Involvement of MAP kinase cascades in Smad7 transcriptional regulation

Current perspectives on inhibitory SMAD7 in health and disease

Transforming growth factor β (TGF-β) family members play an extensive role in cellular communication that orchestrates both early development and adult tissue homeostasis. Aberrant TGF-β family signaling is associated with a pathological outcome in numerous diseases, and in-depth understanding of molecular and cellular processes could result in therapeutic benefit for patients. Canonical TGF-β signaling is mediated by receptor-regulated SMADs (R-SMADs), a single co-mediator SMAD (Co-SMAD), and inhibitory SMADs (I-SMADs). SMAD7, one of the I-SMADs, is an essential negative regulator of the pleiotropic TGF-β and bone morphogenetic protein (BMP) signaling pathways. In a negative feedback loop, SMAD7 inhibits TGF-β signaling by providing competition for TGF-β type-1 receptor (TβRI), blocking phosphorylation and activation of SMAD2. Moreover, SMAD7 recruits E3 ubiquitin SMURF ligases to the type I receptor to promote ubiquitin-mediated proteasomal degradation. In addition to its role in TGF-β and BMP signaling, SMAD7 is regulated by and implicated in a variety of other signaling pathways and functions as a mediator of crosstalk. This review is focused on SMAD7, its function in TGF-β and BMP signaling, and its role as a downstream integrator and crosstalk mediator. This crucial signaling molecule is tightly regulated by various mechanisms. We provide an overview of the ways by which SMAD7 is regulated, including noncoding RNAs (ncRNAs) and post-translational modifications (PTMs). Finally, we discuss its role in diseases, such as cancer, fibrosis, and inflammatory bowel disease (IBD).

Bone morphogenetic proteins mediate crosstalk between cancer cells and the tumour microenvironment at primary tumours and metastases (Review)

Bone morphogenetic proteins (BMP) are pluripotent molecules, co‑ordinating cellular functions from early embryonic and postnatal development to tissue repair, regeneration and homeostasis. They are also involved in tumourigenesis, disease progression and the metastasis of various solid tumours. Emerging evidence has indicated that BMPs are able to promote disease progression and metastasis by orchestrating communication between cancer cells and the surrounding microenvironment. The interactions occur between BMPs and epidermal growth factor receptor, hepatocyte growth factor, fibroblast growth factor, vascular endothelial growth factor and extracellular matrix components. Overall, these interactions co‑ordinate the cellular functions of tumour cells and other types of cell in the tumour to promote the growth of the primary tumour, local invasion, angiogenesis and metastasis, and the establishment and survival of cancer cells in the metastatic niche. Therefore, the present study aimed to provide an informative summary of the involvement of BMPs in the tumour microenvironment.

TGFβ Family Signaling Pathways in Pluripotent and Teratocarcinoma Stem Cells' Fate Decisions: Balancing Between Self-Renewal, Differentiation, and Cancer

The transforming growth factor-β (TGFβ) family factors induce pleiotropic effects and are involved in the regulation of most normal and pathological cellular processes. The activity of different branches of the TGFβ family signaling pathways and their interplay with other signaling pathways govern the fine regulation of the self-renewal, differentiation onset and specialization of pluripotent stem cells in various cell derivatives. TGFβ family signaling pathways play a pivotal role in balancing basic cellular processes in pluripotent stem cells and their derivatives, although disturbances in their genome integrity induce the rearrangements of signaling pathways and lead to functional impairments and malignant transformation into cancer stem cells. Therefore, the identification of critical nodes and targets in the regulatory cascades of TGFβ family factors and other signaling pathways, and analysis of the rearrangements of the signal regulatory network during stem cell state transitions and interconversions, are key issues for understanding the fundamental mechanisms of both stem cell biology and cancer initiation and progression, as well as for clinical applications. This review summarizes recent advances in our understanding of TGFβ family functions in naїve and primed pluripotent stem cells and discusses how these pathways are involved in perturbations in the signaling network of malignant teratocarcinoma stem cells with impaired differentiation potential.

