This AOP is licensed under a Creative Commons Attribution 4.0 International License.
Oxidative stress [MIE] Leading to Decreased Lung Function [AO] via CFTR dysfunction
Point of Contact
- Karsta Luettich
- Hasmik Yepiskoposyan
- Monita Sharma
- Frazer Lowe
- Damien Breheny
|Author status||OECD status||OECD project||SAAOP status|
|Under development: Not open for comment. Do not cite|
This AOP was last modified on September 28, 2021 04:49
|Cystic Fibrosis Transmembrane Regulator Function, Decreased||August 03, 2021 02:31|
|Oxidative Stress||June 19, 2021 14:42|
|Airway Surface Liquid Height, Decreased||September 09, 2021 03:34|
|Mucociliary Clearance, Decreased||September 10, 2021 07:19|
|Cilia Beat Frequency, Decreased||September 10, 2021 01:38|
|Decrease, Lung function||September 08, 2021 04:54|
|Oxidative Stress leads to CFTR Function, Decreased||September 28, 2021 04:27|
|CFTR Function, Decreased leads to ASL Height, Decreased||September 07, 2021 10:14|
|ASL Height, Decreased leads to CBF, Decreased||September 28, 2021 07:54|
|CBF, Decreased leads to MCC, Decreased||September 28, 2021 08:04|
|MCC, Decreased leads to Decreased lung function||September 28, 2021 08:05|
|Acrolein||September 28, 2021 08:23|
|Ozone||September 28, 2021 08:26|
|Cigarette smoke||September 28, 2021 09:07|
This AOP evaluates one of the major processes known to be involved in regulating efficient mucociliary clearance (MCC). MCC is a key aspect of the innate immune defense against airborne pathogens and inhaled chemicals and is governed by the concerted action of its functional components, the cilia and the airway surface liquid (ASL), which is composed of mucus and periciliary layers (Bustamante-Marin and Ostrowski, 2017). For MCC to be efficient, the depth of the ASL has to be constantly adjusted to allow for efficient cilia beating and mucus transport (Antunes and Cohen, 2007). The cystic fibrosis transmembrane regulator (CFTR) is an integral membrane glycoprotein that functions as cAMP-activated and phosphorylation-regulated Cl– channel at the apical membrane of epithelial cells (Farinha et al., 2013) and the major Cl– channel that mediates fluid and electrolyte transport. CFTR function is critical to normal ASL homeostasis. Exposure to inhaled oxidants, such as ozone and cigarette smoke, leads to decreased CFTR gene and protein expression as well as CFTR internalization, thereby reducing or abolishing short-circuit currents (Qu et al., 2009; Cantin et al., 2006a; Cantin et al., 2006b; Clunes et al., 2012; Sloane et al., 2012; Rasmussen et al., 2014). Consequently, ASL height (or volume) decreases. Disturbances in any of the processes regulating ASL volume, mucus production, mucus viscoelastic properties, or ciliary function can cause MCC dysfunction and are linked to airway diseases such as chronic obstructive pulmonary disease (COPD) or asthma, both of which are characterized by decreased lung function and bear a significant risk of increased morbidity and mortality.
With a surface area of ~100 m2 and ventilated by 10,000 to 20,000 liters of air per day (National Research Council, 1988; Frohlich et al., 2016), the lungs are a major barrier that protect the body from a host of external factors that enter the respiratory system and may cause lung pathologies. Mucociliary clearance (MCC) is a key aspect of the innate immune defense against airborne pathogens and inhaled particles and is governed by the concerted action of its functional components, the cilia and the airway surface liquid (ASL), which comprises mucus and the periciliary layer (Bustamante-Marin and Ostrowski, 2017). In healthy subjects, ≥10 mL airway secretions are continuously produced and transported daily by the mucociliary escalator. Disturbances in any of the processes regulating ASL volume, mucus production, mucus viscoelastic properties, or ciliary function can cause MCC dysfunction and are linked to airway diseases such as chronic obstructive pulmonary disease (COPD) or asthma, both of which bear a significant risk of increased morbidity and mortality. The mechanism by which exposure to inhaled toxicants might lead to mucus hypersecretion and thereby impact pulmonary function has already been mapped in AOP148 on decreased lung function. However, whether an exposure-related decline in lung function is solely related to excessive production of mucus is debatable, particularly in light of the close relationship between mucus, ciliary function, and efficient MCC. To date, no single event has been attributed to MCC impairment, and it is likely that events described in this AOP as well as in AOPs 148 and 411 have to culminate to lead to decreased lung function.
