Mucus hypersecretion in the airway

Mucus hypersecretion is a distinguishing feature of chronic inflammation diseases, such as asthma,1 chronic bronchitis,2 bronchiectasis3 and cystic fibrosis.4 Mucus hypersecretion leads to impairment of mucociliary clearance, abnormal bacterial plantation, mucus plug in the airway, and dysfunction of gas exchange.5 To block this vicious cycle, chronic inflammation in the airway must be controlled and mucus hypersecretion must be reduced. It is important for physicians in the prevention of mucus hypersecetion to learn about the characteristics of mucus, mechanism of mucus hypersecretion and transportation of mucus. This review will focus on the data reported by Chinese researchers in the last few years. MUCUS AND MUCIN The airway surface is covered by mucus secreted by goblet cells and the submucosal gland. Mucus protects the underlying airway epithelium from dehydration, pathogens, and chemical and particulate irritants, which consist mainly of water (95%), combined with salts, lipids, proteins, and glycoproteins. Yet glycoproteins, the mucins, provide the viscoelastic properties. Mucins are large molecules composed of 50% to 85% carbohydrates. Each mucin protein contains an apomucin core that is enriched by hydroxyamino acids, serine, and threonine.6 Fifteen mucin genes have been identified and sequenced, and their tissue-specific expression has been assessed. Based on common structural motifs, mucins have been divided into the following two groups: membrane- associated mucins and secreted mucins. Membrane- associated mucin genes include MUC1, MUC3A, MUC3B, MUC4, MUC9, MUC11, and MUC12, as well as Muc14 in rodents. These molecules are thought to have roles in cell-cell communication including cell adhesion, cell recognition, and signaling, and may be involved in tumor cell invasion and metastasis.7 Mucins found in secretions include MUC2, MUC5AC, MUC5B, MUC7, and MUC8. Common to all members of this subgroup is a central region with several repeats of a D-domain. Although details are limited regarding how secreted mucins are processed in the cell, cysteines contained in the D-domain could provide for polypeptide crosslinking and packaging within secretory granules. MUC2, MUC5AC and MUC5B are the main airway mucins produced by two different cell types: goblet cells of the surface epithelium and mucous cells of the submucosal gland. Airway goblet cells produce mainly MUC5AC. MUC5AC mRNA is well expressed in normal airway tissues and its gene products have been identified in lung mucus from patients with asthma, chronic bronchitis, bronchiectasis and cystic fibrosis. MUC5B mRNA is located in the submucosal glands in healthy subjects’ airways. Patients with secondary (such as acute bronchitis) or inflammatory conditions (such as gastroesophageal reflux or allergen/air pollutant exposure) show the expression of MUC5B mRNA both in their submucosal gland cells and in surface goblet cells.8 Obviously, a change in MUC expression by goblet cells is a sequela of pulmonary disease, and therefore is likely to affect the composition and biophysical properties of airway mucus. Mucin gene expression may be altered in pulmonary diseases that have classically been considered disorders of the interstitium. It has been shown that in bacterial infection, more specifical endotoxin released from gram-negative bacteria can stimulate MUC2 gene expression which is normally undetectable in airway goblet cells.9 MUC GENE ABNORMALITIES AND REGULATION MECHANISMS Mucus hypersecretion is accompanied by goblet cell hyperplasia and MUC gene abnormalities. Chen et al10 found that the increase of goblet cell number is due to changes in stored and secreted mucin. The functional consequences of these changes in MUC stores and secretion can contribute to the pathophysiologic mechanisms of multiple abnormalities in patients with chronic inflammatory diseases, including sputum production, airway narrowing, exacerbations, and accelerated loss of lung function. Therefore, the increase of MUC5AC gene expression could be the symbol of goblet cell hyperplasia and mucus hypersecretion.11 In 2004, an investigation12 revealed that gob-5, a member of Ca2+ activated Cl- channel which was cloned from a mouse intestinal cDNA library and strongly