Aop: 136

Title

Each AOP should be given a descriptive title that takes the form “MIE leading to AO”. For example, “Aromatase inhibition [MIE] leading to reproductive dysfunction [AO]” or “Thyroperoxidase inhibition [MIE] leading to decreased cognitive function [AO]”. In cases where the MIE is unknown or undefined, the earliest known KE in the chain (i.e., furthest upstream) should be used in lieu of the MIE and it should be made clear that the stated event is a KE and not the MIE. More help

Intracellular Acidification Induced Olfactory Epithelial Injury Leading to Site of Contact Nasal Tumors

Short name
A short name should also be provided that succinctly summarises the information from the title. This name should not exceed 90 characters. More help
pH Induced Nasal Tumors

Graphical Representation

A graphical summary of the AOP listing all the KEs in sequence, including the MIE (if known) and AO, and the pair-wise relationships (links or KERs) between those KEs should be provided. This is easily achieved using the standard box and arrow AOP diagram (see this page for example). The graphical summary is prepared and uploaded by the user (templates are available) and is often included as part of the proposal when AOP development projects are submitted to the OECD AOP Development Workplan. The graphical representation or AOP diagram provides a useful and concise overview of the KEs that are included in the AOP, and the sequence in which they are linked together. This can aid both the process of development, as well as review and use of the AOP (for more information please see page 19 of the Users' Handbook).If you already have a graphical representation of your AOP in electronic format, simple save it in a standard image format (e.g. jpeg, png) then click ‘Choose File’ under the “Graphical Representation” heading, which is part of the Summary of the AOP section, to select the file that you have just edited. Files must be in jpeg, jpg, gif, png, or bmp format. Click ‘Upload’ to upload the file. You should see the AOP page with the image displayed under the “Graphical Representation” heading. To remove a graphical representation file, click 'Remove' and then click 'OK.'  Your graphic should no longer be displayed on the AOP page. If you do not have a graphical representation of your AOP in electronic format, a template is available to assist you.  Under “Summary of the AOP”, under the “Graphical Representation” heading click on the link “Click to download template for graphical representation.” A Powerpoint template file should download via the default download mechanism for your browser. Click to open this file; it contains a Powerpoint template for an AOP diagram and instructions for editing and saving the diagram. Be sure to save the diagram as jpeg, jpg, gif, png, or bmp format. Once the diagram is edited to its final state, upload the image file as described above. More help

Authors

List the name and affiliation information of the individual(s)/organisation(s) that created/developed the AOP. In the context of the OECD AOP Development Workplan, this would typically be the individuals and organisation that submitted an AOP development proposal to the EAGMST. Significant contributors to the AOP should also be listed. A corresponding author with contact information may be provided here. This author does not need an account on the AOP-KB and can be distinct from the point of contact below. The list of authors will be included in any snapshot made from an AOP. More help

Justin G. Teeguarden, PhD, DABT Pacific Northwest National Laboratory & Oregon State University

Others

Point of Contact

Indicate the point of contact for the AOP-KB entry itself. This person is responsible for managing the AOP entry in the AOP-KB and controls write access to the page by defining the contributors as described below. Clicking on the name will allow any wiki user to correspond with the point of contact via the email address associated with their user profile in the AOP-KB. This person can be the same as the corresponding author listed in the authors section but isn’t required to be. In cases where the individuals are different, the corresponding author would be the appropriate person to contact for scientific issues whereas the point of contact would be the appropriate person to contact about technical issues with the AOP-KB entry itself. Corresponding authors and the point of contact are encouraged to monitor comments on their AOPs and develop or coordinate responses as appropriate.  More help
Justin Teeguarden   (email point of contact)

Contributors

List user names of all  authors contributing to or revising pages in the AOP-KB that are linked to the AOP description. This information is mainly used to control write access to the AOP page and is controlled by the Point of Contact.  More help
  • Justin Teeguarden

Status

The status section is used to provide AOP-KB users with information concerning how actively the AOP page is being developed, what type of use or input the authors feel comfortable with given the current level of development, and whether it is part of the OECD AOP Development Workplan and has been reviewed and/or endorsed. “Author Status” is an author defined field that is designated by selecting one of several options from a drop-down menu (Table 3). The “Author Status” field should be changed by the point of contact, as appropriate, as AOP development proceeds. See page 22 of the User Handbook for definitions of selection options. More help
Author status OECD status OECD project SAAOP status
Open for citation & comment EAGMST Under Review 2.7 Included in OECD Work Plan
This AOP was last modified on June 04, 2021 12:54
The date the AOP was last modified is automatically tracked by the AOP-KB. The date modified field can be used to evaluate how actively the page is under development and how recently the version within the AOP-Wiki has been updated compared to any snapshots that were generated. More help

Revision dates for related pages

Page Revision Date/Time
Decrease, Intracellular pH September 16, 2017 10:16
Increase, Tissue Degeneration, Necrosis & Atrophy September 16, 2017 10:16
Increase, Cell Proliferation June 23, 2021 12:28
Increase, Respiratory or Squamous Metaplasia September 29, 2017 14:47
Increase, Cytotoxicity September 16, 2017 10:16
Increase, Site of Contact Nasal Tumors September 16, 2017 10:16
Increase, Mutations in Critical Genes September 16, 2017 10:16
Decrease, Intracellular pH leads to Increase, Cytotoxicity November 29, 2016 20:42
Increase, Cytotoxicity leads to Increase, Tissue Degeneration, Necrosis & Atrophy November 29, 2016 20:42
Increase, Tissue Degeneration, Necrosis & Atrophy leads to Increase, Respiratory or Squamous Metaplasia December 01, 2016 14:22
Increase, Respiratory or Squamous Metaplasia leads to Increase, Cell Proliferation December 01, 2016 14:23
Increase, Cell Proliferation leads to Increase, Mutations in Critical Genes December 03, 2016 16:38
Increase, Mutations in Critical Genes leads to Increase, Site of Contact Nasal Tumors December 01, 2016 14:34
Increase, Cell Proliferation leads to Increase, Site of Contact Nasal Tumors December 01, 2016 14:35
Vinyl acetate November 29, 2016 18:42

