Regulation of mucin gene expression in human tracheobronchial epithelial cells by thyroid hormone

Triiodothyronine represses MUC5AC expression by antagonizing Sp1 binding to its promoter in human bronchial epithelial HBE16 cells

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.

Apical localization of zinc transporter ZnT4 in human airway epithelial cells and its loss in a murine model of allergic airway inflammation

The apical cytoplasm of airway epithelium (AE) contains abundant labile zinc (Zn) ions that are involved in the protection of AE from oxidants and inhaled noxious substances. A major question is how dietary Zn traffics to this compartment. In rat airways, in vivo selenite autometallographic (Se-AMG)-electron microscopy revealed labile Zn-selenium nanocrystals in structures resembling secretory vesicles in the apical cytoplasm. This observation was consistent with the starry-sky Zinquin fluorescence staining of labile Zn ions confined to the same region. The vesicular Zn transporter ZnT4 was likewise prominent in both the apical and basal parts of the epithelium both in rodent and human AE, although the apical pools were more obvious. Expression of ZnT4 mRNA was unaffected by changes in the extracellular Zn concentration. However, levels increased 3-fold during growth of cells in air liquid interface cultures and decreased sharply in the presence of retinoic acid. When comparing nasal versus bronchial human AE cells, there were significant positive correlations between levels of ZnT4 from the same subject, suggesting that nasal brushings may allow monitoring of airway Zn transporter expression. Finally, there were marked losses of both basally-located ZnT4 protein and labile Zn in the bronchial epithelium of mice with allergic airway inflammation. This study is the first to describe co-localization of zinc vesicles with the specific zinc transporter ZnT4 in airway epithelium and loss of ZnT4 protein in inflamed airways. Direct evidence that ZnT4 regulates Zn levels in the epithelium still needs to be provided. We speculate that ZnT4 is an important regulator of zinc ion accumulation in secretory apical vesicles and that the loss of labile Zn and ZnT4 in airway inflammation contributes to AE vulnerability in diseases such as asthma.

Induction of MUC5AC mucin by conjugated bile acids in the esophagus involves the phosphatidylinositol 3-kinase/protein kinase C/activator protein-1 pathway

BACKGROUND

Bile reflux contributes to the development of esophageal injury and neoplasia. The mucin 5AC (MUC5AC) is absent in the normal squamous epithelium of the esophagus but is strongly expressed in Barrett esophagus (BE). The objective of this study was to determine whether and how bile acids influence the expression of MUC5AC in the esophagus.

METHODS

MUC5AC expression was studied by immunohistochemistry and immunoblotting in human tissues, in tissues from a rat model of BE, and in SKGT-4 cultured esophageal epithelial cells. MUC5AC transcription was studied by real-time polymerase chain reaction and transient transfection assays.

RESULTS

MUC5AC was absent from normal squamous epithelium but was present in 100% of Barrett specimens and in 61.5% of human esophageal adenocarcinoma tissues that were examined. MUC5AC protein expression was induced to a greater degree by conjugated bile acids than by unconjugated bile acids, and this occurred at the transcriptional level. In the rat reflux model, MUC5AC mucin was expressed abundantly in tissues of BE stimulated by duodenoesophageal reflux. Conjugated bile acids induced AKT phosphorylation in SKGT-4 cells but had no effect on extracellular signal-regulated protein kinases 1 and 2, c-Jun N-terminal kinase, or protein-38 kinase phosphorylation. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 and a dominant-negative protein kinase C (AKT) construct prevented the induction of MUC5AC by conjugated bile acids. Transactivation of AP-1 by conjugated bile acids coincided with MUC5AC induction, and cotransfection with a dominant-negative activator protein-1 (AP-1) vector decreased MUC5AC transcription and its induction.

CONCLUSIONS

Conjugated bile acids in the bile refluxate contribute to MUC5AC induction in the esophagus. This occurs at the level of transcription and involves activation of the PI3K/AKT/AP-1 pathway.

