Phytochemicals: Targeting Mitophagy to Treat Metabolic Disorders

Metabolic disorders include metabolic syndrome, obesity, type 2 diabetes mellitus, non-alcoholic fatty liver disease and cardiovascular diseases. Due to unhealthy lifestyles such as high-calorie diet, sedentary and physical inactivity, the prevalence of metabolic disorders poses a huge challenge to global human health, which is the leading cause of global human death. Mitochondrion is the major site of adenosine triphosphate synthesis, fatty acid β−oxidation and ROS production. Accumulating evidence suggests that mitochondrial dysfunction-related oxidative stress and inflammation is involved in the development of metabolic disorders. Mitophagy, a catabolic process, selectively degrades damaged or superfluous mitochondria to reverse mitochondrial dysfunction and preserve mitochondrial function. It is considered to be one of the major mechanisms responsible for mitochondrial quality control. Growing evidence shows that mitophagy can prevent and treat metabolic disorders through suppressing mitochondrial dysfunction-induced oxidative stress and inflammation. In the past decade, in order to expand the range of pharmaceutical options, more and more phytochemicals have been proven to have therapeutic effects on metabolic disorders. Many of these phytochemicals have been proved to activate mitophagy to ameliorate metabolic disorders. Given the ongoing epidemic of metabolic disorders, it is of great significance to explore the contribution and underlying mechanisms of mitophagy in metabolic disorders, and to understand the effects and molecular mechanisms of phytochemicals on the treatment of metabolic disorders. Here, we investigate the mechanism of mitochondrial dysfunction in metabolic disorders and discuss the potential of targeting mitophagy with phytochemicals for the treatment of metabolic disorders, with a view to providing a direction for finding phytochemicals that target mitophagy to prevent or treat metabolic disorders.

Metabolic disorders include metabolic syndrome, obesity, type 2 diabetes mellitus, nonalcoholic fatty liver disease and cardiovascular diseases. Due to unhealthy lifestyles such as high-calorie diet, sedentary and physical inactivity, the prevalence of metabolic disorders poses a huge challenge to global human health, which is the leading cause of global human death. Mitochondrion is the major site of adenosine triphosphate synthesis, fatty acid β−oxidation and ROS production. Accumulating evidence suggests that mitochondrial dysfunction-related oxidative stress and inflammation is involved in the development of metabolic disorders. Mitophagy, a catabolic process, selectively degrades damaged or superfluous mitochondria to reverse mitochondrial dysfunction and preserve mitochondrial function. It is considered to be one of the major mechanisms responsible for mitochondrial quality control. Growing evidence shows that mitophagy can prevent and treat metabolic disorders through suppressing mitochondrial dysfunction-induced oxidative stress and inflammation. In the past decade, in order to expand the range of pharmaceutical options, more and more phytochemicals have been proven to have therapeutic effects on metabolic disorders. Many of these phytochemicals have been proved to activate mitophagy to ameliorate metabolic disorders. Given the ongoing epidemic of metabolic disorders, it is of great significance to explore the contribution and underlying mechanisms of mitophagy in metabolic disorders, and to understand the effects and molecular mechanisms of phytochemicals on the treatment of metabolic disorders. Here, we investigate the mechanism of mitochondrial dysfunction in metabolic disorders and discuss the potential of targeting mitophagy with phytochemicals for the treatment of metabolic disorders, with a view to providing a direction for finding phytochemicals that target mitophagy to prevent or treat metabolic disorders.

INTRODUCTION
Autophagy, an evolutionarily conserved catabolic process, degrades intracellular constituents including lipid, glycogen and protein to maintain cellular energy homeostasis in the absence of nutrients (Vargas et al., 2017). In this process, double-membrane vesicles (autophagosomes) will enfold intracellular constituents and then transfer to lysosomes to form autolysosomes where the lysosomal enzyme can degrade the enveloped cargoes. According to the different methods of delivering cytoplasmic components to lysosomes, autophagy can be divided into three different types: macroautophagy (hereafter referred to as autophagy), microautophagy and chaperonemediated autophagy. Moreover, according to the specificity of the degradation substrate, autophagy can be classified into mitophagy, pexophagy, reticulophagy, ribophagy and xenophagy.
About 50 years ago, autophagy was first described to be triggered to maintain cellular energy balance and cell survival under nutrition-deprived conditions. In the past 10 years, due to the in-depth understanding of the role of autophagy, accumulating study has indicated that autophagy plays a vital role in the physiology and pathology of many diseases, such as metabolic disorders, cancer, Alzheimer's disease and Parkinson's disease. Nowadays, it is commonly accepted that autophagy, especially mitophagy, plays a crucial role in the pathology of metabolic disorders such as non-alcoholic fatty liver, type 2 diabetes and metabolic syndrome (Vásquez-Trincado et al., 2016;Sarparanta et al., 2017;Dombi et al., 2018).
Phytochemicals extracted from natural plants have been widely used to treat metabolic diseases including metabolic syndrome (Cicero and Colletti, 2016), type 2 diabetes (Moreno-Valdespino et al., 2020), obesity , insulin resistance (Mahdavi et al., 2021) and cardiovascular diseases (Pop et al., 2018) due to relative safety and multiple beneficial effects. According to an estimation issued by the World Health Organization in 2008, about 80% of diabetic patients rely on herbal medicine (Bacanli et al., 2019).
In view of the fact that more and more phytochemicals are applied to the treatment of metabolic diseases, it is necessary to have a more comprehensive understanding of the effects and potential mechanisms of phytochemicals on metabolic diseases. Therefore, this review will focus on the regulatory mechanisms of mitophagy in metabolic disorders and further explore the potential of targeting mitophagy with phytochemicals for the prevention or treatment of metabolic disorders.