Role of dual-specificity protein phosphatase DUSP10/MKP-5 in pulmonary fibrosis

Mitogen-activated protein kinase (MAPK) phosphatase 5 (MKP-5) is a member of the dual-specificity family of protein tyrosine phosphatases that negatively regulates p38 MAPK and the JNK. MKP-5-deficient mice exhibit improved muscle repair and reduced fibrosis in an animal model of muscular dystrophy. Here, we asked whether the effects of MKP-5 on muscle fibrosis extend to other tissues. Using a bleomycin-induced model of pulmonary fibrosis, we found that MKP-5-deficient mice were protected from the development of lung fibrosis, expressed reduced levels of hydroxyproline and fibrogenic genes, and displayed marked polarization towards an M1-macrophage phenotype. We showed that the profibrogenic effects of the transforming growth factor-β1 (TGF-β1) were inhibited in MKP-5-deficient lung fibroblasts. MKP-5-deficient fibroblasts exhibited enhanced p38 MAPK activity, impaired Smad3 phosphorylation, increased Smad7 levels, and decreased expression of fibrogenic genes. Myofibroblast differentiation was attenuated in MKP-5-deficient fibroblasts. Finally, we found that MKP-5 expression was increased in idiopathic pulmonary fibrosis (IPF)-derived lung fibroblasts but not in whole IPF lungs. These data suggest that MKP-5 plays an essential role in promoting lung fibrosis. Our results couple MKP-5 with the TGF-β1 signaling machinery and imply that MKP-5 inhibition may serve as a therapeutic target for human lung fibrosis.

CREB1 and Smad3 mediate TGF‑β3‑induced Smad7 expression in rat hepatic stellate cells

Transforming growth factor (TGF)‑β3 has previously been reported to antagonize hepatic fibrosis in vivo and in vitro. The present study aimed to investigate the mechanism underlying the involvement of TGF‑β3 in hepatic fibrosis. Short hairpin (sh)RNA‑cAMP-responsive element binding protein (CREB) 1 and small interfering (si)RNA‑Smad3 were utilized to silence the expression of CREB1 and Smad3 in hepatic stellate cells (HSCs), whereas the vector pRSV‑CREB1 was used to induce CREB1 overexpression in HSCs. Cells were treated with or without exogenous TGF‑β3 or TGF‑β1, and mRNA and protein expression levels were assessed using reverse transcription‑quantitative polymerase chain reaction and western blot analysis. Untreated cells served as the control group. Exogenous TGF‑β3 increased Smad7 mRNA and protein expression levels in rat HSCs, and CREB1 and Smad3 appeared to be implicated in the mechanism of Smad7. CREB1 knockdown inhibited the TGF‑β3‑induced upregulation of Smad7, whereas its overexpression potentiated the Smad7 upregulation in HSCs; conversely, CREB1 manipulations had no effect on Smad7 expression under basal conditions. In addition, TGF‑β3‑induced Smad7 upregulation was blocked when the activity of p38, a kinase upstream of CREB1, was inhibited. Furthermore, silencing Smad3 resulted in decreased Smad7 expression under basal conditions and in TGF‑β3‑stimulated cells. Notably, Smad7 expression appeared to also be induced by exogenous TGF‑β1, independent of CREB1. The present study demonstrated that TGF‑β3 increased Smad7 expression in HSCs, whereas CREB1 and Smad3 appeared to participate in the mechanism of induction. Smad3 is the key regulator whereas CREB‑1 acts as a co‑regulator. These results suggested that this mechanism may underlie the antagonizing effects of TGF‑β3 on hepatic fibrosis.

The JNK Signaling Pathway in Renal Fibrosis

Fibrosis of the glomerular and tubulointerstitial compartments is a common feature of chronic kidney disease leading to end-stage renal failure. This fibrotic process involves a number of pathologic mechanisms, including cell death and inflammation. This review focuses on the role of the c-Jun amino terminal kinase (JNK) signaling pathway in the development of renal fibrosis. The JNK pathway is activated in response to various cellular stresses and plays an important role in cell death and inflammation. Activation of JNK signaling is a common feature in most forms of human kidney injury, evident in both intrinsic glomerular and tubular cells as well as in infiltrating leukocytes. Similar patterns of JNK activation are evident in animal models of acute and chronic renal injury. Administration of JNK inhibitors can protect against acute kidney injury and suppress the development of glomerulosclerosis and tubulointerstitial fibrosis. In particular, JNK activation in tubular epithelial cells may be a pivotal mechanism in determining the outcome of both acute kidney injury and progression of chronic kidney disease. JNK signaling promotes tubular epithelial cell production of pro-inflammatory and pro-fibrotic molecules as well as tubular cell de-differentiation toward a mesenchymal phenotype. However, the role of JNK within renal fibroblasts is less well-characterized. The JNK pathway interacts with other pro-fibrotic pathways, most notable with the TGF-β/SMAD pathway. JNK activation can augment TGF-β gene transcription, induce expression of enzymes that activate the latent form of TGF-β, and JNK directly phosphorylates SMAD3 to enhance transcription of pro-fibrotic molecules. In conclusion, JNK signaling plays an integral role in several key mechanisms operating in renal fibrosis. Targeting of JNK enzymes has therapeutic potential for the treatment of fibrotic kidney diseases.

