Regulation of the ARE-binding proteins, TTP (tristetraprolin) and HuR (human antigen R), in inflammatory response in astrocytes

TERC suppresses PD-L1 expression by downregulating RNA binding protein HuR

TERC is the RNA component of telomerase, and provides a template for TERT to synthesize telomere repeats at chromosome ends. Increasing evidence has revealed that TERC is involved in other biological processes beyond telomerase. Here, we found that the expression level of TERC is negatively correlated with PD-L1 and that ectopic expression of TERC but not TERT in ALT cells significantly inhibits PD-L1, suggesting that TERC suppresses PD-L1 expression in a telomerase-independent manner. Mechanistically, instead of regulating PD-L1 mRNA directly, TERC accelerates PD-L1 mRNA degradation by inhibiting the expression of HuR, which binds to the 3'UTR of PD-L1 mRNA and maintains its stability. We also found that the small molecule AS1842856, a FoxO1 inhibitor, promotes TERC expression and reverses the PD-L1 upregulation caused by chemotherapy, providing a potential combination cancer therapy that avoids cancer immune escape during chemotherapy.

TTP-mediated regulation of mRNA stability in immune cells contributes to adaptive immunity, immune tolerance and clinical applications

Dendritic cells (DCs) form a sentinel network to induce protective immunity against pathogens or self-tolerance. mRNA stability is an important part of the post-transcriptional regulation (PTR) that controls the maturation and function of DCs. In this review, we summarize the effects of TTP-mediated regulation of mRNA stability in DCs, focusing on DC maturation and antigen presentation, T cell activation and differentiation, immune tolerance and inflammation. We also discuss the potential DC-based immune treatment for HIV⁺ patients through regulation of mRNA stability. This review proposes the regulation of mRNA stability as a novel immune therapy for various inflammatory diseases, such as arthritis and dermatitis.

Inhibitor of Hyaluronic Acid Synthesis 4-Methylumbelliferone as an Anti-Inflammatory Modulator of LPS-Mediated Astrocyte Responses

Astrocytes are glial cells that play an important role in neuroinflammation. Astrocytes respond to many pro-inflammatory stimuli, including lipopolysaccharide (LPS), an agonist of Toll-like receptor 4 (TLR4). Regulatory specificities of inflammatory signaling pathways are still largely unknown due to the ectodermal origin of astrocytes. Recently, we have shown that hyaluronic acid (HA) may form part of astrocyte inflammatory responses. Therefore, we tested 4-methylumbelliferone (4-MU), a specific inhibitor of HA synthesis, as a possible regulator of LPS-mediated responses. Rat primary astrocytes were treated with LPS with and without 4-MU and gene expression levels of inflammatory (interleukins 1β, (IL-1β), 6, (IL-6), tumor necrosis factor alpha TNFα,) and resolution interleukin 10 (IL-10) markers were evaluated via real-time PCR and western blot. The release of cytokines and HA was determined by ELISA. Oxylipin profiles were measured by ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) analysis. Our data show that 4-MU (i) has anti-inflammatory effects in the course of TLR4 activation, decreasing the cytokines level TNFα, IL-6 and IL-1β and increasing IL-10, (ii) downregulates prostaglandin synthesis but not via cyclooxygenases COX-1 and COX-2 pathways, (iii) modulates HA synthesis and decreases LPS-induced HA synthase mRNA expression (HAS-1, HAS-2) but does not have an influence on HAS-3, HYAL1 and HYAL2 mRNAs; (iv) the effects of 4-MU are predominantly revealed via JNK but not p38, ERK mitogen-activated protein kinases (MAPKs) or nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) pathways. For the first time, it is shown that 4-MU possesses the useful potential to regulate an inflammatory astrocyte response.

