A suicide gene therapy using herpes simplex virus thymidine kinase (HSV-TK) with ganciclovir (GCV) has been under development as a tumor-targeted therapy. However, the mechanism of cellular GCV uptake, which is prerequisite in the therapy, has not been clarified. In an attempt to resolve this situation and gain information to optimize HSV-TK/GCV system for cancer therapy, we found that human equilibrative nucleobase transporter 1 (ENBT1) can transport GCV with a Michaelis constant of 2.75 mM in Madin-Darby canine kidney II (MDCKII) cells stably transfected with this transporter. In subsequent experiments using green fluorescent protein (GFP)-tagged ENBT1 (GFP-ENBT1) and HSV-TK, the uptake of GCV (30 μM), which was minimal in MDCKII cells and unchanged by their transfection with HSV-TK alone, was found to be increased extensively by their transfection with GFP-ENBT1 together with HSV-TK. Accordingly, cytotoxicity, which was assessed by the WST-8 cell viability assay after the treatment of those cells with GCV (30 μM) for 72 h, was induced in those transfected with GFP-ENBT1 together with HSV-TK but not in those transfected with HSV-TK alone. These results suggest that ENBT1 could facilitate GCV uptake and, thereby, enhance cytotoxicity in HSV-TK/GCV system. We also identified HeLa and HepG2 as cancer cell lines rich with ENBT1, and A549, HCT-15 and MCF-7 as those poor with ENBT1. Accordingly, HSV-TK/GCV system was effective in inducing cytotoxicity in the former but not in the latter. Thus, ENBT1 was found to be a GCV transporter that could enhance the performance of HSV-TK/GCV suicide gene therapy.
- The American Society for Pharmacology and Experimental Therapeutics