The production of gamma-amino butyric acid (GABA) is dependent on glutamate decarboxylases (GAD65 and GAD67), the enzymes that catalyze the decarboxylation of glutamate to GABA. Based on studies suggesting a role of the airway epithelial GABAergic system in asthma-related mucus overproduction, we hypothesized that cigarette smoking, another disorder associated with increased mucus production, may modulate GABAergic system-related gene expression levels in the airway epithelium.
We assessed expression of the GABAergic system in human airway epithelium obtained using bronchoscopy to sample the epithelium and microarrays to evaluate gene expression. RT-PCR was used to confirm gene expression of GABAergic system gene in large and small airway epithelium from heathy nonsmokers and healthy smokers. The differences in the GABAergic system gene was further confirmed by TaqMan, immunohistochemistry and Western analysis.
The data demonstrate there is a complete GABAergic system expressed in the large and small human airway epithelium, including glutamate decarboxylase, GABA receptors, transporters and catabolism enzymes. Interestingly, of the entire GABAergic system, smoking modified only the expression of GAD67, with marked up-regulation of GAD67 gene expression in both large (4.1-fold increase, p < 0.01) and small airway epithelium of healthy smokers (6.3-fold increase, p < 0.01). At the protein level, Western analysis confirmed the increased expression of GAD67 in airway epithelium of healthy smokers compared to healthy nonsmokers (p < 0.05). There was a significant positive correlation between GAD67 and MUC5AC gene expression in both large and small airway epithelium (p < 0.01), implying a link between GAD67 and mucin overproduction in association with smoking.
In the context that GAD67 is the rate limiting enzyme in GABA synthesis, the correlation of GAD67 gene expression with MUC5AC expressions suggests that the up-regulation of airway epithelium expression of GAD67 may contribute to the increase in mucus production observed in association with cigarette smoking.
The Wnt pathway mediates differentiation of epithelial tissues; depending on the tissue types, Wnt can either drive or inhibit the differentiation process. We hypothesized that key genes in the Wnt pathway are suppressed in the human airway epithelium under the stress of cigarette smoking, a stress associated with dysregulation of the epithelial differentiated state.
Microarrays were used to assess the expression of Wnt-related genes in the small airway epithelium (SAE) obtained via bronchoscopy and brushing of healthy nonsmokers, healthy smokers, and smokers with COPD. Thirty-three of 56 known Wnt-related genes were expressed in the SAE. Wnt pathway downstream mediators β-catenin and the transcription factor 7-like 1 were down-regulated in healthy smokers and smokers with COPD, as were many Wnt target genes. Among the extracellular regulators that suppress the Wnt pathway, secreted frizzled-related protein 2 (SFRP2), was up-regulated 4.3-fold in healthy smokers and 4.9-fold in COPD smokers, an observation confirmed by TaqMan Real-time PCR, Western analysis and immunohistochemistry. Finally, cigarette smoke extract mediated up-regulation of SFRP2 and down-regulation of Wnt target genes in airway epithelial cells in vitro.
Smoking down-regulates the Wnt pathway in the human airway epithelium. In the context that Wnt pathway plays an important role in differentiation of epithelial tissues, the down-regulation of Wnt pathway may contribute to the dysregulation of airway epithelium differentiation observed in smoking-related airway disorders.
Rationale: The airway epithelium of smokers is subject to a variety of mechanisms of injury with consequent modulation of epithelial regeneration and disordered differentiation. Several signaling pathways, including the Notch pathway, control epithelial differentiation in lung morphogenesis, but little is known about the role of these pathways in adults.
Objectives: We tested the hypotheses that Notch-related genes are expressed in the normal nonsmoker small airway epithelium of human adults, and that Notch-related gene expression is down-regulated in healthy smokers and smokers with chronic obstructive pulmonary disease (COPD).
Methods: We used microarray technology to evaluate the expression of 55 Notch-related genes in the small airway epithelium of nonsmokers. We used TaqMan quantitative polymerase chain reaction (PCR) to confirm the expression of key genes and we used immunohistochemistry to assess the expression of Notch-related proteins in the airway epithelium. Changes in expression of Notch genes in healthy smokers and smokers with COPD compared with nonsmokers were evaluated by PCR.
Measurements and Main Results: Microarray analysis demonstrated that 45 of 55 Notch-related genes are expressed in the small airway epithelium of adults. TaqMan PCR confirmed the expression of key genes with highest expression of the ligand DLL1, the receptor NOTCH2, and the downstream effector HES1. Immunohistochemistry demonstrated the expression of Jag1, Notch2, Hes1, and Hes5 in airway epithelium. Several Notch ligands, receptors, and downstream effector genes were down-regulated in smokers, with more genes down-regulated in smokers with COPD than in healthy smokers.
