Role of the latent TGF-beta binding protein in the activation of latent TGF-beta by co-cultures of endothelial and smooth muscle cells

Transcriptome and proteome profiling of activated cardiac fibroblasts supports target prioritization in cardiac fibrosis

BACKGROUND

Activated cardiac fibroblasts (CF) play a central role in cardiac fibrosis, a condition associated with most cardiovascular diseases. Conversion of quiescent into activated CF sustains heart integrity upon injury. However, permanence of CF in active state inflicts deleterious heart function effects. Mechanisms underlying this cell state conversion are still not fully disclosed, contributing to a limited target space and lack of effective anti-fibrotic therapies.

MATERIALS AND METHODS

To prioritize targets for drug development, we studied CF remodeling upon activation at transcriptomic and proteomic levels, using three different cell sources: primary adult CF (aHCF), primary fetal CF (fHCF), and induced pluripotent stem cells derived CF (hiPSC-CF).

RESULTS

All cell sources showed a convergent response upon activation, with clear morphological and molecular remodeling associated with cell-cell and cell-matrix interactions. Quantitative proteomic analysis identified known cardiac fibrosis markers, such as FN1, CCN2, and Serpine1, but also revealed targets not previously associated with this condition, including MRC2, IGFBP7, and NT5DC2.

CONCLUSION

Exploring such targets to modulate CF phenotype represents a valuable opportunity for development of anti-fibrotic therapies. Also, we demonstrate that hiPSC-CF is a suitable cell source for preclinical research, displaying significantly lower basal activation level relative to primary cells, while being able to elicit a convergent response upon stimuli.

Latent Transforming Growth Factor β Binding Protein 3 Controls Adipogenesis

Transforming growth factor-beta (TGFβ) is released from cells as part of a trimeric latent complex consisting of TGFβ, the TGFβ propeptides, and either a latent TGFβ binding protein (LTBP) or glycoprotein-A repetitions predominant (GARP) protein. LTBP1 and 3 modulate latent TGFβ function with respect to secretion, matrix localization, and activation and, therefore, are vital for the proper function of the cytokine in a number of tissues. TGFβ modulates stem cell differentiation into adipocytes (adipogenesis), but the potential role of LTBPs in this process has not been studied. We observed that 72 h post adipogenesis initiation Ltbp1, 2, and 4 expression levels decrease by 74-84%, whereas Ltbp3 expression levels remain constant during adipogenesis. We found that LTBP3 silencing in C3H/10T1/2 cells reduced adipogenesis, as measured by the percentage of cells with lipid vesicles and the expression of the transcription factor peroxisome proliferator-activated receptor gamma (PPARγ). Lentiviral mediated expression of an Ltbp3 mRNA resistant to siRNA targeting rescued the phenotype, validating siRNA specificity. Knockdown (KD) of Ltbp3 expression in 3T3-L1, M2, and primary bone marrow stromal cells (BMSC) indicated a similar requirement for Ltbp3. Epididymal and inguinal white adipose tissue fat pad weights of Ltbp3^-/- mice were reduced by 62% and 57%, respectively, compared to wild-type mice. Inhibition of adipogenic differentiation upon LTBP3 loss is mediated by TGFβ, as TGFβ neutralizing antibody and TGFβ receptor I kinase blockade rescue the LTBP3 KD phenotype. These results indicate that LTBP3 has a TGFβ-dependent function in adipogenesis both in vitro and possibly in vivo.

Transforming growth factor β latency: A mechanism of cytokine storage and signalling regulation in liver homeostasis and disease

Transforming growth factor-β (TGF-β) is a potent effector in the liver, which is involved in a plethora of processes initiated upon liver injury. TGF-β affects parenchymal, non-parenchymal, and inflammatory cells in a highly context-dependent manner. Its bioavailability is critical for a fast response to various insults. In the liver – and probably in other organs – this is made possible by the deposition of a large portion of TGF-β in the extracellular matrix as an inactivated precursor form termed latent TGF-β (L-TGF-β). Several matrisomal proteins participate in matrix deposition, latent complex stabilisation, and activation of L-TGF-β. Extracellular matrix protein 1 (ECM1) was recently identified as a critical factor in maintaining the latency of deposited L-TGF-β in the healthy liver. Indeed, its depletion causes spontaneous TGF-β signalling activation with deleterious effects on liver architecture and function. This review article presents the current knowledge on intracellular L-TGF-β complex formation, secretion, matrix deposition, and activation and describes the proteins and processes involved. Further, we emphasise the therapeutic potential of toning down L-TGF-β activation in liver fibrosis and liver cancer.

The role of LTBPs in TGF beta signaling

The purpose of this review is to discuss the transforming growth factor beta (TGFB) binding proteins (LTBP) with respect to their participation in the activity of TGFB. We first describe pertinent aspects of the biology and cell function of the LTBPs. We then summarize the physiological consequences of LTBP loss in humans and mice. Finally, we consider a number of outstanding questions relating to LTBP function.

Exploiting Canonical TGFβ Signaling in Cancer Treatment

Transforming growth factor β (TGFβ) is a pleiotropic cytokine that plays critical roles to define cancer cell phenotypes, construct the tumor microenvironment, and suppress anti-tumor immune responses. As such, TGFβ is a lynchpin for integrating cancer cell intrinsic pathways and communication among host cells in the tumor and beyond that together affect responses to genotoxic, targeted, and immune therapy. Despite decades of preclinical and clinical studies, evidence of clinical benefit from targeting TGFβ in cancer remains elusive. Here, we review the mechanisms by which TGFβ acts to oppose successful cancer therapy, the reported prognostic and predictive value of TGFβ biomarkers, and the potential impact of inhibiting TGFβ in precision oncology. Paradoxically, the diverse mechanisms by which TGFβ impedes therapeutic response are a principal barrier to implementing TGFβ inhibitors because it is unclear which TGFβ mechanism is functional in which patient. Companion diagnostic tools and specific biomarkers of TGFβ targeted biology will be the key to exploiting TGFβ biology for patient benefit.

TGFβ-Neurotrophin Interactions in Heart, Retina, and Brain

Ischemic insults to the heart and brain, i.e., myocardial and cerebral infarction, respectively, are amongst the leading causes of death worldwide. While there are therapeutic options to allow reperfusion of ischemic myocardial and brain tissue by reopening obstructed vessels, mitigating primary tissue damage, post-infarction inflammation and tissue remodeling can lead to secondary tissue damage. Similarly, ischemia in retinal tissue is the driving force in the progression of neovascular eye diseases such as diabetic retinopathy (DR) and age-related macular degeneration (AMD), which eventually lead to functional blindness, if left untreated. Intriguingly, the easily observable retinal blood vessels can be used as a window to the heart and brain to allow judgement of microvascular damages in diseases such as diabetes or hypertension. The complex neuronal and endocrine interactions between heart, retina and brain have also been appreciated in myocardial infarction, ischemic stroke, and retinal diseases. To describe the intimate relationship between the individual tissues, we use the terms heart-brain and brain-retina axis in this review and focus on the role of transforming growth factor β (TGFβ) and neurotrophins in regulation of these axes under physiologic and pathologic conditions. Moreover, we particularly discuss their roles in inflammation and repair following ischemic/neovascular insults. As there is evidence that TGFβ signaling has the potential to regulate expression of neurotrophins, it is tempting to speculate, and is discussed here, that cross-talk between TGFβ and neurotrophin signaling protects cells from harmful and/or damaging events in the heart, retina, and brain.

