Targeted disruption of Smad3 reveals an essential role in transforming growth factor beta-mediated signal transduction

Fusion proteins of retinoid receptors antagonize TGF-beta-induced growth inhibition of lung epithelial cells

Transforming growth factor-beta1 (TGF-beta) is a growth factor that has multiple functions including potent inhibition of cell growth. TGF-beta signals by binding to its cell surface serine/threonine kinase receptors, which in turn phosphorylate downstream signal transducers, Smad2 and Smad3. Phosphorylated Smad2 and Smad3, together with Smad4, enter the nucleus and associate with various transcription factors. This complex of transcription factors regulates transcription of a diverse group of genes, leading to growth arrest at G1 phase. Through a functional expression cloning approach, a gag-retinoid X receptor beta (gag-RXRbeta) fusion protein was found to antagonize TGF-beta-induced growth inhibition of mink lung epithelial cells and the fusion between gag and RXRbeta is essential for resistance to the growth inhibition. Like gag-RXRbeta, the oncogenic PLZF-RARalpha fusion protein also antagonizes TGF-beta-induced growth inhibition, and the fusion between PLZF and RARalpha is essential for resistance to TGF-beta. Moreover, TGF-beta and retinoic acid (RA) cooperatively induce growth inhibition as well as transcription of the p15(ink4b) gene, while PLZF-RARalpha represses TGF-beta-induced expression of the p15(ink4b) gene. Together, these results suggest that the TGF-beta and RA pathways cooperate to inhibit cell growth and that PLZF-RARalpha -mediated resistance to TGF-beta may facilitate the development of the PLZF-RARalpha-induced leukemia.

Reconstitution of Smad3 restores TGF-beta response of tissue inhibitor of metalloprotease-1 upregulation in human choriocarcinoma cells

Extravillous trophoblast (EVT) cells of the human placenta proliferate, migrate, and invade the pregnant uterus and its vasculature in order to nourish the fetus. However, the normal EVT cell proliferation, migration, and invasiveness are exquisitely controlled in situ by decidua-produced transforming growth factor-beta (TGF-beta), whereas EVT cancer (choriocarcinoma) cells are TGF-beta-resistant. We found that these cells lack in expression of Smad3, a key transcription factor involved in TGF-beta signaling pathway. To test whether Smad3 restitution restores TGF-beta response in choriocarcinoma cells, we produced a Smad3-expressing cell line (JAR-smad3/c). Since anti-invasive effect of TGF-beta in the normal EVT cells was partly mediated by an upregulation of tissue inhibitor of metalloprotease (TIMP)-1, we examined whether Smad3-restituted JAR cells have restored TGF-beta response of TIMP-1 upregulation. The expression of TIMP-1 mRNA was found to be low in JAR and JAR-smad3/c cells. Moreover, the basal level of secreted TIMP-1 protein was very low in these cells as compared to the normal EVT cells. TGF-beta1 upregulated TIMP-1 mRNA and secreted protein in Smad3-restituted JAR cells as well as in the normal EVT cells, whereas no effect was detected in Smad3-deficient (wild-type) JAR cells. We had earlier shown that Smad3-restituted JAR cells had also restored TGF-beta response of plasminogen activator inhibitor-1 upregulation. However, in vitro functional analysis revealed that, in contrast to the normal EVT cells, anti-invasive action of TGF-beta was not restored in Smad3-restituted JAR cells. Thus, additional factors (possibly low expression of Smad4 and/or other unknown factors) may contribute to refractoriness to anti-invasive action of TGF-beta in JAR cells.

Menin interacting proteins as clues toward the understanding of multiple endocrine neoplasia type 1

Multiple endocrine neoplasia type 1 (MEN1) is a familial cancer syndrome characterized mostly by tumors of the parathyroids, pancreas and anterior pituitary. The gene responsible, MEN1, encodes Menin, a 610 aminoacid nuclear protein with no sequence homology to other proteins. Although a mouse knock-out model is available, the function of Menin is still elusive. Proteins of known function are shown to interact with Menin: JunD, nuclear factor-KappaB, Smad3, Pem, Nm23H1, glial fibrillary acidic protein, Vimentin, and probably P53. Their partnership with Menin may correspond to a regulation of their activity, but their relevance to the various traits of MEN1 pathogenicity is not established. This raises fundamental issues on the regulation pathways implicated in this complex endocrine disease.

Two major Smad pathways in TGF-beta superfamily signalling

Members of the transforming growth factor-beta (TGF-beta) superfamily bind to two different serine/threonine kinase receptors, i.e. type I and type II receptors. Upon ligand binding, type I receptors specifically activate intracellular Smad proteins. R-Smads are direct substrates of type I receptors; Smads 2 and 3 are specifically activated by activin/nodal and TGF-beta type I receptors, whereas Smads 1, 5 and 8 are activated by BMP type I receptors. Nearly 30 proteins have been identified as members of the TGF-beta superfamily in mammals, and can be classified based on whether they activate activin/TGF-beta-specific R-Smads (AR-Smads) or BMP-specific R-Smads (BR-Smads). R-Smads form complexes with Co-Smads and translocate into the nucleus, where they regulate the transcription of target genes. AR-Smads bind to various proteins, including transcription factors and transcriptional co-activators or co-repressors, whereas BR-Smads interact with other proteins less efficiently than AR-Smads. Id proteins are induced by BR-Smads, and play important roles in exhibiting some biological effects of BMPs. Understanding the mechanisms of TGF-beta superfamily signalling is thus important for the development of new ways to treat various clinical diseases in which TGF-beta superfamily signalling is involved.

Modulation of growth factor/cytokine synthesis and signaling by 1alpha,25-dihydroxyvitamin D(3): implications in cell growth and differentiation

Distinct from its classic functions in the regulation of calcium and phosphorus metabolism as a systemic hormone, 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] is involved in the local control and regulation of cellular growth and differentiation in various tissues, including epidermis (keratinocytes) and bone (osteoblasts and osteoclasts). In this review, the impact of 1alpha,25(OH)(2)D(3) on growth factor/cytokine synthesis and signaling is discussed, particularly as it pertains to bone cells and keratinocytes. 1alpha,25(OH)(2)D(3) not only regulates growth factor/cytokine synthesis but may also alter growth factor signaling. Recently discovered examples for such interactions are the interactions between the vitamin D receptor and the mothers against decapentaplegic-related proteins that function downstream of TGFbeta receptors. Inhibitory effects of 1alpha,25(OH)(2)D(3) on keratinocytes through TGFbeta activation and IL-1alpha, IL-6, and IL-8 suppression may provide a rationale for its beneficial effects in the treatment of hyperproliferative skin disorders, whereas stimulatory effects through the epidermal growth factor-related family members and platelet-derived growth factor may be operative in its beneficial effects in skin atrophy and wound healing. Modulation of cytokines and growth factors by 1alpha,25(OH)(2)D(3) during bone remodeling plays an important role in the coupling of osteoblastic bone formation with osteoclastic resorption to maintain bone mass.

Genetic analysis of the mammalian transforming growth factor-beta superfamily

Members of the TGF-beta superfamily, which includes TGF-betas, growth differentiation factors, bone morphogenetic proteins, activins, inhibins, and glial cell line-derived neurotrophic factor, are synthesized as prepropeptide precursors and then processed and secreted as homodimers or heterodimers. Most ligands of the family signal through transmembrane serine/threonine kinase receptors and SMAD proteins to regulate cellular functions. Many studies have reported the characterization of knockout and knock-in transgenic mice as well as humans or other mammals with naturally occurring genetic mutations in superfamily members or their regulatory proteins. These investigations have revealed that TGF-beta superfamily ligands, receptors, SMADs, and upstream and downstream regulators function in diverse developmental and physiological pathways. This review attempts to collate and integrate the extensive body of in vivo mammalian studies produced over the last decade.