Oxylipin Profiles as Functional Characteristics of Acute Inflammatory Responses in Astrocytes Pre-Treated with IL-4, IL-10, or LPS

Functional phenotypes, which cells can acquire depending on the microenvironment, are currently the focus of investigations into new anti-inflammatory therapeutic approaches. Glial cells, microglia, and astrocytes are major participants in neuroinflammation, but their roles differ, as microglia are cells of mesodermal origin, while astrocytes are cells of ectodermal origin. The inflammatory phenotype of cells can be modulated by ω-6- and ω-3-polyunsaturated fatty acid-derived oxylipins, although data on changes in oxylipin profiles in different cell adaptations to pro- and anti-inflammatory stimuli are scarce. Our study aimed to compare UPLC-MS/MS-measured oxylipin profiles in various rat astrocyte adaptation states. We used cells treated for 24 h with lipopolysaccharide (LPS) for classical pro-inflammatory adaptation and with interleukin 4 (IL-4) or 10 (IL-10) for alternative anti-inflammatory adaptation, with the resulting phenotypes characterized by quantitative real-time PCR (RT-PCR). We also tested long-term, low-concentration LPS treatment (endotoxin treatment) as a model of astrocyte adaptations. The functional response of astrocytes was estimated by acute (4 h) LPS-induced cell reactivity, measured by gene expression markers and oxylipin synthesis. We discovered that, as well as gene markers, oxylipin profiles can serve as markers of pro- (A1-like) or anti-inflammatory (A2-like) adaptations. We observed predominant involvement of ω-6 polyunsaturated fatty acid (PUFA) and the cyclooxygenase branch for classical (LPS) pro-inflammatory adaptations and ω-3 PUFA and the lipoxygenase branch for alternative (IL-4) anti-inflammatory adaptations. Treatment with IL-4, but not IL-10, primes the ability of astrocytes to activate the innate immunity signaling pathways in response to LPS. Endotoxin-treated astrocytes provide an alternative anti-inflammatory adaptation, which makes cells less sensitive to acute LPS stimulation than the IL-4 induced adaptation. Taken together, the data reveal that oxylipin profiles associate with different states of polarization to generate a pro-inflammatory or anti-inflammatory phenotype. This association manifests itself both in native cells and in their responses to a pro-inflammatory stimulus.

Human antigen R: A potential therapeutic target for liver diseases

Human antigen R (HuR), also known as HuA and embryonic lethal abnormal vision-like 1 (ELAVL1), is a ubiquitously expressed RNA binding protein and functions as an RNA regulator and mediates the expression of various proteins by diverse post-transcriptional mechanisms. HuR has been well characterized in the inflammatory responses and in the development of various cancers. The importance of HuR-mediated roles in cell signaling, inflammation, fibrogenesis and cancer development in the liver has attracted a great deal of attention. However, there is still a substantial gap between the current understanding of the potential roles of HuR in the progression of liver disease and whether HuR can be targeted for the treatment of liver diseases. In this review, we introduce the function and mechanistic characterization of HuR, and then focus on the physiopathological roles of HuR in the development of different liver diseases, including hepatic inflammation, alcoholic liver diseases, non-alcoholic fatty liver diseases, viral hepatitis, liver fibrosis and liver cancers. We also summarize existing approaches targeting HuR function. In conclusion, although characterizing the liver-specific HuR function and demonstrating the multi-level regulative networks of HuR in the liver are still required, emerging evidence supports the notion that HuR represents a potential therapeutic target for the treatment of chronic liver diseases.

Regulation of Cell Viability and Anti-inflammatory Tristetraprolin Family Gene Expression in Mouse Macrophages by Cottonseed Extracts

Bioactive plant extracts have been used for the prevention and treatment of various diseases. One of the major classes of bioactive compounds is plant polyphenols. Cottonseed ethanol extracts were determined by HPLC-MS analysis to be essentially free of toxic gossypol. The objective of this study was to investigate the effect of cottonseed ethanol extracts on the cytotoxicity and regulation of anti-inflammatory tristrataprolin (TTP) family gene expression in mouse cells. MTT, qPCR and immunoblotting assays tested the effects of cottonseed extracts in mouse RAW264.7 macrophages and 3T3-L1 adipocytes. No cytotoxicity effect was observed in macrophages treated with extracts from the coat or kernel of glanded and glandless cottonseed. Similarly, the viability of mouse adipocytes was not affected by cottonseed extracts. In contrast, gossypol and lipopolysaccharides were toxic to macrophages but not adipocytes under high concentration or long time treatment. Cottonseed extracts exhibited modest effect on TTP family gene expression in macrophages but glandless cottonseed coat extract significantly increased TTP mRNA and protein levels with a magnitude similar to cinnamon and green tea polyphenol extract and insulin. These results demonstrated that cottonseed extracts are harmless towards the mouse cells and that glandless cottonseed coat extract stimulates TTP gene expression. We propose that glandless cottonseed is a safe source of plant polyphenols with anti-inflammatory property.

