Inhibition of protein kinase CKII activity by euchrestaflavanone B purified from Cudrania tricuspidata

The CKII (EC 2.7.1.37) inhibitory compound was purified from the root barks of Cudrania tricuspidata and identified as (2S)-2-[2,4-dihydroxy-5-(3-methyl-but-2-enyl)-phenyl]-5,7-dihyroxy-6-(3-methyl-but-2-enyl)chroman-4-one (euchrestaflavanone B). Euchrestaflavanone B was shown to inhibit the phosphotransferase activity of CKII with IC50 of about 78 microM. Steady-state studies revealed that euchrestaflavanone B acted as a competitive inhibitor with respect to the substrate ATP. A value of 16.4 microM was obtained for the apparent Ki. Concentration of 0.8 microM euchrestaflavanone B caused 50% growth inhibition of human cancer cells U937 and HeLa. Euchrestaflavanone B-induced cell death was characterized with the cleavage of poly(ADP-ribose) polymerase and procaspase-3, indicating that the inhibitor triggered apoptosis. Because protein kinase CKII is involved in cell proliferation and oncogenesis, these results suggest that euchrestaflavanone B may function by inhibiting oncogenic disease, at least in part, through the inhibition of CKII activity.