Kinetic analysis of Smad nucleocytoplasmic shuttling reveals a mechanism for transforming growth factor beta-dependent nuclear accumulation of Smads

The molecular basis for load-induced skeletal muscle hypertrophy

In a mature (weight neutral) animal, an increase in muscle mass only occurs when the muscle is loaded sufficiently to cause an increase in myofibrillar protein balance. A tight relationship between muscle hypertrophy, acute increases in protein balance, and the activity of the mechanistic target of rapamycin complex 1 (mTORC1) was demonstrated 15 years ago. Since then, our understanding of the signals that regulate load-induced hypertrophy has evolved considerably. For example, we now know that mechanical load activates mTORC1 in the same way as growth factors, by moving TSC2 (a primary inhibitor of mTORC1) away from its target (the mTORC activator) Rheb. However, the kinase that phosphorylates and moves TSC2 is different in the two processes. Similarly, we have learned that a distinct pathway exists whereby amino acids activate mTORC1 by moving it to Rheb. While mTORC1 remains at the forefront of load-induced hypertrophy, the importance of other pathways that regulate muscle mass are becoming clearer. Myostatin, is best known for its control of developmental muscle size. However, new mechanisms to explain how loading regulates this process are suggesting that it could play an important role in hypertrophic muscle growth as well. Last, new mechanisms are highlighted for how β2 receptor agonists could be involved in load-induced muscle growth and why these agents are being developed as non-exercise-based therapies for muscle atrophy. Overall, the results highlight how studying the mechanism of load-induced skeletal muscle mass is leading the development of pharmaceutical interventions to promote muscle growth in those unwilling or unable to perform resistance exercise.

Pharmacology of manipulating lean body mass

Dysfunction and wasting of skeletal muscle as a consequence of illness decreases the length and quality of life. Currently, there are few, if any, effective treatments available to address these conditions. Hence, the existence of this unmet medical need has fuelled large scientific efforts. Fortunately, these efforts have shown many of the underlying mechanisms adversely affecting skeletal muscle health. With increased understanding have come breakthrough disease-specific and broad spectrum interventions, some progressing through clinical development. The present review focuses its attention on the role of the antagonistic process regulating skeletal muscle mass before branching into prospective promising therapeutic targets and interventions. Special attention is given to therapies in development against cancer cachexia and Duchenne muscular dystrophy before closing remarks on design and conceptualization of future therapies are presented to the reader.

Developmental enhancers are marked independently of zygotic Nodal signals in Xenopus

To determine the hierarchy of transcriptional regulation within the in vivo vertebrate embryo, we examined whether developmental enhancers were influenced by Nodal signaling during early embryogenesis in Xenopus tropicalis. We find that developmental enhancers, defined by the active enhancer chromatin marks H3K4me1 and H3K27ac, are established as early as blastula stage and that Smad2/3 only strongly associates with these regions at gastrula stages. Significantly, when we perturb Nodal signaling using the drug SB431542, most enhancers remain marked, including at genes known to be sensitive to Nodal signaling. Overall, as enhancers are in an active conformation prior to Nodal signaling and are established independently of Nodal signaling, we suggest that many developmental enhancers are marked maternally, prior to exposure to extrinsic signals.

The self-limiting dynamics of TGF-β signaling in silico and in vitro, with negative feedback through PPM1A upregulation

The TGF-β/Smad signaling system decreases its activity through strong negative regulation. Several molecular mechanisms of negative regulation have been published, but the relative impact of each mechanism on the overall system is unknown. In this work, we used computational and experimental methods to assess multiple negative regulatory effects on Smad signaling in HaCaT cells. Previously reported negative regulatory effects were classified by time-scale: degradation of phosphorylated R-Smad and I-Smad-induced receptor degradation were slow-mode effects, and dephosphorylation of R-Smad was a fast-mode effect. We modeled combinations of these effects, but found no combination capable of explaining the observed dynamics of TGF-β/Smad signaling. We then proposed a negative feedback loop with upregulation of the phosphatase PPM1A. The resulting model was able to explain the dynamics of Smad signaling, under both short and long exposures to TGF-β. Consistent with this model, immuno-blots showed PPM1A levels to be significantly increased within 30 min after TGF-β stimulation. Lastly, our model was able to resolve an apparent contradiction in the published literature, concerning the dynamics of phosphorylated R-Smad degradation. We conclude that the dynamics of Smad negative regulation cannot be explained by the negative regulatory effects that had previously been modeled, and we provide evidence for a new negative feedback loop through PPM1A upregulation. This work shows that tight coupling of computational and experiments approaches can yield improved understanding of complex pathways.

TGF-β signaling pathway regulates a variety of cellular responses, such as differentiation, migration and apoptosis. Phosphorylated R-Smad, the central signaling protein in this pathway, exhibits self-limiting behaviors: it not only decreases quickly after TGF-β is removed, but it also decreases slowly when TGF-β remains abundant. These two self-limiting behaviors are important to understand clearly because diseases such as cancer and fibrosis might benefit from treatments to decrease Smad signaling. Several negative regulatory effects have been reported previously, and we studied the dynamics of these effects with computational modeling. Analyzing the timing of negative regulation revealed that the three most widely accepted effects were not sufficient to explain the observed declines. After considering and excluding several alternative models, we arrived at a model in which TGF-β upregulated the phosphatase PPM1A. We tested for PPM1A upregulation in cell culture experiments. In addition, our model was able to explain why different durations of TGF-β exposure could cause seemingly opposite results about the importance of Smad degradation.