Cellular Model of Endotoxin Tolerance in Astrocytes: Role of Interleukin 10 and Oxylipins

A phenomenon of endotoxin tolerance where prior exposure of cells to minute amounts of lipopolysaccharide (LPS) causes them to become refractory to a subsequent high-amount endotoxin challenge is well described for innate immune cells such as monocytes/macrophages, but it is still obscure for brain cells. We exposed primary rat cortical astrocytes to a long-term low-grade concentration of LPS, followed by stimulation with a middle-grade concentration of LPS. Inflammatory markers, i.e., pro-inflammatory cytokine TNFα, inducible enzymes COX-2 and iNOS, anti-inflammatory cytokine interleukin 10 (IL-10) detected at the mRNA and protein levels reveal similarities between astrocytes and macrophages in the model, i.e., tolerance in pro-inflammatory markers and priming in IL-10. Long-term or short-term treatment with IL-10 does not change cell sensitivity for LPS, which makes doubtful its involvement in the mechanisms of cell tolerance development. Significant changes occur in the oxylipin profiles measured by UPLC-MS/MS analysis. The priming occurs in the following compounds: 11-HETE, PGD₂, PGE₂, cyclopentenone prostaglandins, and TXB₂. Tolerance is observed for 12-HHT, PGF_2α, and 6-keto-PGF_1α. As far as we know, this is the first report on changes in oxylipin profiles in the endotoxin tolerance model. The data can greatly improve the understanding of oxylipins’ role in inflammatory and resolution processes in the brain and mechanisms of astrocyte involvement in neuroinflammation.

Multiple Sclerosis: Melatonin, Orexin, and Ceramide Interact with Platelet Activation Coagulation Factors and Gut-Microbiome-Derived Butyrate in the Circadian Dysregulation of Mitochondria in Glia and Immune Cells

Recent data highlight the important roles of the gut microbiome, gut permeability, and alterations in mitochondria functioning in the pathophysiology of multiple sclerosis (MS). This article reviews such data, indicating two important aspects of alterations in the gut in the modulation of mitochondria: (1) Gut permeability increases toll-like receptor (TLR) activators, viz circulating lipopolysaccharide (LPS), and exosomal high-mobility group box (HMGB)1. LPS and HMGB1 increase inducible nitric oxide synthase and superoxide, leading to peroxynitrite-driven acidic sphingomyelinase and ceramide. Ceramide is a major driver of MS pathophysiology via its impacts on glia mitochondria functioning; (2) Gut dysbiosis lowers production of the short-chain fatty acid, butyrate. Butyrate is a significant positive regulator of mitochondrial function, as well as suppressing the levels and effects of ceramide. Ceramide acts to suppress the circadian optimizers of mitochondria functioning, viz daytime orexin and night-time melatonin. Orexin, melatonin, and butyrate increase mitochondria oxidative phosphorylation partly via the disinhibition of the pyruvate dehydrogenase complex, leading to an increase in acetyl-coenzyme A (CoA). Acetyl-CoA is a necessary co-substrate for activation of the mitochondria melatonergic pathway, allowing melatonin to optimize mitochondrial function. Data would indicate that gut-driven alterations in ceramide and mitochondrial function, particularly in glia and immune cells, underpin MS pathophysiology. Aryl hydrocarbon receptor (AhR) activators, such as stress-induced kynurenine and air pollutants, may interact with the mitochondrial melatonergic pathway via AhR-induced cytochrome P450 (CYP)1b1, which backward converts melatonin to N-acetylserotonin (NAS). The loss of mitochnodria melatonin coupled with increased NAS has implications for altered mitochondrial function in many cell types that are relevant to MS pathophysiology. NAS is increased in secondary progressive MS, indicating a role for changes in the mitochondria melatonergic pathway in the progression of MS symptomatology. This provides a framework for the integration of diverse bodies of data on MS pathophysiology, with a number of readily applicable treatment interventions, including the utilization of sodium butyrate.

