Model Systems for Investigating Mucin Gene Expression in Airway Diseases

Abstract

Overproduction of mucus and of mucin glycoproteins and goblet cell hyperplasia occurs in chronic obstructive airway diseases, including asthma and cystic fibrosis. Mucus overproduction results from alterations in several cellular processes, including altered regulation of airway mucin genes on exposure to environmental and infectious agents and to inflammatory mediators. Seven of the nine identified MUC genes (which encode the protein backbone of mucins) are normally expressed in human respiratory tract tissues. Several inflammatory mediators have now been shown to regulate expression of MUC2, MUC5AC, and MUC5B genes. Importantly, mucin gene expression can be regulated both transcriptionally and posttranscriptionally. Current information on airway mucin gene expression is summarized in this review along with an overview of airway epithelial model systems. In vitro model systems include airway epithelial carcinoma cell lines and primary normal human bronchial epithelial (NHBE) cells. In vivo systems include human respiratory tract tissues and rodent airways. Our laboratory has begun to investigate the role of cytokines on mucin gene expression in vitro and in vivo and on goblet cell metaplasia in vivo. Because cytokines can alter cell proliferation, we characterized the effect of interleukin (IL)-4 and IL-13 on the proliferation of NHBE cells and three human lung carcinoma cell lines--A549, NCI-H292, and Calu-3--that are frequently used for analyses of airway mucin gene expression. Both IL-4 and IL-13 had cell-specific effects. They increased proliferation moderately (1.2-3.0-fold) in NHBE and Calu-3 cells, but markedly inhibited proliferation of A549 cells in a dose-dependent manner. IL-4 increased proliferation of NCI-H292 cells moderately, although IL-13 had no significant effect. We also examined the role of IL-13 and IL-4 on MUC5AC messenger RNA (mRNA) expression in A549, Calu-3, and H292 cell lines and did not observe any significant effect. However, we recently showed an increase in Muc-5ac mRNA and protein expression in a murine model of ovalbumin-induced allergic asthma and in murine airways when IL-13 was delivered intranasally (Alimam, N.Z., et al. Am J. Respir. Cell Mol. Biol. 22:253--260). Thus, we speculate that IL-13 plays a role in the differentiation of murine airway epithelial cells into goblet cells, which then express Muc-5ac mRNA. A detailed analysis of the role of cytokines in airway cell differentiation and mucin gene expression both in vitro and in vivo is required to elucidate the roles of mucins in airway health and diseases. Identification of Muc-5ac as a major gene and gene product in goblet cell metaplasia should facilitate delineation of the molecular mechanisms underlying the induction and reversal of airway goblet cell metaplasia and goblet cell hyperplasia.

Regulation of Airway Mucin Gene Expression

“…Although the predominant signaling for both these two cytokines may be through IL-4Rα, the association between IL-4's MUC5AC induction is less clear than for IL-13. IL-4's effect on mucous cell metaplasia/hyperplasia and MUC5AC/Muc5ac has been predominantly observed in vivo (50,52), whereas in vitro experiments with defined IL-4 administration to airway epithelial cells have produced conflicting results (97,(100)(101)(102)(103). Hence, IL-4's role in inducing mucous cell differentiation may be due to indirect mechanisms from the inflammatory cells and mediators it recruits to the lung rather than a direct effect on airway epithelial cells.…”

“…Although there are also conflicting data, there is more abundant and stronger evidence for IL-13's role in directly inducing airway epithelial cells to upregulate MUC5AC/Muc5ac and differentiate into mucous cells (97,103,104). However, IL-13's ability to induce mucous cell formation and MUC5AC in culture appears to require a much more prolonged exposure (4 days to 2 weeks) than is typical for most cytokine responses (100, 101).…”

“…Indeed, epithelial denudation and sloughing have long been observed in the respiratory tracts of asthmatic patients (108). IL-13 induces proliferative response in various airway epithelial-type cells, although it does not directly damage epithelial cells, and it is not clear whether this proliferation is related to mucous cell formation (103,104). However, recent evidence suggests that other airway epithelial cells can transdifferentiate (i.e., exhibit metaplasia) into mucous cells (109,110).…”

Cysteine String Protein Interacts with and Modulates the Maturation of the Cystic Fibrosis Transmembrane Conductance Regulator

“…2). This is partly because of the presence of Csp in what appear to be large secretory granules that lack CFTR (44). These granules are likely to contain the secretory products characteristic of airway submucosal gland serous cells, which Calu-3 cells express (e.g.…”

Stimulation of Airway Mucin Gene Expression by Interleukin (IL)-17 through IL-6 Paracrine/Autocrine Loop

“…However, direct treatment of IL-4 on airway epithelial cell culture has produced conflicting results. One group reported a decrease in MUC5AC expression (21), another reported no change (22), and a third reported an increase of MUC2 expression by IL-4 (23).…”