Article pubs.acs.org/JAFC
Possible Antioxidant Mechanism of Melanoidins Extract from Shanxi Aged Vinegar in Mitophagy-Dependent and MitophagyIndependent Pathways Lei Yang,†,§ Xuping Wang,† and Xiaolan Yang*,† †
College of Life Science, Shanxi University, Taiyuan 030006, China College of Life Sciences, Nankai University, Tianjin 300073, China
§
ABSTRACT: Melanoidins are widely reported to have antioxidant activity; however, their mechanism has not been frequently studied. In this study, we found that melanoidins from Shanxi aged vinegar induced mitopahgy, the specific autophagic elimination of mitochondria, as assessed by up-regulation of the autophagy markers LC3-II and Beclin1 as well as degradation of the autophagy substrate p62 and mitochondrial proteins. Melanoidins reduced reactive oxygen species (ROS) in normal human liver cells and mouse livers through a mitophagy-dependent pathway, by the observation that the reducing ROS effect of melanoidins was partially lost when mitophagy was inhibited by chloroquine. Impaired Akt signaling was found in cells treated with melanoidins, which might explain the activation of autophagy induced by melanoidins. These results suggest that in addition to direct free radical scavenging activity, melanoidins decreased ROS levels through mitophagy in which damaged mitochondria, the source of ROS, were degraded. KEYWORDS: antioxidant, melanoidins, autophagy, mitophagy, oxidative stress, reactive oxygen species, vinegar, mouse livers
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INTRODUCTION Interest in the potential health effects of dietary vinegar is increasing worldwide. In China, vinegar has been traditionally produced from cereals for more than 3000 years.1 Studies have reported antioxidative health effects of fruit vinegar,2−4 but little is known about cereal vinegar. Shanxi aged vinegar (SAV) is a traditional Chinese cereal vinegar. Its unique fermentation and roasting processing (6 days at 85 °C) lead to the formation of high amounts of melanoidins,5 which are high molecular weight nitrogenous, brown compounds6 that contribute to the flavor and color of SAV. The potential health impacts of melanoidins include antioxidant, antimicrobial, anticariogenic, anti-inflammatory, antihypertensive, and antiglycative activities.7 The in vitro antioxidant activity of melanoidins that has been evaluated includes 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging activity, reducing power, and inhibitory effects on hydroxyl radicals.8 Protection of cells against oxidative damage by melanoidin has been reported for coffee,9 biscuits,10 and prunes.11 However, because of the complexity of cells, the underlying molecular mechanisms of cellular antioxidant activity remain unclear, and a deeper exploration is needed. Mitochondria are responsible for 90% of cellular energy. In ATP synthesis through mitochondrial oxidative phosphorylation, reactive oxygen species (ROS) are produced by electrons prematurely reacting with O2, especially at respiratory chain complexes I and III.12 Accumulating evidence demonstrates that ROS are involved in liver injuries including alcohol abuse, hepatitis C virus infection, iron overload, and chronic cholestasis.13,14 Antioxidants have been proposed as therapies in experimental models and patients with chronic liver injury.15,16 However, clinical studies have been controversial, with limited or no benefit reported from chronic administration of antioxidants such as vitamin A, vitamin E, or β-carotene.17 © 2014 American Chemical Society
These antioxidants not effective enough to attenuate oxidative stress-induced disorders or diseases, possibly because of an inability to target mitochondria, the primary source of ROS.18 Pharmacological inducers of autophagy have been described as ROS scavengers. For example, resveratrol, a natural polyphenol known to have antioxidative effects since the 1990s,19 was believed to explain the “French paradox,” in which a relatively low incidence of coronary heart disease is seen in a population with a high dietary intake of cholesterol and saturated fats.20 However, the concentrations of resveratrol in red wine are low and cannot entirely account for this phenomenon.21 The possible underlying mechanisms might relate to the autophagic activity of resveratrol. Duan et al. found that in addition to ROS clearance, autophagy is involved in resveratrol prevention of oxidative stress in restrained mice,22 which might explain the antioxidant effect of resveratrol. In this study, we found that melanoidins from SAV induced mitopahgy, the specific autophagic elimination of mitochondria, which reduced ROS in normal human liver cells and mice livers. The ability of melanoidins to reduce ROS and protect cells was lost when mitophagy was inhibited by chloroquine (CQ). Impaired Akt signaling was found in cells treated with melanoidins, which might explain the activation of autophagy induced by melanoidins. Thus, melanoidins and SAV acted as pro-autophagic compounds and could be beneficial in oxidative damage-related disease. Received: Revised: Accepted: Published: 8616
