High progesterone receptor concentration in a variant of the ZR-75-1 human breast cancer cell line adapted to growth in oestrogen free conditions

Abstract

Culture of ZR-75-1 human breast cancer cells for 5 days in the absence of oestrogens (phenol red-free medium supplemented with dextran coated charcoal stripped 5% fetal calf serum) resulted in a slowing of growth rate and loss of progesterone receptors. Oestradiol at 10(-9) M markedly stimulated growth and progesterone receptor synthesis over a 5-day period. While medroxyprogesterone acetate (10(-10) to 10(-6) M) inhibited growth of ZR-75-1 cells growing in complete medium, in the short-term absence of oestrogens low concentrations were growth stimulatory. Cells deprived of oestrogens for 5 days retained sensitivity to growth inhibition by 4-hydroxy tamoxifen. ZR-75-1 cells were also adapted to growth in the absence of oestrogens over a 5-month period. These cells (ZR-PR-LT) failed to express binding sites characteristic of the type 1 oestrogen receptor but progesterone receptor expression was at a level normally associated with oestrogen induction. Adapted cells were growth inhibited by oestradiol, 4-hydroxy tamoxifen and medroxyprogesterone acetate, but despite elevated progesterone receptor expression the progestin was only marginally more inhibitory than in the parent line. Our data indicate a poor quantitative relationship between response to progestins in vitro and progesterone receptor concentration and support previous findings that acquisition of an oestrogen independent phenotype does not necessarily result in resistance to anti-oestrogens.

PIP: After an initial response to hormonal therapy, breast cancer patients whose tumors are positive for the presence of estrogen receptors and progesterone receptors become resistant to this treatment, either because of the development of antiestrogen withdrawal. Within 5 days of transfer to an estrogen-free medium, proliferation of ZR-75-1 cells slowed dramatically and there was a loss of progesterone receptors. Also over a 5-day period, estradiol markedly stimulated growth and progesterone receptor synthesis. Medroxyprogesterone acetate inhibited the growth of ZR-75-1 cells in complete medium, low concentrations in the absence of estrogen were growth stimulatory in the short run. Cells deprived of estrogen for 5 days were still sensitive to growth inhibition by 4-hydroxy tamoxifen. Although ZR-PR-LT cells failed to demonstrate binding sites characteristic of the type 1 estrogen receptor, progesterone receptor expression was at a level consistent with estrogen induction. Adapted cells were growth inhibited by estradiol, 4-hydroxy tamoxifen, and medroxyprogesterone acetate, but the progestin was only slightly more inhibitory than in the parent line. Overall, these findings suggest a poor quantitative relationship between the in vitro response to progestins and progesterone receptor concentration and confirm previous findings that the acquisition of an estrogen-independent phenotype does not necessarily lead to antiestrogen resistance.