Suppression of MyD88- and TRIF-dependent signaling pathways of Toll-like receptor by (-)-epigallocatechin-3-gallate, a polyphenol component of green tea

Green tea catechin, epigallocatechin gallate, suppresses signaling by the dsRNA innate immune receptor RIG-I

BACKGROUND

The Innate immune system constitutes the first line of defense against pathogen infections. The Retinoic acid-inducible gene I (RIG-I) receptor recognizes triphosphorylated ssRNAs and dsRNA to initiate downstream signaling of interferon response. However, unregulated activity of these receptors could lead to autoimmune diseases. We seek to identify small molecules that can specifically regulate RIG-I signaling.

METHODOLOGY/PRINCIPAL FINDINGS

Epigallocatechin gallate (EGCG), a polyphenolic catechin present in green tea, was identified in a small molecule screen. It was found to bind RIG-I and inhibits its signaling at low micromolar concentrations in HEK293T cells. Furthermore, EGCG dose-dependently inhibited the ATPase activity of recombinant RIG-I but did not compete with RIG-I interaction with RNA or with ATP. EGCG did not inhibit signaling by Toll-like receptors 3, 4, 9 or constitutive signaling by the adapter protein IPS-1. Structure activity relationship analysis showed that EGCG, its epimer GCG and a digallate-containing compound, theaflavin 3,3' digallate (TFDG) were potent RIG-I inhibitors. EGCG also inhibited IL6 secretion and IFN- β mRNA synthesis in BEAS-2B cells, which harbors intact endogenous RIG-I signaling pathway.

CONCLUSIONS/SIGNIFICANCE

EGCG and its derivatives could have potential therapeutic use as a modulator of RIG-I mediated immune responses.

TLR4 signaling inhibitory pathway induced by green tea polyphenol epigallocatechin-3-gallate through 67-kDa laminin receptor

Epigallocatechin-3-gallate (EGCG), a major active polyphenol of green tea, has been shown to downregulate inflammatory responses in macrophages; however, the underlying mechanism has not been understood. Recently, we identified the 67-kDa laminin receptor (67LR) as a cell-surface EGCG receptor that mediates the anticancer action of EGCG at physiologically relevant concentrations (0.1-1 microM). In this study, we show the molecular basis for the downregulation of TLR4 signal transduction by EGCG at 1 microM in macrophages. Anti-67LR Ab treatment or RNA interference-mediated silencing of 67LR resulted in abrogation of the inhibitory action of EGCG on LPS-induced activation of downstream signaling pathways and target gene expressions. Additionally, we found that EGCG reduced the TLR4 expression through 67LR. Interestingly, EGCG induced a rapid upregulation of Toll-interacting protein (Tollip), a negative regulator of TLR signaling, and this EGCG action was prevented by 67LR silencing or anti-67LR Ab treatment. RNA interference-mediated silencing of Tollip impaired the TLR4 signaling inhibitory activity of EGCG. Taken together, these findings demonstrate that 67LR plays a critical role in mediating anti-inflammatory action of a physiologically relevant EGCG, and Tollip expression could be modulated through 67LR. These results provide a new insight into the understanding of negative regulatory mechanisms for the TLR4 signaling pathway and consequent inflammatory responses that are implicated in the development and progression of many chronic diseases.

TBK1-targeted suppression of TRIF-dependent signaling pathway of Toll-like receptors by 6-shogaol, an active component of ginger

Toll-like receptors (TLRs) are primary sensors that detect a wide variety of microbial components involving induction of innate immune responses. After recognition of microbial components, TLRs trigger the activation of myeloid differential factor 88 (MyD88) and Toll-interleukin-1 (IL-1) receptor domain-containing adapter inducing interferon-beta (TRIF)-dependent downstream signaling pathways. 6-Shoagol, an active ingredient of ginger, inhibits the MyD88-dependent signaling pathway by inhibiting inhibitor-kappaB kinase activity. Inhibitor-kappaB kinase is a key kinase in nuclear factor kappaB (NF-kappaB) activation. However, it is not known whether 6-shogaol inhibits the TRIF-dependent signaling pathway. Our goal was to identify the molecular target of 6-shogaol in the TRIF-dependent pathway of TLRs. 6-Shogaol inhibited the activation of interferon-regulatory factor 3 (IRF3) induced by lipopolysaccharide (LPS) and by polyriboinosinic polyribocytidylic acid (poly[I:C]), overexpression of TRIF, TANK-binding kinase1 (TBK1), and IRF3. Furthermore, 6-shogaol inhibited TBK1 activity in vitro. Together, these results suggest that 6-shogaol inhibits the TRIF-dependent signaling pathway of TLRs by targeting TBK1, and, they imply that 6-shogaol can modulate TLR-derived immune/inflammatory target gene expression induced by microbial infection.

