Hedgehog and Platelet-derived Growth Factor Signaling Intersect during Postnatal Lung Development

Normal lung development critically depends on HH (Hedgehog) and PDGF (platelet-derived growth factor) signaling, which coordinate mesenchymal differentiation and proliferation. PDGF signaling is required for postnatal alveolar septum formation by myofibroblasts. Recently, we demonstrated a requirement for HH in postnatal lung development involving alveolar myofibroblast differentiation. Given shared features of HH signaling and PDGF signaling and their impact on this key cell type, we sought to clarify their relationship during murine postnatal lung development. Timed experiments revealed that HH inhibition phenocopies the key lung myofibroblast phenotypes of Pdgfa (platelet-derived growth factor subunit A) and Pdgfra (platelet-derived growth factor receptor alpha) knockouts during secondary alveolar septation. Using a dual signaling reporter, Gli1lZ;PdgfraEGFP, we show that HH and PDGF pathway intermediates are concurrently expressed during alveolar septal myofibroblast accumulation, suggesting pathway convergence in the generation of lung myofibroblasts. Consistent with this hypothesis, HH inhibition reduces Pdgfra expression and diminishes the number of Pdgfra-positive and Pdgfra-lineage cells in postnatal lungs. Bulk RNA sequencing data of Pdgfra-expressing cells from Postnatal Day 8 (P8) lungs show that HH inhibition alters the expression not only of well-established HH targets but also of several putative PDGF target genes. This, together with the presence of Gli-binding sites in PDGF target genes, suggests HH input into PDGF signaling. We identified these HH/PDGF targets in several postnatal lung mesenchymal cell populations, including myofibroblasts, using single-cell transcriptomic analysis. Collectively, our data indicate that HH signaling and PDGF signaling intersect to support myofibroblast/fibroblast function during secondary alveolar septum formation. Moreover, they provide a molecular foundation relevant to perinatal lung diseases associated with impaired alveolarization.

A novel Vascular Leak Index identifies sepsis patients with a higher risk for in-hospital death and fluid accumulation

Purpose

Sepsis is a leading cause of morbidity and mortality worldwide and is characterized by vascular leak. Treatment for sepsis, specifically intravenous fluids, may worsen deterioration in the context of vascular leak. We therefore sought to quantify vascular leak in sepsis patients to guide fluid resuscitation.

Methods

We performed a retrospective cohort study of sepsis patients in four ICU databases in North America, Europe, and Asia. We developed an intuitive vascular leak index (VLI) and explored the relationship between VLI and in-hospital death and fluid balance using generalized additive models (GAM).

Results

Using a GAM, we found that increased VLI is associated with an increased risk of in-hospital death. Patients with a VLI in the highest quartile (Q4), across the four datasets, had a 1.61–2.31 times increased odds of dying in the hospital compared to patients with a VLI in the lowest quartile (Q1). VLI Q2 and Q3 were also associated with increased odds of dying. The relationship between VLI, treated as a continuous variable, and in-hospital death and fluid balance was statistically significant in the three datasets with large sample sizes. Specifically, we observed that as VLI increased, there was increase in the risk for in-hospital death and 36–84 h fluid balance.

Conclusions

Our VLI identifies groups of patients who may be at higher risk for in-hospital death or for fluid accumulation. This relationship persisted in models developed to control for severity of illness and chronic comorbidities.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-022-03968-4.

COVID-19 and Respiratory System Disorders: Current Knowledge, Future Clinical and Translational Research Questions

The severe acute respiratory syndrome coronavirus-2 emerged as a serious human pathogen in late 2019, causing the disease coronavirus disease 2019 (COVID-19). The most common clinical presentation of severe COVID-19 is acute respiratory failure consistent with the acute respiratory distress syndrome. Airway, lung parenchymal, pulmonary vascular, and respiratory neuromuscular disorders all feature in COVID-19. This article reviews what is known about the effects of severe acute respiratory syndrome coronavirus-2 infection on different parts of the respiratory system, clues to understanding the underlying biology of respiratory disease, and highlights current and future translation and clinical research questions.

The Hedgehog target Gli1 is not required for bleomycin-induced lung fibrosis

Sonic Hedgehog (SHH) signaling, a developmental pathway promoting lung mesenchymal expansion and differentiation during embryogenesis, has been increasingly recognized as a profibrotic factor in mature lung, where it might contribute to the pathogenesis of lung fibrosis. Pathway inhibition at the level of the downstream Gli transcription factors Gli1 and Gli2 (by GANT61) ameliorates lung fibrosis in the bleomycin model, whereas inhibition proximally at the level of HH ligand (by anti Hh antibody 5E1) or Smo (by GDC-0449) of the canonical pathway does not, implicating Gli1 and/or Gli2 as a key target. The fact that both the Gli1-labelled cell lineage and Gli1 expressing cells expand during fibrosis formation and contribute significantly to the pool of myofibroblasts in the fibrosis scars suggests a fibrogenic role for Gli1. Therefore to further dissect the roles of Gli1 and Gli2 in lung fibrosis we evaluated Gli1 KO and control mice in the bleomycin model. Monitoring of Gli1^+/+ (n = 12), Gli1^lZ/+ (n = 37) and Gli1^lZ/lZ (n = 18) mice did not reveal differences in weight loss or survival. Lung evaluation at the 21-day endpoint did not show differences in lung fibrosis formation (as judged by morphology and trichrome staining), Ashcroft score, lung collagen content, lung weight, BAL protein content or BAL cell differential count. Our data suggest that Gli1 is not required for bleomycin-induced lung fibrosis.

