DAB 2 IP Regulates Autophagy in Prostate Cancer in Response to Combined Treatment of Radiation and a DNA-PKcs Inhibitor 1 , 2

Radiation therapy (RT) is an effective strategy for the treatment of localized prostate cancer (PCa) as well as local invasion. However, some locally advanced cancers develop radiation resistance and recur after therapy; therefore, the development of radiation-sensitizing compounds is essential for treatment of these tumors. DOC-2/DAB2 interactive protein (DAB2IP), which is a novel member of the Ras–GTPase activating protein family and a regulator of phosphatidylinositol 3-kinase–Akt activity, is often downregulated in aggressive PCa. Our previous studies have shown that loss of DAB2IP results in radioresistance in PCa cells primarily because of accelerated DNA double-strand break (DSB) repair kinetics, robust G2/M checkpoint control, and evasion of apoptosis. A novel DNA-PKcs inhibitor NU7441 can significantly enhance the effect of radiation in DAB2IP-deficient PCa cells. This enhanced radiation sensitivity after NU7441 treatment is primarily due to delayed DNA DSB repair. More significantly, we found that DAB2IP-deficient PCa cells show dramatic induction of autophagy after treatment with radiation and NU7441. However, restoring DAB2IP expression in PCa cells resulted in decreased autophagy-associated proteins, such as LC3B and Beclin 1, as well as decreased phosphorylation of S6K andmammalian target of rapamycin (mTOR). Furthermore, the presence of DAB2IP in PCa cells can lead to more apoptosis in response to combined treatment of NU7441 and ionizing radiation. Taken together, NU7441 is a potent radiosensitizer in aggressive PCa cells and DAB2IP plays a critical role in enhancing PCa cell death after combined treatment with NU7441 and radiation. Neoplasia (2012) 14, 1203–1212 Abbreviations: PCa, prostate cancer; DAB2IP, DOC-2/DAB2 interactive protein; mTOR, mammalian target of rapamycin; IR, ionizing radiation; RT, radiation therapy; DSBs,double-strand breaks; 6-TG, 6-thioguanine; BRFS, biochemical recurrence-free survival; IHC, immunohistochemistry; AVOs, acidic vesicular organelles Address all correspondence to: Debabrata Saha, PhD, Department of Radiation Oncology, Division of Molecular Radiation Biology, University of Texas Southwestern Medical Center, 2201 Inwood Road, Dallas, TX 75390-9187. E-mail: debabrata.saha@utsouthwestern.edu This work was supported by the funding from Department of Defense Idea Award W81XWH-11-1-0270 (D.S.), Flight Attendant Medical Research Institute Award grant (D.S.), and a Clinical Research Fellowship (V.T.) from the Doris Duke Charitable Foundation. Conflict of interest: None. This article refers to supplementary material, which is designated by Figure W1 and is available online at www.neoplasia.com. Received 8 August 2012; Revised 10 October 2012; Accepted 12 October 2012 Copyright © 2012 Neoplasia Press, Inc. All rights reserved 1522-8002/12/$25.00 DOI 10.1593/neo.121310 www.neoplasia.com Volume 14 Number 12 December 2012 pp. 1203–1212 1203