Analysis of small latent transforming growth factor-beta complex formation and dissociation by surface plasmon resonance. Absence of direct interaction with thrombospondins

Interaction of TGFβ3 ligand with its receptors type II (TβRII) and type I (TβRI): A unique mechanism of protein-protein association

The proper orchestration of transforming growth factor beta (TGFβ) mediated signal transduction depends upon a delicate set of interactions between specific ligands and their receptors. Here we present an in-depth profiling of the binding mechanism of TGFβ3 ligand with its type II and type I receptors (TβRII and TβRI) using isothermal titration calorimetry (ITC). Studies were carried out in acidic pH as it has great physiological relevance for TGFβ3 activity. Our findings reveal an unusual positive enthalpy (∆H) compensated by a large favourable entropy (∆S) during TGFβ3-TβRII interaction. In addition to the hydrophobic effect, we propose that a distinct conformational switch from "closed" to "open" form as experienced by TGFβ3 on binding to TβRII is contributing significantly to the increase in overall entropy of the system. Binding studies of TGFβ3 and TβRII were carried out at different pH values and salt concentrations to gain further insight into the thermodynamics of the interaction. Furthermore, the importance of hydrophobic interactions on the binding affinity of TβRII with TGFβ3 was confirmed by two TβRII variants (interfacial). Finally, a distinct shift from entropy to enthalpy dominated interaction was observed upon recruitment of TβRI to the binary complex forming the ternary complex.

PSG9 Stimulates Increase in FoxP3+ Regulatory T-Cells through the TGF-β1 Pathway

The pregnancy-specific glycoproteins (PSGs) are a family of proteins secreted by the syncytiotrophoblast of the placenta and are the most abundant trophoblastic proteins in maternal blood during the third trimester. The human PSG family consists of 10 protein-coding genes, some of which have a possible role in maintaining maternal immune tolerance to the fetus. PSG9 was reported as a potential predictive biomarker of pre-eclampsia, a serious complication of pregnancy that has been related to immunological dysfunction at the fetal-maternal interface. Therefore, we hypothesized that PSG9 may have an immunoregulatory role during pregnancy. We found that PSG9 binds to LAP and activates the latent form of TGF-β1. In addition, PSG9 induces the secretion of TGF-β1 from macrophages but not from CD4+ T-cells. TGF-β1 is required for the ex vivo differentiation of regulatory T-cells and, consistent with the ability of PSG9 to activate this cytokine, we observed that PSG9 induces the differentiation of FoxP3+ regulatory T-cells from naïve murine and human T-cells. Cytokines that are associated with inflammatory responses were also reduced in the supernatants of T-cells treated with PSG9, suggesting that PSG9, through its activation of TGFβ-1, could be a potent inducer of immune tolerance.

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.

Structural insights into BMP receptors: Specificity, activation and inhibition

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-β family (TGFβ), which signal through hetero-tetrameric complexes of type I and type II receptors. In humans there are many more TGFβ ligands than receptors, leading to the question of how particular ligands can initiate specific signaling responses. Here we review structural features of the ligands and receptors that contribute to this specificity. Ligand activity is determined by receptor-ligand interactions, growth factor prodomains, extracellular modulator proteins, receptor assembly and phosphorylation of intracellular signaling proteins, including Smad transcription factors. Detailed knowledge about the receptors has enabled the development of BMP-specific type I receptor kinase inhibitors. In future these may help to treat human diseases such as fibrodysplasia ossificans progressiva.

Induction and activation of latent transforming growth factor-β1 are carried out by two distinct domains of pregnancy-specific glycoprotein 1 (PSG1)

