ABSTRACT
To clarify molecular mechanisms underlying liver carcinogenesis induced by aberrant activation of Wnt pathway, we isolated the target genes of beta-catenin from mice exhibiting constitutive activated beta-catenin in the liver. Adenovirus-mediated expression of oncogenic beta-catenin was used to isolate early targets of beta-catenin in the liver. Suppression subtractive hybridization was used to identify the leukocyte cell-derived chemotaxin 2 (LECT2) gene as a direct target of beta-catenin. Northern blot and immunohistochemical analyses demonstrated that LECT2 expression is specifically induced in different mouse models that express activated beta-catenin in the liver. LECT2 expression was not activated in livers in which hepatocyte proliferation was induced by a beta-catenin-independent signal. We characterized by mutagenesis the LEF/TCF site, which is crucial for LECT2 activation by beta-catenin. We further characterized the chemotactic property of LECT2 for human neutrophils. Finally, we have shown an up-regulation of LECT2 in human liver tumors that expressed aberrant activation of beta-catenin signaling; these tumors constituted a subset of hepatocellular carcinomas (HCC) and most of the hepatoblastomas that were studied. In conclusion, our results show that LECT2, which encodes a protein with chemotactic properties for human neutrophils, is a direct target gene of Wnt/beta-catenin signaling in the liver. Since HCC develops mainly in patients with chronic hepatitis or cirrhosis induced by viral or inflammatory factors, understanding the role of LECT2 in liver carcinogenesis is of interest and may lead to new therapeutic perspectives.
Subject(s)
Cytoskeletal Proteins/physiology , Intercellular Signaling Peptides and Proteins/genetics , Nerve Tissue Proteins , Trans-Activators/physiology , Animals , Basic Helix-Loop-Helix Leucine Zipper Transcription Factors , Cell Line , Chemotaxis , DNA-Binding Proteins/metabolism , Gene Expression , Humans , Intercellular Signaling Peptides and Proteins/pharmacology , Intercellular Signaling Peptides and Proteins/physiology , Liver Neoplasms/metabolism , Mice , Mice, Inbred Strains , Mice, Transgenic , Neutrophils/drug effects , Neutrophils/physiology , Nucleic Acid Hybridization/methods , Signal Transduction , TCF Transcription Factors , Transcription Factor 4 , Transcription Factor 7-Like 2 Protein , Transcription Factors/metabolism , Up-Regulation , beta CateninABSTRACT
Inappropriate activation of the Wnt/beta-catenin signaling has been implicated in the development of hepatocellular carcinoma (HCC), but exactly how beta-catenin works remains to be elucidated. To identify, in vivo, the target genes of beta-catenin in the liver, we have used the suppression subtractive hybridization technique and transgenic mice expressing an activated beta-catenin in the liver that developed hepatomegaly. We identified three genes involved in glutamine metabolism, encoding glutamine synthetase (GS), ornithine aminotransferase (OAT) and the glutamate transporter GLT-1. By Northern blot and immunohistochemical analysis we demonstrated that these three genes were specifically induced by activation of the beta-catenin pathway in the liver. In different mouse models bearing an activated beta-catenin signaling in the liver known to be associated with hepatocellular proliferation we observed a marked up-regulation of these three genes. The cellular distribution of GS and GLT-1 parallels beta-catenin activity. By contrast no up-regulation of these three genes was observed in the liver in which hepatocyte proliferation was induced by a signal-independent of beta-catenin. In addition, the GS promoter was activated in the liver of GS(+/LacZ) mice by adenovirus vector-mediated beta-catenin overexpression. Strikingly, the overexpression of the GS gene in human HCC samples was strongly correlated with beta-catenin activation. Together, our results indicate that GS is a target of the Wnt/beta-catenin pathway in the liver. Because a linkage of the glutamine pathway to hepatocarcinogenesis has already been demonstrated, we propose that regulation of these three genes of glutamine metabolism by beta-catenin is a contributing factor to liver carcinogenesis.
