Radiosensitivity Enhancement by Celecoxib, a Cyclooxygenase (COX)-2 Selective Inhibitor, via COX-2–Dependent Cell Cycle Regulation on Human Cancer Cells Expressing Differential COX-2 Levels

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Abstract
To characterize the radiation-enhancing effects on human cancer cells and underlying mechanisms of celecoxib, a cyclooxygenase (COX)-2 selective inhibitor, and to ascertain whether its effects are COX-2 dependent. Clonogenic cytotoxicity assays and radiation survival assays after treatment with celecoxib ± radiation were done on four human cancer cell lines that expressed differential COX-2 levels. Stably COX-2 knocked down or overexpressed cell lines were developed, and clonogenic assays, apoptosis assays, or cell cycle change measurements were conducted after treatment with celecoxib ± radiation. Prostaglandin E2 (PGE2) was applied to medium after treatment with celecoxib ± radiation to determine whether the radiation-enhancing effect associated with celecoxib results from reduced generation of prostaglandin. Celecoxib's radiation-enhancing effect was observed in COX-2–expressing A549 and NCI-H460 cells but was not observed in the COX-2 nonexpressing MCF-7 and HCT-116 cells. Celecoxib's radiation-enhancing effects in A549 cells were shown to disappear after the administration of COX-2 knocked down. In contrast, the HCT-116 cells were radiosensitized by celecoxib after being transfected with COX-2 expression vector. The addition of PGE2 after treatment with celecoxib ± radiation had no significant effects on celecoxib's radiation-enhancing effects in A549 and COX-2 transfected HCT-116 cells. Radiation-induced G2-M arrest was enhanced and sustained in the COX-2–overexpressing cells compared with that seen in COX-2 low-expressing cells. Celecoxib or NS-398 effected no changes or attenuated radiation-induced G2-M arrest in the COX-2–overexpressing cells but further enhanced the radiation-induced G2-M arrest in the COX-2 low-expressing cells. Celecoxib's radiation-enhancing effects seem to occur in a COX-2 expression-dependent manner in the cancer cells. This effect does not seem to be the result of reduced PGE2 generation. Celecoxib may exert an inhibitory effect on enhanced radiation-induced G2-M arrest in the COX-2–overexpressing cells, which may allow the arrested cells to enter mitosis and die after radiation, but may also further enhance radiation-induced G2-M arrest in the COX-2 low-expressing cells, by virtue of another mechanism.