Do extracellular matrix protein expressions change with cyclic reproductive hormones in pelvic connective tissue from women with stress urinary incontinence?

BACKGROUND

To evaluate differential expression of transforming growth factor (TGF-beta1), latent transforming factor-binding proteins (LTBP-1, LTBP-2) and elastin microfibril components (fibrillin-1 and fibrillin-2) in vaginal tissue from women with stress urinary incontinence (SUI).

METHODS

In this case-control study, vaginal tissue from women in both phases of the menstrual cycle was obtained. Messenger RNA (mRNA) expressions of LTBP-1, LTBP-2, fibrillin-1, fibrillin-2 and TGF-beta1 were determined by relative real-time quantification PCR. Tissue localization was analysed by immunohistochemistry, and semiquantitative protein expression was evaluated by Western blot analysis.

RESULTS

Vaginal wall fibroblasts synthesized all proteins tested. LTBP-1, LTBP-2 and TGF-beta1 co-localized with elastin microfibrils, fibrillin-1 and fibrillin-2 in the extracellular matrix. LTBP-1 mRNA and protein expressions were higher in control versus women affected with SUI in the proliferative phase (P = 0.04), while in the secretory phase, mRNA expression in cases was higher (P = 0.04). Fibrillin-1 mRNA was higher in women affected by SUI versus controls in both phases, but no statistical differences in fibrillin-1 protein expression were observed between the two groups in either phase. LTBP-2 and TGF-beta1 mRNA expressions showed the same trends as LTBP-1.

CONCLUSION

LTBP-1, LTBP-2, TGF-beta1, fibrillin-1, and fibrillin-2 expressions are hormonally regulated in vaginal wall fibroblasts and differ in women affected by SUI when compared to controls. These data suggest a mechanism to regulate TGF-beta1 activity in pelvic connective tissue.

Genetic basis of thoracic aortic aneurysms and aortic dissections

Ascending thoracic aortic aneurysms leading to type A dissections (TAAD) can occur in association with a genetic syndrome, such as Marfan syndrome (MFS), or as an autosomal dominant disorder in the absence of syndromic features, termed familial TAAD. Familial TAAD demonstrates genetic heterogeneity, and linkage studies have identified three TAAD loci at 5q13-14 (TAAD1), 11q23 (FAA1), and 3p24-25 (TAAD2). The underlying genetic heterogeneity of TAAD is reflected in the phenotypic variation associated with familial TAAD with respect to age of onset, progression, penetrance, and association with additional cardiac and vascular features. Recently, mutations in the TGFBR2 gene have been identified as the cause of disease linked to the TAAD2 locus, supporting the hypothesis that dysregulation of TGFbeta signaling is a mechanism leading to aneurysms and dissections. The recent identification of the TGFbeta pathway as a key target in the molecular pathogenesis of TAAD has opened new avenues for future genetic and therapeutic research.

Induction and coexpression of latent transforming growth factor beta-binding protein-1 and fibrillin-1 in experimental glomerulonephritis

BACKGROUND

Latent transforming growth factor-beta-binding protein 1 (LTBP-1) and fibrillin-1 were shown to colocalize and interact in the extracellular matrix of the skin and vasculature. This interaction may regulate transforming growth factor-beta (TGF-beta) activity. TGF-beta is an important progression factor for glomerular diseases. We hypothesized that LTBP-1 and fibrillin-1 are coexpressed in the glomerulus and upregulated during glomerulonephritis.

METHODS

Acute anti-Thy1.1 glomerulonephritis was induced with a single intravenous injection (1 mg/kg body weight) of a monoclonal anti-Thy1.1 antibody in rats. Real-time RT-PCR and immunohistochemical analyses for LTBP-1 and fibrillin-1 were performed.

RESULTS

Induction of glomerular LTBP-1 mRNA was detected on day 2 of disease, while mRNA for fibrillin-1 was already upregulated 1 day after induction of disease. Both LTBP-1 and fibrillin-1 showed a mesangial distribution. An expansion of the LTBP-1 and fibrillin-1-positive mesangial area was seen on day 6 of disease, when transient matrix accumulation was most prominent. On day 12 of disease, glomerular LTBP-1 and fibrillin-1 immunoreactivities had returned to control levels. In serial sections, some colocalization of LTBP-1 and fibrillin-1 was detected in control as well as in nephritic glomeruli.

