Ehrlichia chaffeensis induces monocyte inflammatory responses through MyD88, ERK, and NF-κB but not through TRIF, interleukin-1 receptor 1 (IL-1R1)/IL-18R1, or toll-like receptors

Human monocytic ehrlichiosis, an influenza-like illness accompanied by signs of hepatitis, is caused by infection of monocytes/macrophages with a lipopolysaccharide-deficient bacterium, Ehrlichia chaffeensis. The E. chaffeensis strain Wakulla induces diffuse hepatitis with neutrophil infiltration in mice with severe combined immunodeficiency, which is accompanied by strong CXCL2 (mouse functional homolog of interleukin-8 [IL-8]) and tumor necrosis factor alpha (TNF-α) expression in the liver. In this study, we found that expression of IL-1β, CXCL2, and TNF-α was induced by strain Wakulla in mouse bone marrow-derived macrophages; this expression was dependent on MyD88, but not on TRIF, TLR2/4, IL-1R1/IL-18R1, or endosome acidification. When the human leukemia cell line THP-1 was exposed to E. chaffeensis, significant upregulation of IL-8, IL-1β, and TNF-α mRNA and extracellular regulated kinase 2 (ERK2) activation were detected. U0126 (inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1/2 [MEK1/2] upstream of ERK), manumycin A (Ras inhibitor), BAY43-9006 (Raf-1 inhibitor), and NS-50 (inhibitor of NF-κB nuclear translocation) inhibited the cytokine gene expression. A luciferase reporter assay using HEK293 cells, which lack Toll-like receptors (TLRs), showed activation of both the IL-8 promoter and NF-κB by E. chaffeensis. Activation of the IL-8 promoter in transfected HEK293 cells was inhibited by manumycin A, BAY43-9006, U0126, and transfection with a dominant-negative Ras mutant. These results indicate that the E. chaffeensis Wakulla strain can induce inflammatory responses through MyD88-dependent NF-κB and ERK pathways, without the involvement of TRIF and TLRs.     

Mycoplasma genitalium-Derived Lipid-Associated Membrane Proteins Activate NF-κB through Toll-Like Receptors 1, 2, and 6 and CD14 in a MyD88-Dependent Pathway

