Cell Death Induction of Thymidine Kinase Gene Transfer Followed by Ganciclovir Treatment in Oral Squamous Cell Carcinoma Cell Lines

The tumoricidal effect of herpes simplex virus thymidine kinase (HSV-tk) gene transfer followed by ganciclovir (GCV) treatment has been demonstrated in relation to the bystander effect. We examined the mode of cell death after GCV treatment in three human oral squamous cell carcinoma (SCC) cell lines that had been infected with adenovirus possessing HSV-tk gene. Oral SCC cell lines displayed high susceptibility to HSV-tk/GCV treatment despite low transduction. Evidence suggests that apoptosis was not found by internucleosomal DNA fragmentation, cell cycle kinetics or gene expressions indicative of apoptosis. However, we observed different levels of expression of annexin V-positive/propidium iodide-negative cells in all the cell lines and occasional TUNEL-positive cells in one cell line. To address these controversial findings, we further confirmed the morphological phenotypes of cell death with semi-thin and ultra-thin sections, which revealed that the cells undergoing death were consistent with necrosis, i.e. swelling of cytoplasm and intracellular organs with membrane disruption. Furthermore, no apoptotic bodies were detected within cytoplasm of apparently intact cells. A nonapoptotic mechanism may play a central role in the HSV-tk/GCV-induced cell death in oral SCC cell lines.