Antiestrogen resistance of MCF7/IGF-1R cells is induced by high level of IGF-1R signaling. (a) Proliferative behavior of MCF7 versus MCF7/IGF-1R cells in response to 4-hydroxytamoxifen (TAM) in the dose range indicated in combination with estrogen 17β-estradiol (E2) (1 nM), IGF-1 (100 ng/mL) or E2 (1 nM) plus IGF-1 (100 ng/mL). Dimethyl sulfoxide (DMSO) was used as a control. (b) Proliferative behavior of MCF7/IGF-1R versus MCF7 cells in response to fulvestrant (FUL) with the same dose range and treatments as were used for TAM. Each treatment was carried out in triplicate. SRB absorbance values are representative of three independent experiments. Data are expressed as means ± SD.