Iron regulation of mitochondrial aconitase synthesis: A role for iron regulatory proteins (IRP) in cellular citrate metabolism

Diseases of iron metabolism due to nutritional or inherited disorders are a global health concern. Pathological consequences of human diseases of iron metabolism include derangements of energy metabolism and increased oxidative stress. Iron regulatory proteins (IRP) are critical regulators of vertebrate iron homeostasis that sense cellular iron status and act to regulate the synthesis of proteins required for the transport, storage or use of iron. IRP bind to iron responsive elements (IRE) in the 5' or 3' untranslated regions of specific mRNA and regulate their translation or stability, respectively. Mitochondrial aconitase (m-acon) is a tricarboxylic acid (TCA) cycle enzyme that converts citrate to isocitrate and is required for aerobic ATP generation. m-Acon mRNA has been identified as a potential target for IRP action. The physiologic purpose of iron-dependent translational regulation of m-acon by IRP and its role in the adaptive response to iron deficiency is the focus of this dissertation.;Experiments aimed at determining if iron regulates m-acon synthesis in a manner consistent with the action of IRP on m-acon mRNA translation in vivo were conducted. In weanling rat liver the majority of m-acon mRNA was actively directing the synthesis of m-acon protein yet a significant fraction of the mRNA was translationally repressed. In comparison to L-ferritin mRNA, an established target of IRP action, m-acon mRNA was similarly yet less robustly regulated by iron. We conclude that IRP may translationally regulate m-acon expression in order to program changes in citrate use in fuel metabolism and/or antioxidant defense.;To understand the physiological consequences of m-acon regulation by iron, experiments were conducted in hepatocytes and liver mitochondria. Down-regulation of m-acon in iron-deficiency failed to affect TCA cycle capacity. However, iron deficiency resulted in a significant decrease in total cellular citrate level in liver. When the capacity of mitochondria to export citrate was determined in vitro a significant inverse linear relationship between m-acon activity and citrate export capacity was demonstrated. We conclude that iron differentially regulates the translation of IRE-containing mRNA in vivo and that IRP regulate the production of proteins involved in iron and intermediary metabolism.