Genes related to suppression of malignant phenotype induced by Maitake D-Fraction in breast cancer cells

It is already known that the Maitake (D-Fraction) mushroom is involved in stimulating the immune system and activating certain cells that attack cancer, including macrophages, T-cells, and natural killer cells. According to the U.S. National Cancer Institute, polysaccharide complexes present in Maitake mushrooms appear to have significant anticancer activity. However, the exact molecular mechanism of the Maitake antitumoral effect is still unclear. Previously, we have reported that Maitake (D-Fraction) induces apoptosis in breast cancer cells by activation of BCL2-antagonist/killer 1 (BAK1) gene expression. At the present work, we are identifying which genes are responsible for the suppression of the tumoral phenotype mechanism induced by Maitake (D-Fraction) in breast cancer cells. Human breast cancer MCF-7 cells were treated with and without increased concentrations of Maitake D-Fraction (36, 91, 183, 367 μg/mL) for 24 h. Total RNA were isolated and cDNA microarrays were hybridized containing 25,000 human genes. Employing the cDNA microarray analysis, we found that Maitake D-Fraction modified the expression of 4068 genes (2420 were upmodulated and 1648 were downmodulated) in MCF-7 breast cancer cells in a dose-dependent manner during 24 h of treatment. The present data shows that Maitake D-Fraction suppresses the breast tumoral phenotype through a putative molecular mechanism modifying the expression of certain genes (such as IGFBP-7, ITGA2, ICAM3, SOD2, CAV-1, Cul-3, NRF2, Cycline E, ST7, and SPARC) that are involved in apoptosis stimulation, inhibition of cell growth and proliferation, cell cycle arrest, blocking migration and metastasis of tumoral cells, and inducing multidrug sensitivity. Altogether, these results suggest that Maitake D-Fraction could be a potential new target for breast cancer chemoprevention and treatment.

In silico identification of potential therapeutic targets in the TGF-β signal transduction pathway

The transforming growth factor-β (TGF-β) superfamily of cytokines controls fundamental cellular processes, such as proliferation, motility, differentiation, and apoptosis. This fundamental role is emphasized by the widespread presence of mutations of the core components of the TGF-β signal transduction pathway in a number of human diseases. Therefore, there is an increasing interest in the development of therapies to specifically target this pathway. Here we develop a computational approach to identify potential intervention points that are capable of restoring the normal signaling dynamics to the mutated system while maintaining the behavior of normal cells substantially unperturbed. We apply this approach explicitly to the TGF-β pathway to study the signaling dynamics of mutated and normal cells treated with inhibitory drugs and identify the processes in the pathway that are most susceptible to therapeutic intervention.

TGFBR1 and cancer susceptibility

Transforming growth factor beta (TGF-β) is a potent inhibitor of cell growth. TGFBR1 6A is a polymorphism consisting of a 9-base pair in-frame deletion within exon 1 of the type I TGF-β receptor (TGFBR1), which results in a receptor with decreased TGF-β signaling capability. The discovery of an association between TGFBR1*6A and cancer susceptibility led to the hypothesis that hypomorphic variants of the TGF-β signaling pathway may predispose to the development of cancer. This hypothesis was tested in vivo with the development of a mouse model of Tgfbr1 haploinsufficiency. Tgfbr1 (+/-) mice developed twice as many intestinal tumors as Tgfbr1 (+/+). Tgfbr1 haploinsufficiency was also associated with early onset adenocarcinoma and increased tumor cell proliferation. A case control study identified two haplotypes associated with constitutively decreased TGFBR1 and substantially increased colorectal cancer risk indicating that TGFBR1 may act as a potent modifier of cancer risk.

Live-cell single-molecule imaging reveals clathrin and caveolin-1 dependent docking of SMAD4 at the cell membrane

Transforming growth factor β (TGF-β) signaling is important for many biological processes. Although the sequential events of this cascade are known, the dynamics remain speculative. Here, live-cell single-molecule total internal reflection fluorescence microscopy was used to monitor the dynamics of SMAD4, a TGF-β downstream effector, in MDA-MB-231 breast cancer cells. Contrary to previous belief, SMAD4 was detectable at the cytoplasmic membrane, displaying two subpopulations with different membrane docking behaviors. These subpopulations were regulated by clathrin and caveolin-1, and had opposing roles in the nuclear shuttling of SMAD4 and the subsequent transcriptional regulation of genes associated with cell migration. The notion that membrane-docking behaviors of downstream molecules could predict the cellular response to growth factors may revolutionize the way we view cell signaling.

The multiple activities of BMPs during spinal cord development

Bone morphogenetic proteins (BMPs) are one of the main classes of multi-faceted secreted factors that drive vertebrate development. A growing body of evidence indicates that BMPs contribute to the formation of the central nervous system throughout its development, from the initial shaping of the neural primordium to the generation and maturation of the different cell types that form the functional adult nervous tissue. In this review, we focus on the multiple activities of BMPs during spinal cord development, paying particular attention to recent results that highlight the complexity of BMP signaling during this process. These findings emphasize the unique capacity of these signals to mediate various functions in the same tissue throughout development, recruiting diverse effectors and strategies to instruct their target cells.

The roles of TGFβ in the tumour microenvironment

The influence of the microenvironment on tumour progression is becoming clearer. In this Review we address the role of an essential signalling pathway, that of transforming growth factor-β, in the regulation of components of the tumour microenvironment and how this contributes to tumour progression.

Lactoferricin enhances BMP7-stimulated anabolic pathways in intervertebral disc cells

Bone-morphogenetic protein-7 (BMP7) is a well-known anabolic and anti-catabolic growth factor on intervertebral disc (IVD) matrix and cell homeostasis. Similarly, Lactoferricin B (LfcinB) has recently been shown to have pro-anabolic, anti-catabolic, anti-oxidative and/or anti-inflammatory effects in bovine disc cells in vitro. In this study, we investigated the potential benefits of using combined peptide therapy with LfcinB and BMP7 for intervertebral disc matrix repair and to understand cellular and signaling mechanisms controlled by these factors. We studied the effects of BMP7 and LfcinB as individual treatments and combined therapy on bovine nucleus pulposus (NP) cells by assessing proteoglycan (PG) accumulation and synthesis, and the gene expression of matrix protein aggrecan and transcription factor SOX-9. We also analyzed the role of Noggin, a BMP antagonist, in IVD tissue and examined its effect after stimulation with LfcinB. To understand the molecular mechanisms by which LfcinB synergizes with BMP7, we investigated the ERK-SP1 axis as a downstream intracellular signaling regulator involved in BMP7 and LfcinB-mediated activities. Treatment of bovine NP cells cultured in alginate with LfcinB plus BMP7 synergistically stimulates PG synthesis and accumulation in part by upregulation of aggrecan gene expression. The synergism results from LfcinB-mediated activation of Sp1 and SMAD signaling pathways by (i) phosphorylation of SMAD 1/5/8; (ii) downregulation of SMAD inhibitory factors [i.e., noggin and SMAD6 (inhibitory SMAD)]; and (iii) upregulation of SMAD4 (universal co-SMAD). These data indicate that LfcinB-suppression of Noggin may eliminate the negative feedback of BMP7, thereby maximizing biological activity of BMP7 and ultimately shifting homeostasis to a pro-anabolic state in disc cells. We propose that combination growth factor therapy using BMP7 and LfcinB may be beneficial for treatment of disc degeneration.

