Abstract 2815: Sylvestrol inhibits protein synthesis by downregulating the AKT/mTOR pathway

Abstract

Abstract Silvestrol, a cyclopenta[b]benzofuran lignan present in the fruits and twigs of the tropical tree Aglaia foveolata Pannell (Meliaceae), is toxic to human cancer cell lines in culture with a potency similar to those of FDA approved anticancer chemotherapeutic agents. We report here that silvestrol has a mean GI50 of approximately 3 nM in the 60 cell-line panel of human cancer cell lines maintained at the National Cancer Institute. Furthermore, silvestrol is at least as effective as FDA-approved anticancer agents in the treatment of human cancer cells in vivo when tested in the hollow fiber assay or the inhibition of human tumor xenograft tumors propagated in immunedeficient mice. Others as well as our group have reported that silvestrol inhibits protein biosynthesis specifically without affecting the cellular production of either RNA or DNA. Pelletier and colleagues have shown that silvestrol impairs the ribosome recruitment step of translation initiation by affecting the composition of the eukaryotic initiation factor (eIF) 4F complex. In the present study, we test the hypothesis that disruption of protein synthesis by silvestrol can impact cell signaling pathways upstream of translation. Human colon cancer cells designated HT-29 were exposed to silvestrol at concentrations as high as 100 nM for as long as 48 h prior to harvest and evaluation by immunoblot. Protein signaling molecules known to regulate the protein synthesis machinery were evaluated for their abundance and state of phosphorylation as a function of silvestrol exposure. We observed down-regulation of the p110α catalytic subunit of PI3K, which phosphorylates PIP2 to PIP3. PIP3 then recruits PDK1 and AKT to the plasma membrane, where PDK1 can phosphorylate AKT. A decrease in the amount of PDK1 protein and a significant decrease in phosphorylation of AKT at ser 473 occurred. Moreover, a decrease in both the abundance and phosphorylation of mTOR was evident, which is a key component of mTORC2. Consistent with these findings, phosphorylation of AKT at thr 308 was significantly reduced. Finally, silvestrol exposure resulted in a decrease in phosphorylation of the S6 ribosomal protein serine 235/236. These data indicate that silvestrol treatment can disrupt key signaling pathways leading to protein synthesis production in human cancer cells. This work was supported by NIH grants U01 CA52956 and P01 CA125066. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2815. doi:1538-7445.AM2012-2815