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

Differential Interleukin-2 Transcription Kinetics Render Mouse but Not Human T Cells Vulnerable to Splicing Inhibition Early after Activation

T cells are nodal players in the adaptive immune response against pathogens and malignant cells. Alternative splicing plays a crucial role in T cell activation, which is analyzed mainly at later time points upon stimulation. Here we have discovered a 2-h time window early after stimulation where optimal splicing efficiency or, more generally, gene expression efficiency is crucial for successful T cell activation. Reducing the splicing efficiency at 4 to 6 h poststimulation significantly impaired murine T cell activation, which was dependent on the expression dynamics of the Egr1-Nab2-interleukin-2 (IL-2) pathway. This time window overlaps the time of peak IL-2 de novo transcription, which, we suggest, represents a permissive time window in which decreased splicing (or transcription) efficiency reduces mature IL-2 production, thereby hampering murine T cell activation. Notably, the distinct expression kinetics of the Egr1-Nab2-IL-2 pathway between mouse and human render human T cells refractory to this vulnerability. We propose that the rational temporal modulation of splicing or transcription during peak de novo expression of key effectors can be used to fine-tune stimulation-dependent biological outcomes. Our data also show that critical consideration is required when extrapolating mouse data to the human system in basic and translational research.

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.

Effector/memory CD4 T cells making either Th1 or Th2 cytokines commonly co-express T-bet and GATA-3

Naïve CD4 T (NCD4T) cells post-activation undergo programming for inducible production of cytokines leading to generation of memory cells with various functions. Based on cytokine based polarization of NCD4T cells in vitro, programming for either ‘Th1’ (interferon-gamma [IFNg]) or ‘Th2’ (interleukin [IL]-4/5/13) cytokines is thought to occur via mutually exclusive expression and functioning of T-bet or GATA-3 transcription factors (TFs). However, we show that a high proportion of mouse and human memory-phenotype CD4 T (MCD4T) cells generated in vivo which expressed either Th1 or Th2 cytokines commonly co-expressed T-bet and GATA-3. While T-bet levels did not differ between IFNg-expressing and IL-4/5/13-expressing MCD4T cells, GATA-3 levels were higher in the latter. These observations were also confirmed in MCD4T cells from FVB/NJ or aged C57BL/6 or IFNg-deficient mice. While MCD4T cells from these strains showed greater Th2 commitment than those from young C57BL/6 mice, pattern of co-expression of TF was similar. Effector T cells generated in vivo following immunization also showed TF co-expression in Th1 or Th2 cytokine producing cells. We speculated that the difference in TF expression pattern of MCD4T cells generated in vivo and those generated in cytokine polarized cultures in vitro could be due to relative absence of polarizing conditions during activation in vivo. We tested this by NCD4T cell activation in non-polarizing conditions in vitro. Anti-CD3 and anti-CD28-mediated priming of polyclonal NCD4T cells in vitro without polarizing milieu generated cells that expressed either IFNg or IL-4/5/13 but not both, yet both IFNg- and IL-4/5/13-expressing cells showed upregulation of both TFs. We also tested monoclonal T cell populations activated in non-polarizing conditions. TCR-transgenic NCD4T cells primed in vitro by cognate peptide in non-polarizing conditions which expressed either IFNg or IL-4/5/13 also showed a high proportion of cells co-expressing TFs, and their cytokine commitment varied depending on genetic background or priming conditions, without altering pattern of TF co-expression. Thus, the model of mutually antagonistic differentiation programs driven by mutually exclusively expressed T-bet or GATA-3 does not completely explain natural CD4 T cell priming outcomes.

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.

