Colorectal cancer (CRC) is the second leading cause of cancer-related deaths. CRC is largely considered as a preventable malignancy because it has a long latent period of growth and development from initial pre-malignant lesions to clinically evident cancer. Chemopreventive strategies can thus have considerable impact on altering the course of this disease. One of the most frequently observed genetic defects in CRC is deregulated beta-catenin levels and consequently aberrant activation of Wnt/beta-catenin target genes due to mutations in the adenomatous polyposis gene (APC) gene or beta-catenin itself, suggesting that targeting beta-catenin pathway could be a potential translational approach to improve the clinical outcome of CRC. In the present study, we investigated the effect of silibinin, a flavonoid isolated from the seeds of milk thistle extract (Silybum marianum) that is widely consumed as a dietary supplement for strong anti-hepatotoxicity activity, on human colon cancer SW480 cells that harbor mutation in APC. We observed that silibinin treatment at 50-200 \#956;M results in a concentration- and time-dependent inhibition in cell growth. Specifically, silibinin treated cells showed 24-63% inhibition in cell growth after 24 h, 58-80% inhibition after 48 h and 76-90% inhibition after 72 h at 50, 100 and 200 µM concentrations. In addition, a significant increase in dead cell population was also observed but only at the highest concentration of 200 µM at all the three treatment times. Annexin V-PI staining followed by FACS analysis revealed that cell death observed at 200 \#956;M was apoptotic in nature. Additional studies showed that inhibition in cell growth was associated with cell cycle arrest at S-phase. At molecular level, the growth inhibitory effects of silibinin treatment were associated with increased expression of p27, but decreased expression of cyclin A, D1, B and their partners Cdk-2, -4, -6 and phospho- as well as total cdc-2. In addition to causing cell cycle arrest, silibinin treatment also resulted in concentration- and time-dependent decrease in the nuclear as well as total levels of beta-catenin. Beta-catenin dependent TCF-4 transcriptional activity as measured by luciferase reporter construct TOPflash was also found to be inhibited significantly in SW480 cells by silibinin. Consistent with these observations, the expression of downstream beta-catenin target genes such as c-myc and cyclin D1 was also decreased following silibinin treatment. From these results, it is clearly evident that silibinin targets beta-catenin dependent signaling to inhibit the growth of these cells. Together, these findings suggest that silibinin inhibits the proliferation of SW480 cells by multiple mechanisms, and thus it could be an effective chemopreventive agent against CRC carrying APC gene mutation.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 962.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO