Wnt signaling through ␤-catenin and TCF maintains preadipocytes in an un-differentiated proliferative state; however, the molecular pathway has not been completely defined. By integrating gene expression microarray, chromatin immunoprecipitation-chip, and cell-based experimental approaches, we show that Wnt/␤-catenin signaling activates the expression of COUP-TFII which recruits the SMRT corepressor complex to the first introns located downstream from the first exons of both PPAR␥1 and ␥2 mRNAs. This maintains the local chromatin in a hypoacetylated state and represses PPAR␥ gene expression to inhibit adipogenesis. Our experiments define the COUP-TFII/SMRT complex as a previously unappreciated component of the linear pathway that directly links Wnt/␤-catenin signaling to repression of PPAR␥ gene expression and the inhibition of adipogenesis.ChIP-chip ͉ epigenome ͉ histone modification ͉ obesity ͉ Wnt

The proliferation and differentiation of neural precursor cells are mutually exclusive during brain development. Despite its importance for precursor cell self renewal, the molecular linkage between these two events has remained unclear. Fibroblast growth factor 2 (FGF2) promotes neural precursor cell proliferation and concurrently inhibits their differentiation, suggesting a cross talk between proliferation and differentiation signaling pathways downstream of the FGF receptor. We demonstrate that FGF2 signaling through phosphatidylinositol 3 kinase activation inactivates glycogen synthase kinase 3␤ (GSK3␤) and leads to the accumulation of ␤-catenin in a manner different from that in the Wnt canonical pathway. The nuclear accumulated ␤-catenin leads to cell proliferation by activating LEF/TCF transcription factors and concurrently inhibits neuronal differentiation by potentiating the Notch1-RBP-J signaling pathway. ␤-Catenin and the Notch1 intracellular domain form a molecular complex with the promoter region of the antineurogenic hes1 gene, allowing its expression. This signaling interplay is especially essential for neural stem cell maintenance, since the misexpression of dominant-active GSK3␤ completely inhibits the self renewal of neurosphere-forming stem cells and prompts their neuronal differentiation. Thus, the GSK3␤/␤-catenin signaling axis regulated by FGF and Wnt signals plays a pivotal role in the maintenance of neural stem/precursor cells by linking the cell proliferation to the inhibition of differentiation.

Cell fusion has a critical role in various developmental processes, immune response, tissue homeostasis and regeneration, and possibly, in cancer. However, the signals that regulate cell fusion remain poorly understood. In a screen for novel targets of Wnt / β -catenin signalling, we identifi ed glial cells missing 1 ( GCM1 ), which encodes a transcription factor that is involved in epigenetic regulation and is critical for the fusion of syncytiotrophoblast (ST) cells. Here we show that β -catenin / BCL9-Like (BCL9L) / T-cell factor 4 (TCF4) signalling directly targets the GCM1 / syncytin pathway and thereby regulates the fusion of human choriocarcinoma cells. Furthermore, we show that the GCM1 / syncytin-B pathway is signifi cantly downregulated in the placenta of BCL9L-defi cient mice and that the fusion and differentiation of ST-II cells are blocked. Our results demonstrate a signal transduction pathway that regulates cell fusion, and may provide intriguing perspectives into the various biological and pathological processes that involve cell fusion.

Hepatoblastoma (HB) is the most common pediatric liver malignancy; however, hereditary predisposition and acquired molecular aberrations related to HB clinicopathological diversity are not well understood. Here, we perform an integrative genomic profiling of 163 pediatric liver tumors (154 HBs and nine hepatocellular carcinomas) based on the data acquired from a cohort study (JPLT-2). The total number of somatic mutations is precious low (0.52/Mb on exonic regions) but correlated with age at diagnosis. Telomerase reverse transcriptase (TERT) promoter mutations are prevalent in the tween HBs, selective in the transitional liver cell tumor (TLCT, > 8 years old). DNA methylation profiling reveals that classical HBs are characterized by the specific hypomethylated enhancers, which are enriched with binding sites for ASCL2, a regulatory transcription factor for definitive endoderm in Wnt-pathway. Prolonged upregulation of ASCL2, as well as fetal-liver-like methylation patterns of IGF2 promoters, suggests their “cell of origin” derived from the premature hepatoblast, similar to intestinal epithelial cells, which are highly proliferative. Systematic molecular profiling of HB is a promising approach for understanding the epigenetic drivers of hepatoblast carcinogenesis and deriving clues for risk stratification.

Epigenetic regulation is essential in determining cellular phenotypes during differentiation. Although tissue-specific DNA methylation has been studied, the significance of methylation variance for tissue phenotypes remains unresolved, especially for CpG-poor promoters. Here, we comprehensively studied methylation levels of 27 578 CpG sites among 21 human normal tissues from 12 anatomically different regions using an epigenotyping beadarray system. Remarkable changes in tissue-specific DNA methylation were observed within CpG-poor promoters but not CpG-rich promoters. Of note, tissue-specific hypomethylation is accompanied by an increase in gene expression, which gives rise to specialized cellular functions. The hypomethylated regions were significantly enriched with recognition motifs for transcription factors that regulate cell-type-specific differentiation. To investigate the dynamics of hypomethylation events, we analyzed methylation levels of the entire APOA1 gene locus during in vitro differentiation of embryonic stem cells toward the hepatic lineage. A decrease in methylation was observed after day 13, coinciding with alpha-fetoprotein detection, in the vicinity of its transcription start sites (TSSs), and extends up to ∼200 bp region encompassing the TSS at day 21, equivalent to the hepatoblastic stage. This decrease is even more pronounced in the adult liver, where the entire APOA1 gene locus is hypomethylated. Furthermore, when we compared the methylation status of induced pluripotent stem (iPS) cells with their parental cell, IMR-90, we found that fibroblast-specific hypomethylation is restored to a fully methylated state in iPS cells after reprogramming. These results illuminate tissue-specific methylation dynamics in CpG-poor promoters and provide more comprehensive views on spatiotemporal gene regulation in terminal differentiation.