Natural autophagy modulators in non-communicable diseases: from autophagy mechanisms to therapeutic potential

Abstract

Non-communicable diseases (NCDs) are defined as a kind of diseases closely related to bad behaviors and lifestyles, e.g., cardiovascular diseases, cancer, and diabetes. Driven by population growth and aging, NCDs have become the biggest disease burden in the world, and it is urgent to prevent and control these chronic diseases. Autophagy is an evolutionarily conserved process that degrade cellular senescent or malfunctioning organelles in lysosomes. Mounting evidence has demonstrated a major role of autophagy in the pathogenesis of cardiovascular diseases, cancer, and other major human diseases, suggesting that autophagy could be a candidate therapeutic target for NCDs. Natural products/phytochemicals are important resources for drugs against a wide variety of diseases. Recently, compounds from natural plants, such as resveratrol, curcumin, and ursolic acid, have been recognized as promising autophagy modulators. In this review, we address recent advances and the current status of the development of natural autophagy modulators in NCDs and provide an update of the latest in vitro and in vivo experiments that pave the way to clinical studies. Specifically, we focus on the relationship between natural autophagy modulators and NCDs, with an intent to identify natural autophagy modulators with therapeutic potential.

Introduction

Non-communicable diseases (NCDs), also known as chronic diseases, often persist for a long time and are the result of a combination of genetic, physiological, environmental, and behavioral factors. According to the World Health Organization report, NCDs currently pose one of the greatest threats to global health and development, and the main types of NCDs are cardiovascular diseases (CVD), cancer, chronic respiratory diseases, diabetes and neurological disorder [1, 2]. With the rapid growth and aging of the population, NCDs have accounted for three-quarters of all deaths. As estimated, if this trend continues, this proportion will expand to more than 80% globally in 2048 [1]. In low- and lower-middle-income countries, the burden of NCDs continues to rise disproportionately, with 47% (7 million) of premature deaths from NCDs occurring [3]. Due to factors such as aging population, rapid urbanization, the pollution of air, soil, and water and unhealthy lifestyles, it is expected that the global burden of chronic diseases will continue to increase [1, 4, 5].

Autophagy is the major intracellular degradative pathway by which cytoplasmic substances are delivered to and degraded in the lysosome [6]. The purpose of autophagy is not simply to eliminate substances, but rather to act as a dynamic circulatory system that generates new building blocks and energy for cell renewal and homeostasis [7]. It is now widely accepted that autophagy is a primordial determinant of human health, and autophagy regulatory interventions are defined as promising approaches for the prevention or mitigation of phenotypic abnormalities in the most common human diseases [8]. Autophagy can be categorized into macroautophagy, microautophagy, and chaperone-mediated autophagy based on the different pathways by which the substrate enters the lysosome. Macroautophagy is characterized by the isolation of cytoplasmic material in a double-membrane autophagic vesicle, termed the autophagosome, which is subsequently delivered to the lysosome, where they are broken down [9, 10]. Chaperone-mediated autophagy refers to the specific process by which individual cytoplasmic proteins carrying KFERQ-like motifs are delivered to the lysosome for degradation by binding to chaperone proteins [11]. During microautophagy, however, autophagic cargoes are taken up directly by lysosomes and late nuclear endosomes through membrane protrusion and invagination, and autophagic cargoes are degraded in the endolysosomal lumen [11]. Both microautophagy and macroautophagy can be selective or non-selective [12]. Under starvation conditions, non-selective autophagy is used for the turnover of large amounts of cytoplasm, while selective autophagy specifically targets damaged or redundant organelles, such as mitochondrial autophagy/mitophagy [9]. Since macroautophagy is the main form of autophagy, herein we refer to macroautophagy simply as autophagy.

Autophagy is a multi-step process, which can be induced either by inhibiting the nutrient sensor mechanistic target of rapamycin complex 1 (mTORC1) or by activating the energy sensor AMP-activated protein kinase (AMPK). In mammalian cells, this process depends on multiple autophagy-related (ATG) proteins and is typically divided into distinct stages: initiation, nucleation of the autophagosome, elongation and maturation of the autophagosome membrane, fusion with the lysosome, and degradation and recycling of autolysosome products (Fig. 1). The initiation of autophagosome is promoted by the Unc-51 like kinase 1 (ULK1) complex, which involves ULK1, ATG13, FIP200 and ATG101 [13, 14]. Next, it phosphorylates class III phosphatidylinositol 3-kinase (PI3K) complex I, activating it and triggering nucleation [15, 16]. Then PI3KC3 complex I generates phospholipid-phosphatidylinositol (PI3P) to commence phagophore membrane extension with the help of transmembrane protein 41B and ATG2A, which delivers lipids to the PI3P WD repeat domain phosphoinositide-interacting (WIPI2) complex [17]. The elongation of the autophagosome is facilitated by two ubiquitin-like coupling modules, ATG12-ATG5 and LC3-PE complex [15]. The first relies on the activity of ATG7 and ATG10 and is able to form a multi-protein complex consisting of ATG12-ATG5-ATG16L1 [8]. Next, WIPI2 localizes to the autophagosome and recruits ATG12-ATG5-ATG16L1 complex to the autophagosome membrane [17]. Then the complex is sequestered and induces the autophagosome elongation, followed by the dissociation from the intact autophagosomal membrane [18]. In addition, pro-LC3 is cleaved by ATG4 to LC3-I with a C-terminally exposed glycine residue. ATG7 and ATG3 act as E1- and E2-like enzymes, respectively, and the ATG12-ATG5-ATG16L1 complex acts as an E3-like enzyme ultimately catalyzes the binding of LC3-I to PE via ATG3, producing the lipidated form of LC3-II, which is immediately recruited to the membrane [17]. Moreover, ATG4 can also cleave LC3-II to release LC3-I to the cytoplasm for reuse [19]. Additionally, the transmembrane protein ATG9 also facilitates the extension of the phagophore membrane [20]. LC3-II acts as a receptor for LC3-interacting region-containing proteins, including autophagy substrates and receptors such as sequestosome 1 (SQSTM1)/p62, to deliver cargo to autophagosomes [8]. Subsequently, the membrane of the phagosome closes to form an intact double-membrane autophagosomes [21]. The intact autophagosome is then translocated to the lysosome, where the outer membrane of the autophagosome fuses with the lysosomal membrane to form an autolysosome [17, 21]. Ultimately, acidic lysosomal hydrolases degrade autophagic substrates [15]. Most of these steps or ATG proteins in the autophagy process represent potentially druggable targets, which providing ways for regulating autophagy both positively and negatively.

Fig. 1. Overview of the autophagy process.

The initiation of autophagy begins when either the mTORC1 is inhibited or the energy sensor AMPK is activated. Activation of AMPK or inhibition of mTORC1 under stress conditions leads to activation of the ULK1 complex, the first complex in the autophagy pathway. Afterwards, ULK1 complex phosphorylates and activates the BECN1/Beclin1-PI3KC3/Vps34 lipid kinase complex to promote the nucleation of autophagosome. Then, the ATG9 trafficking system, the ATG12-ATG5, and ATG8/LC3-PE ubiquitin-like conjugation systems are required for autophagosome elongation and completion. Autophagosome maturation is followed by fusion with the lysosome, culminating with the degradation of inner autophagosome membrane and its contents, allowing recycling of macromolecules within.

Autophagy exists in both physiologic and pathophysiologic conditions, and the role of autophagy in various NCDs has gradually attracted people’s attention, such as CVD [22], cancer [23], diabetes [24], chronic obstructive pulmonary disease (COPD) [25], and neurogenic diseases [26]. For example, autophagy under baseline conditions, which permits the recycling and clearance of damaged proteins and organelles, is an important homeostatic mechanism for maintaining normal cardiovascular function and morphology. Particularly in the context of cardiac aging, there is an increasing need for degradation of damaged proteins and organelles [27]. In contrast, the protective effects of autophagy and the deleterious effects of over-activation coexist under environmental or intracellular stress [27, 28]. As for cancer, autophagy plays a dual role and its role in preventing early tumor development appears to be opposite to that of maintenance and metabolic adaptation in established and metastatic tumors. Recent studies have explored not only the intrinsic function of autophagy in tumor cells but also the role of autophagy in the tumor microenvironment and associated immune cells [29]. Similarly, autophagy plays an important role in the inflammatory response of the lung to infection and stress. At baseline, autophagy may be critical to enhance primary cilia growth [30]. However, when unregulated, persistent or inefficient autophagy may lead to ciliated cell loss and death [31], which is deleterious to lung epithelial cells and promotes lung injury. Therefore, there is a growing need to discover and to develop potent autophagic modulators for the treatment of NCDs.

Natural products have long been used for medical purposes because of their unique advantages, which include special chemical and structural diversity, low toxicity, and few side effects compared to chemical drugs [32]. Meanwhile, their application in the treatment of diseases is attributed to their powerful pharmacological effects such as anti-oxidant [33], anti-inflammatory [34], anti-bacterial [35], anti-viral [36], and immuno-modulatory effects [36], including resveratrol, polyphenols, terpenoids, alkaloids, polysaccharides, and others. In recent years, there has been an avid search for autophagy regulators in human diseases, and natural compounds have been extensively studied in different autophagy models as an important source for drug discovery.