CONCLUSION

Mesangial expression of LTBP-1 and fibrillin-1 is induced early in experimental nephritis and LTBP-1 and fibrillin-1 are partially colocalized in the nephritic glomerulus. An interaction of these molecules could stabilize latent TGF-beta complexes and thus attenuate the activation of TGF-beta during this self-limited glomerular disease.

Sequential deposition of latent TGF-β binding proteins (LTBPs) during formation of the extracellular matrix in human lung fibroblasts

Latent TGF-beta binding proteins (LTBPs) mediate the targeting of latent TGF-beta complexes into ECM structures, which is important for TGF-beta activation and functions. LTBPs-1, -3 and -4 associate with and regulate the bioavailability of TGF-betas. We investigated whether LTBP-3 and -4 are associated with pericellular fibrillar structures of human lung fibroblast ECM, and which of their domains are important for this function. Immunoblotting analyses of isolated insoluble matrices as well as immunofluorescence analyses and confocal microscopy indicated that both LTBP-3 and -4 get assembled into the ECM. Interestingly, LTBP-4 was not detected until 7-10 days of culture and LTBP-3 until 14 days of culture. This was a major difference from the deposition kinetics of LTBP-1, which was detected already within 2 days of culture. Expression analyses by real time RT-PCR indicated that the slow appearance of LTBP-3 and -4 was due to the low expression levels soon after subculture. Recombinant N-terminal fragments of LTBP-3 and -4 bound readily to fibroblast ECM. The C-terminal domain of LTBP-4, but not of LTBP-3, also associated with the matrix structures. The levels of ECM-associated latent complexes of TGF-beta1 increased in parallel with the increased production and deposition of the LTBPs. The amount of active TGF-beta in the conditioned medium decreased during extended culture. Our results suggest that ECM is an important site of deposition also for LTBP-3 and -4 and that the temporal and spatial targeting of the TGF-beta complexes are associated with ECM maturation.

Involvement of alphavbeta5 integrin-mediated activation of latent transforming growth factor beta1 in autocrine transforming growth factor beta signaling in systemic sclerosis fibroblasts

OBJECTIVE

To confirm the involvement of alphavbeta5 in the self-activation system in systemic sclerosis (SSc) fibroblasts.

METHODS

Levels of alphavbeta5 expression were analyzed by immunoprecipitation. The promoter activity of the human alpha2(I) collagen gene was determined by transient transfection assay. Phosphorylation levels and DNA binding ability of Smad3 were investigated by immunoprecipitation and DNA affinity precipitation, respectively. The localization of active transforming growth factor beta (TGFbeta) was determined by coculture assay using TMLC cells (mink lung epithelial reporter cells that stably express a portion of the plasminogen activator inhibitor 1 promoter). The morphologic features of cells were determined by immunofluorescence analysis.

RESULTS

Levels of alphavbeta5 expression were significantly elevated in SSc fibroblasts compared with normal fibroblasts. Treatment with anti-alphavbeta5 antibody or beta5 antisense oligonucleotide significantly reduced human alpha2(I) collagen gene promoter activity in SSc fibroblasts. In SSc fibroblasts pretreated with TGFbeta1 antisense oligonucleotide, the exogenous latent TGFbeta1 stimulation significantly increased human alpha2(I) collagen gene promoter activity; this effect was significantly reduced in the presence of anti-alphavbeta5 antibody. Phosphorylation levels and DNA binding ability of Smad3 in SSc fibroblasts were significantly reduced by treatment with beta5 antisense oligonucleotide. The luciferase activity of TMLC cells cocultured with SSc fibroblasts was significantly elevated compared with that of TMLC cells cocultured with normal fibroblasts and was significantly reduced in the presence of anti-alphavbeta5 antibody. Anti-alphavbeta5 antibody reversed the myofibroblastic features of SSc fibroblasts.

