Ski/Sno and TGF-beta signaling

Myosin light chain phosphatase is a downstream target of Rho-kinase in endothelin-1-induced transactivation of the TGF-β receptor

BACKGROUND

Rho-kinase (ROCK) regulates actomyosin contraction, coronary vasospasm, and cytoskeleton dynamics. ROCK and of NADPH oxidase (NOX) play an essential role in cardiovascular disease and proteoglycan synthesis, which promotes atherosclerosis by trapping low density lipoprotein. ROCK is activated by endothelin-1 (ET1) and transactivates the transforming growth factor beta receptor (TGFβR1), intensifying Smad signaling and proteoglycan production. This study aimed to identify the role of myosin light chain phosphatase (MLCP) as a downstream target of ROCK in TβR1 transactivation.

METHODS

Vascular smooth muscle cells were treated with ET1 and inhibitors of ROCK and MLCP were added. The phosphorylation levels of Smad2C, myosin light chain (MLC), and MLCP were monitored by western blot, and the mRNA expression of chondroitin 4-O-sulfotransferase 1 (C4ST1) was assessed by quantitative real-time PCR.

RESULTS

We examined ROCK's role in ET1-induced TGFβR1 activation. ROCK phosphorylated MLCP at the MYPT1 T853 residue, blocked by the ROCK inhibitor Y27632. ROCK also increased MLC phosphorylation and actomyosin contraction in response to ET1, enhanced by the phosphatase inhibitor Calyculin A. Calyculin A also increased C4ST1 expression, GAG-chain synthesizing enzymes.

CONCLUSIONS

This work suggests that ROCK is involved in ET1-mediated TβR1 activation through increased MLCP phosphorylation, which leads to Smad2C phosphorylation and stimulates C4ST1 expression.

Combination of high anti-SKI and low anti-TMED5 antibody levels is preferable prognostic factor in esophageal carcinoma

Given that esophageal cancer is highly malignant, the discovery of novel prognostic markers is eagerly awaited. We performed serological identification of antigens by recombinant cDNA expression cloning (SEREX) and identified SKI proto-oncogene protein and transmembrane p24 trafficking protein 5 (TMED5) as antigens recognized by serum IgG antibodies in patients with esophageal carcinoma. SKI and TMED5 proteins were expressed in Escherichia coli, purified by affinity chromatography, and used as antigens. The serum anti-SKI antibody (s-SKI-Ab) and anti-TMED5 antibody (s-TMED5-Ab) levels were significantly higher in 192 patients with esophageal carcinoma than in 96 healthy donors. The presence of s-SKI-Abs and s-TMED5-Abs in the patients' sera was confirmed by western blotting. Immunohistochemical staining showed that the TMED5 protein was highly expressed in the cytoplasm and nuclear compartments of the esophageal squamous cell carcinoma tissues, whereas the SKI protein was localized predominantly in the nuclei. Regarding the overall survival in 91 patients who underwent radical surgery, the s-SKI-Ab-positive and s-TMED5-Ab-negative statuses were significantly associated with a favorable prognosis. Additionally, the combination of s-SKI-Ab-positive and s-TMED5-Ab-negative cases showed an even clearer difference in overall survival as compared with that of s-SKI-Ab-negative and s-TMED5-Ab-positive cases. The s-SKI-Ab and s-TMED5-Ab biomarkers are useful for diagnosing esophageal carcinoma and distinguishing between favorable and poor prognoses.

Genome Analysis Using Whole-Exome Sequencing of Non-Syndromic Cleft Lip and/or Palate from Malagasy Trios Identifies Variants Associated with Cilium-Related Pathways and Asian Genetic Ancestry

Background: Orofacial clefts (OFCs) are common congenital disabilities that can occur as isolated non-syndromic events or as part of Mendelian syndromes. OFC risk factors vary due to differences in regional environmental exposures, genetic variants, and ethnicities. In recent years, significant progress has been made in understanding OFCs, due to advances in sequencing and genotyping technologies. Despite these advances, very little is known about the genetic interplay in the Malagasy population. Methods: Here, we performed high-resolution whole-exome sequencing (WES) on non-syndromic cleft lip with or without palate (nCL/P) trios in the Malagasy population (78 individuals from 26 families (trios)). To integrate the impact of genetic ancestry admixture, we computed both global and local ancestries. Results: Participants demonstrated a high percentage of both African and Asian admixture. We identified damaging variants in primary cilium-mediated pathway genes WNT5B (one family), GPC4 (one family), co-occurrence in MSX1 (five families), WDR11 (one family), and tubulin stabilizer SEPTIN9 (one family). Furthermore, we identified an autosomal homozygous damaging variant in PHGDH (one family) gene that may impact metabiotic activity. Lastly, all variants were predicted to reside on local Asian genetic ancestry admixed alleles. Conclusion: Our results from examining the Malagasy genome provide limited support for the hypothesis that germline variants in primary cilia may be risk factors for nCL/P, and outline the importance of integrating local ancestry components better to understand the multi-ethnic impact on nCL/P.

Differentially hypomethylated cell-free DNA and coronary collateral circulation

BACKGROUND

The factors affecting cardioprotective collateral circulation are still incompletely understood. Recently, characteristics, such as CpG methylation of cell-free DNA (cfDNA), have been reported as markers with clinical utility. The aim of this study was to evaluate whether cfDNA methylation patterns are associated with the grade of coronary collateral circulation (CCC).

RESULT

In this case-control study, clinical and angiographic data were obtained from 143 patients (mean age, 58 years, male 71%) with chronic total coronary occlusion. Enzymatic methyl-sequencing (EM-seq) libraries were prepared using the cfDNA extracted from the plasma. Data were processed to obtain the average methylation fraction (AMF) tables of genomic regions from which blacklisted regions were removed. Unsupervised analysis of the obtained AMF values showed that some of the changes in methylation were due to CCC. Through random forest preparation process, 256 differentially methylated region (DMR) candidates showing strong association with CCC were selected. A random forest classifier was then constructed, and the area under the curve of the receiver operating characteristic curve indicated an appropriate predictive function for CCC. Finally, 20 DMRs were identified to have significantly different AMF values between the good and poor CCC groups. Particularly, the good CCC group exhibited hypomethylated DMRs. Pathway analysis revealed five pathways, including TGF-beta signaling, to be associated with good CCC.

CONCLUSION

These data have demonstrated that differential hypomethylation was identified in dozens of cfDNA regions in patients with good CCC. Our results support the clinical utility of noninvasively obtained epigenetic signatures for predicting collateral circulation in patients with vascular diseases.

A transcription factor DAF-5 functions in Haemonchus contortus development

Background

Abnormal dauer formation gene (daf-5), located downstream of the DAF-7 signalling pathway, mainly functions in dauer formation and reproductive processes in the free-living nematode Caenorhabditis elegans. Although the structure and function of daf-5 have been clarified in C. elegans, they still remain totally unknown in Haemonchus contortus, a socio-economically important parasitic nematode of gastric ruminants.

Methods

A homologue of daf-5, Hc-daf-5, and its inferred product (Hc-DAF-5) in H. contortus were identified and characterized in this study. Then the transcriptional profiles of Hc-daf-5 and the anatomical expression of Hc-DAF-5 in H. contortus were studied using an integrated molecular approach. RNA interference (RNAi) was performed to explore its function in transition from the exsheathed third-stage larvae (xL3s) to the fourth-stage larvae (L4s) in vitro. Finally, the interaction between Hc-DAF-5 and Hc-DAF-3 (a co-Smad) was detected by bimolecular fluorescence complementation (BiFc) in vitro.

