Activation, AhR leads to Increase, Early Life Stage Mortality

The aryl hydrocarbon receptor is commonly known for its involvement in xenobiotic metabolism and clearance, but it also regulates a number of endogenous processes including angiogenesis, immune responses, neuronal processes, metabolism, and development of numerous organ systems (Duncan et al., 1998; Emmons et al., 1999; Hahn et al 2002; Lahvis and Bradfield, 1998).  Strong AHR agonists that cause sustained AHR activation interfere with the receptor's endogenous role in embryogenesis, which causes numerous developmental abnormalities and ultimately leads to embryonic death (Kopf and Walker 2009; Carreira et al 2015).

It's important to note that his relationship only applies to AHR agonists that cause sustained AHR activation.  Strong AHR agonists that are rapidly metabolized, such as polycyclic aromatic hydrocarbons, only cause transient AHR activation leading to an alternate mode of toxicity.

This Key Event Relationship describes the indirect link between the Molecular Initiating Event (activation of the AhR) and the Adverse Outcome (increased early life stage mortality).

Birds:

The predictive ability of an LRG assay measuring induction of AHR1-mediated gene expression in cells transfected with different avian AHR1 expression vectors was demonstrated by linear regression analysis comparing log-transformed LD50 values obtained from the literature to log-transformed PC20 values from the LRG assay (Farmahin et al. 2013b; Manning et al. 2012). PC20 values represent the concentration of DLC that elicited 20% of the TCDD maximal response, and were calculated according to the procedure described in OECD guideline 455 (OECD 2009). LD50 values used in regression analyses were obtained from the literature. As shown in the linear regression analysis (Figure 1), logLD50 values were associated with logPC20 and a significant relationship (R2 = 0.93, p < 0.0001) was observed. Thus, to predict the in ovo LD50 for a given species and DLC, one could use the species’ AHR1 LBD sequence to design an AHR1 expression vector, measure the PC20 of the DLC in the LRG assay, and use the regression to obtain an LD50 value.

Figure 1. Linear regression analysis comparing LD50 values with PC20 (logLD50 = 0.79logPC20 + 0.51) values derived from luciferase reporter gene (LRG) assay concentration-response curves. Open symbols represent LRG data from wild-type chicken, ring-necked pheasant or Japanese quail AHR1 expression vectors. Closed symbols represent LRG data from mutant AHR1 (Source: Manning, G. E. et al. (2012). Toxicol. Appl. Pharmacol. 263(3), 390-399.)

Mammals:

A quantitative model has been developed linking in silico activation of the AhR with acute lethality (measured as dose to cause 50 % lethality; LD50) among 7 species of mammals with an R² of 0.99 (Wang et al 2013). The model is described in detail by Wang et al (2013). The model is described as:

If steric (LJ12-6) < 0 then Log (LD50) = 13.273Log(NOQ) + 5.167Log(-Steric(PLP))-0.157Log(-steric(LJ12-6))-1.799Log(-(H-bond))-24.625

If steric (LJ12-6) > 0 then Log (LD50) = 13.273Log(NOQ) + 5.167Log(-Steric(PLP))+0.157Log(-steric(LJ12-6))-1.799Log(-(H-bond))-24.625

Fishes:

Limited information is currently available across fishes. However, a quantitative model has been developed linking in vitro activation of the AhR2 alpha in transfected COS-7 cells (meaured as concentration to cause 50 % effect; EC50) with early life stage mortality (measured as dose to cause 50 % lethality; LD50) for rainbow trout (Oncorhynchus mykiss) across 6 chemicals with an R² of 0.81 (Abnet et al 1999). The model is described in detail by Abnet et al (1999). The model is described as:

LD50 = 1.57*(EC50)-0.2418

Amphibians and reptiles:

No quantitative models are currently available for amphibians or reptiles.

• Overall, this KER is believed to be applicable to all vertebrates based on mortality as a result of exposure to known agonists of the AhR (Buckler et al 2015; Cohen-Barnhouse et al 2011; Elonen et al 1998; Johnson et al 1998; Jung et al 1997; Kopf & Walker 2009; Park et al 2014; Tillitt et al 2016; Toomey et al 2001; Walker et al 1991; Wang et al 2013; Yamauchi et al 2006; Zabel et al 1995).

• The correlation between AHR-mediated reporter gene activity and embryo death has been demonstrated in species of birds and fishes (Doernig et al 2018).
• Less is known about differences in binding affinity of AhRs and how this relates to sensitivity in reptiles or amphibians.
• Low binding affinity for DLCs of AhR1s of African clawed frog (Xenopus laevis) and axolotl (Ambystoma mexicanum) has been suggested as a mechanism for tolerance of these amphibians to DLCs (Lavine et al 2005; Shoots et al 2015).
• Among reptiles, only AhRs of American alligator (Alligator mississippiensis) have been investigated and little is known about the sensitivity of American alligator or other reptiles to DLCs (Oka et al 2016).
• Among fishes, great differences in sensitivity to DLCs are known both for AhRs and for embryos among species that have been tested (Doering et al 2013; 2014; 2018).
• Differences in binding affinity of the AhR2 have been demonstrated to explain differences in sensitivity to DLCs between sensitive and tolerant populations of Atlantic Tomcod (Microgadus tomcod) (Wirgin et al 2011).