Sonic Hedgehog Signaling Regulates Myofibroblast Function during Alveolar Septum Formation in Murine Postnatal Lung

Sonic Hedgehog (Shh) signaling regulates mesenchymal proliferation and differentiation during embryonic lung development. In the adult lung, Shh signaling maintains mesenchymal quiescence and is dysregulated in diseases such as idiopathic pulmonary fibrosis and chronic obstructive pulmonary disease. Our previous data implicated a role for Shh in postnatal lung development. Here, we report a detailed analysis of Shh signaling during murine postnatal lung development. We show that Shh pathway expression and activity during alveolarization (postnatal day [P] 0-P14) are distinct from those during maturation (P14-P24). This biphasic pattern is paralleled by the transient presence of Gli1⁺;α-smooth muscle actin (α-SMA)⁺ myofibroblasts in the growing alveolar septal tips. Carefully timed inhibition of Hedgehog (Hh) signaling during alveolarization defined mechanisms by which Shh influences the mesenchymal compartment. First, interruption of Hh signaling at earlier time points results in increased lung compliance and wall structure defects of increasing severity, ranging from moderately enlarged alveolar airspaces to markedly enlarged airspaces and fewer secondary septa. Second, Shh signaling is required for myofibroblast differentiation: Hh inhibition during early alveolarization almost completely eliminates Gli1⁺;α-SMA⁺ cells at the septal tips, and Gli1-lineage tracing revealed that Gli1⁺ cells do not undergo apoptosis after Hh inhibition but remain in the alveolar septa and are unable to express α-SMA. Third, Shh signaling is vital to mesenchymal proliferation during alveolarization, as Hh inhibition decreased proliferation of Gli1⁺ cells and their progeny. Our study establishes Shh as a new alveolarization-promoting factor that might be affected in perinatal lung diseases that are associated with impaired alveolarization.

Abstract 710: Identification of autoantibody to ECH1 & HNRNPA2B1 as potential biomarkers in the early detection of lung cancer

Background: Identification of biomarkers for early detection of lung cancer (LC) may lead to more effective treatment and reduction of mortality.

Methods: Serological proteome analysis (SERPA) was used to identify proteins around 34 kDa, which had been previously recognized by autoantibody in sera from LC patients. We have validated autoantibody response in sera from 90 LC patients, 89 normal controls by using immunoassay. Another independent cohort of 25 LC patients with 219 serial serum samples and 56 matched normal controls were examined to evaluate whether the autoantibody can be detected in the preclinical stage.

Results: The proteins with molecular weight of 34 kDa were identified as ECH1, GAPDH and HNRNPA2B1. In the validation study, autoantibody to ECH1 achieved an area under the curve (AUC) of 0.799 with sensitivity of 62.2% and specificity of 95.5% in discriminating LC from normal individuals, and showed negative correlation with tumor size (rs=0.-256, p=0.023). Autoantibody to HNRNPA2B1 performed an AUC of 0.874 with sensitivity of 72.2% and specificity of 95.5%, and showed negative correlation with lymph node metastasis (rs=0.-279, P=0.012). By using longitudinal preclinical samples, autoantibody to ECH1 showed an AUC of 0.763 with sensitivity of 60.0% and specificity of 89.3% in distinguishing LC with matched normal controls, and elevated autoantibody levels could be detected greater than two years prior to LC diagnosis.

Conclusions: ECH1 and HNRNPA2B1 are autoantigens that elicit autoimmune responses in LC and can be used as potential biomarkers for the early detection of LC.

Funding support: This work was supported by the National Natural Science Foundation of China (81672917, 81372371) and the National Institutes of Health (SC1CA166016 and U01CA086137).

Citation Format: Liping Dai, Jitian Li, Jun-Chieh J. Tsay, Xiao Wang, John S. Munger, Harvey Pass, William N. Rom, Eng M. Tan, Jian-Ying Zhang. Identification of autoantibody to ECH1 & HNRNPA2B1 as potential biomarkers in the early detection of lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 710. doi:10.1158/1538-7445.AM2017-710

Identification of autoantibodies to ECH1 and HNRNPA2B1 as potential biomarkers in the early detection of lung cancer

Identification of biomarkers for early detection of lung cancer (LC) is important, in turn leading to more effective treatment and reduction of mortality. Serological proteome analysis (SERPA) was used to identify proteins around 34 kD as ECH1 and HNRNPA2B1, which had been recognized by serum autoantibody from 25 LC patients. In the validation study, including 90 sera from LC patients and 89 sera from normal individuals, autoantibody to ECH1 achieved an area under the curve (AUC) of 0.799 with sensitivity of 62.2% and specificity of 95.5% in discriminating LC from normal individuals, and showed negative correlation with tumor size (r_s = -0.256, p = 0.023). Autoantibody to HNRNPA2B1 performed an AUC of 0.874 with sensitivity of 72.2% and specificity of 95.5%, and showed negative correlation with lymph node metastasis (r_s = -0.279, p = 0.012). By using longitudinal preclinical samples, autoantibody to ECH1 showed an AUC of 0.763 with sensitivity of 60.0% and specificity of 89.3% in distinguishing early stage LC from matched normal controls, and elevated autoantibody levels could be detected greater than 2 y before LC diagnosis. ECH1 and HNRNPA2B1 are autoantigens that elicit autoimmune responses in LC and their autoantibody can be the potential biomarkers for the early detection of LC.