Computational modelling of Smad-mediated negative feedback and crosstalk in the TGF-β superfamily network

The transforming growth factor-β (TGF-β) signal transduction pathway controls many cellular processes, including differentiation, proliferation and apoptosis. It plays a fundamental role during development and it is dysregulated in many diseases. The factors that control the dynamics of the pathway, however, are not fully elucidated yet and so far computational approaches have been very limited in capturing the distinct types of behaviour observed under different cellular backgrounds and conditions into a single-model description. Here, we develop a detailed computational model for TGF-β signalling that incorporates elements of previous models together with crosstalking between Smad1/5/8 and Smad2/3 channels through a negative feedback loop dependent on Smad7. The resulting model accurately reproduces the diverse behaviour of experimental datasets for human keratinocytes, bovine aortic endothelial cells and mouse mesenchymal cells, capturing the dynamics of activation and nucleocytoplasmic shuttling of both R-Smad channels. The analysis of the model dynamics and its system properties revealed Smad7-mediated crosstalking between Smad1/5/8 and Smad2/3 channels as a major determinant in shaping the distinct responses to single and multiple ligand stimulation for different cell types.

Burst BMP triggered receptor kinase activity drives Smad1 mediated long-term target gene oscillation in C2C12 cells

Bone Morphogenetic Proteins (BMPs) are important growth factors that regulate many cellular processes. During embryogenesis they act as morphogens and play a critical role during organ development. They influence cell fates via concentration-gradients in the embryos where cells transduce this extracellular information into gene expression profiles and cell fate decisions. How receiving cells decode and quantify BMP2/4 signals is hardly understood. There is little data on the quantitative relationships between signal input, transducing molecules, their states and location, and ultimately their ability to integrate graded systemic inputs and generate qualitative responses. Understanding this signaling network on a quantitative level should be considered a prerequisite for efficient pathway modulation, as the BMP pathway is a prime target for therapeutic invention. Hence, we quantified the spatial distribution of the main signal transducer of the BMP2/4 pathway in response to different types and levels of stimuli in c2c12 cells. We found that the subcellular localization of Smad1 is independent of ligand concentration. In contrast, Smad1 phosphorylation levels relate proportionally to BMP2 ligand concentrations and they are entirely located in the nucleus. Interestingly, we found that BMP2 stimulates target gene expression in non-linear, wave-like forms. Amplitudes showed a clear concentration-dependency, for sustained and transient stimulation. We found that even burst-stimulation triggers gene-expression wave-like modulations that are detectable for at least 30 h. Finally, we show here that target gene expression oscillations depend on receptor kinase activity, as the kinase drives further expression pulses without receptor reactivation and the target gene expression breaks off after inhibitor treatment in c2c12 cells.

Glucocorticoid receptor concentration and the ability to dimerize influence nuclear translocation and distribution

Glucocorticoid receptor (GR) concentrations and the ability of the GR to dimerize are factors which influence sensitivity to glucocorticoids. Upon glucocorticoid binding, the GR is actively transported into the nucleus, a crucial step in determining GR function. We examined the effects of GR concentration and the ability to dimerize on GR nuclear import, export and nuclear distribution using both live cell microscopy of GFP-tagged GR and immunofluorescence of untagged GR, with both wild type GR (GRwt) and dimerization deficient GR (GRdim). We found that the observed rate of GR nuclear import increases significantly at higher GR concentrations, at saturating concentrations of dexamethasone (10(-6) M) using GFP-tagged GR, while with untagged GR it is only discernable at sub-saturating ligand concentrations (10(-10)-10(-9) M). Loss of dimerization results in a slower observed rate of nuclear import (2.5- to 3.3-fold decrease for GFP-GRdim) as well as a decreased extent of GR nuclear localization (18-27% decrease for untagged GRdim). These results were linked to an increased rate of GR export at low GR concentrations (1.4- to 1.6-fold increase for untagged GR) and where GR dimerization is abrogated (1.5- to 1.7-fold increase for GFP-GRdim). Furthermore, GR dimerization was shown to be required for the appearance of discrete GC-dependent GR nuclear foci, the loss of which may explain the increased rate of GR export for the GRdim. The reduction in the observed rate of nuclear import and increased rate of nuclear export displayed at low GR concentrations and by the GRdim could explain the lowered glucocorticoid response under these conditions.

Comparative study of expression of smad3 in oral lichen planus and normal oral mucosa

Oral lichen planus (OLP) is a chronic inflammatory disease of the oral mucosa which is considered by the World Health Organization (WHO) as a premalignant condition. One step in malignant development is so called epithelial mesenchymal transition (EMT), a process whereby epithelial cells acquire mesenchymal characteristics. A factor known to induce EMT is the transforming growth factor-β (TGF-β), which uses the Smad proteins as mediators for its signaling. The aim of this study was to compare the expression of Smad 3 in Oral Lichen Planus and normal oral mucosa. This descriptive analytic study was performed on 30 patients with OLP (21 women and 9 men with mean age of 45.23± 2.44 years) and 20 normal oral mucosa (14 women and 6 men with mean age of 46.95± 2.21 years). The samples were studied by immunohistochemical staining. Data were analyzed with paired T-test and Wilcoxon test by SPSS software. Expression of Smad3 in OLP samples and normal oral mucosa was different. This difference was statistically significant (P<0.001). The apparently higher expression of Smad 3 in oral lichen planus compared to normal oral mucosa might help to discuss its higher potential for malignant transition.

