Up-regulated transcriptional repressors SnoN and Ski bind Smad proteins to antagonize transforming growth factor-beta signals during liver regeneration

Large induction of type III deiodinase expression after partial hepatectomy in the regenerating mouse and rat liver

The deiodinase types 1 (D1) and 2 (D2) catalyze the activation of T4 to T3, whereas type 3 deiodinase (D3) catalyzes the inactivation of T3 and T4. D3 plays a key role in controlling thyroid hormone bioavailability. It is highly expressed during fetal development, but also in other processes with increased cell proliferation, e.g. in vascular tumors. Because tissue regeneration is dependent on cellular proliferation and is associated with activation of fetal genes, we evaluated deiodinase activities and mRNA expression in rat and mouse liver, as well as the local and systemic thyroid hormone status after partial hepatectomy (PH). We observed that in rats, D3 activity was increased 10-fold at 20 h and 3-fold at 48 h after PH; D3 mRNA expression was increased 3-fold at 20 h. The increase in D3 expression was associated with maximum 2- to 3-fold decreases of serum and liver T3 and T4 levels at 20 to 24 h after PH. In mice, D3 activity was increased 5-fold at 12 h, 8-fold at 24 h, 40-fold at 36 h, 15-fold at 48 h, and 7-fold at 72 h after PH. In correlation with this, D3 mRNA was highest (6-fold increase), and serum T3 and T4 were lowest at 36 h. Furthermore, as a measure for cell proliferation, 5-bromo-2'-deoxyuridine incorporation peaked at 20-24 h after PH in rats and at 36 h in mice. No significant effect on D1 activity or mRNA expression was found after PH. D2 activity was always undetectable. In conclusion, we found a large induction of hepatic D3 expression after PH that was correlated with an increased cellular proliferation and decreased serum and liver T3 and T4 levels. Our data suggest that D3 is important in the modulation of thyroid hormone levels in the regenerating liver, in which a decrease in cellular T3 permits an increase in proliferation.

Impact of rapamycin on liver regeneration

The remarkable capacity of the liver to regenerate after injury and the prospects of organ self-renewal have attracted much interest in the understanding and modulation of the underlying molecular events. We investigated the effect of mammalian target of rapamycin (mTOR) inhibitor rapamycin (RAPA) on liver by correlating intravital microscopy, immunohistochemistry, and reverse transcriptase polymerase chain reaction in a rat model of 2/3 hepatectomy. RAPA significantly retarded proliferation of hepatocytes, endothelial cells, and hepatic stellate cells (HSCs) mostly between days 2 and 4 after hepatectomy and downregulated major cytokines and growth factors (tumor necrosis factor alpha, hepatocyte growth factor, platelet-derived growth factor, platelet-derived growth factor receptor, insulin-like growth factor-1, transforming growth factor beta 1) important for liver regeneration. These effects were almost absent at later time points. RAPA also had a transient, but broad effect on angiogenesis, and impaired sinusoidal density as well as mRNA levels of vascular endothelial growth factor, vascular endothelial growth factor receptor 1, vascular endothelial growth factor receptor 2, and angiopoietin-1. Activation of HSC was also transiently suppressed as observed by smooth muscle protein 1 alpha protein expression and intercellular adhesion molecule-1 mRNA levels. The rate of apoptosis in liver was significantly increased by RAPA between day 3 and day 7. The effect of RAPA on liver repair, angiogenesis, and HSC activation is confined to the phase of active cell proliferation. This transient effect might allow further exploration of mTOR inhibitors in clinical situations that involve liver regeneration, and seems to have implications beyond immunosuppression.

Chromatin-bound p53 anchors activated Smads and the mSin3A corepressor to confer transforming-growth-factor-beta-mediated transcription repression

In hepatic cells, Smad and SnoN proteins converge with p53 to repress transcription of AFP, an oncodevelopmental tumor marker aberrantly reactivated in hepatoma cells. Using p53- and SnoN-depleted hepatoma cell clones, we define a mechanism for repression mediated by this novel transcriptional partnership. We find that p53 anchors activated Smads and the corepressor mSin3A to the AFP distal promoter. Sequential chromatin immunoprecipitation analyses and molecular modeling indicate that p53 and Smad proteins simultaneously occupy overlapping p53 and Smad regulatory elements to establish repression of AFP transcription. In addition to its well-known function in antagonizing transforming growth factor beta (TGF-beta) responses, we find that SnoN actively participates in AFP repression by positively regulating mSin3A protein levels. We propose that activation of TGF-beta signaling restores a dynamic interplay between p53 and TGF-beta effectors that cooperate to effectively target mSin3A to tumor marker AFP and reestablish transcription repression.

