A carboxyl-terminal region of the ski oncoprotein mediates homodimerization as well as heterodimerization with the related protein SnoN

SKI controls MDS-associated chronic TGF-β signaling, aberrant splicing, and stem cell fitness

The transforming growth factor beta (TGF-β) signaling pathway controls hematopoietic stem cell (HSC) behavior in the marrow niche; however, TGF-β signaling becomes chronic in early-stage myelodysplastic syndrome (MDS). Although TGF-β signaling normally induces negative feedback, in early-stage MDS, high levels of microRNA-21 (miR-21) contribute to chronic TGF-β signaling. We found that a TGF-β signal-correlated gene signature is sufficient to identify an MDS patient population with abnormal RNA splicing (eg, CSF3R) independent of splicing factor mutations and coincident with low HNRNPK activity. Levels of SKI messenger RNA (mRNA) encoding a TGF-β antagonist are sufficient to identify these patients. However, MDS patients with high SKI mRNA and chronic TGF-β signaling lack SKI protein because of miR-21 activity. To determine the impact of SKI loss, we examined murine Ski -/- HSC function. First, competitive HSC transplants revealed a profound defect in stem cell fitness (competitive disadvantage) but not specification, homing, or multilineage production. Aged recipients of Ski -/- HSCs exhibited mild phenotypes similar to phenotypes in those with macrocytic anemia. Second, blastocyst complementation revealed a dramatic block in Ski -/- hematopoiesis in the absence of transplantation. Similar to SKI-high MDS patient samples, Ski -/- HSCs strikingly upregulated TGF-β signaling and deregulated expression of spliceosome genes (including Hnrnpk). Moreover, novel single-cell splicing analyses demonstrated that Ski -/- HSCs and high levels of SKI expression in MDS patient samples share abnormal alternative splicing of common genes (including those that encode splicing factors). We conclude that miR-21-mediated loss of SKI activates TGF-β signaling and alternative splicing to impair the competitive advantage of normal HSCs (fitness), which could contribute to selection of early-stage MDS-genic clones.

Transcriptional cofactors Ski and SnoN are major regulators of the TGF-β/Smad signaling pathway in health and disease

The transforming growth factor-β (TGF-β) family plays major pleiotropic roles by regulating many physiological processes in development and tissue homeostasis. The TGF-β signaling pathway outcome relies on the control of the spatial and temporal expression of >500 genes, which depend on the functions of the Smad protein along with those of diverse modulators of this signaling pathway, such as transcriptional factors and cofactors. Ski (Sloan-Kettering Institute) and SnoN (Ski novel) are Smad-interacting proteins that negatively regulate the TGF-β signaling pathway by disrupting the formation of R-Smad/Smad4 complexes, as well as by inhibiting Smad association with the p300/CBP coactivators. The Ski and SnoN transcriptional cofactors recruit diverse corepressors and histone deacetylases to repress gene transcription. The TGF-β/Smad pathway and coregulators Ski and SnoN clearly regulate each other through several positive and negative feedback mechanisms. Thus, these cross-regulatory processes finely modify the TGF-β signaling outcome as they control the magnitude and duration of the TGF-β signals. As a result, any alteration in these regulatory mechanisms may lead to disease development. Therefore, the design of targeted therapies to exert tight control of the levels of negative modulators of the TGF-β pathway, such as Ski and SnoN, is critical to restore cell homeostasis under the specific pathological conditions in which these cofactors are deregulated, such as fibrosis and cancer.

Proteins that repress molecular signaling through the transforming growth factor-beta (TGF-β) pathway offer promising targets for treating cancer and fibrosis. Marina Macías-Silva and colleagues from the National Autonomous University of Mexico in Mexico City review the ways in which a pair of proteins, called Ski and SnoN, interact with downstream mediators of TGF-β to inhibit the effects of this master growth factor. Aberrant levels of Ski and SnoN have been linked to diverse range of diseases involving cell proliferation run amok, and therapies that regulate the expression of these proteins could help normalize TGF-β signaling to healthier physiological levels. For decades, drug companies have tried to target the TGF-β pathway, with limited success. Altering the activity of these repressors instead could provide a roundabout way of remedying pathogenic TGF-β activity in fibrosis and oncology.

SnoN as a novel negative regulator of TGF-β/Smad signaling: a target for tailoring organ fibrosis

Remodeling of the extracellular matrix is beneficial during the acute wound healing stage following tissue injury. In the short term, resident fibroblasts and myofibroblasts regulate the matrix remodeling process through production of matricellular protein components that provide structural support to the damaged tissue. This process is largely governed by the transforming growth factor-β1 (TGF-β1) pathway, a critical mediator of the remodeling process. In the long term, chronic activation of the TGF-β1 pathway promotes excessive synthesis and deposition of matrix proteins, including fibrillar collagens, which ultimately leads to organ failure. SnoN (and its alternatively-spliced isoforms SnoN2, SnoA, and SnoI) is one of four members of a family of negative regulators of TGF-β1 signaling that includes Ski and functional Smad-suppressing elements on chromosomes 15 and 18. SnoN has been shown to be structurally and functionally similar to Ski and has been demonstrated to directly interact with Ski to abrogate gene expression. Despite this, little progress has been made in delineating a specific role for SnoN in the regulation of myofibroblast phenotype and function. This review outlines the current body of knowledge of what we refer to as the “Ski-Sno superfamily,” with a focus on the structural and functional importance of SnoN in mediating the fibrotic response by myofibroblasts following tissue injury.

