Extracellular H2O2 and not superoxide determines the compartment-specific activation of transferrin receptor by iron regulatory protein 1

Iron regulatory protein 1 (IRP1) functions as translational regulator that plays a central role in coordinating the cellular iron metabolism by binding to the mRNA of target genes such as the transferrin receptor (TfR)-the major iron uptake protein. Reactive oxygen species such as H2O2 and O-2(center dot-) that are both co-released by inflammatory cells modulate IRP1 in opposing directions. While H2O2-similar to iron depletion-strongly induces IRP1 via a signalling cascade, O-2(center dot-) inactivates the mRNA binding activity by a direct chemical attack. These findings have raised the question of whether compartmentalization may be an important mechanism for isolating these biological reactants when released from inflammatory cells during the oxygen burst cascade. To address this question, we studied cytosolic IRP1 and its downstream target TfR in conjunction with a tightly controlled biochemical modulation of extracellular O-2(center dot-) and H2O2 levels mimicking the oxygen burst cascade of inflammatory cells. We here demonstrate that IRP1 activity and expression of TfR are solely dependent on H2O2 when co-released with O-2(center dot-) from xanthine oxidase. Our findings confirm that extracellular H2O2 determines the functionality of the IRP1 cluster and its downstream targets while the reactivity of O-2(center dot-) is limited to its compartment of origin.