Analysing the role of UVB-induced translational inhibition and PP2Ac deactivation in NF-κB signalling using a minimal mathematical model

Activation of nuclear factor κB (NF-κB) by interleukin-1β (IL-1) usually results in an anti-apoptotic activity that is rapidly terminated by a negative feedback loop involving NF-κB dependent resynthesis of its own inhibitor IκBα. However, apoptosis induced by ultraviolet B radiation (UVB) is not attenuated, but significantly enhanced by co-stimulation with IL-1 in human epithelial cells. Under these conditions NF-κB remains constitutively active and turns into a pro-apoptotic factor by selectively repressing anti-apoptotic genes. Two different mechanisms have been separately proposed to explain UV-induced lack of IκBα recurrence: global translational inhibition as well as deactivation of the Ser/Thr phosphatase PP2Ac. Using mathematical modelling, we show that the systems behaviour requires a combination of both mechanisms, and we quantify their contribution in different settings. A mathematical model including both mechanisms is developed and fitted to various experimental data sets. A comparison of the model results and predictions with model variants lacking one of the mechanisms shows that both mechanisms are present in our experimental setting. The model is successfully validated by the prediction of independent data. Weak constitutive IKKβ phosphorylation is shown to be a decisive process in IκBα degradation induced by UVB stimulation alone, but irrelevant for (co-)stimulations with IL-1. In silico knockout experiments show that translational inhibition is predominantly responsible for lack of IκBα recurrence following IL-1+UVB stimulation. In case of UVB stimulation alone, cooperation of both processes causes the observed decrease of IκBα. This shows that the processes leading to activation of transcription factor NF-κB upon stimulation with ultraviolet B radiation with and without interleukin-1 costimulation are more complex than previously thought, involving both a cross talk of UVB induced translational inhibition and PP2Ac deactivation. The importance of each of the mechanisms depends on the specific cellular setting.

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.

Dynamics of TGF-β signaling reveal adaptive and pulsatile behaviors reflected in the nuclear localization of transcription factor Smad4

The TGF-β pathway plays a vital role in development and disease and regulates transcription through a complex composed of receptor-regulated Smads (R-Smads) and Smad4. Extensive biochemical and genetic studies argue that the pathway is activated through R-Smad phosphorylation; however, the dynamics of signaling remain largely unexplored. We monitored signaling and transcriptional dynamics and found that although R-Smads stably translocate to the nucleus under continuous pathway stimulation, transcription of direct targets is transient. Surprisingly, Smad4 nuclear localization is confined to short pulses that coincide with transcriptional activity. Upon perturbation, the dynamics of transcription correlate with Smad4 nuclear localization rather than with R-Smad activity. In Xenopus embryos, Smad4 shows stereotyped, uncorrelated bursts of nuclear localization, but activated R-Smads are uniform. Thus, R-Smads relay graded information about ligand levels that is integrated with intrinsic temporal control reflected in Smad4 into the active signaling complex.

BMP2 and mechanical loading cooperatively regulate immediate early signalling events in the BMP pathway

Background

Efficient osteogenic differentiation is highly dependent on coordinated signals arising from growth factor signalling and mechanical forces. Bone morphogenetic proteins (BMPs) are secreted proteins that trigger Smad and non-Smad pathways and thereby influence transcriptional and non-transcriptional differentiation cues. Crosstalk at multiple levels allows for promotion or attenuation of signalling intensity and specificity. Similar to BMPs, mechanical stimulation enhances bone formation. However, the molecular mechanism by which mechanical forces crosstalk to biochemical signals is still unclear.

Results

Here, we use a three-dimensional bioreactor system to describe how mechanical forces are integrated into the BMP pathway. Time-dependent phosphorylation of Smad, mitogen-activated protein kinases and Akt in human fetal osteoblasts was investigated under loading and/or BMP2 stimulation conditions. The phosphorylation of R-Smads is increased both in intensity and duration under BMP2 stimulation with concurrent mechanical loading. Interestingly, the synergistic effect of both stimuli on immediate early Smad phosphorylation is reflected in the transcription of only a subset of BMP target genes, while others are differently affected. Together this results in a cooperative regulation of osteogenesis that is guided by both signalling pathways.

Conclusions

Mechanical signals are integrated into the BMP signalling pathway by enhancing immediate early steps within the Smad pathway, independent of autocrine ligand secretion. This suggests a direct crosstalk of both mechanotransduction and BMP signalling, most likely at the level of the cell surface receptors. Furthermore, the crosstalk of both pathways over longer time periods might occur on several signalling levels.

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.

Computational modeling of signal transduction networks: a pedagogical exposition

We give a pedagogical introduction to computational modeling of signal transduction networks, starting from explaining the representations of chemical reactions by differential equations via the law of mass action. We discuss elementary biochemical reactions such as Michaelis-Menten enzyme kinetics and cooperative binding, and show how these allow the representation of large networks as systems of differential equations. We discuss the importance of looking for simpler or reduced models, such as network motifs or dynamical motifs within the larger network, and describe methods to obtain qualitative behavior by bifurcation analysis, using freely available continuation software. We then discuss stochastic kinetics and show how to implement easy-to-use methods of rule-based modeling for stochastic simulations. We finally suggest some methods for comprehensive parameter sensitivity analysis, and discuss the insights that it could yield. Examples, including code to try out, are provided based on a paper that modeled Ras kinetics in thymocytes.

Mechanism and regulation of nucleocytoplasmic trafficking of smad

Smad proteins are the intracellular mediators of transforming growth factor β (TGF-β) signaling. Smads function as transcription factors and their activities require carboxyl-terminal phosphorylation by TGF-β receptor kinases which are embedded in the cell membrane. Therefore, the translocation of activated Smads from the cytoplasm into the nucleus is a rate-limiting step in TGF-β signal transduction into the nucleus. On the other hand, the export of Smads out of the nucleus turns off TGF-β effect. Such spatial control of Smad ensures a tight regulation of TGF-β target genes. Several cross-talk pathways have been shown to affect TGF-β signaling by impairing nuclear translocation of Smad, exemplifying the biological importance of the nuclear transport process. Many laboratories have investigated the underlying molecular mechanism of Smad nucleocytoplasmic translocation, combining genetics, biochemistry and sophisticated live cell imaging approaches. The last few years have witnessed the elucidation of several key players in Smad nuclear transport, most importantly the karyopherins that carry Smads across the nuclear envelope and nuclear pore proteins that facilitate the trans-nuclear envelope movement. The foundation is now set to further elucidate how the nuclear transport process is regulated and exploit such knowledge to manipulate TGF-β signaling. In this review we will discuss the current understanding of the molecular machinery responsible for nuclear import and export of Smads.