Conclusions: These observations are consistent with the hypothesis that the Notch pathway likely plays a role in the human adult airway epithelium, with down-regulation of Notch pathway gene expression in association with smoking and COPD.
gene expression; microarray analysis; delta-like ligand; basic helix-loop-helix transcription factors; Notch receptors
Background: A common pathological feature of chronic inflammatory airway diseases such as asthma and chronic obstructive pulmonary disease (COPD) is mucus hypersecretion. MUC5AC is the predominant mucin gene expressed in healthy airways and is increased in asthmatic and COPD patients. Recent clinical trials indicate that phosphodiesterase type 4 (PDE4) inhibitors may have therapeutic value for COPD and asthma. However, their direct effects on mucin expression have been scarcely investigated.
Methods: MUC5AC mRNA and protein expression were examined in cultured human airway epithelial cells (A549) and in human isolated bronchial tissue stimulated with epidermal growth factor (EGF; 25 ng/ml). MUC5AC mRNA was measured by real time RT-PCR and MUC5AC protein by ELISA (cell lysates and tissue homogenates), Western blotting (tissue homogenates) and immunohistochemistry.
Results: EGF increased MUC5AC mRNA and protein expression in A549 cells. PDE4 inhibitors produced a concentration dependent inhibition of the EGF induced MUC5AC mRNA and protein expression with potency values (–log IC50): roflumilast (∼7.5) > rolipram (∼6.5) > cilomilast (∼5.5). Roflumilast also inhibited the EGF induced expression of phosphotyrosine proteins, EGF receptor, and phospho-p38- and p44/42-MAPK measured by Western blot analysis in A549 cells. In human isolated bronchus, EGF induced MUC5AC mRNA and protein expression was inhibited by roflumilast (1 µM) as well as the MUC5AC positive staining shown by immunohistochemistry.
Conclusion: Selective PDE4 inhibition is effective in decreasing EGF induced MUC5AC expression in human airway epithelial cells. This effect may contribute to the clinical efficacy of this new drug category in mucus hypersecretory diseases.
Mucus hypersecretion with elevated MUC5B mucin production is a pathologic feature in many airway diseases associated with oxidative stress. In the present work, we evaluated MUC5B expression in airways and in primary cultures of normal human bronchial epithelial (NHBE) cells, as well as the mechanisms involved in its regulation. We found that oxidative stress generated by cigarette smoke or reactive oxygen species (ROS) induces MUC5B up-regulation in airway epithelium from smokers and in NHBE cells, respectively. We have previously shown that ROS-induced MUC5AC expression in NHBE cells is dependent on hyaluronan depolymerization and epidermal growth factor receptor (EGFR)/mitogen-activated protein kinase (MAPK) activation. Since hyaluronan fragments can activate MAPK through the hyaluronan receptor CD44, and CD44 heterodimerizes with EGFR, we tested whether ROS and/or hyaluronan fragments induce MUC5B mRNA and protein expression through CD44/EGFR. We found that ROS promotes CD44/EGFR interaction, EGFR/MAPK activation, and MUC5B up-regulation that are prevented by blocking CD44 and/or EGFR. These results were mimicked by hyaluronan fragments. In summary, our results show that oxidative stress in vivo (cigarette smoke) or in vitro (ROS) induces MUC5B up-regulation. This ROS-induced MUC5B expression requires CD44 as well as EGFR and MAPK activation. In addition, we also provide evidence that hyaluronan fragments are sufficient to induce CD44/EGFR interaction and downstream signaling that results in MUC5B up-regulation, suggesting that hyaluronan depolymerization during inflammatory responses could be directly involved in the induction of mucus hypersecretion.
MUC5B; hyaluronan fragments; CD44; airway epithelium
The toll-like receptors (TLRs) are important components of the respiratory epithelium host innate defense, enabling the airway surface to recognize and respond to a variety of insults in inhaled air. Based on the knowledge that smokers are more susceptible to pulmonary infection and that the airway epithelium of smokers with chronic obstructive pulmonary disease (COPD) is characterized by bacterial colonization and acute exacerbation of airway infections, we assessed whether smoking alters expression of TLRs in human small airway epithelium, the primary site of smoking-induced disease. Microarrays were used to survey the TLR family gene expression in small airway (10th–12th order) epithelium from healthy nonsmokers (n=60), healthy smokers (n=73) and smokers with COPD (n=36). Using the criteria of detection call of present in ≥50%, 6 of 10 TLRs (1, 2, 3, 4, 5 and 8) were expressed. Compared to nonsmokers, the most striking change was for TLR5, which was down-regulated in healthy smokers (1.4-fold, p<10−10) and smokers with COPD (1.6-fold, p<10−11). TaqMan RT-PCR confirmed these observations. Bronchial biopsy immunofluorescence studies showed that TLR5 was expressed mainly on the apical side of the epithelium and was decreased in healthy smokers and smokers with COPD. In vitro, the level of TLR5 downstream genes, IL-6 and IL-8, were highly induced by flagellin in TLR5 high-expressing cells compared to TLR5 low-expressing cells. In the context that TLR5 functions to recognize pathogens and activate innate immune responses, the smoking-induced down-regulation of TLR5 may contribute to smoking-related susceptibility to airway infection, at least for flagellated bacteria.