Transforming growth factor-β/SMAD Target gene SKIL is negatively regulated by the transcriptional cofactor complex SNON-SMAD4

The human SKI-like (SKIL) gene encodes the SMAD transcriptional corepressor SNON that antagonizes TGF-β signaling. SNON protein levels are tightly regulated by the TGF-β pathway: whereas a short stimulation with TGF-β decreases SNON levels by its degradation via the proteasome, longer TGF-β treatment increases SNON levels by inducing SKIL gene expression. Here, we investigated the molecular mechanisms involved in the self-regulation of SKIL gene expression by SNON. Bioinformatics analysis showed that the human SKIL gene proximal promoter contains a TGF-β response element (TRE) bearing four groups of SMAD-binding elements that are also conserved in mouse. Two regions of 408 and 648 bp of the human SKIL gene (∼2.4 kb upstream of the ATG initiation codon) containing the core promoter, transcription start site, and the TRE were cloned for functional analysis. Binding of SMAD and SNON proteins to the TRE region of the SKIL gene promoter after TGF-β treatment was demonstrated by ChIP and sequential ChIP assays. Interestingly, the SNON-SMAD4 complex negatively regulated basal SKIL gene expression through binding the promoter and recruiting histone deacetylases. In response to TGF-β signal, SNON is removed from the SKIL gene promoter, and then the activated SMAD complexes bind the promoter to induce SKIL gene expression. Subsequently, the up-regulated SNON protein in complex with SMAD4 represses its own expression as part of the negative feedback loop regulating the TGF-β pathway. Accordingly, when the SNON-SMAD4 complex is absent as in some cancer cells lacking SMAD4 the regulation of some TGF-β target genes is modified.

SnoN in regulation of embryonic development and tissue morphogenesis

SnoN (Ski-novel protein) plays an important role in embryonic development, tumorigenesis and aging. Past studies largely focused on its roles in tumorigenesis. Recent studies of its expression patterns and functions in mouse models and mammalian cells have revealed that SnoN interacts with multiple signaling molecules at different cellular levels to modulate the activities of several signaling pathways in a tissue context and developmental stage dependent manner. These studies suggest that SnoN may have broad functions in the embryonic development and tissue morphogenesis.

SnoN signaling in proliferating cells and postmitotic neurons

The transcriptional regulator SnoN plays a fundamental role as a modulator of transforming growth factor beta (TGFβ)-induced signal transduction and biological responses. In recent years, novel functions of SnoN have been discovered in both TGFβ-dependent and TGFβ-independent settings in proliferating cells and postmitotic neurons. Accumulating evidence suggests that SnoN plays a dual role as a corepressor or coactivator of TGFβ-induced transcription. Accordingly, SnoN exerts oncogenic or tumor-suppressive effects in epithelial tissues. At the cellular level, SnoN antagonizes or mediates the ability of TGFβ to induce cell cycle arrest in a cell-type specific manner. SnoN also exerts key effects on epithelial-mesenchymal transition (EMT), with implications in cancer biology. Recent studies have expanded SnoN functions to postmitotic neurons, where SnoN orchestrates key aspects of neuronal development in the mammalian brain, from axon growth and branching to neuronal migration and positioning. In this review, we will highlight our understanding of SnoN biology at the crossroads of cancer biology and neurobiology.

Gonadotropin-releasing hormone pulse sensitivity of follicle-stimulating hormone-beta gene is mediated by differential expression of positive regulatory activator protein 1 factors and corepressors SKIL and TGIF1

Gonadotropin synthesis and release is dependent on pulsatile stimulation by the hypothalamic neuropeptide GnRH. Generally, slow GnRH pulses promote FSH production, whereas rapid pulses favor LH, but the molecular mechanism underlying this pulse sensitivity is poorly understood. In this study, we developed and tested a model for FSHβ regulation in mouse LβT2 gonadotropes. By mining a previous microarray data set, we found that mRNA for positive regulators of Fshb expression, such as Fos and Jun, were up-regulated at slower pulse frequencies than a number of potential negative regulators, such as the corepressors Skil, Crem, and Tgif1. These latter corepressors reduced Fshb promoter activity whether driven by transfection of individual transcription factors or by treatment with GnRH and activin. Overexpression of binding or phosphorylation-defective ski-oncogene-like protein (SKIL) and TG interacting factor (TGIF1) mutants, however, failed to repress Fshb promoter activity. Knockdown of the endogenous repressors SKIL and TGIF1, but not cAMP response element-modulator, increased Fshb promoter activity driven by constant GnRH or activin. Chromatin immunoprecipitation analysis showed that FOS, SKIL, and TGIF1 occupy the FSHβ promoter in a cyclical manner after GnRH stimulation. Overexpression of corepressors SKIL or TGIF1 repressed induction of the Fshb promoter at the slow GnRH pulse frequency but had little effect at the fast pulse frequency. In contrast, knockdown of endogenous SKIL or TGIF1 selectively increased Fshb mRNA at the fast GnRH pulse frequency. Therefore, we propose a potential mechanism by which production of gonadotropin Fshb is modulated by positive transcription factors and negative corepressors with different pulse sensitivities.

