Ligand-dependent ubiquitination of Smad3 is regulated by casein kinase 1 gamma 2, an inhibitor of TGF-beta signaling

Receptor-activated transcription factors and beyond: multiple modes of Smad2/3-dependent transmission of TGF-β signaling

Transforming growth factor β (TGF-β) is a pleiotropic cytokine that is widely distributed throughout the body. Its receptor proteins, TGF-β type I and type II receptors, are also ubiquitously expressed. Therefore, the regulation of various signaling outputs in a context-dependent manner is a critical issue in this field. Smad proteins were originally identified as signal-activated transcription factors similar to signal transducer and activator of transcription proteins. Smads are activated by serine phosphorylation mediated by intrinsic receptor dual specificity kinases of the TGF-β family, indicating that Smads are receptor-restricted effector molecules downstream of ligands of the TGF-β family. Smad proteins have other functions in addition to transcriptional regulation, including post-transcriptional regulation of micro-RNA processing, pre-mRNA splicing, and m6A methylation. Recent technical advances have identified a novel landscape of Smad-dependent signal transduction, including regulation of mitochondrial function without involving regulation of gene expression. Therefore, Smad proteins are receptor-activated transcription factors and also act as intracellular signaling modulators with multiple modes of function. In this review, we discuss the role of Smad proteins as receptor-activated transcription factors and beyond. We also describe the functional differences between Smad2 and Smad3, two receptor-activated Smad proteins downstream of TGF-β, activin, myostatin, growth and differentiation factor (GDF) 11, and Nodal.

Targeting SMAD-Dependent Signaling: Considerations in Epithelial and Mesenchymal Solid Tumors

SMADs are the canonical intracellular effector proteins of the TGF-β (transforming growth factor-β). SMADs translocate from plasma membrane receptors to the nucleus regulated by many SMAD-interacting proteins through phosphorylation and other post-translational modifications that govern their nucleocytoplasmic shuttling and subsequent transcriptional activity. The signaling pathway of TGF-β/SMAD exhibits both tumor-suppressing and tumor-promoting phenotypes in epithelial-derived solid tumors. Collectively, the pleiotropic nature of TGF-β/SMAD signaling presents significant challenges for the development of effective cancer therapies. Here, we review preclinical studies that evaluate the efficacy of inhibitors targeting major SMAD-regulating and/or -interacting proteins, particularly enzymes that may play important roles in epithelial or mesenchymal compartments within solid tumors.

Intricacies of TGF-β signaling in Treg and Th17 cell biology

Balanced immunity is pivotal for health and homeostasis. CD4+ helper T (Th) cells are central to the balance between immune tolerance and immune rejection. Th cells adopt distinct functions to maintain tolerance and clear pathogens. Dysregulation of Th cell function often leads to maladies, including autoimmunity, inflammatory disease, cancer, and infection. Regulatory T (Treg) and Th17 cells are critical Th cell types involved in immune tolerance, homeostasis, pathogenicity, and pathogen clearance. It is therefore critical to understand how Treg and Th17 cells are regulated in health and disease. Cytokines are instrumental in directing Treg and Th17 cell function. The evolutionarily conserved TGF-β (transforming growth factor-β) cytokine superfamily is of particular interest because it is central to the biology of both Treg cells that are predominantly immunosuppressive and Th17 cells that can be proinflammatory, pathogenic, and immune regulatory. How TGF-β superfamily members and their intricate signaling pathways regulate Treg and Th17 cell function is a question that has been intensely investigated for two decades. Here, we introduce the fundamental biology of TGF-β superfamily signaling, Treg cells, and Th17 cells and discuss in detail how the TGF-β superfamily contributes to Treg and Th17 cell biology through complex yet ordered and cooperative signaling networks.

Adriamycin induces cardiac fibrosis in mice via PRMT5-mediated cardiac fibroblast activation

