Human airway trypsin-like protease increases mucin gene expression in airway epithelial cells

Amphiregulin Exerts Proangiogenic Effects in Developing Murine Lungs

Interrupted lung angiogenesis is a hallmark of bronchopulmonary dysplasia (BPD); however, druggable targets that can rescue this phenotype remain elusive. Thus, our investigation focused on amphiregulin (Areg), a growth factor that mediates cellular proliferation, differentiation, migration, survival, and repair. While Areg promotes lung branching morphogenesis, its effect on endothelial cell (EC) homeostasis in developing lungs is understudied. Therefore, we hypothesized that Areg promotes the proangiogenic ability of the ECs in developing murine lungs exposed to hyperoxia. Lung tissues were harvested from neonatal mice exposed to normoxia or hyperoxia to determine Areg expression. Next, we performed genetic loss-of-function and pharmacological gain-of-function studies in normoxia- and hyperoxia-exposed fetal murine lung ECs. Hyperoxia increased Areg mRNA levels and Areg+ cells in whole lungs. While Areg expression was increased in lung ECs exposed to hyperoxia, the expression of its signaling receptor, epidermal growth factor receptor, was decreased, indicating that hyperoxia reduces Areg signaling in lung ECs. Areg deficiency potentiated hyperoxia-mediated anti-angiogenic effects. In contrast, Areg treatment increased extracellular signal-regulated kinase activation and exerted proangiogenic effects. In conclusion, Areg promotes EC tubule formation in developing murine lungs exposed to hyperoxia.

Roles of host proteases in the entry of SARS-CoV-2

The spike protein (S) of SARS-CoV-2 is responsible for viral attachment and entry, thus a major factor for host susceptibility, tissue tropism, virulence and pathogenicity. The S is divided with S1 and S2 region, and the S1 contains the receptor-binding domain (RBD), while the S2 contains the hydrophobic fusion domain for the entry into the host cell. Numerous host proteases have been implicated in the activation of SARS-CoV-2 S through various cleavage sites. In this article, we review host proteases including furin, trypsin, transmembrane protease serine 2 (TMPRSS2) and cathepsins in the activation of SARS-CoV-2 S. Many betacoronaviruses including SARS-CoV-2 have polybasic residues at the S1/S2 site which is subjected to the cleavage by furin. The S1/S2 cleavage facilitates more assessable RBD to the receptor ACE2, and the binding triggers further conformational changes and exposure of the S2' site to proteases such as type II transmembrane serine proteases (TTPRs) including TMPRSS2. In the presence of TMPRSS2 on the target cells, SARS-CoV-2 can utilize a direct entry route by fusion of the viral envelope to the cellular membrane. In the absence of TMPRSS2, SARS-CoV-2 enter target cells via endosomes where multiple cathepsins cleave the S for the successful entry. Additional host proteases involved in the cleavage of the S were discussed. This article also includes roles of 3C-like protease inhibitors which have inhibitory activity against cathepsin L in the entry of SARS-CoV-2, and discussed the dual roles of such inhibitors in virus replication.

A common TMPRSS2 variant has a protective effect against severe COVID-19

BACKGROUND

The human protein transmembrane protease serine type 2 (TMPRSS2) plays a key role in SARS-CoV-2 infection, as it is required to activate the virus' spike protein, facilitating entry into target cells. We hypothesized that naturally-occurring TMPRSS2 human genetic variants affecting the structure and function of the TMPRSS2 protein may modulate the severity of SARS-CoV-2 infection.

METHODS

We focused on the only common TMPRSS2 non-synonymous variant predicted to be damaging (rs12329760 C>T, p.V160M), which has a minor allele frequency ranging from 0.14 in Ashkenazi Jewish to 0.38 in East Asians. We analysed the association between the rs12329760 and COVID-19 severity in 2,244 critically ill patients with COVID-19 from 208 UK intensive care units recruited as part of the GenOMICC (Genetics Of Mortality In Critical Care) study. Logistic regression analyses were adjusted for sex, age and deprivation index. For in vitro studies, HEK293 cells were co-transfected with ACE2 and either TMPRSS2 wild type or mutant (TMPRSS2V160M). A SARS-CoV-2 pseudovirus entry assay was used to investigate the ability of TMPRSS2V160M to promote viral entry.

RESULTS

We show that the T allele of rs12329760 is associated with a reduced likelihood of developing severe COVID-19 (OR 0.87, 95%CI:0.79-0.97, p = 0.01). This association was stronger in homozygous individuals when compared to the general population (OR 0.65, 95%CI:0.50-0.84, p = 1.3 × 10-3). We demonstrate in vitro that this variant, which causes the amino acid substitution valine to methionine, affects the catalytic activity of TMPRSS2 and is less able to support SARS-CoV-2 spike-mediated entry into cells.

CONCLUSION

TMPRSS2 rs12329760 is a common variant associated with a significantly decreased risk of severe COVID-19. Further studies are needed to assess the expression of TMPRSS2 across different age groups. Moreover, our results identify TMPRSS2 as a promising drug target, with a potential role for camostat mesilate, a drug approved for the treatment of chronic pancreatitis and postoperative reflux esophagitis, in the treatment of COVID-19. Clinical trials are needed to confirm this.

Trypsin-Like Proteases and Their Role in Muco-Obstructive Lung Diseases

Trypsin-like proteases (TLPs) belong to a family of serine enzymes with primary substrate specificities for the basic residues, lysine and arginine, in the P1 position. Whilst initially perceived as soluble enzymes that are extracellularly secreted, a number of novel TLPs that are anchored in the cell membrane have since been discovered. Muco-obstructive lung diseases (MucOLDs) are characterised by the accumulation of hyper-concentrated mucus in the small airways, leading to persistent inflammation, infection and dysregulated protease activity. Although neutrophilic serine proteases, particularly neutrophil elastase, have been implicated in the propagation of inflammation and local tissue destruction, it is likely that the serine TLPs also contribute to various disease-relevant processes given the roles that a number of these enzymes play in the activation of both the epithelial sodium channel (ENaC) and protease-activated receptor 2 (PAR2). More recently, significant attention has focused on the activation of viruses such as SARS-CoV-2 by host TLPs. The purpose of this review was to highlight key TLPs linked to the activation of ENaC and PAR2 and their association with airway dehydration and inflammatory signalling pathways, respectively. The role of TLPs in viral infectivity will also be discussed in the context of the inhibition of TLP activities and the potential of these proteases as therapeutic targets.

Proteases, Mucus, and Mucosal Immunity in Chronic Lung Disease

Dysregulated protease activity has long been implicated in the pathogenesis of chronic lung diseases and especially in conditions that display mucus obstruction, such as chronic obstructive pulmonary disease, cystic fibrosis, and non-cystic fibrosis bronchiectasis. However, our appreciation of the roles of proteases in various aspects of such diseases continues to grow. Patients with muco-obstructive lung disease experience progressive spirals of inflammation, mucostasis, airway infection and lung function decline. Some therapies exist for the treatment of these symptoms, but they are unable to halt disease progression and patients may benefit from novel adjunct therapies. In this review, we highlight how proteases act as multifunctional enzymes that are vital for normal airway homeostasis but, when their activity becomes immoderate, also directly contribute to airway dysfunction, and impair the processes that could resolve disease. We focus on how proteases regulate the state of mucus at the airway surface, impair mucociliary clearance and ultimately, promote mucostasis. We discuss how, in parallel, proteases are able to promote an inflammatory environment in the airways by mediating proinflammatory signalling, compromising host defence mechanisms and perpetuating their own proteolytic activity causing structural lung damage. Finally, we discuss some possible reasons for the clinical inefficacy of protease inhibitors to date and propose that, especially in a combination therapy approach, proteases represent attractive therapeutic targets for muco-obstructive lung diseases.