Identification of Ski as a target for Aurora A kinase

Ski is a negative regulator of the transforming growth factor-β and other signalling pathways. The absence of SKI in mouse fibroblasts leads to chromosome segregation defects and genomic instability, suggesting a role for Ski during mitosis. At this stage, Ski is phosphorylated but to date little is known about the kinases involved in this process. Here, we show that Aurora A kinase is able to phosphorylate Ski in vitro. In vivo, Aurora A and Ski co-localized at the centrosomes and co-immunoprecipitated. Conversely, a C-terminal truncation mutant of Ski (SkiΔ491-728) lacking a coiled-coil domain, displayed decreased centrosomal localization. This mutant no longer co-immunoprecipitated with Aurora-A in vivo, but was still phosphorylated in vitro, indicating that the Ski-Aurora A interaction takes place at the centrosomes. These data identify Ski as a novel target of Aurora A and contribute to an understanding of the role of these proteins in the mitotic process.

Ski and SnoN, potent negative regulators of TGF-beta signaling

Ski and the closely related SnoN were discovered as oncogenes by their ability to transform chicken embryo fibroblasts upon overexpression. While elevated expressions of Ski and SnoN have also been reported in many human cancer cells and tissues, consistent with their pro-oncogenic activity, emerging evidence also suggests a potential anti-oncogenic activity for both. In addition, Ski and SnoN have been implicated in regulation of cell differentiation, especially in the muscle and neuronal lineages. Multiple cellular partners of Ski and SnoN have been identified in an effort to understand the molecular mechanisms underlying the complex roles of Ski and SnoN. In this review, we summarize recent findings on the biological functions of Ski and SnoN, their mechanisms of action and how their levels of expression are regulated.

Clinical significance of the expression of c-Ski and SnoN, possible mediators in TGF-beta resistance, in primary cutaneous melanoma

BACKGROUND

Loss of TGF-beta growth control is considered as a hallmark of several human neoplasms including melanoma. Resistance of cancer cells to TGF-beta has been linked to mutations in proteins involved in the TGF-beta pathway. In melanoma such mutations have not been observed. C-Ski and SnoN, two structurally and functionally highly homologous proteins, are known as negative regulators in the TGF-beta signaling pathway. C-Ski and SnoN expression levels and subcellular localization have been associated with clinicopathological parameters and tumour progression in several human malignancies. In melanoma cell lines, high c-Ski and SnoN expression levels have been described.

OBJECTIVE

The objective of this study was to evaluate the clinical value of c-Ski and SnoN expression in primary cutaneous melanoma.

METHODS

We evaluated c-Ski and SnoN expression by immunohistochemical staining in 120 primary melanomas. Possible associations between c-Ski and SnoN staining patterns and clinicopathological parameters were analyzed.

RESULTS

Nuclear c-Ski expression was significantly associated with thicker and ulcerated tumours. The percentage of SnoN positivity was higher in ulcerated tumours and in the sentinel node positive group.

CONCLUSION

These results suggest that c-Ski and SnoN, mediators in TGF-beta resistance, might be implicated in melanoma growth and progression.

SnoN expression is differently regulated in microsatellite unstable compared with microsatellite stable colorectal cancers

Background

SnoN is an important regulator of the transforming growth factor beta (TGFβ) signalling pathway and has been shown to exhibit both tumour promotion and suppression activity.

Methods

To further explore the role of this complex molecule in colorectal tumorigenesis, we examined 52 paired normal and tumour colorectal specimens stratified by level of microsatellite instability; 18 with high-level microsatellite instability (MSI-H) and 34 microsatellite stable (MSS). SnoN transcript expression was quantitated by real-time PCR and analysed with respect to clinical indicators of prognosis.

Results

Within the MSI-H subgroup, SnoN was commonly either up-regulated (6/18, 33%) or down-regulated (7/18, 39%). A significantly different distribution of SnoN expression was observed in MSS cancers compared with MSI-H (P ≤ 0.001). Whilst 17/34 (50%) of MSS tumours demonstrated up-regulation, none showed down-regulated expression. Within the MSI-H subgroup, up-regulation was significantly correlated with lack of repeat tract mutation in the TGFβRII gene (P ≤ 0.025), suggesting that SnoN is more frequently up-regulated in the presence of functional TGFβ signalling.

