The possible repositioning of an oral anti-arthritic drug, auranofin, for Nrf2-activating therapy: The demonstration of Nrf2-dependent anti-oxidative action using a zebrafish model

Protein-coated microplastics corona complex: An underestimated risk of microplastics

Traditionally, toxicity of microplastics is ascribed to the chemicals adsorbed on them. However, microplastics can also interact with biomolecules, such as secretory proteins from aquatic organisms, and form protein-coated microplastics corona complex with unknown toxic effects. Here, we investigated the toxic effects of polystyrene microplastics (PS) and bovine serum albumin (BSA) coated PS corona complex (PS + BSA) on adult zebrafish (Danio rerio) intestines. The food intake ratio, accumulation and distribution of microplastics, histopathological changes, and molecular effects related to the antioxidant system in the intestine were studied. For the first time, we observed that PS + BSA aggregated on the inner surface of the zebrafish intestine, whereas PS dispersed. The aggregation of PS + BSA resulted in increased microplastics accumulation and longer residence time in the zebrafish intestine, which inhibited food intake and generated reactive oxygen species (ROS) in the intestine. Furthermore, the functions of the Keap1-Nrf2-ARE antioxidant signaling pathway and the activation of antioxidant enzymes were significantly affected by PS + BSA after a 21-day exposure. Ultimately, a higher accumulation of ROS and stronger inhibition of antioxidants led to more severe intestinal injury. These results suggest that the increased toxicity of protein-coated microplastics corona complex may be affected by oxidative damage and can result in the inhibition of digestion due to their aggregation and longer residence time in the intestine. Therefore, the ecological risk of microplastics may be underestimated owing to the interactive mechanisms of microplastics and protein coronas.

Oxidative stress inducers potentiate 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated pre-cardiac edema in larval zebrafish

We reported the involvement of oxidative stress and prostaglandins including thromboxane and prostacyclin in pre-cardiac edema (early edema) caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). While the involvement of oxidative stress in TCDD-induced toxicity has been frequently reported, the mechanism of its action is still unclear. In the present study, oxidative stress inducers including paraquat, hydrogen peroxide (H₂O₂) and rotenone augmented early edema (edema) induced by a low concentration of TCDD (0.1 ppb) at 55 hr post fertilization (hpf), while each of them alone did not cause edema. Edema caused by TCDD plus oxidative stress inducers was almost abolished by antioxidants, an antagonist for thromboxane receptor (ICI-192,605) and an agonist for prostacyclin receptor (beraprost), suggesting that the site of action of these inducers was in the regular signaling pathway after activation of aryl hydrocarbon receptor type 2 (AHR2) by TCDD. Oxidative stress inducers also enhanced edema caused by an agonist for the thromboxane receptor (U46619), and the enhancement was also inhibited by antioxidants. Sulforaphane and auranofin, activators of Nrf2 that is a master regulator of anti-oxidative response, did not affect U46619-evoked edema but almost abolished TCDD-induced edema and potentiation by paraquat in both TCDD- and U46619-induced edema. Taken together, the results suggest that oxidative stress augments pre-cardiac edema caused by TCDD via activation of thromboxane receptor-mediated signaling in developing zebrafish. As paraquat and other oxidative stress inducers used also are environmental pollutants, interaction between dioxin-like compounds and exogenous source of oxidative stress should also be considered.

The Nrf2 pathway in psychiatric disorders: pathophysiological role and potential targeting

Introduction: All psychiatric disorders exhibit excitotoxicity, mitochondrial dysfunction, inflammation, oxidative stress, and neural damage as their common characteristic. The endogenous nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway is implicated in the defense mechanism against oxidative stress and has a significant role in psychiatric disorders.Areas covered: We explore the role of Nrf2 pathway and its modulators in psychiatric disorders. The literature was searched utilizing various databases such as Embase, Medline, Web of Science, Pub-Med, and Google Scholar from 2010 to 2020. The search included research articles, clinical reports, systematic reviews, and meta-analyses.Expert opinion: Environmental factors and genetic predisposition can be a trigger for the development of psychiatric disorders. Nrf2 downregulates certain inflammatory pathways and upregulates various antioxidant enzymes to maintain a balance. However, its intricate balance with NF-Kβ (Nuclear factor kappa light chain enhancer of activated B cells) and its crosstalk with the transcription factor Nrf2 is critical in severe oxidative stress. Several Nrf2 modulators are now in clinical trials and can help reduce oxidative stress and neuroinflammation. There are immense potential opportunities for these modulators to become a novel therapeutic option.

Myeloid differentiation protein 2 mediates angiotensin II-induced inflammation and mesenchymal transition in vascular endothelium

Angiotensin II (Ang II)-induced vascular inflammation and injury entails endothelial to mesenchymal transition (EndMT). Recent studies have shown that Ang II engages toll-like receptor 4 (TLR4) in the vasculature to mediate adverse effects. Here, we aimed to investigate whether myeloid differentiation protein 2 (MD2), an extracellular molecule indispensable for TLR4 activation, mediates Ang II-induced vascular injury and EndMT. We utilized MD2 knockout mice and wildtype mice treated with a specific MD2 inhibitor to decipher its role in aortas of Ang II-challenged mice. To confirm our results and to provide mechanistic insights, we exposed cultured endothelial cells to Ang II, with or without MD2 silencing. We show that Ang II causes deleterious remodeling and EndMT in aortas of mice within two weeks. These Ang II effects were largely absent in MD2 knockout mice and in wildtype mice treated with a MD2 inhibitor. MD2 silencing in cultured endothelial cells confirmed the essential role of MD2 in Ang II-induced inflammatory factor induction, and EndMT-associated phenotypic change. We also found that Ang II-MD2-EndMT axis involves the activation of nuclear factor-κB. Our studies highlight an essential role of MD2 in Ang II-induced vascular inflammation and EndMT contributing to vascular injury. These results also imply that MD2 may be targeted to dampen inflammatory cardiovascular and EndMT-associated diseases.

