Cyclin E1 Inhibition can Overcome Sorafenib Resistance in Hepatocellular Carcinoma Cells Through Mcl-1 Suppression

Purpose: To clarify the effects of cyclin E1 suppression on antitumor efficacy of sorafenib in hepatocellular carcinoma cells and to explore the potential of combining sorafenib with cyclin-dependent kinase (CDK) inhibition in therapy. Experimental Design: The effects of cyclin E1 suppression on sorafenib-induced apoptosis were tested in both sorafenib-sensitive (Huh-7 and HepG2, IC50 5–6 μmol/L) and sorafenib-resistant (Huh-7R and HepG2R, IC50 14–15 μmol/L) hepatocellular carcinoma cells. The activity of pertinent signaling pathways and the expression of cell cycle and apoptosis-related proteins were measured using Western blotting. Efficacy of sorafenib combined with the pan-CDK inhibitor flavopiridol was tested both in vitro and in xenograft experiments. The pertinent downstream mediators of antitumor efficacy were tested in transient transfection and RNA interference experiments. Results: Cyclin E1 mRNA and protein expressions were suppressed after sorafenib treatment in sorafenib-sensitive but not in sorafenib-resistant hepatocellular carcinoma cells. Changes in cyclin E2 or D1 were not correlated with sorafenib sensitivity. The knockdown of cyclin E1 expression reversed the resistance of hepatocellular carcinoma cells to sorafenib in terms of cell growth and apoptosis induction, whereas the overexpression of cyclin E1 increased the resistance to sorafenib. The growth-inhibitory and apoptosis-inducing effects of sorafenib were enhanced by flavopiridol, and Mcl-1 suppression was determined to play a critical role in mediating this enhancing effect. Conclusions: The cyclin E1 suppression in hepatocellular carcinoma cells may serve as a pharmacodynamic biomarker for predicting sorafenib efficacy. The combination of sorafenib and CDK inhibitors may improve the efficacy of sorafenib in hepatocellular carcinoma. Clin Cancer Res; 22(10); 2555–64. ©2015 AACR.