Green tea: nature's defense against malignancies

The current practice of introducing phytochemicals to support the immune system or fight against diseases is based on centuries old traditions. Nutritional support is a recent advancement in the domain of diet-based therapies; green tea and its constituents are one of the important components of these strategies to prevent and cure various malignancies. The anti-carcinogenic and anti-mutagenic activities of green tea were highlighted some years ago suggesting that it could reduce the prevalence of cancer and even provide protection. The pharmacological actions of green tea are mainly attributed to polyphenols that includes epigallocatechin-3-gallate (EGCG), epicatechin, epicatechin-3-gallate, epigallocatechin. Green tea and its components effectively mitigate cellular damage arising due to oxidative stress. Green tea is supposed to enhance humoral and cell-mediated immunity, decreasing the risk of certain cancers, and may have certain advantage in treating inflammatory disorders. Much of the cancer chemopreventive properties of green tea are mediated by EGCG that induces apoptosis and promotes cell growth arrest, by altering the expression of cell cycle regulatory proteins, activating killer caspases, and suppressing nuclear factor kappa-B activation. Besides, it regulates and promotes IL-23 dependent DNA repair and stimulates cytotoxic T cells activities in a tumor microenvironment. It also blocks carcinogenesis by modulating the signal transduction pathways involved in cell proliferation, transformation, inflammation and metastasis. The review is intended to highlight the chemistry of green tea, its antioxidant potential, its immunopotentiating properties and mode of action against various cancer cell lines that showed its potential as a chemopreventive agent against colon, skin, lung, prostate, and breast cancer.

Inhibition of homodimerization of toll-like receptor 4 by 6-shogaol

Toll-like receptors (TLRs) play a critical role in sensing microbial components and inducing innate immune and inflammatory responses by recognizing invading microbial pathogens. Lipopolysaccharide-induced dimerization of TLR4 is required for the activation of downstream signaling pathways including nuclear factor-kappa B (NF-kappaB). Therefore, TLR4 dimerization may be an early regulatory event in activating ligand-induced signaling pathways and induction of subsequent immune responses. Here, we report biochemical evidence that 6-shogaol, the most bioactive component of ginger, inhibits lipopolysaccharide-induced dimerization of TLR4 resulting in the inhibition of NF-kappaB activation and the expression of cyclooxygenase-2. Furthermore, we demonstrate that 6-shogaol can directly inhibit TLR-mediated signaling pathways at the receptor level. These results suggest that 6-shogaol can modulate TLR-mediated inflammatory responses, which may influence the risk of chronic inflammatory diseases.

Suppression of the TRIF-dependent signaling pathway of Toll-like receptors by luteolin