Abstract

In the abstract section, authors should provide a concise and informative summation of the AOP under development that can stand-alone from the AOP page. Abstracts should typically be 200-400 words in length (similar to an abstract for a journal article). Suggested content for the abstract includes the following: The background/purpose for initiation of the AOP’s development (if there was a specific intent) A brief description of the MIE, AO, and/or major KEs that define the pathway A short summation of the overall WoE supporting the AOP and identification of major knowledge gaps (if any) If a brief statement about how the AOP may be applied (optional). The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance More help

The rodent olfactory epithelium is uniquely sensitive to cytotoxicity induced by exposure to inhaled compounds. Sustained cytotoxicity of the olfactory epithelium is a common precursor for the development of nasal tumors. This AOP describes intracellular pH reduction initiation of cytotoxicity leading to the development of olfactory tumors in rodents. Increased production or reduced buffering of protons in cells comprising the olfactory epithelium that exceed homeostatic control mechanisms and cause intracellular acidification is the MIE for this AOP. Reductions in cellular pH sufficient to denature or alter key cellular apparatus (enzymes, proteins) cause cytotoxicity. Sustained cytotoxicity of cell types comprising the olfactory epithelium, (e.g. olfactory sensory neurons, sustentacular cells, Bowmans glands) causes cell death and a reduction in cell numbers/volume of cells. Tissue necrosis, degeneration (deterioration and loss of function) and atrophy (reduction in tissue mass), are observed. Sustained atrophy/degeneration leads to adaptive tissue remodeling, where cell types unique to olfactory epithelium are replaced by cell types comprising respiratory epithelium or squamous epithelium. Tissue remodeling increases cell division rates. Mutations in critical genes accumulate under the combined influences of increased cell proliferation and cytotoxicity and cellular stress, which together increase the probability of mutation. Continued cell proliferation and accumulation of mutations in critical genes eventually leads to tumors arising from regions of the nasal epithelium that normally are composed of olfactory epithelium.

Background (optional)

This optional subsection should be used to provide background information for AOP reviewers and users that is considered helpful in understanding the biology underlying the AOP and the motivation for its development. The background should NOT provide an overview of the AOP, its KEs or KERs, which are captured in more detail below. Examples of potential uses of the optional background section are listed on pages 24-25 of the User Handbook. More help

Summary of the AOP

This section is for information that describes the overall AOP. The information described in section 1 is entered on the upper portion of an AOP page within the AOP-Wiki. This is where some background information may be provided, the structure of the AOP is described, and the KEs and KERs are listed. More help

Events:

Molecular Initiating Events (MIE)
An MIE is a specialised KE that represents the beginning (point of interaction between a stressor and the biological system) of an AOP. More help
Key Events (KE)
This table summarises all of the KEs of the AOP. This table is populated in the AOP-Wiki as KEs are added to the AOP. Each table entry acts as a link to the individual KE description page.  More help
Adverse Outcomes (AO)
An AO is a specialised KE that represents the end (an adverse outcome of regulatory significance) of an AOP.  More help
Sequence Type Event ID Title Short name
1 MIE 867 Decrease, Intracellular pH Decrease, Intracellular pH
2 KE 868 Increase, Tissue Degeneration, Necrosis & Atrophy Increase, Tissue Degeneration, Necrosis & Atrophy
3 KE 870 Increase, Cell Proliferation Increase, Cell Proliferation
4 KE 869 Increase, Respiratory or Squamous Metaplasia Increase, Respiratory or Squamous Metaplasia
5 KE 768 Increase, Cytotoxicity Increase, Cytotoxicity
6 KE 876 Increase, Mutations in Critical Genes Increase, Mutations in Critical Genes
7 AO 872 Increase, Site of Contact Nasal Tumors Increase, Site of Contact Nasal Tumors

Relationships Between Two Key Events (Including MIEs and AOs)

This table summarises all of the KERs of the AOP and is populated in the AOP-Wiki as KERs are added to the AOP. Each table entry acts as a link to the individual KER description page.To add a key event relationship click on either Add relationship: events adjacent in sequence or Add relationship: events non-adjacent in sequence.For example, if the intended sequence of KEs for the AOP is [KE1 > KE2 > KE3 > KE4]; relationships between KE1 and KE2; KE2 and KE3; and KE3 and KE4 would be defined using the add relationship: events adjacent in sequence button.  Relationships between KE1 and KE3; KE2 and KE4; or KE1 and KE4, for example, should be created using the add relationship: events non-adjacent button. This helps to both organize the table with regard to which KERs define the main sequence of KEs and those that provide additional supporting evidence and aids computational analysis of AOP networks, where non-adjacent KERs can result in artifacts (see Villeneuve et al. 2018; DOI: 10.1002/etc.4124).After clicking either option, the user will be brought to a new page entitled ‘Add Relationship to AOP.’ To create a new relationship, select an upstream event and a downstream event from the drop down menus. The KER will automatically be designated as either adjacent or non-adjacent depending on the button selected. The fields “Evidence” and “Quantitative understanding” can be selected from the drop-down options at the time of creation of the relationship, or can be added later. See the Users Handbook, page 52 (Assess Evidence Supporting All KERs for guiding questions, etc.).  Click ‘Create [adjacent/non-adjacent] relationship.’  The new relationship should be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. To edit a key event relationship, click ‘Edit’ next to the name of the relationship you wish to edit. The user will be directed to an Editing Relationship page where they can edit the Evidence, and Quantitative Understanding fields using the drop down menus. Once finished editing, click ‘Update [adjacent/non-adjacent] relationship’ to update these fields and return to the AOP page.To remove a key event relationship to an AOP page, under Summary of the AOP, next to “Relationships Between Two Key Events (Including MIEs and AOs)” click ‘Remove’ The relationship should no longer be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. More help

Network View

The AOP-Wiki automatically generates a network view of the AOP. This network graphic is based on the information provided in the MIE, KEs, AO, KERs and WoE summary tables. The width of the edges representing the KERs is determined by its WoE confidence level, with thicker lines representing higher degrees of confidence. This network view also shows which KEs are shared with other AOPs. More help

