R-spondin 2 Drives Liver Tumor Development in a Yes-Associated Protein-Dependent Manner

Each year, more than 25,000 people succumb to liver cancer in the United States, and this neoplasm represents the second cause of cancer-related death globally. R-spondins (RSPOs) are secreted regulators of Wnt signaling that function in development and promote tissue stem cell renewal. In cancer, RSPOs 2 and 3 are oncogenes first identified by insertional mutagenesis screens in tumors induced by mouse mammary tumor virus and by transposon mutagenesis in the colonic epithelium of rodents. RSPO2 has been reported to be activated by chromosomal rearrangements in colorectal cancer and overexpressed in a subset of hepatocellular carcinoma. Using human liver tumor gene expression data, we first discovered that a subset of liver cancers were characterized by high levels of RSPO2 in contrast to low levels in adjacent nontumor tissue. To determine if RSPOs are capable of inducing liver tumors, we used an in vivo model from which we found that overexpression of RSPO2 in the liver promoted Wnt signaling, hepatomegaly, and enhanced liver tumor formation when combined with loss of transformation-related protein 53 (Trp53). Moreover, the Hippo/yes-associated protein (Yap) pathway has been implicated in many human cancers, influencing cell survival. Histologic and gene expression studies showed activation of Wnt/β-catenin and Hippo/Yap pathways following RSPO2 overexpression. We demonstrate that knockdown of Yap1 leads to reduced tumor penetrance following RSPO2 overexpression in the context of loss of Trp53. Conclusion: RSPO2 overexpression leads to tumor formation in the mouse liver in a Hippo/Yap-dependent manner. Overall, our results suggest a role for Yap in the initiation and progression of liver tumors and uncover a novel pathway activated in RSPO2-induced malignancies. We show that RSPO2 promotes liver tumor formation in vivo and in vitro and that RSPO2's oncogenic activity requires Hippo/Yap activation in hepatocytes. Both RSPO2 and YAP1 are suggested to represent novel druggable targets in Wnt-driven tumors of the liver.

Emerging signaling pathways in hepatocellular carcinoma

Signaling pathways have become a major source of targets for novel therapies in hepatocellular carcinoma (HCC). Survival benefits achieved with sorafenib, a multikinase inhibitor, are unprecedented and underscore the importance of improving our understanding of how signaling networks interact in transformed cells. Numerous signaling modules are de-regulated in HCC, including some related to growth factor signaling (e.g., IGF, EGF, PDGF, FGF, HGF), cell differentiation (WNT, Hedgehog, Notch), and angiogenesis (VEGF). Intracellular mediators such as RAS and AKT/MTOR may also play a role in HCC development and progression. Different molecular mechanisms have been shown to induce aberrant pathway activation. These include point mutations, chromosomal aberrations, and epigenetically driven down-regulation. The use of novel molecular technologies such as next-generation sequencing in HCC research has enabled the identification of novel pathways previously underexplored in the HCC field, such as chromatin remodeling and autophagy. Considering recent failures of molecular therapies in advanced clinical trials (e.g., sunitinib, brivanib), survey of these and other new pathways may provide alternative therapeutic targets.

Emerging signaling pathways in hepatocellular carcinoma

Signaling pathways have become a major source of targets for novel therapies in hepatocellular carcinoma (HCC). Survival benefits achieved with sorafenib, a multikinase inhibitor, are unprecedented and underscore the importance of improving our understanding of how signaling networks interact in transformed cells. Numerous signaling modules are de-regulated in HCC, including some related to growth factor signaling (e.g., IGF, EGF, PDGF, FGF, HGF), cell differentiation (WNT, Hedgehog, Notch), and angiogenesis (VEGF). Intracellular mediators such as RAS and AKT/MTOR may also play a role in HCC development and progression. Different molecular mechanisms have been shown to induce aberrant pathway activation. These include point mutations, chromosomal aberrations, and epigenetically driven down-regulation. The use of novel molecular technologies such as next-generation sequencing in HCC research has enabled the identification of novel pathways previously underexplored in the HCC field, such as chromatin remodeling and autophagy. Considering recent failures of molecular therapies in advanced clinical trials (e.g., sunitinib, brivanib), survey of these and other new pathways may provide alternative therapeutic targets.

Molecular pathogenesis of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the major causes of death among cirrhotic patients, being viral hepatitis and alcohol abuse, the main risk factors for its development. The introduction of highly sophisticated genomic technologies has spurred extensive research on the molecular pathogenesis of this devastating disease. Several signaling cascades have been consistently found dysregulated in HCC (e.g., WNT-β-catenin, PI3K/AKT/MTOR, RAS/MAPK, IGF, HGF/MET, VEGF, EGFR, and PDGF). In addition, there have been numerous molecular classifications proposed for this disease, what provides an additional hint about its genomic complexity. The importance of knowing the molecular drivers of HCC is underscored by the positive results of a molecular targeted agent, sorafenib, able to improve survival in patients with advanced disease. This review will briefly outline key concepts in alcohol-related hepatocarcinogenesis, and provide some insight regarding current trends in translating HCC genomics into clinical management of the disease.