(A) EDR1 was established from MCF7-E10 cell lines by long-term culture with a steroid-depleted medium. MCF7-E10 cell lines were derived from MCF-7, which had been stably transfected with an ERE-GFP reporter plasmid, and monitored ER expression. EDR1 exhibited ER overexpression. RIBR was established from EDR1 after long-term (7 months) culturing with 1000 nM ribociclib in phenol red–free RPMI medium. (B) Cell proliferation of EDR1 and RIBR1,2 treated with ribociclib for 3 days was measured relative to the negative control. The results are expressed as mean ± SD of three independent experiments; *
P
< 0.05. (C) Colony formation assay of EDR1 and RIBR1,2 cell lines. The control (left) was cultured for 15 days without any drugs. Ribociclib (right) was harvested for 15 days with 1000 nM ribociclib. (D), EDR1 and RIBR1,2 cell lines were treated with DMSO or ribociclib (1000 nM) for 24 h and measured by the fluorescence-activated cell sorting (FACS) cell-cycle analysis. (E), the protein expression levels of CDK6 in EDR1, EDR1 with 1000 nM ribociclib for 24 h, and RIBR1,2 cell lines were analyzed by western blotting.