Toll-like receptors (TLRs) play important roles in induction of innate immune responses for both host defense against invading pathogens and wound healing after tissue injury. Since dysregulation of TLR-mediated immune responses is closely linked to many chronic diseases, modulation of TLR activation by small molecules may have therapeutic potential against such diseases. Expression of the majority of lipopolysaccharide-induced TLR4 target genes is mediated through a MyD88-independent (TRIF-dependent) signaling pathway. In order to evaluate the therapeutic potential of the flavonoid luteolin we examined its effect on TLR-stimulated signal transduction via the TRIF-dependent pathway. Luteolin suppressed activation of Interferon regulatory factor 3 (IRF3) and NFkappaB induced by TLR3 and TLR4 agonists resulting in the decreased expression of target genes such as TNF-alpha, IL-6, IL-12, IP-10, IFNbeta, CXCL9, and IL-27 in macrophages. Luteolin attenuated ligand-independent activation of IRF3 or NFkappaB induced by TLR4, TRIF, or TBK1, while it did not inhibit TLR oligomerization. Luteolin inhibited TBK1-kinase activity and IRF3 dimerization and phosphorylation, leading to the reduction of TBK1-dependent gene expression. Structural analogs of luteolin such as quercetin, chrysin, and eriodictyol also inhibited TBK1-kinase activity and TBK1-target gene expression. These results demonstrate that TBK1 is a novel target of anti-inflammatory flavonoids resulting in the down-regulation of the TRIF-dependent signaling pathway. These results suggest that the beneficial activities of these flavonoids against inflammatory diseases may be attributed to the modulation of TLR-mediated inflammatory responses.

Guggulsterone suppresses the activation of transcription factor IRF3 induced by TLR3 or TLR4 agonists

Toll-like receptors (TLRs) are vital in the induction of innate immune responses. The microbial components trigger the activation of the myeloid differential factor 88 (MyD88)- and toll-interleukin-1 receptor domain-containing adapter inducing interferon-beta (TRIF)-dependent downstream TLR signaling pathways. Guggulsterone, which has been used for centuries to treat many chronic diseases, inhibits the MyD88-dependent pathway by inhibiting the activity of inhibitor-kappaB kinase. However, it is not known whether guggulsterone inhibits the TRIF-dependent pathway. Presently, we sought to identify the molecular targets of guggulsterone in this pathway. Guggulsterone inhibited nuclear factor-kappaB and IRF3 activation induced by lipopolysaccharide or poly[I:C] and activation of IRF3 induced by the overexpression of TRIF, TBK1 or constitutively active IRF3. Guggulsterone also suppressed the lipopolysaccharide-induced phosphorylation of IRF3. These results suggest that guggulsterone can modulate both MyD88- and TRIF-dependent signaling pathways of TLRs leading to decreased inflammatory gene expression.

The adjuvanticity of a mannosylated antigen reveals TLR4 functionality essential for subset specialization and functional maturation of mouse dendritic cells

The evidence that dendritic cell (DC) subsets produce differential cytokines in response to specific TLR stimulation is robust. However, the role of TLR stimulation in Ag presentation and phenotypic maturation among DC subsets is not clear. Through the adjuvanticity of a novel mannosylated Ag, mannosylated dendrimer OVA (MDO), as a pathogen-associated molecular pattern Ag, we characterized the functionality of GM-CSF/IL-4-cultured bone marrow DC and Flt3 ligand (Flt3-L) DC subsets by Ag presentation and maturation assays. It was demonstrated that both bone marrow DCs and Flt3-L DCs bound, processed, and presented MDO effectively. However, while Flt3-L CD24(high) (conventional CD8(+) equivalent) and CD11b(high) (CD8(-) equivalent) DCs were adept at MDO processing by MHC class I and II pathways, respectively, CD45RA(+) plasmacytoid DCs presented MDO poorly to T cells. Successful MDO presentation was largely dependent on competent TLR4 for Ag localization and morphological/phenotypic maturation of DC subsets, despite the indirect interaction of MDO with TLR4. Furthermore, Toll/IL-1 receptor-domain-containing adaptor-inducing IFN-beta, but not MyD88, as a TLR4 signaling modulator was indispensable for MDO-induced DC maturation and Ag presentation. Taken together, our findings suggest that DC subsets differentially respond to a pathogen-associated molecular pattern-associated Ag depending on the intrinsic programming and TLRs expressed. Optimal functionality of DC subsets in Ag presentation necessitates concomitant TLR signaling critical for efficient Ag localization and processing.