Stressors

The stressor field is a structured data field that can be used to annotate an AOP with standardised terms identifying stressors known to trigger the MIE/AOP. Most often these are chemical names selected from established chemical ontologies. However, depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). It can also include non-chemical stressors such as genetic or environmental factors. Although AOPs themselves are not chemical or stressor-specific, linking to stressor terms known to be relevant to different AOPs can aid users in searching for AOPs that may be relevant to a given stressor. More help
Name Evidence Term
Vinyl acetate

Life Stage Applicability

Identify the life stage for which the KE is known to be applicable. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help

Overall Assessment of the AOP

This section addresses the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and WoE for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). The goal of the overall assessment is to provide a high level synthesis and overview of the relative confidence in the AOP and where the significant gaps or weaknesses are (if they exist). Users or readers can drill down into the finer details captured in the KE and KER descriptions, and/or associated summary tables, as appropriate to their needs.Assessment of the AOP is organised into a number of steps. Guidance on pages 59-62 of the User Handbook is available to facilitate assignment of categories of high, moderate, or low confidence for each consideration. While it is not necessary to repeat lengthy text that appears elsewhere in the AOP description (or related KE and KER descriptions), a brief explanation or rationale for the selection of high, moderate, or low confidence should be made. More help

Characterization of the AOP, MIE and AO

Overall, the AOP and the key events comprising the AOP are well characterized experimentally. Most key events have been consistently observed in subchronic and chronic in vivo studies with one or more chemical initiators (cytotoxicity, tissue degeneration/atrophy, metaplasia, cell proliferation). The role of metabolism in the production of protons and subsequent intracellular acidification (The MEI) was established experimentally in cells and nasal tissue extracts exposed the chemical initiator vinyl acetate. By analogy, similar esters subject to ester cleavage in vivo would have some capacity for reducing intracellular pH. These include other listed chemical initiators of several key events proposed in this AOP. The adverse outcome, site of contact nasal tumors, is well understood both qualitatively and quantitatively. Tumors of this kind, and pathology of the rodent nasal tissues in general, are commonly evaluated in almost all inhalation bioassays. There are standards for describing nasal lesions, and accepted practices for quantifying them (morphometry). The general pathobiology of nasal carcinogenesis, including tumors arising from areas of olfactory epithelium, is well understood. The adverse outcome, site of contact nasal tumors, has historically been of regulatory concern for inhalation-route toxicants.

Causal Linkages between Key Events and the Outcome

  • MEI: Intracellular acidification exceeding homeostatic controls is cytotoxic to cells, the first event leading to the proposed series of tissue changes, cell proliferation, mutation and tumor development. In the absence of sustained pH reduction induced cytotoxicity, the remaining events do not occur. The event is necessary, but not sufficient, until it reaches levels that induce cytotoxicity.
  • KE1: Cytotoxicity is directly linked to the remaining key events. All subsequent events are a direct response or consequence of cytotoxicity. The incidence of the AO is zero in the absence of cytotoxicity.
  • KE2: Tissue Necrosis, Degeneration and Atrophy is directly linked to the remaining key events. All subsequent events are a direct response or consequence of this event. The incidence of the AO is zero in the absence of this key event.
  • KE3: Increased respiratory or squamous metaplasia is directly linked to the remaining key events. All subsequent events are a direct response or consequence of this event. Metaplasia induces cell division, which increases the spontaneous mutation rate and influences tumor growth. The incidence of the AO is zero in the absence of this key event.
  • KE4: Increased cell proliferation is directly linked to the remaining key events and the AO. All subsequent events are a direct response or consequence of this event. Cell proliferation increases the spontaneous mutation rate and influences tumor growth. The incidence of the AO is zero in the absence of this key event.
  • KE5: Increased mutation is directly linked development of nasal tumors. Mutation is an obligate step in carcinogenesis, and is therefore causally linked to the adverse outcome.

Limitations in the evidence in support of the AOP

Limitations in the evidence in support of the AOP are sufficiently covered in other sections of the Wiki.

Domain of Applicability

The relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context are defined in this section. Biological domain of applicability is informed by the “Description” and “Biological Domain of Applicability” sections of each KE and KER description (see sections 2G and 3E for details). In essence the taxa/life-stage/sex applicability is defined based on the groups of organisms for which the measurements represented by the KEs can feasibly be measured and the functional and regulatory relationships represented by the KERs are operative.The relevant biological domain of applicability of the AOP as a whole will nearly always be defined based on the most narrowly restricted of its KEs and KERs. For example, if most of the KEs apply to either sex, but one is relevant to females only, the biological domain of applicability of the AOP as a whole would be limited to females. While much of the detail defining the domain of applicability may be found in the individual KE and KER descriptions, the rationale for defining the relevant biological domain of applicability of the overall AOP should be briefly summarised on the AOP page. More help

Tissue, Life-stage or Species Specificity

The AOP applies to all vertebrates with olfactory epithelium, without respect to life-stage or gender.

Essentiality of the Key Events

An important aspect of assessing an AOP is evaluating the essentiality of its KEs. The essentiality of KEs can only be assessed relative to the impact of manipulation of a given KE (e.g., experimentally blocking or exacerbating the event) on the downstream sequence of KEs defined for the AOP. Consequently evidence supporting essentiality is assembled on the AOP page, rather than on the independent KE pages that are meant to stand-alone as modular units without reference to other KEs in the sequence.The nature of experimental evidence that is relevant to assessing essentiality relates to the impact on downstream KEs and the AO if upstream KEs are prevented or modified. This includes: Direct evidence: directly measured experimental support that blocking or preventing a KE prevents or impacts downstream KEs in the pathway in the expected fashion. Indirect evidence: evidence that modulation or attenuation in the magnitude of impact on a specific KE (increased effect or decreased effect) is associated with corresponding changes (increases or decreases) in the magnitude or frequency of one or more downstream KEs.When assembling the support for essentiality of the KEs, authors should organise relevant data in a tabular format. The objective is to summarise briefly the nature and numbers of investigations in which the essentiality of KEs has been experimentally explored either directly or indirectly. See pages 50-51 in the User Handbook for further definitions and clarifications.  More help

