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Relationship: 2688


A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Activation, AhR leads to Decrease, sox9 expression

Upstream event
The causing Key Event (KE) in a Key Event Relationship (KER). More help
Downstream event
The responding Key Event (KE) in a Key Event Relationship (KER). More help

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes. Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

AOP Name Adjacency Weight of Evidence Quantitative Understanding Point of Contact Author Status OECD Status
Aryl hydrocarbon receptor activation leading to early life stage mortality via impeded craniofacial development non-adjacent High High Prarthana Shankar (send email) Under development: Not open for comment. Do not cite
Aryl hydrocarbon receptor activation leading to early life stage mortality via cardiovascular toxicity non-adjacent High High Prarthana Shankar (send email) Under development: Not open for comment. Do not cite

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KER.In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER.  More help
Term Scientific Term Evidence Link
zebrafish Danio rerio High NCBI
human Homo sapiens Moderate NCBI
Salmo salar Salmo salar Moderate NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help
Sex Evidence
Unspecific High

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER.  More help
Term Evidence
Embryo High
Development High

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help
  • The Ahr is a ligand activated transcription factor that is capable of regulating gene expression of several genes, all belonging to the Ahr signaling cascade (Larigot et al., 2018).
  • Canonical Ahr signaling involves receptor translocation from the cytoplasm to the nucleus, followed by Ahr-ARNT heterodimerization. The heterodimer then recognizes Aryl hydrocarbon response elements (AHREs) in the promoter regions of different genes to regulate their expression (Swanson 2002). Indirect gene regulation is also possible, with the downstream target genes interacting with other signaling pathways (Mathew et al., 2008).
  • Sox9 is one proposed indirect gene within the Ahr signaling cascade. Sox9b, one of two paralogs of the sox9 gene in zebrafish, is one of the most reduced transcripts in the jaw upon TCDD exposure in zebrafish (Xiong et al., 2008)Thus, there exists a non-adjacent relationship between Ahr activation and the repression of sox9.

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER.  For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings.  Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help
Biological Plausibility
Addresses the biological rationale for a connection between KEupstream and KEdownstream.  This field can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured.   More help
  • Evidence for biological plausibility comes from Ahr’s ability to interact with several molecular signaling pathways, including the Wnt-beta catenin pathway (Mathew et al. 2008). Sox9 is one important member of the Wnt-beta catenin signaling pathway, specifically as it relates to chondrogenesis (Sinha et al., 2021; Topol et al., 2009).
Uncertainties and Inconsistencies
Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help
  • Whole animal zebrafish exposed to several individual PAHs, many of whom significantly induce cyp1a by 48 hpf, do not cause significant repression of sox9b (Garcia et al., 2018). The PAHs are retene, benzo[j]fluoranthene, benzo[k]fluoranthene, dibenzo[a,h]pyrene, benzo[b]fluoranthene, fluoranthene, phenanthrene, and 9-methylanthracene. Dibenzo[a,i]pyrene was the only PAH from the list that showed a trend for sox9b reduction. One explanation is that the possible tissue-specific sox9b repression was not enough to capture expression changes in this whole-animal study where zebrafish were exposed to Ahr activators not as strong as TCDD.
  • A microarray study investigating gene expression changes in the jaw primordium of zebrafish exposed to TCDD from 1 to 24 hpf did not include either paralog of sox9 in the top downregulated gene list (Planchart and Mattingly 2010). It is possible that sox9 was not present in the microarray.
  • In a human glioblastoma cell culture study, sox9 was repressed when ARNT2 was knocked down, in addition to the study identifying potential binding regions of ARNT2 in the regulatory region of sox9 (Bogeas et al., 2018). While no functional studies were conducted, it is possible that there may be cell-specific direct regulation of sox9 by Ahr/ARNT.
  • In frozen human lung tumor samples, expression of sox9 was significantly higher in smokers compared to in samples from non-smokers. Additionally, in adenocarcinomas in smoking women, sox9 expression was relatively high. Of note, these results were accompanied by the lack of induction of Ahr expression (Szymanowska-Narloch et al., 2013).

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references.  More help
Response-response Relationship
Provides sources of data that define the response-response relationships between the KEs.  More help
Information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). More help
Known Feedforward/Feedback loops influencing this KER
Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help
  • The relationship between Ahr activation and sox9 repression is best studied in developing zebrafish. Some supporting evidence comes from salmon larvae, as well as human lung cells, suggesting that this relationship is highly evolutionarily conserved among vertebrates (at least), but also likely tissue-specific.


List of the literature that was cited for this KER description. More help

Andreasen EA, Mathew LK, Tanguay RL. 2006. Regenerative growth is impacted by tcdd: Gene expression analysis reveals extracellular matrix modulation. Toxicol Sci. 92(1):254-269.

