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Emodin is a natural anthraquinone derivative that occurs in many widely used Chinese medicinal herbs, such as Rheum palmatum, Polygonum cuspidatum and Polygonum multiflorum. Emodin has been used as a traditional Chinese medicine for over 2000 years and is still present in various herbal preparations. Emerging evidence indicates that emodin possesses a wide spectrum of pharmacological properties, including anticancer, hepatoprotective, antiinflammatory, antioxidant and antimicrobial activities. However, emodin could also lead to hepatotoxicity, kidney toxicity and reproductive toxicity, particularly in high doses and with long�term use. Pharmacokinetic studies have demonstrated that emodin has poor oral bioavailability in rats because of its extensive glucuronidation. This review aims to comprehensively summarize the pharmacology, toxicity and pharmacokinetics of emodin reported to date with an emphasis on its biological properties and mechanisms of action.
These pharmacological properties suggest that emodin might be a valuable therapeutic option for the prophylaxis and treatment of various diseases, including constipation, asthma, atopic dermatitis, osteoarthritis, diabetes and diabetic complications, atherosclerosis, Alzheimer's disease (AD), hepatic disease and several types of cancers, such as pancreatic cancer, breast cancer, hepatocellular carcinoma and lung carcinoma. However, an increasing number of recently published studies have reported adverse effects of emodin. The purpose of this review is to provide updated, comprehensive information on the pharmacology, toxicity and pharmacokinetics of emodin in the past few decades to explore the therapeutic potential of this compound and evaluate future research opportunities.
Pharmacology
Anticancer activity
Induction of apoptosis
Emodin significantly inhibited the cell growth of four bladder cancer cell lines by modulating epigenetic modifications in a dose?dependent and time?dependent manner. In another study, emodin (IC50 = 3.70 µm) was shown to modulate the expression of apoptosis?related genes to induce growth inhibition and apoptosis in A549 cells. Treatment with emodin helped to enhance apoptosis in A549 cells through up?regulation of the gene expression of Fas ligand (FASL) and disabled damage repair in A549 cells through down?regulation of the gene expression of C?MYC. Emodin could induce growth inhibition and apoptosis in MCF?7 cells through the modulation of the expression of apoptosis?related genes. The gene expression of FASL was up?regulated, although the expression levels of MCL1, CCND1 and C?MYC were down?regulated.
Emodin effectively blocked the self?renewal activity of glioma stem cells by suppressing crucial stemness signalling pathways involving Notch?1, b?catenin and STAT3. Emodin induced proteosomal degradation of EGFR/EGFRvIII by interfering with its association with Hsp90, leading to partial induction of apoptosis and sensitization of glioma stem cells to ionizing radiation. Wang et al. reported that emodin inhibited human cervical cancer HeLa cell proliferation by inducing apoptosis through the intrinsic mitochondrial and extrinsic death receptor pathways. Furthermore, emodin was also reported to potentiate TNF?related apoptosis?inducing ligand?induced apoptosis through the induction of death receptors and the down?regulation of cell survival proteins in hepatocellular carcinoma cells.
Emodin significantly inhibited breast cancer cell proliferation by down?regulating ERα protein levels. Xie et al. demonstrated that emodin?provoked oxidative stress induced apoptosis in human colon cancer HCT116 cells through a p53?mitochondrial apoptotic pathway. A similar study showed that emodin could inhibit LOVO colorectal cancer cell growth via the regulation of the Bcl?2/Bax ratio and by its effect on the mitochondrial apoptosis pathway.
Wnt signalling is involved in the regulation of cell proliferation, differentiation and apoptosis. Emodin suppressed the Wnt signalling pathway in human colorectal cancer cells (SW480 and SW620) by down?regulating T?cell factor/lymphoid enhancer factor transcriptional activity.
