Smad-dependent and Smad-independent pathways in TGF-beta family signalling

Regulation of functional diversity within the Nedd4 family by accessory and adaptor proteins

Ubiquitination is essential in mediating diverse cellular functions including protein degradation and trafficking. Ubiquitin-protein (E3) ligases determine the substrate specificity of the ubiquitination process. The Nedd4 family of E3 ligases is an evolutionarily conserved family of proteins required for the ubiquitination of a large number of cellular targets. As a result, this family regulates a wide variety of cellular processes including transcription, stability and trafficking of plasma membrane proteins, and the degradation of misfolded proteins. The modular architecture of the proteins, comprising a C2 domain, multiple WW domains and a catalytic domain, enables diverse intermolecular interactions and recruitment to various subcellular locations. The WW domains commonly mediate interaction with substrate proteins; however, an increasing number of Nedd4 targets do not contain obvious WW domain-interaction motifs suggesting the involvement of accessory proteins. This review discusses recent insights into how accessory and adaptor proteins modulate the activities of Nedd4 family members, including recruitment of novel substrates, alteration of subcellular localisation and effects on ubiquitination.

Pro-metastasis function of TGFβ mediated by the smad pathway

The transforming growth factor beta (TGFbeta) signaling pathway plays a vital role in the development and homeostasis of normal tissues. Abnormal function of this pathway contributes to the initiation and progression of cancer. Smad proteins are key signal transducers of the TGFbeta pathway and are essential for the growth suppression function of TGFbeta. Smads are bona fide tumor suppressors whose mutation, deletion, and silencing are associated with many types of human cancer. However, the involvement and functional mechanism of Smad proteins in cancer metastasis are poorly defined. Recent studies using genetically modified cancer cells and mouse tumor models have provided concrete evidence for a Smad-dependent mechanism for metastasis promotion by TGFbeta. Understanding the dual roles of Smad proteins in tumor initiation and progression has important implications for cancer therapeutics.

BMP signaling and stem cell regulation

Stem cells play an essential role in cellular specialization and pattern formation during embryogenesis and in tissue regeneration in adults. This is mainly due to a stem cell's ability to replenish itself (self-renewal) and, at the same time, produce differentiated progeny. Realization of these special stem cell features has changed the prospective of the field. However, regulation of stem cell self-renewal and maintenance of its potentiality require a complicated regulatory network of both extracellular cues and intrinsic programs. Understanding how signaling regulates stem cell behavior will shed light on the molecular mechanisms underlying stem cell self-renewal. In this review, we focus on comparing the progress of recent research regarding the roles of the BMP signaling pathway in different stem cell systems, including embryonic stem cells, germline stem cells, hematopoietic stem cells, and intestinal stem cells. We hope this comparison, together with a brief look at other signaling pathways, will bring a more balanced view of BMP signaling in regulation of stem cell properties, and further point to a general principle that self-renewal of stem cells may require a combination of maintenance of proliferation potential, inhibition of apoptosis, and blocking of differentiation.

Inactivation of ELF/TGF-beta signaling in human gastrointestinal cancer

TGF-beta/Smads regulate a wide variety of biological responses through transcriptional regulation of target genes. ELF, a beta-spectrin, plays a key role in the transmission of TGF-beta-mediated transcriptional response through Smads. ELF was originally identified as a key protein involved in endodermal stem/progenitor cells committed to foregut lineage. Also, as a major dynamic adaptor and scaffolding protein, ELF is important for the generation of functionally distinct membranes, protein sorting and the development of polarized differentiated epithelial cells. Disruption of elf results in the loss of Smad3/Smad4 activation and, therefore, a disruption of the TGF-beta pathway. These observations led us to pursue the function of ELF in gastrointestinal (GI) epithelial cell-cell adhesion and tumor suppression. Here, we show a significant loss of ELF and reduced Smad4 expression in human gastric cancer tissue samples. Also, of the six human gastric cancer cell lines examined, three show deficient ELF expression. Furthermore, we demonstrate the rescue of E-cadherin-dependent homophilic cell-cell adhesion by ectopic expression of full-length elf. Our results suggest that ELF has an essential role in tumor suppression in GI cancers.

Radiation and transforming growth factor-beta cooperate in transcriptional activation of the profibrotic plasminogen activator inhibitor-1 gene

Radiation-induced fibrosis is an important side effect in the treatment of cancer. Profibrotic proteins, such as plasminogen activator inhibitor-1 (PAI-1), transforming growth factor-beta (TGF-beta), and tissue type inhibitor of metalloproteinases-1 (Timp-1), are thought to play major roles in the development of fibrosis via the modulation of extracellular matrix integrity. We did a detailed analysis of transcriptional activation of these profibrotic genes by radiation and TGF-beta. Irradiation of HepG2 cells led to a high increase in PAI-1 mRNA levels and a mild increase in Timp-1 mRNA levels. In contrast, TGF-beta1 and Smad7 were not increased. Radiation and TGF-beta showed strong cooperative effects in transcription of the PAI-1 gene. The TGF-beta1 gene showed a mild cooperative activation, whereas Timp-1 and Smad7 were not cooperatively activated by radiation and TGF-beta. Analysis using the proximal 800 bp of the human PAI-1 promoter revealed a dose-dependent increase of PAI-1 levels between 2 and 32 Gy gamma-rays that was independent of latent TGF-beta activation. Subsequent site-directed mutagenesis of the PAI-1 promoter revealed that mutation of a p53-binding element abolished radiation-induced PAI-1 transcription. In line with this, PAI-1 was not activated in p53-null Hep3B cells, indicating that p53 underlies the radiation-induced PAI-1 activation and the cooperativity with the TGF-beta/Smad pathway. Together, these data show that radiation and TGF-beta activate PAI-1 via partially nonoverlapping signaling cascades that in concert synergize on PAI-1 transcription. This may play a role in patient-to-patient variations in susceptibility toward fibrosis after radiotherapy.

TGFbeta signaling at the summit

Ligands belonging to the transforming growth factor (TGF) beta superfamily have emerged as major regulators of a wide variety of developmental events, ranging from the earliest steps in germ layer patterning of the pre-gastrula embryo to tissue healing, regeneration and homeostasis in the adult. Recently, Caroline Hill and Bob Lechleider organized the third in a bi-annual series of FASEB meetings on TGFbeta signaling and development at Snowmass (CO, USA). This meeting highlighted the ongoing interplay between advances in our understanding of the molecular biology of TGFbeta family signaling and in investigations into its roles in specific developmental events.

Transforming growth factor beta-dependent sequential activation of Smad, Bim, and caspase-9 mediates physiological apoptosis in gastric epithelial cells

Transforming growth factor beta (TGF-beta) has been implicated in the maintenance of homeostasis in various organs, including the gastric epithelium. In particular, TGF-beta-induced signaling was shown to be required for the differentiation-associated physiological apoptosis of gastric epithelial cells, but its mechanism has not been well understood. In this study, the molecular mechanism of TGF-beta-induced apoptosis was analyzed in a human gastric epithelial cell line, SNU16, as an in vitro model. Expression of Smad7 and Bcl-X(L), but not viral FLIP, was shown to prevent TGF-beta-induced apoptosis, indicating an exclusive requirement of the activation of Smad signaling pathway and mitochondrial dysfunction followed by activation of caspase-9. In addition, treatment with TGF-beta induced binding of Bim, a proapoptotic Bcl-2 homology domain 3 (BH3)-only protein, to Bcl-X(L), which is dependent on the activation of Smad, and reduction in the expression of Bim by RNA interference decreased the sensitivity to TGF-beta-induced apoptosis. Moreover, we found abnormalities in the gastric epithelium of both Bim and caspase-9 knockout mice; these abnormalities were associated with a defect of physiological apoptosis in gastric epithelial cells. These results indicate for the first time that TGF-beta is involved in the physiological loss of gastric epithelial cells by activating apoptosis mediated by Smad, Bim, and caspase-9.