Here, we provide an overview of the therapeutic effects of natural autophagy modulators in these NCDs, together with their general mechanisms of action. The structures of these natural autophagy modulators are shown in Figs. 2–4.

Fig. 3.

Structure of autophagy modulators of terpenoids, quinones, partial triterpenes, and others.

Fig. 2.

Structure of autophagy modulators of flavonoids, alkaloids, and polyphenols.

Fig. 4.

Structure of autophagy modulators of triterpenes.

The role of autophagy in NCDs

As a homeostatic pathway that promotes the degradation and recycling of cellular material, the role of autophagy in the physiology and pathology of organisms is gradually being recognized. A growing body of evidence supports the idea that autophagy is involved in a variety of physiological processes, including cellular metabolism, cell survival, and host defense [37, 38]. Besides, under conditions of cellular stress, such as starvation, hypoxia, oxidative stress, and especially nutritional deficiencies, autophagy mechanisms are activated to maintain cellular homeostasis [39–42]. Therefore, the potential pathogenicity of autophagy deficiencies is increasingly being emphasized. For instance, basal autophagy is a homeostatic mechanism that maintains normal function of the cardiovascular system. Under stress conditions, up-regulation of autophagy protects cells from hemodynamics stress. However, autophagy defects, whether caused by pharmacological or genetic manipulation, have been shown to increase the tendency of experimental animals to develop spontaneous CVD [43–46]. For example, myocardial specific deficiency of Atg5 could cause myocardial hypertrophy, left ventricular dilation, and systolic dysfunction in mice [45, 47, 48]. Likely, autophagy is also a key process regulating the homeostatic function of pancreatic β cells. Mechanistically, defective autophagy results in the inability of β cells to make an adequate unfolded protein response, which is essential for maintaining the hypersecretory phenotype of insulin-secreting β cells, while the proficient autophagic response may facilitate the initiation of an antioxidant program of pancreatic β cells to withstand the cumulative oxidative burden associated with high fat diet (HFD) [8]. A study has demonstrated that genetic ablation of Atg7 in β cells resulted in degeneration of islets and impaired glucose tolerance with reduced insulin secretion, suggesting basal autophagy is important for maintenance of normal islet architecture and function [49]. In contrast, restoration of the Atg7 expression increased hepatic autophagy in obese mice and improved systemic insulin sensitivity [50].

However, the link between cancer and autophagy is complex. Autophagy in healthy cells is likely to be a tumor suppressor mechanism used to counteract the effects of pro-carcinogenic stimuli, which may also be one of the reasons why various mTOR inhibitors are currently being used in anticancer therapy. But it has been shown that established tumors require the autophagic process to support uncontrolled cell growth and increased metabolic activity [8, 29]. Thus, inhibiting autophagy could be an effective way for cancer control. Our and others’ studies have identified an important role for ATG4 in autophagy regulation, which is a potential anticancer target, and its effects on tumor growth and proliferation have been widely discussed. Especially, ATG4B is considered as an oncogene in CRC [51], chronic myeloid leukemia (CML) [52], osteosarcoma [53] and human epidermal growth factor receptor 2 (HER2) positive breast cancer cells [54], and knocking down ATG4B or using small molecule inhibitors of ATG4B could reduce the survival rate of these tumor cells [51, 55–58]. Thus, the role of autophagy in cancer development may be context dependent [59–62].

As mentioned above, autophagy is considered to be crucial for host defense, especially the regulation of inflammation. Therefore, the role of autophagy in COPD should not be overlooked. COPD is an irreversible inflammatory lung disease associated with cigarette smoke (CS). Inflammation in the airways of smokers can persist for years even after quitting, while the inflammatory response is prolonged and exacerbated in COPD patients. However, the interpretation of the role of autophagy in the pathogenesis of COPD is controversial [63]. On the one hand, although impaired autophagy could protect against CS-induced cilia shortening and cell apoptosis [31], it might also cause cell senescence and aggregation of ubiquitinated proteins, thereby aggravating the condition of COPD [25, 64]. On the other hand, over-activated autophagy could lead to increased cell death or apoptosis and cilia damage, which in turn triggers and exacerbates airway inflammation [31, 65]. Additionally, autophagy has also been shown to be associated with many of the pathologic changes in neurodegenerative disorders (NDDs) due to its protein degrading function. Indeed, many neurodegenerative diseases are characterized by the accumulation of abnormal protein aggregates (e.g., amyloid β (Aβ), tau, α-synuclein, polyglutamine proteins and SOD1), which are substrates for autophagic degradation [66, 67]. Many studies have shown that mutations in the core ATG gene will trigger neurodegenerative diseases [66, 68, 69]. Heckmann et al. have observed Aβ aggregation and Tau hyperphosphorylation in mice deficient in the WD domain of ATG16L, resulting in spontaneous AD [68]. At the same light, Nilsson et al. have found Aβ secretion was suppressed in Atg7 deficient mice, leading to Aβ accumulation in cells that might cause or exacerbate AD [69]. In addition, ATG7 polymorphism was reported to be associated with early-onset HD [70].

Given that the role of autophagy in the development of various human diseases has already been well summarized [22, 29, 67, 71], this review will not go into too much detail.

Natural products that regulate autophagy in NCDs

Due to its structural diversity and complexity, natural products have provided a valuable resource for drug discovery for a long time. Therefore, in-depth research on the mechanisms of natural compounds in preventing various diseases may have significant therapeutic benefits for medical practice. To date, a growing body of evidence suggests that abundant natural products are involved in the autophagy regulation, inducing or inhibiting autophagy through a variety of signaling pathways and transcriptional factors. Various phytochemicals of natural origin, such as resveratrol, curcumin, ursolic acid, thymoquinone, and trehalose, have also been recognized as promising autophagy modulators. In the following part, we have summarized the effects and molecular mechanisms of these natural autophagy modulators in NCDs.

Natural products that regulate autophagy in CVD

Various natural products act as autophagy activators to induce autophagy and protect myocardial cells from damage, but some also act as autophagy inhibitors to suppress the damage caused by over-activated autophagy. Here, we summarize the role of various natural products in CVD through autophagy regulation and the mechanisms of action was drawn in Fig. 5.

Fig. 5.

Effects of natural autophagy modulators on the different steps of autophagy pathway in different cardiovascular disease states, including myocardial dysfunction, DCM, cardiac I/R injury, myocardial injury, H/R injury, and cardiac remodeling and hypertrophy.

Protection of cardiomyocytes by inducing autophagy

Numerous studies have demonstrated the unique role of melatonin in a variety of CVD. Melatonin, a hormone found in many organisms, from algae to human, was reported to prevent sepsis-induced cardiac dysfunction by inhibiting apoptosis and activating autophagy via activation of sirtuin1 (SIRT1) [72]. Another study suggested that melatonin protected against LPS-induced septic myocardial injury by activating AMPK mediated autophagy pathway [73]. In addition, melatonin could attenuate cardiac ischemia/reperfusion (I/R) injury by activating optic atrophy 1 (OPA1)-related mitochondrial fusion and mitophagy in a manner dependent on the AMPK signaling pathway [74]. In T1DM mice model, melatonin could attenuate diabetic cardiomyopathy (DCM) via increasing the autophagy level of cardiomyocytes, which was manifested by increasing the ratio of LC3-II/LC3-I and down-regulating the expression of p62 [75]. In addition, along with melatonin, asiaticoside, a triterpenoid saponin compound isolated from Centella asiatica, was found to alleviate DCM by enhancing autophagy through activating the AMPK-nuclear factor erythroid 2-related factor 2 (Nrf2) pathway [76].

Polyphenols are another common class of natural products that protect cardiomyocytes through autophagic activation. In diabetic mice, resveratrol, a natural polyphenolic compound, has been shown to significantly inhibit the increase of cardiac SQSTM1/p62, which is associated with autophagic degradation dysfunction, in order to ameliorate myocardial autophagic flux deficits and reduce mortality [77, 78]. Moreover, high-dose resveratrol has been found to partially reverse left ventricular remodeling and significantly improve cardiac function by activating AMPK and enhancing autophagy [79]. Another polyphenolic compound, oleuropein aglycone, has been thought to be an autophagy inducer that activated transcription factor EB (TEFB) to protect against monoamine oxidase-A (MAO-A) enzyme-induced autophagy injury and cardiomyocyte death [80].

