Latent transforming growth factor-beta binding protein domains involved in activation and transglutaminase-dependent cross-linking of latent transforming growth factor-beta

Transglutaminase reactivity of human involucrin

Human involucrin (hINV) is assembled into cornified structures via formation of transglutaminase (TG)-dependent interprotein epsilon-(gamma-glutamyl)lysine bonds. The hINV sequence includes 150 glutamine residues that could function as potential sites of cross-link formation. The present studies were designed to evaluate the extent to which hINV can function as a TG substrate under optimal conditions and in the absence of other substrates. Incubation of hINV with TG results in formation of 4-5 isopeptide bonds per hINV molecule. When the small amine donor (14)C-putrescine is included in the reaction, 48 Q residues are labled. Isotope distribution and sequence analysis suggests that the (14)C-putrescine-labeled sites are located throughout the protein. Our present results show that many hINV Q residues can be utilized for cross-link formation, and that hINV can be cross-linked at very high cross-link densities. These results suggest that, in vivo, factors other than hINV structure limit the number of residues used for cross-link formation.

Potential mechanisms for the plasmin-mediated release and activation of latent transforming growth factor-beta1 from the extracellular matrix of growth plate chondrocytes

Chondrocytes produce latent transforming growth factor-beta1 (TGF-beta1) in a small, circulating form of 100 kDa and also store latent TGF-beta1 in their matrix in a large form of 290 kDa containing the latent TGF-beta1 binding protein 1. As growth plate cartilage cells are exceptionally sensitive to TGF-beta1 and are known to produce plasminogen activator, the role of plasmin in the activation of soluble and matrix-bound latent TGF-beta1 was examined. As is true for other cell types, low-dose plasmin (0.01 U/ml) was found to release both active and latent TGF-beta1 from chondrocyte matrix in a time-dependent manner over 3 h. However, high-dose plasmin (1.0 U/ml) was found to release active TGF-beta1 more rapidly than low-dose plasmin, and this release ceased within 30 min; latent complex continued to be released over time (3 h). When high-dose plasmin was titrated against the serine protease inhibitors, aprotinin and alpha-(2-aminoethyl)benzenesulfonyl fluoride, results similar to low-dose plasmin were obtained, indicating that the effects of high-dose plasmin could be altered to mimic those of low-dose plasmin. No differences were observed on the effects of plasmin on the release of TGF-beta1 from the matrices of either growth zone or resting zone chondrocytes. We examined whether plasmin could further activate the truncated large latent TGF-beta1 complex of 230 kDa that was released into the media by plasmin. It is known that plasmin will activate the small latent complex, so this was compared with the truncated form. Plasmin completely activated the small latent complex, whereas a smaller, but significant, activation of the truncated form of latent TGF-beta1 also occurred. These studies may have relevance to normal physiological conditions, where plasminogen and/or plasmin is present in very small amounts in the cartilage and, therefore, small amounts of active TGF-beta1 would be present, and to pathological conditions such as fractures, where chondroprogenitor cells would be exposed to high concentrations of plasmin and, therefore, to short-term high concentrations of this potent chondrogenic growth factor.

TGF-beta: from latent to active

The transforming growth factor-betas (TGF-betas) are synthesized as precursor proteins that are modified intracellularly prior to secretion. One of the most relevant intracellular modifications is the cleavage of the C-terminal pro-region from the N-terminal portion of the protein. The C-terminal pro-region is referred to as the latency-associated peptide (LAP) while the N-terminal region is called the mature TGF-beta or active TGF-beta. However, with some exceptions the LAP noncovalently associates with the mature TGF-beta prior to secretion. When the mature TGF-beta is associated with the LAP it is called L-TGF-beta and cannot interact with its receptor and has no biological effect. The TGF-betas and their receptors are very ubiquitously expressed, suggesting that the regulation of TGF-beta activity is likely to be complex and multifactorial. However, one of the most important means of controlling the biological effects of TGF-beta is the regulation of converting L-TGF-beta to active TGF-beta. The current literature supports two major mechanisms of activation of L-TGF-beta and suggests that the mechanism of activation of L-TGF-beta may be varied and context-dependent. For TGF-beta to become biologically active the LAP has to be either released from its associations with L-TGF-beta or undergo conformational change such that the LAP is not released from the L-TGF-beta complex but exposes the TGF-beta receptor binding site. Since TGF-beta has been associated with the pathogenesis of numerous diseases, the various mechanisms of activation of L-TGF-beta in context offer the possibility of controlling TGF-beta activity localized to the organ of involvement and to a more specific disease process.

