Abrogation of sterol regulatory element-binding proteins (SREBPs) inhibits the growth of long-term estrogen deprivation (LTED) cells. a Knockdown efficiency of SREBP1 and SREBP2 in MM134 parental and LTED-D cells. mRNA was collected 72 h after reverse transfection with small interfering RNA (siRNA). Data are mean ± standard deviation (SD) of six replicates collected from two independent experiments. Two-tailed Welch’s unequal variances t test, *P ≤0.05, **P <0.01, ***P <0.001. b Growth curve of MM134 parental and LTED cells with SREBP knockdown. Parental and LTED cells in 96-well plates were reverse-transfected with 1 nM siSREBP1 and 1 nM siSREBP2 (SREBP siRNA) or 2 nM non-target siRNA. Parental cells were cultured in their normal growth media (fetal bovine serum, or FBS). Two-way analysis of variance (ANOVA), ***P <0.001. (c, d) Dose response of PF429242, an inhibitor of SREBP1 and SREBP2, in MM134 (c) and SUM44 (d) parental and LTED cells. Parental cells were grown in normal growth media (FBS) and hormone-deprived media (charcoal-stripped fetal bovine serum, or CSS) to control the effect of media on the drug. Plots are representative of two independent experiments. Data are mean ± SD of six replicates. Two-way ANOVA (LTED versus parental in FBS), ***P <0.001. e Dose response of Fatostatin in MM134 parental and LTED cells. Parental cells were grown in normal growth media (FBS) or hormone-deprived media (CSS) to control the effect of media on the drug. Plots are representative of two independent experiments. Data are mean ± SD of six replicates. Two-way ANOVA was used to compare the dose response curves (LTED versus parental in FBS). Two-tailed t tests were performed to compare the inhibition rates of Fatostatin on LTED and parental (FBS) at 10 μm, 35 μm, and 100 μm independently. **P <0.01, ***P <0.001. f The expression change of SREBP1 with 3-month letrozole treatment in letrozole responders (n = 36) and non-responders (n = 14). Black solid lines, increased expression of SREBP1 with 3-month treatment; gray dashed lines, decreased expression of SREBP1 with 3-month treatment. Gene expression data for letrozole-treated patients were downloaded from Gene Expression Omnibus [GSE20181]. Pearson’s chi-squared test. Abbreviation: NS not significant.