Summary of the AOP
Molecular Initiating Events (MIE)
Key Events (KE)
Adverse Outcomes (AO)
|Sequence||Type||Event ID||Title||Short name|
|1||MIE||1392||Oxidative Stress||Oxidative Stress|
|2||KE||1906||Cystic Fibrosis Transmembrane Regulator Function, Decreased||CFTR Function, Decreased|
|3||KE||1907||Airway Surface Liquid Height, Decreased||ASL Height, Decreased|
|4||KE||1908||Cilia Beat Frequency, Decreased||CBF, Decreased|
|5||KE||1909||Mucociliary Clearance, Decreased||MCC, Decreased|
|6||AO||1250||Decrease, Lung function||Decreased lung function|
Relationships Between Two Key Events (Including MIEs and AOs)
|Oxidative Stress leads to CFTR Function, Decreased||adjacent||High||High|
|CFTR Function, Decreased leads to ASL Height, Decreased||adjacent||High||Moderate|
|ASL Height, Decreased leads to CBF, Decreased||adjacent||Moderate||Low|
|CBF, Decreased leads to MCC, Decreased||adjacent||High||Moderate|
|MCC, Decreased leads to Decreased lung function||adjacent||Moderate||Moderate|
Life Stage Applicability
|All life stages||Moderate|
Overall Assessment of the AOP
The experimental evidence to support the biological plausibility of the KERs from MIE to AO is moderate to strong overall for the AOP presented here, while there is a moderate concordance of dose-response relationships. The weakest evidence is for the KER of decreased CFTR function leading to decreased ASL height, due to both insufficient experimental evidence of causality and a scarcity of quantitative data on dose-related responses and temporal concordances. In terms of essentiality, we have rated all KEs either moderate or high.
AOPs such as this one can play a central role in risk assessment strategies for a wide variety of regulatory purposes by providing mechanistic support to an integrated approach to testing and assessment (IATA; (Clippinger et al., 2018)). IATAs are flexibleframeworks that can be adapted to best address the regulatory question or purpose at hand. More specifically, this AOP can be applied to the risk assessment of inhaled toxicants, by enabling the development of testing strategies through the assembly of existing information and the generation of new data where they are currently lacking. Targeted approaches to fill data gaps can be developed using new approach methodologies (NAMs) informed by this AOP.
Domain of Applicability
All KE proposed in this AOP occur and are measurable in several species, including frogs, mice, rats, guinea pigs, ferrets, sheep, and humans. The majority of the supporting empirical evidence derives from studies in rodent and human systems, and—with the exception of CFTR function, which is known to vary from species to species (Higgins, 1992)—experimental findings in animals appear to be highly translatable to humans.
Data regarding the applicability of KE to all life-stages from birth to adulthood are available for the MIE (Oxidative Stress), KE1 (Cystic Fibrosis Transmembrane Regulator Function, Decreased), KE4 (Cilia Beat Frequency, Decreased), KE5 (Mucociliary Clearance, Decreased), and AO (Decreased Lung Function), and indicate that they apply to all life stages. There are no data related to ASL regulation and homeostasis relative to organismal health, but it is reasonable to assume that KE3 (Airway Surface Liquid Height, Decreased), through its impact on MCC, can affect all life-stages. It is also worth noting here that age-dependent decreases in CBF, MCC, and lung function have been demonstrated in several species (e.g., guinea pigs, mice, and humans) and reflect normal physiological aging processes (Bailey et al., 2014; Grubb et al., 2016; Ho et al., 2001; Joki and Saano, 1997; Paul et al., 2013; Sharma and Goodwin, 2006).