CREB mediates prostaglandin F2alpha-induced MUC5AC overexpression

Mucus secretion is an important protective mechanism for the luminal lining of open tubular organs, but mucin overproduction in the respiratory tract can exacerbate the inflammatory process and cause airway obstruction. Production of MUC5AC, a predominant gel-forming mucin secreted by airway epithelia, can be induced by various inflammatory mediators such as prostaglandins. The two major prostaglandins involved in inflammation are PGE(2) and PGF(2alpha). PGE(2)-induced mucin production has been well studied, but the effect of PGF(2alpha) on mucin production remains poorly understood. To elucidate the effect and underlying mechanism of PGF(2alpha) on MUC5AC production, we investigated the signal transduction of PGF(2alpha) associated with this effect using normal human tracheobronchial epithelial cells. Our results demonstrated that PGF(2alpha) induces MUC5AC overproduction via a signaling cascade involving protein kinase C, ERK, p90 ribosomal S6 protein kinase, and CREB. The regulation of PGF(2alpha)-induced MUC5AC expression by CREB was further confirmed by cAMP response element-dependent MUC5AC promoter activity and by interaction between CREB and MUC5AC promoter. The abrogation of all downstream signaling activities via suppression of each signaling molecule along the pathway indicates that a single pathway from PGF(2alpha) receptor to CREB is responsible for inducing MUC5AC overproduction. As CREB also mediates mucin overproduction induced by PGE(2) and other inflammatory mediators, our findings have important clinical implications for the management of airway mucus hypersecretion.

Deoxycholic acid induces the overexpression of intestinal mucin, MUC2, via NF-kB signaling pathway in human esophageal adenocarcinoma cells

Background

Mucin alterations are a common feature of esophageal neoplasia, and alterations in MUC2 mucin have been associated with tumor progression in the esophagus. Bile acids have been linked to esophageal adenocarcinoma and mucin secretion, but their effects on mucin gene expression in human esophageal adenocarcinoma cells is unknown.

Methods

Human esophageal adenocarcinoma cells were treated 18 hours with 50–300 μM deoxycholic acid, chenodeoxycholic acid, or taurocholic acid. MUC2 transcription was assayed using a MUC2 promoter reporter luciferase construct and MUC2 protein was assayed by Western blot analysis. Transcription Nuclear factor-κB activity was measured using a Nuclear factor-κB reporter construct and confirmed by Western blot analysis for Nuclear factor-κB p65.

Results

MUC2 transcription and MUC2 protein expression were increased four to five fold by bile acids in a time and dose-dependent manner with no effect on cell viability. Nuclear factor-κB activity was also increased. Treatment with the putative chemopreventive agent aspirin, which decreased Nuclear factor-κB activity, also decreased MUC2 transcription. Nuclear factor-κB p65 siRNA decreased MUC2 transcription, confirming the significance of Nuclear factor-κB in MUC2 induction by deoxycholic acid. Calphostin C, a specific inhibitor of protein kinase C (PKC), greatly decreased bile acid induced MUC2 transcription and Nuclear factor-κB activity, whereas inhibitors of MAP kinase had no effect.

Conclusion

Deoxycholic acid induced MUC2 overexpression in human esophageal adenocarcinoma cells by activation of Nuclear factor-κB transcription through a process involving PKC-dependent but not PKA, independent of activation of MAP kinase.

Regulation of mucin gene expression by CREB via a nonclassical retinoic acid signaling pathway