Smad4 overexpression causes germ cell ablation and leydig cell hyperplasia in transgenic mice

Members of the transforming growth factor-beta (TGF-beta) superfamily play a variety of important roles in testicular development and function. The tumor suppressor gene, Smad4, is a common mediator of TGF-beta, activin, and bone morphogenetic protein-mediated signaling pathways. To investigate the role of the Smad4 gene during testicular development and function, transgenic mice were generated using a Flag-tagged Smad4 gene driven by 180-bp fragment of the Mullerian inhibiting substance upstream promoter sequence. Three Smad4 transgenic founders (A, B, and G) were detected by Southern blot analysis; line B showed the highest expression of the Smad4 transgene and was further studied. The fertility in F1 generation (B) and F2 generation (BB) of the Smad4 transgenic mice was not impaired. However, in the F3 generation (B2x) all animals were impacted by the overexpression of the Smad4 transgene and two kinds of phenotypes were observed. In one group animals were completely infertile, while in the other group animals were fertile and sired the normal number of pups/litter. These groups are designated as infertile and fertile in the text. Histological evaluation of the testes from the infertile group showed variable degrees of Leydig cell hyperplasia, apoptosis of germ cells, spermatogenic arrest, seminiferous tubule degeneration, and infertility. In the fertile group, there was no apparent change in the histology of the testis except for a slight increase in the number of Leydig cells. Serum follicle-stimulating hormone levels in the adult animals of both groups of Smad4 transgenic male mice were not significantly different from normal littermates; however, testosterone levels in both groups were significantly (P < 0.05) increased. These results suggest that overexpression of Smad4 leads to testicular abnormalities and infertility supporting the hypothesis that the TGF-beta signaling pathways are carefully orchestrated during testicular development. In the absence of normal levels of Smad4 testicular function is compromised.

Disruption of the gene encoding the latent transforming growth factor-beta binding protein 4 (LTBP-4) causes abnormal lung development, cardiomyopathy, and colorectal cancer

Transforming growth factor-betas (TGF-betas) are multifunctional growth factors that are secreted as inactive (latent) precursors in large protein complexes. These complexes include the latency-associated propeptide (LAP) and a latent transforming growth factor-beta binding protein (LTBP). Four isoforms of LTBPs (LTBP-1-LTBP-4) have been cloned and are believed to be structural components of connective tissue microfibrils and local regulators of TGF-beta tissue deposition and signaling. By using a gene trap strategy that selects for integrations into genes induced transiently during early mouse development, we have disrupted the mouse homolog of the human LTBP-4 gene. Mice homozygous for the disrupted allele develop severe pulmonary emphysema, cardiomyopathy, and colorectal cancer. These highly tissue-specific abnormalities are associated with profound defects in the elastic fiber structure and with a reduced deposition of TGF-beta in the extracellular space. As a consequence, epithelial cells have reduced levels of phosphorylated Smad2 proteins, overexpress c-myc, and undergo uncontrolled proliferation. This phenotype supports the predicted dual role of LTBP-4 as a structural component of the extracellular matrix and as a local regulator of TGF-beta tissue deposition and signaling.

The androgen receptor CAG repeat: a modifier of carcinogenesis?

The first exon of the human androgen receptor (AR) contains a translated CAG (poly-glutamine) repeat. The repeat length is polymorphic in the normal population ranging from 8 to 35 repeats. Expansions to over 40 repeats lead to spinal bulbar muscular atrophy (SBMA), a late onset neurodegenerative disease. The repeat is located between the two parts of a bipartite amino-terminal transactivation function and the repeat length, also within in the normal range, is inversely correlated to the transactivation power of the receptor. P160 type co-activators bind more strongly to shorter repeats. A correlation between AR CAG repeat length and total risk, age at diagnosis, recurrence after surgery and aggressive growth has been reported for tumors of classical androgen target tissues. In the prostate, where androgens exert a mitogenic effect, the cancer risk increases with decreasing AR-CAG repeat length. In contrast, in the breast, where the hormone probably acts as anti-mitogen, a higher risk and earlier onset of breast cancer has been reported for carriers of BRCA1 mutations who also have long CAG repeats in the receptor gene. Somatic alterations during carcinogenesis appear to be frequent in endometrial and in colon cancer. In the endometrium the AR CAG repeat prevalently undergoes expansions consistent with the putative protective function of androgens in this tissue. Frequent repeat reductions during colon carcinogenesis would be consistent with a mitogenic effect of androgens. Analysis of AR protein expression by Western blot reveals expression of the AR in healthy and neoplastic colon tissues. Normal mucosa of the colon expresses both AR-isoforms of 110 and 87 kDa, while the tumor samples have lost the expression of the 110-kDa isoform. The 87-kDa isoform is devoid of the amino-terminal portion of the receptor molecule that also contains the poly-glutamine tract. The temporal and causal relation between isoform switch and somatic repeat reductions during colon carcinogenesis is as yet unclear, but the two events could both enhance p160 mediated androgen signaling. The recent finding that smad3 interacts with the AR in a way similar to p160 links the AR to TGFbeta signaling. Interruption of this signaling pathway is a frequent event in colon carcinogenesis.

CD4(+)CD25(+) regulatory T cells can mediate suppressor function in the absence of transforming growth factor beta1 production and responsiveness

CD4(+)CD25(+) regulatory T cells inhibit organ-specific autoimmune diseases induced by CD4(+)CD25(-) T cells and are potent suppressors of T cell activation in vitro. Their mechanism of suppression remains unknown, but most in vitro studies suggest that it is cell contact-dependent and cytokine independent. The role of TGF-beta1 in CD4(+)CD25(+) suppressor function remains unclear. While most studies have failed to reverse suppression with anti-transforming growth factor (TGF)-beta1 in vitro, one recent study has reported that CD4(+)CD25(+) T cells express cell surface TGF-beta1 and that suppression can be completely abrogated by high concentrations of anti-TGF-beta suggesting that cell-associated TGF-beta1 was the primary effector of CD4(+)CD25(+)-mediated suppression. Here, we have reevaluated the role of TGF-beta1 in CD4(+)CD25(+)-mediated suppression. Neutralization of TGF-beta1 with either monoclonal antibody (mAb) or soluble TGF-betaRII-Fc did not reverse in vitro suppression mediated by resting or activated CD4(+)CD25(+) T cells. Responder T cells from Smad3(-/-) or dominant-negative TGF-beta type RII transgenic (DNRIITg) mice, that are both unresponsive to TGF-beta1-induced growth arrest, were as susceptible to CD4(+)CD25(+)-mediated suppression as T cells from wild-type mice. Furthermore, CD4(+)CD25(+) T cells from neonatal TGF-beta1(-/-) mice were as suppressive as CD4(+)CD25(+) from TGF-beta1(+/+) mice. Collectively, these results demonstrate that CD4(+)CD25(+) suppressor function can occur independently of TGF-beta1.