Recruitment of TIF1γ to chromatin via its PHD finger-bromodomain activates its ubiquitin ligase and transcriptional repressor activities

The interplay between sequence-specific DNA-binding transcription factors, histone-modifying enzymes, and chromatin-remodeling enzymes underpins transcriptional regulation. Although it is known how single domains of chromatin "readers" bind specific histone modifications, how combinations of histone marks are recognized and decoded is poorly understood. Moreover, the role of histone binding in regulating the enzymatic activity of chromatin readers is not known. Here we focus on the TGF-β superfamily transcriptional repressor TIF1γ/TRIM33/Ectodermin and demonstrate that its PHD finger-bromodomain constitutes a multivalent histone-binding module that specifically binds histone H3 tails unmethylated at K4 and R2 and acetylated at two key lysines. TIF1γ's ability to ubiquitinate its substrate Smad4 requires its PHD finger-bromodomain, as does its transcriptional repressor activity. Most importantly, TIF1γ's E3 ubiquitin ligase activity is induced by histone binding. We propose a model of TIF1γ activity in which it dictates the residence time of activated Smad complexes at promoters of TGF-β superfamily target genes.

Latent transforming growth factor binding protein 4 (LTBP4) is downregulated in mouse and human DCIS and mammary carcinomas

Background

Transforming growth factor beta (TGF-ß) is able to inhibit the proliferation of epithelial cells and is involved in the carcinogenesis of mammary tumors. Three latent transforming growth factor-ß binding proteins (LTBPs) are known to modulate TGF-ß functions.

Methods

The current study analyses the expression profiles of LTBP4, its isoforms LTBP1 and LTBP3, and TGF-ß1, TGF-ß2, TGF-ß3, and SMAD2, SMAD3 and SMAD4 in human and murine (WAP-TNP8) DCIS compared to invasive mammary tumors. Additionally mammary malignant (MCF7, Hs578T, MDA-MB361) and non malignant cell lines (Hs578BsT) were analysed. Microarray, q-PCR, immunoblot, immunohistochemistry and immunofluorescence were used.

Results

In comparison to non-malignant tissues (n = 5), LTBP4 was downregulated in all human and mouse DCIS (n = 9) and invasive mammary adenocarcinomas (n = 5) that were investigated. We also found decreased expression of bone morphogenic protein 4 (BMP4) and increased expression of its inhibitor gremlin (GREM1). Treatment of the mammary tumor cell line (Hs578T) with recombinant TGF-ß1 rescued BMP4 and GREM1 expression.

Conclusion

We conclude that the lack of LTBP4-mediated targeting in malignant mammary tumor tissues may lead to a possible modification of TGF-ß1 and BMP bioavailability and function.

Electronic supplementary material

The online version of this article (doi:10.1007/s13402-011-0023-y) contains supplementary material, which is available to authorized users.

Insulin-like growth factor 1 synergizes with bone morphogenetic protein 7-mediated anabolism in bovine intervertebral disc cells

OBJECTIVE

We undertook this study to assess the therapeutic benefits of intervertebral disc matrix repair and regeneration by evaluating the potential synergistic effect of insulin-like growth factor 1 (IGF-1) and bone morphogenetic protein 7 (BMP-7) on bovine spine discs and by elucidating the relevant molecular/cellular mechanisms.

METHODS

Bovine nucleus pulposus (NP) cells were treated with BMP-7 and IGF-1. The subsequent anabolic effects driven by NP cells were assessed for proteoglycan (PG) synthesis by (35) S-sulfate incorporation and for PG accumulation by dimethylmethylene blue assays. Matrix formation was visualized by particle exclusion assay. Key matrix components and transcription factors were analyzed by real-time reverse transcription-polymerase chain reaction to determine the signaling pathways by which IGF-1 suppresses noggin, a potent inhibitor of BMP-7. Western blotting and nuclear translocation experiments were performed to assess the activation of Smad proteins.

RESULTS

Stimulation of bovine NP cells by both IGF-1 and BMP-7 greatly potentiated anabolism through complementary and synergistic mechanisms on matrix formation compared with treatment with either growth factor alone. The exogenously added decoy ligand, noggin, attenuated the anabolic effects of BMP-7, and noggin was substantially increased by BMP-7, suggesting a negative feedback regulatory mechanism. In contrast, IGF-1 significantly suppressed noggin expression via the phosphatidylinositol 3-kinase/Akt pathway and thus potentiated BMP-7 signaling in bovine NP cells. Upon combination treatment, IGF-1 activated Smad2, while BMP-7 activated Smad1/5/8 and Smad3, thus inducing all Smad signaling pathways and mimicking the effects of the combination of transforming growth factor β and BMP-7

CONCLUSION

Combination growth factor therapy using BMP-7 and IGF-1 may have considerable promise in the treatment of spine disc degeneration.

Melatonin and vitamin D3 synergistically down-regulate Akt and MDM2 leading to TGFβ-1-dependent growth inhibition of breast cancer cells

Melatonin and vitamin D3 inhibit breast cancer cell growth and induce apoptosis, but they have never been combined as a breast cancer treatment. Therefore, we investigated whether their association could lead to an enhanced anticancer activity. In MCF-7 breast cancer cells, melatonin together with vitamin D3, induced a synergistic proliferative inhibition, with an almost complete cell growth arrest at 144 hr. Cell growth blockade is associated to an activation of the TGFβ-1 pathway, leading to increased TGFβ-1, Smad4 and phosphorylated-Smad3 levels. Concomitantly, melatonin and D3, alone or in combination, caused a significant reduction in Akt phosphorylation and MDM2 values, with a consequent increase of p53/MDM2 ratio. These effects were completely suppressed by adding a monoclonal anti-TGFβ-1 antibody to the culture medium. Taken together, these results indicate that cytostatic effects triggered by melatonin and D3 are likely related to a complex TGFβ-1-dependent mechanism, involving down-regulation of both MDM2 and Akt-phosphorylation.

