Smad3 deficiency attenuates bleomycin-induced pulmonary fibrosis in mice

The molecular genetics of lung morphogenesis and injury repair

Lung development, as well as epithelial injury repair, is tightly coordinated by a fine balance between stimulatory versus inhibitory genes that appear to co-regulate the function of stem/progenitor cells in the lung. Recently, it has been noted that many of the same genes direct development of the respiratory organs (tracheae) in the fruit fly Drosophila as in mice and men. For example, FGF receptor tyrosine kinase signaling is essential for respiratory organogenesis in both fly and mouse and is negatively regulated by the sprouty genes, a family of inducible FGF pathway inhibitors. Additionally, FGF signaling is required for formation of new alveoli, protection of alveolar epithelial cells from injury, as well as migration and proliferation of putative alveolar stem/progenitor cells during lung repair. Conversely, TGFbeta receptor serine-threonine kinase signaling via Smads 2, 3 and 4 inhibits lung morphogenesis and can inhibit postnatal alveolar development, while excessive TGFbeta signaling via Smad3 causes interstitial fibrosis. On the other hand, BMP4 stimulates morphogenesis of intact embryonic lung, while inhibiting proliferation of isolated epithelium. We speculate that evolutionary-developmental, functional conservation of the FGF- FGFR-SPROUTY stimulatory pathway as well as of the TGFbeta/BMP-SMAD modulatory pathways identifies them as potential therapeutic targets for rational therapy. Novel therapy to activate lung stem/progenitor cells, ameliorate lung injury, augment lung repair and/or induce lung regeneration could be highly beneficial in both children and adults with intractable pulmonary insufficiency.

Smad3 as a mediator of the fibrotic response

Transforming growth factor-beta (TGF-beta) plays a central role in fibrosis, contributing to the influx and activation of inflammatory cells, the epithelial to mesenchymal transdifferentiation (EMT) of cells and the influx of fibroblasts and their subsequent elaboration of extracellular matrix. TGF-beta signals through transmembrane receptor serine/threonine kinases to activate novel signalling intermediates called Smad proteins, which modulate the transcription of target genes. The use of mice with a targeted deletion of Smad3, one of the two homologous proteins which signals from TGF-beta/activin, shows that most of the pro-fibrotic activities of TGF-beta are mediated by Smad3. Smad3 null inflammatory cells and fibroblasts do not respond to the chemotactic effects of TGF-beta and do not autoinduce TGF-beta. The loss of Smad3 also interferes with TGF-beta-mediated induction of EMT and genes for collagens, plasminogen activator inhibitor-1 and the tissue inhibitor of metalloprotease-1. Smad3 null mice are resistant to radiation-induced cutaneous fibrosis, bleomycin-induced pulmonary fibrosis, carbon tetrachloride-induced hepatic fibrosis as well as glomerular fibrosis induced by induction of type 1 diabetes with streptozotocin. In fibrotic conditions that are induced by EMT, such as proliferative vitreoretinopathy, ocular capsule injury and glomerulosclerosis resulting from unilateral ureteral obstruction, Smad3 null mice also show an abrogated fibrotic response. Animal models of scleroderma, cystic fibrosis and cirrhosis implicate involvement of Smad3 in the observed fibrosis. Additionally, inhibition of Smad3 by overexpression of the inhibitory Smad7 protein or by treatment with the small molecule, halofuginone, dramatically reduces responses in animal models of kidney, lung, liver and radiation-induced fibrosis. Small moleucule inhibitors of Smad3 may have tremendous clinical potential in the treatment of pathological fibrotic diseases.

Targeted disruption of TGF-beta1/Smad3 signaling protects against renal tubulointerstitial fibrosis induced by unilateral ureteral obstruction

Tubulointerstitial fibrosis is the final common result of a variety of progressive injuries leading to chronic renal failure. Transforming growth factor-beta (TGF-beta) is reportedly upregulated in response to injurious stimuli such as unilateral ureteral obstruction (UUO), causing renal fibrosis associated with epithelial-mesenchymal transition (EMT) of the renal tubules and synthesis of extracellular matrix. We now show that mice lacking Smad3 (Smad3ex8/ex8), a key signaling intermediate downstream of the TGF-beta receptors, are protected against tubulointerstitial fibrosis following UUO as evidenced by blocking of EMT and abrogation of monocyte influx and collagen accumulation. Culture of primary renal tubular epithelial cells from wild-type or Smad3-null mice confirms that the Smad3 pathway is essential for TGF-beta1-induced EMT and autoinduction of TGF-beta1. Moreover, mechanical stretch of the cultured epithelial cells, mimicking renal tubular distention due to accumulation of urine after UUO, induces EMT following Smad3-mediated upregulation of TGF-beta1. Exogenous bone marrow monocytes accelerate EMT of the cultured epithelial cells and renal tubules in the obstructed kidney after UUO dependent on Smad3 signaling. Together the data demonstrate that the Smad3 pathway is central to the pathogenesis of interstitial fibrosis and suggest that inhibitors of this pathway may have clinical application in the treatment of obstructive nephropathy.

