Lipopolysaccharide-dependent interaction between PU.1 and c-Jun determines production of lipocalin-type prostaglandin D synthase and prostaglandin D2 in macrophages

Cell-Type-Specific Gene Regulatory Networks of Pro-Inflammatory and Pro-Resolving Lipid Mediator Biosynthesis in the Immune System

Lipid mediators are important regulators in inflammatory responses, and their biosynthetic pathways are targeted by commonly used anti-inflammatory drugs. Switching from pro-inflammatory lipid mediators (PIMs) to specialized pro-resolving (SPMs) is a critical step toward acute inflammation resolution and preventing chronic inflammation. Although the biosynthetic pathways and enzymes for PIMs and SPMs have now been largely identified, the actual transcriptional profiles underlying the immune cell type-specific transcriptional profiles of these mediators are still unknown. Using the Atlas of Inflammation Resolution, we created a large network of gene regulatory interactions linked to the biosynthesis of SPMs and PIMs. By mapping single-cell sequencing data, we identified cell type-specific gene regulatory networks of the lipid mediator biosynthesis. Using machine learning approaches combined with network features, we identified cell clusters of similar transcriptional regulation and demonstrated how specific immune cell activation affects PIM and SPM profiles. We found substantial differences in regulatory networks in related cells, accounting for network-based preprocessing in functional single-cell analyses. Our results not only provide further insight into the gene regulation of lipid mediators in the immune response but also shed light on the contribution of selected cell types in their biosynthesis.

pKr-2 induces neurodegeneration via upregulation of microglial TLR4 in the hippocampus of AD brain

We recently demonstrated that prothrombin kringle-2 (pKr-2) derived from blood-brain barrier (BBB) disruption could induce hippocampal neurodegeneration and object recognition impairment through neurotoxic inflammatory responses in the five familial Alzheimer's disease mutation (5XFAD) mice. In the present study, we aimed to determine whether pKr-2 induces microglial activation by stimulating toll-like receptor 4 (TLR4) upregulation and examine whether this response contributes to pKr-2-induced neuroinflammatory damage in the hippocampi of mice models. We observed that inflammatory responses induced by pKr-2 administration in the hippocampi of wild-type mice were significantly abrogated in TLR4-deficient mice (TLR4^-/-), and caffeine supply or rivaroxaban treatment that inhibits the overexpression of hippocampal pKr-2 reduced TLR4 upregulation in 5XFAD mice, resulting in the inhibition of neuroinflammatory responses. Similar to the expression patterns of pKr-2, TLR4, and the TLR4 transcription factors, PU.1 and p-c-Jun, seen in the postmortem hippocampal tissues of Alzheimer's disease (AD) patients, our results additionally showed the influence of transcriptional regulation on TLR4 expression following pKr-2 expression in triggering the production of neurotoxic inflammatory mediators. Therefore, we conclude that pKr-2 may play a role in initiating upregulation of microglial TLR4, consequently inducing hippocampal neurodegeneration. Furthermore, the control of pKr-2-induced microglial TLR4 could be a useful therapeutic strategy against hippocampal neurodegeneration in AD.

Biochemical and Structural Characteristics, Gene Regulation, Physiological, Pathological and Clinical Features of Lipocalin-Type Prostaglandin D2 Synthase as a Multifunctional Lipocalin

Lipocalin-type prostaglandin (PG) D₂ synthase (L-PGDS) catalyzes the isomerization of PGH₂, a common precursor of the two series of PGs, to produce PGD₂. PGD₂ stimulates three distinct types of G protein-coupled receptors: (1) D type of prostanoid (DP) receptors involved in the regulation of sleep, pain, food intake, and others; (2) chemoattractant receptor-homologous molecule expressed on T helper type 2 cells (CRTH2) receptors, in myelination of peripheral nervous system, adipocyte differentiation, inhibition of hair follicle neogenesis, and others; and (3) F type of prostanoid (FP) receptors, in dexamethasone-induced cardioprotection. L-PGDS is the same protein as β-trace, a major protein in human cerebrospinal fluid (CSF). L-PGDS exists in the central nervous system and male genital organs of various mammals, and human heart; and is secreted into the CSF, seminal plasma, and plasma, respectively. L-PGDS binds retinoic acids and retinal with high affinities (Kd < 100 nM) and diverse small lipophilic substances, such as thyroids, gangliosides, bilirubin and biliverdin, heme, NAD(P)H, and PGD₂, acting as an extracellular carrier of these substances. L-PGDS also binds amyloid β peptides, prevents their fibril formation, and disaggregates amyloid β fibrils, acting as a major amyloid β chaperone in human CSF. Here, I summarize the recent progress of the research on PGD₂ and L-PGDS, in terms of its “molecular properties,” “cell culture studies,” “animal experiments,” and “clinical studies,” all of which should help to understand the pathophysiological role of L-PGDS and inspire the future research of this multifunctional lipocalin.