Other natural products that have been shown to have cardiovascular protective effects include flavonoids, polysaccharides, and terpenoids. Tanshinone IIA (Tan-IIA) is a representative liposoluble component isolated from Salvia miltiorrhiza, which has a significant protective effect on the cardiovascular system [81]. It was found that Tan-IIA inhibited cell apoptosis and induced autophagy by activating the AMPK-mTOR signaling pathway, thereby protecting myocardial cells and improving cardiac function [82]. Likewise, luteolin, a flavonoid polyphenolic compound, has been found to ameliorate lipopolysaccharide (LPS)-induced myocardial injury by increasing autophagy via AMPK signaling [83]. Similarly, thymoquinone, an active ingredient of black cumin (Nigella sativa) seed oil, has been reported to alleviate I/R injury through activation of autophagy as demonstrated by increasing LC3-II and reducing p62 levels [84]. Trehalose is a kind of natural non-reducing sugar that exists widely in many edible plants, animals, and microorganisms. It was reported that trehalose could induce autophagy by upregulating TFEB to reduce cardiac remodeling, dysfunction, and cardiac hypertrophy caused by chronic myocardial infarction (MI) [85]. In addition, oleanolic acid, a natural triterpenoid compound, has been shown to attenuate aging-induced cardiac remodeling and contractile dysfunction through regulation of FUN14 domain containing 1 (FUNDC1)-dependent mitophagy [86].

Protection of cardiomyocytes by inhibiting autophagy

Unlike the mechanism of activating autophagy mentioned above, many natural products protect the myocardium by inhibiting over-activated autophagy. Berberine, a natural extract from Rhizoma coptidis, could protect against I/R-induced MI by selectively inhibiting excessive autophagy in cardiomyocytes [87]. Additionally, purerarin, the main bioactive ingredient extracted from the root of Pueraria lobata (Willd.) Ohwi could also reduce myocardial hypoxia/reoxygenation injury by autophagy inhibition through the Akt signaling pathway [88]. Another antioxidant polyphenol compound, uridine B, has been reported to inhibit cardiomyocyte autophagy by activating Akt-mTOR-ULK1 pathway and inhibiting I/R-induced autophagy. Thus, the accumulation of p62, the interaction between p62 and kelch-like ECH-associated protein 1 (Keap1), and the nuclear translocation of Nrf2 were promoted, which led to the increase of downstream antioxidant enzyme levels and played a protective role in myocardial injury [89]. Similarly, Hongjingtian injection has been also found to protect against myocardial I/R-induced apoptosis by blocking ROS induced autophagic flux through improving mitochondrial function and regulating AMPK-mTOR pathway [90]. Moreover, Lactone component from Ligusticum chuanxiong is the major constituent of the traditional Chinese herb L. chuanxiong Hort., which could alleviate myocardial ischemia injury by inhibiting autophagy through activation of PI3K-Akt-mTOR signaling pathway [91]. Ginsenoside Rb1 is one of the most abundant pharmacological components in ginseng, which could inhibit autophagy in rat cardiomyocytes by activating the Akt-mTOR pathway, thus exerting its anti-heart failure function [92]. Similarly, allicin, the main biological activity substance in garlic, was found to protect against heart hypertrophy by suppressing autophagy through activating the PI3K-Akt-mTOR signaling pathways [93]. Hydroxysafflor yellow A (HYSA) has been reported to be a representative bioactive component of Luhong formula that could regulate autophagy and protect cardiomyocytes from hypoxia injury [94]. And it could also inhibit autophagy in cardiomyocytes by inhibiting cellular levels of HIF1α-mediated ROS [94]. Overall, most of these natural products act on the PI3K-Akt-mTOR pathway, causing autophagy inhibition to protect cardiomyocytes from damage caused by various stresses, especially oxidative stress.

Collectively, while most studies have identified a role for restoration/activation of autophagic flux in the fight against cardiovascular disease, other studies have observed inhibition of autophagy (Table 1). This may be due to the fact that in addition to their effects on autophagy, these natural compounds may have other beneficial effects such as metabolic regulation, anti-inflammatory and antioxidant modulation. At the same time, over-activation or insufficient autophagy may exist in different stages of a given CVD. For example, autophagy may be protective during ischemia, whereas it may be detrimental during reperfusion. Therefore, whether these natural autophagy activators or inhibitors should be used needs to be judged according to the specific situation.

Table 1.

Study of natural autophagy modulators in CVD.

Natural product	Model	Molecular target/mechanism	Effect on autophagy	Pathophysiological effect	Reference
Melatonin	Male C57BL/6 mice in vivo	Increasing SIRT1 protein expression	Induction	Protecting against sepsis-induced cardiac dysfunction	[72]
	Male C57BL/6 mice in vivo; Primary neonatal rat cardiomyocytes in vitro	AMPK-mediated	Induction	Significantly increasing the survival rate after LPS-induced shock and ameliorating myocardial dysfunction; Decreasing the release of inflammatory cytokines in the sepsis model	[73]
	Cardiac-specific opa1 knockout mice in vivo; Primary cardiomyocytes in vitro	AMPK‐OPA1 signaling pathways activation	Induction	Maintaining myocardial function and cardiomyocyte viability; Reducing cardiac I/R injury	[74]
	T1DM mice in vivo	VEGF-B-GRP78-PERK signaling pathway	Induction	Attenuating DCM by increasing autophagy of cardiomyocytes via regulating VEGF-B-GRP78-PERK signaling pathway	[75]
Asiaticoside	DCM mice in vivo	AMPK-Nrf2 pathway	Induction	Alleviating DCM myocardial injury by enhancing autophagy	[76]
Resveratrol	WT male C57BL/6J mice in vivo	SIRT1-FoxO1-Rab7	Induction	Ameliorating deficient myocardium autophagic flux and attenuating mouse mortality	[78]
	Male C57BL/6J mice in vivo	Activating AMPK	Induction	Reversing left ventricular dilation (reverse remodeling) and significantly improving cardiac function	[79]
Oleuropein aglycone	Cardiomyocytes with overexpression of MAO-A in vitro	Activation of TFEB	Induction	Protecting against MAO-A induced cardiotoxicity	[80]
Tanshinone IIA	SD male rats in vivo	Activation of the AMPK-mTOR signaling pathway	Induction	Protecting against heart failure post-MI	[81, 82]
Luteolin	Male C57BL/6 mice in vivo	AMPK	Induction	Ameliorating LPS-induced myocardial injury	[83]
Thymoquinone	Male Wistar rats in vivo	-	Induction	Reducing myocardial infarct size and myocardial enzyme activities induced by I/R injury	[84]
Trehalose	WT or beclin1+/- mice in vivo	Up-regulation of TFEB	Induction	Improving cardiac remodeling after MI	[85]
Oleanolic acid	Male WT C57BL/6 J mice and male global fundc1 knockout (fundc1-/-) mice in vivo; Cardiomyocytes in vitro	FUNDC1-dependent mitophagy	Induction	Attenuating aging-induced cardiac remodeling and contractile dysfunction	[86]
Berberine	Male C57BL/6 mice in vivo; H9C2 cells in vitro	Dephosphorylation of AMPK and mTORC2； Decreasing the expression level of ATG proteins	Inhibition	Alleviating cardiac I/R injury	[87]
Puerarin	Neonatal rat cardiomyocytes in vitro	Increasing the level of Akt	Inhibition	Attenuating myocardial hypoxia/reoxygenation injury	[88]
Urolithin B	Male adult SD rats in vivo	Increasing p62 accumulation, p62-Keap1 interaction and Nrf2 nuclear translocation	Inhibition	Protecting against myocardial I/R injury	[89]
Hongjingtian injection	C57BL/6J and CAG-RFP-EGFP-LC3 transgenic C57BL/6J mice in vivo; H2O2-induced H9C2 cells in vitro	AMPK-mTOR	Inhibition	Protecting against myocardial I/R-induced apoptosis	[90]
Lactone component from Ligusticum chuanxiong	Male SD rats in vivo; H9C2 cells in vitro	Activation of PI3K-Akt-mTOR	Inhibition	Alleviating myocardial ischemia injury	[91]
Ginsenoside Rb1	Male SD rats in vivo	Activation of RhoA-ROCK and PI3K-Akt-mTOR pathways	Inhibition	Exerting anti-HF function	[92]
Allicin	Male Wistar rats in vivo; Neonatal rat cardiac myocytes in vitro	Activation of PI3K-Akt-mTOR	Inhibition	Protecting against cardiac hypertrophy	[93]
Hydroxysafflor yellow A	Male Wistar rats in vivo; H9C2 and HeLa cells in vitro	Suppressing HIF1α-mediated ROS production	Inhibition	Protecting cardiomyocytes from hypoxia injury	[94]

Natural products that regulate autophagy in cancer

Natural products can usually exert different anti-tumor effects by regulating autophagy. On the one hand, natural autophagy modulators could inhibit tumor cell proliferation and promote cell apoptosis. On the other hand, natural autophagy modulators could overcome cancer drug resistance and improve the effectiveness of chemotherapy drugs [95]. In this section, we recapitulate the reported natural products that regulate autophagy in cancer and illustrate the mechanism of action in Fig. 6.

Fig. 6.

Effects of natural autophagy modulators on the different steps of the autophagic pathway in different types of cancer, including gastrointestinal cancer, lung cancer, breast cancer, cervical cancer, and other cancers.