Vitamin A and its metabolite retinoic acid (RA) are essential elements for normal lung development and the differentiation of lung epithelial cells. We previously showed that RA rapidly activated cyclic AMP response element-binding protein (CREB) in a nonclassical manner in normal human tracheobronchial epithelial (NHTBE) cells. In the present study, we further demonstrated that this nonclassical signaling of RA on the activation of CREB plays a critical role in regulating the expression of airway epithelial cell differentiation markers, the MUC2, MUC5AC, and MUC5B genes. We found that RA rapidly activates the protein kinase Calpha isozyme and transmits the activation signal to CREB via the Raf/MEK/extracellular signal-regulated kinase/p90 ribosomal S6 kinase (RSK) pathway. Activated RSK translocated from the cytoplasm to the nucleus, where it phosphorylates CREB. Activated CREB then binds to a cis-acting replication element motif on the promoter (at nucleotides [nt] -878 to -871) of the MUC5AC gene. The depletion of CREB using small interfering RNA abolished not only the RA-induced MUC5AC but also RA-induced MUC2 and MUC5B. Taken together, our findings demonstrate that CREB activation via this nonclassical RA signaling pathway may play an important role in regulating the expression of mucin genes and mediating the early biological effects of RA during normal mucous differentiation in NHTBE cells.

Nonclassical action of retinoic acid on the activation of the cAMP response element-binding protein in normal human bronchial epithelial cells

Vitamin A (retinol) is essential for normal regulation of cell growth and differentiation. We have shown that the retinol metabolite retinoic acid (RA) induces mucous cell differentiation of normal human tracheobronchial epithelial (NHTBE) cells. However, early biological effects of RA in the differentiation of bronchial epithelia are largely unknown. Here, we showed that RA rapidly activated cAMP response element-binding protein (CREB). However, RA did not use the conventional retinoic acid receptor (RAR)/retinoid X receptor (RXR) to activate CREB. RA activated CREB in NHTBE and H1734 cells in which RARs/RXR were silenced with small interfering RNA (siRNA) targeting RAR/RXR expression or deactivated by antagonist. Inhibition of protein kinase C (PKC) or extracellular regulated kinase (ERK1/2) blocked the RA-mediated activation of CREB. In addition, depletion of p90 ribosomal S6 kinase (RSK) via siRSK1/2 completely abolished the activation, suggesting that PKC, ERK, and RSK are required for the activation. Altogether, this study provides the first evidence that RA rapidly activates CREB transcription factor via PKC, ERK, and RSK in a retinoid receptor-independent manner in normal bronchial epithelial cells. This noncanonical RA signaling pathway may play an important role in mediating early biological effects in the mucociliary differentiation of bronchial epithelia.

Galectin-3 modulates MUC2 mucin expression in human colon cancer cells at the level of transcription via AP-1 activation

BACKGROUND & AIMS

Galectin-3 and MUC2 intestinal mucin each have been correlated with the malignant behavior of colon cancer cells. Galectin-3 modulates expression of MUC2 protein, but the specific regulatory mechanisms are unknown. This study sought to determine how galectin-3 increases MUC2 expression.

METHODS

Galectin-3 levels in human colon cancer cells of high and low metastatic ability were manipulated via expression of galectin-3 complementary DNA in sense or antisense orientation. Galectin-3 and MUC2 protein expression were determined by Western analysis and immunocytochemistry. Transient transfections of promoter reporter constructs were used to monitor MUC2 transcription and AP-1 activity. Electrophoretic mobility shift assays, site-directed mutagenesis, and chromatin immunoprecipitation were used to monitor the participation of AP-1 in MUC2 transcription.

RESULTS

Alterations in galectin-3 levels correlated with both MUC2 protein expression and transcription. By using MUC2 promoter constructs of different lengths, galectin-3 responsiveness was found between 1500 and 2186 bp upstream of the translation start site, a region that contains 1 consensus AP-1 binding site. AP-1 activity paralleled MUC2 transcription in the different cell lines. Mutation in the AP-1 site markedly decreased MUC2 promoter activity, and MUC2 transcription was inhibited by cotransfection with a dominant-negative AP-1 vector. Electrophoretic mobility shift assays, co-immunoprecipitation, and chromatin immunoprecipitation analyses suggested an association between galectin-3, c-Jun, and Fra-1 in forming a complex at the AP-1 site on the MUC2 promoter.

CONCLUSIONS

Galectin-3 up-regulation of MUC2 transcription occurs at the level of transcription through AP-1 activation. This may have important implications for understanding the role of galectin-3 and MUC2 in colon cancer metastasis.

