Cytotoxic activity of gemcitabine in cultured cell lines derived from histologically different types of bladder cancer: role of thymidine kinase 2.

World wide incidence of bladder cancer is rising with nearly 13,760 deaths attributed to bladder cancer in 2007 in the USA. Tumor types of the urothelium include transitional cell carcinomas, squamous cell carcinomas, and adenocarcinomas. This study was undertaken to determine gemcitabine's efficacy against bladder cancer cell lines of different origins (HTB2, a papilloma; HTB3, a squamous cell carcinoma; and HTB4, a transitional cell carcinoma). Roles of nucleoside transporters and key enzymes in gemcitabine pharmacology were examined on the premise that cells originating from different types of bladder cancer exhibit different levels and/or types of nucleoside transporters and enzymes and thus may respond differently to gemcitabine. HTB2 cells had the highest transport efficiency and were also most responsive to gemcitabine. HTB3 and HTB4 cells had similar transport efficiencies, but exhibited different sensitivities to gemcitabine (HTB4 > HTB3). The highest accumulation of [3H]gemcitabine was in HTB2 cells and the lowest was in HTB3 cells. Sequencing experiments revealed no mutations either in coding exons or intron-exon boundaries of the hENT1 genes of the three cell lines. HTB3 cells exhibited high thymidine kinase 2 (TK2) activity whereas HTB2 and HTB4 cells lacked detectable TK2 activity and pretreatment of HTB3 but not of HTB2 and HTB4 cells with extracellular thymidine resulted in enhanced sensitivity to gemcitabine. Our results highlight the importance of hENT1 and TK2 activities in response to gemcitabine. Elevated TK expression in squamous cell carcinomas warrants further study and offers new insights into rational treatment strategies based on bladder cancer phenotype.