Upstream eventActivation, EGFR
Occurrence, Transdifferentiation of ciliated epithelial cells
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|EGFR Activation Leading to Decreased Lung Function||adjacent||Moderate||Moderate|
Life Stage Applicability
Key Event Relationship Description
Airway epithelial injury can be caused by various inhalation exposures (e.g. cigarette smoke, sulfur dioxide, endotoxin, viruses). Subsequent tissue repair processes are thought to initiate the transdifferentiation process, whereby ciliated epithelial cells first dedifferentiate and then redifferentiate to goblet cells, without an apparent increase in the total number of epithelial cells (Lumsden et al., 1984; Shimizu et al., 1996; Reader et al., 2003). EGFR was shown to be a key player in this process in both murine and human airway epithelia (Tyner et al., 2006; Hao et al., 2011; Habibovic et al., 2016).
Evidence Supporting this KER
Transdifferentiation was shown to occur following the activation of EGFR-mediated anti-apoptotic signaling in ciliated epithelial cells (Tyner et al., 2006). Subsequent stimulation by proinflammatory stimuli such as the Th2 cytokines interleukin (IL)-4 and IL-13 then promotes transdifferentiation of ciliated cells into goblet cells, thereby increasing the number of goblet cells (“second hit hypothesis”) in mouse tracheal epithelium and airway epithelia of COPD patients (Curran & Cohn, 2010).
EGFR can be activated by ROS or IL-13 to lead to ciliated cell transdifferentiation. IL-13 stimulates transdifferentiation of ciliated epithelial cells to goblet cells through EGFR activation increasing MMP/ADAM activity and MAPK activation (Casalino-Matsuda et al., 2006; Yoshisue and Hasegawa, 2004; Tyner et al., 2006).
Two studies showed EGFR involvement in a decrease in goblet cell and increase in ciliated cell numbers or cell-specific marker expression (Yoshisue and Hasegawa, 2004; Casalino-Matsuda et al., 2006). Other studies demonstrated ciliated cell transdifferentiation in response to IL13 in an EGFR-dependent manner in a mouse viral infection model and mouse tracheal epithelial cells in vitro (Tyner et al., 2006), rat nasal epithelial cells (Lee et al., 2000), and human airway epithelial cells (Kim et al., 2002; Hao et al., 2011).
Cigarette smoke exposure resulted in increased goblet cell numbers and extensive AB/PAS staining, which were considered signs of goblet cell metaplasia (which is the outcome of transdifferentiation), in the airways of Sprague-Dawley rats. These effects were greatly diminished in animals pre-treated daily with the EGFR kinase inhibitor BIBX 1522 (Takeyama et al., 2001). Similarly, plugging of F344 rat bronchi with agarose resulted in goblet cell metaplasia as evidenced by histology and increased AB/PAS staining, and the extent of airway remodeling was markedly lower in animals treated with BIBX 1522 (Lee et al., 2000). EGFR-dependent goblet cell metaplasia was also observed in primary human bronchial epithelila cells exposed to ROS (Casalino-Matsuda et al., 2006), nasal epithelial cells of asthmatics and lungs of patients with diffuse panbronchiolitis exhibited goblet cell metaplasia and concomitant EGFR expression or activation (Habibovic et al., 2016; Kim et al., 2004).
Uncertainties and Inconsistencies
It is not well-known how ciliated cell transdifferentiation occurs in humans. Under normal conditions, lung epithelial cells (except basal cells) are terminally differentiated (Donnelly et al., 1982; Breuer et al., 1990; Rawlins and Hogan, 2008), and which signals initiate the dedifferentiation/redifferentiation process is not well-understood. The available evidence is indirect or correlative. It also is not in agreement with other studies, which showed that ciliated cells do not give rise to goblet cells during airway remodeling in rodents and humans and with studies that provide evidence for increased goblet cell proliferation and goblet cell hyperplasia (Pardo-Sargenta et al., 2013; Hays et al., 2006; Lawson et al., 2002; Tesfaigzi et al., 2004; Taniguchi et al., 2011; Park et al., 2006; Turner et al., 2011).
Quantitative Understanding of the Linkage
Daily 30-minute treatment of primary human bronchial epithelial cells at the air-liquid interface with 0.6 mM xanthine and 0.5 units xanthine oxidase for 3 days resulted in goblet cell metaplasia as evidenced by an increase in the numbers of MUC5AC-positive cells from 3.3 ± 1.2%to 21.6 ± 3.4% and a decrease in ciliated cell numbers. This effect could be inhibited by EGFR blockade with neutralizing antibodies (Casalino-Matsuda et al., 2006).
