Nrf2-Related Therapeutic Effects of Curcumin in Different Disorders

Curcumin is a natural polyphenol with antioxidant, antibacterial, anti-cancer, and anti-inflammation effects. This substance has been shown to affect the activity of Nrf2 signaling, a pathway that is activated in response to stress and decreases levels of reactive oxygen species and electrophilic substances. Nrf2-related effects of curcumin have been investigated in different contexts, including gastrointestinal disorders, ischemia-reperfusion injury, diabetes mellitus, nervous system diseases, renal diseases, pulmonary diseases, cardiovascular diseases as well as cancers. In the current review, we discuss the Nrf2-mediated therapeutic effects of curcumin in these conditions. The data reviewed in the current manuscript indicates curcumin as a potential activator of Nrf2 and a therapeutic substance for the protection of cells in several pathological conditions.


Introduction
Alternatively named as Nrf2, nuclear factor erythroid-derived 2-like 2 (NFE2L2) is a transcription factor that induces expression of genes in response to stress and decreases levels of reactive oxygen species (ROS) and electrophilic substances, thus being regarded as a modality for the prevention of chronic disorders [1,2].
The natural polyphenol curcumin ( Figure 1) has been found to exert many of its beneficial effects through activation of the Nrf2 pathway [3]. Being extracted from the rhizome of Curcuma longa, curcumin has antioxidant, antibacterial, anti-cancer, and antiinflammation effects.
The poor bioavailability of curcumin has a major obstacle in its clinical application. Several investigations have shown very low or untraceable concentrations of curcumin in the circulation and extraintestinal tissues. This finding is due to poor absorption of curcumin, its fast metabolism, instability of curcumin, and fast systemic removal [4]. Animal studies have shown excretion of more than 90% of oral curcumin in the feces [5]. The main applied strategies to enhance the bioavailability of curcumin are the application of adjuvants such as piperine, making liposomal curcumin formulations, curcumin nanoparticles, curcumin phospholipid complex as well as structural analogs [6]. on the Nrf2 signaling pathway potentiate it as a protective factor against oxidativ age [3]. Curcumin-mediated induction of Nrf2 signaling relies on four putative nisms, namely suppression of Keap1, regulation of activity of upstream mediators regulation of expression of Nrf2 and its targets, and enhancement of Nrf2 nuclea location [3]. Nrf2-related effects of curcumin have been investigated in different co including gastrointestinal disorders, diabetes mellitus, nervous system diseases, re eases, pulmonary diseases, cardiovascular diseases as well as cancers. In the cur view, we discuss the Nrf2-mediated therapeutic effects of curcumin in these cond Figure 1. Chemical structure of curcumin (1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptad dione). Curcumin which is found in the rhizome of Curcuma longa (turmeric) as a strong polyphenol, has a number of biological activities, including antioxidant (could inhibit RO ating enzymes) and anti-inflammatory (could block NF-κB activation) properties, anti-canc obesity, and anti-infertility effects [9].