Nitric oxide as a target of complementary and alternative medicines to prevent and treat inflammation and cancer

Nitric oxide (NO) and associated reactive nitrogen species (RNS) are involved in many physiological functions. There has been an ongoing debate to whether RNS can inhibit or perpetuate chronic inflammation and associated carcinogenesis. Although the final outcome depends on the genetic make-up of its target, the surrounding microenvironment, the activity and localization of nitric oxide synthase (NOS) isoforms, and overall levels of NO/RNS, evidence is accumulating that in general, RNS drive inflammation and cancers associated with inflammation. To this end, many complementary and alternative medicines (CAMs) that work in chemoprevention associated with chronic inflammation, are inhibitors of excessive NO observed in inflammatory conditions. Here, we review recent literature outlining a role of NO/RNS in chronic inflammation and cancer, and point toward NO as one of several targets for the success of CAMs in treating chronic inflammation and cancer associated with this inflammation.

Roles of MAPK and NF-kappaB in interleukin-6 induction by lipopolysaccharide in vascular smooth muscle cells

Toll-like receptor (TLR)-4 signaling promotes cytokine synthesis in vascular smooth muscle cells (VSMC). However, it is unknown how TLR-4 regulates interleukin-6 (IL-6) in VSMC. Therefore, the present study investigated cellular factors involved in TLR-4-mediated IL-6 in VSMC in terms of MAPK and transcription elements. Exposure of aortic smooth muscle cells to TLR4-specific lipopolysaccharide (LPS) not only enhanced IL-6 release but also induced IL-6 transcript via promoter activation. The promoter activation was attenuated by dominant-negative MKK1 and to a lesser extent by dominant-negative MKK3, but not by dominant-negative MKK4. IL-6 promoter activity was diminished by U0126 or SB202190, but not by SP600125. Co-transfection with dominant negative CCAAT/enhancer binding protein or with IkappaB suppressed LPS-induced promoter activation, whereas the promoter activity was not influenced by dominant negative c-Jun. Mutation in the IL-6 promoter region at the binding site of NF-kappaB or C/EBP impaired promoter activation in response to LPS. Further impairment occurred when both NF-kappaB- and C/EBP-binding sites were mutated. LPS-induced IL-6 promoter activation was also prevented by pretreatment with epigallocatechin 3-gallate, curcumin, and resveratrol. The present study reports that TLR4-agonistic LPS induces IL-6 through transcriptional activation in VSMC and ERK1/2, p38 MAPK, NF-kappaB, and C/EBP play active roles in that process.

Inflammatory responses of gingival epithelial cells stimulated with Porphyromonas gingivalis vesicles are inhibited by hop-associated polyphenols

BACKGROUND

Periodontitis is induced by an imbalance between bacterial virulence and host defense ability. Porphyromonas gingivalis, a predominant periodontal pathogen, triggers a series of host inflammatory responses that aggravate the destruction of periodontium. Thus, anti-inflammatory reagents are considered desirable for effective periodontal therapy. In the present study, we examined the inhibitory effects of hop bract polyphenol (HBP) on cellular inflammatory responses induced by P. gingivalis membrane vesicles.

METHODS

Immortalized human gingival epithelial cells were stimulated with P. gingivalis membrane vesicles, and the effects of HBP on mRNA expression of cyclooxygenase (COX)-2, interleukin (IL)-6 and -8, and matrix metalloproteinase (MMP)-1 and -3 were examined using real-time reverse transcription-polymerase chain reaction.

RESULTS

HBP inhibited the mRNA expression of COX-2, IL-6 and -8, and MMP-1 and -3 in a dose-dependent manner, whereas epigallocatechin gallate (a control polyphenol) inhibited COX-2 mRNA expression only. Following further fractionation of HBP to identify the effective components, 2-[(2-methylpropanoyl)-phloroglucinol]1-O-beta-D-glucopyranoside (MPPG) was identified as a significant anti-inflammatory element that completely inhibited the inflammatory mRNA induction. Kaempferol 3-O-beta-glucopyranoside (astragalin) also was found to have anti-inflammatory effects.

CONCLUSIONS

HBP is suggested to be a potent inhibitor of cellular inflammatory responses induced by P. gingivalis vesicles. Further, MPPG and astragalin, identified here as effective components of HBP, also may be useful for the prevention and/or attenuation of periodontitis.

