Modulation of Ionizing Radiation-Induced G₂ Arrest by Cyclooxygenase-2 and its Inhibitor Celecoxib

Purpose: Prolongation or attenuation of ionizing radiation (IR)-induced G₂-M arrest in cyclooxygenase-2 (COX-2) overexpressing or celecoxib-treated cells, respectively, has been previously observed. To better understand the molecular mechanisms involved, we investigated the molecules involved in G₂ checkpoint pathways after treatment with IR ± celecoxib. Methods and Materials: Various molecules in the G₂ checkpoint pathways were investigated in HCT-116-Mock and -COX-2 cells. Western blot, reverse transcriptase polymerase chain reaction, confocal microscopy, and fluorescence activated cell sorter (FACS) analyses were performed to investigate whether expression and activity of the ataxia telangiectasia and rad3-related (ATR) could be modulated by COX-2 and its selective inhibitors. Results: COX-2 overexpression increased expression and activity of ATR after IR exposure. Celecoxib downregulated ATR in all tested cell lines independent of COX-2 expression, but downregulation was greater in COX-2 overexpressing cells after cells were irradiated. Celecoxib pretreatment before radiation caused strongly inhibited G₂ arrest. Conclusions: COX-2 appears to prolong IR-induced G₂ arrest by upregulating ATR. Celecoxib downregulated ATR preferentially in irradiated COX-2 overexpressing cells. Celecoxib may radiosensitize cancer cells by inhibiting G₂ arrest through ATR downregulation.