Nuclear factor erythroid 2–related factor 2 (Nrf2) is an oxidant-responsive transcription factor known to induce detoxifying and antioxidant genes. Cigarette smoke, with its large oxidant content, is a major stress on the cells of small airway epithelium, which are vulnerable to oxidant damage. We assessed the role of cigarette smoke in activation of Nrf2 in the human small airway epithelium in vivo. Fiberoptic bronchoscopy was used to sample the small airway epithelium in healthy-nonsmoker and healthy-smoker, and gene expression was assessed using microarrays. Relative to nonsmokers, Nrf2 protein in the small airway epithelium of smokers was activated and localized in the nucleus. The human homologs of 201 known murine Nrf2-modulated genes were identified, and 13 highly smoking-responsive Nrf2-modulated genes were identified. Construction of an Nrf2 index to assess the expression levels of these 13 genes in the airway epithelium of smokers showed coordinate control, an observation confirmed by quantitative PCR. This coordinate level of expression of the 13 Nrf2-modulated genes was independent of smoking history or demographic parameters. The Nrf2 index was used to identify two novel Nrf2-modulated, smoking-responsive genes, pirin (PIR) and UDP glucuronosyltransferase 1-family polypeptide A4 (UGT1A4). Both genes were demonstrated to contain functional antioxidant response elements in the promoter region. These observations suggest that Nrf2 plays an important role in regulating cellular defenses against smoking in the highly vulnerable small airway epithelium cells, and that there is variability within the human population in the Nrf2 responsiveness to oxidant burden.
Whereas cilia damage and reduced cilia beat frequency have been implicated as causative of reduced mucociliary clearance in smokers, theoretically mucociliary clearance could also be affected by cilia length. Based on models of mucociliary clearance predicting that cilia length must exceed the 6–7 µm airway surface fluid depth to generate force in the mucus layer, we hypothesized that cilia height may be decreased in airway epithelium of normal smokers compared to nonsmokers.
Cilia length in normal nonsmokers and smokers was evaluated in aldehyde-fixed, paraffin-embedded endobronchial biopsies, and air-dried and hydrated samples were brushed from human airway epithelium via fiberoptic bronchoscopy. In 28 endobronchial biopsies, healthy smoker cilia length was reduced by 15% compared to nonsmokers (p<0.05). In 39 air-dried samples of airway epithelial cells, smoker cilia length was reduced by 13% compared to nonsmokers (p<0.0001). Analysis of the length of individual, detached cilia in 27 samples showed that smoker cilia length was reduced by 9% compared to nonsmokers (p<0.05). Finally, in 16 fully hydrated, unfixed samples, smoker cilia length was reduced 7% compared to nonsmokers (p<0.05). Using genome-wide analysis of airway epithelial gene expression we identified 6 cilia-related genes whose expression levels were significantly reduced in healthy smokers compared to healthy nonsmokers.
Models predict that a reduction in cilia length would reduce mucociliary clearance, suggesting that smoking-associated shorter airway epithelial cilia play a significant role in the pathogenesis of smoking-induced lung disease.
The first changes associated with smoking are in the small airway epithelium (SAE). Given that smoking alters SAE gene expression, but only a fraction of smokers develop chronic obstructive pulmonary disease (COPD), we hypothesized that assessment of SAE genome-wide gene expression would permit biologic phenotyping of the smoking response, and that a subset of healthy smokers would have a “COPD-like” SAE transcriptome.
SAE (10th–12th generation) was obtained via bronchoscopy of healthy nonsmokers, healthy smokers and COPD smokers and microarray analysis was used to identify differentially expressed genes. Individual responsiveness to smoking was quantified with an index representing the % of smoking-responsive genes abnormally expressed (ISAE), with healthy smokers grouped into “high” and “low” responders based on the proportion of smoking-responsive genes up- or down-regulated in each smoker. Smokers demonstrated significant variability in SAE transcriptome with ISAE ranging from 2.9 to 51.5%. While the SAE transcriptome of “low” responder healthy smokers differed from both “high” responders and smokers with COPD, the transcriptome of the “high” responder healthy smokers was indistinguishable from COPD smokers.
