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 E 2 (PGE 2) 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 radio-sensitized by celecoxib after being transfected with COX-2 expression vector. The addition of PGE 2 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 G 2-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 G 2-M arrest in the COX-2-overexpressing cells but further enhanced the radiation-induced G 2-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 PGE 2 generation. Celecoxib may exert an inhibitory effect on enhanced radiation-induced G 2-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 G 2-M arrest in the COX-2 low-expressing cells, by virtue of another mechanism.
All Science Journal Classification (ASJC) codes
- Cancer Research