CONCLUSION

Up-regulated expression of alphavbeta5 contributes to the establishment of autocrine TGFbeta signaling in SSc fibroblasts through activation of endogenous latent TGFbeta1.

Parathyroid hormone stimulation and PKA signaling of latent transforming growth factor-beta binding protein-1 (LTBP-1) mRNA expression in osteoblastic cells

Parathyroid hormone (PTH) regulates bone remodeling and calcium homeostasis by acting on osteoblasts. Recently, the gene expression profile changes in the rat PTH (1-34, 10(-8)M)-treated rat osteoblastic osteosarcoma cell line, UMR 106-01, using DNA microarray analysis showed that mRNA for LTBP-1, a latent transforming growth factor (TGF-beta)-binding protein is stimulated by PTH. Latent TGF-beta binding proteins (LTBPs) are required for the proper folding and secretion of TGF-beta, thus modifying the activity of TGF-beta, which is a local factor necessary for bone remodeling. We show here by real time RT-PCR that PTH-stimulated LTBP-1 mRNA expression in rat and mouse preosteoblastic cells. PTH also stimulated LTBP-1 mRNA expression in all stages of rat primary osteoblastic cells but extended expression was found in differentiating osteoblasts. PTH also stimulated TGF-beta1 mRNA expression in rat primary osteoblastic cells, indicating a link between systemic and local factors for intracellular signaling in osteoblasts. An additive effect on LTBP-1 mRNA expression was found when UMR 106-01 cells were treated with PTH and TGF-beta1 together. We further examined the signaling pathways responsible for PTH-stimulated LTBP-1 and TGF-beta1 mRNA expression in UMR 106-01 cells. The PTH stimulation of LTBP-1 and TGF-beta1 mRNA expression was dependent on the PKA and the MAPK (MEK and p38 MAPK) pathways, respectively in these cells, suggesting that PTH mediates its effects on osteoblasts by several intracellular signaling pathways. Overall, we demonstrate here that PTH stimulates LTBP-1 mRNA expression in osteoblastic cells and this is PKA-dependent. This event may be important for PTH action via TGF-beta in bone remodeling.

Induction of human LTBP-3 promoter activity by TGF-beta1 is mediated by Smad3/4 and AP-1 binding elements

Latent TGF-beta binding proteins (LTBPs) are extracellular matrix glycoproteins, which are essential for the targeting and activation of TGF-betas. LTBP-3 regulates the bioavailability of TGF-beta especially in the bone. To understand the regulation of LTBP-3 expression, we have isolated and characterized the promoter region of human LTBP-3 gene. The GC-rich TATA-less promoter contained several transcription initiation sites and putative binding sites for multiple sequence specific transcription factors including Sp1, AP-1, c-Ets, MZF-1, Runx1 and members of the GATA-family. Reporter gene analyses of the promoter indicated that it was more active in MG-63 than in Saos-2 osteosarcoma cells, suggesting that it is regulated as the endogenous gene. TGF-beta1 stimulated the transcriptional activity of LTBP-3 promoter in MG-63 cells, while certain other bone-derived growth factors and hormones were ineffective. TGF-beta1 increased LTBP-3 mRNA levels accordingly. Analyses of deletion constructs of the promoter and mutational deletion of specific transcription factor binding sites indicated that Smad3/4 and AP-1 binding sites mediated the TGF-beta1 response. The involvement of AP-1 activity was further indicated by decreased TGF-beta responsiveness of the LTBP-3 promoter in the presence of a MEK/Erk signaling pathway inhibitor. Our results suggest an important new role for TGF-beta1 in the regulation of its binding protein, LTBP-3.

