Superoxide and hydrogen peroxide‐dependent inhibition of iron regulatory protein activity: a protective stratagem against oxidative injury

Abstract

Cellular iron homeostasis is regulated by the cytoplasmic iron regulatory protein (IRP), which binds to iron-responsive elements (IRE) of mRNAs, modulating iron uptake and sequestration, respectively. When iron is scarce, IRP binds to IRE and coordinately increases the synthesis of transferrin receptor and decreases that of ferritin, thus providing the cell with readily available free iron. When iron is in excess, IRP does not bind and iron sequestration prevails over iron uptake. We have found that incubation of rat liver lysates with xanthine oxidase (XO), which generates superoxide (O2-.) and hydrogen peroxide (H2O2), caused a remarkable but reversible inhibition of IRP activity, as the formation of IRE-IRP decreased by 70-80% but returned to baseline values upon exposure to a reducing agent like 2-mercaptoethanol. IRP inhibition was prevented by separate or simultaneous addition of superoxide dismutase and catalase, showing that both O2-. and H2O2 were involved. By contrast, iron chelators and hydroxyl radical scavengers did not impede the inhibition of IRP, suggesting that O2-. and H2O2 acted independently of free iron sources. Ferritin enhanced IRP inhibition, but this process involved tightly bound iron centers that shunted reducing equivalents from XO and returned them to oxygen, thus increasing the formation of O2-. In agreement with the exclusive role of O2-. and H2O2, XO also inhibited recombinant human IRP in the absence of iron. These results demonstrate that O2-. and H2O2 can directly but reversibly down-regulate the RNA-binding activity of IRP, causing transient decrease of free iron that otherwise would convert them into more potent oxidants such as hydroxyl radicals or equally aggressive iron-peroxo complexes. This establishes a novel protective stratagem against oxidative injury under pathophysiologic conditions characterized by the excessive generation of O2-. and H2O2.

Upregulation of transferrin receptor-1 induces cholangiocarcinoma progression via induction of labile iron pool

“…[23][24][25] Both in vitro and in vivo studies have shown that a number of conditions or agents known to increase cellular levels of ROS induce the reversible inactivation of IRP-1 aimed at decreasing TfR-1 levels while also increasing ferritin levels and hence diminishing the LIP. [26][27][28][29][30] To define that TfR-1 is related with LIP accumulation in CCA, we detected TfR-1 expression and LIP accumulation in CCA cell lines compared with immortal cholangiocyte cell line. Our results showed that TfR-1 expression and LIP levels were significantly increased in CCA cell lines than in the cholangiocyte cell line, suggesting that TfR-1 may be associated with iron uptake and hence induce LIP in CCA.…”

Effects of sham air and cigarette smoke on A549 lung cells: implications for iron-mediated oxidative damage

“…Ferritin serves as a cytoprotective agent against oxidative stress (8), and iron delivered by TfRs or by airborne particles can increase oxidative stress (7,26). Thus repeated exposure of lung tissues to low levels of smoke as are present in environmental pollutants could result in reduced cytoprotection by ferritin at a time when iron uptake is sustained and ROS are present.…”

Regulation of ferritin genes and protein

“…146 Oxidative stress can also contribute to ferritin induction by inactivating IRP1 through reversible oxidation of critical cysteine residues. 147 However, oxidant-mediated inactivation of IRP1 is not always seen. In fact, in other experimental systems, oxidants had the opposite effect: hydrogen peroxide activated the iron responsive protein (IRP1), possibly through induction of a signaling pathway that mobilizes iron from the 4Fe-4S cubane cluster.…”