Activation of the Radiosensitive EGR-1 Promoter Induces Expression of the Herpes Simplex Virus Thymidine Kinase Gene and Sensitivity of Human Glioma Cells to Ganciclovir

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Abstract
Herein we describe experiments showing that the early growth response gene 1 (EGR-1) promoter is sufficient to confer selective expression of the luciferase gene (Luc) in glioma cell lines exposed to ionizing radiation. Activity of the EGR-1 promoter was investigated in human glioblastoma cells using the plasmid vector, pEGR-Luc. The EGR-1 promoter gene directed radiosensitive expression of luciferase. This promoter showed high levels of activity (10-fold) in irradiated glioma cell lines as compared to basal levels of activity in nonirradiated cell lines. Maximum activation was detectable at 1–3 hr after stimulation with 20 Gy. The results also demonstrate that cells modified to contain the herpes simplex virus-thymidine kinase (HSV-tk) gene under control of the EGR-1 promoter become sensitive to treatment with the antiviral agent ganciclovir (GCV), whereas nonirradiated cells and nontransfected cells were unaffected by this agent. This results suggest that therapeutic genes can be expressed selectively in irradiated glioma cells. The results also indicate that the EGR-1 promoter can be used to induce exogenous genes selectively in radiation fields used for the treatment of malignant brain tumors. Early growth response gene 1 (EGR-1) gene expression is increased by ionizing radiation. The present studies demonstrate that the EGR-1 promoter is sufficient to confer selective activation of an exogenous gene in glioma cell lines exposed to ionizing radiation. These regulatory sequences were used to direct transcription of herpes simplex virus-thymidine kinase (HSV-tk) coding sequences within plasmid constructs. We have found that radiation treatment is associated with induction of sensitivity to ganciclovir (GCV) in transfected cells. In contrast, nonirradiated cells were not sensitive to GCV. These results suggest that EGR-1 promoter sequences could be useful in enhancing treatment of brain tumors with radiation by combining this gene therapy approach with GCV administration.