Inhibitory effect of quercetin on bortezomib-induced apoptosis. (A) Fresh CLL cells (10⁶) were pretreated with quercetin (20-60 μM) for 1 hour and then incubated with (bottom panel) or without (top panel) bortezomib (20 nM) for 12 hours. Apoptotic cell death was measured by the DNA content assay using flow cytometry. The percentage of cells in the gate M1 indicates apoptotic or sub-G₀/G₁ cells. The flow cytometric profiles shown are one typical experiment from more than 10 CLL samples. (B) The inhibitory effect of different concentrations of quercetin on bortezomib (20 nM)–induced apoptosis on the EBV transformed HRC57, DLBCL DoHH2, and myeloma cell lines U266 and RPMI-8226. (C) The inhibitory effect of quercetin (20 μM) on different concentrations of bortezomib. Q indicates quercetin; B, bortezomib; Q+B, quercetin added 1 hour before bortezomib; B + Q, quercetin and bortezomib added together. *Statistically significant inhibitory effect of quercetin on bortezomib as analyzed by ANOVA (P < .001). Data are means plus or minus SD from 3 separate experiments.