The SAE transcriptome can be used to classify clinically healthy smokers into subgroups with lesser and greater responses to cigarette smoking, even though these subgroups are indistinguishable by clinical criteria. This identifies a group of smokers with a “COPD-like” SAE transcriptome.
KL-6 is a high-molecular-weight glycoprotein classified as a human MUC1 mucin. It was hypothesized that KL-6 could be detectable in the circulating blood and especially in airway secretions in lung diseases associated with mucus production such as chronic obstructive pulmonary disease (COPD). Additional aims of this study were to investigate whether the levels of KL-6 in plasma and sputum are related to ageing and smoking history.
The concentrations of KL-6 in plasma and induced sputum supernatants from young and/or middle aged/elderly non-smokers, smokers and patients with COPD were assayed by ELISA (n = 201). The subjects were classified into five groups according to age, smoking status and presence of COPD. In addition, KL-6 expression in control and diseased lung i.e. samples from patients with COPD (n = 28), were analyzed by immunohistochemistry and digital image analysis.
The plasma levels of KL-6 increased with age both in non-smokers and smokers. Among middle aged/elderly subjects, plasma KL-6 levels in all smokers regardless of COPD were significantly higher than in non-smokers, whereas sputum levels of KL-6 were significantly higher in COPD compared not only to non-smokers but also to smokers. KL-6 was more prominently expressed in the bronchiolar/alveolar epithelium in COPD than in the control lungs. Plasma and sputum KL-6 levels correlated inversely with obstruction and positively with smoking history and ageing. The linear multiple regression analysis confirmed that age and cigarette smoking had independent effects on plasma KL-6.
KL-6 increases with ageing and chronic smoking history, but prospective studies will be needed to elucidate the significance of KL-6 in chronic airway diseases.
The small airway epithelium and alveolar macrophages are exposed to oxidants in cigarette smoke leading to epithelial dysfunction and macrophage activation. In this context, we asked: what is the transcriptome of oxidant-related genes in small airway epithelium and alveolar macrophages, and does their response differ substantially to inhaled cigarette smoke?
Using microarray analysis, with TaqMan RT-PCR confirmation, we assessed oxidant-related gene expression in small airway epithelium and alveolar macrophages from the same healthy nonsmoker and smoker individuals.
Of 155 genes surveyed, 87 (56%) were expressed in both cell populations in nonsmokers, with higher expression in alveolar macrophages (43%) compared to airway epithelium (24%). In smokers, there were 15 genes (10%) up-regulated and 7 genes (5%) down-regulated in airway epithelium, but only 3 (2%) up-regulated and 2 (1%) down-regulated in alveolar macrophages. Pathway analysis of airway epithelium showed oxidant pathways dominated, but in alveolar macrophages immune pathways dominated.
Thus, the response of different cell-types with an identical genome exposed to the same stress of smoking is different; responses of alveolar macrophages are more subdued than those of airway epithelium. These findings are consistent with the observation that, while the small airway epithelium is vulnerable, alveolar macrophages are not "diseased" in response to smoking.
ClinicalTrials.gov ID: NCT00224185 and NCT00224198
We demonstrate that Pneumocystis reaches a >90% prevalence
peak at 3–5 months of age and associates with increased mucus (MUC5AC), suggesting
airway epithelium stimulation in infants during this age range. Host ability to clear
mucus would determine pathogenic expression.
Background. Pneumocystis without
obvious accompanying pathology is occasionally reported in autopsied infant lungs. Its
prevalence and significance are unknown. Interestingly, this mild infection induces a
strong activation of mucus secretion–related genes in young immunocompetent rodents
that has not been explored in infants. Excess mucus is induced by multiple airway
offenders through nonspecific pathways and would explain a cofactor role of
Pneumocystis in respiratory disease. We undertook characterization of
the prevalence of Pneumocystis and associated mucus in infant lungs.
Methods. Samples from 128 infants (mean age, 101 days)
who died suddenly and unexpectedly in Santiago during 1999–2004 were examined for
Pneumocystis using nested polymerase chain reaction (nPCR)
amplification of the P. jirovecii mtLSU ribosomal RNA gene and
immunofluorescence microscopy (IF). Pneumocystis-negative infants 28 days
and older and their age-closest positives were studied for MUC5AC expression and
Pneumocystis burden by Western blot and quantitative PCR,
Results. Pneumocystis DNA was
detected by nPCR in 105 of the 128 infants (82.0%) and
Pneumocystis organisms were visualized by IF in 99 (94.3%) of
the DNA-positive infants. The infection was commonest at 3–4 months with 40 of 41
(97.6%) infants of that age testing positive. MUC5AC was significantly increased in
Pneumocystis-positive tissue specimens (P =
.013). Death was unexplained in 113 (88.3%) infants; Pneumocystis
was detected in 95 (84.0%) of them vs 10 of 15 (66.7%) with explained death
(P = .28).
