Tanshinone IIA induces apoptosis via inhibition of Wnt/β-catenin/MGMT signaling in AtT-20 cells

A strategy to suppress the expression of the DNA repair enzyme O(6)-methylguanine-DNA methyltransferase (MGMT) by inhibition of Wnt/β-catenin signaling may be useful as a novel treatment for pituitary adenoma. Previous studies have reported that Tanshinone IIA (TSA), a major quinone compound isolated from Salvia miltiorrhiza, had antitumor effects. However, whether TSA has antitumor effects against pituitary adenoma and whether the mechanisms are associated with the Wnt/β-catenin/MGMT pathway remains to be clarified. In the present study, TSA treatment caused apoptosis in AtT-20 cells in a concentration-dependent manner, as demonstrated by cell viability reduction, phophatidylserine externalization detected by Annexin V staining and mitochondrial membrane potential disruption detected by JC-1 staining, which were associated with activation of caspase-3 and DNA fragmentation detected by TUNEL in AtT-20 cells. T-cell factor (TCF)-lymphoid-enhancing factor (LEF) reporter activity was determined by dual luciferase reporter assay and the interaction between β-catenin and TCF-4 were detected using a co-immunoprecipitation kit. The results indicated TSA treatment increased β-catenin phosphorylation, inhibited β-catenin nuclear translocation, reduced β-catenin/TCF-4 complex formation and TCF-LEF luciferase reporter activity, and subsequently reduced the expression of cyclin D1 and MGMT. Notably, overexpression of MGMT in β-catenin knock down AtT-20 cells abrogated the TSA-mediated effects in AtT-20 cells. In conclusion, TSA induced apoptosis via inhibition of Wnt/β-catenin-dependent MGMT expression, which may provide novel insights into the understanding of the mechanism of the antitumor effects of Salvia miltiorrhiza.