Latent TGF-beta binding protein-3 (LTBP-3) requires binding to TGF-beta for secretion

Latent transforming growth factor-beta (TGF-beta) binding protein (LTBP)-1, which is easily secreted, has been shown to enhance the secretion of TGF-beta. Here we show that another member of the LTBP family, LTBP-3, is not secreted by several cell types, but secretion occurs after coexpression with TGF-beta. The secretion of LTBP-3 requires complexing of LTBP-3 with Cys33 of the TGF-beta propeptide.

Beta(2)-adrenoceptor stimulation enhances latent transforming growth factor-beta-binding protein-1 and transforming growth factor-beta1 expression in rat hippocampus after transient forebrain ischemia

A protective capacity of transforming growth factor-beta1 (TGF-beta1) against various insults inducing neurone cell death in vitro and in vivo has been well established. We have recently shown the rapid up-regulation and persistent expression of TGF-beta1 in surviving CA1 pyramidal cells after cerebral ischemia suggesting an endogenous mechanism of neuroprotection by this multifunctional cytokine. In the present study, we demonstrated that intraperitoneal administration of clenbuterol, a lipophilic beta(2)-adrenoceptor agonist, caused an increase in TGF-beta1 expression in non-ischemic rats and further enhanced TGF-beta1 protein levels in rat CA1 pyramidal neurones after transient forebrain ischemia. In the hippocampus neuroprotection by clenbuterol (0.5 mg/kg) was accompanied by increased TGF-beta1 immunoreactivity as early as 3 h, and remained elevated up to 2 days after ischemia. The corresponding increased TGF-beta1 mRNA levels after ischemia were not further enhanced by clenbuterol, suggesting post-transcriptional regulation of TGF-beta1 protein after beta(2)-adrenoceptor stimulation. In saline-treated rats latent TGF-beta-binding protein-1 (LTBP-1) immunoreactivity was moderately elevated 3 and 6 h after ischemia, and returned to control levels after 1 day of reperfusion. In parallel with the up-regulation of TGF-beta1 immunoreactivity, LTBP-1 levels in the hippocampus were considerably increased by clenbuterol from 3 h to 2 days after ischemia. Our data demonstrate a concomitant increase in LTBP-1 and TGF-beta1 expression in the ischemic hippocampus after stimulation of beta(2)-adrenoceptors.

Identification of genes with higher expression in human uterine leiomyomas than in the corresponding myometrium

We used a PCR-based subtraction method, representational difference analysis of cDNA (cDNA--RDA), to identify genes with a higher expression in leiomyomas in comparison with the corresponding myometrium during the proliferative phase of the menstrual cycle. Increased expression of the genes for pregnancy-associated plasma protein A (PAPPA), tomoregulin, cellular retinoid acid binding protein 1 (CRABP1), zinc finger protein 185 (ZFP 185) and latent transforming growth factor beta binding protein 2 (LTBP2) was demonstrated in individual leiomyoma samples compared with corresponding myometrium. Additionally, a specific positive immunostaining of LTBP2 was found in the smooth muscle cells of both leiomyomas and myometrium. These genes may be part of previously unidentified molecular mechanisms responsible for the selective growth advantage of leiomyomas compared with myometrium. This work expands our knowledge about the molecular nature of leiomyomas and provides novel candidate genes to further explore in relation to their function during leiomyoma growth.

Expression of latent TGF-beta binding proteins and association with TGF-beta 1 and fibrillin-1 following arterial injury

OBJECTIVES

Transforming growth factor-beta (TGF-beta), a potent regulator of wound healing and scar formation, is thought to have a key role in the response to arterial injury. Latent TGF-beta binding proteins (LTBPs), members of the fibrillin superfamily, govern TGF-beta1 release, targeting and activation in vitro and also play a role as structural components of fibrillin-rich microfibrils. Despite the potential of LTBPs to modulate the response to arterial injury through either or both of these mechanisms, as yet their expression and function in the injured vasculature remain poorly defined.

METHODS

In this study, a porcine model of coronary angioplasty was used to investigate LTBP-1 and LTBP-2 synthesis and their association with TGF-beta 1 and fibrillin-1.

RESULTS

After angioplasty, increased LTBP-1 and LTBP-2 immunostaining was detected in a similar distribution to increased TGF-beta 1 expression in the neointima and in the neoadventitia. Overnight organ cultures revealed the formation of large latent TGF-beta 1 complexes containing LTBP-1. Increased LTBP-1 proteolysis after arterial injury correlated with increased active and latent TGF-beta levels. LTBP-2 synthesis increased in response to arterial injury but was neither present in large latent complexes nor proteolytically processed. LTBP-1 and LTBP-2 both co-localised to fibrillin-rich fibrillar structures in the neointima and adventitia.