Autoantibodies against tumor-associated antigens in the early detection of lung cancer

OBJECTIVES

Autoantibodies against tumor-associated antigens (TAAs) identified in patients with advanced lung cancer may be detected in subjects with early lung cancer or even predate the diagnosis. The purpose of this study is to address the temporal relationship between lung cancer development and serum autoantibody response.

MATERIALS AND METHODS

Two cohorts of patients with newly diagnosed lung cancer were included. The first cohort included 90 sera from patients with lung cancer (Stages I-III) and 89 normal control sera. In the second cohort, 93 serial serum samples from 25 patients with CT-scan screen-detected stage I lung cancer were collected before the diagnosis of lung cancer (average 32 months) and 56 controls were matched on age, gender, and smoking. Autoantibody levels were measured by immunoassay.

RESULTS

Measurement of autoantibodies against seven TAAs (14-3-3ζ, c-Myc, MDM2, NPM1, p16, p53 and cyclin B1) individually could discriminate lung cancer patients from normal individuals in the first cohort and the area under curve (AUC) was 0.863 based on a panel of seven autoantibodies, with sensitivity of 68.9% and specificity of 79.5%. Autoantibodies in serial pre-diagnostic serum samples against the same panel of seven TAAs were detected prior to lung cancer diagnosis with sensitivity of 76.0% and specificity of 73.2% (AUC) (95%CI): 0.885 (0.797-0.973)). Elevated autoantibody levels could be detected greater than four years prior to lung cancer diagnosis.

CONCLUSION

A panel of seven TAAs may enhance the early detection of lung cancer, consistent with a humoral immune response to TAAs that can be detected months to years prior to the diagnosis.

Reduced lung function in smokers in a lung cancer screening cohort with asbestos exposure and pleural plaques

BACKGROUND

While low dose computed tomography (LDCT) screening for lung cancer is recommended for high-risk smokers, ages 55-74 years, information about asbestos exposure may not be routinely elicited. Asbestos exposure is associated with declining respiratory function over time; however, the effect of a history of asbestos exposure in LDCT screening cohorts is limited. We report the relationship between asbestos exposure and pulmonary function in a cohort of heavy smokers with a history of occupational asbestos exposure, hypothesizing that these subjects will have additional decreased pulmonary function. We also examined relationships between spirometric measurements and the presence of isolated pleural plaques.

METHODS

A cross-sectional study was performed using data from the NYU Lung Cancer Biomarker Center cohort to compare study subjects with a history asbestos exposure primarily in the period since 1970 when tighter federal standards were in place (n = 359) to those without asbestos exposure (n = 1038) with respect to pulmonary function, LDCT lung imaging findings, and clinical symptoms. We further classified individuals with asbestos exposure by length of exposure time to examine the effect of duration of exposure on pulmonary function. Lastly, for asbestos-exposed participants, we examined the association of spirometric measurements with the presence of absence of isolated pleural plaques.

RESULTS

Individuals with asbestos exposure had decreased FVC % predicted compared to those with no asbestos exposure (76% vs. 85% predicted, P < 0.01) and FEV1 % predicted (64% vs. 67% predicted, P < 0.01). Since there was no change in FEV1 /FVC ratio, the findings are consistent with restrictive impairment. Those with ≥20 years of exposure had a lower mean FVC % predicted compared to those with less than 20 years of exposure (74% vs. 78% predicted, P = 0.017). Individuals with asbestos exposure were more likely to have pleural plaques (P < 0.001) on CT. Those with isolated pleural plaques had lower mean % predicted FEV1 (P = 0.005) and FVC (P = 0.001) compared to those without pleural plaques.

CONCLUSIONS

Occupational asbestos exposure in a cohort of heavy smokers was associated with a significant restrictive decline in pulmonary function, with longer duration of exposure associated with greater decline. The presence of isolated pleural plaques was also associated with reduced lung function.

Sonic hedgehog signaling in the lung. From development to disease

Over the past two decades, the secreted protein sonic hedgehog (SHH) has emerged as a critical morphogen during embryonic lung development, regulating the interaction between epithelial and mesenchymal cell populations in the airway and alveolar compartments. There is increasing evidence that the SHH pathway is active in adult lung diseases such as pulmonary fibrosis, asthma, chronic obstructive pulmonary disease, and lung cancer, which raises two questions: (1) What role does SHH signaling play in these diseases? and (2) Is it a primary driver of the disease or a response (perhaps beneficial) to the primary disturbance? In this review we aim to fill the gap between the well-studied period of embryonic lung development and the adult diseased lung by reviewing the hedgehog (HH) pathway during the postnatal period and in adult uninjured and injured lungs. We elucidate the similarities and differences in the epithelial-mesenchymal interplay during the fibrosis response to injury in lung compared with other organs and present a critical appraisal of tools and agents available to evaluate HH signaling.