A road map toward defining the role of Smad signaling in hematopoietic stem cells

The transforming growth factor-beta (TGF-beta) superfamily encompasses the ligands and receptors for TGF-beta, bone morphogenic proteins (BMPs), and Activins. Cellular response to ligand is context-dependent and may be controlled by specificity and/or redundancy of expression of these superfamily members. Several pathways within this family have been implicated in the proliferation, differentiation, and renewal of hematopoietic stem cells (HSCs); however, their roles and redundancies at the molecular level are poorly understood in the rare HSC. Here we have characterized the expression of TGF-beta superfamily ligands, receptors, and Smads in murine HSCs and in the Lhx2-hematopoietic progenitor cell (Lhx2-HPC) line. We demonstrate a remarkable likeness between these two cell types with regard to expression of the majority of receptors and Smads necessary for the transduction of signals from TGF-beta, BMP, and Activin. We have also evaluated the response of these two cell types to various ligands in proliferation assays. In this regard, primary cells and the Lhx2-HPC line behave similarly, revealing a suppressive effect of Activin-A that is similar to that of TGF-beta in bulk cultures and no effect of BMP-4 on proliferation. Signaling studies that verify the phosphorylation of Smad2 (Activin and TGF-beta) and Smad1/5 (BMP) confirm cytosolic responses to these ligands. In addition to providing a thorough characterization of TGF-beta superfamily expression in HSCs, our results define the Lhx2-HPC line as an appropriate model for molecular characterization of Smad signaling.

BMP signaling controls PASMC KV channel expression in vitro and in vivo

Bone morphogenetic proteins (BMPs) have been implicated in the pathogenesis of familial pulmonary arterial hypertension. The type 2 receptor (BMPR2) is required for recognition of all BMPs. Transgenic mice with a smooth muscle cell-targeted mutation in this receptor (SM22-tet-BMPR2(delx4+)) developed increased pulmonary artery pressure, associated with a modest increase in arterial muscularization, after 8 wk of transgene activation (West J, Fagan K, Steudel W, Fouty B, Lane K, Harral J, Hoedt-Miller M, Tada Y, Ozimek J, Tuder R, and Rodman DM. Circ Res 94: 1109-1114, 2004). In the present study, we show that these transgenic mice developed increased right ventricular pressures after only 1 wk of transgene activation, without significant remodeling of the vasculature. We then tested the hypothesis that the increased pulmonary artery pressure due to loss of BMPR2 signaling was mediated by reduced K(V) channel expression. There was decreased expression of K(V)1.1, K(V)1.5, and K(V)4.3 mRNA isolated from whole lung. Western blot confirmed decreased K(V)1.5 protein in these lungs. Human pulmonary artery smooth muscle cells (PASMC) treated with recombinant BMP2 had increased K(V)1.5 protein and macroscopic K(V) current density, which was blocked by anti-K(V)1.5 antibody. In vivo, nifedipine, a selective L-type Ca(2+) channel blocker, reduced RV systolic pressure in these dominant-negative BMPR2 mice to levels seen in control animals. This suggests that activation of L-type Ca(2+) channels caused by reduced K(V)1.5 mediates increased pulmonary artery pressure in these animals. These studies suggest that BMP regulates K(V) channel expression and that loss of this signaling pathway in PASMC through a mutation in BMPR2 is sufficient to cause pulmonary artery vasoconstriction.

TGF-beta control of cell proliferation

This article focuses on recent findings that the type V TGF-beta receptor (TbetaR-V), which co-expresses with other TGF-beta receptors (TbetaR-I, TbetaR-II, and TbetaR-III) in all normal cell types studied, is involved in growth inhibition by IGFBP-3 and TGF-beta and that TGF-beta activity is regulated by two distinct endocytic pathways (clathrin- and caveolar/lipid-raft-mediated). TGF-beta is a potent growth inhibitor for most cell types, including epithelial and endothelial cells. The signaling by which TGF-beta controls cell proliferation is not well understood. Many lines of evidence indicate that other signaling pathways, in addition to the prominent TbetaR-I/TbetaR-II/Smad2/3/4 signaling cascade, are required for mediating TGF-beta-induced growth inhibition. Recent studies revealed that TbetaR-V, which is identical to LRP-1, mediates IGF-independent growth inhibition by IGFBP-3 and mediates TGF-beta-induced growth inhibition in concert with TbetaR-I and TbetaR-II. In addition, IRS proteins and a Ser/Thr-specific protein phosphatase(s) are involved in the TbetaR-V-mediated growth inhibitory signaling cascade. The TbetaR-V signaling cascade appears to cross-talk with the TbetaR-I/TbetaR-II, insulin receptor (IR), IGF-I receptor (IGF-IR), integrin and c-Met signaling cascades. Attenuation or loss of the TbetaR-V signaling cascade may enable carcinoma cells to escape from TGF-beta growth control and may contribute to the aggressiveness and invasiveness of these cells via promoting epithelial-to-mesenchymal transdifferentiation (EMT). Finally, the ratio of TGF-beta binding to TbetaR-II and TbetaR-I is a signal controlling TGF-beta partitioning between two distinct endocytosis pathways and resultant TGF-beta responsiveness. These recent studies have provided new insights into the molecular mechanisms underlying TGF-beta-induced cellular growth inhibition, cross-talk between the TbetaR-V and other signaling cascades, the signal that controls TGF-beta responsiveness and the role of TbetaR-V in tumorigenesis.

Revealing transforming growth factor-beta signaling transduction in human kidney by gene expression data mining

Tumor growth factor-beta (TGF-beta) is a key mediator of glomerular and tubulointerstitial pathobiology in chronic kidney disease. Its signaling transduction controls a diverse number of biological processes in a dynamic and context-dependent manner. We applied a data mining strategy to deconvolute gene expression patterns across hundreds of microarray data sets to reveal members of the TGF-beta signaling network in human kidney. This strategy is composed of three major steps: (i) select genes known to be involved and expressionally regulated in TGF-beta signaling as "bait"; (ii) select microarray data sets in which the bait genes are strongly co-regulated; (iii) identify (or "fish") additional TGF-beta signaling genes by a non-parametric statistic-based gene scoring system (NP score). The 40 genes with highest NP scores and significant permutation p values were selected for in silico validation, and used to identify a network, in which 35 of these genes were found to be connected by literature- derived relationships. Transcription factors were found to be enriched in the top list. Among them, activated transcription factor 3 (ATF3) had the highest NP score, and was proposed to play a pivotal role in TGF-beta signaling in human kidney. Finally, we implemented a non-parametric pathway ranking (NPPR) tool (Mootha et al., 2003) to rank pathways and identified canonical biological pathways associated with the down-stream of TGF-beta signaling.

Cardiovascular malformations with normal smooth muscle differentiation in neural crest-specific type II TGFbeta receptor (Tgfbr2) mutant mice

Previous studies have demonstrated that TGFbeta induces a smooth muscle fate in primary neural crest cells in culture. By crossing a conditional allele of the type II TGFbeta receptor with the neural crest-specific Wnt1cre transgene, we have addressed the in vivo requirement for TGFbeta signaling in smooth muscle specification and differentiation. We find that elimination of the TGFbeta receptor does not alter neural crest cell specification to a smooth muscle fate in the cranial or cardiac domains, and that a smooth muscle fate is not realized by trunk neural crest cells in either control or mutant embryos. Instead, mutant embryos exhibit with complete penetrance two very specific and mechanistically distinct cardiovascular malformations--persistent truncus arteriosus (PTA) and interrupted aortic arch (IAA-B). Pharyngeal organ defects such as those seen in models of DiGeorge syndrome were not observed, arguing against an early perturbation of the cardiac neural crest cell lineage. We infer that TGFbeta is an essential morphogenic signal for the neural crest cell lineage in specific aspects of cardiovascular development, although one that is not required for smooth muscle differentiation.

Drivers of Germ Cell Maturation

Spermatogenesis requires progression of germ line stem cells through a precisely ordered differentiation pathway to form spermatozoa. Diverse and dynamic signals from the transforming growth factor-beta (TGF-beta) superfamily influence many stages of germ cell development. For example, interactions between several TGF-beta superfamily ligands (bone morphogenetic proteins, activin, and glial-derived neurotrophic growth factor [GDNF]) appear to govern the onset of spermatogenesis, and we are exploring how germ cells interpret these competing signals. We examined the in vivo impact of activin on testis development using two mouse models, the inhba-/- mouse (which lacks the gene encoding the activin A subunit and dies at birth) and BK mice, with inhbb (encoding the activin betaB subunit) replacing inhba (which survive to adulthood and show delayed fertility onset in males). Distinct effects on Sertoli cell and germ cell populations during fetal and early postnatal development were measured. We recognize that specific proteins, including downstream targets of TGF-beta signals, such as Smads, must move into the nucleus to implement the gene transcription changes required for development. We hypothesized that changes at the level of cellular nuclear transport machinery may be required to mediate this. Examination of proteins involved in classical nuclear import, the importins, revealed that each importin has a developmentally regulated expression pattern in male germ cells. Because each importin binds a selected range of cargo proteins and mediates their nucleocytoplasmic passage, our findings suggest that each importin ferries cargo required for discrete stages of spermatogenesis.