Inhibiting tumor cell proliferation

The PI3K-Akt-mTOR signaling pathway is important for the survival and proliferation of cancer cells, which has become a target for many natural autophagy modulators to exert anti-tumor effects. Especially, multiple natural compounds have been found to display anti-hepatocellular carcinoma (HCC) activity by promoting autophagy via modulating PI3K-Akt-mTOR signaling pathway, such as apigenin [96], brusatol [97] and mallotucin D [98]. And uvangoletin, a dihydrochalcone extracted from Sarcandra glabra, could induce autophagy by inhibiting Akt-mTOR and mitogen-activated protein kinase (MAPK) signaling pathways, and inhibit migration and invasion of HepG2 cells, thereby exerting anti-HCC activity [99]. In addition, peruvoside could induce apoptosis and autophagy via inhibiting PI3K-Akt-mTOR signaling in breast, lung, and liver cancer cells [100].

There are also many natural autophagy modulators that exert anti-tumor effects at different sites by targeting various targets on the autophagy pathway. Many types of tumors may be amenable to therapeutic effects by modulating autophagy such as colon, breast, lung and cervical cancers. Platycodin D is the main component of triterpene saponins in Platycodi radix, which could induce apoptosis via promoting autophagy in colon cancer cell by increasing the levels of Beclin1 and LC3-II/I [101]. A recent study found that peanut skin extracts could induce autophagy to regulate the cytoprotective effects in melanoma and colorectal cancer cells [102]. Oblongifolin C is a polycyclic polyprenylated acylphroglucinol analog and an autophagy inhibitor that could inhibit autophagy flux by blocking autophagosome-lysosome fusion and inhibiting lysosomal protein hydrolysis activity [103]. And it has been shown to quickly kill nutrient deprived cancer cells through autophagy inhibition [103]. Similarly, liensinine, a major isoquinoline alkaloid, could also inhibit late autophagy/mitophagy by blocking autophagosome-lysosome fusion, and its combination with chemotherapy has been shown to significantly decrease the viability of breast cancer cells and increase apoptosis [104]. Meanwhile, in addition to the above-mentioned anti-hepatocarcinogenic effects, quercetin also inhibits cell migration and glycolysis through autophagy induction mediated by the Akt-mTOR pathway, thereby inhibiting the occurrence and development of breast cancer [105]. Additionally, polyphyllin D, an active component from rhizome of Paris polyphylla Sm., which could induce caspase-dependent apoptosis and cytoprotective autophagy by activation of JNK1-Bcl2 pathway in breast cancer cells [106]. When it comes to HYSA, it has been reported to induce autophagy and reduce HCC cell viability by promoting Beclin1 expression and inhibiting ERK phosphorylation [107]. Similarly, baicalein, a natural compound extracted from Scutellaria baicalensis Georgi, has been found to cause apoptosis and autophagy to exert anti-lung cancer effect by activating AMPK pathway and enhancing fatal dynamin-related protein 1 (DRP1)-mediated mitochondrial fission [108].

One example that illustrates the complex relationship between autophagy regulation and apoptosis is licorice chalcone A (LicA). LicA, an extract of the medicinal plant Licorcice, could promote apoptosis of cervical cancer cells [109]. It has been found that LicA enhanced the expression of LC3-II, Beclin1, ATG5 and ATG7 in human cervical cancer cells, indicating LicA could induce autophagy [110]. However, the combination therapy of LY294002/autophagy inhibitors enhanced LicA-induced cell apoptosis by inhibiting PI3K-Akt or mTOR [110], suggesting a protective role of autophagy in LicA-induced apoptosis. Examples like this include artesunate [111] and aescin [112], both of which have been reported to enhance tumor cell apoptosis by combining with autophagy inhibitors. Similarly, chrysin, a flavonoid extracted from the Bignoniaceae plant oryx, has been found to synergistically inhibit HER2 positive breast cancer cell survival and proliferation in vitro and in vivo by augmenting autophagy in combination treatment with pyrotinib [113]. Besides, artesunate could also induce autophagy in a ROS-dependent manner to inhibit cell proliferation in CRC [114].

Additionally, the anti-tumor effects of resveratrol has been extensively researched. In oral cancer, resveratrol was shown to inhibit cell proliferation and induce autophagy by blocking the expression of SREBP1 in oral cancer cells [115]. In another study, resveratrol-induced autophagy was shown to suppress the epithelial-mesenchymal transition process, tumor invasion, and metastasis of breast cancer by activating the SIRT3/AMPK pathway [116]. In cholangiocarcinoma, cancer associated fibroblasts could inhibit the autophagy of cholangiocarcinoma cells and stimulate their migration by secreting IL-6. However, resveratrol could reverse the pro-migratory effect of IL-6 and induce autophagy in cholangiocarcinoma cells and cholangiocytes [117].

Overcoming the drug resistance of tumor cells

Tumor drug resistance has always been a major challenge in anti-tumor therapy, and autophagy, as one of the mechanisms of tumor drug resistance, is a potential target for combating tumor drug resistance. Interestingly, resveratrol could also serve as an autophagy inhibitor to resist tumor resistance. For example, talazoparib is a third-generation poly (ADP-ribose) polymerase inhibitors (PARPi) that is primarily used to treat cancers with homologous recombination defects. However, it is now known that AKT activation induces resistance to PARPi in cancer cells by activating the pro-survival pathway [118–120]. Whereas resveratrol can sensitize breast cancer to talazoparib through dual inhibition of AKT and autophagy flux [121]. Therefore, the combination of resveratrol and tazopanib could effectively reduce cell proliferation and improve therapeutic efficacy. Similarly, cisplatin could lead to drug resistance in non-small cell lung cancer (NSCLC) cells by inhibiting the Akt-mTOR signaling pathway [122]. Whereas andrographolide (AG) has been reported not only to possess anti-proliferative and anti-angiogenic properties [123], but also to restore cell sensitivity by promoting the activation of the Akt-mTOR signaling pathway in the treatment of cisplatin-resistant NSCLC [122]. Likewise, artesunate could inhibit the growth of cisplatin-resistant bladder cancer cells by inducing apoptosis and autophagy [124]. Furthermore, icariin is one of the main active ingredients of epimedium, which could significantly induce G₀/G₁ phase arrest and apoptosis, and overcome the drug resistance of tamoxifen in the treatment of breast cancer by inhibiting autophagy so as to enhance its efficacy [125]. Apigenin is a flavonoid with anti-cancer properties and a favorable safety profile, and researchers have found that apigenin could induce the expression of miR-520b and inhibit ATG7-dependent autophagy, making the doxorubicin (DOX) resistant HCC cell line BEL-7402/ADM sensitized to DOX [126]. In addition to the natural drugs mentioned above, they also include berberine [127], ursolic acid [128], aloe emodin [129], resveratrol [130], and Tan-IIA [131].

In general, the present findings demonstrate that natural autophagy modulators have context-dependent roles in cancer. In the early stage of tumorigenesis, the activation of autophagy helps to prevent and limit the occurrence and progression of tumors. While in the stage of tumor progression and migration, the application of natural autophagy inhibitors has a unique role. Additionally, autophagy, as one of the tumor drug resistance mechanisms, is a potential target for anti-tumor drug resistance. For drug-resistant patients, the combined application of chemotherapeutic agents and natural autophagy inhibitors may have incomparable therapeutic effects. Finally, the studies of different natural autophagy modulators in different cancers are summarized in Table 2.

Table 2.

Study of natural autophagy modulators in cancer.