Zebrafish Smad7 is regulated by Smad3 and BMP signals

Growth factors of the TGF-beta superfamily such as BMPs and Nodals are important signaling factors during all stages of animal development. Smad proteins, the cytoplasmic mediators of most TGF-beta signals in vertebrates, play central roles not only for transmission but also in controlling inductive TGF-beta signals by feedback regulation. Here, we describe cloning, expression pattern, transcriptional regulation, and functional properties of two novel zebrafish Smad proteins: the TGF-beta agonist Smad3b, and the anti-Smad Smad7. We show that zebrafish Smad3b, in contrast to the related zebrafish Smad2, can induce mesoderm independently of TGF-beta signaling. Although mammalian Smad3 was shown to inhibit expression of the organizer-specific genes goosecoid, zebrafish smad3b activates organizer genes such as goosecoid. Furthermore, we show that Smad3 and BMP signals activate smad7. Because Smad7 blocks distinct TGF-beta signals in early zebrafish development, our data provide hints for new roles of smad3 genes in the regulation and modulation of TGF-beta signals. In summary, our analyses point out differences of Smad3b and Smad2 functions in zebrafish and provide the first link of smad3 and smad7 function in context of vertebrate development.

Restoration of TGF-beta regulation of plasminogen activator inhibitor-1 in Smad3-restituted human choriocarcinoma cells

Proliferation, migration, and invasiveness of the normal placental extravillous trophoblast (EVT) cells are negatively regulated by transforming growth factor-beta (TGF-beta), whereas malignant EVT (JAR and JEG-3 choriocarcinoma) cells are resistant to TGF-beta. These malignant cells were found to have lost the expression of Smad3. Present study examined whether Smad3 restitution in JAR cells could restore TGF-beta response. We produced a stable Smad3 cDNA-transfected clone (JAR-smad3/c) which exhibited further upregulation of Smad3 in the presence of TGF-beta1. Since anti-invasive effects of TGF-beta in the normal EVT cells were shown to be mediated in part by plasminogen activator inhibitor-1 (PAI-1) and urokinase-type plasminogen activator (uPA), we compared the expression of PAI-1 and uPA in the normal EVT, JAR, and JAR-smad3/c cells in the presence or absence of TGF-beta1. The basal levels of PAI-1 mRNA and secreted PAI-1 and uPA proteins were found to be very low in JAR and JAR-smad3/c cells, as compared to the normal EVT cells. However, TGF-beta1 upregulated PAI-1 and downregulated uPA in JAR-smad3/c cells, but not in JAR cells. Thus, resistance of choriocarcinoma cells to anti-invasive effects of TGF-beta may, at least in part, be due to loss of Smad3 expression.

Mechanisms of TGF-beta signaling in regulation of cell growth and differentiation

Transforming growth factor beta (TGF-beta) is a secreted protein that regulates proliferation, differentiation and death of various cell types. All immune cell lineages, including B, T and dendritic cells as well as macrophages, secrete TGF-beta, which negatively regulates their proliferation, differentiation and activation by other cytokines. Thus, TGF-beta is a potent immunosuppressor and perturbation of TGF-beta signaling is linked to autoimmunity, inflammation and cancer. Regulation of cell proliferation and differentiation by TGF-beta is a topic of great basic and clinical importance. We summarize our work on the growth inhibitory pathway downstream of TGF-beta, which is triggered by receptor serine/threonine kinases at the cell surface and downstream effectors of the Smad family. Activated Smads regulate transcription of target genes, including cell cycle inhibitors such as p21, which mediate the anti-proliferative response and partially explain the tumor suppressive action of the TGF-beta pathway. We have described a molecular mechanism of regulation of the p21 gene by Smads and transcription factor Sp1. At late stages of tumor progression, TGF-beta promotes tumorigenesis via suppression of the immune system and changes in cell differentiation of epithelial tumor cells, a phenomenon termed epithelial to mesenchymal transdifferentiation (EMT). We review our work on the role of the Smad pathway in controlling EMT. In conclusion, the molecular pathways that describe the anti-proliferative and transdifferentiating effects of TGF-beta in epithelial cells have been uncovered to great molecular detail; a future challenge will be to test their generality in other systems, including the immune system.

Transforming growth factor beta 1 dysregulation in a human oral carcinoma tumour progression model

A human oral tumour progression model was established that consists of normal epithelial cells and three cell lines representing stages from dysplastic to metastatic cells. To investigate the impact of exogenous transforming growth factor-beta 1 on this model system, we analysed the responsiveness of those cells to transforming growth factor-beta 1 and explored the potential mechanism underlying the transforming growth factor-beta 1 activity. We found that the growth of all cell types, regardless of their stage of tumour progression, is inhibited by transforming growth factor-beta 1, although to different degrees. Transforming growth factor-beta 1 induced the expression of cyclin-dependent kinase inhibitors p15(INK4B), p21WAF1/(CIP1) and p27(KIP1). In contrast, transforming growth factor-beta 1 was found to stimulate the invasive potential of one cell type that represents the most advanced stage of tumour phenotype, suggesting that the impact of transforming growth factor-beta 1 on functional features of tumour cells other than cellular proliferation may play a significant role in the process of oral tumour progression.

Signal transductions induced by bone morphogenetic protein-2 and transforming growth factor-beta in normal human osteoblastic cells

Transforming growth factor beta (TGF-beta) activates Ras/MAPK signaling in many cell types. Because TGF-beta and BMP-2 exert similar effects, we examined if this signaling is stimulated by both factors and analyzed the relationship between this signaling and the Smads in osteoblasts. BMP-2 and TGF-beta stimulated Ras, MAPK, and AP-1 activities. The DNA binding activities of c-Fos, FosB/Delta FosB, Fra-1, Fra-2, and JunB were up-regulated whereas JunD activity was decreased. c-Fos, FosB/Delta FosB, and JunB were associated with Smad4. The stimulation of AP-1 by BMP-2 and TGF-beta was dependent on Smad signaling, and anti-Smad4 antibody interfered with AP-1 activity. Thus, BMP-2 and TGF-beta activate both Ras/MAPK/AP-1 and Smad signaling in osteoblasts with Smads modulating AP-1 activity. To determine the roles of MAPK in BMP-2 and TGF-beta function, we analyzed the effect of ERK and p38 inhibitors on the regulation of bone matrix protein expression and JunB and JunD levels by these two factors. ERK and p38 mediated TGF-beta suppression of osteocalcin and JunD as well as stimulation of JunB. p38 was essential in BMP-2 up-regulation of type I collagen, fibronectin, osteopontin, osteocalcin, and alkaline phosphatase activity whereas ERK mediated BMP-2 stimulation of fibronectin and osteopontin. Thus, ERK and p38 differentially mediate TGF-beta and BMP-2 function in osteoblasts.

Pathway-specific tumor suppression. Reduction of p27 accelerates gastrointestinal tumorigenesis in Apc mutant mice, but not in Smad3 mutant mice

Expression of the cyclin-dependent kinase inhibitor p27(Kip1) (p27) is frequently reduced in human colorectal cancer, and this correlates with poor patient prognosis. To clarify the role of p27 in gastrointestinal (GI) cancer, we measured p27 expression, as well as the effect of germline deletion of p27, in 3 different mouse models of GI neoplasia. p27 expression was frequently reduced in GI tumors arising in 1,2-dimethylhydrazine (DMH) treated mice, and in Apc mutant Min/+ mice, but not in GI tumors arising in Smad3 mutant mice. Germline deletion of p27 resulted in accelerated tumor development and increased tumor cell proliferation in both DMH treated and Min/+ mice, but not in Smad3 mutant mice. p27 deficiency also led to increased adenoma to adenocarcinoma progression. These results indicate that reduction of p27 cooperates with mutations in Apc but not in Smad3 during GI tumorigenesis. Thus, tumor suppression by p27 is contingent on the specific oncogenic pathway that drives tumor development.