CONCLUSIONS

These data suggest that LTBP-1 may have a TGF-beta 1 binding role in the arterial response to injury, and that LTBP-1 and LTBP-2 may have a structural role in association with microfibrils within the developing neointimal lesion. LTBP-1 proteolysis is potentially an important regulatory step for TGF-beta activation in the vasculature and inhibition of proteolysis could represent a novel therapeutic modality for controlling the arterial injury response.

Increased expression of TGF-beta1 and IGF-I in inflammatory stenotic lesions of hemodialysis fistulas

BACKGROUND

Hemodialysis fistula dysfunction due to stenotic lesions remains a frequent cause of hospitalization for hemodialysis patients. Transforming growth factor-beta(TGF-beta) and insulin-like growth factor-I (IGF-I) are known to be involved in atherogenesis. The latent TGF-beta1 binding protein-1 (LTBP-1) targets extracellular matrix (ECM) interactions and is involved in the regulation of TGF-beta latency.

METHODS

We investigated the expression of TGF-beta1, LTBP-1 and IGF-I in 15 occluded or severely narrowed vein segments of primary arteriovenous fistulas, in 29 non-stenosed control veins from uremic, pre-dialysis patients, and in 15 non-stenosed control saphenous veins obtained from patients undergoing aortocoronary bypass grafting. Immunohistochemistry was performed on snap-frozen tissue specimens using antibodies recognizing either the latency-associated peptide of TGF-beta1 (96-1), LTBP-1 (Ab39) or IGF-I. Serum levels of TGF-beta1 and IGF-I were determined by commercially available IRMA.

RESULTS

In stenosed hemodialysis fistulas, a pronounced intimal thickening with deposition of ECM was observed with light and electron microscopy. Infiltrating cells were seen in stenosed vessels, mostly in areas of intimal hyperplasia and in the media. TGF-beta1, LTBP-1 and IGF-I expression were mostly localized in the neointimal and medial layers, and were significantly higher than in the control groups. A positive correlation between the presence of inflammatory cells and the staining intensity for TGF-beta1, LTBP-1 and IGF-I was found in all vessels analyzed.

CONCLUSION

Neointimal thickening of primary arteriovenous fistulas represents a local inflammatory process and appears to be associated with increased protein expression of TGF-beta1 and IGF-I. While local IGF-I is likely to stimulate smooth muscle cell proliferation in this setting, TGF-beta1 may be an important trigger of ECM production and deposition.

Effects of the N2144S mutation on backbone dynamics of a TB-cbEGF domain pair from human fibrillin-1

The calcium-binding epidermal growth factor-like (cbEGF) module and the transforming growth factor beta-binding protein-like (TB) module are the two major structural motifs found in fibrillin-1, the extracellular matrix (ECM) protein defective in the Marfan syndrome (MFS). An MFS-causing mutation, N2144S, which removes a calcium ligand in cbEGF32, does not detectably affect fibrillin-1 biosynthesis, rate of secretion, processing, or deposition of reducible fibrillin-1 into the ECM. Since the residue at position 2144 is normally engaged in calcium ligation, it is unable to mediate intermolecular interactions. We have shown previously that this mutation does not affect the folding properties of the TB or cbEGF domains in vitro, but does decrease calcium-binding in cbEGF and TB-cbEGF domain constructs. Here, we use NMR spectroscopy to probe the effects of the N2144S mutation on backbone dynamic properties of TB6-cbEGF32. Analysis of the backbone (15)N relaxation data of wild-type TB6-cbEGF32 has revealed a flexible inter-domain linkage. Parallel dynamics analysis of the N2144S mutant has shown increased flexibility in the region joining the two domains as well as in the calcium-binding site at the N terminus of cbEGF32. This research demonstrates that a small change in peptide backbone flexibility, which does not enhance proteolytic susceptibility of the domain pair, is associated with an MFS phenotype. Flexibility of the TB-cbEGF linkage is likely to contribute to the biomechanical properties of fibrillin-rich connective tissue microfibrils, and may play a role in the microfibril assembly process.