Inhibitors of Oxidative Phosphorylation Modulate Astrocyte Inflammatory Responses through AMPK-Dependent Ptgs2 mRNA Stabilization

Inflammatory activation of astroglia adds to the pathology of various neurological diseases. Astrocytes respond to microglia-derived cytokines such as interleukin-1α (IL-1α) with enhanced inflammatory signaling. This provokes pro-inflammatory gene expression of, among others, the eicosanoid-generating enzyme prostaglandin endoperoxide synthase 2 (Ptgs2). Whereas metabolic regulation of innate immune cell inflammatory responses is intensely studied, pathways related to how metabolism modulates inflammatory signaling in astrocytes are underexplored. Here, we examined how mitochondrial oxidative phosphorylation affects inflammatory responses towards IL-1α and tumor necrosis factor α in neonatal rat astrocytes. Blocking respiratory complex I and III or adenosine triphosphate (ATP) synthase did not affect activation of inflammatory signaling by IL-1α, but did elicit differential effects on inflammatory gene mRNA expression. Remarkably, mRNA and protein expression of Ptgs2 by IL-1α was consistently up-regulated when oxidative phosphorylation was inhibited. The increase of Ptgs2 resulted from mRNA stabilization. Mitochondrial inhibitors also increased IL-1α-triggered secretion of eicosanoids, such as prostaglandin E₂, prostaglandin F_2α, and 6-keto-prostaglandin F_1α, as assessed by liquid chromatography/mass spectrometry. Mechanistically, attenuating oxidative phosphorylation elevated adenosine monophosphate (AMP) and activated AMP-activated protein kinase (AMPK). AMPK silencing prevented Ptgs2 up-regulation by mitochondrial inhibitors, while AMPK activators recapitulated Ptgs2 mRNA stability regulation. Our data indicate modulation of astrocyte inflammatory responses by oxidative metabolism, with relevance towards eicosanoid production.

High and Low Molecular Weight Hyaluronic Acid Differentially Influences Oxylipins Synthesis in Course of Neuroinflammation

Hyaluronic acid (HA), a major glycosaminoglycan of the extracellular matrix, has cell signaling functions that are dependent on its molecular weight. Anti-inflammatory effects for high-molecular-weight (HMW) HA and pro-inflammatory effects for low-molecular-weight (LMW) HA effects were found for various myeloid cells, including microglia. Astrocytes are cells of ectodermal origin that play a pivotal role in brain inflammation, but the link between HA with different molecular weights and an inflammatory response in these cells is not clear. We tested the effects of LMW and HMW HA in rat primary astrocytes, stimulated with Poly:IC (PIC, TLR3 agonist) and lipopolysaccharide (LPS, TLR4 agonist). Oxylipin profiles were measured by the UPLC-MS/MS analysis and metabolites HDoHEs (from docosahexaenoic acid), -HETEs, prostaglandins (from arachidonic acid), DiHOMEs and HODEs (from linoleic acid) were detected. Both, HMW and LMW HA downregulated the cyclooxygenase-mediated polyunsaturated fatty acids metabolism, LMW also reduced lipoxygenase-mediated fatty acid metabolism. Taken together, the data show that both LMW and HMW (i) influence themselves on cytokines (TNFα, IL-6, IL-10), enzymes iNOS, COX-2, and oxylipin levels in extracellular medium of cultured astrocytes, (ii) induced cellular adaptations in long-term applications, (iii) modulate TLR4- and TLR3-signaling pathways. The effects of HMW and LMW HA are predominantly revealed in TLR4– and TLR3- mediated responses, respectively.

Toll-like receptors control p38 and JNK MAPK signaling pathways in rat astrocytes differently, when cultured in normal or high glucose concentrations

Astrocytes play a vital role in regulating central nervous system inflammation, energy metabolism and brain homeostasis. Unlike macrophages and microglia, which are cells of myeloid ancestry, astrocytes are of ectodermal origin. However, regulatory specificities of signaling pathways connecting inflammatory and metabolic processes are still largely unknown. We analyzed firstly cellular responses to toll-like receptor (TLR) agonists and secondly, modulation of the mRNA of the three isoforms of the transcription factors PPARs (peroxisome proliferator-activated receptors) in primary rat astrocytes exposed to normal glucose (5.5 mM) and high glucose (25 mM). Cell culturing of rat brain astrocytes for 2 days in high glucose did not alter cellular morphology, but i) enhanced the release of TNFα that was induced by TLR4 agonist LPS or TLR3 agonist PIC and the synthesis of prostaglandin E₂ (PGE₂), ii) changed the signaling pathways of TLR4/MAPK (increase in p38 MAPK, and decrease in JNK activities at early stages of TLR activation) and iii) modulated mRNA expression of PPARs. High glucose cultivation reduced PPARα and PPARβ mRNA levels, without altering PPARγ mRNA level and changed the sensitivity of expressions to agonists of TLR1/2 (PGN), TLR4 (LPS), TLR3 (PIC), and TLR5 (FGN). Differences between low and high glucose-adapted cells were obtained for agonists of TLR1/2 (PPARα, PPARβ), TLR4 (PPAR β), TLR3 (PPARα). In the TLR4/p38/PPARβ signaling pathway, there was a stimulatory connection in normal glucose but an inhibitory connection in high glucose. TLR4/JNK/activated PPARβ, TLR4/JNK/inhibited PPARγ both in cells adapted to normal or high glucose, but PPARα expression was not affected. As PPARs in astrocytes are involved in inflammatory processes in the form of the recently published PPAR triad, the changes in expression revealed here are most likely resulting in implications of high glucose in inflammatory processes. Our data underline the complexity of multiple regulatory interactions between inflammatory responses and energy metabolism in astrocytes.

