Polyphenols in the treatment of autoimmune diseases

Abstract

In addition to protecting body from infections and diseases, the immune system produces auto-antibodies that can cause complex autoimmune disorders, such as Type I diabetes, primary biliary cirrhosis, rheumatoid arthritis, and multiple sclerosis, to name a few. In such cases, the immune system fails to recognize between foreign agents and its own body cells. Different factors, such as genetic factors (CD25, STAT4), epigenetic factors (DNA methylation, histone modifications) and environmental factors (xenobiotics, drugs, hormones) trigger autoimmunity. Glucocorticoids, non-steroidal anti-inflammatory drugs (NSAIDs), immunosuppressive and biological agents are currently used to manage autoimmune diseases of different origins. However, complete cure remains elusive. Many dietary and natural products including polyphenols have been widely studied as possible alternative treatment strategies for the management of autoimmune disorders. Polyphenols possess a wide-range of pharmacological and therapeutic properties, including antioxidant and anti-inflammatory activities. As immunomodulatory agents, polyphenols are emerging pharmaceutical tools for management of various autoimmune disorders including vitiligo, ulcerative colitis and multiple sclerosis (MS). Polyphenols activate intracellular pathways such as arachidonic acid dependent pathway, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signalling pathway, mitogen-activated protein kinases (MAPKs) pathway, phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling pathway and epigenetic modulation, which regulate the host’s immune response. This timely review discusses putative points of action of polyphenols in autoimmune diseases, characterizing their efficacy and safety as therapeutic agents in managing autoimmune disorders.

1. Introduction

The immune system protects from infections and diseases resulting from bacterial, viral and other causative agents. However, under certain circumstances, the immune system may produce auto-antibodies against its own cells, leading to autoimmune diseases. In such cases, the immune system fails to recognize between foreign agents and own body cells [1]. Complex systems of innate and adaptive immunity and their interaction with genes and environmental factors trigger the development of systemic autoimmune diseases [2]. Although thought to be rare, more than 80 autoimmune diseases have been identified to date, including autoimmune hepatitis, Type I diabetes, Primary biliary cirrhosis, Rheumatoid arthritis, Multiple sclerosis etc. [3]. Autoimmune diseases are thought to affect 8–10% of the population [4]. The age of onset of symptoms may differ from one autoimmune disease to another. There are also many underlying genetic factors for the development of these diseases.

Conventional and common treatment options for autoimmune diseases include analgesics, non-steroidal anti-inflammatory drugs (NSAIDs) and glucocorticoids [5]. However, in recent years, therapeutic immunosuppression and biological agents have also been shown beneficial in the management of autoimmune disorders [4]. Yet, while attenuating the inflammatory symptoms or progression of the diseases, overall cure remains elusive. Dietary natural products and herbal medicines are also studied widely as possible treatment strategies for autoimmune diseases [6–9].

Dietary natural products play important roles in the maintenance of human health [10]. Polyphenols are secondary metabolites in plants and to date more than 8,000 polyphenols have been identified [11]. They are commonly found in fruits, leafy vegetables, tea, coffee, and legumes, to name a few [11, 12]. Polyphenols can be divided into chemical classes of phenolic acids, flavonoids, tannins, lignans, stilbenes, etc, affording protection in plants against pathogenic organisms [13, 14], predominantly given their antioxidant and anti-inflammatory activities [11, 15].

Many polyphenols, especially flavonoids possess potent anti-inflammatory properties [16–19]. and regulate immunity [6, 7, 17, 20–22]. Several natural products such as epigallocatechin gallate (EGCG) [23], resveratrol [24], curcumin and capsaicin [25] have been well studied for their beneficial effect in autoimmune diseases, and will be the subject of this review, summarizing the clinical data on their efficacy in the treatment of autoimmune diseases.

2. Pathophysiology of autoimmune diseases

The immune system affords a defense mechanism, providing self-tolerance to harmless interactions, while protecting the organism against dangerous predators such as pathogens (bacteria, viruses etc.) and environmental threats. Dysfunction of this system, such as loss of immune tolerance and improper rearrangement of homeostasis may lead to autoimmune diseases (AIDs).

The prevalence of AIDs is around 5% worldwide, and 80 different types of autoimmune diseases have been recognized to date [26]. Genetic, epigenetic and environmental factors (drugs, hormones, nutrition, microbiota, apoptosis, xenobiotics and others) are predisposing factors for autoimmunity [27]. Systemic autoimmune diseases such as systemic lupus erythematosus (SLE), psoriasis and rheumatoid arthritis (RA) are caused as a result of the interaction of many autoantigens, including cell surface molecules and intracellular matrix proteins with reactive autoantibodies. Furthermore, some specific disorders, including type-I diabetes (T1D), multiple sclerosis (MS) and Hashimoto, inflammatory bowel disease (IBD) can also occur due to an immune response to autoantigens localized within a particular organ. Furthermore, the presence of AIDs may affect the development of other chronic diseases. For example, patients with chronic inflammatory diseases (such as SLE, RA) are at greater risk than the general population for developing specific types of lymphoma [28].

