No abstract

Inappropriate activation of the Wnt/b-catenin signaling has been implicated in the development of hepatocellular carcinoma (HCC), but exactly how b-catenin works remains to be elucidated. To identify, in vivo, the target genes of b-catenin in the liver, we have used the suppression subtractive hybridization technique and transgenic mice expressing an activated b-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 b-catenin pathway in the liver. In different mouse models bearing an activated bcatenin signaling in the liver known to be associated with hepatocellular proliferation we observed a marked upregulation of these three genes. The cellular distribution of GS and GLT-1 parallels b-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 b-catenin. In addition, the GS promoter was activated in the liver of GS +/LacZ mice by adenovirus vector-mediated b-catenin overexpression. Strikingly, the overexpression of the GS gene in human HCC samples was strongly correlated with b-catenin activation. Together, our results indicate that GS is a target of the Wnt/b-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 b-catenin is a contributing factor to liver carcinogenesis.

A striking example of the relationship between regulation of transcription and phenotype is the central role of the Y-chromosomal gene Sry in mammalian sex determination. Sry is the founding member of a large family of so-called Sox genes. During murine embryogenesis, the transcriptional activator Sox-4 is expressed at several sites, but in adult mice expression is restricted to immature B and T lymphocytes. Using targeted gene distruption, we have found that SOX-4(-/-) embryos succumb to circulatory failure at day E14. This was a result of impaired development of the endocardial ridges (a specific site of Sox-4 expression) into the semilunar valves and the outlet portion of the muscular ventricular septum. The observed range of septation defects is known as 'common arterial trunk' in man. We studied haemopoiesis in lethally irradiated mice reconstituted with SOX-4(-/-) fetal liver cells and found that a specific block occurred in B-cell development at the pro-B cell stage. In line with this, the frequency and proliferative capacity of IL-7-responsive B cell progenitors in fetal liver were severely decreased in vitro.

In this study we challenge the generally accepted view that cardiac chambers form from an array of segmental primordia arranged along the anteroposterior axis of the linear and looping heart tube. We traced the spatial pattern of expression of genes encoding atrial natriuretic factor, sarcoplasmic reticulum calcium ATPase, Chisel, Irx5, Irx4, myosin light chain 2v, and beta-myosin heavy chain and related these to morphogenesis. Based on the patterns we propose a two-step model for chamber formation in the embryonic heart. First, a linear heart forms, which is composed of "primary" myocardium that nonetheless shows polarity in phenotype and gene expression along its anteroposterior and dorsoventral axes. Second, specialized ventricular chamber myocardium is specified at the ventral surface of the linear heart tube, while distinct left and right atrial myocardium forms more caudally on laterodorsal surfaces. The process of looping aligns these primordial chambers such that they face the outer curvature. Myocardium of the inner curvature, as well as that of inflow tract, atrioventricular canal, and outflow tract, retains the molecular signature originally found in linear heart tube myocardium. Evidence for distinct transcriptional programs which govern compartmentalization in the forming heart is seen in the patterns of expression of Hand1 for the dorsoventral axis, Irx4 and Tbx5 for the anteroposterior axis, and Irx5 for the distinction between primary and chamber myocardium.