expressed in lung tissues from airway hyperreactivity (AHR)-model mice, might be an important gene in inducing airway mucus hypersecretion of asthma. There was a significant association between the increase of gob-5 mRNA expression and that of MUC5AC mRNA expression in the asthmatic group. Niflumic acid (NFA), an inhibitor of gob-5, can inhibit the increase in the cell counting and synthesis of mucus of goblet cells in asthmatic mice, through inhibition of activity and expression gob-5.13 Similarly, others10,14 confirmed CaCC1, which had the high structural homology and similarity of tissue distribution with gob-5, promoted the proliferation and mucin synthesis in airway goblet cells in the NCI-H292 cell line and asthma patients respectively. These studies indicated that the Ca2+-activated Cl- channel gene might regulate MUC5AC gene expression. And the inhibition of CaCC1 might be a potential treatment for mucus hyperproduction in patients with chronic inflammatory diseases. AQP5, another important gene in regulating mucus hypersecretion, is a highly permeable water channel, mainly expressed in type I alveolar epithelial cells and submucosal gland acinar cells. Wang et al15 investigated that the expression of AQP5 in the airway of patients with decreased chronic obstructive pulmonary disease (COPD) compared to control patients. Furthermore, attenuated expression of AQP5 was related to the severity of airflow obstruction and negatively correlated with the expression levels of MUC5AC in the airway epithelium and mucins in the submucosal glands. More than this, Chen et al16,17 found that the AQP5 gene could regulate the expression of MUC5AC gene and proteins reversely. AQP5 mRNA was significantly reduced by 75.1% one day after transfection with specific vector-based short hairpin RNA (shRNA) in the human airway submucosal gland cell line (SPC-A1), named shAQP5. However, the significant suppression of AQP5 protein did not appear until day 5 after transfection. MUC5AC mRNA was remarkably increased by 119.9% on day 3 after shAQP5 transfection, while comparable MUC5AC protein changes were not found in SPC-A1 cells with flow cytometry analysis. This is the first investigation providing evidence demonstrating the regulation of the mucin gene by AQP5 gene silencing. Thus, AQP5 can influence mucus secretion by fluids and mucin. The expressions of mucins in airway are regulated in both transcriptional level and post-transcriptional level (such as glycosylation). Zhou et al18 confirmed that β-1, 4-GT and GLcNAc-6-O-ST mRNA expressions of lung tissue in elder patients with COPD were significantly higher than those of normal lung tissue. The expressions were enhanced about 4 times and 3.5 times levels of the normal respectively in remission stage of COPD and were increased about 6 times and 5 times levels of the normal in the acute attack stage. Thus the high transcriptional levels of β-1, 4-GT and GLcNAc-6-O-ST in the lung tissue of elder patients COPD could be the important molecular basis of intensification of the glycosylated and sulfated degree of the airway hypersecretory mucin. ROLES OF CYTOKINES Airway epithelium cells have emerged as critical regulators of inflammatory networks in the chronic airway inflammation. Being a barrier defending the airway from irritants, the epithelium is also a source of cytokines. Many cytokines can also regulate the expression of mucin. A recent study highlighted that IL-13 acted in combination with the increase of the expression of gob-5 and MUC5AC in asthmatic mice models.19 It suggests that IL-13 is an important cytokine in the pathogenesis of airway inflammation and the induction of mucus hypersecretion. Furthermore, IL-4 and IL-9 play a similar role in the mucus hypersecretion. Huang et al20 found that TNF-α could lead to mucus hypersecretion by PKC signal transduction. Much attention has been paid to the important role of epithelial growth factor receptor (EGFR) involving in the hyperproduction of mucus and epithelial proliferation. Mao et al21 reported that weak EGFR protein signals were detected in the lungs of control subjects in comparison with stronger signal in COPD patients and smokers. EGFR immunoreactivity observed mainly in goblet cells in the controls was higher than that in airways in