Modulation of progestin binding activity in cultured human breast carcinoma cells: the effect of serum type and concentration

Progesterone receptor levels in MCF-7 human breast cancer cells increase as a specific response to estrogen and to some nonsteroidal antiestrogens. In the present study we demonstrate that the type and quantity of serum present during culture of these cells modifies the level of progestin binding activity, but not the level of estradiol binding activity. MCF-7 cells maintained in media supplemented with 5% charcoal-dextran treated calf serum (CDCS) contain 0.3 - 0.4 pmol of cytosol progesterone receptor (PRc) per mg DNA. When cells previously maintained in 5% CDCS-media are shifted to media containing 5% charcoal-dextran treated fetal calf serum (CDFCS), the level of progestin binding increases after day 16, and stabilizes at 2 - 3 pmol/mg DNA at days 30 to 40. Shifting these cells back to 5% CDCS-media, reduces PRc to 0.2 - 0.4 pmol/mg DNA within 3 days. This reduction is dose dependent with a half-optimal decrease at 1% CDCS, and a full decrease at 2% CDCS (4d incubation). Nuclear progestin binding was uniformly low (0.2 - 0.4 pmol/mg DNA) and unaffected by type or concentration of serum, and no consistent change in cytosol or nuclear estrogen receptor levels was observed. These cytoplasmic progestin binding sites are translocated to the nucleus by progesterone, and are similar to estradiol (E2) induced sites by Scatchard binding and sucrose gradient analysis. Similar serum-dependent changes are also observed in the T47D human breast cancer cell line where growth in CDFCS-media results in 4-fold higher progestin binding levels than observed in CDCS-media. Our findings suggest the presence of non-dialyzable stimulatory factor(s) in CDFCS that influence the progestin receptor level and highlight the fact that serum components can alter dramatically the cellular progestin binding activity.