Pharmacology of airway goblet cell mucin release

Physical and barrier changes in gastrointestinal mucus induced by the permeation enhancer sodium 8-[(2-hydroxybenzoyl)amino]octanoate (SNAC)

Drug delivery systems (DDS) for oral delivery of peptide drugs contain excipients that facilitate and enhance absorption. However, little knowledge exists on how DDS excipients such as permeation enhancers interact with the gastrointestinal mucus barrier. This study aimed to investigate interactions of the permeation enhancer sodium 8-[(2-hydroxybenzoyl)amino]octanoate (SNAC) with ex vivo porcine intestinal mucus (PIM), ex vivo porcine gastric mucus (PGM), as well as with in vitro biosimilar mucus (BM) by profiling their physical and barrier properties upon exposure to SNAC. Bulk mucus permeability studies using the peptides cyclosporine A and vancomycin, ovalbumin as a model protein, as well as fluorescein-isothiocyanate dextrans (FDs) of different molecular weights and different surface charges were conducted in parallel to mucus retention force studies using a texture analyzer, rheological studies, cryo-scanning electron microscopy (cryo-SEM), and single particle tracking of fluorescence-labelled nanoparticles to investigate the effects of the SNAC-mucus interaction. The exposure of SNAC to PIM increased the mucus retention force, storage modulus, viscosity, increased nanoparticle confinement within PIM as well as decreased the permeation of cyclosporine A and ovalbumin through PIM. Surprisingly, the viscosity of PGM and the permeation of cyclosporine A and ovalbumin through PGM was unaffected by the presence of SNAC, thus the effect of SNAC depended on the regional site that mucus was collected from. In the absence of SNAC, the permeation of different molecular weight and differently charged FDs through PIM was comparable to that through BM. However, while bulk permeation of neither of the FDs through PIM was affected by SNAC, the presence of SNAC decreased the permeation of FD4 and increased the permeation of FD150 kDa through BM. Additionally, and in contrast to observations in PIM, nanoparticle confinement within BM remained unaffected by the presence of SNAC. In conclusion, the present study showed that SNAC altered the physical and barrier properties of PIM, but not of PGM. The effects of SNAC in PIM were not observed in the BM in vitro model. Altogether, the study highlights the need for further understanding how permeation enhancers influence the mucus barrier and illustrates that the selected mucus model for such studies should be chosen with care.

Bronchipret® syrup containing thyme and ivy extracts suppresses bronchoalveolar inflammation and goblet cell hyperplasia in experimental bronchoalveolitis

BACKGROUND/PURPOSE

Acute bronchitis (AB) is a common lung condition characterized by inflammation of the large bronchi in response to infection. Bronchipret(®) syrup (BRO), a fixed combination of thyme and ivy extracts has been effectively used for the treatment of AB. Combining in vivo and mechanistic in vitro studies we aimed to provide a better understanding of the therapeutic potential of BRO on key aspects of AB and to identify potential mechanisms of action.

METHODS

Bronchoalveolitis in rats was induced by intratracheal LPS instillation. BRO was administered p.o. once daily at 1- to 10-fold equivalents of the human daily dose. Animals were sacrificed 24-72 h post LPS challenge to analyze leukocyte numbers in lung tissue, bronchoalveolar lavage fluid (BALF) and blood as well as goblet cells in bronchial epithelium. Inhibitory effects of BRO analogue on leukotriene (LT) production were determined in human neutrophils and monocytes as well as on isolated 5-lipoxygenase (5-LO).

RESULTS

BRO significantly reversed the LPS-induced increase in leukocyte numbers in lung tissue, BALF and blood as well as goblet cell numbers in bronchial epithelium. In vitro, BRO analogue suppressed cellular release of LTB4 (IC50 = 36 µg⋅ml(-1)) and cysLT (IC50 = 10 µg⋅ml(-1)) and inhibited the activity of isolated 5-LO (IC50 = 19 µg⋅ml(-1)).

CONCLUSION

BRO exerts significant anti-inflammatory effects and attenuates goblet cell metaplasia in LPS-induced bronchoalveolitis in vivo potentially via interference with 5-LO/LT signaling. These effects may contribute to its observed clinical efficacy in AB.

