8 Hepatitis B virus X protein (HBx), a 17.5-kDa nonstructural protein encoded by the X gene, is

a key multifunctional regulatory protein that may participate in viral pathogenesis and carcinogenesis.9 Investigations on tumor samples from HCC patients, cell culture studies in vitro, and transgenic Talazoparib datasheet animal model experiments collectively support the oncogenic role of HBx in HBV-associated hepatocarcinogenesis.10-14 Notch signaling defines an evolutionary conserved local cell interaction mechanism that participates in a variety of cellular processes that involve cell fate determination, differentiation, proliferation, apoptosis, adhesion, epithelial-mesenchymal transition, migration, and angiogenesis.15 In mammals, four Notch molecules (Notch1, Notch2, Notch3, and Notch4) are single-pass, heterodimeric transmembrane proteins that serve as receptors for the five ligands (Jagged-1, Jagged-2, DLL-1, DLL-3, and DLL-4) expressed on the neighboring cells.16 Ligand binding renders the Notch receptor susceptible to two consecutive proteolytic cleavages mediated by tumor necrosis factor-α–converting enzyme (TACE)

and γ-secretase, respectively, which in turn results in the release of truncated constitutively Notch intracellular domain, the active form of the Notch receptor, from the plasma membrane to translocate into the nucleus, leading to transcription of its downstream target genes such as Hes and Herp.17, 18 Experimental data suggesting a tumor suppressive function of Notch1 signaling NVP-BEZ235 in vitro in hepatocarcinogenesis are increasing.19-23 Although the respective oncogenic acetylcholine role of HBx and tumor suppressive function of Notch1 in hepatocarcinogenesis have been studied, the interaction between them remains poorly understood. In this study, we tested the hypothesis of whether HBx could promote HBV-associated hepatocarcinogenesis through inhibition of Notch1 signaling activity. We show for the first

time that HBx expression suppressed Notch1 signaling by decreasing Notch1 cleavage, which contributed to overcoming senescence-like growth arrest of HCC cells in vitro and in vivo. This may serve as an important molecular mechanism for HBV-associated hepatocarcinogenesis. BrdU, 5-bromo-2′-deoxyuridine; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; HBV, hepatitis B virus; HBx, hepatitis B virus X protein; HCC, hepatocellular carcinoma; ICN1, Notch1 intracellular domain; mRNA, messenger RNA; NEXT1, extracellular truncated Notch1; Notch1-FL, full-length Notch1; qRT-PCR, quantitative real-time polymerase chain reaction; Psen1, presenilin1; Psen2, presenilin2; SA-β-gal, senescence-associated β-galactosidase; SD, standard deviation; TACE, tumor necrosis factor-α–converting enzyme.