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Although fractionated irradiation has substantial rationale for the use in clinical radiotherapy, we believe it might also induce adaptive response to cancer cells that results in tolerance to subsequent cytotoxicity of irradiation. Understanding the mechanism by which fractionated irradiation might causes radioresistance in prostate cancer cells should be very important to get the optimal treatment outcome. We have established radioresistant cell lines by high dose fractionated irradiation (2 Gy per fraction, 3 times a week, total 36 Gy) from three parent prostate cancer cell lines (LN-cap, DU-145, PC-3), which have different p53 status each. Clonogenic survival difference, as measured by surviving fraction of 7 Gy, was ranged from 0.15 to 0.3. Interestingly in all the radioresistant prostate cancer cell lines, p21 protein was up regulated and caspase-3 protein activity down regulated, suggesting that these processes might be independent of p53. DNA microarray of radioresistnat cell lines using sham-irradiated cells as a reference also demonstrated same patterns of regulation for p21 and caspase-3 mRNA. Repeated irradiation altered markedly cellular morphology that includes the findings like multinucleated giant cells, swollen cytoplasm, plasma membrane blebbing, apoptotic bodies, etc. With repeated subcultures without irradiation, however, cellular morphology has been nearly normalized like parent cells about 8 to 10 weeks later. More interestingly after the normalization of cellular morphology, the characteristics of radioresistant cell lines, which are increased clonogenic surviving to irradiation, up regulation of p21 protein and down regulation of caspase-3 protein activity, have been still remained. We performed cDNA microarray to compare gene expression profile of each parent and radioresistant cell lines. Between these two cell lines, the difference in cell cycle related genes was the largest among our categorized groups. And the cDNA microanalysis of DU-145 samples normalized morphologically with cessation of irradiation for 8~10 weeks was done to be compared with morphologically abnormal samples, which will be presented in the meeting.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA