Abstract

Indirect, noninvasive imaging of therapeutic gene expression based on levels of reporter gene expression is a powerful tool to devise improved therapeutic strategies in cancer gene therapy. The use of bicistronic vectors carrying internal ribosome entry sites (IRESs) allows the coexpression of multiple gene products from the same promoter but leads to considerable attenuation of the downstream gene. In this study, we describe the use of 10 linked copies of the Gtx (homeodomain protein) IRES (abbreviated as SIRES) in place of the encephalomyocarditis (EMCV) IRES in mediating downstream reporter gene expression in cell culture and in vivo.We constructed several plasmid vectors carrying different upstream and downstream reporter genes (herpes simplex virus type I thymidine kinase [tk], firefly luciferase [fl], and Renilla luciferase [rl]) placed between EMCV IRES and SIRES segments. RL, FL, and TK enzyme activities in N2a, C6, and 293 cells transiently transfected with these vectors were found to be significantly higher for the SIRES vectors than for the EMCV IRES vectors. For in vivo experiments, 4 stably transfected N2a cell lines were implanted in nude mice. The mice were imaged for rl and fl gene expression using a charged-coupled device (CCD) camera. For bioluminescence and microPET imaging of downstream gene expression of fl and tk genes, respectively, mice carrying 4 stably transfected xenografts were imaged using the CCD camera and microPET.In cell culture, using rl as the upstream gene, we demonstrate that the expression of the downstream tk gene is 12-fold greater using SIRES when compared with EMCV IRES. Furthermore, the expression of the 2 genes was highly correlated in N2a cells. In vivo bioluminescence imaging using 4 stably transfected N2a cell lines revealed increasing levels of rl and fl gene expression. Bioluminescence and microPET, respectively, of fl and tk reporter gene expression in nude mice bearing N2a tumor xenografts showed the gene expression mediated by SIRES to be 4- and 8-fold higher, respectively, than EMCV IRES.These findings support the use of SIRES bicistronic vectors for a better assessment of therapeutic gene expression based on reporter gene expression in living subjects.

View details for Web of Science ID 000228200000022

View details for PubMedID 15809490