Pregnancy-specific glycoproteins (PSGs) are a family of Ig-like proteins secreted by specialized placental cells. The PSG1 structure is composed of a single Ig variable region-like N-terminal domain and three Ig constant region-like domains termed A1, A2, and B2. Members of the human and murine PSG family have been shown to induce anti-inflammatory cytokines from monocytes and macrophages and to stimulate angiogenesis. We recently showed that recombinant forms of PSG1 (PSG1-Fc and PSG1-His) and PSG1 purified from the serum of pregnant women are associated with the immunoregulatory cytokine TGF-β1 and activated latent TGF-β1. Here, we sought to examine the requirement of specific PSG1 domains in the activation of latent TGF-β1. Plasmon surface resonance studies showed that PSG1 directly bound to the small latent complex and to the latency-associated peptide of TGF-β1 and that this binding was mediated through the B2 domain. Furthermore, the B2 domain alone was sufficient for activating the small latent complex. In separate experiments, we found that the PSG1-mediated induction of TGF-β1 secretion in macrophages was dependent on the N-terminal domain. Mutagenesis analysis revealed that four amino acids (LYHY) of the CC' loop of the N-terminal domain were required for induction of latent TGF-β1 secretion. Together, our results show that two distinct domains of PSG1 are involved in the regulation of TGF-β1 and provide a mechanistic framework for how PSGs modulate the immunoregulatory environment at the maternal-fetal interface for successful pregnancy outcome.

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.

Surfactant protein A binds TGF-β1 with high affinity and stimulates the TGF-β pathway

We were able to demonstrate reversible, specific and high-affinity binding of radioactively-labelled TGF-β1 (125I-TGF-β1) to immobilized surfactant protein A (SP-A), with an apparent dissociation constant of 53 picomolar at ∼21℃. Addition of a 200-fold molar excess of the latency associated peptide (LAP) prevented and dissociated the binding of 125I-TGF-β1 to SP-A, whereas latent TGF-β1 had no effect. Using a bioassay for TGF-β1 activity—a luciferase reporter assay—we were able to show that SP-A in the presence of TGF-β1 stimulated the TGF-β1 pathway, whereas SP-A alone had no effect. Studies with structural analogues of the distinct SP-A tail domain and head domain indicated that stimulatory activity of SP-A resided in the head domain. No activation of latent TGF-β1 by SP-A was observed. In addition, we observed that SP-A inhibited TGF-β1 inactivation by LAP. These results indicate that SP-A may have a regulatory role in the TGF-β1-mediated processes in the lung.

Latent TGF-beta binding proteins: extracellular matrix association and roles in TGF-beta activation

Transforming growth factor betas (TGF-betas) are multifunctional and pleiotropic growth factors. Their major effects include inhibition of cell proliferation and enhancement of extracellular matrix production. TGF-betas are secreted from cells as latent complexes, consisting of mature dimeric growth factor, the latency-associated propeptide (LAP), and a distinct gene product, latent TGF-beta binding protein LTBP. The secreted complex is targeted to specific locations in the extracellular matrix by the appropriate LTBP. The latent complex needs subsequently to be activated. Most studies describing biological effects of TGF-beta have been carried out in cell cultures using high concentrations of active, soluble TGF-beta, where appropriate targeting of the growth factor is missing. However, TGF-beta is produced and secreted in vivo as a latent complex in a specific and targeted manner. Various experimental approaches have convincingly shown the importance of the activation of latent TGF-beta, as well as the importance of LTBPs as targeting molecules of the effects of TGF-beta. Essential steps in the activation appear to be cellular recognition of extracellular matrix-associated LTBPs and subsequent recognition of the associated latent TGF-beta. Cell recognition by specific molecules like integrins and proteolytic events involving plasminogen activation evidently play multifaceted roles in the regulation of TGF-beta activation.

Expression of the thrombospondin 1 fragment 167-569 in C6 glioma cells stimulates tumorigenicity despite reduced neovascularization

Gliomas are among the most malignant and most highly vascularized human tumors. We studied the therapeutic action of an angiostatic fragment of human thrombospondin 1 (named TSP1ang) on experimental glioma tumor growth. TSP1ang (enclosing amino acids 167-569) comprised the procollagen-homology domain and the three type I repeats of the original molecule. C6 glioma cells that stably express TSP1ang were generated, and their rate of in vitro growth did not appear to differ from that of C6 cells transfected with an empty plasmid. TSP1ang-expressing C6 cells were then injected either subcutaneously or intracerebrally into nude mice. The resulting tumors appeared to be less vascularized, but unexpectedly started to grow earlier and had a much more invasive phenotype than tumors derived from control C6 cells. They were also much more aggressive, since the mice bearing intracerebral TSP1ang-expressing tumor cells died before day 19 post-implantation, whereas all mice bearing control C6 tumors were alive at this time point. These results indicate that careful attention should be paid at designing smaller fragments from the multimodular angiostatic molecule TSP1 since, as observed in this study, it may unmask protumorigenic properties that counteract its antiangiogenic activity.

