Thymidylate synthase (TS) is a dNTP synthetic enzyme and is also a target enzyme of 5-fluorouracil (5-FU). 5-FU is one of the anticancer agents most frequently used for the treatment of colorectal cancers. However, the clinical rate of response to its use as a single agent is not exceptionally high. Therefore, various combination chemotherapies have been devised. The elevated expression of TS in cancer cells is a serious obstacle in the clinical use of 5-FU. In the present study, TS expression was up-regulated by the knockout of the p21WAF1/CIP1 gene in human colorectal cancer HCT116 cells, suggesting that TS expression is mediated through the inhibition of cyclin-dependent kinase (CDK). Based on these findings, we tested whether the CDK inhibitor (CDKI) SU9516, acted as a suppressor of TS. SU9516 effectively reduced the expression of TS in a dose-dependent manner. Furthermore, the reduction of TS expression resulted in enhancement of the sensitivity to 5-FU in human colon cancer DLD-1 cells. Thus, SU9516 might be a promising compound for combination chemotherapy with 5-FU.

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