Nuclear and cytoplasmic c-Ski differently modulate cellular functions

RIKEN, Вако, Saitama, Japan
Genes to Cells (Impact Factor: 2.81). 12/2006; 11(11):1267-80. DOI: 10.1111/j.1365-2443.2006.01018.x
Source: PubMed


c-Ski is a proto-oncogene product that induces morphologic transformation, anchorage independence, and myogenic differentiation when it is over-expressed in mesenchymal cells. c-Ski also inhibits signaling of transforming growth factor-beta (TGF-beta) superfamily members through interaction with Smad proteins. Although c-Ski is predominantly localized in the nucleus, aberrant cytoplasmic localization of it has also been reported in some tumor tissues and cell lines. In the present study, we identified the nuclear localization signal (NLS) in c-Ski. By introducing a mutation to abolish NLS activity, we examined the function of cytoplasmic c-Ski. Although cytoplasmic c-Ski suppressed TGF-beta superfamily-induced Smad signaling through sequestration of activated Smad complex to the cytoplasm, it failed to exhibit some of the activities that require nuclear localization of c-Ski, including suppression of basal transcription of the Smad7 gene. These findings indicate that subcellular localization of c-Ski affects its biologic activities. We also found that c-Ski accumulated in the cytoplasm when proteasome activity was inhibited. Mapping of the regions required for cytoplasmic accumulation by proteasome inhibitors suggests that subcellular localization of c-Ski may be regulated by proteasome-sensitive processes through amino acid residues 94-210 and 491-548.

