Opposite effects of estrogen and the progestin R5020 on cell proliferation and GCDFP-15 expression in ZR-75-1 human breast cancer cells

We have recently demonstrated that physiological concentrations of androgens cause a marked inhibition of basal and 17β-estradiol (E 2)-induced cell growth in ZR-75-1 human breast cancer cells. Moreover, these steroids exert effects on GCDFP-15 (gross cystic disease fluid protein-15) expression that are opposite to their above-indicated actions on cell proliferation. The synthetic progestin R5020 (17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione), on the other hand, causes a potent inhibition of E 2-induced ZR-75-1 cell growth. In order to further characterize the hormonal regulation of GCDFP-15 expression and to better understand the antagonism between progestin and estrogen action in breast cancer cells, we have studied the effect of R5020 on both GCDFP-15 expression and cell growth in ZR-75-1 cells. After a 10-day incubation, the 4-fold stimulatory effect of 1 nM E 2 on cell growth was 60% decreased by maximal effective concentrations of R5020 (> 1 nM) while, in the absence of E 2, R5020 had no effect. The mitogenic action of E 2 was accompanied by a 75% inhibition of GCDFP-15 secretion while nanomolar concentrations of R5020 induced 1.4- and 5.2-fold increases in GCDFP-15 secretion in control and E 2-treated ZR-75-1 cells, respectively. While E 2 caused a marked inhibition of GCDFP-15 mRNA levels, R5020 induced a maximal 2- to 3-fold increase (above control) in GCDFP-15 mRNA accumulation in cells simultaneously incubated with E 2. In the presence of E 2, the effect of R5020 on cell growth and GCDFP-15 secretion was 70–90% blocked by concomitant exposure to the antihormone RU486 while simultaneous exposure to the pure antiandrogen OH-flutamide exerted a small but significant interference with the antiestrogenic effect of R5020 on these two parameters. The present data show that in the presence of E 2, R5020 exerts a potent stimulatory effect on GCDFP-15 expression which is primarily mediated through specific interaction with the progesterone receptor with a minor role of the androgen receptor. In analogy with our previous observations with androgens and estrogens, the effects of the progestin R5020 on GCDFP-15 expression are opposite to their action on cell growth. Such data also suggest that the glycoprotein GCDFP-15 could well be a good biochemical marker to follow the response to androgens and/or progestins as well as antiestrogens in the therapy of advanced breast cancer.