IRP1 Activation by Extracellular Oxidative Stress in the Perfused Rat Liver

Abstract

The expression of several proteins with critical functions in iron metabolism is regulated post-transcriptionally by the binding of iron regulatory proteins, IRP1 and IRP2, to mRNA iron responsive elements (IREs). In iron-deficient tissues and cultured cells, both IRP1 and IRP2 are activated for high affinity IRE binding. Previous work showed that IRP1 is also activated when cultured cells are exposed to H2O2. The well established role of iron and H2O2 in tissue injury (based on Fenton chemistry) suggests that this response may have important pathophysiological implications. This is particularly relevant in inflammation, where cytotoxic immune cells release large amounts of reactive oxygen species. Here, we describe a rat liver perfusion model to study IRP1 activation under H2O2 generation conditions that mimic a physiological inflammatory response, using steady-state concentrations of H2O 2 produced by a glucose/glucose oxidase/catalase system. We show first that stimulated neutrophils are able to increase serum levels of H 2O2 by a factor of 10, even in the presence of H 2O2-degrading erythrocytes. We further show that perfusion of rat liver with glucose oxidase leads to a rapid activation of IRE binding activity in the intact organ. Mobility shift assays with liver extracts and IRP1 or IRP2-specific probes indicate that only IRP1 responds to H 2O2. Our study demonstrates a principal existence of iron regulation by oxidative stress at the intact organ level. It also provides a link between iron metabolism and the inflammatory response, as H 2O2 is a major product of the oxidative burst of neutrophils and macrophages.