Viral ski inhibits retinoblastoma protein (Rb)-mediated transcriptional repression in a dominant negative fashion

Acetylation of nuclear receptors in health and disease: an update

Lysine acetylation is a common reversible post-translational modification of proteins that plays a key role in regulating gene expression. Nuclear receptors (NRs) include ligand-inducible transcription factors and orphan receptors for which the ligand is undetermined, which together regulate the expression of genes involved in development, metabolism, homeostasis, reproduction and human diseases including cancer. Since the original finding that the ERα, AR and HNF4 are acetylated, we now understand that the vast majority of NRs are acetylated and that this modification has profound effects on NR function. Acetylation sites are often conserved and involve both ordered and disordered regions of NRs. The acetylated residues function as part of an intramolecular signalling platform intersecting phosphorylation, methylation and other modifications. Acetylation of NR has been shown to impact recruitment into chromatin, co-repressor and coactivator complex formation, sensitivity and specificity of regulation by ligand and ligand antagonists, DNA binding, subcellular distribution and transcriptional activity. A growing body of evidence in mice indicates a vital role for NR acetylation in metabolism. Additionally, mutations of the NR acetylation site occur in human disease. This review focuses on the role of NR acetylation in coordinating signalling in normal physiology and disease.

Upregulation of proto-oncogene ski by thyroid hormone in the intestine and tail during Xenopus metamorphosis

Thyroid hormone (T3) is important for adult organ function and vertebrate development, particularly during the postembryonic period when many organs develop/mature into their adult forms. Amphibian metamorphosis is totally dependent on T3 and can be easily manipulated, thus offering a unique opportunity for studying how T3 controls postembryonic development in vertebrates. Numerous early studies have demonstrated that T3 affects frog metamorphosis through T3 receptor (TR)-mediated regulation of T3 response genes, where TR forms a heterodimer with RXR (9-cis retinoic acid receptor) and binds to T3 response elements (TREs) in T3 response genes to regulate their expression. We have previously identified many candidate direct T3 response genes in Xenopus tropicalis tadpole intestine. Among them is the proto-oncogene Ski, which encodes a nuclear protein with complex function in regulating cell fate. We show here that Ski is upregulated in the intestine and tail of premetamorphic tadpoles upon T3 treatment and its expression peaks at stage 62, the climax of metamorphosis. We have further discovered a putative TRE in the first exon that can bind to TR/RXR in vitro and mediate T3 regulation of the promoter in vivo. These data demonstrate that Ski is activated by T3 through TR binding to a TRE in the first exon during Xenopus tropicalis metamorphosis, implicating a role of Ski in regulating cell fate during metamorphosis.

Mutations in SKI in Shprintzen-Goldberg syndrome lead to attenuated TGF-β responses through SKI stabilization

Shprintzen-Goldberg syndrome (SGS) is a multisystemic connective tissue disorder, with considerable clinical overlap with Marfan and Loeys-Dietz syndromes. These syndromes have commonly been associated with enhanced TGF-β signaling. In SGS patients, heterozygous point mutations have been mapped to the transcriptional co-repressor SKI, which is a negative regulator of TGF-β signaling that is rapidly degraded upon ligand stimulation. The molecular consequences of these mutations, however, are not understood. Here we use a combination of structural biology, genome editing, and biochemistry to show that SGS mutations in SKI abolish its binding to phosphorylated SMAD2 and SMAD3. This results in stabilization of SKI and consequently attenuation of TGF-β responses, both in knockin cells expressing an SGS mutation and in fibroblasts from SGS patients. Thus, we reveal that SGS is associated with an attenuation of TGF-β-induced transcriptional responses, and not enhancement, which has important implications for other Marfan-related syndromes.

Ski Is Required for Tri-Methylation of H3K9 in Major Satellite and for Repression of Pericentromeric Genes: Mmp3, Mmp10 and Mmp13, in Mouse Fibroblasts

Several mechanisms directing a rapid transcriptional reactivation of genes immediately after mitosis have been described. However, little is known about the maintenance of repressive signals during mitosis. In this work, we address the role of Ski in the repression of gene expression during M/G₁ transition in mouse embryonic fibroblasts (MEFs). We found that Ski localises as a distinct pair of dots at the pericentromeric region of mitotic chromosomes, and the absence of the protein is related to high acetylation and low tri-methylation of H3K9 in pericentromeric major satellite. Moreover, differential expression assays in early G₁ cells showed that the presence of Ski is significantly associated with repression of genes localised nearby to pericentromeric DNA. In mitotic cells, chromatin immunoprecipitation assays confirmed the association of Ski to major satellite and the promoters of the most repressed genes: Mmp3, Mmp10 and Mmp13. These genes are at pericentromeric region of chromosome 9. In these promoters, the presence of Ski resulted in increased H3K9 tri-methylation levels. This Ski-dependent regulation is also observed during interphase. Consequently, Mmp activity is augmented in Ski-/- MEFs. Altogether, these data indicate that association of Ski with the pericentromeric region of chromosomes during mitosis is required to maintain the silencing bookmarks of underlying chromatin.

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.

SUMOylation and phosphorylation cross-talk in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a primary malignancy of the liver and occurs predominantly in patients with underlying chronic liver disease and cirrhosis. The large spectrum of protein post-translational modification (PTM) includes numerous critical signaling events that occur during neoplastic transformation. PTMs occur to nearly all proteins and increase the functional diversity of proteins. We have reviewed the role of two major PTMs, SUMOylation and phosphorylation, in the altered signaling of key players in HCC. SUMOylation is a PTM that involves addition of a small ubiquitin-like modifiers (SUMO) group to proteins. It is known to regulate protein stability, protein-protein interactions, trafficking and transcriptional activity. The major pathways that are regulated by SUMOylation and may influence HCC are regulation of transcription, cell growth pathways associated with B-cell lymphoma 2 (Bcl-2) and methionine adenosyltransferases (MAT), oxidative stress pathways [nuclear erythroid 2-related factor 2 (Nrf2)], tumor suppressor pathways (p53), hypoxia-inducible signaling [hypoxia-inducible factor-1 (HIF-1)], glucose and lipid metabolism, nuclear factor kappa B (NF-κB) and β-Catenin signaling. Phosphorylation is an extensively studied PTM in HCC. The mitogen-activated protein kinase (MAPK), phosphatidyl inositol/AK-strain transforming (PI3K/AKT), and C-SRC pathways have been extensively studied for deregulation of kinases and alteration in signaling of targets through phosphorylation of their substrates. Cross-talk between phosphorylation and SUMOylation is known to influence transcriptional activity of proteins and protein-protein interactions. In HCC, several SUMOylation-dependent phosphorylation events have been studied such as MAPK activation and c-SRC activity that have been reviewed in this work. The drastic effects of site-specific phosphorylation or SUMOylation on enzyme activity of signaling players and its effect on growth and tumorigenesis suggests that these PTMs are novel targets for therapeutic intervention in HCC.

