Another Notch to be added to the list of hepatocellular growth regulatory factors?

Vastatin, an Endogenous Antiangiogenesis Polypeptide That Is Lost in Hepatocellular Carcinoma, Effectively Inhibits Tumor Metastasis

Hepatocellular carcinoma (HCC) is a hypervascular cancer without effective treatment. Here we report that polypeptide of NC1 domain of type VIII collagen (Vastatin) is an endogenous polypeptide expressed in normal liver tissue but lost in the liver of most HCC patients (73.1%). Its expression level is negatively associated with tumor size (P = 0.035) and metastasis (P = 0.016) in HCC patients. To evaluate its potential use as a therapeutic, we constructed a recombinant adeno-associated virus carrying Vastatin (rAAV-Vastatin) to treat HCC in an orthotopic Buffalo rat model. rAAV-Vastatin treatment significantly prolonged the median survival, inhibited tumor growth, and completely prevented metastasis in HCC-bearing rats by decreasing microvessel density and increasing tumor necrosis. No detectable toxicity in nontumor-bearing mice was observed. To investigate its molecular mechanisms, we performed DNA microarray, western blotting assays, and bioinformatic analysis to determine its effect on global gene expression patterns and signal transduction pathways. Our results indicated that rAAV-Vastatin significantly reduced the expressions of Pck1, JAG2, and c-Fos, thus inhibiting the cellular metabolism, Notch and AP-1 signaling pathways, respectively. Hence, we demonstrated for the first time that Vastatin is a novel, safe, and effective antiangiogenic therapeutic and a potential biomarker for HCC.

Hepatitis B virus X protein blunts senescence-like growth arrest of human hepatocellular carcinoma by reducing Notch1 cleavage

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.

Liver-specific inactivation of Notch2, but not Notch1, compromises intrahepatic bile duct development in mice

The Notch pathway is an evolutionary conserved, intercellular signaling pathway that plays an important role in cell fate specification and the embryonic development of many organs, including the liver. In humans, mutations in the Notch receptor ligand Jagged1 gene result in defective intrahepatic bile duct (IHBD) development in Alagille syndrome. Developmental abnormalities of IHBD in mice doubly heterozygous for Jagged1 and Notch2 mutations propose that interactions of Jagged1 and its receptor Notch2 are crucial for normal IHBD development. Because different cell types in the liver are involved in IHBD development and morphogenesis, the cell-specific role of Notch signaling is not entirely understood. We investigated the effect of combined or single targeted disruption of Notch1 and Notch2 specifically in hepatoblasts and hepatoblast-derived lineage cells on liver development using AlbCre transgenic mice. Hepatocyte differentiation and homeostasis were not impaired in mice after combined deletion of Notch1 and Notch2 (N1N2(F/F)AlbCre). However, we detected irregular ductal plate structures in N1N2(F/F)AlbCre newborns, and further postnatal development of IHBD was severely impaired characterized by disorganized ductular structures accompanied by portal inflammation, portal fibrosis, and foci of hepatocyte feathery degeneration in adulthood. Further characterization of mutant mice with single deletion of Notch1 (N1(F/F)AlbCre) or Notch2 (N2(F/F)AlbCre) showed that Notch2 but not Notch1 is indispensable for normal perinatal and postnatal IHBD development. Further reduction of Notch2 gene dosage in Notch2 conditional/mutant (N2(F/LacZ)AlbCre) animals further enhanced IHBD abnormalities and concomitant liver pathology.

CONCLUSION

Notch2 is required for proper IHBD development and morphogenesis.