Regulation of cell surface tissue transglutaminase: effects on matrix storage of latent transforming growth factor-beta binding protein-1

Using a cytochemical approach, we examined the role of tissue transglutaminase (tTgase, Type II) in the incorporation of latent TGF-beta binding protein-1 (LTBP-1) in the extracellular matrix of Swiss 3T3 fibroblasts in which tTgase expression can be modulated through a tetracycline-controlled promoter. Increased tTgase expression led to an increased rate of LTBP-1 deposition in the matrix, which was accompanied by an increased pool of deoxycholate-insoluble fibronectin. Matrix deposition of LTBP-1 could also be reduced by the competitive amine substrate putrescine. Immunolocalization at the fluorescence and electron microscopic level showed that extracellular tTgase is located at the basal and apical surfaces of cells and at cell-cell contacts, and that LTBP-1 is co-distributed with cell surface tTgase, suggesting an early contribution of tTgase to the binding of LTBP-1 to matrix proteins. LTPB-1 was also found to co-localize with both intracellular and extracellular fibronectin, and increased immunoreactivity for LTBP-1 and fibronectin was found in large molecular weight polymers in the deoxycholate-insoluble matrix of fibroblasts overexpressing tTgase. We conclude that regulation of tTgase expression is important for controlling matrix storage of latent TGF-beta1 complexes and that fibronectin may be one extracellular component to which LTBP-1 is crosslinked when LTBP-1 and tTgase interact at the cell surface. (J Histochem Cytochem 47:1417-1432, 1999)

Cell surface localization of tissue transglutaminase is dependent on a fibronectin-binding site in its N-terminal beta-sandwich domain

Increasing evidence indicates that tissue transglutaminase (tTG) plays a role in the assembly and remodeling of extracellular matrices and promotes cell adhesion. Using an inducible system we have previously shown that tTG associates with the extracellular matrix deposited by stably transfected 3T3 fibroblasts overexpressing the enzyme. We now show by confocal microscopy that tTG colocalizes with pericellular fibronectin in these cells, and by immunogold electron microscopy that the two proteins are found in clusters at the cell surface. Expression vectors encoding the full-length tTG or a N-terminal truncated tTG lacking the proposed fibronectin-binding site (fused to the bacterial reporter enzyme beta-galactosidase) were generated to characterize the role of fibronectin in sequestration of tTG in the pericellular matrix. Enzyme-linked immunosorbent assay style procedures using extracts of transiently transfected COS-7 cells and immobilized fibronectin showed that the truncation abolished fibronectin binding. Similarly, the association of tTG with the pericellular matrix of cells in suspension or with the extracellular matrix deposited by cell monolayers was prevented by the truncation. These results demonstrate that tTG binds to the pericellular fibronectin coat of cells via its N-terminal beta-sandwich domain and that this interaction is crucial for cell surface association of tTG.

Tissue transglutaminase is expressed, active, and directly involved in rat dermal wound healing and angiogenesis

Tissue transglutaminase (TG) is an enzyme that stabilizes the structure of tissues by covalently ligating extracellular matrix molecules. Expression and localization of TG are not well established during wound healing. We performed punch biopsy wounds on anesthetized rats and monitored the wound healing process by histological and immunohistochemical methods. The TG antigen and activity are expressed at sites of neovascularization in the provisional fibrin matrix within 24 h of wounding. Endothelial cells, macrophages, and skeletal muscle cells expressed TG throughout the healing process. The TG antigen within the wound was active in vivo based on the detection of isopeptide bonds. The TG antigen increased four- to fivefold by day 3 postwounding and was proteolytically degraded. TG expression occurred in association with TGF-beta, TNF-alpha, IL-6, and VEGF production in the wound. Recombinant TG increased vessel length density (a measure of angiogenesis) when applied topically in rat dorsal skin flap window chambers. We have established that TG is an important tissue stabilizing enzyme that is active during wound healing and can function to promote angiogenesis.

Differing patterns of transforming growth factor-beta expression in normal intestinal mucosa and in active celiac disease

BACKGROUND

Growth-inhibitory autocrine polypeptides such as transforming growth factor (TGF)-beta may play a role in the control of normal epithelial cell proliferation and differentiation. In addition, TGF-beta has a central role in extracellular matrix homeostasis and regulates the immune response at the local level. In this study immunohistochemistry was used to examine the pattern of TGF-beta protein distribution and quantitative reverse transcription-polymerase chain reaction (RT-PCR) to determine levels of TGF-beta messenger RNA expression in normal intestinal mucosa and in the flat mucosa of children with celiac disease.

METHODS

Small intestinal biopsies were performed in children with active celiac disease and in histologically normal control subjects. Frozen sections were single stained using an anti-TGF-beta monoclonal antibody and were double stained for TGF-beta and T cell, macrophages, and the activation marker CD25. Total RNA was extracted from frozen specimens and competitive quantitative RT-PCR performed for TGF-beta mRNA using internal synthetic standard RNA.

RESULTS

In normal intestinal mucosa, by immunohistochemistry, TGF-beta expression was most prominent in the villous tip epithelium, whereas in the lamina propria, weak immunoreactivity was present. The celiac mucosa showed weak and patchy epithelial TGF-beta immunoreactivity. In contrast, an intense staining positivity was present in the lamina propria localized mostly in the subepithelial region where T cells, macrophages, and CD25+ cells were detected by double staining. By quantitative RT-PCR, levels of TGF-beta mRNA transcripts appeared to be increased in celiac intestinal mucosa compared with that in control subjects, although the difference did not reach statistical significance.

CONCLUSIONS

These observations suggest that TGF-beta expression is associated with differentiated enterocyte function. In celiac disease the lower TGF-beta epithelial cell expression could be a consequence of the preponderance of a less differentiated epithelial cell phenotype also present in the surface epithelium. In contrast, the prominent TGF-beta positivity of the subepithelial lamina propria suggests an association with the local immune and inflammatory response, as well as a potential role of these peptides in mesenchymal-epithelial cell interaction.

