T-cell factor 4 and ß-catenin chromatin occupancies pattern zonal liver metabolism in mice.

UNLABELLED: ß-catenin signaling can be both a physiological and oncogenic pathway in the liver. It controls compartmentalized gene expression, allowing the liver to ensure its essential metabolic function. It is activated by mutations in 20%-40% of hepatocellular carcinomas (HCCs) with specific metabolic features. We decipher the molecular determinants of ß-catenin-dependent zonal transcription using mice with ß-catenin-activated or -inactivated hepatocytes, characterizing in vivo their chromatin occupancy by T-cell factor (Tcf)-4 and ß-catenin, transcriptome, and metabolome. We find that Tcf-4 DNA bindings depend on ß-catenin. Tcf-4/ß-catenin binds Wnt-responsive elements preferentially around ß-catenin-induced genes. In contrast, genes repressed by ß-catenin bind Tcf-4 on hepatocyte nuclear factor 4 (Hnf-4)-responsive elements. ß-Catenin, Tcf-4, and Hnf-4α interact, dictating ß-catenin transcription, which is antagonistic to that elicited by Hnf-4α. Finally, we find the drug/bile metabolism pathway to be the one most heavily targeted by ß-catenin, partly through xenobiotic nuclear receptors. CONCLUSIONS: ß-catenin patterns the zonal liver together with Tcf-4, Hnf-4α, and xenobiotic nuclear receptors. This network represses lipid metabolism and exacerbates glutamine, drug, and bile metabolism, mirroring HCCs with ß-catenin mutational activation.

The transforming growth factor-α and cyclin D1 genes are direct targets of ß-catenin signaling in hepatocyte proliferation.

BACKGROUND & AIMS: ß-Catenin is an oncogene frequently mutated in hepatocellular carcinoma. In this study, we investigated target genes of ß-catenin signaling in hepatocyte proliferation. METHODS: We studied transgenic mice displaying either inactivation or activation of the ß-catenin pathway, focusing on analysis of liver proliferation due to aberrant ß-catenin activation, and on the regeneration process during which ß-catenin signaling is transiently activated. We localized in situ the various partners involved in proliferation or identified as targets of ß-catenin in these transgenic and regenerating livers. We also performed comparative transcriptome analyses, using microarrays. Finally, we extracted, from deep-sequencing data, both the DNA regulatory elements bound to the ß-catenin/Tcf nuclear complex and the expression levels of critical targets identified in microarrays. RESULTS: ß-Catenin activation during liver regeneration occurred during G1/S cell cycle progression and allowed zonal extension of the normal territory of active ß-catenin and panlobular proliferation. We found that ß-catenin controlled both cell-autonomous and non-cell-autonomous hepatocyte proliferation, through direct transcriptional and complex control of cyclin D1 gene expression and of the expression of a new target gene, Tgfα. CONCLUSIONS: We propose that ß-catenin controls panlobular hepatocyte proliferation partly by controlling, together with its Tcf4 nuclear partner, expression of the pro-proliferation cyclin D1 and Tgfα genes. This study constitutes a first step toward understanding the oncogenic properties of this prominent signaling pathway in the liver.

DietSee: An on-hand, portable, strip-type biosensor for lipolysis monitoring via real-time amperometric determination of glycerol in blood.