Signaling Cross Talk between TGF-β/Smad and Other Signaling Pathways

Cytokines of the transforming growth factor β (TGF-β) family, including TGF-βs, bone morphogenic proteins (BMPs), activins, and Nodal, play crucial roles in embryonic development and adult tissue homeostasis by regulating cell proliferation, survival, and differentiation, as well as stem-cell self-renewal and lineage-specific differentiation. Smad proteins are critical downstream mediators of these signaling activities. In addition to regulating the transcription of direct target genes of TGF-β, BMP, activin, or Nodal, Smad proteins also participate in extensive cross talk with other signaling pathways, often in a cell-type- or developmental stage-specific manner. These combinatorial signals often produce context-, time-, and location-dependent biological outcomes that are critical for development. This review discusses recent progress in our understanding of the cross talk between Smad proteins and signaling pathways of Wnt, Notch, Hippo, Hedgehog (Hh), mitogen-activated protein (MAP), kinase, phosphoinositide 3-kinase (PI3K)-Akt, nuclear factor κB (NF-κB), and Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathways.

Signaling Cross Talk between TGF-β/Smad and Other Signaling Pathways

Cytokines of the transforming growth factor β (TGF-β) family, including TGF-βs, bone morphogenic proteins (BMPs), activins, and Nodal, play crucial roles in embryonic development and adult tissue homeostasis by regulating cell proliferation, survival, and differentiation, as well as stem-cell self-renewal and lineage-specific differentiation. Smad proteins are critical downstream mediators of these signaling activities. In addition to regulating the transcription of direct target genes of TGF-β, BMP, activin, or Nodal, Smad proteins also participate in extensive cross talk with other signaling pathways, often in a cell-type- or developmental stage-specific manner. These combinatorial signals often produce context-, time-, and location-dependent biological outcomes that are critical for development. This review discusses recent progress in our understanding of the cross talk between Smad proteins and signaling pathways of Wnt, Notch, Hippo, Hedgehog (Hh), mitogen-activated protein (MAP), kinase, phosphoinositide 3-kinase (PI3K)-Akt, nuclear factor κB (NF-κB), and Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathways.

Smad7: not only a regulator, but also a cross-talk mediator of TGF-β signalling

TGF-β (transforming growth factor-β) is a pleiotropic cytokine regulating diverse cellular processes. It signals through membrane-bound receptors, downstream Smad proteins and/or other signalling mediators. Smad7 has been well established to be a key negative regulator of TGF-β signalling. It antagonizes TGF-β signalling through multiple mechanisms in the cytoplasm and in the nucleus. Smad7 can be transcriptionally induced by TGF-β and other growth factors and serves as an important cross-talk mediator of the TGF-β signalling pathway with other signalling pathways. Accordingly, it plays pivotal roles in embryonic development and adult homoeostasis, and altered expression of Smad7 is often associated with human diseases, such as cancer, tissue fibrosis and inflammatory diseases.

Distinctive mechanism for sustained TGF-β signaling and growth inhibition: MEK1 activation-dependent stabilization of type II TGF-β receptors

There are multiple mechanisms by which cells evade TGF-β-mediated growth inhibitory effects. In this report, we describe a novel mechanism by which cells become resistant to TGF-β-mediated growth suppression. Although having all the components of the TGF-β signaling pathway, different cell lines, RL, HaCaT, and BJAB, have different sensitivities toward TGF-β-induced growth suppression. The TGF-β resistance of RL, a B-cell lymphoma cell line, was due to ligand-induced downregulation of TGF-β receptor II (TβRII) and only transient TGF-β induced nuclear translocation of Smad2 and Smad3. With low-dose phorbol 12-myristate 13-acetate (PMA) or anti-IgM treatment, TGF-β sensitivity was restored by stabilizing TβRII expression and sustaining TGF-β signaling. The MEK inhibitor, U0126, blocked both PMA- and anti-IgM-induced upregulation of TβRII. In HaCaT and BJAB, two TGF-β-sensitive cell lines, which had higher basal levels of phospho-MEK and TβRII compared with RL, U0126 induced downregulation of TβRII and blocked subsequent TGF-β signaling. Similar results were also obtained with normal B cells, where MEK1 inhibitor downregulated TβRII and subsequent TGF-β signaling. Constitutively active MEK1, but not constitutively active ERK2, induced upregulation of TβRII. Furthermore, TβRII physically interacted with the constitutively active MEK1, but not with wild-type MEK1, indicating involvement of active MEK1 in stabilizing TβRII. Collectively, our data suggest a novel mechanism for MEK1 in regulating the sensitivity to TGF-β signaling by stabilizing TβRII.

