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      Ras and TGFβ cooperatively regulate epithelial cell plasticity and metastasis : dissection of Ras signaling pathways

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          Abstract

          Multistep carcinogenesis involves more than six discrete events also important in normal development and cell behavior. Of these, local invasion and metastasis cause most cancer deaths but are the least well understood molecularly. We employed a combined in vitro/in vivo carcinogenesis model, that is, polarized Ha-Ras–transformed mammary epithelial cells (EpRas), to dissect the role of Ras downstream signaling pathways in epithelial cell plasticity, tumorigenesis, and metastasis. Ha-Ras cooperates with transforming growth factor β (TGFβ) to cause epithelial mesenchymal transition (EMT) characterized by spindle-like cell morphology, loss of epithelial markers, and induction of mesenchymal markers. EMT requires continuous TGFβ receptor (TGFβ-R) and oncogenic Ras signaling and is stabilized by autocrine TGFβ production. In contrast, fibroblast growth factors, hepatocyte growth factor/scatter factor, or TGFβ alone induce scattering, a spindle-like cell phenotype fully reversible after factor withdrawal, which does not involve sustained marker changes. Using specific inhibitors and effector-specific Ras mutants, we show that a hyperactive Raf/mitogen-activated protein kinase (MAPK) is required for EMT, whereas activation of phosphatidylinositol 3-kinase (PI3K) causes scattering and protects from TGFβ-induced apoptosis. Hyperactivation of the PI3K pathway or the Raf/MAPK pathway are sufficient for tumorigenesis, whereas EMT in vivo and metastasis required a hyperactive Raf/MAPK pathway. Thus, EMT seems to be a close in vitro correlate of metastasis, both requiring synergism between TGFβ-R and Raf/MAPK signaling.

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          Most cited references 45

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          RADIOAUTOGRAPHIC STUDIES OF CHOLINE INCORPORATION INTO PERIPHERAL NERVE MYELIN

          This radioautographic study was designed to localize the cytological sites involved in the incorporation of a lipid precursor into the myelin and the myelin-related cell of the peripheral nervous system. Both myelinating and fully myelinated cultures of rat dorsal root ganglia were exposed to a 30-min pulse of tritiated choline and either fixed immediately or allowed 6 or 48 hr of chase incubation before fixation. After Epon embedding, light and electron microscopic radioautograms were prepared with Ilford L-4 emulsion. Analysis of the pattern of choline incorporation into myelinating cultures indicated that radioactivity appeared all along the length of the internode, without there being a preferential site of initial incorporation. Light microscopic radioautograms of cultures at varying states of maturity were compared in order to determine the relative degree of myelin labeling. This analysis indicated that the myelin-Schwann cell unit in the fully myelinated cultures incorporated choline as actively as did this unit in the myelinating cultures. Because of technical difficulties, it was not possible to determine the precise localization of the incorporated radioactivity within the compact myelin. These data are related to recent biochemical studies indicating that the mature myelin of the central nervous system does incorporate a significant amount of lipid precursor under the appropriate experimental conditions. These observations support the concept that a significant amount of myelin-related metabolic activity occurs in mature tissue; this activity is considered part of an essential and continuous process of myelin maintenance and repair.
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            Transcriptional control by the TGF-beta/Smad signaling system.

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              A mechanism of repression of TGFbeta/ Smad signaling by oncogenic Ras.

              TGFbeta can override the proliferative effects of EGF and other Ras-activating mitogens in normal epithelial cells. However, epithelial cells harboring oncogenic Ras mutations often show a loss of TGFbeta antimitogenic responses. Here we report that oncogenic Ras inhibits TGFbeta signaling in mammary and lung epithelial cells by negatively regulating the TGFbeta mediators Smad2 and Smad3. Oncogenically activated Ras inhibits the TGFbeta-induced nuclear accumulation of Smad2 and Smad3 and Smad-dependent transcription. Ras acting via Erk MAP kinases causes phosphorylation of Smad2 and Smad3 at specific sites in the region linking the DNA-binding domain and the transcriptional activation domain. These sites are separate from the TGFbeta receptor phosphorylation sites that activate Smad nuclear translocation. Mutation of these MAP kinase sites in Smad3 yields a Ras-resistant form that can rescue the growth inhibitory response to TGFbeta in Ras-transformed cells. EGF, which is weaker than oncogenic mutations at activating Ras, induces a less extensive phosphorylation and cytoplasmic retention of Smad2 and Smad3. Our results suggest a mechanism for the counterbalanced regulation of Smad2/Smad3 by TGFbeta and Ras signals in normal cells, and for the silencing of antimitogenic TGFbeta functions by hyperactive Ras in cancer cells.
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                Author and article information

                Journal
                J Cell Biol
                The Journal of Cell Biology
                The Rockefeller University Press
                0021-9525
                1540-8140
                21 January 2002
                : 156
                : 2
                : 299-314
                Affiliations
                [1 ]Institute of Molecular Pathology, A-1030 Vienna, Austria
                [2 ]Signal Transduction Laboratories, Imperial Cancer Research Fund, London WC2A 3PX, United Kingdom
                Author notes

                Address correspondence to Stefan Grünert, Dr. Bohrgasse 7, A-1030, Vienna, Austria. Tel.: 43-1-79730. Fax: 43-1-798-7153. E-mail: grunert@ 123456nt.imp.univie.ac.at

                Article
                0109037
                10.1083/jcb.200109037
                2199233
                11790801
                Copyright © 2002, The Rockefeller University Press
                Categories
                Article

                Cell biology

                pi3k; mapk; tgfβ; emt; metastasis

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