Conclusions. A highly focal
Pneumocystis infection associated to increased mucus expression is
almost universally present in the lungs of infants dying unexpectedly in the community
regardless of autopsy diagnosis.
immunocompetent; non-specific immune response; autopsy; MUC5AC; Sudden Infant Death Syndrome (SIDS)
Lectins are innate immune defense proteins that recognize specific bacterial cell wall components. Based on the knowledge that cigarette smoking is associated with increased risk of bacterial infections, we hypothesized that cigarette smoking may modulate the expression of lectin genes in airway epithelium. Affymetrix microarrays were used to survey expression of lectin genes in large airway epithelium from 9 nonsmokers and 20 healthy smokers and in small airway epithelium from 13 nonsmokers and 20 healthy smokers. There were no changes (>2-fold change, p<0.05) in lectin gene expression among healthy smokers compared to nonsmokers except for a striking down regulation of intelectin 1, a lectin that binds to galactofuranosyl residues in the cell walls of bacteria (large airway epithelium, p<0.01; small airway epithelium, p<0.01). This was confirmed by TaqMan RT-PCR in both large (p<0.05) and small airway epithelium (p<0.02). Immunohistochemistry assessment of airway biopsies demonstrated that intelectin 1 was expressed in secretory cells, while Western analysis confirmed the decreased expression of intelectin 1 in airway epithelium of healthy smokers compared to healthy nonsmokers (p<0.02). Finally, compared to healthy nonsmokers, intelectin 1 expression was also decreased in small airway epithelium of smokers with lone emphysema with normal spirometry (n= 13, p<0.01) and smokers with established COPD (n= 14, p<0.01). In the context that intelectin 1 is an epithelial molecule that likely plays a role in defense against bacteria, its down regulation in response to cigarette smoking is another example of the immunomodulatory effects of smoking on the immune system and may contribute to the increase in susceptibility to infections observed in smokers, including those with COPD.
Cystatin A (gene: CSTA), is up-regulated in non-small-cell lung cancer(NSCLC) and dysplastic vs normal human bronchial epithelium. In the context that chronic obstructive pulmonary disease (COPD), a small airway epithelium (SAE) disorder, is independently associated with NSCLC(especially squamous cell carcinoma, SCC), but only occurs in a subset of smokers, we hypothesized that genetic variation, smoking and COPD modulate CSTA gene expression levels in SAE, with further up-regulation in SCC. Gene expression was assessed by microarray in SAE of 178 individuals [healthy nonsmokers (n=60), healthy smokers (n=82), and COPD smokers (n=36)], with corresponding large airway epithelium (LAE) data in a subset (n=52). Blood DNA was genotyped by SNP microarray. Twelve SNPs upstream of the CSTA gene were all significantly associated with CSTA SAE gene expression(p<0.04 to 5 × 10 −4). CSTA gene expression levels in SAE were higher in COPD smokers (28.4 ± 2.0) than healthy smokers (19.9 ± 1.4, p<10−3), who in turn had higher levels than nonsmokers(16.1 ± 1.1, p<0.04). CSTA LAE gene expression was also smoking-responsive (p<10−3). Using comparable publicly available NSCLC expression data, CSTA was up-regulated in SCC vs LAE (p<10−2) and down-regulated in adenocarcinoma vs SAE (p <10−7). All phenotypes were associated with significantly different proportional gene expression of CSTA to cathepsins. The data demonstrate that regulation of CSTA expression in human airway epithelium is influenced by genetic variability, smoking, and COPD, and is further up-regulated in SCC, all of which should be taken into account when considering the role of CSTA in NSCLC pathogenesis.
cystatin; small airway epithelium; gene expression; genotype; COPD
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 (30 min) 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 (100µM and 1000µM), 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.
PAR-2; mucin; airway; secretion
Mucus overproduction is a characteristic of inflammatory pulmonary diseases including asthma, chronic bronchitis, and cystic fibrosis. Expression of two mucin genes, MUC2 and MUC5AC, and their protein products (mucins), is modulated in certain disease states. Understanding the signaling mechanisms that regulate the production and secretion of these major mucus components may contribute significantly to development of effective therapies to modify their expression in inflamed airways.