Anti?metastasis
Jia et al. demonstrated for the first time that emodin suppressed pulmonary metastasis of breast cancer by inhibiting macrophage recruitment and M2 polarization in the lungs by reducing STAT6 phosphorylation and CCAAT/enhancer?binding protein β expression. In another study, treatment with a combination of emodin (10, 20, 40 and 80 µm) and curcumin (10 µm) inhibited the proliferation and invasion of breast cancer cells by increasing the expression of miR?34a. It has been shown that emodin suppressed human breast cancer cell invasiveness in vitro and in vivo by antagonizing the P2X7Rs. Moreover, emodin dose dependently inhibited the migration and invasion of human breast cancer MDA?MB?231 cells by down?regulating the expression of MMP?2, MMP?9, uPA and uPAR and by decreasing the activity of p38 and extracellular regulated protein kinases (ERK).
Emodin treatment significantly suppressed metastasis to the lungs in an orthotopic hepatocellular carcinoma mice model and CXCR4 expression in tumour tissues. Way et al. demonstrated that emodin significantly inhibited TWIST1?induced cell migration and invasion by inhibiting the β?catenin and Akt pathways.
Cell cycle arrest
An MTT assay, flow cytometry and electron microscopy were used to investigate the inhibitory effect of emodin on the human hepatoma cell line SMMC?7721. The results showed that the proliferation of SMMC?7721 cells was inhibited in a time?dependent and concentration?dependent manner, that cells in G2/M phase increased significantly and that the proportion of S?phase cells gradually declined.
Reversion of multidrug resistance
The overexpression of multidrug resistance (MDR) in tumour cells poses a serious obstacle to successful chemotherapy. Our investigations demonstrate that emodin could sensitize tumour cells to chemotherapeutic agents via inhibiting pathways. Emodin/cisplatin co?treatment inhibited the growth of human ovarian carcinoma cells and gallbladder carcinoma cells in vivo. This mechanism might involve the down?regulation of MRP1 expression and ABCG2 expression. In addition, the combination of emodin (25 µm) and cisplatin (3.0 µm) induced a relatively higher inhibitory effect on gastric cancer cells than that of individual treatment with emodin or cisplatin by inducing apoptosis and cell cycle arrest.
Chen et al. showed a synergistic growth?inhibitory effect of azidothymidine and emodin in K562/ADM cells. This effect might be mediated by S cell cycle arrest and decreased MDR1 mRNA/p?gp protein expression. Furthermore, the gene and protein expression levels of nuclear factor?κB (NF?κB), X?linked inhibitor of apoptosis and survivin were suppressed in Bxpc?3/Gem cells treated with the combination of emodin and gemcitabine.
Emodin inhibits cell growth in several types of cancer cells and regulates genes and proteins related to the control of cell apoptosis, cell invasion, metastasis and cell cycle arrest. Moreover, the synergistic enhancement of apoptosis is important in combination chemotherapeutic agents for cancer and has attracted attention as a promising avenue of treatment.
Antiinflammatory activity
Nuclear factor?κB is involved in the transcription of various pro?inflammatory genes involved in disease progression. The antiinflammatory effect of emodin has been associated with the inhibition of the pro?inflammatory transcription factor NF?κB. Gao et al. elucidated the antiinflammatory active components of Jiashitang scar removal ointment, including saffiomin, emodin, salvianolic acid, tanshinone and triterpenoid saponin derivatives. Among these active ingredients, emodin was predicted to exert NF?κB?inhibiting effects through the mitogen?activated protein kinase (MAPK), PI3K/AKT and NIK?IKK pathways. In a collagen?induced arthritic mouse model, emodin exhibited its anti?arthritis effects through inhibition of the NF?κB pathway and pro?inflammatory mediators. Moreover, emodin could improve the corneal structure and reduce corneal injure by suppressing the activation of NF?κB, c?JunN?terminalkinase (JNK) and the expression of ICAM?1. In a severe acute pancreatitis (SAP) model group, emodin treatment significantly decreased the expression of Bip, inositol?requiring?1α (IRE1α), TNF receptor?associated factor 2 and ASK1 and inhibited phosphorylation of JNK and p38 MAPK by inhibiting ER stress transducers IRE1α and its downstream molecules. Jia et al. found that emodin could halt the transition from simple steatosis to non?alcoholic steatohepatitis by suppressing Erk1/2 and p38 signalling in in vitro experiments.