Mechanisms of glucocorticoid action and insensitivity in airways disease

Glucocorticoids are the mainstay for the treatment of chronic inflammatory diseases including asthma and chronic obstructive pulmonary disease (COPD). However, it has been recognized that glucocorticoids do not work well in certain patient populations suggesting reduced sensitivity. The ultimate biologic responses to glucocorticoids are determined by not only the concentration of glucocorticoids but also the differences between individuals in glucocorticoid sensitivity, which is influenced by multiple factors. Studies are emerging to understand these mechanisms in detail, which would help in increasing glucocorticoid sensitivity in patients with chronic airways disease. This review aims to highlight both classical and emerging concepts of the anti-inflammatory mechanisms of glucocorticoids and also review some novel strategies to overcome steroid insensitivity in airways disease.

Cellular and molecular biology of airway mucins

Airway mucus constitutes a thin layer of airway surface liquid with component macromolecules that covers the luminal surface of the respiratory tract. The major function of mucus is to protect the lungs through mucociliary clearance of inhaled foreign particles and noxious chemicals. Mucus is comprised of water, ions, mucin glycoproteins, and a variety of other macromolecules, some of which possess anti-microbial, anti-protease, and anti-oxidant activities. Mucins comprise the major protein component of mucus and exist as secreted and cell-associated glycoproteins. Secreted, gel-forming mucins are mainly responsible for the viscoelastic property of mucus, which is crucial for effective mucociliary clearance. Cell-associated mucins shield the epithelial surface from pathogens through their extracellular domains and regulate intracellular signaling through their cytoplasmic regions. However, neither the exact structures of mucin glycoproteins, nor the manner through which their expression is regulated, are completely understood. This chapter reviews what is currently known about the cellular and molecular properties of airway mucins.

CFTR, mucins, and mucus obstruction in cystic fibrosis

Mucus pathology in cystic fibrosis (CF) has been known for as long as the disease has been recognized and is sometimes called mucoviscidosis. The disease is marked by mucus hyperproduction and plugging in many organs, which are usually most fatal in the airways of CF patients, once the problem of meconium ileus at birth is resolved. After the CF gene, CFTR, was cloned and its protein product identified as a cAMP-regulated Cl(-) channel, causal mechanisms underlying the strong mucus phenotype of the disease became obscure. Here we focus on mucin genes and polymeric mucin glycoproteins, examining their regulation and potential relationships to a dysfunctional cystic fibrosis transmembrane conductance regulator (CFTR). Detailed examination of CFTR expression in organs and different cell types indicates that changes in CFTR expression do not always correlate with the severity of CF disease or mucus accumulation. Thus, the mucus hyperproduction that typifies CF does not appear to be a direct cause of a defective CFTR but, rather, to be a downstream consequence. In organs like the lung, up-regulation of mucin gene expression by inflammation results from chronic infection; however, in other instances and organs, the inflammation may have a non-infectious origin. The mucus plugging phenotype of the β-subunit of the epithelial Na(+) channel (βENaC)-overexpressing mouse is proving to be an archetypal example of this kind of inflammation, with a dehydrated airway surface/concentrated mucus gel apparently providing the inflammatory stimulus. Data indicate that the luminal HCO(3)(-) deficiency recently described for CF epithelia may also provide such a stimulus, perhaps by causing a mal-maturation of mucins as they are released onto luminal surfaces. In any event, the path between CFTR dysfunction and mucus hyperproduction has proven tortuous, and its unraveling continues to offer its own twists and turns, along with fascinating glimpses into biology.

Analysis of the proteome of human airway epithelial secretions

BACKGROUND

Airway surface liquid, often referred to as mucus, is a thin layer of fluid covering the luminal surface that plays an important defensive role against foreign particles and chemicals entering the lungs. Airway mucus contains various macromolecules, the most abundant being mucin glycoproteins, which contribute to its defensive function. Airway epithelial cells cultured in vitro secrete mucins and nonmucin proteins from their apical surface that mimics mucus production in vivo. The current study was undertaken to identify the polypeptide constituents of human airway epithelial cell secretions to gain a better understanding of the protein composition of respiratory mucus.