Results

It was shown that Hc-DAF-5 was a member of the Sno/Ski superfamily. Hc-daf-5 was transcribed in all developmental stages of H. contortus, with significant upregulation in L3s. Native Hc-DAF-5 was localized in the reproductive organs, cuticle, and intestine via immunohistochemistry. RNAi revealed that specific small interfering RNAs (siRNAs) could retard xL3 development. In addition, the interaction between Hc-DAF-5 and Hc-DAF-3 indicated that the SDS box of Hc-DAF-5 was dispensable for the binding of Hc-DAF-5 to Hc-DAF-3, and the MH2 domain was the binding region between Hc-DAF-3 and Hc-DAF-5.

Conclusions

In summary, these findings show that Hc-daf-5 functions in the developmental processes of H. contortus, and this study is the first attempt to characterize the daf-5 gene in parasitic nematodes.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-05036-2.

Identify differential genes and cell subclusters from time-series scRNA-seq data using scTITANS

Time-series single-cell RNA sequencing (scRNA-seq) provides a breakthrough in modern biology by enabling researchers to profile and study the dynamics of genes and cells based on samples obtained from multiple time points at an individual cell resolution. However, cell asynchrony and an additional dimension of multiple time points raises challenges in the effective use of time-series scRNA-seq data for identifying genes and cell subclusters that vary over time. However, no effective tools are available. Here, we propose scTITANS (https://github.com/ZJUFanLab/scTITANS), a method that takes full advantage of individual cells from all time points at the same time by correcting cell asynchrony using pseudotime from trajectory inference analysis. By introducing a time-dependent covariate based on time-series analysis method, scTITANS performed well in identifying differentially expressed genes and cell subclusters from time-series scRNA-seq data based on several example datasets. Compared to current attempts, scTITANS is more accurate, quantitative, and capable of dealing with heterogeneity among cells and making full use of the timing information hidden in biological processes. When extended to broader research areas, scTITANS will bring new breakthroughs in studies with time-series single cell RNA sequencing data.

Identification, molecular evolution, and expression analysis of the transcription factor Smad gene family in lamprey

Transcription factor small mothers against decapentaplegic (Smad) family SMAD proteins are the essential intracellular signal mediators and transcription factors for transforming growth factor β (TGF-β) signal transduction pathway, which usually exert pleiotropic actions on cell physiology, including immune response, cell migration and differentiation. In this study, the Smad family was identified in the most primitive vertebrates through the investigation of the transcriptome data of lampreys. The topology of phylogenetic tree showed that the four Smads (Smad1, Smad3, Smad4 and Smad6) in lampreys were subdivided into four different groups. Meanwhile, homology analysis indicated that most Smads were conserved with typical Mad Homology (MH) 1 and MH2 domains. In addition, Lethenteron reissneri Smads (Lr-Smads) adopted general Smads folding structure and had high tertiary structural similarity with human Smads (H-Smads). Genomic synteny analysis revealed that the large-scale duplication blocks were not found in lamprey genome and neighbor genes of lamprey Smads presented dramatic differences compared with jawed vertebrates. Importantly, quantitative real-time PCR analysis demonstrated that Smads were widely expressed in lamprey, and the expression level of Lr-Smads mRNA was up-regulated with different pathogenic stimulations. Moreover, depending on the weighted gene co-expression network analysis (WGCNA), four Lr-Smads were identified as two meaningful modules (green and gray). The functional analysis of these two modules showed that they might have a correlation with ployI:C. And these genes presented strong positive correlation during the immune response from the results of Pearson's correlation analysis. In conclusion, our results would not only enrich the information of Smad family in jawless vertebrates, but also lay the foundation for immunity in further study.

Ski: Double roles in cancers

The Ski (Sloan-Kettering Institute) is an evolutionarily conserved protein that plays a dual role as an oncoprotein and tumor suppressor gene in the development of human cancer. The Ski oncogene was first identified as a transforming protein of the avian Sloan-Kettering retrovirus in 1986. Since its discovery, Ski has been identified as a carcinogenic regulator in a variety of malignant tumors. Later, it was reported that Ski regulates the occurrence and development of some cancers by acting as an oncogene. Ski mediates the proliferation, differentiation, metastasis, and invasion of numerous cancer cells through various mechanisms. Several studies have shown that Ski expression is correlated with the clinical characteristics of cancer patients and is a promising biomarker and therapeutic target for cancer. In this review, we summarize the mechanisms and potential clinical implications of Ski in dimorphism, cancer occurrence, and progression in various types of cancer.

Opposing roles and potential antagonistic mechanism between TGF-β and BMP pathways: Implications for cancer progression

The transforming growth factor β (TGF-β) superfamily participates in tumour proliferation, apoptosis, differentiation, migration, invasion, immune evasion and extracellular matrix remodelling. Genetic deficiency in distinct components of TGF-β and BMP-induced signalling pathways or their excessive activation has been reported to regulate the development and progression of some cancers. As more in-depth studies about this superfamily have been conducted, more evidence suggests that the TGF-β and BMP pathways play an opposing role. The cross-talk of these 2 pathways has been widely studied in kidney disease and bone formation, and the opposing effects have also been observed in some cancers. However, the antagonistic mechanisms are still insufficiently investigated in cancer. In this review, we aim to display more evidences and possible mechanisms accounting for the antagonism between these 2 pathways, which might provide some clues for further study in cancer.

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Describe the basics of TGF-β and BMP signalling

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Summarize the potential mechanisms accounting for the antagonism between TGF-β and BMP pathways

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Provide some evidence about the antagonistic effects between pathways observed in some cancers

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.

New traffic light on Th17 Avenue

Activation of STAT3-coupled receptors along with TGF-β signaling are fundamental for Th17 cell differentiation both in vivo and in vitro. A recent paper shows that TGF-β signaling relieves SKI-mediated transcriptional repression of Rorc, the key regulator of the Th17 program.

SETBP1 dysregulation in congenital disorders and myeloid neoplasms

Myeloid malignancies are characterized by an extreme molecular heterogeneity, and many efforts have been made in the past decades to clarify the mechanisms underlying their pathogenesis.

In this scenario SET binding protein 1 (SETBP1) has attracted a lot of interest as a new oncogene and potential marker, in addition to its involvement in the Schinzel-Giedon syndrome (SGS). Our review starts with the analysis of the structural characteristics of SETBP1, and extends to its corresponding physiological and pathological functions. Next, we describe the prevalence of SETBP1 mutations in congenital diseases and in hematologic malignancies, exploring how its alterations might contribute to tumor development and provoke clinical effects. Finally, we consider to understand how SETBP1 activation could be exploited in molecular medicine to enhance the cure rate.

BMP restricts stemness of intestinal Lgr5+ stem cells by directly suppressing their signature genes

The intestinal epithelium possesses a remarkable self-renewal ability, which is mediated by actively proliferating Lgr5⁺ stem cells. Bone morphogenetic protein (BMP) signalling represents one major counterforce that limits the hyperproliferation of intestinal epithelium, but the exact mechanism remains elusive. Here we demonstrate that epithelial BMP signalling plays an indispensable role in restricting Lgr5⁺ stem cell expansion to maintain intestinal homeostasis and prevent premalignant hyperproliferation on damage. Mechanistically, BMP inhibits stemness of Lgr5⁺ stem cells through Smad-mediated transcriptional repression of a large number of stem cell signature genes, including Lgr5, and this effect is independent of Wnt/β-catenin signalling. Smad1/Smad4 recruits histone deacetylase HDAC1 to the promoters to repress transcription, and knockout of Smad4 abolishes the negative effects of BMP on stem cells. Our findings therefore demonstrate that epithelial BMP constrains the Lgr5⁺ stem cell self-renewal via Smad-mediated repression of stem cell signature genes to ensure proper homeostatic renewal of intestinal epithelium.

