Protective Potential of Adiponectin and Quercetin on Alleviating Bisphenol-A-Induced Toxicity in Muscle Cells Through the KEAP1/NRF2 Pathway

Background: Bisphenol A (BPA) is a toxic environmental estrogenic compound which exerts its detrimental effects by increasing oxidative stress and decreasing levels of antioxidants. This study aimed to evaluate benecial effect of adiponectin and quercetin in reducing BPA-induced oxidative stress by assessing the Prooxidant-antioxidant balance (PAB) assay, catalase activity and KEAP1/NRF2 expression in muscle cells. Methods and Results: L6 rat muscle cells were exposed to BPA (50 an100 μM) with and without treatment with different concentrations of adiponectin (10 and 100 ng/ml) and quercetin (10 and 25 ng/ml) for 24 and 48 hours. Cell viability was assessed using MTT assay, and the PAB was evaluated with the ELISA at 540 nm. Catalase level was also evaluated in all groups. Furtheremore, the expression of KEAP1/Nrf2 genes was assessed using qRT-PCR. The results showed a signicant reduction in L6 cells survival after being treated with 100 μM BPA. Adiponectin and quercetin treatment also increased cell survival compared to BPA-treated cells. It was also found that PAB increased with BPA exposure, and quercetin treatment signicantly reduced it compared to BPA treatment. The catalase activity was reduced in BPA-treated cells, which was signicantly increased by treatment with adiponectin and quercetin. A signicant decrease in Nrf2 gene expression was observed in BPA-treated cells compared to the control group. It was further found that cell treatment with quercetin and adiponectin signicantly increased the expression of Nrf2 gene compared to the control group. Conclusions: Taking together, our results implied that adiponectin and quercetin could modulate BPA-induced oxidative stress in muscle cells through KEAP1/Nrf2 pathway. Accordingly, it can be concluded that adiponectin in low dose and quercetin, may have signicant impact in reducing toxicity due to BPA.


Introduction
In the last decade, environmental estrogenic compounds such as bisphenol A (BPA) have raised public health concerns due to their adverse effects in animal models (1). BPA, a potential endocrine-disrupting chemical, is used in factories producing polycarbonate plastics, epoxy resins, food packaging and coating, rubber chemicals, and dental sealants (2). Human studies suggest that BPA may be associated with type 2 diabetes and cardiovascular disease. Recent studies have shown that oxidative stress induced by BPA can lead to insulin resistance (3). Previous studies have shown that BPA exerts diabetogenic effects by diverting insulin signaling to skeletal muscle and adipose tissue. BPA may counteract its toxic effects by increasing oxidative stress and decreasing levels of antioxidants (4).
Cellular metabolism leads to the continuous production of reactive oxygen species (ROS), such as superoxide radicals or non-radical hydrogen peroxide. The imbalance between reactive oxygen species and antioxidant defense mechanisms in a cell leads to oxidative stress emerging (5). Evidence shows that cell damage caused by oxidative stress is involved in the pathogenesis of more than a hundred diseases (6). ROS are produced in skeletal muscle both at rest and during contractile activity. Besides, it has been reported that oxidative stress plays a crucial role in muscle cell pathology through multiple routes including, mitochondrial dysfunction, increased ubiquitin-proteasome system activity, increased myonuclear apoptosis, decreased protein synthesis pathway, and deregulated autophagy. These conditions nally lead to cachexia-skeletal muscle atrophy (7). Myogenic cells are equipped with antioxidant enzymes such as superoxide dismutase, catalase, glutathione peroxidase, and Heme oxygenase 1. They affect the viability and proliferation of muscle satellite cells and affect the differentiation of myoblasts (5).
On the other hand, avonoids such as quercetin delay cell death and oxidative damage by scavenging free radicals (8). Previous studies have shown that quercetin up to 100 mg per day is well tolerated in humans and has no side effects or harmful physiological effects. Moreover, it effectively reduces the risk of cardiovascular disease by reducing oxidative stress (9). Meanwhile, studies have revealed that quercetin, as a powerful antioxidant, concerted its antioxidant effects in the presence of high levels of oxidative stress. Quercetin is able to inhibit oxidant biomolecules that can alter antioxidant defense pathways in vivo and in vitro (10,11). Another powerful antioxidant can be attributed to adiponectin. Adiponectin is an endocrine factor that is expressly released from adipose tissue and suppresses the harmful effects of oxidative stress (12). Adiponectin exerts protective effects on heart tissue against oxidative stress induced by the remodeling process in cardiomyocytes by activating AMP-activated protein kinase (AMPK) and inhibiting regulatory kinases with extracellular signals and the NF-κB pathway (13).
To cope with stressful conditions, mammalian cells have developed complex defense mechanisms to adapt to these conditions, including the Keap1 / Nrf2 pathway (14). Under stress-free conditions, Nrf2 (nuclear factor erythroid 2-related factor 2) binds to the KAEP1 (Kelch-like ECH-associated protein 1) protein in the cytosol. It is ubiquitinated and degraded after a few hours. During cellular stress, several cysteine amino acids are oxidized or covalently oxidized to alter KAEP1 conformity. By the conformation change, Nrf2 is separated from KEAP1 and transported to the nucleus. It will bind to the ARE promoter region, leading to speci c gene expression that causes increased cellular resistance to various stresses (15,16).
Considering the inevitability of human exposure to BPA in current societies and its related health issues, this study aimed to investigate the effect of avonoids quercetin and adiponectin on oxidative stress exerted by BPA on L6 cells (Rat skeletal muscle myoblast) through the KEAP1 / Nrf2 pathway.