Vascular endothelial cell specification in health and disease

There are two vascular networks in mammals that coordinately function as the main supply and drainage systems of the body. The blood vasculature carries oxygen, nutrients, circulating cells, and soluble factors to and from every tissue. The lymphatic vasculature maintains interstitial fluid homeostasis, transports hematopoietic cells for immune surveillance, and absorbs fat from the gastrointestinal tract. These vascular systems consist of highly organized networks of specialized vessels including arteries, veins, capillaries, and lymphatic vessels that exhibit different structures and cellular composition enabling distinct functions. All vessels are composed of an inner layer of endothelial cells that are in direct contact with the circulating fluid; therefore, they are the first responders to circulating factors. However, endothelial cells are not homogenous; rather, they are a heterogenous population of specialized cells perfectly designed for the physiological demands of the vessel they constitute. This review provides an overview of the current knowledge of the specification of arterial, venous, capillary, and lymphatic endothelial cell identities during vascular development. We also discuss how the dysregulation of these processes can lead to vascular malformations, and therapeutic approaches that have been developed for their treatment.

On-Target Anti-TGF-β Therapies Are Not Succeeding in Clinical Cancer Treatments: What Are Remaining Challenges?

Metastasis is the leading cause of death for cancer patients. During cancer progression, the initial detachment of cells from the primary tumor and the later colonization of a secondary organ are characterized as limiting steps for metastasis. Epithelial-mesenchymal transition (EMT) and mesenchymal-epithelial transition (MET) are opposite dynamic multistep processes that enable these critical events in metastasis by altering the phenotype of cancer cells and improving their ability to migrate, invade and seed at distant organs. Among the molecular pathways that promote tumorigenesis in late-stage cancers, transforming growth factor-β (TGF-β) is described as an EMT master inducer by controlling different genes and proteins related to cytoskeleton assembly, cell-cell attachment and extracellular matrix remodeling. Still, despite the successful outcomes of different TGF-β pharmacological inhibitors in cell culture (in vitro) and animal models (in vivo), results in cancer clinical trials are poor or inconsistent at least, highlighting the existence of crucial components in human cancers that have not been properly explored. Here we review most recent findings to provide perspectives bridging the gap between on-target anti-TGF-β therapies in vitro and in pre-clinical models and the poor clinical outcomes in treating cancer patients. Specifically, we focus on (i) the dual roles of TGF-β signaling in cancer metastasis; (ii) dynamic signaling; (iii) functional differences of TGF-β free in solution vs. in exosomes; (iv) the regulatory effects of tumor microenvironment (TME) – particularly by cancer-associated fibroblasts – on TGF-β signaling pathway. Clearly identifying and establishing those missing links may provide strategies to revitalize and clinically improve the efficacy of TGF-β targeted therapies.

On-Target Anti-TGF-β Therapies Are Not Succeeding in Clinical Cancer Treatments: What Are Remaining Challenges?

Metastasis is the leading cause of death for cancer patients. During cancer progression, the initial detachment of cells from the primary tumor and the later colonization of a secondary organ are characterized as limiting steps for metastasis. Epithelial-mesenchymal transition (EMT) and mesenchymal-epithelial transition (MET) are opposite dynamic multistep processes that enable these critical events in metastasis by altering the phenotype of cancer cells and improving their ability to migrate, invade and seed at distant organs. Among the molecular pathways that promote tumorigenesis in late-stage cancers, transforming growth factor-β (TGF-β) is described as an EMT master inducer by controlling different genes and proteins related to cytoskeleton assembly, cell-cell attachment and extracellular matrix remodeling. Still, despite the successful outcomes of different TGF-β pharmacological inhibitors in cell culture (in vitro) and animal models (in vivo), results in cancer clinical trials are poor or inconsistent at least, highlighting the existence of crucial components in human cancers that have not been properly explored. Here we review most recent findings to provide perspectives bridging the gap between on-target anti-TGF-β therapies in vitro and in pre-clinical models and the poor clinical outcomes in treating cancer patients. Specifically, we focus on (i) the dual roles of TGF-β signaling in cancer metastasis; (ii) dynamic signaling; (iii) functional differences of TGF-β free in solution vs. in exosomes; (iv) the regulatory effects of tumor microenvironment (TME) - particularly by cancer-associated fibroblasts - on TGF-β signaling pathway. Clearly identifying and establishing those missing links may provide strategies to revitalize and clinically improve the efficacy of TGF-β targeted therapies.

TGF-β1 in Vascular Wall Pathology: Unraveling Chronic Venous Insufficiency Pathophysiology

Chronic venous insufficiency and varicose veins occur commonly in affluent countries and are a socioeconomic burden. However, there remains a relative lack of knowledge about venous pathophysiology. Various theories have been suggested, yet the molecular sequence of events is poorly understood. Transforming growth factor-beta one (TGF-β1) is a highly complex polypeptide with multifunctional properties that has an active role during embryonic development, in adult organ physiology and in the pathophysiology of major diseases, including cancer and various autoimmune, fibrotic and cardiovascular diseases. Therefore, an emphasis on understanding its signaling pathways (and possible disruptions) will be an essential requirement for a better comprehension and management of specific diseases. This review aims at shedding more light on venous pathophysiology by describing the TGF-β1 structure, function, activation and signaling, and providing an overview of how this growth factor and disturbances in its signaling pathway may contribute to specific pathological processes concerning the vessel wall which, in turn, may have a role in chronic venous insufficiency.

Transforming Growth Factor β1 Function in Airway Remodeling and Hyperresponsiveness. The Missing Link?

The pathogenesis of asthma includes a complex interplay among airway inflammation, hyperresponsiveness, and remodeling. Current evidence suggests that airway structural cells, including bronchial smooth muscle cells, myofibroblasts, fibroblasts, and epithelial cells, mediate all three aspects of asthma pathogenesis. Although studies show a connection between airway remodeling and changes in bronchomotor tone, the relationship between the two remains unclear. Transforming growth factor β1 (TGF-β1), a growth factor elevated in the airway of patients with asthma, plays a role in airway remodeling and in the shortening of various airway structural cells. However, the role of TGF-β1 in mediating airway hyperresponsiveness remains unclear. In this review, we summarize the literature addressing the role of TGF-β1 in airway remodeling and shortening. Through our review, we aim to further elucidate the role of TGF-β1 in asthma pathogenesis and the link between airway remodeling and airway hyperresponsiveness in asthma and to define TGF-β1 as a potential therapeutic target for reducing asthma morbidity and mortality.

Stromal Modulators of TGF-β in Cancer

Transforming growth factor-β (TGF-β) is an intriguing cytokine exhibiting dual activities in malignant disease. It is an important mediator of cancer invasion, metastasis and angiogenesis, on the one hand, while it exhibits anti-tumor functions on the other hand. Elucidating the precise role of TGF-β in malignant development and progression requires a better understanding of the molecular mechanisms involved in its tumor suppressor to tumor promoter switch. One important aspect of TGF-β function is its interaction with proteins within the tumor microenvironment. Several stromal proteins have the natural ability to interact and modulate TGF-β function. Understanding the complex interplay between the TGF-β signaling network and these stromal proteins may provide greater insight into the development of novel therapeutic strategies that target the TGF-β axis. The present review highlights our present understanding of how stroma modulates TGF-β activity in human cancers.