Hedgehog signaling in neonatal and adult lung

Sonic Hedgehog (Shh) signaling is essential during embryonic lung development, but its role in postnatal lung development and adult lung are not known. Using Gli1(nlacZ) reporter mice to identify cells with active Hh signaling, we found that Gli1(nlacZ)-positive mesenchymal cells are densely and diffusely present up to 2 weeks after birth and decline in number thereafter. In adult mice, Gli1(nlacZ)-positive cells are present around large airways and vessels and are sparse in alveolar septa. Hh-stimulated cells are mostly fibroblasts; only 10% of Gli1(nlacZ)-positive cells are smooth muscle cells, and most smooth muscle cells do not have activation of Hh signaling. To assess its functional relevance, we influenced Hh signaling in the developing postnatal lung and adult injured lung. Inhibition of Hh signaling during early postnatal lung development causes airspace enlargement without diminished alveolar septation. After bleomycin injury in the adult lung, there are abundant Gli1(nlacZ)-positive mesenchymal cells in fibrotic lesions and increased numbers of Gli1(nlacZ)-positive cells in preserved alveolar septa. Inhibition of Hh signaling with an antibody against all Hedgehog isoforms does not reduce bleomycin-induced fibrosis, but adenovirus-mediated overexpression of Shh increases collagen production in this model. Our data provide strong evidence that Hh signaling can regulate lung stromal cell function in two critical scenarios: normal development in postnatal lung and lung fibrosis in adult lung.

Cross talk among TGF-β signaling pathways, integrins, and the extracellular matrix

The growth factor TGF-β is secreted in a latent complex consisting of three proteins: TGF-β, an inhibitor (latency-associated protein, LAP, which is derived from the TGF-β propeptide) and an ECM-binding protein (one of the latent TGF-β binding proteins, or LTBPs). LTBPs interact with fibrillins and other ECM components and thus function to localize latent TGF-β in the ECM. LAP contains an integrin-binding site (RGD), and several RGD-binding integrins are able to activate latent TGF-β through binding this site. Mutant mice defective in integrin-mediated activators, and humans and mice with fibrillin gene mutations, show the critical role of ECM and integrins in regulating TGF-β signaling.

Mice that lack activity of alphavbeta6- and alphavbeta8-integrins reproduce the abnormalities of Tgfb1- and Tgfb3-null mice

The arginine-glycine-aspartate (RGD)-binding integrins alphavbeta6 and alphavbeta8 activate latent TGFbeta1 and TGFbeta3 in vivo, but it is uncertain whether other RGD-binding integrins such as integrins alphavbeta5 and alphavbeta3 activate these TGFbeta isoforms. To define the combined role of alphavbeta6- and alphavbeta8-integrin in TGFbeta activation, we analyzed mice lacking function of both integrins by means of gene deletion and/or pharmacologic inhibition. Most Itgb6-/-;Itgb8-/- embryos die at mid-gestation; those that survive develop cleft palate-as observed in Tgfb3-/- mice. Itgb8-/- mice treated with an anti-alphavbeta6-integrin antibody develop severe autoimmunity and lack Langerhans cells-similar to Tgfb1-null mice. These results support a model in which TGFbeta3-mediated palate fusion and TGFbeta1-mediated suppression of autoimmunity and generation of Langerhans cells require integrins alphavbeta6 and alphavbeta8 but not other RGD-binding integrins as TGFbeta activators.

Inhibition of integrin alpha(v)beta6, an activator of latent transforming growth factor-beta, prevents radiation-induced lung fibrosis

RATIONALE

In experimental models, lung fibrosis is dependent on transforming growth factor (TGF)-beta signaling. TGF-beta is secreted in a latent complex with its propeptide, and TGF-beta activators release TGF-beta from this complex. Because the integrin alpha(v)beta6 is a major TGF-beta activator in the lung, inhibition of alpha(v)beta6-mediated TGF-beta activation is a logical strategy to treat lung fibrosis.

OBJECTIVES

To determine, by genetic and pharmacologic approaches, whether murine radiation-induced lung fibrosis is dependent on alpha(v)beta6.

METHODS

Wild-type mice, alpha(v)beta6-deficient (Itgb6-/-) mice, and mice heterozygous for a Tgfb1 mutation that eliminates integrin-mediated activation (Tgfb1(+/RGE)) were exposed to 14 Gy thoracic radiation. Some mice were treated with an anti-alpha(v)beta6 monoclonal antibody or a soluble TGF-beta receptor fusion protein. Alpha(v)beta6 expression was determined by immunohistochemistry. Fibrosis, inflammation, and gene expression patterns were assessed 20-32 weeks postirradiation.

MEASUREMENTS AND MAIN RESULTS

Beta6 integrin expression increased within the alveolar epithelium 18 weeks postirradiation, just before onset of fibrosis. Itgb6-/- mice were completely protected from fibrosis, but not from late radiation-induced mortality. Anti-alpha(v)beta6 therapy (1-10 mg/kg/wk) prevented fibrosis, but only higher doses (6-10 mg/kg/wk) caused lung inflammation similar to that in Itgb6-/- mice. Tgfb1-haploinsufficient mice were also protected from fibrosis.

CONCLUSIONS

Alpha(v)beta6-mediated TGF-beta activation is required for radiation-induced lung fibrosis. Together with previous data, our results demonstrate a robust requirement for alpha(v)beta6 in distinct fibrosis models. Inhibition of alphavbeta6-mediated TGF-beta activation is a promising new approach for antifibrosis therapy.