Emerging roles of SUMO modification in arthritis

Dynamic modification involving small ubiquitin-like modifier (SUMO) has emerged as a new mechanism of protein regulation in mammalian biology. Sumoylation is an ATP-dependent, reversible post-translational modification which occurs under both basal and stressful cellular conditions. Sumoylation profoundly influences protein functions and pertinent biological processes. For example, sumoylation modulates multiple components in the NFkappaB pathway and exerts an anti-inflammatory effect. Likewise, sumoylation of peroxisome proliferator-activated receptor gamma (PPARgamma) augments its anti-inflammatory activity. Current evidence suggests a role of sumoylation for resistance to apoptosis in synovial fibroblasts. Dynamic SUMO regulation controls the biological outcomes initiated by various growth factors involved in cartilage homeostasis, including basic fibroblast growth factors (bFGF or FGF-2), transforming growth factor-beta (TGF-beta) and insulin-like growth factor-1 (IGF-1). The impact of these growth factors on cartilage are through sumoylation-dependent control of the transcription factors (e.g., Smad, Elk-1, HIF-1) that are key regulators of matrix components (e.g., aggrecan, collagen) or cartilage-degrading enzymes (e.g., MMPs, aggrecanases). Thus, SUMO modification appears to profoundly affect chondrocyte and synovial fibroblast biology, including cell survival, inflammatory responses, matrix metabolism and hypoxic responses. More recently, evidence suggests that, in addition to their nuclear roles, the SUMO pathways play crucial roles in mitochondrial activity, cellular senescence, and autophagy. With an increasing number of reports linking SUMO to human diseases like arthritis, it is probable that novel and equally important functions of the sumoylation pathway will be elucidated in the near future.

Enhanced transforming growth factor-beta signaling and fibrogenesis in ovine fetal skeletal muscle of obese dams at late gestation

Maternal obesity (MO) is increasing at an alarming rate. The objective of this study was to evaluate the effect of MO on fibrogenesis in fetal skeletal muscle during maturation in late gestation. Nonpregnant ewes were assigned to a control diet (Con; fed 100% of NRC nutrient recommendations, n = 6) or obesogenic diet (OB; fed 150% of NRC recommendations, n = 6) from 60 days before conception, and fetal semitendenosus (St) muscle was sampled at 135 days of gestation (term 148 days). Total concentration and area of collagen in cross-sections of muscle increased by 27.0 +/- 6.0 (P < 0.05) and 105.1 +/- 5.9% (P = 0.05) in OB compared with Con fetuses. The expression of precursor TGF-beta was 177.3 +/- 47.6% higher, and concentration of phospho-p38 74.7 +/- 23.6% was higher (P < 0.05) in OB than in CON fetal muscle. Increases of 327.9 +/- 168.0 (P < 0.05) and 188.9 +/- 82.1% (P < 0.05), respectively, were observed for mRNA expression of Smad7 and fibronectin in OB compared with Con muscles. In addition, enzymes involved in collagen synthesis, including lysyl oxidase, lysyl hydroxylase 2b, and prolyl 4-hydroxylase-alpha1, were increased by 350.2 +/- 90.0 (P < 0.05), 236.5 +/- 25.2 (P < 0.05), and 82.0 +/- 36.2% (P = 0.05), respectively, in OB muscle. In conclusion, MO-enhanced fibrogenesis in fetal muscle in late gestation was associated with upregulation of the TGF-beta/p38 signaling pathway. Enhanced fibrogenesis at such an early stage of development is expected to negatively affect the properties of offspring muscle because muscle fibrosis is a hallmark of aging.