Egr2 and Egr3 in regulatory T cells cooperatively control systemic autoimmunity through Ltbp3-mediated TGF-β3 production

Systemic lupus erythematosus (SLE) is a prototypical autoimmune disease characterized by multiorgan inflammation induced by autoantibodies. Early growth response gene 2 (Egr2), a transcription factor essential for T-cell anergy induction, controls systemic autoimmunity in mice and humans. We have previously identified a subpopulation of CD4⁺ regulatory T cells, CD4⁺CD25^-LAG3⁺ cells, that characteristically express both Egr2 and LAG3 and control mice model of lupus via TGF-β3 production. However, due to the mild phenotype of lymphocyte-specific Egr2-deficient mice, the presence of an additional regulator has been speculated. Here, we show that Egr2 and Egr3 expressed in T cells cooperatively prevent humoral immune responses by supporting TGF-β3 secretion. T cell-specific Egr2/Egr3 double-deficient (Egr2/3DKO) mice spontaneously developed an early onset lupus-like disease that was more severe than in T cell-specific Egr2-deficient mice. In accordance with the observation that CD4⁺CD25^-LAG3⁺ cells from Egr2/3DKO mice completely lost the capacity to produce TGF-β3, the excessive germinal center reaction in Egr2/3DKO mice was suppressed by the adoptive transfer of WT CD4⁺CD25^-LAG3⁺ cells or treatment with a TGF-β3-expressing vector. Intriguingly, latent TGF-β binding protein (Ltbp)3 expression maintained by Egr2 and Egr3 was required for TGF-β3 production from CD4⁺CD25^-LAG3⁺ cells. Because Egr2 and Egr3 did not demonstrate cell intrinsic suppression of the development of follicular helper T cells, Egr2- and Egr3-dependent TGF-β3 production by CD4⁺CD25^-LAG3⁺ cells is critical for controlling excessive B-cell responses. The unique attributes of Egr2/Egr3 in T cells may provide an opportunity for developing novel therapeutics for autoantibody-mediated diseases including SLE.

Aggregatibacter actinomycetemcomitans outer membrane protein 29 (Omp29) induces TGF-β-regulated apoptosis signal in human gingival epithelial cells via fibronectin/integrinβ1/FAK cascade

Gingival junctional epithelial cell apoptosis caused by periodontopathic bacteria exacerbates periodontitis. This pathological apoptosis is involved in the activation of transforming growth factor β (TGF-β). However, the molecular mechanisms by which microbes induce the activation of TGF-β remain unclear. We previously reported that Aggregatibacter actinomycetemcomitans (Aa) activated TGF-β receptor (TGF-βR)/smad2 signalling to induce epithelial cell apoptosis, even though Aa cannot bind to TGF-βR. Additionally, outer membrane protein 29 kDa (Omp29), a member of the Aa Omps family, can induce actin rearrangements via focal adhesion kinase (FAK) signalling, which also plays a role in the activation of TGF-β by cooperating with integrin. Accordingly, we hypothesized that Omp29-induced actin rearrangements via FAK activity would enhance the activation of TGF-β, leading to gingival epithelial cell apoptosis in vitro. By using human gingival epithelial cell line OBA9, we found that Omp29 activated TGF-βR/smad2 signalling and decreased active TGF-β protein levels in the extracellular matrix (ECM) of cell culture, suggesting the transactivation of TGF-βR. Inhibition of actin rearrangements by cytochalasin D or blebbistatin and knockdown of FAK or integrinβ1 expression by siRNA transfection attenuated TGF-βR/smad2 signalling activity and reduction of TGF-β levels in the ECM caused by Omp29. Furthermore, Omp29 bound to fibronectin (Fn) to induce its aggregation on integrinβ1, which is associated with TGF-β signalling activity. All the chemical inhibitors and siRNAs tested blocked Omp29-induced OBA9 cells apoptosis. These results suggest that Omp29 binds to Fn in order to facilitate Fn/integrinβ1/FAK signalling-dependent TGF-β release from the ECM, thereby inducing gingival epithelial cell apoptosis via TGF-βR/smad2 pathway.

Regulation of the Bioavailability of TGF-β and TGF-β-Related Proteins

The bioavailability of members of the transforming growth factor β (TGF-β) family is controlled by a number of mechanisms. Bona fide TGF-β is sequestered into the matrix in a latent state and must be activated before it can bind to its receptors. Here, we review the molecules and mechanisms that regulate the bioavailability of TGF-β and compare these mechanisms with those used to regulate other TGF-β family members. We also assess the physiological significance of various latent TGF-β activators, as well as other extracellular modulators of TGF-β family signaling, by examining the available in vivo data from knockout mouse models and other biological systems.

Regulation of the Bioavailability of TGF-β and TGF-β-Related Proteins

The bioavailability of members of the transforming growth factor β (TGF-β) family is controlled by a number of mechanisms. Bona fide TGF-β is sequestered into the matrix in a latent state and must be activated before it can bind to its receptors. Here, we review the molecules and mechanisms that regulate the bioavailability of TGF-β and compare these mechanisms with those used to regulate other TGF-β family members. We also assess the physiological significance of various latent TGF-β activators, as well as other extracellular modulators of TGF-β family signaling, by examining the available in vivo data from knockout mouse models and other biological systems.

Genetic analysis of the contribution of LTBP-3 to thoracic aneurysm in Marfan syndrome

Marfan syndrome (MFS) is an autosomal dominant disorder of connective tissue, caused by mutations of the microfibrillar protein fibrillin-1, that predisposes affected individuals to aortic aneurysm and rupture and is associated with increased TGFβ signaling. TGFβ is secreted from cells as a latent complex consisting of TGFβ, the TGFβ propeptide, and a molecule of latent TGFβ binding protein (LTBP). Improper extracellular localization of the latent complex can alter active TGFβ levels, and has been hypothesized as an explanation for enhanced TGFβ signaling observed in MFS. We previously reported the absence of LTBP-3 in matrices lacking fibrillin-1, suggesting that perturbed TGFβ signaling in MFS might be due to defective interaction of latent TGFβ complexes containing LTBP-3 with mutant fibrillin-1 microfibrils. To test this hypothesis, we genetically suppressed Ltbp3 expression in a mouse model of progressively severe MFS. Here, we present evidence that MFS mice lacking LTBP-3 have improved survival, essentially no aneurysms, reduced disruption and fragmentation of medial elastic fibers, and decreased Smad2/3 and Erk1/2 activation in their aortas. These data suggest that, in MFS, improper localization of latent TGFβ complexes composed of LTBP-3 and TGFβ contributes to aortic disease progression.

Mesenchymal stromal cells express GARP/LRRC32 on their surface: effects on their biology and immunomodulatory capacity

Mesenchymal stromal cells (MSCs) represent a promising tool for therapy in regenerative medicine, transplantation, and autoimmune disease due to their trophic and immunomodulatory activities. However, we are still far from understanding the mechanisms of action of MSCs in these processes. Transforming growth factor (TGF)-β1 is a pleiotropic cytokine involved in MSC migration, differentiation, and immunomodulation. Recently, glycoprotein A repetitions predominant (GARP) was shown to bind latency-associated peptide (LAP)/TGF-β1 to the cell surface of activated Foxp3⁺ regulatory T cells (Tregs) and megakaryocytes/platelets. In this manuscript, we show that human and mouse MSCs express GARP which presents LAP/TGF-β1 on their cell surface. Silencing GARP expression in MSCs increased their secretion and activation of TGF-β1 and reduced their proliferative capacity in a TGF-β1-independent manner. Importantly, we showed that GARP expression on MSCs contributed to their ability to inhibit T-cell responses in vitro. In summary, we have found that GARP is an essential molecule for MSC biology, regulating their immunomodulatory and proliferative activities. We envision GARP as a new target for improving the therapeutic efficacy of MSCs and also as a novel MSC marker. Stem Cells2015;33:183–195

Targeting latent TGFβ release in muscular dystrophy

Latent transforming growth factor-β (TGFβ) binding proteins (LTBPs) bind to inactive TGFβ in the extracellular matrix. In mice, muscular dystrophy symptoms are intensified by a genetic polymorphism that changes the hinge region of LTBP, leading to increased proteolytic susceptibility and TGFβ release. We have found that the hinge region of human LTBP4 was also readily proteolysed and that proteolysis could be blocked by an antibody to the hinge region. Transgenic mice were generated to carry a bacterial artificial chromosome encoding the human LTBP4 gene. These transgenic mice displayed larger myofibers, increased damage after muscle injury, and enhanced TGFβ signaling. In the mdx mouse model of Duchenne muscular dystrophy, the human LTBP4 transgene exacerbated muscular dystrophy symptoms and resulted in weaker muscles with an increased inflammatory infiltrate and greater LTBP4 cleavage in vivo. Blocking LTBP4 cleavage may be a therapeutic strategy to reduce TGFβ release and activity and decrease inflammation and muscle damage in muscular dystrophy.

