Abstract 5681: Flavokawain B down-regulates SKP2 through a proteasome-dependent and ubiquitin-independent pathway, and acts synergistically with Bortezomib on inhibition of Skp2 expression and prostate cancer cell growth

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC The E3 ubiquitin ligase SKP2 (S-phase kinase-associated protein 2), an oncogenic protein, was found to be overexpressed in 557 of 622 (89.2%) primary prostate cancer (PCa) samples and in 64 of 74 samples (86.4%) high-grade prostatic intra-epithelial neoplasia lesions. In addition, a transgenic mouse model with prostatic over-expression of SKP2 develops low-grade adenocarcinomas. These results suggest that SKP2 could be a potential target for PCa chemoprevention. In a search for SKP2 targeting agents for PCa chemoprevention, we found that Flavokawain B, a chalcone found in multiple edible plants- including roots of Alpinia speciosa (family Zingiberaceae) and Piper methysticum Forst (kava kava), potently down-regulates the protein expression of SKP2 in several prostate cancer cell lines (LNCaP, C4-2B, DU145 and PC3) at a concentration of 8.8 µM. More importantly, Flavokawain B reduces the cell viability of transfected PC3 cell line stably overexpressing SKP2 compared to vector control-transfected PC3 cells (IC50s are 7 and 26.4 µM, respectively), which suggest that flavokawain B may target SKP2 for its growth inhibitory effect. Flavokawain B treatment did not decrease the promoter activity and mRNA levels of SKP2. Instead, Flavokawain B decreases the protein levels of both a native and a myc-tagged SKP2 in PC3 cells through promoting its degradation. The degradation of the myc tagged SKP2 can be recovered by a proteasome inhibitor MG132. MG132 treatment resulted in an increase of ubquitinated SKP2, whereas Flavokawain B did not affect SKP2 ubquitination. These data suggest that the degradation of SKP2 by Flavokawain B may be through a proteasome-dependent and ubquitination-independent pathway. Interestingly, proteasome inhibitors (MG132 and Bortezomib) also down-regulate SKP2 mRNA expression through transcriptional inhibition, and combination of Bortezomib or MG132 with Flavokawain B exhibits a synergistic effect on SKP2 down-regulation and cell growth inhibition via induction of G2M arrest in PCa cell lines. Taken together, Flavokawain B is a novel SKP2 target agent and deserves further investigation for PCa chemoprevention and therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5681.