MITOCHONDRIAL DYSFUNCTION AND OXIDATIVE STRESS
Mitochondrion regulates many physiological functions such as adenosine triphosphate (ATP) synthesis, free radicals generation, fatty acid β−oxidation, calcium homeostasis, and cell survival and death Kirtonia et al., 2021). Mitochondria are dynamic organelles that can fleetly adapt to changes in cellular energy metabolism by regulating mitochondrial biogenesis, mitochondrial fission, mitochondrial fusion and removal of damaged mitochondria (Vásquez-Trincado et al., 2016;Mills et al., 2017;Keenan et al., 2020). Of note, the uppermost physiological function of mitochondria is to produce ATP via oxidative phosphorylation (OXPHOS) (Bhatti et al., 2017). During OXPHOS, mitochondria will inevitably produce byproduct superoxide anions, which can be further converted into reactive oxygen species (ROS) or reactive nitrogen species (RNS) (Kalyanaraman et al., 2018). Under physiological conditions, ROS and RNS act as the regulatory mechanism for cellular redox homeostasis (Liu and Butow, 2006). Mitochondrial ROS, one of the major sources of cellular ROS, consists of hydrogen peroxide (H 2 O 2 ), superoxide (O − 2 ), and hydroxyl (OH), which can impair lipids, proteins and DNA, resulting in mitochondrial dysfunction and cell apoptosis (Kirtonia et al., 2020;Tan et al., 2021). There are various defense systems to counter ROS-induced oxidative stress, such as catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GSH-PX). However, high levels of ROS will unduly oxidize lipids, DNA and proteins, thereby destroying cell membranes and other cellular structures. In general, mitochondrial ROS can damage mitochondrial DNA to disturb the physiological functions of mitochondria. And the accumulation of damaged mitochondria and the overexpression of mitochondrial ROS will reinforce each other, forming a vicious cycle, and eventually lead to mitochondrial dysfunction. Accumulating evidence has shown that oxidative stress is involved in various pathological conditions including insulin resistance, type 2 diabetes and metabolic syndrome (Yaribeygi et al., 2019). Therefore, the regulation of mitochondrial ROS is of key relevance for cellular homeostasis (Kirtonia et al., 2020).

MITOCHONDRION: STRUCTURE AND FUNCTION
Mitochondrial dysfunction is responsible for various metabolic diseases. Therefore, targeting mitochondrial dysfunction will be a promising therapeutic strategy for metabolic disorders. Nevertheless, due to the high complexity of mitochondrial structure and function, targeting mitochondria for the treatment of metabolic disorders will be an arduous and challenging task. Mitochondrion is a double-membrane organelle with its own unique genome, containing 800 to 1000 copies of mitochondrial DNA (mtDNA) (Chen and Butow, 2005). The mitochondrion is constituted by the outer mitochondrial membrane (OMM), the intermembrane space (IMS), inner mitochondrial membrane (IMM) and mitochondrial matrix (Iacobazzi et al., 2017). Although the outer mitochondrial membrane is more permeable than the inner mitochondrial membrane, only molecules with a molecular weight of 5 kDa or less can cross the outer mitochondrial membrane due to the existence of the voltage-dependent anion channel (VDAC). The IMM and mitochondrial matrix contain various enzymes responsible for electron transport chain (ETC) and ATP generation. The tricarboxylic acid (TCA) cycle, one of the hallmark pathways in metabolism, plays an important role in the oxidation of respiratory substrates for ATP synthesis in the mitochondrial matrix (Sweetlove et al., 2010). During this process, electrons will be released and are absorbed by the ETC to produce ATP, which is the major source of cellular energy (Bhatti et al., 2017).
In IMM, complexes I (NADH ubiquinone reductase), II (Succinate dehydrogenase), III (Ubiquinol-cytochrome c reductase), and IV (Cytochrome c oxidase) constitute the ETC, and complex V is an ATP synthase (Shamsi et al., 2008). The hydrogen atoms released during the TCA cycle and fatty acid β-oxidation processes will be transferred to NAD + or FAD + to form NADH or FADH 2 (St John et al., 2005). Then the electrons provided by NADH and FADH2 will be transferred to complex I and complex II, respectively, and then to complex III and complex IV. Meanwhile, ETC can generate electrochemical gradient by transporting protons into the intermembrane space. The electrochemical gradient serves as a source of potential energy for generating ATP in the complex V (Shamsi et al., 2008). Under physiological conditions, 0.4 to 4% of oxygen consumed by mitochondria is incompletely reduced, which will lead to the generation of ROS, and other reactive species such as nitric oxide (NO) and RNS (Bhatti et al., 2017;Marí and Colell, 2021). Despite there being multiple enzymatic and non-enzymatic antioxidant defense mechanisms in eukaryotic cells, excessive reactive species will inevitably damage mitochondrial proteins/enzymes, mitochondrial membranes and mtDNA, thereby impairing ATP generation (Bhatti et al., 2017).