Leveraging 3D Model Systems to Understand Viral Interactions with the Respiratory Mucosa

Respiratory viruses remain a significant cause of morbidity and mortality in the human population, underscoring the importance of ongoing basic research into virus-host interactions. However, many critical aspects of infection are difficult, if not impossible, to probe using standard cell lines, 2D culture formats, or even animal models. In vitro systems such as airway epithelial cultures at air-liquid interface, organoids, or 'on-chip' technologies allow interrogation in human cells and recapitulate emergent properties of the airway epithelium-the primary target for respiratory virus infection. While some of these models have been used for over thirty years, ongoing advancements in both culture techniques and analytical tools continue to provide new opportunities to investigate airway epithelial biology and viral infection phenotypes in both normal and diseased host backgrounds. Here we review these models and their application to studying respiratory viruses. Furthermore, given the ability of these systems to recapitulate the extracellular microenvironment, we evaluate their potential to serve as a platform for studies specifically addressing viral interactions at the mucosal surface and detail techniques that can be employed to expand our understanding.

Innate Immune Signaling and Proteolytic Pathways in the Resolution or Exacerbation of SARS-CoV-2 in Covid-19: Key Therapeutic Targets?

COVID-19 is caused by the Severe Acute Respiratory Syndrome (SARS) coronavirus (Cov)-2, an enveloped virus with a positive-polarity, single-stranded RNA genome. The initial outbreak of the pandemic began in December 2019, and it is affecting the human health of the global community. In common with previous pandemics (Influenza H1N1 and SARS-CoV) and the epidemics of Middle east respiratory syndrome (MERS)-CoV, CoVs target bronchial and alveolar epithelial cells. Virus protein ligands (e.g., haemagglutinin or trimeric spike glycoprotein for Influenza and CoV, respectively) interact with cellular receptors, such as (depending on the virus) either sialic acids, Dipeptidyl peptidase 4 (DPP4), or angiotensin-converting enzyme 2 (ACE2). Host proteases, e.g., cathepsins, furin, or members of the type II transmembrane serine proteases (TTSP) family, such as Transmembrane protease serine 2 (TMPRSS2), are involved in virus entry by proteolytically activating virus ligands. Also involved are Toll Like Receptor (TLR) family members, which upregulate anti-viral and pro-inflammatory mediators [interleukin (IL)-6 and IL-8 and type I and type III Interferons among others], through the activation of Nuclear Factor (NF)-kB. When these events (virus cellular entry and innate immune responses) are uncontrolled, a deleterious systemic response is sometimes encountered in infected patients, leading to the well-described "cytokine storm" and an ensuing multiple organ failure promoted by a downregulation of dendritic cell, macrophage, and T-cell function. We aim to describe how the lung and systemic host innate immune responses affect survival either positively, through downregulating initial viral load, or negatively, by triggering uncontrolled inflammation. An emphasis will be put on host cellular signaling pathways and proteases involved with a view on tackling these therapeutically.

Retracted Article: Knockdown of TMPRSS11D inhibits the proliferation, migration and invasion of cervical cancer cells

TMPRSS11D is a member of the type II transmembrane serine proteases (TTSPs) family that is implicated in the development and progression of several cancers. However, the biological roles of TMPRSS11D in cervical cancer have not been investigated. In the present study, we detected the expression levels of TMPRSS11D in human cervical cancer tissues and cell lines. The results showed that TMPRSS11D expression was significantly upregulated in cervical cancer tissues as compared to the adjacent normal tissues. Besides, TMPRSS11D was highly expressed in human cervical cancer cell lines. Then we knocked down TMPRSS11D in cervical cancer cell lines to evaluate the effects of TMPRSS11D knockdown on cervical cancer cells. The results showed that knockdown of TMPRSS11D significantly suppressed cell proliferation, migration and invasion in cervical cancer cell lines. Furthermore, the data revealed that TMPRSS11D knockdown prevented epithelial–mesenchymal transition (EMT), as proved by the increased E-cadherin expression, as well as decreased N-cadherin and fibronectin expressions. Additionally, knockdown of TMPRSS11D inhibited the activation of the PI3K/Akt pathway in cervical cancer cells. Furthermore, insulin-like growth factor-1 (IGF-1) treatment reversed the inhibitory effects of TMPRSS11D knockdown on cell proliferation and migration. Collectively, knockdown of TMPRSS11D exerted anti-tumor activity, at least in part, via inhibiting the PI3K/Akt pathway. These findings indicated that TMPRSS11D might serve as a novel therapeutic target for the treatment of cervical cancer.

TMPRSS11D is a member of the type II transmembrane serine proteases (TTSPs) family that is implicated in the development and progression of several cancers.

Human airway trypsin-like protease enhances interleukin-8 synthesis in bronchial epithelial cells by activating protease-activated receptor 2

Human airway trypsin-like protease (HAT) localizes at human bronchial epithelial cells (HBECs). HAT enhanced release of interleukin-8 (IL-8) from HBECs at 10-100 mU/mL and the enhanced release was almost completely abolished by 50 μM leupeptin, a serine protease inhibitor. Previous reports suggested that HAT displays its physiological functions via protease-activated receptor 2 (PAR2). In the present study, we examined the mechanism whereby HAT upregulates IL-8 synthesis in HBECs with a focus on PAR2. Northern blot analysis revealed that HAT enhanced IL-8 mRNA expression at concentrations of 10-100 mU/mL. PAR2 activating peptide (PAR2 AP) also enhanced IL-8 release and IL-8 mRNA expression in HBECs at 50-1,000 μM at similar levels as HAT. Knockdown of PAR2 mRNA by siRNA methods showed that PAR2 mRNA expression was significantly depressed in primary HBECs, and both HAT- and PAR2 AP-induced IL-8 mRNA elevation was significantly depressed in PAR2 siRNA-transfected HBECs. Additionally, HAT cleaved the PAR2 activating site (R³⁶-S³⁷ bond) of synthetic PAR2 N-terminal peptide. These results indicate that HAT stimulates IL-8 synthesis in airway epithelial cells via PAR2 and could help to amplify inflammation in chronic respiratory tract disease.

Membrane-Anchored Serine Proteases: Host Cell Factors in Proteolytic Activation of Viral Glycoproteins

Over one third of all known proteolytic enzymes are serine proteases. Among these, the trypsin-like serine proteases comprise one of the best characterized subfamilies due to their essential roles in blood coagulation, food digestion, fibrinolysis, or immunity. Trypsin-like serine proteases possess primary substrate specificity for basic amino acids. Most of the well-characterized trypsin-like proteases such as trypsin, plasmin, or urokinase are soluble proteases that are secreted into the extracellular environment. At the turn of the millennium, a number of novel trypsin-like serine proteases have been identified that are anchored in the cell membrane, either by a transmembrane domain at the N- or C-terminus or via a glycosylphosphatidylinositol (GPI) linkage. Meanwhile more than 20 membrane-anchored serine proteases (MASPs) have been identified in human and mouse, and some of them have emerged as key regulators of mammalian development and homeostasis. Thus, the MASP corin and TMPRSS6/matriptase-2 have been demonstrated to be the activators of the atrial natriuretic peptide (ANP) and key regulator of hepcidin expression, respectively. Furthermore, MASPs have been recognized as host cell factors activating respiratory viruses including influenza virus as well as severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) coronaviruses. In particular, transmembrane protease serine S1 member 2 (TMPRSS2) has been shown to be essential for proteolytic activation and consequently spread and pathogenesis of a number of influenza A viruses in mice and as a factor associated with severe influenza virus infection in humans.

This review gives an overview on the physiological functions of the fascinating and rapidly evolving group of MASPs and a summary of the current knowledge on their role in proteolytic activation of viral fusion proteins.