To study the differential expression of Muc2 and Muc5ac, a novel monoclonal antibody recognizing guinea pig Muc2 and a commercially-available antibody against human MUC5AC were optimized for recognition of specific guinea pig mucins by enzyme-linked immunosorbent assay (ELISA), Western blot, and immunohistochemistry (IHC). These antibodies were then used to analyze expression of Muc2 and another mucin subtype (likely Muc5ac) in guinea pig tracheal epithelial (GPTE) cells stimulated with a mixture of pro-inflammatory cytokines [tumor necrosis factor-α (TNF-α), interleukin 1β (IL-1β), and interferon- γ (IFN-γ)].
The anti-Muc2 (C4) and anti-MUC5AC (45M1) monoclonal antibodies specifically recognized proteins located in Muc2-dominant small intestinal and Muc5ac-dominant stomach mucosae, respectively, in both Western and ELISA experimental protocols. IHC protocols confirmed that C4 recognizes murine small intestine mucosal proteins while 45M1 does not react. C4 and 45M1 also stained specific epithelial cells in guinea pig lung sections. In the resting state, Muc2 was recognized as a highly expressed intracellular mucin in GPTE cells in vitro. Following cytokine exposure, secretion of Muc2, but not the mucin recognized by the 45M1 antibody (likely Muc5ac), was increased from the GPTE cells, with a concomitant increase in intracellular expression of both mucins.
Given the tissue specificity in IHC and the differential hybridization to high molecular weight proteins by Western blot, we conclude that the antibodies used in this study can recognize specific mucin subtypes in guinea pig airway epithelium and in proteins from GPTE cells. In addition, Muc2 is highly expressed constitutively, modulated by inflammation, and secreted differentially (as compared to Muc5ac) in GPTE cells. This finding contrasts with expression patterns in the airway epithelium of a variety of mammalian species in which only Muc5ac predominates.
Mucus hypersecretion is a distinguished feature of chronic inflammatory airway diseases. Interestingly, in this condition thyroid function is impaired with decreased level of triiodothyronine (T3), indicating potential link between low level of T3 and mucus hypersecretion. But the underlying mechanisms are poorly understood. In this study we aimed to elucidate the effect of T3 on MUC5AC secretion in human bronchial epithelial HBE16 cells and further investigate how T3 regulates MUC5AC gene expression at transcriptional level. By RT-PCR and ELISA we showed that T3 inhibited MUC5AC mRNA expression and protein secretion in HBE16 cells. Furthermore, luciferase assay and site-directed mutagenesis analysis demonstrated that T3 repressed MUC5AC expression at transcriptional level and the mechanism might partly lie in the specific inhibition of Sp1 binding to the promoter. Our results suggest that decreased T3 level leads to the release of repression of MUC5AC expression and thus contributes to mucus hypersecretion.
Mucus hypersecretion contributes to morbidity and mortality in many obstructive lung diseases. Gel-forming mucins are the chief glycoprotein components of airway mucus, and elevated expression of these during mucous metaplasia precedes the hypersecretory phenotype. Five orthologous genes (MUC2, MUC5AC, MUC5B, MUC6, and MUC19) encode the mammalian gel-forming mucin family, and several have been implicated in asthma, cystic fibrosis, and chronic obstructive pulmonary disease pathologies. However, in the absence of a comprehensive analysis, their relative contributions remain unclear. Here, we assess the expression of the entire gel-forming mucin gene family in allergic mouse airways and show that Muc5ac is the predominant gel-forming mucin induced. We previously showed that the induction of mucous metaplasia in ovalbumin-sensitized and -challenged mouse lungs occurs within bronchial Clara cells. The temporal induction and localization of Muc5ac transcripts correlate with the induced expression and localization of mucin glycoproteins in bronchial airways. To better understand the tight regulation of Muc5ac expression, we analyzed all available 5′-flanking sequences of mammalian MUC5AC orthologs and identified evolutionarily conserved regions within domains proximal to the mRNA coding region. Analysis of luciferase reporter gene activity in a mouse transformed Clara cell line demonstrates that this region possesses strong promoter activity and harbors multiple conserved transcription factor–binding motifs. In particular, SMAD4 and HIF-1α bind to the promoter, and mutation of their recognition motifs abolishes promoter function. In conclusion, Muc5ac expression is the central event in antigen-induced mucous metaplasia, and phylogenetically conserved 5′ noncoding domains control its regulation.
