Stimulation of Cell Proliferation and Estrogenic Response by Adrenal C₁₉-Δ⁵-Steroids in the ZR-75-1 Human Breast Cancer Cell Line

Abstract

We have examined the effect of androst-5-ene-3β,17β-diol (Δ⁵-diol) and its precursors, dehydroepiandrosterone (DHEA) and dehydroepiandrosterone 3β-sulfate (DHEAS), on the growth of the estrogen-sensitive human breast cancer cell line, ZR-75-1. While the cell number was increased up to 4-fold by maximal concentrations of estradiol, Δ⁵-diol maximally stimulated cell proliferation by approximately 3-fold. Since the half-maximal stimulation achieved by Δ⁵-diol is observed at 2.5 nm and the normal range of plasma concentrations of this steroid in women is 1 to 3 nm, it is most likely that the stimulatory effect of Δ⁵-diol has physiological significance. DHEA and DHEAS were much less effective than Δ⁵-diol in stimulating the proliferation of ZR-75-1 cells, the maximal effect on cell number being 75% at the maximal dose used, namely 10 µm. The mitogenic effects of estradiol and Δ⁵-diol were competitively inhibited by the antiestrogen LY156758 (keoxifene), while the effects of DHEA and DHEAS were completely abolished by the antiestrogen. The effects of DHEA and Δ⁵-diol on cell proliferation are not likely to be mediated via their conversion to estrone or estradiol, since androstenedione had no effect, while testosterone and dihydrotestosterone decreased cell number by about 20%. The number of specific progesterone binding sites was increased 3.7-, 3.2-, and 2.0-fold by Δ⁵-diol, DHEA, and DHEAS, respectively. The relative potency of the C₁₉-Δ⁵-steroids to increase the number of progesterone-specific binding sites was comparable to their ability to stimulate cell proliferation. Direct competition experiments performed with intact cells in monolayer culture showed that, under conditions of minimal metabolism, only Δ⁵-diol could significantly compete with estradiol for cellular estrogen-specific binding sites with an apparent dissociation constant of 11 nm, thus suggesting that physiological concentrations of C₁₉-Δ⁵-steroids of adrenal origin could exert an estrogenic stimulation of breast tumor growth without involvement of the aromatase pathway. The present data suggest not only that estrone derived from androstenedione could play a role in estrogensensitive breast cancer in women but that Δ⁵-diol could well be the most important estrogen in breast cancer in women.