Autophagy in Oncogenic K-Ras Promotes Basal Extrusion of Epithelial Cells by Degrading S1P

Current biology: CB (Impact Factor: 9.57). 12/2013; 24(1). DOI: 10.1016/j.cub.2013.11.029
Source: PubMed


To maintain a protective barrier, epithelia extrude cells destined to die by contracting a band of actin and myosin. Although extrusion can remove cells triggered to die by apoptotic stimuli, to maintain constant cell numbers, epithelia extrude live cells, which later die by anoikis. Because transformed cells may override anoikis and survive after extrusion, the direction of extrusion has important consequences for the extruded cell's fate. As most cells extrude apically, they are typically eliminated through the lumen; however, cells with upregulated survival signals that extrude basally could potentially invade the underlying tissue and migrate to other sites in the body.
We found that oncogenic K-Ras cells predominantly extrude basally, rather than apically, in a cell-autonomous manner and can survive and proliferate after extrusion. Expression of K-Ras(V12) downregulates the bioactive lipid sphingosine 1-phosphate (S1P) and its receptor S1P2, both of which are required for apical extrusion. Surprisingly, the S1P biosynthetic pathway is not affected because the S1P precursor, sphingosine kinase, and the degradative enzymes S1P lyase and S1PP phosphatase are not significantly altered. Instead, we found that high levels of autophagy in extruding Ras(V12) cells leads to S1P degradation. Disruption of autophagy chemically or genetically in K-Ras(V12) cells rescues S1P localization and apical extrusion.
Oncogenic K-Ras cells downregulate both S1P and its receptor S1P2 to promote basal extrusion. Because live basally extruding cells can survive and proliferate after extrusion, we propose that basal cell extrusion provides a novel mechanism for cells to exit the epithelium and initiate invasion into the surrounding tissues.

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Available from: Jody Rosenblatt, Jan 21, 2014
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    • "S1P expressed by apoptotic cells or RasV12-transformed cells has been reported to be an important regulator for the elimination of those cells from the epithelium (Gu et al., 2011; Slattum et al., 2014). SphKI2 is a specific inhibitor for sphingosine kinase 1 (SphK1) that catalyzes the production of S1P from sphingosine. "
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    ABSTRACT: At the initial step of carcinogenesis, transformation occurs in single cells within epithelia where the newly emerging transformed cells are surrounded by normal epithelial cells. A recent study has revealed that normal epithelial cells have an ability to sense and actively eliminate the neighboring transformed cells, a process named EDAC (Epithelial Defense Against Cancer). However, the molecular mechanism of this tumor-suppressive activity remains largely unknown. In this study, we have investigated a role for the sphingosine-1-phosphate (S1P)-S1P receptor 2 (S1PR2) pathway in EDAC. First, we show that addition of the S1PR2 inhibitor significantly suppresses apical extrusion of RasV12-transformed cells that are surrounded by normal cells. In addition, knockdown of S1PR2 in normal cells induces the same effect, indicating that S1PR2 in the surrounding normal cells plays a positive role in the apical elimination of the transformed cells. Importantly, not endogenous S1P but exogenous S1P is involved in this process. By using FRET analyses, we demonstrate that S1PR2 mediates Rho activation in normal cells neighboring RasV12-transformed cells, thereby promoting accumulation of filamin, a crucial regulator of EDAC. Collectively, these data indicate that S1P is a key extrinsic factor that affects the outcome of cell competition between normal and transformed epithelial cells.
    Preview · Article · Dec 2015 · Molecular biology of the cell
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    • "Therefore, live-cell extrusion and subsequent apoptosis has been proposed to be a tumour-suppressive process that prevents the formation of multilayered epithelia (Eisenhoffer et al., 2012; Eisenhoffer and Rosenblatt, 2013). Events of live-cell extrusion have also been observed upon the activation of different oncogenes (such as K-Ras, H-Ras, Src and ERBB2) within single cells of an epithelium (Hogan et al., 2009; Kajita et al., 2010; Leung and Brugge, 2012; Slattum et al., 2014; Wu et al., 2014). These oncogene-driven extrusion events are subsequently coupled to proliferation, not apoptosis. "
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    ABSTRACT: Epithelial cell extrusion and subsequent apoptosis is a key mechanism to prevent accumulation of excess cells. Conversely, when driven by oncogene expression, apical cell extrusion is followed by proliferation and represents an initial step of tumorigenesis. E-cadherin (E-cad), the main component of adherens junctions, has been shown to be essential for epithelial cell extrusion, but its mechanistic contribution remains unclear. Here, we provide clear evidence that cell extrusion can be driven by E-cad cleavage, both in a wild type and oncogenic environment. We first show that CDC42 activation in a single epithelial cell results in its efficient MMP-sensitive extrusion through MEK signaling activation and is supported by E-cad cleavage. Second, using an engineered cleavable form of E-cad, we demonstrate that sole extracellular E-cad truncation at the plasma membrane promotes apical extrusion. We propose that extracellular cleavage of E-cad generates a rapid change in cell-cell adhesion sufficient to drive apical cell extrusion. Whereas in normal epithelia, extrusion is followed by apoptosis, when combined to active oncogenic signaling, it is coupled to cell proliferation.
    Full-text · Article · Jun 2014 · Journal of Cell Science
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    ABSTRACT: Metastasis is the leading cause of cancer-related deaths, but it is unclear how cancer cells escape their primary sites in epithelia and disseminate to other sites in the body. One emerging possibility is that transformed epithelial cells could invade the underlying tissue by a process called cell extrusion, which epithelia use to remove cells without disrupting their barrier function. Typically, during normal cell turnover, live cells extrude apically from the epithelium into the lumen and later die by anoikis; however, several oncogenic mutations shift cell extrusion basally, towards the tissue that the epithelium encases. Tumour cells with high levels of survival and motility signals could use basal extrusion to escape from the tissue and migrate to other sites within the body.
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