mucin; metaplasia; airway; lung; epithelium
To examine the in vivo effects of the 15-member macrolide, azithromycin (AZM), on mucus hypersecretion, we induced hypertrophic and metaplastic changes of goblet cells in rat nasal epithelium by intranasal instillation of ovalbumin (OVA) in OVA-sensitized rats, or by intranasal lipopolysaccharides (LPS) instillation. Oral administration of AZM (5–10 mg/kg) or clarithromycin (CAM, 5–10 mg/kg) significantly inhibited OVA- and LPS-induced mucus production, whereas josamycin (JM) or ampicillin (ABPC) showed no effect. In vitro effects of AZM on airway epithelial cells were examined using NCI-H292 cells and human nasal epithelial cells cultured in air-liquid interface. Mucus secretion was evaluated by enzyme-linked immunosorbent assay using an anti-MUC5AC monoclonal antibody. AZM or CAM significantly inhibited tumor necrosis factor-α (TNF-α) (20 ng/mL)-induced MUC5AC secretion from NCI-H292 cells at 10−6–10−7 M, whereas JM or ABPC showed no effect. AZM significantly inhibited TNF-α (20 ng/mL)-induced MUC5AC secretion from human nasal epithelial cells at 10−4 M. MUC5AC mRNA expression was also significantly inhibited. These results indicate that the 15-member macrolide, AZM, exerts direct inhibitory effects on mucus secretion from airway epithelial cells and that it may be useful for the treatment of mucus hypersecretion caused by allergic inflammation and LPS stimulation.
The human airway epithelium consists of 4 major cell types: ciliated, secretory, columnar and basal cells. During natural turnover and in response to injury, the airway basal cells function as stem/progenitor cells for the other airway cell types. The objective of this study is to better understand human airway epithelial basal cell biology by defining the gene expression signature of this cell population.
Bronchial brushing was used to obtain airway epithelium from healthy nonsmokers. Microarrays were used to assess the transcriptome of basal cells purified from the airway epithelium in comparison to the transcriptome of the differentiated airway epithelium. This analysis identified the “human airway basal cell signature” as 1,161 unique genes with >5-fold higher expression level in basal cells compared to differentiated epithelium. The basal cell signature was suppressed when the basal cells differentiated into a ciliated airway epithelium in vitro. The basal cell signature displayed overlap with genes expressed in basal-like cells from other human tissues and with that of murine airway basal cells. Consistent with self-modulation as well as signaling to other airway cell types, the human airway basal cell signature was characterized by genes encoding extracellular matrix components, growth factors and growth factor receptors, including genes related to the EGF and VEGF pathways. Interestingly, while the basal cell signature overlaps that of basal-like cells of other organs, the human airway basal cell signature has features not previously associated with this cell type, including a unique pattern of genes encoding extracellular matrix components, G protein-coupled receptors, neuroactive ligands and receptors, and ion channels.
The human airway epithelial basal cell signature identified in the present study provides novel insights into the molecular phenotype and biology of the stem/progenitor cells of the human airway epithelium.
Chronic airway disorders, including chronic obstructive pulmonary disease (COPD), cystic fibrosis, and asthma, are associated with persistent pulmonary inflammation and goblet cell metaplasia and contribute to significant morbidity and mortality worldwide. While the molecular pathogenesis of these disorders is actively studied, little is known regarding the transcriptional control of goblet cell differentiation and mucus hyperproduction. Herein, we demonstrated that pulmonary allergen sensitization induces expression of FOXM1 transcription factor in airway epithelial and inflammatory cells. Conditional deletion of the Foxm1 gene from either airway epithelium or myeloid inflammatory cells decreased goblet cell metaplasia, reduced lung inflammation, and decreased airway resistance in response to house dust mite allergen (HDM). FOXM1 induced goblet cell metaplasia and Muc5AC expression through the transcriptional activation of Spdef. FOXM1 deletion reduced expression of CCL11, CCL24, and the chemokine receptors CCR2 and CX3CR1, resulting in decreased recruitment of eosinophils and macrophages to the lung. Deletion of FOXM1 from dendritic cells impaired the uptake of HDM antigens and decreased cell surface expression of major histocompatibility complex II (MHC II) and costimulatory molecule CD86, decreasing production of Th2 cytokines by activated T cells. Finally, pharmacological inhibition of FOXM1 by ARF peptide prevented HDM-mediated pulmonary responses. FOXM1 regulates genes critical for allergen-induced lung inflammation and goblet cell metaplasia.
Purpose of review
Airway mucus plugging has long been recognized as a principal cause of death in asthma. However, molecular mechanisms of mucin overproduction and secretion have not been understood until recently. These mechanisms are reviewed together with ongoing investigations relating them to lung pathophysiology.