Development of a novel multiplexed assay for quantification of transforming growth factor-β (TGF-β)

Changes in activity or levels of transforming growth factor-β (TGF-β) are associated with a variety of diseases; however, measurement of TGF-β in biological fluids is highly variable. TGF-β is biologically inert when associated with its latency-associated peptide (LAP). Most available immunoassays require exogenous activation by acid/heat to release TGF-β from the latent complex. We developed a novel electrochemiluminescence-based multiplexed assay on the MesoScale Discovery® platform that eliminates artificial activation, simultaneously measures both active TGF-β1 and LAP1 and includes an internal control for platelet-derived TGF-β contamination in blood specimens. We optimized this assay to evaluate plasma levels as a function of activation type and clinical specimen preparation. We determined that breast cancer patients' plasma have higher levels of circulating latent TGF-β (LTGF-β) as measured by LAP1 than healthy volunteers (p < 0.0001). This assay provides a robust tool for correlative studies of LTGF-β levels with disease, treatment outcomes and toxicity with a broad clinical applicability.

TGF- β: an important mediator of allergic disease and a molecule with dual activity in cancer development

The transforming growth factor- β (TGF- β ) superfamily is a family of structurally related proteins that includes TGF- β , activins/inhibins, and bone morphogenic proteins (BMPs). Members of the TGF- β superfamily regulate cellular functions such as proliferation, apoptosis, differentiation, and migration and thus play key roles in organismal development. TGF- β is involved in several human diseases, including autoimmune disorders and vascular diseases. Activation of the TGF- β receptor induces phosphorylation of serine/threonine residues and triggers phosphorylation of intracellular effectors (Smads). Once activated, Smad proteins translocate to the nucleus and induce transcription of their target genes, regulating various processes and cellular functions. Recently, there has been an attempt to correlate the effect of TGF- β with various pathological entities such as allergic diseases and cancer, yielding a new area of research known as "allergooncology," which investigates the mechanisms by which allergic diseases may influence the progression of certain cancers. This knowledge could generate new therapeutic strategies aimed at correcting the pathologies in which TGF- β is involved. Here, we review recent studies that suggest an important role for TGF- β in both allergic disease and cancer progression.

Aortic carboxypeptidase-like protein (ACLP) enhances lung myofibroblast differentiation through transforming growth factor β receptor-dependent and -independent pathways

Idiopathic pulmonary fibrosis (IPF) is a chronic and fatal lung disease characterized by the overgrowth, hardening, and scarring of lung tissue. The exact mechanisms of how IPF develops and progresses are unknown. IPF is characterized by extracellular matrix remodeling and accumulation of active TGFβ, which promotes collagen expression and the differentiation of smooth muscle α-actin (SMA)-positive myofibroblasts. Aortic carboxypeptidase-like protein (ACLP) is an extracellular matrix protein secreted by fibroblasts and myofibroblasts and is expressed in fibrotic human lung tissue and in mice with bleomycin-induced fibrosis. Importantly, ACLP knockout mice are significantly protected from bleomycin-induced fibrosis. The goal of this study was to identify the mechanisms of ACLP action on fibroblast differentiation. As primary lung fibroblasts differentiated into myofibroblasts, ACLP expression preceded SMA and collagen expression. Recombinant ACLP induced SMA and collagen expression in mouse and human lung fibroblasts. Knockdown of ACLP slowed the fibroblast-to-myofibroblast transition and partially reverted differentiated myofibroblasts by reducing SMA expression. We hypothesized that ACLP stimulates myofibroblast formation partly through activating TGFβ signaling. Treatment of fibroblasts with recombinant ACLP induced phosphorylation and nuclear translocation of Smad3. This phosphorylation and induction of SMA was dependent on TGFβ receptor binding and kinase activity. ACLP-induced collagen expression was independent of interaction with the TGFβ receptor. These findings indicate that ACLP stimulates the fibroblast-to-myofibroblast transition by promoting SMA expression via TGFβ signaling and promoting collagen expression through a TGFβ receptor-independent pathway.

Transforming growth factor-beta 1 (TGF-B1) liberation from its latent complex during embryo implantation and its regulation by estradiol in mouse

Transforming growth factor-beta (TGF-B) plays an important role in embryo implantation; however, TGF-B requires liberation from its inactive latent forms (i.e., large latent TGF-B complex [LLC] and small latent TGF-B complex [SLC]) to its biologically active (i.e., monomer or dimer) forms in order to act on its receptors (TGF-BRs), which in turn activate SMAD2/3. Activation of TGF-B1 from its latent complexes in the uterus is not yet deciphered. We investigated uterine latent TGF-B1 complex and its biologically active form during implantation, decidualization, and delayed implantation. Our study, utilizing nonreducing SDS-PAGE followed by Western blotting and immunoblotting with TGF-B1, LTBP1, and latency-associated peptide, showed the presence of LLC and SLC in the uterine extracellular matrix and plasma membranous protein fraction during stages of the implantation period. A biologically active form of TGF-B1 (~17-kDa monomer) was highly elevated in the uterine plasma membranous compartment at the peri-implantation stage (implantation and nonimplantation sites). Administration of hydroxychloroquine (an inhibitor of pro-TGF-B processing) at the preimplantation stage was able to block the liberation of biologically active TGF-B1 from its latent complex at the postimplantation stage; as a consequence, the number of implantation sites was reduced at Day 5 (1000 h), as was the number of fetuses at Day 13. The inhibition of TGF-B1 showed reduced levels of phosphorylated SMAD3. Further, the delayed-implantation mouse model showed progesterone and estradiol coordination to release the active TGF-B1 form from its latent complex in the receptive endometrium. This study demonstrates the importance of liberation of biologically active TGF-B1 during the implantation period and its regulation by estradiol.

Unchaining the beast; insights from structural and evolutionary studies on TGFβ secretion, sequestration, and activation

TGFβ is secreted in a latent state and must be "activated" by molecules that facilitate its release from a latent complex and allow binding to high affinity cell surface receptors. Numerous molecules have been implicated as potential mediators of this activation process, but only a limited number of these activators have been demonstrated to play a role in TGFβ mobilisation in vivo. Here we review the process of TGFβ secretion and activation using evolutionary data, sequence conservation and structural information to examine the molecular mechanisms by which TGFβ is secreted, sequestered and released. This allows the separation of more ancient TGFβ activators from those factors that emerged more recently, and helps to define a potential hierarchy of activation mechanisms.