Requirement for the SnoN oncoprotein in transforming growth factor beta-induced oncogenic transformation of fibroblast cells

Transforming growth factor beta (TGF-beta) was originally identified by virtue of its ability to induce transformation of the AKR-2B and NRK fibroblasts but was later found to be a potent inhibitor of the growth of epithelial, endothelial, and lymphoid cells. Although the growth-inhibitory pathway of TGF-beta mediated by the Smad proteins is well studied, the signaling pathway leading to the transforming activity of TGF-beta in fibroblasts is not well understood. Here we show that SnoN, a member of the Ski family of oncoproteins, is required for TGF-beta-induced proliferation and transformation of AKR-2B and NRK fibroblasts. TGF-beta induces upregulation of snoN expression in both epithelial cells and fibroblasts through a common Smad-dependent mechanism. However, a strong and prolonged activation of snoN transcription that lasts for 8 to 24 h is detected only in these two fibroblast lines. This prolonged induction is mediated by Smad2 and appears to play an important role in the transformation of both AKR-2B and NRK cells. Reduction of snoN expression by small interfering RNA or shortening of the duration of snoN induction by a pharmacological inhibitor impaired TGF-beta-induced anchorage-independent growth of AKR-2B cells. Interestingly, Smad2 and Smad3 play opposite roles in regulating snoN expression in both fibroblasts and epithelial cells. The Smad2/Smad4 complex activates snoN transcription by direct binding to the TGF-beta-responsive element in the snoN promoter, while the Smad3/Smad4 complex inhibits it through a novel Smad inhibitory site. Mutations of Smad4 that render it defective in heterodimerization with Smad3, which are found in many human cancers, convert the activity of Smad3 on the snoN promoter from inhibitory to stimulatory, resulting in increased snoN expression in cancer cells. Thus, we demonstrate a novel role of SnoN in the transforming activity of TGF-beta in fibroblasts and also uncovered a mechanism for the elevated SnoN expression in some human cancer cells.

Sending the right signals: regulating receptor kinase activity

Knowledge of the functions of plant receptor-like-kinases (RLKs) is increasing rapidly, but how their cytoplasmic signalling activity is regulated and how signals are transduced to cytoplasmic or nuclear proteins remain important questions. Recent studies, particularly of the BRASSINOSTEROID INSENSITIVE1 RLK, have begun to shed light on the mechanistic details of RLK activation, including the possible role of ligand binding. Studies of this and other RLKs have also highlighted the potential importance of hetero-oligomerisation and receptor internalisation in RLK signalling. Finally, a range of potential regulatory proteins and putative downstream signalling substrates have been identified for various RLKs. Despite some similarities with animal receptor kinase signalling systems, mechanisms that affect the intracellular behaviour, regulation and interactions of RLKs appear to be very diverse, potentially explaining how signalling specificity is maintained at the cytoplasmic level.

Paclitaxel modulates TGFbeta signaling in scleroderma skin grafts in immunodeficient mice

BACKGROUND

Systemic sclerosis (SSc) is characterized by excessive fibrosis and obliterative vascular lesions. Abnormal TGFbeta activation is implicated in the pathogenesis of SSc. Aberrant TGFbeta/Smad signaling can be controlled by stabilization of microtubules with paclitaxel.

METHODS AND FINDINGS

SSc and healthy human skin biopsies were incubated in the presence or absence of paclitaxel followed by transplantation into severe combined immunodeficient mice. TGFbeta signaling, fibrosis, and neovessel formation were evaluated by quantitative RT-PCR and immunohistochemical staining. Paclitaxel markedly suppressed Smad2 and Smad3 phosphorylation and collagen deposition in SSc grafts. As a result, the autonomous maintenance/reconstitution of the SSc phenotype was prevented. Remarkably, SSc grafts showed a 2-fold increase in neovessel formation relative to normal grafts, regardless of paclitaxel treatment. Angiogenesis in SSc grafts was associated with a substantial increase in mouse PECAM-1 expression, indicating the mouse origin of the neovascular cells.

CONCLUSION

Low-dose paclitaxel can significantly suppress TGFbeta/Smad activity and lessen fibrosis in SCID mice. Transplantation of SSc skin into SCID mice elicits a strong angiogenesis-an effect not affected by paclitaxel. Although prolonged chemotherapy with paclitaxel at higher doses is associated with pro-fibrotic and anti-angiogenic changes, the findings described here indicate that low-dose paclitaxel may have therapeutic benefits for SSc via modulating TGFbeta signaling.

Involvement of the serine/threonine p70S6 kinase in TGF-beta1-induced ADAM12 expression in cultured human hepatic stellate cells

BACKGROUND/AIMS

In chronic liver injury, quiescent hepatic stellate cells change into proliferative myofibroblast-like cells, which are a main source of fibrosis. We have recently reported that these cells synthesize ADAM12, a disintegrin and metalloprotease whose expression is up-regulated by TGF-beta1 in liver cancers. Here, we studied the role of the serine/threonine p70S6 kinase (p70S6K) in regulating TGF-beta1-induced ADAM12 expression.

RESULTS

The phophatidylinositol 3-kinase (PI3K) inhibitor LY294002 and the mitogen-activated protein kinase inhibitor, UO126, decreased the TGF-beta1-dependent ADAM12 expression and prevented the phosphorylation of p70S6K. In addition, TGF-beta1-induced ADAM12 up-regulation was blocked by the Frap/mTOR inhibitor rapamycin, which abrogated the phosphorylation of p70S6K. In untreated cells, LY294002 but not rapamycin diminished the basal ADAM12 expression related to inhibition of Akt and the glycogen synthase kinase-3 phosphorylation.

CONCLUSIONS

The data suggest that TGF-beta1 induces ADAM12 gene expression through both the PI3K/Frap-mTOR/p70S6K and MEK/ERK pathways. In addition, activation of the PI3 pathway might be involved in the basal ADAM12 expression in cultured hepatic stellate cells. The involvement of PI3K in ADAM12 expression, similar to that previously observed for collagen I and fibronectin, suggests common pathways for gene up-regulation in hepatic stellate cells that occur during liver fibrogenesis and contribute to tumor progression.

TGF-beta1 inhibits T-bet induction by IFN-gamma in murine CD4+ T cells through the protein tyrosine phosphatase Src homology region 2 domain-containing phosphatase-1

TGF-beta1 prevents the development of autoimmune disease by restraining the development of autoreactive Th1 cells. TGF-beta1 inhibits Th1 development in part by suppressing the expression of T-bet, an IFN-gamma-induced transcription factor that promotes Th1 differentiation, but how TGF-beta1 suppresses T-bet is not known. In this study we show that TGF-beta1 suppresses IFN-gamma-induced T-bet expression through the hemopoietic protein tyrosine phosphatase (PTP) Src homology region 2 domain-containing phosphatase-1 (Shp-1). In murine CD4+ T cells, IFN-gamma rapidly induced the expression of T-bet as well as of IFN regulatory factor-1, another transcription factor important for Th1 development. TGF-beta1 antagonized the effects of IFN-gamma, inhibiting IFN-gamma's induction of both Th1 transcription factors. In the presence of IFN-gamma, TGF-beta1 rapidly induced in Th cells the synthesis of the PTP Shp-1, but did not induce Shp-2 or several members of the suppressor of cytokine signaling family of Jak-Stat inhibitors. We tested the requirement for Shp-1 by using T cells from the Shp-1-deficient me(v)/me(v) mouse strain. Shp-1 was required for TGF-beta1's suppressive effects, because its suppression of T-bet and IFN regulatory factor-1 was completely abrogated in me(v)/me(v) CD4+ T cells. Receptor-proximal responses to IFN-gamma, such as the induction of Jak-Stat phosphorylation, were inhibited by TGF-beta1 in wild-type T cells, but not in me(v)/me(v) T cells. Consistent with a direct role for Shp-1, TGF-beta1's inhibition of IFN-gamma-induced Stat1 phosphorylation was sensitive to the general PTP inhibitor pervanadate. Together, these data show that TGF-beta1 suppresses IFN-gamma signaling and transcriptional responses in CD4+ T cells through the PTP Shp-1.

