Oestrogen potentiates topoisomerase-II-mediated cytotoxicity in an activated subpopulation of human breast cancer cells: implications for cytotoxic drug resistance in solid tumours.
Primary resistance to chemotherapeutic agents is a major problem in the management of advanced cancer. By using oestrogen to modulate the topoisomerase II content of T-47D human breast cancer cells, we show here that cell subpopulations resistant to the topoisomerase-II-interactive drug VPI6 (etoposide) can be identified and quantified using single-cell analytical techniques. Immunohistochemical studies reveal topoisomerase II to be present in approximately 10% of control cells compared with 30% of oestrogen-stimulated cells, and this difference is reflected in the proportions of cells exhibiting VPI6-induced cell-cycle delay. This moderate increase in overall cell sensitivity is accompanied by massive enhancement of clonogenic cell kill, suggesting that oestrogen enhances VPI6 cytotoxicity by recruiting a clonogenic cell subpopulation characterized by increased topoisomerase II content. Flow cytometry confirms that the increase in topoisomerase II is localized to an activated G1-phase cell subset. We conclude that (i) single-cell analysis of cellular topoisomerase II content is predictive of VPI6 chemosensitivity; (ii) the existence of resistant tumour-cell subpopulations does not necessarily indicate the presence of phenotypically divergent subclones; and (iii) rational strategies for eliminating tumour resistance may be based on biological manipulation of specific cytotoxic drug targets.
Epstein
R J
RJ
MRC Unit, Cambridge.
Smith
P J
PJ
Watson
J V
JV
Waters
C
C
Bleehen
N M
NM
D001943:Breast Neoplasms / Q000473:pathology; D002453:Cell Cycle / Q000187:drug effects; D004250:DNA Topoisomerases, Type II / Q000032:analysis / Q000502:physiology*; D004351:Drug Resistance; D004967:Estrogens / Q000494:pharmacology*; D005047:Etoposide / Q000494:pharmacology*; D005260:Female; D005434:Flow Cytometry; D006801:Humans; D007150:Immunohistochemistry; D014411:Neoplastic Stem Cells / Q000187:drug effects; D014407:Tumor Cells, Cultured
D004967:Estrogens; D005047:Etoposide; D004250:DNA Topoisomerases, Type II