Lung alveolar septation defects in Ltbp-3-null mice

Latent transforming growth factor (TGF)-beta binding proteins (LTBPs) modulate the secretion and activation of latent TGF-beta. To explore LTBP function in vivo, we created an Ltbp-3(-/-) mouse that has developmental emphysema with decreased septation in terminal alveoli. Differences in distal airspace enlargement were obvious at day 6 after birth. Secondary septation was inhibited, so by days 21 to 28 the mean linear intercept was approximately twofold greater in mutant versus control lungs. There were no differences in lung collagen and elastin, visualized by immunohistochemistry, or in myofibroblast numbers, determined by alpha-smooth muscle actin-positive cells, between mutant or wild-type lungs as the animals aged, other than differences associated with altered lung structure in mutant animals. However, from day 10 there was twice the number of alveolar type II cells in mutant alveoli compared to controls. At days 6 and 10, a transient enhancement in cell proliferation in the mutant lungs was observed by both 5-bromo-2'-deoxy-uridine and proliferating cell nuclear antigen labeling, accompanied by enhanced numbers of terminal dUTP nick-end labeling-positive cells at days 4, 6, and 10. Finally, there was a transient decrease in TGF-beta signaling at days 4 to 6 in Ltbp-3(-/-) lungs. These results indicate that in the absence of Ltbp-3, a temporary decrease in TGF-beta signaling in the lungs at days 4 to 6 alters cell proliferation, correlating with inhibition of septation and developmental emphysema.

Apoptosis of human endothelial cells is accompanied by proteolytic processing of latent TGF-beta binding proteins and activation of TGF-beta

Transforming growth factors beta (TGF-betas) are multifunctional cytokines that modulate cell growth, differentiation and apoptosis. Numerous effects initiated by TGF-betas in vitro have been described, but the role of TGF-beta targeting and activation under physiological conditions has gained very little attention and understanding. We report here that apoptosis of human umbilical vein endothelial cells (HUVECs) is accompanied by release of truncated large latent TGF-beta complexes from the pericellular matrix followed by activation of TGF-beta. The activation of TGF-beta during apoptosis was accompanied by enhanced secretion of beta1-LAP protein, and apoptotic HUVECs acquired the capacity to induce the release of latent TGF-beta-binding proteins (LTBPs) from extracellular matrices. Activated TGF-beta, in turn, attenuated apoptotic death of HUVECs. Current results indicate that the activation of TGF-beta accompanies the apoptosis of HUVECs, and may play a protective feedback role against apoptotic cell death. The results suggest a role for TGF-beta as a putative extracellular modulator of apoptosis.

Evolution of distinct EGF domains with specific functions

EGF domains are extracellular protein modules cross-linked by three intradomain disulfides. Past studies suggest the existence of two types of EGF domain with three-disulfides, human EGF-like (hEGF) domains and complement C1r-like (cEGF) domains, but to date no functional information has been related to the two different types, and they are not differentiated in sequence or structure databases. We have developed new sequence patterns based on the different C-termini to search specifically for the two types of EGF domains in sequence databases. The exhibited sensitivity and specificity of the new pattern-based method represents a significant advancement over the currently available sequence detection techniques. We re-annotated EGF sequences in the latest release of Swiss-Prot looking for functional relationships that might correlate with EGF type. We show that important post-translational modifications of three-disulfide EGFs, including unusual forms of glycosylation and post-translational proteolytic processing, are dependent on EGF subtype. For example, EGF domains that are shed from the cell surface and mediate intercellular signaling are all hEGFs, as are all human EGF receptor family ligands. Additional experimental data suggest that functional specialization has accompanied subtype divergence. Based on our structural analysis of EGF domains with three-disulfide bonds and comparison to laminin and integrin-like EGF domains with an additional inter-domain disulfide, we propose that these hEGF and cEGF domains may have arisen from a four-disulfide ancestor by selective loss of different cysteine residues.

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

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

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

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

Intimal hyperplasia and expression of transforming growth factor-beta1 in saphenous veins and internal mammary arteries before coronary artery surgery

BACKGROUND

The development of fibromuscular intimal hyperplasia and subsequent graft failure remains an urgent problem in cardiac surgery. Transforming growth factor-beta1 (TGF-beta1) is involved in the pathogenesis of arteriosclerosis through induction of extracellular matrix proteins. We tested the hypothesis that intimal hyperplasia is already present in human saphenous veins and left internal mammary arteries before coronary artery bypass surgery and is associated with an increased expression of TGF-beta1.