SnoN facilitates ALK1-Smad1/5 signaling during embryonic angiogenesis

In endothelial cells, two type I receptors of the transforming growth factor β (TGF-β) family, ALK1 and ALK5, coordinate to regulate embryonic angiogenesis in response to BMP9/10 and TGF-β. Whereas TGF-β binds to and activates ALK5, leading to Smad2/3 phosphorylation and inhibition of endothelial cell proliferation and migration, BMP9/10 and TGF-β also bind to ALK1, resulting in the activation of Smad1/5. SnoN is a negative regulator of ALK5 signaling through the binding and repression of Smad2/3. Here we uncover a positive role of SnoN in enhancing Smad1/5 activation in endothelial cells to promote angiogenesis. Upon ligand binding, SnoN directly bound to ALK1 on the plasma membrane and facilitated the interaction between ALK1 and Smad1/5, enhancing Smad1/5 phosphorylation. Disruption of this SnoN-Smad interaction impaired Smad1/5 activation and up-regulated Smad2/3 activity. This resulted in defective angiogenesis and arteriovenous malformations, leading to embryonic lethality at E12.5. Thus, SnoN is essential for TGF-β/BMP9-dependent biological processes by its ability to both positively and negatively modulate the activities of Smad-dependent pathways.

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 regulates mammary gland alveologenesis and onset of lactation by promoting prolactin/Stat5 signaling

Mammary epithelial cells undergo structural and functional differentiation at late pregnancy and parturition to produce and secrete milk. Both TGF-β and prolactin pathways are crucial regulators of this process. However, how the activities of these two antagonistic pathways are orchestrated to initiate lactation has not been well defined. Here, we show that SnoN, a negative regulator of TGF-β signaling, coordinates TGF-β and prolactin signaling to control alveologenesis and lactogenesis. SnoN expression is induced at late pregnancy by the coordinated actions of TGF-β and prolactin. The elevated SnoN promotes Stat5 signaling by enhancing its stability, thereby sharply increasing the activity of prolactin signaling at the onset of lactation. SnoN-/- mice display severe defects in alveologenesis and lactogenesis, and mammary epithelial cells from these mice fail to undergo proper morphogenesis. These defects can be rescued by an active Stat5. Thus, our study has identified a new player in the regulation of milk production and revealed a novel function of SnoN in mammary alveologenesis and lactogenesis in vivo through promotion of Stat5 signaling.

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 in mammalian development, function and diseases

SnoN (Ski-novel protein) was discovered as a nuclear proto-oncogene on the basis of its ability to induce transformation of chicken and quail embryonic fibroblasts. As a crucial negative regulator of transforming growth factor-β (TGF-β) signaling and also an activator of p53, it plays an important role in regulating cell proliferation, senescence, apoptosis, and differentiation. Recent studies of its expression patterns and functions in mouse models and mammalian cells have revealed important functions of SnoN in normal epithelial development and tumorigenesis. Evidence suggests that SnoN has both pro-oncogenic and anti-oncogenic functions by modulating multiple signaling pathways. These studies suggest that SnoN may have broad functions in the development and homeostasis of embryonic and postnatal tissues.

The Sno oncogene antagonizes Wingless signaling during wing development in Drosophila

The Sno oncogene (Snoo or dSno in Drosophila) is a highly conserved protein and a well-established antagonist of Transforming Growth Factor-β signaling in overexpression assays. However, analyses of Sno mutants in flies and mice have proven enigmatic in revealing developmental roles for Sno proteins. Thus, to identify developmental roles for dSno we first reconciled conflicting data on the lethality of dSno mutations. Then we conducted analyses of wing development in dSno loss of function genotypes. These studies revealed ectopic margin bristles and ectopic campaniform sensilla in the anterior compartment of the wing blade suggesting that dSno functions to antagonize Wingless (Wg) signaling. A subsequent series of gain of function analyses yielded the opposite phenotype (loss of bristles and sensilla) and further suggested that dSno antagonizes Wg signal transduction in target cells. To date Sno family proteins have not been reported to influence the Wg pathway during development in any species. Overall our data suggest that dSno functions as a tissue-specific component of the Wg signaling pathway with modest antagonistic activity under normal conditions but capable of blocking significant levels of extraneous Wg, a role that may be conserved in vertebrates.

