ER-mediated anti-tumor effects of shikonin on breast cancer

Estrogen receptor (ER) is expressed in most Breast cancer (BC) patients. G protein-coupled estrogen receptor (GPER), which is a membrane-bound estrogen receptor, is associated with the tumor development and progression in BC. Shikonin (SK) is a natural compound that is known to have anti-tumor effects. This study aims to assess the effects of shikonin on the cell proliferation, cell cycle and cell apoptosis of BC and whether the effects are related to ER/GPER signaling pathway. The results demonstrated that shikonin inhibited the cellular proliferation of MCF-7 BC cells via G₀/G₁ arrest and apoptosis in concentration-dependent manner. The anti-proliferative effect of SK on SK-BR-3 BC cells was associated with apoptosis. Both ERα and GPER were expressed in MCF-7 cells, while ERα were negative and GPER were positive in SK-BR-3 cells. Furthermore, shikonin downregulated the expression of ERα and GPER, and this effect was not affected by the estrogen environment. In addition, shikonin downregulated the EGFR and p-ERK expression in MCF-7 and SK-BR-3, which was also not affected by the estrogen environment. EGFR and p-ERK were still suppressed by co-treatment with the selective GPER against G1 or antagonist G15. In conclusion, these results suggest that shikonin shows anti-tumor effects on MCF-7 and SK-BR-3 cells. The effects seem to be associated with EGFR/p-ERK downregulation via ERα and GPER inhibition.