Of the five secreted gel-forming mucins in mammals, only MUC5AC and MUC5B are produced in significant quantities in intrapulmonary airways. MUC5B is the principal gel-forming mucin at baseline in small airways of humans and mice, and therefore likely performs most homeostatic clearance functions. MUC5AC is the principal gel-forming mucin upregulated in airway inflammation and is under negative control by forkhead box a2 and positive control by hypoxia inducible factor-1. Mucin secretion is regulated separately from production, principally by extracellular triphosphate nucleotides that bind P2Y2 receptors on the lumenal surface of airway secretory cells, generating intracellular second messengers that activate the exocytic proteins, Munc13-2 and synaptotagmin-2.
Markedly upregulated production of MUC5AC together with stimulated secretion leads to airflow obstruction in asthma. As MUC5B appears to mediate homeostatic functions, it may be possible to selectively inhibit MUC5AC production without impairing airway function. The precise roles of mucin hypersecretion in asthma symptoms such as dyspnea and cough and in physiologic phenomena such as airway hyperresponsiveness remain to be defined.
airway; asthma; mucin; mucous; mucus
Mucus overproduction in inflammatory and obstructive airway diseases is associated with goblet cell (GC) metaplasia in airways. Although the mechanisms involved in GC metaplasia and mucus hypersecretion are not completely understood, association with oxidative stress and epidermal growth factor receptor (EGFR) signaling has been reported. To explore the mechanisms involved in oxidative stress–induced GC metaplasia, cultures of differentiated normal human bronchial epithelial cells grown at the air–liquid interface were exposed to reactive oxygen species (ROS) generated by xanthine/xanthine oxidase. EGFR activation and signaling was assessed by measuring EGF and transforming growth factor-α release and EGFR and 44/42MAPK phosphorylation. The GC population was evaluated by confocal microscopy. ROS-induced EGFR activation resulted in GC proliferation and increased MUC5AC gene and protein expression. Signaling was due to pro-EGF processing by tissue kallikrein (TK), which was activated by ROS-induced hyaluronan breakdown. It was inhibited by catalase, a TK inhibitor, and EGF-blocking antibodies. Exposure to recombinant TK mimicked the ROS effects, increasing the expression of MUC5AC and lactoperoxidase. In addition, ROS induced the antiapoptotic factor Bcl-2 in a TK-dependent fashion. In conclusion, ROS-induced GC metaplasia in normal human bronchial epithelial cells is associated with HA depolymerization and EGF processing by TK followed by EGFR signaling, suggesting that increases in TK activity could contribute to GC metaplasia and mucus hypersecretion in diseases such as asthma and chronic bronchitis. The data also suggest that increases in GC population could be sustained by the associated upregulation of Bcl-2 in airway epithelial cells.
EGFR; goblet cells; hyaluronan; NHBE cells; tissue kallikrein
Background—The intestinal epithelium is covered
by a continuous layer of mucus which is secreted by well differentiated
epithelial cells. Disregulation of the expression of mucins has been
reported to have possible implications in the neoplastic process which affects intestinal mucosae. It is well known that preneoplastic and
neoplastic tissues can express fetal phenotypic characteristics.
Aims—To assess whether the expression of mucin
genes in the intestinal tract is linked to the stage of cellular
differentiation and tissue development, by studying the expression of
six mucin genes in human fetal small intestine and colon, and also
Methods—In situ hybridisation was used to study
mRNA expression of MUC2, MUC3, MUC4, MUC5B, MUC5AC, and MUC6 in 32 human embryos and fetuses (6.5-27 weeks gestation). Normal adult
mucosae were used as controls.
Results—Three mucin genes, MUC2, MUC4, and
MUC5AC, were differently expressed in fetal intestine compared with
expression in normal adults.
Conclusion—These differences in mucin gene
expression suggest a possible regulatory role for these products in
intestinal epithelial cell differentiation.
mucin genes; mucins; intestine; differentiation; human fetus
The epithelial cells lining the airways serve protective functions. The "barrier function" of the epithelium protects the individual from damage by inhaled irritants. The epithelium produces mucins which become hydrated and form a viscoelastic gel which spreads over the epithelial surface. In healthy individuals inhaled foreign materials become entrapped in the mucus and are cleared by mucociliary transport and by coughing. In many chronic inflammatory airway diseases, however, excessive mucus is produced and is inadequately cleared, leading to mucous obstruction and infection. At present there is no specific treatment for hypersecretion. However, the discovery that an epidermal growth factor receptor (EGFR) cascade is involved in mucin production by a wide variety of stimuli suggests that blockade may provide specific treatment for hypersecretory diseases. EGFR pathways have also been implicated in the repair of damaged airway epithelium. The roles of EGFR in airway epithelial cell hypersecretion and epithelial damage and repair are reviewed and future potential treatments are suggested.