RESULTS

Fifty-five proteins were identified in the high molecular weight fraction of apical secretions collected from in vitro cultures of well-differentiated primary human airway epithelial cells and isolated under physiological conditions. Among these were MUC1, MUC4, MUC5B, and MUC16 mucins. By proteomic analysis, the nonmucin proteins could be classified as inflammatory, anti-inflammatory, anti-oxidative, and/or anti-microbial.

CONCLUSIONS

Because the majority of the nonmucin proteins possess molecular weights less than that selected for analysis, it is theoretically possible that they may associate with the high molecular weight and negatively charged mucins to form a highly ordered structural organization that is likely to be important for maintaining the proper defensive function of airway mucus.

Nucleotide-mediated airway clearance

A thin layer of airway surface liquid (ASL) lines the entire surface of the lung and is the first point of contact between the lung and the environment. Surfactants contained within this layer are secreted in the alveolar region and are required to maintain a low surface tension and to prevent alveolar collapse. Mucins are secreted into the ASL throughout the respiratory tract and serve to intercept inhaled pathogens, allergens and toxins. Their removal by mucociliary clearance (MCC) is facilitated by cilia beating and hydration of the ASL by active ion transport. Throughout the lung, secretion, ion transport and cilia beating are under purinergic control. Pulmonary epithelia release ATP into the ASL which acts in an autocrine fashion on P2Y(2) (ATP) receptors. The enzymatic network describes in Chap. 2 then mounts a secondary wave of signaling by surface conversion of ATP into adenosine (ADO), which induces A(2B) (ADO) receptor-mediated responses. This chapter offers a comprehensive description of MCC and the extensive ramifications of the purinergic signaling network on pulmonary surfaces.

Coupling of airway ciliary activity and mucin secretion to mechanical stresses by purinergic signaling

The mucociliary clearance system is comprised of three components, ion transport activities controlling the height of airway surface liquid (ASL), mucin secretion, and ciliary activity. These activities in humans are controlled principally by local agonists, extracellular nucleotides and nucleosides released from the epithelium. Importantly, mechanical stresses stimulate goblet cell mucin secretion, ciliary beating, and Cl- and fluid secretion through mechanically induced nucleotide release. Emerging evidence also implicates co-secretion of nucleotides and mucin from goblet cells as a source of extracellular agonist. At rest, ATP is released onto airway surfaces at approximately 370fmol/mincm2, but only approximately 3% of released ATP is recovered in ASL. Secreted UTP meets with a similar fate. A wide variety of hydrolytic and transphosphorylating ecto-enzymes convert the triphosphate nucleotides into ADP, AMP, and adenosine, UDP, UMP, and uridine. Of these, ATP, adenosine, UTP, and UDP act as agonists at apical P2Y2 (ATP, UTP), P2Y6 (UDP), and A2B (adenosine) receptors on ciliated and/or goblet cells to regulate mucociliary clearance.

Regulated airway goblet cell mucin secretion

Major advances in understanding regulated mucin secretion from airway goblet cells have been made in the past decade in the areas of pharmacology and basic cell biology. For instance, it is now appreciated that nucleotide agonists acting locally through P2Y purinoceptors on apical membranes of surface goblet cells provide the major regulatory system for mucin secretion. Similarly, Clara cells, the primary secretory cell in the mouse airways (and human small airways), are now recognized as major mucin-secreting cells. In Clara cells, the relative lack of staining for mucosubstances reflects essentially equal baseline rates of mucin synthesis and secretion, with little to no accumulation of mucin granules in storage pools. During mucous metaplasia induced under inflammatory conditions, mucin synthesis is massively upregulated in Clara cells, and stored mucin granules come to dominate the secretory cell phenotype. More importantly, we have seen a transition in the past few years from a pharmacological focus on regulated mucin secretion to a more molecular mechanistic focus that has great promise going forward. In part, these advances are occurring through the use of well-differentiated primary human bronchial epithelial cell cultures, but recent work in mouse models perhaps has had the most important impact. Emerging data from Munc13-2- and synaptotagmin 2-deficient mouse models represent the first direct, molecular-level manipulations of proteins involved in regulated secretory cell mucin secretion. These new data indicate that Munc13-2 is responsible for regulating a baseline mucin secretory pathway in the airways and is not essential for purinergic agonist-induced mucin secretion. In contrast, synaptotagmin 2, a fast Ca2+ sensor for the SNARE complex, is essential for regulated secretion. Interestingly, these early results suggest that there are two pathways for excocytic mucin release from goblet cells.