Conclusion

Together these data support the notion that SnoN has both oncogenic and tumour suppressive properties depending on other genetic changes within the tumour, and that the MSI-H pathway of colorectal tumorigenesis presents an excellent model for the study of these opposing functions.

Sumoylated SnoN represses transcription in a promoter-specific manner

The transcriptional modulator SnoN controls a diverse set of biological processes, including cell proliferation and differentiation. The mechanisms by which SnoN regulates these processes remain incompletely understood. Recent studies have shown that SnoN exerts positive or negative regulatory effects on transcription. Because post-translational modification of proteins by small ubiquitin-like modifier (SUMO) represents an important mechanism in the control of the activity of transcriptional regulators, we asked if this modification regulates SnoN function. Here, we show that SnoN is sumoylated. Our data demonstrate that the SUMO-conjugating E2 enzyme Ubc9 is critical for SnoN sumoylation and that the SUMO E3 ligase PIAS1 selectively interacts with and enhances the sumoylation of SnoN. We identify lysine residues 50 and 383 as the SUMO acceptor sites in SnoN. Analyses of SUMO "loss-of-function" and "gain-of-function" SnoN mutants in transcriptional reporter assays reveal that sumoylation of SnoN contributes to the ability of SnoN to repress gene expression in a promoter-specific manner. Although this modification has little effect on SnoN repression of the plasminogen activator inhibitor-1 promoter and only modestly potentiates SnoN repression of the p21 promoter, SnoN sumoylation robustly augments the ability of SnoN to suppress transcription of the myogenesis master regulatory gene myogenin. In addition, we show that the SnoN SUMO E3 ligase, PIAS1, at its endogenous levels, suppresses myogenin transcription. Collectively, our findings suggest that SnoN is directly regulated by sumoylation leading to the enhancement of the ability of SnoN to repress transcription in a promoter-specific manner. Our study also points to a physiological role for SnoN sumoylation in the control of myogenin expression in differentiating muscle cells.

Expression of a novel Ski-like gene in Xenopus development

Members of the Ski/Sno family of gene products contain a characteristic peptide domain involved in protein-protein or protein-DNA interaction. Here, we characterize the developmental expression of xDawg, in Xenopus laevis, of a new gene, related to the Ski/Sno family of transcription regulators. The Ski/Sno domain of xDawg is predicted to present an electropositive surface, consistent with a role in DNA binding. This gene is expressed in the marginal zone of early gastrulae, and in the brain, sensory vesicles, and cranial neural crest of neurula and tailbud embryos.

Crystal structure of the dachshund homology domain of human SKI

The nuclear protooncoprotein SKI negatively regulates transforming growth factor-beta (TGF-beta) signaling in cell growth and differentiation. It directly interacts with the Smads and, by various mechanisms, represses the transcription of TGF-beta-responsive genes. SKI is a multidomain protein that includes a domain bearing high sequence similarity with the retinal determination protein Dachshund (the Dachshund homology domain, DHD). The SKI-DHD has been implicated in SMAD-2/3, N-CoR, SKIP, and PML-RARalpha binding. The 1.65 A crystal structure of the Dachshund homology domain of human SKI is reported here. The SKI-DHD adopts a mixed alpha/beta structure which includes features found in the forkhead/winged-helix family of DNA binding proteins, although SKI-DHD is not a DNA binding domain. Residues that form a contiguous surface patch on SKI-DHD are conserved within the Ski/Sno family and with Dachshund, suggesting that this domain may mediate intermolecular interactions common to these proteins.

Control of cell cycle-dependent degradation of c-Ski proto-oncoprotein by Cdc34

It is known that excess amounts of Ski, or any member of its proto-oncoprotein family, causes disruption of the transforming growth factor beta signal transduction pathway, thus causing oncogenic transformation of cells. Previous studies indicate that Ski is a relatively unstable protein whose expression levels can be regulated by ubiquitin-mediated proteolysis. Here, we investigate the mechanism by which the stability of Ski is regulated. We show that the steady-state levels of Ski protein are controlled post-translationally by cell cycle-dependent proteolysis, wherein Ski is degraded during the interphase of the cell cycle but is relatively stable during mitosis. Furthermore, we demonstrate that the ubiquitin-conjugating enzyme Cdc34 mediates cell cycle-dependent Ski degradation both in vitro and in vivo. Overexpression of dominant-negative Cdc34 stabilizes Ski and enhances its ability to antagonize TGF-beta signaling. Our data suggest that regulated proteolysis of Ski is one of the key mechanisms that control the threshold levels of this proto-oncoprotein, and thus prevents epithelial cells from becoming TGF-beta resistant.

