Abstract 12528: ALR Protein, a Critical Protein in Cardiac Development, Regulates Cellular Iron Homeostasis by Modulating Mitochondrial Transport of ATP-Binding Cassette (ABC)-B8

Abstract

Introduction: Iron is an essential molecule for normal cellular physiology, and altered cellular iron homeostasis is commonly observed in diseases with disruption of iron/sulfur (Fe/S) cluster maturation, such as cardiomyopathy associated with Friedreich’s ataxia. Inhibition of Augmenter of Liver Regeneration (ALR), a mitochondrial inter-membrane-space protein involved in mitochondrial protein import, results in cardiac developmental defect in zebrafish, and its deletion is associated with increased oxidative stress and cytosolic Fe/S cluster maturation defects. ABCB8 is one of only two mitochondrial membrane proteins known to regulate cytosolic Fe/S cluster maturation. We hypothesized that ALR is critical for cytosolic Fe/S cluster maturation and iron homeostasis by regulating mitochondrial import of ABCB8. Results: Downregulation of ALR in vitro resulted in reduced cytosolic Fe/S cluster-containing enzyme activities and increased cellular iron uptake. Using a knockdown-rescue approach, we further demonstrated that only the mitochondrial, but not the cytosolic, ALR isoform is involved in the maturation of cytosolic Fe/S clusters. Because Fe/S clusters are synthesized in the mitochondria, we then assessed whether ALR can alter the levels or activity of ABCB7 and ABCB8, the two mitochondrial proteins known to regulate the maturation of cytosolic Fe/S clusters. Downregulation of ALR reduced the mitochondrial levels of ABCB8, while ABCB7 levels were not affected. We also identified defects in mitochondrial transport of ABCB8 as the mechanism for reduced mitochondrial ABCB8 levels with ALR knockdown. Finally, we demonstrated that ABCB8 physically interacts with the protein import system consisting of ALR and Mia40, thus providing a mechanism for reduced ABCB8 mitochondrial transport with ALR downregulation. Conclusion: Our results indicate that ALR and its interaction partner Mia40 are involved in the transport of ABCB8 into the mitochondria, which in turn regulates cytoplasmic Fe/S cluster maturation. These findings provide insights into cellular iron regulation, with implications in cardiovascular disease and cardiac development.