Munc13-2-/- baseline secretion defect reveals source of oligomeric mucins in mouse airways

Since the airways of control mouse lungs contain few alcian blue/periodic acid-Schiff's (AB/PAS)+ staining 'goblet' cells in the absence of an inflammatory stimulus such as allergen sensitization, it was surprising to find that the lungs of mice deficient for the exocytic priming protein Munc13-2 stain prominently with AB/PAS under control conditions. Purinergic agonists (ATP/UTP) stimulated release of accumulated mucins in the Munc13-2-deficient airways, suggesting that the other airway isoform, Munc13-4, supports agonist-regulated secretion. Notably, however, not all of the mucins in Munc13-2-deficient airways were secreted, suggesting a strict Munc13-2 priming requirement for a population of secretory granules. AB/PAS+ staining of Munc13-2-deficient airways was not caused by an inflammatory, metaplastic-like response: bronchial-alveolar lavage leucocyte numbers, Muc5ac and Muc5b mRNA levels, and Clara cell ultrastructure (except for increased secretory granule numbers) were all normal. A Muc5b-specific antibody indicated the presence of this mucin in Clara cells of wildtype (WT) control mice, and increased amounts in Munc13-2-deficient mice. Munc13-2 therefore appears to prime a regulated, baseline secretory pathway, such that Clara cell Muc5b, normally secreted soon after synthesis, accumulates in the gene-deficient animals, making them stain AB/PAS+. The defective priming phenotype is widespread, as goblet cells of several mucosal tissues appear engorged and Clara cells accumulated Clara cell secretory protein (CCSP) in Munc13-2-deficient mice. Additionally, because in the human airways, MUC5AC localizes to the surface epithelium and MUC5B to submucosal glands, the finding that Muc5b is secreted by Clara cells under control conditions may indicate that it is also secreted tonically from human bronchiolar Clara cells.

nPKCepsilon, a P2Y2-R downstream effector in regulated mucin secretion from airway goblet cells

Airway goblet cell mucin secretion is controlled by agonist activation of P2Y(2) purinoceptors, acting through Gq/PLC, inositol-1,4,5-trisphosphate (IP(3)), diacylglycerol, Ca(2+) and protein kinase C (PKC). Previously, we showed that SPOC1 cells express cPKCalpha, nPKCdelta, nPKCepsilon, and nPKCeta; of these, only nPKCdelta translocated to the membrane in correlation with mucin secretion (Abdullah LH, Bundy JT, Ehre C, Davis CW. Am J Physiol Lung Physiol 285: L149-L160, 2003). We have verified these results and pursued the identity of the PKC effector isoform by testing the effects of altered PKC expression on regulated mucin release using SPOC1 cell and mouse models. SPOC1 cells overexpressing cPKCalpha, nPKCdelta, and nPKCeta had the same levels of ATPgammaS- and phorbol-1,2-myristate-13-acetate (PMA)-stimulated mucin secretion as the levels in empty retroviral vector expressing cells. Secretagogue-induced mucin secretion was elevated only in cells overexpressing nPKCepsilon (14.6 and 23.5%, for ATPgammaS and PMA). Similarly, only SPOC1 cells infected with a kinase-deficient nPKCepsilon exhibited the expected diminution of stimulated mucin secretion, relative to wild-type (WT) isoform overexpression. ATPgammaS-stimulated mucin secretion from isolated, perfused mouse tracheas was diminished in P2Y(2)-R null mice by 82% relative to WT mice, demonstrating the utility of mouse models in studies of regulated mucin secretion. Littermate WT and nPKCdelta knockout (KO) mice had nearly identical levels of stimulated mucin secretion, whereas mucin release was nearly abolished in nPKCepsilon KO mice relative to its WT littermates. We conclude that nPKCepsilon is the effector isoform downstream of P2Y(2)-R activation in the goblet cell secretory response. The translocation of nPKCdelta observed in activated cells is likely not related to mucin secretion but to some other aspect of goblet cell biology.