Natural product	Model	Molecular target/mechanism	Effect on autophagy	Pathophysiological effect	Reference
Apigenin	The human HCC cell line HepG2 in vitro	PI3K-Akt-mTOR signaling pathway	Induction	Inhibiting cell proliferation and inducing autophagy via suppressing the PI3K-Akt-mTOR pathway	[96]
Brusatol	Human LM3 liver cancer cells in vitro	PI3K-Akt-mTOR signaling pathway	Induction	Inducing apoptosis and autophagy in HCC cells by inhibiting PI3K-Akt-mTOR pathway activation	[97]
Mallotucin D	The human HCC cell line HepG2 in vitro	PI3K-Akt-mTOR signaling pathway	Induction	Suppressing HepG2 cell growth via inducing autophagic cell death	[98]
Uvangoletin	The human HCC cell lines (HepG2, Huh7 and SMMC-7721) and human normal liver cell line HL-7702 in vitro	Akt-mTOR and MAPK signaling pathways	Induction	Inhibiting HepG2 cells migration and invasion by inducing autophagy and apoptosis via Akt-mTOR, MAPK signaling pathways	[99]
Peruvoside	Breast cancer (MCF-7), lung cancer (A549) and hepatocellular carcinoma (HepG2), normal liver cells (WRL68), normal lung (L132) cell lines in vitro	PI3K-Akt-mTOR signaling pathway	Induction	Anti-proliferative activities against human breast, lung, and liver cancer cells	[100]
Platycodin D	HT-29 colon cancer cells in vitro	Beclin1 and LC3-II/I	Induction	Inducing apoptosis via promoting autophagy in colon cancer cell	[101]
Peanut skin extracts	The human melanoma cells A375 and mouse melanoma cells B16F10 and human CRC cell lines, SW480 and HCT15 in vitro	-	Induction	Regulating the cytoprotective effects in melanoma and colorectal cancer cells by inducing autophagy	[102]
Oblongifolin C	BALB/c nude mice in vivo; Human cervical carcinoma HeLa cells in vitro	Blocking autophagosome-lysosome fusion	Inhibition	Enhancing antitumor efficacy of nutrient deprivation	[103]
Liensinine	Female nude mice in vivo; MDA-MB-231 and MCF-7 cells in vitro	Blocking autophagosome-lysosome fusion and lysosomal cathepsin maturation	Inhibition	Sensitizing breast cancer cells to chemotherapy	[104]
Quercetin	Female BALB/c nude mice in vivo; Human breast cancer cell lines MCF-7 and MDA-MB-231 in vitro	Akt-mTOR pathway	Induction	Suppressing the progression of breast cancer by inhibiting cell mobility and glycolysis	[105]
Polyphyllin D	BALB/c nude mice in vivo; Human breast cancer cell lines and nontumorigenic epithelial cell line in vitro	JNK1-Bcl2 pathway	Induction	Inducing caspase-dependent apoptosis and cyto-protectvie autophagy	[106]
Hydroxysafflor yellow A	HepG2 liver cancer cell line in vitro	Promoting the expression of Beclin1 and inhibiting the phosphorylation of ERK	Induction	Preventing metastasis, inducing apoptosis and reducing viability	[107]
Baicalein	Male C57BL/6 mice in vivo; Lung cancer cell lines and WST-1 cell in vitro	AMPK pathway	Induction	Anti-lung cancer effect	[108]
Licochalcone A	BALB/c female athymic mice in vivo; Human cervical cancer cell lines in vitro	Inhibition of PI3K-Akt/mTOR pathway	Induction	Inhibiting growth and inducing apoptosis in human cervical cancer cells	[109, 110]
Chrysin	BALB/c nude mice in vivo; Human breast cancer cell lines in vitro	miR-16-5p-ZBTB16-G6D axis	Induction	Inhibiting HER2 positive breast cancer cell survival and proliferation	[113]
Artesunate	CRC cell lines in vitro	ROS generation	Induction	Over-activating autophagy in a ROS-dependent manner to inhibit cell viability	[114]
Resveratrol	Nude mouse model of Ca9-22 in vivo; Human oral squamous cell carcinoma cell lines (HSC-2, HSC-3, HSC-4, Ca9-22 in vitro;	Blocking SREBP1	Induction	Inhibiting proliferation and inducing autophagy by blocking SREBP1 expression in oral cancer cells	[115]
	Mouse xenograft model in vivo; Breast cancer cell line 4T1 in vitro;	SIRT3-AMPK	Induction	Inducing autophagy through the SIRT3-AMPK pathway and suppressing the TGF-β1-induced invasion and metastasis of 4T1 cells	[116]
	Human cholangiocarcinoma cell lines KKU-213 and KKU-100 in vitro	-	Induction	Reverting the pro-migratory effect of IL-6 while inducing autophagy in cholangiocarcinoma cells	[117]
	Breast adenocarcinoma MCF-7 (BRCA WT) and MDA-MB-231 (BRCA WT) in vitro	Dual-inhibition of AKT-signaling and the fusion of autophagosome and lysosome	Inhibition	Attenuating fusion of autophagosome and lysosome, and sensitizing breast cancer to talazoparib	[121]
Andrographolide	NCR-nu/nu (nude) female mice in vivo; The cisplatin-sensitive human NSCLC cell line A549 and its cisplatin-resistant derivate A549/DDP in vitro	Activation of the Akt-mTOR pathway	Inhibition	Resensitizing cisplatin-resistant NSCLC cells	[122, 123]
Artesunate	The cell lines RT4, RT112, T24, and TCCSup and the cisplatin-resistant sublines in vitro	-	Induction	Impairing growth in cisplatin-resistant bladder cancer cells	[124]
Icariin	Human breast cancer cell lines, MCF-7, T47D, and the corresponding TAM-resistant cell lines in vitro	Decreasing ATG5, Beclin1, and conversion of LC3-I to LC3-II, but increasing the level of p62	Inhibition	Enhancing cell viability and apoptosis	[125]
Apigenin	The nude mice in vivo; Human HCC cell line BEL-7402 and ADM-resistant HCC cell line BEL-7402/ADM in vitro	Inducing miR-520b expression and inhibiting ATG7-dependent autophagy	Inhibition	Sensitizing HCC cells to DOX	[126]

Natural products that regulate autophagy in diabetes

Many studies have shown that natural products inhibit or promote autophagy through different mechanisms, playing a role in the treatment of diabetes and the prevention of diabetes complications. Here, we summarize the function of diverse natural autophagy modulators in diabetes, and illustrate the mechanism of action in Fig. 7.

Fig. 7.

Effects of natural autophagy modulators on the different steps of the autophagic pathway in the treatment of diabetes and the prevention of diabetic complications.

On the role of the treatment of diabetes

Morus alba leaves ethanol extract has been found to protect islet cells against dysfunction and death by inducing AMPK-mTOR-mediated autophagy in type 2 diabetes mellitus (T2DM) [132]. Similarly, pectic bee pollen polysaccharide and ginsenoside Rg1 have also been shown to have therapeutic effects on diabetes by inducing AMPK-mTOR-mediated autophagy [133, 134]. In addition, pectic bee pollen polysaccharide was shown to alleviate insulin resistance [133] and ginsenoside Rg1 was shown to ameliorate pancreatic injuries in type 1 diabetes mellitus (T1DM) [134]. Besides, the water extract of Scutellaria baicalensis Georgi also shown therapeutic effects on diabetes, it could increase insulin secretion and reduce apoptosis under high glucose by inducing autophagy through the AMPK pathway [135]. Cinnamtannin D1, one of the A-type procyanidin oligomers, has been found to protect pancreatic β cells from apoptosis via AMPK-mTOR-ULK1-mediated autophagy activation and have a hypoglycemic effect in type 2 diabetes (T2D [db/db]) mice [136]. In addition, luteolin, a polyphenolic bioflavonoid, has been discovered to improve islet β cell survival and function by promoting autophagy and marginally ameliorating the symptoms of T2D mice [137]. However, elevated levels of FFAs (lipotoxicity) and ectopic lipid accumulation contributed to impaired β cell function and aggravated the progression of diabetes [138]. Therefore, protecting β cells from lipid attack may be beneficial for T2DM [139]. Kaempferol, a natural flavonol, was shown to prevent ectopic lipid accumulation to restore β cell function and protect pancreatic β cells against lipotoxicity by activating autophagy through the AMPK-mTOR pathway [139, 140]. Similarly, two main active ingredients (ginsenoside Rb2 and ginsenoside Rg2) of ginseng were also shown to alleviate lipid accumulation [141, 142]. Specifically, ginsenoside Rb2 could alleviate hepatic lipid accumulation by restoring autophagy via inducing SIRT1 and activating AMPK, resulting in improved glucose tolerance [141]. And ginsenoside Rg2 could ameliorate HFD-induced lipid droplet deposition and insulin resistance through activating autophagy, suggesting the therapeutic effects of ginsenoside Rg2 in the progression of HFD-induced T2DM [142]. Another major active ingredient of ginseng, ginsenoside CK, has been suggested to improve skeletal muscle insulin resistance via activating DRP1-PINK1-mediated mitophagy [143].

In contrast, the following natural products were found to show therapeutic benefits for diabetes by inhibiting autophagy. For example, in tacrolimus-induced DM, Korean red ginseng extract was shown to suppress autophagy to lower blood sugar and improve pancreatic islet function [144]. Resveratrol was shown to mitigate β cell death by inhibiting autophagy and alleviating acute glucose and insulin intolerance in T1DM, suggesting that resveratrol may have therapeutic potential for T1DM prevention [145]. In addition, the methanolic extract of L. flavescens was found to protect pancreatic β islets by inhibiting β cell autophagy, thereby increasing cell viability and insulin production, thus showing potential for anti-diabetic activity [146].

On the role of the prevention of diabetes complications

Furthermore, there are also several natural products that play a role in preventing diabetes complications by regulating autophagy. Lycopene is a carotenoid that has been reported to reduce autophagy flux in endothelial progenitor cells (EPCs) caused by advanced glycation end products, thus protecting EPCs and possibly helping to prevent diabetic vascular complications [147]. Conversely, resveratrol might help prevent diabetic cardiovascular complications by inducing autophagy via the AMPK-mTOR pathway [148]. However, resveratrol was also shown to protect T1DM-induced germ cell apoptotic death along with the increase in testicular Nrf2 expression and function by promoting the p62-dependent autophagic degradation of Keap1 [149]. Furthermore, resveratrol could prevent diabetic muscle atrophy by inhibiting the activation of mitophagy [150]. Korean red ginseng, the modified form of Panax ginseng C. A. Mey, was shown to attenuate hyperglycemia-induced renal inflammation and fibrosis via accelerating autophagy and protecting against diabetic kidney disease [151]. Likewise, berberine might also help prevent diabetic kidney disease through enhancing autophagy and protecting podocytes from high glucose-induced injury by promoting AMPK activation and inhibiting the p70 ribosomal protein S6 kinase (p70S6K)-4EBP1 signaling pathway [152, 153]. Most recently, tea polyphenols, the main active compounds in tea, have been found to improve memory in the aged T2DM rats to prevent memory decline in elderly with T2DM by enhancing mitophagy in hippocampal neurons [154].