The effector mechanisms involved in the production of pro-inflammatory cytokines and the self-reactive T helper cells (Th) are commonly responsible for the initiation of autoimmune pathogenesis. T helper cells present the T cells to autoreactive B cells and as a consequence, the autoreactive B cells elaborate autoantibodies that cause tissue inflammation. The cells grow after interaction with an antigen and proceed to differentiate into effector (Th1, Th2, Th9 and Th17) and regulator (Treg) subsets [29]. To maintain an effective immunological homeostasis, a balance between Th cells activation and Treg cells mediated suppression is required. The disruption of this balance causes development of an immune response by lymphocyte and/or antibody production against its own cells and tissues [30]. Moreover, the aforementioned disbalance in favor of Th17 cells causes a significant increase in the severity of AID. Treg cells modulate the activity of Th cells and secrete immunomodulatory cytokines (interleukin (IL)-35 and 10) and transforming growth factor (TGF)-β), thus inducing tolerance to antigens and leading to the release of cytotoxic molecules such as granzyme and perforin. In addition, IL-6 and IL-21 have an active role in maintaining the balance between Th17 cells and Treg cells [31].

T cell subgroups involved in inflammatory responses are mainly Th1 and Th17. Th1 cells provide protection against intracellular infections by secreting tumor necrosis factor (TNF)-α and interferon (IFN)-γ. Th2 cells secrete cytokines, including IL-4, IL-5, IL-10, IL-13, which in turn target parasitic organisms and cause allergic diseases. Th17 cells, one of the major pathogenic Th cell population, require a number of transcription factors (such as NF-κB, STAT3) and specific cytokines (such as TGF-β, IL-6, IL-23) for activation and proliferation. In addition, they secrete TNF-α and interleukins including IL-17A, IL-17F, IL-21, IL-22. IL-17A and IL-17F, which play a key role in the pathophysiology of AIDs, such as RA, SLE and IBD, by inhibiting extracellular matrix production in chondrocytes and osteoblasts and by activating the production of matrix metalloproteinases [32, 33].

IL-17 and TNF-α-induced increase in intestinal barrier permeability facilitate the pathophysiology of Crohn’s disease (CD), IBD (ulcerative colitis (UC) and MS [34, 35]. Similarly, in intestinal barrier-microbiome and brain connectivity, it was found that increased plasma LPS levels in MS patients were related to IL-6 production, proliferation of Th17 cells in the intestine and differences in expression in genes associated with NF-kB signaling [36, 37].

IBD and SLE are autoimmune diseases characterized by increased levels of the pro-inflammatory cytokine IL-1 and LTB4 [38]. Furthermore, overexpression of IL-6 leads to the development of AIDs by stimulating B-cell differentiation. Although serum IL-6 concentration is higher in CD compared to UC, the production of macrophage-derived cytokines (IL-1, IL-6 and TNF-α) in both diseases is increased. Similarly, IL-6 levels also elevated in patients with T1D, RA and psoriasis [39].

In addition, heterodimer structure IL-23 that regulates the development of IL-17 and IL-22 producing T cells is also associated with the etiology of the autoimmune diseases. For example, psoriasis is known to result from the activation of the IL-23/Th17 cytokine axis [40, 41]. Recent studies have also corroborated a role for IL-27p28 in diseases associated with CNS (central nervous system) autoimmunity, such as MS, where it suppresses the functions of Th1 and Th17 effector cells [42].

AIDs result from a combination of genetic predisposition and environmental factors. Indeed, the genetic background of AIDs is closely related to the major histocompatibility complex (MHC) referred to as human leukocyte antigen (HLA). The relationship between MHC class II molecules (HLA-DR2b and HLA-DR4) and increased susceptibility to diseases, such as MS and RA has been documented [43]. Similarly, common AIDs, such as T1D, SLE, psoriasis, are associated with specific HLA alleles [44]. Moreover, AIRE (autoimmune regulator), TNFRSF6 (TNF receptor super family member 6), Foxp3, CD25, PTPN2, IRF5, STAT4, ICAM3, BANK1 gene mutations were shown to be strongly related with AIDs [45, 46].

