Abstract 1467: Inhibition of Cdk4 radiosensitizes breast cancer cells by increasing PP2A levels, and decreasing Bad136 phosphorylation.
Authors: Katie R. Hagen, Xiangbin Zeng, Shannon Tucker-Kahn, Mary Kathryn Harrison, Sandra S. Zaky, Tongrui Liu, Yuan Liu, Ruth M. O'Regan, Xingming Deng, Harold I. Saavedra
Journal: Cancer Research (2012)

Abstract Treatment of breast cancers by targeting molecular markers associated with various subtypes has improved the outcome of breast cancer patients. Unfortunately, triple negative breast cancers lack these markers; missing estrogen and progesterone receptors and HER2. These breast cancers are invulnerable to the most effective treatments. Albeit triple-negative breast cancers and Her2+ER-PR- breast cancers respond well to initial therapies, including radiation, those cancers are prone to recurrence and patients show decreased survival rates. Increasing evidence suggests that this resistance is in part related to deregulation of G1 phase regulatory mechanisms. G1/S phase regulatory molecules, including cyclins, Cdks and the E2Fs may contribute to lower survival rates, higher recurrence, and resistance to adjuvant therapies. Radiotherapy is one of the foundations of breast cancer treatments. However breast tumors can acquire radioresistance. Consequently, re-sensitizing malignant cells to radiation would increase the effectiveness of radiotherapy. To address whether deregulation contributes to radiotherapy resistance, a normal human mammary epithelial cell line, an ER-PR-Her2+ and an ER-PR-Her2- breast cancer cell line were analyzed in terms of molecular markers and radioresistance. Cyclin D1 and Cyclin A1 correlated with radioresistance and were overexpressed in all cell lines. Each influence Cdk2 and Cdk4 activities, respectively. We directly addressed the role played by those Cdks in radioresistance by generating cell lines stably expressing small hairpin RNAs (shRNA) targeting Cdk2 or Cdk4. Colony forming assays established that cell lines knocked down for Cdk2 did not display radiosensitization, however cell lines knocked down for Cdk4 were radiosensitized. Data demonstrated that silencing Cdk4 significantly increases radiation induced cell apoptosis in all ER-PR-Her2- cell lines without considerably altering cell proliferation, cell cycle progression or DNA repair. Cdk4 silencing in conjunction with ionizing radiation caused lower levels of phosphorylation of BCL-2 family member, BAD, a pro-apoptotic member of the Bcl-2 family that promotes cell death when hypophosphorylated at at Ser-136. Elevated levels of PP2A, a phosphatase that dephosphorylates Bad at ser136 were also found. This is further supported by the data showing over expression of PP2A in ER-PR-Her2- cell lines increase apoptosis and encouraged radiosensitivity in colony formations experiments. We conclude that inhibition of Cdk4 activity sensitizes breast cancer cells to radiation by activating PP2A/Bad-dependent cell death pathways. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1467. doi:1538-7445.AM2012-1467