SnoN Stabilizes the SMAD3/SMAD4 Protein Complex

TGF-β signaling regulates cellular processes such as proliferation, differentiation and apoptosis through activation of SMAD transcription factors that are in turn modulated by members of the Ski-SnoN family. In this process, Ski has been shown to negatively modulate TGF-β signaling by disrupting active R-SMAD/Co-SMAD heteromers. Here, we show that the related regulator SnoN forms a stable complex with the R-SMAD (SMAD3) and the Co-SMAD (SMAD4). To rationalize this stabilization at the molecular level, we determined the crystal structure of a complex between the SAND domain of SnoN and the MH2-domain of SMAD4. This structure shows a binding mode that is compatible with simultaneous coordination of R-SMADs. Our results show that SnoN, and SMAD heteromers can form a joint structural core for the binding of other transcription modulators. The results are of fundamental importance for our understanding of the molecular mechanisms behind the modulation of TGF-β signaling.

Role of the RB-Interacting Proteins in Stem Cell Biology

Human retinoblastoma gene RB1 is the first tumor suppressor gene (TSG) isolated by positional cloning in 1986. RB is extensively studied for its ability to regulate cell cycle by binding to E2F1 and inhibiting the transcriptional activity of the latter. In human embryonic stem cells (ESCs), only a minute trace of RB is found in complex with E2F1. Increased activity of RB triggers differentiation, cell cycle arrest, and cell death. On the other hand, inactivation of the entire RB family (RB1, RBL1, and RBL2) in human ESC induces G2/M arrest and cell death. These observations indicate that both loss and overactivity of RB could be lethal for the stemness of cells. A question arises why inactive RB is required for the survival and stemness of cells? To shed some light on this question, we analyzed the RB-binding proteins. In this review we have focused on 27 RB-binding partners that may have potential roles in different aspects of stem cell biology.

How microRNA and transcription factor co-regulatory networks affect osteosarcoma cell proliferation

Osteosarcomas (OS) are complex bone tumors with various genomic alterations. These alterations affect the expression and function of several genes due to drastic changes in the underlying gene regulatory network. However, we know little about critical gene regulators and their functional consequences on the pathogenesis of OS. Therefore, we aimed to determine microRNA and transcription factor (TF) co-regulatory networks in OS cell proliferation. Cell proliferation is an essential part in the pathogenesis of OS and deeper understanding of its regulation might help to identify potential therapeutic targets. Based on expression data of OS cell lines divided according to their proliferative activity, we obtained 12 proliferation-related microRNAs and corresponding target genes. Therewith, microRNA and TF co-regulatory networks were generated and analyzed regarding their structure and functional influence. We identified key co-regulators comprising the microRNAs miR-9-5p, miR-138, and miR-214 and the TFs SP1 and MYC in the derived networks. These regulators are implicated in NFKB- and RB1-signaling and focal adhesion processes based on their common or interacting target genes (e.g., CDK6, CTNNB1, E2F4, HES1, ITGA6, NFKB1, NOTCH1, and SIN3A). Thus, we proposed a model of OS cell proliferation which is primarily co-regulated through the interactions of the mentioned microRNA and TF combinations. This study illustrates the benefit of systems biological approaches in the analysis of complex diseases. We integrated experimental data with publicly available information to unravel the coordinated (post)-transcriptional control of microRNAs and TFs to identify potential therapeutic targets in OS. The resulting microRNA and TF co-regulatory networks are publicly available for further exploration to generate or evaluate own hypotheses of the pathogenesis of OS (http://www.complex-systems.uni-muenster.de/co_networks.html).

Expression of Ski can act as a negative feedback mechanism on retinoic acid signaling

BACKGROUND

Retinoic acid signaling is essential for many aspects of early development in vertebrates. To control the levels of signaling, several retinoic acid target genes have been identified that act to suppress retinoic acid signaling in a negative feedback loop. The nuclear protein Ski has been extensively studied for its ability to suppress transforming growth factor-beta (TGF-β) signaling but has also been implicated in the repression of retinoic acid signaling.

RESULTS

We demonstrate that ski expression is up-regulated in response to retinoic acid in both early Xenopus embryos and in human cell lines. Blocking retinoic acid signaling using a retinoic acid antagonist results in a corresponding decrease in the levels of ski mRNA. Finally, overexpression of SKI in human cells results in reduced levels of CYP26A1 mRNA, a known target of retinoic acid signaling.

CONCLUSIONS

Our results, coupled with the known ability of Ski to repress retinoic acid signaling, demonstrate that Ski expression is a novel negative feedback mechanism acting on retinoic acid signaling.

Chromatin remodeling in cancer: a gateway to regulate gene transcription

Cancer cells are remarkably adaptive to diverse survival strategies, probably due to its ability to interpret signaling cues differently than the normal cells. It appears as if cancer cells are constantly sampling, selecting and adapting signaling pathways to favor its proliferation. This process of successful adaptive evolution eventually renders a retractile nature to therapeutic regimens, fueling to the process of cancer progression. Based on plethora of available information, it is now evident that multiple signaling pathways eventually converge, perhaps, in a tempo-spatial manner, onto DNA template-dependent dynamic processes. Considering the complexity and packaging of eukaryotic genome, this process involves energy-dependent sub-events mediated by chromatin remodelers. Chromatin remodeler proteins function as gatekeepers and constitute a major determinant of accessibility of accessory factors to nucleosome DNA, allowing a wide repertoire of biological functions. And thus, aberrant expression or epigenetic modulation of remodeler proteins confers a unique ability to cancer cells to reprogram its genome for the maintenance of oncogenic phenotypes. Cancer cells can uniquely select a multi-subunit remodeler proteome for oncogenic advantage. This review summarizes our current understanding and importance of remodeler and chromatin proteins in cancer biology and also highlights the paradoxical role of proteins with or without dual-regulator functions. It is our hope that an in-depth understanding of these events is likely to provide a next set of opportunities for novel strategies for targeted cancer therapeutics.

