Abstract 436: Enhanced induction of apoptosis by PLK1 inhibition and radiation is associated with the additive, but not supra-additive, killing effect of the combined treatment.
Authors: Wong Nelson , Misri Ripen , Khan Mohamed 
Published: August 2014

Chemoradiotherapy has been employed to improve the clinical outcomes of cancer patients. The rationale for the combined usage of chemotherapeutics and radiation is the benefits of the additive or synergistic (supra-additive) effect of the two agents, leading to better treatment efficacy. Recent studies have shown that inhibition of Polo-like kinase1 (PLK1) function can sensitize head-and-neck squamous cell carcinoma and medulloblastoma cells to radiation. PLK1 is an important regulator of mitotic functions that is over-expressed in many cancers. It also plays a role in DNA double-strand break repair through phosphorylation of Rad51 recombinase. Therefore, PLK1 presents as an attractive target and we set out to determine if PLK1 inhibition can be used as a general strategy for radiosensitization.

The half-maximal inhibitory concentrations (IC50) of the PLK1 inhibitor BI2536 were first determined for various cancer cell lines as indicators of sensitivity using XTT assay. The IC50 values of six different cancer (breast, lungs, pancreas, and colon) cell lines were &lt; 6 nM while that of primary human fibroblasts was ∼ 25 nM. Results of western blots suggest that the protein levels of PLK1 do not correlate with sensitivity. Clonogenic assays were performed to evaluate if synergistic effect (radiosensitization) exists with combined treatment of BI2536 and radiation. Cells were treated with BI2536 for 24 hr before irradiation with various dosages. Additive killing effect was observed with all the cell lines tested; however, radiosensitization (supra-additive) effect was observed in only some cell lines. To determine if the additive or supra-additive killing is associated with apoptosis, cells treated with BI2536 and radiation were incubated with a caspase 3/7 substrate that emits a fluorescent signal upon cleavage. The induction of the fluorescent signal was followed for 72 hrs and the results suggest that enhanced induction of apoptosis occur in all cell lines exposed to the combined treatment. Therefore, enhanced induction of apoptosis is associated with additive, but not supra-addictive, killing of the combined treatment.PLK1 inhibition causes synergistic radiosensitization in some, but not all, cancer cell lines. However, additive killing of the combined treatment was observed with all the cancer cells tested. Our current effort is to decipher why some cancer cells are radiosensitized synergistically by PLK1 inhibition while some are not. We hope the results will give further insights into the mechanism of radiosensitization and identify novel targets for chemoradiotherpy.

Citation Format: Nelson Wong, Ripen Misri, Mohamed Khan. Enhanced induction of apoptosis by PLK1 inhibition and radiation is associated with the additive, but not supra-additive, killing effect of the combined treatment. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 436. doi:10.1158/1538-7445.AM2013-436