Digestive System Diseases
Tetrahydrocurcumin (THC) and octahydrocurcumin (OHC) are the primary nal hydrogenated metabolites of curcumin. These curcumin metabolites have hig oxidant activities and can ameliorate acetaminophen-induced hepatotoxicity and histopathological changes. In addition, these curcumin metabolites could restore dant conditions in the liver by decreasing MDA and ROS levels and increasing GSH CAT, and T-AOC levels. Moreover, they have decreased activity and expres CYP2E1, induced the Keap1-Nrf2 pathway, and promoted expressions of Nrf2-t genes. The latter effect has been exerted through suppression of Keap1 expressi inhibition of Keap1/Nrf2 interaction [10]. Another study has shown the effect of cu on decreasing amounts of hepatic steatosis and inflammatory responses in adult Sp Dawley rats. Moreover, curcumin could decrease serum levels of ALT, AST, and and amend insulin resistance. Serum and hepatic amounts of TNF-α, IL-6, and MD also been decreased following the administration of curcumin. Curcumin could crease Nrf2 levels in nuclei of liver cells, indicating that this substance has a pote fect in the prevention and amelioration of nonalcoholic steatohepatitis through a tion in lipid levels and inflammatory responses, improvement of insulin resistan enhancement of antioxidants, possibly through induction of Nrf2 [11].
This natural substance has potential applications in a variety of disorders, such as neurodegenerative diseases, renal diseases, and diabetes mellitus. In addition, its effects on the Nrf2 signaling pathway potentiate it as a protective factor against oxidative damage [3]. Curcumin-mediated induction of Nrf2 signaling relies on four putative mechanisms, namely suppression of Keap1, regulation of activity of upstream mediators of Nrf2, regulation of expression of Nrf2 and its targets, and enhancement of Nrf2 nuclear translocation [3]. Nrf2-related effects of curcumin have been investigated in different contexts, including gastrointestinal disorders, diabetes mellitus, nervous system diseases, renal diseases, pulmonary diseases, cardiovascular diseases as well as cancers. In the current review, we discuss the Nrf2-mediated therapeutic effects of curcumin in these conditions.

Digestive System Diseases
Tetrahydrocurcumin (THC) and octahydrocurcumin (OHC) are the primary and final hydrogenated metabolites of curcumin. These curcumin metabolites have high antioxidant activities and can ameliorate acetaminophen-induced hepatotoxicity and amend histopathological changes. In addition, these curcumin metabolites could restore antioxidant conditions in the liver by decreasing MDA and ROS levels and increasing GSH, SOD, CAT, and T-AOC levels. Moreover, they have decreased activity and expression of CYP2E1, induced the Keap1-Nrf2 pathway, and promoted expressions of Nrf2-targeted genes. The latter effect has been exerted through suppression of Keap1 expression and inhibition of Keap1/Nrf2 interaction [10]. Another study has shown the effect of curcumin on decreasing amounts of hepatic steatosis and inflammatory responses in adult Sprague-Dawley rats. Moreover, curcumin could decrease serum levels of ALT, AST, and lipids and amend insulin resistance. Serum and hepatic amounts of TNF-α, IL-6, and MDA have also been decreased following the administration of curcumin. Curcumin could also increase Nrf2 levels in nuclei of liver cells, indicating that this substance has a potential effect in the prevention and amelioration of nonalcoholic steatohepatitis through a reduction in lipid levels and inflammatory responses, improvement of insulin resistance, and enhancement of antioxidants, possibly through induction of Nrf2 [11].
Curcumin has also been shown to prevent hepatic injury induced by post-intrahepatic inoculation of trophozoites. Macroscopic and microscopic evaluations in animal models have shown that this substance can decrease ALT, ALP, and γ-GTP activities. Curcumin could also ameliorate the amoebic damage-induced reduction in glycogen content and suppress NF-κB activity and IL-1β levels while inducing a tendency toward up-regulation of Nrf2 production [12]. Moreover, curcumin could attenuate ethanol-induced hepatic steatosis via modulation of the Nrf2/FXR axis [13]. Table 1 summarizes Nrf2-related therapeutic effects of curcumin in gastrointestinal diseases.