Mitophagy-Mediated Mitochondrial Quality Control
Considering the crucial role of mitochondria in cellular homeostasis, monitoring the quality of mitochondrial is important to avoid adverse effects (Ni et al., 2015;Pickles et al., 2018;Chan, 2020). Mitochondrial quality control can be acted in many forms including molecular, organellar and cellular levels. It's worth noting that CAT, SOD and GSH-PX, as molecular-level oxidative stress defense mechanisms, can effectively decelerate the pace of oxidative damage to mitochondria (Naraki et al., 2021). However, the ROS-scavenging system cannot completely prevent excessive ROS-mediated damage to mitochondria. Accordingly, inhibiting the excessive production of ROS is gradually considered as a more effective way to prevent oxidative damage (Simona et al., 2019).
It is known that the clearance mechanism of damaged mitochondria, an important source of mitochondrial ROS, will be the potent therapeutic strategy for oxidative stress (Angajala et al., 2018;Wang R. et al., 2019). It is now generally accepted that damaged and dysfunctional mitochondria will be removed through mitophagy. In this process, damaged and dysfunctional mitochondria will be captured by autophagic membranes and further delivered to lysosomes, in which mitochondria will be degraded and the degradation products will be used as energy source for metabolism (Figure 1). With the deepening of the understanding of the physiological and pathological role of mitophagy, impaired mitophagy has been proved to be related to the development of various diseases including insulin resistance, type 2 diabetes, metabolic syndrome and non-alcoholic fatty liver disease (Su et al., 2019).

Other Mitochondrial Quality Control Mechanisms
In addition to mitophagy-dependent mitochondrial quality control, mitochondrion also has its own regulatory mechanisms for mitochondrial quality control, such as the mitochondrial unfolded protein response (UPR mt ) and mitochondrial fusion and fission (Roque et al., 2020).
The mitochondrial unfolded protein response (UPR mt ) is a stress response pathway that can be activated by multiple stress conditions including mitochondrial DNA defects, decreased mitochondrial membrane potential, accumulated unfolded mitochondrial proteins and ROS detoxification (Hernando-Rodríguez and Artal-Sanz, 2018; Roque et al., 2020). At present, the molecular mechanism of UPR mt in Caenorhabditis elegans (C. elegans) is better understood than in mammals. Activating transcription factor associated with stress (ATFS-1), a key regulatory factor for UPR mt in Caenorhabditis elegans, has an N-terminal mitochondrial targeting sequence. Normally, ATFS-1 is imported into mitochondria and is degraded by the matrix-localized protease LON. However, in the case of mitochondrial stress such as respiratory chain dysfunction and ROS, a portion of ATFS-1 will be transferred to the nucleus to activate UPR mt to regulate the abundance of mitochondrial chaperones and proteases to preserve mitochondrial homeostasis (Wrobel et al., 2015;Melber and Haynes, 2018). It's worth noting that if mitochondrial dysfunction continues to worsen, other mitochondrial quality control mechanisms, such as mitophagy, will be activated to clear away those dysfunctional mitochondria (Ma X. et al., 2020).
Mitochondria are dynamic organelles that continuously undergo fission and fusion to maintain the balance of small fragmented mitochondria and long interconnected mitochondrial network, which is essential for cell growth, division, and distribution of mitochondria during differentiation (van der Bliek et al., 2013;Roque et al., 2020). Mitochondrial fission will split a mitochondrion into a healthy mitochondrion with increased mitochondrial membrane potential and a dysfunctional one containing diminished mitochondrial membrane potential, damaged proteins and damaged mtDNA. And then the dysfunctional mitochondrion will be targeted and degraded by mitochondrial quality control mechanisms such as mitophagy (Tahrir et al., 2019). Various regulatory factors involved in mitochondrial fission include dynamin-related protein 1 (DRP1), fission 1 (Fis1), mitochondria fission factor (Mff), mitochondrial dynamics protein of 49 kDa (MID49) and MID51 (Ni et al., 2015). When mitochondrial fission occurs, DRP1 will translocate to the outer mitochondrial membrane from cytoplasm via actin and microtubule mechanisms, and then interacts with Fis1, MFF, MID49 and MID51 to cleave mitochondrion (Anzell et al., 2018). On the other hand, mitochondrial fusion allows damaged mitochondria to fuse with healthy mitochondria to facilitate the equilibration of mitochondrial components, such as mtDNA, proteins and FIGURE 1 | Diagram illustrates the mechanisms of mitophagy. Damaged and dysfunctional mitochondria will be segregated by autophagic membranes. And the autophagosome fuses with lysosome to form autophagolysosome, in which mitochondria will be degraded by lysosomal enzyme and the degradation products will be used as substrates for energy metabolism. metabolites, thereby enhancing respiratory chain activity and maintaining mitochondrial homeostasis (Baker et al., 2011). Mitochondrial fusion is regulated by dynamin-related GTPase proteins mitofusin 1 (MFN1), mitofusin 2 (MFN2) and optic atrophy 1 (OPA1). MFN1 and MFN2 are responsible for the fusion of the outer mitochondrial membranes, and OPA1 is responsible for the fusion of the inner mitochondrial membranes (Ni et al., 2015). Therefore, outer mitochondrial membrane fusion is carried out in an MFN1/MFN2-dependent manner and inner mitochondrial membrane fusion is performed in an OPA1-dependent manner, and GTP hydrolysis provides energy for the process (Roque et al., 2020). However, when the stimulation severely interferes with mitochondria and causes the mitochondrial membrane potential to dissipate, mitochondrial fusion will be halted to prevent those mitochondria from fusing with the healthy network, thereby limiting the damage caused by dysfunctional mitochondria (Baker et al., 2011).