The essential role for c-Ski in mediating TGF-beta1-induced bi-directional effects on skin fibroblast proliferation through a feedback loop

The bi-directional regulation of TGF-beta1 (transforming growth factor-beta1) on fibroblast proliferation with stimulation at low concentration, but inhibition at high concentration, has important significance during tissue repair. The mechanism has not been defined. c-Ski is a major co-repressor of TGF-beta1/Smad3 signalling; however, the exact role of c-Ski in the bi-directional regulation of fibroblast proliferation remains to be determined. In the present study, we established a dose-effect relationship of bi-directional regulation of TGF-beta1-mediated proliferation in rat skin fibroblasts, and found that c-Ski overexpression promoted fibroblast proliferation by inhibiting Smad3 activity. Importantly, c-Ski expression was decreased at the high concentration of TGF-beta1, but increased at the low concentration of TGF-beta1. This dose-dependent change in TGF-beta1 action did not affect Smad3 phosphorylation or nuclear translocation, but altered Smad3 DNA-binding activity, transcriptional activity and expression of the downstream gene p21 that both increased at the high concentration and decreased at the low concentration. Furthermore, c-Ski overexpression exerted synergistic stimulation with TGF-beta1 at the low concentration, but reversed the inhibitory effect of TGF-beta1 at high concentrations, while knockdown of c-Ski by RNA interference abrogated bi-directional role of TGF-beta1 on fibroblast proliferation. Thus our data reveal a new mechanism for this bi-directional regulation, i.e. c-Ski expression change induced by low or high TGF-beta1 concentration in turn determines the promoting or inhibiting effects of TGF-beta1 on fibroblast proliferation, and suggests an important role of c-Ski that modulates the local availability of TGF-beta1 within the wound repair microenvironment.

Advances in signaling in vertebrate regeneration as a prelude to regenerative medicine

While all animals have evolved strategies to respond to injury and disease, their ability to functionally recover from loss of or damage to organs or appendages varies widely damage to skeletal muscle, but, unlike amphibians and fish, they fail to regenerate heart, lens, retina, or appendages. The relatively young field of regenerative medicine strives to develop therapies aimed at improving regenerative processes in humans and is predicated on >40 years of success with bone marrow transplants. Further progress will be accelerated by implementing knowledge about the molecular mechanisms that regulate regenerative processes in model organisms that naturally possess the ability to regenerate organs and/or appendages. In this review we summarize the current knowledge about the signaling pathways that regulate regeneration of amphibian and fish appendages, fish heart, and mammalian liver and skeletal muscle. While the cellular mechanisms and the cell types involved in regeneration of these systems vary widely, it is evident that shared signals are involved in tissue regeneration. Signals provided by the immune system appear to act as triggers of many regenerative processes. Subsequently, pathways that are best known for their importance in regulating embryonic development, in particular fibroblast growth factor (FGF) and Wnt/beta-catenin signaling (as well as others), are required for progenitor cell formation or activation and for cell proliferation and specification leading to tissue regrowth. Experimental activation of these pathways or interference with signals that inhibit regenerative processes can augment or even trigger regeneration in certain contexts.

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.

Deletion of Smad2 in mouse liver reveals novel functions in hepatocyte growth and differentiation

Smad family proteins Smad2 and Smad3 are activated by transforming growth factor beta (TGF-beta)/activin/nodal receptors and mediate transcriptional regulation. Although differential functional roles of Smad2 and Smad3 are apparent in mammalian development, the relative functional roles of Smad2 and Smad3 in postnatal systems remain unclear. We used Cre/loxP-mediated gene targeting for hepatocyte-specific deletion of Smad2 (S2HeKO) in adult mice and generated hepatocyte-selective Smad2/Smad3 double knockouts by intercrossing AlbCre/Smad2(f/f) (S2HeKO) and Smad3-deficient Smad3ex8/ex8 (S3KO) mice. All strains were viable and had normal adult liver. However, necrogenic CCL4-induced hepatocyte proliferation was significantly increased in S2HeKO compared to Ctrl and S3KO livers, and transplanted S2HeKO hepatocytes repopulated recipient liver at dramatically increased rates compared to Ctrl hepatocytes in vivo. Using primary hepatocytes, we found that TGF-beta-induced G1 arrest, apoptosis, and epithelial-to-mesenchymal transition in Ctrl and S2HeKO but not in S3KO hepatocytes. Interestingly, S2HeKO cells spontaneously acquired mesenchymal features characteristic of epithelial-to-mesenchymal transition (EMT). Collectively, these results demonstrate that Smad2 suppresses hepatocyte growth and dedifferentiation independent of TGF-beta signaling. Smad2 is not required for TGF-beta-stimulated apoptosis, EMT, and growth inhibition in hepatocytes.