Reports also have shown that emodin has a protective role in rats with acute necrotizing pancreatitis by increasing the expression levels of mCD14 and inhibiting the serum expression levels of TNF?α, IL?6 and IL?1β. In lipopolysaccharide (LPS)?induced mouse mastitis, emodin could reduce LPS?induced mammary gland injuries and inflammatory cell infiltration, decrease myeloperoxidase (MPO) activation in the mammary gland, down?regulate the expression of TNF?a, IL?6 and IL?1β and activate PPAR?γ in a dose?dependent manner. A similar study showed that emodin considerably decreased the levels of TNF?α, IL?6, MPO, malondialdehyde (MDA), CINC?1, MIP?2 and ICAM?1. However, the levels of nitric oxide and inducible nitric oxide synthase were markedly increased.
Emodin could attenuate pulmonary oedema and inflammation, enhance alveolar epithelial barrier function and promote the expression of claudin?4, claudin?5 and occludin in lung tissues. Furthermore, emodin treatment effectively inhibited allergen?induced inflammation by reducing the Th2 immune response, suppressing MMP?9 expression and inducing HO?1 expression in a murine model of asthma. LTs synthesized by the 5?LOX reaction are believed to play an important role in atopic dermatitis, and Jin et al. demonstrated that emodin possessed a 5?LOX inhibitory action.
The study results reflect that emodin is capable of inhibiting the activation of NF?κB, JNK, p38 MAPK, Erk1/2 and 5?LOX, suppressing the expression of TNF?α, IL?6, IL?1β, MPO, MDA, CINC?1, MIP?2, ICAM?1 and MMP?9 and enhancing the levels of mCD14, nitric oxide and inducible nitric oxide synthase. Emodin has been shown to exhibit potential therapeutic effects in the treatment and prevention of various inflammatory disorders.
Antivirus activity
Severe acute respiratory syndrome (SARS) is an emerging infectious disease caused by a novel coronavirus. In a model of Xenopus oocytes, emodin could inhibit the 3a ion channel of coronavirus SARS?CoV and HCoV?OC43 and inhibit virus release from HCoV?OC43 with a K1/2 value of approximately 20 µm. Moreover, emodin significantly blocked the S protein and ACE2 interactions in a dose?dependent manner and inhibited the infectivity of S protein?pseudotyped retrovirus to Vero E6 cells. The results suggested that emodin might be considered as a potential lead therapeutic agent in the treatment of SARS. Dang et al., for the first time, demonstrated that emodin had a weak but long?lasting inhibitory effect on HBV replication in vivo. In vitro experiments showed that exposure of HepG?2 cells to emodin resulted in time?dependent and concentration?dependent inhibition of hepatitis B virus DNA replication and hepatitis B surface antigen secretion. Similarly, emodin reduced Coxsackievirus B4 entry and replication on HepG?2 cells in a concentration?dependent and time?dependent manner. The results implied that emodin might act as a biological synthesis inhibitor against Coxsackievirus B4 infection rather than directly inactivating the viruses or blocking their absorption to the susceptible cells. In a biochemical assay, emodin specifically inhibited the nuclease activity of herpes simplex virus?1 UL12 alkaline nuclease. Furthermore, emodin possessed antiviral activities through the disruption of the phospholipid bilayer and the inhibition of CK2.
Therefore, these results suggest that emodin might be a potent viral inhibitor with a broad spectrum of antiviral activities, indicating that emodin might act as an antiviral drug by blocking virus infection and replication in a time?dependent and concentration?dependent manner.
Antibacterial activity
Emodin exhibited a remarkable bacteriostatic effect on the Gram?positive bacteria tested, especially Bacillus subtilis and Staphylococcus aureus. The bioactive minimum inhibitory concentration (MIC) values of emodin were 28.9 and 14.4 µm. However, emodin was not active against two Gram?negative bacteria (Klebsiella pneumoniae and Escherichia coli) at the highest concentration (1851.9 µm) tested. Similarly, emodin has been reported to exhibit antimycobacterial and broad spectrum antibacterial activity, particularly against M. tuberculosis (lowest MIC = 0.9 µm) and Gram?positive bacteria (lowest MIC < 14.8 µm) of clinical origin. Cao et al. investigated the anti?methicillin?resistant S. aureus (anti?MRSA) activity and chemical compositions of ether extracts from Rhizoma Polygoni Cuspidati (ET?RPC). The results showed that ET?RPC could significantly inhibit bacterial growth of MRSA strains. Moreover, emodin was identified as the major compound with anti?MRSA activity, which was related to the destruction of the integrity of the cell wall and cell membrane.