Latent transforming growth factor beta-binding protein-3 and fibulin-1C interact with the extracellular domain of the heparin-binding EGF-like growth factor precursor

BACKGROUND

The membrane-bound cell-surface precursor and soluble forms of heparin-binding epidermal growth factor-like growth factor (HB-EGF) contribute to many cellular developmental processes. The widespread occurrence of HB-EGF in cell and tissue types has led to observations of its role in such cellular and tissue events as tumor formation, cell migration, extracellular matrix formation, wound healing, and cell adherence. Several studies have reported the involvement of such extracellular matrix proteins as latent transforming growth factor beta-binding protein, TGF-beta, and fibulin-1 in some of these processes. To determine whether HB-EGF interacts with extracellular matrix proteins we used the extracellular domain of proHB-EGF in a yeast two-hybrid system to screen a monkey kidney cDNA library. cDNA clones containing nucleotide sequences encoding domains of two proteins were obtained and their derived amino acid sequences were evaluated.

RESULTS

From approximately equal to 3 x 10(6) screened monkey cDNA clones, cDNA clones were recovered that contained nucleotide sequences encoding domains of the monkey latent transforming growth factor-beta binding protein-3 (MkLTBP-3) and fibulin-1C protein. The amino acid sequence derived from the MkLTBP-3 gene shared 98.6% identity with human LTBP-3 and 86.7% identity with mouse LTBP-3 amino acid sequences. The amino acid sequence derived from the monkey fibulin-1C gene shared 97.2% identity with human fibulin-1C. Yeast two-hybrid screens indicate that LTBP-3 and fibulin-1C interact with proHB-EGF through their calcium-binding EGF-like modules.

CONCLUSIONS

The interactions of the extracellular domain of proHB-EGF with LTBP-3 and fibulin-1C suggest novel functions for HB-EGF between cell and tissue surfaces.

Bone abnormalities in latent TGF-[beta] binding protein (Ltbp)-3-null mice indicate a role for Ltbp-3 in modulating TGF-[beta] bioavailability

The TGF-betas are multifunctional proteins whose activities are believed to be controlled by interaction with the latent TGF-beta binding proteins (LTBPs). In spite of substantial effort, the precise in vivo significance of this interaction remains unknown. To examine the role of the Ltbp-3, we made an Ltbp-3-null mutation in the mouse by gene targeting. Homozygous mutant animals develop cranio-facial malformations by day 10. At 2 mo, there is a pronounced rounding of the cranial vault, extension of the mandible beyond the maxilla, and kyphosis. Histological examination of the skulls from null animals revealed ossification of the synchondroses within 2 wk of birth, in contrast to the wild-type synchondroses, which never ossify. Between 6 and 9 mo of age, mutant animals also develop osteosclerosis and osteoarthritis. The pathological changes of the Ltbp-3-null mice are consistent with perturbed TGF-beta signaling in the skull and long bones. These observations give support to the notion that LTBP-3 is important for the control of TGF-beta action. Moreover, the results provide the first in vivo indication for a role of LTBP in modulating TGF-beta bioavailability.

The first stage of transforming growth factor beta1 activation is release of the large latent complex from the extracellular matrix of growth plate chondrocytes by matrix vesicle stromelysin-1 (MMP-3)