High TMPRSS11D protein expression predicts poor overall survival in non-small cell lung cancer

TMPRSS11D (HAT) belongs to the large type II transmembrane serine protease (TTSP) family, participating in various biological and physiological processes. TMPRSS11D expression has been reported during squamous cell carcinogenesis, however, its expression during non-small cell lung cancer (NSCLC) development has not been studied. In this study, we determined the mRNA and protein expression of TMPRSS11D in NSCLC tumorous and matched adjacent normal tissues by quantitative reverse transcription PCR (qRT-PCR) and tissue microarray immunohistochemistry analysis (TMA-IHC) respectively. TMPRSS11D protein expression in tumorous tissues were correlated with NSCLC patients’ clinical characteristics and overall survival. Both TMPRSS11D mRNA and protein expression levels were significantly higher in NSCLC tumorous tissues than in adjacent normal tissues. High TMPRSS11D protein expression was associated with high TNM stages, and high TMPRSS11D protein expression is an independent prognostic marker in NSCLC. Based on our results, we conclude that TMPRSS11D could play a role in NSCLC development and progression. Because of its role in proteolysis of extracellular matrix, targeting TMPRSS11D may prevent the development of metastasis in NSCLC.

Protease-activated receptor 2 activates airway apical membrane chloride permeability and increases ciliary beating

Mucociliary clearance, driven by the engine of ciliary beating, is the primary physical airway defense against inhaled pathogens and irritants. A better understanding of the regulation of ciliary beating and mucociliary transport is necessary for identifying new receptor targets to stimulate improved clearance in airway diseases, such as cystic fibrosis and chronic rhinosinusitis. In this study, we examined the protease-activated receptor (PAR)-2, a GPCR previously shown to regulate airway cell cytokine and mucus secretion, and transepithelial Cl^- current. PAR-2 is activated by proteases secreted by airway neutrophils and pathogens. We cultured various airway cell lines, primary human and mouse sinonasal cells, and human bronchial cells at air-liquid interface and examined them using molecular biology, biochemistry, and live-cell imaging. We found that PAR-2 is expressed basolaterally, where it stimulates both intracellular Ca²⁺ release and Ca²⁺ influx, which activates low-level nitric oxide production, increases apical membrane Cl^- permeability ∼3-5-fold, and increases ciliary beating ∼20-50%. No molecular or functional evidence of PAR-4 was observed. These data suggest a novel and previously overlooked role of PAR-2 in airway physiology, adding to our understanding of the role of this receptor in airway Ca²⁺ signaling and innate immunity.-McMahon, D. B., Workman, A. D., Kohanski, M. A., Carey, R. M., Freund, J. R., Hariri, B. M., Chen, B., Doghramji, L. J., Adappa, N. D., Palmer, J. N., Kennedy, D. W., Lee, R. J. Protease-activated receptor 2 activates airway apical membrane chloride permeability and increases ciliary beating.

Inhibition of Protease-Epithelial Sodium Channel Signaling Improves Mucociliary Function in Cystic Fibrosis Airways

RATIONALE

In cystic fibrosis (CF) a reduction in airway surface liquid (ASL) height compromises mucociliary clearance, favoring mucus plugging and chronic bacterial infection. Inhibitors of the epithelial sodium channel (ENaC) have therapeutic potential in CF airways to reduce hyperstimulated sodium and fluid absorption to levels that can restore airway hydration.

OBJECTIVES

To determine whether a novel compound (QUB-TL1) designed to inhibit protease/ENaC signaling in CF airways restores ASL volume and mucociliary function.

METHODS

Protease activity was measured using fluorogenic activity assays. Differentiated primary airway epithelial cell cultures (F508del homozygotes) were used to determined ENaC activity (Ussing chamber recordings), ASL height (confocal microscopy), and mucociliary function (by tracking the surface flow of apically applied microbeads). Cell toxicity was measured using a lactate dehydrogenase assay.

MEASUREMENTS AND MAIN RESULTS

QUB-TL1 inhibits extracellularly located channel activating proteases (CAPs), including prostasin, matriptase, and furin, the activities of which are observed at excessive levels at the apical surface of CF airway epithelial cells. QUB-TL1-mediated CAP inhibition results in diminished ENaC-mediated Na(+) absorption in CF airway epithelial cells caused by internalization of a prominent pool of cleaved (active) ENaCγ from the cell surface. Importantly, diminished ENaC activity correlates with improved airway hydration status and mucociliary clearance. We further demonstrate QUB-TL1-mediated furin inhibition, which is in contrast to other serine protease inhibitors (camostat mesylate and aprotinin), affords protection against neutrophil elastase-mediated ENaC activation and Pseudomonas aeruginosa exotoxin A-induced cell death.

CONCLUSIONS

QUB-TL1 corrects aberrant CAP activities, providing a mechanism to delay or prevent the development of CF lung disease in a manner independent of CF transmembrane conductance regulator mutation.

Human airway trypsin-like protease, a serine protease involved in respiratory diseases

More than 2% of all human genes are coding for a complex system of more than 700 proteases and protease inhibitors. Among them, serine proteases play extraordinary, diverse functions in different physiological and pathological processes. The human airway trypsin-like protease (HAT), also referred to as TMPRSS11D and serine 11D, belongs to the emerging family of cell surface proteolytic enzymes, the type II transmembrane serine proteases (TTSPs). Through the cleavage of its four major identified substrates, HAT triggers specific responses, notably in epithelial cells, within the pericellular and extracellular environment, including notably inflammatory cytokine production, inflammatory cell recruitment, or anticoagulant processes. This review summarizes the potential role of this recently described protease in mediating cell surface proteolytic events, to highlight the structural features, proteolytic activity, and regulation, including the expression profile of HAT, and discuss its possible roles in respiratory physiology and disease.

Role of Proteases in Lung Disease: A Brief Overview

Proteases play an important role in health and disease of the lung. In the normal lungs, proteases maintain their homeostatic functions that regulate processes like its regeneration and repair. Dysregulation of proteases–antiproteases balance is crucial in the manifestation of different types of lung diseases. Chronic inflammatory lung pathologies are associated with a marked increase in protease activities. Thus, in addition to protease activities, inhibition of anti-proteolytic control mechanisms are also important for effective microbial infection and inflammation in the lung. Herein, we briefly summarize the role of different proteases and to some extent antiproteases in regulating a variety of lung diseases.

Cell Surface Human Airway Trypsin-Like Protease Is Lost During Squamous Cell Carcinogenesis

Cancer progression is accompanied by increased levels of extracellular proteases that are capable of remodeling the extracellular matrix, as well as cleaving and activating growth factors and receptors that are involved in pro‐cancerous signaling pathways. Several members of the type II transmembrane serine protease (TTSP) family have been shown to play critical roles in cancer progression, however, the expression or function of the TTSP Human Airway Trypsin‐like protease (HAT) in carcinogenesis has not been examined. In the present study we aimed to determine the expression of HAT during squamous cell carcinogenesis. HAT transcript is present in several tissues containing stratified squamous epithelium and decreased expression is observed in carcinomas. We determined that HAT protein is consistently expressed on the cell surface in suprabasal/apical layers of squamous cells in healthy cervical and esophageal epithelia. To assess whether HAT protein is differentially expressed in normal tissue versus tissue in different stages of carcinogenesis, we performed a comprehensive immunohistochemical analysis of HAT protein expression levels and localization in arrays of paraffin embedded human cervical and esophageal carcinomas compared to the corresponding normal tissue. We found that HAT protein is expressed in the non‐proliferating, differentiated cellular strata and is lost during the dedifferentiation of epithelial cells, a hallmark of squamous cell carcinogenesis. Thus, HAT expression may potentially be useful as a marker for clinical grading and assessment of patient prognosis in squamous cell carcinomas. J. Cell. Physiol. 231: 1476–1483, 2016. © 2015 Wiley Periodicals, Inc.

