Molecular Mechanisms of Action of Emodin: As an Anti-Cardiovascular Disease Drug

Emodin is a natural occurring anthraquinone derivative isolated from roots and barks of numerous plants, molds, and lichens. It is found to be an active ingredient in different Chinese herbs including Rheum palmatum and Polygonam multiflorum, and it is a pleiotropic molecule with diuretic, vasorelaxant, anti-bacterial, anti-viral, anti-ulcerogenic, anti-inflammatory, and anti-cancer effects. Moreover, emodin has also been shown to have a wide activity of anti-cardiovascular diseases. It is mainly involved in multiple molecular targets such as inflammatory, anti-apoptosis, anti-hypertrophy, anti-fibrosis, anti-oxidative damage, abnormal, and excessive proliferation of smooth muscle cells in cardiovascular diseases. As a new type of cardiovascular disease treatment drug, emodin has broad application prospects. However, a large amount of evidences detailing the effect of emodin on many signaling pathways and cellular functions in cardiovascular disease, the overall understanding of its mechanisms of action remains elusive. In addition, by describing the evidence of the effects of emodin in detail, the toxicity and poor oral bioavailability of mice have been continuously discovered. This review aims to describe a timely overview of emodin related to the treatment of cardiovascular disease. The emphasis is to summarize the pharmacological effects of emodin as an anti-cardiovascular drug, as well as the targets and its potential mechanisms. Furthermore, the treatment of emodin compared with conventional cardiovascular drugs or target inhibitors, the toxicity, pharmacokinetics and derivatives of emodin were discussed.


INTRODUCTION
Emodin (1, 3, 8-trihydroxy-6-methylanthraquinone) is a natural anthraquinone derivative, and is the main ingredient of many herbs including rhubarb, polygonum multiflorum, and polygonum cuspidatum (Dong et al., 2016;Monisha et al., 2016). Pharmacological studies have shown that emodin, as a polyvalent molecule, has a variety of biological functions, such as antiinflammatory Xu et al., 2020), anti-bacterial (Ji et al., 2020), anti-cancer (Wu C. et al., 2019;Ma et al., 2020;Rossi et al., 2020) and inhibition of oxidative stress (Lee et al., 2020). Emodin has also been shown to have potent anti-cardiovascular activities in vitro and in vivo, including anti-inflammatory , antioxidant damage (Du and Ko, 2005), inhibition of myocardial cell fibrosis (Evans et al., 2020) and other pharmacological effects. Based on these pharmacological effects, emodin has been shown to be functional in myocarditis, myocardial hypertrophy, ischemia-reperfusion injury, and myocardial infarction. This indicates that emodin plays an important role in the treatment of cardiovascular diseases (Shrimali et al., 2013). Numerous studies have also made huge strides in revealing the mechanisms behind these effects. In addition, comparable therapeutic effect and better assisting effect were also shown in the comparative study between emodin and conventional therapeutic drugs or target inhibitors (Du and Ko, 2005;Du and Ko, 2006;Liu et al., 2013;Evans et al., 2020). However, whether emodin has a specific therapeutic effect in cardiovascular diseases is still unclear, which limits its further research and application. In addition, although there is abundant evidence detailing the effects of emodin on various signaling pathways and cellular functions, the overall understanding of its mechanisms of action remains elusive. Therefore, the purpose of this paper is to provide an overview of emodin's role in the fight against cardiovascular disease timely. The emphasis is to summarize the pharmacological effects of emodin as a possible anti-cardiovascular drug, as well as its targets and mechanisms of action.

TARGETS AND STUDIES OF EMODIN AGAINST ANTI-CARDIOVASCULAR DISEASE
Emodin has been shown to be active in a variety of cardiovascular diseases in vivo and in vitro. And numerous studies have explored the underlying mechanisms of these effects ( Table 1).
The results and potential mechanisms of cardiovascular protection are shown below.
Immunomodulation NF-kB essential for the injury and necrosis of cardiovascular cells (Ait-Oufella et al., 2019). Emodin has been found to have effective antiinflammatory effects (Shrimali et al., 2013;Ding et al., 2020), and it also has the activity in cardiovascular diseases. It is reported that emodin can control the expression and secretion of inflammatory factors by inhibiting the transcription factor nuclear factor kappa-B (NF-kB) in a variety of inflammatory diseases. For example, emodin (30 mg/kg) significantly inhibited the expression of tumor necrosis factor-a (TNF-a) and the activation of NF-kB in the local myocardial infarction area of acute myocardial infarction (AMI) model mice, protected myocardial cell injury by inhibiting local inflammation, reduced the range of myocardial infarction induced by occluding the left anterior descending coronary artery in BALB/c mice in a dose-dependent manner .