Long-term treatment with adriamycin (ADR) is associated with higher incidences of cumulative cardiotoxicity manifest as heart failure. ADR-induced cardiomyopathy is characterized by extensive fibrosis that is caused by cardiac fibroblast activation. To date, however, no specific treatment is available to alleviate ADR-induced cardiotoxicity. Protein arginine methyltransferase 5 (PRMT5), a major enzyme responsible for methylation of arginine, regulates numerous cellular processes such as cell differentiation. In the present study we investigated the role of PRMT5 in cardiac fibrosis. Mice were administered ADR (3 mg/kg, i.p., every 2 days) for 2 weeks. We showed that aberrant PRMT5 expression was largely co-localized with α-SMA-positive activated cardiac fibroblasts in ADR-injected mice and in ADR-treated cardiac fibroblasts in vitro. PRMT5-overexpression exacerbated, whereas PRMT5 knockdown alleviated ADR-induced cardiac fibrosis in vivo and TGF-β1-induced cardiac fibroblast activation in vitro. We demonstrated that PRMT5-overexpression enhanced methylated-Smad3 levels in vivo and in vitro. Pretreatment with a specific PRMT5 inhibitor EPZ015666 (5 nM) or overexpression of a catalytically inactive mutant of PRMT5, PRMT5(E444Q), reduced PRMT5-induced methylation of Smad3, thus suppressing PRMT5-mediated cardiac fibroblast activation in vitro. Furthermore, ADR activated cardiac fibroblasts was depending on autocrine TGF-β1. Taken together, our results demonstrate that PRMT5 promotes ADR-induced cardiac fibrosis via activating cardiac fibroblasts, suggesting that it may be a potential therapeutic target of ADR-caused cardiotoxicity.

RNAseq Analysis of FABP4 Knockout Mouse Hippocampal Transcriptome Suggests a Role for WNT/β-Catenin in Preventing Obesity-Induced Cognitive Impairment

Microglial fatty-acid binding protein 4 (FABP4) is a regulator of neuroinflammation. We hypothesized that the link between lipid metabolism and inflammation indicates a role for FABP4 in regulating high fat diet (HFD)-induced cognitive decline. We have previously shown that obese FABP4 knockout mice exhibit decreased neuroinflammation and cognitive decline. FABP4 knockout and wild type mice were fed 60% HFD for 12 weeks starting at 15 weeks old. Hippocampal tissue was dissected and RNA-seq was performed to measure differentially expressed transcripts. Reactome molecular pathway analysis was utilized to examine differentially expressed pathways. Results showed that HFD-fed FABP4 knockout mice have a hippocampal transcriptome consistent with neuroprotection, including associations with decreased proinflammatory signaling, ER stress, apoptosis, and cognitive decline. This is accompanied by an increase in transcripts upregulating neurogenesis, synaptic plasticity, long-term potentiation, and spatial working memory. Pathway analysis revealed that mice lacking FABP4 had changes in metabolic function that support reduction in oxidative stress and inflammation, and improved energy homeostasis and cognitive function. Analysis suggested a role for WNT/β-Catenin signaling in the protection against insulin resistance, alleviating neuroinflammation and cognitive decline. Collectively, our work shows that FABP4 represents a potential target in alleviating HFD-induced neuroinflammation and cognitive decline and suggests a role for WNT/β-Catenin in this protection.

To Ub or not to Ub: a regulatory question in TGF-β signaling

The transforming growth factor β (TGF-β) superfamily controls a wide spectrum of biological processes in metazoans, including cell proliferation, apoptosis, differentiation, cell-fate determination, and embryonic development. Deregulation of TGF-β-Smad signaling contributes to developmental anomalies and a variety of disorders and diseases such as tumorigenesis, fibrotic disorders, and immune diseases. In cancer, TGF-β has dual effects through its antiproliferative and prometastatic actions. At the cellular level, TGF-β functions mainly through the canonical Smad-dependent pathway in a cell type-specific and context-dependent manner. Accumulating evidence has demonstrated that ubiquitination plays a vital role in regulating TGF-β-Smad signaling. We summarize current progress on ubiquitination (Ub) and the ubiquitin ligases that regulate TGF-β-Smad signaling.

WINNER: A network biology tool for biomolecular characterization and prioritization

Background and contribution

In network biology, molecular functions can be characterized by network-based inference, or “guilt-by-associations.” PageRank-like tools have been applied in the study of biomolecular interaction networks to obtain further the relative significance of all molecules in the network. However, there is a great deal of inherent noise in widely accessible data sets for gene-to-gene associations or protein-protein interactions. How to develop robust tests to expand, filter, and rank molecular entities in disease-specific networks remains an ad hoc data analysis process.

Results

We describe a new biomolecular characterization and prioritization tool called Weighted In-Network Node Expansion and Ranking (WINNER). It takes the input of any molecular interaction network data and generates an optionally expanded network with all the nodes ranked according to their relevance to one another in the network. To help users assess the robustness of results, WINNER provides two different types of statistics. The first type is a node-expansion p-value, which helps evaluate the statistical significance of adding “non-seed” molecules to the original biomolecular interaction network consisting of “seed” molecules and molecular interactions. The second type is a node-ranking p-value, which helps evaluate the relative statistical significance of the contribution of each node to the overall network architecture. We validated the robustness of WINNER in ranking top molecules by spiking noises in several network permutation experiments. We have found that node degree–preservation randomization of the gene network produced normally distributed ranking scores, which outperform those made with other gene network randomization techniques. Furthermore, we validated that a more significant proportion of the WINNER-ranked genes was associated with disease biology than existing methods such as PageRank. We demonstrated the performance of WINNER with a few case studies, including Alzheimer's disease, breast cancer, myocardial infarctions, and Triple negative breast cancer (TNBC). In all these case studies, the expanded and top-ranked genes identified by WINNER reveal disease biology more significantly than those identified by other gene prioritizing software tools, including Ingenuity Pathway Analysis (IPA) and DiAMOND.