Absence of integrin-mediated TGFbeta1 activation in vivo recapitulates the phenotype of TGFbeta1-null mice

The multifunctional cytokine transforming growth factor (TGF) β1 is secreted in a latent complex with its processed propeptide (latency-associated peptide [LAP]). TGFβ1 must be functionally released from this complex before it can engage TGFβ receptors. One mechanism of latent TGFβ1 activation involves interaction of the integrins αvβ6 and αvβ8 with an RGD sequence in LAP; other putative latent TGFβ1 activators include thrombospondin-1, oxidants, and various proteases. To assess the contribution of RGD-binding integrins to TGFβ1 activation in vivo, we created a mutation in Tgfb1 encoding a nonfunctional variant of the RGD sequence (RGE). Mice with this mutation (Tgfb1^RGE/RGE) display the major features of Tgfb1^−/− mice (vasculogenesis defects, multiorgan inflammation, and lack of Langerhans cells) despite production of normal levels of latent TGFβ1. These findings indicate that RGD-binding integrins are requisite latent TGFβ1 activators during development and in the immune system.

A genetic screen to identify latent transforming growth factor beta activators

The mechanisms by which latent transforming growth factor beta (TGFbeta) is converted to the active cytokine are largely unknown. Here we present a genetic screen that combines retroviral mutagenesis and cDNA expression cloning to reveal proteins involved in the extracellular regulation of latent TGFbeta activation. The screen employs a cell line engineered to express green fluorescent protein (GFP) in response to TGFbeta. The cells produce their own latent TGFbeta. Therefore, after transduction with a retroviral cDNA library that contains an insert for an activator of latent TGFbeta, cells expressing the activator are GFP-bright. These cells are enriched by fluorescence-activated cell sorting and grown as individual clones. The isolated clones are cocultured with a second TGFbeta reporter cell line that produces luciferase in response to TGFbeta. Cells that have acquired the ability to activate latent TGFbeta induce luciferase expression in the absence but not in the presence of neutralizing antibodies to TGFbeta. The activator expressed by the positive clones can be identified by retrieval of the retrovirus cDNA insert.

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.

Making sense of latent TGFbeta activation

TGFbeta is secreted as part of a latent complex that is targeted to the extracellular matrix. A variety of molecules, 'TGFbeta activators,' release TGFbeta from its latent state. The unusual temporal discontinuity of TGFbeta synthesis and action and the panoply of TGFbeta effects contribute to the interest in TGF-beta. However, the logical connections between TGFbeta synthesis, storage and action are obscure. We consider the latent TGFbeta complex as an extracellular sensor in which the TGFbeta propeptide functions as the detector, latent-TGFbeta-binding protein (LTBP) functions as the localizer, and TGF-beta functions as the effector. Such a view provides a logical continuity for various aspects of TGFbeta biology and allows us to appreciate TGFbeta biology from a new perspective.

Integrin alpha8beta1 mediates adhesion to LAP-TGFbeta1

The development of fibrosis is a common response to a variety of injuries and results in the net accumulation of matrix proteins and impairment of normal organ function. We previously reported that the integrin alpha8beta1 is expressed by alveolar interstitial cells in normal lung and is upregulated during the development of fibrosis. TGFbeta1 is an important mediator of the inflammatory response in pulmonary fibrosis. TGFbeta1 is secreted as a latent protein that is non-covalently associated with latency-associated peptide (LAP) and requires activation to exert its effects. LAP-TGFbeta1 and LAP-TGFbeta3 contain the tripeptide sequence, arginine-glycine-aspartic acid (RGD), a known integrin recognition motif. The integrin alpha8beta1 binds to several ligands such as fibronectin and vitronectin through the RGD sequence. Recent reports demonstrate that the integrins alphavbeta1, alphavbeta6 and alphavbeta8 adhere to LAP-TGFbeta1 through the RGD site. Therefore, we asked whether LAP-TGFbeta1 might be a ligand for alpha8beta1 and whether this may be important in the development of fibrosis. We found that cell lines transfected with alpha8 subunit were able to spread on and adhere to recombinant LAP-TGFbeta1 significantly better than mock transfected cell lines. alpha8-transfected cells were also able to adhere to LAP-TGFbeta3 significantly better than mock transfected cells. Adhesion to LAP-TGFbeta1 was enhanced by activation of alpha8beta1 by Mn(2+), or 8A2, an integrin beta1 activating antibody. Furthermore, cell adhesion was abolished when we used a recombinant LAP-TGFbeta1 protein in which the RGD site was mutated to RGE. alpha8beta1 binding to LAP-TGFbeta1 increased cell proliferation and phosphorylation of FAK and ERK, but did not activate of TGFbeta1. These data strongly suggest that LAP-TGFbeta1 is a ligand of alpha8beta1 and interaction of alpha8beta1 with LAP-TGFbeta1 may influence cell behavior.