Support for Essentiality

Support for the essentiality of the key events was provided by in vitro metabolism studies, in vitro cytotoxicity studies in cells and nasal tissue explants, concepts of multistage carcinogenesis, and a series of rodent in vivo subchronic and chronic inhalation toxicity studies:

  1. Evidence for the essentiality of metabolic proton production for pH reduction[24], and the essentiality for proton production/pH reduction for induction of cytotoxicity was provided in several vitro studies exposing olfactory and respiratory nasal tissue explants to the chemical initiator vinyl acetate[25]. Reducing metabolic production of protons reduced cytotoxicity, but scavenging acetaldehyde did not. Acetic acid, but not the other predominant cytotoxic metabolite, acetaldehyde, was shown to be cytotoxic at the tested concentrations[26].
  2. The essentiality of increased mutations secondary to cell proliferation was inferred from fundamental concepts of multistage carcinogenesis[27] and broad evidence that increased cell proliferation increases replication error and mutation rates[28]In vitro studies demonstrated that acetaldehyde was not genotoxic at non cytotoxic cell concentrations[29], supporting the inference of mutation secondary to increased cell proliferation.
  3. Evidence for the essentiality of cytotoxicity, tissue degeneration, necrosis, and atrophy and subsequent respiratory and squamous cell metaplasia was provided by subchronic and chronic in vivo rodent studies consistently showing the absence of site of contact tumors in the absence of these events[30].
  4. Evidence for the essentiality of tissue degeneration, necrosis and atrophy in the development of respiratory and olfactory metaplasia was provided by fundamental understanding of the pathobiology of metaplastic change in these tissues derived from studies nasal toxicants [31] and from multiple subchronic and chronic inhalation studies of the chemical initiator vinyl acetate [32]

Rationale for essentiality calls:

  • Intracellular Acidification: Acetic acid shown to be cytotoxic, cytotoxicity dependent on metabolic production of protons, acetaldehyde shown to be less cytotoxic than acetic acid.
  • Increased Cytotoxicity: Obligate step in increased tissue degeneration, necrosis, and atrophy. Tumors absent at non-cytotoxic exposures.
  • Increased Tissue Degeneration, Necrosis and Atrophy: Obligate step in the induction of respiratory or squamous metaplasia, which is an adaptive response to chronic tissue injury.
  • Increased Respiratory or Squamous Metaplasia: Induces division of stem cells with the potential progress from early lesions to tumors. Induces chronic cell proliferation. Tumors absent at exposures where metaplasia does not occur.
  • Increased Cell Proliferation: Mechanistically linked to increased probability of mutational events in stem cells.
  • Increased Mutation: Acquisition of specific mutations is an obligate step in tumorigenesis.

Evidence Assessment

The biological plausibility, empirical support, and quantitative understanding from each KER in an AOP are assessed together.  Biological plausibility of each of the KERs in the AOP is the most influential consideration in assessing WoE or degree of confidence in an overall hypothesised AOP for potential regulatory application (Meek et al., 2014; 2014a). Empirical support entails consideration of experimental data in terms of the associations between KEs – namely dose-response concordance and temporal relationships between and across multiple KEs. It is examined most often in studies of dose-response/incidence and temporal relationships for stressors that impact the pathway. While less influential than biological plausibility of the KERs and essentiality of the KEs, empirical support can increase confidence in the relationships included in an AOP. For clarification on how to rate the given empirical support for a KER, as well as examples, see pages 53- 55 of the User Handbook.  More help

Concordance of exposure-response relationships

Exposure-response relationships for nearly all of the key events have been established in vitro and/or in vitro, in some cases in two species. The most complete set of studies establishing the exposure-response relationships have been conducted using the chemical initiator vinyl acetate.

KE1(MEI): Concentration dependent intracellular acidification has been observed in hepatocytes and olfactory tissue explants exposed in vitro to vinyl acetate[1]. Acetic acid was shown to be toxic to olfactory tissue explants in vitro[2].

KE 2,3,4: Concentration dependent increases in cytotoxicity, respiratory and squamous metaplasia and cell proliferation were observed in SD rats exposed to vinyl acetate for 65 days[33]. Increases were observed almost exclusively at tumorigenic exposure concentrations (>50 ppm).

KE 2,3,4: Concentration dependent increases in cytotoxicity, respiratory and squamous metaplasia and cell proliferation were observed in SD rats exposed to vinyl acetate for 65 days[3]. Increases were observed almost exclusively at tumorigenic exposure concentrations (>50 ppm).

KE 2,3,4: Concentration dependent increases in cytotoxicity, respiratory and squamous metaplasia were observed in SD rats and ICR mice exposured for up to 104 weeks to vinyl acetate. Increases were observed exclusively at tumorigenic exposure concentrations (200 and 600 ppm).

KE5: Concentration dependent increases in olfactory tissue cell proliferation were observed after 1 day, but not 5 or 20 days inhalation exposure to vinyl acetate in rats[4]. Sustained cell proliferation occurs after longer term exposure, but only at exposures causing toxicity and metaplasia [5].

KE6: There are no studies measuring dose-dependent increases in mutation rates secondary to cell proliferation in vivo following exposure to chemical initiators of this AOP. Increased mutation secondary to increased cell proliferation would only occur at doses inducing cell proliferation. There is strong experimental and theoretical basis for a role of cell proliferation in enhancing rates of mutation that would otherwise be managed by protective cellular controls. Together, this provides moderate evidence for a dose dependency for this key event.

KE6: In vitro dose-response data for the site of contact nasal carcinogen vinyl acetate and its metabolite acetaldehyde show no increases in adducts or mutations up to ~200 µm, concentrations higher than those attainable at tumorigenic exposures to these compounds.

KE7 (AO): Concentration dependent increases in tumors derived from olfactory epithelium were observed in a rat two-year inhalation-route bioassay[6]. Tumors only occurred at exposures > than 200 ppm.

Temporal concordance among the key events and adverse effect

The temporal relationship between the MEI, key events and the AO has been thoroughly established through in vitro and in vivo studies. The most complete data set establishing the relationship between the key events is from metabolism and toxicity studies of the chemical initiator vinyl acetate. Supporting studies comprise short term in vitro metabolism studies, in vitro adduct and mutation studies, and short (1,5, 20, 65 day) and chronic (52 and 104 week ) rodent inhalation studies. Consistency with the fundamental concepts of multiage carcinogenesis was also considered.

