Oleanolic acid inhibits epithelial–mesenchymal transition of hepatocellular carcinoma by promoting iNOS dimerization

Oleanolic acid (OA) exhibits extensive pharmacological activities and takes significant anti-tumor effects. Its pharmacological mechanism, however, are still remained to be further clarified. In this study, we demonstrated that OA attenuated the migration and invasion abilities, resulting in the suppression of the EMT process in liver cancer cells, and inhibited the tumor growth of the PLC-bearing mice. We further proved that inducible nitric oxide synthase (iNOS) may be the potential target of OA. We confirmed that OA could promote the dimerization of iNOS, activating it and subsequently increasing the production of nitric oxide (NO). Further experiments indicated that OA promoted the nitration of specific proteins and consequently suppressed their epithelial–mesenchymal transition (EMT)-related biological functions. Furthermore, it has been confirmed that OA enhanced the anti-tumor effects of regorafenib in liver cancer treatment. These results deepened our understanding of the pharmacological mechanism of the antitumor effect OA, and the importance of NOS as a therapeutic target for liver cancer treatment. PLC-PRF-5 cells were treated with OA (50 μM) or DMSO for 24 h. TRIzol Reagent (Invitrogen) was then used to lyse cells and extract the total RNA.