Inhibition of protein kinase CK2 by condensed polyphenolic derivatives. An in vitro and in vivo study

ATP site-directed inhibitors that can target individual kinases are powerful tools for use in signal transduction research, all the more so in the case of a pleiotropic, constitutively active protein kinase such as CK2, which is not turned on in response to specific stimuli. By screening a library of more than 200 derivatives of natural polyphenolic compounds, we have identified 16 molecules which inhibit CK2 with IC50 values of less than or equal to 1 muM. They belong to the classes of anthraquinones (six compounds), xanthenones (two compounds), fluorenones (one compound), and coumarins (seven compounds), and their inhibitory potency correlates with the presence of nitro, amino, or halogen substituents at specific positions. Three of the most potent inhibitors, MNX (1,8-dihydroxy-4-nitroxanthen-9-one), NBC (8-hydroxy-4-methyl-9-nitrobenzo[g]chromen-2-one), and DBC (3,8-dibromo-7-hydroxy-4-methylchromen-2-one), whose IC50 values range between 0.13 and 0.36 muM, are quite specific toward CK2 within a panel of 33 protein kinases tested. Treatment of Jurkat cells with these compounds promotes inhibition of endogenous CK2 and induction of apoptosis. A correlation is observed between their efficacy as CK2 inhibitors (as judged from IC50 values) and their capacity to induce cell death (DC50 values). Mutations of the unique CK2alpha residues Val66 and/or IIe174 to alanine have a detrimental effect on inhibition by these compounds with 16-67-fold increases in IC50 values. The combined usage of these reagents can be exploited to gain information about cellular functions mediated by CK2.