Ischemia-Reperfusion (IR) Injury
An antioxidant mono-carbonyl analog of curcumin (MACs) has been shown to exert a protective effect against ischemia/reperfusion (I/R)-induced cardiac injury. This structurally modified formulation of curcumin does not have the β-diketone moiety and possesses improved in vitro stability and better in vivo pharmacokinetic profiles. Based on the results obtained from the in vitro cell-based screening experiments, pre-treatment of H9c2 cells with a certain curcumin analog could activate the Nrf2 signaling pathway, reduce H 2 O 2 -induced up-regulation of MDA and SOD levels, suppress TBHP-induced cell death, and reduce the activity of Bax/Bcl-2-caspase-3 axis. Subsequent studies in animal models of myocardial I/R have also verified the effect of curcumin on the reduction in infarct size and myocardial apoptosis [27]. Another study has shown that the combination of the immunomodulatory drug dimethyl fumarate (DMF) and curcumin has a prominent ameliorative effect in I/R-induced hepatic injury, as is evident by a remarkable decrease in serum ALT and AST activity and improvement of histopathological features. The protective effects of DMF have been shown to be exerted through activation of Nrf2/HO-1 signaling and enhancement of GSH and TAC levels. Curcumin could influence levels of inflammatory markers and infiltration of neutrophils. Besides, curcumin has enhanced DMF-induced Nrf2/HO-1 activation [28]. Finally, another experiment has shown that curcumin could reduce neurological dysfunction and brain edema after cerebral I/R through elevation of Nrf2 and down-regulation of NF-κB [29] (Figure 2). Table 2 shows Nrf2-related therapeutic effects of curcumin in IR injury.

Diabetes Mellitus and Its Related Complications
Curcumin has a synthetic derivative, namely (2E,6E)-2,6-bis(2-(trifluoromethyl) benzylidene) cyclohexanone or C66. This curcumin derivative has been shown to ameliorate diabetes-induced pathogenic alterations in the aorta through activation of Nrf2. This substance could amen to diabetes-associated oxidative stress in the aorta and reverse the effects of diabetes on apoptosis, inflammation, and fibrosis of the aorta. While either C66 or JNK2 deletion could activate Nrf2, C66 had no additional influence on diabetic aortic injury or Nrf2 activity without JNK2. Taken together, C66 can protect against diabetes-associated pathological alterations in the aorta through suppression of JNK2 and enhancement of Nrf2 levels and function [30]. Another study has shown that curcumin analog A13 can reduce the histological changes in the myocardial tissues of diabetic rats. A13 is a hydrosoluble mono-carbonyl analogue of curcumin with the following formulation: (2E,5E)-2,5bis(4-(3-(dimethylamino)-propoxy)benzylidene)cyclopentanone. Curcumin and A13 could also decrease MDA levels and enhance SOD activity in this tissue through activation of the Nrf2/ARE pathway [31]. The protective effect of curcumin against diabetes-related retinopathy is also related to its role in the activation of the Nrf2/HO-1 axis [32]. Table 3 summarizes Nrf2-related therapeutic effects of curcumin in diabetes.

Nervous System Diseases
Curcumin has been shown to ameliorate traumatic brain injury (TBI)-induced abnormal changes in the brain, as evident by amendment of the water content of the brain, oxidative stress, neurological severity score, and apoptosis of neurons. The anti-apoptotic effects of curcumin have been verified through the observed elevation of Bcl-2 levels and reduction in cleaved caspase-3 levels. Notably, curcumin could increase the nuclear translocation of Nrf2, enhance levels of HO1 and NADPH: NQO1, and preclude the reduction in antioxidant enzymes activity. Taken together, curcumin has been found to enhance the activity of antioxidant enzymes and decrease brain injury, most probably through enhancing the activity of the Nrf2/ARE axis [36]. Another study has shown that the administration of curcumin can reduce ipsilateral cortex injury, infiltration of neutrophils, and activation of microglia in animal models of TBI. These effects have led to improvement of neuron survival and reduction in TBI-associated apoptosis and degeneration. Nrf2 has been found to be the main mediator of these effects since Nrf2 deletion has diminished the neuroprotective impact of curcumin [37]. Curcumin has also been shown to ameliorate radiation-induced cerebral injury through up-regulation of Nrf2 [38]. In addition, curcumin could amend motor dysfunction and enhance the activity of tyrosine hydroxylase in the substantia nigra pars compacta of rotenone-injured rats. This natural substance could increase GSH levels and decrease ROS activity and MDA level. The effects of curcumin in amelioration of rotenone-associated oxidative damage in dopaminergic neurons have been shown to be exerted through activation of the Akt/Nrf2 axis [39]. Table 4 shows Nrf2-related therapeutic effects of curcumin in nervous system disorders.

