Volume 18 Issue 6
Jun.  2020
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The pharmacology, toxicology and therapeutic potential of anthraquinone derivative emodin

  • Corresponding author: Tel: 86-28-85503040, E-mail: lijuan@scu.edu.cn
  • Received Date: 01-Oct.-2019
    Available Date: 14-Feb.-2020
  • Emodin (1, 3, 8-trihydroxy-6-methylanthraquinone) is a derived anthraquinone compound extracted from roots and barks of pharmaceutical plants, including Rheum palmatum, Aloe vera, Giant knotweed, Polygonum multiflorum and Polygonum cuspidatum. The review aims to provide a scientific summary of emodin in pharmacological activities and toxicity in order to identify the therapeutic potential for its use in human specific organs as a new medicine. Based on the fundamental properties, such as anticancer, anti-inflammatory, antioxidant, antibacterial, antivirs, anti-diabetes, immunosuppressive and osteogenesis promotion, emodin is expected to become an effective preventive and therapeutic drug of cancer, myocardial infarction, atherosclerosis, diabetes, acute pancreatitis, asthma, periodontitis, fatty livers and neurodegenerative diseases. This article intends to provide a novel insight for further development of emodin, hoping to reveal the potential of emodin and necessity of further studies in this field.
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The pharmacology, toxicology and therapeutic potential of anthraquinone derivative emodin

    Corresponding author: Tel: 86-28-85503040, E-mail: lijuan@scu.edu.cn
  • 1. State Key laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University, Chengdu 610041, China
  • 2. Department of orthodontics, West China School of Stomatology Sichuan University, Chengdu 610041, China
  • 3. West China Hospital of Clinical Medicine, Sichuan University, Chengdu 610041, China

Abstract: Emodin (1, 3, 8-trihydroxy-6-methylanthraquinone) is a derived anthraquinone compound extracted from roots and barks of pharmaceutical plants, including Rheum palmatum, Aloe vera, Giant knotweed, Polygonum multiflorum and Polygonum cuspidatum. The review aims to provide a scientific summary of emodin in pharmacological activities and toxicity in order to identify the therapeutic potential for its use in human specific organs as a new medicine. Based on the fundamental properties, such as anticancer, anti-inflammatory, antioxidant, antibacterial, antivirs, anti-diabetes, immunosuppressive and osteogenesis promotion, emodin is expected to become an effective preventive and therapeutic drug of cancer, myocardial infarction, atherosclerosis, diabetes, acute pancreatitis, asthma, periodontitis, fatty livers and neurodegenerative diseases. This article intends to provide a novel insight for further development of emodin, hoping to reveal the potential of emodin and necessity of further studies in this field.

    • Emodin (1, 3, 8-trihydroxy-6-methylanthraquinone) is a derived anthraquinone compound extracted from roots and barks of pharmaceutical plants, including Rheum palmatum, Aloe vera, Giant knotweed, Polygonum multiflorum and Polygonum cuspidatum [1-5], which has a history of over 2000 years and is still existing in multiple herbal preparations. It is well-known that emodin has various pharmacological properties, including anticancer, anti-inflammatory, antioxidant, antibacterial, antivirus, anti-diabetes, immunosuppressive and osteogenesis promotion activity [6]. These beneficial effects of emodin indicate that it might be a valuable medicine for preventing and treating multiple diseases in human body. However, the specific mechanisms of emodin are discrepant and sophisticated in various body cells, tissues and organs, such as cerebrum, angiocarpy, lung, liver, pancreas, oral cavity and skeleton. The anticancer activity of emodin is the focus of studies all the time. Interestingly, recent studies report that emodin may have a potential promotion effect on bone tissue regeneration. Emodin can induce osteoblasts formation and inhibit osteoclasts activation and maturation in bone microenvironment [7]. It also has a potential therapeutic action on pancreas and lung tissues, which mainly relies on the anti-inflammation and antioxidant activities. Furthermore, emodin exerts potential therapeutic action on mutiple chronic diseases, including myocardial infarction, atherosis, diabetes and alzheimer disease [8-11]. However, emodin has several disadvantages, such as nephrotoxicity, hepatotoxicity, genetoxicity as well as low bioactivity via oral administration [12-15]. Therefore, numerous challenges are still to go through in order to minimize the toxity of emodin to human body. Although the studies concerning emodin are still on the road accompanied with quite a few controversies, emodin is more likely to be healpful to public health in future. This review focus on the fundamental properties of emodin and its therapeutic action on specific body tissue, at the same time, we review the potential and sophisticated mechanisms of emodin in mutiple diseases, including cancer, bone regeneration and cardiovascular system etc., hoping to provide several suggestions on the prospective studies of emodin in clinical application.