Structure and function of the mammalian fibrillin gene family: implications for human connective tissue diseases

Fibrillins and latent transforming growth factor β binding proteins (LTBPs) are components of the extracellular matrix of connective tissue. While fibrillins are integral to the 10nm microfibrils, and often associated with elastin, all family members are likely to have an additional role in regulating the bioavailability of transforming growth factor β (TGBβ). Both fibrillins and LTBPs are large glycoproteins, containing a series of calcium binding epidermal growth factor domains as well as a number of copies of a unique 8 cysteine domain found only in this protein superfamily. There are three mammalian fibrillins and four LTBPs. Fibrillin monomers link head to tail in microfibrils which can then form two and three dimensional structures. In some tissues elastin is recruited to the fibrillin microfibrils to provide elasticity to the tissue. LTBPs are part of the TGBβ large latent complex which sequesters TGBβ in the extracellular matrix. Fibrillin-1 appears to bind to LTBPs to assist in this process and is thus involved in regulating the bioavailability of TGBβ. Mutation of fibrillin genes results in connective tissue phenotypes which reflect both the increased level of active TGBβ and the structural failure of the extracellular matrix due to the absence or abnormality of fibrillin protein. Fibrillinopathies include Marfan syndrome, familial ectopia lentis, familial thoracic aneurysm (mutations of FBN1) and congenital contractural arachnodactyly (mutation of FBN2). There are no diseases currently associated with mutation of FBN3 in humans, and this gene is no longer active in rodents. Expression patterns of fibrillin genes are consistent with their role in extracellular matrix structure of connective tissue. FBN1 expression is high in most cell types of mesenchymal origin, particularly bone. Human and mouse FBN2 expression is high in fetal cells and has more restricted expression in mesenchymal cell types postnatally. FBN3 is expressed early in development (embryonic and fetal tissues) in humans. The fibrillins are thus important in maintaining the structure and integrity of the extracellular matrix and, in combination with their sequence family members the LTBPs, also contribute to the regulation of the TGFβ family of major growth factors.

Specificity of latent TGF-β binding protein (LTBP) incorporation into matrix: role of fibrillins and fibronectin

Fibrillin microfibrils are extracellular matrix structures with essential functions in the development and the organization of tissues including blood vessels, bone, limbs and the eye. Fibrillin-1 and fibrillin-2 form the core of fibrillin microfibrils, to which multiple proteins associate to form a highly organized structure. Defining the components of this structure and their interactions is crucial to understand the pathobiology of microfibrillopathies associated with mutations in fibrillins and in microfibril-associated molecules. In this study, we have analyzed both in vitro and in vivo the role of fibrillin microfibrils in the matrix deposition of latent TGF-β binding protein 1 (LTBP-1), -3 and -4; the three LTBPs that form a complex with TGF-β. In Fbn1(-/-) ascending aortas and lungs, LTBP-3 and LTBP-4 are not incorporated into a matrix lacking fibrillin-1 microfibrils, whereas LTBP-1 is still deposited. In addition, in cultures of Fbn1(-/-) smooth muscle cells or lung fibroblasts, LTBP-3 and LTBP-4 are not incorporated into a matrix lacking fibrillin-1 microfibrils, whereas LTBP-1 is still deposited. Fibrillin-2 is not involved in the deposition of LTBP-1 in Fbn1(-/-) extracellular matrix as cells deficient for both fibrillin-1 and fibrillin-2 still incorporate LTBP-1 in their matrix. However, blocking the formation of the fibronectin network in Fbn1(-/-) cells abrogates the deposition of LTBP-1. Together, these data indicate that LTBP-3 and LTBP-4 association with the matrix depends on fibrillin-1 microfibrils, whereas LTBP-1 association depends on a fibronectin network.

Measurement of human latent Transforming Growth Factor-β1 using a latency associated protein-reactive ELISA

Human Transforming Growth Factor (TGF)-β1, one of three TGF-β isoforms, is a pleotropic cytokine critical for many physiological and immunological processes. TGF-β1 is secreted in a latent form, linked to Latency Associated Protein (LAP). Analysis of Latent TGF-β1 by TGF-β1 ELISA requires dissociation of TGF-β1 from LAP, e.g. by acidification of samples. The ELISA then measures total TGF-β1, equivalent to dissociated Latent TGF-β1 plus any free TGF-β1 present prior to acidification. Evolutionary conservation of TGF-β1 across mammals also renders TGF-β1 ELISAs reactive with TGF-β1 in bovine serum often used in human cell cultures. To enable a direct analysis of Latent TGF-β1, monoclonal antibodies were made against LAP from human Latent TGF-β1 and used to develop a LAP ELISA detecting Latent TGF-β1. The ELISA did not react with LAP from human Latent TGF-β2 or 3, respectively, nor with Latent TGF-β in bovine serum. EDTA-containing plasma from healthy subjects (n=20) was analyzed by conventional TGF-β1 ELISA and LAP ELISA. By TGF-β1 ELISA, total TGF-β1 were detected in all samples (median 133 pM, range 34-348 pM); low levels of free TGF-β1 found in 8/20 non-acidified samples showed that >98.5% of the total TGF-β1 derived from Latent TGF-β1. Latent TGF-β1 found in non-acidified samples by LAP ELISA (median 154 pM, range 48-403 pM) was comparable in molar levels to, and correlated with, total TGF-β1 (r(s) 0.96, p<0.0001). A similar agreement between the total TGF-β1 and the LAP ELISA was found with citrate- and heparin-containing plasma. The LAP ELISA facilitates analysis of Latent TGF-β1 without sample acidification and is not compromised by the presence of bovine serum in human cell supernatants.

Expression of the Tgfβ2 gene during chick embryogenesis

We performed a comprehensive analysis of the expression of transforming growth factor (TGF) β2 during chick embryogenesis from stage 6 to 30 (Hamburger and Hamilton, J Morphol 1951;88:49-92) using in situ hybridization. During cardiogenesis, Tgfβ2 was expressed in the endothelial/mesenchymal cells of the valvulo-septal endocardial cushion tissue and in the epicardium until the end of embryogenesis. During the formation of major arteries, Tgfβ2 was localized in smooth muscle progenitors but not in the vascular endothelium. During limb development, Tgfβ2 was expressed in the mesenchymal cells in the presumptive limb regions at stage 16, and thereafter it was localized in the skeletal muscle progenitors. In addition, strong Tgfβ2 expression was seen in the mesenchymal cells in the pharyngeal arches. Tgfβ2 mRNA was also detected in other mesoderm-derived tissues, such as the developing bone and pleura. During ectoderm development, Tgfβ2 was expressed in the floor plate of the neural tube, lens, optic nerve, and otic vesicle. In addition, Tgfβ2 was expressed in the developing gut epithelium. Our results suggest that TGFβ2 plays an important role not only in epithelial-mesenchymal interactions but also in cell differentiation and migration and cell death during chick embryogenesis. We also found that chick and mouse Tgfβ2 RNA show very similar patterns of expression during embryogenesis. Chick embryos can serve as a useful model to increase our understanding in the roles of TGFβ2 in cell-cell interactions, cell differentiation, and proliferation during organogenesis.

Choreographing metastasis to the tune of LTBP

Latent Transforming Growth Factor beta (TGFβ) Binding Proteins (LTBPs) are chaperones and determinants of TGFβ isoform-specific secretion. They belong to the LTBP/Fibrillin family and form integral components of the fibronectin and microfibrillar extracellular matrix (ECM). LTBPs serve as master regulators of TGFβ bioavailability, functioning to incorporate and spatially pattern latent TGFβ at regular intervals within the ECM, and actively participate in integrin-mediated stretch activation of TGFβ in vivo. In so doing they create a highly patterned sensory system where local changes in ECM tension can be detected and transduced into focal signals. The physiological role of LTBPs in the mammary gland remains largely unstudied, however both loss and gain of LTBP expression is found in breast cancers and breast cancer cell lines. Importantly, elevated LTBP1 levels appear in two gene signatures predictive of enhanced metastatic behavior. LTBP may promote metastasis by providing the bridge between structural and signaling components of the epithelial to mesenchymal transition (EMT).

