Quinocetone-induced Nrf2/HO-1 pathway suppression aggravates hepatocyte damage of Sprague–Dawley rats

Advances in Metabolism and Metabolic Toxicology of Quinoxaline 1,4-Di-N-oxides

Quinoxaline 1,4-di-N-Oxides (QdNOs) have been used as synthetic antimicrobial agents in animal husbandry and aquaculture. The metabolism and potential toxicity have been also concerns in recently years. The metabolism investigations showed that there were 8 metabolites of Carbadox (CBX), 34 metabolites of Cyadox (CYA), 33 metabolites of Mequindox (MEQ), 35 metabolites of Olaquindox (OLA), and 56 metabolites of Quinocetone (QCT) in different animals. Among them, Cb3 and Cb8, M6, and O9 are metabolic residual markers of CBX, MEQ and OLA, which are associated with N → O reduction. Toxicity studies revealed that QdNOs exhibited severe tumorigenicity, cytotoxicity, and adrenal toxicity. Metabolic toxicology showed that toxicity of QdNOs metabolites might be related to the N → O group reduction, and some metabolites exhibited higher toxic effects than the precursor, which could provide guidance for further research on the metabolic toxicology of QdNOs and provide a wealth of information for food safety evaluation.

Quercetin Attenuates Quinocetone-Induced Cell Apoptosis In Vitro by Activating the P38/Nrf2/HO-1 Pathway and Inhibiting the ROS/Mitochondrial Apoptotic Pathway

Quinocetone (QCT), a member of the quinoxaline 1,4-di-N-oxides (QdNOs) family, can cause genotoxicity and hepatotoxicity, however, the precise molecular mechanisms of QCT are unclear. This present study investigated the protective effect of quercetin on QCT-induced cytotoxicity and the underlying molecular mechanisms in human L02 and HepG2 cells. The results showed that quercetin treatment (at 7.5–30 μM) significantly improved QCT-induced cytotoxicity and oxidative damage in human L02 and HepG2 cells. Meanwhile, quercetin treatment at 30 μM significantly inhibited QCT-induced loss of mitochondrial membrane potential, an increase in the expression of the CytC protein and the Bax/Bcl-2 ratio, and an increase in caspases-9 and -3 activity, and finally improved cell apoptosis. Quercetin pretreatment promoted the expression of the phosphorylation of p38, Nrf2, and HO-1 proteins. Pharmacological inhibition of p38 significantly inhibited quercetin-mediated activation of the Nrf2/HO-1 pathway. Consistently, pharmacological inhibitions of the Nrf2 or p38 pathways both promoted QCT-induced cytotoxicity and partly abolished the protective effects of quercetin. In conclusion, for the first time, our results reveal that quercetin could improve QCT-induced cytotoxicity and apoptosis by activating the p38/Nrf2/HO-1 pathway and inhibiting the ROS/mitochondrial apoptotic pathway. Our study highlights that quercetin may be a promising candidate for preventing QdNOs-induced cytotoxicity in humans or animals.

Combination of Oxytetracycline and Quinocetone Synergistically Induces Hepatotoxicity via Generation of Reactive Oxygen Species and Activation of Mitochondrial Pathway

Oxytetracycline (OTC) and Quinocetone (QCT) are antimicrobials, whose residues have been found in food and environment. These two are sometimes used simultaneously in livestock and aquaculture, potentially resulting in the simultaneous consumption of multi-antimicrobials by consumers. However, the combined toxic effects of this phenomenon have yet to be addressed. Since the liver is a major target of both OTC and QCT, we tested their hepatotoxic effect using both cell cultures and animal models. Results showed that the QCT (5-25 μM) or OTC (20-100 μM) treatments alone caused dose-dependent reductions in cell numbers, while their combination strongly further enhanced inhibitory effects. Mechanistically, the combination enhanced the generation of reactive oxygen species (ROS) and activated mitochondrial cell death pathways. It also showed that the combination of OTC (800 mg/kg, i.g., 5d) and QCT (5000 mg/kg, i.g., 5d) resulted in significantly enhanced liver toxicity in C57BL/6N mice, the serum alanine transaminase (ALT) and aspartate transaminase (AST) were significantly increased by the OTC/QCT. These findings indicate the necessity of considering the combined toxicity of these two antimicrobials in safety assessments.