Emodin also significantly attenuated the growth, acid production and insoluble glucan synthesis of Streptococcus mutans in vitro and suppressed the development of dental caries in rats. The results suggested that the natural compound emodin might be a novel pharmacological agent for the prevention and treatment of dental caries. Chen et al. reported for the first time that emodin functioned as a competitive inhibitor against Helicobacter pylori, which might be associated with occupying the entrance of the tunnel or embedding into the tunnel to prevent the substrate from accessing the active site.
Overall, like many herbal monomers, emodin has a potential antimicrobial property. Emodin itself could be used as a potential lead compound for further anti?bacterial drug discovery.
Anti?allergic activity
Allergic reactions are triggered when allergens cross?link with the high?affinity IgE receptor (FcεRI) on mast cells. To assess the anti?allergic activity of emodin, an in vivo passive anaphylaxis animal model and in vitro mouse bone marrow?derived mast cells were used to investigate the mechanism of its action on mast cells. The results demonstrated that emodin attenuated the mast cell?dependent passive anaphylactic reaction in IgE?sensitized mice. Therefore, emodin inhibited mast cell activation and thereby the anaphylactic reaction through the suppression of the receptor?proximal Syk?dependent signalling pathways.
An increase in the intracellular calcium (Ca2+) concentration also triggers degranulation, bypassing receptor activation. Emodin significantly inhibited TNF?α production and degranulation through the regulation of IKK2 and PKC activation, suggesting that emodin inhibits mast cell activation through the FcεRI?mediated proximal signalling pathway and through the Ca2+ influx?mediated downstream signalling pathway. In rat basophilic leukaemia (RBL?2H3) cells, Wang et al. evaluated the inhibitory effects of emodin on the IgE?mediated allergic response by measuring the release of granules and cytokines. Emodin suppressed degranulation and cytokine production by increasing the stability of the cell membrane and inhibiting extracellular Ca2+ influx.
Emodin significantly suppresses mast cell degranulation by either FcεRI or calcium ionophore stimulation, suggesting that emodin could be further developed as a therapeutic agent for immediate and chronic allergic diseases.
Neuroprotective activity
The accumulation of β?amyloid protein (Aβ) in the brain plays an important role in the pathogenesis of AD. Emodin up?regulated Bcl?2 and also blockaded amyloid?β25–35?induced autophagy through the activation of the ER/PI3K/Akt pathway and the class III phosphatidylinositol 3?kinase/Beclin?1/B?cell lymphoma 2 pathway, respectively. The results provided confirmatory evidence for the application of emodin in the prevention and treatment of AD. Furthermore, emodin raised the survival rate of oxygen?glucose?deprived neuron?like cells, increased activin A expression and decreased caspase?3 expression, indicating that emodin could inhibit neuronal apoptosis and alleviate nerve cell injury after oxygen?glucose deprivation through the activin A pathway. Yang et al. observed the interventional effects of emodin in epileptic rats and elucidated a possible mechanism of action. The findings suggested that emodin could influence the effects of cyclooxygenase?2 in brain tissue and block N?methyl?d?aspartate?mediated overexpression of MDR1, thereby achieving anti?epileptic effects.