West Nile virus envelope protein inhibits dsRNA-induced innate immune responses

The immune response against viral infection relies on the early production of cytokines that induce an antiviral state and trigger the activation of immune cells. This response is initiated by the recognition of virus-associated molecular patterns such as dsRNA, a viral replication intermediate recognized by TLR3 and certain RNA helicases. Infection with West Nile virus (WNV) can lead to lethal encephalitis in susceptible individuals and constitutes an emerging health threat. In this study, we report that WNV envelope protein (WNV-E) specifically blocks the production of antiviral and proinflammatory cytokines induced by dsRNA in murine macrophages. This immunosuppressive effect was not dependent on TLR3 or its adaptor molecule Trif. Instead, our experiments show that WNV-E acts at the level of receptor-interacting protein 1. Our results also indicate that WNV-E requires a certain glycosylation pattern, specifically that of dipteran cells, to inhibit dsRNA-induced cytokine production. In conclusion, these data show that the major structural protein of WNV impairs the innate immune response and suggest that WNV exploits differential vector/host E glycosylation profiles to evade antiviral mechanisms.

Garlic (Allium sativum) extract inhibits lipopolysaccharide-induced Toll-like receptor 4 dimerization

Garlic has long been used as a folk medicine. Numerous studies have demonstrated that a garlic extract and its sulfur-containing compounds inhibited nuclear factor kappa B (NF-kappaB) activation induced by various receptor agonists including lipopolysaccharide (LPS). Toll-like receptors (TLRs) play a key role in sensing diverse microbial products and inducing innate immune responses. The dimerization of TLR4 is required for the activation of downstream signaling pathways, including NF-kappaB. Therefore, TLR4 dimerization may be one of the first lines of regulation in activating LPS-induced signaling pathways. We report here biochemical evidence that the ethyl acetate fraction of garlic inhibited the LPS-induced dimerization of TLR4, resulting in the inhibition of NF-kappaB activation and the expression of cyclooxygenase 2 and inducible nitric oxide synthase. Our results demonstrate for the first time that a garlic extract can directly inhibit the TLRs-mediated signaling pathway at the receptor level. These results shed a new insight into understanding how garlic modulates the immune responses that could modify the risk of many chronic diseases.

Inhibition of lipopolysaccharide-induced inducible nitric oxide synthase and cyclooxygenase-2 expression by xanthanolides isolated from Xanthium strumarium

Three sesquiterpenoids, xanthatin (1), xanthinosin (2), and 4-oxo-bedfordia acid (3) were isolated from Xanthium strumarium as inhibitors of nitric oxide synthesis in activated microglia (IC(50) values: 0.47, 11.2, 136.5 microM, respectively). Compounds 1 and 2 suppressed the expression of iNOS and COX-2 and the activity of NF-kappaB through the inhibition of LPS-induced I-kappaB-alpha degradation in microglia.

Selenium suppresses the activation of transcription factor NF-kappa B and IRF3 induced by TLR3 or TLR4 agonists

Toll-like receptors (TLRs) play an important role in recognition of microbial components and induce innate immune responses by recognizing invading microbial pathogens leading to the activation of the adaptive immune responses. The microbial components trigger the activation of two downstream signaling pathways of TLRs; MyD88- and TRIF-dependent pathways leading to the expression of pro-inflammatory cytokines and type I interferons (IFNs). The MyD88- and TRIF-dependent pathways lead to the activation of NF-kappa B and IRF3 through the activation of IKK-beta and TBK1, respectively. Selenium is an essential trace element nutrient possessing anticarcinogenic properties. Here, we attempted to identify the molecular targets of selenium in TLR signaling pathways. Selenium inhibited NF-kappaB activation induced by poly[I:C] (TLR3 agonist), LPS (TLR4 agonist) or overexpression of MyD88 or IKK-beta which is the key kinase of MyD88-dependent signaling pathway. Selenium inhibited IRF3 activation induced by poly[I:C], LPS or the overexpression of TRIF or TBK1. Selenium also suppressed the expression of COX-2 and iNOS and the endogenous IFN beta mRNA induced by poly[I:C] or LPS. Therefore, our results suggest that selenium can modulate both MyD88- and TRIF-dependent signaling pathways of TLRs leading to decreased inflammatory gene expression.