KE1: Intracellular acidification following exposure to vinyl acetate has been shown to occur on a time scale of seconds in hepatocytes and no more than 5 minutes for olfactory explants exposed in vitro[7]

KE2,3: A single day of exposure 600 ppm or greater concentrations of vinyl acetate is sufficient to cause cytotoxicity, tissue degeneration of the olfactory epithelium[8] in rats exposed by inhalation.

KER3: Regenerative hyperplasia of the olfactory epithelium is observed after 5, or 20 days of exposure by inhalation to vinyl acetate, but not after a single day of exposure[9].

KE4:Loss of olfactory epithelium, evidenced by reductions in olfactory marker protein stained nerve bundles, reduced tissue thickness, and loss of olfactory marker protein stained epithelium, occurs as early as 5 days in rats after inhalation exposure to vinyl acetate and continues through the latest exposure period measured, 65 days (600 and 1000 ppm)[10]. By day 65, markers of olfactory tissue are absent at exposures equal to or greater than 600 ppm. This indicates metaplastic change from olfactory epithelium to a respiratory or squamous epithelial cell type.

KE5: Evidence of respiratory metaplasia in the form of appearance of AB/PAS positive mucoussubstances in areas of olfactory epithelium occurs as early as 5 days of exposure to greater than or equal to 200 ppm vinyl acetate[11].

KE5: Increases in cell proliferation in the olfactory epithelium following inhalation exposure to concentrations of vinyl acetate greater than 50 ppm occur after 1[12], 5, 20 and 65 exposures (90 day study)[13]. Cell proliferation is temporally related to tissue regeneration (resolving as hyperplasia or olfactory to respiratory epithelial transitioning) secondary to cytotoxicity.

KE7: Tumors arising from areas of the olfactory epithelium are observed after 104 weeks of exposure to vinyl acetate, but not after subchronic exposures of 28-90 days[14].

Strength, consistency, and specificity of association of final adverse outcome and MIE

The experimental evidence and pathophysiological basis for the association between the MIE (pH reduction) and the sequence of events leading to the AO is overall very strong. Each key event, with the exception of mutation, has been demonstrated using at least one chemical initiator in the target tissue (olfactory epithelium). Some or all of the key events have been observed in three in vivo studies, demonstrating consistency. Consistent dose-response data for almost all of the key events in vitro and in vivo in the selected target cells/tissues provide strong evidence in support of the postulated AOP and the linkages between the key events. Several reviews and other reports provide good summaries of the strength of the data[15]. While pH reduction is probable in any tissue where metabolic production of protons exceed homeostatic control, evidence supports this MIE in the olfactory epithelium, which appears more susceptible compared to other epithelial tissue types in the nose[16].

Biological plausibility, coherence, and consistency of the experimental evidence

In general, the biological plausibility and coherence of the linkages between pH reduction and site of contact tumors of the olfactory epithelium expressed in this AOP is very robust. More than two decades of research support the key events and key event relationships comprising the AOP. The AOP is consistent with our broad understanding of the pathobiology of the rodent nasal tissues following inhalation of toxicants[17], consistent with the concepts of multistage carcinogenesis[18], and consistent with the weak genotoxicity and cytotoxicity of acetaldehyde and high toxicity of acetic acid and pH reduction[19]. The coherence and consistency of the experimental data in support of the AOP was discussed in the section “Strength, consistency and specificity of the association of final adverse outcome and MIE”

Alternative mechanism(s)

Within the context of this AOP, the key events are well substantiated. The only additional or alternative mechanistic element that could be considered is a role for DNA alkylation in the key event “increased mutation.” For some chemical initiators, for example those that reduce pH and produce an alkylating agent, it is possible a direct mutational key event is also operative. DNA alkylation was not considered a key event in this AOP, which focuses on initiators that do not produce alkylating agents, or where there is evidence that DNA alkylation sufficient to cause mutation does not occur (e.g. vinyl acetate).

This AOP proposes pH reduction as one of many plausible MEI’s leading to the first of the five key events leading to site of contact olfactory epithelial tumors. Chemicals that cause site of contact tumor of the olfactory epithelium, but do not cause pH reduction, may have other MEI’s. For example, formaldehyde causes site of contact tumors in the olfactory epithelium, also involving cytotoxicity and metaplasia, but may trigger the initial cytotoxicity through alkylative damage. Acetaldehyde shows a similar set of key events and produces tumors of the olfactory epithelium, and produces both alkylative damage and has the potential to produce protons upon metabolism. In the latter case, both pH reduction and alkylative damage are plausible MEI’s. However, it was shown in olfactory and respiratory tissue explants that pH reduction was more cytotoxic than acetaldehyde exposure[20], suggesting that for the current AOP, pH reduction is the operative MEI.

In general, we expect that the key events of and the AO of this AOP will be linked to additional chemical initiators via different MEIs leading to cytotoxicity and the subsequent key events.

Uncertainties, inconsistencies and data gaps

There are two data gaps which impact the overall strength of the AOP. First, while there is a strong theoretical and broad experimental support for the influence of cell proliferation rates on spontaneous mutation, there are no specific data on this key event/key event relationship for the chemical initiators of the AOP (vinyl acetate, ethyl acetate, methyl methacrylate, etc. Second, there are no in vivo data demonstrating increases in mutations leading to tumors arising in the olfactory epithelium. This uncertainty should be viewed in the context of our understanding of carcinogenesis, in which the presence of tumors is a biomarker of cellular mutation.

