Simple SummaryHepatocellular carcinoma (HCC) represents the fourth leading cause of cancer-related death in the world. Sorafenib, a first-line chemotherapeutic drug that inhibits cell growth and angiogenesis, has become the standard therapy for inoperable advanced HCC. However, while sorafenib improves survival and is cost-effective in advanced HCC patients, its efficacy is severely restricted by the development of drug resistance. As a result, more research is needed to investigate the underlying molecular process of sorafenib resistance and to identify novel molecular targets. This study aimed to determine the mechanisms underlying sorafenib resistance. Herein, we found activation of RBM38 significantly reverses the resistance of HCC cells to sorafenib by restoring the expression of the lncRNA GAS5, indicating that the RBM38–GAS5 signaling pathway is involved in sorafenib resistance. Therefore, RBM38 and the GAS5 may serve as promising therapeutic targets for enhancing the responsiveness of patients with advanced HCC to sorafenib.AbstractBackground: Hepatocellular carcinoma (HCC) is a life-threatening human malignancy and the fourth leading cause of cancer-related deaths worldwide. Patients with HCC are often diagnosed at an advanced stage with a poor prognosis. Sorafenib is a multikinase inhibitor used as the first-line treatment for patients with advanced HCC. However, acquired resistance to sorafenib in HCC leads to tumor aggression and limits the drug’s survival benefits; the underlying molecular mechanisms for this resistance remain unclear. Methods: This study aimed to examine the role of the tumor suppressor RBM38 in HCC, and its potential to reverse sorafenib resistance. In addition, the molecular mechanisms underlying the binding of RBM38 and the lncRNA GAS5 were examined. The potential involvement of RBM38 in sorafenib resistance was examined using both in vitro and in vivo models. Functional assays were performed to assess whether RBM38: binds to and promotes the stability of the lncRNA GAS5; reverses the resistance of HCC to sorafenib in vitro; and suppresses the tumorigenicity of sorafenib-resistant HCC cells in vivo. Results: RBM38 expression was lower in HCC cells. The IC50value of sorafenib was significantly lower in cells with RBM38 overexpression than in control cells. RBM38 overexpression improved sorafenib sensitivity in ectopic transplanted tumors and suppressed the growth rate of tumor cells. RBM38 could bind to and stabilize GAS5 in sorafenib-resistant HCC cells. In addition, functional assays revealed that RBM38 reversed sorafenib resistance both in vivo and in vitro in a GAS5-dependent manner. Conclusions: RBM38 is a novel therapeutic target that can reverse sorafenib resistance in HCC by combining and promoting the lncRNA GAS5.