Transforming growth factor beta-1 (TGF-beta1) is secreted in a biologically inactive form and stored in the extracellular matrix as a 290 kDa complex consisting of the mature TGF-beta1 homodimer (Mr 25 kDa), the latency-associated peptide (LAP; Mr 75 kDa), and the latent TGF-beta1 binding protein-1 (LTBP1; Mr 190 kDa). Latent TGF-beta1, composed of these three components, is known as the "large latent TGF-beta1 complex." In contrast, latent TGF-beta1 without LTBP1 is known as "small latent TGF-beta1." For all latent forms, dissociation of the TGF-beta1 homodimer from LAP is necessary for growth factor activation and acquisition of biological activity. Matrix vesicles produced by growth plate chondrocytes contain matrix metalloproteinases that can activate small latent TGF-beta1. The enzyme responsible for this is matrix metalloproteinase-3 (MMP-3), although matrix vesicles also contain MMP-2 and plasminogen activator. The present study tested the hypothesis that matrix vesicle enzymes are also involved in the release of the large latent TGF-beta1 complex stored in the extracellular matrix. Matrix vesicles were isolated from cultures of resting zone and growth zone chondrocytes and metalloproteinases present in the matrix vesicles extracted with guanidine-HCl. Chondrocyte extracellular matrices were prepared by lysing confluent cultures and removing the lysed cells. The matrices were incubated with matrix vesicle extracts and the release of total and active TGF-beta1 was determined. To determine if MMP-2 or MMP-3 was involved in the release, matrix vesicle extracts were preincubated with anti-MMP-2 antibody or anti-MMP-3 antibody to selectively deplete the enzyme activity. Matrices were also treated with rhMMP-2 or rhMMP-3. To determine the identity of the released protein(s), digests were separated on SDS-polyacrylamide gels and Western blotting analysis was performed using a specific antibody to LTBP1. Matrix vesicle extracts released both active and total (=latent + active) TGF-beta1 in a time-dependent manner, with peak release after 1 hour of incubation. The amount of total TGF-beta1 released was 10 times higher than the release of active TGF-beta1. The effect of the matrix vesicle extracts was dose-dependent; in addition, the amount and ratio of active to total TGF-b1 released was very similar, irrespective of the source of matrix or matrix vesicle extracts. Pre-incubation of matrix vesicle extracts with anti-MMP-3 antibody blocked the release of active and total TGF-beta1, whereas pre-incubation with pre-immune IgG or anti-MMP-2 antibody had no effect. The addition of rhMMP-3, but not rhMMP-2, caused a dose-dependent increase in the release of total, but not active, TGF-beta1. Western analysis confirmed that both matrix vesicle extracts and rhMMP-3 released the large latent TGF-beta1 complex from the matrix. In addition to the expected 290, 230, and 190 kDa bands, samples run without reduction also contained proteins of molecular weights 110 and 50 kDa that reacted with the anti-LTBP1 antibody. When these same samples were electrophoresed after reduction, the high molecular weight immunoreactive bands disappeared and three bands of molecular weight 75, 32, and 25 kDa were observed. These results indicate that matrix vesicles contain enzymes, especially MMP-3, which are responsible for the release of TGF-beta1 from the matrix, most of which is in latent form. Further, the data suggest that release of the large complex occurs via cleavage at several novel sites in the 130 kDa LTBP1 molecule. Since matrix vesicle MMP-3 is also able to activate small latent TGF-beta1, these results suggest that the large latent TGF-beta1 complex protects against activation of the small latent TGF-beta1. Thus, the data suggest that release of the large latent TGF-bl complex from the matrix and activation of the latent growth factor are only two steps of what must be at least a three-step process.

Role of transforming growth factor-beta1 and its latent form binding protein in pseudoexfoliation syndrome

Pseudoexfoliation (PEX) syndrome is a common and clinically important systemic condition characterized by the pathologic production and accumulation of an abnormal fibrillar extracellular material in many intra- and extraocular tissues. Recent evidence suggests that it is a type of elastosis associated with the excess synthesis of elastic microfibrillar components such as fibrillin-1. Since transforming growth factor (TGF)-beta is a major modulator of extracellular matrix formation, the potential involvement of TGF-beta and its latent form binding protein (LTBP) in this aberrant matrix process was investigated. The expression of various isoforms of TGF-beta and LTBP was investigated in the anterior segment tissues of PEX and control eyes on the protein and mRNA level by light and electron microscopic immunohistochemistry, in situ hybridization, and semiquantitative RT-PCR. TGF-beta1 and TGF-beta2 levels were measured in aqueous humor and serum of PEX and control patients by ELISA. Cultures of Tenon's capsule fibroblasts were established to study the effect of TGF-beta1 on fibrillin-1 mRNA expression by Northern blot analysis. Significantly increased concentrations of both total and active TGF-beta1 were measured in the aqueous humor of PEX eyes without and with glaucoma as compared to control eyes, whereas levels of TGF-beta2 were not significantly different. The expression of TGF-beta1, LTBP-1, and LTBP-2, but not TGF-beta2, was markedly increased in anterior segment tissues of PEX eyes, particularly in the non-pigmented epithelium of the ciliary body, on both the mRNA and the protein level. Latent TGF-beta1 staining was consistently associated with PEX material deposits and could be released by proteolytic processing. Double immunolabeling revealed clear co-localization of LTBP-1 and -2 with latent TGF-beta1 and with fibrillin-1 on PEX fibrils. The expression of mRNA coding for fibrillin-1 was up-regulated in vitro by TGF-beta1. This study provides evidence for a significant role of TGF-beta1 and the LTBPs 1 and 2 in PEX syndrome. The results suggest that increased levels of latent and active TGF-beta1 in the aqueous humor of PEX patients, derived from enhanced local synthesis and activation, promote the buildup of the abnormal extracellular elastic material characteristic of PEX syndrome. They further support a dual role for LTBPs, both as integral structural components of PEX fibers and as a means of matrix anchorage of latent TGF-beta1, representing one possible mechanism for the regulation of TGF-beta1 activity in PEX eyes. Future therapeutic strategies might focus on TGF-beta1 antagonistic approaches.