Differential role of Sloan-Kettering Institute (Ski) protein in Nodal and transforming growth factor-beta (TGF-β)-induced Smad signaling in prostate cancer cells

Transforming growth factor-beta (TGF-β) signaling pathways contain both tumor suppressor and tumor promoting activities. We have demonstrated that Nodal, another member of the TGF-β superfamily, and its receptors are expressed in prostate cancer cells. Nodal and TGF-β exerted similar biological effects on prostate cells; both inhibited proliferation in WPE, RWPE1 and DU145 cells, whereas neither had any effect on the proliferation of LNCaP or PC3 cells. Interestingly, Nodal and TGF-β induced migration in PC3 cells, but not in DU145 cells. TGF-β induced predominantly phosphorylation of Smad3, whereas Nodal induced phosphorylation of only Smad2. We also determined the expression and differential role of Ski, a corepressor of Smad2/3, in Nodal and TGF-β signaling in prostate cancer cells. Similar levels of Ski mRNA were found in several established prostate cell lines; however, high levels of Ski protein were only detected in prostate cancer cells and prostate cancer tissue samples. Exogenous Nodal and TGF-β had no effects on Ski mRNA levels. On the other hand, TGF-β induced a rapid degradation of Ski protein mediated by the proteasomal pathway, whereas Nodal had no effect on Ski protein. Reduced Ski levels correlated with increased basal and TGF-β-induced Smad2/3 phosphorylation. Knockdown of endogenous Ski reduced proliferation in DU145 cells and enhanced migration of PC3 cells. We conclude that high levels of Ski expression in prostate cancer cells may be responsible for repression of TGF-β and Smad3 signaling, but Ski protein levels do not influence Nodal and Smad2 signaling.

Overexpression of SKI oncoprotein leads to p53 degradation through regulation of MDM2 protein sumoylation

Protooncogene Ski was identified based on its ability to transform avian fibroblasts in vitro. In support of its oncogenic activity, SKI was found to be overexpressed in a variety of human cancers, although the exact molecular mechanism(s) responsible for its oncogenic activity is not fully understood. We found that SKI can negatively regulate p53 by decreasing its level through up-regulation of MDM2 activity, which is mediated by the ability of SKI to enhance sumoylation of MDM2. This stimulation of MDM2 sumoylation is accomplished through a direct interaction of SKI with SUMO-conjugating enzyme E2, Ubc9, resulting in enhanced thioester bond formation and mono-sumoylation of Ubc9. A mutant SKI defective in transformation fails to increase p53 ubiquitination and is unable to increase MDM2 levels and to increase mono-sumoylation of Ubc9, suggesting that the ability of SKI to enhance Ubc9 activity is essential for its transforming function. These results established a detailed molecular mechanism that underlies the ability of SKI to cause cellular transformation while unraveling a novel connection between sumoylation and tumorigenesis, providing potential new therapeutic targets for cancer.

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.

The crystal structure of the Dachshund domain of human SnoN reveals flexibility in the putative protein interaction surface

The human SnoN is an oncoprotein that interacts with several transcription-regulatory proteins such as the histone-deacetylase, N-CoR containing co-repressor complex and Smad proteins. This study presents the crystal structure of the Dachshund homology domain of human SnoN. The structure reveals a groove composed of conserved residues with characteristic properties of a protein-interaction surface. A comparison of the 12 monomers in the asymmetric unit reveals the presence of two major conformations: an open conformation with a well accessible groove and a tight conformation with a less accessible groove. The variability in the backbone between the open and the tight conformations matches the differences seen in previously determined structures of individual Dachshund homology domains, suggesting a general plasticity within this fold family. The flexibility observed in the putative protein binding groove may enable SnoN to recognize multiple interaction partners.

The phosphatidylinositol 3-kinase/Akt pathway regulates transforming growth factor-{beta} signaling by destabilizing ski and inducing Smad7

Ski is an oncoprotein that negatively regulates transforming growth factor (TGF)-beta signaling. It acts as a transcriptional co-repressor by binding to TGF-beta signaling molecules, Smads. Efficient TGF-beta signaling is facilitated by rapid proteasome-mediated degradation of Ski by TGF-beta. Here we report that Ski is phosphorylated by Akt/PKB kinase. Akt phosphorylates Ski on a highly conserved Akt motif at threonine 458 both in vitro and in vivo. The phosphorylation of Ski at threonine 458 is induced by Akt pathway activators including insulin, insulin-like growth factor-1, and hepatocyte growth factor. The phosphorylation of Ski causes its destabilization and reduces Ski-mediated inhibition of expression of another negative regulator of TGF-beta, Smad7. Induction of Smad7 levels leads to inactivation of TGF-beta receptors and TGF-beta signaling cascade, as indicated by reduced induction of TGF-beta target p15. Therefore, Akt modulates TGF-beta signaling by temporarily adjusting the levels of two TGF-beta pathway negative regulators, Ski and Smad7. These novel findings demonstrate that Akt pathway activation directly impacts TGF-beta pathway.

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.

Ski co-repressor complexes maintain the basal repressed state of the TGF-beta target gene, SMAD7, via HDAC3 and PRMT5

The products encoded by ski and its related gene, sno, (Ski and Sno) act as transcriptional co-repressors and interact with other co-repressors such as N-CoR/SMRT and mSin3A. Ski and Sno mediate transcriptional repression by various repressors, including Mad, Rb and Gli3. Ski/Sno also suppress transcription induced by multiple activators, such as Smads and c-Myb. In particular, the inhibition of TGF-beta-induced transcription by binding to Smads is correlated with the oncogenic activity of Ski and Sno. However, the molecular mechanism by which Ski and Sno mediate transcriptional repression remains unknown. In this study, we report the purification and characterization of Ski complexes. The Ski complexes purified from HeLa cells contained histone deacetylase 3 (HDAC3) and protein arginine methyltransferase 5 (PRMT5), in addition to multiple Smad proteins (Smad2, Smad3 and Smad4). Chromatin immunoprecipitation assays indicated that these components of the Ski complexes were localized on the SMAD7 gene promoter, which is the TGF-beta target gene, in TGF-beta-untreated HepG2 cells. Knockdown of these components using siRNA led to up-regulation of SMAD7 mRNA. These results indicate that Ski complexes serve to maintain a TGF-beta-responsive promoter at a repressed basal level via the activities of histone deacetylase and histone arginine methyltransferase.