Profiling of anti-fibrotic signaling by hepatocyte growth factor in renal fibroblasts

Hepatocyte growth factor (HGF) is a multifunctional growth factor affecting cell proliferation and differentiation. Due to its mitogenic potential, HGF plays an important role in tubular repair and regeneration after acute renal injury. However, recent reports have shown that HGF also acts as an anti-inflammatory and anti-fibrotic factor, affecting various cell types such as renal fibroblasts and triggering tubulointerstitial fibrosis of the kidney. The present study provides evidence that HGF stimulation of renal fibroblasts results in the activation of both the Erk1/2 and the Akt pathways. As previously shown, Erk1/2 phosphorylation results in Smad-linker phosphorylation, thereby antagonizing cellular signals induced by TGFbeta. By siRNA mediated silencing of the Erk1/2-Smad linkage, however, we now demonstrate that Akt signaling acts as an auxiliary pathway responsible for the anti-fibrotic effects of HGF. In order to define the anti-fibrotic function of HGF we performed comprehensive expression profiling of HGF-stimulated renal fibroblasts by microarray hybridization. Functional cluster analyses and quantitative PCR assays indicate that the HGF-stimulated pathways transfer the anti-fibrotic effects in renal interstitial fibroblasts by reducing expression of extracellular matrix proteins, various chemokines, and members of the CCN family.

Role of Radiation-induced TGF-beta Signaling in Cancer Therapy

TGF-β signaling regulates several different biological processes involving cell-growth, differentiation, apoptosis, motility, angiogenesis, epithelial mesenchymal transition and extracellular matrix production that affects embryonic development and pathogenesis of various diseases, including cancer, its effects depending on the cellular context and physiological environment. Growth suppression mediated by TGF-β signaling often associated with inhibition of c-myc, cdks and induction of p15, p27, Bax and p21. Despite its growth inhibitory effect, in certain conditions TGF-β may act as a promoter of cell proliferation and invasion. Loss of responsiveness to growth suppression by TGF-β due to mutation or loss of TGF-beta type II receptor (TβRII) and Smad4 in several different cancer cells are reported. In addition, TGF-β binding to its receptor activates many non-canonical signaling pathways. Radiation induced TGF-β is primarily involved in normal tissue injury and fibrosis. Seminal studies from our group have used radio-adjuvant therapies, involving classical components of the pathway such as TβRII and SMAD4 to overcome the growth promoting effects of TGF-β. The main impediment in the radiation-induced TGF-β signaling is the induction of SMAD7 that blocks TGF-β signaling in a negative feedback manner. It is well demonstrated from our studies that the use of neutralizing antibodies against TGF- β can render a robust radio-resistant effect. Thus, understanding the functional interactions of TGF-β signaling components of the pathway with other molecules may help tailor appropriate adjuvant radio-therapeutic strategies for treatment of solid tumors.

Signaling cross-talk between TGF-beta/BMP and other pathways

Transforming growth factor-beta (TGF-beta)/bone morphogenic protein (BMP) signaling is involved in the vast majority of cellular processes and is fundamentally important during the entire life of all metazoans. Deregulation of TGF-beta/BMP activity almost invariably leads to developmental defects and/or diseases, including cancer. The proper functioning of the TGF-beta/BMP pathway depends on its constitutive and extensive communication with other signaling pathways, leading to synergistic or antagonistic effects and eventually desirable biological outcomes. The nature of such signaling cross-talk is overwhelmingly complex and highly context-dependent. Here we review the different modes of cross-talk between TGF-beta/BMP and the signaling pathways of Mitogen-activated protein kinase, phosphatidylinositol-3 kinase/Akt, Wnt, Hedgehog, Notch, and the interleukin/interferon-gamma/tumor necrosis factor-alpha cytokines, with an emphasis on the underlying molecular mechanisms.

ATF2 on the double - activating transcription factor and DNA damage response protein

Signal transduction pathways play a key role in the regulation of key cellular processes, including survival and death. Growing evidence points to changes in signaling pathway that occur during skin tumor development and progression. Such changes impact the activity of downstream substrates, including transcription factors. The activating transcription factor 2 (ATF2) has been implicated in malignant and non-malignant skin tumor developments. ATF2 mediates both transcription and DNA damage control, through its phosphorylation by JNK/p38 or ATM/ATR respectively. Here, we summarize our present understanding of ATF2 regulation, function and contribution to malignant and non-malignant skin tumor development.

Enhanced TGF-beta/Smad signaling in the early stage of diabetic nephropathy is independent of the AT1a receptor

BACKGROUND

Angiotensin II (AII) and transforming growth factor-beta (TGF-beta) are closely involved in the pathogenesis of diabetic nephropathy (DN). AII is known to induce TGF-beta production in resident renal cells, including glomerular mesangial cells and tubular epithelial cells. TGF-beta receptor types I and II (TGF-betaRI, II) are up-regulated in the diabetic kidney. The aim of this study was to clarify the role of AII in the regulation of the TGF-beta system in the early stage of DN using AII type1a receptor-deficient(AT1a(-/-)) mice.