METHODS

Forty-six segments of saphenous veins and 27 of left internal mammary arteries were collected from 50 patients undergoing coronary artery bypass surgery. Morphometric analysis was performed by microscopic computer analysis. Immunohistochemistry was performed with antibodies directed against TGF-beta1, its latent binding protein (LTBP-1) and its type 2 receptor (RII).

RESULTS

The incidence of intimal hyperplasia was significantly higher in saphenous veins (67.4%) than in mammary arteries (29.6%; p < 0.05). Saphenous veins and mammary arteries with intimal hyperplasia expressed more TGF-beta1 (endothelial and intimal layers) and LTBP-1 (intimal and medial layers) when compared with corresponding vessels without hyperplasia (both groups p < 0.05). Endothelial and intimal RII expression was significantly higher in saphenous veins with intimal hyperplasia as compared with saphenous veins without hyperplasia (p < 0.05). Transforming growth factor-beta1 staining in the intima correlated with the presence of an intimal hyperplasia in saphenous veins (rho = 0.317) and mammary arteries (rho = 0.428).

CONCLUSIONS

Local TGF-beta1 expression is associated with the presence of intimal hyperplasia in the examined vessels. Preexisting intimal hyperplasia is more prevalent and serious in saphenous veins than in left internal mammary arteries, giving further explanation to the superior long-term results of left internal mammary grafts.

The prodomain of BMP-7 targets the BMP-7 complex to the extracellular matrix

Biochemical and biophysical methods are used to show that BMP-7 is secreted as a stable complex consisting of the processed growth factor dimer noncovalently associated with its two prodomain propeptide chains and that the BMP-7 complex is structurally similar to the small transforming growth factor beta (TGFbeta) complex. Because the prodomain of TGFbeta interacts with latent TGFbeta-binding proteins, a family of molecules homologous to the fibrillins, the prodomain of BMP-7 was tested for binding to fibrillin-1 or to LTBP-1. The BMP-7 prodomain and BMP-7 complex, but not the separated growth factor dimer, interact with N-terminal regions of fibrillin-1. This interaction may target the BMP-7 complex to fibrillin microfibrils in the extracellular matrix. Immunolocalization of BMP-7 in tissues like the kidney capsule and skin reveals co-localization with fibrillin. However, BMP-7 immunolocalization in other tissues known to be active sites for BMP-7 signaling is not apparent, suggesting that immunolocalization of BMP-7 in certain tissues represents specific extracellular storage sites. These studies suggest that the prodomains of TGFbeta-like growth factors are important for positioning and concentrating growth factors in the extracellular matrix. In addition, they raise the possibility that prodomains of other TGFbeta-like growth factors interact with fibrillins and/or LTBPs and are also targeted to the extracellular matrix.

Expression of truncated latent TGF-beta-binding protein modulates TGF-beta signaling

Transforming growth factor-beta is released from most cells as an inactive complex consisting of transforming growth factor-beta, the transforming growth factor-beta propeptide and the latent transforming growth factor-beta-binding protein. We studied the role of latent transforming growth factor-beta-binding protein in modulating transforming growth factor-beta availability by generating transgenic mice that express a truncated form of latent transforming growth factor-beta-binding protein-1 that binds to transforming growth factor-beta but is missing the known N- and C-terminal matrix-binding sequences. As transforming growth factor-beta is an inhibitor of keratinocyte proliferation and is involved in the control of hair cycling, we over-expressed the mutated form of latent transforming growth factor-beta-binding protein under the control of the keratin 14-promoter. Transgenic animals displayed a hair phenotype due to a reduction in keratinocyte proliferation, an abbreviated growth phase and an early initiation of the involution (catagen) phase of the hair cycle. This phenotype appears to result from excess active transforming growth factor-beta, as enhanced numbers of pSmad2/3-positive nuclei are observed in transgenic animal skin. These data suggest that the truncated form of latent transforming growth factor-beta-binding protein-1 competes with wild-type latent transforming growth factor-beta-binding protein for binding to latent transforming growth factor-beta, resulting in latent transforming growth factor-beta complexes that fail to be targeted correctly in the extracellular matrix. The mis-localization of the transforming growth factor-beta results in inappropriate activation and premature initiation of catagen, thereby illustrating the significance of latent transforming growth factor-beta-binding protein interaction with transforming growth factor-beta in the targeting and activation of latent transforming growth factor-beta in addition to previously reported effects on small latent complex secretion.