Hyperhomocysteinemia-induced Nrf2/HO-1 pathway suppression aggravates cardiac remodeling of hypertensive rats

ABJECTIVE

Interaction of hypertension and hyperhomocysteinemia (HHcy) leads to enhanced cardiac remodeling in hypertensive heart disease. However, the mechanism of collagen accumulation and cardiac remodeling remains unclear. In this study, we attempted to evaluate the relationship between hypertension and HHcy in the context of cardiac remodeling and to explore its mechanism of action.

METHODS

Wistar Kyoto (WKY) and spontaneous hypertension rats (SHR) were randomly divided into four groups, namely WKY group, WKY + HHcy group, SHR group and SHR + HHcy group. We measured blood pressure (BP), plasma homocysteine (Hcy), serum superoxide dismutase (SOD) and serum malondialdehyde (MDA). We also examined cardiac histopathology and gene and protein expression levels of Nrf2 and HO-1.

RESULTS

Compared with the WKY group, myocardial interstitial and perivascular collagen deposition in the WKY + HHcy group, the SHR group and the SHR + HHcy group increased successively, indicating that cardiac remodeling gradually increased, and HHcy aggravated cardiac remodeling was more serious in hypertensive rats. SOD decreased gradually in the four groups, while MDA was on the contrary. WKY + HHcy and SHR + HHcy groups both suppressed Nrf2 and HO-1 expression and inhibited the translocation of Nrf2 from cytoplasm to nucleus compared with their control groups, and the SHR + HHcy group had a stronger inhibitory effect.

CONCLUSION

HHcy enhanced cardiac remodeling in rats by enhancing oxidative stress, suppressing the Nrf2/HO-1 pathway and Nrf2 nuclear transport, and this inhibitory effect was stronger in the context of hypertension.

Induction of HO-1 by 5, 8-Dihydroxy-4',7-Dimethoxyflavone via Activation of ROS/p38 MAPK/Nrf2 Attenuates Thrombin-Induced Connective Tissue Growth Factor Expression in Human Cardiac Fibroblasts

Heme oxygenase-1 (HO-1) has been shown to exert as an antioxidant and anti-inflammatory enzyme in cardiovascular inflammatory diseases. Flavonoids have been demonstrated to display anti-inflammatory and antioxidant effects through the induction of HO-1. 5,8-Dihydroxy-4',7-dimethoxyflavone (DDF), one of the flavonoid compounds, is isolated from Reevesia formosana. Whether DDF induced HO-1 expression on human cardiac fibroblasts (HCFs) remained unknown. Here, we found that DDF time- and concentration-dependently induced HO-1 protein and mRNA expression, which was attenuated by pretreatment with reactive oxygen species (ROS) scavenger N-acetyl cysteine (NAC) in HCFs. DDF-enhanced ROS generation was attenuated by NAC, but not by either diphenyleneiodonium chloride (DPI, Nox inhibitor) or MitoTempol (mitochondrial ROS scavenger). Interestingly, pretreatment with glutathione (GSH) inhibited DDF-induced HO-1 expression. The ratio of GSH/GSSG was time-dependently decreased in DDF-treated HCFs. DDF-induced HO-1 expression was attenuated by an inhibitor of p38 MAPK (p38i VIII) or siRNA, but not by MEK1/2 (PD98059) or JNK1/2 (SP600125). DDF-stimulated p38 MAPK phosphorylation was inhibited by GSH or p38i VIII. Moreover, DDF-induced HO-1 expression was mediated through Nrf2 phosphorylation and translocation into the nucleus which was attenuated by NAC or p38 siRNA. DDF also stimulated antioxidant response element (ARE) promoter activity which was inhibited by NAC, GSH, or p38i VIII. Interaction between Nrf2 and the ARE-binding sites on the HO-1 promoter was revealed by chromatin immunoprecipitation assay, which was attenuated by NAC, GSH, or p38i VIII. We further evaluated the functional effect of HO-1 expression on the thrombin-induced fibrotic responses. Our result indicated that the induction of HO-1 by DDF can attenuate the thrombin-induced connective tissue growth factor expression. These results suggested that DDF-induced HO-1 expression is, at least, mediated through the activation of the ROS-dependent p38 MAPK/Nrf2 signaling pathway in HCFs. Thus, the upregulation of HO-1 by DDF could be a candidate for the treatment of heart fibrosis.