Quantitative Understanding

Some proof of concept examples to address the WoE considerations for AOPs quantitatively have recently been developed, based on the rank ordering of the relevant Bradford Hill considerations (i.e., biological plausibility, essentiality and empirical support) (Becker et al., 2017; Becker et al, 2015; Collier et al., 2016). Suggested quantitation of the various elements is expert derived, without collective consideration currently of appropriate reporting templates or formal expert engagement. Though not essential, developers may wish to assign comparative quantitative values to the extent of the supporting data based on the three critical Bradford Hill considerations for AOPs, as a basis to contribute to collective experience.Specific attention is also given to how precisely and accurately one can potentially predict an impact on KEdownstream based on some measurement of KEupstream. This is captured in the form of quantitative understanding calls for each KER. See pages 55-56 of the User Handbook for a review of quantitative understanding for KER's. More help

There is a rich data set providing substantial support for establishing quantitative relationships between exposure to the chemical initiator and each of the MIE and key events and the AO. These are detailed in the dose concordance table(s). Studies reported in the tables provide quantitative data for each of the key events[21].

Data establishing the quantitative relationship between any two key events, is incomplete, in part because of the semi-qualitative nature of pathology data for key events measured in whole animal studies. For example, while the linkage between cytotoxicity and necrosis, degeneration and atrophy is clear, the available data are not sufficient to determine a specific threshold for cytotoxicity that would trigger significant tissue degeneration. More qualitative relationships can be readily described as the extent of cytotoxicity and tissue degeneration observed. There is no quantitative data establishing the number or relative increase in indirect mutations leading to site of contact tumors of the nasal cavity. There are several quantitative relationships that have been described for key events that are critical to this AOP. For example, estimates of proton production and tissue pH change associated with cellular cytotoxicity have been described[22]. Thresholds for several key events, each related to a threshold for cytotoxicity, have been described for the chemical initiator vinyl acetate[23].

Considerations for Potential Applications of the AOP (optional)

At their discretion, the developer may include in this section discussion of the potential applications of an AOP to support regulatory decision-making. This may include, for example, possible utility for test guideline development or refinement, development of integrated testing and assessment approaches, development of (Q)SARs / or chemical profilers to facilitate the grouping of chemicals for subsequent read-across, screening level hazard assessments or even risk assessment. While it is challenging to foresee all potential regulatory application of AOPs and any application will ultimately lie within the purview of regulatory agencies, potential applications may be apparent as the AOP is being developed, particularly if it was initiated with a particular application in mind. This optional section is intended to provide the developer with an opportunity to suggest potential regulatory applications and describe his or her rationale.To edit the “Considerations for Potential Applications of the AOP” section, on an AOP page, in the upper right hand menu, click ‘Edit.’ This brings you to a page entitled, “Editing AOP.” Scroll down to the “Considerations for Potential Applications of the AOP” section, where a text entry box allows you to submit text. In the upper right hand menu, click ‘Update AOP’ to save your changes and return to the AOP page or 'Update and continue' to continue editing AOP text sections.  The new text should appear under the “Considerations for Potential Applications of the AOP” section on the AOP page. More help