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from low molecular weight material (LMW). The resulting filtrate (MW 10 kDa, HMW, 26.4% of SAVE) were frozen and freeze-dried. HMW was subjected to a second purification by size exclusion chromatography with a Sephadex G-75 column (60 × 1.6 cm). Freezedried HMW was dissolved to 10 mg/mL in distilled water and filtered with 0.45 μm microporous membranes to 2 mL of filtrate for chromatographic separation. The eluent was distilled water at a flow rate of 0.5 mL/min, continuously monitored by absorbance at 280 nm. Collection was 3 mL, and total sugars were determined by using the phenol−sulfuric acid method at 490 nm.24 A color index was determined by absorbance at 420 nm. Column exclusion (V0) was determined with blue dextran-2000 (20000 kDa) and total volume (Vt) was determined with L-tyrosine (181 Da). Fractions 1−4 (F1− F4) (F1, 6.8% of HMW; F2, 82.4% of HMW; F3, 7.2% of HMW; and F4, 3.6% of HMW) were collected, and the molecular weight was calculated by log MW − Ve protein standard curves: lg MW = −0.0288Ve + 5.9402, 3r = 0.996, which was calculated by four protein markers: bovine serum albumin (67 kDa), chicken egg albumin (43 kDa), curd protease A (25 kDa), and cytochrome c (12.4 kDa). DPPH Radical-Scavenging Activity Assays. DPPH radicalscavenging activity was determined according to the method of Slusarczyk et al.25 with slight changes. Test samples were mixed with DPPH radical solution in ethanol (1:2 v/v) and incubated at room temperature in the dark for 30 min, and absorbance was measured at 517 nm. Each test was repeated three times. DPPH radical-scavenging activity was calculated as DPPH radical scavenging (%) = [A0 − (A1 − A2)]/A0 × 100, where A0 is the absorbance of the DPPH solution, A1 is the absorbance of the mixture of sample and DPPH solution after incubation, and A2 is the absorbance of the sample. Total Reducing Power Assays. Total reducing power was determined as in Zhu et al.26 with slight changes. A test sample of 0.5 mL was mixed with 2.5 mL of phosphate buffer (0.2 mol/L, pH 6.6) and 2.5 mL of potassium ferricyanide (1%), incubated at 50 °C for 20 min, mixed with 2.5 mL of trichloroacetic acid (10%), blended, and centrifuged at 3000 rpm for 10 min. A 2.5 mL supernatant sample was mixed with 2.5 mL of distilled water and 1 mL of FeCl3 (0.1%) and reacted for 10 min, and absorbance was measured at 700 nm. Determination of Cell Viability. Cell viability was determined by using the MTT assay as previously described.27 Cells were cultured in 96-well microtiter plates and exposed to H2O2 or F2 for 24 h. After treatment, 5 mg/mL MTT solution was added and incubated for 4 h at 37 °C. After a washing with PBS, purple-blue formazan was dissolved in 1 mL of DMSO, and absorbance was determined at 550 nm. Determination of Cellular ROS Level. Samples were incubated with 20 μM DCFH-DA for 30 min. Intracellular ROS-mediated oxidation of DCFH-DA to the fluorescent compound DCF was measured.28 Cells were harvested, and pellets were suspended in 1 mL of PBS and analyzed by flow cytometer (Becton Dickinson, Franklin Lakes, NJ, USA) with excitation of 488 nm and emission of 535. Western Blotting. Cells were washed with ice-cold PBS, harvested, and lysed for 1 h in ice-cold lysis buffer (1% NP40, 0.5% sodium deoxycholate, 0.1% SDS, 1 mM EDTA, 1 mM EGTA, 1 mM Na3VO4, 20 mM NaF, 0.5 mM DTT, 1 mM PMSF, protease inhibitor cocktail in PBS, pH 7.4). After centrifugation at 4 °C (12000g, 10 min), supernatants were collected and solubilized in 6× loading buffer (0.5 M Tris-HCl, pH 6.8, 30% glycerol, 10% SDS, 5% βmercaptoethanol, 0.012% bromophenol blue). Lysates were separated by 10 or 15% SDS-PAGE and transferred to PVDF membranes (Millipore, Billerica, MA, USA). Proteins were detected with antibodies and HRP-conjugated secondary antibodies. ECL Western Blotting Substrate (Thermo Fisher Scientific) was the HRP substrate. Antibodies were against β-actin (Sigma), beclin1, p62, LC3, tom20, tim23, cytochrome c (BD), phosphor-Akt (Ser47), pan-Akt, and the C subunit of protein phosphatase 2A (PP2A-C) (Cell Signaling Technology). Animals and Treatment. Hepatotoxicity assays were performed with 48 male Kunming mice from Shanxi Medical University Animal Research Center. Mice were housed in a temperature- and humidity-
