THE BYSTANDER EFFECT EXERTED BY TUMOR-CELLS EXPRESSING THE HERPES-SIMPLEX VIRUS THYMIDINE KINASE (HSVTK) GENE IS DEPENDENT ON CONNEXIN EXPRESSION AND CELL COMMUNICATION VIA GAP-JUNCTIONS

To elucidate the role gap junctions play in the bystander effect, we e xamined the cytotoxic effect of herpes simplex virus thymidine kinase (HSVtk) modified tumor cells on gap communication-deficient tumor cell s and their connexin transfectants. Communication competent Walker 256 tumor cells engineered to express the HSVtk gene (Walker-tk(+)) when cocultured with N2A mouse neuroblastoma and PC12 rat pheochromocytoma cells with absent endogenous junctional conductance showed no bystande r cyctotoxicity. Transfection of N2A cells with the rat connexin37 gen e (5Q) and PC12 cells with the human connexin43 gene rendered them sus ceptible to bystander cell death. Additionally, communication-deficien t N2A cells transfected with the HSVtk gene failed to exert a bystande r. effect, whereas N2A transfectants coexpressing the connexin37 and H SVtk genes (5Qtk(+) cells) exerted bystander cytotoxicity on gap junct ion communication-competent 5Q but not on communication-deficient N2A cells in vitro. In vivo experiments also showed tumor growth inhibitio n of communication-competent 5Q but not communication-incompetent N2A cells by 5Qtk(+) cells. hn conclusion, these results indicate that in several cellular environments the bystander effect is dependent on con nexin expression and gap junctional communication between HSVtk-positi ve and HSVtk-negative cells.