References

List the bibliographic references to original papers, books or other documents used to support the AOP. More help
  1. Bogdanffy (2002). Vinyl acetate-induced intracellular acidification: implications for risk assessment. Toxicol Sci. 66: 320-326, Lantz, Orozco and Bogdanffy (2003). Vinyl acetate decreases intracellular pH in rat nasal epithelial cells. Toxicol Sci. 75: 423-431
  2. Kuykendall, Taylor and Bogdanffy (1993). Cytotoxicity and DNA-protein crosslink formation in rat nasal tissues exposed to vinyl acetate are carboxylesterase-mediated. Toxicol Appl Pharmacol. 123: 283-292
  3. Hotchkiss, Krieger, Harkema and Mahoney (2014). VINYL ACETATE: EVALUATION OF VINYL ACETATE-SPECIFIC DNA ADDUCTS, HISTOPATHOLOGY AND EPITHELIAL CELL PROLIFERATION IN NASAL AIRWAYS OF Crl:CD(SD) RATS REPEATEDLY EXPOSED TO VINYL ACETATE VAPORS, The Dow Chemical Company
  4. Bogdanffy, Gladnick, Kegelman and Frame (1997). FOUR-WEEK INHALATION CELL PROLIFERATION STUDY OF THE EFFECTS OF VINYL ACETATE ON RAT NASAL EPITHELIUM. Inhalation Toxicology, Taylor & Francis. 9: 331-350
  5. Hotchkiss, Krieger, Harkema and Mahoney (2013). Draft Report: VINYL ACETATE: EVALUATION OF VINYL ACETATE-SPECIFIC DNA ADDUCTS, HISTOPATHOLOGY AND EPITHELIAL CELL PROLIFERATION IN NASAL AIRWAYS OF Crl:CD(SD) RATS REPEATEDLY EXPOSED TO VINYL ACETATE VAPORS. Washington, DC, The Vinyl Acetate Council
  6. Bogdanffy, Dreef-van der Meulen, Beems, Feron, Cascieri, Tyler, Vinegar and Rickard (1994). Chronic toxicity and oncogenicity inhalation study with vinyl acetate in the rat and mouse. Fundam Appl Toxicol. 23: 215-229
  7. Bogdanffy (2002). Vinyl acetate-induced intracellular acidification: implications for risk assessment. Toxicol Sci. 66: 320-326, Lantz, Orozco and Bogdanffy (2003). Vinyl acetate decreases intracellular pH in rat nasal epithelial cells. Toxicol Sci. 75: 423-431
  8. Bogdanffy, Gladnick, Kegelman and Frame (1997). FOUR-WEEK INHALATION CELL PROLIFERATION STUDY OF THE EFFECTS OF VINYL ACETATE ON RAT NASAL EPITHELIUM. Inhalation Toxicology, Taylor & Francis. 9: 331-350
  9. Bogdanffy, Gladnick, Kegelman and Frame (1997). FOUR-WEEK INHALATION CELL PROLIFERATION STUDY OF THE EFFECTS OF VINYL ACETATE ON RAT NASAL EPITHELIUM. Inhalation Toxicology, Taylor & Francis. 9: 331-350
  10. Hotchkiss, Krieger, Harkema and Mahoney (2013). Draft Report: VINYL ACETATE: EVALUATION OF VINYL ACETATE-SPECIFIC DNA ADDUCTS, HISTOPATHOLOGY AND EPITHELIAL CELL PROLIFERATION IN NASAL AIRWAYS OF Crl:CD(SD) RATS REPEATEDLY EXPOSED TO VINYL ACETATE VAPORS. Washington, DC, The Vinyl Acetate Council
  11. Hotchkiss, Krieger, Harkema and Mahoney (2013). Draft Report: VINYL ACETATE: EVALUATION OF VINYL ACETATE-SPECIFIC DNA ADDUCTS, HISTOPATHOLOGY AND EPITHELIAL CELL PROLIFERATION IN NASAL AIRWAYS OF Crl:CD(SD) RATS REPEATEDLY EXPOSED TO VINYL ACETATE VAPORS. Washington, DC, The Vinyl Acetate Council
  12. Bogdanffy, Gladnick, Kegelman and Frame (1997). FOUR-WEEK INHALATION CELL PROLIFERATION STUDY OF THE EFFECTS OF VINYL ACETATE ON RAT NASAL EPITHELIUM. Inhalation Toxicology, Taylor & Francis. 9: 331-350
  13. Hotchkiss, Krieger, Harkema and Mahoney (2013). Draft Report: VINYL ACETATE: EVALUATION OF VINYL ACETATE-SPECIFIC DNA ADDUCTS, HISTOPATHOLOGY AND EPITHELIAL CELL PROLIFERATION IN NASAL AIRWAYS OF Crl:CD(SD) RATS REPEATEDLY EXPOSED TO VINYL ACETATE VAPORS. Washington, DC, The Vinyl Acetate Council
  14. Bogdanffy, Gladnick, Kegelman and Frame (1997). Cell proliferation responses in rat nasal eipithelium following repeated exposures to vinly acetate vapor. Inhalation Toxicology. 9: 331-350, Bogdanffy, Gladnick, Kegelman and Frame (1997). FOUR-WEEK INHALATION CELL PROLIFERATION STUDY OF THE EFFECTS OF VINYL ACETATE ON RAT NASAL EPITHELIUM. Inhalation Toxicology, Taylor & Francis. 9: 331-350, Hotchkiss, Krieger, Harkema and Mahoney (2013). Draft Report: VINYL ACETATE: EVALUATION OF VINYL ACETATE-SPECIFIC DNA ADDUCTS, HISTOPATHOLOGY AND EPITHELIAL CELL PROLIFERATION IN NASAL AIRWAYS OF Crl:CD(SD) RATS REPEATEDLY EXPOSED TO VINYL ACETATE VAPORS. Washington, DC, The Vinyl Acetate Council
  15. Bogdanffy, Sarangapani, Plowchalk, Jarabek and Andersen (1999). A biologically based risk assessment for vinyl acetate-induced cancer and noncancer inhalation toxicity. Toxicol Sci. 51: 19-35, Bogdanffy (2002). Vinyl acetate-induced intracellular acidification: implications for risk assessment. Toxicol Sci. 66: 320-326, Bogdanffy and Valentine (2003). Differentiating between local cytotoxicity, mitogenesis, and genotoxicity in carcinogen risk assessments: the case of vinyl acetate. Toxicol Lett. 140-141: 83-98
  16. Hardisty, Garman, Harkema, Lomax and Morgan (1999). Histopathology of nasal olfactory mucosa from selected inhalation toxicity studies conducted with volatile chemicals. Toxicol Pathol. 27: 618-627
  17. Hardisty, Garman, Harkema, Lomax and Morgan (1999). Histopathology of nasal olfactory mucosa from selected inhalation toxicity studies conducted with volatile chemicals. Toxicol Pathol. 27: 618-627
  18. Weisburger (1978). Mechanisms of chemical carcinogenesis. Annu Rev Pharmacol Toxicol. 18: 395-415, Cohen and Ellwein (1991). Genetic errors, cell proliferation, and carcinogenesis. Cancer Res. 51: 6493-6505, Pitot (1993). Multistage carcinogenesis--genetic and epigenetic mechanisms in relation to cancer prevention. Cancer Detection & Prevention. 17: 567-573, Cohen (1995). Role of cell proliferation in regenerative and neoplastic disease. Toxicol Lett. 82-83: 15-21, Bertram (2000). The molecular biology of cancer. Mol Aspects Med. 21: 167-223
  19. Lantz, Orozco and Bogdanffy (2003). Vinyl acetate decreases intracellular pH in rat nasal epithelial cells. Toxicol Sci. 75: 423-431
  20. Lantz, Orozco and Bogdanffy (2003). Vinyl acetate decreases intracellular pH in rat nasal epithelial cells. Toxicol Sci. 75: 423-431