Overall, the role of natural autophagy modulators in the fight against diabetes is mainly to protect pancreatic β-cells and improve insulin resistance. However, natural autophagy modulators have different regulatory mechanisms in diabetes models, and further studies are needed to determine when and whether to use them. The studies of different natural autophagy modulators in different diabetes models are summarized in Table 3.

Table 3.

Study of natural autophagy modulators in diabetes.

Natural product	Model	Molecular target/mechanism	Effect on autophagy	Pathophysiological effect	Reference
Morus alba leaves ethanol extract	Male SD rats in vivo; The INS-1 rat insulinoma cells in vitro	AMPK-mTOR pathway	Induction	Protecting islet cells against dysfunction and death in T2DM	[132]
Pectic bee pollen polysaccharide	HFD-induced obesity mice in vivo; High glucose and fatty acids-treated HepG2 cells in vitro	AMPK-mTOR pathway	Induction	Alleviating insulin resistance	[133]
Ginsenoside Rg1	The streptozotocin-induced DM mice in vivo; The rat pancreatic β cell line RIN-m5F in vitro	AMPK-mTOR pathway	Induction	Ameliorating pancreatic injuries in T1DM	[134]
Scutellaria baicalensis Georgi	The mouse pancreatic β cell MIN-6 in vitro	AMPK pathway	Induction	Increasing insulin secretion and reducing apoptosis under high glucose	[135]
Cinnamtannin D1	C57BKSdb/db mice in vivo; The INS-1 rat insulinoma cell line in vitro	AMPK-mTOR-ULK1 pathway	Induction	Protecting pancreatic β cells from apoptosis; Hypoglycemic effect	[136]
Luteolin	T2D (db/db, ob/ob) mice in vivo; The INS-1E (rat insulinoma) cell line in vitro	-	Induction	Improving islet β cell survival and function; Marginally ameliorating the symptoms	[137]
Kaempferol	Female SD rats in vivo; PA-stressed RIN-5F cells and murine pancreatic islets/RIN-5F cell line in vitro	AMPK-mTOR pathway	Induction	Preventing ectopic lipid accumulation to restore β cell function and protecting pancreatic β cells against lipotoxicity	[139, 140]
Ginsenoside Rb2	C57BL/KsJ-Lepdb (db/db) mice and their lean WT littermates in vivo; HepG2 cells and primary mouse hepatocytes in vitro	SIRT1; AMPK	Induction	Alleviating hepatic lipid accumulation; Improving glucose tolerance	[141]
Ginsenoside-Rg2	HFD-fed mice in vivo;	-	Induction	Ameliorating HFD-induced lipid droplet deposition and insulin resistance	[142]
Ginsenoside CK	Male C57BL/6J and db/db mice in vivo; The murine myoblast cell line C2C12 in vitro	DRP1-PINK1-mediated mitophagy	Induction	Improving skeletal muscle insulin resistance	[143]
Korean red ginseng extract	Male BALB/c mice in vivo; The rat insulinoma cell line INS-1 in vitro	-	Inhibition	Lowering blood sugar and improving pancreatic islet function	[144]
Resveratrol	Streptozotocin induced BALB/c albino mice in vivo	-	Inhibition	Mitigating β cell death; Extenuating acute glucose and insulin intolerance in T1DM	[145]
	The HFD mice in vivo; Human aortic endothelial cells in vitro	AMPK-mTOR pathway	Induction	Inhibiting intracellular ROS production and ameliorating endothelial dysfunction; Diabetic cardiovascular complication prevention	[148]
	T1DM male FVB mice in vivo	p62; Keap1-Nrf2	Induction	Protecting T1D-induced germ cell apoptotic death along with the increase in testicular Nrf2 expression and function	[149]
	Streptozocin-induced diabetic mice in vivo	-	Induction	Preventing diabetic muscle atrophy by inhibiting the activation of mitophagy	[150]
The methanolic extract of L. flavescens	SD rats in vivo; INS-1 rat insulinoma β cells in vitro	-	Inhibition	Protecting pancreatic β islets; Increasing β cell viability and insulin production	[146]
Lycopene	DM rats in vivo; EPCs from T2D mellitus rats in vitro	-	Inhibition	Protecting EPCs and may help prevent diabetic vascular complications	[147]
Korean red ginseng	Male SD rats in vivo; Human kidney proximal tubular cells in vitro	-	Induction	Attenuating hyperglycemia-induced renal inflammation and fibrosis; Protecting against diabetic kidney disease	[151]
Berberine	The conditionally immortalized mouse podocytes and mouse podocytes in vitro	AMPK; mTOR-P70S6K-4EBP1 pathway	Induction	Protecting from high glucose-induced injury in podocytes; Preventing diabetic kidney disease	[152, 153]
Tea polyphenols	Aged T2DM model rats in vivo; PC12 cells in vitro	Endoplasmic reticulum stress; Mitophagy	Induction	Improving memory in the aged T2DM rats	[154]

Natural products that regulate autophagy in COPD

Although current drugs for treating COPD can produce effective bronchodilator effects, they cannot treat patients with underlying inflammatory diseases. Numerous studies have suggested that oxidative stress plays a central role in the progression of COPD. Therefore, there is an urgent need for effective therapeutic antioxidant measures to control and alleviate local and systemic blood oxygen outbreaks in COPD patients [155]. Surprisingly, some natural products are currently widely recognized due to their enormous antioxidant capacity. Here, we summarize how different natural products affect autophagy in COPD and demonstrate the mechanism of action in Fig. 8.

Fig. 8. Effects of natural autophagy modulators on the different steps of the autophagic pathway in COPD.

Natural autophagy modulators can reduce lung injury mainly by regulating CS-induced over-activated autophagy or impaired basal autophagy.

A recent study has described the therapeutic potential of resveratrol in mouse model of LPS and CS-induced COPD, and its mechanism of action may be related to the inhibition of total Beclin1 levels in mouse lungs [156]. Besides, in human umbilical vein endothelial cells, resveratrol has been suggested to induce autophagy in a Notch1 dependent manner to weaken cigarette smoke extract (CSE) induced endothelial cell apoptosis [157], which can support the therapeutic potential of resveratrol in preventing and treating CSE related diseases such as COPD. Similarly, melatonin has been reported to inhibit ROS-induced NLRP3 inflammasome activation by up-regulating cellular antioxidant status and mitophagy, demonstrating the improved effect of melatonin on the progression of COPD [158]. Another natural product, puerarin, has been reported to inhibit CSE-induced mitophagy and apoptosis of human bronchial epithelial cells (HBECs) by activating the PI3K-Akt-mTOR signaling pathway, reducing ROS generation and achieving cellular protective effects, providing new ideas for the treatment of COPD [159]. Another study suggested that taurine served as an autophagy inhibitor to suppress autophagy caused by particulate matter (PM) and restore mitochondrial gene expression levels, thereby improving PM induced emphysema [160]. In addition, silymarin [161], monascus adlay [162], and vam3 [163] were found to improve lung inflammation through regulating autophagy. Silymarin, a flavonoid compound extracted from the milk thistle (Silybum marianum), was found to attenuate CSE-induced inflammatory responses through simultaneously inhibiting the activation of autophagy and the activity of ERK/p38 MAPK pathway in HBECs [161]. Monascus adlay is produced by inoculating cooked adlay with Monascus spp, which was found to reduce CS-induced acute lung injury by inhibiting autophagy [162]. And vam3, a derivative of resveratrol, has been found to decrease CS-induced autophagy via up-regulating/restoring the levels of SIRT1 and FoxO3a and inhibiting the induced oxidative stress, suggesting that it could prevent the progression of COPD [163].

Overall, the role of natural autophagy modulators in COPD is mainly to regulate autophagy dysregulation and oxidative stress caused by CS, protect cells from CS damage, and delay the progression of COPD. For the over-activation of autophagy caused by CS, these mentioned above natural autophagy inhibitors can be considered. While, for the impaired autophagy, it is necessary to further study the degree of autophagy damage, and then consider whether to use autophagy inducers. The studies of different natural autophagy modulators in different COPD models are summarized in Table 4.

Table 4.

Study of natural autophagy modulators in COPD.