Transforming growth factor-beta regulator SnoN modulates mammary gland branching morphogenesis, postlactational involution, and mammary tumorigenesis

SnoN is an important negative regulator of transforming growth factor-beta (TGF-beta) signaling that was originally identified as a transforming oncogene in chicken embryonic fibroblasts. Both pro-oncogenic and antioncogenic activities of SnoN have been reported, but its function in normal epithelial cells has not been defined. In the mouse mammary gland, SnoN is expressed at relatively low levels, but it is transiently upregulated at late gestation before being downregulated during lactation and early involution. To assess the effects of elevated levels of SnoN, we generated transgenic mice expressing a SnoN fragment under the control of the mouse mammary tumor virus promoter. In this model system, SnoN elevation increased side-branching and lobular-alveolar proliferation in virgin glands, while accelerating involution in postlactation glands. Increased proliferation stimulated by SnoN was insufficient to induce mammary tumorigenesis. In contrast, elevated levels of SnoN cooperated with polyoma middle T antigen to accelerate the formation of aggressive multifocal adenocarcinomas and to increase the formation of pulmonary metastases. Our studies define functions of SnoN in mammary epithelial cell proliferation and involution, and provide the first in vivo evidence of a pro-oncogenic role for SnoN in mammalian tumorigenesis.

Regulated production of SnoN2 is a feature of testicular differentiation

Transforming growth factor betas (TGF beta s) and activins are key regulators of male fertility, affecting somatic and germ cell proliferation and differentiation in the developing and adult testis. Several studies have shown that these ligands influence discrete developmental stages, suggesting that temporal expression of modifying factors may determine their specific signaling outcomes. Upon binding to cell surface receptors, TGFbeta and activin signals are transduced intracellularly by the phosphorylation and nuclear accumulation of SMAD2 and SMAD3 transcription factors. The objective of this study was to determine the cellular localization of phosphorylated SMAD2/3 and the transcriptional repressor SnoN (Ski-like), a modifier of SMAD2/3 transcriptional activity, in mouse testes. Western blot established that only the smaller SnoN isoform, SnoN2, is produced in the testis. By immunohistochemistry, widespread phospho-SMAD2/3 distribution was observed in somatic and germ cells at all ages. In contrast, SnoN2 production was highly regulated, being detected only in gonocytes and interstitial cells at birth and in pachytene spermatocytes at puberty. In the adult, SnoN2 expression differed to that during the first wave, being ubiquitously expressed but exhibiting regulated nuclear localization. In another model of spermatogenic differentiation, the irradiated rat testis, widespread phospho-SMAD2/3 contrasted with restricted SnoN2 expression. SnoN2 was limited to interstitial cells, with reduced staining intensity observed associated with the timing of spermatogenesis resumption. We conclude that somatic and germ cells at all differentiation stages are actively transducing TGFbeta superfamily signals but that responses to these ligands may be selectively modulated by controlled production and nuclear localization of SnoN2.

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.

Arvanil and anandamide up-regulate CD36 expression in human peripheral blood mononuclear cells

In this study we analysed the regulation of gene expression by arvanil and anandamide in human peripheral blood mononuclear cells (PBMCs) to clarify their immunosuppressive properties. PBMCs were activated, leading to CD36 down regulation, that was normalized by arvanil and anandamide. We used microarray technology to identify a regulatory pattern associated with cell proliferation in the presence of both substances. CD3-CD28 stimulated PBMCs showed a pattern of up-regulated and down-regulated genes after treatment with these substances. We selected and analysed several genes chosen by their function in the regulation of cell proliferation. We showed a transcriptional control of the CD36 gene by arvanil and anandamide associated with an increased protein expression, thus suggesting a possible role of CD36 in anandamide and arvanil anti-inflammatory pattern.

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.