Ski regulates muscle terminal differentiation by transcriptional activation of Myog in a complex with Six1 and Eya3

Overexpression of the Ski pro-oncogene has been shown to induce myogenesis in non-muscle cells, to promote muscle hypertrophy in postnatal mice, and to activate transcription of muscle-specific genes. However, the precise role of Ski in muscle cell differentiation and its underlying molecular mechanism are not fully understood. To elucidate the involvement of Ski in muscle terminal differentiation, two retroviral systems were used to achieve conditional overexpression or knockdown of Ski in satellite cell-derived C2C12 myoblasts. We found that enforced expression of Ski promoted differentiation, whereas loss of Ski severely impaired it. Compromised terminal differentiation in the absence of Ski was likely because of the failure to induce myogenin (Myog) and p21 despite normal expression of MyoD. Chromatin immunoprecipitation and transcriptional reporter experiments showed that Ski occupied the endogenous Myog regulatory region and activated transcription from the Myog regulatory region upon differentiation. Transactivation of Myog was largely dependent on a MEF3 site bound by Six1, not on the binding site of MyoD or MEF2. Activation of the MEF3 site required direct interaction of Ski with Six1 and Eya3 mediated by the evolutionarily conserved Dachshund homology domain of Ski. Our results indicate that Ski is necessary for muscle terminal differentiation and that it exerts this role, at least in part, through its association with Six1 and Eya3 to regulate the Myog transcription.

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.

Expression and localization of Ski determine cell type-specific TGFbeta signaling effects on the cell cycle

Transforming growth factor β (TGFβ) promotes epithelial cell differentiation but induces Schwann cell proliferation. We show that the protooncogene Ski (Sloan-Kettering viral oncogene homologue) is an important regulator of these effects. TGFβ down-regulates Ski in epithelial cells but not in Schwann cells. In Schwann cells but not in epithelial cells, retinoblastoma protein (Rb) is up-regulated by TGFβ. Additionally, both Ski and Rb move to the cytoplasm, where they partially colocalize. In vivo, Ski and phospho-Rb (pRb) appear to interact in the Schwann cell cytoplasm of developing sciatic nerves. Ski overexpression induces Rb hyperphosphorylation, proliferation, and colocalization of both proteins in Schwann cell and epithelial cell cytoplasms independently of TGFβ treatment. Conversely, Ski knockdown in Schwann cells blocks TGFβ-induced proliferation and pRb cytoplasmic relocalization. Our findings reveal a critical function of fine-tuned Ski levels in the control of TGFβ effects on the cell cycle and suggest that at least a part of Ski regulatory effects on TGFβ-induced proliferation of Schwann cells is caused by its concerted action with Rb.

Ski/SnoN expression in the sequence metaplasia-dysplasia-adenocarcinoma of Barrett's esophagus

Barrett's esophagus (BE) is a precancerous condition. However, the mechanisms underlying the transformation from metaplastic to dysplastic to adenocarcinomatous epithelium are still poorly understood. As loss of transforming growth factor-beta growth inhibition is considered a hallmark of several human neoplasms, we evaluated the expression of Ski and SnoN (proteins that antagonize transforming growth factor-beta signaling through physical interaction with Smad complex and by recruiting histone deacetylases), as markers of the transforming growth factor-beta signaling pathway, in BE with and without dysplasia. Biopsy samples from 37 patients (26 men, aged 60 +/- 8 years) with histologically proven BE were evaluated; 10 patients had concomitant low-grade dysplasia, 7 high-grade dysplasia (HGD), and 6 HGD associated with adenocarcinoma. Ski and SnoN expression was assessed immunohistochemically. Neither Ski nor SnoN was expressed in normal esophageal epithelium, but both were strongly expressed in BE tissue, with intense cytoplasmic positivity. Expression of these proteins decreased markedly in dysplastic areas in patients with low-grade dysplasia and was absent in those with HGD or HGD/adenocarcinoma. Ski and SnoN proteins are overexpressed in BE and may be involved in abnormal signaling elicited by transforming growth factor-beta in this epithelium, enhancing the tumorigenesis process. These observations might help to elucidate the molecular mechanisms involved in the BE tumorigenesis process.

Ski can negatively regulates macrophage differentiation through its interaction with PU.1

In the hematopoietic cell system, the oncoprotein Ski dramatically affects growth and differentiation programs, in some cases leading to malignant leukemia. However, little is known about the interaction partners or signaling pathways involved in the Ski-mediated block of differentiation in hematopoietic cells. Here we show that Ski interacts with PU.1, a lineage-specific transcription factor essential for terminal myeloid differentiation, and thereby represses PU.1-dependent transcriptional activation. Consistent with this, Ski inhibits the biological function of PU.1 to promote myeloid cells to differentiate into macrophage colony-stimulating factor receptor (M-CSFR)-positive macrophages. Using a Ski mutant deficient in PU.1 binding, we demonstrate that Ski-PU.1 interaction is critical for Ski's ability to repress PU.1-dependent transcription and block macrophage differentiation. Furthermore, we provide evidence that Ski-mediated repression of PU.1 is due to Ski's ability to recruit histone deacetylase 3 to PU.1 bound to DNA. Since inactivation of PU.1 is closely related to the development of myeloid leukemia and Ski strongly inhibits PU.1 function, we propose that aberrant Ski expression in certain types of myeloid cell lineages might contribute to leukemogenesis.

Characterization of dSnoN and its relationship to Decapentaplegic signaling in Drosophila

Vertebrate members of the ski/snoN family of proto-oncogenes antagonize TGFbeta and BMP signaling in a variety of experimental situations. This activity of Ski/SnoN proteins is related to their ability to interact with Smads, the proteins acting as key mediators of the transcriptional response to the TGFbeta superfamily members. However, despite extensive efforts to identify the physiological roles of the Ski/SnoN proteins, it is not yet clear whether they participate in regulating Activin and/or BMP signaling during normal development. It is therefore crucial to examine their roles in vivo mostly because of the large number of known Ski/SnoN-interacting proteins and the association between the up-regulation of these genes and cancer progression. Here we characterize the Drosophila homolog to vertebrate ski and snoN genes. The Drosophila dSnoN protein retains the ability of its vertebrate counterparts to antagonize BMP signaling in vivo and in cultured cells. dSnoN does not interfere with Mad phosphorylation but it interacts genetically with Mad, Medea and dSmad2. Mutations in either the Smad2-3 or Smad4 putative binding sites of dSnoN prevent the antagonism of dSnoN towards Dpp signaling, although homozygous flies for these mutations or for a genetic deficiency of the locus are viable and have wings of normal size and pattern.

