Fig. 6: The underlying mechanism of PP in regulating β-catenin.

A The protein levels of AKT, p-AKT, GSK3β, p-GSK3β (S9), p-GSK3β (Y216), p-β-catenin (S552), and p-β-catenin (S33/37/T41) were evaluated through western blotting following PP treatment at different contents. B Effect of pretreatment with GSK3β inhibitor CHIR-99021 on PP-induced β-catenin down-regulation (top panel). Densitometric analysis of β-catenin/β-actin expression fold change (bottom panel). C Schematic diagram for the underlying mechanism by which PP regulated β-catenin. PP inhibits the activation of AKT, thereby increasing the activity of GSK3β, leading to increased degradation of β-catenin by multi-protein destruction complex, which mainly includes Axin, APC, and GSK-3β. β-catenin has at least two distinct pools. One is located in cytoplasm which binds with E-cadherin, and the other one is involved in the Wnt signaling pathway so that shuttles between cytoplasm and nucleus [45]. AKT mediates the phosphorylation of β-catenin associated with plasma membrane, and then induces the dissociation of β-catenin from E-cadherin, which accumulates in cytoplasm and nucleus [30]. In the case of PP addition, PP reduces the p-AKT protein level and further down-regulates p-β-catenin (S552) expression in the cells. Consequently, dissociation of β-catenin from E-cadherin within cell membrane is reduced, so that the combination of β-catenin with the LEF/TCF transcription factors in cell nucleus is reduced, either, which hinders the activation of MGMT gene. *p < 0.05; **p < 0.01; ***p < 0.005.