In chronic unpredictable mild stress mice, emodin treatment (20, 40 and 80 mg/kg) reversed the behavioural deficiency. Emodin normalized the change of the plasma corticosterone level and up?regulated the hippocampal glucocorticoid receptor (GR) (mRNA and protein) and brain?derived neurotrophic factor (BDNF) (mRNA) expression levels. It was shown for the first time that emodin treatment led to induced neurite outgrowth through the PI3K/Akt/GSK?3β signalling pathway in Neuro2a cells. Zhou et al. demonstrated that emodin induced BV?2 cell apoptosis and consequently eliminated inflammatory microglia through the induction of TRB3, suggesting that emodin is a candidate for treating intracerebral haemorrhages and other neurodegenerative disorders that involve microglial activation. The P2X2/3 receptors play a crucial role in facilitating the transmission of pain in neuropathic pain states. Emodin could decrease the expression of the P2X2/3 receptor during chronic pain and inhibit primary afferent transmission mediated by the P2X2/3 receptor.
Based on the aforementioned studies, emodin could act as an effective neuroprotective drug for treating AD and epilepsy and could antagonize ischemia?reperfusion brain injury and glutamate?induced neuronal damage.
Immunosuppressive activity
Emodin has been developed as an immunosuppressive agent. Emodin (1, 10 and 100 µm) was observed to exert immunosuppressive actions by inhibiting the growth of human T cells and inducing apoptosis in a dose?dependent and time?dependent manner. Furthermore, emodin disrupted the mitochondrial membrane potential and increased the cytosolic level of cytochrome C and levels of activated cleavage fragments of caspase?3, ?4, and ?9 in human T cells. In vivo and in vitro experiments showed that emodin ameliorated the proliferation of peripheral blood mononuclear cells, promoted Th2?type cytokine IL?4 and reduced the Th1?type cytokine IL?2, showing immunosuppressive potential.
Emodin played a key role in the inhibition of the differentiation and maturation of dendritic cells and enhancement of Tregs production, which might be helpful for the modulation of immune rejection after liver transplantation. Emodin treatment also prolonged the liver allograft survival time and inhibited histopathologic changes of acute graft rejection. The mechanisms underlying this effect might be associated with polarizing the Th1/Th2 paradigm to Th2.
The studies demonstrated that emodin exerts a broad range of actions on the immune system. The potential immunosuppressive mechanism might be the suppression of lymphocyte proliferation and cytokine production, which might be helpful for the modulation of immune suppression and the induction of immune tolerance.
Anti?osteoporotic activity
Emodin has been used to treat bone diseases for thousands of years. In the mouse model of LPS?mediated osteoporosis, Kang et al. demonstrated that emodin treatment showed the anti?osteoporotic effect by the marked suppression of bone resorption. Furthermore, emodin could increase the number of osteoblasts, the bone mineral density and the connectivity density in ovariectomized mice. The gene and protein expression levels of osteogenesis markers, such as Runx2, osterix, collagen type I, osteocalcin or ALP, were up?regulated. A similar study showed that emodin also increased alizarin red?mineralization activity and the expression of osteogenesis markers (Runx2, osteocalcin and ALP) and activated the p38?Runx2 pathway, which enhanced osteoblast differentiation. However, emodin suppressed receptor activator of NF?κB ligand (RANKL)?induced osteoclast differentiation of bone marrow macrophages and the bone?resorbing activity of mature osteoclasts by inhibiting RANKL?induced NF?κB, c?Fos and NFATc1 expression. In another study, Lee et al. reported that emodin at low concentrations could accelerate the osteoblast differentiation by the induction of the BMP?2 gene by activating the PI3K?Akt and/or MAP kinase–NF?κB signalling pathways.
Summarizing the beneficial effect of emodin on bone health, our studies elucidate the pharmacological roles of emodin in the prevention of osteoporosis and provide the initiative in the early drug discovery and development for osteoporosis.
Anti?diabetic activity
Recent studies have suggested that emodin has a PPARγ?activating effect. Intraperitoneal injections with emodin for 3 weeks notably ameliorated the symptoms of diabetic animals, and this effect was likely associated with the regulation of the PPARγ pathway. A similar study showed that emodin effectively ameliorated p38 over?activation and hypocontractility in high glucose?induced mesangial cells via activation of PPARγ. In differentiated 3 T3?L1 adipocytes, emodin exhibited a high binding affinity to PPARγ by inducing an increase in glucose uptake and increases in GLUT1 and GLUT4 mRNA expression.
