Flavokawain B, A Kava Chalcone, Acts Synergistically With Proteasome Inhibitors (I.E., Bortezomib And Mg132) To Reduce The Growth Of Prostate Cancer Cells Via Downregulation Of Skp2

Flavokawain B, a chalcone isolated from kava root extracts, has been shown to inhibit the in vitro and in vivo growth of various cancer cell lines via induction of apoptosis. However, the mechanism of flavokawain B9s action remains largely unknown. Here we have demonstrated that flavokawain B caused Skp2 degradation in an ubiquitin and proteasome dependent manner. In addition, flavokawain B inhibited NEDD8 conjugations to both Cullin1 and Ubc12 in PC3 cells and Ubc12 NEDDylation in an in vitro assay. Overexpression of dominant-negative cullin1 (1-452) or K720R mutant cullin1 rescued the effect of flavokawain B on Skp2 degradation, but further increased the expression of p27. However, siRNA knock-down of Skp1, CSN5, and UBC12 expression resulted in down-regulation of Skp2 expression and further enhanced the effect of flavokawain B on Skp2 degradation. siRNA knock-down of Cdh1, a known E3 ligase of Skp2 for targeted degradation, also didn9t rescue the effect of flavokawain B on Skp2 degradation, but overexpression of F-box deleted Skp2 can reverse the effect of flavokawain B on Skp2 degradation. These results suggest that degradation of Skp2 by flavokawain B is involved in Cullin1. Furthermore, flavokawain B selectively inhibited the growth of pRb deficient cell lines and PC3 cells with Skp2 overexpression. Flavokawain B also acts synergistically with Bortezomib and MG132 to inhibit the growth of prostate cancer cell lines via down-regulation of Skp2 and up-regulation of p27 and p21 expression. These finding provide rationale for combination of flavokawain B and Bortezomib in treatment of prostate cancer. Citation Format: Xuesen Li, Victor Pham, Matthew Tippin, Shan Xu, Dongjun Fu, Liankun Song, Bang H. Hoang, Xiaolin Zi. Flavokawain B, a kava chalcone, acts synergistically with proteasome inhibitors (i.e., bortezomib and MG132) to reduce the growth of prostate cancer cells via downregulation of Skp2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 275.