New targets of beta-catenin signaling in the liver are involved in the glutamine metabolism

Abstract

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.