Silymarin as an herbal medicine has shown anticancer effects on tumor cells, while having low toxicity in normal cells. In this study, the effects of Silymarin on proliferation and apoptosis of colorectal cancer cells and its impact on immune response against cancer cells were evaluated in vitro and in vivo.

The effect of Silymarin on CT-26 and Caco-2 cells proliferation and apoptosis were demonstrated by MTT assay and PI staining. A subcutaneous tumor of colorectal cancer was developed. Silymarin and Doxorubicin were administrated by intravenous injection. qRT-PCR analyses was performed on blood samples and tumor tissues. Spleen tissue was used to evaluate CD8+ T cell immune responses. Histological study was carried out on tumor tissues.

Silymarin showed anti-proliferative effects on CT-26 and Caco-2 cells. The markers of immunogenic cell death (Calreticulin exposure, ATP secretion, and HMGB1 secretion) significantly increased in both cell lines in the presence of silymarin. The expression of genes related to cell proliferation particularly β-Catenin and Cycline D1, and also anti-apoptotic ones such as Bcl-2 significantly reduced in mice treated with Silymarin while the expression of pro-apoptotic Bax increased. The RNA level of PD-L1 decreased in tumor tissues exposed by Silymarin. Moreover, the number of CTLs increased in the spleen of mice treated with Silymarin in comparison with untreated mice. Decreased tumor size and also survival of colorectal cancer cells in Silymarin-treated mice were observed in histological analysis.

Silymarin treatment showed a suppressive role on colorectal cancer cells almost as much as Doxorubicin. Our study indicated that having a low toxicity profile, cost-effectiveness, and availability of raw materials, plant-derived Silymarin can be a good candidate for further investigation to treat CRC.