Transforming growth factor beta signaling is disabled early in human endometrial carcinogenesis concomitant with loss of growth inhibition

Transforming growth factor beta (TGF-beta), a potent ubiquitous endogenous inhibitor of epithelial cell growth, is secreted as a latent molecule (LTGF-beta)requiring activation for function. TGF-beta signals through the type I(TbetaRI) and type II (TbetaRII) receptors, which cooperate to phosphorylate/activate Smad2/3, the transcriptional regulators of genes that induce cell cycle arrest. That carcinomas grow in vivo suggests that they are refractory to TGF-beta. However, this has been difficult to prove because of an inability to analyze the functional status of TGF-beta in vivo as well as lack of close physiological paradigms for carcinoma cells in vitro. The current studies demonstrate that whereas primary cultures of endometrial epithelial cells derived from normal proliferative endometrium (PE; n = 10) were dose-dependently and maximally growth inhibited by 55% +/- 5.3% with 10 pM TGF-beta1, endometrial epithelial cells derived from endometrial carcinomas (ECAs; n = 10) were unresponsive (P < or = 0.0066). To determine the mechanism of TGF-beta resistance in ECAs, we analyzed the TGF-beta signaling pathway in vivo by immunohistochemistry using specific antibodies to TbetaRI and TbetaRII, Smads, and to the phosphorylated form of Smad2 (Smad2P), an indicator of cells responding to bioactive TGF-beta. Smad2P was expressed in all of the normal endometria (n = 25), and was localized to the cytoplasm and nucleus in PE, and only nuclear in the secretory endometrium. In marked contrast, Smad2P immunostaining was weak or undetectable in ECA (n = 22; P < or = 0.001) and reduced in glandular hyperplasia (n = 25) compared with normal endometrium. However, total Smad2 and Smad7 (which inhibits Smad2 activation) levels were identical in ECA and normal tissue. Consistent with loss of downstream signaling, both TbetaRI (n = 19) and TbetaRII (n = 22) protein expression were significantly reduced in ECA compared with PE (n = 11; P < or = 0.05). By in situ hybridization, the mRNA levels of TbetaRI and TbetaRII were decreased in the carcinoma cells compared with normal PE glands, suggesting that receptor down-regulation occurs at the transcriptional level. Furthermore, a somatic frameshift mutation in the polyadenine tract at the 5' end of the TbetaR-II gene was detected in two of six cases examined. Finally, the ability of explants of ECA to activate endogenous LTGF-beta was deficient compared with normal tissue (23.5% versus 7.4%). Therefore, our results suggest that loss of Smad2 signaling in ECA may be because of down-regulation of TbetaRI and TbetaRII, and/or decreased activation of LTGF-beta. Because disruption of TGF-beta signaling occurred independent of grade or degree of invasion and was evident in premalignant hyperplasia, we conclude that inactivation of TGF-beta signaling leading to escape from normal growth control occurs at an early stage in endometrial carcinogenesis, thereby defining novel molecular targets for cancer prevention.

The integrin alpha(v)beta8 mediates epithelial homeostasis through MT1-MMP-dependent activation of TGF-beta1

Integrins, matrix metalloproteases (MMPs), and the cytokine TGF-beta have each been implicated in homeostatic cell behaviors such as cell growth and matrix remodeling. TGF-beta exists mainly in a latent state, and a major point of homeostatic control is the activation of TGF-beta. Because the latent domain of TGF-beta1 possesses an integrin binding motif (RGD), integrins have the potential to sequester latent TGF-beta (SLC) to the cell surface where TGF-beta activation could be locally controlled. Here, we show that SLC binds to alpha(v)beta8, an integrin expressed by normal epithelial and neuronal cells in vivo. This binding results in the membrane type 1 (MT1)-MMP-dependent release of active TGF-beta, which leads to autocrine and paracrine effects on cell growth and matrix production. These data elucidate a novel mechanism of cellular homeostasis achieved through the coordination of the activities of members of three major gene families involved in cell-matrix interactions.

The integrin alphaVbeta6 binds and activates latent TGFbeta3

Transforming growth factors-beta (TGFbeta1, 2 and 3) are secreted in a complex with their propeptides (latency-associated peptide 1 (LAP1), 2 and 3). TGFbeta signaling requires the dissociation of LAP and TGFbeta, a process termed latent TGFbeta activation. This process is a critical but incompletely understood step in the regulation of TGFbeta function. In particular, the extent to which activation mechanisms differ among the three TGFbeta isoforms is relatively unexplored. We show here that alphaVbeta6 binds and activates latent TGFbeta3.

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.

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.

Latent transforming growth factor-beta: structural features and mechanisms of activation

Transforming growth factor-beta are cytokines with a wide range of biological effects. They play a pathologic role in inflammatory and fibrosing diseases such as nephrosclerosis. TGF-beta s are secreted in a latent form due to noncovalent association with latency associated peptide (LAP), which is a homodimer formed from the propeptide region of TGF-beta. LAP is disulfide linked to another protein, latent TGF-beta binding protein (LTBP). LTBP has features in common with extracellular matrix proteins, and targets latent TGF-beta to the matrix. Activation of latent TGF-beta can be accomplished in vitro by denaturing treatments, plasmin digestion, ionizing radiation and interaction with thrombospondin. The mechanisms by which latent TGF-beta is activated physiologically are not well understood. Results to date suggest an important role for proteases, particularly plasmin, although other mechanisms probably exist. A general model of activation is proposed in which latent TGF-beta is released from the extracellular matrix by proteases, localized to cell surfaces, and activated by cell-associated plasmin.

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.