MicroRNA-150 targets PU.1 and regulates macrophage differentiation and function in experimental autoimmune encephalomyelitis

PU.1 is a transcription factor which is expressed in myeloid cells. Herein, we investigated the expression of PU.1 and its potentially targeting miRNAs in the central nervous system (CNS) of mice with experimental autoimmune encephalitis (EAE) and in cultured primary macrophages. PU.1 levels where highly induced in EAE spinal cords and in activated macrophages; this was associated with a significant reduction in miR-150-5p levels at chronic phase of disease and in activated cells. Luciferase assays confirmed the PU.1-miR-150-5p interaction. Overexpression of miR-150-5p in macrophages decreased the expression of proinflammatory cytokines and shifted the polarization of macrophages away from the M1-like phenotype.

Suppression of lung inflammation by the methanol extract of Spilanthes acmella Murray is related to differential regulation of NF-κB and Nrf2

ETHNOPHARMACOLOGICAL RELEVANCE

Although Spilanthes acmella has been used to relieve inflammation, fever, pain, or infection in traditional Asian medicine, experimental evidence supporting these functions is scarce. Here, we examined an anti-inflammatory function and a possible underlying mechanism of S. acmella Murray (SAM).

MATERIALS AND METHOD

The methanol extract of SAM was fingerprinted by HPLC. C57BL/6 mice were administered with a single intratracheal (i.t.) LPS and 2 h later with a single i.t. SAM. The effect of SAM on lung inflammation was assessed by histology, semi-quantitative RT-PCR, and MPO assay of lung tissue. The effects of SAM on a pro-inflammatory factor NF-κB and an anti-inflammatory factor Nrf2 were analyzed by immunoblotting of nuclear proteins and by semi-quantitative RT-PCR analysis of mRNA of the genes governed by these transcription factors. V5-Nrf2 was precipitated by an anti-V5 antibody and the ubiquitinated V5-Nrf2 was revealed by immunoblotting of HA-tagged ubiquitin.

RESULTS

The i.t. SAM robustly diminished a neutrophilic lung inflammation induced by i.t. LPS treatment of mice. In RAW 264.7 cells, SAM suppressed the nuclear localization of NF-κB and the expression of NF-κB-dependent cytokine genes. SAM increased the level of Nrf2 in the nucleus and the expression of Nrf2-dependent genes while suppressing ubiquitination of Nrf2.

CONCLUSION

Our results suggest that SAM can suppress a neutrophilic inflammation in mouse lungs, which is associated with suppressed NF-κB and activated Nrf2. Our results provide experimental evidence supporting the anti-inflammatory function of S. acmella.

Suppressed ubiquitination of Nrf2 by p47phox contributes to Nrf2 activation

Although critical in phagocytosis in innate immunity, reactive oxygen species (ROS) collaterally inflict damage to host phagocytes because they indiscriminate targets. Since Nrf2 increases the expression of anti-oxidant enzymes that nullifies ROS, ROS activating Nrf2 is a critical negative regulatory step for countering the deleterious effects of ROS. Here, we postulate whether, along with ROS activating Nrf2, NADPH oxidase components also participate in direct activation of Nrf2, contributing to protection from ROS. Our results show that the p47^phox of the NADPH oxidase, but not p65^phox or p40^phox, physically binds to Nrf2, activating the Nrf2 function. p47^phox binding to Nrf2/Keap1 complex suppresses the ubiquitination of Nrf2, while p47^phox becomes ubiquitinated by Keap1. p47^phox increases the nuclear translocation of Nrf2 and the expression of Nrf2-dependent genes, whereas genetic ablation of p47^phox decreases the expression of those genes. In a lipopolysaccharide-induced acute lung inflammation mouse model, selective expression of p47^phox in mouse lungs induces the expression of Nrf2-dependent genes and is sufficient to suppress neutrophilic lung inflammation. Therefore, our findings suggest that p47^phox is a novel regulator of Nrf2 function.