Expression of mRNA isoforms of latent transforming growth factor-β binding protein-1 in coronary atherosclerosis and human tissues

Latent transforming growth factor-β binding protein-1 (LTBP1) has been implicated in the control of secretion, localization, and activation of TGFβ (transforming growth factor-β). We developed a quantitative reverse-transcriptase polymerase chain reaction (Q-RT-PCR) assay using an RNA internal standard to examine the expression of three alternatively spliced isoforms of LTBP1 (LTBP1Δ41, LTBP1Δ53, and LTBP1Δ55) in a variety of human tissues. The assays were also used to determine the expression of LTBP1L and LTBP1S isoforms and total LTBP1. The Q-RT-PCR assays were highly reproducible and showed that in most tissues LTBP1Δ55 and LTBP1L were minor components of LTBP1. The proportion of LTBP1Δ41 ranged from 2% of total LTBP1 mRNA in early coronary atherosclerotic lesions to 54% in advanced lesions.

TGFbeta-induced endothelial podosomes mediate basement membrane collagen degradation in arterial vessels

Podosomes are specialized plasma-membrane actin-based microdomains that combine adhesive and proteolytic activities to spatially restrict sites of matrix degradation in in vitro assays, but the physiological relevance of these observations remain unknown. Inducible rings of podosomes (podosome rosettes) form in cultured aortic cells exposed to the inflammatory cytokine TGFbeta. In an attempt to prove the existence of podosomes in living tissues, we developed an ex vivo endothelium observation model. This system enabled us to visualize podosome rosettes in the endothelium of native arterial vessel exposed to biologically active TGFbeta. Podosomes induced in the vessel appear similar to those formed in cultured cells in terms of molecular composition, but in contrast to the latter, arrange in a protruding structure that is similar to invadopodia. Local degradation of the basement membrane scaffold protein collagen-IV, is observed underneath the structures. Our results reveal for the first time the presence of podosome rosettes in the native endothelium and provide evidence for their capacity to degrade the basement membrane, opening up new avenues to study their role in vascular pathophysiology. We propose that podosome rosettes are involved in arterial vessel remodeling.

Latent TGF-beta-binding protein 4 modifies muscular dystrophy in mice

Most single-gene diseases, including muscular dystrophy, display a nonuniform phenotype. Phenotypic variability arises, in part, due to the presence of genetic modifiers that enhance or suppress the disease process. We employed an unbiased mapping approach to search for genes that modify muscular dystrophy in mice. In a genome-wide scan, we identified a single strong locus on chromosome 7 that influenced two pathological features of muscular dystrophy, muscle membrane permeability and muscle fibrosis. Within this genomic interval, an insertion/deletion polymorphism of 36 bp in the coding region of the latent TGF-beta-binding protein 4 gene (Ltbp4) was found. Ltbp4 encodes a latent TGF-beta-binding protein that sequesters TGF-beta and regulates its availability for binding to the TGF-beta receptor. Insertion of 12 amino acids into the proline-rich region of LTBP4 reduced proteolytic cleavage and was associated with reduced TGF-beta signaling, decreased fibrosis, and improved muscle pathology in a mouse model of muscular dystrophy. In contrast, a 12-amino-acid deletion in LTBP4 was associated with increased proteolysis, SMAD signaling, and fibrosis. These data identify Ltbp4 as a target gene to regulate TGF-beta signaling and modify outcomes in muscular dystrophy.

Articular cartilage tissue engineering

Articular cartilage repair remains a challenge to surgeons and basic scientists. The field of tissue engineering allows the simultaneous use of material scaffolds, cells and signalling molecules to attempt to modulate the regenerative tissue. This review summarises the research that has been undertaken to date using this approach, with a particular emphasis on those techniques that have been introduced into clinical practice, via in vitro and preclinical studies.

Latent transforming growth factor-beta-binding protein-4 regulates transforming growth factor-beta1 bioavailability for activation by fibrogenic lung fibroblasts in response to bleomycin

Recent evidence suggests that subsets of lung fibroblasts differentially contribute to fibrogenic progression. We have previously shown that a subset of rat lung fibroblasts with fibrogenic characteristics [Thy-1 (-) fibroblasts] responds to stimuli (bleomycin, interleukin-4, etc) with increased latent transforming growth factor (TGF)-beta activation, whereas non-fibrogenic Thy-1-expressing [Thy-1 (+)] fibroblasts do not. Activation of latent TGF-beta1 by interstitial lung fibroblasts is critical for fibrogenic responses. To better understand the susceptibility of fibrogenic fibroblasts to the stimulation of TGF-beta activation, we examined the role of latent TGF-beta-binding proteins (LTBPs), key regulators of TGF-beta bioavailability and activation, in TGF-beta1 activation by these fibroblasts. Treatment of fibroblasts with bleomycin up-regulated LTBP-4 mRNA, protein, and soluble LTBP-4-bound large latent TGF-beta1 complexes in Thy-1 (-) fibroblasts to significantly higher levels than in Thy-1 (+) fibroblasts. Bleomycin-induced TGF-beta1 activation required LTBP-4, since lung fibroblasts deficient in LTBP-4 did not activate TGF-beta1. Expression of LTBP-4 restored TGF-beta1 activation in response to bleomycin, but expression either of LTBP-4 lacking the TGF-beta-binding site or only the TGF-beta-binding domain did not. Bleomycin treatment of mice increased LTBP-4 expression in the lung. Thy-1 knockout mice had increased levels of both LTBP-4 expression and TGF-beta activation, as well as enhanced Smad3 phosphorylation compared with wild-type mice. Together, these data identify a critical role for LTBP-4 in the regulation of latent TGF-beta1 activation in bleomycin-induced lung fibrosis.

Intracellular localization of Cthrc1 characterizes differentiated smooth muscle

OBJECTIVE

We recently reported expression of collagen triple helix repeat containing-1 (Cthrc1) in injured arteries and proteolytic cleavage of Cthrc1 in smooth muscle cells in vitro. The present study characterizes Cthrc1 processing and determines its biological significance.

METHODS AND RESULTS

Domain-specific antibodies localized full-length Cthrc1 in the cytoplasm of vascular, gastrointestinal, and uterine smooth muscle as well as in some neurons. Unlike smooth muscle alpha-actin, Cthrc1 was not expressed in the embryonic myocardium. Intracellular localization of full-length Cthrc1 was sharply reduced in dedifferentiated smooth muscle of the developing neointima despite the previously shown increase in mRNA levels with accompanying extracellular Cthrc1 immunoreactivity. Immunoblotting suggested an apparent covalent association of monomeric full-length Cthrc1 with a cytoplasmic protein present in differentiated smooth muscle. Plasmin was identified as a protease that cleaved a putative propeptide generating an N-terminally truncated form of Cthrc1 with increased inhibitory activity of procollagen synthesis.

CONCLUSIONS

Our data show that the differentiated smooth muscle cell phenotype is associated with the intracellular localization of noncleaved Cthrc1 despite the presence of a signal peptide. On arterial injury, increased Cthrc1 expression with apparent extracellular localization of N-terminally truncated Cthrc1 occurs. Removal of the propeptide correlated with increased activity of the molecule.

