Experimental studies on PNP suicide gene therapy of hepatoma

To investigate the killing effect of PNP/MeP-dR suicide gene system on hepatoma cells, pcDNA3.0/PNP, an eukaryotic expression vector harboring E. coli PNP gene, was transfected into human hepatoma HepG2 cells by liposome-mediated method. A HepG2 cell line with stable PNP gene expression, HepG2/PNP, was established with presence of G418 selection. The cell growth curves were determined with trypan blue staining. The sensitivity of HepG2/PNP to MeP-dR and bystander effects were assayed by MTT and FCM methods. The enzymatic activity of the product of PNP gene was determined by HPLC method. The cytotoxic effects of MeP-dR on HepG2/PNP cells were obvious (IC50 = 4.5 micromol/L) and all HepG2/PNP cells were killed 4 days after the treatment with 100 micromol/L MeP-dR. In mixed cultures containing increasing percentages of HepG2/PNP cells, total population killing was demonstrated when HepG2/PNP cells accounted for as few as 5% of all HepG2 cells 8 days after the treatment with 100 micromol MeP-dR. High-pressure liquid chromatography (HPLC) demonstrated that the PNP enzyme could convert MeP-dR into 6-MP. PNP/MeP-dR suicide gene system had an advantage over traditional suicide gene systems for hepatoma gene therapy. Our e results suggest that high-level bystander effects of this system result in significant anti-tumor responses to hepatoma gene therapy, especially in vivo.