Bone morphogenetic protein (BMP) maintains intestinal homeostasis by restricting its hyperproliferation but whether it directly regulates the stem cells is unknown. Here the authors show that BMP constrains the Lgr5⁺ stem cell expansion under both homeostatic and injury conditions through Smad-mediated repression of stem cell signature genes.

Endothelin-1 (ET-1) stimulates carboxy terminal Smad2 phosphorylation in vascular endothelial cells by a mechanism dependent on ET receptors and de novo protein synthesis

OBJECTIVE

G protein-coupled receptor (GPCR) agonists through their receptors can transactivate protein tyrosine kinase receptors such as epidermal growth factor receptor and serine/threonine kinase receptors most notably transforming growth factor (TGF)-β receptor (TβRI). This signalling mechanism represents a major expansion in the cellular outcomes attributable to GPCR signalling. This study addressed the role and mechanisms involved in GPCR agonist, endothelin-1 (ET-1)-mediated transactivation of the TβRI in bovine aortic endothelial cells (BAECs).

METHOD

The in-vitro model used BAECs. Signalling intermediate phospho-Smad2 in the carboxy terminal was detected and quantified by Western blotting.

KEY FINDING

ET-1 treatment of BAECs resulted in a time and concentration-dependent increase in pSmad2C. Peak phosphorylation was evident with 100 nm treatment of ET-1 at 4-6 h. TβRI antagonist, SB431542 inhibited ET-1-mediated pSmad2C. In the presence of bosentan, a mixed ET_A and ET_B receptor antagonist ET-1-mediated pSmad2C levels were inhibited. The ET-mediated pSmad2C was blocked by the protein synthesis inhibitor, cycloheximide.

CONCLUSION

In BAECs, ET-1 via the ETB receptor is involved in transactivation of the TβRI. The transactivation-dependent response is dependent upon de novo protein synthesis.

P144, a Transforming Growth Factor beta inhibitor peptide, generates antitumoral effects and modifies SMAD7 and SKI levels in human glioblastoma cell lines

Glioblastoma (GBM) is the most prevalent malignant primary brain tumor, accounting for 60-70% of all gliomas. Current median patient survival time is 14-16 months after diagnosis. Numerous efforts in therapy have not significantly altered the nearly uniform lethality of this malignancy. The Transforming Growth Factor beta (TGF-β) signaling pathway plays a key role in GBM and is implicated in proliferation, invasion and therapy resistance. Several inhibitors of the TGF-β pathway have entered clinical trials or are under development. In this work, the therapeutic potential of P144, a TGF-β inhibitor peptide, was analyzed. P144 decreased proliferation, migration, invasiveness, and tumorigenicity in vitro, whereas apoptosis and anoikis were significantly increased for GBM cell lines. SMAD2 phosphorylation was reduced, together with a downregulation of SKI and an upregulation of SMAD7 at both transcriptional and translational levels. Additionally, P144 was able to impair tumor growth and increase survival in an in vivo flank model. Our findings suggest a potential effect of P144 in vitro and in vivo that is mediated by regulation of transcriptional target genes of the TGF-β pathway, suggesting a therapeutic potential of P144 for GBM treatment.

The downregulation of SnoN expression in human renal proximal tubule epithelial cells under high-glucose conditions is mediated by an increase in Smurf2 expression through TGF-β1 signaling

Transforming growth factor (TGF)-β1 is a profibrotic cytokine that plays a critical role in the progression of diabetic nephropathy (DN). Previous studies have demonstrated that the Smad transcriptional co-repressor, Ski-related novel protein N (SnoN), an antagonizer of TGF-β1/Smad signaling, is downregulated in the kidneys of diabetic rats; however, the underlying molecular mechanisms remain elusive. In the present study, we demonstrated that the upregulation of Smad ubiquitination regulatory factor-2 (Smurf2), through TGF-β1/Smad signaling, contributes to the downregulation of SnoN under high-glucose conditions in primary human renal proximal tubule epithelial cells (hRPTECs). The hRPTECs were cultured in high-glucose (30 mmol/l D-glucose) medium in the presence or absence of either the proteasome inhibitor, MG132, or the TGF-β type I receptor kinase inhibitor, SB-431542. Small interfering RNA (siRNA) was used to silence Smurf2. The expression levels of SnoN, Smurf2, Smad2 and phosphorylated (p-)Smad2 were measured by western blot analysis and RT-qPCR. The protein levels of SnoN were markedly downregulated, while its mRNA levels were increased in the hRPTECs cultured under high-glucose conditions. The protein and mRNA levels of Smurf2 were significantly increased under high-glucose conditions. The knockdown of Smurf2 increased SnoN expression in the hRPTECs cultured in high-glucose medium. Moreover, MG132 partially inhibited SnoN degradation in the hRPTECs under high-glucose conditions and SB-431542 decreased the phosphorylation of Smad2 and the expression of Smurf2 induced under high-glucose conditions. Taken together, the findings of this study demonstrate that the downregulation of SnoN expression in hRPTECs under high-glucose conditions is mediated by the increased expression of Smurf2 through the TGF-β1/Smad signaling pathway.

Bone morphogenetic proteins in tumour associated angiogenesis and implication in cancer therapies

Bone morphogenetic protein (BMP) belongs to transforming growth factor-β superfamily. To date, more than 20 BMPs have been identified in humans. BMPs play a critical role in embryonic and postnatal development, and also in maintaining homeostasis in different organs and tissues by regulating cell differentiation, proliferation, survival and motility. They play important roles in the development and progression of certain malignancies, including prostate cancer, breast cancer, lung cancer, etc. Recently, more evidence shows that BMPs are also involved in tumour associated angiogenesis. For example BMP can either directly regulate the functions of vascular endothelial cells or indirectly influence the angiogenesis via regulation of angiogenic factors, such as vascular endothelial growth factor (VEGF). Such crosstalk can also be reflected in the interaction with other angiogenic factors, like hepatocyte growth factor (HGF) and basic fibroblast growth factor (bFGF). All these factors are involved in the orchestration of the angiogenic process during tumour development and progression. Review of the relevant studies will provide a comprehensive prospective on current understanding and shed light on the corresponding therapeutic opportunity.

Invited review: mesenchymal progenitor cells in intramuscular connective tissue development

The abundance and cross-linking of intramuscular connective tissue contributes to the background toughness of meat, and is thus undesirable. Connective tissue is mainly synthesized by intramuscular fibroblasts. Myocytes, adipocytes and fibroblasts are derived from a common pool of progenitor cells during the early embryonic development. It appears that multipotent mesenchymal stem cells first diverge into either myogenic or non-myogenic lineages; non-myogenic mesenchymal progenitors then develop into the stromal-vascular fraction of skeletal muscle wherein adipocytes, fibroblasts and derived mesenchymal progenitors reside. Because non-myogenic mesenchymal progenitors mainly undergo adipogenic or fibrogenic differentiation during muscle development, strengthening progenitor proliferation enhances the potential for both intramuscular adipogenesis and fibrogenesis, leading to the elevation of both marbling and connective tissue content in the resulting meat product. Furthermore, given the bipotent developmental potential of progenitor cells, enhancing their conversion to adipogenesis reduces fibrogenesis, which likely results in the overall improvement of marbling (more intramuscular adipocytes) and tenderness (less connective tissue) of meat. Fibrogenesis is mainly regulated by the transforming growth factor (TGF) β signaling pathway and its regulatory cascade. In addition, extracellular matrix, a part of the intramuscular connective tissue, provides a niche environment for regulating myogenic differentiation of satellite cells and muscle growth. Despite rapid progress, many questions remain in the role of extracellular matrix on muscle development, and factors determining the early differentiation of myogenic, adipogenic and fibrogenic cells, which warrant further studies.