Molecular mechanism of olaquindox-induced hepatotoxicity and the hepatic protective role of curcumin

Olaquindox (OLA) is a chemosynthetic growth promoter, which could promote the treatment of bacterial infections and improve feed energy efficiency. Hepatotoxicity is still a poor feature associated with the adverse effects of OLA. The present study aimed to investigate the molecular mechanism of OLA-induced hepatotoxicity and the protective role of curcumin in mice and HepG2 cells. The result showed that representative biomarkers involved in mitochondrial pathway, p53 pathway, mitogen-activated protein kinase (MAPK) pathway, autophagy and antioxidant pathway were activated. Furthermore, curcumin attenuated OLA-induced serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) and liver damage in mice. In addition, cell viability of HepG2 was enhanced by curcumin pretreatment at 5, 10 and 20 μM. Meanwhile, curcumin markedly ameliorated OLA-induced oxidative stress, apoptosis and mitochondrial dysfunction. Moreover, curcumin pretreatment significantly up-regulated the expressions of nuclear factor erythroid-2-related factor 2 (Nrf2) and heme oxygenase-1(HO-1) and down-regulated the expressions of nuclear factor-kappaB (NF-kB) and p53 through reduced the nuclear translocation of NF-kB induced by OLA. In summary, our findings indicated that OLA-induced hepatotoxicity involved in mitochondrial apoptosis, autophagy, p53 pathway, Nrf2/HO-1 pathways, and curcumin regulated OLA-induced liver damage, oxidative stress and apoptosis via activation of Nrf2/HO-1 pathway and suppression of p53 and NF-kB pathway.

Heme Oxygenase-1 Suppresses Wnt Signaling Pathway in Nonalcoholic Steatohepatitis-Related Liver Fibrosis

Methods

Mice were fed with a methionine-choline-deficient (MCD) diet for 8 weeks to induce steatohepatitis-related liver fibrosis and were treated with HO-1 inducer Hemin and inhibitor ZnPP. Mouse sera were collected for the biochemical analysis, and livers were obtained for further histological observation and gene expression analysis. HSC-T6 cells were cultured for the in vitro study and were administrated with Hemin and si-HO-1 to induce or inhibit the expression of HO-1. qPCR and Western blot were used to assess the mRNA and protein levels of genes.

Results

MCD-fed mice developed marked macrovesicular steatosis, focal necrosis, and inflammatory infiltration and pericellular fibrosis in liver sections. Administration of Hemin could significantly ameliorate the severity of steatosis, inflammation, and fibrosis and also could decrease the serum ALT and AST. We demonstrated that HO-1 induction was able to downregulate the key regulator of the canonical Wnt pathway Wnt1 and the noncanonical Wnt pathway Wnt5a. The downstream factors of the Wnt pathway β-catenin and NFAT5 were inhibited by Hemin, but GSK-3β was upregulated compared to the MCD group, which were consistent with the in vitro study. Hemin markedly inhibited the TGF-β1/Smad signaling pathway in both in vivo and in vitro studies.

Conclusion

Our study demonstrated that HO-1 inhibited the activation of canonical and noncanonical Wnt signaling pathways in NASH-related liver fibrosis. Thus, these results may suggest a new therapeutic strategy for NASH-related liver fibrosis.

Hepatocyte-specific deficiency of Nrf2 exacerbates carbon tetrachloride-induced liver fibrosis via aggravated hepatocyte injury and subsequent inflammatory and fibrogenic responses

BACKGROUND

Liver fibrosis, in which hepatocyte damage and inflammatory response play critical roles, is a physiological response to chronic or iterative liver injury and can progress to cirrhosis over time. Nuclear factor E2-related factor 2 (Nrf2) is a master transcription factor that regulates oxidative and xenobiotic stress responses as well as inflammation.

METHOD

To ascertain the cell-specific roles of Nrf2 in hepatocytes and myeloid lineage cells in the progression of liver fibrosis, mice lacking Nrf2 specifically in hepatocytes [Nrf2(L)-KO] and myeloid lineage cells [Nrf2(M)-KO] were generated to evaluate carbon tetrachloride (CCl₄)-induced liver injury, subsequent inflammation and fibrosis. In addition, mouse primary hepatocytes were used to investigate the underlying mechanisms.

