PDGF essentially links TGF-β signaling to nuclear β-catenin accumulation in hepatocellular carcinoma progression

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Abstract
The cooperation of Ras – extracellular signal-regulated kinase/mitogen-activated protein kinase and transforming growth factor (TGF)-β signaling provokes an epithelial to mesenchymal transition (EMT) of differentiated p19ARF null hepatocytes, which is accompanied by a shift in malignancy and gain of metastatic properties. Upon EMT, TGF-β induces the secretion and autocrine regulation of platelet-derived growth factor (PDGF) by upregulation of PDGF-A and both PDGF receptors. Here, we demonstrate by loss-of-function analyses that PDGF provides adhesive and migratory properties in vitro as well as proliferative stimuli during tumor formation. PDGF signaling resulted in the activation of phosphatidylinositol-3 kinase, and furthermore associated with nuclear β-catenin accumulation upon EMT. Hepatocytes expressing constitutively active β-catenin or its negative regulator Axin were employed to study the impact of nuclear β-catenin. Unexpectedly, active β-catenin failed to accelerate proliferation during tumor formation, but in contrast, correlated with growth arrest. Nuclear localization of β-catenin was accompanied by strong expression of the Cdk inhibitor p16INK4A and the concomitant induction of the β-catenin target genes cyclin D1 and c-myc. In addition, active β-catenin revealed protection of malignant hepatocytes against anoikis, which provides a prerequisite for the dissemination of carcinoma. From these data, we conclude that TGF-β acts tumor progressive by induction of PDGF signaling and subsequent activation of β-catenin, which endows a subpopulation of neoplastic hepatocytes with features of cancer stem cells.