Conclusion

WINNER ranking strongly correlates to other ranking methods when the network covers sufficient node and edge information, indicating a high network quality. WINNER users can use this new tool to robustly evaluate a list of candidate genes, proteins, or metabolites produced from high-throughput biology experiments, as long as there is available gene/protein/metabolic network information.

Cu/Zn Superoxide Dismutase (Sod1) regulates the canonical Wnt signaling pathway

Cu/Zn Superoxide Dismutase (Sod1) catalyzes the disproportionation of cytotoxic superoxide radicals (O2•-) into oxygen (O2) and hydrogen peroxide (H2O2), a key signaling molecule. In Saccharomyces cerevisiae, we previously discovered that Sod1 participates in an H2O2-mediated redox signaling circuit that links nutrient availability to the control of energy metabolism. In response to glucose and O2, Sod1-derived H2O2 stabilizes a pair of conserved plasma membrane kinases - yeast casein kinase 1 and 2 (Yck1/2) - that signal glycolytic growth and the repression of respiration. The Yck1/2 homolog in humans, casein kinase 1-γ (CK1γ), is an integral component of the Wingless and Int-1 (Wnt) signaling pathway, which is essential for regulating cell fate and proliferation in early development and adult tissue and is dysregulated in many cancers. Herein, we establish the conservation of the SOD1/YCK1 redox signaling axis in humans by finding that SOD1 regulates CK1γ expression in human embryonic kidney 293 (HEK293) cells and is required for canonical Wnt signaling and Wnt-dependent cell proliferation.

Highly multiplexed quantitative proteomic and phosphoproteomic analyses in vascular smooth muscle cell dedifferentiation

Restenosis, re-narrowing of arterial lumen following intervention for cardiovascular disease, remains a major issue limiting the long-term therapeutic efficacy of treatment. The signaling molecules, TGFβ (transforming growth factor-beta) and Smad3, play important roles in vascular restenosis, but very little is yet known about the down-stream dynamics in global protein expression and phosphorylation. Here, we develop a highly multiplexed quantitative proteomic and phosphoproteomic strategy employing 12-plex N,N-dimethyl leucine (DiLeu) isobaric tags and The DiLeu Tool software to globally assess protein expression and phosphorylation changes in smooth muscle cells (SMCs) treated with TGFβ/Smad3 and/or SDF-1α (stromal cell-derived factor). A total of 4086 proteins were quantified in the combined dataset of proteome and phosphoproteome across 12-plex DiLeu-labeled SMC samples. 2317 localized phosphorylation sites were quantified, corresponding to 1193 phosphoproteins. TGFβ/Smad3 induced up-regulation of 40 phosphosites and down-regulation of 50 phosphosites, and TGFβ/Smad3-specific SDF-1α exclusively facilitated up-regulation of 27 phosphosites and down-regulation of 47 phosphosites. TGFβ/Smad3 inhibited the expression of contractile-associated proteins including smooth muscle myosin heavy chain, calponin, cardiac muscle alpha-actin, and smooth muscle protein 22α. Gene ontology and pathway enrichment analysis revealed that elevated TGFβ/Smad3 activated cell proliferation and TGFβ signaling pathway, sequentially stimulating phosphorylation of CXCR4 (C-X-C chemokine receptor 4). SDF-1α/CXCR4 activated extracellular signal-regulating kinase signaling pathway and facilitated the expression of synthetic marker, osteopontin, which was validated through targeted analysis. These findings provide new insights into the mechanisms of TGFβ regulated SMC dedifferentiation, as well as new avenues for designing effective therapeutics for vascular disease.

miR-615-3p promotes the epithelial-mesenchymal transition and metastasis of breast cancer by targeting PICK1/TGFBRI axis

Background

Increasing evidence indicates that epithelial-mesenchymal transition (EMT) can be regulated by microRNAs (miRNAs). miR-615-3p was shown to be involved in tumor development. However, the role of miR-615-3p in the metastasis of breast cancer remains largely unknown.

Methods

The expression of miR-615-3p in breast cancer cells and tissues was assessed by qRT-PCR and situ hybridization assays. Effects of miR-615-3p on tumor metastasis were evaluated with experiments in vitro and mouse model. EMT markers were detected by western blot and immunofluorescence assays. Molecular mechanism of miR-615-3p in the regulation of breast cancer cell metastasis was analyzed by Western Blot, Co-immunoprecipitation, and Luciferase assay.

