Figure 4.

Continuous tamoxifen treatment induces an increase in BrdU incorporation in breast cancer cells stimulated with tamoxifen. Untreated cells (A) or continuously exposed to tamoxifen (B) were grown on 96-well plates, synchronized and stimulated with 2,000 nM tamoxifen, 10 nM 17β-estradiol (E2) or 100 nM G1 for 24 h. BrdU incorporation was determined by a cell proliferation immunoassay and by measuring absorbance at 450 nm. The effect produced by tamoxifen on BrdU incorporation was significantly reduced by preincubation of cells with 1,000 nM G15 in both, untreated cells and cells continuously exposed to the drug. Results are shown as mean ± SEM (n = 3) *P < 0.05; **P < 0.001 versus unstimulated cells or preincubated with G15.