Tissue transglutaminase: an enzyme with a split personality

Tissue transglutaminase (tTG) belongs to the family of transglutaminase enzymes that catalyze the posttranslational modification of proteins via Ca(2+)-dependent cross-linking reactions. The catalytic action of tTG results in the formation of an isopeptide bond that is of great physiological significance since it is highly resistant to proteolysis and denaturants. Although tTG-mediated cross-linking reactions have been implicated to play a role in diverse biological processes, the precise physiological function of the enzyme remains unclear. Recent data, however, suggest that the protein polymers resulting from tTG-catalyzed reactions may play a role in commitment of cells to undergo apoptosis. On the same token, tTG-mediated formation of insoluble protein aggregates may underlie the markers of numerous pathological conditions, such as the senile plaques in Alzheimer's disease and the Lewy bodies in Parkinson's disease. In addition to catalyzing Ca(2+)-dependent cross-linking reactions, tTG can also bind and hydrolyze guanosine triphosphate and adenosine triphosphate. By virtue of this ability, tTG has been identified as a novel G-protein that interacts and activates phospholipase C following stimulation of the alpha-adrenergic receptor. The ability of tTG to mediate signal transduction may contribute to its involvement in the regulation of cell cycle progression. The following review summarizes the important features of this multifunctional enzyme that have emerged as a result of recent work from different laboratories.

Latent transforming growth factor-beta binding proteins (LTBPs)--structural extracellular matrix proteins for targeting TGF-beta action

Growth factors of the transforming growth factor-beta family are potent regulators of the extracellular matrix formation, in addition to their immunomodulatory and regulatory roles for cell growth. TGF-beta s are secreted from cells as latent complexes containing TGF-beta and its propeptide, LAP (latency-associated peptide). In most cells LAP is covalently linked to an additional protein, latent TGF-beta binding protein (LTBP), forming the large latent complex. LTBPs are required for efficient secretion and correct folding of TGF-beta s. The secreted large latent complexes associate covalently with the extracellular matrix via the N-termini of the LTBPs. LTBPs belong to the fibrillin-LTBP family of extracellular matrix proteins, which have a typical repeated domain structure consisting mostly of epidermal growth factor (EGF)-like repeats and characteristic eight cysteine (8-Cys) repeats. Currently four different LTBPs and two fibrillins have been identified. LTBPs contain multiple proteinase sensitive sites, providing means to solubilize the large latent complex from the extracellular matrix structures. LTBPs are now known to exist both as soluble molecules and in association with the extracellular matrix. An important consequence of this is LTBP-mediated deposition and targeting of latent, activatable TGF-beta into extracellular matrices and connective tissues. LTBPs have a dual function, they are required both for the secretion of the small latent TGF-beta complex as well as directing bound latent TGF-beta to extracellular matrix microfibrils. However, it is not known at present whether LTBPs are capable of forming microfibrils independently, or whether they are a part of the fibrillin-containing fibrils. Most LTBPs possess RGD-sequences, which may have a role in their interactions with the cell surface. At least LTBP-1 is chemotactic to smooth muscle cells, and is involved in vascular remodelling. Analyses of the expressed LTBPs have revealed considerable variations throughout the molecules, generated both by alternative splicing and utilization of multiple promoter regions. The significance of this structural diversity is mostly unclear at present.

Pathogenetic sequence for aneurysm revealed in mice underexpressing fibrillin-1

Dissecting aortic aneurysm is the hallmark of Marfan syndrome (MFS) and the result of mutations in fibrillin-1, the major constituent of elastin-associated extracellular microfibrils. It is yet to be established whether dysfunction of fibrillin-1 perturbs the ability of the elastic vessel wall to sustain hemodynamic stress by disrupting microfibrillar assembly, by impairing the homeostasis of established elastic fibers, or by a combination of both mechanisms. The pathogenic sequence responsible for the mechanical collapse of the elastic lamellae in the aortic wall is also unknown. Targeted mutation of the mouse fibrillin-1 gene has recently suggested that deficiency of fibrillin-1 reduces tissue homeostasis rather than elastic fiber formation. Here we describe another gene-targeting mutation, mgR, which shows that underexpression of fibrillin-1 similarly leads to MFS-like manifestations. Histopathological analysis of mgR/mgR specimens implicates medial calcification, the inflammatory-fibroproliferative response, and inflammation-mediated elastolysis in the natural history of dissecting aneurysm. More generally, the phenotypic severity associated with various combinations of normal and mutant fibrillin-1 alleles suggests a threshold phenomenon for the functional collapse of the vessel wall that is based on the level and the integrity of microfibrils.

Serum immunoglobulin A from patients with celiac disease inhibits human T84 intestinal crypt epithelial cell differentiation

BACKGROUND & AIMS

Celiac disease is characterized by disturbed jejunal crypt-villus axis biology with immunoglobulin (Ig) A deposits underlining the epithelium. The aim of this study was to test whether celiac disease serum IgA (reticulin/endomysial autoantibodies) interferes with the mesenchymal-epithelial cell cross-talk.

METHODS

Differentiation of T84 epithelial cells was induced with IMR-90 fibroblasts or transforming growth factor beta in three-dimensional collagen gel cultures. The effects of purified celiac IgA and monoclonal tissue transglutaminase antibodies (CUB7402) were studied by adding the antibodies to the cocultures.

