Novel roles for iron regulatory proteins in the adaptive response to iron deficiency.

Iron regulatory proteins (IRP) modulate the use of mRNA-encoding proteins that are involved in the transport, storage and use of iron. Several new potential mRNA targets for IRP were recently identified: divalent metal transporter-1 (DMT-1) and ferroportin, which are critical regulators of iron absorption in the gut and of iron cycling between various tissues of the body. Although this may extend the reach of IRP to other processes that are important for maintaining body iron homeostasis, the extent to which IRP modulate other physiological processes that are altered in response to changes in iron availability is not clear. However, in the past several years, targets for IRP and IRP-like proteins were identified in eukaryotes and prokaryotes in the tricarboxylic acid (TCA) cycle and electron-transport chain. In mammals, this includes the mRNA that encodes the TCA-cycle enzyme mitochondrial aconitase (m-acon). Recent work established that m-acon expression is translationally regulated by iron in a manner that is strongly correlated with IRP RNA-binding activity. Interestingly, these studies also demonstrate that IRP regulate their mRNA targets in a hierarchical manner. The changes in m-acon synthesis and abundance in liver during iron deficiency fail to affect TCA-cycle capacity but are associated with a significant upregulation of mitochondrial export of radiolabeled citrate. We conclude that IRP are required for the regulation of physiological pathways that include but are not limited to iron metabolism, and as such, IRP are critical factors in the adaptive response to iron deficiency.