A bulge uridine in the HIF2α IRE allows IRP1 but not IRP2 to selectively regulate HIF2α expression and ensuing EPO...

Iron regulatory proteins (IRP1 and IRP2) play a pivotal role in maintaining cellular iron homeostasis by binding to iron-responsive elements (IREs) of target messenger RNAs and regulating the expression of these iron-related genes. Mice and humans who lack functional IRP1 develop erythrocytosis due to erythropoietin (EPO) overproduction, whereas those who lack IRP2 develop microcytic anemia, believed to result from iron deficiency of erythroblasts. Here, we discovered that IRP2 deficiency reduced the expression of hypoxia-inducible factor 2α (HIF2α) and its transcriptional target, EPO, thereby compromising the stress erythropoiesis response to generate red blood cells upon anemia. The distinct consequences of IRP2 and IRP1 on EPO result from the higher binding affinity of the HIF2α IRE for IRP1 than IRP2. This difference in binding affinity arises from a bulge uridine in the upper stem of HIF2α IRE that impairs the ability of IRP2 to bind the IRE. These results reveal that IRP1 and IRP2 play distinct roles in erythropoiesis and unveil an unsuspected IRE binding preference that contributes to the divergent phenotypes observed in IRP1- and IRP2-deficient mammals.