Differential sensitivity to methylated DNA by ETS-family transcription factors is intrinsically encoded in their DNA-binding domains

Transactivation by the ETS family of transcription factors, whose members share structurally conserved DNA-binding domains, is variably sensitive to methylation of their target genes. The mechanism by which DNA methylation controls ETS proteins remains poorly understood. Uncertainly also pervades the effects of hemi-methylated DNA, which occurs following DNA replication and in response to hypomethylating agents, on site recognition by ETS proteins. To address these questions, we measured the affinities of two sequence-divergent ETS homologs, PU.1 and Ets-1, to DNA sites harboring a hemi- and fully methylated CpG dinucleotide. While the two proteins bound unmethylated DNA with indistinguishable affinity, their affinities to methylated DNA are markedly heterogeneous and exhibit major energetic coupling between the two CpG methylcytosines. Analysis of simulated DNA and existing co-crystal structures revealed that hemi-methylation induced non-local backbone and groove geometries that were not conserved in the fully methylated state. Indirect readout of these perturbations was differentially achieved by the two ETS homologs, with the distinctive interfacial hydration in PU.1/DNA binding moderating the inhibitory effects of DNA methylation on binding. This data established a biophysical basis for the pioneering properties associated with PU.1, which robustly bound fully methylated DNA, but not Ets-1, which was substantially inhibited.

Close teamwork between Nrf2 and peroxiredoxins 1 and 6 for the regulation of prostaglandin D2 and E2 production in macrophages in acute inflammation

Inflammation is a complex biological self-defense reaction triggered by tissue damage or infection by pathogens. Acute inflammation is regulated by the time- and cell type-dependent production of cytokines and small signaling molecules including reactive oxygen species and prostaglandins. Recent studies have unveiled the important role of the transcription factor Nrf2 in the regulation of prostaglandin production through transcriptional regulation of peroxiredoxins 1 and 6 (Prx1 and Prx6) and lipocalin-type prostaglandin D synthase (L-PGDS). Prx1 and Prx6 are multifunctional proteins important for cell protection against oxidative stress, but also work together to facilitate production of prostaglandins E2 and D2 (PGE2 and PGD2). Prx1 secreted from cells under mild oxidative stress binds Toll-like receptor 4 and induces NF-κB activation, important for the expression of cyclooxygenase-2 and microsomal PGE synthase-1 (mPGES-1) expression. The activated MAPKs p38 and ERK phosphorylate Prx6, leading to NADPH oxidase-2 activation, which contributes to production of PGD2 by hematopoietic prostaglandin D synthase (H-PGDS). PGD2 and its end product 15-deoxy-∆(12,14)-prostaglandin J2 (15d-PGJ2) activate Nrf2 thereby forming a positive feedback loop for further production of PGD2 by L-PGDS. Maintenance of cellular glutathione levels is an important role of Nrf2 not only for cell protection but also for the synthesis of prostaglandins, as mPGES-1 and H-PGDS require glutathione for their activities. This review is aimed at describing the functions of Prx1 and Prx6 in the regulation of PGD2 and PGE2 production in acute inflammation in macrophages and the importance of 15d-PGJ2 as an intrinsic Nrf2 activator.

Silencing of Kir2 channels by caveolin-1: cross-talk with cholesterol

A growing number of studies show that different types of ion channels localize in caveolae and are regulated by the level of membrane cholesterol. Furthermore, it has been proposed that cholesterol-induced regulation of ion channels might be attributed to partitioning into caveolae and association with caveolin-1 (Cav-1). We tested, therefore, whether Cav-1 regulates the function of inwardly rectifying potassium channels Kir2.1 that play major roles in the regulation of membrane potentials of numerous mammalian cells. Our earlier studies demonstrated that Kir2.1 channels are cholesterol sensitive. In this study, we show that Kir2.1 channels co-immunoprecipitate with Cav-1 and that co-expression of Kir2.1 channels with Cav-1 in HEK293 cells results in suppression of Kir2 current indicating that Cav-1 is a negative regulator of Kir2 function. These observations are confirmed by comparing Kir currents in bone marrow-derived macrophages isolated from Cav-1(-/-) and wild-type animals. We also show, however, that Kir2 channels maintain their sensitivity to cholesterol in HEK293 cells that have very low levels of endogenous Cav-1 and in bone marrow-derived macrophages isolated from Cav-1(-/-) knockout mice. Thus, these studies indicate that Cav-1 and/or intact caveolae are not required for cholesterol sensitivity of Kir channels. Moreover, a single point mutation of Kir2.1, L222I that abrogates the sensitivity of the channels to cholesterol also abolishes their sensitivity to Cav-1 suggesting that the two modulators regulate Kir2 channels via a common mechanism.

