Fig. 4: Suppression of IL-6 in palbociclib-sensitive hormone receptor–positive breast cancer cells delays palbociclib resistance.

A, B Dose-response curves in MCF7 and T47D parental (Par) control scrambled shRNA (shSCR), shIL-6 #1, and shIL-6 #2 cells, depicting the effect of treatment with 0.01–12 μM palbociclib for 6 days followed by 6 days of recovery (A) and after 24 h of estrogen deprivation followed by re-addition of 10 nM beta-estradiol and treatment with varying concentrations (0.01–12 μM) of fulvestrant for 2 days (B). Dashed line depicts half-maximal inhibitor concentration (IC₅₀) values. C, D Bar graphs depicting the average IC₅₀ values in MCF7 and T47D parental shSCR, shIL-6 #1, and shIL-6 #2 cells treated with palbociclib (C) and fulvestrant (D) as indicated in A, B. E, F Doubling time every 3 days for 21 days showing the effect of shRNA knockdown of IL-6 on the proliferation of MCF7 and T47D parental cell lines. G, H MCF7 parental (G) and T47D (H) shSCR, shIL-6 #1, and shIL-6 #2 cells were treated with 1 μM palbociclib every 3 days for 21 days with measurements taken every 3 days as indicated in G for cell growth and IL-6 secreted protein levels (ELISA). I Representative images of senescence-associated ß-galactosidase (SA-βgal) staining in MCF7 parental shSCR, shIL-6 #1, and shIL-6 #2 cells treated with or without 1 μM palbociclib every 3 days for 21 days. Quantification of SA-βgal+ cells was calculated every 3 days relative to number of cells. J Representative images of SA-βgal staining in MCF7 resistant (Res) and T47D Res shSCR, shIL-6 #1, and shIL-6 #2 cells treated with or without 1 μM palbociclib every 3 days for 6 days. Quantification of SA-βgal+ cells was calculated at day 6 relative to number of cells. *p < 0.05; **p < 0.01; ***p < 0.001.