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Available from: Keiji Miyazawa, Mar 27, 2015
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    • "The protooncoprotein Ski has a complex set of functions and is linked to skeletal muscle hypertrophy (Sutrave et al., 1990a; Sutrave et al., 1990b), as well as developmental defects in the neural tube and cranial mesenchyme (Berk et al., 1997). Ski binds to R-Smad proteins (Sun et al., 1999; Suzuki et al., 2004; Ueki and Hayman, 2003; Xu et al., 2000) and might inhibit TGF-b-mediated effects through both nuclear and cytosolic mechanisms (Akiyoshi et al., 1999; Ferrand et al., 2010; Nagata et al., 2006; Suzuki et al., 2004). We have previously demonstrated the anti-fibrotic and anti-contractile effects of overexpression of Ski in TGF-b1- stimulated primary cardiac myofibroblasts, and have shown these inhibitory effects to be orchestrated at the nuclear level (Cunnington et al., 2011). "
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    ABSTRACT: Background: Cardiac fibrosis is linked to fibroblast to myofibroblast phenoconversion and proliferation; mechanisms underlying this phenoconversion are poorly understood. c-Ski (Ski) is a negative regulator of TGF-β/Smad signaling in myofibroblasts, and may redirect the myofibroblast phenotype back to fibroblasts. Meox2 may alter TGF-β-mediated cellular processes and is repressed by Zeb2. Hypothesis: Ski diminishes the myofibroblast phenotype by de-repressing Meox2 expression and function via repression of Zeb2 expression. Results: Meox1 and Meox2 mRNA expression, Meox2 protein expression are reduced during phenoconversion of fibroblasts to myofibroblasts. Meox2 over-expression shifts the myofibroblasts to fibroblasts, whereas the Meox2 DNA-binding mutant has no effect on myofibroblast phenotype. Ski over-expression partially restores Meox2 mRNA expression levels to those in cardiac fibroblasts. Expression of Zeb2 increased during phenoconversion and Ski over-expression reduces Zeb2 expression in first-passage myofibroblasts. Meox2 expression is decreased in scar following myocardial infarction, whereas Zeb2 protein expression increases in the infarct scar. Thus Ski modulates the cardiac myofibroblast phenotype and function via suppression of Zeb2 by up-regulating Meox2. This cascade may regulate cardiac myofibroblast phenotype and presents therapeutic options for treatment of cardiac fibrosis.
    Full-text · Article · Oct 2013 · Journal of Cell Science
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    • "It is likely that Ski levels increase in HKc/DR because TGF-β signaling is reduced in these cells, and TGF-β is known to promote Ski degradation (Baldwin et al., 2004; Le Scolan et al., 2008; Sun et al., 1999; Nagano et al., 2010). At the same time, however, Ski can contribute to further decreasing TGF-β signaling in HKc/DR through its many interactions with components of the TGF-β signaling pathway (Nagata et al., 2006; Jacob et al., 2008; Kokura et al., 2003; Prunier et al., 2003; Ferrand et al., 2010). In addition, increased levels of Ski are likely to promote E6/E7 expression: We have previously demonstrated that Ski cooperates with NF1 to increase HPV16 URR activity in HKc/HPV16 and HKc/DR, leading to increased expression of E6 and E7 (Baldwin et al., 2004). "
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    ABSTRACT: We compared the levels of the Ski oncoprotein, an inhibitor of transforming growth factor-beta (TGF-β) signaling, in normal human keratinocytes (HKc), HPV16 immortalized HKc (HKc/HPV16), and differentiation resistant HKc/HPV16 (HKc/DR) in the absence and presence of TGF-β. Steady-state Ski protein levels increased in HKc/HPV16 and even further in HKc/DR, compared to HKc. TGF-β treatment of HKc, HKc/HPV16, and HKc/DR dramatically decreased Ski. TGF-β-induced Ski degradation was delayed in HKc/DR. Ski and phospho-Ski protein levels are cell cycle dependent with maximal Ski expression and localization to centrosomes and mitotic spindles during G2/M. ShRNA knock down of Ski in HKc/DR inhibited cell proliferation. More intense nuclear and cytoplasmic Ski staining and altered Ski localization were found in cervical cancer samples compared to adjacent normal tissue in a cervical cancer tissue array. Overall, these studies demonstrate altered Ski protein levels, degradation and localization in HPV16-transformed human keratinocytes and in cervical cancer.
    Full-text · Article · Jun 2013 · Virology
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    • "As c-Ski binds competitively to Smads with the transcriptional coactivator p300 (Akiyoshi et al. 1999), an excess of c-Ski within the nucleus may cause a reduced response to TGF-b. Other studies using mutant c-Ski proteins that lack nuclear localization signals show the capacity of c- Ski to inhibit TGF-b within the cytoplasm (Nagata et al. 2006), suggesting that the antagonizing ability of c-Ski is not limited to transcriptional repression. c-Ski is a phosphorylatable protein (Sutrave et al. 1990a) that is suggested to act as a substrate for the cyclin-dependent protein kinase cdc2 / cyclinB (Marcelain and Hayman 2005), and it is therefore likely that the phosphorylation state of c-Ski regulates its function and (or) location within the cell. "
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    ABSTRACT: Recent studies demonstrate the critical role of the extracellular matrix in the organization of parenchymal cells in the heart. Thus, an understanding of the modes of regulation of matrix production by cardiac myofibroblasts is essential. Transforming growth factor beta (TGF-beta) signaling is transduced through the canonical Smad pathway, and the involvement of this pathway in matrix synthesis and other processes requires precise control. Inhibition of Smad signaling may be achieved at the receptor level through the targeting of the TGF-beta type I receptors with an inhibitory Smad7/Smurf2 complex, or at the transcriptional level through c-Ski/receptor-Smad/co-mediator Smad4 interactions. Conversely, Arkadia protein intensifies TGF-beta-induced effects by marking c-Ski and inhibitory Smad7 for destruction. The study of these TGF-beta mediators is essential for future treatment of fibrotic disease, and this review highlights recent relevant findings that may impact our understanding of cardiac fibrosis.
    Preview · Article · Oct 2009 · Canadian Journal of Physiology and Pharmacology
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