Expression and possible mechanism of c-ski, a novel tissue repair-related gene during normal and radiation-impaired wound healing

C-ski is a complicated regulating factor for fibroblast proliferation and an important co-repressor of Smad3. Although inhibiting Smad3 activity can markedly promote wound healing because Smad3 mediates the role of transforming growth factor-beta in inhibiting cell proliferation and inducing cell apoptosis; there has been no report on whether c-ski is expressed during wound healing and the relationship between its expression and wound healing. By establishing animal models of normal and radiation-impaired wound healing and using immunohistochemistry, in situ hybridization, and reverse transcription-polymerase chain reaction, we found that c-ski was expressed after wounding and reached its peak on day 9 and then significantly decreased. C-ski was present in all repair cells, and was especially prominent in fibroblasts. Compared with the control side, the irradiated side showed a lower expression of c-ski on postwound days 3-9, but higher on day 15, and not significantly different after the wound was healed. The expression of Smad3 was in contrast to the c-ski and cellular proliferation was similar to that of c-ski expression. The apoptosis index was significantly higher on the irradiated side on days 3-9 compared with the control side. In vitro, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide results showed that c-ski could reverse the inhibitory role of Smad3 on fibroblast proliferation. Flow cytometry analysis found that c-ski also diminished fibroblast apoptosis induced by Smad3 transfection. These results suggest that there is not only obvious expression of this regulatory protein but there is also a significant change in the levels of c-ski during wound healing. Its in vivo expression pattern and experiments in vitro suggest that c-ski may be involved in tissue repair by repressing Smad3 activity. Radiation can reduce c-ski and increase Smad3 expression, resulting in elevated Smad3 activity, resulting in diminished cell proliferation, cell apoptosis, and wound-healing delays.

Pocket protein function in melanocyte homeostasis and neoplasia

Melanoma is the most lethal of human skin cancers and its incidence is increasing worldwide [L.K. Dennis (1999). Arch. Dermatol. 135, 275; C. Garbe et al. (2000). Cancer 89, 1269]. Melanomas often metastasize early during the course of the disease and are then highly intractable to current therapeutic regimens [M.F. Demierre and G. Merlino (2004). Curr. Oncol. Rep. 6, 406]. Consequently, understanding the factors that maintain melanocyte homeostasis and prevent their neoplastic transformation into melanoma is of utmost interest from the perspective of therapeutic interdiction. This review will focus on the role of the pocket proteins (PPs), Rb1 (retinoblastoma protein), retinoblastoma-like 1 (Rbl1 also known as p107) and retinoblastoma-like 2 (Rbl2 also known as p130), in melanocyte homeostasis, with particular emphasis on their functions in the cell cycle and the DNA damage repair response. The potential mechanisms of PP deregulation in melanoma and the possibility of PP-independent pathways to melanoma development will also be considered. Finally, the role of the PP family in ultraviolet radiation (UVR)-induced melanoma and the precise contribution that each PP family member makes to melanocyte homeostasis will be discussed in the context of a number of genetically engineered mouse models.

SKI pathways inducing progression of human melanoma

The proteins SKI and SnoN are implicated in processes as diverse as differentiation, transformation and tumor progression. Until recently, SKI was solely viewed as a nuclear protein with a principal function of inhibiting TGF-beta signaling through its association with the Smad proteins. However, new studies suggest that SKI plays additional roles not only inside but also outside the nucleus. In normal melanocytes and primary non-invasive melanomas, SKI localizes predominantly in the nucleus, whereas in primary invasive melanomas SKI displays both nuclear and cytoplasmic localization. Intriguingly, metastatic melanoma tumors display nuclear and cytoplasmic or predominantly cytoplasmic SKI distribution. Cytoplasmic SKI is functional, as it associates with Smad3 and prevents its nuclear localization mediated by TGF-beta. SKI can also function as a transcriptional activator, targeting the beta -catenin pathway and activating MITF and NrCAM, two proteins involved in survival, migration and invasion. Intriguingly, SKI appears to live a dual life, one as a tumor suppressor and another as a transforming protein. Loss of one copy of mouse ski increases susceptibility to tumorigenesis in mice, whereas its overexpression is associated with cancer progression of human melanoma, esophageal, breast and colon. The molecular reasons for such dramatic change in SKI function appear to result from new acquired activities. In this review, we discuss the mechanisms by which SKI regulates crucial pathways involved in the progression of human malignant melanoma.

Rb enhances p160/SRC coactivator-dependent activity of nuclear receptors and hormone responsiveness

The retinoblastoma tumor suppressor protein (Rb) is best known as a repressor of genes involved in cell cycle progression. Rb has also been implicated in activation of transcription, in particular by nuclear receptors (NRs) and by differentiation-related transcription factors, but the relevance of this activity is unclear. We show that Rb and the related proteins p107 and p130 enhance the activity of NRs related to NGFI-B (Nur factors) through direct interactions with NGFI-B and SRC-2. Although recruitment of SRC/p160 coactivators to the NGFI-B AF1 domain is independent of Rb, its presence enhances SRC-dependent transcription. Rb potentiation of SRC coactivators is exerted on a subset (Nur factors, hepatocyte nuclear factor-4 (HNF-4), SF-1, and ER) but not all NRs. The levels of Rb-related proteins modulate hormone responsiveness of the NGFI-B-dependent pituitary proopiomelanocortin gene and HNF-4-dependent transcription during enterocyte differentiation. Increased Rb expression upon cell differentiation may promote differentiated functions, at least in part, by potentiation of NR activity.

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.

Ski negatively regulates erythroid differentiation through its interaction with GATA1

The Ski oncoprotein dramatically affects cell growth, differentiation, and/or survival. Recently, Ski was shown to act in distinct signaling pathways including those involving nuclear receptors, transforming growth factor beta, and tumor suppressors. These divergent roles of Ski are probably dependent on Ski's capacity to bind multiple partners with disparate functions. In particular, Ski alters the growth and differentiation program of erythroid progenitor cells, leading to malignant leukemia. However, the mechanism underlying this important effect has remained elusive. Here we show that Ski interacts with GATA1, a transcription factor essential in erythropoiesis. Using a Ski mutant deficient in GATA1 binding, we show that this Ski-GATA1 interaction is critical for Ski's ability to repress GATA1-mediated transcription and block erythroid differentiation. Furthermore, the repression of GATA1-mediated transcription involves Ski's ability to block DNA binding of GATA1. This finding is in marked contrast to those in previous reports on the mechanism of repression by Ski, which have described a model involving the recruitment of corepressors into DNA-bound transcription complexes. We propose that Ski cooperates in the process of transformation in erythroid cells by interfering with GATA1 function, thereby contributing to erythroleukemia.

