Insulin activates hepatic Wnt/ß-catenin signaling through stearoyl-CoA desaturase 1 and Porcupine.

The Wnt/ß-catenin pathway plays a pivotal role in liver structural and metabolic homeostasis. Wnt activity is tightly regulated by the acyltransferase Porcupine through the addition of palmitoleate. Interestingly palmitoleate can be endogenously produced by the stearoyl-CoA desaturase 1 (SCD1), a lipogenic enzyme transcriptionally regulated by insulin. This study aimed to determine whether nutritional conditions, and insulin, regulate Wnt pathway activity in liver. An adenoviral TRE-Luciferase reporter was used as a readout of Wnt/ß-catenin pathway activity, in vivo in mouse liver and in vitro in primary hepatocytes. Refeeding enhanced TRE-Luciferase activity and expression of Wnt target genes in mice liver, revealing a nutritional regulation of the Wnt/ß-catenin pathway. This effect was inhibited in liver specific insulin receptor KO (iLIRKO) mice and upon wortmannin or rapamycin treatment. Overexpression or inhibition of SCD1 expression regulated Wnt/ß-catenin activity in primary hepatocytes. Similarly, palmitoleate added exogenously or produced by SCD1-mediated desaturation of palmitate, induced Wnt signaling activity. Interestingly, this effect was abolished in the absence of Porcupine, suggesting that both SCD1 and Porcupine are key mediators of insulin-induced Wnt/ß-catenin activity in hepatocytes. Altogether, our findings suggest that insulin and lipogenesis act as potential novel physiological inducers of hepatic Wnt/ß-catenin pathway.

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.

PNPLA3, a genetic marker of progressive liver disease, still hiding its metabolic function?

Non-alcoholic fatty liver disease (NAFLD) is an emerging epidemic disease. It represents a large spectrum of liver diseases, and affects both adults and children. The etiology of NAFLD is multifactorial. Indeed, several events such as caloric imbalance including sedentary lifestyle, obesity and/or a predisposing genetic background are key players in the increasing risk for NAFLD development and its progression. Recently, a sequence variation within the gene encoding for patatin-like phospholipase containing 3 (PNPLA3, rs738409) was found to modulate steatosis, inflammation and fibrosis in NAFLD. It was also demonstrated as a novel genetic marker associated with progressive ALD (alcoholic liver disease). In this mini-review, we summarize the current knowledge on (i) PNPLA3 variant(s) in the pathogenesis of liver diseases, and (ii) PNPLA3 gene regulation and potential function in liver.

O-GlcNAcylation increases ChREBP protein content and transcriptional activity in the liver.

OBJECTIVE: Carbohydrate-responsive element-binding protein (ChREBP) is a key transcription factor that mediates the effects of glucose on glycolytic and lipogenic genes in the liver. We have previously reported that liver-specific inhibition of ChREBP prevents hepatic steatosis in ob/ob mice by specifically decreasing lipogenic rates in vivo. To better understand the regulation of ChREBP activity in the liver, we investigated the implication of O-linked ß-N-acetylglucosamine (O-GlcNAc or O-GlcNAcylation), an important glucose-dependent posttranslational modification playing multiple roles in transcription, protein stabilization, nuclear localization, and signal transduction. RESEARCH DESIGN AND METHODS: O-GlcNAcylation is highly dynamic through the action of two enzymes: the O-GlcNAc transferase (OGT), which transfers the monosaccharide to serine/threonine residues on a target protein, and the O-GlcNAcase (OGA), which hydrolyses the sugar. To modulate ChREBP(OG) in vitro and in vivo, the OGT and OGA enzymes were overexpressed or inhibited via adenoviral approaches in mouse hepatocytes and in the liver of C57BL/6J or obese db/db mice. RESULTS: Our study shows that ChREBP interacts with OGT and is subjected to O-GlcNAcylation in liver cells. O-GlcNAcylation stabilizes the ChREBP protein and increases its transcriptional activity toward its target glycolytic (L-PK) and lipogenic genes (ACC, FAS, and SCD1) when combined with an active glucose flux in vivo. Indeed, OGT overexpression significantly increased ChREBP(OG) in liver nuclear extracts from fed C57BL/6J mice, leading in turn to enhanced lipogenic gene expression and to excessive hepatic triglyceride deposition. In the livers of hyperglycemic obese db/db mice, ChREBP(OG) levels were elevated compared with controls. Interestingly, reducing ChREBP(OG) levels via OGA overexpression decreased lipogenic protein content (ACC, FAS), prevented hepatic steatosis, and improved the lipidic profile of OGA-treated db/db mice. CONCLUSIONS: Taken together, our results reveal that O-GlcNAcylation represents an important novel regulation of ChREBP activity in the liver under both physiological and pathophysiological conditions.

Distinct regulation of adiponutrin/PNPLA3 gene expression by the transcription factors ChREBP and SREBP1c in mouse and human hepatocytes.

BACKGROUND & AIMS: The adiponutrin/PNPLA3 (patatin-like phospholipase domain-containing protein 3) variant I148M has recently emerged as an important marker of human fatty liver disease. In order to understand the role of the adiponutrin/PNPLA3 protein, we investigated the regulation of its expression in both human and mouse hepatocytes. METHODS: Adiponutrin/PNPLA3 and lipogenic enzyme expression was determined by real-time PCR analysis in a wide panel of analysis in vivo in the mouse liver and in vitro in murine hepatocytes and human hepatocyte cell lines infected with ChREBP or SREBP1c-expressing adenoviruses. RESULTS: We show that in the mouse liver, adiponutrin/PNPLA3 gene expression is under the direct transcriptional control of ChREBP (carbohydrate-response element-binding protein) and SREBP1c (sterol regulatory element binding protein1c) in response to glucose and insulin, respectively. In silico analysis revealed the presence of a ChoRE (carbohydrate response element) and of a SRE (sterol response element) binding site on the mouse adiponutrin/PNPLA3 gene promoter. Point mutation analysis in reporter gene assays identified the functional response of these two binding sites in the mouse adiponutrin/PNPLA3 promoter. In contrast, in human immortalized hepatocytes and in HepG2 hepatoma cells, only SREBP1c was able to induce adiponutrin/PNPLA3 expression, whereas ChREBP was unable to modulate its expression. CONCLUSIONS: All together, our results suggest that adiponutrin/PNPLA3 is regulated by two key factors of the glycolytic and lipogenic pathways, raising the question of its implication in the metabolism of carbohydrates and lipids.