Apigenin blocks IKKα activation and suppresses prostate cancer progression.

Sanjeev Shukla,Rajnee Kanwal,Pingfu Fu,Sanjay Gupta,Manish Datt,Eswar Shankar,Mark R Chance,Gregory T Maclennan

doi:10.18632/oncotarget.5157

Sanjeev Shukla, Rajnee Kanwal + Show 6 more

Open Access

https://doi.org/10.18632/oncotarget.5157

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Abstract

IKKα has been implicated as a key regulator of oncogenesis and driver of the metastatic process; therefore is regarded as a promising therapeutic target in anticancer drug development. In spite of the progress made in the development of IKK inhibitors, no potent IKKα inhibitor(s) have been identified. Our multistep approach of molecular modeling and direct binding has led to the identification of plant flavone apigenin as a specific IKKα inhibitor. Here we report apigenin, in micro molar range, inhibits IKKα kinase activity, demonstrates anti-proliferative and anti-invasive activities in functional cell based assays and exhibits anticancer efficacy in experimental tumor model. We found that apigenin directly binds with IKKα, attenuates IKKα kinase activity and suppresses NF-ĸB/p65 activation in human prostate cancer PC-3 and 22Rv1 cells much more effectively than IKK inhibitor, PS1145. We also showed that apigenin caused cell cycle arrest similar to knockdown of IKKα in prostate cancer cells. Studies in xenograft mouse model indicate that apigenin feeding suppresses tumor growth, lowers proliferation and enhances apoptosis. These effects correlated with inhibition of p-IKKα, NF-ĸB/p65, proliferating cell nuclear antigen and increase in cleaved caspase 3 expression in a dose-dependent manner. Overall, our results suggest that inhibition of cell proliferation, invasiveness and decrease in tumor growth by apigenin are mediated by its ability to suppress IKKα and downstream targets affecting NF-ĸB signaling pathways.

Highlights

The IĸB kinases IKKα and IKKβ are critical in activating the NF-ĸB pathway [1, 2]
Apigenin suppresses tumor growth in athymic nude mouse xenograft model Apigenin has been shown to be effective in cell culture, inhibiting IKKα/β phosphorylation and down stream NF-ĸB signaling in human prostate cancer PC-3 and 22Rv1 cells; we extended our study to determine whether these events occur in vivo using xenograft mouse model
We present several lines of evidence that IKKα plays an important role in cancer progression and that apigenin blocks IKKα activation, leading to decreased proliferation and enhanced apoptosis in prostate cancer cells

Summary

Introduction

The IĸB kinases IKKα and IKKβ are critical in activating the NF-ĸB pathway [1, 2]. IKKα/β are catalytic subunits of the heterotrimeric IKK complex bound to the non-catalytic subunit IKKγ/Nemo [1,2,3,4]. Blocking IKK-mediated IĸBα degradation and NF-ĸB activation, repression of NF-ĸB transactivation potential and stabilization of IĸB has been shown to inhibit aberrant gene expression, malignant phenotypes and therapeutic resistance in pre-clinical models of prostate cancer [11,12,13,14]. Clinical trials using bortezomib alone in advanced stage prostate cancer and in combination with androgen blockade and chemotherapy showed limited clinical efficacy due to incomplete targeting of NF-ĸB/Rel subunits and other signaling pathways mediated through androgen receptor and β-catenin, which contribute to the resistance to bortezomib and castration in prostate cancer [17, 18] Another recent study showed proteasome inhibition by bortezomib increases IL-8 expression and nuclear accumulation of IKKα [19]. A better understanding of the target specificity and dosage required for inhibition of NF-ĸB signaling through IKK inhibitors may lead to the development of more specific and efficacious inhibitors of NF-ĸB signaling

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