Mtt Assay
The L6 cells were cultured in DMEM medium supplemented with 10% FBS (Gibco) to reach suitable con uency. 7000 cells were cultured into 96 wells plate. 24 h later, each medium was replaced by the prepared mediums containing BPA, quercetin, and adiponectin according to the mentioned groups. 24 and 48 h after treatment, cells were incubated with (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) MTT solution (5mg/ml of DMEM) for 3 h. Next, the supernatant was taken away, and DMSO solvent was added. The absorbance of suspension was then read at 570 nm using a microplate reader.

Prooxidant-antioxidant Balance (Pab) Assay
The PAB test has been performed in two different reactions, including enzymatic and chemical reactions.
In the enzymatic reaction, the pro-oxidants oxidize the chromogen TMB to a color cation, while in the chemical reaction, the antioxidants reduce the TMB cation to a colorless compound. First, by dissolving the TMB powder in DMSO, the TMB/DMSO solution was prepared. Next, 1 ml of TMB/DMSO solution was added to acetate buffer (50 mL, pH 4.5). Then, chloramine T (175 µL, 100 mM) to prepare the TMB cation solution incubated in a dark place for 2 hours at room temperature. Also, 16.5 µL of peroxidase enzyme solution was added and placed at -20 ºC.
TMB solution was prepared by adding 200 µL of TMB/DMSO to the acetate buffer (10 mL, pH 5.6). As the working solution, the third solution was prepared by adding 1 ml of TMB cation solution to 10 ml of TMB solution and, after 6 min incubation in the dark place at room temperature, freshly used. Also, the standard solutions are obtained by mixing different ratios of hydrogen peroxide at a concentration of 500 µM (as peroxidants) with a solution of 3 mM Uric acid in 10 mM NaOH (as antioxidants).
To perform the PAB test and evaluate the oxidant and antioxidant activity of the cells, 50,000 cells were cultured in 24 well plates according to the mentioned groups for 24 and 48 h. 200µL of the working solution was added to each well and incubated in a dark place at 37 o C for 12 min. Then, 5 µL of HCL 2N was added to wells, and the optical density (OD) was read at 450 nm (the reference wavelength was 620 nm).

Catalase Activity Assay
For catalase activity evaluation, hydrogen peroxide, and ammonium molybdate solutions were prepared.
Brie y, 102 µl of 30% hydrogen peroxide solution (Mw: 34.01, 9.79 N) was diluted with PBS to 50 ml, and 10 g of ammonium molybdate was diluted to 250 ml with distilled water to prepare 20 mM hydrogen peroxide solution and 32.4 mM ammonium molybdate, respectively. Different catalase reagents were designed and are shown in Table 1. The reagents were gently mixed with the vortex. After incubation at 37 ° C for 3 minutes, 4 ml of ammonium molybdate was added to the tubes and read at 374 nm using a spectrophotometer. Formula 1 is used to obtain the activity of the catalase enzyme based on (KU / L) KU.  Table 2.
Real time-PCR was performed by the 10-min at 95 ˚ C for enzyme activation, followed by 35 cycles of 15 seconds for denaturation at 95 ˚ C, 60 seconds for annealing at 54.5 ˚ C, and 25 seconds for the extension at 72 ˚ C. The 2 −ΔΔCt method was used to calculate relative expression levels.  Also, according to the results shown in Figure 1C, it was found that BPA (100 µM) led to no signi cant effect on cell survival in any A and Q treatments after 24 hours.

Investigation Of Catalase Activity
The catalase activity of L6 cells was examined and the results are shown in Figure 2. The results showed that the presence of BPA decreased the catalase activity of L6 cells in all treated groups. But Q (25 ng/ml) and A (10 ng/ml) along with Q (25 ng/ml) had opposite effects on BPA impact. (Figure 2A).

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The toxic effect of BPA on catalase activity also was seen after 48 h. In this situation, Q (25 ng/ml) and A (10 ng/ml) with Q (10, 25 ng/ml) relieved BPA effect ( Figure 2B).

Investigation Of Keap1 And Nrf2 Gene Expression
The expression of KEAP1 and Nrf2 genes in L6 cells was examined after treatment with BPA (50 and 100 µM) and in the presence of different concentrations of adiponectin and quercetin, and the results are shown in Figure 4. The results showed that the expression of the KEAP1 gene treated with 50 µM BPA had a signi cant increase only in BPA (50 µM) + A (100 ng/ml) group compared to the control group (p <0.05) ( Figure 4A). Nrf2 gene expression in BPA (50 µM) + Q (25 ng/ml) and BPA (50 µM) + A (10 ng/ml) with Q (25ng/ml) groups was signi cantly higher than the control group (p <0.05) ( Figure 4B).

