Epstein-Barr virus-based vector improves the tumor cell killing effect of pituitary tumor in HVJ-liposome-mediated transcriptional targeting suicide gene therapy

Although tissue-specific promoters offer a promising approach to the targeting of gene therapy, the activity of such promoters is generally low, which is thus a major limitation, especially when using non-viral vectors. To establish effective transcriptional targeting gene therapy for growth hormone (GH) producing pituitary tumors, an Epstein-Barr virus (EBV) based vector system expressing herpes simplex virus type 1 thymidine kinase (HSV1-TK) driven by a rat GH promoter (pEBGTK) was developed. This harbors an EBV nuclear antigen-1 (EBNA-1) gene with an origin of the latent viral DNA replication (OriP) gene of EBV. We constructed an EBV-based luciferase plasmid (pEBGL) as a reporter plasmid. We also generated pGTK and pGL, which are non-EBV counterparts. Metastatic GH3 (mGH3) cells were used in this study. The transfection of pEBGL to mGH3 resulted in approximately a 39 times greater luciferase activity than pGL in vitro. Its expression was also prolonged 144 h after transfection. According to the results of pEBGL gene transfer in in vivo experiments, the luciferase activity was only observed in the tumors, but not detected in other normal tissues. The luciferase activities in tumor tissues were found until day 25 post transfection. During in vitro gene therapy, the transfection by pEBGTK using hemmaglutinating virus of Japan (HVJ) liposome enhances the susceptibility of mGH3 to gancyclovir (GCV) 110 times more than that by pGTK. The in vivo anti-tumor effects of pEBGTK on mGH3-tumor-bearing nude mice were evaluated. The intratumoral injection of HVJ anionic lipososme-enveloped pEBGTK followed by the intra-peritoneal injection of GCV demonstrated a significant growth inhibition against tumors without toxicity, while the tumors treated by other treatment modalities grew progressively. These results demonstrated that the EBV-based vector system can therefore contribute to the improvement of the anti-tumor effects for the HVJ-liposome-mediated transcriptional targeting suicide gene therapy, suggesting that this paradigm may thus be a potentially effective approach for the treatment of uncontrollable pituitary tumors.