The Protooncogene Ski Controls Schwann Cell Proliferation and Myelination

Schwann cell proliferation and subsequent differentiation to nonmyelinating and myelinating cells are closely linked processes. Elucidating the molecular mechanisms that control these events is key to the understanding of nerve development, regeneration, nerve-sheath tumors, and neuropathies. We define the protooncogene Ski, an inhibitor of TGF-beta signaling, as an essential component of the machinery that controls Schwann cell proliferation and myelination. Functional Ski overexpression inhibits TGF-beta-mediated proliferation and prevents growth-arrested Schwann cells from reentering the cell cycle. Consistent with these findings, myelinating Schwann cells upregulate Ski during development and remyelination after injury. Myelination is blocked in myelin-competent cultures derived from Ski-deficient animals, and genes encoding myelin components are downregulated in Ski-deficient nerves. Conversely, overexpression of Ski in Schwann cells causes an upregulation of myelin-related genes. The myelination-regulating transcription factor Oct6 is involved in a complex modulatory relationship with Ski. We conclude that Ski is a crucial signal in Schwann cell development and myelination.

Ski and SnoN: negative regulators of TGF-beta signaling

Ski and SnoN are unique proto-oncoproteins in that they can induce both oncogenic transformation and terminal muscle differentiation when expressed at high levels. Recent studies using in vitro and in vivo approaches have begun to unravel the complex roles of Ski and SnoN in tumorigenesis and embryonic development. The identification of Ski and SnoN as important negative regulators of signal transduction by the transforming growth factor-beta superfamily of cytokines provides a valuable molecular basis for the complex functions of Ski and SnoN.

Increased expression of c-Ski as a co-repressor in transforming growth factor-beta signaling correlates with progression of esophageal squamous cell carcinoma

Transforming growth factor-beta (TGF-beta) regulates cell growth inhibition, and inactivation of the TGF-beta signaling pathway contributes to tumor development. In our previous study, altered expression of TGF-beta, TGF-beta-specific receptors and Smad4 was shown to correlate with tumor progression in esophageal squamous cell carcinoma (SCC). These components, however, were maintained normally in some patients with esophageal SCC. In our study, the mechanism by which aggressive esophageal SCC maintains these components was investigated, with particular emphasis on the participation of c-Ski and SnoN as transcriptional co-repressors in TGF-beta signaling. Immunohistochemistry for c-Ski and SnoN was carried out on surgical specimens obtained from 80 patients with esophageal SCC. The expression of c-Ski and SnoN was also studied in 6 established cell lines derived from esophageal SCC and compared to an immortalized human esophageal cell line by Western blotting. High levels of expression of c-Ski, detected immunohistologically, were found to correlate with depth of invasion (p = 0.0080) and pathologic stage (p = 0.0447). There was, however, no significant correlation between expression of SnoN and clinicopathologic characteristics. A significant correlation between c-Ski and TGF-beta expression was observed. Moreover, in patients with TGF-beta negative expression, the survival rates of patients with c-Ski positive expression were significantly lower than those of patients with c-Ski negative expression (p = 0.0486). c-Ski was expressed at a high level in 5 of 6 cell lines derived from esophageal SCC compared to immortalized esophageal keratinocytes. Furthermore, the cyclin-dependent kinase (CDK) inhibitor, p21 that was up-regulated by TGF-beta signaling was expressed at a low level in the 5 cell lines. The expression of c-Ski protein as a transcriptional co-repressor in TGF-beta signaling seems to be correlated with tumor progression of esophageal SCC.

Oncogenic activation of c-Myb correlates with a loss of negative regulation by TIF1beta and Ski

The c-myb proto-oncogene product (c-Myb) regulates proliferation of hematopoietic cells by inducing the transcription of a group of target genes. Removal or mutations of the negative regulatory domain (NRD) in the C-terminal half of c-Myb leads to increased transactivating capacity and oncogenic activation. Here we report that TIF1beta directly binds to the NRD and negatively regulates the c-Myb-dependent trans-activation. In addition, three corepressors (Ski, N-CoR, and mSin3A) bind to the DNA-binding domain of c-Myb together with TIF1beta and recruit the histone deacetylase complex to c-Myb. Furthermore, the Drosophila TIF1beta homolog, Bonus, negatively regulates Drosophila Myb activity. The Ski corepressor competes with the coactivator CBP for binding to c-Myb, indicating that the selection of coactivators and corepressors is a key event for c-Myb-dependent transcription. Mutations or deletion of the NRD of c-Myb and the mutations found in the DNA-binding domain of v-Myb decrease the interaction with these corepressors and weaken the corepressor-induced negative regulation of Myb activity. These observations have conceptual implications for understanding how the nuclear oncogene is activated.

Requirement of the co-repressor homeodomain-interacting protein kinase 2 for ski-mediated inhibition of bone morphogenetic protein-induced transcriptional activation

Multiple co-repressors such as N-CoR/SMRT, mSin3, and the c-ski proto-oncogene product (c-Ski) mediate the transcriptional repression induced by Mad and the thyroid hormone receptor by recruiting the histone deacetylase complex. c-Ski also binds directly to Smad proteins, which are transcriptional activators in the transforming growth factor-beta (TGF-beta)/bone morphogenetic protein (BMP) signaling pathways, and inhibits TGF-beta/BMP-induced transcriptional activation. However, it remains unknown whether other co-repressor(s) are also involved with Ski in the negative regulation of the TGF-beta/BMP signaling pathways. Here, we report that the co-repressor homeodomain-interacting protein kinase 2 (HIPK2) directly binds to both c-Ski and Smad1. HIPK2 efficiently inhibited Smad1/4-induced transcription from the Smad site-containing promoter. A dominant negative form of HIPK2, in which the ATP binding motif in the kinase domain and the putative phosphorylation sites were mutated, enhanced Smad1/4-dependent transcription and the BMP-induced expression of alkaline phosphatase. Furthermore, the c-Ski-induced inhibition of the Smad1/4-dependent transcription was suppressed by a dominant negative form of HIPK2. The HIPK2 co-repressor activity may be regulated by an uncharacterized HIPK2 kinase. These results indicate that HIPK2, together with c-Ski, plays an important role in the negative regulation of BMP-induced transcriptional activation.