  21. Bogdanffy, Dreef-van der Meulen, Beems, Feron, Cascieri, Tyler, Vinegar and Rickard (1994). Chronic toxicity and oncogenicity inhalation study with vinyl acetate in the rat and mouse. Fundam Appl Toxicol. 23: 215-229, Bogdanffy, Gladnick, Kegelman and Frame (1997). FOUR-WEEK INHALATION CELL PROLIFERATION STUDY OF THE EFFECTS OF VINYL ACETATE ON RAT NASAL EPITHELIUM. Inhalation Toxicology, Taylor & Francis. 9: 331-350, Albertini (2013). Vinyl acetate monomer (VAM) genotoxicity profile: relevance for carcinogenicity. Crit Rev Toxicol. 43: 671-706, Budinsky, Gollapudi, Albertini, Valentine, Stavanja, Teeguarden, Fensterheim, Rick, Lardie, McFadden, Green and Recio (2013). Nonlinear responses for chromosome and gene level effects induced by vinyl acetate monomer and its metabolite, acetaldehyde in TK6 cells. Environ Mol Mutagen. 54: 755-768, Hotchkiss, Krieger, Harkema and Mahoney (2013). Draft Report: VINYL ACETATE: EVALUATION OF VINYL ACETATE-SPECIFIC DNA ADDUCTS, HISTOPATHOLOGY AND EPITHELIAL CELL PROLIFERATION IN NASAL AIRWAYS OF Crl:CD(SD) RATS REPEATEDLY EXPOSED TO VINYL ACETATE VAPORS. Washington, DC, The Vinyl Acetate Council
  22. Bogdanffy, Dreef-van der Meulen, Beems, Feron, Cascieri, Tyler, Vinegar and Rickard (1994). Chronic toxicity and oncogenicity inhalation study with vinyl acetate in the rat and mouse. Fundam Appl Toxicol. 23: 215-229, Bogdanffy, Gladnick, Kegelman and Frame (1997). FOUR-WEEK INHALATION CELL PROLIFERATION STUDY OF THE EFFECTS OF VINYL ACETATE ON RAT NASAL EPITHELIUM. Inhalation Toxicology, Taylor & Francis. 9: 331-350, Albertini (2013). Vinyl acetate monomer (VAM) genotoxicity profile: relevance for carcinogenicity. Crit Rev Toxicol. 43: 671-706, Budinsky, Gollapudi, Albertini, Valentine, Stavanja, Teeguarden, Fensterheim, Rick, Lardie, McFadden, Green and Recio (2013). Nonlinear responses for chromosome and gene level effects induced by vinyl acetate monomer and its metabolite, acetaldehyde in TK6 cells. Environ Mol Mutagen. 54: 755-768, Hotchkiss, Krieger, Harkema and Mahoney (2013). Draft Report: VINYL ACETATE: EVALUATION OF VINYL ACETATE-SPECIFIC DNA ADDUCTS, HISTOPATHOLOGY AND EPITHELIAL CELL PROLIFERATION IN NASAL AIRWAYS OF Crl:CD(SD) RATS REPEATEDLY EXPOSED TO VINYL ACETATE VAPORS. Washington, DC, The Vinyl Acetate Council
  23. Bogdanffy, Sarangapani, Plowchalk, Jarabek and Andersen (1999). A Biologically-Based Risk Assessment for Vinyl Acetate-Induced Cancer and Non-Cancer Inhalation Toxicity. Toxicological Sciences. 51: 19-35, Bogdanffy, Plowchalk, Sarangapani, Starr and Andersen (2001). Mode-of-action-based dosimeters for interspecies extrapolation of vinyl acetate inhalation risk. Inhal Toxicol. 13: 377-396
  24. Bogdanffy (2002). Vinyl acetate-induced intracellular acidification: implications for risk assessment. Toxicol Sci. 66: 320-326, Lantz, Orozco and Bogdanffy (2003). Vinyl acetate decreases intracellular pH in rat nasal epithelial cells. Toxicol Sci. 75: 423-431
  25. Kuykendall, Taylor and Bogdanffy (1993). Cytotoxicity and DNA-protein crosslink formation in rat nasal tissues exposed to vinyl acetate are carboxylesterase-mediated. Toxicol Appl Pharmacol. 123: 283-292
  26. Kuykendall, Taylor and Bogdanffy (1993). Cytotoxicity and DNA-protein crosslink formation in rat nasal tissues exposed to vinyl acetate are carboxylesterase-mediated. Toxicol Appl Pharmacol. 123: 283-292
  27. Bertram (2000). The molecular biology of cancer. Mol Aspects Med. 21: 167-223, Hanahan and Weinberg (2000). The hallmarks of cancer. Cell. 100: 57-70
  28. Preston-Martin, Pike, Ross, Jones and Henderson (1990). Increased cell division as a cause of human cancer. Cancer Res. 50: 7415-7421, Cohen, Purtilo and Ellwein (1991). Ideas in pathology. Pivotal role of increased cell proliferation in human carcinogenesis. Mod Pathol. 4: 371-382
  29. Albertini (2013). Vinyl acetate monomer (VAM) genotoxicity profile: relevance for carcinogenicity. Crit Rev Toxicol. 43: 671-706, Budinsky, Gollapudi, Albertini, Valentine, Stavanja, Teeguarden, Fensterheim, Rick, Lardie, McFadden, Green and Recio (2013). Nonlinear responses for chromosome and gene level effects induced by vinyl acetate monomer and its metabolite, acetaldehyde in TK6 cells. Environ Mol Mutagen. 54: 755-768
  30. Bogdanffy, Dreef-van der Meulen, Beems, Feron, Cascieri, Tyler, Vinegar and Rickard (1994). Chronic toxicity and oncogenicity inhalation study with vinyl acetate in the rat and mouse. Fundam Appl Toxicol. 23: 215-229, Bogdanffy, Gladnick, Kegelman and Frame (1997). FOUR-WEEK INHALATION CELL PROLIFERATION STUDY OF THE EFFECTS OF VINYL ACETATE ON RAT NASAL EPITHELIUM. Inhalation Toxicology, Taylor & Francis. 9: 331-350, Hotchkiss, Krieger, Harkema and Mahoney (2013). Draft Report: VINYL ACETATE: EVALUATION OF VINYL ACETATE-SPECIFIC DNA ADDUCTS, HISTOPATHOLOGY AND EPITHELIAL CELL PROLIFERATION IN NASAL AIRWAYS OF Crl:CD(SD) RATS REPEATEDLY EXPOSED TO VINYL ACETATE VAPORS. Washington, DC, The Vinyl Acetate Council
  31. Hardisty, Garman, Harkema, Lomax and Morgan (1999). Histopathology of nasal olfactory mucosa from selected inhalation toxicity studies conducted with volatile chemicals. Toxicol Pathol. 27: 618-627
  32. Bogdanffy, Dreef-van der Meulen, Beems, Feron, Cascieri, Tyler, Vinegar and Rickard (1994). Chronic toxicity and oncogenicity inhalation study with vinyl acetate in the rat and mouse. Fundam Appl Toxicol. 23: 215-229, Bogdanffy, Gladnick, Kegelman and Frame (1997). FOUR-WEEK INHALATION CELL PROLIFERATION STUDY OF THE EFFECTS OF VINYL ACETATE ON RAT NASAL EPITHELIUM. Inhalation Toxicology, Taylor & Francis. 9: 331-350, Hotchkiss, Krieger, Harkema and Mahoney (2013). Draft Report: VINYL ACETATE: EVALUATION OF VINYL ACETATE-SPECIFIC DNA ADDUCTS, HISTOPATHOLOGY AND EPITHELIAL CELL PROLIFERATION IN NASAL AIRWAYS OF Crl:CD(SD) RATS REPEATEDLY EXPOSED TO VINYL ACETATE VAPORS. Washington, DC, The Vinyl Acetate Council
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