METHODS

We investigated the expression of TGF-beta1, TGF-betaRI, II, and Smad signaling in AT1a(-/-) mice with streptozotocin (STZ)-induced DN. Mice were killed 10 and 20 days after the induction of hyperglycemia. The expression of TGF-beta receptors was analyzed by immunohistochemical staining and reverse transcriptase-polymerase chain reaction (RT-PCR). TGF-beta-specific Smad signaling was analyzed by electrophoretic mobility shift assay and Western blotting.

RESULTS

The expression of both TGF-betaRI and RII was up-regulated in the glomerular tufts and vasculature in diabetic AT1a(+/+) mice kidney by immunohistochemistry. RT-PCR revealed that mRNAs for TGF-betaRI and RII were also up-regulated. Smad2 and 4 protein levels were reduced in the renal cortex after the induction of diabetes, with an increase of Smad 3/4 complex in the nucleus. The expression of TGF-beta receptors increased in both diabetic AT1a(-/-) and AT1a(+/+) mice. Smad signaling in AT1a(-/-) mice was also enhanced.

CONCLUSIONS

Our results suggest that the complete blockade of the AT1a-mediated pathway has a minimal effect on the enhanced TGF-beta/Smad signaling in the early stage of DN, at least in the AT1a(-/-) model.

Crosstalk mechanisms between the mitogen-activated protein kinase pathways and Smad signaling downstream of TGF-beta: implications for carcinogenesis

Transforming growth factor-beta (TGF-beta) superfamily members signal via membrane-bound heteromeric serine-threonine kinase receptor complexes. Upon ligand-binding, receptor activation leads to phosphorylation of cytoplasmic protein substrates of the Smad family. Following phosphorylation and oligomerization, the latter move into the nucleus to act as transcription factors to regulate target gene expression. TGF-beta responses are not solely the result of the activation Smad cascade, but are highly cell-type specific and dependent upon interactions of Smad signaling with a variety of other intracellular signaling mechanisms, initiated or not by TGF-beta itself, that may either potentiate, synergize, or antagonize, the rather linear TGF-beta/Smad pathway. These include, (a), regulation of Smad activity by mitogen-activated protein kinases (MAPKs), (b), nuclear interaction of activated Smads with transcriptional cofactors, whether coactivators or corepressors, that may be themselves be regulated by diverse signaling mechanisms, and (c), negative feedback loops exerted by inhibitory Smads, transcriptional targets of the Smad cascade. This review focuses on how MAPKs modulate the outcome of Smad activation by TGF-beta, and how cross-signaling mechanisms between the Smad and MAPK pathways may take place and affect cell fate in the context of carcinogenesis.

Transforming growth factor-beta-activated kinase-1 (TAK1), a MAP3K, interacts with Smad proteins and interferes with osteogenesis in murine mesenchymal progenitors

TAK1 (transforming growth factor-beta-activated kinase-1), a MAP3K with considerable sequence similarity to Raf-1 and MEKK-1, has been identified as a transforming growth factor-beta/bone morphogenetic protein (BMP)-activated cytosolic component of the MAPK pathways. In this investigation, the molecular interactions between TAK1 and Smad proteins were characterized as well as their influence on BMP-mediated mesenchymal cell differentiation along the osteogenic/chondrogenic pathway. In co-immunoprecipitations we found an interaction of TAK1 with all Smads tested, R-Smads Smads1-5, the co-Smad Smad4, and the inhibitory Smads (I-Smad6 and I-Smad7). Smad interaction with TAK1 takes place through their MH2 domain. This interaction is dependent on the presence of an active kinase domain in TAK1. TAK1 dramatically interferes with R-Smad transactivation in reporter assays and affects subcellular distribution of Smad proteins. Activated TAK1 also interferes with BMP-dependent osteogenic development in murine mesenchymal progenitor cells (C3H10T 1/2). A potential TAK1-mediated apoptosis process could be excluded for these cells. Both synergistic and interfering influences of TAK1 on BMP-mediated Smad-signaling have been reported previously. We suggest that TAK1 is a factor that is involved in the fine-tuning of BMP effects during osteogenic development.

Discovery of mammalian genes that participate in virus infection

Background

Viruses are obligate intracellular parasites that rely upon the host cell for different steps in their life cycles. The characterization of cellular genes required for virus infection and/or cell killing will be essential for understanding viral life cycles, and may provide cellular targets for new antiviral therapies.