The role of a Williams-Beuren syndrome-associated helix-loop-helix domain-containing transcription factor in activin/nodal signaling

We investigated the regulation of the activin/nodal-inducible distal element (DE) of the Xenopus goosecoid (gsc) promoter. On the basis of its interaction with the DE, we isolated a Xenopus homolog of the human Williams-Beuren syndrome critical region 11 (XWBSCR11), and further, show that it interacts with pathway-specific Smad2 and Smad3 in a ligand-dependent manner. Interestingly, we also find that XWBSCR11 functions cooperatively with FoxH1 (Fast-1) to stimulate DE-dependent transcription. We propose a mechanism in which FoxH1 functions together with Smads as a cofactor for the recruitment of transcription factors like XWBSCR11 in the process of activin/nodal-mediated gsc-specific induction. This mechanism provides considerable opportunities for modulation of transcription across a variety of activin/nodal-inducible genes, increasing diversity in promoter selection, thus leading to the differential induction of activin/nodal target genes.

Regulation of cell proliferation by Smad proteins

Transforming growth factor-beta (TGF-beta) family members which include TGF-betas, activins, and bone morphogenetic proteins (BMPs) regulate a broad spectrum of biological responses on a large variety of cell types. TGF-beta family members initiate their cellular responses by binding to distinct receptors with intrinsic serine/threonine kinase activity and activation of specific downstream intracellular effectors termed Smad proteins. Smads relay the signal from the cell membrane to the nucleus, where they affect the transcription of target genes. Smad activation, subcellular distribution, and stability have been found to be intricately regulated and a broad array of transcription factors have been identified as Smad partners. Important activities of TGF-beta are its potent anti-mitogenic and pro-apoptotic effects that, at least in part, are mediated via Smad proteins. Escape from TGF-beta/Smad-induced growth inhibition and apoptosis is frequently observed in tumors. Certain Smads have been found to be mutated in specific types of cancer and gene ablation of particular Smads in mice has revealed increased rate of tumorigenesis. In late stage tumors, TGF-beta has been shown to function as a tumor promoter. TGF-beta can stimulate the de-differentiation of epithelial cells to malignant invasive and metastatic fibroblastic cells. Interestingly, TGF-beta may mediate these effects directly on tumor cells via subverted Smad-dependent and/or Smad-independent pathways.

Suppressor and oncogenic roles of transforming growth factor-beta and its signaling pathways in tumorigenesis

Transforming growth factor-beta (TGF-beta) has been implicated in oncogenesis since the time of its discovery almost 20 years ago. The complex, multifunctional activities of TGF-beta endow it with both tumor suppressor and tumor promoting activities, depending on the stage of carcinogenesis and the responsivity of the tumor cell. Dysregulation or alteration of TGF-beta signaling in tumorigenesis can occur at many different levels, including activation of the ligand, mutation or transcriptional suppression of the receptors, or alteration of downstream signal transduction pathways resulting from mutation or changes in expression patterns of signaling intermediates or from changes in expression of other proteins which modulate signaling. New insights into signaling from the TGF-beta receptors, including the identification of Smad signaling pathways and their interaction with mitogen-activated protein (MAP) kinase pathways, are providing an understanding of the changes involved in the change from tumor suppressor to tumor promoting activities of TGF-beta. It is now appreciated that loss of sensitivity to inhibition of growth by TGF-beta by most tumor cells is not synonymous with complete loss of TGF-beta signaling but rather suggests that tumor cells gain advantage by selective inactivation of the tumor suppressor activities of TGF-beta with retention of its tumor promoting activities, especially those dependent on cross talk with MAP kinase pathways and AP-1.

Mice lacking Smad3 are protected against cutaneous injury induced by ionizing radiation

Transforming growth factor-beta (TGF-beta) plays a central role in the pathogenesis of inflammatory and fibrotic diseases, including radiation-induced fibrosis. We previously reported that mice null for Smad3, a key downstream mediator of TGF-beta, show accelerated healing of cutaneous incisional wounds with reduced inflammation and accumulation of matrix. To determine if loss of Smad3 decreases radiation-induced injury, skin of Smad3+/+ [wild-type (WT)] and -/- [knockout (KO)] mice was exposed to a single dose of 30 to 50 Gy of gamma-irradiation. Six weeks later, skin from KO mice showed significantly less epidermal acanthosis and dermal influx of mast cells, macrophages, and neutrophils than skin from WT littermates. Skin from irradiated KO mice exhibited less immunoreactive TGF-beta and fewer myofibroblasts, suggesting that these mice will have a significantly reduced fibrotic response. Although irradiation induced no change in the immunohistochemical expression of the TGF-beta type I receptor, the epidermal expression of the type II receptor was lost after irradiation whereas its dermal expression remained high. Primary keratinocytes and dermal fibroblasts prepared from WT and KO mice showed similar survival when irradiated, as did mice exposed to whole-body irradiation. These results suggest that inhibition of Smad3 might decrease tissue damage and reduce fibrosis after exposure to ionizing irradiation.

Overgrowth of a mouse model of the Simpson-Golabi-Behmel syndrome is independent of IGF signaling

The type 1 Simpson-Golabi-Behmel overgrowth syndrome (SGBS1) is caused by loss-of-function mutations of the X-linked GPC3 gene encoding glypican-3, a cell-surface heparan sulfate proteoglycan that apparently plays a negative role in growth control by an unknown mechanism. Mice carrying a Gpc3 gene knockout exhibited several phenotypic features that resemble clinical hallmarks of SGBS1, including somatic overgrowth, renal dysplasia, accessory spleens, polydactyly, and placentomegaly. In Gpc3/DeltaH19 double mutants (lacking GPC3 and also carrying a deletion around the H19 gene region that causes bialellic expression of the closely linked Igf2 gene by imprint relaxation), the Gpc3-null phenotype was exacerbated, while additional SGBS1 features (omphalocele and skeletal defects) were manifested. However, results from a detailed comparative analysis of growth patterns in double mutants lacking GPC3 and also IGF2, IGF1, or the type 1 IGF receptor (IGF1R) provided conclusive genetic evidence inconsistent with the hypothesis that GPC3 acts as a growth suppressor by sequestering or downregulating an IGF ligand. Nevertheless, our data are compatible with a model positing that there is downstream convergence of the independent signaling pathways in which either IGFs or (indirectly) GPC3 participate.

Smad3 deficiency attenuates bleomycin-induced pulmonary fibrosis in mice

Transforming growth factor-beta (TGF-beta) signaling plays an important regulatory role during lung fibrogenesis. Smad3 was identified in the pathway for transducing TGF-beta signals from the cell membrane to the nucleus. Using mice without Smad3 gene expression, we investigated whether Smad3 could regulate bleomycin-induced pulmonary fibrosis in vivo. Mice deficient in Smad3 demonstrated suppressed type I procollagen mRNA expression and reduced hydroxyproline content in the lungs compared with wild-type mice treated with bleomycin. Furthermore, loss of Smad3 greatly attenuated morphological fibrotic responses to bleomycin in the mouse lungs, suggesting that Smad3 is implicated in the pathogenesis of pulmonary fibrosis. These results show that Smad3 contributes to bleomycin-induced lung injury and that Smad3 may serve as a novel target for potential therapeutic treatment of lung fibrosis.