PPAR-g and eNOS
Abnormal expression of endothelial inflammatory cytokines and mediators is a critical factor leading to myocardial ischemiareperfusion (I-R) injury (Sager and Nahrendorf, 2016). Peroxisome proliferator-activated receptor g (PPAR-g) is identified as a cardio-protective factor (Agrawal et al., 2014). I-R can inhibit the activation of PPAR-g signaling pathway by reducing the phosphorylation of PPAR-protein . PPAR-g agonists can reduce I-R cell damage in multiple organs, including brain, liver, heart, and lung, and further activate endothelial nitric oxide synthase (eNOS), which plays a key role in maintaining and improving endothelial function (Kobayashi et al., 2008). In addition, eNOS inhibited the production of inflammatory cytokines by NO dependent mechanisms (Wang et al., 2017). Shou et al. (2018) established an in vitro hypoxia-reoxygenation model of human aortic endothelial cells (HAECs). The inactivation of PPAR-g and eNOS signaling pathways is essential for the expression of inflammatory cytokines in endothelial cells induced by hypoxia-reoxygenation. The intervention of emodin reversed the levels of endothelial inflammatory cytokines and the expression of mediators in a concentration-dependent manner. Emodin recovered the PPAR-g activity of hypoxia-reoxygenated endothelial cells by the molecular interaction between Heat Shock Proteins90 (HSP90) and eNOS. Therefore, it advances the production of NO, inhibits the inflammatory response, and has a protective effect on hypoxia-reoxygenation injury.

MiR-223
Studies have reported that MicroRNA (miR)-223 can be invoked as a new potential target for the diagnosis and treatment of inflammatory diseases, such as thrombophlebitis , acute peritonitis (Brook et al., 2019), viral myocarditis (Gou et al., 2018), and acute lung injury . It can inhibit the inflammatory response and prevent the indirect damage in the process of infection (Yuan et al., 2018;Cavallari et al., 2019;Kim et al., 2019). Jun kinase enzyme (JNK) signaling regulates a wide range of cellular processes, including cell proliferation, differentiation, survival, apoptosis, and inflammation (Hammouda et al., 2020).  explored the effects of miR-223 and JNK signaling pathways on LPS-induced H9c2 cells, and found that LPS exposure significantly up-regulated the expression levels of miR-223 and JNK-related proteins (p-JNK and p-c-Jun), promoted apoptosis and the release of inflammatory factors IL-1b, IL-6, TNFa. Furthermore, 20 mM of emodin intervention eliminated the upregulation. Emodin alleviated the LPS-induced myocarditis damage by inhibiting the over expression of miR-223. And by down-regulating miR-223 to inactivate the JNK signaling pathway. Therefore, miR-223 is considered as a possible target of emodin.

Anti-Apoptosis
Caspase-3 and Caspase-9 Effector caspase-3 and the initiator caspase-9 have been recorded as important agents of apoptosis (Cao et al., 2002;Yao et al., 2007). Emodin has been repeatedly reported to down-regulate caspase-3 and caspase-9 due to myocardial cell injury in vivo and in vitro Yang et al., 2019). Liu et al. (2013) first reported that the apoptosis rate of hep-2 cells treated with emodin was significantly lower than that of the control group infected with the virus. The 50% effective concentration (EC50) of the pro-apoptotic gene caspase-3 in the heart tissue of mice with coxsackievirus B4 (CVB4)-induced viral myocarditis down-regulated by emodin was 12.06 mM, and the selectivity index (SI) was 5.08. At the same time, the apoptosis inhibitor gene bcl-2 was up-regulated to play an antiapoptotic role. Furthermore, survival rate of mice or cells was improved, MTD was prolonged, and HW/BW, viral titer and myocardial pathological score caused by viral infection were decreased. Studies Zhang et al., 2019) also showed that H9c2 cells with myocardial ischemia (hypoxiainduced) after treatment with emodin (15 mM or 20 mM) had the effect of down-regulating caspase-3 and caspase-9. Regarding the role of emodin, Leung et al. (2020) also reported that emodin can regulate the kinase (ERK)-1/2 signaling pathway in vivo and in vitro by activating extracellular signaling, increasing the expression of Bcl-2 and glutamate transporter-1 (GLT-l), and inhibiting the activated level of caspase-3.