    Pharmacological Properties
    • For all the time, the anticancer activity of emodin attracts extensive attention. There are findings suggesting that emodin has extensive therapeutic effects on tumors, such as lung cancer, pancreatic cancer, breast cancer [16-18], as well as its possible mechanisms are related to inhibiting tumor proliferation, angiogenesis, invasion, metastasis, promoting tumor apoptosis, reversing multidrug resistance and increasing sensitivity to chemotherapy in a concentration-dependent manner [19]. Orchestral signaling cascades play an important role in tumor cell biological behaviors. Some recent studies have shown emodin can effectively inhibit the activation of multiple signaling cascades that are closely associated with proliferation of tumor cells. For example, Chang et al. [20] reported the cyclin D1 transcription activity in TE1 cells was lessened with emodin treatment (2.5, 5, 10, 20 μmol·L−1), indicating that emodin could inhibit the proliferation and differentiation of esophageal cancer cells in a does-dependent manner via inhibiting the phosphorylation levels of AKT and ERK. The effect of emodin on inhibiting tumor cells growth is related to the NF-κB signal pathway, which also effectively also effectively reversed drug resistance during the treatment with gemcitabine [21]. Lu et al. [22] found emodin inhibited the growth and induced the apoptosis of SMMC-7721 cells in hepatocellular carcinoma, and its possible molecular mechanism correlated with the inhibition of MAPK and PI3K/AKT pathways. Moreover, increasing evidence has demonstrated that emodin induced tumor apoptosis on account of activating apoptosis-related pathways, including mitochondria-mediated, death receptor-mediated and ER stress-initiating apoptosis pathways. Emodin induced HepG2 cells apoptosis through a multilevel cascade, which involved in the production of mitochondrial debris (e.g. cytochrome c) as well as apoptosis-related molecules (e.g. caspase-3, caspase-8 and caspase-9). Meanwhile, the cytotoxicity effect of emodin on HepG2 cells may relate to oxidative stress injury of intracellular mitochondria induced by the BCL/BAX ratio and the NF-κB pathway [12-23]. Membrane receptor FAS (CD95/APO-1) and its ligand FASL (CD95L) are both belonging to TNF receptor (TNFR) superfamily, which major and best known character is apoptotic cell death [24]. Recent studies have addressed that emodin induced growth inhibition and apoptosis of lung cancer A549 cells through up-regulating the expression levels of FASL [25]. Additionally, new evidence indicates that endoplasmic reticulum (ER) stress is responsible for emodin-induced apoptosis of A549 cells, and underlying mechanism may relate to inhibition of TRIB3/NF-κB signal pathway hway [26]. Furthermore, one of key characteristics of malignant tumors depends on its capability of invasion and metastasis, which may cause postoperative recidivation and poor prognosis of cancer patients [27]. Emodin was reported to poss a potential effect on inhibiting metastasis and invasion of multiple malignant neoplasm, such as human pancreatic cancer, high metastatic breast cancer, tongue cancer and neuroblastoma, which main mechanism may relate to inhibiting epithelial-mesenchymal transition (EMT) of tumor cells associated with the expression levels of matrix metalloproteinases (MMPs) and E-cadherin [28-32]. Meanwhile, aloe-emodin could also enhance radiosensitivity and chemosensitivity, inhibit angiogenesis and drug resistance of tumors, but its underlying mechanisms are still indistinct [33-35].

      As a traditional Chinese medicine extract, emodin has attracted great attention for its prominent antitumor effect in vitro. Nonetheless, there are still several problems about its effects in vivo and the treatment of different diseases. For example, the anticancer action of emodin tightly relies on its high concentration, whereas the effect of the intake quantity of emodin of our eating aloe is significantly little in normal life and the oral bioavailability in vivo is low as well [36]. Furthermore, it is impossible to ignore the fact that emodin would produce kidney toxicity, hepatotoxicity and cytotoxicity in high doses and with long-term use. Therefore, more clinical trials are still required for the application of emodin in malignant neoplasm (Fig. 1).