Absence of alphavbeta6 integrin is linked to initiation and progression of periodontal disease

Integrin alphavbeta6 is generally not expressed in adult epithelia but is induced in wound healing, cancer, and certain fibrotic disorders. Despite this generalized absence, we observed that alphavbeta6 integrin is constitutively expressed in the healthy junctional epithelium linking the gingiva to tooth enamel. Moreover, expression of alphavbeta6 integrin was down-regulated in human periodontal disease, a common medical condition causing tooth loss and also contributing to the development of cardiovascular diseases by increasing the total systemic inflammatory burden. Remarkably, integrin beta6 knockout mice developed classic signs of spontaneous, chronic periodontal disease with characteristic inflammation, epithelial down-growth, pocket formation, and bone loss around the teeth. Integrin alphavbeta6 acts as a major activator of transforming growth factor-beta1 (TGF-beta1), a key anti-inflammatory regulator in the immune system. Co-expression of TGF-beta1 and alphavbeta6 integrin was observed in the healthy junctional epithelium. Moreover, an antibody that blocks alphavbeta6 integrin-mediated activation of TGF-beta1 initiated inflammatory periodontal disease in a rat model of gingival inflammation. Thus, alphavbeta6 integrin is constitutively expressed in the epithelium sealing the gingiva to the tooth and plays a central role in protection against inflammatory periodontal disease through activation of TGF-beta1.

Identification of a novel pool of extracellular pro-myostatin in skeletal muscle

Myostatin, a transforming growth factor-beta superfamily ligand, negatively regulates skeletal muscle growth. Generation of the mature signaling peptide requires cleavage of pro-myostatin by a proprotein convertase, which is thought to occur constitutively in the Golgi apparatus. In serum, mature myostatin is found in an inactive, non-covalent complex with its prodomain. We find that in skeletal muscle, unlike serum, myostatin is present extracellularly as uncleaved pro-myostatin. In cultured cells, co-expression of pro-myostatin and latent transforming growth factor-beta-binding protein-3 (LTBP-3) sequesters pro-myostatin in the extracellular matrix, and secreted pro-myostatin can be cleaved extracellularly by the proprotein convertase furin. Co-expression of LTBP-3 with myostatin reduces phosphorylation of Smad2, and ectopic expression of LTBP-3 in mature mouse skeletal muscle increases fiber area, consistent with reduction of myostatin activity. We propose that extracellular pro-myostatin constitutes the major pool of latent myostatin in muscle. Post-secretion activation of this pool by furin family proprotein convertases may therefore represent a major control point for activation of myostatin in skeletal muscle.

Extracellular control of TGFbeta signalling in vascular development and disease

The intracellular mechanism of transforming growth factor-beta (TGFbeta) signalling via kinase receptors and SMAD effectors is firmly established, but recent studies of human cardiovascular syndromes such as Marfan syndrome and pre-eclampsia have refocused attention on the importance of regulating the availability of active extracellular TGFbeta. It seems that elastic extracellular matrix (ECM) components have a crucial role in controlling TGFbeta signalling, while soluble and membrane bound forms of TGFbeta co-receptors add further layers of regulation. Together, these extracellular interactions determine the final bioavailability of TGFbeta to vascular cells, and dysregulation is associated with an increasing number of vascular pathologies.

Long form of latent TGF-beta binding protein 1 (Ltbp1L) is essential for cardiac outflow tract septation and remodeling

Latent TGF-beta binding protein 1 (LTBP1) is a member of the LTBP/fibrillin family of extracellular proteins. Due to the usage of different promoters, LTBP1 exists in two major forms, long (L) and short (S), each expressed in a temporally and spatially unique fashion. Both LTBP1 molecules covalently interact with latent TGF-beta and regulate its function, presumably via interaction with the extracellular matrix (ECM). To explore the in vivo role of Ltbp1 in mouse development, at the time when only the L isoform is expressed, we mutated the Ltbp1L locus by gene targeting. Ltbp1L-null animals die shortly after birth from defects in heart development, consisting of the improper septation of the cardiac outflow tract (OFT) and remodeling of the associated vessels. These cardiac anomalies present as persistent truncus arteriosus (PTA) and interrupted aortic arch (IAA), which are associated with the faulty function of cardiac neural crest cells (CNCCs). The lack of Ltbp1L in the ECM of the septating OFT and associated vessels results in altered gene expression and function of CNCCs and decreased Tgf-beta activity in the OFT. This phenotype reveals a crucial role for Ltbp1L and matrix as extracellular regulators of Tgf-beta activity in heart organogenesis.

Potential role for heparan sulfate proteoglycans in regulation of transforming growth factor-beta (TGF-beta) by modulating assembly of latent TGF-beta-binding protein-1

Latent transforming growth factor-beta-binding proteins (LTBPs) are extracellular matrix (ECM) glycoproteins that play a major role in storage of latent TGF-beta in the ECM and regulate its availability. We have previously identified fibronectin as a key molecule for incorporation of LTBP1 and TGF-beta into the ECM of osteoblasts and fibroblasts. Here we provide evidence that heparan sulfate proteoglycans may mediate binding between LTBP1 and fibronectin. We have localized critical domains in the N terminus of LTBP1 that are required for co-localization with fibronectin in osteoblast cultures and have identified heparin binding sites in the N terminus of LTBP1 between residues 345 and 487. Solid-phase binding assays suggest that LTBP1 does not bind directly to fibronectin but that the binding is indirect. Heparin coupled to bovine serum albumin (heparin-BSA) was able to mediate binding between fibronectin and LTBP1. Treatment of primary osteoblast cultures with heparin or heparin-BSA but not with chondroitin sulfate impaired LTBP1 deposition onto fibronectin without inhibiting expression of LTBP1. Inhibition of LTBP1 incorporation was accompanied by reduced incorporation of latent TGF-beta into the ECM, with increased amounts of soluble latent TGF-beta. Inhibition of attachment of glycosaminoglycans to the core proteins of proteoglycans by beta-d-xylosides also reduced incorporation of LTBP1 into the ECM. These studies suggest that heparan sulfate proteoglycans may play a critical role in regulating TGF-beta availability by controlling the deposition of LTBP1 into the ECM in association with fibronectin.

BMP1 controls TGFbeta1 activation via cleavage of latent TGFbeta-binding protein

Transforming growth factor beta1 (TGFbeta1), an important regulator of cell behavior, is secreted as a large latent complex (LLC) in which it is bound to its cleaved prodomain (latency-associated peptide [LAP]) and, via LAP, to latent TGFbeta-binding proteins (LTBPs). The latter target LLCs to the extracellular matrix (ECM). Bone morphogenetic protein 1 (BMP1)-like metalloproteinases play key roles in ECM formation, by converting precursors into mature functional proteins, and in morphogenetic patterning, by cleaving the antagonist Chordin to activate BMP2/4. We provide in vitro and in vivo evidence that BMP1 cleaves LTBP1 at two specific sites, thus liberating LLC from ECM and resulting in consequent activation of TGFbeta1 via cleavage of LAP by non-BMP1-like proteinases. In mouse embryo fibroblasts, LAP cleavage is shown to be predominantly matrix metalloproteinase 2 dependent. TGFbeta1 is a potent inducer of ECM formation and of BMP1 expression. Thus, a role for BMP1-like proteinases in TGFbeta1 activation completes a novel fast-forward loop in vertebrate tissue remodeling.