Relationship between Sloan-Kettering virus expression and mammalian follicular development

Sloan-Kettering virus gene, a product of a cellular proto-oncogene c-Ski is a unique nuclear pro-oncoprotein and belongs to the Ski/Sno proto-oncogene family. The aim of the present study was to locate Ski protein in rat ovaries in order to find insights into the possible involvement of Ski in follicular development. First, expression of c-Ski mRNA in the ovaries of adult female rats was confirmed by RT-PCR. Then, ovaries obtained on the day of estrus were subjected to immunohistochemical analysis for Ski and proliferating cell nuclear antigen (PCNA) in combination with terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). RT-PCR and in situ hybridization revealed that c-Ski mRNA was expressed in the ovaries of the adult rat on the day of estrous and localized mainly in the granulose cells. Ski was expressed in granulosa cells that were positive for TUNEL, but negative for PCNA, regardless of the shape and size of follicles. Expression of Ski in TUNEL-positive granulosa cells, but not in PCNA-positive granulosa cells, was also verified in rats having atretic follicles with double staining. These results indicate that Ski is profoundly expressed in the granulosa cells of atretic follicles, but not in growing follicles. Based on the present findings, Ski may play a role in the apoptosis of granulosa cells during follicular atresia.

Mechanisms of action of acetaldehyde in the up-regulation of the human α2(I) collagen gene in hepatic stellate cells: key roles of Ski, SMAD3, SMAD4, and SMAD7

Alcohol-induced liver fibrosis and eventually cirrhosis is a leading cause of death. Acetaldehyde, the first metabolite of ethanol, up-regulates expression of the human α2(I) collagen gene (COL1A2). Early acetaldehyde-mediated effects involve phosphorylation and nuclear translocation of SMAD3/4-containing complexes that bind to COL1A2 promoter to induce fibrogenesis. We used human and mouse hepatic stellate cells to elucidate the mechanisms whereby acetaldehyde up-regulates COL1A2 by modulating the role of Ski and the expression of SMADs 3, 4, and 7. Acetaldehyde induced up-regulation of COL1A2 by 3.5-fold, with concomitant increases in the mRNA (threefold) and protein (4.2- and 3.5-fold) levels of SMAD3 and SMAD4, respectively. It also caused a 60% decrease in SMAD7 expression. Ski, a member of the Ski/Sno oncogene family, is colocalized in the nucleus with SMAD4. Acetaldehyde induces translocation of Ski and SMAD4 to the cytoplasm, where Ski undergoes proteasomal degradation, as confirmed by the ability of the proteasomal inhibitor lactacystin to blunt up-regulation of acetaldehyde-dependent COL1A2, but not of the nonspecific fibronectin gene (FN1). We conclude that acetaldehyde up-regulates COL1A2 by enhancing expression of the transactivators SMAD3 and SMAD4 while inhibiting the repressor SMAD7, along with promoting Ski translocation from the nucleus to cytoplasm. We speculate that drugs that prevent proteasomal degradation of repressors targeting COL1A2 may have antifibrogenic properties.

Downregulation of SnoN oncoprotein induced by antibiotics anisomycin and puromycin positively regulates transforming growth factor-β signals

BACKGROUND

SnoN and Ski proteins function as Smad transcriptional corepressors and are implicated in the regulation of diverse cellular processes such as proliferation, differentiation and transformation. Transforming growth factor-β (TGF-β) signaling causes SnoN and Ski protein degradation via proteasome with the participation of phosphorylated R-Smad proteins. Intriguingly, the antibiotics anisomycin (ANS) and puromycin (PURO) are also able to downregulate Ski and SnoN proteins via proteasome.

METHODS

We explored the effects of ANS and PURO on SnoN protein downregulation when the activity of TGF-β signaling was inhibited by using different pharmacological and non-pharmacological approaches, either by using specific TβRI inhibitors, overexpressing the inhibitory Smad7 protein, or knocking-down TβRI receptor or Smad2 by specific shRNAs. The outcome of SnoN and Ski downregulation induced by ANS or PURO on TGF-β signaling was also studied.

RESULTS

SnoN protein downregulation induced by ANS and PURO did not involve the induction of R-Smad phosphorylation but it was abrogated after TGF-β signaling inhibition; this effect occurred in a cell type-specific manner and independently of protein synthesis inhibition or any other ribotoxic effect. Intriguingly, antibiotics seem to require components of the TGF-β/Smad pathway to downregulate SnoN. In addition, SnoN protein downregulation induced by antibiotics favored gene transcription induced by TGF-β signaling.

CONCLUSIONS

ANS and PURO require TGF-β/Smad pathway to induce SnoN and Ski protein downregulation independently of inducing R-Smad2 phosphorylation, which facilitates TGF-β signaling.

GENERAL SIGNIFICANCE

Antibiotic analogs lacking ribotoxic effects are useful as pharmacological tools to study TGF-β signaling by controlling Ski and SnoN protein levels.

Cooperative involvement of NFAT and SnoN mediates transforming growth factor-β (TGF-β) induced EMT in metastatic breast cancer (MDA-MB 231) cells

Epithelial to mesenchymal transition (EMT) is a secondary phenomenon concomitantly associated with the tumor progression. The regulatory signals and mechanistic details of EMT are not fully elucidated. Here, we shared a TGF-β mediated mechanism of EMT in breast cancer (MDA-MB 231) cells. Initial exposure of TGF-β for 48 h, enhanced the rate of cell proliferation and associated with EMT of MDA-MB 231 cells. The EMT was characterized by observing the increased N-cadherin, fibronectin, Snail expression and associated with the morphological change with a reduced E-cadherin expression. NFAT, a transcription factor, alters tumor suppressive function of TGF-β towards tumor progression. Up regulation of NFAT, coupled with a foremost translocation of one oncogenic protein SnoN from cytoplasm to nucleus was noticed during this TGF-β mediated EMT. Silencing of NFAT also showed the inhibition of TGF-β mediated EMT characterized by down regulation of N-cadherin and associated with reduced expression of SnoN. In addition, it was also observed that NFAT sequestering the Smad3 prevents the proteasome mediated degradation of SnoN and this SnoN has a role on the regulation of MMP-2, MMP-9 activity. Increased Smad3-SnoN interaction and proteasome mediated degradation of SnoN were detected after silencing of NFAT with a reduced MMP-2, MMP-9 activity. All of these observations provide a fresh mechanism in which by a twofold involvement of NFAT and SnoN plays a crucial role in TGF-β mediated EMT by recruiting the effector molecules N-cadherin and MMP-2, MMP-9.

Ski protein levels increase during in vitro progression of HPV16-immortalized human keratinocytes and in cervical cancer

We compared the levels of the Ski oncoprotein, an inhibitor of transforming growth factor-beta (TGF-β) signaling, in normal human keratinocytes (HKc), HPV16 immortalized HKc (HKc/HPV16), and differentiation resistant HKc/HPV16 (HKc/DR) in the absence and presence of TGF-β. Steady-state Ski protein levels increased in HKc/HPV16 and even further in HKc/DR, compared to HKc. TGF-β treatment of HKc, HKc/HPV16, and HKc/DR dramatically decreased Ski. TGF-β-induced Ski degradation was delayed in HKc/DR. Ski and phospho-Ski protein levels are cell cycle dependent with maximal Ski expression and localization to centrosomes and mitotic spindles during G2/M. ShRNA knock down of Ski in HKc/DR inhibited cell proliferation. More intense nuclear and cytoplasmic Ski staining and altered Ski localization were found in cervical cancer samples compared to adjacent normal tissue in a cervical cancer tissue array. Overall, these studies demonstrate altered Ski protein levels, degradation and localization in HPV16-transformed human keratinocytes and in cervical cancer.