RESULTS

Active celiac disease IgA, reactive for tissue transglutaminase, significantly inhibited T84 epithelial cell differentiation (P < 0.001) and increased epithelial cell proliferation (P = 0.024). Similar effects were obtained with antibodies against tissue transglutaminase.

CONCLUSIONS

Celiac disease-associated IgA class antibodies disturb transforming growth factor beta-mediated fibroblast-epithelial cell cross-talk in this in vitro crypt-villus axis model. This primary finding indicates that celiac disease-specific autoantibodies may also contribute to the formation of the gluten-triggered jejunal mucosal lesion in celiac disease.

The integrin alpha v beta 6 binds and activates latent TGF beta 1: a mechanism for regulating pulmonary inflammation and fibrosis

Transforming growth factor beta (TGF beta) family members are secreted in inactive complexes with a latency-associated peptide (LAP), a protein derived from the N-terminal region of the TGF beta gene product. Extracellular activation of these complexes is a critical but incompletely understood step in regulation of TGF beta function in vivo. We show that TGF beta 1 LAP is a ligand for the integrin alpha v beta 6 and that alpha v beta 6-expressing cells induce spatially restricted activation of TGF beta 1. This finding explains why mice lacking this integrin develop exaggerated inflammation and, as we show, are protected from pulmonary fibrosis. These data identify a novel mechanism for locally regulating TGF beta 1 function in vivo by regulating expression of the alpha v beta 6 integrin.

Proteolytic control of growth factor availability

Most growth factors are released from cells in a form that does not permit immediate interaction with their high affinity receptors. An important mechanism for presentation of these released latent growth factors is activation by the plasminogen activator-plasmin system. The involvement of this system in the biology of Transforming Growth Factor-beta (TGF-beta) is reviewed.

Cellular and extracellular biology of the latent transforming growth factor-beta binding proteins

The latent transforming growth factor-beta binding proteins (LTBP) are a recently identified family of widely expressed multidomain glycoproteins that range in size from 125 kDa to 240 kDa. Four LTBP genes have been described, and the homology of latent transforming growth factor-beta binding proteins molecules to the fibrillins has resulted in their inclusion in the so-called 'fibrillin superfamily'. They form intracellular covalent complexes with latent transforming growth factor-beta and target these growth factors to the extracellular matrix. This review describes their structure, summarizes current understanding of their dual roles as growth factor binding proteins and components of the extracellular matrix, and highlights their significance in tissue development and disease.

Autoantibodies to tissue transglutaminase as predictors of celiac disease

BACKGROUND & AIMS

Immunoglobulin A (IgA) autoantibodies to endomysium (EMA) are highly specific and sensitive markers for celiac disease. Recently, we identified tissue transglutaminase (tTG) as the major if not sole endomysial autoantigen.

METHODS

An enzyme-linked immunosorbent assay (ELISA) was established to measure IgA anti-tTG titers in serum samples from 106 celiac patients with partial or subtotal villous atrophy, 43 celiac patients on a gluten-free diet, and 114 diseased and healthy controls. Results were correlated with clinical and histological data and with EMA titers.

RESULTS

In patients with biopsy-proven celiac disease consuming a normal, gluten-containing diet, 98.1% of the serum samples had elevated IgA titers against tTG, whereas 94.7% of the control sera were negative. IgA anti-tTG correlated positively with semiquantitative IgA EMA titers (r = 0.862; P < 0.0001).

CONCLUSIONS

An ELISA based on tTG allows diagnosis of celiac disease with a high sensitivity and specificity. IgA anti-tTG and IgA EMA show an excellent correlation, further confirming the enzyme as the celiac disease autoantigen. Because the assay is quantitative, not subjected to interobserver variation, and easy to perform, it will be a useful tool for population screening of a hitherto underdiagnosed disease.

The cutaneous microfibrillar apparatus contains latent transforming growth factor-beta binding protein-1 (LTBP-1) and is a repository for latent TGF-beta1

The transforming growth factors-beta1 and beta2 (TGF-beta) stimulate synthesis of extracellular matrix proteins in vitro and appear upregulated in fibrotic conditions, in scar formation, and in wound healing. The extracellular matrix in turn might also act as a scavenger or repository for TGF-beta. We therefore studied the in situ distribution of latent TGF binding protein-1 (LTBP-1) and latent TGF-beta1 on extracellular matrix elements of normal human skin and skin regenerating from cultured keratinocyte autografts. We localized both LTBP-1 and latent TGF-beta1 to fibrillin-containing (elastic) microfibrils. Both LTBP-1 and latent TGF-beta1 were already present during the earliest stages of the de novo formation of the microfibrillar apparatus, i.e., on fusiform, randomly oriented microfibrils that later coalesced to form the typical candelabra-like structures in the papillary dermis. We show herewith that LTBP-1 exerts a dual role as a component of fibrillin-microfibrils of the skin and in targeting latent TGF-beta1 to the cutaneous microfibrillar apparatus. Thus, this major connective tissue structure does not only serve as a force bearing element and scaffold for elastin deposition in the dermis, but also as an important repository for latent TGF-beta in the skin.

