Marie-Annick Buendia

Hepatic Stem-like Phenotype and Interplay of Wnt/β-Catenin and Myc Signaling in Aggressive Childhood Liver Cancer

Hepatoblastoma, the most common pediatric liver cancer, is tightly linked to excessive Wnt/beta-catenin signaling. Here, we used microarray analysis to identify two tumor subclasses resembling distinct phases of liver development and a discriminating 16-gene signature. beta-catenin activated different transcriptional programs in the two tumor types, with distinctive expression of hepatic stem/progenitor markers in immature tumors. This highly proliferating subclass was typified by gains of chromosomes 8q and 2p and upregulated Myc signaling. Myc-induced hepatoblastoma-like tumors in mice strikingly resembled the human immature subtype, and Myc downregulation in hepatoblastoma cells impaired tumorigenesis in vivo. Remarkably, the 16-gene signature discriminated invasive and metastatic hepatoblastomas and predicted prognosis with high accuracy.

TAK1 Suppresses a NEMO-Dependent but NF-κB-Independent Pathway to Liver Cancer

The MAP3-kinase TGF-beta-activated kinase 1 (TAK1) critically modulates innate and adaptive immune responses and connects cytokine stimulation with activation of inflammatory signaling pathways. Here, we report that conditional ablation of TAK1 in liver parenchymal cells (hepatocytes and cholangiocytes) causes hepatocyte dysplasia and early-onset hepatocarcinogenesis, coinciding with biliary ductopenia and cholestasis. TAK1-mediated cancer suppression is exerted through activating NF-kappaB in response to tumor necrosis factor (TNF) and through preventing Caspase-3-dependent hepatocyte and cholangiocyte apoptosis. Moreover, TAK1 suppresses a procarcinogenic and pronecrotic pathway, which depends on NF-kappaB-independent functions of the I kappaB-kinase (IKK)-subunit NF-kappaB essential modulator (NEMO). Therefore, TAK1 serves as a gatekeeper for a protumorigenic, NF-kappaB-independent function of NEMO in parenchymal liver cells.

Activation of c-myc by woodchuck hepatitis virus insertion in hepatocellular carcinoma

Two hepatocellular carcinomas, induced in woodchucks chronically infected with woodchuck hepatitis virus, were characterized for viral integration near c-myc and alterations of c-myc expression. In one tumor, viral integration within the untranslated region of c-myc exon 3 resulted in overexpression of a long c-myc viral cotranscript. In the second tumor, a single insertion of highly rearranged viral sequences 600 bp upstream of c-myc exon 1 was associated with increased levels of normal c-myc mRNA. In both cases, viral enhancer insertion and disruption of normal c-myc transcriptional or posttranscriptional control appear to be involved in c-myc activation. These results demonstrate that integration of woodchuck hepatitis virus near a cellular proto-oncogene, as in several retroviral models, can contribute to the genesis of liver tumors.

Stem cell-like micro-RNA signature driven by Myc in aggressive liver cancer.

Myc activation has been implicated in the pathogenesis of hepatoblastoma (HB), a rare embryonal neoplasm derived from liver progenitor cells. Here, microRNA (miR) expression profiling of 65 HBs evidenced differential patterns related to developmental stage and Myc activity. Undifferentiated aggressive HBs overexpressed the miR-371–3 cluster with concomitant down-regulation of the miR-100/let-7a-2/miR-125b-1 cluster, evoking an ES cell expression profile. ChIP and Myc inhibition assays in hepatoma cells demonstrated that both miR clusters are regulated by Myc in an opposite manner. We show that the two miR clusters exert antagonistic effects on cell proliferation and tumorigenicity. Moreover, their combined deregulation cooperated in modulating the hepatic tumor phenotype, implicating stem cell-like regulation of Myc-dependent miRs in poorly differentiated HBs. Importantly, a four-miR signature representative of these clusters efficiently stratified HB patients, and when applied to 241 hepatocellular carcinomas (HCCs), it identified invasive tumors with a poor prognosis. Our data argue that Myc-driven reprogramming of miR expression patterns contributes to the aggressive phenotype of liver tumors originating from hepatic progenitor cells.

