Combined treatment with peroxisome proliferator-activated receptor (PPAR) gamma ligands and gamma radiation induces apoptosis by PPARγ-independent up-regulation of reactive oxygen species-induced deoxyribonucleic acid damage signals in non-small cell lung cancer cells

Purpose: To investigate possible radiosensitizing activities of the well-known Peroxisome Proliferator-activated Receptor (PPAR)γ ligand ciglitazone and novel PPARγ ligands CAY10415 and CAY10506 in non-small cell lung Cancer (NSCLC) cells.

Methods and materials: Radiosensitivity was assessed using a clonogenic cell survival assay. To investigate the mechanism underlying PPARγ ligand-induced radiosensitization, the subdiploid cellular DNA fraction was analyzed by flow cytometry. Activation of the Caspase pathway by combined PPARγ ligands and γ-radiation treatment was detected by immunoblot analysis. Reactive Oxygen Species (ROS) were measured using 2,7-dichlorodihydrofluorescein diacetate and flow cytometry.

Results: The 3 PPARγ ligands induced cell death and ROS generation in a PPARγ-independent manner, enhanced γ-radiation-induced Apoptosis and caspase-3-mediated poly (ADP-ribose) polymerase (PARP) cleavage in vitro. The combined PPARγ ligand/γ-radiation treatment triggered Caspase-8 activation, and this initiator Caspase played an important role in the combination-induced Apoptosis. Peroxisome proliferator-activated receptor-γ ligands may enhance the γ-radiation-induced DNA damage response, possibly by increasing γ-H2AX expression. Moreover, the combination treatment significantly increased ROS generation, and the ROS scavenger N-acetylcysteine inhibited the combined treatment-induced ROS generation and apoptotic cell death.

Conclusions: Taken together, these results indicated that the combined treatment of PPARγ ligands and γ-radiation synergistically induced DNA damage and Apoptosis, which was regulated by ROS.