Results

Candidate genes required for lytic reovirus infection were identified by tagged sequence mutagenesis, a process that permits rapid identification of genes disrupted by gene entrapment. One hundred fifty-one reovirus resistant clones were selected from cell libraries containing 2 × 10⁵ independently disrupted genes, of which 111 contained mutations in previously characterized genes and functionally anonymous transcription units. Collectively, the genes associated with reovirus resistance differed from genes targeted by random gene entrapment in that known mutational hot spots were under represented, and a number of mutations appeared to cluster around specific cellular processes, including: IGF-II expression/signalling, vesicular transport/cytoskeletal trafficking and apoptosis. Notably, several of the genes have been directly implicated in the replication of reovirus and other viruses at different steps in the viral lifecycle.

Conclusions

Tagged sequence mutagenesis provides a rapid, genome-wide strategy to identify candidate cellular genes required for virus infection. The candidate genes provide a starting point for mechanistic studies of cellular processes that participate in the virus lifecycle and may provide targets for novel anti-viral therapies.

Targeted disruption of TGF-beta/Smad3 signaling modulates skin fibrosis in a mouse model of scleroderma

Transforming growth factor-beta (TGF-beta) is a potent stimulus of connective tissue accumulation, and is implicated in the pathogenesis of scleroderma and other fibrotic disorders. Smad3 functions as a key intracellular signal transducer for profibrotic TGF-beta responses in normal skin fibroblasts. The potential role of Smad3 in the pathogenesis of scleroderma was investigated in Smad3-null (Smad3(-/-)) mice using a model of skin fibrosis induced by subcutaneous injections of bleomycin. At early time points, bleomycin-induced macrophage infiltration in the dermis and local TGF-beta production were similar in Smad3(-/-) and wild-type mice. In contrast, at day 28, lesional skin from Smad3(-/-) mice showed attenuated fibrosis, lower synthesis and accumulation of collagen, and reduced collagen gene transcription in situ, compared to wild-type mice. Connective tissue growth factor and alpha-smooth muscle actin expression in lesional skin were also significantly attenuated. Electron microscopy revealed an absence of small diameter collagen fibrils in the dermis from bleomycin-treated Smad3(-/-) mice. Compared to fibroblasts derived from wild-type mice, Smad3(-/-) fibroblasts showed reduced in vitro proliferative and profibrotic responses elicited by TGF-beta. Together, these results indicate that ablation of Smad3 is associated with markedly altered fibroblast regulation in vivo and in vitro, and confers partial protection from bleomycin-induced scleroderma in mice. Reduced fibrosis is due to deregulated fibroblast function, as the inflammatory response induced by bleomycin was similar in wild-type and Smad3(-/-) mice.

Extracellular signal regulated kinase and SMAD signaling both mediate the angiotensin II driven progression towards overt heart failure in homozygous TGR(mRen2)27

Angiotensin (Ang) II is a key player in left ventricular (LV) remodeling and cardiac fibrosis. Its effects are thought to be transferred at least in part by mitogen-activated protein kinases (MAPK), transforming growth factor (TGF) beta1, and the Smad pathway. In this study we sought to elucidate whether Ang II related effects on LV dysfunction and fibrosis in vivo are mediated via MAPK or rather via Smad stimulation. We treated homozygous REN2 rats (7-11 weeks) with placebo, Ang II type 1 (AT1) receptor blocker or tyrphostin A46 (TYR), an inhibitor of epidermal growth factor receptor tyrosine kinase that blocks extracellular signal-regulated kinase (ERK) activity. REN2 rats had LV hypertrophy (LVH) and LV dysfunction that progressed to heart failure between 10 and 13 weeks. Blood pressure normalized over time. Renin, N-terminal atrial natriuretic peptide (N-ANP), and ERK were activated while p38 MAPK was not. Treatment with AT1 receptor blockade prevented LVH and right ventricular hypertrophy, normalized systolic and diastolic d P/d t, N-ANP levels, and reduced collagen apposition. Similarly, TYR reduced LVH, N-ANP levels, and collagen apposition. Myocardial ERK activation did not depend on AT1 receptor signaling as it was not affected by AT1 receptor blockade. TYR abolished myocardial ERK activity. Smad2 activation was inhibited by AT1 receptor blockade but was unaltered by TYR. Ang II induced LV remodeling and fibrosis are dependent on both ERK and Smad2 activation. This process is prevented by both AT1 receptor blockade and TYR, and therefore inhibition of either pathway is equally efficacious in restoring LV function and architecture.

