Abstract. In order to investigate the malignant phenotype of cyclooxygenase (COX)-2 overexpressing cancer cells, a human epidermoid KB carcinoma cell line minimally expressing COX-2 protein was transfected with human COX-2 cDNA. In this study, we used a COX-2 transfected clone KB/COX-2 and a neomycin-transfected clone KB/neo as the control. When we examined the susceptibility to anticancer agents, there was no difference between these two clones in vincristine, bleomycin and 5-fluorouracil, although KB/ COX-2 showed a 2.5-fold resistance to cisplatin (CDDP) as compared with KB/neo. The IC 50 for CDDP was 4.3 μM in KB/COX-2 and 1.7 μM in KB/neo. Treatment with small interfering RNA (siRNA) mediated the inhibition of COX-2 significantly increasing the level of susceptibility to CDDP in COX-2 siRNA as compared to that of the control siRNA. The expression of MRP1 and MRP2 was stronger in KB/COX-2 than in KB/neo by Western blot analysis. In addition, apoptosis induction by CDDP was at a lower level in KB/COX-2 (31%) than in KB/neo (38%). These results suggested that the overexpression of COX-2 increases the intracellular production of MRP1 and MRP2 and causes drug resistance to CDDP. IntroductionCyclooxygenase (COX) catalyzes the synthesis of prostaglandins from arachidonic acid. Two isoforms of the COX enzyme exist, COX-1 and COX-2 (1-4). While COX-1 is expressed constitutively in many organs including the alimentary canal, COX-2 is induced by stimuli such as cytokines and growth factors and is involved in various biological responses. COX-2 is up-regulated at the sites of inflammation and in various cancer tissues; colon (2), stomach (3), breast (4), lung (5), esophagus (6), pancreas (7), urinary bladder (8), prostate (9) and skin (10). We demonstrated that the overexpression of COX-2 elevated cell migration in vitro and tumorigenicity and local tumor invasion in vivo via up-regulating MMP and Rho family small GTPases and down-regulating TIMP activities in the human KB carcinoma cell line (11).Cisplatin (CDDP) is one of the most potent anticancer agents, displaying clinical activity against a wide variety of human malignancies, including head and neck, testicular, lung, ovarian and colon cancers (12,13). Drug resistance is the major limitation on the use of this anticancer agent. The mechanisms responsible for CDDP resistance are multifactorial and thereby make the treatment difficult.In this study, the human KB carcinoma cell line minimally expressing COX-2 protein was transfected with COX-2 cDNA and an isolated clone with high COX-2 expression was compared with a mock-infected clone in the susceptibility to anticancer agents of COX-2 transfected cells in vitro. In particular, the purpose of this study was to investigate the relationship between COX-2 expression and the level of susceptibility to CDDP. Materials and methodsCell line and cell culture. The human KB carcinoma cell line (14) derived from the epidermoid carcinoma of the floor of the mouth was used in this study. KB cells were grown in Dulbecco's modifi...
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