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JNCI Journal of the National Cancer Institute 1993 85(1):48-53; doi:10.1093/jnci/85.1.48
© 1993 by Oxford University Press
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Journal of the National Cancer Institute, Vol. 85, No. 1, 48-53, January 6, 1993
© 1993 Oxford University Press

Dominant Expression of Multiple Drug Resistance After In Vitro X-irradiation Exposure in Intraspecific Chinese Hamster Ovary Hybrid Cells

Siobhán McClean, Louise K. Hosking, Bridget T. Hill

Laboratory of Cellular Chemotherapy, Imperial Cancer Research Fund London, England

Correspondence to: Bridget T. Hill, Ph.D., Laboratory of Cellular Chemotherapy, Imperial Cancer Research Fund, 44, Lincoln's Inn Fields, London WC2A 3PX, England.

Background: Exposure of Chinese hamster ovary (CHO) cells to fractionated x irradiation in vitro has led to expression of a distinctive multiple-drug-resistant phenotype. This phenotype is characterized by overexpression of P-glycoprotein without an increase in P-glycoprotein messenger RNA or gene amplification; increased glutathione S-transferase (GST) activity; and resistance to vincristine, colchicine, and etoposide but not to doxorubicin. Purpose: To investigate whether this phenotype is dominant or recessive, we established intraspecific hybrids by funsion of x-ray-treated, drug-resistant CHO cells (DXR-10I or DXR-10II) with drug-sensitive CHO cells (E29). Methods: Drug resistance levels were determined in the wild-type CHO cell line AuxB1, the drug-sensitive E29 line, the x-ray-pretreated lines, and the hybrid lines by colony-forming assay of cells grown in increasing concentrations of colchicine, vincristine, or doxorubicin. The hybrids were characterized by analysis of DNA content, P-glycoprotein expression by Western blotting, GST activity by use of 1-chloro-2, 4-dinitrobenzene as substrate, and sensitivity to reversal of resistance to vincristine by exposure to verapamil. Results: These hybrids proved resistant to colchicine (two-fold) and vincristine (five- to seven-fold) but not to doxorubicin. After the hybrids were exposed to verapamil, vincristine cytotoxicity was increased 10- to 12-fold. The hybrid lines exhibited levels of P-glycoprotein comparable to those of the unfused x-ray-treated parent cell line, suggesting that P-glycoprotein overexpression is a dominant trait in these hybrid lines. Interpretation of the role of increased GST activity in these hybrids was inconclusive because of the very high levels of GST in the drug-sensitive cell-fusion partner. Conclusions: The multiple-drug-resistant phenotype following x-ray treatment of CHO cells in vitro was dominantly expressed. Overall, these data are consistent with the hypothesis that this phenotype is a consequence of the dominant genetic alteration resulting from exposure to xirradiation. Implications: This work adds weight to our hypothesis that there is a biological basis for the expression of clinical drug resistance in certain patients whose tumors have been previously irradiated. [J Natl Cancer Inst 85: 48–53, 1993]


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