m6A Reader YTHDF2 Regulates LPS-Induced Inflammatory Response

N6-methyladenosine (m6A) is an abundant mRNA modification that affects multiple biological processes, including those involved in the cell stress response and viral infection. YTH domain family 2 (YTHDF2) is an m6A-binding protein that affects the localization and stability of targeted mRNA. RNA-binding proteins (RBPs) can regulate the stability of inflammatory gene mRNA transcripts, thus participating in the regulation of inflammatory processes. As an RBP, the role of YTHDF2 in the LPS-induced inflammatory reaction has not been reported. To elucidate the function of YTHDF2 in the inflammatory response of macrophages, we first detected the expression level of YTHDF2 in RAW 264.7 cells, and found that it was upregulated after LPS stimulation. YTHDF2 knockdown significantly increased the LPS-induced IL-6, TNF-α, IL-1β, and IL-12 expression and the phosphorylation of p65, p38, and ERK1/2 in NF-κB and MAPK signaling. Moreover, the upregulated expression of TNF-α and IL-6 in cells with silenced YTHDF2 expression was downregulated by the NF-κB, p38, and ERK inhibitors. YTHDF2 depletion increased the expression and stability of MAP2K4 and MAP4K4 mRNAs. All of these results suggest that YTHDF2 knockdown increases mRNA expression levels of MAP2K4 and MAP4K4 via stabilizing the mRNA transcripts, which activate MAPK and NF-κB signaling pathways, which promote the expression of proinflammatory cytokines and aggravate the inflammatory response in LPS-stimulated RAW 264.7 cells.

The Interleukin-10 Family of Cytokines and Their Role in the CNS

Resident cells of the central nervous system (CNS) play an important role in detecting insults and initiating protective or sometimes detrimental host immunity. At peripheral sites, immune responses follow a biphasic course with the rapid, but transient, production of inflammatory mediators giving way to the delayed release of factors that promote resolution and repair. Within the CNS, it is well known that glial cells contribute to the onset and progression of neuroinflammation, but it is only now becoming apparent that microglia and astrocytes also play an important role in producing and responding to immunosuppressive factors that serve to limit the detrimental effects of such responses. Interleukin-10 (IL-10) is generally considered to be the quintessential immunosuppressive cytokine, and its ability to resolve inflammation and promote wound repair at peripheral sites is well documented. In the present review article, we discuss the evidence for the production of IL-10 by glia, and describe the ability of CNS cells, including microglia and astrocytes, to respond to this suppressive factor. Furthermore, we review the literature for the expression of other members of the IL-10 cytokine family, IL-19, IL-20, IL-22 and IL-24, within the brain, and discuss the evidence of a role for these poorly understood cytokines in the regulation of infectious and sterile neuroinflammation. In concert, the available data indicate that glia can produce IL-10 and the related cytokines IL-19 and IL-24 in a delayed manner, and these cytokines can limit glial inflammatory responses and/or provide protection against CNS insult. However, the roles of other IL-10 family members within the CNS remain unclear, with IL-20 appearing to act as a pro-inflammatory factor, while IL-22 may play a protective role in some instances and a detrimental role in others, perhaps reflecting the pleiotropic nature of this cytokine family. What is clear is that our current understanding of the role of IL-10 and related cytokines within the CNS is limited at best, and further research is required to define the actions of this understudied family in inflammatory brain disorders.