Studying Smad2 intranuclear diffusion dynamics by mathematical modelling of FRAP experiments

We combine Fluorescence Recovery After Photobleaching (FRAP) experiments with mathematical modelling to study the dynamics inside the nucleus of both the TGF-β-sensitive transcriptional regulator Smad2, and Green-Fluorescent Protein (GFP). We show how combining modelling with bleaching strips of different areas allows a rigorous test of whether or not a protein is moving via diffusion as a single species. As noted recently by others, it is important to consider diffusion during the bleaching process. Neglecting it can cause serious error. Also, it is possible to use the bleaching process itself to provide an extra consistency test to the models predicting the recovery. With our method we show that the dynamics of GFP are consistent with it diffusing as a single species in a uniform environment in which flow is negligible. In contrast, the dynamics of the intracellular signal transducer Smad2 are never consistent with it moving as a single species via simple diffusion in a homogeneous environment without flow. Adding TGF-β slows down the dynamics of Smad2 but even without TGF-β, the Smad2 dynamics are influenced by one or more of: association, flow, and inhomogeneity in space of the dynamics. We suggest that the dynamics inside cells of many proteins may be poorly described by simple diffusion of a single species, and that our methodology provides a general and powerful way to test this hypothesis.

Analysis of ligand-dependent nuclear accumulation of Smads in TGF-beta signaling

The growth inhibition of dividing cells and most of the transcriptional responses upon TGF-beta treatment depend on the Smad2, Smad3, and Smad4 transcription factors. These proteins shuttle continuously between the cytoplasm and the nucleus, transmitting the ligand status of the TGF-beta receptors to the nuclear transcription machinery. In the absence of TGF-beta ligand, Smads 2/3/4 reside predominantly in the cytoplasm. Following ligand binding to the TGF-beta receptors, the dynamic equilibrium of shuttling Smads 2/3/4 shifts toward a predominantly nuclear state, where a high concentration of these transcription factors drives transcriptional activation and repression of genes required for proper cellular response. Here, we describe live cell imaging and immunofluorescence microscopy methods for tracking Smads subcellular localization in response to TGF-beta and leptomycin B treatment. In addition, a method of fractionating nuclear and cytoplasmic proteins used to confirm the imaging results was presented. Our results support the notion that the R-Smad shuttling mechanism is distinct from Co-Smad.

Nodal morphogens

Nodal signals belong to the TGF-beta superfamily and are essential for the induction of mesoderm and endoderm and the determination of the left-right axis. Nodal signals can act as morphogens-they have concentration-dependent effects and can act at a distance from their source of production. Nodal and its feedback inhibitor Lefty form an activator/inhibitor pair that behaves similarly to postulated reaction-diffusion models of tissue patterning. Nodal morphogen activity is also regulated by microRNAs, convertases, TGF-beta signals, coreceptors, and trafficking factors. This article describes how Nodal morphogens pattern embryonic fields and discusses how Nodal morphogen signaling is modulated.

Regulation of TGF-beta signalling by protein phosphatases

Tight regulation of TGF-beta (transforming growth factor-beta) superfamily signalling is important for normal cellular functions and tissue homoeostasis. Since TGF-beta superfamily signalling pathways are activated by a short phosphorylation cascade, from receptor phosphorylation to subsequent phosphorylation and activation of downstream signal transducer R-Smads (receptor-activated Smads), reversible phosphorylation serves as a critical step to assure proper TGF-beta signalling. The present article will review the current progress on the understanding of dynamic phosphorylation in TGF-beta signalling and the essential role of protein phosphatases in this process.

Increased expression of Smad proteins, and in particular Smad3, in oral lichen planus compared to normal oral mucosa

BACKGROUND

Oral lichen planus (OLP) is a chronic inflammatory disease of the oral mucosa which the World Health Organisation (WHO) considers a premalignant condition. One step in malignant development is so called epithelial mesenchymal transition (EMT), a process whereby epithelial cells acquire mesenchymal characteristics. EMT occurs during embryogenesis and wound healing but also in some human diseases such as cancer and fibrosis. A factor known to induce EMT is transforming growth factor-β (TGF-β), which uses the Smad proteins as mediators for its signalling. TGF-β is also often over-expressed in squamous cell carcinoma of the head and neck (SCCHN).

METHODS

In the present study we mapped expression of Smad proteins in OLP lesions by immunohistochemistry, and compared to expression in normal and sensitive oral mucosa. The latter group of patients had developed SCCHN after shorter or longer periods of diffuse oral symptoms. The aim was to see if there were any signs of EMT related changes in the OLP lesions, as judged by changes in the TGF-β pathway.

CONCLUSION

Changes in the TGF-β pathway related to EMT are seen in the very earliest stages of oral malignancy and become more severe as lesions progress.

Intravital imaging illuminates transforming growth factor beta signaling switches during metastasis

Transforming growth factor beta (TGFbeta) has seemingly contradictory roles in tumor progression: it can promote metastatic invasion but also act as a tumor suppressor. Recently, two studies have used intravital imaging to unravel the role of TGFbeta at different stages of the metastatic process. TGFbeta promotes single cell motility, which enables invasion into blood vessels. However the activation of TGFbeta signaling is a transient event and is not maintained at distant sites. The downregulation of TGFbeta signaling at secondary sites then permits growth of secondary tumors. In the absence of TGFbeta, cells are restricted to collective movement and lymphatic spread. Here, we discuss these findings and their potential implications.

Coupling of dephosphorylation and nuclear export of Smads in TGF-beta signaling

In eukaryotes, regulation of signaling mediators/effectors in the nucleus is one of the principal mechanisms that govern duration and strength of signaling. Smads are a family of structurally related intracellular proteins that serve as signaling effectors for transforming growth factor beta (TGF-beta) and TGF-beta-related proteins. Accumulating evidence demonstrates that Smads possess intrinsic nucleocytoplasmic shuttling capacity, which enables them to transmit TGF-beta signals from cell membrane to nucleus. We recently identified two important steps in the termination of nuclear Smad signaling. The first step is initiated by a serine/threonine phosphatase PPM1A that dephosphorylates Smad2/3 in the nucleus, thereby shutting down signaling capacity of phosphorylated Smad2/3. The second step involves nuclear export of dephosphorylated Smad2/3 with the aid of nuclear protein RanBP3 to terminate Smad signaling. This chapter introduces methods for examining nuclear export of Smad2/3 in TGF-beta signaling.