Evidence for the decreased expression of the latent TGF-beta binding protein and its splice form in human liver tumours

BACKGROUND

Recently, a splice form of the latent TGF-beta binding protein (LTBP-1) was identified in the liver lacking potential important sequences for matrix association and proteinase cleavage (LTBP-1D, -1delta53). For a better understanding of the unknown (patho)physiological role, the expression levels of LTBP-1D and LTBP-1 (full length) were investigated in normal and malignant human liver on the mRNA and protein level.

METHODS

Normal liver (5 specimens), hepatocellular carcinoma (4 specimens) and fibrolamellar carcinoma (2 specimens) were examined by quantitative reverse transcription-polymerase chain reaction and immunohistochemistry, for which specific antibodies were generated.

RESULTS

The mRNA levels of LTBP-1/-1D in malignant liver tissues are decreased in comparison to normal liver--more so in HCC than in FLC. This finding was confirmed by a strong decrease of immunostaining of LTBP-1/-1D in neoplastic parenchymal cells of HCC and FLC. However, the intensity of LTBP-1 (full length) protein staining was increased in the extracellular matrix of the carcinomas, while LTBP-1D was not detectable in the matrix.

CONCLUSION

Since TGF-beta is known to be over-expressed in liver tumours, the results suggest its enhanced synthesis without binding to LTBP-1. This probably influences the availability of bioactive TGF-beta in the tumour tissue. The missing matrix localization of LTBP-1D indicates that the hinge region containing a heparin-binding site is essential for the binding of LTBP-1 in the extracellular matrix. LTBP-1D may fulfil specific functions for the latency of matrix-unbound TGF-beta.

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.

Overexpression of latent transforming growth factor-beta 1 (TGF-beta 1) binding protein 1 (LTBP-1) in association with TGF-beta 1 in ovarian carcinoma

Using the differential display method, latent transforming growth factor‐β (TGF‐β1) binding protein 1 (LTBP‐1) mRNA was identified as one of the enriched mRNAs in ovarian carcinoma tissues after isolation of genes responsible for the development of ovarian cancer. Semi‐quantitative reverse transcription (RT)‐PCR analysis showed that expression of LTBP‐1 and TGF‐β1 mRNAs was much higher in both serous and mucinous adenocarcinomas than in their benign counterparts, including serous and mucinous cystadenomas and cystadenomas of low malignant potential (LMPs). Immunohistochemical analysis demonstrated that only proliferating benign adenoma cells were immunoreactive for both LTBP‐1 and TGF‐β1 proteins. In contrast, most serous and mucinous adenocarcinoma cells and their surrounding stroma were intensely immunoreactive for LTBP‐1 and TGF‐β1. LTBP‐1 and TGF‐β1 proteins, and their complex forms were identified in ovarian carcinoma cell lines and in their culture media by western blot analysis, suggesting these products were produced in ovarian carcinoma cells. RT‐PCR analysis demonstrated that LTBP‐1L, one of the LTBP‐1 transcripts that has a strong activity in targeting the latent form of TGF‐β1 to extracellular matrix (ECM), was predominantly expressed in ovarian carcinomas. Taken together, the results suggest that upregulation of LTBP‐1 in ovarian carcinoma cells may have an important role in distributing TGF‐β1 in the stromal tissues surrounding carcinoma cells.

Novel non-TGF-β-binding splice variant of LTBP-4 in human cells and tissues provides means to decrease TGF-β deposition

Small latent TGF-β consists of latency associated peptide (LAP) bound to the 25 kDa TGF-β by noncovalent interactions. Small latent TGF-β is secreted from cells and deposited into the extracellular matrix as covalent complexes with its binding proteins, LTBPs. Four LTBPs have been molecularly cloned and their structures contain repetitive sequences. The 3rd 8-Cys repeats of LTBP-1, -3 and -4 are able to associate with small latent TGF-β. We analyzed by RT-PCR the expression of LTBPs 1-4 in a panel of cultured human cell lines including fibroblasts of different origin, endothelial cells and immortalized keratinocytes. LTBPs were expressed in an overlapping manner, but differences in their expression levels were detected. SV-40 transformed human embryonic lung fibroblasts contained less of the mRNAs for the LTBPs, suggesting that malignant transformation leads to decrease in LTBP expression. A novel alternatively spliced form of LTBP-4 lacking the 3rd 8-Cys repeat (LTBP-4Δ8-Cys3rd) was identified. LTBP-4Δ8-Cys3rd does not bind TGF-β and it was found to be expressed in the same tissues as the full length LTBP-4. The exon-intron structure of LTBP-4 around the 3rd 8-Cys repeat was similar to those of LTBP-2 and -3. LTBP-4Δ8-Cys3rd was produced by alternative splicing over two exons. In addition, HL-60 promyelocytic leukemia cells expressed a splice variant lacking only one exon of this region. The expression of the non-TGF-β-binding variant of LTBP-4 may be important for the regulation of TGF-β deposition in tissues. Since LTBPs are a part of the extracellular matrix microfibrils, the LTBP-4Δ8-Cys3rd protein may also be involved in various structural functions not related to TGF-β signaling.