Renal Diseases
Co-administration of thymoquinone and curcumin has been shown to ameliorate cisplatin-induced kidney toxicity. These two substances have synergistic effects in the reduction of cisplatin-induced apoptosis in HEK-293 cells. They could also amend the effects of cisplatin on antioxidant enzyme concentrations and mitochondrial ATPases. Akt, Nrf2, and HO-1 levels have been increased by thymoquinone and curcumin. Moreover, these agents could decrease cleaved caspase 3 and NF-κB levels in kidney homogenates [46]. An animal study has shown that the impact of curcumin in the treatment of chronic kidney disease (CKD) is mediated via the Keap1/Nrf2 axis [47]. However, a randomized, doubleblind placebo-controlled clinical trial in patients with nondiabetic or diabetic proteinuric CKD has shown no significant effect of curcumin on proteinuria, estimated glomerular filtration rate, or lipid profile. Yet, curcumin could attenuate lipid peroxidation in plasma of patients with nondiabetic proteinuric CKD and improve antioxidant activity in patients with diabetic proteinuric CKD [48]. Table 5 summarizes the Nrf2-related therapeutic effects of curcumin in renal disorders.

Pulmonary Diseases
An experiment in lung mesenchymal stem cells (LMSCs) has shown that curcumin could decrease ROS levels while increasing mitochondrial membrane potential levels. Moreover, curcumin has decreased expression levels of cleaved caspase-3, enhanced Nrf2 and HO-1 levels, and increased Bcl-2/Bax and p-Akt/Akt ratios. Taken together, curcumin has been shown to protect against H 2 O 2 -associated LMSC damage via regulation of the Akt/Nrf2/HO-1 signaling [50]. Another study has shown that the potential effect of curcumin in the prevention of high altitude pulmonary edema is exerted via up-regulation of Nrf2 and HIF1-α [51]. Moreover, it has been shown to attenuate airway inflammation in asthma via activating Nrf2/HO-1 axis [52]. Table 6 shows Nrf2-related therapeutic effects of curcumin in pulmonary disorders.

Other Conditions
Therapeutic effects of curcumin have also been investigated in a variety of disorders such as osteoporosis, temporomandibular joint osteoarthritis, muscle damage, skin damage, heat-or H 2 O 2 -induced oxidative stress, and cystopathy (Table 7). Moreover, curcumin could effectively inhibit quinocetone-induced apoptosis via inhibiting the NF-κB and activating Nrf2/HO-1 axis [54]. Besides, curcumin could protect osteoblasts against oxidative stress-induced dysfunction through the GSK3b-Nrf2 axis [55]. Table 8 summarizes Nrf2-related therapeutic effects of curcumin in diverse conditions. Table 7. Nrf2-related therapeutic effects of curcumin in other conditions.

Figure 2.
Oxidative stress formed by a number of disorders or external factors such as chemical drugs, heat stress, and so on could induce both extrinsic and intrinsic apoptotic pathways [75]. However, during abnormal conditions, endoplasmic reticulum (ER) stress could also activate the intrinsic apoptotic pathway [76], leading to cell death. On the other hand, disruption of Bax/Bcl-2 balance by mitochondrial dysfunction leads to ROS elevation [75]. Afterward, ROS activate the NF-κB signaling pathway, subsequently increasing the release of inflammatory cytokines. Interestingly, antioxidant sources-such as curcumin-could decrease ROS production and cell death rate, finally. In this regard, curcumin via activating the Nrf2 pathway could increase the levels of cellular antioxidants [43,48], such as SOD, GPx, and CAT, and by activating the expression of HO-1 (Heme Oxygenase-1), as an Nrf2-regulated gene which is involved in the prevention of vascular inflammation, could directly or indirectly decrease the generation of ROS as well as inflammation [28,54]. On the

