Since sex steroid hormones and growth factors are known to modulate the proliferation of breast tumors, we have studied the effects of estrogen and progestin, their antagonists, and growth factors on the regulation of estrogen receptor (ER) mRNA and protein levels in T47D breast cancer cells, which contain low levels of ER, and in two sublines of MCF-7 cells which contain high ER levels. The mRNA levels were measured by Northern blot analysis using lambda OR8, a cDNA probe for ER, and protein levels were measured by hormone binding or Western blot analysis. Treatment of T47D cells with estradiol (E2) caused a 2.5-fold increase in ER mRNA (6.6 kilobases) levels after 48 h. The progestin R5020 evoked a marked decrease in ER mRNA and protein levels to 20% of control values, while the antiprogestin RU38,486 caused no change in ER. In MCF-7 cells, the effect of E2 on ER levels was dependent on the prior growth history of the cells. In cells grown in low estrogen [5% charcoal-dextran-treated calf serum with phenol red for 8 yr (MCF-7-K2)], which are still E2 responsive, treatment with E2, the antiestrogen LY117018, or both produced little change in ER mRNA or protein; in contrast, ER mRNA and protein were reduced by E2 to 40% and 50% of control levels, respectively, in MCF-7 cells (denoted MCF-7-K1) which had been maintained routinely in medium containing 5% calf serum. This decrease in ER mRNA was dose dependent; 10(-11) E2 reduced levels to 60%, and 10(-10) M E2 evoked the maximal drop to 40% of the control level in 2 days. LY117018 alone did not alter ER mRNA levels in these cells, but it completely prevented the down-regulation of ER by E2. Administration of progestin, but not antiprogestin, along with E2 partially prevented the decrease in ER evoked by E2. Addition of epidermal growth factor or insulin-like growth factor-I to MCF-7-K1 cells, which increased cell proliferation, had no detectable effect on ER levels. Treatment with transforming growth factor-beta, which decreased cell proliferation, reduced ER by about 20%.(ABSTRACT TRUNCATED AT 400 WORDS)