HER2/Neu- and TAK1-mediated up-regulation of the transforming growth factor beta inhibitor Smad7 via the ETS protein ER81

The cytokine transforming growth factor beta (TGF-beta) plays an important role in preventing tumor formation by blocking cell cycle progression. Accordingly, many cancers demonstrate mutations in TGF-beta signaling components or enhanced expression of inhibitors of the TGF-beta pathway such as Smad7. In this report we show that the oncoprotein HER2/Neu is able to collaborate with the ETS transcription factor ER81 to activate Smad7 transcription in breast, endometrial, and ovarian cancer cell lines. ER81 binds to two ETS sites within the Smad7 promoter, and mutation of one of these ETS sites greatly decreases Smad7 induction by HER2/Neu and ER81. Furthermore, we show that Smad7 activation involves the processing of signals from HER2/Neu to ER81 via the ERK mitogen-activated protein kinase pathway. Thus, we have uncovered a novel mechanism by which oncogenic HER2/Neu, in collaboration with ER81, can induce carcinogenesis through Smad7 up-regulation. Moreover, we show that TAK1, a TGF-beta-activated protein kinase, stimulates ER81 via the p38 mitogen-activated protein kinase pathway and thereby induces the Smad7 promoter. This suggests that attenuation of TGF-beta signaling by activating Smad7 transcription may proceed not only through TGF-beta receptor-regulated Smad proteins but also through an independent pathway involving ER81 and TAK1.

Possible involvement of mitogen-activated protein kinase in the angiotensin II-induced fibronectin synthesis in renal interstitial fibroblasts

Angiotensin II (AT II) is thought to be associated with the development of renal interstitial fibrosis. However, the molecular mechanisms of the interstitial fibrosis have not been extensively studied. We have examined the role of mitogen-activated protein kinases (MAPKs) on fibronectin (FN) accumulation in cultured normal rat kidney interstitial fibroblasts (NRK 49F cell line). AT II caused dose-dependent increases in FN accumulation and FN mRNA in these cells. AT II also activated the extracellular signal-regulated kinase (ERK) and p38 MAPK in the presence of AT II. These increases in FN accumulation and activation of MAPKs were inhibited with AT I receptor antagonist (ARB; CV-11974) in renal interstitial fibroblasts. The inhibitors against ERK (PD98059) and p38 MAPK (SB203580) significantly inhibited AT II-induced increases in FN mRNA. These findings suggest that the MAPKs play an important role in AT II-mediated renal interstitial fibrosis and that ARB may be useful for preventing renal interstitial fibrosis.

Estradiol reverses TGF-beta1-induced mesangial cell apoptosis by a casein kinase 2-dependent mechanism

BACKGROUND

The slower rate of progression of chronic renal disease in women than in men is explained in part by the ability of estradiol to reverse the stimulatory effect of transforming growth factor-beta1 (TGF-beta1) on collagen IV synthesis at the level of casein kinase 2 activation. Casein kinase 2 also phosphorylates and activates the pro-apoptotic protein, p53. We hypothesized that estradiol would reverse TGF-beta1-induced mesangial cell apoptosis by antagonizing the stimulatory effects of TGF-beta1 on casein kinase 2 activity, thereby preventing p53 activation.

METHODS

The effects of TGF-beta1 on mesangial cell apoptosis, p53 phosphorylation, Bax and Bcl-2 levels, caspase 9 activity, and cleavage of PARP were examined. The abilities of estradiol and a specific inhibitor of CK2 (5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole) (DRB) to modulate the effects of TGF-beta1 on these processes were also examined.

RESULTS

TGF-beta1 (2 ng/mL), which up-regulates CK2 activity, induces apoptosis in murine mesangial cells together with p53 serine389 phosphorylation, up-regulation of Bax, suppression of Bcl-2, destabilization of mitochondrial permeability transition pores, stimulation of caspase 9 activity and activation of PARP. TGF-beta1-induced p53 activation and all the intermediary steps leading to mesangial cell apoptosis were reversed by estradiol (10-9 mol/L) and by DRB, potent inhibitors of CK2 activity, but not by inhibitors of the p38 MAPK, ERK or JNK signaling cascades. In contrast, TGF-beta1 failed to induce apoptosis in p53 knockout mesangial cells.

CONCLUSIONS

Our data suggest that CK2 mediates the stimulatory effects of TGF-beta1 on mesangial cell apoptosis via a p53-dependent mechanism. The ability of estradiol to reverse TGF-beta1-induced apoptosis may contribute to the protective effects of female gender on the course of chronic renal disease.