Inhibitor of Nuclear Factor-κB kinase (IKK) complex is the master regulator of the NF-κB signaling pathway. IKK complex activation contributes to major cellular processes and plays a key role in the pathogenesis various human malignancies including prostate cancer. Recently, catalytic IKKs represent attractive targets for intervention with small molecule kinase inhibitors. We aimed to develop apigenin, a plant flavone, as a potent inhibitor of the IKK-kinase domain. IKK complex consist of structurally similar IKKα and IKKβ kinases. Both contains a kinase domain, a leucine-zipper motif through which the catalytic subunits dimerize, and a helix-loop-helix domain followed by a C-terminal tail that, when unphosphorylated interacts with IKKγ, the non-catalytic subunit. Screening of human prostate tumors demonstrated an increase in IKKα in low and high Gleason grade, whereas, IKKβ was higher in benign and low grade tumors. Phosphorylation of IKKα (Ser176/180) and IKKβ (Ser177/181) was significantly increased in prostate tumors compared to benign tissues. Treatment of human prostate cancer 22Rv1 and PC-3 cells to apigenin resulted in a decreased phosphorylation of IKKα/β in dose-dependent fashion, which correlated with lower proliferation and invasiveness. Ex Vivo IKK complex pull down assay using sepharose beads also confirmed direct binding of apigenin with IKK complex. In vivo studies with PC-3 and 22Rv1 xenografts in athymic nude mice further demonstrated that oral intake of apigenin at doses of 20 and 50μg/mouse/day over an 8-wk period resulted in a marked reduction in tumor growth, IKK activity, IKK phosphorylation, and NF-κB/p65 at both doses of apigenin. Furthermore, 3D structural in silico modeling analysis with PS1145, a known IKK inhibitor and apigenin were assessed for IKKα and IKKβ separately for protein-ligand docking studies using glide score. Apigenin was well anchored by hydrogen bonds with amino acid resides in the IKK complex protein at both ends. All three hydroxyl groups of apigenin were favorably oriented around different hydrogen bond acceptor atoms in the protein. Glide score generated by apigenin and PS1145 docking to IKK-α were -8.40 and -5.79, whereas IKK-β were -11.07 and -9.11; lower glide score represents better binding to IKK complex pocket. These results suggest that inhibition of cell proliferation and decrease in tumor growth by apigenin are mediated, at least in part, by its ability to suppress IKK complex and NF-κB signaling pathways.

Citation Format: Sanjeev Shukla, Manish Datt, Mark R. Chance, Sanjay Gupta. Apigenin suppresses IKK activation and downstream signaling leading to prostate cancer inhibition. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3660. doi:10.1158/1538-7445.AM2013-3660

©2013 American Association for Cancer Research
2013