Nuclear ferritin: a ferritoid-ferritin complex in corneal epithelial cells.

Iron is essential for life in all eukaryotes and most prokaryotes; however, free iron (Fe2+), in excess, can exacerbate oxidative damage through the Fenton reaction, which generates hydroxyl radicals, the most energetic and deleterious reactive oxygen species (ROS).1–3 Therefore, iron-sequestering proteins such as ferritin have evolved as one of the cellular mechanisms of detoxification.4–7 Although it was generally believed that the subcellular localization of ferritin is exclusively cytoplasmic, recent studies have reported cells with ferritin in a nuclear location. For tissues in vivo, these include avian embryonic corneal epithelium (CE) and nucleated red blood cells.8,9 In developing rats, these include the brain.10 For cells in culture, these include astrocytoma and glial cell lines and cells subjected to iron overloading and other pathologic conditions.11–13 Several functions for nuclear ferritin have been suggested. In CE cells, we have considerable evidence that the nuclear ferritin affords protection from UV-and H2O2-induced damage to DNA.14–16 In other cell types, nuclear ferritin has also been suggested to protect DNA and, in addition, to provide iron for nuclear enzymes and to regulate the initiation of transcription.11,12,17 Similarly, for the nuclear transport of ferritin, at least two mechanisms have been suggested. One, in CE cells, involves a tissue-specific nuclear transporter protein for ferritin and another, in astrocytoma cells, involves posttranslational modifications of the ferritin H-chain.18,19 Cytoplasmic mammalian ferritin complexes are heteropolymers composed of two types of subunits, H and L, assembled in different ratios to form a 24-mer supramolecular complex capable of sequestering approximately 4500 atoms of iron.20,21 In addition, the cytoplasmic ferritin complex has been reported to associate with nonferritin proteins that deliver iron to the ferritin core22 and others that are involved in the subcellular distribution of ferritin.8,23 However, in avian species, only the H-subunit has been detected. In chicken CE cells, we have previously identified a novel protein, ferritoid, that binds to ferritin and translocates it into the nucleus. Ferritoid consists of two domains. One ferritin-like domain is involved in its binding to ferritin, and the other domain has a consensus SV40-type nuclear localization signal that is responsible for the nuclear transport.24 Other than this, however, little was known concerning the association between ferritoid and ferritin, such as the type of complexes formed between these two components, the subcellular localization(s) of these complexes, and whether they are transient—that is, present only during the transport process—or whether, once formed, they remain stable. In addition, if the ferritoid-ferritin complexes are stable, do they have unique characteristics/properties that distinguish them from other multimeric ferritin complexes? In the present study we have determined certain of the characteristics of the nuclear ferritoid-ferritin complexes.