Latent TGF-beta binding proteins

Latent TGF-beta binding proteins are multidomain proteins with a common, highly repetitive structural organization and partially overlapping expression patterns. Latent TGF-beta binding protein-1, -3 and -4 bind latent TGF-beta. TGF-betas are normally secreted as latent complexes, consisting of the mature TGF-beta dimer non-covalently bound to its processed propeptide dimer plus a latent TGF-beta binding protein. The latent TGF-beta binding protein is covalently bound to the propeptide. These binding proteins may perform at least two functions: structural, as components of the matrix, and regulatory, as modulators of TGF-beta availability.

Amino acid requirements for formation of the TGF-beta-latent TGF-beta binding protein complexes

Transforming growth factor beta (TGF-beta) is secreted primarily as a latent complex consisting of the TGF-beta homodimer, the TGF-beta propeptides (called the latency-associated protein or LAP) and the latent TGF-beta binding protein (LTBP). Mature TGF-beta remains associated with LAP by non-covalent interactions that block TGF-beta from binding to its receptor. Complex formation between LAP and LTBP is mediated by an intramolecular disulfide exchange between the third 8-cysteine (8-Cys3) domain of LTBP with a pair of cysteine residues in LAP. Only the third 8-Cys domains of LTBP-1, -3, and -4 bind LAP. From comparison of the 8-Cys3(LTBP-1) structure with that of the non-TGF-beta-binding 8-Cys6(fibrillin-1), we observed that a two-residue insertion in 8-Cys3(LTBP-1) increased the potential for disulfide exchange of the 2-6 disulfide bond. We further proposed that five negatively charged amino acid residues surrounding this bond mediate initial protein-protein association. To validate this hypothesis, we monitored binding by fluorescence resonance energy transfer (FRET) analysis and co-expression assays with TGF-beta1 LAP (LAP-1) and wild-type and mutant 8-Cys3 domains. FRET experiments demonstrated ionic interactions between LAP-1 and 8-Cys3. Mutation of the five amino acid residues revealed that efficient complex formation is most dependent on two of these residues. Although 8-Cys3(LTBP-1) binds proTGF-betas effectively, the domain from LTBP-4 does so poorly. We speculated that this difference was due to the substitution of three acidic residues by alanine, serine, and arginine in the LTBP-4 sequence. Additional experiments with 8-Cys3(LTBP-4) indicated that enhanced binding of LAP to 8-Cys3(LTBP-4) is achieved if the residues A, S, and R are changed to those in 8-Cys3(LTBP1) (D, D, and E) and the QQ dipeptide insertion of LTBP-4 is changed to the FP in 8-Cys3(LTBP-1). These studies identify surface residues that contribute to the interactions of 8-Cys3 and LAP-1 and may yield information germane to the interaction of 8-Cys domains and additional TGF-beta superfamily propeptides, an emerging paradigm for growth factor regulation.

Fibronectin regulates latent transforming growth factor-beta (TGF beta) by controlling matrix assembly of latent TGF beta-binding protein-1

Latent transforming growth factor-beta-binding proteins (LTBPs) are extracellular matrix (ECM) glycoproteins that play a major role in the storage of latent TGF beta in the ECM and regulate its availability. Here we show that fibronectin is critical for the incorporation of LTBP1 and transforming growth factor-beta (TGF beta) into the ECM of osteoblasts and fibroblasts. Immunolocalization studies suggested that fibronectin provides an initial scaffold that precedes and patterns LTBP1 deposition but that LTBP1 and fibronectin are later localized in separate fibrillar networks, suggesting that the initial template is lost. Treatment of fetal rat calvarial osteoblasts with a 70-kDa N-terminal fibronectin fragment that inhibits fibronectin assembly impaired incorporation of LTBP1 and TGFbeta into the ECM. Consistent with this, LTBP1 failed to assemble in embryonic fibroblasts that lack the gene for fibronectin. LTBP1 assembly was rescued by full-length fibronectin and superfibronectin, which are capable of assembly into fibronectin fibrils, but not by other fibronectin fragments, including a 160-kDa RGD-containing fragment that activates alpha5beta1 integrins. This suggests that the critical event for LTBP1 assembly is the formation of a fibronectin fibrillar network and that integrin ligation by fibronectin molecules alone is not sufficient. Not only was fibronectin essential for the initial incorporation of LTBP1 into the ECM, but the continued presence of fibronectin was required for the continued assembly of LTBP1. These studies highlight a nonredundant role for fibronectin in LTBP1 assembly into the ECM and suggest a novel role for fibronectin in regulation of TGF beta via LTBP1 interactions.