Several environmental factors stimulate epigenetic and intrinsic components that can alter gene function, and thus, are associated with immune cell expressions/functions. Epigenetic mechanisms (DNA methylation, histone acetylation, microRNAs), in the influence of environmental factors, affect the prevalence of many AIDs. These mechanisms play an important role in embryogenesis, cellular differentiation, X-chromosome inactivation, and genomic imprinting. In a recent methylation study of >1,200 genes, methylation patterns of osteoarthritis patients were found to be dissimilar to those in patients with RA [47].

Numerous AIDs are more common in women of reproductive age than men due to sex hormones and X chromosome inactivation [48]. SLE represents an AID of epigenetic origin characterized by the deterioration of T-cell DNA methylation. The reason for the higher incidence in women is demethylation of CD40LG on the inactive X chromosome and over-stimulated of the B cells to produce immunoglobulin G (IgG) [49].

The factors that influence the development of autoimmunity are summarized in Figure 1.

Figure 1.

The factors that influence the development of autoimmunity.

3. Therapeutic significance of polyphenols

Polyphenols are the most abundant phytochemicals in human diet with antioxidant potential [50]. Polyphenols have dynamic role as antimicrobials (against viruses, bacteria and fungi) [51], cardioprotective [52], antiasthma [53], antidepressant and anxiolytic [54], antidiabetic [55], neuroprotective [56] anticarcinogenic [57], to name a few. Their antioxidant properties render then invulnerable in the management of age related diseases [58, 59].

Given their immunomodulatory activities, polyphenols have emerged as pharmaceutical tools for the treatment of various autoimmunity disorders [60]. Currently, a combination of drugs is used in the management of autoimmune disorders such as rheumatic arthritis [61]. Such combinations of drugs e.g. DMARDs in rheumatic arthritis provides synergistic effects and reduces side effects associated with individual drugs; yet, the results have been disappointing. Additionally, vaccination has been shown efficacious in the management of several autoimmune disorders [62], therefore, more effects are required in .

4. Clinical status of polyphenols in autoimmune disorders

Autoimmune disorders have been categorized into several types, and approximately 80 such diseases have been identified. Various clinical trials have been conducted on the beneficial effect of polyphenolics on autoimmune diseases and its associated complications (Table 1).

Table 1:

List of clinical trials (completed/ongoing) on the effect of polyphenolic on autoimmune diseases

S.No.	Plant/polyphenols	Autoimmune disorders	Study design (age, sex, doses)	Status	Reference (clinical trial number)
1	MEMOREX 60 mg BID (Gingko biloba) and narrow band UVB	Vitiligo	Randomized double-bling placebo controlled trial (160 participants, 12–65 years)	RECRUITING	NCT01006421
2	60 mg Ginkgo biloba BID for 12 weeks	Vitiligo vulgaris	Non-randomized single group assignment (12 participants, 12–18 years)	completed	NCT00907062
3	5 aminosalicylicacid +curcumin (50–100 mg BID)	ULCERATIV E COLITIS	Randomized parallel assignment (50 participants, 18–70 years)	RECRUITING	NCT02683759
4	5 aminosalicylicacid +curcumin (500 mg BID)	ULCERATIV E COLITIS	Randomized parallel assignment (50 participants, 18–70 years)	completed	NCT01320436
5	Aloe barbadnsis (200 ml per day for three months)	Inflammation in patients with mild ulcerative colitis	Randomized parallel assignment (60 participants, 18–59 years)	recruiting	NCT01783119
6	CURCUMIN (1–2 g BID)	Pediatric ulcerative colitis	Randomized double-blind parallel assignment (60 participants, 6–18 years)	Not yet recruiting	NCT02277223
7	Curcumin (3g per day)	ULCERATIVE COLITIS IN REMISSION	Randomized double-blind parallel assignment (172 participants, 18 years and older)	RECRUITING	NCT03122613
8	Quercetin (1000 mg per day)	Sarcoidosis	Randomized double-blind parallel assignment (18 participants)	Completed (reduced markers of oxidative stress and inflammati on in sarcoidosis patients)	NCT00402623
9	Ashwagandha (3 ml for 5 days)	Inflammation, cancer and autoimmune disease	Non-randomized single group assignment (25 participants)	Completed	NCT00817752
10	Red Yeast Rice and Tea	Rheumatoid Arthritis	Randomized parallel assignment (120 participants, 18–75 years)	Recruiting	NCT02257047
11	Resveratrol (80 mg per day for 2 months)	Spastic Paraplegia	Randomized crossover assignment (12 participants, 18 years and older)	Completed	NCT02314208
12	High Flavanol Cocoa drink (450 mg cocoa flavanols BID for 7 days)	Peripheral and cerebral blood flow in diabetes type −2	Randomized crossover assignment (18 participants, 35–70 years)	Completed	NCT01654172
13	Green tea extract (capsule containing 160 mg teavigo (94% epigallocatechin-3-gallate) DAILY FOR 3 MONTHS	Multiple sclerosis	Randomized crossover assignment (20 participants, 20–60 years)	completed	NCT01417312
14	Green Tea (1000 mg aqueous green tea extract per day for 3 months)	Lupus	Randomized double-blind parallel assignment (68 participants, 15 years and older)	Completed	NCT02875691
15	Antioxidant cocktail (vitamin C, E, alpha lipoic acid) and Resveratrol (270mg)	Type 1 diabetes	Randomized double-blind parallel assignment (198 participants, 18–40 years)	Recruiting	NCT03436992
16	Curcumin	Pelvic inflammatory disease, Endometritis, wound infection	Randomized parallel assignment (180 participants, 18–52 years)		NCT03016039
17	Liquorice (150 g glycyrrhizinic acid) and Grapefruit juice (200 ml three times a day)	Addison Disease	Randomized crossover assignment (17 participants, 18–80 years)	Completed	NCT01271296
18	Epigallocatechinga llate (600mg per day)	Multiple Sclerosis	Randomized parallel assignment (60 participants, 18–52 years)	Recruiting	NCT03740295
19	Migh Tea flow (green tea, jaborandi extract, xylitol) 4–6 times daily for 8 weeks	Sjogren syndrome	Randomized double-blind parallel assignment (60 participants, 18–75 years)	Completed	NCT01647737

