NARINGENIN - A WEAKLY ESTROGENIC BIOFLAVONOID THAT EXHIBITS ANTIESTROGENIC ACTIVITY

Treatment of immature 21-day-old female Sprague-Dawley rats with 17 be ta-estradiol (E2) (0.5 mu g/rat) caused a significant increase in uter ine wet weight, DNA synthesis, progesterone receptor (PR) binding, and peroxidase activity. At doses as high as 40 mg/rat, the bioflavonoid naringenin did not cause a significant increase in any of these E2-ind uced responses. However, in rats cotreated with E2 (0.5 mu g/rat) plus naringenin (30 mg/rat), there was a significant decrease in E2-induce d uterine wet weight, DNA synthesis, PR binding, and peroxidase activi ty, indicating that naringenin exhibits antiestrogenic activity in the immature rodent uterus. The binding of uterine nuclear extracts to a P-32-labeled estrogen responsive element (ERE) or progesterone respons ive element (PRE) was determined using gel electrophoretic band shift assays. Incubation of [P-32]ERE with uterine nuclear extracts from rat s treated with naringenin or E2 resulted in the formation of estrogen receptor (ER):ERE complexes; a higher mobility complex was prominent i n the extracts from E2-treated rats, whereas a lower mobility complex was observed using nuclear extracts from naringenin-treated animals. T here was a significant decrease in the intensity of the E2-induced com plex using nuclear extracts from rats treated with E2 plus naringenin. In contrast, transformed cytosol from control rats gave an intense ER :ERE complex, whereas the intensity of the band was decreased markedly using transformed uterine cytosol from treated rats. Formation of a P R:PRE complex was also determined using transformed uterine cytosol. C ytosol from E2-treated rats gave an intense retarded band, whereas onl y weak bands were observed using cytosols from DMSO- (solvent), naring enin-, or naringenin plus E2-treated cells. The results of in vitro st udies showed that 1 nM E2 increased (3- to 4-fold) the growth of MCF-7 human breast cancer cells, whereas 1-1000 nM naringenin had no effect on cell proliferation. In cells cotreated with 1 nM E2 plus 1000 nM n aringenin, there was a significant decrease in E2-induced cell growth. In MCF-7 cells transiently transfected with a pS2 promoter-regulated luciferase reporter gene, naringenin exhibited weak estrogenic activit y. In cells cotreated with 0.1 or 1.0 mu M naringenin plus 1 nM E2, na ringenin inhibited E2-induced luciferase activity. The results of thes e studies confirmed that naringenin is a weak estrogen that also exhib its partial antiestrogenic activity in the female rat uterus and MCF-7 human breast cancer cells.