      Figure 1.  Emodin for different treatments

    • Emodin has been reported to poss remarkable anti-inflammation effects in many researches. It is a chain coordinating process of inflammatory response activation, in which a large number of immune cells aggregate at the inflammatory sites, release multiple proinflammatory cytokines, accumulate oxygen/nitrogen species (ROS/RNS) and initiate oxidative stress [37]. Macrophages are usually distinguished as pro-inflammatory M1 macrophages and anti-inflammatory M2 macrophages, which play an important role in many stages of inflammatory response [38]. Interestingly, the M1 or M2 polarization of macrophages was inhibited with treatment of emodin on account of the removal of H3K27 trimethylation (H3K27m3) marks and the addition of H3K27 acetylation (H3K27ac) marks on genes, as well as the production of proinflammatory factors was decreased [39-40]. Recent study has shown that emodin had protective effects against cell injury and inflammation of AR42J cells that were induced by mitochondrial damage and ROS-mediated pathway, and the levels of inflammatory cytokines, including TNF-α and IL-6, were inhibited [41]. Furthermore, with the establishment of emodin combined with silver nanoparticles (E/S), the compound significantly down-regulated the expression levels of IL-8, TNF-α and lactic dehydrogenase (LDH) compared to simple emodin, and exerted an anti-sepsis protection via inhibition of NF-κB, p38 pathways as well as inflammatory cells infiltration [42]. Therefore, emodin with the establishment of drug delivery pathway might effectively ameliorate the poor bioavailability via oral administration and increase the anti-inflamation property.

      In conclusion, it can clearly know that emodin could restore dynamic homeostasis between M1 and M2 macrophages in various inflammation-induced pathologies as well as modulate macrophage phagocytosis, migration and the production of pro-inflammation factors. Moreover, the anti-inflammation activity of emodin is associated with activation process of inflammation-related signal transductions, such as ROS-mediated, NF-κB and p38 pathways [43, 44]. It is possible that emodin could be extensively applied to inflammation-mediating tissue damages and diseases in future.

    • It is impossible to ignore the antioxidant action of emodin in human body. Excessive free-radical and reactive oxygen species produced in cellular metabolic process were detrimental to human health, which might give rise to DNA injury and cytotoxicity [45]. Recent study demonstrates that dietary emodin (30 mg·kg−1) significantly increases the antioxidant-related mRNA expressions of GPx1, GSTM and HSP70 in liver of megalobrama amblycephala as well as inhibits the expression of reactive oxygen species via SOD, H2O2, superoxide and NF-κB pathway . However, there are some experiments pointing out that emodin of high concentration (1.25−2.5 mmol·L−1) may alter the subcellular redox equilibrium and produce cytotoxicity induced by reactive oxygen species. For example, emodin (1−25 μg·mL−1 induces mitochondria-induced cell apoptosis associated with △ψm disruption, ROS and SDH generation [46]. It can not ignore characters of emodin at high concentration resulting in DNA injury, as well as inducing a pro-oxidant effect that reducing power of ions transcends the scavenging activity of hydroxyl free radical [47]. Therefore, the antioxidant and pro-oxidative activities of emodin may be affected by dose and concentration in a sophisticated way, and more underlying mechanisms need further investigations.

      From above study results, we can know that the phenol rings of emodin structure exert a potential antioxidant activity related to donating electrons, scavenging of free radicals, inhibiting reactive oxygen species (ROS) and reducing oxidative stress damages [48], but detailed biochemical mechanisms tolerant to oxadation need to take a further exploration.

    • Increasing studies have demonstrated that emodin might exert a broad range of inhibiting effects on immune system. After (1, 10, 100 μmol·L−1) emodin treatment for 72 h or (100 μmol·L−1) emodin treatment for 24, 48, 72 h, Qu et al. found the growth of human T cells as well as cell adhision were increasingly inhibited, thus we can know that emodin could inhibit the growth of human T cells and induce apoptosis in does- and time- dependent manner by triggering ROS-mediated ER stress system, disturbing mitochondrial membrane potential of releasing cytochrome C as well as activating cleavage fragments, including caspase-3, caspase-4, caspase-9, ROA and MDA MDA [49]. Besides, emodin inhibited the proliferation of T cells in vitro through blocking the mTOR signaling pathway, as well as hindered DC maturation, inhibited alloimmunity and alloantibody production mainly induced by both CD4+ FoxP3+ and CD8+ CD122+ Tregs [50]. However, emodin (100 μmol·L−1) when down-regulating the expression ratios of CD80 and CD83 in dendritic cells increased the number of Tregs with lower levels of HLA-DR, GITR, CTLA-4 [51]. Therefore, emodin may exert extensively immunosuppression via modulating T lymphocytes, dendritic cells and Tregs.