Novel functional single nucleotide polymorphisms in the latent transforming growth factor-beta binding protein-1L promoter: effect on latent transforming growth factor-beta binding protein-1L expression level and possible prognostic significance in ovarian cancer

Latent transforming growth factor (TGF)-beta binding proteins (LTBPs) play important roles in the secretion and activation of TGF-beta. We previously reported that LTBP-1L is overexpressed in some patients with ovarian cancer. To clarify the molecular mechanism of LTBP-1L regulation, we analyzed DNA sequences in the promoter region of LTBP-1L and identified two novel single nucleotide polymorphisms, -202G/C and +20A/C. While the alleles with -202C and +20C were initially reported, our data demonstrated that -202G and +20A are common in both ovarian cancer patients and healthy patients in the Japanese population. Luciferase reporter assays revealed that the G-A haplotype induced transcriptional activation in a Sp1-dependent manner. Electrophoretic mobility shift assays showed that increased binding affinity of Sp1 to the promoter with -202G and +20A. Interestingly, ovarian cancer patients (n = 42) with G-A/G-A homozygous genotype had increased expression of LTBP-1 and apparently poorer survival than those with other genotypes (P = 0.02). These findings suggest that the single nucleotide polymorphisms -202G/C and +20A/C on the LTBP-1L promoter may affect the clinical outcome of ovarian cancer patients, probably via up-regulating protein expression. Further studies using a larger number of samples will definitively determine the correlation between LTBP-1 haplotype and clinical behavior of ovarian cancer.

New insights into extracellular matrix assembly and reorganization from dynamic imaging of extracellular matrix proteins in living osteoblasts

The extracellular matrix (ECM) has been traditionally viewed as a static scaffold that supports cells and tissues. However, recent dynamic imaging studies suggest that ECM components are highly elastic and undergo continual movement and deformation. Latent transforming growth factor beta (TGFbeta) binding protein-1 (LTBP1) is an ECM glycoprotein that binds latent TGFbeta and regulates its availability and activity. LTBP1 initially co-distributes with fibronectin in the extracellular matrix of osteoblasts, and depends on fibronectin for its assembly. To gain further insights into the mechanisms of assembly of LTBP1 and its spatial and temporal interactions with fibronectin, we have performed dual fluorescence time-lapse imaging of these two proteins in living osteoblasts using fluorescent probes. Time-lapse movies showed surprisingly large fibril displacements associated with cellular movement as well as occasional breaking of LTBP1 or fibronectin-containing fibrils. Individual fibrils stretched to as much as 3.5 times or contracted to as much as one fourth of their original length. Motile cells appeared to actively mediate extracellular matrix assembly by adding 'globules' or 'packets' of matrix material onto existing fibrils. They also actively reorganized the extracellular matrix by shunting matrix material from one location to another and exchanging fibrillar material between fibrils. This cell-mediated matrix reorganization was primarily associated with the assembly and remodeling of the initial (early) matrix, whereas mature, established ECM was more stable. Displacement vector mapping showed that different matrix fibrillar networks within the same cultures can show different dynamic motion in response to cell movement and showed that the motion of fibrils was correlated with cell motion. These data suggest novel cell-mediated mechanisms for assembly and reorganization of the extracellular matrix and highlight a role for cell motility in the assembly process.

Fibronectin is required for integrin alphavbeta6-mediated activation of latent TGF-beta complexes containing LTBP-1

Transforming growth factor-betas (TGF-beta) are secreted as latent complexes consisting of the TGF-beta dimer, the TGF-beta propeptide dimer, and the latent TGF-beta binding protein (LTBP). Although the bonds between TGF-beta and its propeptide are cleaved intracellulary, the propeptide associates with TGF-beta by electrostatic interactions, thereby conferring latency to the complex. We reported that a specific sequence of LTBP-1 is required for latent TGF-beta activation by the integrin alphavbeta6. Here we describe a 24 amino acid sequence from the hinge domain required for activation. The LTBP-1 polypeptide rL1N, which includes the hinge, associates with fibronectin in binding assays. We present evidence that fibronectin null cells minimally activate latent TGF-beta and poorly incorporate the active hinge sequence into their matrix. In addition, cells missing the fibronectin receptor alpha5beta1 exhibit defective activation of latent TGF-beta by alphavbeta6 and decreased matrix incorporation. The results indicate specificity for integrin-mediated latent TGF-beta activation that include unique sequences in LTBP-1 and an appropriate matrix molecule.

Dynamics of Assembly and Reorganization of Extracellular Matrix Proteins

This chapter will review advances in our understanding of the dynamics of assembly and reorganization of extracellular matrix (ECM) proteins and will highlight the role of fibronectin as a key orchestrator for the assembly of multiple ECM proteins. The dynamic rather than static nature of the ECM will be emphasized by reviewing time-lapse imaging studies in living cell and embryo systems, with a particular focus on fibronectin and members of the fibrillin superfamily. These studies have provided new insights into the assembly and reorganization of ECM fibrillar networks, suggesting that fibril assembly is a hierarchical process, with increasingly larger fibrillar structures formed by the progressive aggregation of smaller units. These studies have also revealed that motile cells appear to be actively involved in the assembly and reorganization of ECM fibrillar networks by shunting fibrillar material from one location to another, adding fibrillar material to the ends of growing fibrils, and exchanging material between fibrils. A common theme emerging from these studies is that cell- and tissue-generated mechanical forces are critical in the assembly and remodeling of the ECM.

Expression and regulation of latent TGF-beta binding protein-1 transcripts and their splice variants in human glomerular endothelial cells

Latent transforming growth factor (TGF)-beta-binding protein (LTBP) is required for the assembly, secretion, matrix association, and activation of latent TGF-beta complex. To elucidate the cell specific expression of the genes of LTBP-1 and their splice variants and the factors that regulate the gene expression, we cultured primary human glomerular endothelial cells (HGEC) under different conditions. Basal expression of LTBP-1 mRNA was suppressed in HGEC compared to WI-38 human embryonic lung fibroblasts. High glucose, H(2)O(2), and TGF-beta1 upregulated and vascular endothelial growth factor (VEGF) further downregulated LTBP-1 mRNA in HGEC. RT-PCR with a primer set for LTBP-1S produced many clones but no clone was gained with a primer set for LTBP-1L. Of 12 clones selected randomly, Sca I mapping and DNA sequencing revealed that only one was LTBP-1S and all the others were LTBP-1Sdelta53. TGF-beta1, but not high glucose, H(2)O(2) or VEGF, tended to increase LTBP-1Sdelta53 mRNA. In conclusion, HGEC express LTBP-1 mRNA which is suppressed at basal state but upregulated by high glucose, H(2)O(2), and TGF-beta1 and downregulated by VEGF. Major splice variant of LTBP-1 in HGEC was LTBP-1S 53. Modification of LTBP-1S 53 gene in HGEC may abrogate fibrotic action of TGF-beta1 but this requires confirmation.

Osteopetrosis-like phenotype in latent TGF-beta binding protein 3 deficient mice

LTBPs are extracellular matrix proteins resembling fibrillins. LTBP-1, 3, and 4 covalently bind latent TGF-beta and modulate tissue levels of this potent cytokine through regulation of its secretion, localization, and/or activation. To address LTBP function in vivo, we generated Ltbp-3 null mice. Ltbp-3-/- animals developed craniofacial abnormalities due to early ossification of the skull base synchondroses and displayed reduced body size. In addition, histological examination of Ltbp-3-/- skeletons revealed an increase in bone mass. The osteoblast numbers and mineral apposition rates were decreased in Ltbp-3-/- mice, whereas the osteoclast numbers were similar in null and wild type mice. Histological examination revealed persistence of cartilage remnants in Ltbp-3-/- trabecular bone. Taken together, these results indicate that the Ltbp-3-/- high bone mass phenotype was due to a defect in bone resorption. We hypothesize that lack of Ltbp-3 results in decreased levels of TGF-beta in bone and cartilage, which leads to compromised osteoclast function and decreased bone turnover.