Glycerol is a clinical biomarker of lipolysis that is mainly produced by adipose tissues. Blood glycerol content increases in pathological conditions such as metabolic and cardiovascular diseases or cancer cachexia, but also in response to energetic stress such as physical exercise. Accurate glycerol monitoring is therefore important in a range of healthcare contexts. However, current methods available for the quantification of glycerol are expensive, time-consuming, and require the extraction of plasma from blood, from which blood glycerol content is then extrapolated. Here, we report the development of a new point-of-care glycerometer device, DietSee, based on a strip-type biosensor that enables the quantification of glycerol directly from whole blood in 6 s. The performance of the biosensor was first evaluated using buffer solutions and spiked human and mouse plasma samples, and its response was compared with that of the gold-standard colorimetric method. The results obtained using DietSee correlated strongly with those from the reference method and demonstrated a linear response to glycerol levels across a wide range of concentrations (40-750 µM) that were representative of those in the human body. Next, the biosensor was validated using spiked human blood samples over a range of 30-55% hematocrit; it also demonstrated a strong correlation with reference measurements under these conditions (R2 = 0.97). In addition, the biosensor was only minimally affected by a variety of potential interferents (endogenous and exogenous) and was highly stable in storage (more than 2 years when strips were stored dry at 4 °C). Finally, we investigated the application of the biosensor to real-time monitoring of lipolysis and found that the DietSee is well adapted for this purpose in both human and mouse samples. To conclude, the novel DietSee glycerometer is a sensitive, selective, and rapid tool that enables characterization of the metabolic status of an individual by measuring the glycerol concentration from a single fingertip blood drop.

Wnt/beta-catenin pathway in hepatocellular carcinoma pathogenesis and liver physiology.

The Wnt/beta-catenin pathway is a key developmental pathway for which alterations have been described in various human cancers. The aberrant activation of this pathway is a major event in human hepatocellular carcinoma. Several laboratories have shown that the Wnt/beta-catenin pathway plays an essential role in all phases of liver development and maturation, and is required for the metabolic function of this organ. In this review, we summarize current knowledge regarding the role of the Wnt/beta-catenin pathway in hepatocellular carcinoma pathogenesis and liver biology, and the possibilities for developing new therapeutic interventions based on this knowledge.

Transcription dynamics in a physiological process: ß-catenin signaling directs liver metabolic zonation.

The liver displays a remarkable phenomenon known as metabolic zonation. Highly specialized hepatocytes fulfill different metabolic functions that depend on their position along the porto-central axis, distinguishing "periportal" hepatocytes from "pericentral" hepatocytes. The mechanisms by which zonation is established have been extensively investigated since its initial discovery. Using murine models with ß-catenin conditional activation or invalidation in the liver, a major role for the Wnt/ß-catenin developmental pathway has been demonstrated in this functional heterogeneity of hepatocytes. Under physiological conditions, this pathway is activated in pericentral hepatocytes. This is partly due to the absence in the pericentral area of adenomatous polyposis coli, a negative regulator also known as the "zonation-keeper" of the liver lobule. The Wnt pathway induces a pericentral genetic program and represses a periportal genetic program in these hepatocytes. In mice with aberrant activation of ß-catenin signaling, Wnt signaling also controls hepatocyte proliferation through a non-cell-autonomous mechanism. This pathway therefore controls metabolism and proliferation in liver cells, its role in proliferation being consistent with its involvement in liver cancer. Finally, the hepatic-enriched transcription factor Hnf4 has been shown to play a role in the Wnt-dependent transcription of zonated genes. From these findings, it now appears that the combinatorial interplay of different transcription factors with ß-catenin supports liver metabolic zonation. We propose that genome-wide approaches using chromatin immunoprecipitation will allow to further explore the molecular determinants of ß-catenin-dependent liver zonation.

Molecular determinants of liver zonation.

The phenomenon of "liver zonation" is a remarkable process by which the liver fulfills its metabolic functions, involving highly dynamic transcriptional mechanisms. Its understanding is therefore a challenging issue. Zonation is reflected in heterogeneity of hepatocytes along the porto-central axis of the liver: periportal hepatocytes, located in the vicinity of the afferent portal vein, do not express the same metabolic enzymes than pericentral hepatocytes located near the efferent central vein. This is mainly dictated at the transcriptional level by specific pericentral versus periportal genetic programs. The mechanisms by which zonation is established have been extensively investigated since its initial discovery 40 years ago. The discovery in 2006 that Wnt/ß-catenin pericentral signaling was a master regulator of this complex liver topology has been a major breakthrough. A major current priority in the field is the integration of the ß-catenin pathway with other determinants that govern zonation of the liver.