MITOCHONDRIAL DYSFUNCTION AND INSULIN RESISTANCE
Insulin resistance is a pathological state in which target cells including hepatic cells, adipose cells and skeletal muscle cells are insensitive to the physiological level of insulin (Vazirani et al., 2016;Archer et al., 2018;Nishida et al., 2021). Mounting evidence reveals that insulin resistance is a common risk factor for various metabolic diseases such as metabolic syndrome, obesity, type 2 diabetes, non-alcoholic fatty liver disease and cardiovascular diseases (Patel et al., 2016;Gluvic et al., 2017;Yazıcı and Sezer, 2017;Czech, 2020;Fujii et al., 2020). Of note, a growing number of studies have established the causal relationship between mitochondrial dysfunction and insulin resistance (Ba Razzoni et al., 2012;Yazıcı and Sezer, 2017;Yaribeygi et al., 2019). Now, the complexity of mitochondrial function and the complicated relationship between mitochondrial dysfunction and the pathogenesis of insulin resistance have led to the development of many theories describing the mechanism connecting mitochondrial dysfunction and insulin resistance.

Ectopic Lipid Accumulation and Insulin Resistance
When the causal relationship between mitochondrial dysfunction and insulin resistance was first confirmed, more and more theories describing the relationship between mitochondrial dysfunction and insulin resistance were proposed. Mitochondria are the main sites of fatty acid β-oxidation, which is the main degradation mechanism of fatty acids in cells (Su et al., 2019). Therefore, mitochondrial dysfunction will cause inefficient fatty acid oxidation, which will lead to ectopic lipid accumulation in non-adipose tissues including liver, muscle and pancreas (Martin and McGee, 2014). Ectopic lipid accumulation will result in the remarkable increase in lipid metabolites such as ceramide and diacylglycerol, which are verified to impair insulin signaling pathway and cause insulin resistance (Montgomery et al., 2019). Now, many theories describing the potential mechanism of ceramide-induced insulin resistance have been put forward (Petersen and Shulman, 2017). First, evidence shows that ceramide induces insulin resistance via suppressing ATK activation through two mechanisms: increases proteinphosphatase-2A (PP2A) and PKCζ activities, thereby disturbing AKT translocation (Petersen and Shulman, 2017). Moreover, the connection among ceramide, adipose inflammation and NLRP3 inflammasome also provides another evidence for ceramide-induced insulin resistance (Turpin et al., 2014;Xia et al., 2015;Petersen and Shulman, 2017). One putative mechanism is that diacylglycerol promotes the membrane translocation of PKCε, which in turn phosphorylates insulin receptor Thr1160 to impair insulin receptor kinase (IRK) activity, thereby inducing insulin resistance (Petersen et al., 2016). In conclusion, mitochondrial dysfunction will cause ectopic lipid accumulation, thereby resulting in the significant increase of ceramide and diacylglycerol, which will directly or indirectly suppress insulin signaling pathway to induce insulin resistance.

Mitochondrial ROS and Insulin Resistance
Mitochondrial ROS has been recognized as the leading cause of insulin resistance. Bloch-Damti and Bashan (2005) brought forward a viewpoint that mitochondrial ROS should be accepted as a possible cause of insulin resistance in animal models of diabetes. However, these initial evidence don't specify whether ROS that induces insulin resistance originates from mitochondria. Since mitochondria are the major sources of cellular ROS, it is necessary to explore the role of mitochondrial ROS in insulin resistance. Indeed, subsequent studies verified the causal relationship of mitochondrial ROS and insulin resistance (Anderson et al., 2009). Anderson et al. believe that mitochondrial ROS is not only an indicator of energy balance, but also a regulator of cellular redox environment, linking cellular metabolic balance with the control of insulin sensitivity (Anderson et al., 2009). There are several potential mechanisms linking mitochondrial ROS and insulin resistance. First, mitochondrial ROS activates various serine kinases that phosphorylate IRS protein and suppresses serine/threonine phosphatase activity to inhibit insulin signaling pathway (Kim et al., 2008;Fisher-Wellman and Neufer, 2012). Furthermore, mitochondrial ROS activates apoptosis signal-regulating kinase 1 (ASK1) and c-jun NH2-terminal kinases (JNK), increases serine phosphorylation of IRS-1, and decreases insulin-stimulated tyrosine phosphorylation of IRS-1, resulting in insulin resistance (Nishikawa and Araki, 2007). However, the only available experimental data has just begun to support those mechanisms, and the detailed mechanism of mitochondrial ROS-induced insulin resistance still needs further exploration.