MiR-138
MicroRNA (miRNA) is essential in a variety of biological processes, including cell differentiation and proliferation, apoptosis, and metabolism (Brennecke et al., 2003;Chen et al., 2004). MiRNA deregulation often disrupts critical cellular processes, leading to the onset and progression of various human diseases. The identified miRNA can target various target genes, and the target miRNA can also contain multiple miRNA binding sequences. Currently, it have shown hope of miRNA-based therapies in treating human diseases Cheng et al., 2019;Wang et al., 2019;Pottoo et al., 2020;Wu et al., 2020). Among them, miR-138 has been reported to protect cardiomyocytes from hypoxia-induced apoptosis (He et al., 2013;Xiong H. et al., 2016). Emodin (10 mM) has been demonstrated to inhibit hypoxia-induced injury. And it can significantly reduce the decline in cell viability and apoptosis caused by hypoxia in a dose-dependent manner Fan et al., 2018). Subsequently, it was reported that emodin can protect cardiomyocytes by regulating miRNA expression (Hua et al., 2015;Xiang et al., 2017). Overexpression of miR-138 enhanced the protective effect of emodin, while miR-138 silencing weakened the protective effect of emodin on hypoxiainduced injury, suggesting that emodin played a protective effect on cardiomyocytes by upregulation of miR-138 (Rukov and Shomron, 2011;Zhang et al., 2019). In addition, in hypoxiainduced damaged cells, the overexpression of Mixed Lineage Kinase 3 (MLK3), which is synergistic with miR-138, inactivates the Sirt1/AKT and Wnt/wp-catenin pathways (He et al., 2013). The activation of Sirt1/AKT and Wnt/w1-catenin pathways plays an important role in anti-apoptosis (Lehwald et al., 2011). Zhang et al. (2019) showed that emodin (15 and 20 mM) inhibited the hypoxia-induced damage of H9c2 cells by up-regulated miR-138 and activating Sirt1/AKT and Wnt/wp-catenin pathways. It suggested that emodin may promote hypoxia-induced cell viability and inhibit cell apoptosis by de-regulating the expression of miR-138.

MiR-26a
MiR-26a plays a crucial role in the regulation of cardiomyocytes . Huang et al. found that the up-regulation of miR-26a promoted the apoptosis of hypoxia-treated H9c2 cells, while emodin (15 and 20 mM) could negatively regulate the expression of miR-26a and reduce the apoptosis by regulating miR-26a . Meanwhile, survivin is a member of the inhibitor of apoptosis protein (IAPs) family. It promotes cell survival by interfering with various cycle-related proteins, such as inner centromere protein and Aurora B kinase (Cheung et al., 2011). In addition, the relationship between hypoxic treatment and survivin expression has also been found in previous studies (Zhu et al., 2001;Zhang et al., 2007). Huang et al. (2019) revealed that hypoxia decreased the accumulated level of survivin while emodin enhanced the expression of survivin, which provided a reason for emodin to inhibit cell apoptosis due to the antiapoptosis effects of survival. It is observed that MiR-26a achieves its functions via targeting survivin.