Smad3 mediates transforming growth factor-beta-induced alpha-smooth muscle actin expression

Transforming growth factor-beta (TGF-beta)-induced alpha-smooth muscle actin (ASMA) expression is a key indicator of myofibroblast differentiation from fibroblasts. Recent studies suggest that a TGF-beta control element is important in the regulation of the ASMA gene promoter by TGF-beta. In this study, the role of Smad3, a key component of the Smad pathway that mediates TGF-beta signaling in regulation of ASMA gene expression, is investigated. All members of the Smad family were expressed in rat lung fibroblasts, and Smad3 expression was elevated upon TGF-beta 1 treatment. Transfection with a Smad3-expressing plasmid markedly increased Smad3 and ASMA protein expression, whereas transfection with an antisense Smad3 plasmid suppressed Smad3 and ASMA expression. Similar effects were noted when the cloned rat ASMA promoter-luciferase reporter gene construct was used to monitor transcriptional activation of the ASMA gene. Electrophoretic mobility shift assays and DNA affinity precipitation indicated Smad3 binding to at least two regions of the promoter containing CAGA motifs, termed Smad3-binding elements (SBEs). Mutation of one of the SBEs decreased promoter activity significantly, indicative of a functional role for this SBE. Taken together, these findings suggest a role for Smad3 in TGF-beta regulation of ASMA gene expression in myofibroblast differentiation.

Injury and repair in lung and airways

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are common causes of morbidity and mortality in the intensive care unit. ALI/ARDS occurs as a result of systemic inflammation, usually triggered by a microorganism. Activation of leukocytes and release of proinflammatory mediators from multiple cellular sources result in both local and distant tissue injury. Tumor necrosis factor-alpha and interleukin-1 beta are the best characterized of the proinflammatory cytokines contributing to ALI/ARDS and subsequent fibrosis. The ultimate clinical course of ALI/ARDS often is determined by the ability of the injured lung to repopulate the alveolar epithelium with functional cells. Death may occur when fibrosis predominates the healing response, as it results in worsening lung compliance and oxygenation. The rodent bleomycin model of lung fibrosis allows the use of molecular tools to dissect the cellular and subcellular processes leading to fibrosis. The elements of this response may provide therapeutic targets for the prevention of this devastating complication of ALI/ARDS.

Sustained activation of fibroblast transforming growth factor-beta/Smad signaling in a murine model of scleroderma

Transforming growth factor-beta is responsible for triggering a cascade of events leading to fibrosis in scleroderma. The Smads are intracellular signal transducers recently shown to mediate fibroblast activation and other profibrotic responses elicited by transforming growth factor-betain vitro. To understand better the involvement of Smads in the pathogenesis of fibrosis, we examined Smad expression and activation in situ in a murine model of scleroderma. Bleomycin injections induced striking dermal infiltration with macrophages by 3 d, and progressive fibrosis by 2 wk. Infiltrating macrophages and resident fibroblasts expressed Smad3, the positive mediator for transforming growth factor-beta responses. Importantly, in bleomycin-injected skin, fibroblasts showed predominantly nuclear localization of Smad3 and intense staining for phospho-Smad2/3. Furthermore, phosphorylated Smad2/3 in fibroblasts was detected even after the resolution of inflammation. Expression of Smad7, the endogenous inhibitor of transforming growth factor-beta/Smad signaling, was strongly induced in dermal cells by transforming growth factor-beta, but not by bleomycin injections. Collectively, these results indicate that bleomycin-induced murine scleroderma is associated with rapid and sustained induction of transforming growth factor-beta/Smad signaling in resident dermal fibroblasts. Despite apparent activation of the intracellular transforming growth factor-beta signaling pathway in the lesional dermis, the expression of transforming growth factor-beta-inducible Smad7 was not upregulated. In light of the critical function of Smad7 as an endogenous inhibitor of Smad signaling that restricts the duration and magnitude of transforming growth factor-beta responses, and as a mediator of apoptosis, relative Smad7 deficiency observed in the present studies may account for sustained activation of transforming growth factor-beta/Smad signaling in lesional tissues. These findings raise the possibility that Smads plays an important part in the pathogenesis of fibrosis, and may therefore represent targets for selective anti-fibrotic interventions.