Tamoxifen and estrogen effects on TGF-beta formation: role of thrombospondin-1, alphavbeta3, and integrin-associated protein

We have found that the enhanced activation of latent TGF-beta by human breast carcinoma cell lines either treated with tamoxifen or deprived of estrogen is dependent upon thrombospondin-1 (TSP-1) since activation was blocked by anti-TSP-1 antibodies or by a TSP antagonist peptide. However, TGF-beta formation upon tamoxifen exposure to estrogen withdrawal is associated with decreased levels of soluble TSP-1. A concomitant increase in the expression of the TSP-1 receptors alphavbeta3 and integrin-associated protein (IAP) occurs under these conditions, and antibodies to TSP-1 or to these receptors inhibit increased TGF-beta formation. Therefore, increased cell surface associated TSP-1 enhances latent TGF-beta activation.

Unilateral or asymmetric pseudoexfoliation syndrome? An ultrastructural study

BACKGROUND

Clinically, most patients with pseudoexfoliation (PEX) syndrome reveal only unilateral ocular involvement. However, the generalized nature of the disorder suggests that PEX syndrome is clinically asymmetric rather than strictly unilateral.

OBJECTIVE

To perform an ultrastructural study of the contralateral eyes in patients with unilateral PEX syndrome.

METHODS

Five pairs of donor eyes with slitlamp microscopic, macroscopic, and light microscopic evidence of unilateral PEX syndrome and 6 normal control eyes were investigated by transmission electron microscopy and light and electron microscopic immunohistochemistry using antibodies against the human natural killer (HNK-1) epitope and against latent transforming growth factor beta1-binding protein, both markers for the identification of PEX deposits.

RESULTS

Ultrastructural alterations were observed in anterior segment tissues of all apparently not involved fellow eyes. These included (1) deposits of typical PEX fibrils on the iris and ciliary epithelia and in the dilator muscle of the iris; (2) increased accumulation of extracellular matrix, including microfibrils and reduplicated basement membrane material in the periphery of iris vessels, in the dilator muscle and in the juxtacanalicular tissue of the trabecular meshwork; and (3) degenerative changes of the iris pigment epithelium and dilator muscle cells. Latent transforming growth factor beta1-binding protein- and HNK-1-positive deposits indicating PEX material accumulations were detected in the periphery of iris vessels and in the dilator muscle in all affected and contralateral eyes, but not in the control eyes.

CONCLUSIONS

These subclinical alterations of contralateral eyes in clinically so-called unilateral PEX syndrome support the concept that PEX syndrome is a generalized basically bilateral disorder with a clinically marked asymmetric manifestation. The iris changes may account for the clinical signs characteristic of early stages, such as melanin dispersion, peripupillary atrophy, trabecular meshwork pigmentation, and insufficient asymmetric mydriasis. The findings should be considered in the clinical management of the patients.

CLINICAL RELEVANCE

In view of the fact that PEX syndrome is the most common identifiable cause of open-angle glaucoma worldwide and as it is an important risk factor for a wide spectrum of ocular complications, particularly during cataract surgery, the potential involvement of both eyes in the PEX process is of clinical significance.

Release of biologically active TGF-beta from airway smooth muscle cells induces autocrine synthesis of collagen

In severe or chronic asthma, there is an increase in airway smooth muscle cell (ASMC) mass as well as an increase in connective tissue proteins in the smooth muscle layer of airways. Transforming growth factor-beta (TGF-beta) exists in three isoforms in mammals and is a potent regulator of connective tissue protein synthesis. Using immunohistochemistry, we had previously demonstrated that ASMCs contain large quantities of TGF-beta1-3. In this study, we demonstrate that bovine ASMC-derived TGF-beta associates with the TGF-beta latency binding protein-1 (LTBP-1) expressed by the same cells. The TGF-beta associated with LTBP-1 localizes TGF-beta extracellularly. Furthermore, plasmin, a serine protease, regulates the secretion of a biologically active form of TGF-beta by ASMCs as well as the release of extracellular TGF-beta. The biologically active TGF-beta released by plasmin induces ASMCs to synthesize collagen I in an autocrine manner. The autocrine induction of collagen expression by ASMCs may contribute to the irreversible fibrosis and remodeling seen in the airways of some asthmatics.

