One of the serious sequelae of chronic hepatitis B virus (HBV) infection is hepatocellular carcinoma (HCC). Among all the proteins encoded by the HBV genome, hepatitis B virus X protein (HBx) is highly associated with the development of HCC. Although Notch1 signaling has been found to exert a tumor-suppressive function during HCC development, the mechanism of interaction between HBx expression and Notch1 signaling needs to be explored. In this study, we report that HBx expression in hepatic and hepatoma cells resulted in decreased endogenous protein levels of Notch1 intracellular domain (ICN1) and messenger RNA levels of its downstream target genes. These effects were due to a reduction of Notch1 cleavage by HBx through the suppression of presenilin1 (Psen1) transcription rather than inhibition of Notch1 transcription or its ligands' expression. Through transient HBx expression, decreased ICN1 resulted in enhanced cell proliferation, induced G1-S cell cycle progression, and blunted cellular senescence in vitro. Furthermore, the effect of blunted senescence-like growth arrest by stable HBx expression through suppression of ICN1 was shown in a nude mouse xenograft transplantation model. The correlation of inhibited Psen1-dependent Notch1 signaling and blunted senescence-like growth arrest was also observed in HBV-associated HCC patient tumor samples. Conclusion: Our results reveal a novel function of HBx in blunting senescence-like growth arrest by decreasing Notch1 signaling, which could be a putative molecular mechanism mediating HBV-associated hepatocarcinogenesis. Copyright © 2010 by the American Association for the Study of Liver Diseases.