MITOPHAGY SIGNALING PATHWAYS
Mitophagy, a mitochondrial quality control mechanism, selectively removes dysfunctional mitochondria to preserve mitochondrial function and maintain cellular energy homeostasis. In mammalian cells, there are three main signaling pathways that regulate mitophagy: PINK1/Parkin-dependent mitophagy, BNIP3/NIX-dependent mitophagy and FUNDC1dependent mitophagy. Lemasters et al. (1998) discovered that dysfunctional mitochondria were engulfed into autophagosome and then fused with lysosome, in which the dysfunctional mitochondria were degraded. Then Lemasters (2005) further proposed the concept "mitophagy" for the first time to describe the process of eliminating damaged mitochondria. Narendra et al. (2009) confirmed that PTEN-induced putative kinase 1 (PINK1) and the E3 ubiquitin ligase Parkin are two crucial mediators regulating mitophagy in mammalian cells. In general, PINK1 is located in the outer mitochondrial membrane, but PINK1 cannot be detected in healthy mitochondria. Because, after being located into the mitochondrial matrix, PINK1 is cleaved by intramembrane-cleaving protease PARL and then the truncated form of PINK1 is released into the cytoplasm, in which PINK1 is further degraded by the ubiquitin proteasome system to remain at a low basal level (Wu et al., 2015;Wang H. et al., 2019). However, in dysfunctional mitochondria, PINK1 cannot be transported into the inner mitochondrial membrane, thereby avoiding cleavage by intramembranecleaving protease PARL. Subsequently, PINK1 located in the outer mitochondrial membrane will recruit autophagy receptors including SQSTM1/p62, nuclear dot protein 52 (NDP52) and optineurin (OPTN), which can bind to LC3 to combine the dysfunctional mitochondria and autophagosomes, eventually dysfunctional mitochondria will be degraded in autolysosome (Moreira et al., 2017). To further activate mitophagy, PINK1 also phosphorylates Ser65 in the ubiquitin and ubiquitin-like domain of Parkin, and further facilitates Parkin localization from the cytosol to the outer mitochondrial membrane of dysfunctional mitochondria Ma X. et al., 2020).

PINK1/Parkin-Dependent Mitophagy
Additionally, Parkin can also promote the ubiquitination of the mitochondrial fusion proteins mitofusin 1 (MFN1) and mitofusin 2 (MFN2), the mitochondrial adapter protein Miro1, translocase of outer mitochondrial membrane 20 (TOM20), and voltage-dependent anion channel (VDAC) to induce mitophagy (Nardin et al., 2016;Bradshaw et al., 2020). Normally, there are two major mechanisms involved in Parkin-dependent mitophagy: the first mechanism is that Parkin ubiquitinates MFN1 and MFN2, and further degraded by the proteasome, resulting in mitochondrial fission. Mitochondrial fission contributes to the separation of dysfunctional mitochondria from healthy network, and then dysfunctional mitochondria will be engulfed by autophagosomes and further degraded in autolysosomes (Eid et al., 2016;Ma X. et al., 2020). Additionally, Parkin-mediated the ubiquitination of mitochondrial outer membrane proteins VDAC will promote the recognition of VDAC by autophagy receptors such as histone deacetylase 6 (HDAC6) and p62. Subsequently, p62 will bind to LC3 positive autophagosomes to promote dysfunctional mitochondria to be captured by autophagosomes and then be degraded in autolysosomes (Figure 2; Moreira et al., 2017).

BNIP3/NIX-Dependent Mitophagy
In order to cope with hostile environments such as hypoxia and nutritional deficiencies, mitochondrial fission will enhance and mitophagy activation will also increase to degrade excessive mitochondria, and finally adaptively reducing mitochondrial quantity and maintaining cellular energy homeostasis. Generally speaking, this kind of stress-induced mitophagy is mediated by BCL-2/adenovirus E1B interacting protein 3 (BNIP3) and Niplike protein X (NIX) (Lampert et al., 2019;Xu et al., 2020). Of note, contrary to PINK1/Parkin-mediated indirect connection between damaged mitochondria and autophagosomes, BNIP3 and NIX induce mitophagy by direct connecting damaged mitochondria to autophagosomes (Moreira et al., 2017). The phosphorylation of Ser17 and Ser24 on BNIP3 can promote the affinity of BNIP3 and LC3, thereby activating mitophagy (Zhu et al., 2013). Furthermore, Beclin1 induces mitophagy in the form of Beclin1-Vps34 -Vps15 complexes (Ma et al., 2014;Wang et al., 2017). However, Bcl-2 and Bcl-XL can bind to the BH3 domain of Beclin1 to form Beclin1-Bcl-2-Bcl-XL complexes to inhibit mitophagy. It is interesting to note that BNIP3 and NIX are easier to bind to Bcl-2 and Bcl-XL than Beclin1, which release Beclin1 from Beclin1-Bcl-2-Bcl-XL complexes and subsequently induces mitophagy (M Chiara et al., 2014;Chiang et al., 2018). What's more, rashomolog enriched in brain (Rheb) can activate the mammalian target of rapamycin (mTOR) to inhibit mitophagy, but BNIP3 can suppress Rheb/mTOR activation (Gong et al., 2018). Similar to BNIP3, NIX also can directly interact with LC3, and the phosphorylation of Ser34 and Ser35 on NIX further promotes this interaction. Moreover, Parkinmediated NIX ubiquitination will recruit NBR1, an autophagy cargo receptor, to enhance mitophagy-mediated degradation of dysfunctional mitochondria. Glick et al. (2012) have confirmed that elevated lipid synthesis, reduced fatty acids β-oxidation, impaired glucose tolerance, and elevated ROS levels, inflammation response and steatohepatitis are found in liver-specific BNIP3 gene knockout mice. Further research finds that elevated numbers of mitochondria but impaired mitochondrial function are also found in liver-specific BNIP3 gene knockout mice, which is characterized by loss of mitochondrial membrane potential, dysfunctional oxidative phosphorylation and reduced fatty acids β-oxidation (Glick et al., 2012;Boland et al., 2014). These results confirm the critical role of BNIP3 in maintaining mitochondrial integrity, which contributes to the prevention and treatment of metabolic diseases (Figure 2).
Under normoxic conditions, FUNDC1 is located in the endoplasmic reticulum-mitochondria contact sites. In order to cope with hypoxic stress, FUNDC1 associates with the endoplasmic reticulum (ER) protein calnexin (CANX) in mitochondria-associated ER membranes and then recruits DRP1 to promote mitochondrial fission and further activates mitophagy (Palikaras et al., 2018). Although FUNDC1, BNIP3 and NIX can directly interact with LC3 to activate mitophagy, the interaction among FUNDC1, BNIP3 and NIX is still not well explained, and their synergy is very important for mitophagy-mediated mitochondrial quality control (Figure 2).