Results

In the present study, we found that miR-615-3p was significantly elevated in breast cancer cells and tissues, especially in those with metastasis. In breast cancer cell lines, stable overexpression of miR-615-3p was sufficient to promote cell motility in vitro, and pulmonary metastasis in vivo, accompanied by the reduced expression of epithelial markers and the increased levels of mesenchymal markers. Further studies revealed that the reintroduction of miR-615-3p increased the downstream signaling of TGF-β, the type I receptor (TGFBRI) by targeting the 3′-untranslated regions (3′-UTR) of PICK1. PICK1 inhibits the binding of DICER1 to Smad2/3 and the processing of pre-miR-615-3p to mature miR-615-3p in breast cancer cells, thus exerting a negative feedback loop.

Conclusions

Our data highlight an important role of miR-615-3p in the molecular etiology of breast cancer, and implicate the potential application of miR-615-3p in cancer therapy.

TGF-β Signaling

Transforming growth factor-β (TGF-β) represents an evolutionarily conserved family of secreted polypeptide factors that regulate many aspects of physiological embryogenesis and adult tissue homeostasis. The TGF-β family members are also involved in pathophysiological mechanisms that underlie many diseases. Although the family comprises many factors, which exhibit cell type-specific and developmental stage-dependent biological actions, they all signal via conserved signaling pathways. The signaling mechanisms of the TGF-β family are controlled at the extracellular level, where ligand secretion, deposition to the extracellular matrix and activation prior to signaling play important roles. At the plasma membrane level, TGF-βs associate with receptor kinases that mediate phosphorylation-dependent signaling to downstream mediators, mainly the SMAD proteins, and mediate oligomerization-dependent signaling to ubiquitin ligases and intracellular protein kinases. The interplay between SMADs and other signaling proteins mediate regulatory signals that control expression of target genes, RNA processing at multiple levels, mRNA translation and nuclear or cytoplasmic protein regulation. This article emphasizes signaling mechanisms and the importance of biochemical control in executing biological functions by the prototype member of the family, TGF-β.

Phosphorylation of IRS4 by CK1γ2 promotes its degradation by CHIP through the ubiquitin/lysosome pathway

IRS4, a member of the insulin receptor substrate protein family, can induce constitutive PI3K/AKT hyperactivation and cell proliferation even in the absence of insulin or growth factors and promote tumorigenesis, but its regulation has only been explored at the transcriptional level.

Methods: Scansite was used to predict the potential protein kinases that may regulate the functions of IRS4, and mass spectrometry was used to identify the E3 ligase for IRS4. The protein interaction was carried out by immunoprecipitation, and protein stability was measured by cycloheximide treatment. In vitro kinase assay was used to determine the phosphorylation of IRS4 by casein kinase 1γ2 (CK1γ2). Colony formation assay and xenograft-bearing mice were employed to assess the cancer cell growth in vitro and in vivo, respectively. Immunohistochemistry was performed to examine protein levels of both IRS4 and CK1γ2 in osteosarcoma specimens and their relationship was evaluated by χ² test. Two-tailed Student's t-test or the Mann-Whitney U test were used to compare the differences between subgroups.

Results: IRS4 was phosphorylated at Ser859 by CK1γ2 in vitro and in vivo, which promoted the polyubiquitination and degradation of IRS4 through the ubiquitin/lysosome pathway by the carboxyl terminus of Hsc70-interacting protein(CHIP). Using osteosarcoma cell lines, the ectopic nonphosphorylated mutant of IRS4 by CK1γ2 triggered higher level of p-Akt and displayed faster cell proliferation and cancer growth in vitro and in nude mice. In addition, a negative correlation in protein levels between CK1γ2 and IRS4 was observed in osteosarcoma cell lines and tissue samples.

Conclusions: IRS4, as a new substrate of CHIP, is negatively regulated by CK1γ2 at the posttranslational level, and specific CK1γ2 agonists may be a potentially effective strategy for treating patients with osteosarcoma.