Divergent Inhibitor Susceptibility among Airway Lumen-Accessible Tryptic Proteases

Tryptic serine proteases of bronchial epithelium regulate ion flux, barrier integrity, and allergic inflammation. Inhibition of some of these proteases is a strategy to improve mucociliary function in cystic fibrosis and asthmatic inflammation. Several inhibitors have been tested in pre-clinical animal models and humans. We hypothesized that these inhibitors inactivate a variety of airway protease targets, potentially with bystander effects. To establish relative potencies and modes of action, we compared inactivation of human prostasin, matriptase, airway trypsin-like protease (HAT), and β-tryptase by nafamostat, camostat, bis(5-amidino-2-benzimidazolyl)methane (BABIM), aprotinin, and benzamidine. Nafamostat achieved complete, nearly stoichiometric and very slowly reversible inhibition of matriptase and tryptase, but inhibited prostasin less potently and was weakest versus HAT. The IC₅₀ of nafamostat’s leaving group, 6-amidino-2-naphthol, was >10⁴-fold higher than that of nafamostat itself, consistent with suicide rather than product inhibition as mechanisms of prolonged inactivation. Stoichiometric release of 6-amidino-2-naphthol allowed highly sensitive fluorometric estimation of active-site concentration in preparations of matriptase and tryptase. Camostat inactivated all enzymes but was less potent overall and weakest towards matriptase, which, however was strongly inhibited by BABIM. Aprotinin exhibited nearly stoichiometric inhibition of prostasin and matriptase, but was much weaker towards HAT and was completely ineffective versus tryptase. Benzamidine was universally weak. Thus, each inhibitor profile was distinct. Nafamostat, camostat and aprotinin markedly reduced tryptic activity on the apical surface of cystic fibrosis airway epithelial monolayers, suggesting prostasin as the major source of such activity and supporting strategies targeting prostasin for inactivation.

The serine protease inhibitor camostat inhibits influenza virus replication and cytokine production in primary cultures of human tracheal epithelial cells

BACKGROUND

Serine proteases act through the proteolytic cleavage of the hemagglutinin (HA) of influenza viruses for the entry of influenza virus into cells, resulting in infection. However, the inhibitory effects of serine protease inhibitors on influenza virus infection of human airway epithelial cells, and on their production of inflammatory cytokines are unclear.

METHODS

Primary cultures of human tracheal epithelial cells were treated with four types of serine protease inhibitors, including camostat, and infected with A/Sendai-H/108/2009/(H1N1) pdm09 or A/New York/55/2004(H3N2).

RESULTS

Camostat reduced the amounts of influenza viruses in the supernatants and viral RNA in the cells. It reduced the cleavage of an influenza virus precursor protein, HA0, into the subunit HA1. Camostat also reduced the concentrations of the cytokines interleukin (IL)-6 and tumor necrosis factor (TNF)-α in the supernatants. Gabexate and aprotinin reduced the viral titers and RNA levels in the cells, and aprotinin reduced the concentrations of TNF-α in the supernatants. The proteases transmembrane protease serine S1 member (TMPRSS) 2 and HAT (human trypsin-like protease: TMPRSS11D), which are known to cleave HA0 and to activate the virus, were detected at the cell membrane and in the cytoplasm. mRNA encoding TMPRSS2, TMPRSS4 and TMPRSS11D was detectable in the cells, and the expression levels were not affected by camostat.

CONCLUSIONS

These findings suggest that human airway epithelial cells express these serine proteases and that serine protease inhibitors, especially camostat, may reduce influenza viral replication and the resultant production of inflammatory cytokines possibly through inhibition of activities of these proteases.

Respiratory protease/antiprotease balance determines susceptibility to viral infection and can be modified by nutritional antioxidants

The respiratory epithelium functions as a central orchestrator to initiate and organize responses to inhaled stimuli. Proteases and antiproteases are secreted from the respiratory epithelium and are involved in respiratory homeostasis. Modifications to the protease/antiprotease balance can lead to the development of lung diseases such as emphysema or chronic obstructive pulmonary disease. Furthermore, altered protease/antiprotease balance, in favor for increased protease activity, is associated with increased susceptibility to respiratory viral infections such as influenza virus. However, nutritional antioxidants induce antiprotease expression/secretion and decrease protease expression/activity, to protect against viral infection. As such, this review will elucidate the impact of this balance in the context of respiratory viral infection and lung disease, to further highlight the role epithelial cell-derived proteases and antiproteases contribute to respiratory immune function. Furthermore, this review will offer the use of nutritional antioxidants as possible therapeutics to boost respiratory mucosal responses and/or protect against infection.

Hepatocyte growth factor activator inhibitor type 1 maintains the assembly of keratin into desmosomes in keratinocytes by regulating protease-activated receptor 2-dependent p38 signaling

Hepatocyte growth factor activator inhibitor type 1 (HAI-1; official symbol SPINT1) is a membrane-associated serine proteinase inhibitor abundantly expressed in epithelial tissues. Genetically engineered mouse models demonstrated that HAI-1 is critical for epidermal function, possibly through direct and indirect regulation of cell surface proteases, such as matriptase and prostasin. To obtain a better understanding of the role of HAI-1 in maintaining epidermal integrity, we performed ultrastructural analysis of Spint1-deleted mouse epidermis and organotypic culture of an HAI-1 knockdown (KD) human keratinocyte cell line, HaCaT. We found that the aggregation of tonofilaments to desmosomes was significantly reduced in HAI-1-deficient mouse epidermis with decreased desmosome number. Similar findings were observed in HAI-1 KD HaCaT organotypic cultures. Immunoblot and immunohistochemical analyses revealed that p38 mitogen-activated protein kinase was activated in response to HAI-1 insufficiency. Treatment of HAI-1 KD HaCaT cells with a p38 inhibitor abrogated the above-observed ultrastructural abnormalities. The activation of p38 induced by the loss of HAI-1 likely resulted from enhanced signaling of protease-activated receptor-2 (PAR-2), because its silencing abrogated the enhanced activation of p38. Consequently, treatment of HAI-1 KD HaCaT cells with a serine protease inhibitor, aprotinin, or PAR-2 antagonist alleviated the abnormal ultrastructural phenotype in organotypic culture. These results suggest that HAI-1 may have a critical role in maintaining normal keratinocyte morphology through regulation of PAR-2-dependent p38 mitogen-activated protein kinase signaling.

Nuclear erythroid 2 p45-related factor-2 Nrf2 ameliorates cigarette smoking-induced mucus overproduction in airway epithelium and mouse lungs

BACKGROUND AND OBJECTIVE

Nuclear erythroid 2 p45-related factor-2 (Nrf2) is known to play important roles in airway disorders, whereas little has been investigated about its direct role in airway mucus hypersecretion. The aim of this study is to determine whether this factor could protect pulmonary epithelium and mouse airway from cigarette-induced mucus overproduction.

METHODS

Using genetic approaches, the role of Nrf2 on cigarette smoking extracts (CSE) induced MUC5AC expression was investigated in lung A549 cells. Nrf2 deficiency mice were smoked for various periods, and the airway inflammation and mucus production was characterized.

RESULTS

Acute smoking exposure induced expression of MUC5AC and Nrf2 in both A549 cells and mouse lungs. Genetic ablation of Nrf2 augmented, whereas overexpression of this molecule ameliorated CSE-induced expression of MUC5AC. Nrf2 knockout mice, after exposure to cigarette smoking, displayed enhanced airway inflammation and mucus production.

CONCLUSION

Nrf2 negatively regulated smoking-induced mucus production in vitro and in vivo, suggesting therapeutic potentials of this factor in airway diseases with hypersecreted mucus.

Non-hematopoietic PAR-2 is essential for matriptase-driven pre-malignant progression and potentiation of ras-mediated squamous cell carcinogenesis

The membrane-anchored serine protease, matriptase, is consistently dysregulated in a range of human carcinomas, and high matriptase activity correlates with poor prognosis. Furthermore, matriptase is unique among tumor-associated proteases in that epithelial stem cell expression of the protease suffices to induce malignant transformation. Here, we use genetic epistasis analysis to identify proteinase-activated receptor (PAR)-2-dependent inflammatory signaling as an essential component of matriptase-mediated oncogenesis. In cell-based assays, matriptase was a potent activator of PAR-2, and PAR-2 activation by matriptase caused robust induction of nuclear factor (NF)κB through Gαi. Importantly, genetic elimination of PAR-2 from mice completely prevented matriptase-induced pre-malignant progression, including inflammatory cytokine production, inflammatory cell recruitment, epidermal hyperplasia and dermal fibrosis. Selective ablation of PAR-2 from bone marrow-derived cells did not prevent matriptase-driven pre-malignant progression, indicating that matriptase activates keratinocyte stem cell PAR-2 to elicit its pro-inflammatory and pro-tumorigenic effects. When combined with previous studies, our data suggest that dual induction of PAR-2-NFκB inflammatory signaling and PI3K-Akt-mTor survival/proliferative signaling underlies the transforming potential of matriptase and may contribute to pro-tumorigenic signaling in human epithelial carcinogenesis.