RhoA modulates Smad signaling during transforming growth factor-beta-induced smooth muscle differentiation

We recently reported that transforming growth factor (TGF)-beta induced the neural crest stem cell line Monc-1 to differentiate into a spindle-like contractile smooth muscle cell (SMC) phenotype and that Smad signaling played an important role in this phenomenon. In addition to Smad signaling, other pathways such as mitogen-activated protein kinase (MAPK), phosphoinositol-3 kinase, and RhoA have also been shown to mediate TGF-beta actions. The objectives of this study were to examine whether these signaling pathways contribute to TGF-beta-induced SMC development and to test whether Smad signaling cross-talks with other pathway(s) during SMC differentiation induced by TGF-beta. We demonstrate here that RhoA signaling is critical to TGF-beta-induced SMC differentiation. RhoA kinase (ROCK) inhibitor Y27632 significantly blocks the expression of multiple SMC markers such as smooth muscle alpha-actin, SM22alpha, and calponin in TGF-beta-treated Monc-1 cells. In addition, Y27632 reversed the cell morphology and abolished the contractility of TGF-beta-treated cells. RhoA signaling was activated as early as 5 min following TGF-beta addition. Dominant negative RhoA blocked nuclear translocation of Smad2 and Smad3 because of the inhibition of phosphorylation of both Smads and inhibited Smad-dependent SBE promoter activity, whereas constitutively active RhoA significantly enhanced SBE promoter activity. Consistent with these results, C3 exotoxin, an inhibitor of RhoA activation, significantly attenuated SBE promoter activity and inhibited Smad nuclear translocation. Taken together, these data point to a new role for RhoA as a modulator of Smad activation while regulating TGF-beta-induced SMC differentiation.

Uptake of host cell transforming growth factor-beta by Trypanosoma cruzi amastigotes in cardiomyocytes: potential role in parasite cycle completion

The cytokine transforming growth factor-beta (TGF-beta) plays various functions in the control of Trypanosoma cruzi infectivity and in the progression of Chagas' disease. When we immunostained T. cruzi-infected cardiomyocytes (after either in vivo or in vitro infections) for TGF-beta, we observed stronger immunoreactivity in parasites than in host cells. TGF-beta immunoreactivity evolved during parasite cycle progression, with intense staining in amastigotes versus very faint staining in trypomastigotes. TGF-beta was present on the surface of amastigotes, in the flagellar pocket, and in intraparasitic vesicles as revealed by electron microscopy. However, no ortholog TGF-beta gene could be identified in the genome of T. cruzi by in silico analysis or by extensive polymerase chain reaction and reverse transcriptase-polymerase chain reaction studies. Immunoreactive TGF-beta was most probably taken up by the parasite from the host cell cytoplasm because such an internalization process of biotinylated TGF-beta could be observed in axenic amastigotes in vitro. These observations represent the first example of a novel mechanism by which a primitive unicellular protozoan can use host cell TGF-beta to control its own intracellular life cycle.

The ETV6-NTRK3 chimeric tyrosine kinase suppresses TGF-beta signaling by inactivating the TGF-beta type II receptor

An emerging theme in cancer biology is that although some malignancies occur through the sequential acquisition of different genetic alterations, certain dominantly acting oncoproteins such as those associated with chromosomal translocations have multiple functions and do not require additional mutations for cell transformation. The ETV6-NTRK3 (EN) chimeric tyrosine kinase, a potent oncoprotein expressed in tumors derived from multiple cell lineages, functions as a constitutively active protein tyrosine kinase. Here, we show that EN suppresses TGF-beta signaling by directly binding to the type II TGF-beta receptor, thereby preventing it from interacting with the type I TGF-beta receptor. This activity requires a functional EN protein tyrosine kinase, and type II TGF-beta receptor appears to be a direct target of EN. Our findings provide evidence for a previously undescribed mechanism by which oncogenic tyrosine kinases can block TGF-beta tumor suppressor activity.

Advanced glycation end-product-induced mitogenesis and collagen production are dependent on angiotensin II and connective tissue growth factor in NRK-49F cells

Diabetic nephropathy (DN) is characterized by glomerulopathy and tubulointerstitial expansion followed by renal fibrosis. Angiotensin II (Ang II) and connective tissue growth factor (CTGF) are involved in the pathogenesis of DN, while Janus kinase 2 (JAK2) is important in advanced glycation end-product (AGE)-induced effects in renal interstitial (NRK-49F) fibroblasts. Thus, we studied the role of Ang II, CTGF, and JAK2 in AGE-induced effects in NRK-49F cells. We found that AGE (150 microg/ml) increased mitogenesis and type I collagen production at 7 days while Ang II (10(-7)M) increased mitogenesis and type I collagen production at 3 days. We also found that AGE (150 microg/ml) increased angiotensinogen protein at 2 days, which was attenuated by AG-490 (a JAK2 inhibitor). AGE (150 microg/ml) increased CTGF mRNA and protein expression at 3 and 5 days, respectively. Ang II (10(-7)M) increased CTGF mRNA and protein expression at 1 and 2 days, respectively, which were attenuated by AG-490. Moreover, losartan (a type I angiotensin receptor blocker) and captopril (an angiotensin converting enzyme inhibitor) attenuated AGE-induced CTGF mRNA/protein expression while attenuating AGE-induced mitogenesis and type I collagen production. AG-490 and CTGF antisense (but not sense) oligodeoxynucleotide (ODN) attenuated Ang II (10(-7)M) and AGE-induced mitogenesis and type I collagen production at 3 and 7 days, respectively. We concluded that AGE (150 microg/ml)-induced mitogenesis and type I collagen production are dependent on the Ang II-JAK2-CTGF pathway in NRK-49F cells. Moreover, Ang II-induced mitogenesis and type I collagen production are dependent on the JAK2-CTGF pathway.

Transforming growth factor-beta signaling in stem cells and cancer

Transforming growth factor-beta (TGF-beta) and TGF-beta-related proteins, such as the bone morphogenetic proteins, have emerged as key regulators of stem cell renewal and differentiation. These proteins have disparate roles in regulating the biology of embryonic stem cells and tumor suppression, and they help define the selection of cell fate and the progression of differentiation along a lineage. Here we illustrate their roles in embryonic stem cells and in the differentiation of neural, hematopoietic, mesenchymal, and gastrointestinal epithelial stem cells.

The BMP type II receptor is located in lipid rafts, including caveolae, of pulmonary endothelium in vivo and in vitro

Polymorphic mutations in the Bone Morphogenetic Protein type II receptor (BMPrII) gene have been implicated in the development of familial primary pulmonary hypertension (PPH) however, the role BMPrII mutations play in the development of PH has not yet been elucidated. Endothelial caveolae are an important domain of hemodynamics containing eNOS, the serotonin transporter, and endothelin receptors. In this study we show by standard immunohistochemistry (IHC) that BMPrII is widely distributed in the vasculature of the rat lung, and more specifically distributed to both apical and basal membranes of the arteriolar endothelium by fluorescent IHC. We also examined compartmentalization of BMPrII in lipid fractions of plasma membranes isolated by silica based extraction from human pulmonary artery endothelial cells and rat lung endothelium. Density gradient centrifugation demonstrated BMPrII in separate caveolin-1 (cav-1) and non-cav-1 lipid rich fractions. Electron microscopy co-localized cav-1 and BMPrII in flask shaped membrane fragments. Three-dimensional fluorescence microscopy demonstrated BMPrII in discrete membrane foci, a portion of which were co-localized with cav-1, as well as in Golgi. Our findings indicate that BMPrII is located within lipid-dense fractions of pulmonary endothelial cell membranes with a portion present in caveolae suggesting potential dynamic regulatory structural relationships.