(NCT: numbers refer to the source of www.clinicalTrails.gov).

Vitiligo is one of the common skin disorders characterized by hypopigmentation with significant psychiatric issues and problems if occurring before adulthood. Recently, an open label pilot clinical trial with 12 adolescents (age 12 to 18 years) was conducted using Ginkgo biloba powder [63]. Ginkgo biloba is known to be a rich source of polyphenolics [64]. G. biloba was administered in dose of 60 mg BID, i.e. two times per day for 12 weeks. The Vitiligo European Task Force (VETF) and Vitiligo Area Scoring Index (VASI) were used to access the effectiveness of the G. biloba extract whereas serum coagulation factors (platelets, PTT, INR) at baseline and week 12 were used to access other parameters including. Treatment results in completely depigmentation, 0.4% decrease in VETF total vitiligo lesion area and improvement of total VASI score, VETF staging score, VETF spreading score by 0.5, 0.7 and 3.9 respectively. No significant changes in coagulation factors were reported.

The efficacy of G. biloba treatment was tested in slowly spreading vitiligo in double-blind placebo-controlled trial. A total number of 47 patients (including placebo) were given 40mg of G. biloba extract three times a day [65]. As compared to placebo, G. biloba extract was associated with disease arrest, noting it arrested the progression of vitiligo disease by reducing depigmentation and promoting repigmentation.

Curcumin a natural phenolic compound has multiple uses. Curcumin was examined for its effectiveness in maintaining remission of ulcerative colitis [66]. In a randomized double-blind placebo-controlled study, 50 patients with active mild-to-moderate ulcerative colitis already treated with mesalamine, were given curcumin capsule 3g/day for 1 month. It was concluded that 53.8% i.e. 14 patients achieved clinical remission of disease at week 4, as compared to none in placebo treatment. Clinical response was achieved by 65.3% (17 patients) in curcumin treated group as compared to 12.5% (3 patients) in the placebo group.

Quercetin, a flavonoid class of polyphenolic compound was tested for its beneficial effect to reduce oxidative stress and inflammation in sarcoidosis. A double-blind randomized placebo controlled clinical trial on 18 non-treated sarcoidosis patients was conducted. Quercetin at a dose of 4×500mg was administered within 24h. Quercetin treatment was increased total plasma antioxidant capacity. The oxidative and inflammatory markers (malondialdehyde, TNFα/IL-10 and IL-8/IL-10) were also downregulated in sarcoidosis patients after curcumin treatment.

Multiple sclerosis (MS) is one of the autoimmune diseases of neurologic origin characterized by fatigue and muscle weakness. In one of the study, metabolic response to EGCG and substrate utilization in patients with MS were examined [67]. A randomized, double-blind, placebo-controlled trial was conducted in 8 patients with relapsing-remitting multiple sclerosis. A dose of EGCG (600 mg/d) was given over 12 weeks. At rest, postprandial energy expenditure and carbohydrate oxidation, as well as glucose supply and adipose tissue perfusion were significantly lower in men, but higher in women receiving EGCG as compared with placebo. During exercise, postprandial energy expenditure was reported to be lower after intake of EGCG compared to placebo. After placebo, exercise EE was mainly fueled by fat oxidation in both men and women. After EGCG, there was a shift to a higher and more stable carbohydrate oxidation during exercise in men, but not in women.