Mycoplasma genitalium is a leading pathogen of nongonoccocal chlamydia-negative urethritis, which has been implicated directly in numerous other genitourinary and extragenitourinary tract pathologies. The pathogenesis of infection is attributed in part to excessive immune responses. M. genitalium-derived lipid-associated membrane proteins (LAMPs) are a mixture of bacterial lipoproteins, exposed at the surface of mycoplasma, that are potent inducers of the host innate immune system. However, the interaction of M. genitalium-derived LAMPs as pathogenic agents with Toll-like receptors (TLRs) and the signaling pathways responsible for active inflammation and NF-κB activation have not been fully elucidated. In this study, LAMPs induced the production of tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) in a dose-dependent manner. Blocking assays showed that TLR2- and CD14-neutralizing antibodies reduced the expression of TNF-α and IL-6 in THP-1 cells. Furthermore, LAMP-induced NF-κB activation was increased in 293T cells transfected with TLR2 plasmid. The activity of NF-κB was synergically augmented by cotransfected TLR1, TLR6, and CD14. Additionally, LAMPs were shown to inhibit NF-κB expression by cotransfection with dominant-negative MyD88 and TLR2 plasmids. These results suggest that M. genitalium-derived LAMPs activate NF-κB via TLR1, TLR2, TLR6, and CD14 in a MyD88-dependent pathway.Mycoplasmas are the smallest of the known self-replicating parasitic microorganisms, and they lack a cell wall in cell-free medium. They are pervasive in nature, humans, and animals, but most mycoplasmas are nonpathogenic to the host (36). Lipid-associated membrane proteins (LAMPs) are a mixture of bacterial lipoproteins that are expressed on the surface, and they are the main structures of interaction with the host cells (37). They have been demonstrated to be biologically active and are the most potent initiator of inflammatory reactions in mycoplasma infection (9, 11, 34). Since Mycoplasma genitalium was first isolated from humans in 1981 (51), it has been considered an important pathogen that could emerge in sexually transmitted diseases, including acute endometritis, salpingitis, mucopurulent cervicitis, tubal factor infertility, and pelvic inflammatory disease, as well as a range of other pathologies, such as arthritis, AIDS progression, chronic fatigue, and autoimmune disorders (4, 10, 19). Recent evidence has demonstrated that M. pneumoniae-derived LAMPs can induce NF-κB activation in human acute monocytic leukemia cell lines (THP-1 cells) (42, 43).Toll-like receptors (TLRs) play an essential role in initiating the inflammatory reactions, such as NF-κB activation (2, 22, 26). To date, 13 types of human TLRs have been identified and shown to be critical for signaling transduction in response to a number of pathogen-associated molecular patterns (PAMPs) (8, 44). PAMPs are recognized by different TLRs to form heteromeric complexes, among which the family of TLRs prominently features, for the cell to distinguish successfully among different pathogens. Among TLRs, TLR2, in tandem with TLR1 or TLR6, has been identified as a receptor that is important to the innate immune response against several gram-positive bacteria and for cellular signaling by components of gram-positive bacteria, such as peptidoglycan, lipoteichoic acid, and lipoproteins. The receptors transmit signaling pathways via interleukin-1R (IL-1R)-associated signal molecules, including myeloid differentiation factor 88 (MyD88), IL-1R-activated kinase, tumor necrosis factor (TNF) receptor-associated factor 6, and mitogen-activated protein kinases, leading to the activation of NF-κB and activating protein 1 (AP-1), which in turn triggers the expression of many proinflammatory cytokines, such as TNF-α, IL-1β, IL-6, and IL-8 (1, 20, 29, 47). MyD88 serves as the essential adaptor for other IL-1/TLR family members, including IL-1R and IL-18R. The activation of the intracellular pathway through MyD88 results in the activation of NF-κB. CD14 is a glycosylphosphatidylinositol surface-anchored molecule expressed in many kinds of cells as another accessory receptor of TLR2. It lacks an intracellular segment and functions as a coreceptor for numerous bacterial products, including peptidoglycan, lipopolysaccharide (LPS), and bacterial lipoproteins, thereby facilitating signaling through other receptors (23). Several studies have suggested that TLR2 functions as a signaling receptor for LPS in the presence of CD14 (50). Recently published data support the possibility that the multifunctional B class scavenger receptor CD36 also is involved in TLR2 transmembrane signal transducing (16). However, data available also have underscored the current lack of a complete understanding of the molecular mechanisms that link M. genitalium infection to the activation of the innate immune system, which is essential to the induction of the inflammation.The LAMPs from Mycoplasma fermentans and Mycoplasma penetrans activate human immunodeficiency virus long terminal repeats through TLRs (41). To further understand how LAMPs activate the immune system of the cell, including THP-1 cells and human embryonic kidney cells (HEK293T cells), the present study was designed to investigate the interaction of M. genitalium-derived LAMPs with TLRs and CD14 and to clarify the role of MyD88 in activating the NF-κB signaling pathway.     

LFP-20, a porcine lactoferrin peptide,ameliorates LPS-induced inflammation via the MyD88/NF-κB and MyD88/MAPK signaling pathways

LFP-20 is one of the 20 amino acid anti-microbial peptides identified in the N terminus of porcine lactoferrin. Apart from its extensively studied direct anti-bacterial activity, its potential as an activator of immune-related cellular functions is unknown. Therefore, this study investigated its anti-inflammatory effects in lipopolysaccharide (LPS)-stimulated pig alveolar macrophages in vitro and systemic inflammation in an in vivo mouse model. We found that the inhibitory effects of LFP-20 on production of pro-inflammatory cytokines were independent of its LPS-binding activity. However, they were associated with NF-κB and MAPK-dependent signaling. Furthermore, LFP-20 might directly influence MyD88 levels to block its interaction with NF-κB and MAPK-dependent signaling molecules that might alter LPS-mediated inflammatory responses in activated macrophages. Taken together, our data indicated that LFP-20 prevents the LPS-induced inflammatory response by inhibiting MyD88/NF-κB and MyD88/MAPK signaling pathways, and sheds light on the potential use of LFP-20 in the therapy of LPS-mediated sepsis.     