Direct interaction of Ski with either Smad3 or Smad4 is necessary and sufficient for Ski-mediated repression of transforming growth factor-beta signaling

The oncoprotein Ski represses transforming growth factor (TGF)-beta signaling in an N-CoR-independent manner. However, the molecular mechanism(s) underlying this event has not been elucidated. Here, we identify an additional domain in Ski that mediates interaction with Smad3 which is important for this repression. This domain is distinct from the previously reported N-terminal Smad3 binding domain in Ski. Individual alanine substitution of several residues in the domain significantly affected Ski-Smad3 interaction. Furthermore, combined mutations within this domain, together with those in the previously identified Smad3 binding domain, can completely abolish the interaction of Ski with Smad3, while mutation in each domain alone retained partial interaction. By introducing those mutations that abolish direct interaction with Smad3 or Smad4 individually, or in combination, we show that interaction of Ski with either Smad3 or Smad4 is sufficient for Ski-mediated repression of TGF-beta signaling. Furthermore our results clearly demonstrate that Ski does not disrupt Smad3-Smad4 heteromer formation, and recruitment of Ski to the Smad3/4 complex through binding to either Smad3 or Smad4 is both necessary and sufficient for repression.

The transforming activity of Ski and SnoN is dependent on their ability to repress the activity of Smad proteins

The regulation of cell growth and differentiation by transforming growth factor-beta (TGF-beta) is mediated by the Smad proteins. In the nucleus, the Smad proteins are negatively regulated by two closely related nuclear proto-oncoproteins, Ski and SnoN. When overexpressed, Ski and SnoN induce oncogenic transformation of chicken embryo fibroblasts. However, the mechanism of transformation by Ski and SnoN has not been defined. We have previously reported that Ski and SnoN interact directly with Smad2, Smad3, and Smad4 and repress their ability to activate TGF-beta target genes through multiple mechanisms. Because Smad proteins are tumor suppressors, we hypothesized that the ability of Ski and SnoN to inactivate Smad function may be responsible for their transforming activity. Here, we show that the receptor regulated Smad proteins (Smad2 and Smad3) and common mediator Smad (Smad4) bind to different regions in Ski and SnoN. Mutation of both regions, but not each region alone, markedly impaired the ability of Ski and SnoN to repress TGF-beta-induced transcriptional activation and cell cycle arrest. Moreover, when expressed in chicken embryo fibroblasts, mutant Ski or SnoN defective in binding to the Smad proteins failed to induce oncogenic transformation. These results suggest that the ability of Ski and SnoN to repress the growth inhibitory function of the Smad proteins is required for their transforming activity. This may account for the resistance to TGF-beta-induced growth arrest in some human cancer cell lines that express high levels of Ski or SnoN.

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.

The oncoprotein Ski acts as an antagonist of transforming growth factor-beta signaling by suppressing Smad2 phosphorylation

The phosphorylation of Smad2 and Smad3 by the transforming growth factor (TGF)-beta-activated receptor kinases and their subsequent heterodimerization with Smad4 and translocation to the nucleus form the basis for a model how Smad proteins work to transmit TGF-beta signals. The transcriptional activity of Smad2-Smad4 or Smad3-Smad4 complexes can be limited by the corepressor Ski, which is believed to interact with Smad complexes on TGF-beta-responsive promoters and represses their ability to activate TGF-beta target genes by assembling on DNA a repressor complex containing histone deacetylase. Here we show that Ski can block TGF-beta signaling by interfering with the phosphorylation of Smad2 and Smad3 by the activated TGF-beta type I receptor. Furthermore, we demonstrate that overexpression of Ski induces the assembly of Smad2-Smad4 and Smad3-Smad4 complexes independent of TGF-beta signaling. The ability of Ski to engage Smad proteins in nonproductive complexes provides new insights into the molecular mechanism used by Ski for disabling TGF-beta signaling.

Signal-dependent N-CoR requirement for repression by the Ski oncoprotein

The oncoprotein Ski represses transforming growth factor-beta (TGF-beta) and nuclear receptor signaling. To achieve this, Ski has been proposed to recruit the corepressor N-CoR to either the TGF-beta-regulated Smad transcription factors or nuclear receptors. Here we define the role of the Ski/N-CoR interaction in Ski-mediated repression of TGF-beta and vitamin D signaling. We show that Ski can negatively regulate vitamin D-mediated transcription by directly interacting with the vitamin D receptor. More importantly, a Ski single point mutant lacking N-CoR binding revealed that the Ski/N-CoR interaction is essential for repression of vitamin D signaling, but, surprisingly, not TGF-beta signaling. Thus, Ski modulates transcription in either an N-CoR-dependent or -independent manner depending on the signaling pathways targeted.

Generation of Ski-knockdown mice by expressing a long double-strand RNA from an RNA polymerase II promoter

We have developed a new vector, named pDECAP, to express long double-strand RNA (ds-RNA) from an RNA polymerase II (Pol II) promoter. Because the transcripts from pDECAP lack both the 5'-cap structure and the 3'-poly(A) tail that facilitate ds-RNA export to the cytoplasm, long ds-RNA from pDECAP does not induce the interferon response. Transgenic mice embryos expressing long ds-RNA for the transcriptional corepressor Ski from this vector exhibited phenotypes that were remarkably similar to those of Ski-deficient embryos, including defects of neural tube closure and eye formation. Thus, this vector provides a new tool to efficiently generate tissue-specific knockdown mice for studying gene function in whole animal systems.