Posttranslational Regulation of Smads

Transforming growth factor β (TGF-β) family signaling dictates highly complex programs of gene expression responses, which are extensively regulated at multiple levels and vary depending on the physiological context. The formation, activation, and destruction of two major functional complexes in the TGF-β signaling pathway (i.e., the TGF-β receptor complexes and the Smad complexes that act as central mediators of TGF-β signaling) are direct targets for posttranslational regulation. Dysfunction of these complexes often leads or contributes to pathogenesis in cancer and fibrosis and in cardiovascular, and autoimmune diseases. Here we discuss recent insights into the roles of posttranslational modifications in the functions of the receptor-activated Smads in the common Smad4 and inhibitory Smads, and in the control of the physiological responses to TGF-β. It is now evident that these modifications act as decisive factors in defining the intensity and versatility of TGF-β responsiveness. Thus, the characterization of posttranslational modifications of Smads not only sheds light on how TGF-β controls physiological and pathological processes but may also guide us to manipulate the TGF-β responses for therapeutic benefits.

Phosphorylation of Smads by Intracellular Kinases

Smad proteins transduce the TGF-ß family signal at the cell surface into gene regulation in the nucleus. In addition to being phosphorylated by the TGF-ß family receptors, Smads are phosphorylated by a variety of intracellular kinases. The most studied are by cyclin-dependent kinases, the MAP kinase family members, and GSK-3. Phosphorylation by these kinases regulates Smad activities, leading to various biological effects. This chapter describes the methods for analyzing Smad phosphorylation by these kinases.

Akt suppression of TGFβ signaling contributes to the maintenance of vascular identity in embryonic stem cell-derived endothelial cells

The ability to generate and maintain stable in vitro cultures of mouse endothelial cells (ECs) has great potential for genetic dissection of the numerous pathologies involving vascular dysfunction as well as therapeutic applications. However, previous efforts at achieving sustained cultures of primary stable murine vascular cells have fallen short, and the cellular requirements for EC maintenance in vitro remain undefined. In this study, we have generated vascular ECs from mouse embryonic stem (ES) cells and show that active Akt is essential to their survival and propagation as homogeneous monolayers in vitro. These cells harbor the phenotypical, biochemical, and functional characteristics of ECs and expand throughout long-term cultures, while maintaining their angiogenic capacity. Moreover, Akt-transduced embryonic ECs form functional perfused vessels in vivo that anastomose with host blood vessels. We provide evidence for a novel function of Akt in stabilizing EC identity, whereby the activated form of the protein protects mouse ES cell-derived ECs from TGFβ-mediated transdifferentiation by downregulating SMAD3. These findings identify a role for Akt in regulating the developmental potential of ES cell-derived ECs and demonstrate that active Akt maintains endothelial identity in embryonic ECs by interfering with active TGFβ-mediated processes that would ordinarily usher these cells to alternate fates.

Blockade of Smad signaling by 3'-deoxyadenosine: a mechanism for its anti-fibrotic potential

Cordyceps militaris has been used in Eastern countries for the treatment of various diseases including chronic kidney diseases. However, there are no reports that identified its active entities and molecular mechanisms underlying its therapeutic effectiveness. 3'-Deoxyadenosine is a major nucleoside derivative isolated from C. militaris. Some reports suggested that both C. militaris and 3'-deoxyadenosine have anti-inflammatory and anti-fibrotic effects. In the present report, we investigated whether and how 3'-deoxyadenosine interferes with fibrogenic processes in the kidney. For this purpose, we examined effects of 3'-deoxyadenosine on the expression of collagens triggered by transforming growth factor-β (TGF-β1) and bone morphogenetic protein-4 (BMP-4), especially focusing on the regulation of Smad signaling in vitro and in vivo. We found that 3'-deoxyadenosine suppressed expression of collagens induced by TGF-β1 and BMP-4 dose dependently. This suppression occurred at the transcriptional level and was correlated with blunted activation of the CAGA box and the BMP-responsive element. The suppressive effect on the TGF-β/BMP signaling was mediated mainly by adenosine transporter and partially by the A3 adenosine receptor, but not A1/A2 adenosine receptors. 3'-Deoxyadenosine reduced levels of both phosphorylated and total Smad proteins (Smad1, 2 and 3) dose dependently. It was mainly ascribed to transcriptional suppression, but not to enhanced protein degradation and eIF2α-mediated translational suppression. Consistent with the in vitro results, in vivo administration with 3'-deoxyadenosine reduced the levels of phosphorylated and total Smad proteins, as well as the levels of Smad mRNAs, in the kidney subjected to unilateral ureteral obstruction. It was associated with blunted induction of type I collagen and α-smooth muscle actin, a decrease in the number of interstitial myofibroblasts and reduced fibrotic area. These results suggest that 3'-deoxyadenosine interferes with the TGF-β and BMP signaling via downregulation of Smads, which may underlie the anti-fibrotic effect of this agent. 3'-Deoxyadenosine may be useful for therapeutic intervention in various TGF-β-related fibrotic disorders.