Amphiregulin

Amphiregulin (AREG) is a ligand of the epidermal growth factor receptor (EGFR), a widely expressed transmembrane tyrosine kinase. AREG is synthesized as a membrane-anchored precursor protein that can engage in juxtacrine signaling on adjacent cells. Alternatively, after proteolytic processing by cell membrane proteases, mainly TACE/ADAM17, AREG is secreted and behaves as an autocrine or paracrine factor. AREG gene expression and release is induced by a plethora of stimuli including inflammatory lipids, cytokines, hormones, growth factors and xenobiotics. Through EGFR binding AREG activates major intracellular signaling cascades governing cell survival, proliferation and motility. Physiologically, AREG plays an important role in the development and maturation of mammary glands, bone tissue and oocytes. Chronic elevation of AREG expression is increasingly associated with different pathological conditions, mostly of inflammatory and/or neoplastic nature. Here we review the essential aspects of AREG structure, function and regulation, discuss the basis for its differential role within the EGFR family of ligands, and identify emerging aspects in AREG research with translational potential.

Activation of influenza viruses by proteases from host cells and bacteria in the human airway epithelium

Influenza is an acute infection of the respiratory tract, which affects each year millions of people. Influenza virus infection is initiated by the surface glycoprotein hemagglutinin (HA) through receptor binding and fusion of viral and endosomal membranes. HA is synthesized as a precursor protein and requires cleavage by host cell proteases to gain its fusion capacity. Although cleavage of HA is crucial for virus infectivity, little was known about relevant proteases in the human airways for a long time. Recent progress in the identification and characterization of HA‐activating host cell proteases has been considerable however and supports the idea of targeting HA cleavage as a novel approach for influenza treatment. Interestingly, certain bacteria have been demonstrated to support HA activation either by secreting proteases that cleave HA or due to activation of cellular proteases and thereby may contribute to virus spread and enhanced pathogenicity. In this review, we give an overview on activation of influenza viruses by proteases from host cells and bacteria with the main focus on recent progress on HA cleavage by proteases HAT and TMPRSS2 in the human airway epithelium. In addition, we outline investigations of HA‐activating proteases as potential drug targets for influenza treatment.

The authors, who are leading experts in this field, present a timely, authoritative review on the proteolytic cleavage of the influenza hemagglutinin (HA), an activation mechanism that is essential for the infectivity of influenza viruses, including the recently emerged H7N9. They also address the potential of host proteases as targets for developing new influenza drugs. This review will be of considerable interest to virologists, microbiologists and pharmaceutical companies alike.

Genetic determinants of reovirus pathogenesis in a murine model of respiratory infection

Many viruses invade mucosal surfaces to establish infection in the host. Some viruses are restricted to mucosal surfaces, whereas others disseminate to sites of secondary replication. Studies of strain-specific differences in reovirus mucosal infection and systemic dissemination have enhanced an understanding of viral determinants and molecular mechanisms that regulate viral pathogenesis. After peroral inoculation, reovirus strain type 1 Lang replicates to high titers in the intestine and spreads systemically, whereas strain type 3 Dearing (T3D) does not. These differences segregate with the viral S1 gene segment, which encodes attachment protein σ1 and nonstructural protein σ1s. In this study, we define genetic determinants that regulate reovirus-induced pathology following intranasal inoculation and respiratory infection. We report that two laboratory isolates of T3D, T3D(C) and T3D(F), differ in the capacity to replicate in the respiratory tract and spread systemically; the T3D(C) isolate replicates to higher titers in the lungs and disseminates, while T3D(F) does not. Two nucleotide polymorphisms in the S1 gene influence these differences, and both S1 gene products are involved. T3D(C) amino acid polymorphisms in the tail and head domains of σ1 protein influence the sensitivity of virions to protease-mediated loss of infectivity. The T3D(C) polymorphism at nucleotide 77, which leads to coding changes in both S1 gene products, promotes systemic dissemination from the respiratory tract. A σ1s-null virus produces lower titers in the lung after intranasal inoculation and disseminates less efficiently to sites of secondary replication. These findings provide new insights into mechanisms underlying reovirus replication in the respiratory tract and systemic spread from the lung.

P. aeruginosa lipopolysaccharide-induced MUC5AC and CLCA3 expression is partly through Duox1 in vitro and in vivo

BACKGROUND

We have previously found that reactive oxygen species (ROS) are involved in Pseudomonas aeruginosa lipopolysaccharide (PA-LPS) induced MUC5AC in airway epithelial cells. Dual oxidase1 (Duox1), a member of NADPH oxidase(Nox), is known to be responsible for ROS production in respiratory tract epithelial cells. Our aim was to clarify whether Duox1 was also involved in the PA-LPS-induced MUC5AC and calcium dependent chloride channel 3(Clca3), another recognized marker of goblet cell hyperplasia and mucus hyper-production.

METHODS

PA-LPS-induced Duox1 mRNA levels were examined in A549 cells, primary mouse tracheal epithelial cells (mTECS) and lung tissues of mice. Nox inhibitors diphenyleneiodonium chloride (DPI) and Duox1 siRNA were used to investigate whether Duox1 is involved in PA-LPS-induced MUC5AC and Clca3 expression both in vitro and in vivo.

RESULTS

Duox1 is induced by PA-LPS in A549 cells, primary mTECs and lung tissues of mice. DPI significantly inhibited PA-LPS-induced up-regulation of Duox1, Muc5ac and Clca3 in primary mouse trachea epithelial cells and lung tissues of mice. Knockdown of Duox1 markedly inhibited PA-LPS-induced MUC5AC expression via a ROS-TGF-α cascade in A549 cells. Furthermore, DPI significantly inhibited PA-LPS-induced increases in inflammatory cells accumulated in mouse lungs.

CONCLUSIONS

We demonstrate for the first time that PA-LPS-induced MUC5AC and Clca3 expression is partly through Duox1, and provide supportive evidence for Duox1 as a potential target in treatments of mucin over-production diseases.

Human airway trypsin-like protease induces mucin5AC hypersecretion via a protease-activated receptor 2-mediated pathway in human airway epithelial cells

Mucus hypersecretion is a common feature in chronic airway diseases, and serine proteases play a critical role in this process. However, the mechanisms by which serine proteases induce mucin5AC hypersecretion have not been fully explored. In this study, we characterized human airway trypsin-like protease (HAT), a serine protease that is found in the mucoid sputum of patients with chronic airway diseases and is an agonist of protease-activated receptor 2 (PAR2)-induced cellular responses in human bronchial epithelial cells (16HBE). We also investigated the potential involvement of PAR2 in this process. We found that both HAT and PAR2-AP enhance the exocytosis of mucin5AC protein, whereas HAT, but not PAR2-AP, enhances the expression of mucin5AC mRNA. PAR2 is expressed at a much higher level in the cells than the other three PARs. Transfection with an siRNA against the PAR2 receptor or Gαq/11 protein or pretreatment with the Gαq/11 protein inhibitor YM-254890, the PLC inhibitor U73122 or the intracellular Ca(2+) chelator BAPTA-AM all effectively attenuated the HAT-induced cellular responses. Taken together, these results indicate that HAT can stimulate mucin5AC hypersecretion through a PAR2-mediated signaling pathway in 16HBE cells. Thus, PAR2 could represent a novel therapeutic target for chronic airway diseases with mucus hypersecretion.