RESULTS

Nrf2-mediated antioxidant response in the liver is responsive to acute CCl₄ exposure in mice. With repeated CCl₄ administration, Nrf2(L)-KO, but not Nrf2(M)-KO, mice showed more severe liver fibrosis than Nrf2-LoxP control mice. In addition, in response to acute CCl₄ exposure, Nrf2(L)-KO mice displayed aggravated liver injury, elevated lipid peroxidation and inflammatory response compared to control mice. In mouse primary hepatocytes, deficiency of Nrf2 resulted in more severe CCl₄-induced lipid oxidation and inflammatory response.

CONCLUSION

Deficiency of Nrf2 in hepatocytes sensitizes the cells to CCl₄-induced oxidative damage and inflammatory response, which are initiator and enhancer of subsequent hepatic inflammation and fibrosis. Thus, Nrf2 is a critical determinant of liver injury and fibrosis in response to CCl₄, suggesting that Nrf2 might be a valuable target for the intervention.

Distribution and elimination of quinocetone and its major metabolites in Cherry Valley ducks

We examined the tissue distribution and elimination of quinocetone (QCT) and its major metabolites 1-desoxyquinocetone (1-DQCT), di-desoxyquinocetone (BDQCT), and 3-methyl-quinoxaline-2-carboxylic (MQCA) in ducks. The analytes were simultaneously quantitated using a UPLC-MS/MS method after oral administration of QCT at 100 mg·kg-1 day-1 for 7 days. We found that QCT and its major metabolites were widely distributed in duck tissues. The concentrations indicated that the primary compound in the liver, kidney, and heart was MQCA and the primary compound in the stomach, intestine, spleen, and lung was QCT. We also identified that MQCA was the most appropriate compound for QCT residue monitoring. The liver and kidney are the primary QCT target organs in ducks, and this study provides clear monitoring tools and important data to evaluate its safety.

CO-Releasing Molecule-2 Induces Nrf2/ARE-Dependent Heme Oxygenase-1 Expression Suppressing TNF-α-Induced Pulmonary Inflammation

The upregulation of heme oxygenase-1 (HO-1) by the carbon monoxide-releasing molecule (CORM)-2 may be mediated through the activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases [Nox] and reactive oxygen species (ROS) generation, which could provide cytoprotection against various cellular injuries. However, the detailed mechanisms of CORM-2-induced HO-1 expression in human pulmonary alveolar epithelial cells (HPAEpiCs) remain largely unknown. Therefore, we dissected the mechanisms underlying CORM-2-induced HO-1 expression in HPAEpiCs. We found that the administration of mice with CORM-2 attenuated the tumor necrosis factor-alpha (TNF-α)-induced intercellular adhesion molecule-1 (ICAM-1) expression and leukocyte count as revealed by immunohistochemical staining, western blot, real-time polymerase chain reaction (PCR), and cell count. Furthermore, TNF-α-induced ICAM-1 expression associated with monocyte adhesion to HPAEpiCs was attenuated by infection with adenovirus (adv)-HO-1 or incubation with CORM-2. These inhibitory effects of HO-1 were reversed by pretreatment with hemoglobin (Hb). Moreover, CORM-2-induced HO-1 expression was mediated via the phosphorylation of p47^phox, c-Src, epidermal growth factor receptor (EGFR), Akt, and NF-E2-related factor 2 (Nrf2), which were inhibited by their pharmacological inhibitors, including diphenyleneiodonium (DPI) or apocynin (APO), ROS [N-acetyl-L-cysteine (NAC)], PP1, AG1478, PI3K (LY294002), or Akt (SH-5), and small interfering RNAs (siRNAs). CORM-2-enhanced Nrf2 expression, and anti-oxidant response element (ARE) promoter activity was also inhibited by these pharmacological inhibitors. The interaction between Nrf2 and AREs was confirmed with a chromatin immunoprecipitation (ChIP) assay. These findings suggest that CORM-2 increases the formation of the Nrf2 and AREs complex and binds with ARE-binding sites via Src, EGFR, and PI3K/Akt, which further induces HO-1 expression in HPAEpiCs. Thus, the HO-1/CO system might suppress TNF-α-mediated inflammatory responses and exert a potential therapeutic strategy in pulmonary diseases.