MATERIALS AND METHODS
Chemicals, Cell Lines, and Culture Conditions. SAV was from local supermarkets. Blue dextran-2000, L-tyrosine, bovine serum albumin, chicken egg albumin, curd protease A, cytochrome c, 3(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), dimethyl sulfoxide (DMSO), hydrogen peroxide (H2O2) CQ, 2′,7′dichlorofluorescein diacetate (DCFH-DA), and bromobenzene (BB) were from Sigma-Aldrich (St. Louis, MO, USA). Immortalized human liver LO2 cells and HepG2 cells were cultured according to the method of Zhang et al.23 Cells were seeded in 6- or 96-well plates and adapted to the culture substrate for 12−24 h before experiments. Melanoidins Isolation and Purification. Components of SAV were prepared as in Figure 1. Briefly, 2 L of SAV was diluted with 2 L
Figure 1. Scheme for melanoidin isolation and purification from Shanxi aged vinegar (SAV). of distilled water. The resulting dispersion was filtered through Whatman filter paper 4 to remove insoluble matter. The filtrate was concentrated under vacuum using an RE-52 rotary evaporator (Yarong Experimental Instrument Co., Ltd., Shanghai, China) and freeze-dried to obtain Shanxi aged vinegar extract (SAVE), which was 21.3% of the SAV. To obtain high molecular weight material (HMW), 100 g of SAVE was dissolved in 2 L of distilled water and fractionated through ultrafiltration membranes (AMFOR, 10 kDa cutoff) with two washings with 2 L of distilled water to reduce contamination in the retentate 8617
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controlled room with a 12 h light−dark cycle and free access to a standard pellet diet and water. All animal protocols were approved by the Animal Care Committee of Shanxi University. After 1 week of adaptation, mice were randomly divided into six groups of eight mice per group: control, BB, low-dose SAVE (ig 500 mg/kg SAVE + BB), medium-dose SAVE (ig 1000 mg/kg SAVE + BB), high-dose SAVE (ig 2000 mg/kg SAVE + BB), and CQ+SAVE (ip 60 mg/kg CQ + ig 1000 mg/kg SAVE + BB). Mice were administered compounds or distilled water for 30 days and challenged ig with BB dispersed in 0.2 mL of plant oil at 0.47 mg kg−1 body weight, followed by an 18−22 h recovery stage before determination of liver biochemical parameters. All biochemical parameters, thiobarbituric acid reactive substances (TBARS), superoxide dismutase (SOD), and glutathione peroxidase (GSH-Px) were measured as previously reported.29 Statistical Analysis. All data were expressed as the mean ± standard deviation (SD) and analyzed using SPSS statistical software (SPSS Inc., Chicago, IL, USA). One-way ANOVA with Duncan’s test was used for intergroup comparison. P values of 80 kDa; F2, 80−10 kDa; F3, 10−3 kDa; and F4, 10 kDa) were more effective as antioxidants than other fractions because of the direct scavenging activity against radicals and a stronger metalchelating capability.31 However, larger molecular weight melanoidins do not necessarily have stronger antioxidative ability. Our purification to produce HMW showed that F2 (10−80 kDa) had higher DPPH radical-scavenging activity and total reducing power than F1 (>80 kDa). Although the mechanism remains unclear, this result might be related to differences in the composition of melanoidins in fractions with different molecular weights, with different melanoidin populations exhibiting antioxidative capacities through different mechanisms.7 We observed that the antioxidative ability of melanoidins of different molecular weights correlated with their polyphenol content (data not shown). Further study of melanoidin structures will improve the understanding of their antioxidant properties. To evaluate the ROS-reducing effect of F2 in cells, we used H2O2 to activate mitochondrial oxidative stress. H2O2, a specific ROS, was sufficient and essential for inducing oxidative stress. H2O2 interacts with metal ions such as Fe2+, forming metalcentered oxygen radicals and hydroxyl radicals (OH•) that are highly reactive and oxidatively damage mitochondria.35 Mitochondria are the major site of ROS production and the major target of their detrimental effects.36 We found a significantly increased ROS level in H2O2-treated groups, indicating that mitochondria were damaged and produced excessive ROS. Consistent with these studies, we observed a decrease in cell survival rates in H2O2-treated groups. Treatment with F2 reduced ROS levels and improved cell survival, indicating that F2 had protective effects on LO2 cells. Autophagy can be nonselective or selective. Nonselective autophagy is an evolutionarily conserved mechanism in which 8620