      These studies indicate emodin may play a significant role in modulating immune rejection through inhibiting differentiation and maturation of lymphocytes. The possible mechanism of emodin inducing immunosuppression may relate to suppressing activation of multiple proinflammatory factors that usually cause immune rejection or immune tolerance, finally inhibiting differentiation and maturation of Tcells, DC cells and Tregs [52].

    Antibacterial
    • Emodin effectively inhibits the proliferation of pathogenic microorganisms via acting on cell membrane and DNA. Li et al. [53] demonstrated that emodin showed potent inhibitory effect against Haemophilus parasuis that is the main pathogen of Glasser’s disease. Emodin was confirmed to produce significant cell membrane depolarization due to potassium leakage and damage to the selective permeability of bacterial cell membrane, resulting in cell death of bacterials. Emodin can induce DNA condensation because of a strong electrostatic interaction with bacterial DNA. It is worth noting that emodin inhibits the growth of common drug-resistant bacterial strains, such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus faecium (VRE) [54]. Furthermore, there are findings showing that emodin could effectively stop hyphal formation of clinical Candida strains at a low concentration and show a distinct antibiotic potential [55]. And also, candida culture added emodin inhibited the phosphorylation of many cellular proteins owing to the occupation of the ATP-binding of CK2 [56].

      Some researches have addressed emodin could destruct the constructing of membrane protein, increase membrane permeability and destroy cell membrane integrity of mutiple bacteriums. What may be concerned is if the anti-proliferation effect is also found in other pathogenic microorganisms. Cutaneous leishmaniasis is caused by parasites, which is endemic in some parts of Iran. Recent study has reported topical application of aloe-emodin could inhibit the growth of Leishmania amastigotes and induced apoptosis in promastigotes, thus reducing the ulcer size [57].

    • Emodin can effectively inhibit viral infection to some extent. It is well known that viral infection is a common public health problem worldwide, especially in developing countries. Recent experiments have shown aloe-emodin increased the expression of antiviral genes, such as IFN-β, IFN-γ, PKR and 2′5′–OAS alic > γ, PKR and 2′5′–OAS [58, 59]. Meanwhile, the antiviral effect of emodin also associates with the inhibition of viral replication and virus-induced cytopathic effect, for example, increasing studies reports that emodin significantly controlls influenza infection induced by influenza A H7N9 virus in humans via inhibiting the replication of influenza A and virus-induced cytopathic effect in MDCK cells [60]. Zhong et al. [61] proved that emodin (29.6 μmol·L−1) effectively protected MRC5 cells from EV71-induced cytopathic effect due to inhibiting viral replication and maturation on genomic and protein expression levels. Therefore, it is possible that emodin could become a potentially prophylactic and therapeutic drug to control virus infection via inhibiting virus replication on gene level, reducing virus-induced cytopathic effect, and increasing antiviral genes expression as well.

    • In recent years, emodin has been found to poss a potential promotion effect on osteogenesis. Bone tissue remolding is a complex, interlocking physiological process of bone formation through adult life, in which new bone formed by osteoblasts is coupling with the primordial bone resorbed by osteoclasts and both are in a dynamic balance [62]. However, It is occurring of bone resorption while osteoclastogenesis exceeds osteoblastogenesis, leading to bone metabolism-related disease, such as osteoporosis, rheumatoid arthritis (RA) and progressing periodontitis [63]. Osteoporosis is a severe disease characterized by decrease in bone density and degeneration in bone fibrous. Kang et al. [64] has demonstrated that emodin treatment showed the anti-osteoporotic effect via the marked inhibition of bone resorption in the mouse model of LPS-mediated osteoporosis. Increasing researches prove that emodin is closely linked with bone metabolism [65]. Some studies have indicated that emodin accelerated osteoblast differentiation and bone mineral density through acting on phosphatidylinositol 3-kinase (PI3K) pathway and promoting the expression of bone morphogenetic protein-2 (BMP-2) gene [66], whereas it inhibited osteoclast differentiation of hematopoietic stem cell (HSC) and the bone-resorbing activity of mature osteoclasts via inhibiting receptor activator of nuclear transformation factor (NF-κB) ligand (RANKL) and RANKL-induced NF-κB [67].

      These study results suggest that emodin could induce osteoblasts formation and inhibit osteoclasts activation and maturation in bone microenvironment. The osteogenesis promotion activity of emodin may be closely correlated to maintaining the dynamic balance of bone microenvironment during the bone reconstruction and bone remolding process. Therefore, it can be concluded that emodin might be a promising drug in the precautions and treatments of bone metabolism-related diseases in future.

