TGF-beta repressors SnoN and Ski are implicated in human colorectal carcinogenesis

Alterations in TGFβ signaling during prostate cancer progression

During prostate cancer progression, TGF-β acts as both a tumor suppressor and tumor promoter. TGF-β inhibits cell proliferation in normal and early-stage prostate cancer cells, but during later stages of the disease the cancer cells develop resistance to inhibitory effects on cell proliferation. In these cells, TGF-β promotes cancer progression due to its effects on epithelial to mesenchymal transition (EMT), cell migration and invasion, and immune suppression. The intracellular mechanisms involved in the development of resistance to TGF-β effects on cell proliferation are largely unknown. In this review, we summarized the roles of several intracellular proteins including PTEN, Id1 and JunD, which may play a role in this transition. The role of Ski/SnoN proteins in inhibition of Smad2/3 signaling is highlighted.

Increased ski expression levels are associated with a higher risk and poor prognosis in patients with gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) are the most commonly diagnosed primary mesenchymal tumors of the gastrointestinal tract and 30% of GISTs are associated with a high recurrence risk or metastasis. The current risk classification criteria of the National Comprehensive Cancer Network are based on tumor size, mitotic activity and localization. Investigating additional biomarkers associated with clinical risk may aid in the diagnosis of GIST and improves prediction of patient prognosis. In the present study, the value of using the expression levels of the oncoprotein ski as a prognostic predictor for GISTs was investigated. The results demonstrated that high ski expression levels were correlated with high risk and recurrence rates and indicated poor prognosis regarding median disease-free survival. Overall, the present study suggests that ski expression levels may serve as a predictor for clinical risk and prognosis of patients with GISTs.

Combined cistrome and transcriptome analysis of SKI in AML cells identifies SKI as a co-repressor for RUNX1

SKI is a transcriptional co-regulator and overexpressed in various human tumors, for example in acute myeloid leukemia (AML). SKI contributes to the origin and maintenance of the leukemic phenotype. Here, we use ChIP-seq and RNA-seq analysis to identify the epigenetic alterations induced by SKI overexpression in AML cells. We show that approximately two thirds of differentially expressed genes are up-regulated upon SKI deletion, of which >40% harbor SKI binding sites in their proximity, primarily in enhancer regions. Gene ontology analysis reveals that many of the differentially expressed genes are annotated to hematopoietic cell differentiation and inflammatory response, corroborating our finding that SKI contributes to a myeloid differentiation block in HL60 cells. We find that SKI peaks are enriched for RUNX1 consensus motifs, particularly in up-regulated SKI targets upon SKI deletion. RUNX1 ChIP-seq displays that nearly 70% of RUNX1 binding sites overlap with SKI peaks, mainly at enhancer regions. SKI and RUNX1 occupy the same genomic sites and cooperate in gene silencing. Our work demonstrates for the first time the predominant co-repressive function of SKI in AML cells on a genome-wide scale and uncovers the transcription factor RUNX1 as an important mediator of SKI-dependent transcriptional repression.

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.

Next generation sequencing analysis of miRNAs: MiR-127-3p inhibits glioblastoma proliferation and activates TGF-β signaling by targeting SKI

Glioblastoma (GBM) proliferation is a multistep process during which the expression levels of many genes that control cell proliferation, cell death, and genetic stability are altered. MicroRNAs (miRNAs) are emerging as important modulators of cellular signaling, including cell proliferation in cancer. In this study, using next generation sequencing analysis of miRNAs, we found that miR-127-3p was downregulated in GBM tissues compared with normal brain tissues; we validated this result by RT-PCR. We further showed that DNA demethylation and histone deacetylase inhibition resulted in downregulation of miR-127-3p. We demonstrated that miR-127-3p overexpression inhibited GBM cell growth by inducing G1-phase arrest both in vitro and in vivo. We showed that miR-127-3p targeted SKI (v-ski sarcoma viral oncogene homolog [avian]), RGMA (RGM domain family, member A), ZWINT (ZW10 interactor, kinetochore protein), SERPINB9 (serpin peptidase inhibitor, clade B [ovalbumin], member 9), and SFRP1 (secreted frizzled-related protein 1). Finally, we found that miR-127-3p suppressed GBM cell growth by inhibiting tumor-promoting SKI and activating the tumor suppression effect of transforming growth factor-β (TGF-β) signaling. This study showed, for the first time, that miR-127-3p and its targeted gene SKI, play important roles in GBM and may serve as potential targets for GBM therapy.

c-Ski activates cancer-associated fibroblasts to regulate breast cancer cell invasion