PHYTOCHEMICALS
Phytochemicals derived from natural plants are often used to prevent and/or treat metabolic disorders due to their unique therapeutic properties and safety (Bacanli et al., 2019). Plentiful medicinal and non-medicinal natural plants have been used to treat diseases from time immemorial in the world on account of the accessibility and low cost. Studies have shown that phytochemicals such as akebia saponin D, quercetin, cyanidin-3-O-glucoside, corilagin, notoginsenoside R1, scutellarin, salvianolic acid B, resveratrol and curcumin show protective effects against metabolic diseases, and their plant origins, effects and molecular mechanisms on metabolic diseases are provided in Table 1.

Non-alcoholic Fatty Liver Disease
Non-alcoholic fatty liver disease (NAFLD) refers to the accumulation of fat in the liver of an individual, which is not caused by excessive alcohol consumption (Younossi et al., 2016;Geng et al., 2021). There is a consensus that NAFLD is the risk factor of non-alcoholic steatohepatitis, cirrhosis and hepatocellular carcinoma. Currently, more and more phytochemicals have been found to target mitophagy to prevent/treat metabolic diseases. For example, akebia saponin D, the major active component of Dipsacus asper Wall. ex Henry, activates BNIP3-mediated mitophagy to alleviate hepatic steatosis in oleic acid-induced buffalo rat liver cells. Liu et al. have observed that quercetin, the most common flavonoid from FIGURE 2 | Diagram illustrates the signaling pathway regulating mitophagy. In damaged mitochondria, PINK1 located in the outer mitochondrial membrane, can phosphorylate Ser65 in the ubiquitin and ubiquitin-like domain of Parkin and further facilitates its localization from the cytosol to the outer mitochondrial membrane. Moreover, Parkin can also promote the ubiquitination of MFN1, MFN2, TOM20 and VDAC to further induce mitophagy. In response to hypoxia and nutritional deprivation, BNIP3 and NIX-mediated mitophagy is induced. In this process, BNIP3 and NIX directly interact with LC3 to enhance mitophagy. And BNIP3 and NIX also bind to the BH3 domain of Beclin1 to activate mitophagy. What's more, similar to BNIP3/NIX, FUNDC1 interacts with LC3 through its LIR to activate mitophagy under hypoxic conditions. • Type 2 diabetes mellitus; • Cardiovascular; • Non-alcoholic fatty liver disease; • Obesity; • Alcohol-induced liver injury; • Ethanol-induced liver steatosis.

Obesity
In 2020, the last report of World Health Organization (WHO) categorizes 1.9 billion adults as overweight and more than 650 million as obese worldwide. Accumulating evidence confirms that mitophagy plays an important role in the regulation of mitochondrial content and function in the setting of obesity. In Parkin −/− mice, Gouspillou et al. show that Parkin ablation causes decreased mitochondrial respiration and increased ROS production in skeletal muscles (Gouspillou et al., 2018;Pileggi et al., 2021). In liver-specific Parkin knockout mice, hepatic steatosis increased by 45% compared with wild-type mice (Edmunds et al., 2020). Although there were no differences in the number of mitochondria in liver between wild-type mice and liver-specific Parkin knockout mice, mitochondrial respiratory were obviously decreased in liver-specific Parkin knockout mice. And whole-body insulin resistance and hepatic insulin resistance were observed in high-fat diet-fed liver-specific Parkin knockout mice (Edmunds et al., 2020). In high-fat dietfed mice, cyanidin-3-O-glucoside, a kind of anthocyanins mainly found in black rice, black beans and purple potatoes, significantly reduces body weight by activating PINK1-mediated mitophagy . Quercetin is proved to inhibit the body weight gain in high-fat diet-fed mice, simultaneously suppress the level of hepatic or serum cholesterol and triglyceride, and inhibit the expression of lipogenic gene fatty acid synthase (FAS) . Quercetin also prevents oleate/palmitateinduced lipid accumulation in HepG2 cells . Mechanistically, Quercetin activates PINK1/Parkin-dependent mitophagy to improve lipid metabolism in the setting of obesity. Since obesity is considered to be a concomitant symptom of various diseases such as metabolic syndrome and NAFLD, only a few studies have focused on it. Therefore, it will be very meaningful to clarify the role of mitophagy-mediated mitochondrial quality control in the prevention and treatment of obesity in future studies (Table 2 and Figure 3).