Drosophila SnoN modulates growth and patterning by antagonizing TGF-beta signalling

Signalling by TGF-beta ligands through the Smad family of transcription factors is critical for developmental patterning and growth. Disruption of this pathway has been observed in various cancers. In vertebrates, members of the Ski/Sno protein family can act as negative regulators of TGF-beta signalling, interfering with the Smad machinery to inhibit the transcriptional output of this pathway. In some contexts ski/sno genes function as tumour suppressors, but they were originally identified as oncogenes, whose expression is up-regulated in many tumours. These growth regulatory effects and the normal physiological functions of Ski/Sno proteins have been proposed to result from changes in TGF-beta signalling. However, this model is controversial and may be over-simplified, because recent findings indicate that Ski/Sno proteins can affect other signalling pathways. To address this issue in an in vivo context, we have analyzed the function of the Drosophila Ski/Sno orthologue, SnoN. We found that SnoN inhibits growth when overexpressed, indicating a tumour suppressor role in flies. It can act in multiple tissues to selectively and cell autonomously antagonise signalling by TGF-beta ligands from both the BMP and Activin sub-families. By contrast, analysis of a snoN mutant indicates that the gene does not play a global role in TGF-beta-mediated functions, but specifically inhibits TGF-beta-induced wing vein formation. We propose that SnoN normally functions redundantly with other TGF-beta pathway antagonists to finely adjust signalling levels, but that it can behave as an extremely potent inhibitor of TGF-beta signalling when highly expressed, highlighting the significance of its deregulation in cancer cells.

dSno facilitates baboon signaling in the Drosophila brain by switching the affinity of Medea away from Mad and toward dSmad2

A screen for modifiers of Dpp adult phenotypes led to the identification of the Drosophila homolog of the Sno oncogene (dSno). The dSno locus is large, transcriptionally complex and contains a recent retrotransposon insertion that may be essential for dSno function, an intriguing possibility from the perspective of developmental evolution. dSno is highly transcribed in the embryonic central nervous system and transcripts are most abundant in third instar larvae. dSno mutant larvae have proliferation defects in the optic lobe of the brain very similar to those seen in baboon (Activin type I receptor) and dSmad2 mutants. This suggests that dSno is a mediator of Baboon signaling. dSno binds to Medea and Medea/dSno complexes have enhanced affinity for dSmad2. Alternatively, Medea/dSno complexes have reduced affinity for Mad such that, in the presence of dSno, Dpp signaling is antagonized. We propose that dSno functions as a switch in optic lobe development, shunting Medea from the Dpp pathway to the Activin pathway to ensure proper proliferation. Pathway switching in target cells is a previously unreported mechanism for regulating TGFbeta signaling and a novel function for Sno/Ski family proteins.

FRNK, the autonomously expressed C-terminal region of focal adhesion kinase, is uniquely regulated in vascular smooth muscle: analysis of expression in transgenic mice

FRNK, the autonomously expressed carboxyl-terminal region of focal adhesion kinase (FAK), is expressed in tissues that are rich in vascular smooth muscle cells (VSMCs). Here we report the generation of transgenic mice harboring the putative FRNK promoter fused to LacZ and examine the promoter activity in situ via expression of beta-galactosidase. The transgenic mice exhibited expression of beta-galactosidase predominantly in arterial VSMCs in large and small blood vessels of major organs. Upregulation of beta-galactosidase activity was observed in tunica media following carotid injury, indicating that the FRNK promoter is activated in VSMCs in response to injury. Robust expression of beta-galactosidase in blood vessels was also detected in the developing embryo. However, expression was also observed in the midline, the nose and skin epidermis, indicating distinct transcriptional regulation of the FRNK promoter in embryogenesis. To analyze FRNK expression in vitro, we identified a 116 bp sequence in the FRNK promoter that was sufficient to function as an enhancer when fused to the minimal actin promoter and expressed in cultured smooth muscle cells. Mutation of AP-1 and NF-E2 binding consensus sequences within this element abrogated enhancer activity, supporting the involvement of this promoter element in VSMC expression of FRNK.