Natural product	Model	Molecular target/mechanism	Effect on autophagy	Pathophysiological effect	Reference
Resveratrol	Male Kunming mice in vivo	Beclin1	Inhibition	Inhibiting inflammatory cell infiltration, fibrosis response, and mucus hypersecretion	[156]
	HUVECs in vitro	Notch1	Induction	Weakening CSE-induced endothelial cell apoptosis	[157]
Melatonin	Male Swiss albino mice in vivo; The human lung alveolar epithelium cell line in vitro	Mitophagy; ER stress	Induction	Inhibiting ROS induced NLRP3 inflammasome activation and alleviating CS exposed lung damage by suppressing oxidative injury	[158]
Puerarin	CES-induced HBECs in vitro	PI3K-Akt-mTOR pathway	Inhibition	Reducing ROS generation; Achieving cellular protective effects	[159]
Taurine	The HBE cell line in vitro	-	Inhibition	Restoring mitochondrial gene expression levels and improving PM induced emphysema	[160]
Silymarin	Male BALB/c mice in vivo; Human bronchial epithelial cell line in vitro	-	Inhibition	Attenuating CSE-induced inflammation	[161]
Monascus adlay	Female Wistar rats in vivo	Beclin1	Inhibition	Reducing CS-induced acute lung injury	[162]
Vam3	Male BALB/c mice in vivo; HBECs in vitro	SIRT1-FoxO3a	Induction	Preventing COPD progression	[163]

Natural products that regulate autophagy in NDDs

Numerous studies have demonstrated that natural products target distinct autophagy processes to provide therapeutic benefit for NDDs. Here, we summarize the role of diverse natural products in NDDs via autophagy regulation and depict the mechanism of action in Fig. 9.

Fig. 9.

Effects of natural autophagy modulators on the different steps of autophagy pathway in NDDs, including AD, PD, HD, and ALS.

In Alzheimer’s disease (AD), natural products play a therapeutic role mainly by regulating the levels of Aβ and tau and improving dysfunction. Panax notoginseng saponins is the primary active ingredient extracted from P. notoginseng, which was shown to promote parkin-mediated mitophagy and defend PC-12 cells from damage caused by Aβ [164]. Similarly, resveratrol was found to attenuate Aβ protein fragment 25–35-induced neurotoxicity in PC-12 cells by activating autophagy via the Tyrosyl transfer-RNA synthetase (TyrRS)-Auto-poly-ADP-ribosylation of poly polymerase 1 (PARP1)-SIRT1 signaling pathway [165]. Moreover, resveratrol was also found to increase the degradation of aged proteins through the autophagy pathways to reduce the amount of aggregated Aβ and prevent Aβ-induced cytotoxicity [166]. In addition, resveratrol was shown to activate AMPK, leading to mTOR inhibition and thus initiating autophagy for lysosomal clearance of Aβ, indicating it has therapeutic potential against AD [167]. Oxyresveratrol, a well-known polyphenolic, was reported to trigger the AMPK-ULK1-mTOR dependent autophagy pathway to reduce the production of amyloid precursor protein (APP), suggesting oxyresveratrol might have the effect of delaying AD progression [168]. Arctigenin, a biologically active lignan, has been shown to promote Aβ clearance by enhancing autophagy through Akt-mTOR signaling inhibition and AMPK-Raptor pathway activation [169]. Similarly, curcumin, a natural polyphenolic compound isolated from Curcuma longa, could not only induce autophagy by down-regulating the PI3K-Akt-mTOR signaling pathway to reduce Aβ production and promote Aβ clearance but also improve memory and cognitive function in mice [170]. Likewise, ginsenoside Rg2 has been suggested to improve cognitive functions in a mouse model of AD by lowering intracellular Aβ levels via activating the AMPK-ULK1 pathway to induce autophagy [142]. Dehydropachymic acid is one of the major triterpenoids isolated from Poria cocos that has been indicated to promote the fusion of autophagosomes with lysosomes to restore autophagic flux and lysosomal acidification, thereby reducing the accumulation of Aβ [171]. Moreover, it has been reported that long-term treatment of berberine could not only promote autophagolysosomal fusion and improve autophagy flux but also enhance autophagic activity through the PI3KC3-Beclin1-Bcl-2 signaling pathway, thereby promoting the clearance of tau to alleviate cognitive decline [172]. Another natural product, celastrol, a triterpenoid isolated from Tripterygium wilfordii, has been found to improve memory in AD animal models by promoting the degradation of phosphorylated tau aggregates through enhancing TFEB-mediated autophagy [173]. And esculentoside A, a triterpenoid isolated from Phytolacca esculenta, has been found to attenuate cognitive decline by modulating Akt-glycogen synthase kinase-3β activity to reduce hyperphosphorylation of tau and enhance autophagic clearance of hyperphosphorylated tau in an AMPK-dependent manner [174].

A variety of natural products also play a neuroprotective role through regulating autophagy. Dendrobium nobile Lindl alkaloid is an alkaloid extracted from Dendrobium nobile Lindl that has been reported to induce autophagic flux by promoting the formation and degradation of autophagosomes in hippocampus neurons to protect axonal degeneration from Aβ peptide cytotoxicity [175]. However, resveratrol has been found to exert neuroprotective effects by suppressing neuronal autophagy in AD [176]. Moreover, resveratrol and other two other polyphenols (quercetin and apigenin) have also been reported to protect nerve cells. They could inhibit 7-ketocholesterol-induced oxiapoptophagy and normalize autophagy activity to prevent neurodegeneration [177]. Trehalose, a natural disaccharide, has been found to ameliorate dopaminergic and tau pathology through autophagy activation, suggesting that trehalose has a neuroprotective effect in AD and Parkinson’s disease (PD) [178]. Similarly, piperine, an alkaloid extracted from Piper longum L, has been shown to induce autophagy by inhibiting the Akt-mTORC1 signaling pathway, playing a neuroprotective role in PD models [179]. While celastrol was shown to inhibit intracellular α-synuclein aggregates and exert neuroprotective effects by inducing autophagy [180].

In addition to their neuroprotective effects on PD, natural products also exert anti-PD effects by regulating α-synuclein levels and ameliorating pathology and dysfunction. Curcumin has been reported to efficiently reduce α-synuclein accumulation in PD by down-regulating mTOR-p70S6K signaling and restoring autophagy that had been inhibited [181]. While resveratrol has been shown to induce LC3 deacetylation by activating SIRT1, thereby mediating α-synuclein autophagic degradation and improving motor deficits and pathological changes in PD mice [182]. Moreover, resveratrol could also promote heme oxygenase-1 (HO-1) expression and HO-1-dependent autophagic flux to prevent against rotenone-induced neuronal apoptosis, which may provide therapeutic targets for the treatment of PD [183]. Isorhynchophylline is an alkaloid isolated from the Chinese herbal medicine Uncaria rhynchophylla (Miq.) Jacks. (Gouteng in Chinese), which has been found to promote the degradation of α-synuclein in neuronal cells via inducing autophagy [184]. Corynoxine B (Cory B) is also an alkaloid isolated from Uncaria rhynchophylla (Miq.) Jacks., and Cory B and corynoxine are enantiomers of each other. Cory B was reported to enhance the activity of PI3KC3 complex I and increase autophagy by promoting the interaction between Beclin1 and HMGB1/2, and corynoxine was reported to induce autophagy through the Akt-mTOR pathway, promoting the degradation of α-synuclein [185, 186]. Piperlongumine, an alkaloid extracted from Piper longum L, has been shown to induce autophagy by increasing Ser70 phosphorylation of Bcl-2 via JNK1 activation, thereby alleviating rotenone-induced dyskinesia and loss of midbrain dopaminergic neurons in a mouse model of PD [187]. Morever, harmol, a β-carboline alkaloid, has been reported to activate the autophagy-lysosome pathway through the AMPK-mTOR-TFEB pathway to promote α-synuclein clearance [188].

Additionally, certain natural products can exert therapeutic or neuroprotective effects on both PD and Huntington’s disease (HD). For example, hederagenin and α-hederin are triterpenoid saponins derived from the hedera helix, which could induce autophagy by activating the AMPK-mTOR signaling pathway, reducing the levels of mutant Htt protein and α-synuclein, and inhibiting the aggregation of α-synuclein and inclusion formation of Htt [189]. Similarly, onjisaponin B, a triterpenoid derived from Radix Polygalae, was reported to activate autophagy in an ATG7, AMPK-mTOR dependent manner and enhance the clearance of mutant Htt and α-synuclein in PC-12 cells [190]. While trehalose was reported to induce autophagy in a mTOR-independent manner and accelerate the clearance of mutant Htt and α-synuclein [191]. Furthermore, several natural products play a role in HD and amyotrophic lateral sclerosis (ALS) mainly by clearing mutant Htt and SOD1. For instance, neferine is isolated from the lotus seed embryo of Nelumbo nucifera, which could induce autophagy through an AMPK-mTOR-dependent pathway and then reduce both the protein level and toxicity of mutant Htt through an ATG7-dependent mechanism to play a neuroprotective role in HD [192]. Trehalose has been reported to mediate the degradation of mutant SOD1 and delay the progression of ALS by enhancing autophagy in motoneuron [193].

