Cytoplasmically “Sequestered” Wild Type p53 Protein Is Resistant to Mdm2-mediated Degradation*
Abstract
The Mdm2 oncoprotein mediates p53 degradation at cytoplasmic proteasomes and is the principal regulator for maintaining low, often undetectable levels of p53 in unstressed cells. However, a subset of human tumors including neuroblastoma constitutively harbor high levels of wild type p53 protein localized to the cytoplasm. Here we show that the abnormal p53 accumulation in such cells is due to a profound resistance to Mdm2-mediated degradation. Overexpression of Mdm2 in neuroblastoma (NB)1cell lines failed to decrease the high steady state levels of endogenous p53. Moreover, exogenous p53, when introduced into these cells, was also resistant to Mdm2-directed degradation. This resistance is not due to a lack of Mdm2 expression in NB cells or a lack of p53-Mdm2 interaction, nor is it due to a deficiency in the ubiquitination state of p53 or proteasome dysfunction. Instead, Mdm2-resistant p53 from NB cells is associated with covalent modification of p53 and masking of the modification-sensitive PAb 421 epitope. This system provides evidence for an important level of regulation of Mdm2-directed p53 destruction in vivo that is linked to p53 modification.
Footnotes
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↵* This work was supported by National Institutes of Health Grant R01 CA60664.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‡ To whom correspondence should be addressed: Dept. of Pathology, State University of New York at Stony Brook, Health Science Center, Stony Brook, NY 11794. Tel.: 516-444-2459; Fax: 516-444-3424; E-mail: umoll@path.som.sunysb.edu.
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↵2 A. Zaika and U. M. Moll, unpublished results.
- Abbreviations:
- NB
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neuroblastoma
- GFP
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green fluorescent protein
- CBP
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cAMP response element-binding protein
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- Received April 21, 1999.
- Revision received July 14, 1999.
- The American Society for Biochemistry and Molecular Biology, Inc.