TGF-beta 1 plays an important role in the mechanism of CD4+CD25+ regulatory T cell activity in both humans and mice

In previous studies, we have shown that murine CD4+CD25+ regulatory T cells produce high levels of TGF-beta1 in a cell surface and/or secreted form, and blockade of such TGF-beta1 by anti-TGF-beta curtails the ability of these cells to suppress CD25- T cell proliferation and B cell Ig production in in vitro suppressor assays. In further support for the role of TGF-beta1 in suppression by CD4+CD25+ T cells, we show in this study that another TGF-beta1-blocking molecule, recombinant latency-associated peptide of TGF-beta1 (rLAP), also reverses suppression by mouse CD4+CD25+ T cells as well as their human counterparts, CD4+CD25(high) T cells. In addition, we show that CD25- T cells exposed to CD4+CD25+ T cells in vitro manifest activation of Smad-2 and induction of CD103, the latter a TGF-beta-inducible surface integrin. In further studies, we show that while CD4+CD25+ T cells from TGF-beta1-deficient mice can suppress CD25- T cell proliferation in vitro, these cells do not protect recipient mice from colitis in the SCID transfer model in vivo, and, in addition, CD4+LAP+, but not CD4+LAP- T cells from normal mice protect recipient mice from colitis in this model. Together, these studies demonstrate that TGF-beta1 produced by CD4+CD25+ T cells is involved in the suppressor activity of these cells, particularly in their ability to regulate intestinal inflammation.

Activation of transforming growth factor beta by malaria parasite-derived metalloproteinases and a thrombospondin-like molecule

Much of the pathology of malaria is mediated by inflammatory cytokines (such as interleukin 12, interferon γ, and tumor necrosis factor α), which are part of the immune response that kills the parasite. The antiinflammatory cytokine transforming growth factor (TGF)-β plays a crucial role in preventing the severe pathology of malaria in mice and TGF-β production is associated with reduced risk of clinical malaria in humans. Here we show that serum-free preparations of Plasmodium falciparum, Plasmodium yoelii 17XL, and Plasmodium berghei schizont-infected erythrocytes, but not equivalent preparations of uninfected erythrocytes, are directly able to activate latent TGF-β (LatTGF-β) in vitro. Antibodies to thrombospondin (TSP) and to a P. falciparum TSP-related adhesive protein (PfTRAP), and synthetic peptides from PfTRAP and P. berghei TRAP that represent homologues of TGF-β binding motifs of TSP, all inhibit malaria-mediated TGF-β activation. Importantly, TRAP-deficient P. berghei parasites are less able to activate LatTGF-β than wild-type parasites and their replication is attenuated in vitro. We show that activation of TGF-β by malaria parasites is a two step process involving TSP-like molecules and metalloproteinase activity. Activation of LatTGF-β represents a novel mechanism for direct modulation of the host response by malaria parasites.

Identification of a novel domain of fibroblast growth factor 2 controlling its angiogenic properties

Fibroblast growth factor 2 (FGF-2) is a potent factor modulating the activity of many cell types. Its dimerization and binding to high affinity receptors are considered to be necessary steps to induce FGF receptor phosphorylation and signaling activation. A structural analysis was carried out and a region encompassing residues 48-58 of human FGF-2 was identified, as potentially involved in FGF-2 dimerization. A peptide (FREG-48-58) derived from this region strongly and specifically inhibited FGF-2 induced proliferation and migration of primary bovine aorta endothelial cells (BAEC) in vitro, and markedly reduced FGF-2-dependent angiogenesis in two distinct in vivo assays. To further investigate the role of region 48-58, a polyclonal antibody raised against FREG-(48-58) was tested and was found to block FGF-2 action in vitro. Human FGF-2 has three histidine residues, one falling within the region 48-58. Chemical modification of histidine residues blocked FGF-2 activity and FREG-(48-58) inhibitory effect in vitro, indicating that histidine residues, in particular the one within FREG-(48-58) region, play a crucial role in the observed activity. Additional experiments showed that FREG-(48-58) specifically interacted with FGF-2, impaired FGF-2-interaction with itself, with heparin and with FGF receptor 1, and inhibited FGF-2-induced receptor phosphorylation and FGF-2 internalization. These data indicate for the first time that region 48-58 of FGF-2 is a functional domain controlling FGF-2 activity.

