Effect of young apple (Malus domestica Borkh. cv. Red Fuji) polyphenols on alleviating insulin resistance
Graphical abstract
Introduction
The incidence of type 2 diabetes (T2DM) has significantly increased around the world (Abu Bakar et al., 2015). Its development is closely related to high blood glucose level. The methods of controlling high blood glucose level mainly involve three mechanisms, including (i) inhibiting small intestine glucose transporters, such as the sodium-dependent glucose transporter 1 (SGLT1) and glucose transporter 2 (GLUT2); (ii) postponing the starch digestion through inhibiting the activities of carbohydrate-hydrolyzing enzymes, such as α-amylase and α-glucosidase; and (iii) improving insulin resistance (IR). Accumulated evidence has suggested that IR has an important role in the development of T2DM. IR can lead to hyperglycemia by disturbing the homeostasis of the glycometabolism of the target organ such as the liver (Liu et al., 2015).
The possible mechanisms of IR both in vitro and in vivo have been studied (Manzano et al., 2016; Ren et al., 2018). IR can be induced by impairing insulin signal transduction, which leads to overproduction of glucose that cannot be effectively utilized by other organs. Activating the IR signal pathway could effectively improve IR, and the insulin receptor substrate/phosphatidylinositol 3-kinase/phosphorylation of the protein kinase B (IRS/PI3K/AKT) signal pathway has an important role (Gao et al., 2015). Besides, glycogen synthase kinase 3β (GSK3β), an important regulatory kinase, and forkhead transcription factor O1 (FOXO1), a member of the forkhead family of transcription factors, can be used to regulate glycogen synthesis and gluconeogenesis, respectively. The expression of GSK3β and FOXO1 can be adjusted by the IRS/PI3K/AKT signal pathway (Yan, Dai, & Zheng, 2016). In addition, mitochondria are recognized as the heart of cells, and can be used to modulate various metabolic processes such as cell proliferation and oxidative stress (Reed, 2000). Oxidative stress is linked to the pathogenesis of diabetes and IR, thus, deteriorated mitochondrial functions are associated with impaired insulin sensitivity (Abu Bakar & Tan, 2017). Therefore, it is of interest to explore compounds that alleviate IR for the potential treatment of T2DM.
Given the potential side effects of existing drugs, it is potentially beneficial to explore safe and natural products from fruits and vegetables for the prevention or treatment of IR (Madrigal-Santillan et al., 2014). A number of phytochemicals have been considered for this purpose. For example, anthocyanins extracted from mulberry and eriodictyol were shown to improve glucose metabolism and IR by activating the PI3K/AKT signal pathway in HepG2 cells and 3T3-L1 adipocytes (Yan et al., 2016; Zhang et al., 2012). Flavonoid glycosides from fenugreek seeds were found to regulate glycolipid metabolism and IR by activating the AKT and the AMP-activated protein kinase (AMPK) signal pathways and improving mitochondrial function (Luan et al., 2018).
Apples are rich in bioactive compounds that are beneficial to human health. Every year, thinning of young apples is done to improve fruit quality and to increase yield, and about 1.6 million tonnes of thinned young apples are discarded (Sun, Guo, Fu, Li, & Li, 2013). Due to much higher polyphenol content than matured fresh apples, thinned young apples are a potentially valuable food resource (Zheng, Kim, & Chung, 2012). It will add value to agricultural and food industry by recycling these thinned young apples and exploring their potential applications.
In a previous study, young apple polyphenols (YAP) extracted from thinned young apples has been reported to have potential anti-hyperglycemic activity by inhibiting starch digestive enzymes including α-amylase and α-glucosidase in vitro and in vivo (Li et al., 2019). The study showed a positive correlation between YAP and its potential anti-hyperglycemic activity. Therefore, to further understand the anti-hyperglycemic effect of YAP, the effects of YAP on IR as well as the relevant mechanisms were investigated in this study through activating the AKT phosphorylation signal pathway and improving mitochondrial dysfunction using insulin resistant HepG2 cells.
Section snippets
Chemical reagents
HepG2 cells (Kilton Biotechnology Co., Ltd., Shanghai, China), Dulbecco's modified Eagle's medium (DMEM), fetal bovine serum (FBS), penicillin (10000 U/ml), streptomycin (10 mg/ml), 2′,7′- dichlorofluorescein diacetate (DCFH-DA), lipophilic cationic probe 5,5′,6,6′-terachloro-1,1′,3,3′-tetraethyl-imidacarbocyanine iodide (JC-1) assay kit, cell lysis buffer and ECL plus kit (Solebao Technology Co. Ltd., Beijing, China), 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) (MCE,
Composition of phenolic compounds in YAP
The composition of phenolic compounds in YAP has been analyzed (Li et al., 2019). As shown in Table S1, a total of 15 peaks were found in YAP. Chlorogenic acid (peak 6) and phlorizin (peak 15) were the two main phenolic compounds, and other phenolic compounds only account for a small proportion. Therefore, in this study, YAP, phlorizin and chlorogenic acid were chosen for the following experiments.
Determination of cell viability of HepG2 cells
To confirm the safe dosage of IR related experiments, the effect of YAP, phlorizin or chlorogenic
Discussion
IR is considered as a predictor for development of T2DM. It has been reported that polyphenols offer resistance against the development of diabetes through regulation of insulin release, insulin sensitivity, insulin dependent signaling pathways, and mitochondrial function (Arts & Hollman, 2005; Graf, Milbury, & Blumberg, 2005). Thinned young apples are a valuable and easily accessible natural source, in which the polyphenol content is much higher than matured fresh apples (Zheng et al., 2012).
Conclusions
The protective effect of YAP on high glucose plus PA induced IR in HepG2 cells was studied and the possible mechanisms were explored. The results showed that hepatic IR could be improved by YAP through activating the IRS2/PI3K/AKT/GSK3β signal pathway to accelerate glycogen synthesis and the IRS2/PI3K/AKT/FOXO1 signal pathway to inhibit gluconeogenesis, as well as improving mitochondrial function in the insulin resistant HepG2 cells, and phlorizin had the best effect (Fig. 6). However, further
CRediT authorship contribution statement
Dan Li: Conceptualization, Methodology, Writing - original draft, Visualization. Yongli Yang: Validation, Software, Writing - original draft. Lijun Sun: Writing - review & editing. Zhongxiang Fang: Writing - review & editing. Lei Chen: Software, Formal analysis. Pengtao Zhao: Data curation. Zichao Wang: Conceptualization, Investigation, Visualization. Yurong Guo: Resources, Project administration, Funding acquisition.
Declaration of competing interest
The authors claim no conflict of interests in this work.
Acknowledgements
This study was supported by the China Agriculture Research System (CARS-27) from the Ministry of Agriculture of the People's Republic of China.
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They contributed equally to this work.