The roles of Akt and MAPK family members in silymarin's protection against UV-induced A375-S2 cell apoptosis

Silymarin is a polyphenolic flavonoid derived from milk thistle (Silybum marianum) and has anti-inflammatory, cytoprotective as well as anticarcinogenic effects [Manna, S.K., Mukhopadlhyay, A., Van, N.T., Aggarwal, B., Silymarin suppresses TNF-induced activation of NF-kappaB, c-Jun N-terminal kinase, and apoptosis. J. Immunol. 1999; 163, 6800-6809.]. In this study, we assessed the effect of silymarin on ultraviolet light (UV)-induced cell apoptosis in human malignant melanoma, A375-S2 cells. Silymarin pre-treatment reversed the effect of UV irradiation on the expression of phosphorylated Akt and phosphorylated p53 (regulated by Akt activation), followed by down-regulation of Bax and up-regulated expressions of Bcl-2 and Bcl-xL proteins in UV-irradiated A375-S2 cells. Akt inhibitor decreased the viability of UV-irradiated cells which was treated with silymarin. In addition, the effect of UV irradiation on the phosphorylation of mitogen-activated protein kinase (MAPK) family members [extracellular signal-regulated kinase (ERK), p38 and c-Jun N-terminal kinase (JNK)] was also reversed by silymarin. Moreover, ERK inhibitor (PD98059) and p38 inhibitor (SB203580) augmented UV-induced apoptosis in silymarin treated A375-S2 cells. Consequently, silymarin partially reduced UV-induced apoptosis by activating the Akt pathway, and silymarin's protective effect was also exerted by MAPK family members.