Role of Aryl Hydrocarbon Receptor and Oxidative Stress in the Regioselective Toxicities of Hydroxychrysenes in Embryonic Japanese Medaka (Oryzias latipes)

by Selleckchem | Jun 23 2023

Oxygenated polycyclic aromatic hydrocarbons (oxy-PAHs) are environmental contaminants that can be created through oxidation of parent PAHs. Previous studies have found that 2-hydroxychrysene (2-OHCHR) caused anemia in embryonic Japanese medaka whereas 6-hydroxychrysene (6-OHCHR) did not, an example of regioselective toxicity. Anemia was prevented by cytochrome P450 (CYP) inhibition, which reduced the formation of the potential oxidatively active metabolite, 1,2-catechol, from 2-OHCHR. 2-OHCHR has also been found to be a four-fold more potent aryl hydrocarbon receptor (AhR) agonist compared with 6-OHCHR. These findings led us to hypothesize that AhR activation and/or oxidative stress play an important role in 2-OHCHR toxicity. Although treatments with the AhR agonists polychlorinated biphenyl (PCB)126 and 2-methoxychrysene (2-MeOCHR) did not cause significant toxicity, pretreatments with the AhR antagonist, CH-223191, reduced anemia by 97.2 ± 0.84% and mortality by 96.6 ± 0.69%. Aryl hydrocarbon receptor inhibition by the antagonist was confirmed by significant reductions (91.0 ± 9.94%) in induced ethoxyresorufin-O-deethylase activity. Thiobarbituric acid reactive substances concentrations were 32.9 ± 3.56% higher (p < 0.05) in 2-OHCHR treatments at 100 hours postfertilization compared with controls. Staining 2-OHCHR-treated embryos with the reactive oxygen species (ROS) scavenger 2',7'-dichlorofluorescin diacetate revealed 32.6 ± 2.69% of 2-OHCHR-treated embryos exhibiting high concentrations of ROS in caudal tissues, which is a site for embryonic hematopoiesis in medaka. Pretreatment with antioxidants, N-acetylcysteine (NAC) or vitamin E (Vit E) significantly reduced 2-OHCHR-induced anemia (NAC: 80.7 ± 1.12% and Vit E: 99.1 ± 0.43%) and mortality (NAC: 67.1 ± 1.69% and Vit E: 98.9 ± 0.66%). These results indicate that AhR may mediate 2-OHCHR toxicity through canonical signaling by up-regulating CYP1, enhancing the formation of reactive metabolites of 2-OHCHR that generate ROS within caudal hematopoietic tissues, potentially disrupting hematopoiesis, leading to anemia and subsequent mortality. Environ Toxicol Chem 2023;42:698-706. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Comments:

The passage you provided describes a study on the toxicity of 2-hydroxychrysene (2-OHCHR), an oxygenated polycyclic aromatic hydrocarbon (oxy-PAH), in embryonic Japanese medaka. The study found that 2-OHCHR caused anemia and mortality in the medaka embryos, while another oxy-PAH called 6-hydroxychrysene (6-OHCHR) did not. This suggests that the toxicity of these compounds is regioselective, meaning it depends on the specific position of the hydroxyl group.

The study further investigated the mechanisms underlying the toxicity of 2-OHCHR. It was found that 2-OHCHR is a more potent activator of the aryl hydrocarbon receptor (AhR) than 6-OHCHR. AhR activation is known to be involved in the toxicity of certain environmental contaminants. However, direct treatment with AhR agonists, such as polychlorinated biphenyl (PCB)126 and 2-methoxychrysene (2-MeOCHR), did not cause significant toxicity in the medaka embryos.

To understand the role of AhR activation in 2-OHCHR toxicity, the researchers used an AhR antagonist called CH-223191. Pretreatment with this antagonist significantly reduced anemia and mortality caused by 2-OHCHR, suggesting that AhR activation contributes to the toxic effects of 2-OHCHR.

The study also investigated oxidative stress as a potential mechanism of 2-OHCHR toxicity. Oxidative stress occurs when there is an imbalance between the production of reactive oxygen species (ROS) and the body's ability to detoxify them. Treatment with 2-OHCHR resulted in higher levels of ROS in the caudal tissues of the medaka embryos, which are involved in hematopoiesis (blood cell formation). This suggests that ROS generated by 2-OHCHR may disrupt hematopoiesis, leading to anemia and mortality.

The researchers further confirmed the role of oxidative stress by treating the embryos with antioxidants, specifically N-acetylcysteine (NAC) and vitamin E (Vit E). Pretreatment with these antioxidants significantly reduced 2-OHCHR-induced anemia and mortality, indicating that oxidative stress contributes to the toxic effects of 2-OHCHR.

Overall, the study suggests that the toxicity of 2-OHCHR in medaka embryos is mediated, at least in part, by AhR activation and the subsequent up-regulation of cytochrome P450 (CYP) enzymes, which generate reactive metabolites of 2-OHCHR. These reactive metabolites lead to the production of ROS, causing oxidative stress and disrupting hematopoiesis, ultimately resulting in anemia and mortality.