Structure and activation of the large latent transforming growth factor-beta complex

Most cell types express transforming growth factor-beta (TGF-beta) as a large latent TGF-beta complex that must be converted to an active form before TGF-beta can interact with cell surface TGF-beta receptors. This conversion involves the release of mature TGF-beta from the complex by disrupting noncovalent interactions between mature TGF-beta and its propeptide, latency associated peptide. A critical step in regulating TGF-beta effects may be the activation of the large latent TGF-beta complex. Activation of the complex can be achieved by chemical and enzymatic treatments, or by various cell systems. We have identified that coculturing bovine endothelial and smooth muscle cells generates active TGF-beta. Coculture activation of the large latent TGF-beta complex occurs through a plasmin-dependent mechanism that requires concentration of reactants on the cell surface and/or extracellular matrix. The mechanism of latent TGF-beta activation self-regulates through effectors of plasmin generation.

Lysosomal processing of amyloid precursor protein to A beta peptides: a distinct role for cathepsin S

To investigate the potential contribution of the lysosomal compartment in the processing of amyloid precursor protein (APP) to amyloid beta-peptides (A beta s), we stably overexpressed a series of lysosomal proteases (the cysteine proteases, cathepsins B, L and S, and the aspartic protease, cathepsin D) in a human kidney epithelial cell line (293) transfected to express high levels of beta APP. Preliminary experiments indicated that 293 cells endogenously synthesize cathepsins B, L and D, but not cathepsin S. A beta secretion was assessed by immunoprecipitation and ELISA and found to be increased approximately 2-fold following cathepsin S expression, but to be unchanged (cathepsins B, L) or decreased (cathepsin D) in the other double transfectants. E-64d, an inhibitor of lysosomal cysteine proteases, significantly reduced A beta secretion by the cathepsin S transfectants, but had no effect on cells expressing the other proteases. Radiosequencing of A beta secreted by cathepsin S-expressing cells revealed that a previously unreported variant beginning at Met -1 (relative to the most common A beta N-terminus, Asp -1) accounted for most of the increase in A beta secretion. Immunostaining of human brain sections revealed cathepsin S in cortical neurons and glia in samples of brain from patients with Alzheimer's disease. These results provide evidence in living cells for a pathway in which cathepsin S generates A beta from amyloidogenic fragments of beta APP in the endosomal/lysosomal compartment. This pathway appears to be inducible, distinct from a constitutive pathway used by 293 and other cells to generate A beta, and may be relevant to the pathogenesis of Alzheimer's disease.

The lysosomal cysteine protease, cathepsin S, is increased in Alzheimer's disease and Down syndrome brain. An immunocytochemical study

Expression of cathepsin (cat) S, a lysosomal cysteine protease, has recently been shown to cause an increase in production of amyloid beta-peptides in transfected human cells. In this study, we examined the presence and localization of cat S by immunocytochemistry in 21 control, 24 Alzheimer's disease (AD), and 10 Down syndrome (DS) postmortem brains. An antiserum to a human cat S fusion protein was affinity purified and its specificity confirmed by abolition of immunoreactivity after adsorption with cat S but not cat L fusion protein. A small minority of control cases showed light, focal staining of scattered cortical neurons. Many control cases, as well as most AD and DS cases, showed prominent staining of vascular smooth muscle cells, particularly in leptomeningeal vessels. Both AD and DS brain tissue showed increased immunoreactivity in a subset of neocortical and hippocampal neurons and glia. Cat S immunoreactivity occurred in a granular, cytoplasmic pattern in some neurons or in a more dense staining pattern in certain neurofibrillary tangle-bearing neurons. Cat S-positive neurons were also present in amygdala and basal forebrain in AD brains. A subset of astrocytes were immunoreactive with the cat S antibody in AD and DS but not in control brains. In rare AD cases, cat S immunostaining was observed in astrocytes in the periphery of amyloid-beta-containing plaques. These results suggest that cat S is up-regulated in AD and DS brain. The association of cat S immunoreactivity with tangle-bearing neurons, astrocytes, and rare senile plaques implies a role for altered cat S activity in the pathogenesis of AD.

The role of thiol proteases in tissue injury and remodeling

Human lung macrophages express all four of the known lysosomal thiol proteases: cathepsins B, H, L, and S. These enzymes share a similar size and targeting mechanism for lysosomal accumulation and all have relatively indiscriminate substrate specificity in comparison with such highly selective serine proteases as urokinase or thrombin. These enzymes do have distinctive properties: only cathepsin B has C-terminal dipeptidase activity, only cathepsin H has potent aminopeptidase activity, and only cathepsin L and S are elastolytic. Cathepsin S is unique in that it is stable at neutral pH; indeed, at neutral pH it has elastolytic activity roughly comparable with that of neutrophil elastase. Recent studies of the differential expression of these cathepsins suggest they not only cooperate in terminal degradation of endocytized protein but also have specific functions such as proenzyme activation, antigen processing, and tissue remodeling, especially bone matrix resorption. Lysates of lung macrophages degrade elastin at neutral pH, suggesting that necrosis of macrophages at sites of macrophage accumulation, e.g., caseation necrosis, could contribute to tissue destruction. Tissue destruction and remodeling by thiol proteases expressed by live macrophages, however, is limited by tight compartmentalization of cathepsins to lysosomes. Nonetheless, macrophages accumulate at sites of known injury in cigarette smokers. Because these cells contain potent elastases, and because lysosomal enzyme release and cell surface acidification are regulated events, dysregulation of thiol protease expression in stimulated macrophages may contribute to the injury observed in cigarette smokers with non-alpha-1-protease inhibitor-type emphysema.

