DIFFERENTIAL GANCICLOVIR-MEDIATED CYTOTOXICITY AND BYSTANDER KILLING IN HUMAN COLON-CARCINOMA CELL-LINES EXPRESSING HERPES-SIMPLEX VIRUS THYMIDINE KINASE

The two human colon carcinoma cell lines HT-29 and SW620, which stably express herpes simplex virus thymidine kinase (HSV-TK), are sensitize d to the cytotoxic effects of the antiviral drug ganciclovir (GCV), Co mpared with HT-29 cells, SW620 cells ware more sensitive to lower GCV concentrations (<1 mu M), accumulated GCV triphosphate more rapidly, a nd incorporated higher levels of GCV irate DNA, Following a 24-hr expo sure to 10 mu M GCV, bystander killing was as much as sixfold greater in SW620 cells than HT-29 cells. This bystander effect was dependent o n the level of HSV-TK expression, the number of cells expressing HSV-T K, and the overall confluency of the cells. However, bystander killing did not correlate with gap junctional intercellular communication as determined by microinjection of Lucifer Yellow fluorescent dye. SW620 cells were coupled to <3% adjacent cells (compared with = 50% for HT-2 9 cells), but were still able to transfer phosphorylated GCV to bystan der cells as soon as 4 hr after drug was added. These results emphasiz e the importance of cell-specific metabolism in HSV-TK/GCV-mediated cy totoxicity and may suggest a novel mechanism for bystander killing.