Ubiquitin ligase Nedd4L targets activated Smad2/3 to limit TGF-beta signaling

TGF-beta induces phosphorylation of the transcription factors Smad2 and Smad3 at the C terminus as well as at an interdomain linker region. TGF-beta-induced linker phosphorylation marks the activated Smad proteins for proteasome-mediated destruction. Here, we identify Nedd4L as the ubiquitin ligase responsible for this step. Through its WW domain, Nedd4L specifically recognizes a TGF-beta-induced phosphoThr-ProTyr motif in the linker region, resulting in Smad2/3 polyubiquitination and degradation. Nedd4L is not interchangeable with Smurf1, a ubiquitin ligase that targets BMP-activated, linker-phosphorylated Smad1. Nedd4L limits the half-life of TGF-beta-activated Smads and restricts the amplitude and duration of TGF-beta gene responses, and in mouse embryonic stem cells, it limits the induction of mesoendodermal fates by Smad2/3-activating factors. Hierarchical regulation is provided by SGK1, which phosphorylates Nedd4L to prevent binding of Smad2/3. Previously identified as a regulator of renal sodium channels, Nedd4L is shown here to play a broader role as a general modulator of Smad turnover during TGF-beta signal transduction.

Informatics approaches to understanding TGFbeta pathway regulation

In recent years, informatics studies have predicted several new ways in which the transforming growth factor beta (TGFbeta) signaling pathway can be post-translationally regulated. Subsequently, many of these predictions were experimentally validated. These approaches include phylogenetic predictions for the phosphorylation, sumoylation and ubiquitylation of pathway components, as well as kinetic models of endocytosis, phosphorylation and nucleo-cytoplasmic shuttling. We review these studies and provide a brief ;how to' guide for phylogenetics. Our hope is to stimulate experimental tests of informatics-based predictions for TGFbeta signaling, as well as for other signaling pathways, and to expand the number of developmental pathways that are being analyzed computationally.

Photoactivatable fluorescent proteins for diffraction-limited and super-resolution imaging

Photoactivatable fluorescent proteins (PA-FPs) are molecules that switch to a new fluorescent state in response to activation to generate a high level of contrast. Over the past eight years, several types of PA-FPs have been developed. The PA-FPs fluoresce green or red, or convert from green to red in response to activating light. Others reversibly switch between 'off' and 'on' in response to light. The optical "highlighting" capability of PA-FPs has led to the rise of novel imaging techniques providing important new biological insights. These range from in cellulo pulse-chase labeling for tracking subpopulations of cells, organelles or proteins under physiological settings, to super-resolution imaging of single molecules for determining intracellular protein distributions at nanometer precision. This review surveys the expanding array of PA-FPs, including their advantages and disadvantages, and highlights their use in novel imaging methodologies.

Increased cAMP levels modulate transforming growth factor-beta/Smad-induced expression of extracellular matrix components and other key fibroblast effector functions

cAMP is a key messenger of many hormones and neuropeptides, some of which modulate the composition of extracellular matrix. Treatment of human dermal fibroblasts with dibutyryl cyclic AMP and forskolin antagonized the inductive effects of transforming growth factor-beta (TGF-beta) on the expression of collagen, connective tissue growth factor, tissue inhibitor of matrix metalloproteinase-1, and plasminogen activator inhibitor type I, four prototypical TGF-beta-responsive genes. Increased intracellular cAMP prevented TGF-beta-induced Smad-specific gene transactivation, although TGF-beta-mediated Smad phosphorylation and nuclear translocation remained unaffected. However, increased cAMP levels abolished TGF-beta-induced interaction of Smad3 with its transcriptional co-activator cAMP-response element-binding protein (CREB)-binding protein (CBP)/p300. Overexpression of the transcriptional co-activator CBP/p300 rescued Smad-specific gene transcription in the presence of cAMP suggesting that sequestration of limited amounts of CBP/p300 by the activated cAMP/CREB pathway is the molecular basis of this inhibitory effect. These findings were extended by two functional assays. Increased intracellular cAMP levels suppressed the inductive activity of TGF-beta to contract mechanically unloaded collagen lattices and resulted in an attenuation of fibroblast migration of mechanically induced cell layer wounds. Of note, cAMP and TGF-beta synergistically induced hyaluronan synthase 2 (HAS2) expression and hyaluronan secretion, presumably via putative CREB-binding sites adjacent to Smad-binding sites within the HAS2 promoter. Our findings identify the cAMP pathway as a potent but differential and promoter-specific regulator of TGF-beta-mediated effects involved in extracellular matrix homeostasis.

Localized and reversible TGFbeta signalling switches breast cancer cells from cohesive to single cell motility

Here we use intravital imaging to demonstrate a reversible transition to a motile state as breast cancer cells spread. Imaging primary tumours reveals heterogeneity in cell morphology and motility. Two distinct modes of motility are observed: collective and single-celled. By monitoring the localisation of Smad2 and the activity of a TGFβ-dependent reporter gene during breast cancer cell dissemination we demonstrate that TGFβ signalling is transiently and locally activated in motile single cells. TGFβ1 switches cells from cohesive to single cell motility through a transcriptional programme involving Smad4, EGFR, Nedd9, M-RIP, FARP and RhoC. Blockade of TGFβ signalling prevents cells moving singly in vivo but does not inhibit cells moving collectively. Cells restricted to collective invasion are capable of lymphatic invasion but not blood-borne metastasis. Constitutive TGFβ signalling promotes single cell motility and intravasation but reduces subsequent growth in the lungs. Thus, transient TGFβ signalling is critical for blood-borne metastasis.

The structure of myostatin:follistatin 288: insights into receptor utilization and heparin binding

Myostatin is a member of the transforming growth factor-beta (TGF-beta) family and a strong negative regulator of muscle growth. Here, we present the crystal structure of myostatin in complex with the antagonist follistatin 288 (Fst288). We find that the prehelix region of myostatin very closely resembles that of TGF-beta class members and that this region alone can be swapped into activin A to confer signalling through the non-canonical type I receptor Alk5. Furthermore, the N-terminal domain of Fst288 undergoes conformational rearrangements to bind myostatin and likely acts as a site of specificity for the antagonist. In addition, a unique continuous electropositive surface is created when myostatin binds Fst288, which significantly increases the affinity for heparin. This translates into stronger interactions with the cell surface and enhanced myostatin degradation in the presence of either Fst288 or Fst315. Overall, we have identified several characteristics unique to myostatin that will be paramount to the rational design of myostatin inhibitors that could be used in the treatment of muscle-wasting disorders.