Expression of alternatively spliced human latent transforming growth factor beta binding protein-1

Latent transforming growth factor beta binding protein-1 (LTBP1) is important in regulating the localisation and activation of transforming growth factor beta(TGFbeta). Three forms of LTBP1 mRNA have previously been described, LTBP1L, LTBP1S and LTBPdelta53. Here, we have analysed the LTBP1 coding sequence and identified two other spliced forms, LTBP1delta55 and LTBP1delta41. LTBP1delta55 is a short form of LTBPIL which lacks 55 amino acids including two consensus N-glycosylation sites and LTBP1delta41 is a form of LTBP1 which lacks the 12th EGF-like repeat. Furthermore, sequencing of genomic clones showed that splicing to generate LTBP1L occurs using an intra-exonic 3' splice acceptor site in the first coding exon of LTBP1S and that LTBP1delta55 arises from the alternative use of an exonic 3' splice acceptor site at the end of the following intron. LTBP1delta41 arises from skipping the exon which encodes the 12th EGF-like repeat. LTBP1delta55 and LTBP1delta41 mRNA are expressed in a wide variety of human tissues but the proportions of each splice form vary in the tissues.

Interactions between growth factors and integrins: latent forms of transforming growth factor-beta are ligands for the integrin alphavbeta1

The multipotential cytokine transforming growth factor-beta (TGF-beta) is secreted in a latent form. Latency results from the noncovalent association of TGF-beta with its processed propeptide dimer, called the latency-associated peptide (LAP); the complex of the two proteins is termed the small latent complex. Disulfide bonding between LAP and latent TGF-beta-binding protein (LTBP) produces the most common form of latent TGF-beta, the large latent complex. The extracellular matrix (ECM) modulates the activity of TGF-beta. LTBP and the LAP propeptides of TGF-beta (isoforms 1 and 3), like many ECM proteins, contain the common integrin-binding sequence RGD. To increase our understanding of latent TGF-beta function in the ECM, we determined whether latent TGF-beta1 interacts with integrins. A549 cells adhered and spread on plastic coated with LAP, small latent complex, and large latent complex but not on LTBP-coated plastic. Adhesion was blocked by an RGD peptide, and cells were unable to attach to a mutant form of recombinant LAP lacking the RGD sequence. Adhesion was also blocked by mAbs to integrin subunits alphav and beta1. We purified LAP-binding integrins from extracts of A549 cells using LAP bound to Sepharose. alphavbeta1 eluted with EDTA. After purification in the presence of Mn2+, a small amount of alphavbeta5 was also detected. A549 cells migrated equally on fibronectin- and LAP-coated surfaces; migration on LAP was alphavbeta1 dependent. These results establish alphavbeta1 as a LAP-beta1 receptor. Interactions between latent TGF-beta and alphavbeta1 may localize latent TGF-beta to the surface of specific cells and may allow the TGF-beta1 gene product to initiate signals by both TGF-beta receptor and integrin pathways.

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.

Extracellular matrix-associated transforming growth factor-beta: role in cancer cell growth and invasion

Growth factors of the transforming growth factor-beta (TGF-beta) family inhibit the proliferation of epithelial, endothelial, and hematopoietic cells, and stimulate the synthesis of extracellular matrix components. TGF-beta s are secreted from cells in high-molecular-mass protein complexes that are composed of three proteins, the mature TGF-beta-dimer, the TGF-beta propeptide dimer, or latency-associated protein (LAP), and the latent TGF-beta binding protein (LTBP). Mature TGF-beta is cleaved from its propeptide during secretion, but the proteins remain associated by noncovalent interactions. LTBP is required for efficient secretion and processing of latent TGF-beta and it binds to LAP via disulfide bond(s). LTBP is a component of extracellular matrix microfibrils, and it targets the latent TGF-beta complex to the extracellular matrix. TGF-beta signaling is initiated by proteolytic cleavage of LTBP that results in the release of the latent TGF-beta complex from the extracellular matrix. TGF-beta is activated by dissociation of LAP from the mature TGF-beta. Subsequent signaling involves binding of active TGF-beta to its type II cell surface receptors, which phosphorylate and activate type I TGF-beta receptors. Type I receptors, in turn, phosphorylate cytoplasmic transcriptional activator proteins Smad2 and Smad3, inducing their translocation to the nucleus. Recent evidence suggests that acquisition of resistance to TGF-beta growth inhibition plays a major role in the progression of epithelial and hematopoietic cell malignancies. The role of secretion of TGF-beta in tumorigenesis is more complex. The secretion of TGF-beta s by tumor cells may contribute to autocrine growth inhibition, but on the other hand, it may also promote invasion, metastasis, angiogenesis, and even immunosuppression. Tumor cells may also fail to deposit LTBP:TGF-beta complexes to the extracellular matrix. The elucidation of the mechanisms of the release of TGF-beta from the matrix and its subsequent activation aids the understanding of the pathophysiologic roles of TGF-beta in malignant growth, and allows the development of therapeutic agents that regulate the activity of TGF-beta.