Structure–function analysis of the Drosophila retinal determination protein Dachshund

Dachshund (Dac) is a highly conserved nuclear protein that is distantly related to the Ski/Sno family of corepressor proteins. In Drosophila, Dac is necessary and sufficient for eye development and, along with Eyeless (Ey), Sine oculis (So), and Eyes absent (Eya), forms the core of the retinal determination (RD) network. In vivo and in vitro experiments suggest that members of the RD network function together in one or more complexes to regulate the expression of downstream targets. For example, Dac and Eya synergize in vivo to induce ectopic eye formation and they physically interact through conserved domains. Dac contains two highly conserved domains, named DD1 and DD2, but no function has been assigned to either of them in an in vivo context. We performed structure-function studies to understand the relationship between the conserved domains of Dac and the rest of the protein and to determine the function of each domain during development. We show that only DD1 is essential for Dac function and while DD2 facilitates DD1, it is not absolutely essential in spite of more than 500 million years of conservation. Moreover, the physical interaction between Eya and DD2 is not required for the genetic synergy between the two proteins. Finally, we show that DD1 also plays a central role for nuclear localization of Dac.

Defective T-cell activation is associated with augmented transforming growth factor Beta sensitivity in mice with mutations in the Sno gene

The proto-oncogene Sno has been shown to be a negative regulator of transforming growth factor beta (TGF-beta) signaling in vitro, using overexpression and artificial reporter systems. To examine Sno function in vivo, we made two targeted deletions at the Sno locus: a 5' deletion, with reduced Sno protein (hypomorph), and an exon 1 deletion removing half the protein coding sequence, in which Sno protein is undetectable in homozygotes (null). Homozygous Sno hypomorph and null mutant mice are viable without gross developmental defects. We found that Sno mRNA is constitutively expressed in normal thymocytes and splenic T cells, with increased expression 1 h following T-cell receptor ligation. Although thymocyte and splenic T-cell populations appeared normal in mutant mice, T-cell proliferation in response to activating stimuli was defective in both mutant strains. This defect could be reversed by incubation with either anti-TGF-beta antibodies or exogenous interleukin-2 (IL-2). Together, these findings suggest that Sno-dependent suppression of TGF-beta signaling is required for upregulation of growth factor production and normal T-cell proliferation following receptor ligation. Indeed, both IL-2 and IL-4 levels are reduced in response to anti-CD3 epsilon stimulation of mutant T cells, and transfected Sno activated an IL-2 reporter system in non-T cells. Mutant mouse embryo fibroblasts also exhibited a reduced cell proliferation rate that could be reversed by administration of anti-TGF-beta. Our data provide strong evidence that Sno is a significant negative regulator of antiproliferative TGF-beta signaling in both T cells and other cell types in vivo.

Repression of TGF-beta signaling by the oncogenic protein SKI in human melanomas: consequences for proliferation, survival, and metastasis

Transforming growth factor-beta (TGF-beta ) has dual and paradoxical functions as a tumor suppressor and promoter of tumor progression and metastasis. TGF-Ji-mediated growth inhibition is gradually lost during melanoma tumor progression, but there are no measurable defects at the receptor level. Furthermore, melanoma cells release high levels of TGF-beta to the microenvironment, which upon activation induces matrix deposition, angiogenesis, survival, and transition to more aggressive phenotypes. The SKI and SnoN protein family associate with and repress the activity of Smad2, Smad3, and Smad4, three members of the TGF-fl signaling pathway. SKI also facilitates cell-cycle progression by targeting the RB pathway by at least two ways: it directly associates with RB and represses its activity when expressed at high levels, and indirectly, it represses Smad-mediated induction of p21(Waf-1) This results in increased CDK2 activity, RB phosphorylation,and inactivation. Therefore, high levels of SKI result in lesions to the RB pathway in a manner similar to p16 (INK4a) loss. SKI mRNA and protein levels dramatically increase during human melanoma tumor progression. In addition,the SKI protein shifts from nuclear localization in intraepidermal melanoma cells to nuclear and cytoplasmic in invasive and metastatic melanomas. Here, I discuss the basis for repression of intracellular TGF-beta signaling by SKI, some additional activities of this protein, and propose that by disrupting multiple tumor suppressor pathways, SKI functions as a melanoma oncogene.