    Toxicity
    • It can not deny that emodin posses various pharmacological properties, however, several studies have reported the toxic effect of emodin, including hepatotoxicity, nephrotoxicity, genotoxicity, reproductive toxicity and phototoxicity [68-71]. Chen et al. established a cell metabonomic method based on nuclear magnetic resonance (NMR), which identified the toxicity of emodin in HepG2 cells [12]. Nesslany et al. [72] reported aloe emodin (AE) induced DNA fragmentation in kidney and colon cells, indicating the target organ of AE may be kidney as it is one of major route in excretion. Uchino et al. [73] found emodin exerted cytotoxity and genotoxicity to Human peripheral blood lymphocytes (HPBLs) at 150 μg·mL−1 and 200 μg·mL−1, which caused cell death and DNA damage because of excessive oxidative stress. Furthermore, emodin resulted in hypospermatogenesis, eosinophilic change and apoptosis of germ cell associated with the IGF-1 receptor signaling pathway and case in kinase II (CK2) expression [74]. Increasing researches find that emodin exerts side-effects in embryonic development by inhibiting retinoic acid receptors and inducing injury in mouse blastocysts, indicating it may have a potential effect on inducing reproductive toxicity [75]. Interestingly, Brkanac et al. [76] reported the phototoxicity of aloe emodin, which may depend on its direct photo-oxidative damage to DNA or RNA and reactive oxygen species generated through energy transfer or electron transfer.

      The research results reflect that emodin can exert hepatotoxity, nephrotoxicity, genotoxicity, reproductive toxicity and phototoxicity associated with the process of metabolism and excretion, oxidative stress damage and related receptor-mediated signaling pathways. Therefore, it is necessary to comprehend mechanism and feature of the toxity in different tissues of emodin so as to ongoing treatment potential.

    The Application of Emodin in Specific Tissues
    • All the time, emodin has been demonstrated to have a potential protective action on cardiovascular system. Currently, mutiple studies substantiate that emodin has several properties, including protecting acute myocardial infarction cells, inhibiting proliferation of VSMC and endothelial cells, dilating blood vessels through two ways of non-endothelium-dependent and endothelium-dependent and stabilizing atherosclerotic plaque [77-80]. Acute myocardial infarction (AMI) mainly involves in the process of partial myocardial ischemic necrosis caused by acute coronary artery occlusion, which is one of primary reasons of human death caused by cardiovascular diseases [81]. Several cytokines, such as TNF, IL-1 and IL-6, play an important role in the inflammatory phenomenon of AMI associated with promoting the formation and rupture of coronary atherosclerotic plaques of the initial factors in AMI. And tumor necrosis factor (TNF), interleukin-6 (IL-6) and other pro-inflammatory cytokines in serum or plasma of AMI patients at acute stage are higher than normal and their levels are closely related to cardiac function classification [82]. Additionally, NF-κB is a nuclear transcription factor commonly associated with inflammatory responses. Recently, Song et al. report that emodin can significantly suppress the expression of tumor necrosis factor (TNF-α) and the activation of NF-κB in the area of local myocardial infarction and play a protective role in mouse models of myocardial ischemia [83]. It is possible that emodin may play a protective role in myocardium by regulating the release of local inflammatory mediators and inhibiting inflammation-mediated tissue damage. Moreover, increasing evidence indicates emodin exerts an inhibitory effect on the formation and progress of vulnerable atherosclerotic plaque (VAP) [84]. For instance, Zhou et al. [85] found that emodin effectively inhibited the expression of GM-CSF and MMP-9 and stabilized the VAP in the aortic root of ApoE-knockout mice. Meanwhile, emodin has vasodilatory functions by inhibiting the contractile effect of 5-hydroxytryptamine and synergizing the diastolic effect of acetylcholine related to up-regulation of free radicals, hydrogen peroxidation and cGMP, as well as its possible mechanisms of improving microcirculation are associated with inhibiting platelet aggregation and reducing blood viscosity [86, 87].

      In conclusion, emodin has a protective effect on the cardiovascular system, and the main mechanism is connected with its anti-inflammation and antioxidation resistance. Although beneficial effects of emodin on the cardiovascular system were proved in numerous experiments in vivo and in vitro, how to apply emodin to the corresponding treatment needs more clinical tests (Table 1).

