Abstract 16497: Characterization of ATP Binding Cassette Protein B8 (ABCB8) as a Mitochondrial Iron and Glutathione Exporter
Background: Mitochondrial iron is required for the synthesis of heme and Fe/S clusters, but can also be cytotoxic via production of reactive oxygen species (ROS). The process of mitochondrial iron regulation, though essential, is not well understood. We hypothesized that ABCB8, a mitochondrial protein with unknown function, is a mitochondrial iron exporter.
Methods and Results: To study the role of ABCB8 in mitochondrial iron homeostasis, we generated cardiac-specific ABCB8 knockout (KO) mice. ABCB8 KO in the heart led to cardiac dysfunction, increased fibrosis, ROS-induced damage and mitochondrial disarray. Furthermore, mitochondrial iron content was elevated in these mice. Consistent with in vivo studies, downregulation of ABCB8 in neonatal rat cardiomyocytes (NRCM) led to an increase, while its overexpression reduced mitochondrial iron levels. To test whether ABCB8 functions in mitochondrial iron export, we measured the release of radioactive 55Fe from isolated mitochondria following ABCB8 modulation. ABCB8 siRNA increased mitochondrial 55Fe content, but reduced the amount of radioactive iron in the soluble fraction, indicating impaired export. This also led to decreased activity of cytosolic, but not mitochondrial, Fe/S cluster-containing proteins. Furthermore, overexpression of ABCB8 increased iron export out of the mitochondria. These results establish ABCB8 as the first mitochondrial iron exporter and suggest that mitochondrial iron export through ABCB8 is required for the maturation of cytosolic Fe/S cluster proteins. To provide further mechanistic insights into mitochondrial iron homeostasis, we assessed the role of ABCB8 in the export of glutathione (GSH). GSH is essential for Fe/S cluster formation in yeast, and participates in nitric oxide (NO)-iron complex transport in mammals. Using similar export assay, we found mitochondrial GSH levels to be elevated and GSH release from mitochondria to be impaired with ABCB8 knockdown, suggesting a functional association between GSH and iron export from mitochondria.
Conclusions: We identify ABCB8 as a mitochondrial iron and GSH exporter which is essential for cardiac health. Furthermore, mitochondrial iron export by ABCB8 is needed for the maturation of cytosolic Fe/S proteins.
- © 2011 by American Heart Association, Inc.