Aberrant expression of c-Ski oncoprotein in some tumor cells has been shown to be associated with cancer development. However, the role of c-Ski in cancer-associated fibroblasts (CAFs) of tumor microenvironment has not been characterized. In the current study, we found that c-Ski is highly expressed in CAFs derived from breast carcinoma microenvironment and this CAF-associated c-Ski expression is associated with invasion and metastasis of human breast tumors. We showed that c-Ski overexpression in immortalized breast normal fibroblasts (NFs) induces conversion to breast CAFs by repressing p53 and thereby upregulating SDF-1 in NFs. SDF-1 treatment or p53 knockdown in NFs had similar effects on the activation of NFs as c-Ski overexpression. The c-Ski-activated CAFs show increased proliferation, migration, invasion and contraction compared with NFs. Furthermore, c-Ski-activated CAFs facilitated the migration and invasion of MDA-MB-231 breast cancer cells. Our data suggest that c-Ski is an important regulator in the activation of CAFs and may serve as a potential therapeutic target to block breast cancer progression.

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.

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.

Expression and clinical significance of the transforming growth factor-β signalling pathway in endometrial cancer

AIMS

To evaluate the components of the transforming growth factor (TGF)-β-Smad signalling pathway in human endometrial cancer (EC).

METHODS AND RESULTS

TGF-β1, TGF-β receptor type I, TGF-β receptor type II, Smad2, Smad3, Smad4, Skil and Disabled-2 (DAB2) mRNA levels were determined by reverse transcriptase polymerase chain reaction on EC cell lines and in 70 EC tissues. Immunohistochemistry for Skil and DAB2 antibodies was performed on 362 EC cases. Decreased mRNA levels of all eight components of the TGF-β pathway tested were found in the majority of 70 cases. For DAB2, the mRNA level was correlated with protein expression level (P = 0.04). The Skil mRNA level was associated with tumour stage (P = 0.03), and the Smad2/3/4 mRNA level with tumour grade (P = 0.03, P = 0.02, and P = 0.00, respectively). The Smad4 mRNA level was also associated with tumour size (P = 0.05), subtype (P = 0.04), and disease-free survival (DFS) (P = 0.05). The TGF-β1 mRNA level was associated with DFS (P = 0.04). Finally, tumours with positive Skil protein expression had a shorter recurrence time, whereas, those with positive DAB2 protein expression had a longer recurrence time.

CONCLUSIONS

Down-regulation of the TGF-β-Smad signalling pathway might be responsible for the pathogenesis of human EC, and some of its components appeared to be prognostic factors. Exploration of future therapy targeting the TGF-β-Smad pathway is warranted in EC.

Enhancement of TGF-β signaling responses by the E3 ubiquitin ligase Arkadia provides tumor suppression in colorectal cancer

TGF-β signaling provides tumor protection against colorectal cancer (CRC). Mechanisms that support its tumor-suppressive properties remain unclear. The ubiquitin ligase Arkadia/RNF111 enhances TGF-β signaling responses by targeting repressors of the pathway for degradation. The corepressors SnoN/Ski, critical substrates of Arkadia, complex with the activated TGF-β signaling effectors Smad2/3 (pSmad2/3) on the promoters of target genes and block their transcription. Arkadia degrades this complex including pSmad2/3 and unblocks the promoter. Here, we report that Arkadia is expressed highly in the mouse colonic epithelium. Heterozygous Akd(+/-) mice are normal but express less Arkadia. This leads to reduced expression of several TGF-β target genes, suggesting that normal levels of Arkadia are required for efficient signaling responses. Critically, Akd(+/-) mice exhibit increased susceptibility to azoxymethane/dextran sodium sulfate carcinogen-induced CRC, as they develop four-fold more tumors than wild-type mice. Akd(+/-) tumors also exhibit a more aggressive pathology, higher proliferation index, and reduced cytostasis. Therefore, Arkadia functions as a tumor suppressor whose peak expression is required to suppress CRC development and progression. The accumulation of nuclear SnoN and pSmad2, along with the downregulation of TGF-β target genes observed in Akd(+/-) colon and tumors, suggest that tumor-suppressing properties of Arkadia are mediated by its ability to derepress TGF-β signaling. Consistent with this likelihood, we identified mutations in primary colorectal tumors from human patients that reduce Arkadia function and are associated with the accumulation of nuclear SNON. Collectively, our findings reveal that Arkadia enhances TGF-β signaling responses and supports its tumor-suppressing properties in CRC.