Discussion
The potential role of oxidants such as environmentally toxic compound in the pathogenesis of many diseases suggests that antioxidants may be an optimal option for encountering them. The ROS produced can disrupt metabolic regulation and enhance metabolic diseases such as insulin resistance locally in adipose tissue and insulin target tissues such as skeletal muscle and liver (17). Flavonoids, such as quercetin, can postpone cell death and oxidative damage through free radical scavenging. Quercetin can improve the body's metabolism through ROS uptake (18-21). According to previous studies, adiponectin plays a protective role in the metabolism of many body tissues, including heart tissue, skeletal muscle, liver tissue, and adipose tissue through reducing ROS levels (22,23).
Based on the results of our study, the toxicity effect of BPA on L6 cells was dependent on concentration and time. 50 µM BPA did not have any signi cant impact on cell viability for up to 48 hours. On the other hand, while 100 µM did not show any toxicity on the cells for 24 hours, it signi cantly reduced cells survival after 48 hours. Increased BPA concentrations are associated with an increased risk of cardiovascular, hepatic, and renal diseases such as coronary heart disease, fatty liver disease, and chronic kidney disease. In this regard, BPA has been shown to cause various cytotoxic, genotoxic, and carcinogenic effects in vitro and in vivo. In line with our ndings, Podlich et al. studied the cytotoxic effects of BPA on HEK293 cells. They showed that incubation of HEK293 with 100 µM or 1000 µM BPA resulted in a signi cant dose-dependent reduction in renal cell viability (24). Moreover, BPA-treated cells incubated with adiponectin and quercetin showed that treatments with quercetin (at both concentrations) were able to overcome BPA-induced toxicity after 48 hours. Sangai NP et al. showed that quercetin decreased ROS production and increased total antioxidant capacity in rat kidneys (25). The use of quercetin can reduce the production of free radicals and play an essential role in improving the antioxidant/oxidant balance (26). However, quercetin has another therapeutic property that inhibits lipid peroxidation and has been shown to inhibit H 2 O 2 -induced lipid peroxidation even at a low concentration (2 µM) (27).
Catalase activity was also evaluated in our study, and the results showed that cell treatment with BPA at both 50 and 100 µM signi cantly reduced the cells' catalase activity at 24 and 48 hours. Shirani et al.
also showed that BPA reduced catalytic activity in all rats (28). In another study, Aboul Ezz et al. Reported similar results for BPA-treated rats after six weeks (27). Catalase is an enzyme found in almost all living organisms exposed to oxygen that catalyzes the breakdown of hydrogen peroxide into water and oxygen. This enzyme is crucial in protecting cells against oxidative damage by ROS. Our study showed that quercetin is almost more compelling than adiponectin in increasing cell catalase. Shirani et al. had reported similar results on the effect of quercetin on catalase activity (28). Demkovych et al. also reported increased catalytic activity due to quercetin treatment in rats (29).
PAB was evaluated in BPA-treated cells. The results showed that BPA at both concentrations signi cantly increased PAB. BPA disrupts cellular oxidative homeostasis by altering the dynamic balance between oxidative mediators and the activity of antioxidant enzymes, thereby predisposing the cell to apoptosis (30). Moghaddam et al. Reported that exposure to BPA (0.5 and 2 mg/kg /day) for four weeks in male mice signi cantly disrupted the dynamic balance of enzymatic antioxidants (31). Regardless of the presence of the antioxidant system, excessive or unbalanced production of ROS due to exposure to chemicals may lead to several clinical disorders. For example, BPA can cause oxidative stress by disrupting the oxidation state of cells (32). Therefore, our study con rmed that BPA exposure induced oxidative stress by upsetting the balance between ROS and the antioxidant defense system in L6 cells.
Our results also showed that quercetin reduced the effects of BPA on PAB. It seems that the use of compounds with potent antioxidant properties such as quercetin can reduce the production of free radicals due to BPA toxicity in the body and play an essential role in improving the antioxidant/oxidant balance (33). In a study, Mahdavinia et al. examined the effect of quercetin on the oxidative toxicity of BPA in the liver. They reported that potential hepatotoxicity of BPA could be prevented with quercetin, which protects the body against oxidative stress (33).
In this study, we also investigated the expression of KEAP1 and Nrf2 genes and found that BPA at 100 µM concentration increased KEAP1 expression and decreased Nrf2 expression. Nrf2 is a transcription factor that mediates an essential signaling pathway that prevents injury and disease. Under normal circumstances, Nrf2 is found and degraded in the cytoplasm attached to KEAP1. Following oxidative stress or drug intervention, Nrf2 is cleaved from KEAP1 and transported to the nucleus, activating transcription of protective genes to defend against organ damage (34 Author Contribution DM and MH make substantial contributions to conception and design. FJM, DGh,KN and ZR participate in drafting the article or revising it critically for important intellectual content. All authors give nal approval of the version to be submitted and any revised version.