Post translational activation of latent transforming growth factor beta (L-TGF-beta): clinical implications

Transforming growth factor-betas (TGF-betas) are multifunctional cytokines that exist in 3 isoforms in mammals. The TGF-betas are ubiquitously expressed and all isoforms are secreted as biologically inactive precursors called latent TGF-beta (L-TGF-beta). L-TGF-betas are generally not effective molecules because they are unable to interact with their receptors. However, the removal of or conformational change of the precursor protein called the latency associated peptide (LAP) results in the generation of biologically active TGF-beta. In vitro active TGF-beta has many biological effects but from a clinical point of view one of the most recognized associations of aberrant TGF-beta production is with diseases characterized by enhanced connective tissue synthesis. Recently a number of observations in the context of fibrotic disorders suggest mechanisms of activation of L-TGF-beta1 in vivo. The recognition of mechanisms that activate L-TGF-beta1 in vivo offers the possibility of interfering with the activation of L-TGF-beta1 for therapeutic purposes.

Expression of isoforms and splice variants of the latent transforming growth factor beta binding protein (LTBP) in cultured human liver myofibroblasts

BACKGROUND/AIMS

The activation of hepatic stellate cells (HSC) to extracellular matrix (ECM) producing myofibroblasts (MFB) is the key pathogenetic event in human liver fibrogenesis. Latent transforming growth factor beta binding protein (LTBP), a component of the profibrogenic large latent transforming growth factor (TGF)-beta complex, is suggested to be important for secretion, latency, storage and activation of TGF-beta in the ECM. This study was performed to identify the expression profile of all hitherto known LTBP isoforms and LTBP splice variants in conjunction with that of TGF-beta isoforms in cultured human liver MFB.

METHODS

Cultured human MFB were analyzed for TGF-beta and LTBP using reverse-transcription polymerase chain reaction (RT-PCR), sequence analysis, immunofluorescence staining, metabolic labeling, immunoprecipitation, and enzyme-linked immunosorbent assay (ELISA).

RESULTS

Transcripts of all three TGF-beta isoforms, of all four LTBP isoforms and of nearly all splice variants of LTBP-1 and LTBP-4 so far known were detected. Metabolic labeling followed by immunoprecipitation with anti-LTBP-1 antibody revealed the synthesis of LTBP proteins. Secretion of free LTBP and LTBP integrated into the large latent TGF-beta complex was demonstrated by size-exclusion chromatography. Co-localization of LTBP-1 and -2 with fibronectin and collagen type I was observed by double immunofluorescence staining.

CONCLUSION

The expression of a complete profile of hitherto known LTBP proteins by cultured human MFB suggests a role in modulating the bioactivity of TGF-beta in the diseased liver.

Accumulation of latent transforming growth factor-beta binding protein-1 and TGF beta 1 in extracellular matrix of filtering bleb and of cultured human subconjunctival fibroblasts

PURPOSE

To examine immunohistochemically whether extracellular matrix (ECM) of the filtering bleb and of cultured human subconjunctival fibroblasts contains latent TGF beta binding protein-1 (LTBP-1) and TGF beta.

METHODS

An enucleated human eye that had undergone trabeculectomy and cultured human subconjunctival fibroblasts were processed for light microscopic immunohistochemistry. Antibodies against LTBP-1, collagen types, fibrillin-1 and TGF beta s were used. TGF beta 1 was located by detecting beta 1-latency associated peptide (LAP).

RESULTS

LTBP-1, beta 1-LAP and fibrillin-1 were all located in the subepithelial ECM as well as in the basal epithelial cells of the conjunctiva over the filtering bleb. TGF beta 2 and beta 3 were immunolocated to epithelium and/or fibroblasts/keratocytes. ECM deposited in confluent fibroblast cultures was positive for beta 1-LAP, LTBP-1 and fibrillin-1, whereas sparse cells were negative.

CONCLUSIONS

LTBP-1, beta 1-LAP and fibrillin-1 are co-localized to the ECM of the filtering bleb and of cultured conjunctival fibroblasts. Both conjunctival epithelium and fibroblasts are considered to be the source of TGF beta in healing bleb. ECM secreted by in vivo and in vitro subconjunctival fibroblasts may works as a scavenger or repository of TGF beta.