Gender-specific data relevant to the AOP are not as widely available as species-specific data, and to our knowledge, the role of gender has not been systematically evaluated for all KE described here. Informative evidence on gender differences stems from patients with chronic pulmonary diseases, such as cystic fibrosis, asthma, COPD, and bronchiectasis, that are characterized by decreased lung function. For example, epidemiological data indicate more rapid lung function decline and shorter life expectancy in females with cystic fibrosis (genetic CFTR dysfunction; Corey and Farewell, 1996; Harness-Brumley et al., 2014; Olesen et al., 2010; Rosenfeld et al., 1997), and earlier disease onset, more severe disease and more rapid lung function decline in females with COPD (acquired CFTR dysfunction; Prescott et al., 1997; Sørheim et al., 2010) but higher prevalence of COPD in males although this gender gap is closing (Ntritsos et al., 2018). Considering the expression pattern of CFTR and its function as well as the importance of efficient MCC—brought about by the interactions of ciliary function, ASL homeostasis and mucus properties—for normal physiological function, we consider this AOP applicable to both genders.
Essentiality of the Key Events
The definition of essentiality implies that the modulation of upstream KEs impacts the downstream KEs in an expected fashion. If blocked or failing to occur, the KEs in the current AOP will not necessarily stop the progression to subsequent KEs. Due to the complex biology of motile cilia formation and function, ASL homeostasis, mucus properties and MCC, the KEs and AO may be triggered because of alternative pathways or biological redundancies. However, when exacerbated, the KEs promote the occurrence of downstream events eventually leading to the AO. The causal pathway starting from the exposure to oxidants and leading to decreased lung function involves parallel routes with KEs (also see AOP411), each of which is sufficient to cause the downstream KE to occur. Oxidant-induced decreases in ASL height via CFTR function decline lead to decreased CBF and decreased MCC. Based on the evidence we judge the key events MIE (Oxidative Stress), KE1 (Cystic Fibrosis Transmembrane Regulator Function, Decreased), KE4 (Cilia Beat Frequency, Decreased), and KE5 (Mucociliary Clearance, Decreased) as highly essential and suggest moderate essentiality for KE3 (Airway Surface Liquid Height, Decreased).
We judge the overall biological plausibility of this AOP as strong. Several KER (i.e., Oxidative stress leading to decreased CFTR function, Decreased CFTR function leading to decreased ASL height) are supported by multiple studies across different species with ample empirical evidence reflecting both dose-response and time concordance. Other KER, such as Decreased ASL height leading to reduced CBF, lack this expanse of empirical evidence, or the evidence does not fully support the causality between the KE (Decreased CBF leading to decreased MCC) even though the relationship is logical and plausible.
Overall, our quantitative understanding of the AOP network is moderate. There is robust evidence that provides an insight into several KER presented here, and the dose response and temporal relationship between the two KE in question are well described and quantified for different stressors across different test systems (Oxidative stress leading to decreased CFTR function, Decreased CFTR function leading to decreased ASL height; Decreased CBF leading to decreased MCC). In some instances, we are less confident in our quantitative understanding. For example, for the KER Decreased ASL height leading to decreased CBF, empirical evidence supporting causality between the two KE is lacking as is quantitative evidence. Dose response data as well as data supportive of the KE causality are limited for the KER Decreased MCC leading to decreased lung function.
Considerations for Potential Applications of the AOP (optional)
Given the individual and public health burden of the consequences of lung function impairment, gaining a greater understanding of the underlying mechanisms is extremely important in the risk assessment of respiratory toxicants. An integrated assessment of substances with the potential to be inhaled, either intentionally or unintentionally, could incorporate inhalation exposure and dosimetry modelling to inform an in vitro approach with appropriate exposure techniques and cell systems to assess KEs in this AOP (EPA’s Office of Chemical Safety and Pollution Prevention, 2019). Standardization and robustness testing of assays against explicit performance criteria using suitable reference materials can greatly increase the level of confidence in their use for KE assessment (Petersen et al., In Press). Much of the empirical evidence that supports the KERs in the qualitative AOP described here was obtained from in vitro studies using well-established methodologies for biological endpoint assessment. Being chemical agnostic, this AOP can be applied to a variety of substances that share the AO. For example, impaired MCC and decreased lung function have a long-known relationship with smoking, but little is known about the consequences of long-term use of alternative inhaled nicotine delivery products such as electronic cigarettes and heated tobacco products. This AOP can form the basis of an assessment strategy to evaluate the effects of exposure to aerosol from these products based on the KEs identified here.
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