Tranilast strongly sensitizes pancreatic cancer cells to gemcitabine via decreasing protein expression of ribonucleotide reductase 1.

Gemcitabine (Gem) is a dFdC analogue with activity against several solid tumors. Gem is intracellularly phosphorylated by dCK, leading to the production of the metabolite dFdCDP. dFdCDP exhibits the cytotoxic effect by inactivating ribonucleotide reductase larger subunit 1 (RRM1), which is a rate limiting enzyme for de novo DNA synthesis. To date, RRM1 expression is believed to determine sensitivity to Gem in pancreatic and non-small cell lung cancer. In the present study, we found that an anti-allergic drug, tranilast strongly enhanced the sensitivity of pancreatic cancer cell line KP4 to Gem. In growth inhibition assay, 100 microM of tranilast plus 1 microM of Gem more strongly suppressed the growth of KP4 at 12.7-fold in IC50 than single Gem treatment, while this compound no longer affected the sensitivity to other drugs such as 5-fluorouracil, irinotecan or paclitaxel. FACS and TUNEL analysis demonstrated the increased apoptotic population in KP4 cells under tranilast plus Gem, compared with single Gem treatment. In Western blot analysis, tranilast treatment decreased RRM1 expression at protein level with dose-dependency in KP4 cells. Proteasome inhibitor MG132 disturbed the reduction of RRM1 expression in tranilast treated KP4 cells, indicating protein degradation by the activated proteasome. Transfection using siRNA against RRM1 increased the sensitivity of KP4 to Gem, suggesting that RRM1 suppression is an important step in increasing Gem efficacy. Finally, we demonstrated that tranilast reduced RRM1 protein and increased Gem efficacy in 4 other pancreatic cell lines. In a future, a novel chemotherapeutic strategy by Gem along with tranilast might improve Gem efficacy against pancreatic cancer.