Human placental trophoblast as an in vitro model for tumor progression

The human placenta is a highly invasive tumor-like structure in which a subpopulation of placental trophoblast cells known as the "extravillous trophoblast" (EVT) invades the uterine decidua and its vasculature to establish adequate fetal-maternal exchange of molecules. By utilizing in vitro-propagated short-lived EVT cell lines we found that molecular mechanisms responsible for their invasiveness are identical to those of cancer cells; however, unlike cancer cells, their proliferation, migration, and invasiveness in situ are stringently controlled by decidua-derived transforming growth factor (TGF)-beta. By SV40T antigen transfection of normal EVT cells followed by a forced crisis regimen in culture we produced an immortalized premalignant derivative that is hyperproliferative, hyperinvasive, and deficient in gap-junctional intercellular communication. Both premalignant and malignant EVT (JAR and JEG-3 choriocarcinoma) cell lines were found to be TGF-beta-resistant. Using these cell lines, we investigated genetic changes responsible for transition of the normal EVT cells to premalignant and malignant phenotype. Hyperinvasiveness in both cases resulted from a downregulation of tissue inhibitor of metalloprotease (TIMP)-1 and plasminogen activator inhibitor (PAI)-1 genes. In contrast to normal EVT cells, both cell types failed to upregulate these genes in response to TGF-beta. Loss of TGF-beta response in malignant EVT cells was explained by the loss of expression of Smad3 gene. Differential mRNA display of normal and premalignant EVT cells identified up- and down-regulation of numerous known or novel genes in premalignant EVT cells, with potential oncogenic and (or) tumor-suppressor functions, e.g., loss of fibronectin and insulin-like growth factor binding protein (IGFBP-5). Premalignant EVT cells also lost IGF receptor type 2 (IGFR-II). IGFBP-5 was shown to be a negative regulator of IGF-1-induced proliferation of premalignant EVT cells, so that loss of IGFBP-5 as well as IGFR-II permitted their unrestricted proliferation in an IGF-I-rich microenvironment of the fetal-maternal interface. The present model may be a good prototype for identifying genetic changes underlying epithelial tumor progression.

Physical and functional interaction between GATA-3 and Smad3 allows TGF-beta regulation of GATA target genes

BACKGROUND

Members of the GATA family of zinc finger transcription factors are genetically controlled "master" regulators of development in the hematopoietic and nervous systems. Whether GATA factors also serve to integrate epigenetic signals on target promoters is, however, unknown. The transforming growth factor-beta (TGF-beta) superfamily is a large group of phylogenetically conserved secreted factors controlling cell proliferation, differentiation, migration, and survival in multiple tissues.

RESULTS

GATA-3, a key regulator of T helper cell development, was found to directly interact with Smad3, an intracellular signal transducer of TGF-beta. Complex formation required a central region in GATA-3 and the N-terminal domain of Smad3. GATA-3 mediated recruitment of Smad3 to GATA binding sites independently of Smad3 binding to DNA, and the two factors cooperated synergistically to regulate transcription from the IL-5 promoter in a TGF-beta-dependent manner. Treatment of T helper cells with TGF-beta promoted the formation of an endogenous Smad3/GATA-3 nuclear complex and stimulated production of the Th2 cytokine IL-10 in a Smad3- and GATA-3-dependent manner.

CONCLUSIONS

Although Smad proteins are known to interact with a number of general transcription factors, these are insufficient to explain the tissue-specific biology of TGF-beta proteins. Through its interaction with Smad3, GATA-3 is able to integrate a genetic program of cell differentiation with an extracellular signal, providing a molecular framework for the effects of TGF-beta on the development and function of specific subsets of immune cells and possibly other cell types.

Targeted mutagenesis of Smad1 reveals an essential role in chorioallantoic fusion

The Smad family of intracellular signaling intermediates transduce signals downstream from the transforming growth factor beta (TGF-beta) family of receptor serine threonine kinases. The original member of this family, Smad1, has been shown to mediate signals from receptors for the bone morphogenetic proteins (BMPs), a large group of ligands in the TGF-beta superfamily that mediate important developmental events. We have targeted the Smad1 gene in mice and created mutants null at this locus. Smad1 mutant mice die at approximately 9.5 days postcoitum due to defects in allantois formation. In Smad1 mutant mice, the allantois fails to fuse to the chorion, resulting in a lack of placenta and failure to establish a definitive embryonic circulation. Although vasculogenesis is initiated in the mutant allantois, the vessels formed are disorganized, and VCAM-1 protein, a marker for distal allantois development, is not expressed. Smad1 null fibroblasts are still able to respond to BMP2, however, suggesting that the defect observed in the developing extraembryonic tissue is caused by a very specific loss of transcriptional activity regulated by Smad1. Our data further demonstrate that although highly similar structurally, Smad proteins are not functionally homologous.

TGF-beta inhibits muscle differentiation through functional repression of myogenic transcription factors by Smad3

Transforming growth factor-beta (TGF-beta) is a potent inhibitor of skeletal muscle differentiation, but the molecular mechanism and signaling events that lead to this inhibition are poorly characterized. Here we show that the TGF-beta intracellular effector Smad3, but not Smad2, mediates the inhibition of myogenic differentiation in MyoD-expressing C3H10T1/2 cells and C2C12 myoblasts by repressing the activity of the MyoD family of transcriptional factors. The Smad3-mediated repression was directed at the E-box sequence motif within muscle gene enhancers and the bHLH region of MyoD, the domain required for its association with E-protein partners such as E12 and E47. The repression could be overcome by supplying an excess of E12, and covalent tethering of E47 to MyoD rendered the E-box-dependent transcriptional activity refractory to the effects of Smad3 and TGF-beta. Smad3 physically interacted with the HLH domain of MyoD, and this interaction correlated with the ability of Smad3 to interfere with MyoD/E protein heterodimerization and binding of MyoD complexes to oligomerized E-box sites. Together, these results reveal a model for how TGF-beta, through Smad3-mediated transcriptional repression, inhibits myogenic differentiation.

Nodal signaling in early vertebrate embryos: themes and variations

The nodal family of TGFbeta-related ligands have emerged as critical regulators of early vertebrate embryogenesis. Recent studies in mice, fish, and frogs of nodals and their intracellular transducers allow a comparison of how this signaling pathway is used in the patterning of early embryos of these different vertebrates.

Transforming growth factor-beta repression of matrix metalloproteinase-1 in dermal fibroblasts involves Smad3

Enhanced production of matrix metalloproteinase-1 (MMP-1, collagenase-1) is implicated in pathological tissue destruction. Transforming growth factor-beta (TGF-beta) prevents cytokine-induced MMP-1 gene expression in fibroblasts. In these studies, we examined the hypothesis that repression of MMP-1 may be mediated through the Smad signaling pathway. The results showed that Smad3 and Smad4, but not Smad1 or Smad2, mimicked the inhibitory effect of TGF-beta and abrogated interleukin-1beta (IL-1beta)-induced stimulation of MMP-1 promoter activity and NFkappaB-specific gene transcription in dermal fibroblasts. Experiments with truncation mutants indicated that both MH1 and MH2 domains of Smad3 were necessary for inhibitory activity. Dominant negative mutants of Smad3 or Smad4 and antagonistic Smad7, which disrupts ligand-induced Smad3 phosphorylation, abrogated the repression of MMP-1 transcription by TGF-beta. Similar results were obtained using immunoblot and Northern analysis. Furthermore, TGF-beta failed to repress MMP-1 promoter activity in Smad3-deficient murine embryonic fibroblasts. These results implicated cellular Smads in mediating the inhibitory effects of TGF-beta. Overexpression of the transcriptional co-activator p300, but not its histone acetyltransferase (HAT)-deficient mutant, was able to relieve repression of MMP-1 gene expression, suggesting that Smad-dependent inhibition may be due to increased competition between Smad proteins and IL-1beta signaling pathways for limiting amounts of cellular p300. Together, these results demonstrate that MMP-1 is a target for negative regulation by TGF-beta through cellular Smad3 and Smad4. Smad-mediated repression of MMP-1 gene expression may be important for preventing excessive matrix degradation induced by inflammatory cytokines; disruption of Smad signaling, as occurs in certain cancer cells, may thus be causally linked to uncontrolled tissue destruction mediated through MMP-1.

