Fibroblast Growth Factor Receptor-Mediated Activation of AKT-β-Catenin-CBP Pathway Regulates Survival and Proliferation of Murine Hepatoblasts and Hepatic Tumor Initiating Stem Cells

Nirmala Mavila,Kasper S Wang,Nguyen Cu,Sarah Utley,Michael Kahn,C Bart Rountree,David James,Katrina Mak,Orly Coblens,Saverio Bellusci

doi:10.1371/journal.pone.0050401

Abstract

Fibroblast Growth Factor (FGF)-10 promotes the proliferation and survival of murine hepatoblasts during early stages of hepatogenesis through a Wnt-β-catenin dependent pathway. To determine the mechanism by which this occurs, we expanded primary culture of hepatoblasts enriched for progenitor markers CD133 and CD49f from embryonic day (E) 12.5 fetal liver and an established tumor initiating stem cell line from Mat1a−/− livers in media conditioned with recombinant (r) FGF10 or rFGF7. FGF Receptor (R) activation resulted in the downstream activation of MAPK, PI3K-AKT, and β-catenin pathways, as well as cellular proliferation. Additionally, increased levels of nuclear β-catenin phosphorylated at Serine-552 in cultured primary hepatoblasts, Mat1a−/− cells, and also in ex vivo embryonic liver explants indicate AKT-dependent activation of β-catenin downstream of FGFR activation; conversely, the addition of AKT inhibitor Ly294002 completely abrogated β-catenin activation. FGFR activation-induced cell proliferation and survival were also inhibited by the compound ICG-001, a small molecule inhibitor of β-catenin-CREB Binding Protein (CBP) in hepatoblasts, further indicating a CBP-dependent regulatory mechanism of β-catenin activity. Conclusion: FGF signaling regulates the proliferation and survival of embryonic and transformed progenitor cells in part through AKT-mediated activation of β-catenin and downstream interaction with the transcriptional co-activator CBP.

Highlights

Despite progress in characterizing the physiology of hepatic progenitor/stem cells, numerous aspects of signaling pathways regulating these progenitor/stem cells remain undefined
Fibroblast Growth Factor (FGF) Signaling in E12.5 Murine Hepatoblasts We previously showed that E12.5 murine hepatoblasts, which exhibit FGF10-dependent b-catenin, co-express Albumin, Cytokeratin-19, and Fgfr2IIIb [36]
We previously reported that FGF10, secreted by embryonic hepatic stellate cells, promotes the proliferation and survival of hepatoblasts via activation of b-catenin pathway after the initial liver bud formation [36]

Summary

Introduction

Despite progress in characterizing the physiology of hepatic progenitor/stem cells, numerous aspects of signaling pathways regulating these progenitor/stem cells remain undefined. Under circumstances where liver regeneration via hepatocyte proliferation is impaired, there is evidence of FGF signaling activation promoting the proliferation of epithelial progenitor cells in response to liver injury [10] Further elucidation of these pathways could provide insight into repair mechanisms involving progenitor cells, and into the biology of liver tumors such as hepatoblastoma, which are theorized to arise from clonal expansion of malignantly transformed progenitor/stem cells [11,12]. Calmont et al demonstrated that during the initial stages of hepatogenesis, the FGF-mediated MAPK pathway regulates endodermal cell specification with induction of hepatic genes, such as Albumin, whereas PI3K pathway activation promotes liver growth during early embryonic liver development [17]

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Discussion

Conclusion