A direct intersection between p53 and transforming growth factor beta pathways targets chromatin modification and transcription repression of the alpha-fetoprotein gene

We purified the oncoprotein SnoN and found that it functions as a corepressor of the tumor suppressor p53 in the regulation of the hepatic alpha-fetoprotein (AFP) tumor marker gene. p53 promotes SnoN and histone deacetylase interaction at an overlapping Smad binding, p53 regulatory element (SBE/p53RE) in AFP. Comparison of wild-type and p53-null mouse liver tissue by using chromatin immunoprecipitation (ChIP) reveals that the absence of p53 protein correlates with the disappearance of SnoN at the SBE/p53RE and loss of AFP developmental repression. Treatment of AFP-expressing hepatoma cells with transforming growth factor-beta1 (TGF-beta1) induced SnoN transcription and Smad2 activation, concomitant with AFP repression. ChIP assays show that TGF-beta1 stimulates p53, Smad4, P-Smad2 binding, and histone H3K9 deacetylation and methylation, at the SBE/p53RE. Depletion, by small interfering RNA, of SnoN and/or p53 in hepatoma cells disrupted repression of AFP transcription. These findings support a model of cooperativity between p53 and TGF-beta effectors in chromatin modification and transcription repression of an oncodevelopmental tumor marker gene.

SnoN is a cell type-specific mediator of transforming growth factor-beta responses

The transforming growth factor-beta (TGF-beta) family of secreted proteins have pleiotropic functions that are critical to normal development and homeostasis. However, the intracellular mechanisms by which the TGF-beta proteins elicit cellular responses remain incompletely understood. The Smad proteins provide a major means for the propagation of the TGF-beta signal from the cell surface to the nucleus, where the Smad proteins regulate gene expression leading to TGF-beta-dependent cellular responses including the inhibition of cell proliferation. Recent studies have suggested that a nuclear Smad-interacting protein termed SnoN, when overexpressed in cells, suppresses TGF-beta-induced Smad signaling and TGF-beta inhibition of cell proliferation. However, the physiologic function of endogenous SnoN in TGF-beta-mediated biological responses remained to be elucidated. Here, we determined the effect of genetic knock-down of SnoN by RNA interference on TGF-beta responses in mammalian cells. Unexpectedly, we found that SnoN knock-down specifically inhibited TGF-beta-induced transcription in the lung epithelial cell line Mv1Lu but not in HeLa or HaCaT cells. SnoN knock-down was also found to block TGF-beta-dependent cell cycle arrest in Mv1Lu cells. Collectively, these data indicate that rather than suppressing TGF-beta-induced responses, endogenous SnoN acts as a positive mediator of TGF-beta-induced transcription and cell cycle arrest in lung epithelial cells. Our study also shows that SnoN couples the TGF-beta signal to gene expression in a cell-specific manner.

Resistance to CD4+CD25+ regulatory T cells and TGF-beta in Cbl-b-/- mice

Cbl-b(-/-) mice have signaling defects that result in CD28-independent T cell activation, increased IL-2 production, hyper-reactive T cells, and increased autoimmunity. Although the increased autoimmunity in these mice is believed to result from the hyper-reactive T cells, the mechanisms leading from T cell hyper-reactivity to autoimmunity remain unclear. Specifically, the function and interaction of CD4(+)CD25(+) regulatory T cells (T(reg)) and CD4(+)CD25(-) effector T cells (T(eff)) in Cbl-b(-/-) mice have not been examined. We now report that Cbl-b(-/-) CD4(+)CD25(+) T(reg) exhibit normal regulatory function in vitro. In contrast, the in vitro response of Cbl-b(-/-) CD4(+)CD25(-) T(eff) is abnormal, in that it is not inhibited by either Cbl-b(-/-) or wild-type T(reg). This resistance of Cbl-b(-/-) T(eff) to in vitro regulation is seen at the levels of both DNA synthesis and cell division. In addition to this resistance to CD4(+)CD25(+) T(reg), Cbl-b(-/-) T(eff) demonstrate in vitro resistance to inhibition by TGF-beta. This second form of resistance in Cbl-b(-/-) T(eff) is seen despite the expression of normal levels of type II TGF-beta receptors and normal levels of phosphorylated Smad3 after TGF-beta stimulation. Coupled with recent reports of resistance to T(reg) in T(eff) exposed to LPS-treated dendritic cells, our present findings suggest that resistance to regulation may be a relevant mechanism in both normal immune function and autoimmunity.

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.