STAT3
It has been reported that signal transducer and activator of transcription (STAT)3 is an anti-apoptotic gene (Gu et al., 2020;Turovskaya et al., 2020). The activation of STAT3 may prevent cardiomyocyte apoptosis and enhance regulatory cardiovascular protective genes in vivo and in vitro during hypoxiareoxygenation and I-R therapy (Mascareno et al., 2005;Ke et al., 2019). In a study , emodin can activate the inactivated JAK1/STAT3 pathway after hypoxia. Further results indicated that emodin (at the concentration of 15 and 20 mM) increased the phosphorylation of pathway-related proteins and that the JNK/STAT3 pathway inhibitor AG490 could weaken the protective effect of emodin on hypoxia-induced damage. This indicates that emodin alleviates hypoxia-induced cell damage through activation of the JAK1/STAT3 signaling pathway.

MTA3
Cardiac fibrosis has been identified as the cause of heart failure (Jin et al., 2018). Emodin can improve multi-tissue fibrosis (Guan et al., 2016;Wang et al., 2016). Therefore, Xiao et al. (2019) evaluated the beneficial effects and potential mechanisms of emodin on cardiac fibrosis. Firstly, it was observed that emodin can reduce cardiac fibrosis by TAC-induced and cardiac fibroblast activation by AngII-induced. Then, it was found that the emodin/MTA3 axis play a crucial role in regulating cardiac fibrosis. This is consistent with the recognized role of MTA3 in regulating a variety of cellular events (Jiao et al., 2017). As a transcription factor, MTA3 becomes a cancer suppressor by inhibiting the proliferation, invasion, and migration of cancer cells (Ma et al., 2016). In addition, MTA3 also has anti-fibrosis effect (Qin et al., 2015). Xiao et al. (2019) further demonstrated that the upregulation of MTA3 may be molecular mechanisms of emodin inhibiting cardiac fibrosis.

HDAC
Lysine acetylation on histone tails is a reversible epigenetic modification that is regulated by two enzymes: histone deacetylases (HDACs) and histone acetyltransferases (HATs) (Yang and Seto, 2007). It has been reported that inhibition of class I and class II HDAC can effectively treat pathological myocardial hypertrophy in animal models (Antos et al., 2003). The pan-HDAC inhibitor TSA increases the acetylation of histones and attenuates cardiac hypertrophy (Yoshimura et al., 1993;Nakagawa et al., 1995). Recently, it has been reported that emodin can inhibit HDAC activity in a test tube (Godoy et al., 2017). Levi W. Evans et al. (Evans et al., 2020) has reported that emodin (10 mM) normalized cardiac gene expression with changes similar to the well-known HDAC inhibitor TSA. Emodin has been shown to block pathological cardiac hypertrophy in vivo and in vitro, which is consistent with its role as an HDAC inhibitor.

SIRT3
SIRT3 is a member of the Class III HDACs family (Zheng et al., 2019). It has been shown to play a key role in maintaining normal mitochondrial biological function through reversible deacetylation of protein lysine. Inhibition of SIRT3 can lead to defects in mitochondrial function, leading to the development of various cardiovascular diseases. The activation of SIRT3 may represent a promising therapeutic strategy to improve mitochondrial function and treat related cardiovascular diseases (Mihanfar et al., 2018;Wu J. et al., 2019). Gao et al. (2020) extracted rhubarb ingredients and cardiac hypertrophy targets based on machine learning and the network pharmacology method, and the optimal componenttarget combination (emodin-SIRT3) was obtained and further functional analysis was carried out on the combination. The results showed that emodin could treat or prevent agonistinduced and pressure-overload mediated cardiac hypertrophy. This suggested that activation of SIRT3 may be potential mechanisms for emodin to prevent mitochondrial dysfunction.

