Although both virus-induced and immunologically-mediated hepatocytes damage play an important role in pathogenesis, the mechanisms underlying HCV persistence and pathogenesis have been poorly understood. HCV core protein has been reported to play a critical role in HCV-associated liver diseases by activation of several cellular signal transduction, direct interaction with proto-oncogenes 
, and to transform cell lines and induce tumors in transgenic mouse models 
. Recently, HCC-derived core protein has been shown to shift TGF-β responses from tumor suppression to epithelial-mesenchymal transition (EMT), which contributes to the promotion of cell invasion and metastasis 
In the present study, we sought to investigate whether HCV core protein directly activates Wnt/β-catenin signaling pathway and hence is involved in HCV-induced liver pathogenesis. We found that HCV core protein synergizes Wnt3A-mediated β-catenin-dependent transcriptional activity in hepatocytes and HCC cells. Moreover, co-expression of core and Wnt3A gene induces stabilization and nuclear translocation of β-catenin, contributing to the up-regulation of Wnt-target gene expression, such as c-Myc
and cyclin D1
. We further demonstrated that cells transduced with core and Wnt3A exhibited increased proliferation, promoted cell cycle progression and accelerated tumor formation in athymic nude mice. Our results are supported by several studies on interaction between virus-encoded proteins and Wnt/β-catenin tumorigenic signaling. For example, HCV NS5A protein was shown to activate β-catenin signaling cascades through increasing the stability of β-catenin 
. HBV X protein along with Wnt-1 activated Wnt/β-catenin signaling in Huh7 cells 
The stabilization and accumulation of β-catenin can be achieved by two possible mechanisms. One mechanism is Wnt-dependent stabilization of β-catenin, which was modulated by several components upstream of β-catenin molecule 
. The other mechanism is phosphorylation of GSK3β at the Ser9 residue. GSK3β is a critical component that controls the distribution balance of β-catenin between cytoplasm and nucleus. Inactivation of GSK3β leads to the accumulation of β-catenin in the cytoplasm. Many growth factors, including IGF-1, EGF and HGF have been implicated in GSK3β inhibition 
. It has been reported that GSK3β activity can be modulated by virus-encoded proteins. For example, the latent membrane protein 2A of Epstein-Barr virus and Hepatitis B virus X protein have been shown to enhance β-catenin accumulation through Ser9 phosphorylation of GSK3β 
. Erk-mediated inactivation of GSK3β may be involved in HBx- and growth factor-induced β-catenin stabilization 
. Recently, the nonstructural 5A (NS5A) protein of HCV has been shown to inactivate GSK3β activity and subsequently increase accumulation of β-catenin in hepatoma cells 
. Here, we demonstrated that the phosphorylation of GSK3β was increased in core-expression cells, indicating core may up-regulate β-catenin through inactivation of GSK-3β by phosphorylation at Serine-9. Further studies should be directed to explore the molecular mechanism behind stabilization of β-catenin involving GSK3β phosphorylation.
Exogenous core expression was shown to enhance Wnt3A-stimulated HCC tumor growth, possibly by inducing β-catenin accumulation and oncogene overexpression. Interestingly, in xenogarft animal model nuclear translocation of β-catenin in tumor cells was less prominent. There are several possible explanations. First, previous data showed that neoplastic hepatoma cells with nuclear accumulation of β-catenin were restricted to the periphery of tumor nodules or were in small tumor regions detached from large nodules 
. The liver tissue sampling we used may not include this region; therefore, nuclear accumulation of β-catenin can not be readily observed. Secondly, Huh7 cells were classified as “well-differentiated” hepatoma cells according to the epithelial gene expression profile 
, while nuclear accumulation of β-catenin was correlated with the dedifferentiation of tumor cells to immature hepatocyte progenitors 
. Thus, the differentiation status of xenograft tumors may determine the subcellular distribution of β-catenin. Lastly some components upstream of β-catenin may be negatively modulated in Huh7 cells, leading to inhibition of β-catenin translocation. It is conceivable that some if not all of the above possibilities may be responsible for the β-catenin immunostaining results in xenograft tumors.
In summary, by co-expressing HCV core protein and Wnt3A in hepatocytes and HCC cell lines, we have found that the HCV core protein can potentiate and synergize with Wnt/β-catenin signaling pathway in promoting cell proliferation and tumor growth. These results strongly suggest that HCV core, possibly through synergizing Wnt-induced stabilization and accumulation of β-catenin, may play an important role in HCV pathogenesis. Nonetheless, the exact role of HCV core in Wnt/β-catenin-induced HCC tumorigenesis remains to be fully investigated. Ultimately, this line of investigation may lead to the development of novel anti-HCV therapies by targeting both HCV itself and the canonical Wnt signaling pathway.