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Research Articles: Therapeutics

Vorinostat, a histone deacetylase inhibitor, enhances the response of human tumor cells to ionizing radiation through prolongation of -H2AX foci

¹ Departments of Experimental Radiation Oncology and ² Biomathematics and Applied Statistics, The University of Texas M.D. Anderson Cancer Center, Houston, Texas and ³ Merck & Co., Inc., Boston, Massachusetts

Requests for reprints: Anupama Munshi, Department of Experimental Radiation Oncology, The University of Texas M.D. Anderson Cancer Center, Box 066, 1515 Holcombe Boulevard, Houston, TX 77030. Phone: 713-792-4864; Fax: 713-794-5369. E-mail: amunshi{at}mdanderson.org

Vorinostat (suberoylanilide hydroxamic acid) is the prototype of a family of hybrid polar compounds that can induce growth arrest in transformed cells and shows promise for the treatment of cancer. Vorinostat specifically binds to and inhibits the activity of histone deacetylases resulting in acetylation of nucleosomal histones and an activation of gene transcription. Because histone deacetylases modulate chromatin structure and gene expression, both of which can influence radioresponse, this study was designed to examine the capacity of Vorinostat to influence radiation response in human tumor cells and investigate the mechanism underlying these interactions. Vorinostat induced hyperacetylation of histone H4 in a dose-dependent manner. We tested its ability to radiosensitize three human tumor cell lines (A375, MeWo, and A549) using clonogenic cell survival assays. Clonogenic cell survival assay showed that Vorinostat significantly radiosensitized all three tumor cell lines, substantially reducing the surviving fraction at 2 Gy. We examined potential mechanisms that may contribute to the enhanced radiation response induced by Vorinostat. Vorinostat and radiation alone did not induce apoptosis in the melanoma cell line. However, enhanced apoptosis was observed when cells were exposed to both Vorinostat and radiation, suggesting that Vorinostat renders tumor cells more susceptible to radiation-induced apoptosis. Results from DNA damage repair analysis in cultured A375 cells showed that Vorinostat had a strong inhibitory effect on the nonhomologous end joining pathway after radiation. A detailed examination of the involvement of the DNA repair pathway following Vorinostat treatment showed that Vorinostat reduced the expression of the repair-related genes Ku70, Ku80, and Rad50 in A375 cells as detected by Western blot analysis. We also examined -H2AX phosphorylation as a predictive marker of radiotherapy response to Vorinostat and observed that the combination of Vorinostat and radiation caused a prolongation of expression of DNA repair proteins such as -H2AX. Overall, we conclude that Vorinostat enhances tumor radioresponse by multiple mechanisms that may involve antiproliferative growth inhibition and effects on DNA repair after exposure to radiation. [Mol Cancer Ther 2006;5(8):1967–74]

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.

⁴ StataCorp. Stata statistical software: release 8.0. College Station (TX): Stata Corporation; 2004.

Received 1/13/06; revised 6/ 7/06; accepted 6/ 7/06.

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