Recombinant latent transforming growth factor beta-binding protein 2 assembles to fibroblast extracellular matrix and is susceptible to proteolytic processing and release

Latent transforming growth factor beta-binding protein 2 (LTBP-2) belongs to the fibrillin-LTBP gene family and is a component of 10-nm microfibrils. LTBP-2 consists mainly of domains of 8-cysteine and EGF-like repeats linked by proline-rich regions. To characterize the biochemical properties of LTBP-2, its assembly to the extracellular matrix, and its proteolytic release from the matrix, LTBP-2 was expressed recombinantly in Chinese hamster ovary cells and purified to homogeneity under nondenaturing conditions. Purified LTBP-2 bound calcium and was glycosylated at the central domain of EGF-like repeats. Antibodies made against the recombinant LTBP-2 decorated fibrillar structures in fibroblast extracellular matrix. Treatment of matrices with plasmin or elastase released a soluble approximately 160-kDa LTBP-2 fragment. Processing of LTBP-2 was studied by treating purified LTBP-2 with plasmin or porcine pancreatic elastase. LTBP-2 was processed with these proteases initially to a approximately 160-kDa fragment, and with higher concentrations to a protease-resistant approximately 120-kDa fragment. Processing sites were localized by amino acid sequencing to proline-rich regions at the N-terminal part of LTBP-2, suggesting that the matrix binding sites locate to the N-terminal approximately 500 amino acids of LTBP-2. Purified and biotinylated LTBP-2 could be assembled to fibrillar structures in fibroblast extracellular matrix during cell cultivation, indicating that LTBP-2 assembly to the matrix is not strictly linked to cells that make it and suggesting that microfibril assembly may involve soluble intermediates.

The involvement of aryl hydrocarbon receptor in the activation of transforming growth factor-beta and apoptosis

The aryl hydrocarbon receptor (AHR) is believed to mediate many of the toxic, carcinogenic, and teratogenic effects of environmental contaminants such as dioxins, polycyclic aromatic hydrocarbons, and polyhalogenated biphenyls. Ligands for the AHR have been shown to influence cell proliferation, differentiation, and apoptosis, but the mechanism by which the AHR affects the cell cycle is not known. Increased levels of mature transforming growth factor-beta (TGFbeta) has been correlated with reduced cell proliferation and increased rates of apoptosis and fibrosis. Based on the increase in portal fibrosis and small liver size observed in AHR-null (Ahr-/-) mice, the relationship between TGFbeta expression and apoptosis in this mouse line was analyzed. Livers from Ahr-/- mice had marked increase in active TGFbeta1 and TGFbeta3 proteins and elevated numbers of hepatocytes undergoing apoptosis compared with wild-type mice. Furthermore, increases in TGFbeta and apoptotic cells were found in the portal areas of the liver, where fibrosis is found in the Ahr-/- mice. In vitro, primary hepatocyte cultures from Ahr-/- mice exhibited a high number of cells in later stages of apoptosis and an elevated secretion of active TGFbeta into the media compared with cultures from wild-type mice, which have previously been shown to secrete only latent forms of the molecule. Conditioned media from Ahr-/- hepatocytes stimulated apoptosis in cultured hepatocytes from wild-type mice. Taken together, these findings suggest that the phenotypic abnormalities in Ahr-/- mice could be mediated in part by abnormal levels of active TGFbeta and altered cell cycle control.

Thrombospondin-1 is a major activator of TGF-beta1 in vivo

The activity of TGF-beta1 is regulated primarily extracellularly where the secreted latent form must be modified to expose the active molecule. Here we show that thrombospondin-1 is responsible for a significant proportion of the activation of TGF-beta1 in vivo. Histological abnormalities in young TGF-beta1 null and thrombospondin-1 null mice were strikingly similar in nine organ systems. Lung and pancreas pathologies similar to those observed in TGF-beta1 null animals could be induced in wild-type pups by systemic treatment with a peptide that blocked the activation of TGF-beta1 by thrombospondin-1. Although these organs produced little active TGF-beta1 in thrombospondin null mice, when pups were treated with a peptide derived from thrombospondin-1 that could activate TGF-beta1, active cytokine was detected in situ, and the lung and pancreatic abnormalities reverted toward wild type.

Analysis of the expression pattern of the latent transforming growth factor beta binding protein isoforms in normal and diseased human liver reveals a new splice variant missing the proteinase-sensitive hinge region

Latent transforming growth factor beta binding protein (LTBP), a component of the extracellular matrix (ECM) of various tissues, is important for the secretion of TGF-beta and, furthermore, for the storage of TGF-beta in ECM. The proteolytic cleavage of LTBP is assumed to be the prerequisite for the activation of TGF-beta. We investigated the mRNA expression pattern of the three LTBP isoforms (LTBP-1, -2, -3) and the protein distribution of the components of the large latent TGF-beta complex, namely LTBP-1 and -2, latency-associated protein (LAP), and TGF-beta, in human liver using reverse-transcription polymerase chain reaction (RT-PCR) and immunohistochemical alkaline phosphatase anti-alkaline phosphatase (APAAP) staining. Parts of explanted livers diagnosed as hepatitis B, hepatitis C, primary biliary cirrhosis (PBC), and primary sclerosing cholangitis (PSC) and normal liver tissue were examined. LTBP transcripts were detected in the same manner in all liver specimens. Interestingly, we found a new splice variant of LTBP-1 (LTBP-1D), in which the sequence coding for the proteinase-sensitive hinge region is deleted. The corresponding parts of the human LTBP-2 and LTBP-3 cDNA coding for the hinge region were sequenced and show neither similar proteinase cleavage sites nor deleted cDNA sequences. The proposed proteinase cleavage site of mouse LTBP-3 seems not to be conserved in the human LTBP-3 gene. By immunohistochemistry, LTBP-1, -2, and LAP were detectable in normal and diseased livers and showed a different staining pattern for both LTBP isoforms. By contrast, TGF-beta showed a spotted staining pattern in diseased livers only, predominantly in the area of parenchymal cells that are close to fibrotic tissue. This strongly suggests the release of active TGF-beta from preexisting latent complexes. The LTBP-1D splice variant, which is probably less sensitive against proteolytic degradation and therefore may protect TGF-beta from activation, may have importance for modulating the biological activity of TGF-beta in normal and diseased liver.

