Abstract 5692: Anti-inflammatory effects of silibinin in colorectal cancer: Targeting the activation of NF-κB signaling
Authors: Agarwal Chapla , Raina Komal , Agarwal Rajesh 
Published: February 2011

Chronic inflammation is one of the primary causes of colorectal cancer (CRC) owing to increased CRC risk with longer duration of inflammatory bowel diseases and other inflammatory responses. Anti-inflammatory drugs are, thus, the drugs of choice to prevent CRC; however, there are several side effects associated with their long term use suggesting that more efforts are needed to identify non-toxic agents possibly from dietary/non-dietary sources that can be used to prevent/intervene CRC. Major inflammatory pathways implicated in CRC are COX2 and iNOS; both regulated by activated NF-κB pathway, suggesting that agents inhibiting these pathways could be ideal against CRC. Silibinin, a polyphenolic flavonolignan isolated from the seeds of milk thistle (Silybum marianum), is one such agent that has shown promising anti-cancer efficacy in several epithelial cancers and is devoid of any toxicity in both rodents and humans. In terms of its chemopreventive efficacy in CRC, recent studies by us have shown that silibinin feeding inhibits both chemically-induced and spontaneous tumorigenesis in rodent models, which in part was associated with decreased COX2 and iNOS levels. Accordingly, here we assessed whether silibinin targets NF-κB activation and associated signaling as a mechanism of its anti-inflammatory and anti-cancer effects in CRC. Treatment of human CRC HT29, LoVo and SW40 cells with silibinin strongly inhibited TNFα-induced NF-κB activation together with decreased nuclear levels of both p65 and p50 sub-units concomitant with increased cytosolic levels. Additional mechanistic studies in LoVo and SW480 cells showed that silibinin treatment significantly increases IκBα level with a concomitant decrease in phospho-IκBα, indicating inhibitory effect of silibinin on IKKα kinase activity. Next we assessed the effect of 6 weeks of oral silibinin feeding on NF-κB pathway in SW480 (COX-2 negative) and LoVo (COX-2 positive) tumor xenografts in nude mice. Together with its inhibitory efficacy on tumor growth and progression, silibinin also inhibited NF-κB activation in xenografts from both the cell lines as determined by IHC and EMSA analyses. Since NF-κB is an inducible transcription factor regulating transcription of diverse array of genes involved in proliferation, apoptosis, inflammation, angiogenesis and metastasis; we also assessed silibinin effect on various NF-κB-regulated molecules involved in these processes. The protein levels of cyclin D1, Bcl-2, COX2, iNOS, VEGF and MMPs were decreased by silibinin in both cell culture studies and xenograft analyses, further suggesting its inhibitory effect on NF-κB transcriptional activity. Together, these findings are both novel and highly significant in establishing for the first time that silibinin suppresses CRC growth and progression by interfering with NF-κB activation and thus has potential against human CRC (NCI RO1 grant CA112304).

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 5692.