Emodin has been reported to be a novel AMP?activated protein kinase (AMPK) activator. Emodin had a positive effect on glucose metabolism in 3 T3?L1 adipocytes, which was primarily due to increases in glycolysis and might be mediated by the AMPK signalling pathway. Chen and Song et al. also demonstrated that emodin regulated glucose utilization through activating the AMPK signalling pathway. Moreover, emodin displayed an inhibitory effect on the high?level glucose?induced phosphorylation of ERK 1/2 and p38 MAPK. Wang et al. demonstrated that long?term emodin (3 µm) administration improved glucose tolerance and ameliorated other metabolic disorders in ob/ob mice by the inhibition of 11β?HSD1 activity in adipose tissue. Emodin has been reported to exhibit protective effects against diabetic cardiomyopathy by regulating the AKT/GSK?3β signalling pathway.
Diabetic nephropathy (DN) is a major cause of end?stage renal disease. Emodin (40 mg/kg/day) was efficient to ameliorate renal dysfunction in DN rats likely by its inhibition of the activation of the p38 MAPK pathway and down?regulation of the expression of fibronectin. Yang et al. also reported that emodin significantly decreased NF?κB?mediated TGF?β1 and fibronectin expression. Furthermore, emodin (20 mg/kg/day) repressed renal integrin?linked kinase and desmin expression, preserved nephrin expression and ameliorated albuminuria in streptozotocin?induced diabetic rats. In rat mesangial cells cultured under high glucose, emodin suppressed hyperglycaemia?induced cell proliferation and fibronectin expression by inhibiting the p38MAPK pathway and cellular FLICE?inhibitory protein (cFLIP), suggesting a potential role of emodin in the treatment of DN.
In summary, our results show that emodin ameliorates the symptoms of diabetic animals, and this effect is likely associated with the activation of the PPARγ and AMPK signalling pathways. Therefore, emodin holds great promise for the eventual use of a therapeutic agent to treat diabetes mellitus and its complications.
Hepatoprotective activity
Emodin was found to ameliorate ethanol?mediated liver steatosis and decrease alanine aminotransferase (ALT), aspartate aminotransferase and hepatic triglycerides. Moreover, emodin significantly reduced liver α?smooth muscle actin and collagen type I, whereas it increased the mRNA levels of PPAR?γ. In another experiment, emodin was shown to attenuate the ethanol?induced cytotoxicity of HepG2/CYP2E1 cells by the inhibition of Gamma?glutamyltransferase (GGT) activity and CYP2E1 protein expression in a dose?dependent manner, indicating that emodin might be beneficial in patients with alcoholic liver disease. In a rat model with CCl4?induced liver fibrogenesis, the activities of serum ALT and aspartate aminotransferase and the hepatic hydroxyproline content were significantly reduced by the administration of emodin. The mRNA and protein levels of TGF?β1, Smad4 and α?SMA were also down?regulated in liver tissues. Lee et al. demonstrated that emodin might be valuable for the protection of CCl4?induced liver injury by reducing lipid peroxidation and by positively modulating inflammation. Emodin also had the ability to prevent CCl4?induced liver damage through reversing hepatic oxidative insults, CYP enzymatic activity and ultrastructural changes. In addition, emodin at a dose of 30 mg/kg (po) possessed optimum hepatoprotective ability against acetaminophen (APAP)?induced toxicity via diminishing oxidative stress.
Yin et al. showed that emodin could effectively prevent LPS?induced fulminant hepatic failure, and this beneficial effect might occur by the blockade of TLR4/MD2 complex expression on the cell surface of macrophages, which could lead to the deactivation of the p38 MAPK and NF?κB signalling pathways and inhibition of TNF?α production. Similarly, emodin treatment protected against concanavalin A?induced liver injury by inhibiting the activation of the p38 MAPK and NF?κB signalling pathways in CD4+ T cells and macrophages, indicating that emodin can be applied as a potential candidate for the prevention and intervention of clinical fulminant hepatic failure.
We may be able to conclude from these findings that emodin exhibits hepatoprotective effects in ethanol?induced hepatosteatosis and CCl4 or APAP?induced liver injury, indicating that emodin might have therapeutic applications in the prevention of fulminant hepatitis.