TGF-β activation by traction?

Transforming growth factor (TGF)-betas are powerful cytokines that are secreted as inactive (latent) precursors into the extracellular space. To exert their pleiotropic functions, latent TGF-betas require activation. This requisite restricts TGF-beta signaling to tissues that express TGF-beta-activating proteins such as the adhesion molecule alphavbeta6 integrin. Recent work has uncovered the molecular mechanism by which alphavbeta6 integrin activates latent TGF-beta. Latent-TGF-beta-binding protein 1 has been identified as being the major component of this process, and the integrin-interacting region has been mapped to a poorly conserved sequence stretch called the hinge region.

Liver portal fibrosis in dioxin receptor-null mice that overexpress the latent transforming growth factor-beta-binding protein-1

Mice lacking aryl hydrocarbon (dioxin) receptor (AhR) had variable degree of hepatic fibrosis and altered liver architecture. Transforming growth factor-beta (TGF-beta), a major profibrogenic molecule in the liver, is localized to the extracellular matrix by its association to the latent TGF-beta-binding protein-1 (LTBP-1). Very recently, LTBP-1 has been shown to be negatively regulated by the AhR. Embryonic fibroblasts from AhR-null (AhR(-/-)) mice overexpress LTBP-1 and secrete four times more active TGF-beta than wild-type fibroblasts. To test whether TGF-beta and LTBP-1 overexpression colocalize within the fibrotic nodule of AhR(-/-) liver, we have characterized this hepatic portal fibrosis using collagen protein staining, immunohistochemistry and in situ hybridization. LTBP-1 mRNA and protein were overexpressed in the fibrotic region and colocalized with other indicators of fibrosis such as collagen and fibronectin and the fibroblast marker proteins alpha-actin and vimentin. TGF-beta protein also colocalized with fibrosis, although in contrast, TGF-beta mRNA expression, rather than restricted to the fibrotic compartment, was present throughout the hepatic parenchyma and exhibited similar levels in wild-type and AhR(-/-) mice. These results suggest that LTBP-1 targets TGF-beta to specific areas of the liver and that the AhR could be a negative regulator of liver fibrosis, possibly through the control of LTBP-1 and TGF-beta activities.

Disruption of LTBP-4 function reduces TGF-beta activation and enhances BMP-4 signaling in the lung

Disruption of latent TGF-β binding protein (LTBP)–4 expression in the mouse leads to abnormal lung development and colorectal cancer. Lung fibroblasts from these mice produced decreased amounts of active TGF-β, whereas secretion of latent TGF-β was significantly increased. Expression and secretion of TGF-β2 and -β3 increased considerably. These results suggested that TGF-β activation but not secretion would be severely impaired in LTBP-4 −/− fibroblasts. Microarrays revealed increased expression of bone morphogenic protein (BMP)–4 and decreased expression of its inhibitor gremlin. This finding was accompanied by enhanced expression of BMP-4 target genes, inhibitors of differentiation 1 and 2, and increased deposition of fibronectin-rich extracellular matrix. Accordingly, increased expression of BMP-4 and decreased expression of gremlin were observed in mouse lung. Transfection of LTBP-4 rescued the −/− fibroblast phenotype, while LTBP-1 was inefficient. Treatment with active TGF-β1 rescued BMP-4 and gremlin expression to wild-type levels. Our results indicate that the lack of LTBP-4–mediated targeting and activation of TGF-β1 leads to enhanced BMP-4 signaling in mouse lung.

