Mechanisms of cell death induced by suicide genes encoding purine nucleoside phosphorylase and thymidine kinase in human hepatocellular carcinoma cells in vitro

For gene therapy of hepatocellular carcinoma (HCC), the Escherichia coli pu rine nucleoside phosphorylase (PNP)/fludarabine suicide gene system may be more useful than the herpes simplex virus thymidine kinase/ganciclovir (HSV -tk/GCV) system as a result of a stronger bystander effect. To analyze the molecular mechanisms involved in PNP/fludarabine-mediated cell death in hum an HCC cells in comparison with HSV-tk/GCV, we transduced human HCC cells o f the cell lines, HepG2 and Hep3B, with PNP or HSV-tk using adenoviral vect ors, followed by prodrug incubation. Both systems predominantly induced apo ptosis in HepG2 and Hep3B cells. PNP/fludarabine induced strong p53 accumul ation and a more rapid onset of apoptosis in p53-positive HepG2 cells as co mpared with p53-negative Hep3B cells, but efficiency of tumor cell killing was similar in both cell lines. In contrast, HSV-tk/GCV-induced apoptosis w as reduced in p53-negative Hep3B cells as compared with p53-positive HepG2 cells. HSV-tk/GCV, but not PNP/fludarabine, caused up-regulation of Fas in p53-positive HepG2 cells and of Fas ligand (FasL) in both HCC cell lines. T hese results demonstrate cell line-specific differences in response to trea tment with PNP/fludarabine and HSV-tk/GCV, respectively, and indicate that PNP/fludarabine may be superior to HSV-tk/GCV for the treatment of human HC C because of its independence from p53 and the Fas/FasL system.