Transgene Expression in Malignant Glioma Using a Replication-Defective Adenoviral Vector Containing the Egr-1 Promoter: Activation by Ionizing Radiation or Uptake of Radioactive Iododeoxyuridine

One approach to improving the specificity of gene therapy involves using radiosensitive promoters to activate gene expression selectively in the radiation field. In this study, we evaluated the ability of irradiation to regulate the transcription of a recombinant replication-defective adenovirus vector, Ad.Egr-1/lacZ, containing the radiation-inducible Egr-1 promoter driving the β-galactosidase reporter gene in glioma cells. Transcripts of the Egr-1 gene in human and rat glioma cells were induced following irradiation with as little as 2 Gy. This dose was 10-fold less than previously reported, and comparable to doses of irradiation used clinically in standard fractionated radiotherapy for brain tumors. When 9L rat gliosarcoma cells were infected with Ad.Egr-1/lacZ in vitro and exposed to 2 Gy of external beam irradiation, there was a threefold increase in β-galactosidase expression. Irradiation of intracerebral 9L tumors infected with the Ad.Egr-1/lacZ virus, using either external beam radiotherapy (2 Gy) or the thymidine analog 5-iodo-2′-deoxyuridine radiolabeled with the Auger electron emitter iodine-125 ([¹²⁵I]IdUrd), also resulted in increased β-galactosidase activity of the tumor cells. These results indicate that the use of viral vectors containing radiation-inducible promoters represents a novel therapeutic approach that enables gene therapy to be spatially and temporally regulated by ionizing radiation. These findings also support a potential role for radiation-inducible promoters in the treatment of malignant brain tumors.

Overview summary

Gene therapy using viral vectors containing the radiation-inducible Egr-1 promoter provides a potential approach for controlling gene transcription by ionizing radiation. We herein demonstrate that Egr-1 transcription is increased in glioma cells in vitro after irradiation with as little as 2 Gy. Irradiation of intracerebral 9L tumors infected with the Ad.Egr-1/lacZ virus, using either external beam radiotherapy (2 Gy) or the thymidine analog 5-iodo-2′-deoxyuridine radiolabeled iodine-125 ([¹²⁵I]IdUrd), resulted in significantly increased β-galactosidase activity of tumor cells. These results suggest the gene therapy using radiation-inducible promoters may have a role in the treatment of brain tumors.