ENHANCED EXPRESSION OF THYMIDINE KINASE IN HUMAN-CELLS FOLLOWING IONIZING-RADIATION

Purpose: We investigated the induction of thymidine kinase transcripti on and enzymatic activity, and the activation of transcription factors binding to the thymidine kinase promoter, in human normal compared to tumor cells in culture before and after ionizing radiation. Methods a nd Materials: Northern blot, dot-blot, and thymidine kinase enzyme ass ays were used to observe thymidine kinase transcript and enzymatic cha nges before and after radiation. Temporal expression of thymidine kina se transcripts following an optimal induction dose of radiation was al so studied. Gel mobility shift assays were performed using a 95-base p air fragment of the thymidine kinase promoter (containing the CCAAT bo x) to analyze transcription factor binding. Results: Thymidine kinase transcript and enzymatic levels were higher in human tumor compared to normal cells. In contrast, levels of x-ray-activated thymidine kinase transcription factors were not significantly different in human neopl astic compared to normal cells. Conclusion: Elevated x-ray-induced thy midine kinase transcripts, enzymatic levels, and transcription factors are consistent with the loss of stringent cell growth regulation asso ciated with neoplastic cells. The induction of thymidine kinase follow ing ionizing radiation may be exploited in chemotherapeutic strategies which use halogenated pyrimidines and/or in various gene therapy stra tegies.