Increased susceptibility to tumorigenesis of ski-deficient heterozygous mice

The c-ski proto-oncogene product (c-Ski) acts as a co-repressor and binds to other co-repressors N-CoR/SMRT and mSin3A which form a complex with histone deacetylase (HDAC). c-Ski mediates the transcriptional repression by a number of repressors, including nuclear hormone receptors and Mad. c-Ski also directly binds to, and recruits the HDAC complex to Smads, leading to inhibition of tumor growth factor-beta (TGF-beta) signaling. This is consistent with the function of ski as an oncogene. Here we show that loss of one copy of c-ski increases susceptibility to tumorigenesis in mice. When challenged with a chemical carcinogen, c-ski heterozygous mice showed an increased level of tumor formation relative to wild-type mice. In addition, c-ski-deficient mouse embryonic fibroblasts (MEFs) had increased proliferative capacity, whereas overexpression of c-Ski suppressed the proliferation. Furthermore, the introduction of activated Ki-ras into c-ski-deficient MEFs resulted in neoplastic transformation. These findings demonstrate that c-ski acts as a tumor suppressor in some types of cells. The level of cdc25A mRNA, which is down regulated by two tumor suppressor gene products, Rb and Mad, was upregulated in c-ski-deficient MEFs, whereas it decreased by overexpressing c-Ski in MEFs. This is consistent with the fact that c-Ski acts as a co-repressor of Mad and Rb. These results support the view that the decreased activities of Mad and Rb in ski-deficient cells at least partly contribute to enhanced proliferation and susceptibility to tumorigenesis. Human c-ski gene was mapped to a region close to the p73 tumor suppressor gene at the 1p36.3 locus, which is already known to contain multiple uncharacterized tumor suppressor genes.

Possible role for the c-ski gene in the proliferation of myogenic cells in regenerating skeletal muscles of rats

Skeletal muscle regeneration after injury involves various processes, such as infiltration by inflammatory cells, the proliferation of satellite cells and fusion to myotubes. The c-ski nuclear protein has been implicated in the control of cell proliferation and/or terminal differentiation in the growth of skeletal muscle. However, there have been no reports concerning the involution of c-ski in the regeneration of injured skeletal muscle in mammals. A possible role for c-ski in the proliferation of myogenic cells in rat skeletal muscle during regeneration has been investigated with the assistance of in vitro experiments with L6 skeletal muscle cells. The expression levels of c-ski mRNA in regenerating tissues increased to approximately threefold that of intact tissues at 2 days after injury and decreased to normal levels at 2 weeks after injury. Many mononuclear cells among the Ski-positive cells expressed desmin and proliferating cell nuclear antigen, indicating that Ski-producing cells include the proliferating myogenic cells. The proliferation of L6 cells was significantly retarded by expression of the antisense ski gene. The results of the present study reveal that the c-ski gene plays an important role in the proliferation of myogenic cells in the regeneration of injured skeletal muscle.

Ski/Sno and TGF-beta signaling

Transforming growth factor-beta is a potent inhibitor of epithelial cell proliferation. Proteins involved in TGF-beta signaling are bona fide tumor suppressors and many tumor cells acquire the ability to escape TGF-beta growth inhibition through the loss of key signaling transducers in the pathway or through the activation of oncogenes. Recent studies indicate that there is a specific connection between the TGF-beta signaling pathway and the Ski/SnoN family of oncoproteins. We summarize evidence that Ski and SnoN directly associate with Smad proteins and block the ability of the Smads to activate expression of many if not all TGF-beta-responsive genes. This appears to cause abrogation of TGF-beta growth inhibition in epithelial cells.

Ectopic expression of c-ski disrupts gastrulation and neural patterning in zebrafish

The c-ski proto-oncogene encodes a transcriptional regulator that has been implicated in the development of different tissues at different times during vertebrate development. We identified two novel paralogues of the c-ski gene family, skiA and skiB in zebrafish (Danio rerio). The skiA protein is maternal and ubiquitous while skiB is zygotic. Overexpression of SkiA or SkiB disrupted gastrulation and resulted in a dorsalized phenotype. In situ analyses suggested that overexpression of Ski leads to a slight expansion of dorsal-axial mesoderm, diminishment or loss of ventral mesoderm and radialization of dorsal neuroectoderm. The dorsalized phenotype could be rescued by the ventral specifying factor, BMP4. These results provide evidence that Ski proteins participate in dorsal-ventral specification of both neuroectoderm and mesoderm.

The sno gene, which encodes a component of the histone deacetylase complex, acts as a tumor suppressor in mice

The Ski and Sno oncoproteins are components of a macromolecular complex containing the co-repressor N-CoR/SMRT, mSin3 and histone deacetylase. This complex has been implicated in the transcriptional repression exerted by a number of repressors including nuclear hormone receptors and Mad. Further more, Ski and Sno negatively regulate transforming growth factor-beta (TGF-beta) signaling by recruiting this complex to Smads. Here we show that loss of one copy of sno increases susceptibility to tumorigenesis in mice. Mice lacking sno died at an early stage of embryogenesis, and sno was required for blastocyst formation. Heterozygous (sno(+/-)) mice developed spontaneous lymphomas at a low frequency and showed an increased level of tumor formation relative to wild-type mice when challenged with a chemical carcinogen. sno(+/-) embryonic fibroblasts had an increased proliferative capacity and the introduction of activated Ki-ras into these cells resulted in neoplastic transformation. The B cells, T cells and embryonic fibroblasts of sno(+/-) mice had a decreased sensitivity to apoptosis or cell cycle arrest. These findings demonstrate that sno acts as a tumor suppressor at least in some types of cells.

