Two unique 5' untranslated regions in mRNAs encoding human 14-3-3 zeta: differential expression in hemopoietic cells

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 (serine³¹ was mutated into alanine³¹ or aspartic acid³¹), we found that the phosphorylated serine³¹ with its near amino acid residues constituted a RPLphosphoS³¹GP (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.

Reduction of 14-3-3 proteins correlates with increased sensitivity to killing of human lung cancer cells by ionizing radiation

The 14-3-3 proteins have a wide range of ligands and are involved in a variety of biological pathways. Importantly, 14-3-3 proteins are known to be overexpressed in some human lung cancers, suggesting that they may play a role in tumorigenesis. Here we examined 14-3-3 expression in several lung cancer-derived cell lines and found that four of the seven 14-3-3 isoforms, beta, epsilon, theta and zeta, were highly expressed in both lung cancer cell lines and normal lung fibroblasts. Two isoforms, sigma and gamma, were present only at very low levels. Immunoprecipitation data showed 14-3-3zeta could bind to CDC25C in irradiated A549 cells, and suppression of 14-3-3zeta in A549 cells with antisense resulted in a decrease in CDC25C localization in cytoplasm and CDC2 phosphorylation on Tyr15. As a consequence, CDC2 activity remained elevated which resulted in release from radiation-induced G(2)/M-phase arrest. Moreover, 16% 14-3-3zeta antisense-transfected cells underwent apoptosis when exposed to 10 Gy ionizing radiation. These data indicate that 14-3-3zeta is involved in G(2) checkpoint activation and that inhibition of 14-3-3 may be a useful approach to sensitize human lung cancers to ionizing radiation.