Sustained oxidative stress (H2O2) increases hepatic iron accumulation via translational stimulation of transferrin receptor 1 synthesis independent of the IRE/IRP network

Aims: Various inflammatory conditions of the liver are associated with intracellular iron accumulation, e.g. in chronic hepatitis C and alcoholic steatohepatitis which are independent risk factors for disease progression and fibrosis. Previous work showed that the cellular iron master switch iron regulatory protein 1 (IRP1) is rapidly activated when cultured hepatocytes or the whole liver are exposed to H₂O₂. However, it is poorly understood how hepatic iron metabolism responds to sustained exposure of H₂O₂ which more likely represents pathophysiological oxidative stress conditions.

METHODS AND Results: We employed a novel enzymatic system of glucose oxidase (GOX) from Aspergillus niger and catalase for a sustained release of H₂O₂ at steady state levels that mimic various cellular H₂O₂ sources. We demonstrate that the prolonged exposure (24 to 48 hours) of cultured hepatoma cells (HepG2, HCT116, Huh7) to non-toxic H₂O₂ concentrations of less than 5 micromol/liter elicits a delayed induction of transferrin receptor 1 (TfR1) protein expression in vitro – the major iron uptake protein of the liver. TfR1 induction is associated with increased uptake of 59Fe-labeled transferrin and the intracellular accumulation of iron as assessed by the calcein technique. Interestingly, IRP1 is only transiently activated and TfR1 mRNA was not increased suggesting an IRP1-independent pathway of TfR1 indcution. Pulse chase experiments with [S35] labeled methionine and immunoprecipitated TfR1 indicate that chronic H₂O₂ exposure directly increases TfR1 synthesis at the translational level while not effecting its stability. We finally show that exposure of rat liver to non-toxic H₂O₂ by GOX-loaded Kupffer cells for 24h confirms TfR1 upreguation in an in vivo setting without concomitant TfR1 mRNA stabilization, IRP1 induction and IL6/hepcidin involvement.

Conclusions: Our results suggest a novel mechanism of hepatic iron accumulation by sustained non-toxic H₂O₂ that is based on the translational activation of major iron uptake protein TfR1. We suggest that this mechanism contributes to pathological iron accumulation in chronic liver disease such as HCV and ASH.

Literatur: Mueller S, Afdhal NH, Schuppan D. Iron, HCV, and liver cancer: hard metal setting the pace? Gastroenterology. 2006 Jun;130(7):2229-34.
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Sureda A, Hebling U, Pons A, Mueller S. Extracellular H2O2 and not superoxide determines the compartment-specific activation of transferrin receptor by iron regulatory protein 1. Free Radic Res. 2005 Aug;39(8):817-24 Mutze S, Hebling U, Stremmel W, Wang J, Arnhold J, Pantopoulos K, Mueller S. Myeloperoxidase-derived hypochlorous acid antagonizes the oxidative stress-mediated activation of iron regulatory protein 1. J Biol Chem. 2003 Oct 17;278(42):40542-9.

HCV - alcoholic hepatitis - hydrogen peroxide - iron - oxidative stress - transferrin receptor