Background: The survival benefit of sorafenib, the most used drug for advanced hepatocellular carcinoma (HCC), is unsatisfactory due to the development of adaptive resistance. Exploring the mechanisms underlying sorafenib resistance is important to develop sensitizing strategy. Sphingomyelin synthase (SMS) plays a critical role in sphingolipid metabolism which is involved in oncogenesis and drug resistance.
Methods: SMS1 and SMS2 levels in HCC cells in response to prolonged chemotherapy were analyzed using ELISA. mRNA and protein levels of SMS in HCC and adjacent normal tissues were analyzed by ELISA and real-time PCR. The roles of SMS and its downstream targets were investigated using cellular and biochemical assays and mass spectrometry.
Results: SMS1, but not SMS2, was upregulated in HCC in response to sorafenib treatment, although HCC displayed similar RNA and protein level of SMS1 compared to adjacent normal liver tissues. Overexpression of SMS1 promoted HCC growth and migration, and alleviated sorafenib's toxicity. SMS1 inhibition via genetic and pharmacological approaches consistently resulted in inhibition of growth and migration, and apoptosis induction in sorafenib-resistance HCC cells. SMS1 inhibition also augmented the efficacy of sorafenib in sensitive HCC cells. SMS1 inhibition disrupted sphingolipid metabolism via accumulating ceramide and decreasing sphingomyelin, inducing mitochondrial dysfunction and oxidative stress, and decreasing Ras activity in resistant cells. Overexpression of constitutively active Ras reversed the inhibitory effects of SMS1 inhibition. Although SMS1 overexpression did not affect Ras expression and activity, Pearson correlation coefficient analysis of SMS1 and Ras expression demonstrated that there was positive correlation between SMS1 and RAS (NRAS, R = 0.55, p < 0.01; KRAS, R = 0.44, p < 0.01).
Conclusions: Our work is the first to suggest that SMS1 plays a more important role in sorafenib resistance than tumorigenesis, and provides preclinical evidence to overcome sorafenib resistance with SMS1 inhibition in HCC.
Keywords: Hepatocellular carcinoma; Ras; SMS1; Sorafenib resistance; Sphingolipid.