5. Polyphenols targeting various signaling pathways

Polyphenols stand out for their antioxidant capacity, which allows them to reduce the negative effects of the reactive species produced by the over activation of the immune system when there is an autoimmune disease. In addition, polyphenols are pharmacologically active compounds with immunomodulatory activity [60]. However, one of the problems faced in evaluating their pharmacological activity is the great structural diversity of this group of compounds, and the high variability in their bioavailability and secondary metabolism to which they are subjected. Moreover, each type of polyphenol targets different immune cells and, therefore, triggers a plethora of diverse intracellular signaling pathways that ultimately regulate the host’s immune response. The modulation of several signaling pathways leads to alterations in the expression of proinflammatory genes, for instance those that code for multitude of cytokines, cyclooxygenase (COX), lipoxygenase (COX), phospholipase A₂ (PLA₂), inducible isoform of nitric oxide synthetase (iNOS)-, which combined with their ability to modulate the population and differentiation of specific immune cells and their direct antioxidant and anti-inflammatory activity allow the regulation of the inflammatory process (Table 2) [68, 69]. To date, the majority of the mechanistic studies were performed in cell cultures given their greater simplicity and reproducibility; however, they fail to recapitulate many of the problems that are inherent to oral administration, bioavailability and the intense secondary metabolism to which polyphenols are subject. Proposed points of action of polyphenols in autoimmune disorders are summarized in Figure 2. The structures of most widely studied polyphenols are given in Figure 3.

Table 2:

Polyphenols targeting signaling pathways

Signaling Pathways	Polyphenols	Actions	References
Arachidonic acid dependent pathway	Quercetin, kaempferol, galangin, anthocyanidins1, Catechol (1,2-dihydroxybenzen), resveratrol, p-coumaric, honokiol,	Inhibiting PLA2	[71, 72, 74–76]
	Luteolin, galangin, morin, apigenin	Inhibiting COX enzyme	[78, 79]
	Quercetin, kaempferol, myricetin, morin	Inhibiting LOX enzyme	[80]
NF-κB signaling pathway	Resveratrol, quercetin, genistein, EGCG	Modulating NF-κB signaling pathway which alters the genetic regulation of COX enzyme, pro-inflammatory cytokines and chemokines	[86, 92, 94, 138]
MAPKs pathway	Quercetin, EGCG, resveratrol, apigenin, luteolin	Suppressed the phosphorylation of NF-κB, ERK1/2, JNK and p38 pathway proteins which alters the production and expression of proinflammatory cytokines	[98, 99, 101, 106, 108]
	Hesperidin, naringin,	Inhibit the p38 MAPK signaling pathway2	[107]
	Kaempferol, chrysin	Attenuation of the JNK activity	[108]
Phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling pathway	Resveratrol, quercetin, EGCG	Downregulating the expression of proinflammatory cytokines by alteration of PI3K/Akt signaling pathway	[112–114, 116]
	Silibinin	Reduced the expression of pro-inflammatory cytokines and COX-2 by PI3K/Akt inhibition	[115]
	Isorhapontigenin	Cytokine inhibitory effects through modulation of corticosteroid insensitive PI3K/Akt pathway	[117]
	Ellagic acid	Protect HUVECs from apoptosis mediated by PI3K/Akt activation	[118]
	Resveratrol	Neuroprotection in rats undergoing ischemia induced cerebral damage by activating the PI3K/Akt survival pathway	[119]
Epigenetic modulation (DNA methylation, histone modifications and posttranscriptional regulation by miRNAs)	Resveratrol	Activation of SIRT1 resulting in the inhibition of NFκB and its downstream genes such as COX-2 and iNOS	[123, 124]
	Fisetin	Increase SIRT1 and SIRT3 expression	[58, 126]
	Curcumin	Inhibiting DNMTs Regulation of HATs and HDACs Regulate miRNA expression (miR-181b) Reactivate the neprilysin gene leading to inhibition of NFκB	[127, 132, 134]
	Quercetin, myricetin	Downregulate c-Myc, PHD2 and β-catenin expressions via SIRT1 activation in a manner that mimics hypoxic preconditioning	[128]
	EGCG	Hypoacetylation of p65 by inhibiting the activity of HAT which results in reduction the activity of NFκB	[129]
	Resveratrol	Restore LINE1 methylation in human retinal pigment epithelial cells through modulation of SIRT1 and DNMT Regulate miRNA expression (miR-21, miR-Let7A)	[131, 135, 136]

