Apoptotic cell death through inhibition of protein kinase CKII activity by 3,4-dihydroxybenzaldehyde purified from Xanthium strumarium

The CKII inhibitory compound was purified from the fruit of Xanthium strumarium by organic solvent extraction and silica gel chromatography. The inhibitory compound was identified as 3,4-dihydroxybenzaldehyde by analysis with FT-IR, FAB-Mass, EI-Mass, ¹H-NMR and ¹³C-NMR. 3,4-dihydroxybenzaldehyde inhibited the phosphotransferase activity of CKII with IC₅₀ of about 783 μM. Steady-state studies revealed that the inhibitor acts as a competitive inhibitor with respect to the substrate ATP. A value of 138.6 μM was obtained for the apparent K_i. Concentration of 300 μM 3,4-dihydroxybenzaldehyde caused 50% growth inhibition of human cancer cell U937. 3,4-dihydroxybenzaldehyde-induced cell death was characterised with the cleavage of poly(ADP-ribose) polymerase and procaspase-3. Furthermore, the inhibitor induced the fragmentation of DNA into multiples of 180 bp, indicating that it triggered apoptosis. This induction of apoptosis by 3,4-dihydroxybenzaldehyde was also confirmed by using flow cytometry analysis. Since CKII is involved in cell proliferation and oncogenesis, these results suggest that 3,4-dihydroxybenzaldehyde may function by inhibiting oncogenic disease, at least in part, through the inhibition of CKII activity.