AXIN deficiency in human and mouse hepatocytes induces hepatocellular carcinoma in the absence of ß-catenin activation.

BACKGROUND & AIMS: The Wnt/ß-catenin pathway is the most frequently deregulated pathway in hepatocellular carcinoma (HCC). Inactivating mutations of the gene encoding AXIN1, a known negative regulator of the Wnt/ß-catenin signaling pathway, are observed in about 10% of HCCs. Whole-genome studies usually place HCC with AXIN1 mutations and CTNNB1 mutations in the group of tumors with Wnt/ß-catenin activated program. However, it has been shown that HCCs with activating CTNNB1 mutations form a group of HCCs, with a different histology, prognosis and genomic signature to those with inactivating biallelic AXIN1 mutations. We aimed to elucidate the relationship between CTNNB1 mutations, AXIN1 mutations and the activation level of the Wnt/ß-catenin program. METHODS: We evaluated two independent human HCC datasets for the expression of a 23-ß-catenin target genes program. We modeled Axin1 loss of function tumorigenesis in two engineered mouse models and performed gene expression profiling. RESULTS: Based on gene expression, we defined three levels of ß-catenin program activation: strong, weak or no activation. While more than 80% CTNNB1-mutated tumors were found in the strong or in the weak activation program, most of the AXIN1-mutated tumors (>70%) were found in the subgroup with no activation. We validated this result by demonstrating that mice with a hepatocyte specific AXIN1 deletion developed HCC in the absence of ß-catenin induction. We defined a 329-gene signature common in human and mouse AXIN1 mutated HCC that is highly enriched in Notch and YAP oncogenic signatures. CONCLUSIONS: AXIN1-mutated HCCs occur independently of the Wnt/ß-catenin pathway and involve Notch and YAP pathways. These pathways constitute potentially interesting targets for the treatment of HCC caused by AXIN1 mutations. LAY SUMMARY: Liver cancer has a poor prognosis. Defining the molecular pathways involved is important for developing new therapeutic approaches. The Wnt/ß-catenin pathway is the most frequently deregulated pathway in hepatocellular carcinoma (HCC). Mutations of AXIN1, a member of this pathway, represent about 10% of HCC mutations. Using both human HCC collections and engineered mouse models of liver cancers with AXIN1 mutation or deletion, we defined a common signature of liver tumors mutated for AXIN1 and demonstrate that these tumors occur independently of the activation of the Wnt/ß-catenin pathway.

LKB1 and Notch Pathways Interact and Control Biliary Morphogenesis.

BACKGROUND: LKB1 is an evolutionary conserved kinase implicated in a wide range of cellular functions including inhibition of cell proliferation, regulation of cell polarity and metabolism. When Lkb1 is inactivated in the liver, glucose homeostasis is perturbed, cellular polarity is affected and cholestasis develops. Cholestasis occurs as a result from deficient bile duct development, yet how LKB1 impacts on biliary morphogenesis is unknown. METHODOLOGY/PRINCIPAL FINDINGS: We characterized the phenotype of mice in which deletion of the Lkb1 gene has been specifically targeted to the hepatoblasts. Our results confirmed that lack of LKB1 in the liver results in bile duct paucity leading to cholestasis. Immunostaining analysis at a prenatal stage showed that LKB1 is not required for differentiation of hepatoblasts to cholangiocyte precursors but promotes maturation of the primitive ductal structures to mature bile ducts. This phenotype is similar to that obtained upon inactivation of Notch signaling in the liver. We tested the hypothesis of a functional overlap between the LKB1 and Notch pathways by gene expression profiling of livers deficient in Lkb1 or in the Notch mediator RbpJκ and identified a mutual cross-talk between LKB1 and Notch signaling. In vitro experiments confirmed that Notch activity was deficient upon LKB1 loss. CONCLUSION: LKB1 and Notch share a common genetic program in the liver, and regulate bile duct morphogenesis.

The Wnt/ß-catenin pathway as a therapeutic target in human hepatocellular carcinoma.

Hepatocellular carcinoma (HCC) is the third most common cause of cancer-related death worldwide. However, targeted therapies are still at their beginning for the treatment of this poor-prognosis tumor. Among the signaling cascades deregulated in HCC, the Wnt/ß-catenin signaling pathway plays a key role in hepatic oncogenesis. Although it has been shown, using HCC cell lines, that inhibition of the ß-catenin signaling has anti-tumoral effect, no molecules targeting the Wnt pathway are currently tested in clinical trials for the treatment of HCC. Here we review our current knowledge about the role of the Wnt/ß-catenin pathway in hepatocellular carcinoma pathogenesis and the benefits and limits of targeting this pathway in HCC.