2-Phenylamino-6-cyano-1H-benzimidazole-based isoform selective casein kinase 1 gamma (CK1γ) inhibitors

Screening of the Amgen compound library led to the identification of 2-phenylamino-6-cyano-1H-benzimidazole 1a as a potent CK1 gamma inhibitor with excellent kinase selectivity and unprecedented CK1 isoform selectivity. Further structure-based optimization of this series resulted in the discovery of 1h which possessed good enzymatic and cellular potency, excellent CK1 isoform and kinase selectivity, and acceptable pharmacokinetic properties.

TGF-β control of stem cell differentiation genes

The canonical TGF-β/Smad signaling pathway was delineated in the mid 90s and enriched over the past decade with many findings about its specificity, regulation, networking, and malfunctions in disease. However, a growing understanding of the chromatin status of a critical class of TGF-β target genes - the genes controlling differentiation of embryonic stem cells - recently prompted a reexamination of this pathway and its critical role in the regulation of stem cell differentiation. The new findings reveal master regulators of the pluripotent state set the stage for Smad-mediated activation of master regulators of the next differentiation stage. Furthermore, a novel branch of the TGF-β/Smad pathway has been identified in which a chromatin-reading Smad complex makes the master differentiation genes accessible to canonical Smad complexes for transcriptional activation. These findings provide exciting new insights into the global role of TGF-β signaling in the regulators of stem cell fate.

Casein kinase iγ2 impairs fibroblasts actin stress fibers formation and delays cell cycle progression in g1

Actin cytoskeleton remodeling is under the regulation of multiple proteins with various activities. Here, we demonstrate that the γ2 isoform of Casein Kinase I (CKIγ2) is part of a novel molecular path regulating the formation of actin stress fibers. We show that overexpression of CKIγ2 in fibroblasts alters cell morphology by impairing actin stress fibers formation. We demonstrate that this is concomitant with increased phosphorylation of the CDK inhibitor p27^Kip and lower levels of activated RhoA, and is dependent on CKIγ2 catalytic activity. Moreover, we report that roscovitine, a potent inhibitor of cyclin-dependent kinases, including Cdk5, decreases p27^Kip protein levels and restores actin stress fibers formation in CKIγ2 overexpressing cells, suggesting the existence of a CKIγ2-Cdk5-p27^Kip-RhoA pathway in regulating actin remodeling. On the other hand, we also show that in a manner independent of its catalytic activity, CKIγ2 delays cell cycle progression through G1. Collectively our findings reveal that CKIγ2 is a novel player in the control of actin cytoskeleton dynamics and cell proliferation.

Phosphatases in SMAD regulation

SMAD transcription factors are key mediators of the transforming growth factor-beta (TGFß) family of cytokines. Reversible phosphorylation of SMAD proteins plays a key role in regulating their function. Several phosphatases have been proposed to act on SMAD proteins to influence TGFß/BMP signalling. Here we provide an overview of the SMAD regulation by different protein phosphatases and review the evidence supporting each phosphatase as a candidate SMAD-phosphatase.

Four-gene expression model predictive of lymph node metastases in oral squamous cell carcinoma

BACKGROUND

Previous knowledge of cervical lymph node compromise may be crucial to choose the best treatment strategy in oral squamous cell carcinoma (OSCC). Here we propose a set four genes, whose mRNA expression in the primary tumor predicts nodal status in OSCC, excluding tongue.

MATERIAL AND METHODS

We identified differentially expressed genes in OSCC with and without compromised lymph nodes using Differential Display RT-PCR. Known genes were chosen to be validated by means of Northern blotting or real time RT-PCR (qRT-PCR). Thereafter we constructed a Nodal Index (NI) using discriminant analysis in a learning set of 35 patients, which was further validated in a second independent group of 20 patients.

RESULTS

Of the 63 differentially expressed known genes identified comparing three lymph node positive (pN +) and three negative (pN0) primary tumors, 23 were analyzed by Northern analysis or RT-PCR in 49 primary tumors. Six genes confirmed as differentially expressed were used to construct a NI, as the best set predictive of lymph nodal status, with the final result including four genes. The NI was able to correctly classify 32 of 35 patients comprising the learning group (88.6%; p = 0.009). Casein kinase 1alpha1 and scavenger receptor class B, member 2 were found to be up regulated in pN + group in contrast to small proline-rich protein 2B and Ras-GTPase activating protein SH3 domain-binding protein 2 which were upregulated in the pN0 group. We validated further our NI in an independent set of 20 primary tumors, 11 of them pN0 and nine pN + with an accuracy of 80.0% (p = 0.012).

CONCLUSIONS

The NI was an independent predictor of compromised lymph nodes, taking into the consideration tumor size and histological grade. The genes identified here that integrate our "Nodal Index" model are predictive of lymph node metastasis in OSCC.