SnoN co-repressor binds and represses smad7 gene promoter

SnoN and Ski oncoproteins are co-repressors for Smad proteins and repress TGF-beta-responsive gene expression. The smad7 gene is a TGF-beta target induced by Smad signaling, and its promoter contains the Smad-binding element (SBE) required for a positive regulation by the TGF-beta/Smad pathway. SnoN and Ski co-repressors also bind SBE but regulate negatively smad7 gene. Ski along with Smad4 binds and represses the smad7 promoter, whereas the repression mechanism by SnoN is not clear. Ski and SnoN overexpression inhibits smad7 reporter expression induced through TGF-beta signaling. Using chromatin immunoprecipitation assays, we found that SnoN binds smad7 promoter at the basal condition, whereas after a short TGF-beta treatment for 15-30 min SnoN is downregulated and no longer bound smad7 promoter. Interestingly, after a prolonged TGF-beta treatment SnoN is upregulated and returns to its position on the smad7 promoter, functioning probably as a negative feedback control. Thus, SnoN also seems to regulate negatively the TGF-beta-responsive smad7 gene by binding and repressing its promoter in a similar way to Ski.

Inability of transforming growth factor-beta to cause SnoN degradation leads to resistance to transforming growth factor-beta-induced growth arrest in esophageal cancer cells

It is well established that loss of a growth inhibitory response to transforming growth factor-beta (TGF-beta) is a common feature of epithelial cancers including esophageal cancer. However, the molecular basis for the abrogation of this key homeostatic mechanism is poorly understood. In esophageal cancer cell lines that are resistant to TGF-beta-induced growth inhibition, TGF-beta also fails to decrease transcription of c-myc despite the presence of functional signaling components. Consequently, to gain a better understanding of the mechanisms leading to resistance to TGF-beta-induced growth arrest, the basis for the inability to decrease c-myc transcription was investigated. Regardless of sensitivity to TGF-beta-induced growth arrest, TGF-beta enhanced the ability of Smad3-protein complexes to bind c-myc regulatory elements. However, in a growth inhibition-resistant esophageal cancer cell line, the Smad3-protein complexes contained the SnoN oncoprotein. Furthermore, in esophageal cancer cell lines that are resistant to TGF-beta-induced growth arrest, TGF-beta does not cause degradation of SnoN. Analyses of the effect of modulating SnoN expression in both growth inhibition-sensitive and growth inhibition-resistant cell lines showed that degradation of SnoN is a prerequisite for both TGF-beta-induced repression of c-myc transcription and growth arrest. The data indicate that SnoN-Smad3 complexes do not cause repression of c-myc transcription but rather prevent functionality of active repressor complexes. Thus, these studies reveal a novel mechanism for resistance to TGF-beta-induced growth inhibition in esophageal cancer, namely the failure to degrade SnoN. In addition, they show that SnoN can block TGF-beta repression of gene transcription.

Downregulation of Ski and SnoN co-repressors by anisomycin

Proteasome pathway regulates TGF-beta signaling; degradation of activated Smad2/3 and receptors turns TGF-beta signal off, while degradation of negative modulators such as Ski and SnoN maintains the signal. We have found that anisomycin is able to downregulate Ski and SnoN via proteasome as TGF-beta does, but through a mechanism independent of Smad activation. The mechanism used by anisomycin to downregulate Ski and SnoN is also independent of MAPK activation and protein synthesis inhibition. TGF-beta signal was the only pathway described causing Ski and SnoN degradation, thus this new effect of anisomycin on endogenous Ski and SnoN proteins suggests alternative processes to downregulate these negative modulators of TGF-beta signaling.

Negative regulation of Smad2 by PIASy is required for proper Xenopus mesoderm formation

Mesoderm induction and patterning are primarily regulated by the concentration of locally expressed morphogens such as members of the TGFβsuperfamily. Smad2 functions as a transcription factor to regulate expression of mesodermal genes downstream of such morphogens. We have identified Xenopus PIASy (XPIASy), a member of the PIAS family, by yeast two-hybrid screening using Xenopus Smad2 (XSmad2) as a bait. During mesoderm induction, XPIASy is expressed in the animal half of embryos with a ventral high-dorsal low gradient at the marginal zone. XPIASyexpression is positively and negatively regulated by activities of the XSmad2 and Wnt pathways, respectively. Interestingly, inhibition of XPIASy by morpholinos induces elongation of animal caps with induction of mesoderm genes even in the absence of their morphogen-mediated activation. In addition, their introduction into the ventral marginal zone results in a secondary axis formation. Gain-of-function analysis revealed that XPIASy inhibits mesoderm induction by specific and direct downregulation of XSmad2 transcriptional activity. These observations indicate that XPIASy functions as an essential negative regulator of the XSmad2 pathway to ensure proper mesoderm induction at the appropriate time and in the appropriate region, and suggest that both the initial step of morphogen-mediated activation of the XSmad2 pathway and regulation of the final downstream transcription step have crucial roles in mesoderm induction and patterning.

