IDENTIFICATION OF A DISTINCTIVE P-GLYCOPROTEIN-MEDIATED RESISTANCE PHENOTYPE IN HUMAN OVARIAN-CARCINOMA CELLS AFTER THEIR IN-VITRO EXPOSURETO FRACTIONATED X-IRRADIATION

Background. Clinical drug resistance is recognized in patients previou sly treated with radiotherapy and after chemotherapy. In vitro exposur e of mammalian tumor cells to fractionated X-irradiation also resulted in the expression of drug resistance. Analysis of the resistance phen otype of irradiated Chinese hamster ovary sublines revealed P-glycopro tein overexpression, without any concomitant increase in P-glycoprotei n messenger RNA, under posttranslational regulation. This study aimed to determine whether this distinctive resistance phenotype could also be identified in irradiated human tumor cells. Methods. Irradiated sub lines established from two human ovarian tumor cell lines, SK-OV-3 and JA-T, which showed resistance to vincristine and to etoposide, were s tudied. Protein and RNA expression were quantitated by Western and Nor thern blotting or RNase protection assays. P-glycoprotein turnover was measured after immunoprecipitation of metabolically labelled cells. R esults. Significant P-glycoprotein overexpression was detected using t he C219 and C494 monoclonal antibodies in the two irradiated human ova rian tumor sublines. No concomitant increase in P-glycoprotein messeng er RNA was detectable in the SK-OV-3/DXR10 subline, contrasting with t he increased message characteristic of vincristine-selected SKVCR subl ines. In addition, turnover of P-glycoprotein was significantly reduce d in these DXR10 cells when compared with that measured in a vincristi ne-selected subline. These irradiated sublines showed reduced levels o f epidermal growth factor receptors and unchanged levels of topoisomer ase II, but they overexpressed c-erbB2 marginally and heat shock prote in 27 significantly. These latter elevations in protein levels, howeve r, were associated with concomitant increases in their respective mess enger RNAs, implicating regulation at the transcriptional level. Concl usions. Exposure of human ovarian tumor cells to fractionated X-irradi ation in vitro resulted in the expression of a distinctive multiple dr ug resistance phenotype unusually involving posttranslational regulati on of P-glycoprotein. Monitoring tumor biopsies for P-glycoprotein-ass ociated drug resistance in patients treated with radiotherapy should e valuate protein levels rather than, or as well as, MDR1 mRNA expressio n.