A bulge uridine in the HIF2a IRE allows IRP1 but not IRP2 to selectively regulate HIF2a expression and ensuing EPO levels.

ABSTRACT

Iron regulatory proteins (IRP1 and IRP2) play a pivotal role in maintaining cellular iron homeostasis by binding to iron-responsive elements (IREs) of target mRNAs and regulating the expression of these iron-related genes. Mice and humans that lack functional IRP1 develop erythrocytosis due to erythropoietin overproduction, whereas those that 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 2 alpha (HIF2a) and its transcriptional target, erythropoietin (EPO), thereby compromising the stress erythropoiesis response to generate RBCs upon anemia. The distinct consequences of IRP2 and IRP1 on EPO result from the higher binding affinity of the HIF2a IRE for IRP1 compared to IRP2. This difference in binding affinity arises from a bulge uridine in the upper stem of HIF2a 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.