Inhibition of MEK suppresses hepatocellular carcinoma growth through independent MYC and BIM regulation

Hepatocellular carcinoma (HCC) is an aggressive malignancy. In HCC, mitogen-activated protein kinase (MAPK) signaling is overactivated. The MAPK kinase (MEK) inhibitor trametinib has been approved to treat several types of advanced cancers with a BRAF mutation. Herein, we examined whether trametinib has efficacy against HCC. The effects of trametinib on cell viability, proliferation and tumor growth were assessed in HCC-derived cell lines and mouse xenograft models. Western blot analysis and immunohistochemistry were used to identify key regulators critical for HHC cell proliferation and tumor growth. We found that trametinib dose-dependently inhibited the viability and proliferation of HCC cells. We also found that a strong suppression of MEK by trametinib downregulated the pro-survival protein MYC, but upregulated the pro-apoptotic protein BIM. This dual differential regulation of MYC and BIM was found to be accompanied by upregulation of a MYC-targeted cyclin dependent kinase inhibitor, p27kip1 (p27), and an apoptosis marker, cleaved poly (ADP ribose) polymerase 1 (PARP), indicating a concurrent modulation of cell cycle- and apoptosis-related pathways. Importantly, we found that MYC overexpression did not block increased BIM in trametinib-treated HCC cells, indicating that MAPK signaling independently regulates MYC and BIM. Finally, we found that trametinib in vivo inhibited HepG2 xenograft tumor growth and attenuated tumor invasion into surrounding tissues. Consistent with the in vitro findings, MYC expression was found to be reduced, while p27 expression was found to be elevated, and BIM expression and cleaved PARP levels were found to be increased in trametinib-treated xenograft tumors. Collectively, our data indicate that trametinib exhibits efficacy in treating HCC cells via distinct regulation of the MYC and BIM pathways. As such, targeting MEK to block MAPK signaling with trametinib may provide novel treatment opportunities for HCC.