c-Ski acts as a transcriptional co-repressor in transforming growth factor-beta signaling through interaction with smads

Smads are intracellular signaling mediators of the transforming growth factor-beta (TGF-beta) superfamily that regulates a wide variety of biological processes. Among them, Smads 2 and 3 are activated specifically by TGF-beta. We identified c-Ski as a Smad2 interacting protein. c-Ski is the cellular homologue of the v-ski oncogene product and has been shown to repress transcription by recruiting histone deacetylase (HDAC). Smad2/3 interacts with c-Ski through its C-terminal MH2 domain in a TGF-beta-dependent manner. c-Ski contains two distinct Smad-binding sites with different binding properties. c-Ski strongly inhibits transactivation of various reporter genes by TGF-beta. c-Ski is incorporated in the Smad DNA binding complex, interferes with the interaction of Smad3 with a transcriptional co-activator, p300, and in turn recruits HDAC. c-Ski is thus a transcriptional co-repressor that links Smads to HDAC in TGF-beta signaling.

Transformation of hematopoietic cells by the Ski oncoprotein involves repression of retinoic acid receptor signaling

The Ski oncogene has dramatic effects on the differentiation of several different cell types. It induces the differentiation of quail embryo cells into myoblasts and arrests the differentiation of chicken hematopoietic cells. The mechanism that Ski uses to carry out these disparate biological activities is unknown. However, we were struck by the similarity of these effects to those of certain members of the nuclear hormone receptor family. Both Ski and the thyroid hormone receptor-derived oncogene v-ErbA can arrest the differentiation of avian erythroblasts, and v-Ski-transformed avian multipotent progenitor cells resemble murine hematopoietic cells that express a dominant-negative form of the retinoic acid receptor, RARalpha. In this paper, we have tested the hypothesis that v-Ski and its cellular homologue c-Ski exert their effects by interfering with nuclear hormone receptor-induced transcription. We demonstrate that Ski associates with the RAR complex and can repress transcription from a retinoic acid response element. The physiological significance of this finding is demonstrated by the ability of high concentrations of a RARalpha-specific ligand to abolish v-Ski-induced transformation of the multipotent progenitors. These results strongly suggest that the ability of Ski to alter cell differentiation is caused in part by the modulation of RAR signaling pathways.

Mammalian and Drosophila dachshund genes are related to the Ski proto-oncogene and are expressed in eye and limb

We have isolated mammalian homologues of the Drosophila dachshund gene. Two domains of high conservation, one of which contains an alpha-helical, coiled-coil motif, show similarity to the Ski family of genes. We therefore propose that Dachshund belongs to a superfamily including these genes. Mouse Dachshund (Dach) is expressed in the eye and limb, structures affected by the Drosophila loss-of-function mutant, and rib primordia, CNS and genital eminence. Pax6 and Dach show overlapping but non-identical expression patterns. Dach expression is unaffected in smalleye mouse brain, indicating that Pax6 is not directly activating Dach. In Drosophila eye development dachshund is a component of an interacting network of proteins. Genes homologous to many of these exist in mammals; Dach joins this expanding group.

Transcriptional repression by v-Ski and c-Ski mediated by a specific DNA binding site

The Ski oncoprotein has been shown to bind DNA and activate transcription in conjunction with other cellular factors. Because tumor cells or myogenic cells were used for those studies, it is not clear that those activities of Ski are related to its transforming ability. In this study, we use a nuclear extract of c-ski-transformed cells to identify a specific DNA binding site for Ski with the consensus sequence GTCTAGAC. We demonstrate that both c-Ski and v-Ski in nuclear extracts are components of complexes that bind specifically to this site. By evaluating the features of the sequence that are critical for binding, we show that binding is cooperative. Although Ski cannot bind to this sequence on its own, we use cross-linking with ultraviolet light to show that Ski binds to this site along with several unidentified cellular proteins. Furthermore, we find that Ski represses transcription either through upstream copies of this element or when brought to the promoter by a heterologous DNA binding domain. This is the first demonstration that Ski acts as a repressor rather than an activator and could provide new insights into regulation of gene expression by Ski.

High affinity dimerization by Ski involves parallel pairing of a novel bipartite alpha-helical domain

c-Ski protein possesses a C-terminal dimerization domain that was deleted during the generation of v-ski, and has been implicated in the increased potency of c-ski in cellular transformation compared with the viral gene. The domain is predicted to consist of an extended alpha-helical segment made up of two motifs: a tandem repeat (TR) consisting of five imperfect repeats of 25 residues each and a leucine zipper (LZ) consisting of six heptad repeats. We have examined the structure and dimerization of TR or LZ individually or the entire TR-LZ domain. Using a quenched chemical cross-linking method, we show that the TR dimerizes with moderate efficiency (Kd = 4 x 10(-6) M), whereas LZ dimerizes poorly (Kd > 2 x 10(-5) M). However, the entire TR-LZ domain dimerizes efficiently (Kd = 2 x 10(-8) M), showing a cooperative effect of the two motifs. CD analyses indicate that all three proteins contain predominantly alpha-helices. Limited proteolysis of the TR-LZ dimer indicates that the two helical motifs are linked by a small loop. Interchain disulfide bond formation indicates that both the LZ and TR helices are oriented in parallel. We propose a model for the dimer interface in the TR region consisting of discontinuous clusters of hydrophobic residues forming "leucine buttons."