Bogeas A, Morvan-Dubois G, El-Habr EA, Lejeune FX, Defrance M, Narayanan A, Kuranda K, Burel-Vandenbos F, Sayd S, Delaunay V et al. 2018. Changes in chromatin state reveal arnt2 at a node of a tumorigenic transcription factor signature driving glioblastoma cell aggressiveness. Acta Neuropathol. 135(2):267-283.

Garcia GR, Goodale BC, Wiley MW, La Du JK, Hendrix DA, Tanguay RL. 2017. In vivo characterization of an ahr-dependent long noncoding rna required for proper sox9b expression. Mol Pharmacol. 91(6):609-619.

Garcia GR, Shankar P, Dunham CL, Garcia A, La Du JK, Truong L, Tilton SC, Tanguay RL. 2018. Signaling events downstream of ahr activation that contribute to toxic responses: The functional role of an ahr-dependent long noncoding rna (slincr) using the zebrafish model. Environ Health Perspect. 126(11):117002.

Hofsteen P, Plavicki J, Johnson SD, Peterson RE, Heideman W. 2013. Sox9b is required for epicardium formation and plays a role in tcdd-induced heart malformation in zebrafish. Molecular Pharmacology. 84(3):353-360.

Jenny MJ, Karchner SI, Franks DG, Woodin BR, Stegeman JJ, Hahn ME. 2009. Distinct roles of two zebrafish ahr repressors (ahrra and ahrrb) in embryonic development and regulating the response to 2,3,7,8-tetrachlorodibenzo-p-dioxin. Toxicological Sciences. 110(2):426-441.

Larigot L, Juricek L, Dairou J, Coumoul X. 2018. Ahr signaling pathways and regulatory functions. Biochim Open. 7:1-9.

Mathew LK, Sengupta SS, Ladu J, Andreasen EA, Tanguay RL. 2008. Crosstalk between ahr and wnt signaling through r-spondin1 impairs tissue regeneration in zebrafish. FASEB J. 22(8):3087-3096.

Olufsen M, Arukwe A. 2011. Developmental effects related to angiogenesis and osteogenic differentiation in salmon larvae continuously exposed to dioxin-like 3,3',4,4'-tetrachlorobiphenyl (congener 77). Aquat Toxicol. 105(3-4):669-680.

Planchart A, Mattingly CJ. 2010. 2,3,7,8-tetrachlorodibenzo-p-dioxin upregulates foxq1b in zebrafish jaw primordium. Chem Res Toxicol. 23(3):480-487.

Prochazkova J, Strapacova S, Svrzkova L, Andrysik Z, Hyzdalova M, Hruba E, Pencikova K, Libalova H, Topinka J, Klema J et al. 2018. Adaptive changes in global gene expression profile of lung carcinoma a549 cells acutely exposed to distinct types of ahr ligands. Toxicol Lett. 292:162-174.

Shi X, He C, Zuo Z, Li R, Chen D, Chen R, Wang C. 2012. Pyrene exposure influences the craniofacial cartilage development of sebastiscus marmoratus embryos. Mar Environ Res. 77:30-34.

Simeckova P, Marvanova S, Kulich P, Kralikova L, Neca J, Prochazkova J, Machala M. 2019. Screening of cellular stress responses induced by ambient aerosol ultrafine particle fraction pm0.5 in a549 cells. Int J Mol Sci. 20(24).

Sinha A, Fan VB, Ramakrishnan AB, Engelhardt N, Kennell J, Cadigan KM. 2021. Repression of wnt/beta-catenin signaling by sox9 and mastermind-like transcriptional coactivator 2. Sci Adv. 7(8).

Swanson HI. 2002. DNA binding and protein interactions of the ahr/arnt heterodimer that facilitate gene activation. Chem-Biol Interact. 141(1-2):63-76.

Szymanowska-Narloch A, Jassem E, Skrzypski M, Muley T, Meister M, Dienemann H, Taron M, Rosell R, Rzepko R, Jarzab M et al. 2013. Molecular profiles of non-small cell lung cancers in cigarette smoking and never-smoking patients. Adv Med Sci. 58(2):196-206.

Topol L, Chen W, Song H, Day TF, Yang Y. 2009. Sox9 inhibits wnt signaling by promoting beta-catenin phosphorylation in the nucleus. J Biol Chem. 284(5):3323-3333.

Xiong KM, Peterson RE, Heideman W. 2008. Aryl hydrocarbon receptor-mediated down-regulation of sox9b causes jaw malformation in zebrafish embryos. Mol Pharmacol. 74(6):1544-1553.