The loss of Smad3 results in a lower rate of bone formation and osteopenia through dysregulation of osteoblast differentiation and apoptosis

Smad3 is a well-characterized intracellular effector of the transforming growth factor beta (TGF-beta) signaling pathway and was implicated recently in the potentiation of vitamin D receptor (VDR)-mediated signaling. Given that both TGF-beta and vitamin D are important regulators of bone remodeling, it is expected that Smad3 plays an integral role in normal maintenance of bone. However, the exact mechanisms by which Smad3 functions in bone remodeling are unknown. Here, we show that mice with targeted deletion of Smad3 are osteopenic with less cortical and cancellous bone compared with wild-type littermates. Decreases in bone mineral density (BMD) in Smad3 null mice reflect the inability of osteoblasts to balance osteoclast activity, although osteoclast numbers are normal and vitamin D mediated serum calcium homeostasis is maintained. The osteopenia of Smad3 null mice is attributed to a decreased rate of bone formation associated with increased osteocyte number and apoptosis. These findings are supported by studies with isolated primary osteoblasts that show TGF-beta can no longer inhibit the differentiation of osteoblasts in the absence of Smad3; yet, TGF-beta-stimulated proliferation remains intact. Together these data support a model that a loss of Smad3 increases the osteocyte fate of the osteoblast and decreases the duration of osteoblast function by shortening lifespan, ultimately resulting in osteopenia.

TGF-beta signaling in tumor suppression and cancer progression

Epithelial and hematopoietic cells have a high turnover and their progenitor cells divide continuously, making them prime targets for genetic and epigenetic changes that lead to cell transformation and tumorigenesis. The consequent changes in cell behavior and responsiveness result not only from genetic alterations such as activation of oncogenes or inactivation of tumor suppressor genes, but also from altered production of, or responsiveness to, stimulatory or inhibitory growth and differentiation factors. Among these, transforming growth factor beta (TGF-beta) and its signaling effectors act as key determinants of carcinoma cell behavior. The autocrine and paracrine effects of TGF-beta on tumor cells and the tumor micro-environment exert both positive and negative influences on cancer development. Accordingly, the TGF-beta signaling pathway has been considered as both a tumor suppressor pathway and a promoter of tumor progression and invasion. Here we evaluate the role of TGF-beta in tumor development and attempt to reconcile the positive and negative effects of TGF-beta in carcinogenesis.

Expression of TGF-beta signaling genes in the normal, premalignant, and malignant human trophoblast: loss of smad3 in choriocarcinoma cells

We had earlier shown that TGF-beta controls proliferation, migration, and invasiveness of normal human trophoblast cells, whereas premalignant and malignant trophoblast cells are resistant to TGF-beta. To identify signaling defects responsible for TGF-beta resistance in premalignant and malignant trophoblasts, we have compared the expression of TGF-beta signaling molecules in a normal trophoblast cell line (HTR-8), its premalignant derivative (RSVT2/C), and two choriocarcinoma cell lines (JAR and JEG-3). RT-PCR analysis revealed that all these cell lines expressed the mRNA of TGF-beta1, -beta2, and -beta3, TGF-beta receptors type I, II, and III, and post-receptor signaling genes smad2, smad3, smad4, smad6, and smad7 with the exception that TGF-beta2 and smad3 were undetectable in JAR and JEG-3 cells. Immunoblot analysis confirmed the absence of smad3 protein in choriocarcinoma cells. Treatment with TGF-beta1 induced smad3 phosphorylation and smad3 translocation to the nucleus in the normal and premalignant trophoblast cells. These results suggest that loss of smad3 may account for a functional disruption in the TGF-beta signaling pathway in choriocarcinomas, but not in the premalignant trophoblast.

New developments for TGFbeta

A recent FASEB meeting was held in Tucson, Arizona that encompassed TGFbeta superfamily signaling pathways and their roles in development. This review focuses on the developmental biology presented at the meeting.

Invasive candidiasis stimulates hepatocyte and monocyte production of active transforming growth factor beta

Candida albicans is an opportunistic fungal pathogen and a major cause of morbidity and mortality in patients with compromised immune function. The cytokine response to tissue invasion by C. albicans can influence the differentiation and function of lymphocytes and other mononuclear cells that are critical components of the host response. While the production of transforming growth factor beta (TGF-beta) has been documented in mice infected with C. albicans and is known to suppress phagocyte function, the cellular source and role of this cytokine in the pathogenesis of systemic candidiasis are not well understood. We have investigated the source of production of TGF-beta by immunohistochemical studies in tissue samples from patients with an uncommon complication of lymphoreticular malignancy, chronic disseminated candidiasis (CDC), and from a neutropenic-rabbit model of CDC. Liver biopsy specimens from patients with documented CDC demonstrated intense staining for extracellular matrix-associated TGF-beta1 within inflammatory granulomas, as well as staining for TGF-beta1 and TGF-beta3 within adjacent hepatocytes. These results correlate with the immunolocalization of TGF-beta observed in livers of infected neutropenic rabbits, using a neutralizing antibody that recognizes the mature TGF-beta protein. Human peripheral blood monocytes incubated with C. albicans in vitro release large amounts of biologically active TGF-beta1. The data demonstrate that local production of active TGF-betas by hepatocytes and by infected mononuclear cells is a component of the response to C. albicans infection that most probably contributes to disease progression in the immunocompromised host.

Overexpression of Smad2 reveals its concerted action with Smad4 in regulating TGF-beta-mediated epidermal homeostasis

Members of the transforming growth factor-beta (TGF-beta) superfamily are critical regulators for epithelial growth and can alter the differentiation of keratinocytes. Transduction of TGF-beta signaling depends on the phosphorylation and activation of Smad proteins by heteromeric complexes of ligand-specific type I and II receptors. To understand the function of TGF-beta and activin-specific Smad, we generated transgenic mice that overexpress Smad2 in epidermis under the control of keratin 14 promoter. Overexpression of Smad2 increases endogenous Smad4 and TGF-beta 1 expression while heterozygous loss of Smad2 reduces their expression levels, suggesting a concerted action of Smad2 and -4 in regulating TGF-beta signaling during skin development. These transgenic mice have delayed hair growth, underdeveloped ears, and shorter tails. In their skin, there is severe thickening of the epidermis with disorganized epidermal architecture, indistinguishable basement membrane, and dermal fibrosis. These abnormal phenotypes are due to increased proliferation of the basal epidermal cells and abnormalities in the program of keratinocyte differentiation. The ectodermally derived enamel structure is also abnormal. Collectively, our study presents the first in vivo evidence that, by providing an auto-feedback in TGF-beta signaling, Smad2 plays a pivotal role in regulating TGF-beta-mediated epidermal homeostasis.

Smad proteins and hepatocyte growth factor control parallel regulatory pathways that converge on beta1-integrin to promote normal liver development

Smads serve as intracellular mediators of transforming growth factor beta (TGF-beta) signaling. After phosphorylation by activated type I TGF-beta receptors, Smad proteins translocate to the nucleus, where they serve as transcription factors and increase or decrease expression of TGF-beta target genes. Mice lacking one copy each of Smad2 and Smad3 suffered midgestation lethality due to liver hypoplasia and anemia, suggesting essential dosage requirements of TGF-beta signal components. This is likely due to abnormal adhesive properties of the mutant hepatocytes, which may result from a decrease in the level of the beta1-integrin and abnormal processing and localization of E-cadherin. Culture of mutant livers in vitro revealed the existence of a parallel developmental pathway mediated by hepatocyte growth factor (HGF), which could rescue the mutant phenotype independent of Smad activation. These pathways merge at the beta1-integrin, the level of which was increased by HGF in the cultured mutant livers. HGF treatment reversed the defects in cell proliferation and hepatic architecture in the Smad2(+/-); Smad3(+/-) livers.