Latency, activation, and binding proteins of TGF-beta

The TGF-beta superfamily of growth factors consists of an increasing number of different polypeptide modulators of cell growth, differentiation, and morphogenesis. Three mammalian isoforms have been molecularly cloned. Numerous ways to regulate the expression of the TGF-beta genes have been identified. TGF-betas are, for example, subject to regulation by retinoids, steroid hormones, and vitamin D. A characteristic feature in the biology of TGF-betas is that they are usually secreted from cells in latent forms. The large latent complex consists of the small latent complex (TGF-beta and its propeptide) and a high molecular weight protease resistant binding protein, latent TGF-beta binding protein (LTBP). LTBPs are required for the proper folding and secretion of TGF-beta. TGF-beta is not just secreted from cultured cells but is deposited via LTBPs to the pericellular space, namely to the extracellular matrix. Release of these complexes and activation by proteases is under tight regulation and provides a means to rapidly increase local concentrations of TGF-beta. Biological events, where enhanced or focal proteolysis and activation of latent TGF-beta takes place, include cell invasion, tissue remodeling, and wound healing.

Expression and matrix deposition of latent transforming growth factor beta binding proteins in normal and fibrotic rat liver and transdifferentiating hepatic stellate cells in culture

Latent transforming growth factor beta binding protein (LTBP), a high-molecular-weight glycoprotein of the large latent TGF-beta complex is suggested to serve as an anchor for latent TGF-beta in the extracellular matrix and as a component of microfibrillar structures. Proteolytic cleavage of LTBP is supposed to be a prerequisite for the release and generation of bioactive (mature) TGF-beta. We investigated the expression of LTBP isoforms in normal and fibrotic rat liver and in cultured rat hepatic stellate cells (HSC) transdifferentiating to myofibroblasts (MFB). We further determined their interaction with the matrix and some of their basic functions. Immunostainings of normal and fibrotic livers demonstrate intense signals for LTBP-1 and -2, preferably in parenchymal, but also nonparenchymal, cells and in fibrotic extracellular matrix. However, in situ hybridization points to a restriction of transcripts to nonparenchymal cells from fibrotic livers, whereas hepatocytes were always devoid of LTBP transcripts. The findings were confirmed by real-time quantitative reverse-transcription polymerase chain reaction (RT-PCR), which showed isoform-specific increases of LTBP transcripts in cultured stellate cells transdifferentiating to MFB and by Northern blot analyses showing the absence of LTBP-1 mRNA in freshly isolated hepatocytes. Using a cell enzyme-linked immunosorbent assay (ELISA), a differential increase of partly deoxycholate (DOC)-resistant, matrix-bound LTBP-1 and -2 was measured in cultured stellate cells. Treatment with plasmin generated soluble LTBP-1 and bioactive TGF-beta, which was able to induce Smad7 expression in an autocrine fashion. Our data propose (transdifferentiating) stellate cells, respectively MFB, as the major source of LTBP in normal and fibrotic liver, which here probably fulfills structural and TGF-beta-regulating functions as suggested for nonhepatic tissues.

Latent TGF-beta binding protein LTBP-1 contains three potential extracellular matrix interacting domains

Latent TGF-beta binding proteins (LTBPs) are components of the extracellular matrix (ECM). They belong to the fibrillin/LTBP-superfamily, and are high molecular weight glycoproteins characterized by EGF-like repeats and 8-Cys repeats. Most LTBPs associate with the small latent forms of TGF-beta. Their roles include to facilitate the secretion of latent TGF-beta and to target it to the ECM. In order to identify new matrix-binding domains of LTBP-1 and to characterize their association with the extracellular matrix, we have produced (in a mammalian expression system) partly overlapping recombinant fragments of its shorter form, LTBP-1S, and analyzed the binding of the purified fusion proteins to extracellular matrices of cultured human dermal and lung fibroblasts. Recombinant fragments from three different regions of the N- and C-termini showed affinity to the matrix. These interacting regions contain either the first (hybrid), second or fourth 8-Cys domains of the LTBP-1S molecule. They bound independently to the matrix. Each of them had an ability to inhibit the association of native exogenous LTBP-1 with fibroblast extracellular matrix. The interactions of the LTBP-1 fragments with the extracellular matrix resisted treatment with sodium deoxycholate, suggesting strong, possibly covalent binding. The binding occurred in a time- and dose-dependent fashion. The N-terminal fragments bound more readily to the matrices. With all fragments the binding took place both with intact fibroblast matrices and with matrices isolated by sodium deoxycholate. When using CHO cell layers, which form sparse matrices, only the N-terminal fragment of LTBP-1 was efficiently incorporated. The association of the binding fragments with isolated matrices was enhanced by soluble, cell-derived factors. The current data suggest that LTBP-1 contains three different domains with an ability to associate with the extracellular matrix.