**5. Anti-oxidation effect of selenium in HCC**

Oxidative stress is characterized by the excessive production of oxidants or ROS and insufficient elimination by protective antioxidants [40]. This persistent imbalance may lead to somatic mutations and neoplastic transformation [40]. In cancer, ROS is commonly found elevated and is a key constituent to cancer survival [61, 62]. Most of the effects of dietary selenium on oxidation are attributable to the insertion of this element to selenoproteins, mostly its cofactor glutathione peroxidase. In the active oxidative metabolism, selenium-dependent glutathione peroxidase acts with tripeptide glutathione (GSH) and competes for the catalyzation for hydrogen peroxide as a substrate, scavenging reactive oxygen species (ROS) and lipid peroxide (LPO) [63]. In 2007, a study by Katzenellenbogen et al. investigated the effect of selenomethionine on oxidative stress and inflammation or lipid metabolism with cDNA microarrays in HCC development in Mdr2 knockout mice model [47]. The results showed that selenomethionine alters the expression of commonly upregulated genes found in response to inflammation, oxidative stress, and cancer [47]. Further, the inhibitory effect on gene expression of selenomethionine is positively correlated with its role to reduce the incidence of large tumors [47]. These results provide a theoretical basis for the anticancer mechanism of selenium.

### **6. Anti-metastasis effect of selenium in HCC**

HCC is characterized by its invasive and metastatic potential [64]. In liver cancer, it has been suggested that SBP1, a Se-containing protein, and its primary function is Se transport, plays a role in metastasis and if found to be highly expressed in healthy liver tissue but was nearly non-detectable in highly metastatic liver cancer cell lines [65]. Epithelial-mesenchymal transition (EMT) is a process that plays a vital role in HCC metastasis cascade and had been suggested to be closely related to the initiation of HIF-1α [64]. SBP-1 is a downstream target of HIF-1α and has been found that loss of SBP1 promoted liver cancer cell migration and increased GPX1 activity, which further suppresses in hydrogen peroxide and other reactive oxygen species, leading to the inhibition of HIF-1α [64]. Recently study by Gao et al. identified 186 differentially expressed genes among control and SBP1 expressing HCC cells [66]. Further investigation showed C-X-C motif chemokine receptor 4 (CXCR4) expression was inhibited by SBP1 and is closely related to the migration and invasion ability of HCC cells through activation of AKT signaling [66]. These results suggested the potential application of selenium in liver cancer metastasis prevention and treatment; however, more data from *in vitro* and *in vivo* are warranted to solidify effect and mechanism of action.