Growth factor signaling in lung morphogenetic centers: automaticity, stereotypy and symmetry

Lung morphogenesis is stereotypic, both for lobation and for the first several generations of airways, implying mechanistic control by a well conserved, genetically hardwired developmental program. This program is not only directed by transcriptional factors and peptide growth factor signaling, but also co-opts and is modulated by physical forces. Peptide growth factors signal within repeating epithelial-mesenchymal temporospatial patterns that constitute morphogenetic centers, automatically directing millions of repetitive events during both stereotypic branching and nonstereotypic branching as well as alveolar surface expansion phases of lung development. Transduction of peptide growth factor signaling within these centers is finely regulated at multiple levels. These may include ligand expression, proteolytic activation of latent ligand, ligand bioavailability, ligand binding proteins and receptor affinity and presentation, receptor complex assembly and kinase activation, phosphorylation and activation of adapter and messenger protein complexes as well as downstream events and cross-talk both inside and outside the nucleus. Herein we review the critical Sonic Hedgehog, Fibroblast Growth Factor, Bone Morphogenetic Protein, Vascular Endothelial Growth Factor and Transforming Growth Factorbeta signaling pathways and propose how they may be functionally coordinated within compound, highly regulated morphogenetic gradients that drive first stereotypic and then non-stereotypic, automatically repetitive, symmetrical as well as asymmetrical branching events in the lung.

Mice lacking Smad3 are protected against streptozotocin-induced diabetic glomerulopathy

Transforming growth factor-beta (TGF-beta) has been implicated in the development of diabetic glomerulopathy. In order to evaluate a role of Smad3, one of the major signaling molecules downstream of TGF-beta, in the pathogenesis of diabetic glomerulopathy, Smad3-null mice were made diabetic with streptozotocin injection and analyzed 4 weeks after induction of diabetes. Electron microscopy revealed that the thickness of glomerular basement membrane (GBM) in wild-type diabetic mice was significantly higher than that in non-diabetic mice, whereas no appreciable GBM thickening was found in Smad3-null diabetic mice. Urinary albumin excretion was dramatically increased in wild-type diabetic mice, whereas Smad3-null diabetic mice did not show any overt albuminuria. Northern blotting revealed that mRNA levels of fibronectin and alpha 3 chain of type IV collagen (alpha 3Col4) in renal cortex of wild-type diabetic mice were approximately twice as much as those of non-diabetic mice, whereas their mRNA levels were not increased in Smad3-null diabetic mice. Real-time polymerase chain reaction (PCR) also confirmed diabetes-induced upregulation of fibronectin and alpha 3Col4 in glomeruli of wild-type mice. Glomerular expression of TGF-beta 1, as assessed by real-time PCR, was enhanced to a similar degree in wild-type and smad3-null diabetic mice, indicating that the observed differences between wild-type and Smad3-null mice are not attributable to difference in the expression of TGF-beta 1. These data clearly demonstrate a critical role of Smad3 in the early phase of diabetic glomerulopathy. This may be due at least partly to the present findings that diabetes-induced upregulation of fibronectin and alpha 3Col4 is dependent on Smad3 function.

TGF-beta1-induced Smad 3 binding to the Smad 7 gene: knockout of Smad 7 gene transcription by sense phosphorothioate oligos, autoregulation, and effect on TGF-beta1 secretion: bleomycin acts through TGF-beta1

