Hepatocytes convert to a fibroblastoid phenotype through the cooperation of TGF-β1 and Ha-Ras: Steps towards invasiveness

Institute of Cancer Research, University of Vienna, Borschke-Gasse 8a, A-1090 Vienna, Austria.
Journal of Cell Science (Impact Factor: 5.43). 04/2002; 115(Pt 6):1189-202.
Source: PubMed

ABSTRACT In hepatocarcinogenesis, it is an open question whether transforming growth factor (TGF)-beta1 provides a tumor-suppressive or a tumor-promoting role. To address this question, we employed immortalized murine hepatocytes, which display a high degree of differentiation and, expectedly, arrest in the G1 phase under exposure to TGF-beta1. These hepatocytes maintain epithelial polarization upon expression of oncogenic Ha-Ras. However, Ras-transformed hepatocytes rapidly convert to a spindle-shaped, fibroblastoid morphology upon treatment with TGF-beta1, which no longer inhibits proliferation. This epithelial to fibroblastoid conversion (EFC) is accompanied by disruption of intercellular contacts and remodeling of the cytoskeletal framework. Fibroblastoid derivatives form elongated branching cords in collagen gels and grow to severely vascularized tumors in vivo, indicating their increased malignancy and even invasive phenotype. Additionally, fibroblastoid cells secrete strongly enhanced levels of TGF-beta1, suggesting an autocrine regulation of TGF-beta signaling. Expression profiling further revealed that the loss of the adhesion component E-cadherin correlates with the upregulation of its transcriptional repressor Snail in fibroblastoid cells. Moreover, the phosphoinositide 3-OH (PI3) kinase pathway was required for the maintenance of EFC, as inhibition of PI3 kinase reverted fibroblastoid cells to an epithelial-like phenotype. Taken together, these data indicate a dual role of TGF-beta1 in hepatocytes: it induces proliferation arrest but provides a crucial function in promoting late malignant events in collaboration with activated Ha-Ras.

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    • "Multiple signaling cascades are activated when TGFβ binds to its cognate receptor. These include Smad [17], RhoA-related signals [18], mitogen-activated protein kinase (MAPK)-Erk-1 and −2 [19], stress kinases (i.e., c-Jun N-terminal kinase [JNK]) [20,21], p38 mitogen-actiated protein kinase (p38MAPK) [22,23], phosphatase 2A [24], and phosphoinositide 3-kinase/AKT (PI3K/AKT) [25,26]. The pathways involved in cellular differentiation or transformation are Smad, Rho proteins, and PI3-kinase. "
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    • "Pre-malignant cells frequently gain their ability to proliferate and clonally expand due to constitutive activation of receptor tyrosine kinases and downstream effectors such as RAS. Several studies have demonstrated cooperation between growth factors and RAS signalling in the induction of EMT [10], [11], [12] suggesting that coordinated activation of multiple pathways is essential for EMT to occur. "
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    • "Loss of E-cadherin expression is a hallmark of epithelial-mesenchymal transition (EMT), during which epithelial cells lose many of their epithelial characteristics and acquire certain properties of mesenchymal cells, conferring increased motility and invasiveness. Mounting evidence suggests that cancer cells exhibit a dynamic plasticity between epithelial and mesenchymal states that allows them to survive at distinct steps of metastasis [18], [19], [20], [21]. While the consequence of the manipulation of EMT-related factors has been studied extensively, cycling of EMT has not been directly visualized in vitro or in vivo in real-time. "
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