The Smad3 protein is involved in TGF-beta inhibition of class II transactivator and class II MHC expression

TGF-beta is a immunoregulatory cytokine that inhibits class II MHC expression in a variety of cell types. Previous studies have shown that the class II MHC transactivator (CIITA), a master regulator that controls class II MHC expression, is targeted by TGF-beta for repression of IFN-gamma-induced class II MHC expression in astrocytes. The mechanism(s) underlying the TGF-beta inhibitory effect is not understood. In this study, we demonstrate that TGF-beta inhibition of CIITA expression occurs at the transcriptional level, and that both constitutive and IFN-gamma-induced human CIITA type IV promoter activity is inhibited by TGF-beta. TGF-beta does not affect the signaling events that mediate IFN-gamma activation of CIITA expression; i.e, TGF-beta does not inhibit IFN-gamma-induced STAT-1alpha phosphorylation and/or DNA binding ability, nor is IFN-gamma induction of IFN regulatory factor affected. The inhibitory effect of TGF-beta on the type IV CIITA promoter is mediated through a promoter region within 80 bp from the transcription start site. Elimination of TGF-beta inhibition of class II MHC and CIITA expression in Smad3-deficient astrocytes, as well as restoration of the inhibitory effect by overexpression of the Smad3 protein, demonstrates that Smad3 is essential in mediating TGF-beta inhibition of CIITA and class II MHC expression.

Transforming growth factor-beta signal transduction in epithelial cells

Transforming growth factor (TGF)-beta is a natural and potent growth inhibitor of a variety of cell types, including epithelial, endothelial, and hematopoietic cells. The ability of TGF-beta to potently inhibit the growth of many solid tumors of epithelial origin, including breast and colon carcinomas, is of particular interest. However, many solid tumor cells become refractory to the growth inhibitory effects of TGF-beta due to defects in TGF-beta signaling pathways. In addition, TGF-beta may stimulate the invasiveness of tumor cells via the paracrine effects of TGF-beta. Accordingly, in order to develop more effective anticancer therapeutics, it is necessary to determine the TGF-beta signal transduction pathways underlying the growth inhibitory effects and other cellular effects of TGF-beta in normal epithelial cells. Thus far, two primary signaling cascades downstream of the TGF-beta receptors have been elucidated, the Sma and mothers against decapentaplegic homologues and the Ras/mitogen-activated protein kinase pathways. The major objective of this review is to summarize TGF-beta signaling in epithelial cells, focusing on recent advances involving the Sma and mothers against decapentaplegic homologues and Ras/mitogen-activated protein kinase pathways. This review is particularly timely in that it provides a comprehensive summary of both signal transduction mechanisms and the cell cycle effects of TGF-beta.

The transcriptional role of Smads and FAST (FoxH1) in TGFbeta and activin signalling

The Smad family of proteins are critical components of the TGFbeta superfamily signalling pathway. Ligand addition induces phosphorylation of specific receptor-regulated Smads, which then form heteromeric complexes with the common mediator Smad, Smad4. This complex then translocates from the cytoplasm into the nucleus. Once there, the R-Smad/Smad4 complex interacts with a variety of DNA binding proteins and is thereby targetted to a diverse array of gene promoters. The Smad-containing DNA binding complex can then positively or negatively regulate gene expression through the recruitment of co-activators and co-repressors. Xenopus FAST (now known as FoxH1) was the first Smad DNA binding partner identified and the FoxH1 family now includes related proteins from mouse, human and Zebrafish. In all organisms examined, FoxH1 is expressed primarily during the earliest stages of development and thus FoxH1 is thought to play a critical role in mediating TGFbeta superfamily signals during these early developmental stages. Other Smad partners range from those that are ubiquitously expressed to others that are present only in specific cell types or developmental stages. Thus, it is the interaction of Smads with a wide range of specific transcriptional partners that is important for the generation of diverse biological responses to TGFbeta superfamily members.

Evidence for activin A and follistatin involvement in the systemic inflammatory response

The inflammatory cascade is a multifactorial process regulated by interwoven cytokine and growth factor networks. This review summarizes the emerging evidence that implicate activin A and follistatin in inflammatory processes. Our recent studies have determined that activin A is released early in the cascade of circulatory cytokines during systemic inflammatory episodes, roughly coincident with tumour necrosis factor (TNF)-alpha and before interleukin (IL)-6 and follistatin. The source(s) of this activin A are not yet established, but prime candidates are monocytes/macrophages, other immune cell types or vascular endothelial cells. Clinical data are limited, but activin beta(A) subunit mRNA or activin A protein is elevated in inflammatory bowel diseases and inflammatory arthropathies, and circulating concentrations of follistatin are elevated in patients with sepsis. In more mechanistic approaches, in vitro studies show that activin A can have both pro- and anti-inflammatory actions on key inflammatory mediators such as TNFalpha, IL-1beta and IL-6. Furthermore, there is emerging understanding of how the intracellular signaling pathway for activin A, incorporating Smads, may interact with and be modulated by other key regulatory cytokines and growth factors.

Functional characterization of transforming growth factor beta signaling in Smad2- and Smad3-deficient fibroblasts

A prominent pathway of transforming growth factor (TGF)-beta signaling involves receptor-dependent phosphorylation of Smad2 and Smad3, which then translocate to the nucleus to activate transcription of target genes. To investigate the relative importance of these two Smad proteins in TGF-beta1 signal transduction, we have utilized a loss of function approach, based on analysis of the effects of TGF-beta1 on fibroblasts derived from mouse embryos deficient in Smad2 (S2KO) or Smad3 (S3KO). TGF-beta1 caused 50% inhibition of cellular proliferation in wild-type fibroblasts as assessed by [(3)H]thymidine incorporation, whereas the growth of S2KO or S3KO cells was only weakly inhibited by TGF-beta1. Lack of Smad2 or Smad3 expression did not affect TGF-beta1-induced fibronectin synthesis but resulted in markedly suppressed induction of plasminogen activator inhibitor-1 by TGF-beta1. Moreover, TGF-beta1-mediated induction of matrix metalloproteinase-2 was selectively dependent on Smad2, whereas induction of c-fos, Smad7, and TGF-beta1 autoinduction relied on expression of Smad3. Investigation of transcriptional activation of TGF-beta-sensitive reporter genes in the different fibroblasts showed that activation of the (Smad binding element)(4)-Lux reporter by TGF-beta1 was dependent on expression of Smad3, but not Smad2, whereas activation of the activin response element-Lux reporter was strongly suppressed in S2KO fibroblasts but, on the contrary, enhanced in S3KO cells. Our findings indicate specific roles for Smad2 and Smad3 in TGF-beta1 signaling.

TGF-beta-induced repression of CBFA1 by Smad3 decreases cbfa1 and osteocalcin expression and inhibits osteoblast differentiation

Transforming growth factor-beta (TGF-beta), a secreted factor present at high levels in bone, inhibits osteoblast differentiation in culture; yet, the mechanism of this inhibition remains unclear. We studied the effects of TGF-beta and its effectors, the Smads, on the expression and function of the osteoblast transcription factor CBFA1. TGF-beta inhibited the expression of the cbfa1 and osteocalcin genes, whose expression is controlled by CBFA1 in osteoblast-like cell lines. This inhibition was mediated by Smad3, which interacts physically with CBFA1 and represses its transcriptional activity at the CBFA1-binding OSE2 promoter sequence. The repression of CBFA1 function by Smad3 contrasts with previous observations that Smads function as transcription activators. This repression occurred in mesenchymal but not epithelial cells, and depended on the promoter sequence. Smad3-mediated repression of CBFA1 provides a central regulatory mechanism for the inhibition of osteoblast differentiation by TGF-beta, since it inhibits both cbfa1 transcription and transcriptional activation of osteoblast differentiation genes by CBFA1. Altering Smad3 signaling influenced osteoblast differentiation in the presence or absence of TGF-beta, implicating Smad3/TGF-beta-mediated repression in autocrine regulation of osteoblast differentiation.

