS formation [ 27 ]. Therefore, mitochondria have been repeatedly suggested as the most important target for anthracyclines cardiotoxicity [ 24 ]. In this context, the availability of intracellular reactive iron, which catalyzes ROS formation, appears to be determinant for Dox cardiotoxicity, as shown by the protective effect of iron chelation in patients and in experimental models [ 28 ], whereas iron overload have been shown to exacerbate the cardiotoxic effects of the drug [ 29 – 32 ]. In particular, mitochondrial iron may play a significant role in leading to elevated mitochondrial ROS formation, and it has been recently shown that overexpression of FtMt can protect HeLa cells from Dox toxicity [ 33 ] and that Dox treatment strongly induces FtMt expression in neonatal rat cardiomyocytes [ 34 ]. Moreover, a recent study showed that Dox cardiotoxicity is mediated by mitochondrial iron accumulation and that heart-specific deletion of mitochondrial iron exporter ABCB8 increased mice sensitivity to Dox cardiotoxicity, whereas its overexpression was protective [ 34 ].