Platelet-derived growth factor-BB and basic fibroblast growth factor directly interact in vitro with high affinity

Platelet-derived growth factor-BB (PDGF-BB) and basic fibroblast growth factor (bFGF) are potent growth factors active on many cell types. The present study indicates that they directly interact in vitro. The interaction was investigated with overlay experiments, surface plasmon resonance experiments, and solid-phase immunoassays by immobilizing one factor or the other and by steady-state fluorescence analysis. The interaction observed was specific, dose-dependent, and saturable, and the bFGF/PDGF-BB binding stoichiometry was found to be 2:1. K(D)(1) for the first step equilibrium and the overall K(D) values were found to be in the nanomolar and in the picomolar range, respectively. Basic FGF/PDGF-BB interaction was strongly reduced as a function of time of PDGF-BB proteolysis. Furthermore, docking analysis suggested that the PDGF-BB region interacting with bFGF may overlap, at least in part, with the PDGF-BB receptor-binding site. This hypothesis was supported by surface plasmon resonance experiments showing that an anti-PDGF-BB antibody, known to inhibit PDGF-BB binding with its receptor, strongly reduced bFGF/PDGF-BB interaction, whereas a control antibody was ineffective. According to these data, the observed bFGF.PDGF-BB complex formation might explain, at least in part, previous observations showing that PDGF-BB chemotactic and mitogenic activity on smooth muscle cells are strongly inhibited in the presence of bFGF.

Thrombospondin-1 as an endogenous inhibitor of angiogenesis and tumor growth

Thrombospondin-1 (TSP-1) is a matricellular glycoprotein that influences cellular phenotype and the structure of the extracellular matrix. These effects are important components of the tissue remodeling that is associated with angiogenesis and neoplasia. The genetic mutations in oncogenes and tumor suppressor genes that occur within tumor cells are frequently associated with decreased expression of TSP-1. However, the TSP-1 that is produced by stromal fibroblasts, endothelial cells and immune cells suppresses tumor progression. TSP-1 inhibits angiogenesis through direct effects on endothelial cell migration and survival and through indirect effects on growth factor mobilization. TSP-1 that is present in the tumor microenvironment also acts to suppress tumor cell growth through activation of transforming growth factor beta in those tumor cells that are responsive to TGF beta. In this review, the molecular basis for the role of TSP-1 in the inhibition of tumor growth and angiogenesis is summarized.

Latent-TGF-beta: an overview

The latency associated with the transforming growth factor-betas (TGF-betas) was discovered in 1984. Since the two publications on this subject in that year, there has been on average over sixty reports in which latency was the dominant theme for each of the past 10 years, proof enough of the interest in this field of growth factor research. As the mature 25 kD forms of the TGF-betas are required for them to exert their many, diverse biological effects, it was inevitable that an explanation of the structure and of the activation of the latent complexes be sought. This overview provides a description of these essential points. Now that it has been clearly shown that dysregulation of particular components of the TGF-beta signalling pathway is implicated in many human diseases, the activation of the latent TGF-beta complexes has taken on added importance. Technical improvements enable the distinction of active and latent TGF-beta proteins in vivo and have started to reveal anomalies in the control of activation in relation to various pathological situations.