Anti-Oxidative Damage
Emodin can act on the antioxidant system (Huang et al., 1991;Yi et al., 2004). Due to the ability of single electron transfer, emodin is considered as a generator of reactive oxygen species (Shi et al., 1994). Studies have showed that the formation of myocardial oxygen free radicals (oxygen free radicals) increased during I-R, and mitochondria were the main source of oxygen free radicals. Myocardial I-R injury was associated with decreased mitochondrial ATP production and antioxidant level/activity (Munro and Treberg, 2017;Ryoo and Kwak, 2018). The decrease of mitochondrial ATP production might be due to the oxidative damage of the protein complexes involved in the electron transfer process, while the consumption of non-enzymatic antioxidants, such as GSH and atoc and the inhibition of antioxidant enzymes such as GRD and SOD will aggravate the oxidative damage of mitochondria (Yim et al., 1998). Du and Ko (2005) studied the sensitivity of emodin in a single dose treatment to mitochondrial ATP production and antioxidant composition in male and female rats and ischemia reperfusion (IR) injury. The results showed that a single dose of emodin can enhance the antioxidant components of mitochondria and prevent IR damage. Early experimental results have demonstrated the increased formation of oxygen-derived free radicals (oxy-radicals) in the myocardium during reperfusion after ischemia, wherein mitochondria are the major source of oxy-radical production. Pharmacological pretreatment of myocardium with low dose emodin chronic therapy can significantly enhance the antioxidant components of mitochondria. Du and Ko (2006) demonstrated that the combination of emodin in a low dose (25 mM/kg/day ×15) and IPC pretreatment produced a protective effect on the heart in a semiadditive manner. It protects the heart muscle from I-R damage.

Anti-Proliferation
Atherosclerosis is the main cause of ischemic heart disease, and percutaneous coronary intervention is the main method to treat coronary artery occlusion (Fischman et al., 1994;Serruys et al., 1994). However, progressive arterial restenosis caused by abnormal proliferation and migration of vascular smooth muscle cells (VSMC) impedes effective treatment (Hao et al., 2003). Therefore, the concern is inhibition of migration and proliferation of vascular smooth muscle cells. It has been reported that emodin can inhibit the proliferation of VSMC, thereby inhibiting the hyperplasia of the intima (Xu et al., 2018a). It is reported by  that emodin in a dose-dependent induced apoptosis of hVSMC and two additional forms of injury: autophagy and senescence. It may be regulated by ROS and p53 pathways. Subsequently, Wang S. J. et al. (2008) also proved that emodin directly inhibited VSMCs proliferation induced by Ang II in a dose-dependent manner. And inhibiting the expression of PCNA and c-myc may contribute to its anti-proliferative activity. Further evidence suggested that the proto-oncogene c-myc plays a key role in the cardiac protective properties of emodin.

Other Functions
The cardiovascular protective effects of emodin are still under further exploration. Zhou et al. (2014) demonstrated that emodin (30 mM) increased ANP secretion via inhibition of L-type Ca(2+) channels through an activation of K(+)ATP channel in isolated beating rabbit atria. The results also provided role of emodin in the treatment of cardiovascular homeostasis. Wu et al. (2014) has reported that emodin may have great therapeutic potential for DCM in the treatment of diabetes-induced cardiomyopathy by Akt/gsk-3 signaling pathway. A study has demonstrated that emodin significantly prevented DEP induced cardiac inflammation and oxidative stress, and thrombotic complications in vivo and in vitro (Nemmar et al., 2015). This suggested that the use of functional food such as emodin can be considered a useful agent and may have the potential to protect people of all ages against the cardiovascular events in cities with high concentrations of particulate air pollution which was pending further studies.

POTENTIAL APPLICATIONS IN CARDIOVASCULAR DISEASES THERAPY
Emodin targets multiple features of cardiovascular diseases through a series of pathways and has a variety of targets. Antioxidant, antiviral, and anti-fibrotic effects are similar to conventional therapeutic methods or target protein inhibitors. In addition, it also showed good synergistic therapeutic effect. All these make emodin have broad application prospects in the treatment of cardiovascular diseases ( Table 2). Among them, emodin has antioxidant properties. Pretreatment in Sprague-Dawley isolated mouse hearts (male and female) with I-R injury improved their antioxidant capacity, which was similar to the antioxidant a-tocopherol (a-TOC) and a-lipoic acid (a-LA) (Du and Ko, 2005). Emodin antiviral, reported by Liu et al. (2013) in BALB/c mice with viral myocarditis (CVB4), 15 mg/kg/d emodin treatment was equivalent to 10 mg/kg/d ribavirin can reduce viral cardiomyopathy. Antifibrosis role in hypertensive rats with LV fibrosis in Goldblatt (2K1C), Emodin, irbesartan or two drugs together can potentially inhibit the ventricular fibrosis in Goldblatt hypertensive rats. Furthermore, the combination of these two drugs may provide a better anti-fibrosis effect than the single application . In neonatal rat ventricular myocytes (NRVMs) with myocardial hypertrophy (phenylephrine induced), emodin blocks pathological cardiac hypertrophy and reverses stress-induced transcriptome changes in cardiomyocytes, consistent with HDAC inhibitor TSA (Evans et al., 2020). Emodin has a protective effect on myocardial I-R injury. In Sprague-Dawley isolated rat hearts with I-R injury, the protective effect of emodin pretreatment on myocardial I-R injury is consistent with that of ischemic preconditioning (IPC), and the protective effect of emodin and IPC combined pretreatment on myocardium is semi-additive (Du and Ko, 2006).