Protein kinase A modulates transforming growth factor-β signaling through a direct interaction with Smad4 protein

Transforming growth factor β (TGFβ) signaling normally functions to regulate embryonic development and cellular homeostasis. It is increasingly recognized that TGFβ signaling is regulated by cross-talk with other signaling pathways. We previously reported that TGFβ activates protein kinase A (PKA) independent of cAMP through an interaction of an activated Smad3-Smad4 complex and the regulatory subunit of the PKA holoenzyme (PKA-R). Here we define the interaction domains of Smad4 and PKA-R and the functional consequences of this interaction. Using a series of Smad4 and PKA-R truncation mutants, we identified amino acids 290-300 of the Smad4 linker region as critical for the specific interaction of Smad4 and PKA-R. Co-immunoprecipitation assays showed that the B cAMP binding domain of PKA-R was sufficient for interaction with Smad4. Targeting of B domain regions conserved among all PKA-R isoforms and exposed on the molecular surface demonstrated that amino acids 281-285 and 320-329 were required for complex formation with Smad4. Interactions of these specific regions of Smad4 and PKA-R were necessary for TGFβ-mediated increases in PKA activity, CREB (cAMP-response element-binding protein) phosphorylation, induction of p21, and growth inhibition. Moreover, this Smad4-PKA interaction was required for TGFβ-induced epithelial mesenchymal transition, invasion of pancreatic tumor cells, and regulation of tumor growth in vivo.

Recurrent SETBP1 mutations in atypical chronic myeloid leukemia

Atypical chronic myeloid leukemia (aCML) shares clinical and laboratory features with CML, but it lacks the BCR-ABL1 fusion. We performed exome sequencing of eight aCMLs and identified somatic alterations of SETBP1 (encoding a p.Gly870Ser alteration) in two cases. Targeted resequencing of 70 aCMLs, 574 diverse hematological malignancies and 344 cancer cell lines identified SETBP1 mutations in 24 cases, including 17 of 70 aCMLs (24.3%; 95% confidence interval (CI) = 16-35%). Most mutations (92%) were located between codons 858 and 871 and were identical to changes seen in individuals with Schinzel-Giedion syndrome. Individuals with mutations had higher white blood cell counts (P = 0.008) and worse prognosis (P = 0.01). The p.Gly870Ser alteration abrogated a site for ubiquitination, and cells exogenously expressing this mutant exhibited higher amounts of SETBP1 and SET protein, lower PP2A activity and higher proliferation rates relative to those expressing the wild-type protein. In summary, mutated SETBP1 represents a newly discovered oncogene present in aCML and closely related diseases.

The brake within: Mechanisms of intrinsic regulation of axon growth featuring the Cdh1-APC pathway

Neurons of the central nervous system (CNS) form a magnificent network destined to control bodily functions and human behavior for a lifetime. During development of the CNS, neurons extend axons that establish connections to other neurons. Axon growth is guided by extrinsic cues and guidance molecules. In addition to environmental signals, intrinsic programs including transcription and the ubiquitin proteasome system (UPS) have been implicated in axon growth regulation. Over the past few years it has become evident that the E3 ubiquitin ligase Cdh1-APC together with its associated pathway plays a central role in axon growth suppression. By elucidating the intricate interplay of extrinsic and intrinsic mechanisms, we can enhance our understanding of why axonal regeneration in the CNS fails and obtain further insight into how to stimulate successful regeneration after injury.

Ski inhibits TGF-β/phospho-Smad3 signaling and accelerates hypertrophic differentiation in chondrocytes

Since transforming growing factor-β (TGF-β)/Smad signaling inhibits chondrocyte maturation, endogenous negative regulators of TGF-β signaling are likely also important regulators of the chondrocyte differentiation process. One such negative regulator, Ski, is an oncoprotein that is known to inhibit TGF-β/Smad3 signaling via its interaction with phospho-Smad3 and recruitment of histone deacetylases (HDACs) to the DNA binding complex. Based on this, we hypothesized that Ski inhibits TGF-β signaling and accelerates maturation in chondrocytes via recruitment of HDACs to transcriptional complexes containing Smads. We tested this hypothesis in chick upper sternal chondrocytes (USCs), where gain and loss of Ski expression experiments were performed. Over-expression of Ski not only reversed the inhibitory effect of TGF-β on the expression of hypertrophic marker genes such as type X collagen (colX) and osteocalcin, it induced these genes basally as well. Conversely, knockdown of Ski by RNA interference led to a reduction of colX and osteocalcin expression under basal conditions. Furthermore, Ski blocked TGF-β induction of cyclinD1 and caused a basal up-regulation of Runx2, consistent with the observed acceleration of hypertrophy. Regarding mechanism, not only does Ski associate with phospho-Smad2 and 3, but its association with phospho-Smad3 is required for recruitment of HDAC4 and 5. Implicating this recruitment of HDACs in the phenotypic effects of Ski in chondrocytes, the HDAC inhibitor SAHA reversed the up-regulation of colX and osteocalcin in Ski over-expressing cells. These results suggest that inhibition of TGF-β signaling by Ski, which involves its association with phospho-Smad3 and recruitment of HDAC4 and 5, leads to accelerated chondrocyte differentiation.

fussel (fuss)--A negative regulator of BMP signaling in Drosophila melanogaster

The TGF-β/BMP signaling cascades control a wide range of developmental and physiological functions in vertebrates and invertebrates. In Drosophila melanogaster, members of this pathway can be divided into a Bone Morphogenic Protein (BMP) and an Activin-ß (Act-ß) branch, where Decapentaplegic (Dpp), a member of the BMP family has been most intensively studied. They differ in ligands, receptors and transmitting proteins, but also share some components, such as the Co-Smad Medea (Med). The essential role of Med is to form a complex with one of the two activating Smads, mothers against decapentaplegic (Mad) or dSmad, and to translocate together to the nucleus where they can function as transcriptional regulators of downstream target genes. This signaling cascade underlies different mechanisms of negative regulation, which can be exerted by inhibitory Smads, such as daughters against decapentaplegic (dad), but also by the Ski-Sno family. In this work we identified and functionally analyzed a new member of the Ski/Sno-family, fussel (fuss), the Drosophila homolog of the human functional suppressing element 15 (fussel-15). fuss codes for two differentially spliced transcripts with a neuronal expression pattern. The proteins are characterized by a Ski-Sno and a SAND homology domain. Overexpression studies and genetic interaction experiments clearly reveal an interaction of fuss with members of the BMP pathway, leading to a strong repression of BMP-signaling. The protein interacts directly with Medea and seems to reprogram the Smad pathway through its influence upon the formation of functional Mad/Medea complexes. This leads amongst others to a repression of downstream target genes of the Dpp pathway, such as optomotor blind (omb). Taken together we could show that fuss exerts a pivotal role as an antagonist of BMP signaling in Drosophila melanogaster.

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.

Dynamics of TGF-β/Smad signaling

The physiological responses to TGF-β stimulation are diverse and vary amongst different cell types and environmental conditions. Even though the principal molecular components of the canonical and the non-canonical TGF-β signaling pathways have been largely identified, the mechanism that underlies the well-established context dependent physiological responses remains a mystery. Understanding how the components of TGF-β signaling function as a system and how this system functions in the context of the global cellular regulatory network requires a more quantitative and systematic approach. Here, we review the recent progress in understanding TGF-β biology using integration of mathematical modeling and quantitative experimental analysis. These studies reveal many interesting dynamics of TGF-β signaling and how cells quantitatively decode variable doses of TGF-β stimulation.

Regulation of early xenopus embryogenesis by Smad ubiquitination regulatory factor 2

BACKGROUND

Smad ubiquitination regulatory factor (Smurf) 1 and 2 are E3 ubiquitin ligases originally identified as inhibitors of transforming growth factor beta signaling and are shown to modulate multiple cellular activities. The roles of Smurfs in vertebrate embryogenesis, however, are not completely understood.