Smad3 signalling plays an important role in keloid pathogenesis via epithelial-mesenchymal interactions

Smad signalling plays important roles in developmental and cancer biology as well as in fibropathogenesis. Its role in keloid biology is not known. Epithelial-mesenchymal interactions, originally described in normal skin, have recently been established to play a significant role in keloid pathogenesis, and demonstrate the important influence of keratinocyte paracrine factor signalling on fibroblast behaviour. The present study investigated the role of downstream Smad cascade induction in this interaction. Normal fibroblasts (NF) and keloid fibroblasts (KF) were co-cultured in serum-free medium with normal keratinocytes (NK) or keloid keratinocytes (KK) for 5 days, after which fibroblast cell lysates were subjected to western blot and immunoprecipitation analysis to quantify the levels of Smad and Smad2/3/4 binding complex. In another set of experiments, wild-type (wt), Smad2-null (Smad2-/-) and Smad3-null (Smad3-/-) mouse embryonic fibroblasts (MEF) were assayed for cell proliferation and collagen production after serum-free co-culture with KK or exposure to conditioned media collected from serum-free KK/KF co-culture. Compared to normal skin, keloids expressed high basal levels of TGFbetaR1 and TGFbetaR2, Smad2, 3 and 4 and phospho-Smad2. Upregulation of TGFbetaR1 and TGFbetaR2, Smad3 and p-Smad2 was observed in KF co-cultured with KK, together with enhanced Smad3 phosphorylation and Smad2/3/4 binding complex production. When MEF-wt, MEF-Smad2-/- or MEF-Smad3-/- were co-cultured with KK or exposed to KK/KF co-culture conditioned media, enhanced proliferation and collagen production were seen in MEF-wt and MEF-Smad2-/- but not in MEF-Smad3-/- cells. The activation of Smad signalling, importantly that of Smad3, appears to be one facet of the complex epithelial-mesenchymal interactions in keloid pathogenesis, resulting in active KF proliferation and collagen-ECM production in co-culture with KK. This finding suggests the suppression of Smad signalling as a novel approach in keloid therapy.

Requirement of km23 for TGFβ-mediated growth inhibition and induction of fibronectin expression

We previously identified km23 as a novel TGFbeta receptor-interacting protein. Here we show that km23 is ubiquitously expressed in human tissues and that cell-type specific differences in endogenous km23 protein expression exist. In addition, we demonstrate that the phosphorylation of km23 is TGFbeta-dependent, in that EGF was unable to phosphorylate km23. Further, the kinase activity of both TGFbeta receptors appears to play a role in the TGFbeta-mediated phosphorylation of km23, although TGFbeta RII kinase activity is absolutely required for km23 phosphorylation. Blockade of km23 using small interfering RNAs significantly decreased key TGFbeta responses, including induction of fibronectin expression and inhibition of cell growth. Thus, our results demonstrate that km23 is required for TGFbeta induction of fibronectin expression and is necessary, but not sufficient, for TGFbeta-mediated growth inhibition.

Current view of the role of transforming growth factor beta 1 in skin carcinogenesis

Previously, we have shown that transforming growth factor beta 1 (TGFbeta1) overexpression in suprabasal epidermis suppresses skin carcinogenesis at early stages, but promotes tumor invasion at later stages. To elucidate the role of TGFbeta1 overexpression in naturally occurring human squamous cell carcinomas (SCC), we screened TGFbeta1 expression patterns in human skin SCC samples and found that TGFbeta1 was overexpressed with two distinct patterns: either predominantly in suprabasal layers or throughout tumor epithelia including basal proliferative cells. To determine the effect of TGFbeta1 overexpression in basal keratinocytes, we generated transgenic mice expressing wild-type TGFbeta1 in basal keratinocytes and hair follicles using the K5 promoter (K5.TGFbeta1(wt)). Surprisingly, these mice developed a severe inflammatory skin disorder. Inflammation was also observed in head and neck tissue when TGFbeta1 transgene expression was inducibly expressed in head and neck epithelia in our gene-switch-TGFbeta1 transgenic mice. Given the importance of inflammation in cancer development, our data suggest that TGFbeta1-induced inflammation may override its tumor-suppressive effect even at early stages of skin carcinogenesis. This notion is further suggested by our recent study that Smad3 knockout mice were resistant to skin chemical carcinogenesis at least in part via abrogation of endogenous TGFbeta1-induced inflammation.

SPECIFICITY AND VERSATILITY IN TGF-β SIGNALING THROUGH SMADS

The TGF-beta family comprises many structurally related differentiation factors that act through a heteromeric receptor complex at the cell surface and an intracellular signal transducing Smad complex. The receptor complex consists of two type II and two type I transmembrane serine/threonine kinases. Upon phosphorylation by the receptors, Smad complexes translocate into the nucleus, where they cooperate with sequence-specific transcription factors to regulate gene expression. The vertebrate genome encodes many ligands, fewer type II and type I receptors, and only a few Smads. In contrast to the perceived simplicity of the signal transduction mechanism with few Smads, the cellular responses to TGF-beta ligands are complex and context dependent. This raises the question of how the specificity of the ligand-induced signaling is achieved. We review the molecular basis for the specificity and versatility of signaling by the many ligands through this conceptually simple signal transduction mechanism.

Functions of the superfamily in human embryonic stem cells

The establishment of human embryonic stem (ES) cells has opened possibilities for cell replacement therapy to treat diseases such as diabetes, Parkinson's disease and cardiac myopathies. Self-renewal is one of the essential defining characteristics of stem cells. If stem cells are to have widespread therapeutic applications, it is essential to identify the extrinsic and intrinsic factors maintaining self-renewal, particularly in culture. Insight into the regulation of known self-renewal transcription factors and cross-talk between their upstream signalling pathways is important for a better understanding of how stem cell self-renewal and differentiation are related to downstream target genes. This may lead to the establishment of protocols for obtaining a large supply of ES cells. Here, we review the role that TGFbeta superfamily members are thought to play in self-renewal and differentiation of human and mouse ES cells. We focus on the prototype TGFbeta, TGFbeta1, activin A, nodal and bone morphogenetic proteins and their expression, activity and function in embryonic stem cells.

Emerging drugs for idiopathic pulmonary fibrosis

Pulmonary fibrosis is often the end stage of chronic, persistent, low-level lung injury, either of known or unknown cause. The most severe form of pulmonary fibrosis is idiopathic pulmonary fibrosis (IPF), a disease process of unknown aetiology and one that often leads to respiratory failure and death. At present there are no proven or effective drug therapies for IPF. Recent advances in understanding of disease pathogenesis have focused attention on drug targeting of fibrogenic pathways, as opposed to traditional anti-inflammatory approaches. In this report, the present status of drug development of a number of emerging antifibrotic strategies and agents that may prove more effective in the therapy of this progressive, debilitating and fatal disease are reviewed.

Adenovirus-mediated gene transfer of mutated IkappaB kinase and IkappaBalpha reveal NF-kappaB-dependent as well as NF-kappaB-independent pathways of HAS1 activation

It has become increasingly clear that hyaluronan is more than the simple matrix molecule it was once thought to be but instead takes part in a multitude of biological functions. Three genes encode for hyaluronan synthases (HAS). We demonstrated earlier that HAS2 and HAS3 are constitutively activated in type-B synoviocytes (fibroblast-like synoviocytes) and, furthermore, that the only gene that readily responds to stimulation with a series of proinflammatory cytokines is HAS1. Here we probe the involvement of the transcription factor NF-kappaB in induced and noninduced HAS activation. Transforming growth factor (TGF) beta1 as well as interleukin (IL)-1beta are both strong inducers of HAS1 transcription. Stimulation of fibroblast-like synoviocytes with IL-1beta resulted in rapid degradation of IkappaBalpha, an event that was preceded by IkappaBalpha phosphorylation. Interestingly, TGFbeta1 neither affected IkappaBalpha levels, nor did it cause phosphorylation of IkappaBalpha. In addition, TGFbeta1 had no effect on IkappaBbeta and IkappaBepsilon levels. Electrophorectic mobility shift assays demonstrate that IL-1beta is a potent inducer of NF-kappaB translocation; however, TGFbeta1 treatment did not result in shifting bands. Two adenovirus constructs were used to further clarify differences in TGFbeta1- and IL-1beta-induced HAS1 activation. Overexpressing IkappaBalpha completely abolished the IL-1beta effect on HAS1 but did not interfere with TGFbeta1-induced HAS1 mRNA accumulation. Identical results were obtained when a dominant negative IKK was overexpressed. Interestingly, neither overexpression of IkappaBalpha nor of IKK had any effect on HAS2 and HAS3 mRNA levels. Taken together, HAS1 can be activated by distinct pathways; IL-1beta utilizes NF-kappaB, and TGFbeta1 does not. Furthermore, HAS2 and HAS3 are activated without the involvement of NF-kappaB.