Mequindox induces apoptosis, DNA damage, and carcinogenicity in Wistar rats

Mequindox (MEQ) is a synthetic antibacterial agent. Recent studies showed that MEQ and its primary metabolites exhibit strong genotoxicity to mammalian cells, and MEQ induced carcinogenicity in mice. These findings suggest that chronic exposure to MEQ could lead to an increased risk of cancer later in life. In the present study, four groups of Wistar rats (55 rats/sex/group) were fed with diets containing MEQ (0, 25, 55, and 110 mg/kg) for 2 years. The results showed that the hematological system, liver, kidneys, and adrenal glands, as well as the developmental and reproductive systems, were the main targets for MEQ. Liver toxicity mediated by MEQ was associated with apoptosis and the nuclear factor κB (NF-κB) signaling pathway. In addition, MEQ increased the incidence of tumors in rats. Phosphorylated histone H2AX (γ-H2AX) is identified as a biomarker of cellular response to DNA double-strand breaks (DSB). Our data demonstrated that γ-H2AX expression was significantly increased in tumors. Thus, high levels of DSB might be responsible for carcinogenesis in rats, and further investigation is absolutely required to clarify the exact molecular mechanisms for carcinogenicity caused by MEQ in vivo.

Benzyl sulforaphane is superior to sulforaphane in inhibiting the Akt/MAPK and activating the Nrf2/ARE signalling pathways in HepG2 cells

Objectives

Sulforaphane (SFN), an isothiocyanate found in cruciferous vegetables, has been reported to own anticarcinogenic, antiinflammatory and cancer chemopreventive properties. Benzyl sulforaphane (BSFN) was a derivative of SFN which was designed and synthesized by our laboratory. Here, the cancer prevention and anticancer effects of BSFN on human hepatoma (HepG2) cells were investigated.

Methods

The following effects of BSFN on components of the mitochondrial apoptotic pathway were examined: generation of reactive oxygen species and mitochondrial membrane potential (ΔΨm) changes by flow cytometry, the expression changes of Bcl-2 family proteins and Akt/MAPK proteins by western blot. The protein levels of Nrf2 and Keap1 were also tested via Western blot. The effects of BSFN on Nrf2 nuclear translocation and ARE-reporter gene activity were examined by fluorescence microscope and multifunctional spectrophotometer.

Key findings

Benzyl sulforaphane could induce cell apoptosis by mitochondrion-dependent pathway, which inhibited HepG2 cells growth in a manner of time- and concentration -dependent. Furthermore, BSFN could inhibit the Akt/MAPK and activate the Nrf2/ARE pathway in HepG2 cells.

Conclusions

Benzyl sulforaphane was superior to SFN in inhibiting Akt/MAPK and activating Nrf2/ARE signalling pathways in HepG2 cells, which indicated that BSFN could be a safe therapeutic strategy for the prevention and treatment of liver cancer.

Mequindox-Induced Kidney Toxicity Is Associated With Oxidative Stress and Apoptosis in the Mouse

Mequindox (MEQ), belonging to quinoxaline-di-N-oxides (QdNOs), is a synthetic antimicrobial agent widely used in China. Previous studies found that the kidney was one of the main toxic target organs of the QdNOs. However, the mechanisms underlying the kidney toxicity caused by QdNOs in vivo still remains unclear. The present study aimed to explore the molecular mechanism of kidney toxicity in mice after chronic exposure to MEQ. MEQ led to the oxidative stress, apoptosis, and mitochondrial damage in the kidney of mice. Meanwhile, MEQ upregulated Bax/Bcl-2 ratio, disrupted mitochondrial permeability transition pores, caused cytochrome c release, and a cascade activation of caspase, eventually induced apoptosis. The oxidative stress mediated by MEQ might led to mitochondria damage and apoptosis in a mitochondrial-dependent apoptotic pathway. Furthermore, upregulation of the Nrf2-Keap1 signaling pathway was also observed. Our findings revealed that the oxidative stress, mitochondrial dysfunction, and the Nrf2-Keap1 signaling pathway were associated with the kidney apoptosis induced by MEQ in vivo.