Activation of β-catenin in epithelial and mesenchymal hepatoblastomas

Wnt/β-catenin signaling is frequently activated in cancer cells by stabilizing mutations of β-catenin or loss-of-function mutations of the APC tumor suppressor gene. We have analysed the role of β-catenin in the pathogenesis of hepatoblastoma (HB), an embryonic liver tumor occurring mainly in children under 2 years of age. Sequence analysis of the β-catenin NH2-terminal domain in 18 epithelial and mixed HBs revealed missense mutations in the GSK3β phosphorylation motif or interstitial deletions in 12 tumors (67%). In the remaining cases, no truncating mutation of APC could be evidenced. Immunohistochemical analysis of β-catenin in 11 HBs demonstrated nuclear/cytoplasmic accumulation of the protein in all tumors analysed, with predominant nuclear β-catenin immunostaining in undifferentiated cells. Membranous β-catenin localization was preserved only in fetal-type tumoral hepatocytes and was associated with E-cadherin expression. Moreover, we show that β-catenin is aberrantly overexpressed in a large spectrum of tumor components, including hepatocyte-like cells at various differentiation stages and heterologous elements such as squamous, osteoid and chrondroid tissues, and in occasional other mesenchymally-derived cells. These data strongly suggest that activation of β-catenin signaling is an obligatory step in HB pathogenesis, and raise the possibility that it interferes with developmental signals that specify different tissue types at early stages of hepatic differentiation.

Identification of the LIM protein FHL2 as a coactivator of β-catenin

β-Catenin is a key mediator of the Wnt pathway, which plays a critical role in embryogenesis and oncogenesis. As a transcriptional activator, β-catenin binds the transcription factors, T-cell factor and lymphoid enhancer factor, and regulates gene expression in response to Wnt signaling. Abnormal activation of β-catenin has been linked to various types of cancer. In a yeast two-hybrid screen, we identified the four and a half of LIM-only protein 2 (FHL2) as a novel β-catenin-interacting protein. Here we show specific interaction of FHL2 with β-catenin, which requires the intact structure of FHL2 and armadillo repeats 1–9 of β-catenin. FHL2 cooperated with β-catenin to activate T-cell factor/lymphoid enhancer factor-dependent transcription from a synthetic reporter and the cyclin D1 and interleukin-8 promoters in kidney and colon cell lines. In contrast, coexpression of β-catenin and FHL2 had no synergistic effect on androgen receptor-mediated transcription, whereas each of these two coactivators independently stimulated AR transcriptional activity. Thus, the ability of FHL2 to stimulate the trans-activating function of β-catenin might be dependent on the promoter context. The detection of increased FHL2 expression in hepatoblastoma, a liver tumor harboring frequent β-catenin mutations, suggests that FHL2 might enforce β-catenin transactivation activity in cancer cells. These findings reveal a new function of the LIM coactivator FHL2 in transcriptional activation of Wnt-responsive genes.

Acetylation of beta-catenin by p300 regulates beta-catenin-Tcf4 interaction.

ABSTRACT Lysine acetylation modulates the activities of nonhistone regulatory proteins and plays a critical role in the regulation of cellular gene transcription. In this study, we showed that the transcriptional coactivator p300 acetylated β-catenin at lysine 345, located in arm repeat 6, in vitro and in vivo. Acetylation of this residue increased the affinity of β-catenin for Tcf4, and the cellular Tcf4-bound pool of β-catenin was significantly enriched in acetylated form. We demonstrated that the acetyltransferase activity of p300 was required for efficient activation of transcription mediated by β-catenin/Tcf4 and that the cooperation between p300 and β-catenin was severely reduced by the K345R mutation, implying that acetylation of β-catenin plays a part in the coactivation of β-catenin by p300. Interestingly, acetylation of β-catenin had opposite, negative effects on the binding of β-catenin to the androgen receptor. Our data suggest that acetylation of β-catenin in the arm 6 domain regulates β-catenin transcriptional activity by differentially modulating its affinity for Tcf4 and the androgen receptor. Thus, our results describe a new mechanism by which p300 might regulate β-catenin transcriptional activity.