Jellyfish collagen stimulates production of TNF-α and IL-6 by J774.1 cells through activation of NF-κB and JNK via TLR4 signaling pathway

We previously reported that jellyfish collagen stimulates both the acquired and innate immune responses. In the acquired immune response, jellyfish collagen enhanced immunoglobulin production by lymphocytes in vitro and in vivo. Meanwhile, in the innate immune response jellyfish collagen promoted cytokine production and phagocytotic activity of macrophages. The facts that jellyfish collagen plays several potential roles in stimulating cytokine production by macrophages have further attracted us to uncover its mechanisms. We herein describe that the cytokine production-stimulating activity of jellyfish collagen was canceled by a Toll-like receptor 4 (TLR4) inhibitor. Moreover, jellyfish collagen stimulated phosphorylation of inhibitor of κBα (IκBα), promoted the translocation of nucleus factor-κB (NF-κB), and activated c-Jun N-terminal kinase (JNK). A JNK inhibitor also abrogated the cytokine production-stimulating activity of jellyfish collagen. These results suggest that jellyfish collagen may facilitate cytokine production by macrophages through activation of NF-κB and JNK via the TLR4 signaling pathways.     

SIRT1 negatively regulates amyloid-beta-induced inflammation via the NF-κB pathway

Chronic inflammation induced by amyloid-beta (Aβ) plays a key role in the development of age-related macular degeneration (AMD), and matrix metalloproteinase-9 (MMP-9), interleukin (IL)-6, and IL-8 may be associated with chronic inflammation in AMD. Sirtuin 1 (SIRT1) regulates inflammation via inhibition of nuclear factor-kappa B (NF-κB) signaling, and resveratrol has been reported to prevent Aβ-induced retinal degeneration; therefore, we investigated whether this action was mediated via activation of SIRT1 signaling. Human adult retinal pigment epithelial (RPE) cells were exposed to Aβ, and overactivation and knockdown of SIRT1 were performed to investigate whether SIRT1 is required for abrogating Aβ-induced inflammation. We found that Aβ-induced RPE barrier disruption and expression of IL-6, IL-8, and MMP-9 were abrogated by the SIRT1 activator SRT1720, whereas alterations induced by Aβ in SIRT1-silenced RPE cells were not attenuated by SRT1720. In addition, SRT1720 inhibited Aβ-mediated NF-κB activation and decrease of the NF-κB inhibitor, IκBα. Our findings suggest a protective role for SIRT1 signaling in Aβ-dependent retinal degeneration and inflammation in AMD.     

Andrographolide attenuates microglia-mediated Aβ neurotoxicity partially through inhibiting NF-κB and JNK MAPK signaling pathway

Neuroinflammation plays a critical role in the pathogenesis of several neurodegenerative diseases, including Alzheimer’s disease (AD). Microglial cells after activated play critical roles in development of neuroinflammation, and may accelerate the progression of AD. Andrographolide (ANDRO), a potent naturally extracted substance, has been demonstrated to exert suppressive effects on LPS-induced inflammation by modulating macrophage and microglia overactivation. Whereas in AD, β-amyloid (Aβ) peptides have been considered as a potent activator of neuroinflammation, the effect of ANDRO on Aβ-induced neuroinflammation has not been examined. In this study, we investigated the effects of ANDRO on Aβ(1–42)-induced neuroinflammation. We found that ANDRO significantly protected neuronal cells against microglia-mediated Aβ(1–42) toxicity and attenuated the release of preinflammatory productions such as tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), nitric oxide (NO), and prostaglandin E₂ (PGE₂). It also downregulated the protein levels of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in microglial cells. Further the involved mechanism study demonstrated that ANDRO inhibited the nuclear translocation of nuclear factor-κB (NF-κB) by affecting IκB phosphorylation, and attenuated Aβ(1–42)-induced JNK–MAPK overactivation. In summary, this study, for the first time, revealed ANDRO reduced inflammation-mediated neuronal damage by blocking inflammatory responses of microglial cells to Aβ(1–42), suggesting ANDRO may be an effective agent in modulating neuroinflammatory process in AD.     