Carthami Flos suppresses neutrophilic lung inflammation in mice, for which nuclear factor-erythroid 2-related factor-1 is required

Carthami Flos (CF) is used in traditional Asian medicine to treat blood stagnation and its associated diseases in patients. While the underlying mechanism for this effect remains unknown, CF has been reported to activate Nrf2, a transcription factor that is critical in protecting from various inflammatory lung diseases including acute lung injury (ALI). Here, we examined whether CF has a therapeutic effect on lung inflammation and assessed the impact of Nrf2 on the effect of CF using an ALI mouse model. Treatment of bone marrow derived macrophages with standardized aqueous extract of CF (AECF) activated Nrf2, resulting in the expression of Nrf2 dependent genes including GCLC, NQO-1 and HO-1. While intranasal LPS treatment of wild type mice resulted in neutrophilic infiltration and a concomitant expression of pro-inflammatory cytokine genes in the lung, the hallmarks of ALI, an intratracheal spraying of AECF to the lung 2h after LPS treatment suppressed the inflammatory response. By contrast, similar treatment in nrf2(-/-) mice with AECF failed to attenuate the inflammatory response. Thus, our results show that AECF attenuated neutrophilic lung inflammation in mice, which required Nrf2. Since AECF administration abrogates lung inflammation after LPS treatment, we propose CF as a potential therapeutics in the management of ALI.

Nrf2 is essential for the expression of lipocalin-prostaglandin D synthase induced by prostaglandin D2

Nrf2 is a transcription factor that protects against inflammatory diseases, but the underlying mechanism of this effect remains unclear. Here, we report that Nrf2 uses lipocalin-prostaglandin D synthase (L-PGDS) as a mechanism for suppressing inflammation. Exogenously added prostaglandin D2 (PGD2) induced L-PGDS expression in bone-marrow-derived macrophages (BMDMs), suggesting a positive feedback loop between L-PGDS expression and PGD2. Unlike lipopolysaccharide (LPS)-induced L-PGDS expression, PGD2-mediated expression was independent of MAPK, PU.1, or TLR4. Sequence analysis located a putative Nrf2 binding site in the murine L-PGDS promoter, to which Nrf2 bound when treated with PGD2. Chemical activation, or overexpression, of Nrf2 was sufficient to induce L-PGDS expression in macrophages, BMDMs, or lungs of Nrf2-knockout (KO) mice, but treatment with PGD2 failed to do so, suggesting a pivotal role for Nrf2 in the expression of L-PGDS. Consistent with this, expression of Nrf2 in the lungs of Nrf2-KO mice was sufficient to induce the expression of L-PGDS and to reduce neutrophilic lung inflammation elicited by LPS. Furthermore, expression of L-PGDS in mouse lungs decreased neutrophilic infiltration, ameliorating lung inflammation in mice. Together, our results show that Nrf2, activated by PGD2, induced L-PGDS expression, resulting in decreased inflammation. We suggest that the positive feedback induction of L-PGDS by PGD2 is part of the mechanism by which Nrf2 regulates inflammation.

PGD synthase and PGD2 in immune resposne

PGD₂ is formed from arachidonic acid by successive enzyme reactions: oxygenation of arachidonic acid to PGH₂, a common precursor of various prostanoids, catalyzed by cyclooxygenase, and isomerization of PGH₂ to PGD₂ by PGD synthases (PGDSs). PGD₂ can be either pro- or anti-inflammatory depending on disease process and etiology. The anti-inflammatory and immunomodulatory attributes of PGDS/PGD₂ provide opportunities for development of novel therapeutic approaches for resistant infections and refractory inflammatory diseases. This paper highlights the role of PGD synthases and PGD2 in immune inflammatory response.

Unexpected role of lipocalin-type prostaglandin D synthase in brain: regulation of glial cell migration and morphology

Lipocalin-type prostaglandin D synthase (L-PGDS) is one of the most abundant proteins in the cerebrospinal fluid. Nevertheless, its role in the central nervous system is far from clear. Here, we present evidence that L-PGDS induces glial cell migration and morphological changes in vitro and in vivo. We also identified myristoylated alanine-rich C-kinase substrate (MARCKS), heat shock proteins and actin as L-PGDS-binding proteins, demonstrating that MARCKS/Akt/Rho/Jnk pathways are involved in the L-PGDS actions in glia. We further show that the cell migration-promoting activity of L-PGDS is independent of PGD 2 production. The results suggest a novel non-enzymatic function of L-PGDS protein in brain inflammation, and may have an impact on glial cell biology and brain pathology related with reactive gliosis. L-PGDS is a potential drug target that can be exploited for therapeutic intervention of glia-driven neuroinflammation and related diseases.

