Sustained Hydrogen Peroxide Induces Iron Uptake by Transferrin Receptor-1 Independent of the Iron Regulatory Protein/Iron-responsive Element Network

Abstract

Local and systemic inflammatory conditions are characterized by the intracellular deposition of excess iron, which may promote tissue damage via Fenton chemistry. Because the Fenton reactant H 2 O 2 is continuously released by inflammatory cells, a tight regulation of iron homeostasis is required. Here, we show that exposure of cultured cells to sustained low levels of H 2 O 2 that mimic its release by inflammatory cells leads to upregulation of transferrin receptor 1 (TfR1), the major iron uptake protein. The increase in TfR1 results in increased transferrin-mediated iron uptake and cellular accumulation of the metal. Although iron regulatory protein 1 is transiently activated by H 2 O 2 , this response is not sufficient to stabilize TfR1 mRNA and to repress the synthesis of the iron storage protein ferritin. The induction of TfR1 is also independent of transcriptional activation via hypoxia-inducible factor 1␣ or significant protein stabilization. In contrast, pulse experiments with 35 Slabeled methionine/cysteine revealed an increased rate of TfR1 synthesis in cells exposed to sustained low H 2 O 2 levels. Our results suggest a novel mechanism of iron accumulation by sustained H 2 O 2 , based on the translational activation of TfR1, which could provide an important (patho)physiological link between iron metabolism and inflammation.Systemic iron homeostasis undergoes typical changes during inflammatory or infectious conditions. A decrease in plasma iron concentration limits the availability of the metal for erythropoiesis, ultimately leading to the so-called anemia of chronic disease (1). In addition to iron retention within the reticuloendothelial system, parenchymal cells such as hepatocytes also accumulate iron under inflammatory conditions (2-8), and this iron deposition has been identified as an important factor in tissue damage by free radicals (9). In addition, hepatic iron accumulation appears to be an important cofactor in the development of fibrosis and end stage liver disease in such common chronic liver pathologies such as hepatitis C or alcoholic steatohepatitis (4 -8).Significant progress has been made toward understanding the molecular basis of iron retention within the reticuloendothelial system during inflammation (10 -12). The mechanism involves the interleukin-6-mediated induction of the iron-regulatory peptide hepcidin (13, 14), which inhibits iron efflux from macrophages and intestinal enterocytes (15, 16) by binding to and promoting the degradation of the transporter ferroportin 1 (IREG1 or MTP1) (17). The ensuing hypoferremia is thought to be part of a physiological defense strategy to deplete invading bacteria from the growth-essential iron. Thus far, the possibility that inflammation-mediated accumulation of iron in parenchymal cells may also contribute to hypoferremia has not received much attention. Nevertheless, the expression of transferrin receptor 1 (TfR1), 2 the major iron uptake protein, is induced in several models of inflammation (2, 18).Upon activation, inflammatory cells su...

Redox control of iron regulatory protein 2 stability

“…2B). These responses are consistent with stabilization of TfR1 mRNA and translational inhibition of ferritin mRNA by activated IRP2 [1][2][3]; the increase of TfR1 expression is also congruous with the reported IRP-independent stimulation of TfR1 mRNA translation that is elicited during mild sustained oxidative stress [28]. As the catabolism of H 2 O 2 by catalases is associated with generation of oxygen [22], the degradation of a 1 mM H 2 O 2 bolus may lead to hyperoxia, which would be expected to upregulate FBXL5 and antagonize the stabilization of IRP2.…”

Evaluation of standard liver volume formulae for Chinese adults

“…Preliminary data from ALD patients and ethanol-treated mice models sug gest that hepatic iron uptake pathways are increased in the liver and potential mechanisms involve an increase of the transferrin receptor (TfR)1 and repression of the systemic iron hormone hepcidin that controls duodenal iron absorption and RES-mediated iron release via the iron exporter ferroportin [119,120] . Using novel in vitro and in vivo models [121,122] , we have recently demonstrated that H2O2 alone increases TfR1 via posttranscriptional and translation mechanisms ultimately leading to cellular accumulation of iron [123,124] . These data show that chronic exposure of cells to non toxic levels of H2O2 lead to accumulation of iron via distinct regulatory mechanisms promoting Fenton chemistry.…”

Sustained Submicromolar H2O2 Levels Induce Hepcidin via Signal Transducer and Activator of Transcription 3 (STAT3)

“…and catalase (CAT) were purchased from Sigma-Aldrich. Actual activities of GOX and CAT were determined at very low H 2 O 2 concentrations prior to the experiment using a sensitive chemiluminescence technique (19,22,25,26). During all experiments, k GOX was kept at 4 ϫ 10 Ϫ8 M/s, whereas k CAT was adjusted to reach defined H 2 O 2 ss concentrations between 0.15 and 6 M. Because GOX metabolizes glucose and oxygen stochiometrically to H 2 O 2 and ␦-gluconolactone, k GOX was selected to not decrease glucose levels during 24 h by more than 3 mM, which does not influence glucose metabolism.…”

“…Previous studies showed direct regulatory control of iron homeostasis by H 2 O 2 ; the iron regulatory protein 1 (IRP1), which usually stabilizes the mRNA of TfR1 by binding to iron-responsive elements in the 3Ј-untranslated region (16), shows increased binding activity under persistently elevated H 2 O 2 concentrations (17,18). In addition, TfR1 is up-regulated independent of IRP-1 at the translational level under sustained levels of H 2 O 2 (19). Both pathways finally lead to an iron shift into cells and removal of circulating free iron.…”

“…However, a detailed mechanistic participation of individual ROS on hepcidin down-regulation was not demonstrated, and some reported observations were performed under artificially high ROS conditions. Miura et al (21) (19,(22)(23)(24)(25). This system allows mimicking the continuous H 2 O 2 flux from inflammatory cells.…”