PURPOSE: Estrogen, various polypeptide hormones and growth factors are associated with the development and progression of breast cancer. Coregulatory proteins are also associated with estrogen receptor (ER) transcriptional activity and tamoxifen resistance. Therefore, it is necessary to investigate the change of coregulator mRNAs and various cell proliferation proteins and cell cycle-related proteins after treatment with estrogen or antiestrogen. METHODS: MCF-7 cells were maintained in dextran-coated charcoal stripped 10% Dulbecco's Modified Eagle Medium (DMEM). To measure the change of the coactivators' (src-1, P/CAF, CBP, AIB1) mRNAs and corepressors' (SMRT, N-coR) mRNAs, multiple PCR was carried out using specific primers. In addition, intracellular proteins related to cell proliferation and cell cycle regulation were measured by performing Western blotting after treatment with estrogen or tamoxifen. The change of mitogen activated protein kinases was also measured by performing Western after tamoxifen treatment for 4 weeks. RESULTS: Coactivator mRNAs expression rapidly decreased in 15 min after estrogen treatment but this recovered to the initial level in 3 hr. The pattern was similar for the case of tamoxifen treatment. Corepressor mRNAs expression rapidly decreased in 15 min after estrogen treatment and it remained at a lower level until 24 hr after estrogen treatment. With tamoxifen treatment, the initial response was similar to the cases of estrogen treatment, but the xpression gradually increased 3 hr after tamoxifen treatment. Treatment of estrogen induced intracellular concentrations of c-myc and Ki-67 and it increased nuclear translocation of NF-kappaB and phosphor-ERK and it decreased the intracellular cell cycle suppressor p27/kip1. Tamoxifen treatment increased nuclear p27/kip1 but it decreased c-myc, NF-kappaB and phosphor-ERK. Long-term (4 weeks) treatment of tamoxifen was associated with decrease of activated ERK and p38 but there was no change in phospho-Akt level. CONCLUSION: Estrogen induced cell proliferation and the survival pathway-related factors, but it decreased the cell cycle suppressor p27/kip1. Long-term treatment with antiestrogen tamoxifen might decrease the MAPK activities in ERalpha-expressing tumor cells.