ROS-mediated oligomerization of VDAC2 is associated with quinocetone-induced apoptotic cell death

Quinocetone (QCT) has been approved and widely used as an animal feed additive in China since 2003. However, investigations indicate that QCT shows potential toxicity both in vitro and in vivo. Although voltage dependent anion channel 1 (VDAC1) involved in regulating QCT-induced apoptotic cell death has been established, the role of voltage dependent anion channel 2 (VDAC2) in QCT-induced toxicity remains unclear. In this study, we showed that QCT-induced cell death was coupled to VDAC2 oligomerization. Moreover, VDAC inhibitor 4, 4'-diisothiocyano stilbene-2, 2'-disulfonic acid (DIDS) alleviated QCT-induced cell death and VDAC2 oligomerization. Meanwhile, overexpression VDAC2 aggravated QCT-induced VDAC2 oligomerization. In addition, caspase inhibitor Z-VAD-FMK and reactive oxidative species (ROS) scavenger N-acetyl-l-cysteine (NAC) apparently blocked QCT-induced cell death and VDAC2 oligomerization. Finally, overexpression N-terminal truncated VDAC2 attenuated QCT-induced VDAC2 oligomerization but had no influence on its localization to mitochondria when comparing to the full length of VDAC2. Taken together, our results reveal that ROS-mediated VDAC2 oligomerization is associated with QCT-induced apoptotic cell death. The N-terminal region of VDAC2 is required for QCT-induced VDAC2 oligomerization.

Embryotoxicity Caused by DON-Induced Oxidative Stress Mediated by Nrf2/HO-1 Pathway

Deoxynivalenol (DON) belongs to the type B group of trichothecenes family, which is composed of sesquiterpenoid metabolites produced by Fusarium and other fungi in grain. DON may cause various toxicities, such as cytotoxicity, immunotoxicity, genotoxicity as well as teratogenicity and carcinogenicity. In the present study, we focus on a hypothesis that DON alters the expressions of Nrf2/HO-1 pathway by inducing embryotoxicity in C57BL/6 mouse (5.0, 2.5, 1.0, and 0 mg/kg/day) and BeWo cell lines (0 and 50 nM; 3 h, 12 h and 24 h). Our results indicate that DON treatment in mice during pregnancy leads to ROS accumulation in the placenta, which results in embryotoxicity. At the same time Nrf2/HO-1 pathway is up-regulated by ROS to protect placenta cells from oxidative damage. In DON-treated BeWo cells, the level of ROS has time–effect and dose–effect relationships with HO-1 expression. Moderate increase in HO-1 protects the cell from oxidative damage, while excessive increase in HO-1 aggravates the oxidative damage, which is called in some studies the “threshold effect”. Therefore, oxidative stress may be the critical molecular mechanism for DON-induced embryotoxicity. Besides, Nrf2/HO-1 pathway accompanied by the “threshold effect” also plays an important role against DON-induced oxidative damage in this process.

Quinocetone induces mitochondrial apoptosis in HepG2 cells through ROS-dependent promotion of VDAC1 oligomerization and suppression of Wnt1/β-catenin signaling pathway

Quinocetone (QCT) has been used as an animal feed additive in China since 2003. However, investigations indicate that QCT has potential toxicity due to the fact that it shows cytotoxicity, genotoxicity, hepatotoxicity, nephrotoxicity and immunotoxicity in vitro and animal models. Although QCT-induced mitochondrial apoptosis has been established, the molecular mechanism remains unclear. This study was aimed to investigate the role of voltage-dependent anion channel 1 (VDAC1) oligomerization and Wnt/β-catenin pathway in QCT-induced mitochondrial apoptosis. The results showed VDAC inhibitor 4, 4-diisothiocyano stilbene-2, 2-disulfonic acid (DIDS) partly compromised QCT-induced cell viability decrease (from 34.1% to 68.5%) and mitochondrial apoptosis accompanied by abating VDAC1 oligomerization, cytochrome c (Cyt c) release and the expression levels of cleaved caspase-9, -3 and poly (ADP-ribose) polymerase (PARP). Meanwhile, overexpression VDAC1 exacerbated QCT-induced VDAC1 oligomerization and Cyt c release. In addition, lithium chloride (LiCl), an activator of Wnt/β-catenin pathway, markedly attenuated QCT-induced mitochondrial apoptosis by partly restoring the expression levels of Wnt1 and β-catenin. Finally, reactive oxygen species (ROS) scavenger N-acetyl-l-cysteine (NAC) obviously blocked QCT-induced VDAC1 oligomerization and the inhibition of Wnt1/β-catenin pathway. Taken together, our results reveal that QCT induces mitochondrial apoptosis by ROS-dependent promotion of VDAC1 oligomerization and suppression of Wnt1/β-catenin pathway.