Extracellular ADP augments microglial inflammasome and NF-κB activation via the P2Y12 receptor

The NLRP3 inflammasome is a molecular complex that translates signals from pathogens and tissue damage into inflammatory responses, and plays crucial roles in numerous neurological diseases. Activation of the NLRP3 inflammasome leads to caspase-1 dependent cleavage of pro-IL-1β to form mature IL-1β. By acting on the P2X7 purinergic receptor, extracellular ATP is one of the major stimuli that activates the NLRP3 inflammasome. Although microglia express multiple purinergic receptors, their roles in inflammasome-mediated inflammation are largely unknown. We studied the role of the P2Y12 receptor, a metabotropic P2Y receptor enriched in microglia, on inflammation in vitro. Inhibition of the microglial P2Y12 receptor by PSB0739 or siRNA knockdown suppressed IL-1β release. P2Y12 receptor-deficient microglia displayed markedly attenuated IL-1β mRNA expression and release. P2Y12 receptor blockade also suppressed IL-6 production. Both IL-1β and IL-6 responses were augmented by extracellular ADP or ADP-βS and were abrogated by PSB0739. Mechanistically, ADP-βS potentiated NF-κB activation. In addition, ADP altered mitochondrial membrane potential in combination with ATP and increased the number of caspase-1 positive cells through the P2Y12 receptor. These results elucidate a novel inflammatory mechanism by which extracellular ADP acts on the P2Y12 receptor to activate NF-κB and the NLRP3 inflammasome to enhance microglial inflammation.     

ML365 inhibits lipopolysaccharide-induced inflammatory responses via the NF-κB signaling pathway

ML365 is a selective inhibitor of the twik-related acid-sensitive potassium channel 1/two-pore domain channel subfamily k member 3 two-pore domain potassium channel. There are no functional studies of the relationship between ML365 and inhibition of inflammation. In this study, we evaluated the anti-inflammatory effect of ML365 on lipopolysaccharide (LPS)-induced inflammation and elucidated the possible mechanism. ML365 showed no cytotoxicity and did not induce apoptosis on RAW264.7 cells and inhibited nitric oxide production. ML365 suppressed the release of tumor necrosis factor-alpha, interleukin (IL)-6 and IL-1β measured using enzyme-linked immunosorbent assay and quantitative polymerase chain reaction assays. LPS-induced activation and co-localization of NF-κB was inhibited by ML365 pre-treatment. ML365 inhibited the protein expression of Erk, p38 and Jnk. In vivo, ML365 appeared to prevent pathological damages in the LPS-induced endotoxin shock model. These findings suggest that ML365 inhibits LPS-induced inflammatory responses by regulating the NF-κB signaling pathway.     

AGEs/RAGE complex upregulates BACE1 via NF-κB pathway activation

Although the pathogenesis of sporadic Alzheimer disease (AD) is not clearly understood, it is likely dependent on several age-related factors. Diabetes is a risk factor for AD, and multiple mechanisms connecting the 2 diseases have been proposed. Hyperglycemia enhances the formation of advanced glycation end products (AGEs) that result from the auto-oxidation of glucose and fructose. The interaction of AGEs with their receptor, named RAGE, elicits the formation of reactive oxygen species that are also believed to be an early event in AD pathology. To investigate a functional link between the disorders diabetes and AD, the effect of 2 AGEs, pentosidine and glyceraldehydes-derived pyridinium (GLAP), was studied on BACE1 expression both in vivo, in streptozotocin treated rats, and in vitro in differentiated neuroblastoma cells. We showed that pentosidine and GLAP were able to upregulate BACE1 expression through their binding with RAGE and the consequent activation of NF-κB. In addition, both pentosidine and GLAP were found to be increased in the brain in sporadic AD patients. Our findings demonstrate that activation of the AGEs/RAGE axis, by upregulating the key enzyme for amyloid-β production, provides a pathologic link between diabetes mellitus and AD.     

