COMPLEX RESPONSE OF BREAST EPITHELIAL-CELL LINES TO TOPOISOMERASE INHIBITORS

The topoisomerase inhibitors, camptothecin and etoposide target the ac tivity of topoisomerase I and II respectively. These agents, or their analogues, are undergoing clinical trials for the treatment of metasta tic breast cancer. In this study, we examined the response of eight br east epithelial cell lines, including six lines derived from breast ca ncers and two immortalized normal epithelial lines to camptothecin and etoposide. The lines varied by 700 fold in their sensitivity to the g rowth inhibiting effects of camptothecin and 30 fold in their response to etoposide. The BT474 line was the most resistant to both agents. T he other cell lines did not have uniform sensitivity to both drugs, i. e., some lines were sensitive to one drug but relatively resistant to the other. A variety of parameters in these lines were analyzed to elu cidate mechanisms of resistance including S phase, doubling time, expr ession and activity of topoisomerase I and II, expression of mdr-1, p5 3 status, cell cycle an est, level of apoptosis, and expression of the apoptotic proteins Bcl-2 and Bax. We found that low levels of the top o I protein and its enzymatic activity were associated with increased resistance to camptothecin. This was not true for topo II activity and etoposide. Increased apoptotic responses were generally observed in c ell lines that were sensitive to etoposide and this correlated with lo w ratios of Bcl-2/Bax protein. No single parameter was entirely predic tive of response. However, the BT474 line displayed a series of charac teristics including slow growth, the presence of mutant p53, low topo I activity, and a high Bcl-2/Bax ratio which together likely contribut ed to the resistance of this line to both etoposide and camptothecin.