Toxic metabolites, MAPK and Nrf2/Keap1 signaling pathways involved in oxidative toxicity in mice liver after chronic exposure to Mequindox

Mequindox (MEQ) is a synthetic antimicrobial agent of quinoxaline-1,4-dioxide group (QdNOs). The liver is regarded as the toxicity target of QdNOs, and the role of N → O group-associated various toxicities mediated by QdNOs is well recognized. However, the mechanism underlying the in vivo effects of MEQ on the liver, and whether the metabolic pathway of MEQ is altered in response to the pathophysiological conditions still remain unclear. We now provide evidence that MEQ triggers oxidative damage in the liver. Moreover, using LC/MS-ITTOF analysis, two metabolites of MEQ were detected in the liver, which directly confirms the potential connection between N → O group reduction metabolism of MEQ and liver toxicity. The gender difference in MEQ-induced oxidative stress might be due to adrenal toxicity and the generation of M4 (2-isoethanol 1-desoxymequindox). Furthermore, up-regulation of the MAPK and Nrf2-Keap1 family and phase II detoxifying enzymes (HO-1, GCLC and NQO1) were also observed. The present study demonstrated for the first time the protein peroxidation and a proposal metabolic pathway after chronic exposure of MEQ, and illustrated that the MAPK, Nrf2-Keap1 and NF-кB signaling pathways, as well as the altered metabolism of MEQ, were involved in oxidative toxicity mediated by MEQ in vivo.

GADD45a Regulates Olaquindox-Induced DNA Damage and S-Phase Arrest in Human Hepatoma G2 Cells via JNK/p38 Pathways

Olaquindox, a quinoxaline 1,4-dioxide derivative, is widely used as a feed additive in many countries. The potential genotoxicity of olaquindox, hence, is of concern. However, the proper mechanism of toxicity was unclear. The aim of the present study was to investigate the effect of growth arrest and DNA damage 45 alpha (GADD45a) on olaquindox-induced DNA damage and cell cycle arrest in HepG2 cells. The results showed that olaquindox could induce reactive oxygen species (ROS)-mediated DNA damage and S-phase arrest, where increases of GADD45a, cyclin A, Cdk 2, p21 and p53 protein expression, decrease of cyclin D1 and the activation of phosphorylation-c-Jun N-terminal kinases (p-JNK), phosphorylation-p38 (p-p38) and phosphorylation-extracellular signal-regulated kinases (p-ERK) were involved. However, GADD45a knockdown cells treated with olaquindox could significantly decrease cell viability, exacerbate DNA damage and increase S-phase arrest, associated with the marked activation of p-JNK, p-p38, but not p-ERK. Furthermore, SP600125 and SB203580 aggravated olaquindox-induced DNA damage and S-phase arrest, suppressed the expression of GADD45a. Taken together, these findings revealed that GADD45a played a protective role in olaquindox treatment and JNK/p38 pathways may partly contribute to GADD45a regulated olaquindox-induced DNA damage and S-phase arrest. Our findings increase the understanding on the molecular mechanisms of olaquindox.

The critical role of oxidative stress in the toxicity and metabolism of quinoxaline 1,4-di-N-oxides in vitro and in vivo

Quinoxaline 1,4-dioxide derivatives (QdNOs) have been widely used as growth promoters and antibacterial agents. Carbadox (CBX), olaquindox (OLA), quinocetone (QCT), cyadox (CYA) and mequindox (MEQ) are the classical members of QdNOs. Some members of QdNOs are known to cause a variety of toxic effects. To date, however, almost no review has addressed the toxicity and metabolism of QdNOs in relation to oxidative stress. This review focused on the research progress associated with oxidative stress as a plausible mechanism for QdNO-induced toxicity and metabolism. The present review documented that the studies were performed over the past 10 years to interpret the generation of reactive oxygen species (ROS) and oxidative stress as the results of QdNO treatment and have correlated them with various types of QdNO toxicity, suggesting that oxidative stress plays critical roles in their toxicities. The major metabolic pathways of QdNOs are N→O group reduction and hydroxylation. Xanthine oxidoreductase (XOR), aldehyde oxidase (SsAOX1), carbonyl reductase (CBR1) and cytochrome P450 (CYP) enzymes were involved in the QdNOs metabolism. Further understanding the role of oxidative stress in QdNOs-induced toxicity will throw new light onto the use of antioxidants and scavengers of ROS as well as onto the blind spots of metabolism and the metabolizing enzymes of QdNOs. The present review might contribute to revealing the QdNOs toxicity, protecting against oxidative damage and helping to improve the rational use of concurrent drugs, while developing novel QdNO compounds with more efficient potentials and less toxic effects.