Integrin alphaVbeta6-mediated activation of latent TGF-beta requires the latent TGF-beta binding protein-1

Transforming growth factor-βs (TGF-β) are secreted as inactive complexes containing the TGF-β, the TGF-β propeptide, also called the latency-associated protein (LAP), and the latent TGF-β binding protein (LTBP). Extracellular activation of this complex is a critical but incompletely understood step in TGF-β regulation. We have investigated the role of LTBP in modulating TGF-β generation by the integrin α_Vβ₆. We show that even though α_vβ₆ recognizes an RGD on LAP, LTBP-1 is required for α_Vβ₆-mediated latent TGF-β activation. The domains of LTBP-1 necessary for activation include the TGF-β propeptide-binding domain and a basic amino acid sequence (hinge domain) with ECM targeting properties. Our results demonstrate an LTBP-1 isoform-specific function in α_Vβ₆-mediated latent TGF-β activation; LTBP-3 is unable to substitute for LTBP-1 in this assay. The results reveal a functional role for LTBP-1 in latent TGF-β activation and suggest that activation of specific latent complexes is regulated by distinct mechanisms that may be determined by the LTBP isoform and its potential interaction with the matrix.

Hepatitis C virus NS5A-regulated gene expression and signaling revealed via microarray and comparative promoter analyses

Most individuals exposed to hepatitis C virus (HCV) become chronically infected and are predisposed to liver disease. The mechanisms underlying viral persistence and disease progression are unknown. A role for the HCV NS5A protein in viral replication and interferon resistance has been demonstrated. To identify mechanisms affected by NS5A, we analyzed the gene expression of Huh7 cells expressing NS5A and control cells using oligonucleotide microarrays. A set of 103 genes (43 up-regulated, 60 down-regulated) whose expression was modified by at least twofold was selected. These included genes involved in cell adhesion and motility, calcium homeostasis, lipid transport and metabolism, and genes regulating immune responses. The finding of modulated expression of genes related to the TGF-beta superfamily and liver fibrosis was observed. Interestingly, both the tumor necrosis factor and lymphotoxin beta receptors were down-regulated by NS5A. Similar data were obtained following expression of four NS5A mutants obtained from patients who were not responsive or were sensitive to interferon therapy. Through computational analysis, we determined that 39 of the 43 genes up-regulated by NS5A contained one or more nuclear factor kappaB (NF-kappaB) binding sites within their promoter region. Using the Gibbs sampling method, we also detected enrichment of NF-kappaB consensus binding sites in the upstream regions of the 43 coexpressed genes. Activation of NF-kappaB by NS5A was subsequently demonstrated in luciferase reporter assays. Adenovirus-mediated expression of IkappaBalpha reverted NS5A mediated up-regulation of gene expression. In conclusion, this study suggests a role of NS5A and NF-kappaB in HCV pathogenesis and related liver disease. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Latent TGF-beta binding protein LTBP-2 decreases fibroblast adhesion to fibronectin

We have analyzed the effects of latent TGF-beta binding protein 2 (LTBP-2) and its fragments on lung fibroblast adhesion. Quantitative cell adhesion assays indicated that fibroblasts do not adhere to full-length LTBP-2. Interestingly, LTBP-2 had dominant disrupting effects on the morphology of fibroblasts adhering to fibronectin (FN). Fibroblasts plated on LTBP-2 and FN substratum exhibited less adherent morphology and displayed clearly decreased actin stress fibers than cells plated on FN. These cells formed, instead, extensive membrane ruffles. LTBP-2 had no effects on cells adhering to collagen type I. Fibroblasts adhered weakly to the NH2-terminal fragment of LTBP-2. Unlike FN, this fragment did not augment actin stress fiber formation. Interestingly, the adhesion-mediating and cytoskeleton-disrupting effects were localized to the same NH2-terminal proline-rich region of LTBP-2. LTBP-2 and its antiadhesive fragment bound to FN in vitro, and the antiadhesive fragment associated with the extracellular matrix FN fibrils. These observations reveal a potentially important role for LTBP-2 as an antiadhesive matrix component.