Generation and characterization of keap1a- and keap1b-knockout zebrafish

The Keap1–Nrf2 pathway is an evolutionarily conserved mechanism that protects cells from oxidative stress and electrophiles. Under homeostatic conditions, Keap1 interacts with Nrf2 and leads to its rapid proteasomal degradation, but when cells are exposed to oxidative stress/electrophiles, Keap1 senses them, resulting in an improper Keap1–Nrf2 interaction and Nrf2 stabilization. Keap1 is therefore considered both an “inhibitor” of and “stress sensor” for Nrf2 activation. Interestingly, fish and amphibians have two Keap1s (Keap1a and Keap1b), while there is only one in mammals, birds and reptiles. A phylogenetic analysis suggested that mammalian Keap1 is an ortholog of fish Keap1b, not Keap1a. In this study, we investigated the differences and similarities between Keap1a and Keap1b using zebrafish genetics. We generated zebrafish knockout lines of keap1a and keap1b. Homozygous mutants of both knockout lines were viable and fertile. In both mutant larvae, the basal expression of Nrf2 target genes and antioxidant activity were up-regulated in an Nrf2-dependent manner, suggesting that both Keap1a and Keap1b can function as Nrf2 inhibitors. We also analyzed the effects of the Nrf2 activator sulforaphane in these mutants and found that keap1a-, but not keap1b-, knockout larvae responded to sulforaphane, suggesting that the stress/chemical-sensing abilities of the two Keap1s are different.

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Fish and amphibians have two Keap1s: Keap1a and Keap1b.

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Mammalian Keap1 is an ortholog of fish Keap1b, not Keap1a.

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Both Keap1a and Keap1b can function as Nrf2 inhibitors.

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The sulforaphane-sensing abilities of Keap1a and Keap1b are different.

Evaluation of Antioxidant Activity of Spice-Derived Phytochemicals Using Zebrafish

Various dietary phytochemicals seem to display antioxidant activity through the NF-E2-related factor 2 (Nrf2) pathway. However, few studies have demonstrated its antioxidant effect and Nrf2 dependency at the animal level. We constructed a zebrafish-based assay system to analyze the in vivo antioxidant activity of phytochemicals and examined the activity of 10 phytochemicals derived from spices, using this system as a pilot study. Hydrogen peroxide and arsenite were used as oxidative stressors, and Nrf2 dependency was genetically analyzed using an Nrf2-mutant zebrafish line. The activities of curcumin, diallyl trisulfide and quercetin were involved in the reduction of hydrogen peroxide toxicity, while those of cinnamaldehyde, isoeugenol and 6-(methylsulfinyl)hexyl isothiocyanate were involved in the reduction of arsenite toxicity. The antioxidant activities of these phytochemicals were all Nrf2 dependent, with the exception of cinnamaldehyde, which showed strong antioxidant effects even in Nrf2-mutant zebrafish. In summary, we succeeded in constructing an assay system to evaluate the in vivo antioxidant activity of various phytochemicals using zebrafish larvae. Using this system, we found that each spice-derived phytochemical has its own specific property and mechanism of antioxidant action.

The effects of dietary selenomethionine on tissue-specific accumulation and toxicity of dietary arsenite in rainbow trout (Oncorhynchus mykiss) during chronic exposure

Selenomethionine facilitated arsenic deposition in the brain and likely in other tissues, possibly via bio-complexation. Elevated dietary selenomethionine can increase the tissue-specific accumulation and toxicity of As³⁺ in fish during chronic dietary exposure.

Increased susceptibility to oxidative stress-induced toxicological evaluation by genetically modified nrf2a-deficient zebrafish

INTRODUCTION

Oxidative stress plays an important role in drug-induced toxicity. Oxidative stress-mediated toxicities can be detected using conventional animal models but their sensitivity is insufficient, and novel models to improve susceptibility to oxidative stress have been researched. In recent years, gene targeting methods in zebrafish have been developed, making it possible to generate homozygous null mutants. In this study, we established zebrafish deficient in the nuclear factor erythroid 2-related factor 2a (nrf2a), a key antioxidant-responsive gene, and its potential to detect oxidative stress-mediated toxicity was examined.

METHODS

Nrf2a-deficient zebrafish were generated using the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 technique. The loss of nrf2a function was confirmed by the tolerability to hydrogen peroxide and hydrogen peroxide-induced gene expression profiles being related to antioxidant response element (ARE)-dependent signaling. Subsequently, vulnerability of nrf2a-deficient zebrafish to acetaminophen (APAP)- or doxorubicin (DOX)-induced toxicity was investigated.

RESULTS

Nrf2a-deficient zebrafish showed higher mortality than wild type accompanied by less induction of ARE-dependent genes with hydrogen peroxide treatment. Subsequently, this model showed increased severity and incidence of APAP-induced hepatotoxicity or DOX-induced cardiotoxicity than wild type.

DISCUSSION

Our results demonstrated that anti-oxidative response might not fully function in this model, and resulted in higher sensitivity to drug-induced oxidative stress. Our data support the usefulness of nrf2a-deficient model as a tool for evaluation of oxidative stress-related toxicity in drug discovery research.