Quinocetone triggered ER stress-induced autophagy via ATF6/DAPK1-modulated mAtg9a trafficking

The present study is undertaken to explore quinocetone-induced autophagy and its possible mechanism. Western blotting and green fluorescence protein (GFP)-LC3 vector transfection were performed to determine the ratio of LC3 conversion and its subcellular localization. Results revealed that the quinocetone induced autophagy in time- and dose-dependent manners. Besides, we tested the expressions of immunoglobulin heavy chain binding protein (BiP) and C/EBP homologous protein (CHOP) and the transcription of BiP, HerpUD, and sec24D by western blotting and RT-PCR, respectively. Results showed that quinocetone also induced endoplasmic reticulum (ER) stress during quinocetone-induced autophagy. Furthermore, we observed the cleavage of ATF6, the phosphorylation of MRLC, and the expression of death-associated protein kinase (DAPK1) by western blotting; the transcription of DAPK1 by RT-PCR; and the subcellular localization of ATF6 and mAtg9 by immunofluorescence. These results suggest that quinocetone stimulates the MRLC-mediated mAtg9 trafficking, which is critical for autophagosome formation, via the ATF6 upregulated expression of DAPK1. Last, we generated ATF6 and DAPK1 stable knockdown HepG2 cell lines and found that the conversion ratios of LC3 were decreased upon the treatment of quinocetone. Together, we propose that quinocetone induces autophagy through ER stress signaling pathway-induced cytoskeleton activation.

Genotoxic risk of quinocetone and its possible mechanism in in vitro studies

Quinoxalines possessing the quinoxaline-1,4-dioxide (QdNOs) basic structure are used for their antibacterial action, although their mechanism of genotoxicity is not clear. After comparing the sensitivity of V79 cells and HepG2 cells to quinocetone (QCT) and other QdNOs, it was found that HepG2 cells are more sensitive. The results show that QCT induces the generation of O2˙- and OH˙ during metabolism. Free radicals could then attack guanine and induce 8-hydroxy-deoxyguanine (8-OHdG) generation, causing DNA strand breakage, the inhibition of topoisomerase II (topo II) activity, and alter PCNA, Gadd45 and topo II gene expression. QCT also caused mutations in the mtDNA genes COX1, COX3 and ATP6, which might affect the function of the mitochondrial respiratory chain and increase the production of reactive oxygen species (ROS). Nuclear extracts from HepG2 cells treated with QCT had markedly reduced topo II activity, as judged by the inability to convert pBR322 DNA from the catenated to the decatenated form by producing stable DNA-topo II complexes. This study suggests that QCT electrostatically bound to DNA in a groove, affecting the dissociation of topo II from DNA and impacting DNA replication. Taken together, these data reveal that DNA damage induced by QCT resulted from O2˙- and OH˙ generated in the metabolism process. This data throws new light onto the genotoxicity of quinoxalines.

Mechanism of adrenocortical toxicity induced by quinocetone and its bidesoxy-quinocetone metabolite in porcine adrenocortical cells in vitro

Quinocetone (QCT) is a new feeding antibacterial agent in the QdNOs family. The mechanism of its adrenal toxicity is far from clear. This study was conducted to estimate the adrenal cell damage induced by QCT and its bidesoxy-quinocetone (B-QCT) metabolite and to further investigate their mechanisms. Following doses of QCT increasing from 5 to 50 μM, cell apoptosis and necrosis, mitochondrial dysfunction and redox imbalance were observed in porcine adrenocortical cells. The mRNA levels of the six components of intermediary enzymes and the adrenal renin-angiotensin-aldosterone system (RAAS) displayed a dysregulation induced by QCT, indicating that QCT might influence aldosterone secretion not only through the upstream of the production but also through the downstream of the adrenal RAAS pathway. In contrast, B-QCT had few toxic effects on the cell apoptosis, mitochondrial dysfunction and redox imbalance. Moreover, LCMS-IT-TOF analysis showed that no desoxy metabolites of QCT were found in either cell lysate or supernatant samples. In conclusion, we reported on the cytotoxicity in porcine adrenocortical cells exposed to QCT via oxidative stress, which raised awareness that its toxic effects resulted from N→O groups, and its toxic mechanism might involve the interference of the steroid hormone biosynthesis pathway.