Nf2/Merlin controls progenitor homeostasis and tumorigenesis in the liver.

The molecular signals that control the maintenance and activation of liver stem/progenitor cells are poorly understood, and the role of liver progenitor cells in hepatic tumorigenesis is unclear. We report here that liver-specific deletion of the neurofibromatosis type 2 (Nf2) tumor suppressor gene in the developing or adult mouse specifically yields a dramatic, progressive expansion of progenitor cells throughout the liver without affecting differentiated hepatocytes. All surviving mice eventually developed both cholangiocellular and hepatocellular carcinoma, suggesting that Nf2(-/-) progenitors can be a cell of origin for these tumors. Despite the suggested link between Nf2 and the Hpo/Wts/Yki signaling pathway in Drosophila, and recent studies linking the corresponding Mst/Lats/Yap pathway to mammalian liver tumorigenesis, our molecular studies suggest that Merlin is not a major regulator of YAP in liver progenitors, and that the overproliferation of Nf2(-/-) liver progenitors is instead driven by aberrant epidermal growth factor receptor (EGFR) activity. Indeed, pharmacologic inhibition of EGFR blocks the proliferation of Nf2(-/-) liver progenitors in vitro and in vivo, consistent with recent studies indicating that the Nf2-encoded protein Merlin can control the abundance and signaling of membrane receptors such as EGFR. Together, our findings uncover a critical role for Nf2/Merlin in controlling homeostasis of the liver stem cell niche.

Apc tumor suppressor gene is the "zonation-keeper" of mouse liver.

The molecular mechanisms by which liver genes are differentially expressed along a portocentral axis, allowing for metabolic zonation, are poorly understood. We provide here compelling evidence that the Wnt/beta-catenin pathway plays a key role in liver zonation. First, we show the complementary localization of activated beta-catenin in the perivenous area and the negative regulator Apc in periportal hepatocytes. We then analyzed the immediate consequences of either a liver-inducible Apc disruption or a blockade of Wnt signaling after infection with an adenovirus encoding Dkk1, and we show that Wnt/beta-catenin signaling inversely controls the perivenous and periportal genetic programs. Finally, we show that genes involved in the periportal urea cycle and the perivenous glutamine synthesis systems are critical targets of beta-catenin signaling, and that perturbations to ammonia metabolism are likely responsible for the death of mice with liver-targeted Apc loss. From our results, we propose that Apc is the liver "zonation-keeper" gene.

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.

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.

Reptin regulates DNA double strand breaks repair in human hepatocellular carcinoma.

Reptin/RUVBL2 is overexpressed in most hepatocellular carcinomas and is required for the growth and viability of HCC cells. Reptin is involved in several chromatin remodeling complexes, some of which are involved in the detection and repair of DNA damage, but data on Reptin involvement in the repair of DNA damage are scarce and contradictory. Our objective was to study the effects of Reptin silencing on the repair of DNA double-strand breaks (DSB) in HCC cells. Treatment of HuH7 cells with etoposide (25 µM, 30 min) or γ irradiation (4 Gy) increased the phosphorylation of H2AX by 1.94 ± 0.13 and 2.0 ± 0.02 fold, respectively. These values were significantly reduced by 35 and 65 % after Reptin silencing with inducible shRNA. Irradiation increased the number of BRCA1 (3-fold) and 53BP1 foci (7.5 fold). Depletion of Reptin reduced these values by 62 and 48%, respectively. These defects in activation and/or recruitment of repair proteins were not due to a decreased number of DSBs as measured by the COMET assay. All these results were confirmed in the Hep3B cell line. Protein expression of ATM and DNA-PKcs, the major H2AX kinases, was significantly reduced by 52 and 61 % after Reptin depletion whereas their mRNA level remained unchanged. Phosphorylation of Chk2, another ATM target, was not significantly altered. Using co-immunoprecipitation, we showed an interaction between Reptin and DNA-PKcs. The half-life of newly-synthesized DNA-PKcs was reduced when Reptin was silenced. Finally, depletion of Reptin was synergistic with etoposide or γ irradiation to reduce cell growth and colony formation. In conclusion, Reptin is an important cofactor for the repair of DSBs. Our data, combined with those of the literature suggests that it operates at least in part by regulating the expression of DNA-PKcs by a stabilization mechanism. Overexpression of Reptin in HCC could be a factor of resistance to treatment, consistent with the observed overexpression of Reptin in subgroups of chemo-resistant breast and ovarian cancers.

YAP Inhibition Restores Hepatocyte Differentiation in Advanced HCC, Leading to Tumor Regression.

Defective Hippo/YAP signaling in the liver results in tissue overgrowth and development of hepatocellular carcinoma (HCC). Here, we uncover mechanisms of YAP-mediated hepatocyte reprogramming and HCC pathogenesis. YAP functions as a rheostat in maintaining metabolic specialization, differentiation, and quiescence within the hepatocyte compartment. Increased or decreased YAP activity reprograms subsets of hepatocytes to different fates associated with deregulation of the HNF4A, CTNNB1, and E2F transcriptional programs that control hepatocyte quiescence and differentiation. Importantly, treatment with small interfering RNA-lipid nanoparticles (siRNA-LNPs) targeting YAP restores hepatocyte differentiation and causes pronounced tumor regression in a genetically engineered mouse HCC model. Furthermore, YAP targets are enriched in an aggressive human HCC subtype characterized by a proliferative signature and absence of CTNNB1 mutations. Thus, our work reveals Hippo signaling as a key regulator of the positional identity of hepatocytes, supports targeting of YAP using siRNA-LNPs as a paradigm of differentiation-based therapy, and identifies an HCC subtype that is potentially responsive to this approach.