TOXICITY
Although emodin has beneficial pharmacology activities, it has been found that hepatotoxicity, nephrotoxicity, and reproductive toxicity limit its further study. First, it was found in studies that emodin has hepatotoxic effects . Chen et al. (2020b) found that long-term or high-dose emodin showed a cytotoxic effect in hepatocytes and inhibited hepatocyte nuclear factor 4a expression, thereby reducing UDP-glucuronosyltransferase 2B7 (UGT2B7) expression and causing hepatotoxicity.  also showed that there were gender differences in the hepatotoxicity and toxicology of emodin, which may be mediated by the coupling of UGT2B7 and multidrug-resistant-protein 2 in vivo. Second, with regard to the nephrotoxicity of emodin, the results of the United States National Toxicology Program showed that mice taking emodin at 22 mg·kg −1 were associated with tubular injury (increased incidence of tubular pigmentation) and increased incidence of kidney disease in female mice (National Toxicology Program, 2001). After 24 h of emodin was applied (40 and 80 µm), cell viability was significantly reduced by approximately 29% and 45%, respectively, which was mediated by the induction of apoptosis in the caspase 3-dependent pathway and the mitochondrial pathway (Wang C. et al., 2007. Wang C. et al. (2008) have shown that emodin has cytotoxic effects on HK-2 cells, partially via induction of cathepsin B (CB) protein and activation of CB protease. And then, the CK2a' knockout mice were given emodin orally for 5 days (1000 mg/kg/day), and it was found that emodin had testosterone toxicity due to disruption of testicular gene expression (Oshida et al., 2011). The different doses of emodin (25-400 mM) were applied to ejaculated human sperm, showed that sperm functions were inhibited by reducing sperm [Ca(2+)]i and prohibiting tyrosine phosphorylation in vitro (Luo et al., 2015). In addition, it was found that 24 h emodin (100 mM) interrupted several metabolites and biological processes including amino acid metabolism, purine metabolism, and Krebs cycle disruption . Since the ROS scavenging agent NAC can counteract the toxic effect of emodin, the increase in mitochondrial reactive oxygen production was also considered as an aspect of emodin toxicity (Kolitsida and Abeliovich, 2017).

PHARMACOKINETICS
Emodin has a wide range of biological activities, but its bioavailability is low when taken orally . Shia et al. (2010) reported that after intragastric administration at doses of 20 or 40 mg/kg, emodin rapidly underwent phase II metabolism to form its glucuronide, and the parent form of emodin was almost undetectable in vivo. Even at higher doses (82.4 mg/kg), the levels of emodin detected in vivo remained very low. The appearance of glucuronidation metabolism and oxidative conversion into rhein may be the main reasons for the extremely poor oral bioavailability of emodin (Liu et al., 2012;Li et al., 2017;Cheng et al., 2020). Li et al. (2019) revealed that the absorption of emodin was faster, and the effect was rapider when used in combination with aloeemodin, rhein, chrysophanol, or physcion. Trans-2,3,5,4′tetrahydroxystilbene-2-O-b-D-glucopyranoside (TSG) can promote the absorption of emodin and increase its plasma concentration to enhance the efficacy of emodin (Xing et al., 2019). A similar study revealed that piperine significantly increased the C max and area under concentration-time curve (AUC) of emodin and decreased the C max and AUC of emodin glucuronide (Di et al., 2015). Therefore, the application of emodin is greatly limited, because of its poor intestinal absorption, rapid elimination and low bioavailability in vivo after oral administration. In future studies, it is important and urgent to improve oral bioavailability in clinical practice.