Bleomycin produces its fibrogenic effect, at least in part, by TGF-beta1 secretion. Treatment of IMR-90 human embryonic lung fibroblasts with bleomycin at 0.5 microg/ml results in a 1.6-fold increase of TGF-beta1 as determined by a specific ELISA assay for TGF-beta1 after acidification of the conditioned media. This elevation of TGF-beta1 secretion is furthermore enhanced in vivo by TGF-beta1 autoinduction of the TGF-beta1 gene. To demonstrate TGF-beta1 autoinduction, the fibroblasts were pretreated with 12.5 ng/ml TGF-beta1, washed extensively to remove any residual TGF-beta1, and then allowed to incubate for 24 h in AIM V synthetic serum-free media. The media when assayed using the ELISA assay contained a 1.6-fold increase of TGF-beta1. The distal promoter of the human TGF-beta1 gene contains a Smad 3 element (CAGGACA), which is homologous to the Smad 3 binding element motif (CAGA). The nuclear extracts of human embryonic lung fibroblasts treated for either 15 min or 24 h with TGF-beta1 did not demonstrate specificity of binding of a protein(s) to the homologous Smad 3 element as determined by cold wild-type oligodeoxynucleotide competition experiments. However, specific Smad 3 binding to the Smad 3 element (GTCTAGAC) found in proximal promoter of the Smad 7 gene was observed by cold oligo competition and supershift assays using a goat polyclonal Smad 3 antibody in the presence and absence of an N-terminal Smad 3 peptide. To determine the functionality of this Smad 3 binding to the Smad 3 element in the proximal promoter of the Smad 7 inhibitory gene to TGF-beta1 secretion, fibroblasts were transiently pretransfected with double-stranded phosphorothioate oligo "decoys" containing the Smad 7/Smad 3 element in the presence of plasmin to convert latent TGF-beta1 to active TGF-beta1. Under these conditions, which simulate the in vivo situation of 2.2-fold increase of total active TGF-beta1 was observed. Fibroblasts were also pretransfected with these double-stranded oligo "decoys," washed, then treated with TGF-beta1, washed and incubated in AIM V for an additional 24 h. In this latter experiment, a superinduction of TGF-beta1 secretion was observed. We propose that these oligo "decoys" bind Smad 3 preventing this initiation factor from binding to the Smad 7/Smad 3 element thereby decreasing the transcription of the Smad 7 gene. The decrease of the inhibitory Smad 7 would result in less binding of this Smad inhibitor to the Type I TGF-beta receptor and less antagonism of active TGF-beta1, more autoinduction of the TGF-beta1 gene, and more of the fibrogenic effects of TGF-beta1.

Role of TGF-beta signaling in extracellular matrix production under high glucose conditions

BACKGROUND

Hyperglycemia has been shown to play an important role in diabetic renal and vascular complications. Some studies show that high glucose may mediate diabetic complications by stimulating extracellular matrix (ECM) production. We hypothesize that this may be mediated by activating transforming growth factor-beta (TGF-beta)/Smads signaling.

METHODS

Renal and vascular cells were cultured under high glucose conditions in the presence or absence of a neutralizing TGF-beta antibody and examined for activation of Smad signaling and collagen production. The regulating role of Smad signaling in high glucose-induced collagen synthesis was determined by inducing overexpression of the inhibitory Smad7 in a stable Smad7-expressing tubular cell line.

RESULTS

Activation of Smad signaling, as evidenced by Smad2 and Smad3 nuclear translocation and phosphorylation, was found in renal and vascular cells at 24 hours after high glucose stimulation (up to 55% increased). This was associated with de novo synthesis of collagen I at day 3 by all cell types. High glucose-induced activation of Smad signaling and collagen synthesis were TGF-beta-dependent since these were associated with a significant increase in TGF-beta production at 24 hours (P < 0.01) and were blocked by a neutralizing TGF-beta antibody. Importantly, overexpression of Smad7 resulted in marked inhibition of high glucose-induced Smad2 and Smad3 activation and type I collagen synthesis, suggesting that Smad signaling is a key pathway in high glucose-mediated renal and vascular scarring.

CONCLUSION

High glucose acts by activating the TGF-beta dependent Smad signaling pathway to stimulate collagen synthesis by renal and vascular cells. Smad signaling plays a critical role in regulating high-glucose-mediated diabetic renal and vascular complications.

Impact of interleukin-13 responsiveness on the synthetic and proliferative properties of Th1- and Th2-type pulmonary granuloma fibroblasts

Interleukin-13 (IL-13) has emerged as a major cytokine mediator of fibroblast activation and pulmonary fibrosis. Normal (from noninflamed lung), Th1-type (induced by the pulmonary embolization of purified peptide derivative-coated beads in mice sensitized to purified peptide derivative), and Th2-type (induced by the pulmonary embolization of Schistosoma mansoni egg antigen-coated beads in mice sensitized with S. mansoni eggs) primary fibroblast cell lines all exhibited constitutive gene expression of two receptor chains that bind and signal IL-13-mediated cellular events: IL-4Ralpha and IL-13Ralpha1. However, all three fibroblast cell lines exhibited divergent synthetic and proliferative responses to the exogenous addition of either recombinant IL-13 or a chimeric protein comprised of IL-13 and a truncated version of Pseudomonas exotoxin (IL13-PE), which targets and kills IL-13 receptor overexpressing cells. The exogenous addition of IL-13 to Th1-type and Th2-type fibroblast cultures significantly increased the cellular expression of IL-13Ralpha2, which may function as an IL-13 decoy receptor. After a 24-hour exposure to IL-13, the total collagen generation and cellular proliferation by Th2-type fibroblasts were significantly higher than that observed in similar numbers of normal and Th1-type fibroblasts. In addition IL13-PE, which binds with highest affinity to IL-13Ralpha2, exhibited down-regulatory effects on proliferation and matrix generation expression by Th1- and Th2-type, but not normal, fibroblasts. Thus, these data demonstrate that fibroblasts derived from murine pulmonary granulomas exhibit divergent expression of functional IL-13 receptor and this expression dictates the responsiveness and susceptibility to recombinant IL-13 and IL-13 immunotoxin, respectively.