Modulation of nucleocytoplasmic trafficking by retention in cytoplasm or nucleus

Nuclear protein transport processes have largely been studied using in vitro semi-intact cell systems where high concentrations of nuclear localizing substrates are used, and cytoplasmic components such as the microtubule (MT) network, are either absent or damaged. Here we use the fluorescence recovery after photobleaching (FRAP) technique to analyze the nucleocytoplasmic flux of distinct fluorescently tagged proteins over time in living cultured cells. FRAP was performed in different parts of the cell to analyze the kinetics of nucleocytoplasmic trafficking and intranuclear/cytoplasmic mobility of the tumor suppressor Rb protein and a SV40 large tumor antigen (T-ag) derivative containing the nuclear localization sequence (NLS), both fused to green fluorescent protein (GFP). The results indicate that proteins carrying the T-ag NLS are highly mobile in the nucleus and cytoplasm. Rb, in contrast, is largely immobile in both cellular compartments, with similar nuclear import and export kinetics. Rb nuclear export was CRM-1-mediated, with its reduced mobility in the cytoplasm in part due to association with MTs. Overall our results show that nuclear and cytoplasm retention modulates the rates of nuclear protein import and export in intact cells.

A detailed analysis of 3D subcellular signal localization

Detection and localization of fluorescent signals in relation to other subcellular structures is an important task in various biological studies. Many methods for analysis of fluorescence microscopy image data are limited to 2D. As cells are in fact 3D structures, there is a growing need for robust methods for analysis of 3D data. This article presents an approach for detecting point-like fluorescent signals and analyzing their subnuclear position. Cell nuclei are delineated using marker-controlled (seeded) 3D watershed segmentation. User-defined object and background seeds are given as input, and gradient information defines merging and splitting criteria. Point-like signals are detected using a modified stable wave detector and localized in relation to the nuclear membrane using distance shells. The method was applied to a set of biological data studying the localization of Smad2-Smad4 protein complexes in relation to the nuclear membrane. Smad complexes appear as early as 1 min after stimulation while the highest signal concentration is observed 45 min after stimulation, followed by a concentration decrease. The robust 3D signal detection and concentration measures obtained using the proposed method agree with previous observations while also revealing new information regarding the complex formation.

PP2A regulates BMP signalling by interacting with BMP receptor complexes and by dephosphorylating both the C-terminus and the linker region of Smad1

Phosphorylation of Smads is a crucial regulatory step in the signal transduction pathway initiated by bone morphogenetic proteins (BMPs). Although the dephosphorylation events terminating the pathway in the nucleus have been characterized, little is known about the dephosphorylation of Smads in the cytoplasm. In a proteomic screen for proteins interacting with the BMP type-II receptor, we found the regulatory Bbeta subunit of PP2A. PP2A is one of the major serine/threonine phosphatases involved in cell-cycle regulation and signal transduction. Here, we present data showing that the Bbeta subunit of PP2A interacts with both BMP type-I and type-II receptors. Furthermore, we demonstrate that several B subunits can associate with the BMP type-II receptor, independently of the kinase activity of the receptor and the catalytic subunit of PP2A. By contrast, the PP2A catalytic subunit is required for PP2A function at the receptor complex. This function of PP2A is the dephosphorylation of Smad1, mainly in the linker region. PP2A-mediated dephosphorylation of the BMP-Smad linker region leads to increased nuclear translocation of Smads and overall amplification of the BMP signal. Although other phosphatases identified within the BMP pathway are all shown to inhibit signalling, PP2A is the first example for a signalling stimulatory phosphatase within this pathway.

A negative feedback control of transforming growth factor-beta signaling by glycogen synthase kinase 3-mediated Smad3 linker phosphorylation at Ser-204

Through the action of its membrane-bound type I receptor, transforming growth factor-beta (TGF-beta) elicits a wide range of cellular responses that regulate cell proliferation, differentiation, and apo ptosis. Many of these signaling responses are mediated by Smad proteins. As such, controlling Smad activity is crucial for proper signaling by TGF-beta and its related factors. Here, we show that TGF-beta induces phosphorylation at three sites in the Smad3 linker region in addition to the two C-terminal residues, and glycogen synthase kinase 3 is responsible for phosphorylation at one of these sites, namely Ser-204. Alanine substitution at Ser-204 and/or the neighboring Ser-208, the priming site for glycogen synthase kinase 3 in vivo activity, strengthened the affinity of Smad3 to CREB-binding protein, suggesting that linker phosphorylation may be part of a negative feedback loop that modulates Smad3 transcriptional activity. Thus, our findings reveal a novel aspect of the Smad3 signaling mechanism that controls the final amplitude of cellular responses to TGF-beta.

Quantitative modeling and analysis of the transforming growth factor beta signaling pathway

Transforming growth factor beta (TGF-beta) signaling, which regulates multiple cellular processes including proliferation, apoptosis, and differentiation, plays an important but incompletely understood role in normal and cancerous tissues. For instance, although TGF-beta functions as a tumor suppressor in the premalignant stages of tumorigenesis, paradoxically, it also seems to act as a tumor promoter in advanced cancer leading to metastasis. The mechanisms by which TGF-beta elicits such diverse responses during cancer progression are still not entirely clear. As a first step toward understanding TGF-beta signaling quantitatively, we have developed a comprehensive, dynamic model of the canonical TGF-beta pathway via Smad transcription factors. By describing how an extracellular signal of the TGF-beta ligand is sensed by receptors and transmitted into the nucleus through intracellular Smad proteins, the model provides quantitative insight into how TGF-beta-induced responses are modulated and regulated. Subsequent model analysis shows that mechanisms associated with Smad activation by ligand-activated receptor, nuclear complex formation among Smad proteins, and inactivation of ligand-activated Smad (e.g., degradation, dephosphorylation) may be critical for regulating TGF-beta-targeted functional responses. The model was also used to predict dynamic characteristics of the Smad-mediated pathway in abnormal cells, from which we generated four testable hypotheses regarding potential mechanisms by which TGF-beta's tumor-suppressive roles may appear to morph into tumor-promotion during cancer progression.