Production, characterization and functional activities of v-Ski in cultured cells

The v-ski oncogene was introduced into mammalian cells in order to study its biochemical and biological properties. v-Ski, produced at relatively high levels by mouse L cells stably transfected with this DNA, was localized to the cell nucleus, was of correct apparent molecular mass, and was capable of complexing with DNA. Transient transfection of reporter plasmids into control or Ski producing mouse L cells revealed that Ski acts as a transcriptional activator of various transcriptional regulatory elements, including CMVie, RSV LTR and SV40. These results indicate that mouse L cells contain the nuclear cofactor(s) required for the ability of v-Ski to bind to DNA and also suggest that the v-Ski present within the cells is functional.

DNA binding and transcriptional activation by the Ski oncoprotein mediated by interaction with NFI

The Ski oncoprotein has been found to bind non-specifically to DNA in association with unindentified nuclear factors. In addition, Ski has been shown to activate transcription of muscle-specific and viral promoters/enhancers. The present study was undertaken to identify Ski's DNA binding and transcriptional activation partners by identifying specific DNA binding sites. We used nuclear extracts from a v-Ski-transduced mouse L-cell line and selected Ski-bound sequences from a pool of degenerate oligonucleotides with anti-Ski monoclonal antibodies. Two sequences were identified by this technique. The first (TGGC/ANNNNNT/GCCAA) is the previously identified binding site of the nuclear factor I (NFI) family of transcription factors. The second (TCCCNNGGGA) is the binding site of Olf-1/EBF. By electophoretic mobility shift assays we find that Ski is a component of one or more NFI complexes but we fail to detect Ski in Olf-1/EBF complexes. We show that Ski binds NFI proteins and activates transcription of NFI reporters, but only in the presence of NFI. We also find that homodimerization of Ski is essential for co-activation with NFI. However, the C-terminal dimerization domain of c-Ski, which is missing in v-Ski, can be substituted by the leucine zipper domain of GCN4.

A transient increase of snoN transcript by growth arrest upon serum deprivation and cell-to-cell contact

To analyze the possible involvement of c-ski and c-sno during the course of in vitro myogenesis, expression of their transcripts during differentiation of a murine muscle cell line (C2C12) was monitored by competitive reverse transcription-polymerase chain reaction (RT-PCR). The transcripts of c-snoN were temporarily increased 25-fold above basal level at 12 h prior to the onset of transcription of muscle-specific gene, e.g. myogenin and muscle creatine kinase, whereas c-ski was expressed invariably. The transient increase of c-snoN was blocked when myogenesis was interrupted by the presence of fetal calf serum in culture medium, probably due to growth factors being included; basic fibroblast growth factor (b-FGF) blocked the transient increase whereas epidermal growth factor (EGF) did not, consistent with the inhibitory effect of b-FGF and no effect of EGF on myotube formation of C2C12. In fibroblastic C3H10T1/2 cells, snoN exhibited a similar transient increase of transcript when growth arrested under the same conditions as for in vitro myogenesis, indicating that the expression of snoN is not sufficient to induce the onset of muscle differentiation and an unknown factor involved in myogenic cells is necessary. The transient increase of snoN transcript may represent a common entrance step of cells into the G0 phase where muscle differentiation is substantiated, considering that it was observed upon growth arrest of fibroblastic C3H10T1/2 cells and prior to the elevation of MCK in C2C12 but undetected when entry into G0 was blocked by b-FGF.

Cloning and characterization of the murine homolog of the sno proto-oncogene reveals a novel splice variant

The cellular function(s) of the SNO protein remain undefined. To gain a better understanding of possible developmental roles of this cellular proto-oncogene, we have cloned two murine sno cDNAs and have investigated their expression patterns in embryonic and postnatal tissues. A single major transcript of 7.5 kb is detected in multiple tissues by Northern blot. However, reverse transcriptase polymerase chain reaction (RT-PCR) and RNAse protection assays revealed a novel splice variant in every tissue examined. Two isoforms, termed sno N and sno-dE3 (dE3, deletion within exon 3), were identified. The sno-dE3 isoform employs a novel 5' splice site located within the coding region of the third exon and deletes potential kinase recognition motifs. Transcripts of both sno isoforms accumulate ubiquitously but are most abundant in the developing central nervous system. The in situ hybridization patterns of sno expression during murine development suggest potential roles in tissues with a high degree of cellular proliferation. Expression in terminally differentiated tissues such as muscle and neurons indicates that SNO may have multiple functional activities.