Regulation of airway goblet cell mucin secretion by tyrosine phosphorylation signaling pathways

Mucus hyperproduction in pulmonary obstructive diseases results from increased goblet cell numbers and possibly increased cellular mucin synthesis, occurring in response to inflammatory mediators acting via receptor tyrosine kinases (RYK) and tyrosine phosphorylation (Y-Pi) signaling pathways. Yet, increased mucin synthesis does not lead necessarily to increased secretion, as mucins are stored in secretory granules and secreted in response to extracellular signals, commonly assumed to be mediated by G protein-coupled receptors (GPCRs). We asked whether activation 1) of Y-Pi signaling pathways, in principal, and 2) of the novel PKC isoform, nPKCdelta, by Y-Pi, specifically, might lead to regulated mucin secretion. nPKCdelta in SPOC1 cells was tyrosine phosphorylated by exposure to purinergic agonist (ATPgammaS) or PMA, actions that were blocked by the Src kinase inhibitor, PP1. Mucin secretion, however, was not affected by PP1. Hence, activation of nPKCdelta by Y-Pi is unlikely to participate in GPCR-related mucin secretion. Mucin secretion from both SPOC1 and normal human bronchial epithelial (NHBE) cells was stimulated by generalized protein Y-Pi induced by the tyrosine phosphatase inhibitor, pervanadate (PV). PV-induced SPOC1 cell mucin secretion was not affected by inhibition of Src kinases (genistein or PP1), or of PI3 kinase (LY-294002). MAP kinase pathway inhibitors, RAF1 kinase inhibitor-I and U0126 (MEK), inhibited SPOC1 cell PV-induced secretion by approximately 50%. Significantly, the phospholipase C (PLC) inhibitor, U-73122, essentially abolished PV- and ATPgammaS-induced mucin secretion from both SPOC1 and NHBE cells. Hence, PLC signaling may play a key role in regulated mucin secretion, whether the event is initiated by mediators interacting with GPCRs or RYKs.

The signaling pathway involved in neutrophil elastase stimulated MUC1 transcription

We previously reported that neutrophil elastase (NE) stimulated MUC1 gene expression in A549 lung epithelial cells through binding of Sp1 to the MUC1 promoter element. The current study was undertaken to elucidate the complete signaling pathway leading to Sp1 activation. Using a combination of pharmacologic inhibitors, dominant-negative mutant, RNA interference, and soluble receptor blocking techniques, we identified a protein kinase Cdelta (PKCdelta) --> dual oxidase 1 (Duox1) --> reactive oxygen species (ROS) --> TNF-alpha-converting enzyme (TACE) --> TNF-alpha --> TNF receptor (TNFR)1 --> extracellular signal-regulated kinase (ERK)1/2 --> Sp1 pathway as responsible for NE-activated MUC1 transcription. This cascade was identical up to the point of TACE with the signaling pathway previously reported for NE-stimulated MUC5AC production. However, unlike the MUC5AC pathway, TNF-alpha, TNFR1, ERK1/2, and Sp1 were unique components of the MUC1 pathway. Given the anti-inflammatory role of MUC1 during airway bacterial infection, up-regulation of MUC1 by inflammatory mediators such as NE and TNF-alpha suggests a crucial role for MUC1 in the control of excessive inflammation during airway bacterial infection.

