Apigenin inhibits CK2 kinase activity and induces growth inhibition and cell cycle arrest in MM cells. (A) CK2 kinase activity was detected in U266 and RPMI 8226 cells exposed to TBB or apigenin for 24 h. Results from 1 of 3 different experiments are shown; data are means ± SD. *Significant difference from control by ANOVA, *p < 0.05, **p < 0.01. (B) CK2α protein levels were detected by western blot analysis in both cell lines treated with different doses of apigenin or TBB for 24 h, and the levels of β-actin serves as the loading control. (C) and (D) U266 and RPMI 8226 cells were grown for 24-48 h in the absence or presence of increasing concentrations of apigenin or TBB respectively. Cell growth inhibition was measured by MTS and is expressed as a percentage of vehicle-treated control; results are presented as means ± SD of three independent experiments. *Significant difference from control by ANOVA, **p < 0.01. (E) and (F) Cell cycle analysis by flow cytometry after apigenin or TBB treatment for 24 h respectively in U266 and RPMI 8226 cells.