Type 2 Diabetes
Growing evidence has proved that mitochondrial dysfunction is a risk factor of type 2 diabetes and its complications such as hyperglycemia, diabetic nephropathy and diabetic retinopathy (Rovira-Llopis et al., 2017). Therefore, targeting mitophagy is a promising pharmaceutical strategy for type 2 diabetes and its complications. The Astragalus mongholicus Bunge and Panax notoginseng (Burk.) F.H. Chen formula (APF) consists of Astragalus mongholicus Bunge, Panax notoginseng (Burkill) F.H. Chen, Angelica sinensis (Oliv.) Diels, Achyranthes bidentata Blume, and Ecklonia kurome Okamura. APF significantly improves the blood urea nitrogen, serum creatinine and 24-h albuminuria of mice with diabetic nephropathy, and prevents inflammatory response in high glucose-induced renal mesangial cells and the kidney tissue of mice with diabetic nephropathy. Furthermore, increased expressions of PINK1, Parkin, Beclin1 and LC3-II are recorded in renal mesangial cells and the kidney tissue of

Liu et al., 2018
Cyanidin Frontiers in Cell and Developmental Biology | www.frontiersin.org   FIGURE 3 | Phytochemical activates mitophagy to treat metabolic disorders. Phytochemical activates PINK1-Parkin-dependent mitophagy and BNIP3/NIX-dependent mitophagy to treat metabolic disorders. In PINK1-Parkin-dependent mitophagy, PINK1 phosphorylates Ser65 in the ubiquitin and ubiquitin-like domain of Parkin and further facilitates its localization from the cytosol to the outer mitochondrial membrane of dysfunctional mitochondria. Moreover, Parkin can further promote the ubiquitination of MFN1, MFN2, TOM20 and VDAC, which can be identified by autophagy receptors p62 and then bind to LC3 positive autophagosomes to promote dysfunctional mitochondria to be captured by autophagosomes. Additionally, BNIP3 and NIX are easier to bind to Bcl-2 and Bcl-XL than Beclin1, which causes the release of Beclin-1 from Beclin1-Bcl-2-Bcl-XL complexes and subsequently induces mitophagy.
mice with diabetic nephropathy. These results remind that APF activates PINK1/Parkin-mediated mitophagy to protect the kidney from inflammatory injury in the setting of diabetes mellitus (Wen et al., 2020). Diabetic retinopathy is a serious complication of diabetes and remains the leading cause of blindness worldwide (Zhou et al., 2019a). Notoginsenoside R1, a saponin from Panax notoginseng, is proved to prevent oxidative stress and inflammatory response in high glucosetreated rat retinal Müller cells and the retinas of diabetic db/db mice. Mechanistically, Notoginsenoside R1 improves diabetic retinopathy through activating PINK1-mediated mitophagy (Zhou et al., 2019a). There is a consensus that endothelial cell injury is a critical pathophysiological basis of diabetesrelated vascular disease. Scutellarin, a main composition of Scutellaria baicalensis Georgi, inhibits mitochondrial-mediated apoptosis to increase cell viability of high glucose-induced human umbilical vein endothelial cells (HUVECs). Moreover, scutellarin can activate PINK1/Parkin-mediated mitophagy to improve mitochondrial function characterized by reduced ROS production, enhanced mitochondrial membrane potential and increased SOD activity. Many mechanisms involved in this effect include LC3-II, Atg5, P62, PINK1, Parkin, and MFN2. However, the effect is weakened by PINK1 gene knockdown. These results remind that scutellarin suppresses vascular endothelial cell damage caused by hyperglycemia through activating PINK1/Parkin-dependent mitophagy ( Table 2 and Figure 3; Xi et al., 2021).

