Mechanism of autophagy induction and role of autophagy in antagonizing mitomycin C-induced cell apoptosis in silibinin treated human melanoma A375-S2 cells.

The aim of this study was to elucidate the molecular mechanisms mediating silibinin-induced autophagy in A375-S2 cells. In the present study it was found that silibinin-induced autophagy through increasing the conversion of LC3 I to LC3 II and up-regulating Beclin-1 expression, which was concomitant with p53 suppression and NF-κB activation. P53 inhibitor, pifithrin-α (PFT-α), increased autophagy and enhanced the expression of NF-κB. Moreover, inducing p53 accumulation with MG132 reduced autophagic ratio, and repressed the expression and activation of NF-κB expression. NF-κB inhibitor, pyrrolidine dithiocarbamate (PDTC) suppressed autophagy. Autophagic specific inhibitor 3-methyladenine (3-MA) treatment reversed silibinin-induced p53 suppression as well as NF-κB activation, suggesting that there was a positive feedback loop between p53 inhibition-mediated NF-κB activation and autophagy. In addition, we also found that 3-MA efficiently abrogated silibinin's cyto-protective effect against mitomycin C-induced cell death, and reversed the suppressive efficacy of silibinin on p53 expression, suggesting that autophagy contributed to silibinin's cyto-protective effect against mitomycin C-induced cell death in A375-S2 cells.