Figure 2.
Oxidative stress formed by a number of disorders or external factors such as chemical drugs, heat stress, and so on could induce both extrinsic and intrinsic apoptotic pathways [75]. However, during abnormal conditions, endoplasmic reticulum (ER) stress could also activate the intrinsic apoptotic pathway [76], leading to cell death. On the other hand, disruption of Bax/Bcl-2 balance by mitochondrial dysfunction leads to ROS elevation [75]. Afterward, ROS activate the NF-κB signaling pathway, subsequently increasing the release of inflammatory cytokines. Interestingly, antioxidant sources-such as curcumin-could decrease ROS production and cell death rate, finally. In this regard, curcumin via activating the Nrf2 pathway could increase the levels of cellular antioxidants [43,48], such as SOD, GPx, and CAT, and by activating the expression of HO-1 (Heme Oxygenase-1), as an Nrf2-regulated gene which is involved in the prevention of vascular inflammation, could directly or indirectly decrease the generation of ROS as well as inflammation [28,54]. On the one hand, it has been reported that curcumin via activating the ERK and MAPK could ease the oxidative damage [32,60].

Cancers
The beneficial effects of curcumin have also been investigated in malignant conditions. In animal models of ovarian cancer, curcumin could prevent epithelial-mesenchymal transition (EMT)-mediated progression through modulation of Nrf2/ETBR/ET-1 axis [77]. In colorectal cancer cells, curcumin could affect multidrug resistance via modulation of Nrf2 [78]. Curcumin can inhibit the proliferation of breast cancer cells via Nrf2-mediated down-regulation of Fen1 [79].
A single study in an immortalized lymphoblastoid cell line has shown that treatment of cells with a proapoptotic dose of curcumin can lead to nuclear accumulation of Nrf2 and the expression of Nrf2 targets at early phases, while at late phases it total and nuclear protein levels of Nrf2 have been decreased and Nrf2 targets have been down-regulated in the absence of p53 activity. Thus, apoptosis-associated inactivation of Nrf2 can happen in a p53 dysfunctional context [80]. Table 8 shows Nrf2-related therapeutic effects of curcumin in cancers.

Discussion
Curcumin is a natural substance that has been shown to increase nuclear levels of Nrf2 and enhance the biological function of this nuclear factor through interacting with Cys151 in Keap1 [82]. This substance is an important therapeutic modality for a variety of oxidative stress-related disorders such as diabetes mellitus, brain disorders, cardiovascular disorders, and malignancies. In addition to modulating antioxidant enzymes and inflammatory responses, curcumin can affect the activity of NF-κB. Other pathways modulated by curcumin should also be investigated in different contexts. This information would help in better understanding the mechanism of the therapeutic effects of curcumin.
The impact of curcumin on Nrf2 expression has been vastly investigated in cell lines as well as animal models. In cell lines, both curcumin and its analogs could exert functional effects through modulation of Nrf2 expression. Experiments in animal models of different disorders have shown its beneficial effects on animal health and amelioration of pathological events during the course of malignant or non-malignant disorders.
A single randomized, double-blind placebo-controlled clinical trial in patients with CKD has shown no significant impact of curcumin on Nrf2 expression, despite its effectiveness in the improvement of antioxidant activity [48]. Thus, it is necessary to conduct sufficient studies in human subjects to verify the results of in vitro and animal studies.
Taken together, Nrf2-related effects of curcumin have not been thoroughly and systematically investigated in human subjects. Thus, there is no predictor for assessment of response to this natural substance in different diseases. Moreover, the Nrf2-related effects of this substance have been less investigated in neoplastic conditions. Based on the complexity of gene networks in these conditions, it is necessary to find exact targets of curcumin in cancers.

Conflicts of Interest:
The authors declare no conflict of interest.