Cancer pharmacogenomics: current and future applications

Heterogeneity in patient response to chemotherapy is consistently observed across patient populations. Pharmacogenomics is the study of inherited differences in interindividual drug disposition and effects, with the goal of selecting the optimal drug therapy and dosage for each patient. Pharmacogenomics is especially important for oncology, as severe systemic toxicity and unpredictable efficacy are hallmarks of cancer therapies. In addition, genetic polymorphisms in drug metabolizing enzymes and other molecules are responsible for much of the interindividual differences in the efficacy and toxicity of many chemotherapy agents. This review will discuss clinically relevant examples of gene polymorphisms that influence the outcome of cancer therapy, and whole-genome expression studies using microarray technology that have shown tremendous potential for benefiting cancer pharmacogenomics. The power and utility of the mouse as an experimental system for pharmacogenomic discovery will also be discussed in the context of cancer therapy.

Intratracheal gene transfer of decorin reduces subpleural fibroproliferation induced by bleomycin

Decorin, a small leucin-rich proteoglycan, is a negative regulator of transforming growth factor-beta, but the antifibrotic effect of decorin gene transfer has not been examined in a mouse model of usual interstitial pneumonia (UIP). We constructed a replication-defective recombinant adenovirus harboring human decorin gene (AdCMV.DC) and administered 1 x l0(9) plaque-forming units of AdCMV.DC intratracheally or intravenously to C57BL/6 mice with intraperitoneal injection of bleomycin, which induces a subpleural fibroproliferation, mimicking UIP, by day 28. Only intratracheal administration of AdCMV.DC increased decorin mRNA expression in the lung and decreased the hydroxyproline content augmented in bleomycin-induced pulmonary fibrosis (1.13 +/- 0.02 to 0.96 +/- 0.02, P = 0.006). In contrast, intravenous administration of AdCMV.DC increased the decorin expression only in the liver, but not in the lung, and without reducing lung fibrosis. These results indicate that adenoviral decorin gene transfer is effective only by direct administration to fibrosing lungs.

Smad3 mediates the TGF-beta-induced contraction of type I collagen gels by mouse embryo fibroblasts

TGF-beta signals through TGF-beta receptors and Smad proteins. TGF-beta also augments fibroblast-mediated collagen gel contraction, an in vitro model of connective tissue remodeling. To investigate the importance of Smad2 or Smad3 in this augmentation process, embryo-derived fibroblasts from mice lacking expression of Smad2 or Smad3 genes were cast into native type I collagen gels. Fibroblast-populated gels were then released into 0.2% FCS-DMEM alone or with recombinant human TGF-beta1, beta2, beta3, or recombinant rat PDGF-BB. Gel contraction was determined using an image analyzer. All three isoforms of TGF-beta significantly augmented contraction of collagen gels mediated by fibroblasts with genotypes of Smad2 knockout (S2KO), Smad2 wildtype (S2WT), and Smad3 wildtype (S3WT), but not Smad3 knockout (S3KO) mice. PDGF-BB augmented collagen gel contraction by all fibroblast types. These results suggest that expression of Smad3 but not Smad2 may be critical in TGF-beta augmentation of fibroblast-mediated collagen gel contraction. Thus, the Smad3 gene could be a target for blocking contraction of fibrotic tissue induced by TGF-beta.

Pathogenesis of fibrosis: role of TGF-beta and CTGF

Transforming growth factor (TGF)-beta regulates diverse biologic activities including cell growth, cell death or apoptosis, cell differentiation, and extracellular matrix (ECM) synthesis. TGF-beta is believed to be a key mediator of tissue fibrosis as a consequence of ECM accumulation in pathologic states such as systemic sclerosis. TGF-beta is known to induce the expression of ECM proteins in mesenchymal cells, and to stimulate the production of protease inhibitors that prevent enzymatic breakdown of the ECM. Connective tissue growth factor (CTGF), which is induced by TGF-beta, has been reported to mediate stimulatory actions of TGF-beta ECM synthesis. This review focuses on the possible role of TGF-beta and CTGF in the pathogenesis of fibrosis.