title: Dominant expression of multiple drug resistance after in vitro X-irradiation exposure in intraspecific Chinese hamster ovary hybrid cells.
abstract: 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.
  
  
  OBJECTIVE
  To investigate whether this phenotype is dominant or recessive, we established intraspecific hybrids by fusion 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 x irradiation.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Laboratory of Cellular Chemotherapy, Imperial Cancer Research Fund, London, England.
authors.forename: S
authors.initials: S
authors.lastname: McClean
authors.forename: L K
authors.initials: LK
authors.lastname: Hosking
authors.forename: B T
authors.initials: BT
authors.lastname: Hill
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D015153:Blotting, Western; D016466:CHO Cells / Q000528:radiation effects; D002459:Cell Fusion; D003078:Colchicine / Q000494:pharmacology; D003114:Colony-Forming Units Assay; D006224:Cricetinae; D004317:Doxorubicin / Q000494:pharmacology; D004351:Drug Resistance / Q000235:genetics*; D005799:Genes, Dominant; D005982:Glutathione Transferase / Q000378:metabolism; D006822:Hybrid Cells / Q000201:enzymology; D008562:Membrane Glycoproteins / Q000096:biosynthesis; D010641:Phenotype; D011831:Radiation Genetics; D014700:Verapamil / Q000494:pharmacology; D014750:Vincristine / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000970:Antineoplastic Agents; D008562:Membrane Glycoproteins; D014750:Vincristine; D004317:Doxorubicin; D014700:Verapamil; D005982:Glutathione Transferase; D003078:Colchicine