LIM Kinase1 controls synaptic stability downstream of the type II BMP receptor

Here, we demonstrate that the BMP receptor Wishful Thinking (Wit) is required for synapse stabilization. In the absence of BMP signaling, synapse disassembly and retraction ensue. Remarkably, downstream Smad-mediated signaling cannot fully account for the stabilizing activity of the BMP receptor. We identify LIM Kinase1 (DLIMK1)-dependent signaling as a second, parallel pathway that confers the added synapse-stabilizing activity of the BMP receptor. We show that DLIMK1 binds a region of the Wit receptor that is necessary for synaptic stability but is dispensable for Smad-mediated synaptic growth. A genetic analysis demonstrates that DLIMK1 is necessary, presynaptically, for synapse stabilization, but is not necessary for normal synaptic growth or function. Furthermore, presynaptic expression of DLIMK1 in a wit or mad mutant significantly rescues synaptic stability, growth, and function. DLIMK1 localizes near synaptic microtubules and functions independently of ADF/cofilin, highlighting a novel requirement for DLIMK1 during synapse stabilization rather than actin-dependent axon outgrowth.

Smad4 dependency defines two classes of transforming growth factor {beta} (TGF-{beta}) target genes and distinguishes TGF-{beta}-induced epithelial-mesenchymal transition from its antiproliferative and migratory responses

In response to transforming growth factor beta (TGF-beta), Smad4 forms complexes with activated Smad2 and Smad3, which accumulate in the nucleus, where they both positively and negatively regulate TGF-beta target genes. Mutation or deletion of Smad4 is found in about 50% of pancreatic tumors and in about 15% of colorectal tumors. As Smad4 is a central component of the TGF-beta/Smad pathway, we have determined whether Smad4 is absolutely required for all TGF-beta responses, to evaluate the effect of its loss during human tumor development. We have generated cell lines from the immortalized human keratinocyte cell line HaCaT or the pancreatic tumor cell line Colo-357, which stably express a tetracyline-inducible small interfering RNA targeted against Smad4. In response to tetracycline, Smad4 expression is effectively silenced. Large-scale microarray analysis identifies two populations of TGF-beta target genes that are distinguished by their dependency on Smad4. Some genes absolutely require Smad4 for their regulation, while others do not. Functional analysis also indicates a differential Smad4 requirement for TGF-beta-induced functions; TGF-beta-induced cell cycle arrest and migration, but not epithelial-mesenchymal transition, are abolished after silencing of Smad4. Altogether our results suggest that loss of Smad4 might promote TGF-beta-mediated tumorigenesis by abolishing tumor-suppressive functions of TGF-beta while maintaining some tumor-promoting TGF-beta responses.

Smad4 dependency defines two classes of transforming growth factor {beta} (TGF-{beta}) target genes and distinguishes TGF-{beta}-induced epithelial-mesenchymal transition from its antiproliferative and migratory responses

In response to transforming growth factor beta (TGF-beta), Smad4 forms complexes with activated Smad2 and Smad3, which accumulate in the nucleus, where they both positively and negatively regulate TGF-beta target genes. Mutation or deletion of Smad4 is found in about 50% of pancreatic tumors and in about 15% of colorectal tumors. As Smad4 is a central component of the TGF-beta/Smad pathway, we have determined whether Smad4 is absolutely required for all TGF-beta responses, to evaluate the effect of its loss during human tumor development. We have generated cell lines from the immortalized human keratinocyte cell line HaCaT or the pancreatic tumor cell line Colo-357, which stably express a tetracyline-inducible small interfering RNA targeted against Smad4. In response to tetracycline, Smad4 expression is effectively silenced. Large-scale microarray analysis identifies two populations of TGF-beta target genes that are distinguished by their dependency on Smad4. Some genes absolutely require Smad4 for their regulation, while others do not. Functional analysis also indicates a differential Smad4 requirement for TGF-beta-induced functions; TGF-beta-induced cell cycle arrest and migration, but not epithelial-mesenchymal transition, are abolished after silencing of Smad4. Altogether our results suggest that loss of Smad4 might promote TGF-beta-mediated tumorigenesis by abolishing tumor-suppressive functions of TGF-beta while maintaining some tumor-promoting TGF-beta responses.

Morphogens and synaptogenesis in Drosophila

During development and adult life synapses are remodeled in response to genetic programs and environmental cues. This synaptic plasticity is thought to be the basis of learning and memory. The larval neuromuscular junction of Drosophila is established during embryogenesis and grows during larval development to accommodate muscle growth and maintain synaptic homeostasis. This growth is dependent on bidirectional communication between the motoneuron and the muscle fiber. The best-characterized retrograde signaling pathway is defined by Glass bottom boat (Gbb), a morphogen of the transforming growth factor-beta (TGF-beta) superfamily. Gbb acts as a muscle-derived retrograde signal that activates the TGF-beta pathway presynaptically. This pathway includes the type II receptor Wishful thinking, type I receptors Thick veins and Saxophone, and the second messenger Smads Mothers against dpp (Mad) and Medea. Mutations that block this pathway result in small synapses that are morphologically aberrant and severely impaired functionally. An emerging anterograde signaling pathway is defined by Wingless, a morphogen of the Wnt family that acts as a motoneuron-derived anterograde signal required for both pre- and postsynaptic development. In the absence of Wingless the neuronal microtubule cytoskeleton regulator Futsch is down-regulated and synaptic growth impaired. Some of these morphogens have conserved roles in mammalian synaptogenesis, and genetic analysis suggests that additional signaling molecules are required for synaptic growth at the Drosophila neuromuscular junction.

Parathyroid hormone stimulation and PKA signaling of latent transforming growth factor-beta binding protein-1 (LTBP-1) mRNA expression in osteoblastic cells

Parathyroid hormone (PTH) regulates bone remodeling and calcium homeostasis by acting on osteoblasts. Recently, the gene expression profile changes in the rat PTH (1-34, 10(-8)M)-treated rat osteoblastic osteosarcoma cell line, UMR 106-01, using DNA microarray analysis showed that mRNA for LTBP-1, a latent transforming growth factor (TGF-beta)-binding protein is stimulated by PTH. Latent TGF-beta binding proteins (LTBPs) are required for the proper folding and secretion of TGF-beta, thus modifying the activity of TGF-beta, which is a local factor necessary for bone remodeling. We show here by real time RT-PCR that PTH-stimulated LTBP-1 mRNA expression in rat and mouse preosteoblastic cells. PTH also stimulated LTBP-1 mRNA expression in all stages of rat primary osteoblastic cells but extended expression was found in differentiating osteoblasts. PTH also stimulated TGF-beta1 mRNA expression in rat primary osteoblastic cells, indicating a link between systemic and local factors for intracellular signaling in osteoblasts. An additive effect on LTBP-1 mRNA expression was found when UMR 106-01 cells were treated with PTH and TGF-beta1 together. We further examined the signaling pathways responsible for PTH-stimulated LTBP-1 and TGF-beta1 mRNA expression in UMR 106-01 cells. The PTH stimulation of LTBP-1 and TGF-beta1 mRNA expression was dependent on the PKA and the MAPK (MEK and p38 MAPK) pathways, respectively in these cells, suggesting that PTH mediates its effects on osteoblasts by several intracellular signaling pathways. Overall, we demonstrate here that PTH stimulates LTBP-1 mRNA expression in osteoblastic cells and this is PKA-dependent. This event may be important for PTH action via TGF-beta in bone remodeling.

Induction of human LTBP-3 promoter activity by TGF-beta1 is mediated by Smad3/4 and AP-1 binding elements

Latent TGF-beta binding proteins (LTBPs) are extracellular matrix glycoproteins, which are essential for the targeting and activation of TGF-betas. LTBP-3 regulates the bioavailability of TGF-beta especially in the bone. To understand the regulation of LTBP-3 expression, we have isolated and characterized the promoter region of human LTBP-3 gene. The GC-rich TATA-less promoter contained several transcription initiation sites and putative binding sites for multiple sequence specific transcription factors including Sp1, AP-1, c-Ets, MZF-1, Runx1 and members of the GATA-family. Reporter gene analyses of the promoter indicated that it was more active in MG-63 than in Saos-2 osteosarcoma cells, suggesting that it is regulated as the endogenous gene. TGF-beta1 stimulated the transcriptional activity of LTBP-3 promoter in MG-63 cells, while certain other bone-derived growth factors and hormones were ineffective. TGF-beta1 increased LTBP-3 mRNA levels accordingly. Analyses of deletion constructs of the promoter and mutational deletion of specific transcription factor binding sites indicated that Smad3/4 and AP-1 binding sites mediated the TGF-beta1 response. The involvement of AP-1 activity was further indicated by decreased TGF-beta responsiveness of the LTBP-3 promoter in the presence of a MEK/Erk signaling pathway inhibitor. Our results suggest an important new role for TGF-beta1 in the regulation of its binding protein, LTBP-3.

