CELL-CYCLE ARREST, MICRONUCLEUS FORMATION, AND CELL-DEATH IN GROWTH-INHIBITION OF MCF-7 BREAST-CANCER CELLS BY TAMOXIFEN AND CISPLATIN

The induction of cell death along with cell-cycle arrest is one of the foremost mechanisms regulating cell growth. In the human breast carci noma cell line MCF-7 we investigated two chemotherapeutic agents, the antiestrogen tamoxifen and the DNA-damaging drug cisplatin, for the re lative contribution of these mechanisms to growth inhibition in cultur e. Growth kinetics and flow cytometry confirmed that tamoxifen at 1 mu M acts mainly by arresting cells in the G0/G1 phase of the cell cycle . Compared to untreated controls, only a few more cells were detached from the monolayer and dead after a 5-day incubation. On the other han d, cisplatin at 1 mu M did not induce the well-defined G2/M-arrest rep orted for other cell types, but resulted in a marked increase in the r ate of cell death. A morphological feature observed, especially with c isplatin-treated MCF-7 cells, was the formation of numerous micronucle i (in up to 30% of the cells) and an increase in the number of binucle ate cells (up to 20%). In both tamoxifen- and cisplatin- treated cultu res, cell death appeared to occur by apoptosis, as indicated morpholog ically by cellular and nuclear shrinkage accompanied by DNA-condensati on and ultimately the formation of DNA containing apoptotic bodies. Ho wever, no internucleosomal DNA degradation or endogenous endonuclease activity could be detected in the cells of the monolayer or in the mai nly dead and detached cells of the culture supernatant. DNA fragmentat ion was only observed when isolated MCF-7 nuclei were incubated with e xogenous endonucleases. However, as determined by reverse transcriptas e/polymerase chain reaction amplification, MCF-7 cells do express the mRNA for DNase I, an endonuclease known to be involved in apoptosis, T hus, apoptosis is part of the growth-inhibitory process and occurs wit hout apparent internucleosomal DNA fragmentation in MCF-7 cell culture s.