Inhibitory influence of natural flavonoids on human protein kinase CK2 isoforms: effect of the regulatory subunit

Abstract

CK2 is a pleiotropic, constitutively active protein kinase responsible for the phosphorylation of more than 300 physiological substrates. Typically, this enzyme is found in tetrameric form consisting of two regulatory subunits CK2β and two catalytic subunits CK2α or CK2α'. Several natural occurring flavonoids were tested for their ability to inhibit both CK2 holoenzymes, CK2α₂β₂ and CK2α'₂β₂. We identified few substances selectively inhibiting only the α' subunit. Other compounds showed similar effect towards all four isoforms. In some cases, like chrysoeriol, pedalitin, apigenin, and luteolin, the α₂β₂ holoenzyme was at least six times better inhibited than the free α subunit. Otherwise, we have found a luteolin derivative decreased the kinase activity of CK2α' with an IC₅₀ value of 0.8 μM, but the holoenzyme only with 9.5 µM.

Keywords: CK2 holoenzymes; Flavonoids; Inhibitors; Phosphorylation.

Fig. 1

Structures of flavonoids tested in this study

Fig. 2

Inhibition of different CK2 isoforms. Comparison of chrysoeriol (a) and pedalitin (b) and their influence on CK2α (black circle), CK2α′ (black square), CK2α₂β₂ (white circle), and CK2α′₂β₂ (white square) phosphorylating activity. The experiment was carried out using 10 μM P2B as protein substrate and 20 μM ATP as phospho-donor. The results are the means of three independent experiments

Fig. 3

Comparison of compounds chrysoeriol (1), apigenin (4), luteolin 7-O-glucoside (5a), and 6-hydroxyluteolin 7-O-glucoside (5b) towards their inhibitory potential on CK2α (white), CK2α₂β₂ (dark gray), CK2α′ (light grey), and CK2α′₂β₂ (black) phosphorylation. The experiments were carried out in the presence of 50 μM of the synthetic peptide and in the presence of 20 μM ATP. The results are the means of three independent experiments

Fig. 4

Comparison of the binding modes of chrysoeriol (white), apigenin (blue), and quercetin (green) to CK2α (a), CK2α′ (b), and CK2α₂β₂ (c) obtained with molecular docking. The docking was provided using the Swissdock web server and analysis was done with UCSF Chimera 1.12rc software. (Color figure online)