SnoN: bridging neurobiology and cancer biology

The transcriptional regulator SnoN has been the subject of growing interest due to its diverse functions in normal and pathological settings. A large body of evidence has established a fundamental role for SnoN as a modulator of signaling and responses by the transforming growth beta (TGFbeta) family of cytokines, though how SnoN regulates TGFbeta responses remains incompletely understood. In accordance with the critical and complex roles of TGFbeta in tumorigenesis and metastasis, SnoN may act as a tumor promoter or suppressor depending on the stage and type of cancer. Beyond its role in cancer, SnoN has also been implicated in the control of axon morphogenesis in postmitotic neurons in the mammalian brain. Remarkably, signaling pathways that control SnoN functions in the divergent cycling cells and postmitotic neurons appear to be conserved. Identification of novel SnoN regulatory and effector mechanisms holds the promise of advances at the interface of cancer biology and neurobiology.

Efficient TGF-β/SMAD signaling in human melanoma cells associated with high c-SKI/SnoN expression

Background

SKI and SnoN proteins have been shown to inhibit TGF-β signaling, acting both as transcriptional co-repressors in the cell nucleus, and as sequestrators of SMAD proteins in the cytoplasm. TGF-β, on the other hand, induces rapid, proteasome-mediated, degradation of both proteins. How elevated SKI and SnoN protein levels co-exist with active autocrine TGF-β signaling in cancer cells is yet to be understood.

Results

In this study, we found elevated SKI and SnoN protein levels in a panel of melanoma cell lines, as compared to normal melanocytes. There was no correlation between SKI protein content and the capacity of melanoma cells to invade Matrigel™, to form subcutaneous tumors, or to metastasize to bone after intracardiac inoculation into nude mice. Nor did we find a correlation between SKI expression and histopathological staging of human melanoma. TGF-β induced a rapid and dose-dependent degradation of SKI protein, associated with SMAD3/4 specific transcriptional response and induction of pro-metastatic target genes, partially prevented by pharmacologic blockade of proteasome activity. SKI knockdown in 1205Lu melanoma cells did not alter their invasive capacity or transcriptional responses to TGF-β, and did not allow p21 expression in response to TGF-β or reveal any growth inhibitory activity of TGF-β.

Conclusions

Despite high expression in melanoma cells, the role of SKI in melanoma remains elusive: SKI does not efficiently interfere with the pro-oncogenic activities of TGF-β, unless stabilized by proteasome blockade. Its highly labile nature makes it an unlikely target for therapeutic intervention.

SKI promotes Smad3 linker phosphorylations associated with the tumor-promoting trait of TGFbeta

The transcriptional co-regulator SKI is a potent inhibitor of TGFbeta-growth inhibitory signals. SKI binds to receptor-activated Smads in the nucleus, forming repressor complexes containing HDACs, mSin3, NCoR, and other protein partners. Alternatively, SKI binds to activated Smads in the cytoplasm, preventing their nuclear translocation. SKI is necessary for anchorage-independent growth of melanoma cells in vitro, and most important, for human melanoma xenograft growth in vivo. We recently identified a novel role of SKI in TGFbeta signaling. SKI promotes the switch of Smad3 from repressor of proliferation to activator of oncogenesis by facilitating phosphorylations in the linker domain. High levels of endogenous SKI are required by the tumor promoting trait of TGFbeta to induce expression of the plasminogen-activator inhibitor-1 (PAI-1), sustained expression of C-Myc and for aborting upregulation of p21(Waf-1). Here we discuss how SKI diversifies and amplifies its functions by associating with multiple protein partners and by promoting Smad3 linker phosphorylation(s) in response to TGFbeta signaling in melanoma cells.

SnoN functions as a tumour suppressor by inducing premature senescence

SnoN represses TGF-beta signalling to promote cell proliferation and has been defined as a proto-oncogene partly due to its elevated expression in many human cancer cells. Although the anti-tumourigenic activity of SnoN has been suggested, the molecular basis for this has not been defined. We showed here that high levels of SnoN exert anti-oncogenic activity by inducing senescence. SnoN interacts with the promyelocytic leukaemia (PML) protein and is recruited to the PML nuclear bodies where it stabilizes p53, leading to premature senescence. Furthermore, overexpression of SnoN inhibits oncogenic transformation induced by Ras and Myc in vitro and significantly blocks papilloma development in vivo in a carcinogen-induced skin tumourigenesis model. The few papillomas that were developed displayed high levels of senescence and spontaneously regressed. Our study has revealed a novel Smad-independent pathway of SnoN function that mediates its anti-oncogenic activity.