Anti-latent TGF-beta binding protein-1 antibody or synthetic oligopeptides inhibit extracellular matrix expression induced by stretch in cultured rat mesangial cells

Transforming growth factor-beta (TGF-beta) is usually secreted as a large latent complex associated with latent TGF-beta binding protein-1 (LTBP-1), which is known to bind to extracellular matrix (ECM) components. Although the LTBP-ECM interaction has been suggested to play a role in the activation and biological action of TGF-beta, the precise mechanism is still unclear. In glomerular hypertension mesangial cells are believed to perceive the increased cyclic strain and we have recently reported that cyclic mechanical stretch in vitro enhances the expression of ECM components via an autocrine/paracrine secretion of TGF-beta in cultured rat mesangial cells. Therefore, in this study we examined the role of LTBP-1 in the stretch-induced, TGF-beta-mediated ECM expression. Mesangial cells expressed mRNA for short and long forms of LTBP-1 (LTBP-1S and LTBP-1L, respectively). Mesangial cells were subjected to cyclic stretch to provide a maximal elongation of 20% at a rate of 60 cycles/min for 24 to 36 hours in the presence of polyclonal antibody raised against human LTBP-1 or synthetic oligopeptides corresponding to the N-terminal portions of human LTBP-1, which may work as competitive inhibitors against the LTBP-ECM association. Both anti-LTBP-1 antibody (Ab39) and synthetic oligopeptides inhibited the stretch-induced mRNA expression of type I collagen and fibronectin in a dose-dependent manner, but the inhibition by Ab39 or the oligopeptides was recovered by adding recombinant TGF-beta. Ab39 or the oligopeptides did not change the effect of exogenously added TGF-beta, such as growth-inhibition in mink lung epithelial cells. These results suggest that mesangial cells secrete TGF-beta as a large latent complex, and the LTBP-ECM interaction may be a pivotal step in TGF-beta action and ECM accumulation, providing a new therapeutic strategy against progression of glomerulosclerosis and other fibrotic diseases.

Role of latent TGF-beta 1 binding protein in vascular remodeling

Transforming growth factor-beta (TGF-beta) is secreted as a latent, high molecular weight complex, which is composed of TGF-beta, a latency associated peptide (LAP) and a latent TGF-beta binding protein (LTBP). In this study, we report on the role of LTBP in vascular remodeling. 0.01-5 ng/ml of LTBP stimulated the migration activities of cultured rat arterial smooth muscle cells (SMC) about 4-7 fold compared with control in vitro. The maximal activity of SMC migration by LTBP was 75% of that by 10 ng/ml of PDGF-BB. A checker board analysis showed that the migration by LTBP was chemotactic, not chemokinetic. By cross-linking experiment, LTBP associated with 80-120 kd cell surface protein of SMC, suggesting that a part of LTBP can bind with SMC. Furthermore, LTBP was more strongly expressed in the intimal layer than in the medial layer of BCI artery. These results suggest that LTBP plays an important role in the initial stage of arterial intimal thickening through the acceleration of SMC migration from the medial to intimal layer and is one of the essential factors influencing vascular remodeling.

Mannose 6-phosphate/insulin-like growth factor-II receptor targets the urokinase receptor to lysosomes via a novel binding interaction

The urokinase-type plasminogen activator receptor (uPAR) plays an important role on the cell surface in mediating extracellular degradative processes and formation of active TGF-beta, and in nonproteolytic events such as cell adhesion, migration, and transmembrane signaling. We have searched for mechanisms that determine the cellular location of uPAR and may participate in its disposal. When using purified receptor preparations, we find that uPAR binds to the cation-independent, mannose 6-phosphate/insulin-like growth factor-II (IGF-II) receptor (CIMPR) with an affinity in the low micromolar range, but not to the 46-kD, cation-dependent, mannose 6-phosphate receptor (CDMPR). The binding is not perturbed by uPA and appears to involve domains DII + DIII of the uPAR protein moiety, but not the glycosylphosphatidylinositol anchor. The binding occurs at site(s) on the CIMPR different from those engaged in binding of mannose 6-phosphate epitopes or IGF-II. To evaluate the significance of the binding, immunofluorescence and immunoelectron microscopy studies were performed in transfected cells, and the results show that wild-type CIMPR, but not CIMPR lacking an intact sorting signal, modulates the subcellular distribution of uPAR and is capable of directing it to lysosomes. We conclude that a site within CIMPR, distinct from its previously known ligand binding sites, binds uPAR and modulates its subcellular distribution.

Plasminogen/plasminogen activator and growth factor activation

The plasminogen/plasminogen activator system is widely used in extracellular proteolysis. In this review the involvement of this system in tumour invasion, cell migration, growth factor presentation and inhibition of angiogenesis are discussed.