Real-time monitoring of the interactions of transforming growth factor-beta (TGF-beta ) isoforms with latency-associated protein and the ectodomains of the TGF-beta type II and III receptors reveals different kinetic models and stoichiometries of binding

Mature transforming growth factor-beta (TGF-beta) is proteolytically derived from the C terminus of a precursor protein. Latency-associated protein (LAP), the N-terminal remnant of the TGF-beta precursor, is able to bind and neutralize TGF-beta. Mature TGF-beta exerts its activity by binding and complexing members of two subfamilies of receptors, the type I and II receptors. In addition to these signaling receptors, TGF-beta can also interact with an accessory receptor termed the type III receptor. Using a surface plasmon resonance-based biosensor (BIAcore), we determined the mechanisms of interaction of four binding proteins (LAP, the type II and III receptor ectodomains (EDs), and a type II receptor ED/Fc chimera) with three TGF-beta isoforms, and we quantified their related kinetic parameters. Using global fitting based on a numerical integration data analysis method, we demonstrated that LAP and the type II receptor/Fc chimera interacted with the TGF-beta isoforms with a 1:1 stoichiometry. In contrast, the type II ED interactions with TGF-beta were best fit by a kinetic model assuming the presence of two independent binding sites on the ligand molecule. We also showed that the type III ED bound two TGF-beta molecules. Further experiments revealed that LAP was able to block the interactions of TGF-beta with the two EDs, but that the two EDs did not compete or cooperate with each other. Together, these results strongly support the existence of a cell-surface complex consisting of one type III receptor, two TGF-beta molecules, and four type II receptors, prior to the recruitment of the type I receptor for signal transduction. Additionally, our results indicate that the apparent dissociation rate constants are more predictive of the neutralizing potency of these TGF-beta-binding proteins (LAP, the type II and III receptor EDs, and the type II receptor/Fc chimera) than the apparent equilibrium constants.

The latent transforming growth factor-beta-binding protein-1 promotes in vitro differentiation of embryonic stem cells into endothelium

The latent transforming growth factor-beta-binding protein-1 (LTBP-1) belongs to a family of extracellular glycoproteins that includes three additional isoforms (LTBP-2, -3, and -4) and the matrix proteins fibrillin-1 and -2. Originally described as a TGF-beta-masking protein, LTBP-1 is involved both in the sequestration of latent TGF-beta in the extracellular matrix and the regulation of its activation in the extracellular environment. Whereas the expression of LTBP-1 has been analyzed in normal and malignant cells and rodent and human tissues, little is known about LTBP-1 in embryonic development. To address this question, we used murine embryonic stem (ES) cells to analyze the appearance and role of LTBP-1 during ES cell differentiation. In vitro, ES cells aggregate to form embryoid bodies (EBs), which differentiate into multiple cell lineages. We analyzed LTBP-1 gene expression and LTBP-1 fiber appearance with respect to the emergence and distribution of cell types in differentiating EBs. LTBP-1 expression increased during the first 12 d in culture, appeared to remain constant between d 12 and 24, and declined thereafter. By immunostaining, fibrillar LTBP-1 was observed in those regions of the culture containing endothelial, smooth muscle, and epithelial cells. We found that inclusion of a polyclonal antibody to LTBP-1 during EB differentiation suppressed the expression of the endothelial specific genes ICAM-2 and von Willebrand factor and delayed the organization of differentiated endothelial cells into cord-like structures within the growing EBs. The same effect was observed when cultures were treated with either antibodies to TGF-beta or the latency associated peptide, which neutralize TGF-beta. Conversely, the organization of endothelial cells was enhanced by incubation with TGF-beta 1. These results suggest that during differentiation of ES cells LTBP-1 facilitates endothelial cell organization via a TGF-beta-dependent mechanism.

The functions of thrombospondin-1 and-2

Considerable progress has been made towards understanding the function of thrombospondin-1 and-2. The description of the phenotype of mice with thrombospondin-1 and-2 knocked-out supports in vitro biochemical and cell-biological data and has opened new avenues of research. Recently, our understanding of the roles of thrombospondins in the activation of TGFbeta, inhibition of angiogenesis and the initiation of signal transduction has advanced.