Human cathepsin S: chromosomal localization, gene structure, and tissue distribution

The human lysosomal cysteine proteinases, cathepsins H, L, and B, have been mapped to chromosomes 15, 9, and 8, respectively, and the genomic structures of cathepsins L and B have been determined. We report here the chromosomal localization and partial gene structure for a recently sequenced human cysteine proteinase, cathepsin S. A 20-kilobase pair genomic clone of the human cathepsin S gene was isolated from a human fibroblast genomic library and used to map the human cathepsin S gene to chromosome 1q21 by fluorescence in situ hybridization. This clone contains exons 1 through 5, introns 1 through 4, part of intron 5, and > 7 kilobase pairs of the 5'-flanking sequence. The gene structure of human cathepsin S is similar to that of cathepsin L through the first 5 exons, except that cathepsin S introns are substantially larger. Sequencing of the 5'-flanking region revealed, similar to human cathepsin B, no classical TATA or CAAT box. In contrast to cathepsin B, cathepsin S contains only two SP1 and at least 18 AP1 binding sites that potentially could be involved in regulation of the gene. This 5'-flanking region also contains CA microsatellites. The presence of AP1 sites and CA microsatellites suggest that cathepsin S can be specifically regulated. Results of Northern blotting using probes for human cathepsins B, L, and S are consistent with this hypothesis; only cathepsin S shows a restricted tissue distribution, with highest levels in spleen, heart, and lung. In addition, immunostaining of lung tissue demonstrated detectable cathepsin S only in lung macrophages. The high level of expression in the spleen and in phagocytes suggests that cathepsin S may have a specific function in immunity, perhaps related to antigen processing.

Molecular cloning and expression of human alveolar macrophage cathepsin S, an elastinolytic cysteine protease

Human alveolar macrophages (HAM) express an elastase activity of acidic pH optimum inhibitable by cysteine protease inhibitors. Recent studies indicate that the only known eukaryotic elastinolytic cysteine protease, cathepsin L, cannot completely account for this activity. In order to search for additional cysteine proteases with elastinolytic activity, low degeneracy oligonucleotide primers based on regions of strong homology among the known cysteine proteases were used to screen reverse-transcribed HAM RNA for cysteine proteases by the polymerase chain reaction. Among the cDNA sequences generated was a 493-base pair product highly homologous to bovine cathepsin S. Screening of a HAM cDNA eukaryotic expression library with this cDNA yielded a 1.7-kilobase full-length cDNA highly homologous to bovine cathepsin S (approximately 85% identical). This cDNA predicts a 331-amino acid preprocathepsin. Expression of this cDNA in COS cells revealed the active enzyme to be a single chain 28-kDa protease, as judged by active site labeling with a novel iodinated analogue of N-(L-3-trans-carboxyoxirane-2-carbonyl)-L-leucylamido-(4-gua nido)butane (E-64). The recombinant enzyme was found to be elastinolytic toward 3H-labeled elastin (bovine ligamentum nuchae) at pH 5.5 but with 25% of this activity retained at pH 7.0. Labeling of HAM with the active site probe revealed these cells express a 28-kDa cysteine protease, and Northern blot analysis revealed the presence of a approximately 1.7-kilobase cathepsin S mRNA. These data establish that human macrophages express at least two cysteine proteases with elastinolytic activity. The relatively broad pH range of human cathepsin S activity suggests this enzyme may contribute to the contact-dependent elastase activity of live human alveolar macrophages.

A serine esterase released by human alveolar macrophages is closely related to liver microsomal carboxylesterases

We identified a 60-kDa diisopropylfluorophosphate-(DFP) reactive protein in human bronchoalveolar lavage fluid, at a yield of 50-100 pmol/lavage. The protein is associated with the cell-free lavage fluid sediment, which consists mainly of surfactant. [3H]DFP labeling is inhibited by heating to 56 degrees C, 2 mM phenylmethylsulfonylfluoride and 1 mM bis(4-nitrophenyl)-phosphate. An identical 60-kDa [3H]DFP-reactive protein is present in the insoluble fraction of alveolar macrophage-conditioned culture medium and in total membrane preparations of alveolar macrophages. The [3H]DFP-labeled protein was purified approximately 30-fold from lavage fluid sediment by size-exclusion (Sephacryl S-200) and ion-exchange (Mono-Q) chromatography. Cyanogen bromide treatment of the partially purified protein produced a major labeled peptide of 14 kDa with an NH2-terminal sequence 90% identical to a region of form 1 rabbit liver microsomal carboxylesterase. Esterase activity in unlabeled starting material, detected using p-nitrophenyl valerate as substrate, copurified with the [3H]DFP-labeled enzyme. Degenerate oligonucleotide primers were designed based on the partial amino acid sequence and on a highly conserved region of known liver carboxylesterase sequences. Polymerase chain reaction using these primers and reverse-transcribed human alveolar macrophage mRNA yielded a 354-base pair product which was then used to screen a human alveolar macrophage cDNA library. A complete esterase sequence was obtained from two incomplete, overlapping clones, and is virtually identical to human liver carboxylesterase partial sequences. Northern blot analysis demonstrated a single approximately 1.7-kilobase transcript in human monocytes and alveolar macrophages, with much higher levels in the latter. These data indicate that human alveolar macrophages both contain and release a serine esterase that is apparently identical to liver microsomal carboxylesterase. Its enzymatic profile suggests it is a major component of alveolar macrophage-nonspecific esterase activity. We hypothesize that it acts as a detoxication enzyme in the lung.