Influence of surface charge and inner composition of nanoparticles on intracellular delivery of proteins in airway epithelial cells

The delivery of protein in the airway using nanoparticles (NP) is an emerging strategy that shows encouraging results in vivo for several applications. However, the mechanisms by which NP deliver proteins to the inside of cells remain poorly understood. In this study, we investigated the intracellular delivery of ovalbumin (OVA) in human airway cells by two porous cationic polysaccharides nanoparticles. These NP have the same surface charge density but differ in that their inner core contains either cationic or anionic charges (respectively: NP(+) and DGNP(+)). Confocal microscopy showed a rapid uptake of both NP by human airway cells, followed by a significant accumulation in clathrin vesicles and early endosomes. Both NP were found to associate OVA in a quantitative manner, and this association was stable even in presence of serum proteins. We observed that the two NP greatly increased OVA uptake by human airway cells, meanwhile FRET studies using FITC-labelled NP and TRITC-labelled OVA showed a gradual release of OVA from NP within cells, and this was much faster with DGNP(+) than NP(+). These results were confirmed using OVA-DQ to follow OVA degradation fragments within cells. Both NP increased intracellular proteolysis of OVA, however DGNP(+) facilitated OVA escape from endosomes. Studies with trypsin and pepsin at different pH strongly suggested that both NP can protect (in the extracellular medium) or promote (in acidic endosomes) protein proteolysis, depending on the environment. Interestingly, the mechanisms involved could be explained as a function of protein global charge at different pH. All these results confirm the importance of not only the surface charge but also the inner composition of NP in determining their efficacy as tools for the delivery of proteins to different cellular compartments.

Protease-activated receptor 2 mediates mucus secretion in the airway submucosal gland

Protease-activated receptor 2 (PAR2), a G protein-coupled receptor expressed in airway epithelia and smooth muscle, plays an important role in airway inflammation. In this study, we demonstrated that activation of PAR2 induces mucus secretion from the human airway gland and examined the underlying mechanism using the porcine and murine airway glands. The mucosa with underlying submucosal glands were dissected from the cartilage of tissues, pinned with the mucosal side up at the gas/bath solution interface of a physiological chamber, and covered with oil so that secretions from individual glands could be visualized as spherical bubbles in the oil. Secretion rates were determined by optical monitoring of the bubble diameter. The Ca(2+)-sensitive dye Fura2-AM was used to determine intracellular Ca(2+) concentration ([Ca(2+)](i)) by means of spectrofluorometry. Stimulation of human tracheal mucosa with PAR2-activating peptide (PAR2-AP) elevated intracellular Ca(2+) and induced glandular secretion equal to approximately 30% of the carbachol response in the human airway. Porcine gland tissue was more sensitive to PAR2-AP, and this response was dependent on Ca(2+) and anion secretion. When the mouse trachea were exposed to PAR2-AP, large amounts of secretion were observed in both wild type and ΔF508 cystic fibrosis transmembrane conductance regulator mutant mice but there is no secretion from PAR-2 knock out mice. In conclusion, PAR2-AP is an agonist for mucus secretion from the airway gland that is Ca(2+)-dependent and cystic fibrosis transmembrane conductance regulator-independent.

Influenza A induces the major secreted airway mucin MUC5AC in a protease-EGFR-extracellular regulated kinase-Sp1-dependent pathway

Mucins, the main glycoproteins present within mucus, modulate the rheologic properties of airways and participate in lung defense. They are thought to be able to trap and eliminate microorganisms from the lung. Among the mucins secreted in the lung, MUC5AC is the most prominent factor secreted by surface epithelial cells. Although much is known about the signaling pathways involved in the regulation of MUC5AC by host factors such as cytokines or proteases, less is known about the pathways triggered by microorganisms and, specifically, by influenza A virus (IAV). We therefore set up experiments to dissect the molecular mechanisms responsible for the potential modulation of MUC5AC by IAV. Using epithelial cells, C57/Bl6 mice, and IAV strains, we measured MUC5AC expression at the RNA and protein levels, specificity protein 1 (Sp1) activation, and protease activity. Intermediate molecular partners were confirmed using pharmacological inhibitors, blocking antibodies, and small interfering (si)RNAs. We showed in vitro and in vivo that IAV up-regulates epithelial cell-derived MUC5AC and Muc5ac expression in mice, both at transcriptional (through the induction of Sp1) and translational levels. In addition, we determined that this induction was dependent on a protease-epithelial growth factor receptor-extracellular regulated kinase-Sp1 signaling cascade, involving in particular the human airway trypsin. Our data point to MUC5AC as a potential modulatory mechanism by which the lung epithelia respond to IAV infection, and we dissect, for the first time to the best of our knowledge, the molecular partners involved. Future experiments using MUC5AC-targeted strategies should help further unravel the pathophysiological consequences of IAV-induced MUC5AC expression for lung homeostasis.

Impact of host proteases on reovirus infection in the respiratory tract

Virion uncoating is an essential early event in reovirus infection. In natural enteric infections, rapid proteolytic uncoating of virions is mediated by pancreatic serine proteases. The proteases that promote reovirus disassembly and cell entry in the respiratory tract remain unknown. In this report, we show that endogenous respiratory and inflammatory proteases can promote reovirus infection in vitro and that preexisting inflammation augments in vivo infection in the murine respiratory tract.

Hepatocyte growth factor activator inhibitor type 1 inhibits protease activity and proteolytic activation of human airway trypsin-like protease

Hepatocyte growth factor activator inhibitor type 1 (HAI-1) is a Kunitz-type transmembrane serine protease inhibitor initially identified as a potent inhibitor of hepatocyte growth factor activator (HGFA), a serine protease that converts pro-HGF to the active form. HAI-1 also has inhibitory activity against serine proteases such as matriptase, hepsin and prostasin. In this study, we examined effects of HAI-1 on the protease activity and proteolytic activation of human airway trypsin-like protease (HAT), a transmembrane serine protease that is expressed mainly in bronchial epithelial cells. A soluble form of HAI-1 inhibited the protease activity of HAT in vitro. HAT was proteolytically activated in cultured mammalian cells transfected with its expression vector, and a soluble form of active HAT was released into the conditioned medium. The proteolytic activation of HAT required its own serine protease activity. Co-expression of the transmembrane full-length HAI-1 inhibited the proteolytic activation of HAT. In addition, full-length HAI-1 associated with the transmembrane full-length HAT in co-expressing cells. Like other target proteases of HAI-1, HAT converted pro-HGF to the active form in vitro. These results suggest that HAI-1 functions as a physiological regulator of HAT by inhibiting its protease activity and proteolytic activation in airway epithelium.

Expression and genetic loss of function analysis of the HAT/DESC cluster proteases TMPRSS11A and HAT

Genome mining at the turn of the millennium uncovered a new family of type II transmembrane serine proteases (TTSPs) that comprises 17 members in humans and 19 in mice. TTSPs phylogenetically belong to one of four subfamilies: matriptase, hepsin/TMPRSS, corin and HAT/DESC. Whereas a wealth of information now has been gathered as to the physiological functions of members of the hepsin/TMPRSS, matriptase, and corin subfamilies of TTSPs, comparatively little is known about the functions of the HAT/DESC subfamily of proteases. Here we perform a combined expression and functional analysis of this TTSP subfamily. We show that the five human and seven murine HAT/DESC proteases are coordinately expressed, suggesting a level of functional redundancy. We also perform a comprehensive phenotypic analysis of mice deficient in two of the most widely expressed HAT/DESC proteases, TMPRSS11A and HAT, and show that the two proteases are dispensable for development, health, and long-term survival in the absence of external challenges or additional genetic deficits. Our comprehensive expression analysis and generation of TMPRSS11A- and HAT-deficient mutant mouse strains provide a valuable resource for the scientific community for further exploration of the HAT/DESC subfamily proteases in physiological and pathological processes.

