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Molecular and Cellular Biology, February 2004, p. 954-965, Vol. 24, No. 3
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.3.954-965.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Iron-Mediated Degradation of IRP2, an Unexpected Pathway Involving a 2-Oxoglutarate-Dependent Oxygenase Activity

Lady Davis Institute for Medical Research, Sir Mortimer B. Davis Jewish General Hospital,¹ Department of Medicine, McGill University, Montreal, Quebec, Canada,³ European Molecular Biology Laboratory, D-69117 Heidelberg, Germany²

Received 18 June 2003/ Returned for modification 3 September 2003/ Accepted 28 October 2003

Iron regulatory protein 2 (IRP2), a central posttranscriptional regulator of cellular and systemic iron metabolism, undergoes proteasomal degradation in iron-replete cells. The prevailing model postulates that the mechanism involves site-specific oxidation of 3 cysteine residues (C168, C174, and C178) within a 73-amino-acid (73-aa) degradation domain. By expressing wild-type and mutated versions of IRP2 in H1299 cells, we find that a C168S C174S C178S triple mutant, or a deletion mutant lacking the entire "73-aa domain," is sensitive to iron-mediated degradation, like wild-type IRP2. The antioxidants N-acetylcysteine, ascorbate, and -tocopherol not only fail to stabilize IRP2 but, furthermore, promote its proteasomal degradation. The pathway for IRP2 degradation is saturable, which may explain earlier data supporting the "cysteine oxidation model," and shows remarkable similarities with the degradation of the hypoxia-inducible factor 1 (HIF-1): dimethyl-oxalylglycine, a specific inhibitor of 2-oxoglutarate-dependent oxygenases, stabilizes IRP2 following the administration of iron to iron-deficient cells. Our results challenge the current model for IRP2 regulation and provide direct pharmacological evidence for the involvement of 2-oxoglutarate-dependent oxygenases in a pathway for IRP2 degradation.

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