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Molecular and Cellular Biology, November 2000, p. 8018-8025, Vol. 20, No. 21
Divisions of Medical
Oncology1 and
Genetics,4 Department of Medicine, and
Division of Hematology/Oncology, Department of
Pediatrics,2 Stanford University School of
Medicine, Stanford, California 94305, and Ludwig Institute for
Cancer Research, Imperial College of Medicine at St. Mary's, London W2
1PG, United Kingdom3
Received 13 January 2000/Returned for modification 17 March
2000/Accepted 3 August 2000
p53 is an important mediator of the cellular stress response with
roles in cell cycle control, DNA repair, and apoptosis. 53BP2, a
p53-interacting protein, enhances p53 transactivation, impedes cell
cycle progression, and promotes apoptosis through unknown mechanisms.
We now demonstrate that endogenous 53BP2 levels increase following UV
irradiation induced DNA damage in a p53-independent manner. In
contrast, we found that the presence of a wild-type (but not mutant)
p53 gene suppressed 53BP2 steady-state levels in cell lines with
defined p53 genotypes. Likewise, expression of a tetracycline-regulated
wild-type p53 cDNA in p53-null fibroblasts caused a reduction in 53BP2
protein levels. However, 53BP2 levels were not reduced if the
tetracycline-regulated p53 cDNA was expressed after UV damage in these
cells. This suggests that UV damage activates cellular factors that can
relieve the p53-mediated suppression of 53BP2 protein. To address the
physiologic significance of 53BP2 induction, we utilized stable cell
lines with a ponasterone A-regulated 53BP2 cDNA. Conditional expression
of 53BP2 cDNA lowered the apoptotic threshold and decreased clonogenic
survival following UV irradiation. Conversely, attenuation of
endogenous 53BP2 induction with an antisense oligonucleotide resulted
in enhanced clonogenic survival following UV irradiation. These results
demonstrate that 53BP2 is a DNA damage-inducible protein that promotes
DNA damage-induced apoptosis. Furthermore, 53BP2 expression is highly
regulated and involves both p53-dependent and p53-independent
mechanisms. Our data provide new insight into 53BP2 function and open
new avenues for investigation into the cellular response to genotoxic stress.
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Proapoptotic p53-Interacting Protein 53BP2 Is
Induced by UV Irradiation but Suppressed by p53

*
Corresponding author. Mailing address: Department of
Medicine, Division of Medical Oncology, 269 Campus Dr., CCSR, Stanford University School of Medicine, Stanford, CA 94305. Phone: (650) 498-5221. Fax: (650) 736-0195. E-mail:
cdlopez{at}leland.stanford.edu.
Present address: School of Natural and Applied Sciences, University
of Houston-Clear Lake, Houston, TX 77058.
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