Potential complex formation of bortezomib with catechols. (A) Effect of flavonoids on bortezomib-induced apoptosis. DoHH2 cells were pretreated with 30 μM myricetin, 20 μM kaempferol, or 5 μM apigenin individually for 1 hour and then incubated with or without 20 nM bortezomib or 1 μM MG-132 for 12 hours. Apoptosis was measured by the annexin V assay. Data shown (mean ± SD) were from 3 separate experiments. Inhibition of myricetin on bortezomib-induced apoptosis was statistically analyzed by the t test. *P < .001. (B) Chemical structures of quercetin, myricetin, kaempferol, and apigenin. The B-rings of quercetin and myricetin are substituted catechols. (C) Chemical structure of bortezomib (Ci), MG-262 or MG-132 (Cii), and complex formation of catechol derivatives with boronic acids to form boronate esters (Ciii). (D) Inhibitory effect quercetin on MG-262–induced apoptosis. DoHH2 cells were preincubated with or without 20 μM quercetin for 1 hour and then treated with MG-262 for 12 hours. *Significant inhibition as analyzed by ANOVA (P < .001). (E) Effect of quercetin on lactacystin-induced apoptosis. DoHH2 cells were preincubated with 20 μM quercetin for 1 hour and then treated with lactacystin for 12 hours. Apoptosis was determined by DNA content assay with flow cytometry. Data shown (mean ± SD) were from 3 separate experiments. (F) Detection of chemical reactions by Raman spectroscopy. Chemical reactions between quercetin and bortezomib (Fi), quercetin and MG-262 (Fii), and quercetin and boric acid (Fiii,Fiv). Boxes indicate significantly altered spectra.