The Ski-binding protein C184M negatively regulates tumor growth factor-beta signaling by sequestering the Smad proteins in the cytoplasm

Ski is a transcriptional co-repressor and is involved in the negative regulation of tumor growth factor-beta (TGF-beta) signaling. To understand more fully the role of Ski in TGF-beta signaling, we searched for novel Ski-interacting proteins. The identified C184M protein consists of 189 amino acids and contains the leucine-rich region. An association between Ski and C184M involving the leucine-rich region of C184M and the C-terminal coiled-coil motif of Ski was confirmed by glutathione S-transferase pull-down and immunoprecipitation assays. The C184M protein is located in the cytosol, and the C184M and Ski signals are co-localized in the cytoplasm when C184M was co-expressed with Ski in CV-1 cells. The cytoplasmic C184M-Ski complex inhibited the nuclear translocation of Smad2. Consistent with this, the activity of promoter containing the Smad-binding sites was repressed by C184M, and the TGF-beta-induced growth inhibition of mink lung Mv1Lu cells was attenuated by the ectopic expression of C184M. Thus, C184M inhibits TGF-beta signaling in concert with Ski. In hepatocytes, which express significant levels of C184M, the Ski signals were found only in the cytoplasm, supporting the notion that C184M forms a complex with Ski in the cytosol.

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.

Repression of E2F1-mediated transcription by the ErbB3 binding protein Ebp1 involves histone deacetylases

Ebp1, an ErbB3 binding protein that is a member of the proliferation-associated PA2G4 family, inhibits the proliferation and induces the differentiation of human ErbB positive breast and prostate cancer cell lines. Ebp1 binds the tumor suppressor retinoblastoma protein (Rb) both in vivo and in vitro, and Rb and Ebp1 cooperate to inhibit the transcription of the E2F1-regulated cyclin E promoter. We show here that Ebp1 can inhibit the transcription of other E2F-regulated reporter genes and of several endogenous E2F-regulated genes important in cell cycle progression in both Rb positive and Rb null cells. The Ebp1-mediated transcriptional repression depended on the presence of an E2F1 consensus element in the promoters. A fusion of Ebp1 with the GAL4 DNA binding domain protein had independent transcriptional repression activity that mapped to the C-terminal region of Ebp1. This C-terminal region of Ebp1 bound functional histone deacetylase (HDAC) activity and inhibitors of HDAC significantly reduced Ebp1-mediated repression. Ebp1 bound HDAC2, but not HDAC1, in vitro. An Ebp1 mutant lacking the HDAC binding domain failed to inhibit transcription. Our results suggest that Ebp1 can repress transcription of some E2F-regulated promoters and that one mechanism of Ebp1- mediated transcriptional repression is via its ability to recruit HDAC activity.

Two short segments of Smad3 are important for specific interaction of Smad3 with c-Ski and SnoN

c-Ski and SnoN are transcriptional co-repressors that inhibit transforming growth factor-beta signaling through interaction with Smad proteins. Among receptor-regulated Smads, c-Ski and SnoN bind more strongly to Smad2 and Smad3 than to Smad1. Here, we show that c-Ski and SnoN bind to the "SE" sequence in the C-terminal MH2 domain of Smad3, which is exposed on the N-terminal upper side of the toroidal structure of the MH2 oligomer. The "QPSMT" sequence, located in the vicinity of SE, supports the interaction with c-Ski and SnoN. Sequences similar to SE and QPSMT are found in Smad2, but not in Smad1. The N-terminal MH1 domain and linker region of Smad3 protrude from the N-terminal upper side of the MH2 oligomer toroid. Smurf2 induces ubiquitin-dependent degradation of SnoN, since it appears to be located close to SnoN through binding to the linker region of Smad2. In contrast, transcription factors Mixer and FoxH3 (FAST1) bind to the bottom side of the Smad3 MH2 toroid; therefore, c-Ski does not affect the interaction of Smads with these transcription factors. Our findings thus demonstrate the stoichiometry of how multiple molecules can associate with the Smad oligomers and how the Smad-interacting proteins functionally interact with each other.

Role of the RB tumor suppressor in cancer

Apart from their coordinated inactivation by DNA tumor viral oncoproteins, the pRB and p53 tumor suppressor pathways were not known to be connected ten years ago. Within the last decade, our appreciation of how these pathways are interconnected has grown substantially. The checks and balances that exist between pRB and p53 involve the regulation of the G1/S transition and its checkpoints, and much of this is under the control of the E2F transcription factor family. Following DNA damage, the p53-dependent induction of p21CIP1 regulates cyclin E/Cdk2 and cyclin A/Cdk2 complexes both of which phosphorylate pRB, leading to E2F-mediated activation. Similarly, E2F1-dependent induction of p19ARF antagonizes the ability of mdm2 to degrade p53, leading to p53 stabilization and potentially p53-mediated apoptosis or cell cycle arrest. From the existing mouse models discussed above, we also know that proliferation, cell death and differentiation of distinct tissues are also intimately linked through entrance and exit from the cell cycle, and thus through pRB and p53 pathways. Virtually all human tumors deregulate either the pRB or p53 pathway, and often times both pathways simultaneously, which is critical for crippling cellular defense against neoplasia. The next decade of cancer research will likely see these two tumor suppressor pathways only merge even more.

Skip interacts with the retinoblastoma tumor suppressor and inhibits its transcriptional repression activity

Ski interacting protein (Skip) plays an important role in the transforming activity of both v-Ski and EBNA2 (Epstein-Barr virus encoded latency protein) and is involved in EBNA2 and NotchIC activation of CBF1-repressed promoters. We have previously shown that Skip acts as a transcriptional co-activator on a number of cellular and viral promoters. Here, we report that Skip also interacts with pRb and, in cooperation with Ski, can overcome pRb-induced transcriptional repression. We show a strong and direct interaction between pRb and Skip, and we map the site of interaction to amino acid residues 171-353 of the evolutionarily conserved SNW domain of Skip. Furthermore, the combination of Skip and Ski can successfully overcome the G1 arrest and flat cell phenotype induced by pRb. Taken together, these studies suggest that one potential function of the Skip-Ski complex is to overcome the growth-suppressive activities of pRb.

Structural mechanism of Smad4 recognition by the nuclear oncoprotein Ski: insights on Ski-mediated repression of TGF-beta signaling

The Ski family of nuclear oncoproteins represses TGF-beta signaling through interactions with the Smad proteins. The crystal structure of the Smad4 binding domain of human c-Ski in complex with the MH2 domain of Smad4 reveals specific recognition of the Smad4 L3 loop region by a highly conserved interaction loop (I loop) from Ski. The Ski binding surface on Smad4 significantly overlaps with that required for binding of the R-Smads. Indeed, Ski disrupts the formation of a functional complex between the Co- and R-Smads, explaining how it could lead to repression of TGF-beta, activin, and BMP responses. Intriguingly, the structure of the Ski fragment, stabilized by a bound zinc atom, resembles the SAND domain, in which the corresponding I loop is responsible for DNA binding.