RESULTS

Here we investigate the function of Smurf2 during early Xenopus development. We show that distinctly from Smurf1, overexpression of Smurf2 in presumptive mesoderm interfered with mesoderm induction and caused axial defects, whereas knockdown of Smurf2 with antisense morpholino oligonucleotides resulted in expansion of the mesoderm. These results imply that Smurf2 may modulate nodal-mediated mesodermal induction. Consistently, ventral expression of Smurf2 induced a partial secondary axis with head structures. In the ectoderm, Smurf2 resembled Smurf1 in controlling neural and epidermal marker expression and influencing head formation. Smurf1, but not Smurf2, additionally affected neural tube closure. Interestingly, both Smurfs could enhance as well as repress neural crest markers, implying that they modulate their targets dynamically during neural plate border specification.

CONCLUSION

Our data demonstrate that Smurf1 and Smurf2 have overlapping and distinct functionalities during early frog embryogenesis; collectively, they regulate ectodermal and mesodermal induction and patterning to ensure normal development of Xenopus embryos.

Induction of Ski Protein Expression upon Luteinization in Rat Granulosa Cells

Ski protein is implicated in proliferation/differentiation in a variety of cells. We had previously reported that Ski protein is present in granulosa cells of atretic follicles, but not in preovulatory follicles, suggesting that Ski has a role in apoptosis of granulosa cells. The alternative fate of granulosa cells other than apoptosis is to differentiate to luteal cells; however, it is unknown whether Ski is expressed and has a role in granulosa cells undergoing luteinization. Thus, the aim of the present study was to locate Ski protein in the rat ovary during luteinizationto predict the possible role of Ski. In order to examine the expression pattern of Ski protein along with the progress of luteinization, follicular growth was induced by administration of equine chorionic gonadtropin to immature female rats, and luteinization was induced by human chorionic gonadtropin treatment to mimic luteinizing hormone (LH) surge. While no Ski-positive granulosa cells were present in preovulatory follicle, Ski protein expression was induced in response to LH surge, and was maintained after the formation of the corpus luteum (CL). Though Ski protein is absent in granulosa cells of preovulatory follicle, its mRNA (c-Ski) was expressed and the level was unchanged even after LH surge. Taken together, these results demonstrated that Ski protein expression is induced in granulosa cells upon luteinization, and suggests that its expression is regulated post-transcriptionally.

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.

Induction of Ski protein expression upon luteinization in rat granulosa cells without a change in its mRNA expression

The Ski protein is implicated in the proliferation/differentiation of a variety of cells. We previously reported that the Ski protein is present in granulosa cells of atretic follicles, but not in preovulatory follicles, suggesting that Ski has a role in apoptosis of granulosa cells. However, granulosa cells cannot only undergo apoptosis but can alternatively differentiate into luteal cells. It is unknown whether Ski is expressed and has a role in granulosa cells undergoing luteinization. Thus, the aim of the present study was to determine the localization of the Ski protein in the rat ovary during luteinization to examine if Ski might play a role in this process. In order to examine the Ski protein expression during the progression of luteinization, follicular growth was induced in immature female rats by administration of equine chorionic gonadotropin, and luteinization was induced by human chorionic gonadotropin treatment to mimic the luteinizing hormone (LH) surge. While no Ski-positive granulosa cells were present in the preovulatory follicle, Ski protein expression was induced in response to the LH surge and was maintained after formation of the corpus luteum (CL). Although the Ski protein is absent from the granulosa cells of the preovulatory follicle, its mRNA (c-ski) was expressed, and the level of c-ski mRNA was unchanged even after the LH surge. The combined results demonstrated that Ski protein expression is induced in granulosa cells upon luteinization, and suggested that its expression is regulated posttranscriptionally.

Dynamics and feedback loops in the transforming growth factor β signaling pathway

Transforming growth factor β (TGF-β) ligands activate a signaling cascade with multiple cell context dependent outcomes. Disruption or disturbance leads to variant clinical disorders. To develop strategies for disease intervention, delineation of the pathway in further detail is required. Current theoretical models of this pathway describe production and degradation of signal mediating proteins and signal transduction from the cell surface into the nucleus, whereas feedback loops have not exhaustively been included. In this study we present a mathematical model to determine the relevance of feedback regulators (Arkadia, Smad7, Smurf1, Smurf2, SnoN and Ski) on TGF-β target gene expression and the potential to initiate stable oscillations within a realistic parameter space. We employed massive sampling of the parameters space to pinpoint crucial players for potential oscillations as well as transcriptional product levels. We identified Smad7 and Smurf2 with the highest impact on the dynamics. Based on these findings, we conducted preliminary time course experiments.

Relationship between Sloan-Kettering virus expression and mouse follicular development

Sloan-Kettering virus gene product (Ski) is an unique nuclear pro-oncoprotein and belongs to the ski/sno proto-oncogene family. Ski plays multiple roles in a variety of cell types, it can induce both oncogenic transformation and terminal muscle differentiation when expressed at high levels. Ski/SnoN are important transcription regulators of the transforming growth factor-β (TGF-β) superfamily and function mainly through heterodimers. Since TGF-β superfamily are key regulators of follicle development and it has been previously shown that SnoN is also vital to follicle development, this research was conducted to clarify the relationship between Ski expression and mouse follicular development, in ovaries of neonatal and gonadotropin-induced immature mice by immunohistochemical and real-time PCR techniques. In postnatal mice, positive staining for Ski was highly detected in oocyte nuclei at postnatal day 1. With follicular development, the localization moved gradually from oocyte nuclei to perinuclear space and the total levels decreased. During the estrous cycle, Ski expression was apparent at proestrus and estrus, faint at metestrus, highest at diestrus. After injection of gonadotropin, Ski was found in perinuclear space and weak in oocyte nuclei. Following the initiation of luteinization, the expression of Ski was found in corpus luteum. Real-time PCR results also showed that Ski mRNA expression was opposite to ovulation-related genes during the cumulus expansion, with the development of the follicles, its expression level decreased. Ski is expressed in a specific manner during follicle development, ovulation and luteinization. So Ski might play essential roles in these processes especially during early follicular development.

Quantitative analysis of transient and sustained transforming growth factor-β signaling dynamics

Mathematical modeling and experimental analyses reveal that TGF-β ligand depletion has an important role in converting short-term graded signaling responses to long-term switch-like responses.

Cells respond in real time to the absolute number of TGF-β molecules in their environment.

A single pulse of TGF-β stimulation results in transient SMAD activation whereas repeated short pulses of stimulation result in sustained SMAD activation.

Ligand-induced short-term TGF-β/SMAD signaling activation is graded while long-term signaling response is switch-like or ultrasensitive.

TGF-β ligand depletion is a major cause of conversion from graded short-term responses to ultrasensitive long-term responses.

The transforming growth factor-β (TGF-β) pathway is a prominent signaling pathway that regulates diverse aspects of cellular homeostasis, including proliferation, differentiation, migration, and death (Massague, 1998). Remarkably, the pleiotropic biological effects of TGF-β are mediated by a relatively simple signaling module (Clarke and Liu, 2008). An interesting question is how such an apparently straightforward and simple cascade can generate a wide array of biological responses depending on the cellular context.

Members of the TGF-β superfamily are frequently used as morphogens in early embryo development (Green, 2002). The best-studied examples include Dpp in Drosophila and Activin in Xenopus (Gurdon and Bourillot, 2001; Lander, 2007). In the developmental context, cells can respond to a graded ligand concentration and produce discrete biological responses (e.g., transcription of certain genes, proliferation, or differentiation; Green, 2002). To convert continuous morphogen stimulation into discrete responses, mechanisms must exist to provide a threshold for the cellular response. How variable TGF-β ligand doses quantitatively control intracellular signaling dynamics and how continuous ligand doses are translated into discontinuous cellular fate decisions remains poorly understood.