Cardiovascular Disease
Cardiovascular disease is the leading cause of death in the world, including cardiac failure, coronary heart disease, myocardial infarction and atherosclerosis (Mir et al., 2021). Atherosclerosis is one of the most common causes of cardiovascular disease (Poznyak et al., 2021). It is known that mitochondrial dysfunction is an important cause of inflammatory response, lipid accumulation and oxidative stress, which are the pathogenic factors of atherosclerosis (Poznyak et al., 2021). Given the critical role of mitophagy in maintaining mitochondrial function in cardiovascular disease, the regulatory mechanism of mitophagy has captured the attention of researchers around the world. Among various pathogenic factors, vascular endothelial injury is the driving force for the development of atherosclerosis, and oxidized low-density lipoprotein (ox-LDL) plays a key role in this process (Fang et al., 2014). Delphinidin-3-O-β-glucoside, a natural anthocyanin, is abundant in black soybean, bilberries and cereals . Jin et al. have demonstrated that delphinidin-3-O-β-glucoside suppresses ox-LDL-induced cell death in human umbilical vein endothelial cells. Further studies have suggested that activated AMPK/SIRT1-dependent mitophagy is the pivotal molecular mechanism for the protective effect of delphinidin-3-O-β-glucoside on human umbilical vein endothelial cells (Jin et al., 2014). Myocardial ischemic injury, a common kind of cardiovascular disease, can cause irreversible damage to heart. It is known that NLRP3 inflammasomemediated inflammatory response is the pivotal mechanism of the development of myocardial ischemic injury (Zhu et al., 2015;Yao et al., 2020). Salvianolic acid B, an active constituent of Salvia miltiorrhiza Bge, significantly prevents acute myocardial ischemic injury in rats with isoproterenol-induced acute myocardial ischemia. In lipopolysaccharide + adenosine triphosphate-administrated H9C2 cells, salvianolic acid B markedly prevents ROS production, NLRP3 inflammasomemediated inflammatory response and cell apoptosis, and enhances mitochondrial membrane potential. Further evidence has confirmed that salvianolic acid B promotes SIRT1-mediated mitophagy to restore mitochondrial function, thereby preventing myocardial ischemic injury . Accumulating evidence has demonstrated that NLRP3 inflammasomeregulated inflammatory responses are responsible for the development of atherosclerosis. Recent evidence reminds that mitochondrion is a critical regulator for inflammatory responses because mitochondrial ROS is considered as a potent activator of NLRP3 inflammasome (Gurung et al., 2015;Jin et al., 2017). It is known that mitophagy plays a pivotal role in maintaining mitochondrial function though selectively eliminating dysfunctional mitochondria. Therefore, mitophagy is primarily considered as a regulator for NLRP3 inflammasome-mediated inflammatory response through scavenging mitochondrial ROS (Zhou et al., 2011;Kim et al., 2016). Ma et al. verify that melatonin activates Sirt3/FOXO3/Parkin-mediated mitophagy to scavenge excessive mitochondrial ROS, thereby suppressing NLRP3 inflammasome activation and then ameliorating atherosclerosis . Chen et al. also confirm that melatonin protects vascular smooth muscle cells against calcification by promoting AMPK/OPA1-dependent mitophagy.
In ox-LDL-administrated human umbilical vein endothelial cells, cell apoptosis, cell proliferation arrest, impaired mitochondrial respiration, excessive mitochondrial ROS and mitochondrial dysfunction are observed. However, resveratrol, a potent natural antioxidant from Vitis vinifera L., enhances the expressions of hypoxia-inducible factor-1 (HIF-1) and AMPK protein to activate BNIP3-related mitophagy, thereby promoting mitochondrial respiration, scavenging excessive mitochondrial ROS and favoring endothelial cell survival . The increased senescence of vascular endothelial cells evoked by high glucose and palmitate will cause endothelial dysfunction, leading to diabetic cardiovascular complications. The traditional Chinese medicine Ginseng-Sanqi-Chuanxiong (GSC) is composed of Panax ginseng C. A. Mey., Panax notoginseng (Burk.) F. H. Chen, and Ligusticum chuanxiong Hort. at a ratio of 2:3:4. Wang et al. find that GSC extract inhibits high glucose and palmitate-induced vascular endothelial cell senescence via activating AMPK-dependent mitophagy to suppress mitochondrial ROS production (Table 2 and Figure 3; Wang et al., 2020). At present, more and more clinical trials have confirmed the efficacies and mechanisms of phytochemicals for metabolic diseases, such as quercetin, melatonin and resveratrol (Supplementary Table 1). However, as more and more phytochemicals have been used to prevent or treat metabolic disorders, their side effects should also be taken seriously. For example, high-dose of quercetin can cause liver damage in an animal model of hyperhomocysteinemia (Meng, 2013). High concentration of resveratrol (50 µM) suppresses the cell viability of transformed macrophages and carcinoma cells. However, low concentration of resveratrol (5 µM) promotes the cell viability of these cells (Shaito et al., 2020). Moreover, low concentration of resveratrol (0.5-5µM) has no significant effect on the viability or function of rat pancreatic cells, while high concentration of resveratrol (50 µM) increases pancreatic FIGURE 4 | Mechanisms of phytochemical-mediated mitophagy in the treatment of metabolic disorders. Phytochemical activates mitophagy to degrade damaged mitochondria to prevent the production of mitochondrial ROS which can trigger oxidative stress and inflammatory response, and eventually preventing and treating metabolic diseases. cell apoptosis (Shaito et al., 2020). Surprisingly, resveratrol can stimulate oxidative stress to induce mitochondrial-mediated cancer cell apoptosis (Ashrafizadeh et al., 2020). In conclusion, exploring the optimal phytochemical dosage and the different molecular mechanisms shown in different disease models can maximize its health benefits without causing toxicity problems, which will be an area worthy of in-depth research.

SUMMARY AND FUTURE PERSPECTIVES
Mitochondrion is an important organelle responsible for various cellular processes including ATP generation, energy metabolism, ROS production and Ca 2+ homeostasis, cell survival and death. Accumulating evidence proves that mitochondrial dysfunction is involved in various metabolic disorders such as NAFLD, obesity, type 2 diabetes and cardiovascular disease. Mitophagy, a major mechanism of mitochondrial quality control, can selectively degrade dysfunctional mitochondria to maintain mitochondrial integrity and function. Generally speaking, mitophagy selectively degrades damaged mitochondria to suppress damaged mitochondria-derived ROS which will significantly damage healthy mitochondria and ultimately resulting in mitochondrial dysfunction. Despite increasing evidence has greatly improved our understanding of the underlying mechanisms involved in the regulation of mitophagy in metabolic disorders, some adverse results are also reported. Montgomery et al. consider that mitophagy-mediated elimination of damaged mitochondria inevitably decreases mitochondrial number, leading to decreased substrate oxidation, and finally impairing mitochondrial function (Montgomery and Turner, 2015). Now the association between mitophagy and mitochondrial homeostasis is still not fully understood, in part due to various methods for detecting mitochondrial function, different cell models and disease states (Chow et al., 2010;Montgomery and Turner, 2015). Fortunately, current studies have demonstrated that activated mitophagy will prevent damaged mitochondria-derived ROS triggering oxidative stress and inflammatory response, which are the leading causes of metabolic diseases. However, the research focused on the role of phytochemical-mediated mitophagy in the prevention and treatment of metabolic diseases is limited, and the definite relationship between phytochemical-regulated mitophagy and the treatment of metabolic diseases still needs further experimental confirmation. One thing is for sure, looking for natural compounds with mitophagic activities will provide new insights into the therapeutic intervention for mitochondrial dysfunction-related metabolic diseases (Figure 4).

AUTHOR CONTRIBUTIONS
ZS, YG, XH, GZ, and YZ conceived of the topic for the review. All authors listed have made a substantial direct and intellectual contribution to the work, and approved it for publication.