Interleukin-1beta-induced mucin production in human airway epithelium is mediated by cyclooxygenase-2, prostaglandin E2 receptors, and cyclic AMP-protein kinase A signaling

We reported recently that interleukin (IL)-1beta exposure resulted in a prolonged increase in MUC5AC mucin production in normal, well differentiated, human tracheobronchial epithelial (NHTBE) cell cultures, without significantly increasing MUC5AC mRNA (Am J Physiol 286:L320-L330, 2004). The goal of the present study was to elucidate the signaling pathways involved in IL-1beta-induced MUC5AC production. We found that IL-1beta increased cyclooxygenase-2 (COX-2) mRNA expression and prostaglandin (PG) E(2) production and that the COX-2 inhibitor celecoxib suppressed IL-1beta-induced MUC5AC production. Addition of exogenous PGE(2) to NHTBE cultures also increased MUC5AC production and IL-1beta-induced Muc5ac hypersecretion in tracheas from wild-type but not from COX-2-/- mice. NHTBE cells expressed all four E-prostanoid (EP) receptor subtypes and misoprostol, an EP2 and EP4 agonist, increased MUC5AC production, whereas sulprostone, an EP1 and EP3 agonist, did not. Furthermore, specific protein kinase A (PKA) inhibitors blocked IL-1beta and PGE(2)-induced MUC5AC production. However, neither inhibition of epidermal growth factor receptor (EGFR) activation with the tyrosine kinase inhibitor 4-(3-chloroanilino)-6,7-dimethoxyquinazoline HCl (AG-1478) or EGFR blocking antibody nor inhibition of extracellular signal-regulated kinase/P-38 mitogen activated protein kinases with specific inhibitors blocked IL-1beta stimulation of MUC5AC mucin production. We also observed that tumor necrosis factor (TNF)-alpha, platelet activating factor (PAF), and lipopolysaccharide (LPS) induced COX-2 and increased MUC5AC production that was blocked by celecoxib, suggesting a common signaling pathway of inflammatory mediator-induced MUC5AC production in NHTBE cells. We conclude that the induction of MUC5AC by IL-1beta, TNF-alpha, PAF, and LPS involves COX-2- generated PGE(2), activation of EP2 and/or EP4 receptor(s), and cAMP-PKA-mediated signaling.

Interleukin-1beta induces MUC2 and MUC5AC synthesis through cyclooxygenase-2 in NCI-H292 cells

Interleukin-1beta (IL-1beta) has been implicated in the pathogenesis of inflammatory diseases of the airway. In this study, we investigated the regulation of MUC2 and MUC5AC expression and of their regulatory mechanisms through cyclooxygenase-2 (COX-2) and prostaglandin E(2) (PGE(2)). Cells activated by IL-1beta showed increased COX-2, MUC2, and MUC5AC expressions at both the mRNA and protein levels. Mucin production was blocked by the selective COX-2 inhibitor NS398, and PGE(2) directly induced MUC2 and MUC5AC expression at both the mRNA and protein levels in a dose-dependent manner. These results suggest a role for PGE(2) in IL-1beta-induced mucin synthesis in NCI-H292 cells. To investigate the roles of molecules upstream of COX-2 in mucin regulation, we examined the role of mitogen-activated protein kinases (MAPKs). Cells activated by IL-1beta showed increased extracellular signal-regulated kinase (ERK)1/2 and p38 phosphorylation, and IL-1beta-induced MUC2 and MUC5AC production was blocked by the ERK pathway inhibitor PD98059 or the p38 inhibitor SB203580. The inhibition of both MAPKs reduced IL-1beta-induced COX-2 expression and PGE(2) synthesis. Furthermore, the addition of PGE(2) to cells overcame the inhibitory effects of both MAPK inhibitors in IL-1beta-induced mucin production. These results indicate that in human pulmonary epithelial cells, IL-1beta activates ERK or p38 to induce COX-2 production, which in turn induces MUC2 and MUC5AC production.