Tbx3 Is a Downstream Target of the Wnt/β-Catenin Pathway and a Critical Mediator of β-Catenin Survival Functions in Liver Cancer

Tbx3 encodes a transcriptional repressor that is important for diverse patterning events during development, and Tbx3 mutation in humans causes the ulnar-mammary syndrome. Here, we describe the identification of Tbx3 in array-based search for genes downstream Wnt/beta-catenin that are implicated in liver tumorigenesis. Overexpression of Tbx3 is closely associated with the mutational status of beta-catenin in murine liver tumors induced by Myc as well as in human hepatocellular carcinomas and hepatoblastomas. Moreover, Tbx3 transcription is activated by ectopic expression of beta-catenin in mouse liver and in human tumor cell lines. Evidence that Tbx3 transcription is directly regulated by beta-catenin is provided by chromatin immunoprecipitation and reporter assays. Although HepG2 cells stably transfected with Tbx3 display moderately enhanced growth rate, the dominant negative mutant Tbx3-Y149S drastically inhibits hepatoma cell growth in vitro and in vivo. Moreover, small interfering RNAs (siRNA) directed against Tbx3 inhibit anchorage-independent growth of liver and colon carcinoma cells. We further show that inhibition of Tbx3 expression by specific siRNAs blocks beta-catenin-mediated cell survival and renders cells sensitive to doxorubicin-induced apoptosis. Conversely, ectopic expression of Tbx3 inhibits apoptosis induced by beta-catenin depletion. Marked overexpression of Tbx3 in a subset of hepatoblastomas is associated with chemotherapy-resistant phenotype and unfavorable patient outcome. These results reveal an unsuspected role of Tbx3 as a mediator of beta-catenin activities on cell proliferation and survival and as an important player in liver tumorigenesis.

Chapter 4 Hepatitis B Virus X Protein: Molecular Functions and Its Role in Virus Life Cycle and Pathogenesis

Despite the existence of effective vaccines, HBV infection remains a major health problem with 2 billion people infected worldwide. Among them, 350 million are chronically infected, a major risk factor for the development of hepatocellular carcinoma (HCC). There is a strong need to develop new and efficient treatments against chronic infection and HCC. It is therefore important to understand HBV replication and persistence as well as the role of HBV in liver carcinogenesis. This chapter focuses on the regulatory protein HBx which is thought to play a central role in HBV regulation and pathogenesis. HBx has been shown to modulate a myriad of viral and cellular functions, yet its role in virus replication and pathogenesis in infected individuals remains far from being completely understood.

Interaction and Functional Cooperation between the LIM Protein FHL2, CBP/p300, and β-Catenin

ABSTRACT Transcriptional activation of gene expression by Wnt signaling is driven by the association of β-catenin with TCF/LEF factors and the recruitment of transcriptional coactivators. It has been shown that the LIM protein FHL2 and the acetyltransferase CBP/p300 individually stimulate β-catenin transactivating activity and that β-catenin is acetylated by p300. Here, we report that FHL2 and CBP/p300 synergistically enhanced β-catenin/TCF-mediated transcription from Wnt-responsive promoters and that the acetyltransferase activity of CBP/p300 was involved in the cooperation. CBP/p300 interacted directly with FHL2, predominantly through the CH3 domain but not the histone acetyltransferase domain, and different regions of CBP/p300 were involved in FHL2 and β-catenin binding. We provided evidence for the formation of a ternary complex by FHL2, CBP/p300, and β-catenin and for colocalization of the three proteins in the nucleus. In murine FHL2−/− embryo fibroblasts, the transactivation activity of β-catenin/TCF was markedly reduced, and this defect could be restored by exogenous expression of FHL2. However, CBP/p300 were still able to coactivate the β-catenin/TCF complex in FHL2−/− cells, suggesting that FHL2 is dispensable for the coactivator function of CBP/p300 on β-catenin. Furthermore, we found that FHL2 significantly increased acetylation of β-catenin by p300 in vivo. Finally, we showed that FHL2, CBP/p300, and β-catenin could synergistically activate androgen receptor-mediated transcription, indicating that the synergistic coactivator function is not restricted to TCF/LEF.