14-3-3ζ Binds to Hepatitis B Virus Protein X and Maintains Its Protein Stability in Hepatocellular Carcinoma Cells

14-3-3ζ, a phosphopeptide-binding molecule, is reportedly overexpressed in the cancerous tissues of patients with hepatocellular carcinoma (HCC). Hepatitis B virus (HBV) protein X (HBx) draws intensive attention in HBV-related HCC because it not only regulates HBV replication, but also promotes carcinogenesis by interacting with various tumor or antitumor molecules. This study is performed to investigate whether and how 14-3-3ζ interacts with HBx. The coimmunoprecipitation (Co-IP) results showed that 14-3-3ζ bond to HBx in HBV-infected Hep3B HCC cells and CSQT-2 portal vein tumor thrombosis (PVTT) cells. By performing Co-IP assay in HBV-free Huh7 cells expressing wild-type HBx, mutant HBx-S31A, or HBx-S31D (serine was mutated into alanine or aspartic acid ), we found that the phosphorylated serine with its near amino acid residues constituted a RPLphosphoS GP (R, arginine; P, proline; L, leucine; S, serine; G, glycine) motif in HBx for 14-3-3ζ docking. This 14-3-3ζ-HBx interaction was partly impaired when Akt signaling transduction was blocked by LY294002. Furthermore, 14-3-3ζ silencing augmented HBx ubiquitination and decreased its expression in cancer cells and xenograft tumor. The migratory and invasive abilities of CSQT-2 cells were inhibited upon 14-3-3ζ silencing, whereas partly restored by HBx overexpression. Additionally, 14-3-3ζ positively correlated with HBx to be overexpressed in the primary HCC tissues (r = 0.344) and metastatic PVTT (r = 0.348). In summary, findings of this study reveal a novel 14-3-3ζ-HBx interaction in HCC cells and suggest 14-3-3ζ as a candidate target for treating HBV-related HCC.