The selective cyclooxygenase (COX)-2 inhibitor, celecoxib, alone and in combination with radiation was investigated in vitro and in vivo. Murine mammary tumor line (MCa-35) and human lung carcinoma line (A549) have high and low basal levels of COX-2 protein, respectively. Treatment of both tumor cells with celecoxib alone resulted in a dose- and time-dependent reduction of cell number (clonogenic cell death) and tumor cell growth rate in vitro; however, inhibition of tumor cell growth by celecoxib was not correlated with the reduction of COX-2 protein in tumor cells. Although both tumor cell types had similar DNA damage after celecoxib treatment, significant induction of tumor cell apoptosis was only observed in MCa-35. Celecoxib-mediated radiation sensitization also occurred in MCa-35 cells determined by clonogenic assay, in part due to a G2/M arrest at 8 to 24 hours after treatment. The tumor growth inhibitory effects of celecoxib were also studied in vivo. It was found that celecoxib inhibited both tumor growth after intragastric administration of celecoxib (5 daily doses of 50 mg/kg). Combined with a single 30-Gy dose of radiation, celecoxib resulted in additive effects on A549 tumors. Celecoxib-treated A549 tumors had marginal reduction of total and perfused blood vessels compared with untreated controls. Reduction of tumor angiogenic cytokine and growth factor mRNA was associated with decreased perfused vessels. Finally, reduction of vascular endothelial growth factor protein after celecoxib was also observed in both tumor lines by Western blot. Our results indicate that the selective inhibition of COX-2 combined with radiation has potential application in radiotherapy, and celecoxib-mediated antitumor effects may act through different mechanisms including direct inhibition of tumor cell proliferation, alteration of tumor cell cycle, and antiangiogenesis.
Cyclooxygenase (COX) is a critical enzyme involved in mammalian physiology and several disease conditions. 1-3 COX-mediated prostaglandin production has recently been implicated in cancer development and tumor angiogenesis. 2,4-6 Two isoforms, cyclooxygenase (COX-1 and COX-2), are coexpressed in both normal and tumor tissues. COX-1 is constitutively expressed in most tissues producing prostaglandins required for normal physiologic function, whereas COX-2 is expressed at relatively low levels but is induced by a variety of agents, including cytokines, growth factors, radiation, and stress-related stimuli. 7,8 There is also considerable evidence that suggests a causal relationship between COX-2 overexpression and tumor formation in human and animal tumor models. Therefore, selective overexpression of COX-2 in tumors versus normal tissues makes this enzyme a potential target for cancer therapy. COX inhibitors also participate in radiation-mediated antitumor effects. 9 Milas et al. 10 and Furuta et al. 11 have shown that indomethacin enhances the antitumor efficacy of ionizing radiation against prostaglandin-producing, transplanted murine sarcomas. Others recently also reported that other COX inhibitors enhanced the radiosensitivity of human prostate carcinoma cells. 12,13 Because of the overexpression of COX-2 in many human malignancies, the recently developed selective COX-2 inhibitors have been extensively investigated in antineoplastic therapy both alone and in combination with radiation. 2,14 In support of this concept, we recently also reported that celecoxib caused a dramatic enhancement of the in vivo radiation response of esophageal carcinoma cells, and found that celecoxib protected normal soft tissue against damage by radiation. 15
The molecular basis of antitumor effects mediated by COX-2 inhibitors has not been well defined. Antitumor effects of COX-2 inhibitors have been documented, which includes 1) direct reduction of tumor cell proliferation, 2) the inhibition of angiogenesis and reduction of angiogenic growth factor and cytokine production, 3) the regulation of cytokine and growth factor production by reducing prostaglandin production and indirectly affecting tumor cell growth, and 4) increased intrinsic radiosensitivity of tumor cells by alteration of the cell cycle. Celecoxib-mediated antitumor effects could be COX-2 dependent or COX-2 independent. 16,17 To investigate the effects of celecoxib on the radiosensitivity of other types of human tumors, we used two carcinoma cell lines with different endogenous COX-2 expression levels in this study. We now report on the in vitro and in vivo effects of celecoxib alone and in combination with radiation on breast MCa-35 and lung A549 tumor cells.