Cyclooxygenase-2 gene induction causes CDDP resistance in colon cancer cell line, HCT-15

Drug resistance to cisplatin (CDDP) would represent a major obstacle for cancer therapy. The adenosine triphosphate (ATP) binding cassette (ABC) family of transport proteins, such as the 170 kDa P-glycoprotein (multidrug resistance gene-1; MDR- 1) and the 190 kDa multidrug resistance-associated proteins (MRPs), are associated with multidrug resistance, including resistance to CDDP. The purpose of the present study was to investigate the relationship between cyclooxygenase-2 (COX-2) expression and the level of chemosensitivity to CDDP. We established the COX-2-overexpressed colon cancer cell line TR-5 from HCT-15 cells. We quantified the expression of m-RNA for MRP-1 and MDR-1 by a real-time PCR method, determining that the values of each gene/standardized GAPDH in HCT-15 and TR-5 were 2.3±0.4 and 6.1±0.5 in MRP-1 (p<0.02) and 9.0±4.8 and 3.6±0.5 in MDR-1, respectively. With respect to chemosensitivity, survival rates for 3 Ig/ml and 10 Ig/ml of CDDP were 81.5±12.2% and 26.1±11.7% (IC50=6.5 Ig/ml) for HCT- 15 and 96.6±1.7% and 77.4±4.9% (IC50=18.5 Ig/ml) for TR- 5, respectively, thus TR-5 showed higher resistance to CDDP than HCT-15 did with statistical differences. We also demonstrated a successful re-sensitization to CDDP toxicity in TR-5 by means of the COX-2 selective inhibitor JTE-522, 4-(4-cyclohexyl-2-methyl- 1, 3-oxazol-5-yl)-2-fluorobenzene sulfonamide, which markedly decreased the IC50 of CDDP for TR-5 (from 17.3±2.6 Ig/ml to 8.6±2.5 Ig/ml). In conclusion, COX-2 overexpression induced increased MRP-1 expression in a colon cancer cell line, TR-5, resulting in chemoresistance to CDDP that was approximately triple the level of chemoresistance observed in the original HCT- 15 cells line, as measured by calculation of the IC50. We also confirmed the efficacy of pretreatment of TR-5 cells with the COX-2 selective inhibitor JTE-522 in restoring chemosensitivity of these cells to CDDP, suggesting a strategy for overcoming drug resistance to CDDP. Since cis-diamminedichloroplatinum II (cisplatin; CDDP) is one of the most effective drugs against colon cancer, the development of drug resistance to CDDP would represent a major obstacle to the effective treatment of this cancer. Multidrug resistance has been shown to be conferred by different integral membrane proteins, including the 170 kDa