      Tissue/OrgansDiseaseAction MechanismReference
      Epithelium Epithelial ovarian cancer Inhibits integrin-linked kinase and epithelial–mesenchymal transition associated factors [21]
      Esophageal cancer
      (TE1 cells)
      Inhibits AKT and ERK associated with cell proliferation and differentiation [20]
      Tongue cancer Induces DNA damage and inhibits DNA repair gene expression; Inhibits gene expression of metalloproteinase-9 [31]
      Cardiovascular system Myocardial infarction Regulates the release of local inflammatory mediators and inhibits oxidative stress injury [84]
      Atherosclerosis Stabilizes vulnerable atherosclerotic plaque; Inhibits platelet aggregation and blood viscosity [85]
      Diabetic cardiomyopathy Induces phosphorylation of Akt and GSK-3β in myocardium. [93]
      Eyes
      Kidney
      Diabetic retinopathy Inhibits aldose reductase (AR) and revascularization [94]
      Diabetic nephropathy (DN) Inhibits inflammation-related factors and oxidative stress damages [92]
      Pancreas Pancreatic cancer Inhibits pancreatic cancer cell growth and angiogenesis, induces apoptosis [23]
      Acute pancreatitis Induces apoptosis of inflammation-related lymphocytes; Inhibits excessive oxidative stress and inflammatory cytokines expression [97]
      [98]
      Oral cavity Dental Caries Inhibits the growth, insoluble glucans synthesis and acid production of S. mutans [101]
      Periodontitis Inhibits the levels of NO, inflammatory response and alveolar bone absorption [106]
      Lung Lung cancer (A549 and H1299 cells) Induced ER stress and TRIB3/NF-κB pathway [18]
      Asthma Restricts Th2-related macrophage polarization and airway inflammation action [40]
      Acute Lung Injury Exerts anti-fibrotic activity, reverses epithelial-mesenchymal transition (EMT) and inhibits oxidative stress damage [108]
      Pulmonary edema [109]
      Nervous system Alzheimer’s disease (AD) Inhibits β-amyloid-induced toxicity by PI3K/Akt and the class III phosphatidylinositol-3-kinase pathways; Exerts direct neuroprotective effect via regulation of hormones, nerve growth factors (NGF) and related-signaling pathway [111]
      Parkinson’s disease (PD) [113]
      [114]
      [115]
      Liver Hepatocellular carcinoma Inhibits MAPK and PI3K/AKT pathways
      Fatty Liver IInhibits SREBP1 activity via the CaMKK-AMPK-mTOR-p70S6K signaling pathway; Decreases alanine aminotransferase (ALT), hepatic triglycerides and aspartate aminotransferase [24]
      [116]
      Hepatitis B virus (HBV) Inhibits HBV DNA replication [118]
      Skeleton Osteoporosis Inhibits bone resorption by OPG and RANKL/RANK pathway [62]

      Table 1.  Emodin for different treatents

    • Recently, emodin has been implicated as a therapeutic medicine for diabetes and its severe complications. Diabetes mellitus (DM) is a chronic disease with increasing prevalence in population growth, aging, urbanization and obesity, which complications increasingly contribute to the mortality of diabetes patients [88]. There are findings suggesting that emodin can protect against diabetic nephropathy (DN), which underlying mechanism may involve inhibition of inflammation-related factors and oxidative stress damages [89]. However, what can be concerned is how to ameliorate the hyposensitive anti-diabetes property because of poor oral bioavailability of emodin. With establishment of the self-microemulsifying drug delivery system (SMEDDS) based on emodin, it attained better suppressive effects on the protein level of renal fibrosis compositions in AGEs-induced GMCs and NRK-52E cells [90]. Emodin that was encapsulated into nanoparticle significantly supressed currents activation of HEK293 cells transfected with the P2X3 receptor mediating DN in the dorsal root ganglia [91]. Furthermore, emodin may have potential therapeutic effects on diabetic retinopathy and cataract patients through inhibiting the activity of aldose reductase and ameliorating retinal neovascularization [92, 93]. For example, it is well-known that emodin has good selectively inhibitory activity against aldose reductase (AR) (IC50 = 2.69 ± 0.90 μmol·L−1) and is stable at 37 °C for at least 7 days, and the 3-hydroxy group of emodin that interacts with Ser302 through hydrogen bonding in the specificity pocket of AR plays an essential role. Meanwhile, recent studies show that aloe-emodin prevents hypoxia-induced retinal neovascularization through inhibition of VEGF, prolyl hydroxylase-2 and HIF-1α [ 94].

      From above research results, we can know that emodin could have a distinct therapeutic effect on the complications of diabetes, including diabetic nephropathy, retinopathy and cataract, which potantial mechanisms may link to inhibiting oxidation-related enzyme, revascularization, tissue fibrosis and damages, etc.

