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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: 5.33). 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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    • "Snail induction by TGFβ is dependent on cooperation with active Ras signals. The H-Ras mediated induction of Snail depends on both MAPK and phosphatidylinositol 3- kinase (PI3K) activities (Gotzmann et al., 2002; Peinado et al., 2003). These results are consistent with the findings that Snail expression is triggered by constitutively active Akt, a kinase downstream of PI3K (Peinado et al., 2003). "
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    ABSTRACT: The Snail superfamily of zinc-finger transcription factors are involved in processes that result in pronounced cell movement during embryonic development and are also associated with the acquisition of invasive and migratory properties during tumor progression. Snail was first described in Drosophila melanogaster, where it was shown to be essential for the formation of the mesoderm. The absence of Snail is lethal, causing severe defects during the gastrula stage of development. Snail is mainly expressed in neoplastic epithelial cells. Understanding the regulation and functional roles of Snail will shed new light on the mechanisms involved in tumor progression and help with the development of novel cancer therapies. https://www.novapublishers.com/catalog/product_info.php?products_id=35363
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    • "Thus, we created an in vitro scenario with the Ishikawa EEC cell line infected with lentiviruses carrying the V600 BRAF mutation. Some studies have reported that, in many different cell systems, Ras downstream signaling is required in the process of EMT [19] [20] [21] [22]. The BRAF missense mutation V600E leads to a constitutive activation of the kinase activity of BRAF and, thus, to the activation of MEKs and ERKs [21] [22]. "
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    • "In tumours, canonical TGFb signalling is often suppressed, and cell-cycle arrest and apoptosis are bypassed by reduced TGFb receptor II (TGFbRII) expression or by mutational inactivation of Smad proteins (Massague, 2008). Yet, cancer cells utilize TGFb to promote tumour progression and survival by non-canonical TGFb signalling, which mainly results in the activation of the MAPK and the PI3K pathways (Gotzmann et al, 2002; Lee et al, 2007b). A total loss of TGFb signalling impairs late stage tumour progression and metastasis formation, demonstrating a critical role of TGFb signalling for cancer malignancy (Cui et al, 1996; Oft et al, 1998; Moustakas and Heldin, 2005). "
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    ABSTRACT: Acquiring resistance against transforming growth factor β (TGFβ)-induced growth inhibition at early stages of carcinogenesis and shifting to TGFβ's tumour-promoting functions at later stages is a pre-requisite for malignant tumour progression and metastasis. We have identified the transcription factor distal-less homeobox 2 (Dlx2) to exert critical functions during this switch. Dlx2 counteracts TGFβ-induced cell-cycle arrest and apoptosis in mammary epithelial cells by at least two molecular mechanisms: Dlx2 acts as a direct transcriptional repressor of TGFβ receptor II (TGFβRII) gene expression and reduces canonical, Smad-dependent TGFβ signalling and expression of the cell-cycle inhibitor p21(CIP1) and increases expression of the mitogenic transcription factor c-Myc. On the other hand, Dlx2 directly induces the expression of the epidermal growth factor (EGF) family member betacellulin, which promotes cell survival by stimulating EGF receptor signalling. Finally, Dlx2 expression supports experimental tumour growth and metastasis of B16 melanoma cells and correlates with tumour malignancy in a variety of human cancer types. These results establish Dlx2 as one critical player in shifting TGFβ from its tumour suppressive to its tumour-promoting functions.
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