We have previously reported that ligand molecules per cell is the input variable to which the cells respond, and ligand number per cell is the best predictor of signaling responses (Zi and Klipp, 2007a; Clarke et al, 2009). Here, we developed an improved mathematical model to predict TGF-β signaling responses by calibrating the model with various experimental data sets from different TGF-β stimulations. Using a combined experimental and mathematical modeling approach, we showed that TGF-β pulse stimulation results in transient activation of the pathway while repeated short pulses at short time intervals lead to a sustained activation similar to persistent ligand exposure.

We next investigate the system response to variable doses of TGF-β in HaCaT cells. Our mathematical model predicts that the short-term Smad2 phosphorylation (after 45 min of TGF-β stimulation) is a graded response, while long-term Smad2 activation (after 24 h of TGF-β stimulation) is a switch-like response (Figure 5A and B). As shown in Figure 5A–D, both short- and long-term Smad2 phosphorylation can be saturated but doses of TGF-β that cause maximum response are quite different. Additionally, the shapes of response curves were different. The short-term Smad2 activation was a graded (Michaelis–Menten-like) response with a very low apparent Hill coefficient of about 0.8 (Figure 5A and C) while the long-term Smad2 activation (P-Smad2 at 24 h) yielded a switch-like response with an apparent Hill coefficient of about 4.5 (Figure 5B and D). Thus, the Smad2 response is initially graded and sharpens over time to become ultrasensitive. To address whether TGF-β-inducible gene expression responses are graded or switch-like in the short and long term, we measured mRNA levels of Smad7, an early responsive gene of TGF-β and protein levels of p21 and PAI-1 whose inductions are delayed and late, respectively. The experimental data show that Smad7 induction exhibits a graded response with corresponding Hill coefficients of about 1.3 (Figure 5E), which is consistent with the graded P-Smad2 response at 45 min (Figure 5A and C). PAI-1 induction in response to variable doses of TGF-β for 24 h is highly ultrasensitive with an apparent Hill coefficient of ∼5.3. Compared with Smad7 and PAI-1, p21 induction is only modest ultrasensitive (n_Hill≈2) (Figure 5G). These results suggest short-term gene induction by TGF-β appears to be graded while long-term targets are more switch-like. Finally, we measured the growth inhibitory response of HaCaT cells to variable doses of TGF-β. The level of BrdU incorporation is also ultrasensitive with an apparent Hill coefficient of about 4.3 (Figure 5H). Therefore, the long-term TGF-β growth inhibitory response also shows a switch-like behavior. Finally, we show that TGF-β depletion affects long-term Smad phosphorylation and switch-like response of TGF-β signaling system. These findings shed new light on how continuous ligand doses are translated into discontinuous cell fate decisions in biological systems.

In summary, we have shown that the dose and time course of TGF-β stimulation have profound effects on Smad signaling dynamics. The rate of ligand depletion controls the duration of Smad2 phosphorylation. Cells can respond to a short pulse of TGF-β stimulation, and periodic short ligand exposures are sufficient to generate long-term signaling responses. Short-term TGF-β stimulation causes only transient pathway activation and can be terminated by ligand depletion. TGF-β-induced Smad2 phosphorylation is graded in the short-term but ultrasensitive (switch-like) in the long-term (Figure 7). Additionally, cell growth arrest in response to TGF-β shows switch-like rather than graded behavior. Our modeling and experimental analyses suggest that ligand depletion is likely to be involved in sharpening a graded response into a switch-like response.

Mammalian cells can decode the concentration of extracellular transforming growth factor-β (TGF-β) and transduce this cue into appropriate cell fate decisions. How variable TGF-β ligand doses quantitatively control intracellular signaling dynamics and how continuous ligand doses are translated into discontinuous cellular fate decisions remain poorly understood. Using a combined experimental and mathematical modeling approach, we discovered that cells respond differently to continuous and pulsating TGF-β stimulation. The TGF-β pathway elicits a transient signaling response to a single pulse of TGF-β stimulation, whereas it is capable of integrating repeated pulses of ligand stimulation at short time interval, resulting in sustained phospho-Smad2 and transcriptional responses. Additionally, the TGF-β pathway displays different sensitivities to ligand doses at different time scales. While ligand-induced short-term Smad2 phosphorylation is graded, long-term Smad2 phosphorylation is switch-like to a small change in TGF-β levels. Correspondingly, the short-term Smad7 gene expression is graded, while long-term PAI-1 gene expression is switch-like, as is the long-term growth inhibitory response. Our results suggest that long-term switch-like signaling responses in the TGF-β pathway might be critical for cell fate determination.

Peroxisome proliferator-activated receptor γ (PPARγ) mediates a Ski oncogene-induced shift from glycolysis to oxidative energy metabolism

Overexpression of the Ski oncogene induces oncogenic transformation of chicken embryo fibroblasts (CEFs). However, unlike most other oncogene-transformed cells, Ski-transformed CEFs (Ski-CEFs) do not display the classical Warburg effect. On the contrary, Ski transformation reduced lactate production and glucose utilization in CEFs. Compared with CEFs, Ski-CEFs exhibited enhanced TCA cycle activity, fatty acid catabolism through β-oxidation, glutamate oxidation, oxygen consumption, as well as increased numbers and mass of mitochondria. Interestingly, expression of PPARγ, a key transcription factor that regulates adipogenesis and lipid metabolism, was dramatically elevated at both the mRNA and protein levels in Ski-CEFs. Accordingly, PPARγ target genes that are involved in lipid uptake, transport, and oxidation were also markedly up-regulated by Ski. Knocking down PPARγ in Ski-CEFs by RNA interference reversed the elevated expression of these PPARγ target genes, as well as the shift to oxidative metabolism and the increased mitochondrial biogenesis. Moreover, we found that Ski co-immunoprecipitates with PPARγ and co-activates PPARγ-driven transcription.

Fascin protein is critical for transforming growth factor β protein-induced invasion and filopodia formation in spindle-shaped tumor cells

Fascin, an actin-bundling protein overexpressed in all carcinomas, has been associated with poor prognosis, shorter survival, and more metastatic diseases. It is believed that fascin facilitates tumor metastasis by promoting the formation of invasive membrane protrusions. However, the mechanisms by which fascin is overexpressed in tumors are not clear. TGFβ is a cytokine secreted by tumor and mesenchymal cells and promotes metastasis in many late stage tumors. The pro-metastasis mechanisms of TGFβ remain to be fully elucidated. Here we demonstrated that TGFβ induced fascin expression in spindle-shaped tumor cells through the canonical Smad-dependent pathway. Fascin was critical for TGFβ-promoted filopodia formation, migration, and invasion in spindle tumor cells. More importantly, fascin expression significantly correlates with TGFβ1 and TGFβ receptor I levels in a cohort of primary breast tumor samples. Our results indicate that elevated TGFβ level in the tumor microenvironment may be responsible for fascin overexpression in some of the metastatic tumors. Our data also suggest that fascin could play a central role in TGFβ-promoted tumor metastasis.

