Silymarin inhibits cell cycle progression and mTOR activity in activated human T cells: therapeutic implications for autoimmune diseases.

Silymarin, a complex flavonolignan from the 'milk thistle' (Silybum marianum) plant, exhibits anticarcinogenic, anti-inflammatory and cytoprotective effects. Several reports have demonstrated immunosuppressive activity of silymarin; however, the molecular mechanisms involved in immunomodulatory activities of silymarin are not fully understood yet. In this study, the effect of silymarin on cell cycle and PI3K/Akt/mTOR signalling pathway of activated T lymphocytes was investigated in vitro. Peripheral blood mononuclear cells (PBMC) were isolated from healthy volunteers and activated with anti-CD3 (5 μg/ml) and anti-CD28 (2 μg/ml) monoclonal antibodies. Cells were cultured in Complete RPMI medium with 10, 50 and 100 μM silymarin or dimethyl sulphoxide (DMSO) and incubated for 24-96 hrs. Cell cycle analysis was performed by PI staining and flow cytometry. The effect of silymarin on PI3K/AKT signalling pathway and mTOR activity was determined in activated T cells after 72-hr incubation with silymarin or DMSO. A significant G1 arrest in cell cycle of activated T lymphocytes was found after 96-hr incubation with 100 μM silymarin without causing cell death. Silymarin also significantly inhibited the level of phospho-S6 ribosomal protein and mTOR activity in cell lysates of activated T cells after 72-hr incubation in comparison with DMSO. This study shows that silymarin inhibits cell proliferation through G1 cell cycle arrest and also through the suppression of the mTOR signalling pathway in human activated T lymphocytes in vitro. Characterizing molecular mechanism of such immunomodulatory effects may have a great potential in future practical application of silymarin in transplantation and auoimmunity.