Induction of mucin and MUC5AC expression by the protease activity of Aspergillus fumigatus in airway epithelial cells

Allergic bronchopulmonary mycosis, characterized by excessive mucus secretion, airflow limitation, bronchiectasis, and peripheral blood eosinophilia, is predominantly caused by a fungal pathogen, Aspergillus fumigatus. Using DNA microarray analysis of NCI-H292 cells, a human bronchial epithelial cell line, stimulated with fungal extracts from A. fumigatus, Alternaria alternata, or Penicillium notatum, we identified a mucin-related MUC5AC as one of the genes, the expression of which was selectively induced by A. fumigatus. Quantitative RT-PCR, ELISA, and histochemical analyses confirmed an induction of mucin and MUC5AC expression by A. fumigatus extracts or the culture supernatant of live microorganisms in NCI-H292 cells and primary cultures of airway epithelial cells. The expression of MUC5AC induced by A. fumigatus extracts diminished in the presence of neutralizing Abs or of inhibitors of the epidermal growth factor receptor or its ligand, TGF-α. We also found that A. fumigatus extracts activated the TNF-α-converting enzyme (TACE), critical for the cleavage of membrane-bound pro-TGF-α, and its inhibition with low-molecular weight inhibitors or small interfering RNA suppressed the expression of MUC5AC. The protease activity of A. fumigatus extracts was greater than that of other fungal extracts, and treatment with a serine protease inhibitor, but not with a cysteine protease inhibitor, eliminated its ability to activate TACE or induce the expression of MUC5AC mRNA in NCI-H292. In conclusion, the prominent serine protease activity of A. fumigatus, which caused the overproduction of mucus by the bronchial epithelium via the activation of the TACE/TGF-α/epidermal growth factor receptor pathway, may be a pathogenetic mechanism of allergic bronchopulmonary mycosis.

Prostaglandin E2receptor subtype EP2- and EP4-regulated gene expression profiling in human ciliary smooth muscle cells

Prostanoids are an important class of intraocular pressure (IOP)-lowering antiglaucoma agents that act primarily via increased uveo-scleral aqueous humor outflow through the ciliary body. We have developed two novel PGE2analogs that are specific agonists for the PGE2receptor subtypes EP2 and EP4, respectively. To identify gene regulatory networks and key players that mediate the physiological effects observed in vivo, we performed genomewide expression studies using human ciliary smooth muscle cells. Quantitative real-time RT-PCR confirmed a largely overlapping gene expression profile subsequent to EP2 and EP4 agonist treatment, with 65 significantly regulated genes identified overall, 5 being specific for the EP2 agonist and 6 specific for the EP4 agonist. We found predicted functional cAMP-response elements in promoter regions of a large fraction of the predominantly upregulated genes, which suggests that the cAMP signaling pathway is the most important intracellular signaling pathway for these agonists in these cells. Several target genes were identified that, as part of complex regulatory networks, are implicated in tissue remodeling processes and osmoregulation (e.g., AREG, LOXL3, BMP2, AQP3) and thus may help elucidate the mechanism of action of these IOP-lowering drugs involving the uveo-scleral outflow path.

Cleavage of influenza virus hemagglutinin by airway proteases TMPRSS2 and HAT differs in subcellular localization and susceptibility to protease inhibitors

Proteolytic cleavage of the influenza virus surface glycoprotein hemagglutinin (HA) by host cell proteases is crucial for infectivity and virus spread. The proteases HAT (human airway trypsin-like protease) and TMPRSS2 (transmembrane protease serine S1 member 2) known to be present in the human airways were previously identified as proteases that cleave HA. We studied subcellular localization of HA cleavage and cleavage inhibition of seasonal influenza virus A/Memphis/14/96 (H1N1) and pandemic virus A/Hamburg/5/2009 (H1N1) in MDCK cells that express HAT and TMPRSS2 under doxycycline-induced transcriptional activation. We made the following observations: (i) HA is cleaved by membrane-bound TMPRSS2 and HAT and not by soluble forms released into the supernatant; (ii) HAT cleaves newly synthesized HA before or during the release of progeny virions and HA of incoming viruses prior to endocytosis at the cell surface, whereas TMPRSS2 cleaves newly synthesized HA within the cell and is not able to support the proteolytic activation of HA of incoming virions; and (iii) cleavage activation of HA and virus spread in TMPRSS2- and HAT-expressing cells can be suppressed by peptide mimetic protease inhibitors. The further development of these inhibitors could lead to new drugs for influenza treatment.

MDCK cells that express proteases TMPRSS2 and HAT provide a cell system to propagate influenza viruses in the absence of trypsin and to study cleavage of HA and its inhibition

Cleavage of the influenza virus hemagglutinin (HA) by host cell proteases is essential for virus infectivity and, therefore, relevant proteases may present promising new drug targets. We recently demonstrated that serine proteases TMPRSS2 and HAT from human airways activate influenza virus HA with monobasic cleavage site in vitro. In the present study we generated MDCK cells with inducible expression of either TMPRSS2 or HAT. MDCK-TMPRSS2 and MDCK-HAT cells supported growth of human and avian influenza viruses of different subtypes in the absence of exogenous trypsin. Further, we used these cell lines to investigate the efficacy of protease inhibitors to prevent proteolytic activation of HA by TMPRSS2 and HAT. Multicycle viral replication in both cell lines was markedly suppressed in the presence of serine protease inhibitors and we found that particularly in MDCK-HAT cells proteolytic activation of progeny viruses was very susceptible to inhibitor treatment. Taken together, our data demonstrate that MDCK-HAT and MDCK-TMPRSS2 cells are useful experimental systems to study cleavage of HA by host cell protease and its inhibition and in addition represent applicable cell lines to propagate influenza viruses in the absence of trypsin.

Reconstitution of amphiregulin-epidermal growth factor receptor signaling in lung squamous cell carcinomas activates PTHrP gene expression and contributes to cancer-mediated diseases of the bone

Parathyroid hormone-related protein (PTHrP) is the causative factor of the paraneoplastic syndrome humoral hypercalcemia of malignancy (HHM) and it also contributes to osteolytic metastases, both of which are common complications of squamous carcinomas of the lung. Inhibition of autocrine epidermal growth factor receptor (EGFR) signaling has been shown to reduce plasma calcium and PTHrP concentrations in two lung squamous cell carcinoma xenograft models of HHM. The purpose of this study was to investigate the mechanism by which EGFR is activated and stimulates PTHrP gene expression in lung squamous carcinoma cell lines. Amphiregulin (AREG) was the only EGFR ligand that could be consistently detected in conditioned media from the SCC lines, and reduction of its expression either by siRNA or by precipitating antibody reduced PTHrP mRNA expression as effectively as EGFR-targeted inhibition. Using siRNA knockdown or inhibitors to upstream regulators of AREG shedding including TACE, Src/Lck, and G(i/o), also reduced PTHrP mRNA expression. We determined that blockade of autocrine AREG-EGFR signaling does not affect PTHrP mRNA stability. Of the three PTHrP promoters (P1, P2, and P3), P1 mRNA could be reduced by nearly 100% with an EGFR inhibitor, and both epidermal growth factor and AREG stimulated P1 mRNA by approximately 5-fold. Finally, ectopic expression of EGFR in a receptor-low but AREG-expressing cell line increased PTHrP mRNA levels in vitro, and induced the capability to cause HHM and rapid osteolytic growth in vivo. Taken together, we provide evidence that AREG stimulation of EGFR results in high levels of PTHrP gene expression, contributing to cancer-associated bone pathology.

