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...

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In iron-replete cells the posttranscriptional regulator IRP2 undergoes ubiquitination and proteasomal degradation. A similar response occurs in cells exposed to sodium nitroprusside (SNP), an NO-releasing drug. It has been proposed that nitroprusside ([Fe(CN) 5 NO]2؊ ) fails to donate iron into cells and that it promotes IRP2 degradation via S nitrosylation at C178. This residue is located within a stretch of 73 amino acids, earlier proposed to define an iron-dependent degradation domain. Surprisingly, we show that IRP2 bearing a C178S mutation or a ⌬73 deletion is sensitive to degradation not only by ferric ammonium citrate (FAC) but also by SNP. Moreover, FAC and SNP attenuate the RNA-binding activities of IRP2 and its homologue IRP1 with similar kinetics. Actinomycin D, cycloheximide, succinylacetone, and dimethyl-oxalylglycine antagonize IRP2 degradation in response to both FAC and SNP, suggesting a common mechanistic basis. IRP2 is not only sensitive to fresh, but also to photodegraded SNP and remains unaffected by S-nitrosoglutathione (GSNO), an established nitrosation agent. Importantly, both fresh and photodegraded SNP, but not GSNO, promote a >4-fold increase in the calcein-accessible labile iron pool. Collectively, these results suggest that IRP2 degradation by SNP does not require S nitrosylation but rather represents a response to iron loading.Iron regulatory proteins, IRP1 and IRP2, are posttranscriptional regulators of iron metabolism (13, 32). They coordinately control the expression of mRNAs containing iron-responsive elements (IREs), such as those encoding transferrin receptor 1 (TfR1) and ferritin. In iron-deficient cells, IRPs are activated for high-affinity IRE-binding, and IRE-IRP interactions stabilize TfR1 mRNA and inhibit ferritin mRNA translation. These homeostatic responses stimulate acquisition of extracellular iron via TfR1 and prevent its storage in ferritin.IRPs control the expression of additional IRE-containing mRNAs encoding proteins with crucial functions in body iron homeostasis, such as the erythroid-specific enzyme of the heme biosynthetic pathway aminolevulinate synthase 2 and the iron transporters ferroportin 1 and DMT1 (13, 32). IRP1/IRP2 double-knockout mice exhibit early embryonic lethality (38). The targeted inactivation of IRP1 has yielded only minor phenotypic abnormalities in the kidney and in brown fat (29), whereas the disruption of IRP2 has been associated with a progressive adult onset neurodegenerative disorder (28) and/or microcytosis (5, 10).Experiments with IRP1 Ϫ/Ϫ and IRP2 Ϫ/Ϫ cells showed that IRP1 and IRP2 have similar iron-sensing capacities under typical tissue culture conditions with 21% oxygen; however, when oxygen concentration was reduced to 3 to 6%, which is believed to mimic physiological conditions in tissues, only IRP2 responded to alterations in iron levels (30). It appears that under these conditions (30) and in animal tissues (29), IRP1 predominates in the cytosolic aconitase form and does not very efficiently respond to iron deficiency, whic...

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