Calcium signaling in human airway goblet cells following purinergic activation

Despite the general importance of Ca(2+) signaling in signal transduction, and of goblet cell mucin hypersecretion in inflammatory pulmonary diseases, measurement of airway goblet cell intracellular Ca(2+) (Ca(i)(2+)) has not been reported. In this article, we describe the results of experiments measuring Ca(i)(2+) in primary cultures of human bronchial goblet cells after stimulation with the purinergic agonist adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) and phorbol 12-myristate 13-acetate (PMA). Ca(2+) signaling in human goblet cells after purinergic stimulation follows the classic paradigm of a Ca(i)(2+) transient from a basal activity of 110 nM to a peak response of 260.1 +/- 41.2 nM within 2 min, followed by a long superbasal plateau (155.3 +/- 0.2 nM) between 10 and 15 min. The rise in Ca(i)(2+) appears to result from a mobilization of intracellular stores, because the transient was nearly abolished by inhibition of PLC with the phosphatidylinositol-specific PLC inhibitor U-73122, and it was not affected significantly by removal of extracellular Ca(2+). Loading goblet cells with BAPTA inhibited the ATPgammaS-induced Ca(2+) transient by 86.0 +/- 13.1%, relative to control. Finally, in contrast to the massive effects of high doses of PMA (300 nM) on mucin secretion from goblet cells, phorbol ester stimulated a small (27.1 +/- 7% of the ATPgammaS control peak), brief rise in Ca(i)(2+). This diminutive signal likely denotes a local Ca(2+) gradient, which may be associated with the mucin granule exocytotic process.

Ciliostasis is a key early event during colonization of canine tracheal tissue by Bordetella bronchiseptica

The primary site of infection for Bordetella bronchiseptica, Bordetella pertussis and Bordetella parapertussis is the ciliated respiratory epithelium. Previous studies have implicated adherence of bacteria to cilia, induction of mucus production, induction of ciliostasis and damage to the ciliated epithelium in Bordetella pathogenesis. This paper describes the use of an air-interface organ culture system using canine tracheal tissue infected with B. bronchiseptica to assess the temporal relationship between these pathologies. Ciliostasis occurs very early during the host tissue-pathogen interaction, before mucus production and obvious signs of epithelial damage occur. A B. bronchiseptica bvg mutant does not colonize the organ culture model, induce ciliostasis or cause damage to the epithelial cell layer, but it does induce similar amounts of mucus release as does infection by wild-type bacteria. The authors propose that ciliostasis is a key early event during the B. bronchiseptica-host tissue interaction that abrogates the muco-ciliary defences of the host tissue, renders it susceptible to colonization by the bacteria and allows subsequent damage to the epithelium. The organ culture model described offers a physiologically relevant tool with which to characterize the molecular basis for interactions between Bordetella and its primary site of infection, the ciliated respiratory epithelium.

Effects of dexamethasone on Muc5ac mucin production by primary airway goblet cells

Mucus hypersecretion associated with airway inflammation is reduced by glucocorticoids. Two mechanisms of glucocorticoid-mediated inhibition of mucus production have been proposed, direct inhibition of mucus production by airway epithelial cells and indirectly through inhibition of proinflammatory mediators that stimulate mucus production. In this study, we examined the effect of dexamethasone (DEX) on mRNA expression and synthesis of MUC5AC by A549 human lung adenocarcinoma cells as well as Muc5ac and total high-molecular-weight (HMW) mucins by primary rat tracheal surface epithelial (RTSE) cells. Our results showed that in primary RTSE cells, DEX 1) dose dependently suppressed Muc5ac mRNA levels, but the levels of cellular Muc5ac protein and HMW mucins were unaffected; 2) did not affect constitutive or UTP-stimulated mucin secretion; 3) enhanced the translation of Muc5ac; and 4) increased the stability of intracellular Muc5ac protein by a mechanism other than the inhibition of the proteasomal degradation. In A549 cells, however, DEX suppressed both MUC5AC mRNA levels and MUC5AC protein secretion in a dose-dependent manner. We conclude that whereas DEX inhibits the levels of Muc5ac mRNA in primary RTSE cells, the levels of Muc5ac protein remain unchanged as a consequence of increases in both translation and protein stability. Interestingly, some of the effects of DEX were opposite in a cell line.