¹cyanidin, delphinidin malvidin, peonidin and petunidin

²No effect on ERK and JNK

Phospholipases A2 (PLA₂); Cyclooxygenase (COX); Lipoxygenase (LOX); nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB signalling pathway); epigallocatechin-3-gallate (EGCG); Mitogen-activated protein kinases (MAPKs pathway); extracellular signal–regulated kinases (ERK1/2); c-Jun N-terminal kinase pathway (JNK); human umbilical vein endothelial cells (HUVECs); members of sirtuin family (SIRT1 and SIRT3); DNA methyltransferase (DNMTs); Histone acetyltransferases (HATs); Histone deacetylases (HDACs); micro RNAs (miRNAs); oncogenes (c-Myc); prolyl hydroxylase domain-containing protein 2 (PHD2); long interspersed nuclear element-1 (LINE1).

Figure 2.

Main proposed points of action of polyphenols in autoimmune diseases (+ means activation and - inhibition. Akt, protein kinase B; COX, cyclooxygenase; ERK-1/2, extracellular-signal-regulated kinases 1/2; IκBs, inhibitory proteins of κB; IKK, IκBs kinase; JNKs, c-Jun amino-terminal kinases; LOX, lipoxygenase; MAPKs, mitogen-activated protein kinases; p38, p38-mitogen-activated protein kinase; PI3K, phosphatidylinositol 3-kinase; PLA₂, phospholipase A2; PTEN, phosphatase and tensin homolog; SIRT1, sirtuin 1.

Figure 3.

Chemical structures of polyphenols used in clinical studies

5.1. Arachidonic acid dependent pathway

Arachidonic acid or eicosatetraenoic acid (AA) is a polyunsaturated fatty acid of the omega-6 series present in membrane phospholipids, from where it is released by the action of PLA₂. Once in the cytoplasm, AA is targeted by various enzymes such as COX and LOX to generate prostaglandins (PGs) and thromboxanes A₂ or hydroxyeicosatetraenoic acids (HETEs) and leukotrienes (LTs), respectively[69]. These molecules are lipid mediators that contribute to the inflammatory process, and by inference, their inhibition represents a therapeutic target for the reduction of inflammation. Accordingly, one of the mechanisms by which polyphenols can modulate the immune response is via direct inhibition of these pro-inflammatory enzymes [70]. Considering PLA 2 as the first enzyme in the AA cascade, it has been evidenced the inhibitory capabilities by polyphenols such as quercetin, kaempferol, and galangin, as well as some anthocyanidins (cyanidin, delphinidin malvidin, peonidin and petunidin) [71–73] Catechol (1,2-dihydroxybenzen) binds to PLA2 preventing the substrate from entering into the active site [74]. Furthermore, resveratrol and p-coumaric acid interact directly with catalytic residues of PLA2, blocking its catalytic activity [75]. An interesting study showed that quercetin and honokiol inhibited cytosolic PLA2 phosphorylation and activation in differentiated SH-SY5Y neuroblastoma cells [76]. As for COX and LOX enzymes, flavones are predominantly COX inhibitors, whereas flavanols are preferentially LOX inhibitors [77]. Thus, flavonoids, such as luteolin, galangin, morin and apigenin exert notable COX inhibitory effects [78, 79]. Flavanols, including quercetin, kaempferol, myricetin and morin, in turn, inhibit LOX enzymes [80].