A Smad action turnover switch operated by WW domain readers of a phosphoserine code

When directed to the nucleus by TGF-β or BMP signals, Smad proteins undergo cyclin-dependent kinase 8/9 (CDK8/9) and glycogen synthase kinase-3 (GSK3) phosphorylations that mediate the binding of YAP and Pin1 for transcriptional action, and of ubiquitin ligases Smurf1 and Nedd4L for Smad destruction. Here we demonstrate that there is an order of events-Smad activation first and destruction later-and that it is controlled by a switch in the recognition of Smad phosphoserines by WW domains in their binding partners. In the BMP pathway, Smad1 phosphorylation by CDK8/9 creates binding sites for the WW domains of YAP, and subsequent phosphorylation by GSK3 switches off YAP binding and adds binding sites for Smurf1 WW domains. Similarly, in the TGF-β pathway, Smad3 phosphorylation by CDK8/9 creates binding sites for Pin1 and GSK3, then adds sites to enhance Nedd4L binding. Thus, a Smad phosphoserine code and a set of WW domain code readers provide an efficient solution to the problem of coupling TGF-β signal delivery to turnover of the Smad signal transducers.

Role of Smads in TGFβ signaling

Transforming growth factor-β (TGFβ) is the prototype for a large family of pleiotropic factors that signal via heterotetrameric complexes of type I and type II serine/threonine kinase receptors. Important intracellular mediators of TGFβ signaling are members of the Smad family. Smad2 and 3 are activated by C-terminal receptor-mediated phosphorylation, whereafter they form complexes with Smad4 and are translocated to the nucleus where they, in cooperation with other transcription factors, co-activators and co-repressors, regulate the transcription of specific genes. Smads have key roles in exerting TGFβ-induced programs leading to cell growth arrest and epithelial-mesenchymal transition. The activity and stability of Smad molecules are carefully regulated by a plethora of post-translational modifications, including phosphorylation, ubiquitination, sumoylation, acetylation and poly(ADP)-ribosylation. The Smad function has been shown to be perturbed in certain diseases such as cancer.

Phosphorylation of the anaphase-promoting complex/Cdc27 is involved in TGF-beta signaling

Loss of TGF-β-induced growth inhibition is a hallmark of many human tumors. Previous studies implied that activation of the anaphase-promoting complex (APC/cyclosome) is involved in the TGF-β signaling pathway, which facilitates the destruction of SnoN, a transcriptional co-suppressor, which leads in turn to the transactivation of TGF-β-responsive genes for cell cycle arrest. The function of APC was demonstrated in TGF-β signal transduction, but the mechanism by which it is activated in response to TGF-β signaling remains unclear. We report here that phosphorylation of Cdc27, a core subunit of APC, in response to TGF-β signaling can facilitate the activation of APC. We have demonstrated that casein kinase II (CKII) is involved in the phosphorylation of Cdc27 in response to TGF-β signaling. Depletion of CKII by shRNA abolishes the TGF-β-induced phosphorylation of Cdc27 and subsequent degradation of SnoN. Disruptive mutation of Cdc27 (S154A) attenuates TGF-β-induced SnoN degradation. In addition, expression of a phosphorylation-resistant Cdc27 mutant significantly attenuates TGF-β-induced growth inhibition. Together, the results suggest that phosphorylation of Cdc27 by CKII is involved in TGF-β-induced activation of APC.

DEDD negatively regulates transforming growth factor-beta1 signaling by interacting with Smad3

Transforming growth factor-beta1 (TGF-beta1) regulates a wide variety of cellular responses, such as proliferation, differentiation, migration and apoptosis. Here we report that death effector domain-containing DNA-binding protein (DEDD) physically interacts with Smad3. The inhibition of Smad3 by DEDD resulted in a reduction in TGF-beta1/Smad3-mediated transcription. DEDD inhibited the functions of Smad3 by preventing Smad3 phosphorylation, which led to the reduced expression of TGF-beta1/Smad3-targeted genes. TGF-beta1 inhibited DEDD expression, and DEDD inhibited TGF-beta1-mediated invasion. Therefore, our findings suggest that through its interaction with Smad3, DEDD is a novel negative regulator of the TGF-beta1 signaling pathway.

Fate-determining mechanisms in epithelial-myofibroblast transition: major inhibitory role for Smad3

Smad3 inhibits activation of the smooth muscle actin promoter and functions as a timer for myogenic programming in the epithelium.

