The Roles of GSK-3 in Regulation of Retinoid Signaling and Sorafenib Treatment Response in Hepatocellular Carcinoma

Rationale: Glycogen synthase kinase-3 beta (GSK-3 beta) plays key roles in metabolism and many cellular processes. It was recently demonstrated that overexpression of GSK-3 beta can confer tumor growth. However, the expression and function of GSK-3 beta in hepatocellular carcinoma (HCC) remain largely unexplored. This study is aimed at investigating the role and therapeutic target value of GSK-3 beta in HCC. Methods: We firstly clarified the expression of GSK-3 beta in human HCC samples. Given that deviated retinoid signalling is critical for HCC development, we studied whether GSK-3 beta could be involved in the regulation. Since sorafenib is currently used to treat HCC, the involvement of GSK-3 beta in sorafenib treatment response was determined. Co-immunoprecipitation, GST pull down, in vitro kinase assay, luciferase reporter and chromatin immunoprecipitation were used to explore the molecular mechanism. The biological readouts were examined with MTT, flow cytometry and animal experiments. Results: We demonstrated that GSK-3 beta is highly expressed in HCC and associated with shorter overall survival (OS). Overexpression of GSK-3 beta confers HCC cell colony formation and xenograft tumor growth. Tumor-associated GSK-3 beta is correlated with reduced expression of retinoic acid receptor-beta (RAR beta), which is caused by GSK-3 beta-mediated phosphorylation and heterodimerization abrogation of retinoid X receptor (RXR alpha) with RAR alpha on RAR beta promoter. Overexpression of functional GSK-3 beta impairs retinoid response and represses sorafenib anti-HCC effect. Inactivation of GSK-3 beta 3 by tideglusib can potentiate 9-cis-RA enhancement of sorafenib sensitivity (tumor inhibition from 48.3% to 93.4%). Efficient induction of RAR beta by tideglusib/9-cis-RA is required for enhanced therapeutic outcome of sorafenib, which effect is greatly inhibited by knocking down RAR beta. Conclusions: Our findings demonstrate that GSK-3 beta is a disruptor of retinoid signalling and a new resistant factor of sorafenib in HCC. Targeting GSK-3 beta may be a promising strategy for HCC treatment in clinic.