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Research Articles: Therapeutics, Targets, and Development

Rad51 overexpression contributes to chemoresistance in human soft tissue sarcoma cells: a role for p53/activator protein 2 transcriptional regulation

Departments of ¹ Surgical Oncology, ² Cancer Biology, ³ Experimental Therapeutics, and ⁴ Pathology, The University of Texas M. D. Anderson Cancer Center, Houston, Texas

Requests for reprints: Dina Lev, Department of Cancer Biology, The University of Texas M. D. Anderson Cancer Center, Unit 173, 1515 Holcombe Boulevard, Houston, TX 77030. Phone: 713-792-1637; Fax: 713-792-0722. E-mail: dlev{at}mdanderson.org

Abstract

We investigated whether Rad51 overexpression plays a role in soft tissue sarcoma (STS) chemoresistance as well as the regulatory mechanisms underlying its expression. The studies reported here show that Rad51 protein is overexpressed in a large panel of human STS specimens. Human STS cell lines showed increased Rad51 protein expression, as was also observed in nude rat STS xenografts. STS cells treated with doxorubicin exhibited up-regulation of Rad51 protein while arrested in the S-G₂ phase of the cell cycle. Treatment with anti-Rad51 small interfering RNA decreased Rad51 protein expression and increased chemosensitivity to doxorubicin. Because we previously showed that reintroduction of wild-type p53 (wtp53) into STS cells harboring a p53 mutation led to increased doxorubicin chemosensitivity, we hypothesized that p53 participates in regulating Rad51 expression in STS. Reintroduction of wtp53 into STS cell lines resulted in decreased Rad51 protein and mRNA expression. Using luciferase reporter assays, we showed that reconstitution of wtp53 function decreased Rad51 promoter activity. Deletion constructs identified a specific Rad51 promoter region containing a p53-responsive element but no p53 consensus binding site. Electrophoretic mobility shift assays verified activator protein 2 (AP2) binding to this region and increased AP2 binding to the promoter in the presence of wtp53. Mutating this AP2 binding site eliminated the wtp53 repressive effect. Furthermore, AP2 knockdown resulted in increased Rad51 expression. In light of the importance of Rad51 in modulating STS chemoresistance, these findings point to a potential novel strategy for molecular-based treatments that may be of relevance to patients burdened by STS. [Mol Cancer Ther 2007;6(5):1650–60]

Grant support: National Cancer Institute/M. D. Anderson Cancer Center Core grant P30-CA16672 and NIH grants RO1 CA67802 (R.E. Pollock) and CA60488 (D. Yu).

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 10/18/06; revised 2/23/07; accepted 3/29/07.

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