Collectively, natural autophagy modulators can treat or delay the progression of neurodegenerative diseases by acting on different stages of autophagy, clearing mutants or aggregates and exerting neuroprotective effects. For clearance of mutants or aggregates, the use of natural autophagy inducers may be considered. The studies of different natural autophagy modulators in different NDDs models are summarized in Table 5.

Table 5.

Study of natural autophagy modulators in NDDs.

Disease	Natural product	Model	Molecular target/mechanism	Effect on autophagy	Pathophysiological effect	Reference
AD	Ginsenoside Rg2	The 5XFAD transgenic mice in vivo; HeLa cell lines in vitro	AMPK-ULK1 pathway	Induction	Improving cognitive functions; Lowering intracellular Aβ levels	[142]
	Panax	PC12 cells line in vitro	Promoting parkin-mediated mitophagy	Induction	Defending PC-12 cells from damage caused by Aβ	[164]
	Resveratrol	PC12 cells in vitro	TyrRS-PARP1-SIRT1 signaling pathway	Induction	Attenuating Aβ protein fragment 25–35-induced neurotoxicity in PC-12 cells by activating autophagy via the TyrRS-PARP1-SIRT1 signaling pathway	[165]
		The transgenic Caenorhabditis elegans strain CL2006 in vitro	-	Induction	Increasing the degradation of aged proteins through the autophagy pathways to reduce the amount of aggregated Aβ and prevent Aβ-induced cytotoxicity	[166]
		Male APP/PS1 transgenic mice in vivo; HEK293 (APP-HEK293) and N2a (APP-N2a) cells stably transfected with human APP695 in vitro	AMPK-mTOR	Induction	Activating AMPK resulted in mTOR inhibition and initiation of autophagy and lysosomal clearance of Aβ	[167]
		3×Tg AD mice in vivo	-	Inhibition	Neuroprotective effects	[176]
		Neuronal N2a cells in vitro	-	Inhibition	Preventing neurodegeneration; Protecting nerve cells	[177]
	Oxyresveratrol	Mouse cortical astrocyte in vitro	AMPK-ULK1-mTOR pathway	Induction	Reducing the production of APP	[168]
	Arctigenin	APP/PS1 mice in vivo; HEK293 cells expressing APP Swedish mutant K595N/M596L in vitro	Akt-mTOR signaling; AMPK-Raptor pathway	Induction	Promoting Aβ clearance	[169]
	Curcumin	APP/PS1 double transgenic AD mice in vivo	PI3K-Akt-mTOR pathway	Induction	Reducing Aβ production and promoting Aβ clearance; Improving memory and cognitive function in mice	[170]
	Dehydropachymic acid	PC12 cells stable transfected with pCB6-APP in vitro	Restoring the lysosomal acidification	Induction	Reducing the accumulation of Aβ	[171]
	Berberine	3×Tg AD mice in vivo	Activation of Akt and inhibition of GSK3β; PI3KC3-Beclin1 pathway	Induction	Promoting the clearance of tau to alleviate cognitive decline	[172]
	Celastrol	P301S Tau and 3×Tg mice in vivo; HeLa, N2a, and HEK293 cells in vitro	TFEB	Induction	Improving memory; Promoting the degradation of phosphorylated tau aggregates	[173]
	Esculentoside A	3×Tg-AD model mice in vivo	Akt-glycogen synthase kinase-3β pathway; AMPK	Induction	Attenuating cognitive decline; Reducing hyperphosphorylation of tau and enhancing autophagic clearance of hyperphosphorylated tau	[174]
	Dendrobium nobile Lindl alkaloid	Hippocampus primary neurons of rats in vivo and in vitro	Promoting the formation and degradation of autophagosomes	Induction	Protecting axonal degeneration from Aβ peptide cytotoxicity	[175]
AD/PD	Trehalose	Human-mutated tau over-expressing mice (TauVLW) and the parkin-null mutant mice (PK-/-) in vivo	-	Induction	Ameliorating dopaminergic and tau pathology; Neuroprotective effect	[178]
PD	Piperine	Rotenone-induced primary rat cortical neurons and a mouse in vivo; Rotenone-induced SK-N-SH cells in vitro	PP2A-Akt-mTORC1 pathway	Induction	Neuroprotective effect	[179]
	Celastrol	Human neuroblastoma SH-SY5Y cells in vitro	-	Induction	Inhibiting intracellular α-synuclein aggregates; Neuroprotective effect	[180]
	Curcumin	A53T α-synuclein cell in vitro	Down-regulating mTOR-p70S6K signaling	Induction	Reducing α-synuclein accumulation	[181]
	Resveratrol	MPTP-treated male C57BL/6 mice in vivo	LC3; SIRT1	Induction	α-synuclein degradation and improving motor deficits and pathological changes	[182]
		Dopaminergic SH-SY5Y cells in vitro	HO-1	Induction	Preventing rotenone-induced neuronal apoptosis	[183]
	Isorhynchophylline	Neuronal cell lines (N2a, SH-SY5Y and PC12 cells) and primary cortical neurons in vitro	Beclin1	Induction	Promoting the degradation of α-synuclein	[184]
	Corynoxine	Neuronal cell lines (N2a, PC12 and SHSY-5Y cells) in vitro	Akt-mTOR pathway	Induction	Promoting the degradation of α-synuclein	[185]
	Corynoxine B	Prp-α-syn A53T transgenic mice in vivo; N2a cells and PC12 cells in vitro	HMGB1/2; PI3KC3 complex I	Induction	Enhancing the activity of PI3KC3 complex I and increasing autophagy by promoting the interaction between Beclin1 and HMGB1/2	[186]
	Piperlongumine	Rotenone-induced PD C57BL male mice in vivo; Primary neurons and SK-N-SH cells in vitro	JNK1-Bcl2 pathway	Induction	Alleviating rotenone-induced dyskinesia and loss of midbrain dopaminergic neurons	[187]
	Harmol	α-syn transgenic mice in vivo	Activating ALP and AMPK-mTOR-TFEB pathway	Induction	Promoting α-synuclein degradation and improving motor impairment	[188]
PD/HD	Hederagenin and α-hederin	MPTP-induced PD mice in vivo; PC12 cells in vitro	AMPK-mTOR pathway	Induction	Reducing the levels of mutant Htt protein and α-synuclein and inhibiting the aggregation of α-synuclein and inclusion formation of Htt	[189]
	Onjisaponin B	PC-12 cells and WT atg7 and atg7-deficient MEF in vitro	atg7; AMPK-mTOR pathway	Induction	Enhancing the clearance of mutant Htt and α-synuclein	[190]
	Trehalose	Human neuroblastoma cells (SKN-SH), stable HeLa cells expressing EGFP-LC3 and WT atg5 (atg5+/+) and atg5-deficient (atg5-/-) MEFs in vitro	mTOR	Induction	Accelerating the clearance of mutant Htt and α-synuclein	[191]
HD	Neferine	PC-12 cells and WT atg7 and atg7-deficient MEF in vitro	AMPK-mTOR pathway; atg7	Induction	Reducing both the protein level and toxicity of mutant Htt; Neuroprotective effect	[192]
ALS	Trehalose	Mutant SOD1 transgenic mice in vivo; Mutant SOD1 in NSC34 motoneuron cells in vitro	Activation of FoxO1	Induction	Mediating the degradation of mutant SOD1 and delaying the progression of ALS	[193]

Conclusions and perspectives

Autophagy, as an evolutionarily conserved mechanism, exists in both physiologic and pathophysiologic conditions, and dysregulated autophagy has implications in health and disease. In 2016, the Nobel Prize in Physiology or Medicine was awarded to Yoshinori Ohsumi for the discoveries of mechanisms governing autophagy. In the past decade, multiple natural products are found with good autophagy-regulating activity. As summarized in this review, most of these natural product autophagy modulators are derived from dietary and herbal sources and thus have unique advantages. Moreover, natural products neither over-induce nor over-inhibit autophagy, making them potential therapeutic agents suitable for long-term administration and regulating the balance of cellular functions. Therefore, more in-depth studies are needed for further discovery and development of autophagic modulators for the treatment of NCDs. However, the current research on the regulation of autophagy by natural products is not sufficiently advanced. For example, many papers rely on LC3-only data, which, as we now know, can indicate either canonical autophagy or other forms of non-cannical autophagy [e.g., Conjugation of ATG8 to single membranes (CASM)] [194]. Furthermore, the relationship between autophagy, apoptosis, and oxidative stress is intricate, and many natural products can act on two or even all three of these processes at the same time, which is one of the reasons for the discrepancy in the findings. Therefore, further studies are needed to explore the exact mechanisms by which natural products regulate autophagy and to illustrate the application of these natural autophagy modulators in NCDs.

In this review, we have summarized the pharmacological effects of the reported natural products on NCDs from an autophagy mechanistic point of view. Based on our summary, the role of many natural autophagy modulators in different NCDs may be context dependent and this controversy may result from complexity of autophagy in disease development, variability in disease models, altervation in disease processes and stages, and diversity of natural product actions. This review may help to discover novel natural autophagy modulators for the treatment of NCDs in the future.

Competing interests

The authors declare no competing interests.