Cigarette smoke exposure at 8 cigarettes per day for 5 days markedly increased AB-PAS staining in airway epithelia and goblet cell numbers from 40 ± 19 to 167 ± 19 cells/mm of epithelium. Treatment with the EGFR inhibitor BIBX1522 during exposure dose-dependently decreased goblet cell numbers, with a maximal decrease seen for 3 mg/kg inhibitor (51 ± 19 cells/mm epithelium) (Tamiguchi et al., 2001).
Induction of airway inflammation with 50 microg house dust mite (1.27 endotoxin units/mg) for 5 days/week for 3 weeks resulted in a 3-fold increase of pEGFR-positive cells in the bronchiolar epithelium of C57Bl/6 mice. Six-week treatment led to goblet cell metaplasia as evidenced by extensive AB staining and an approx. 10-fold increase in Clca3-positive cells in the animals' airways. Concomitant treatment with 100 mg/kg erlotinib six times a week for 6 weeks reduced the number of Clca3-positive cells by ca. 5-fold (Le Cras et al., 2011). Using the same model with a 3-week treatment demonstrated goblet cell metaplasia as judged by increased PAS staining in the airway epithelium and ca. 10-, 5-, and 4-fold increases in expression of goblet cell metaplasia-related genes Muc5ac, Clca1, and Postn, respectively (Habibovic et al., 2016).
Instillation of agarose plugs (0.7-0.8 mm diameter, 4% agarose II) in Fischer rats caused a time-dependent increase in goblet cell area (by AB/PAS staining), which was detectable as early as 24 hours and was greatest 72 hours post-instillation. The AB/PAS-stained area increased from 0.1 ± 0.1% in control animals to 4.7 ± 1.4, 13.3 ± 0.7, and to 19.1 ± 0.7% at 24, 48, and 72 hours post-instillation, respectively. Goblet cell numbers increased from 0 to 13.1 ± 5.6, 25.7 ± 15.0, and 51.5 ± 9.0 cells/mm basal lamina at 24, 48, and 72 hours post-instillation, respectively. Treatment of the animals prior and after instillation with 80 mg/kg/day BIBX1522 resulted in a marked decrease in the AB/PAS-stained area (<5% at 72 hours). Of note, the AB/PAS staining in the airway epithelia coincided with EGFR staining (Lee et al., 2000).
Pyocyanin, a redox-active exotoxin of Pseudomonas aeruginosa, caused goblet cell metaplasia in C57Bl/6 mice after 3 weeks (25 microg/day). PAS staining increased by ca. 30%; the percentage of Muc5ab-positive cell in bronchial epithelium increased 6.4-fold and in bronchiolar epithelium 11.4-fold. This was accompanied by increased EGFR phosphorylation coincident with AB/PAS staining. Moreover, 24-hour pyocyanin treatment of H292 and 16-HBE cells with physiologically relevant concentrations from 1.3 to 25 microg/mL significantly increased MUC5B mRNA expression 3.8- to 13.4-fold and increased levels of pEGFR 11.8- to 18.3-fold (1.6 to 12.5 microg/mL pyocyanin) (Hao et al., 2012).
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Two mouse studies demonstrated ciliated cell transdifferentiation and goblet metaplasia in response to virus and/or IL13 (Tyner et al., 2006; Fujisawa et al., 2008). Indirect evidenc is also available from rat studies and studies on human cells and clinical samples.
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Casalino-Matsuda, S., Monzón, M., and Forteza, R. (2006). Epidermal Growth Factor Receptor Activation by Epidermal Growth Factor Mediates Oxidant-Induced Goblet Cell Metaplasia in Human Airway Epithelium. Am J Respir Cell Mol Biol 34, 581–591.
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Lawson, G.W., Van Winkle, L.S., Toskala, E., Senior, R.M., Parks, W.C., and Plopper, C.G. (2002). Mouse strain modulates the role of the ciliated cell in acute tracheobronchial airway injury-distal airways. Am J Pathol 160, 315–327.
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Pardo-Saganta, A., Law, B.M., Gonzalez-Celeiro, M., Vinarsky, V., and Rajagopal, J. (2013). Ciliated cells of pseudostratified airway epithelium do not become mucous cells after ovalbumin challenge. Am. J. Respir. Cell Mol. Biol. 48, 364–373.
Park, K.-S., Wells, J.M., Zorn, A.M., Wert, S.E., Laubach, V.E., Fernandez, L.G., and Whitsett, J.A. (2006). Transdifferentiation of ciliated cells during repair of the respiratory epithelium. Am J Respir Cell Mol Biol 34, 151–157.
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Reader, J.R., Tepper, J.S., Schelegle, E.S., Aldrich, M.C., Putney, L.F., Pfeiffer, J.W., and Hyde, D.M. (2003). Pathogenesis of mucous cell metaplasia in a murine asthma model. Am J Pathol 162, 2069-2078.
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