ESTROGEN-DEPLETED CONDITION INDUCES APOPTOSIS OF RAT MAMMARY-CANCER CELLS AFTER ENTERING THE S-PHASE OF THE CELL-CYCLE

To elucidate the relationship of estrogen-depleted condition to apopto sis and tumor regression, 7,12-dimethylbenz[a]anthracene-induced mamma ry cancers of Sprague-Dawley rats were ovariectomized or treated with the anti-estrogenic agent epitiostanol after which proliferative activ ity and the incidence of apoptosis were investigated using the nick en d labeling method, agarose gel electrophoresis of DNA, electron micros copy, the BrdU-labeling method and mitotic count. Tumor regression was found after 7-day treatment, and apoptosis induced by the agent on th e 3rd day was clearly shown in both agarose gel electrophoresis of DNA and electron microscopy, which are two major methods used to judge ap optosis. The incidence of apoptosis revealed by the nick end labeling method reached its maximum, about threefold the control level, on the 3rd day of epitiostanol treatment compared with control tumors (P<0.01 ). The incidence of the cells incorporating BrdU reached its maximum o f 9.7% on the 2nd day of the treatment, while the incidence in tumors without treatment was 7.5% (P<0.05). Subsequently, the incidence of ap optosis was reduced after 7-day treatment, and the incidence of BrdU-p ositive cells was significantly reduced to about 3% after 5-day treatm ent. The incidence of mitosis did not change until the 3rd day of the treatment and was reduced after 5-day treatment. Similarly, chronologi cal changes of the incidences of BrdU-labeled cells, apoptotic cells a nd mitosis were observed in the tumors after ovariectomy. BrdU-labeled apoptotic bodies were detected in the tumors on the 3rd day in epitio stanol-treated rats that received a 6-hr bolus of BrdU before sacrific e. These findings indicate that, in a hormone-dependent rat mammary ca ncer model, treatment with this anti-estrogenic agent causes at least some cancer cells to fall into apoptosis after entering the S-phase of the cell cycle, resulting in the regression of mammary tumors.