Oestrogen potentiates topoisomerase‐II‐mediated cytotoxicity in an activated subpopulation of human breast cancer cells: Implications for cytotoxic drug resistance in solid tumours

Publisher Website
Google Scholar
Abstract
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 subpopu-lations resistant to the topoisomerase-ll-interactive drug VP16 (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 VP16-induced cell-cycle delay. This moderate increase in overall cell sensitivity is accompanied by massive enhancement of clonogenic cell kill, suggesting that oestrogen enhances VP16 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 VP16 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.