Fibrinogen binding to ICAM-1 promotes EGFR-dependent mucin production in human airway epithelial cells

Mucous hypersecretion is a serious feature of chronic airway diseases such as asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis. Although mucins are produced via activation of an EGF receptor (EGFR) signaling cascade, the mechanisms leading to exaggerated mucin production in mucous hypersecretory diseases are unknown. Because expression of ICAM-1 and of the ICAM-1 ligand fibrinogen is increased in the airways of subjects with mucous hypersecretory diseases, we hypothesized that fibrinogen binding to ICAM-1 could increase EGFR-dependent mucin production in human airway (NCI-H292) epithelial cells. Consistent with this hypothesis, we found that an ICAM-1 neutralizing antibody and an ICAM-1(8-22) peptide that binds fibrinogen decreased mucin production induced by the EGFR ligand transforming growth factor (TGF)-alpha dose-dependently. Exogenous fibrinogen and a fibrinogen(117-133) peptide that binds ICAM-1 rescued mucin production in cells treated with the ICAM-1(8-22) peptide. Surprisingly, the ICAM-1(8-22) peptide increased EGFR phosphotyrosine and phospho-ERK1/2 in cells treated with TGF-alpha. The ICAM-1(8-22) peptide-induced increases in EGFR phosphotyrosine and phospho-ERK1/2 were prevented by exogenous fibrinogen, by the fibrinogen(117-133) peptide, and by selective inhibitors of phospholipase C (PLC), protein kinase C (PKC)-alpha/beta, and metalloproteases. These results suggest that fibrinogen binding to ICAM-1 promotes mucin production by decreasing TGF-alpha-induced EGFR and ERK1/2 activation and that the fibrinogen-ICAM-1-dependent decrease in EGFR and ERK1/2 activation occurs via inhibition of an early positive feedback pathway involving PLC- and PKC-alpha/beta-dependent metalloprotease activation and subsequent metalloprotease-dependent EGFR reactivation.

Matriptase-2 (TMPRSS6): a proteolytic regulator of iron homeostasis

Maintaining the body's levels of iron within precise boundaries is essential for normal physiological function. Alterations of these levels below or above the healthy limit lead to a systemic deficiency or overload in iron. The type-two transmembrane serine protease (TTSP), matriptase-2 (also known as TMPRSS6), is attracting significant amounts of interest due to its recently described role in iron homeostasis. The finding of this regulatory role for matriptase-2 was originally derived from the observation that mice deficient in this protease present with anemia due to elevated levels of hepcidin and impaired intestinal iron absorption. Further in vitro analysis has demonstrated that matriptase-2 functions to suppress bone morphogenetic protein stimulation of hepcidin transcription through cell surface proteolytic processing of the bone morphogenetic protein co-receptor hemojuvelin. Consistently, the anemic phenotype of matriptase-2 knockout mice is mirrored in humans with matripase-2 mutations. Currently, 14 patients with iron-refractory iron deficiency anemia (IRIDA) have been reported to harbor various genetic mutations that abrogate matriptase-2 proteolytic activity. In this review, after overviewing the membrane anchored serine proteases, in particular the TTSP family, we summarize the identification and characterization of matriptase-2 and describe its functional relevance in iron metabolism.

Mucins, mucus, and sputum

Normal airway mucus lines the epithelial surface and provides an important innate immune function by detoxifying noxious molecules and by trapping and removing pathogens and particulates from the airway via mucociliary clearance. The major macromolecular constituents of normal mucus, the mucin glycoproteins, are large, heavily glycosylated proteins with a defining feature of tandemly repeating sequences of amino acids rich in serine and threonine, the linkage sites for large carbohydrate structures. The mucins are composed of two major families: secreted mucins and membrane-associated mucins. Membrane-associated mucins have been reported to function as cell surface receptors for pathogens and to activate intracellular signaling pathways. The biochemical and cellular functions for secreted mucin glycoproteins have not been definitively assigned. In contrast to normal mucus, sputum production is the hallmark of chronic inflammatory airway diseases such as asthma, chronic bronchitis, and cystic fibrosis (CF). Sputum has altered macromolecular composition and biophysical properties which vary with disease, but unifying features are failure of mucociliary clearance, resulting in airway obstruction, and failure of innate immune properties. Mucin glycoprotein overproduction and hypersecretion are common features of chronic inflammatory airway disease, and this has been the underlying rationale to investigate the mechanisms of mucin gene regulation and mucin secretion. However, in some pathologic conditions such as CF, airway sputum contains little intact mucin and has increased content of several macromolecules including DNA, filamentous actin, lipids, and proteoglycans. This review will highlight the most recent insights on mucus biology in health and disease.

Nafamostat mesilate, a potent serine protease inhibitor, inhibits airway eosinophilic inflammation and airway epithelial remodeling in a murine model of allergic asthma

To clarify the involvement of serine proteases in the development of allergic airway inflammation, we investigated the effect of nafamostat mesilate, a serine protease inhibitor, in a murine model of allergic asthma. Mice were sensitized to ovalbumin (OA) with alum and then exposed to 1% OA for 30 min, three times every 4th day. Nafamostat mesilate was administered orally for 10 days during the allergen challenge. In sensitized mice, repeated allergen challenge induced an increase in tryptase proteolytic activity in bronchoalveolar lavage fluid (BALF). In addition, marked increases in the numbers of inflammatory cells, levels of T helper type 2 (Th2) cytokines and eotaxin in BALF, numbers of goblet cells in the epithelium, and level of OA-specific IgE in serum were observed after repetitive allergen inhalation. Treatment with nafamostat mesilate significantly inhibited not only increased proteolytic activities, but also increases in the numbers of eosinophils and lymphocytes in the BALF. Nafamostat mesilate also dose-dependently inhibited increases in the levels of interleukin-13 and eotaxin in BALF and goblet cell hyperplasia. These findings suggest that increased serine protease activity in the airways is involved in the development of antigen-induced allergic eosinophilic inflammation and epithelial remodeling in bronchial asthma.

Protease-activated receptor-2 (PAR-2) is a weak enhancer of mucin secretion by human bronchial epithelial cells in vitro

PAR-2, a member of a family of G-protein-coupled receptors, can be activated by serine proteases via proteolytic cleavage. PAR-2 expression is known to be upregulated in respiratory epithelium subsequent to inflammation in asthma and chronic obstructive pulmonary disease (COPD). Since these diseases also are characterized by excessive mucus production and secretion, we investigated whether PAR-2 could be linked to mucin hypersecretion by airway epithelium. Normal human bronchial epithelial (NHBE) cells in primary culture or the human bronchial epithelial cell lines, NCI-H292 and HBE-1, were used. NHBE, NCI-H292, and HBE-1 cells expressed prominent levels of PAR-2 protein. Short-term (30min) exposure of cells to the synthetic PAR-2 agonist peptide (SLIGKV-NH2) elicited a small but statistically significant increase in mucin secretion at high concentrations (100microM and 1000microM), compared to a control peptide with reversed amino acid sequence (VKGILS-NH2). Neither human lung tryptase nor bovine pancreatic trypsin, both PAR-2 agonists, affected NHBE cell mucin secretion when added over a range of concentrations. Knockdown of PAR-2 expression by siRNA blocked the stimulatory effect of the AP. The results suggest that, since PAR-2 activation only weakly increases mucin secretion by human airway epithelial cells in vitro, PAR-2 probably is not a significant contributor to mucin hypersecretion in inflamed airways.

A novel therapeutic target in various lung diseases: Airway proteases and protease-activated receptors

Protease-activated receptors (PAR), which are G protein-coupled receptors, have 4 members, PAR-1 to PAR-4. PARs are activated by proteolysis of a peptide bond at the N-terminal domain of the receptor. PARs are widely distributed throughout the airways. Their activity is modulated by airway proteases of endogenous and exogenous origin, which can either activate or disable the receptors. The regulation of PAR activity by proteases is important under pathological conditions when the activity of proteases is increased. Moreover, various inflammatory mediators, such as cytokines, growth factors, or prostanoids, alter the PAR expression level. Elevated PAR levels are observed in various lung disorders, and their significance in the development of pathological situations in the lung is currently intensively investigated. Consequences of PAR activation can be either beneficial or deleterious, depending on the PAR subtype. PAR-1 has been shown to be an important player in the development of pulmonary fibrosis. Thus, PAR-1 represents an exciting target for clinical intervention in fibrotic diseases. PAR-2 contributes to allergic airway inflammation. However, the question whether the impact of PAR-2 is beneficial or deleterious is still under intensive discussion. Therefore, precise information concerning the participation of PAR-2 in various lesions is required. Moreover, it is necessary to generate selective PAR- and organ-targeted approaches for treating the diseases. A thorough understanding of PAR-induced cellular events and the consequences of receptor blockade may help in the development of novel therapeutic strategies targeted to prevent lung destruction and to avoid deterioration of conditions of patients with inflammatory or fibrotic lung diseases.