Downregulation of Raf-1 kinase inhibitory protein as a sorafenib resistance mechanism in hepatocellular carcinoma cell lines.

325 Background: Despite enhancing overall survival, sorafenib resistance usually occurs and is a significant limiting factor of improved prognosis in hepatocellular carcinoma (HCC) patients. Therefore, it is important to identify the mechanism of sorafenib resistance. This study is designed to identify that causative factor for sorafenib resistance and to suggest way for overcoming sorafenib resistance. Methods: The sensitivity of sorafenib was compared in human HCC lines and patient-derived HCC primary cells. Based on its cytotoxicity, analysis was performed to examine the altered signaling pathway by sorafenib and its causative factor. It was further investigated that how sorafenib modify the sorafenib resistant inducer through gene or protein expression or stability. We also design the treatment option to overcome sorafenib resistance. Results: Sorafenib activates the Raf/MEK/ERK pathway, causing sorafenib resistance in HCC cell lines and patient-derived HCC primary cells. Reactivation of the MAPK pathway by sorafenib occurs via down-regulation of RKIP at a post-translational level. Knockdown of RKIP increased phosphorylated ERK and consequently suppressed sorafenib-mediated cell death. We also found that sorafenib-reactivated ERK maybe an attractive target for second-line therapy in patients with sorafenib resistance. Sequential combination of sorafenib and PD98059 significantly reduced cell viability and proliferation, while increasing apoptosis efficacy in sorafenib-resistant cells. Conclusions: Our findings suggest that reactivation of the Raf/MEK/ERK pathway through aberrant expression of RKIP is one of the mechanisms for sorafenib resistance in HCC. Sequential combination treatment of sorafenib and PD98059 could provide a new strategy to overcome sorafenib resistance in future clinical studies.