5.2. NF-κB signaling pathway

NF-κB regulates the expression of a wide range of genes implicated in the inflammatory process including the inducible form of COX (COX-2), pro-inflammatory cytokines [TNFα, IL-1, IL-2, IL-6 and IL-8], chemokines [MCP-1 i.e. monocyte chemoattractant protein 1, MIP-1α, i.e. macrophage inflammatory protein, CXCL1 i.e. (C-X-C motif) ligand 1 and IL-18], adhesion molecules [VCAM-1, i.e vascular cell adhesion protein 1, ICAM-1 i.e. intercellular adhesion molecule 1] and diverse growth factors and immuno-receptors [81, 82]. Polyphenols can alter the NFκB pathway along multiple steps in the signalling cascade. Given their antioxidant action, polyphenols can reduce the levels of reactive species, mainly H₂O₂, thus reducing the activation of the pathway [83]. Thus, it can be assumed that these species can act as a secondary messengers facilitating the activation of the NFκB pathway by a redox-sensitive process [84]. In addition, polyphenols can block the phosphorylation and subsequent degradation of inhibitory proteins (IκBs) by altering the activation of the IκBs kinase (IKK) and preventing nuclear translocation and binding of the factor to DNA [85]. Different polyphenols including resveratrol, quercetin, genistein or epigallocatechin-3-gallate (EGCG) can inhibit the NFκB pathway. Resveratrol blocks the activation of NF-κB in macrophage RAW 264.7 cells when stimulated with LPS through avoiding IKK activation and IκB phosphorylation [86]. Furthermore, resveratrol can interfere with TLR4 oligomerization, a necessary step in the activation of the receptor [86]. In addition, resveratrol as well as oleuropein aglycone and hydroxytyrosol significantly reduce the activation of NFκB in LPS-stimulated human umbilical vein endothelial cells (HUVECs) as determined by electrophoretic mobility shift assay [87]. Studying various immune cell models (RAW264.7 macrophages and bone marrow-derived macrophages, HMC-1 human mast cells, mouse BV-2 microglia and HUVECs) the inhibitory effects of quercetin on NFκB activation has been reported, including a reduction in nuclear translocation of p50 and p65 subunits, an inhibition of the phosphorylation of IκBα and their consequent degradation, and a blockage of the IKK activation. Genistein has been shown to reduce the overproduction of TNFα and IL-6 in RAW 264.7 macrophages stimulated by LPS via inhibition of NFκB activation [92]. The mechanism of action was associated with blockage of the IKK expression, IκBα degradation and p65 translocation into the nucleus. Analogous results were noted in human synoviocyte MH7A cells stimulated with TNFα by preventing IKK phosphorylation and NFκB translocation [93]. EGCG also prevented the IKK activation and inhibited phosphorylation of the p65 subunit of NFκB in a human respiratory epithelium A549 cells stimulated by IL-1β [94]. This effect seems to be initially mediated by the inhibition of the IRAK (IL-1β-mediated IL-1β receptor-associated kinase) degradation, which prevents IKK activation. Inhibition in NFκB activation has also been documented in primary osteoarthritis chondrocytes stimulated with IL-1β [95].

5.3. Mitogen-activated protein kinases (MAPKs) pathway

MAPKs comprise a group of serine/threonine protein kinases mainly activated by stress, mitogens and growth factors with a central role in many cellular processes including the regulation of genes related to inflammation [96]. The potential effect of polyphenols on MAPKs depends on the polyphenol itself as well as the cell target. Several studies have reported an inhibitory effect of polyphenols on JNKs (c-Jun amino-terminal kinases), ERK-1/2 (extracellular-signal-regulated kinases), p38/SAPKs (stress-activated protein kinases). These inhibitory effects result in decreased expression and release of proinflammatory mediators, such as TNFα or adhesion molecules. For example, quercetin has been shown to interfere with the phosphorylation and activation of JNK on LPS-treated RAW 264.7 macrophages, thus preventing the activator protein 1 (AP-1) from binding to ADN, and inhibiting TNFα transcription [98]. Moreover, inhibition of ERK1/2 and p38 activation has been also reported in response to quercetin [98, 99]. Corroborating this study, a reduction in phosphorylated forms of ERK1/2 and p38 has been reported in an experimental rat model of autoimmune myocarditis study upon treatment with quercetin [100].

Additional studies reported on the inhibitory effects of EGCG against MAPKs by reducing their phosphorylation and activation, including LPS-activated macrophages or LPS-induced murine dendritic cells maturation [101–103]. EGCG have also been shown to attenuate symptoms associated with a murine model for human autoimmune Sjogren’s syndrome by reducing the phosphorylation of p38 in salivary acinar cells [104]. Resveratrol treatment inhibited p38 and JNK signalling pathways in IL-1β-stimulated rat RSC-364 synovial cells and in HUVECs treated with hydrogen peroxide [105, 106]. Similarly, other polyphenols, such as luteolin, chrysin, hesperidin, naringin and kaempferol have been shown to reduce inflammation by interfering with MAPKs pathways [107–109]. However, opposite effects, reporting on the activation of ERK and JNK signaling pathways in cultured human coronary artery endothelial cells by quercetin and catechin resulting in a downregulation of plasminogen activator inhibitor 1 (PAI-1) [110], are noteworthy and emphasize the need for future characterization on the efficacy of polyphenols in affecting MAPK signaling.