    • Increasing studies address that emodin plays a potential role in the treatment of pancreatitis. Acute pancreatitis is a common disease and mainly ruled by its complications and recurrent attacks. Interestingly, emodin can decrease the expression of preBcell colonyenhancing factor and promote the apoptosis of polymorphonuclear leukocyte neutrophil in severe acute pancreatitis (SAP)-associated acute lung injury (ALI) [95]. Meanwhile, lung oedema induced by SPA of rats was significantly alleviated with the combined treatment of emodin and dexamethasone (DEX) [96]. Excessive oxidative stress induced by acute inflammatory response usually causes tissue damages and plays an important role in the development course of acute pancreatitis. Jin et al. found emodin with the strategy of high concentration effectively diminished oxidative stress in SAP rats, and shown lower levels of serum amylase, HMGB 1 and COX-2 and higher levels of PPAR-γ [97]. In another experiment, Wu et al. [98] found that treatment with emodin obviously ameliorated pancreatic injury and decreased the release of amylase and inflammatory cytokines such as TNF-α and IL-6.

      Therefore, above study results have shown that emodin might become the beneficial treatment medicine in severe acute pancreatitis and its multiple complications via suppressing oxidative stress, immune and inflammatory responses. Although the inhibiting effect of emodin on the complications and recurrent attacks of pancreatitis has been reported in numerous researches, the clinical treatment of emodin in SPA patients is little reported.

    • The application of emodin in the field of oral treatment mainly presumes on its anti-inflammatory, antibacterial effects and inhibition of osteoclast-induced bone resorption. Dental caries is a bacterial infection-related oral disease prevalent across the world. All the time, Streptococcus mutans (S. mutans) is considered as a crucial pathogen in the pathogenesis of dental caries [99]. The major mechanism of being responsible for the cariogenicity of S. mutans relies on its ability of producing glucosyltransferases (Gtfs), synthesizing insoluble glucans, generating acids and surviving at low pH environment [100]. Interestingly, Pandit et al. [101] demonstrated that polygonum cuspidatum extract F1, mainly composed of resveratrol, emodin and physcion (approximately 16.2%, 18.9% and 2.07% of the weight of F1, respectively), might be useful in control of dental biofilms and improving the cariostatic properties of fluoride without increasing its exposure. According to study results, emodin (0.5−2 mg·mL−1) effectively inhibits the growth, insoluble glucans synthesis and acid production of S. mutans, and reduces the incidence and severity of carious lesions in rats [102]. These experiments suggest that the natural compound emodin might have a potential prevention and treatment effect on dental caries. Additionally, very recent researches show that periodontitis is a bacteria-induced inflammatory bone loss disease. One of important reasons of alveolar bone destruction is closely associated with inflammatory response caused by oral pathogens, including Porphyromonas gingivalis (P.g), Actinobacillus actinomycetemcomitans (A. a) and Tannerella forsythia (T. f) [103-105]. Interestingly, it was found that the levels of NO in the peripheral blood and gingival tissue were decreased with emodin treatment, and subsequent inflammatory response and alveolar bone absorption were inhibited as well [106]. From above study results, it is obvious that emodin exerts suppressing effects on cariogenic bacteria and metabolic bone resorption, this compound might have the possibility in prevention and treatment of dental diseases, thus further studies on it are still needed.

    • Emodin has potential therapeutic effects on asthma and several lung damage diseases. The primary mechanism of emodin on asthma may associate with inhibition of airway inflammation reaction. For example, Song et al. [107]found emodin effectively decreased pulmonary inflammatory cells infiltration, mucus secretion and serum IgE production, as well as IL-4-mediated macrophage polarization and STAT6 phosphorylation. Additionally, There are findings showing that emodin alleviates lung injury by suppressing and reversing epithelial-mesenchymal transition (EMT)-like shifts, reducing the accumulation of p-IκBα, NF-κB and TGF-β1, and activating the Nrf2-antioxidant signaling pathway [108]. Furthermore, it is impossible to ignore the fact that emodin may have an anti-fibrotic effect on COPD patients, for instance, Guan et al. [109] proved emodin exerted anti-fibrotic activity via inhibiting the levels of TNF-α, IL-6, TGF-β1 and heat shock protein (HSP)-47 induced by Smad2/3 and STAT3 signaling molecules in the lungs of BLM-treated rats.

      Therefore, the emodin reduces lung structural distortion and damages through inhibition of massive inflammatory cells infiltration and pro-inflammatory cytokines expansion, and its possible mechanisms on asthma and lung damages may closely associate with its superiority of anti-inflammation, anti-oxidation and immunosuppressive activity.

