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

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 F 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 F radiation. Prostaglandin E 2 (PGE 2 ) was applied to medium after treatment with celecoxib F 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 PGE 2 after treatment with celecoxib F 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. (Cancer Res 2005; 65(20): 9501-9)

Cyclooxygenase-2 (COX-2) Negatively Regulates Expression of Epidermal Growth Factor Receptor and Causes Resistance to Gefitinib in COX-2–Overexpressing Cancer Cells

“…Both HCT-116 colon adenocarcinoma cells and NCI-H460 lung large cell carcinoma cells express low levels of endogenous COX-2 (21). Interestingly, we found that ectopically overexpressed COX-2 down-regulated EGFR expression in both of these cancer cell lines, whereas knockdown of endogenous COX-2 in MOR-P cells resulted in increased EGFR expression (Fig.…”

“…Therefore, we also developed a NCI-H460 cell line constitutively overexpressing COX-2 (H460-COX-2; refs. 21,22) and did the same comparison. EGFR expression in H460-COX-2 cells was also found to be down-regulated compared with mock cells much as it was in the HCT-116-COX-2 cells (Fig.…”

Cyclooxygenase-2 Up-Regulates Ataxia Telangiectasia and Rad3 Related through Extracellular Signal-Regulated Kinase Activation

“…Therefore, our observations that COX-2 enhances the activity of ATR and its substrates, including Chk1 and p53, may be important to understand the various roles of COX-2 in cells. In addition, it may serve as an underlying mechanism for our previous observations that COX-2-overexpressing cells showed prolonged IR-induced G 2 arrest (29). Prolongation of G 2 arrest after IR has been reported in AT cells and is known as "G 2 accumulation" (37).…”

“…To investigate whether PGE 2 , a major end product of COX-2, is associated with ERK activation, cells were treated with 500 nmol/L PGE 2 . A concentration of 500 nmol/L PGE 2 is equivalent to the maximum inducible concentration in A549 cells, which have higher expression levels of COX-2 compared with those of HCT116 cells, and this amount, although sufficient to exert a considerable effect, does not exceed its physiologic range (29).…”

“…In our previous study, we found that COX-2 overexpression caused prolonged G 2 arrest after exposure to IR and suggested that COX-2 may contribute to the regulation of the G 2 checkpoint (29). Therefore, we have been interested in discovering and characterizing target molecules of COX-2 in the G 2 checkpoint pathway.…”