Evolution of the TGF-β signaling pathway and its potential role in the ctenophore, Mnemiopsis leidyi

The TGF-β signaling pathway is a metazoan-specific intercellular signaling pathway known to be important in many developmental and cellular processes in a wide variety of animals. We investigated the complexity and possible functions of this pathway in a member of one of the earliest branching metazoan phyla, the ctenophore Mnemiopsis leidyi. A search of the recently sequenced Mnemiopsis genome revealed an inventory of genes encoding ligands and the rest of the components of the TGF-β superfamily signaling pathway. The Mnemiopsis genome contains nine TGF-β ligands, two TGF-β-like family members, two BMP-like family members, and five gene products that were unable to be classified with certainty. We also identified four TGF-β receptors: three Type I and a single Type II receptor. There are five genes encoding Smad proteins (Smad2, Smad4, Smad6, and two Smad1s). While we have identified many of the other components of this pathway, including Tolloid, SMURF, and Nomo, notably absent are SARA and all of the known antagonists belonging to the Chordin, Follistatin, Noggin, and CAN families. This pathway likely evolved early in metazoan evolution as nearly all components of this pathway have yet to be identified in any non-metazoan. The complement of TGF-β signaling pathway components of ctenophores is more similar to that of the sponge, Amphimedon, than to cnidarians, Trichoplax, or bilaterians. The mRNA expression patterns of key genes revealed by in situ hybridization suggests that TGF-β signaling is not involved in ctenophore early axis specification. Four ligands are expressed during gastrulation in ectodermal micromeres along all three body axes, suggesting a role in transducing earlier maternal signals. Later expression patterns and experiments with the TGF-β inhibitor SB432542 suggest roles in pharyngeal morphogenesis and comb row organization.

Signaling networks in human hepatocarcinogenesis--novel aspects and therapeutic options

Hepatocellular carcinoma (HCC) represents one of the most common human malignancies with poor prognosis. Because therapeutic strategies are insufficient for most HCC patients, there is a great need to determine the central molecular mechanisms and pathways in order to derive novel targets for systemic therapy. There is vast evidence that not only the dysregulation of distinct signaling cascades, but also their interactions at different levels, affect tumor cell function. Through these interactions, the effects of pathways can be increased, and even new tumor-supporting qualities acquired that further facilitate HCC progression. Although several approaches for the modulation of these relevant pathways are under development, future therapeutic strategies should take into account that oncogenic stimuli cannot be understood in a monodimensional manner. In order to avoid escape mechanisms during therapy, strategies based on comprehensive knowledge of the interactive regulatory network in hepatocarcinogenesis are necessary.

Transforming growth factor-β and the hallmarks of cancer

Tumorigenesis is in many respects a process of dysregulated cellular evolution that drives malignant cells to acquire six phenotypic hallmarks of cancer, including their ability to proliferate and replicate autonomously, to resist cytostatic and apoptotic signals, and to induce tissue invasion, metastasis, and angiogenesis. Transforming growth factor-β (TGF-β) is a potent pleiotropic cytokine that functions as a formidable barrier to the development of cancer hallmarks in normal cells and tissues. Paradoxically, tumorigenesis counteracts the tumor suppressing activities of TGF-β, thus enabling TGF-β to stimulate cancer invasion and metastasis. Fundamental gaps exist in our knowledge of how malignant cells overcome the cytostatic actions of TGF-β, and of how TGF-β stimulates the acquisition of cancer hallmarks by developing and progressing human cancers. Here we review the molecular and cellular mechanisms that underlie the ability of TGF-β to mediate tumor suppression in normal cells, and conversely, to facilitate cancer progression and disease dissemination in malignant cells.

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.

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.

Effects of Arkadia on airway remodeling through enhancing TGF-beta signaling in allergic rats

Upregulation of transforming growth factor-beta (TGF-beta) signaling is interrelated with the development of airway remodeling. In this study, we examined the role of two E3 ubiquitin ligases, Arkadia and Smurf2, which are critically required for TGF-beta signaling in airway remodeling. Rats were immunized with ovalbumin (OVA) and then challenged with an OVA aerosol. In in vitro experiments, normal human bronchial epithelial cells were stimulated with TGF-beta(1) with or without the preincubation of Arkadia/Smurf2 small interfering RNA (siRNA) or lactacystin (an inhibitor of proteasomal degradation). In the lungs of OVA-treated rats, a large number of inflammatory cells were present near the airways. An increased subepithelial collagen deposition was associated with high expression levels of Smad7, SnoN and Ski mRNAs, Arkadia, Smurf2, and TGF-beta type I receptor (TbetaRI), but low expression levels of Smad7, SnoN and Ski proteins. Smad7, SnoN and Ski interacted with both Arkadia and Smurf2 while TbetaRI only interacted with Smurf2 but not with Arkadia. In in vitro experiments, the inhibitory effect of TGF-beta(1) on the expression of Smad7, SnoN and Ski was reversed by Arkadia siRNA and lactacystin, whereas the stimulatory effect of TGF-beta(1) on the expression of TbetaRI protein and Smad7/SnoN/Ski mRNAs was not affected. In contrast, Smurf2 siRNA did not influence the effects of TGF-beta(1) on the expression of the above proteins. Our results suggest that Arkadia may contribute to the pathogenesis of airway remodeling through enhancing TGF-beta signaling by inducing the reduction of Smad7, SnoN and Ski proteins in OVA-sensitized and -challenged rats.

Nuclear factor I-C links platelet-derived growth factor and transforming growth factor beta1 signaling to skin wound healing progression

Transforming growth factor beta (TGF-beta) and platelet-derived growth factor A (PDGFAlpha) play a central role in tissue morphogenesis and repair, but their interplay remain poorly understood. The nuclear factor I C (NFI-C) transcription factor has been implicated in TGF-beta signaling, extracellular matrix deposition, and skin appendage pathologies, but a potential role in skin morphogenesis or healing had not been assessed. To evaluate this possibility, we performed a global gene expression analysis in NFI-C(-/-) and wild-type embryonic primary murine fibroblasts. This indicated that NFI-C acts mostly to repress gene expression in response to TGF-beta1. Misregulated genes were prominently overrepresented by regulators of connective tissue inflammation and repair. In vivo skin healing revealed a faster inflammatory stage and wound closure in NFI-C(-/-) mice. Expression of PDGFA and PDGF-receptor alpha were increased in wounds of NFI-C(-/-) mice, explaining the early recruitment of macrophages and fibroblasts. Differentiation of fibroblasts to contractile myofibroblasts was also elevated, providing a rationale for faster wound closure. Taken together with the role of TGF-beta in myofibroblast differentiation, our results imply a central role of NFI-C in the interplay of the two signaling pathways and in regulation of the progression of tissue regeneration.

The expression of SnoN in normal human skin and cutaneous keratinous neoplasms

BACKGROUND

SnoN is a member of the ski family of proto-oncogenes. It has been revealed that SnoN plays a role in the regulation of cell growth, vertebrate development, and tumorigenesis. This study investigated the expression and significance of SnoN protein in normal human skin and in the development of seborrheic keratosis (SK), basal cell carcinoma (BCC), and squamous cell carcinoma (SCC) of the skin.

METHODS

Six frozen sections of normal human skin, three of SK (acanthotic type), six of BCC, six of intraepidermal SCC (actinic keratosis, AK), and six each of poorly and well-differentiated SCC were immunohistochemically stained with a polyclonal antibody against SnoN.

RESULTS

In normal epidermis, strong positive staining was observed in the suprabasal layers, whereas the basal cell layer was entirely unstained. Expression was observed in tumor cells with a squamoid phenotype in SK, but not in BCC. In intraepidermal SCC, although a strong signal was seen in the well-differentiated keratinocytes of the superficial epidermal cell layers, no signal was seen in the poorly differentiated atypical cells situated in the lower epidermis. In invasive SCC, a few scattered cells were positive for SnoN in the well-differentiated sample, but much larger numbers of positive cells were observed in the poorly differentiated sample.

CONCLUSIONS

On the basis of our results, it is suggested that SnoN is involved in differentiation in normal skin and benign and nonmetastatic skin tumors, but plays a proto-oncogenic role in undifferentiated SCC.