17β-Estradiol promotes metastasis in triple-negative breast cancer through the Calpain/YAP/β-catenin signaling axis.

β-catenin is an important regulator of malignant progression. 17β-Estradiol (E2), an important sex hormone in women, promotes the growth and metastasis of triple-negative breast cancer (TNBC). However, whether β-catenin is involved in E2-induced metastasis of TNBC remains unknown. In this study, we show that E2 induces the proliferation, migration, invasion, and metastasis of TNBC cells. E2 induces β-catenin protein expression and nuclear translocation, thereby regulating the expression of target genes such as Cyclin D1 and MMP-9. The inhibition of β-catenin reversed the E2-induced cell malignant behaviors. Additionally, E2 activated Calpain by increasing intracellular Ca2+ levels and reducing calpastatin levels. When Calpain was inhibited, E2 did not induce the proliferation, migration, invasion, or metastasis of TNBC cells. In addition, E2 promoted translocation of YAP into the nucleus by inhibiting its phosphorylation. Calpain inhibition reversed the E2-induced YAP dephosphorylation. Inhibition of YAP transcriptional activity reversed the effects of E2 on the proliferation, migration, invasion, and β-catenin of TNBC cells. In conclusion, we demonstrated that E2 induced metastasis-related behaviors in TNBC cells and this effect was mediated through the Calpain/YAP/β-catenin signaling pathway.