Insulin-like growth factor binding protein (IGFBP)-3 and IGFBP-5 mediate TGF-beta- and myostatin-induced suppression of proliferation in porcine embryonic myogenic cell cultures

We have previously shown that cultured porcine embryonic myogenic cells (PEMC) produce both insulin-like growth factor binding protein (IGFBP)-3 and IGFBP-5 and secrete these proteins into their media. Exogenously added recombinant porcine (rp) IGFBP-3 and rpIGFBP-5 act via IGF-dependent and IGF-independent mechanisms to suppress proliferation of PEMC cultures. Furthermore, immunoneutralization of endogenous IGFBP-3 and IGFBP-5 in the PEMC culture medium results in increased DNA synthesis rate suggesting that endogenous IGFBP-3 and IGFBP-5 suppress PEMC proliferation. TGF-beta superfamily members myostatin and TGF-beta1 have also been shown to suppress proliferation of myogenic cells, and treatment of cultured PEMC with either TGF-beta1 or myostatin significantly (P < 0.01) increases levels of IGFBP-3 and -5 mRNA. We have previously shown that immunoneutralization of IGFBP-3 decreases the proliferation-suppressing activity of TGF-beta1 and myostatin. Here, we show that immunoneutralization of IGFBP-5 also significantly (P < 0.05) decreases the DNA synthesis-suppressing activity of these molecules. Simultaneous immunoneutralization of both IGFBP-3 and IGFBP-5 in TGF-beta1 or myostatin-treated PEMC cultures restores Long-R3-IGF-I-stimulated DNA synthesis rates to 90% of the levels observed in control cultures receiving no TGF-beta1 or myostatin treatment (P < 0.05). Even though immunoneutralization of IGFBP-3 and -5 increased DNA synthesis rates in TGF-beta1 or myostatin-treated PEMC cultures, phosphosmad2 levels in these cultures were not affected. These findings strongly suggest that IGFBP-3 and IGFBP-5 affect processes downstream from receptor-mediated Smad phosphorylation that facilitate the ability of TGF-beta and myostatin to suppress proliferation of PEMC.

TGF-β Concentration Specifies Differential Signaling Profiles of Growth Arrest/Differentiation and Apoptosis in Podocytes

Podocyte depletion occurs in most progressive glomerular diseases and is thought to result from podocyte loss while the remaining podocytes are unable to proliferate. The underlying mechanisms for podocyte growth arrest/differentiation and depletion remain poorly understood but may involve TGF-beta, which is typically upregulated in injured glomeruli. The TGF-beta are multifunctional cytokines that regulate growth, differentiation, and apoptosis in most cells. Determinants of functional specificity of TGF-beta signaling in cell-cycle control and apoptosis remain poorly understood. Using a unique system of conditionally immortalized podocytes, it is demonstrated that autocrine TGF-beta2 induces G0/G1 arrest and differentiation under nonpermissive culture through Smad3-dependent induction of the cyclin-dependent kinase inhibitor p15(Ink4b) (Cdkn2b). When exposed to recombinant TGF-beta1 (or TGF-beta2), nonpermissive culture podocytes switch to G2/M arrest and apoptosis, selectively at advanced TGF-beta concentrations and specifically in association with suppression of Cdkn2b and activation of proapoptotic p38 mitogen-activated protein kinase. Thus, distinct signaling profiles activated in a concentration-dependent manner by TGF-beta were identified. Autocrine TGF-beta2/Smad3/Cdkn2b signaling in podocytes specifies G0/G1 arrest associated with podocyte differentiation, whereas increasing TGF-beta concentrations beyond a critical threshold induces G2/M block and apoptosis associated with selective p38 mitogen-activated protein kinase activation and with suppression of Cdkn2b. In summary, the results suggest a new functional requirement of TGF-beta2 in growth arrest and differentiation of murine podocytes in vitro and demonstrate that a critical TGF-beta concentration threshold may specify a molecular switch to proapoptotic signaling profiles and apoptosis.

Role of Smad3 in the regulation of rat telomerase reverse transcriptase by TGFβ

Telomerase is induced in certain pathological conditions such as cancer and tissue injury and repair. This induction in fibroblasts from injured lung is repressed by transforming growth factor beta (TGFbeta) via yet unknown mechanisms. In this study, the role of Smad3 in the inhibition of telomerase reverse transcriptase (TERT) gene transcription by TGFbeta was investigated. The rat TERT (rTERT) gene promoter was cloned by PCR amplification and fused with a luciferase reporter gene. This construct was used to analyse regulation of promoter activity in fibroblasts isolated from bleomycin-injured lung with induced telomerase activity. The results showed that TGFbeta inhibited rTERT transcription while stimulating Smad3 expression. Interestingly, TGFbeta also inhibited the expression of c-myc. Cotransfection with a Smad3 expressing plasmid further repressed rTERT transcription and c-myc expression, while cotransfection with the corresponding antisense Smad3 construct had the opposite effect. Mutation of an E-box in the rTERT promoter suppressed its activity, which could be further reduced by TGFbeta treatment. In contrast, mutation at a Smad binding element enhanced promoter activity whose inhibition was impaired by TGFbeta treatment. Thus TGFbeta inhibition of rTERT gene expression was directly mediated by Smad3 via the Smad binding element, while c-myc appears to primarily regulate its constitutive or induced expression.

Dpp-responsive Silencers Are Bound by a Trimeric Mad-Medea Complex

Transcriptional regulation by transforming growth factor-beta signaling is mediated by the Smad family of transcription factors. It is generally accepted that Smads must interact with other transcription factors to bind to their targets. However, recently it has been shown that a complex of the Drosophila Smad proteins, Mad and Medea, binds with high affinity to silencer elements that repress brinker and bag of marbles in response to Dpp signaling. Here we report that these silencers are bound by a heterotrimer containing two Mad subunits and one Medea subunit. We found that the MH1 domains of all three subunits contributed directly to sequence-specific DNA contact, thus accounting for the exceptionally high stability of the Smad-silencer complex. The Medea MH1 domain binds to a canonical Smad box (GTCT), whereas the Mad MH1 domains bind to a GC-rich sequence resembling Mad binding sites previously identified in Dpp-responsive enhancer elements. The consensus for this sequence, GRCGNC, differs from that of the canonical Smad box, but we found that Mad binding nonetheless required the same beta-hairpin amino acids that mediate base-specific contact with GTCT. Binding was also affected by alanine substitutions in Mad and Med at a subset of basic residues within and flanking helix 2, indicating a contribution to binding of the GRCGNC and GTCT sites. The slight alteration of the Dpp silencers caused them to activate transcription in response to Dpp signaling, indicating that the potential for Smad complexes to recognize specific targets need not be limited to repression.

Transforming growth factor-beta1 to the bone

TGF-beta1 is a ubiquitous growth factor that is implicated in the control of proliferation, migration, differentiation, and survival of many different cell types. It influences such diverse processes as embryogenesis, angiogenesis, inflammation, and wound healing. In skeletal tissue, TGF-beta1 plays a major role in development and maintenance, affecting both cartilage and bone metabolism, the latter being the subject of this review. Because it affects both cells of the osteoblast and osteoclast lineage, TGF-beta1 is one of the most important factors in the bone environment, helping to retain the balance between the dynamic processes of bone resorption and bone formation. Many seemingly contradictory reports have been published on the exact functioning of TGF-beta1 in the bone milieu. This review provides an overall picture of the bone-specific actions of TGF-beta1 and reconciles experimental discrepancies that have been reported for this multifunctional cytokine.