TGF-beta/Smad3 signals repress chondrocyte hypertrophic differentiation and are required for maintaining articular cartilage

Endochondral ossification begins from the condensation and differentiation of mesenchymal cells into cartilage. The cartilage then goes through a program of cell proliferation, hypertrophic differentiation, calcification, apoptosis, and eventually is replaced by bone. Unlike most cartilage, articular cartilage is arrested before terminal hypertrophic differentiation. In this study, we showed that TGF-beta/Smad3 signals inhibit terminal hypertrophic differentiation of chondrocyte and are essential for maintaining articular cartilage. Mutant mice homozygous for a targeted disruption of Smad3 exon 8 (Smad3(ex8/ex8)) developed degenerative joint disease resembling human osteoarthritis, as characterized by progressive loss of articular cartilage, formation of large osteophytes, decreased production of proteoglycans, and abnormally increased number of type X collagen-expressing chondrocytes in synovial joints. Enhanced terminal differentiation of epiphyseal growth plate chondrocytes was also observed in mutant mice shortly after weaning. In an in vitro embryonic metatarsal rudiment culture system, we found that TGF-beta1 significantly inhibits chondrocyte differentiation of wild-type metatarsal rudiments. However, this inhibition is diminished in metatarsal bones isolated from Smad3(ex8/ex8) mice. These data suggest that TGF-beta/Smad3 signals are essential for repressing articular chondrocyte differentiation. Without these inhibition signals, chondrocytes break quiescent state and undergo abnormal terminal differentiation, ultimately leading to osteoarthritis.

Interaction between Smad anchor for receptor activation and Smad3 is not essential for TGF-beta/Smad3-mediated signaling

Regulation of subcellular localization of Smad proteins is supposed to be critical for the effective initiation and maintenance of TGF-beta signaling. Recently, Smad anchor for receptor activation (SARA) has been identified as a Smad2 binding protein. SARA regulates the subcellular localization of Smad2 and is required for TGF-beta/Smad2-mediated signaling. In this study, we determined whether the interaction between SARA and Smad3 is essential for TGF-beta/Smad3-mediated signaling. We found that a mutant Smad3 (Smad3NS) that lacked the binding to SARA was phosphorylated by TGF-beta type I receptor at the similar level to that in wild-type Smad3 (Smad3WT). Smad3NS also formed complexes with Smad4 and translocalized into the nucleus. Moreover, Smad3NS and Smad3WT equally enhanced TGF-beta-induced transcription. Therefore, these findings indicate that, in contrast to SARA/Smad2 interaction, SARA/Smad3 interaction is not essential for TGF-beta/Smad3-mediated signaling.

TGF-beta signaling by Smad proteins

Members of the transforming growth factor-beta (TGF-beta) family bind to type II and type I serine/threonine kinase receptors, which initiate intracellular signals through activation of Smad proteins. Receptor-regulated Smads (R-Smads) are anchored to the cell membrane by interaction with membrane-bound proteins, including Smad anchor for receptor activation (SARA). Upon ligand stimulation, R-Smads are phosphorylated by the receptors and form oligomeric complexes with common-partner Smads (Co-Smads). The oligomeric Smad complexes then translocate into the nucleus, where they regulate the transcription of target genes by direct binding to DNA, interaction with various DNA-binding proteins, and recruitment of transcriptional coactivators or corepressors. A third class of Smads, inhibitory Smads (I-Smads), inhibits the signals from the serine/threonine kinase receptors. Since the expression of I-Smads is induced by the TGF-beta superfamily proteins, Smads constitute an autoinhibitory signaling pathway. The functions of Smads are regulated by other signaling pathways, such as the MAP kinase pathway. Moreover, Smads interact with and modulate the functions of various transcription factors which are downstream targets of other signaling pathways. Loss of function of certain Smads is involved in tumorigenesis, e.g., pancreatic and colorectal cancers. Analyses by gene targeting revealed pivotal roles of Smads in early embryogenesis, angiogenesis, and immune functions in vivo.

Transforming growth factor-beta signaling in cancer

Transforming growth factor (TGF-beta) is a multifunctional polypeptide implicated in the regulation of a variety of cellular processes including growth, differentiation, apoptosis, adhesion, and motility. Abnormal activation or inhibition of these TGF-beta regulated processes is implicated in many diseases, including cancer. Cancers can develop through selective exploitation of defects in TGF-beta signaling that occur at several different levels in the pathway. The TGF-beta signal transduction cascade is initiated when TGF-beta binds to transmembrane receptors. The TGF-beta receptors then phosphorylate and activate Smad proteins, which transduce the signal from the cytoplasm to the nucleus. In the nucleus, Smads can bind directly to DNA and cooperate with other transcription factors to induce transcription of TGF-beta target genes. Mutations in target genes, Smads, or the TGF-beta receptor are associated with certain human cancers.

Immunohistochemical expression of Smads 1-6 in the 15-day gestation mouse embryo: signaling by BMPs and TGF-betas

The eight mammalian Smad proteins mediate cellular signaling from members of the transforming growth factor-beta (TGF-beta), bone morphogenetic protein (BMP), and activin families. Smads 1, 5, and 8 transmit signals from BMPs, while Smads 2 and 3 transmit signals from TGF-betas and activin. Smad 4 is a common mediator of both pathways, while Smads 6 and 7 inhibit signaling. Signal transduction involves translocation of Smad complexes to the nucleus and subsequent gene activation. Little is known about the expression of endogenous Smad proteins during development. We identified commercially available Smad antibodies that specifically recognize a unique Smad protein and are suitable for immunohistochemistry. Here we compare the localization of Smads 1, 2, 3, 4, 5, and 6 in tissues of the 15-day gestation mouse embryo. Immunoreactive Smad proteins are seen in many tissues with differences in the localization being dependent upon the cell type. All tissues express Smad 4 and at least one each of the BMP-specific and TGF-beta-specific Smads, while expression of Smad 6 is more restricted. Differences are observed in the nuclear versus cytoplasmic localization among the Smads in different cell types or tissues, suggesting selective activation of Smads during this stage of development.

Nodal signaling uses activin and transforming growth factor-beta receptor-regulated Smads

Nodal, a member of the transforming growth factor beta (TGF-beta) superfamily, is implicated in many events critical to the early vertebrate embryo, including mesoderm formation, anterior patterning, and left-right axis specification. Here we define the intracellular signaling pathway induced by recombinant nodal protein treatment of P19 embryonal carcinoma cells. Nodal signaling activates pAR3-Lux, a luciferase reporter previously shown to respond specifically to activin and TGF-beta. However, nodal is unable to induce pTlx2-Lux, a reporter specifically responsive to bone morphogenetic proteins. We also demonstrate that nodal induces p(CAGA)(12), a reporter previously shown to be specifically activated by Smad3. Expression of a dominant negative Smad2 significantly reduces the level of luciferase reporter activity induced by nodal treatment. Finally, we show that nodal signaling rapidly leads to the phosphorylation of Smad2. These results provide the first direct biochemical evidence that nodal signaling is mediated by both activin-TGF-beta pathway Smads, Smad2 and Smad3. We also show here that the extracellular cripto protein is required for nodal signaling, making it distinct from activin or TGF-beta signaling.