MEK inhibition by cobimetinib suppresses hepatocellular carcinoma and angiogenesis in vitro and in vivo.

Hepatocellular carcinoma which is featured with the extensive vascularization is the third most frequent cause of cancer-related deaths with limited therapeutic options, particularly for advanced disease. Cobimetinib, a MEK inhibitor, has been approved for the treatment of melanomas with a BRAF mutation. In this work, we investigated the efficacy of cobimetinib in sensitive and resistant HCC cells. Using a panel of HCC cell lines and normal hepatocellular cells as control, we showed that cobimetinib is active against HCC cells and spare normal hepatocellular cells. Cobimetinib at nanomolar concentration inhibited proliferation and induced apoptosis in sorafenib-resistant HCC cells (Hep3B-r), suggesting its ability to overcome HCC resistance to standard of care. This was further demonstrated by our results that cobimetinib significantly augmented the inhibitory effects of sorafenib and doxorubicin in HCC cells. Notably, cobimetinib dose-dependently inhibited tumor angiogenesis by inhibiting HCC endothelial cell (HCCEC) growth, survival and capillary network work formation. Cobimetinib suppressed ERK/RSK without affecting JNK or p38 signaling pathways in Hep3B-r and HCCEC cells. In addition, cobimetinib negatively influenced the apoptosis pathways by increasing pro-apoptotic protein Bim and decreasing anti-apoptotic proteins Mcl-1 and Bcl-2. In addition, we validated the in vitro findings in HCC xenograft mouse model and demonstrated that cobimetinib inhibited ERK signaling, promoted apoptosis, and was active against resistant HCC growth and angiogenesis in vivo, without causing significant toxicity in mice. Our findings support the clinical trials of cobimetinib for HCC treatment and highlight the therapeutic value of inhibiting MEK/ERK/RSK to overcome HCC resistance.