Smad7 induces G0/G1 cell cycle arrest in mesenchymal cells by inhibiting the expression of G1 cyclins

The major Smad pathways serve in regulating the expression of genes downstream of TGFbeta signals. In this study, we examined the effects of sustained Smad7 expression in cultured cells. Interestingly, Smad7 caused various mesenchymal cells, including NIH3T3 fibroblast and ST2 bone-marrow stromal cells, to undergo a marked morphological alteration into a flattened cell shape, but kept them alive for as long as 60 days. Furthermore, Smad7 arrested the proliferation of the cells even before they reached confluence. These cells became quiescent in G0/G1 phase and accumulated a hypophosphorylated form of retinoblastoma. The cytostatic effect of Smad7 was closely associated with a preceding decrease in the levels of G1 cyclins, such as cyclin D1 and cyclin E. Accordingly, ectopic cyclin E was able to overcome the Smad7-induced arrest of proliferation. These results indicate that Smad7 functions upstream of G1 cyclins and suggest a novel role for Smad7 as an antiproliferative factor. In contrast to the growth of mesenchymal cells, that of epithelial cells was little susceptible to Smad7. The present findings raise the possibility that a link between Smad7 and the G1 to S phase transition may also contribute to the cell cycle control by certain Smad7-inducing stimuli in a cell-type-dependent fashion.

Activated type I TGFbeta receptor kinase enhances the survival of mammary epithelial cells and accelerates tumor progression

We have examined the effects of transforming growth factor-beta (TGFbeta) signaling on mammary epithelial cell survival. Transgenic mice expressing an active mutant of Alk5 in the mammary gland (MMTV-Alk5(T204D)) exhibited reduced apoptosis in terminal endbuds and during postlactational involution. Transgene-expressing mammary cells contained lower Smad2/3 and higher c-myc levels than controls, high ligand-independent phosphatidylinositol-3 kinase (PI3K) and Akt activities, and were insensitive to TGFbeta-mediated growth arrest. Treatment with a proteasome inhibitor increased Smad2/3 levels and ligand-independent Smad transcriptional reporter activity, as well as reduced both c-myc protein and basal cell proliferation. Treatment with an Alk5 kinase small-molecule inhibitor upregulated Smad2/3 levels, reduced PI3K activity, P-Akt, and c-myc, and inhibited cell survival. Although Alk5(T204D)-expressing mice did not develop mammary tumors, bigenic MMTV-Alk(T204D) x Neu mice developed cancers that were more metastatic than those occurring in MMTV-Neu transgenics. These data suggest that (1) TGFbeta can signal to PI3K/Akt and enhance mammary epithelial cell survival in vivo before cytological or histological evidence of transformation, and (2) TGFbeta signaling can provide epithelial cells with a 'gain-of-function' effect that synergizes with oncogene-induced transformation.

SKI pathways inducing progression of human melanoma

The proteins SKI and SnoN are implicated in processes as diverse as differentiation, transformation and tumor progression. Until recently, SKI was solely viewed as a nuclear protein with a principal function of inhibiting TGF-beta signaling through its association with the Smad proteins. However, new studies suggest that SKI plays additional roles not only inside but also outside the nucleus. In normal melanocytes and primary non-invasive melanomas, SKI localizes predominantly in the nucleus, whereas in primary invasive melanomas SKI displays both nuclear and cytoplasmic localization. Intriguingly, metastatic melanoma tumors display nuclear and cytoplasmic or predominantly cytoplasmic SKI distribution. Cytoplasmic SKI is functional, as it associates with Smad3 and prevents its nuclear localization mediated by TGF-beta. SKI can also function as a transcriptional activator, targeting the beta -catenin pathway and activating MITF and NrCAM, two proteins involved in survival, migration and invasion. Intriguingly, SKI appears to live a dual life, one as a tumor suppressor and another as a transforming protein. Loss of one copy of mouse ski increases susceptibility to tumorigenesis in mice, whereas its overexpression is associated with cancer progression of human melanoma, esophageal, breast and colon. The molecular reasons for such dramatic change in SKI function appear to result from new acquired activities. In this review, we discuss the mechanisms by which SKI regulates crucial pathways involved in the progression of human malignant melanoma.

TGF beta-induced focal complex formation in epithelial cells is mediated by activated ERK and JNK MAP kinases and is independent of Smad4

Advanced malignancies often exhibit increased concentrations of transforming growth factor-beta (TGF beta), which has been suggested to promote invasion and metastasis. While inhibition of epithelial cell proliferation in response to TGF beta is mainly mediated by the well-characterised Smad pathway, the molecular mechanism leading to TGF beta-induced invasiveness and metastasis are largely unknown. To elucidate these mechanisms, we compared TGF beta1 signalling in MCF-7 and the Smad4-negative MDA-MB-468 breast cancer cells. Both cell lines react to TGF beta1 treatment with decreased subcortical actin and increased numbers of focal contacts. TGF beta1-induced cell migration was strongly dependent on the activation of extracellular signal-regulated kinase (ERK) and Jun N-terminal kinase (JNK). These mitogen-activated protein kinases were phosphorylated in response to TGF beta and subsequently translocated into focal contacts. Inhibition of the TGF beta type I receptor ALK5 slightly reduced phosphorylation of ERK in MCF-7 cells, but neither inhibited phosphorylation of ERK in MDA-MB-468 cells nor TGF beta1-induced migration of both cell lines. In contrast, ALK5 inhibition effectively blocked Smad2 phosphorylation. In addition to ERK and JNK, the monomeric GTPase RhoA was activated by TGF beta1 and necessary for TGF beta-induced migration. Taken together, our study identifies a role of ERK and JNK activation and association of activated MAPKs with focal complexes in TGF beta1-induced cell migration in epithelial cells. These TGF beta-dependent processes were mediated independently of Smad4.

Activation of protein kinase C alpha is required for TPA-triggered ERK (MAPK) signaling and growth inhibition of human hepatoma cell HepG2

The signaling mechanisms for most of the antiproliferative processes are not fully understood. We have demonstrated that ERK(MAPK) signaling was involved in the induction of both p15(INK4b)and p16(INK4a) CDK inhibitors and growth inhibition of hepatoma cell HepG2 triggered by the tumor promoter tetradecanoyl phorbol acetate (TPA). In this study, the upstream signal mechanism for TPA-induced ERK(MAPK) activation was investigated. In HepG2 cells only one of the cPKC isozymes, PKCalpha, but not cPKCbetaII, nPKCepsilon or aPKCzeta was activated by TPA as demonstrated by its membrane translocation within 10-30 min and down-regulation at 24 h after TPA treatment. Pretreatment of 0.2-2.0 microM Bisindolylmaleimides, an inhibitor of PKC, attenuated the TPA-induced phosphorylation of ERK, gene expressions of p15(INK4b) and p16(INK4a), and growth inhibition of HepG2 cell in a dose-dependent manner. Consistently, transfection of HepG2 with 1.0-3.0 microM antisense (AS) PKCalpha, but not (AS) PKCbetaII, or nPKCepsilon oligonucleotides (ODN), for 36 h prior to TPA treatment also prevented the TPA-induced molecular and cellular effects described above. Taken together, we concluded that PKCalpha is specifically required for TPA-induced ERK(MAPK) signaling to trigger gene expressions of p15(INK4b) and p16(INK4a) leading to HepG2 growth inhibition.

Autoantibodies to Fibrillin-1 Activate Normal Human Fibroblasts in Culture through the TGF-β Pathway to Recapitulate the “Scleroderma Phenotype”

Fibroblasts from patients with systemic sclerosis (SSc) are activated producing excessive amounts of extracellular matrix (ECM) components. Recently, we identified a new SSc-specific autoantibody against portions of fibrillin-1, a major component of ECM microfibrils and regulator of TGF-beta1 signaling. To examine a potential pathogenic role of anti-fibrillin-1 autoantibodies, normal human fibroblasts were treated with affinity-purified autoantibodies isolated from SSc sera and then examined for alterations in gene and protein expression levels using microarrays, quantitative RT-PCR, immunoblots, and immunofluorescence. Compared with fibroblasts cultured in normal medium or in medium containing normal human IgG, anti-fibrillin-1 autoantibody-treated normal dermal fibroblasts showed increased expression of COL and several other ECM components characteristically overexpressed in SSc fibroblasts. This was accompanied by phosphorylation and nuclear translocation of Smad3. Neutralization of TGF-beta1 with anti-TGF-beta1 Abs significantly diminished the activation of fibroblasts by anti-fibrillin-1 autoantibodies. These data indicate that anti-fibrillin-1 autoantibodies can induce the activation of normal dermal fibroblasts into a profibrotic phenotype resembling that of SSc by potentially causing the release of sequestered TGF-beta1 from fibrillin-1-containing microfibrils in the ECM.