HIV-1 gp120-mediated increases in IL-8 production in astrocytes are mediated through the NF-κB pathway and can be silenced by gp120-specific siRNA

Background  

The exact mechanism underlying HIV-associated neurocognitive disorders still remains largely unresolved. However, viral genes (for example gp120 and tat) and their effect on cytokine/chemokine expressions have been linked with neuroinflammation. Conversely, interlekin-8 (IL-8) is a known proinflammatory chemokine and is known to be over-expressed in human brain microvascular endothelial cells in response to gp120. In this study, we sought to address whether HIV-1gp120 could affect IL-8 expression in astrocytes and whether the NF-κB pathway is involved in this phenomenon.     

MD-2 regulates LPS-induced NLRP3 inflammasome activation and IL-1beta secretion by a MyD88/NF-κB-dependent pathway in alveolar macrophages cell line

Myeloid differentiation protein 2 (MD-2) is required in the recognition of lipopolysaccharide (LPS) by toll-like receptor 4 (TLR4), and participates in LPS-induced alveolar macrophage (AM) inflammation during acute lung injury (ALI). Activation of the NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome aggravates inflammation in LPS-induced ALI. However, there is currently little known about the relationship between MD-2 signaling and the NLRP3 inflammasome. This study showed that NLRP3 expression, IL-1beta (IL-1β) secretion, and pyroptosis were up-regulated after LPS stimulation in the NR8383 AM cell-line. MD-2 gene knock-down reduced LPS-induced mRNA and protein expression of NLRP3 and IL-1β secretion in NR8383 cells, and inhibited the MyD88/NF-κB signaling pathway. Conversely, over-expression of MD-2 not only heightened NLRP3, MyD88, and NF-κB p65 protein expression, it also aggravated the LPS-induced inflammatory response. Furthermore, the NF-κB inhibitor SN50 had a beneficial role in decreasing NLRP3 and caspase-1 mRNA and protein expression. The observations suggest that MD-2 helps to regulate LPS-induced NLRP3 inflammasome activation and the inflammatory response in NR8383 cells, and likely does so by affecting MyD88/NF-κB signaling.     

Lipopolysaccharide and IL-1β coordinate a synergy on cytokine production by upregulating MyD88 expression in human gingival fibroblasts

Both lipopolysaccharide (LPS) and interleukin (IL)-1β activate the MyD88-dependent signaling pathways to stimulate proinflammatory cytokine expression. However, it remains unknown how LPS and IL-1β interact with each other to coordinate the stimulation. In this study, we sought to investigate the interaction between LPS and IL-1β on MyD88-dependent signaling pathways in human gingival fibroblasts (HGFs). Results showed that LPS derived from Porphyromonas gingivalis (Pg LPS) and IL-1β cooperatively stimulated mitogen-activated protein kinase (MAPK) and nuclear factor kappa B (NFκB) signaling pathways, and subsequent expression of proinflammatory cytokine expression. Furthermore, our results showed that Pg LPS and IL-1β exerted a synergy on MyD88 expression and knockdown of MyD88 expression by small interfering RNA diminished the synergistic effect of Pg LPS and IL-1β on IL-6 expression, suggesting that upregulation of MyD88 is involved in the coordinated stimulation by Pg LPS and IL-1β of proinflammatory cytokine expression. Finally, our results showed that pharmacological inhibitors for MAPK and NFκB significantly reduced IL-6 secretion stimulated by Pg LPS and IL-1β, indicating that the MyD88-dependent MAPK and NFκB signaling pathways are essential for the upregulation of proinflammatory cytokine expression by Pg LPS and IL-1β. Taken together, this study showed that LPS and IL-1β coordinate a synergy on cytokine production by upregulating MyD88 expression in HGFs.