Submitted on January 24, 2008
Accepted on March 2, 2008
CUGBP2 Downregulation By Prostaglandin E2 Protects Colon Cancer Cells from Radiation-Induced Mitotic Catastrophe
Gopalan Natarajan1, Satish Ramalingam2, Ilangovan Ramachandran3, Randal May4, Lurdes Queimado5, Courtney W Houchen4, and Shrikant Anant6*
1 Medicine/Digestive Diseases and Nutrition, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States; Medicine, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States
2 Medicine, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States; Medicine/Digestive Diseases and Nutrition, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States
3 Otorhinolaryngology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States
4 Medicine/Digestive Diseases and Nutrition, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States
5 Otorhinolaryngology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States; Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States
6 Medicine/Digestive Diseases and Nutrition, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States; Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States
* To whom correspondence should be addressed. E-mail: shrikant-anant{at}ouhsc.edu.
Prostaglandin E2 (PGE2) is a potent inhibitor of ionizing radiation (IR) induced cell death. Exposure of colon cancer cells to IR leads to increased CUGBP2 expression. Therefore, we tested the hypothesis that PGE2 radioprotects colon cancer cells by inhibiting CUGBP2 expression. Exposure of HCT116 cells to 
-IR (0-12 Gy) resulted in a dose-dependent reduction in cell growth and an increase in G2-M phase of cell cycle. Western blot analyses demonstrated increased levels of activated caspase 9 and caspase 3. In addition, while Bax expression is increased, that of Bcl-2 and Bcl-XL was reduced. Further analyses demonstrated increased activation of Chk1 and Chk2 kinases, coupled with higher levels of nuclear cyclin B1 and Cdc2. Pretreatment with PGE2 suppressed the activation of caspase 3 and capase 7 and inhibited Bax expression. In addition, PGE2 treatment restored growth and colony formation to control levels. IR significantly upregulated the expression of CUGBP2 in the cells, which was suppressed when cells were pretreated with PGE2. Ectopic overexpression of CUGBP2 also induced apoptosis. Furthermore, it reversed the PGE2-mediated protection from IR-induced mitotic catastrophe. Furthermore, there was an increase in nuclear localization of cyclin B1 and Cdc2 coupled with increased phosphorylation of p53, Chk1, Chk2 and Cdc25c proteins. Cell cycle analysis also demonstrated increased G2-M transition. In contrast, siRNA mediated suppression of CUGBP2 expression restored normal cell cycle progression and decreased IR-induced apoptosis. Taken together, these data demonstrate that PGE2 protects colon cancer cells from IR induced mitotic catastrophe in part through suppression of CUGBP2 expression.