Activation of latent TGF-beta by thrombospondin-1: mechanisms and physiology

Regulation of the activation of latent TGF-beta is essential for health as too much or too little TGF-beta activity can have serious, deleterious consequences. The processes that control conversion of the precursor to the biologically active form of TGF-beta in vivo are not well characterized. We have identified a mechanism for the activation of latent TGF-beta that involves binding of the secreted and extracellular matrix protein, thrombospondin-1 (TSP-1), to the latent precursor. Specific sequences in TSP-1 and in the precursor portion (the latency associate peptide-LAP) have been determined to be essential for activation of latent TGF-beta by TSP-1. It is thought that binding of TSP-1 to the latent complex induces a conformational rearrangement of the LAP in such a manner as to prevent the LAP from conferring latency on the mature domain of TGF-beta. A TSP-dependent mechanism of activation may be locally important during wound healing and in post-natal development of epithelial structures. The possible involvement of TSP-1 in TGF-beta activation during several disease processes is also discussed.

Bovine thrombospondin-2: complete complementary deoxyribonucleic acid sequence and immunolocalization in the external zones of the adrenal cortex

Given the variety of biological functions in the adrenal cortex that are controlled by ACTH, we hypothesized that some extracellular proteins act as biological relays for this systemic hormone. One candidate protein [corticotropin-induced secreted protein (CISP)] was purified from the conditioned medium of bovine adrenocortical cells on the basis of a 5- to 14-fold increase in its synthesis after the addition of ACTH. We report here the cloning of overlapping complementary DNAs that span the sequence encoding the full-length protein (1170 amino acids). The deduced CISP protein sequence is 89% identical to that of human thrombospondin-2 (TSP2), but only 61% identical to that of bovine TSP1, confirming that CISP is the bovine ortholog of TSP2. The bovine TSP2 sequence aligned perfectly with human, mouse, and chicken TSP2 sequences, except for a gap of 2 amino acids located in a linker region. All 58 cysteine residues that are conserved in other species were present in the bovine sequence as well as most of the functional domains. Most endocrine tissues (adrenal cortex, testis, ovary, and placenta) appeared to express TSP2, as determined by Western blot analysis. The highest levels of TSP2 protein were found in the adrenal cortex, followed by the heart, spleen, brain, and kidney. A differential extent of N-glycosylation or tissular proteolytic maturation may be responsible for the mol wt differences observed between bovine TSP2 detected in the medium from primary cultures and that in fresh tissue extracts. The immunohistochemical analysis of the distribution of TSP2 in the bovine adrenal gland revealed that the protein is much more abundant in the external zones (zona glomerulosa and zona fasciculata) than in the internal reticularis zone, a pattern similar to that reported for ACTH receptors. This distribution clearly suggests that TSP2 is a candidate relay protein for a subset of ACTH actions in the adrenal cortex.

The activation sequence of thrombospondin-1 interacts with the latency-associated peptide to regulate activation of latent transforming growth factor-beta

One of the primary points of regulation of transforming growth factor-beta (TGF-beta) activity is control of its conversion from the latent precursor to the biologically active form. We have identified thrombospondin-1 as a major physiological regulator of latent TGF-beta activation. Activation is dependent on the interaction of a specific sequence in thrombospondin-1 (K412RFK415) with the latent TGF-beta complex. Platelet thrombospon-din-1 has TGF-beta activity and immunoreactive mature TGF-beta associated with it. We now report that the latency-associated peptide (LAP) of the latent TGF-beta complex also interacts with thrombospondin-1 as part of a biologically active complex. Thrombospondin.LAP complex formation involves the activation sequence of thrombospondin-1 (KRFK) and a sequence (LSKL) near the amino terminus of LAP that is conserved in TGF-beta1-5. The interactions of LAP with thrombospondin-1 through the LSKL and KRFK sequences are important for thrombospondin-mediated activation of latent TGF-beta since LSKL peptides can competitively inhibit latent TGF-beta activation by thrombospondin or KRFK-containing peptides. In addition, the association of LAP with thrombospondin-1 may function to prevent the re-formation of an inactive LAP.TGF-beta complex since thrombospondin-bound LAP no longer confers latency on active TGF-beta. The mechanism of TGF-beta activation by thrombospondin-1 appears to be conserved among TGF-beta isoforms as latent TGF-beta2 can also be activated by thrombospondin-1 or KRFK peptides in a manner that is sensitive to inhibition by LSKL peptides.

