Identification of dihydrotanshinone I as an ERp57 inhibitor with anti-breast cancer properties via the UPR pathway.

Salvia miltiorrhiza (Danshen) is a well-known traditional Chinese medicine for treating various diseases, such as breast cancer. However, knowledge regarding its mechanisms is scant. Herein, the active ingredient dihydrotanshinone I (DHT) in Salvia miltiorrhiza extract (SME), which binds ERp57 was identified and verified by an enzymatic solid-phase method combined with LC-MS/MS. DHT potentially inhibited ERp57 activity and suppressed ERp57 expression at both the RNA and protein levels. Molecular docking simulation indicated that DHT could form a hydrogen bond with catalytic site of ERp57. Moreover, ERp57 overexpression decreased DHT-induced cytotoxicity in MDA-MB-231 cells. Thereafter, the signaling pathway downstream of ERp57 was investigated by Western blot analysis. The mechanistic study revealed that DHT treatment resulted in activation of endoplasmic reticulum (ER) stress, the unfolded protein response (UPR), and cellular apoptosis. In conclusion, our data implied that DHT targeted ERp57 for inhibition and induced ER stress and UPR activation, which in turn triggered breast cancer cell apoptosis.