Epithelial–myofibroblast (MF) transition (EMyT) is a critical process in organ fibrosis, leading to α–smooth muscle actin (SMA) expression in the epithelium. The mechanism underlying the activation of this myogenic program is unknown. We have shown previously that both injury to intercellular contacts and transforming growth factor β (TGF-β) are indispensable for SMA expression (two-hit model) and that contact disruption induces nuclear translocation of myocardin-related transcription factor (MRTF). Because the SMA promoter harbors both MRTF-responsive CC(A/T)-rich GG element (CArG) boxes and TGF-β–responsive Smad-binding elements, we hypothesized that the myogenic program is mobilized by a synergy between MRTF and Smad3. In this study, we show that the synergy between injury and TGF-β exclusively requires CArG elements. Surprisingly, Smad3 inhibits MRTF-driven activation of the SMA promoter, and Smad3 silencing renders injury sufficient to induce SMA expression. Furthermore, Smad3 is degraded under two-hit conditions, thereby liberating the myogenic program. Thus, Smad3 is a critical timer/delayer of MF commitment in the epithelium, and EMyT can be dissected into Smad3-promoted (mesenchymal) and Smad3-inhibited (myogenic) phases.

The regulation of TGFbeta signal transduction

Transforming growth factor beta (TGFbeta) pathways are implicated in metazoan development, adult homeostasis and disease. TGFbeta ligands signal via receptor serine/threonine kinases that phosphorylate, and activate, intracellular Smad effectors as well as other signaling proteins. Oligomeric Smad complexes associate with chromatin and regulate transcription, defining the biological response of a cell to TGFbeta family members. Signaling is modulated by negative-feedback regulation via inhibitory Smads. We review here the mechanisms of TGFbeta signal transduction in metazoans and emphasize events crucial for embryonic development.

Mechanism of TGF-beta signaling to growth arrest, apoptosis, and epithelial-mesenchymal transition

Members of the transforming growth factor-beta (TGF-beta) family have important roles during embryogenesis, as well as in the control of tissue homeostasis in the adult. They exert their cellular effects via binding to serine/threonine kinase receptors. Members of the Smad family of transcription factors are important intracellular messengers, and recent studies have shown that the ubiquitin ligase TRAF6 mediates other specific signals. TGF-beta signaling is tightly controlled by post-translational modifications, which regulate the activity, stability, and subcellular localization of the signaling components. The aim of this review is to summarize some of the recent findings on the mechanism of TGF-beta signaling to growth arrest, apoptosis, and epithelial-mesenchymal transition.

Phospho-control of TGF-beta superfamily signaling

Members of the transforming growth factor-beta (TGF-beta) family control a broad range of cellular responses in metazoan organisms via autocrine, paracrine, and endocrine modes. Thus, aberrant TGF-beta signaling can play a key role in the pathogenesis of several diseases, including cancer. TGF-beta signaling pathways are activated by a short phospho-cascade, from receptor phosphorylation to the subsequent phosphorylation and activation of downstream signal transducers called R-Smads. R-Smad phosphorylation state determines Smad complex assembly/disassembly, nuclear import/export, transcriptional activity and stability, and is thus the most critical event in TGF-beta signaling. Dephosphorylation of R-Smads by specific phosphatases prevents or terminates TGF-beta signaling, highlighting the need to consider Smad (de)phosphorylation as a tightly controlled and dynamic event. This article illustrates the essential roles of reversible phosphorylation in controlling the strength and duration of TGF-beta signaling and the ensuing physiological responses.

Casein kinase I{gamma}2 down-regulates trafficking of ceramide in the synthesis of sphingomyelin

Intracellullar trafficking of lipids is fundamental to membrane biogenesis. For the synthesis of sphingomyelin, ceramide is transported from the endoplasmic reticulum to the Golgi apparatus by the ceramide transfer protein CERT. CERT is phosphorylated by protein kinase D at S132 and subsequently multiple times in a serine-repeat motif, resulting in its inactivation. However, the kinase involved in the multiple phosphorylation remains unclear. Here, we identify the gamma2 isoform of casein kinase I (CKIgamma2) as a kinase whose overexpression confers sphingomyelin-directed toxin-resistance to Chinese hamster ovary cells. In a transformant stably expressing CKIgamma2, CERT was hyperphosphorylated, and the intracellular trafficking of ceramide was retarded, thereby reducing de novo sphingomyelin synthesis. The reduction in the synthesis of sphingomyelin caused by CKIgamma2 was reversed by the expression of CERT mutants that are not hyperphosphorylated. Furthermore, CKIgamma2 directly phosphorylated CERT in vitro. Among three gamma isoforms, only knockdown of gamma2 isoform caused drastic changes in the ratio of hypo- to hyperphosphorylated form of CERT in HeLa cells. These results indicate that CKIgamma2 hyperphosphorylates the serine-repeat motif of CERT, thereby inactivating CERT and down-regulating the synthesis of sphingomyelin.