5.4. Phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling pathway

PI3K/Akt signaling pathway play an important role in the expression of pro-inflammatory mediators by inducing the degradation of IκB and the following activation of NFκB [111]. Diverse polyphenols mediate their anti-inflammatory effects, in part, through PI3K/Akt inhibition. The expression of the pro-inflammatory cytokine IL-17 was reduced by resveratrol treatment in cardiac fibroblasts in a process mediated by PI3K/Akt inhibition [112]. Resveratrol also supressed PI3K/Akt activity in IL-1β-activated human tenocytes [113]. Quercetin suppressed the phosphorylation of Akt by direct binding and inhibition of PI3K in JB6 mouse epidermal cells [114]. Silibinin, applied topically to inflamed murine ears suppressed the expression of pro-inflammatory cytokines and COX-2 by PI3K/Akt inhibition [115]. EGCG inhibited epithelial-mesenchymal transition and inflammation via the PI3K/AKT pathway by upregulating the expression of phosphatase and tensin homolog (PTEN). Since this enzyme blocks the activation of PI3K through dephosphorylation of the signaling lipid phosphatidylinositol-3,4,5-triphosphate [116]. Another study reported on the protective efficacy of isorhapontigenin against airway epithelial cell inflammation by suppressing the PI3K/Akt pathway and reducing the activation of NFκB and the release of IL-6 and chemokine CXCL8 i.e. (C-X-C motif) ligand 8 [117].

It is noteworthy that the favourable effects of polyphenols may also be associated with increased signaling in the PI3K/Akt pathway. In these studies, the activation of the pathway as a signaling mechanism for survival prevails. For instance, ellagic acid was shown to protect HUVECs from apoptosis induced by oxidized low-density lipoprotein treatment in a process mediated by PI3K/Akt activation [118]. Also, resveratrol induced neuroprotection in rats undergoing ischemia/reperfusion induced cerebral damage by activating the PI3K/Akt survival pathway [119].

5.5. Epigenetic modulation

A novel point for cellular control for polyphenols is secondary to their ability to modulate modular epigenetic mechanisms such as DNA methylation, histone modifications and posttranscriptional regulation by microRNAs, modulating the activation and differentiation of immune cells. Changes in epigenetic patterns have been evidenced in diseases including inflammatory disorders [120]. Indeed, a series of studies have reported the existence of polyphenol-induced epigenetic modifications, leading to gene activation or silencing in the absence of changes in DNA sequences [121, 122]. Among the various polyphenols, resveratrol has been shown to be a strong activator of SIRT1, resulting in the inhibition of NFκB and its downstream genes, such as COX-2 and iNOS [123, 124]. In addition, several other polyphenols, such as quercetin, curcumin, myricetin and fisetin also activate SIRT1 [125–128]. EGCG has been shown to reduce the activity of NFκB via hypoacetylation of p65 by inhibiting the activity of histone acetyl transferase [129, 130]. In addition to SIRT1, resveratrol also acts on DNA methyltransferases, increasing their expression and activities [131]. Curcumin has also been reported to reactivate the neprilysin gene (a strong inhibitor of Akt) through CpG demethylation, leading to Akt inhibition and the subsequent inhibition of NFκB in mouse neuroblastoma N2a cells [132].

MicroRNAs are small and non-coding regulatory RNAs with the capability to regulate the translocation and/or degradation of messenger RNAs [133]. Diverse studies reported the modulatory effects of polyphenols on microRNAs. However, although some studies have shown anti-inflammatory effects of the modulation of microRNAs, most research has focussed in this context on cancer outcomes. Curcumin has been found to upregulate the expression of miR-181b, which, in turn, reduces the expression of the pro-inflammatory chemokines CXCL1 and CXCL2 [134]. Another interesting microRNA is the miR-21, as it has been associated with the activity of NFκB [133]. Resveratrol has been shown to decrease miR-21 expression and NFκB activity in U251 brain tumour cells [135]. Resveratrol has also been reported to increase miR-Let7A in LPS-stimulated THP-1 cells, resulting in the reduced expression of IL-6, IL-10 and TNFα [136]. In another study performed in A/J mice lung adenoma model, EGCG has been shown to increase 12 microRNAs and to decrease 9 target genes associated with key elements of the Akt and MAPK signaling pathways [137].

6. Conclusion

Polyphenols, one of the main secondary metabolites in plants, are found to be beneficial in prevention and treatment of various diseases. In autoimmune diseases, many plant extracts or isolated individual phenolic compounds have been evaluated for their efficacy in clinical studies. Among them, extracts of G. biloba and green tea were studied for their effectiveness in vitiligo, ulcerative colitis, Multiple Sclerosis, etc. Similarly, curcumin, resveratrol, epigallocatechin gallate were also studied for their efficacy in various autoimmune diseases. Taken together, polyphenols have been shown as promising candidates for the development of novel therapeutics in autoimmune diseases. While few mechanism-based studies have been performed on their mechanism of action in autoimmune diseases, they have yet to be fully delineated.