Key Event Title
|Level of Biological Organization|
Key Event Components
|epidermal growth factor-activated receptor activity||epidermal growth factor receptor||occurrence|
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP|
|Decreased lung function||MolecularInitiatingEvent|
Key Event Description
The EGF receptor family comprises 4 members, EGFR (also referred to as ErbB1/HER1), ErbB2/Neu/HER2, ErbB3/HER3 and ErbB4/HER4, all of which are transmembrane glycoproteins with an extracellular ligand binding site and an intracellular tyrosine kinase domain. Receptor-ligand binding induces dimerization and internalization, subsequently leading to activation of the receptor through autophosphorylation (Higashiyama et al., 2008).
EGFR signaling is central to airway epithelial maintenance and mucin production (Burgel and Nadel, 2004), and EGFR expression has been demonstrated in lung epithelial cells under physiological (albeit weakly) as well as pathological conditions in vitro and in vivo (Aida et al., 1994; Burgel and Nadel, 2008; O’Donnell et al., 2004; Polosa et al., 1999). Of note, lung epithelial EGFR phosphorylation (i.e., activation) was increased under conditions of oxidative stress including exposure to H2O2 (Goldkorn et al., 1998), naphthalene (Van Winkle et al., 1997), cigarette smoke (de Boer et al., 2006; Marinaş et al., 2011) and in the presence of neutrophils or neutrophil elastase (Kohri et al., 2002; Shao et al., 2004; Shao and Nadel, 2005; Shim et al., 2001; Takeyama et al., 2000). EGFR activation by oxidative stress may have a number of root causes: ROS were shown to increase production of EGF, the prime EGFR ligand, by lung epithelial cells (Casalino-Matsuda et al., 2004). Similarly, expression and secretion of TGF-α and AREG, also EGFR ligands, were elevated in human bronchial epithelial cells in response to fine particulate matter (PM2.5), diesel particulate matter and cigarette smoke exposure (Blanchet et al., 2004; Lemjabbar et al., 2003; Rumelhard et al., 2007). Mechanistically, this process is dependent on ROS-mediated activation of metalloproteinases or ADAMs which cleave membrane-bound EGFR ligand precursors, making them locally available to bind to and transactivate EGFR in an autocrine manner (Deshmukh et al., 2009; Kim et al., 2004b; Val et al., 2012; Yoshisue and Hasegawa, 2004). Furthermore, ligand binding to EGFR itself was shown to lead to H2O2 production, thereby facilitating receptor activation and downstream signaling (DeYulia et al., 2005; DeYulia and Cárcamo, 2005; Truong and Carroll, 2012). While it is tempting to speculate that the increase in H2O2 would perpetuate EGFR activation via the continuous proteolytic shedding of pro-ligands in an autocrine loop, multiple lines of evidence suggest that oxidative modification, specifically EGFR sulfenylation, contributes to enhanced tyrosine phosphorylation of the receptor and downstream signaling (Paulsen et al., 2011; Ravid et al., 2002; Truong and Carroll, 2012; Truong et al., 2016).
Classical EGFR downstream signaling involves activation of Ras which subsequently initiates signal transduction through the Raf-1/MEK/ERK pathway. MAP kinase activation in turn promotes airway epithelial cell proliferation and differentiation (Hackel et al., 1999; Kim et al., 2005; Lemjabbar et al., 2003) and facilitates epithelial wound repair (Allahverdian et al., 2010; Burgel and Nadel, 2004; Van Winkle et al., 1997).
How It Is Measured or Detected
Proof of EGFR activation can be derived from Western blots of e.g. untreated and treated cell or tissue lysates using specific antibodies targeting the phosphorylated EGFR epitopes. Densitometric evaluation of the colorimetrically stained, chemiluminescent or radioactive bands on the blot then permit a (semi-)quantitative measure of activation. Moreover, the addition of EGFR inhibitors such as AG1478 and BIBX 1522 or neutralizing antibodies is well suited to demonstrate causality.
Domain of Applicability
EGFR activation in human, mouse and rat is well documented, and EGF ligands and EGFR are orthologous within these species. EGFR is a driver of human cancer in various tissues and numerous drugs are approved that inhibit EGFR activation (Ciardiello and Tortora, 2008). Although EGFR and its ligands are expressed in human, mouse and rat (Chen et al., 2007), species differences have been noted in binding and structure (Nexø and Hansen, 1985), and even can have opposite downstream effects in mouse and rat (Kiley and Chevalier, 2007).
Evidence for Perturbation by Stressor
Overview for Molecular Initiating Event
EGFR activation in lung epithelial cells can be triggered by exposure to H2O2 (Goldkorn et al., 1998; Takeyama et al., 2000), naphthalene (Van Winkle et al., 1997), cigarette smoke (Takeyama et al., 2001; de Boer et al., 2006; Marinaş et al., 2011; Yu et al., 2011; Yu et al., 2015), acrolein (Deshmukh et al., 2008), and TCDD (Lee et al., 2011). Mechanistically, this process is dependent on ROS-mediated activation of metalloproteinases or ADAMs which cleave membrane-bound EGFR ligand precursors, making them locally available to bind to and transactivate EGFR in an autocrine manner (Deshmukh et al., 2009; Kim et al., 2004b; Val et al., 2012; Yoshisue and Hasegawa, 2004). Furthermore, ligand binding to EGFR itself was shown to lead to H2O2 production, thereby facilitating receptor activation and downstream signaling (DeYulia et al., 2005; DeYulia and Cárcamo, 2005; Truong and Carroll, 2012).
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