Intact signaling by transforming growth factor beta is not required for termination of liver regeneration in mice

Transforming growth factor beta (TGF-beta) is a potent inhibitor of hepatocyte proliferation in vitro and is suggested to be a key negative regulator of liver growth. To directly address the role of TGF-beta signaling in liver regeneration in vivo, the TGF-beta type II receptor gene (Tgfbr2) was selectively deleted in hepatocytes by crossing "floxed" Tgfbr2 conditional knockout mice with transgenic mice expressing Cre under control of the albumin promoter. Hepatocytes isolated from liver-specific Tgfbr2 knockout (R2LivKO) mice were refractory to the growth inhibitory effects of TGF-beta1. The peak of DNA synthesis after 70% partial hepatectomy occurred earlier (36 vs. 48 hours) and was 1.7-fold higher in R2LivKO mice compared with controls. Accelerated S-phase entry by proliferating R2LivKO hepatocytes coincided with the hyperphosphorylation of Rb protein and the early upregulation of cyclin D1 and cyclin E. However, by 120 hours after partial hepatectomy, hepatocyte proliferation was back to baseline in both control and R2LivKO liver. Regenerating R2LivKO liver showed evidence of increased signaling by activin A and persistent activity of the Smad pathway. Blockage of activin A signaling by the specific inhibitor follistatin resulted in increased hepatocyte proliferation at 120 hours, particularly in R2LivKO livers. In conclusion, TGF-beta regulates G(1) to S phase transition of hepatocytes, but intact signaling by TGF-beta is not required for termination of liver regeneration. Increased signaling by activin A may compensate to regulate liver regeneration when signaling through the TGF-beta pathway is abolished, and may be a principal factor in the termination of liver regeneration.

Liver regeneration: from myth to mechanism

The unusual regenerative properties of the liver are a logical adaptation by organisms, as the liver is the main detoxifying organ of the body and is likely to be injured by ingested toxins. The numerous cytokine- and growth-factor-mediated pathways that are involved in regulating liver regeneration are being successfully dissected using molecular and genetic approaches. So what is known about this process at present and which questions remain?

Systematic interactome mapping and genetic perturbation analysis of a C. elegans TGF-beta signaling network

To initiate a system-level analysis of C. elegans DAF-7/TGF-beta signaling, we combined interactome mapping with single and double genetic perturbations. Yeast two-hybrid (Y2H) screens starting with known DAF-7/TGF-beta pathway components defined a network of 71 interactions among 59 proteins. Coaffinity purification (co-AP) assays in mammalian cells confirmed the overall quality of this network. Systematic perturbations of the network using RNAi, both in wild-type and daf-7/TGF-beta pathway mutant animals, identified nine DAF-7/TGF-beta signaling modifiers, seven of which are conserved in humans. We show that one of these has functional homology to human SNO/SKI oncoproteins and that mutations at the corresponding genetic locus daf-5 confer defects in DAF-7/TGF-beta signaling. Our results reveal substantial molecular complexity in DAF-7/TGF-beta signal transduction. Integrating interactome maps with systematic genetic perturbations may be useful for developing a systems biology approach to this and other signaling modules.

TGFbeta1 signaling via alphaVbeta6 integrin

BACKGROUND

Transforming growth factor beta1 (TGFbeta1) is a potent inhibitor of epithelial cell growth, thus playing an important role in tissue homeostasis. Most carcinoma cells exhibit a reduced sensitivity for TGFbeta1 mediated growth inhibition, suggesting TGFbeta1 participation in the development of these cancers. The tumor suppressor gene DPC4/SMAD4, which is frequently inactivated in carcinoma cells, has been described as a key player in TGFbeta1 mediated growth inhibition. However, some carcinoma cells lacking functional SMAD4 are sensitive to TGFbeta1 induced growth inhibition, thus requiring a SMAD4 independent TGFbeta1 pathway.

RESULTS

Here we report that mature TGFbeta1 is a ligand for the integrin alphaVbeta6, independent of the common integrin binding sequence motif RGD. After TGFbeta1 binds to alphaVbeta6 integrin, different signaling proteins are activated in TGFbeta1-sensitive carcinoma cells, but not in cells that are insensitive to TGFbeta1. Among others, interaction of TGFbeta1 with the alphaVbeta6 integrin resulted in an upregulation of the cell cycle inhibitors p21/WAF1 and p27 leading to growth inhibition in SMAD4 deleted as well as in SMAD4 wildtype carcinoma cells.

CONCLUSIONS

Our data provide support for the existence of an alternate TGFbeta1 signaling pathway that is independent of the known SMAD pathway. This alternate pathway involves alphaVbeta6 integrin and the Ras/MAP kinase pathway and does not employ an RGD motif in TGFbeta1-sensitive tumor cells. The combined action of these two pathways seems to be necessary to elicit a complete TGFbeta1 signal.

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.