Loss of a consensus heparin binding site by alternative splicing of latent transforming growth factor-beta binding protein-1

Latent transforming growth factor-beta binding protein-1 (LTBP-1), plays an important role in controlling localisation and activation of transforming growth factor-beta (TGF-beta). We show that alternative splicing generates a form of mRNA which lacks bases 1277-1435 (termed LTBP-1delta53). The 53 amino acids encoded by these bases include the eighth cysteine of the first cysteine repeat and a consensus heparin binding sequence. Sequencing of genomic clones showed that alternative splicing resulted from the use of an intra-exonic 3' splice acceptor site. The loss of the heparin binding site implies that LTBP-1delta53 will bind to the extracellular matrix less efficiently than LTBP-1.

Cell-associated activation of latent transforming growth factor-beta by calpain

Transforming growth factor-beta (TGF-beta) is normally secreted in a latent form, and plasmin-mediated proteolytic cleavage of latency-associated peptide (LAP), a component of latent TGF-beta complex that makes the complex inactive, activates latent TGF-beta. In the present study, we investigated the possible involvement of calpain, one of the cysteine proteases, in the activation of latent TGF-beta. When recombinant latent TGF-beta was incubated with calpain (1-10 u/ml) in a test tube, calpain cleaved LAP and released mature TGF-beta from the latent complex. When calpain was applied to cultured bovine capillary endothelial (BCE) cells, a low concentration of calpain (0.05-0.1 u/ml) inhibited the migration and proliferation of the cells, and these inhibitory effects were abrogated by anti-TGF-beta antibody as well as by calpain inhibitor peptide, but not by alpha2-antiplasmin, a specific inhibitor of plasmin. Active TGF-beta was detected in the conditioned medium of BCE cells collected in the presence of calpain. Chemical cross-linking of (125)I-calpain to BCE cells followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis indicated that calpain bound to the cell surface through chondroitinase ABC-sensitive proteoglycan. In addition, treatment of the BCE cells with chondroitinase ABC abrogated the inhibitory effect of calpain on the migration of these cells. Our data thus suggest that calpain is able to activate latent TGF-beta through a mechanism independent of plasmin. This activation is efficient in the presence of cells, and calpain binds to the cell surface via proteoglycan and activates latent TGF-beta, which is targeted to the same surface.

Modulation of the in situ activity of tissue transglutaminase by calcium and GTP

Tissue transglutaminase (tTG) is a calcium-dependent enzyme that catalyzes the posttranslational modification of proteins by transamidation of specific polypeptide-bound glutamine residues. Previous in vitro studies have demonstrated that the transamidating activity of tTG requires calcium and is inhibited by GTP. To investigate the endogenous regulation of tTG, a quantitative in situ transglutaminase (TG) activity assay was developed. Treatment of human neuroblastoma SH-SY5Y cells with retinoic acid (RA) resulted in a significant increase in tTG levels and in vitro TG activity. In contrast, basal in situ TG activity did not increase concurrently with RA-induced increased tTG levels. However, stimulation of cells with the calcium-mobilizing drug maitotoxin (MTX) resulted in increases in in situ TG activity that correlated (r2 = 0.76) with increased tTG levels. To examine the effects of GTP on in situ TG activity, tiazofurin, a drug that selectively decreases GTP levels, was used. Depletion of GTP resulted in a significant increase in in situ TG activity; however, treatment of SH-SY5Y cells with a combination of MTX and tiazofurin resulted in significantly less in situ TG activity compared with treatment with MTX alone. This raised the possibility of calcium-dependent proteolysis due to the effects of tiazofurin, because in vitro GTP protects tTG against proteolysis by trypsin. Studies with a selective membrane permeable calpain inhibitor indicated that tTG is likely to be an endogenous substrate of calpain, and that depletion of GTP increases tTG degradation after elevation of intracellular calcium levels. TG activity was also increased in response to activation of muscarinic cholinergic receptors, which increases intracellular calcium through inositol 1,4,5-trisphosphate generation. The results of these experiments demonstrate that selective changes in calcium and GTP regulate the activity and levels of tTG in situ.

TGF-beta signal transduction

The transforming growth factor beta (TGF-beta) family of growth factors control the development and homeostasis of most tissues in metazoan organisms. Work over the past few years has led to the elucidation of a TGF-beta signal transduction network. This network involves receptor serine/threonine kinases at the cell surface and their substrates, the SMAD proteins, which move into the nucleus, where they activate target gene transcription in association with DNA-binding partners. Distinct repertoires of receptors, SMAD proteins, and DNA-binding partners seemingly underlie, in a cell-specific manner, the multifunctional nature of TGF-beta and related factors. Mutations in these pathways are the cause of various forms of human cancer and developmental disorders.

TGF-beta latency: biological significance and mechanisms of activation

Transforming growth factor (TGF-) beta is secreted as a latent complex in which the mature growth factor remains associated with its propeptide. In order to elicit a biological response, the cytokine must be released from the latent complex, a process termed latent TGF-beta activation or TGF-beta formation. Although latent TGF-beta activation is a critical step in the regulation of its activity, little is known about the molecular mechanisms that lead to the production of active TGF-beta. In this article, we present an overview of the data available on this topic, and we propose a tentative model for the mechanism of TGF-beta formation based upon the observations with different cell systems and on recent findings on the structure of the latent TGF-beta complex.