Conformation and self-association of human recombinant transforming growth factor-beta3 in aqueous solutions

The transforming growth factors-beta (TGF-beta) are important regulatory peptides for cell growth and differentiation with therapeutic potential for wound healing. Among the several TGF-beta isoforms TGF-beta3 has a particularly low solubility at physiological pH and easily forms aggregates. A spectroscopic structural analysis of TGF-beta3 in solution has thus been difficult. In this study, circular dichroism spectroscopy was used to determine the secondary structural elements of TGF-beta3. In addition, the aggregation of TGF-beta3 was investigated systematically as a function of pH and salt concentration using a rapid screening method. Sedimentation equilibrium and sedimentation velocity analysis revealed that TGF-beta3 exists predominantly in two major forms: (i) monomers in solution at low pH and (ii) large precipitating aggregates at physiological pH. Under acidic conditions (pH < 3.8) the protein was not aggregated. At pH approximately 3.9, a monomer right arrow over left arrow dimer equilibrium could be detected that transformed into larger aggregates at pH > 4.1. Aggregation was pronounced in the pH range of 4.3 < pH < 9.8 with the aggregation maximum between pH 6.5 and 8. 5. The aggregation process was accompanied by a structural change of the protein. The CD spectra were characterized by an isodichroic point at 209.5 nm indicating a two-state equilibrium between TGF-beta3 dissolved in solution and aggregated TGF-beta3. Aggregated TGF-beta3 showed a higher beta-sheet content and lower beta-turn and random coil contributions compared with monomeric TGF-beta3. Both the solution structure and the aggregate structure of TGF-beta3 were different from the crystal structure. This was in contrast to TGF-beta2, which showed very similar crystal and solution structures. Under alkaline conditions (pH > 9.8) the turbidity disappeared and a further conformational change was induced. The pH dependence of the TGF-beta3 conformation in solution in the range of 2.3 < pH < 11. 0 was reversible. Aggregation of TGF-beta3 was, furthermore, influenced by the presence of salt. For pH > 3.8 the addition of salt greatly enhanced the tendency to aggregate, even in the very basic domain. Under physiological conditions (pH 7.4, cNaCl = 164 mM) TGF-beta3 has almost the highest tendency to aggregate and will remain in solution only at nanomolar concentrations.

Retinoids potentiate transforming growth factor-beta activity in bovine endothelial cells through up-regulating the expression of transforming growth factor-beta receptors

Retinoic acid (RA) induces the activation of latent transforming growth factor-beta (TGF-beta) in bovine aortic endothelial cells (BAECs) via enhancement of cellular plasminogen activator (PA)/plasmin levels. The resultant TGF-beta suppresses the excessive fibrinolytic activity by decreasing PA expression and stimulating expression of the PA inhibitor, PA inhibitor-1 (PAI-1), and inhibits cell proliferation. Here, we report that, in this regulatory system, RA simultaneously up-regulates the expression of TGF-beta receptor types I and II, resulting in enhancement of TGF-beta activity in the cells. RA increased the numbers of high- and low-affinity binding sites for 125I-TGF-beta1 2.1-fold and 1.5-fold, respectively, without alteration of their Kd values. Affinity labeling and Western and Northern blotting studies showed that, following RA treatment, surface levels of both type I and type II receptors increased due to augmentation in their mRNA levels. The effect was dose- and time-dependent. Treatment with 1 microM RA for 15 hr increased mRNA levels of type I and II receptor threefold and eightfold, respectively. Pretreatment of BAECs with either RA or retinol lowered the concentration of TGF-beta1 required to suppress PA levels, to enhance PAI-1 levels, and to inhibit cell proliferation. Thus, retinoids may regulate cellular functions of BAECs not only by inducing the formation of active TGF-beta but also by stimulating TGF-beta receptor expression. This regulatory mechanism may sustain TGF-beta-mediated regulation of EC function at a focal site where RA is acting.