Identification of a distinctive p‐glycoprotein‐mediated resistance phenotype in human ovarian carcinoma cells after their in vitro exposure to fractionated x‐irradiation

Background. Clinical drug resistance is recognized in patients previously treated with radiotherapy and after chemotherapy. In vitro exposure of mammalian tumor cells to fractionated X‐irradiation also resulted in the expression of drug resistance. Analysis of the resistance phenotype of irradiated Chinese hamster ovary sublines revealed P‐glycoprotein overexpression, without any concomitant increase in P‐glycoprotein 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 sublines established from two human ovarian tumor cell lines, SK‐OV‐3 and JA‐T, which showed resistance to vincristine and to etoposide, were studied. Protein and RNA expression were quantitated by Western and Northern blotting or RNase protection assays. P‐glycoprotein turnover was measured after immunoprecipitation of metabolically labelled cells. Results. Significant P‐glycoprotein overexpression was detected using the C219 and C494 monoclonal antibodies in the two irradiated human ovarian tumor sublines. No concomitant increase in P‐glycoprotein messenger RNA was detectable in the SK‐OV‐3/DXR10 subline, contrasting with the increased message characteristic of vincristine‐selected SKVCR sublines. In addition, turnover of P‐glycoprotein was significantly reduced in these DXR10 cells when compared with that measured in a vincristine‐selected subline. These irradiated sublines showed reduced levels of epidermal growth factor receptors and unchanged levels of topoisomerase II, but they overexpressed c‐erbB2 marginally and heat shock protein 27 significantly. These latter elevations in protein levels, however, were associated with concomitant increases in their respective messenger RNAs, implicating regulation at the transcriptional level. Conclusions. Exposure of human ovarian tumor cells to fractionated X‐irradiation in vitro resulted in the expression of a distinctive multiple drug resistance phenotype unusually involving posttranslational regulation of P‐glycoprotein. Monitoring tumor biopsies for P‐glycoprotein‐associated drug resistance in patients treated with radiotherapy should evaluate protein levels rather than, or as well as, MDR1 mRNA expression. Cancer 1994; 73:2990–9.