    • Recently, emodin has been reported to have a potential nervous system protection effect. The protein misfolding resulting from β-amyloid protein (Aβ) in brain plays an important role in the pathogenesis of neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD) [110]. Liu et al. [111] found that emodin prevented the cultured cortical neurons from β-amyloid-induced toxicity, which was closely associated with increasing the Bcl-2 regulated by the ER stress and PI3K/Akt pathway. Meanwhile, Sun et al. [112] found emodin blockaded the β-amyloid-induced autophagy with the activation of the class III phosphatidylinositol-3-kinase and downstream signaling molecules ules ules. In a word, these results provided adequate evidence for the application of emodin in the prevention and treatment of neurodegenerative diseases associated with the misfolding of Aβ. Moreover, Ma et al. proved that emodin normalized the change of the plasma corticosterone level and up-regulated the mRNA and protein expression levels of hippocampal glucocorticoid receptor (GR) and brain-derived neurotrophic factor (BDNF) [113]. Additionally, emodin treatment effectively reduced OGD/R-lead to neurotoxicity through potentiating Nrf2/ARE-regulated neuroprotection associated with the AMPK/GSK3β pathway in SH-SY5Y cells as well [114]. Therefore, emodin may produce direct neuroprotective effect via regulation of hormones, nerve growth factors (NGF) and related-signaling pathway [115]. Although these studies in vitro and in vivo indicate that emodin might be useful for treating neurodegenerative disorders, more laboratory and clinical experiments are needed for its clinical treatments.

    • It is well-known that emodin generates hepatocyte protection action via suppressing fibration. Liver is one of the primary organs participating in the metabolism of human body, there would be plenty of adverse effects produced with disorders of its detoxification, metabolism, excretion and other fuctions. Interestingly, emodin may have hepatoprotective activity at low concentration, for example, Ding et al. [116] found emodin can alleviate intrahepatic cholestasis by regulating the expression of mRNA and protein of FXR, SHP, UGT2B4, and BSEP. Besides, Feng et al. [117] demonstrated that emodin significantly alleviated CCl4-induced liver fibrosis by suppressing epithelial-mesenchymal transition (EMT) and transforming growth factor-β1 (TGF-β1) in rats rats. There are findings showing that emodin effectively ameliorates hepatic steatosis through the CaMKK–AMPK–mTOR–p70S6K–SREBP1 signaling pathway, which indicates emodin might be beneficial in patients with non-alcoholic fatty liver diseases [4]. Besides, Liu et al. [118] found emodin can also ameliorate ethanol-mediated liver steatosis and treat alcoholic liver by down-regulating the levels of alanine aminotransferase (ALT), hepatic triglycerides and aspartate aminotransfeansferase. Thus, we can know that emodin may have a potential prevention and treatment action on hepatofibrosis via inhibiting epithelial-mesenchymal transition as well as the expression levels of several cascade signaling pathways.

      However, it is impossible to ignore the phenomenon that emodin might produce hepatotoxicity at a high concentration. Recent study has demonstrated that high-concentration emodin might have potential risk of the hepatotoxicity based on bilirubin metabolism mediated by glucuronidation of UGT1A1 enzyme [119]. Therefore, there are more studies needed to explore effects of emodin on both hepatoprotective activity and hepatotoxicity.

    Future Perspectives
    • Emodin exhibits a variety of pharmacological benefits, including anticancer, anti-inflammatory, antioxidant, antimicrobial, antivirus, anti-diabetes, immunosuppressive and osteogenesis promotion activity, indicating it could have the potential to become a preventive and therapeutic drug for treatment of related diseases. For the aspect of application, emodin combined with chemotherapy drugs for anticancer treatment can produce synergistic and protective effect. Moreover, emodin has great potentials in inflammation-induced diseases and immunoregulation. However, the effect mechanisms of emodin in different diseases are intricate and indistinct. It still needs further research on how concentration affects the pharmacological activity of emodin. Notwithstanding the fact that the properties of emodin have been gradually clarified, there still exists quite a few controversies, for example, mechanisms of emodin in hepatoprotective activity and hepatotoxicity are uncertain. With high doses and long-term use, emodin may lead to hepatotoxicity, kidney toxicity and cytotoxicity. Meanwhile, oral administration of emodin has low bioavailability because of its extensive glucuronidation, and its effects are easily influenced by concentration, derivatives and other factors. Therefore, further studies are needed to attenuate the toxicity effect and oral bioavailability of emodin for further clinical treatment.

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