EMODIN DERIVATIVES
In order to apply emodin safely and effectively, the research on emodin derivatives has been carried out continuously. Chen et al. (2016) reported that the emodin derivatives, madagascine, has been ameliorated vasospasm related cardiovascular diseases. It exerted vasodilatation effect through activating adenosine 5′monophosphate (AMP)-activated protein kinase (AMPK), leading to the activation of eNOS in endothelium and inhibition of rho-associated protein kinase/myosine phosphatase target subunit 1(ROCK/MYPT1) in vascular smooth muscle (VSM). Furthermore, under the same experimental conditions, compared with emodin, it has higher biological activity and safety. Huang et al. (2014) provided evidence that PPemd26, an anthraquinone derivative, suppressed angiogenesis through inhibiting the vascular Endothelial Growth Factor Receptor 2 (VEGFR2) signaling pathway, suggesting that it is a potential drug candidate for the development of anti-angiogenic agents for angiogenesis-related diseases, including coronary heart disease. Ji et al. (2020)  reported that haloemodin had strong antibacterial activity against Gram-positive bacterial strains. Compared with emodin, haloemodin more rapidly increases the bacterial plasma membrane permeability for potassium ions. More importantly, compared with emodin, halomodin binds to protein tighter and more stable. The results partially explain the stronger antibacterial activities of haloemodin over their parent nucleus, emodin. Chen et al. (2020a) disclosed that Aloe-Emodin (AE) could alleviate high-fat diet/palmitic acid-induced cardiac inflammation via inhibition of the TLR4/NF-kB signaling pathway. Thus, AE may be a promising therapeutic strategy to prevent myocardial injury kB and p-P65l caused by hyperlipidemia. Bai's findings suggested a novel pharmacological role of AE in high-fat diet-induced cardiac electrical remodelling (Bai et al., 2017). Yu et al. (2019) findings indicated that AE can protect against myocardial infarction via the upregulation of miR-133, inhibition of ROS production and suppression of caspase-3 apoptotic signaling pathway. Zhang et al. (2020) results revealed that inhibiting the activation of NLRP3 inflammasome and reducing the release of HMGB1 by promoting NLRP3 ubiquitination, thereby restoring the endothelial tight junction proteins and permeability, which indicated that AE exhibited immense potential therapeutic value in hypertensionrelated cardiovascular disease and the development of innovative drugs ( Table 3).

SUMMARY AND OUTLOOKS
In summary, it is reported that emodin has a wide range of anticardiovascular activities. It is involved in multiple molecular targets, such as inflammatory regulation, anti-apoptosis, antihypertrophy, anti-myocardial fibrosis, inhibition of oxidative damage, and abnormal and excessive proliferation of smooth muscle cells. It showed a good therapeutic prospect in the treatment of myocardial hypertrophy, coronary heart disease, and myocardial ischemia/reperfusion injury. Numerous studies have also made huge strides in revealing the mechanisms behind these effects. In addition, similar therapeutic effects and better synergistic effects were also shown in studies comparing emodin with conventional therapeutic methods or target protein inhibitors. However, despite the encouraging results in this research field, the current research only exists in the animal research stage, and a systematic and overall understanding of its mechanisms of action has not yet been formed. More importantly, attentions should be paid to the toxicity of emodin due to the limitation of its use caused by its increased content and low bioavailability. They are the most important factor limiting in the future research. With the deepening of research on emodin, its toxicity has been continuously discovered, and it is necessary to search for more bioavailability and toxicity neutralization solutions.

AUTHOR CONTRIBUTIONS
Authors' contributions were as follows: conception and design of the study (QL, JG, YW); combing the literature (QL, XP); drafting the article (QL); revisions for content (QL, AC); and critical revisions for important intellectual content (QL, YW). All authors contributed to the article and approved the submitted version.

FUNDING
We acknowledge the financial supports from the National Natural Science Foundation of China (81573832)  Suppresses angiogenesis via inhibiting VEGFR2 signaling.  Madagascine 3-isopentenyloxyemodin In vitro and in vivo: cell HUVECs and HCASMCs; Male Wistar rats.