Excess capacity of the iron regulatory protein system

Iron regulatory proteins (IRP1 and IRP2) are master regulators of cellular iron metabolism. IRPs bind to iron response elements (IREs) present in the untranslated regions of mRNAs encoding proteins of iron storage, uptake, transport, and export. Because simultaneous knockout of IRP1 and IRP2 is embryonic lethal, it has not been possible to use dual knockouts to explore the consequences of loss of both IRP1 and IRP2 in mammalian cells. In this report, we describe the use siRNA to assess the relative contributions of IRP1 and IRP2 in epithelial cells. Stable cell lines were created in which either IRP1, IRP2, or both were knocked down. Knockdown of IRP1 decreased IRE binding activity but did not affect ferritin H and TfR1 expression, while knockdown of IRP2 marginally affected IRE binding activity but caused an increase in ferritin H and a decrease in TfR1. Knockdown of both IRPs resulted in a greater reduction of IRE binding activity and more severe perturbation of ferritin H and TfR1 expression compared to single IRP knockdown. Even though the knockdown of IRP-1, IRP-2 or both was efficient, resulting in non-detectable protein and under 5% of wildtype levels of mRNA, all stable knockdowns retained an ability to modulate ferritin H and TfR1 appropriately in response to iron challenge. However, further knockdown of IRPs accomplished by transient transfection of siRNA in stable knockdown cells completely abolished the response of ferritin H and TfR1 to iron challenge, demonstrating an extensive excess capacity of the IRP system.