the COPD patients or the smokers. In contrast, airways in the COPD patients or smokers showed more expression of EGFR mainly in basal cells than in control airways. There was a significant positive correlation between EGFR immunoreactivity and the area of MUC5AC positive staining in all subjects, suggesting that EGFR activation was involved in mucin expression in COPD airways. Moreover, TNF-α can upregulate the expression of EGFR, which increases the EGF and TNF-α’s induction of expression of MUC5AC. SIGNAL TRANSDUCTION IN MUCUS HYPERSECRETION Myristoylated alanine-rich C kinase substrate (MARCKS) is a central regulatory molecule linking secretagogue stimulation at the cell surface to mucin granule release by differentiated normal human bronchial epithelial cells in vitro. It was found that in the rat model of airway mucus hypersecretion induced by aerosol acrolein inhalation, intratracheal instillation of MARKS-related peptide can attenuate mucus secretion. The intracellular mechanism controlling this secretory process involves cooperative action of two separate protein kinases, protein kinase C and cGMP-dependent protein kinase. Upon stimulation, activated protein kinase C phosphorylates MARCKS, causing translocation of MARCKS from the plasma membrane to the cytoplasm, where it is then dephosphorylated by a protein phosphatase 2A that is activated by cGMP-dependent protein kinase, and associates with both actin and myosin. Dephosphorylated cytoplasmic MARCKS would also be free to interact with mucin granule membranes and thus could link granules to the contractile cytoskeleton, mediating their movement to the cell periphery and subsequent exocytosis. These findings suggest that there are several novel intracellular targets for pharmacological intervention in disorders involving aberrant secretion of respiratory mucin and they may relate to other lesions involving exocytosis of membrane-bound granules in various cells and tissues. It was investigated that silaenafil could increase the expression of cGMP and decrease the expression of TNF-α, which could lead to the decrease of the expression of MUC5AC stimulated by acrolein, involved in TNF-α/EGFR signaling. Ou et al22 found that LPS significantly induced the expression of MUC5AC mRNA and protein in bronchial epithelia, the staining of NF-κB in cytoplasm, and the nuclear translocation ratio in airway epithelial cells. The upregulated expression of MUC5AC mRNA was positively correlated with NF-κB activation and cytokine concentration. Roxithromycin significantly suppressed bronchial MUC5AC expression, and NF-κB nuclear translocation was stimulated by LPS. There was a significant correlation between the ratio of NF-κB nuclear translocation and the expression level of MUC5AC mRNA. Roxithromycin inhibits pulmonary inflammatory response and airway mucus hypersecretion induced by LPS, which might be ascribed to inhibition of NF-κB activation. The effect of NF-κB activation was also confirmed by Wang et al,23 and Liu et al24 investigated that rosiglitazone, an agonist of peroxisome proliferators activated receptor gamma (PPAR-γ), could also decrease the expressions of TNF-α, IL-8 and suppress mucus hypersecretion in rat airways in the way of NF-κB signaling. Xu et al25 realized that suppressive effect on mucus of macrolides was indirectly realized by inhibiting NE release because of macrolides endocytosis in neutrohpils. It was found that Toll-like receptor 4 mediated endotoxin-induced airway mucus hypersecretion and dexamethasone could inhibit this effect. PROSPECTS The mechanism of mucus secretion and the regulation of expression of mucins are obscure. The ultimate goal for study of airway mucins and mucus is to circumvent the processes that result in mucin overproduction and thus to prevent from mucus obstruction of the airways. Some ordinary drugs, such as ipratropium bromide, glucocorticosteroid and roxithromycin are the confirmable inhibitors of mucus hypersecretion. Ambroxol hydrochloride, bromhexim and broncholysin are the ordinary apophlegmatisants used. But it is difficult to maintain the balance between “healthy” mucus secretion and hypersecretion. At the present, most results come from animal models or cells test. More work should be focused on the hypersecretion of mucus.