Gene therapy of hepatoma: bystander effects and non-apoptotic cell death induced by thymidine kinase and ganciclovir

A retroviral vector carrying herpes simplex virus thymidine kinase gene was constructed, and transfected into the ψ2 packaging cells. The replication-defective retrovirus produced by this cell line (ψ2tkn cells) was transduced into XC rat hepatoma cells, from which a cell line (XCtkn2) highly sensitive to ganciclovir was cloned. Ganciclovir suppressed the growth of XCtkn2 hepatoma and ψ2tkn cells. Both of these HSV-tk-carrying cells treated with ganciclovir showed potent ‘bystander effect’ on co-culturing with genetically unmodified XC hepatoma cells. In addition, intratumoral injection of XCtkn2 and ψ2tkn cells into the XC hepatomas transplanted in nude mice and subsequent ganciclovir administration suppressed in vivo growth of the hepatomas. Flow cytometry disclosed that the ganciclovir-treatment increased the relative number of XCtkn2 hepatoma and ψ2tkn cells at the G2 phase of the cell cycle. However, the nuclear fragmentation and internucleosomal DNA cleavage were not observed, indicating that the death of XCtkn2 hepatoma and ψ2tkn cells treated with ganciclovir was not apoptotic.