We have investigated the effects of the pure antiestrogen EM-139 and the nonaromatizable androgen dihydrotestosterone (DHT) alone or in combination with estradiol (E2) on cell proliferation and cell kinetic parameters in human ZR-75-1 breast cancer cells. Following a 24- to 30-h exposure to E2, a decrease in the proportion of G0-G1 cells was observed, this effect being accompanied by the well-known stimulatory effect of the estrogen on cell proliferation at later time intervals. By contrast, DHT or EM-139 alone inhibited basal cell proliferation without a significant influence on cell cycle distribution. Moreover, pretreatment with DHT for 8 days, while decreasing ZR-75-1 cell number, did not cause a loss in E2 sensitivity. In fact, as early as after 24 h of E2 treatment, a decrease in the G0-G1 cell fraction accompanied by a corresponding increase of the S-phase was observed in both control and DHT-pretreated cells. When added concomitantly with E2, DHT or EM-139 inhibited the E2 stimulatory effect on cell proliferation, but only EM-139 significantly reversed the G0-G1 decrease induced by E2. Although DHT and EM-139 did not affect the distribution of ZR-75-1 cells between the different phases of the cell cycle, continuous labeling with 5'-bromodeoxyuridine showed that EM-139 and DHT had a global slowing effect on the duration of the cell cycle, thus explaining the potent inhibitory effect of these compounds on cell proliferation. The present data demonstrate that DHT and EM-139 are both potent inhibitors of the stimulatory effect of E2 on cell proliferation, their main action being related to a general increase in the duration of the cell cycle.