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yeast by eliminating mitochondria to a basal level and preventing excess ROS production.44 Han et al. reported that curcumin protects human endothelial cells from oxidative damage via autophagy.45 Our results also support these findings. We observed that activation of autophagy by F2 protected cells from oxidative stress and decreased cell survival and that CQ treatment blocked this protective effect. Akt kinase (protein kinase B) is a serine/threonine protein kinase that negatively regulates autophagy.46 Because melanoidins impair the Akt signaling pathway34 and down-regulation of phospho-Akt (P-Akt) activates autophagy, we hypothesized that autophagic activity by F2 might be because of Akt kinase inhibition. F2 decreased the Akt protein kinase activity by impaired Akt phosphorylation at serine 473. Akt phosphorylation is determined by a balance between kinases and phosphatases.47 Therefore, we investigated F2 effects on PP2A and P-Akt. PP2A is in a family of serine/threonine phosphatases that down-regulate Akt phosphorylation. Increasing evidence shows that PP2A is important in autophagy promotion.48 PP2A contains a highly active core dimer of a catalytic C subunit (PP2A-C) and a structural A subunit (PP2A-A) that recruits one of several regulatory B subunits (PP2A-B) to form a PP2A heterotrimeric complex.49,50 F2 treatment notably up-regulated PP2A-C expression, indicating that F2 increased PP2A activity. These results indicated that F2 might have activated autophagy through the PP2A/Akt signaling pathway. Autophagy is protective against liver injury.51−53 We did not obtain enough F2 for animal tests, so we used SAVE. We imitated pathological hepatic conditions using a BB-induced liver injury model to evaluate the antioxidative activities of SAVE. We previously demonstrated that BB is a strong oxidant that causes hepatic lipid peroxidation.29 Liver oxidative damage was evaluated by lipid peroxidation production of TBARS and two antioxidant enzymes, SOD and GSH-px. The hepatic protective effect of SAVE was partially inhibited by CQ (Table 1). Similar to our results in LO2 cells, which showed that F2 had mitophagy-dependent pathways to lower ROS production (Figure 4B−D), the in vivo experiment also showed that SAVE had mitophagy-dependent pathways to lower liver oxidative damage, which suggested that F2 might contribute to the protective effect of SAVE in vivo, through mitophagydependent pathways. Taken together, these results suggested that F2-mediated autophagy might be important for preventing liver injury in vivo. We demonstrated that mitophagy was involved in the ability of melanoidins extracted from SAV to protect the liver from oxidative stress in vitro and in vivo. Melanoidins might activate autophagy through the PP2A/Akt signaling pathway. This study provides novel insights into the cellular mechanisms of the antioxidative effect of melanoidins and potential future therapeutic strategies.
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ACKNOWLEDGMENTS
We are grateful to International Science Editing for providing language editing assistance.
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ABBREVIATIONS USED SAV, Shanxi aged vinegar; SAVE, Shanxi aged vinegar extracts; HMW, high molecular weight material; LMW, low molecular weight material; F1−F4, fractions 1−4; ROS, reactive oxygen species; CQ, chloroquine; BB, bromobenzene
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AUTHOR INFORMATION
Corresponding Author
*(X.Y.) Phone: +86-351-7011590. Fax: +86-351-7011590. Email:
[email protected]. Funding
This work was supported by the National Natural Science Foundation of China (Grant 31171748). Notes
The authors declare no competing financial interest. 8621
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