Lipocalin-type prostaglandin D2 synthase protein regulates glial cell migration and morphology through myristoylated alanine-rich C-kinase substrate: prostaglandin D2-independent effects

Prostaglandin D synthase (PGDS) is responsible for the conversion of PGH(2) to PGD(2). Two distinct types of PGDS have been identified: hematopoietic-type PGDS (H-PGDS) and lipocalin-type PGDS (L-PGDS). L-PGDS acts as both a PGD(2)-synthesizing enzyme and as an extracellular transporter of various lipophilic small molecules. Although L-PGDS is one of the most abundant proteins in the cerebrospinal fluid, little is known about the function of L-PGDS in the central nervous system (CNS). To better understand the role of L-PGDS in the CNS, effects of L-PGDS on the migration and morphology of glial cells were investigated. The L-PGDS protein accelerated the migration of cultured glial cells. Expression of the L-pgds gene was detected in glial cells and neurons. L-PGDS protein also induced morphological changes in glia similar to the characteristic phenotypic changes in reactive gliosis. L-PGDS-induced cell migration was associated with augmented formation of actin filaments and focal adhesion, which was accompanied by activation of AKT, RhoA, and JNK pathways. L-PGDS protein injected into the mouse brain promoted migration and accumulation of astrocytes in vivo. Furthermore, the cell migration-promoting effect of L-PGDS on glial cells was independent of the PGD(2) products. The L-PGDS protein interacted with myristoylated alanine-rich protein kinase C substrate (MARCKS) to promote cell migration. These results demonstrate the critical role of L-PGDS as a secreted lipocalin in the regulation of glial cell migration and morphology. The results also indicate that L-PGDS may participate in reactive gliosis in an autocrine or paracrine manner, and may have pathological implications in neuroinflammatory diseases.

Functional PU.1 in macrophages has a pivotal role in NF-κB activation and neutrophilic lung inflammation during endotoxemia

Although the role of ETS family transcriptional factor PU.1 is well established in macrophage maturation, its role in mature macrophages with reference to sepsis- related animal model has not been elucidated. Here, we report the in vivo function of PU.1 in mediating mature macrophage inflammatory phenotype by using bone marrow chimera mice with conditional PU.1 knockout. We observed that the expression of monocyte/macrophage-specific markers CD 11b, F4/80 in fetal liver cells, and bone marrow-derived macrophages were dependent on functional PU.1. Systemic inflammation as measured in terms of NF-κB reporter activity in lung, liver, and spleen tissues was significantly decreased in PU.1-deficient chimera mice compared with wild-type chimeras on lipopolysaccharide (LPS) challenge. Unlike wild-type chimera mice, LPS challenge in PU.1-deficient chimera mice resulted in decreased lung neu-trophilic inflammation and myeloperoxidase activity. Similarly, we found attenuated inflammatory gene expression (cyclooxygenase-2, inducible nitric-oxide synthase, and TLR4) and inflammatory cytokine secretion (IL-6, MCP-1, IL-1β, TNF-α, and neutrophilic chemokine keratinocyte-derived chemokine) in PU.1-deficient mice. Most importantly, this attenuated lung and systemic inflammatory phenotype was associated with survival benefit in LPS-challenged heterozygotic PU.1-deficient mice, establishing a novel protective mechanistic role for the lineage-specific transcription factor PU.1.

Biosynthesis of hemiketal eicosanoids by cross-over of the 5-lipoxygenase and cyclooxygenase-2 pathways

The prostaglandin and leukotriene families of lipid mediators are formed via two distinct biosynthetic pathways that are initiated by the oxygenation of arachidonic acid by either cyclooxygenase-2 (COX-2) or 5-lipoxygenase (5-LOX), respectively. The 5-LOX product 5S-hydroxyeicosatetraenoic acid, however, can also serve as an efficient substrate for COX-2, forming a bicyclic diendoperoxide with structural similarities to the arachidonic acid-derived prostaglandin endoperoxide PGH(2) [Schneider C, et al. (2006) J Am Chem Soc 128:720-721]. Here we identify two cyclic hemiketal (HK) eicosanoids, HKD(2) and HKE(2), as the major nonenzymatic rearrangement products of the diendoperoxide using liquid chromatography-mass spectrometry analyses as well as UV and NMR spectroscopy. HKD(2) and HKE(2) are furoketals formed by spontaneous cyclization of their respective 8,9-dioxo-5S,11R,12S,15S-tetrahydroxy- or 11,12-dioxo-5S,8S,9S,15S-tetrahydroxy-eicosadi-6E,13E-enoic acid precursors, resulting from opening of the 9S,11R- and 8S,12S-peroxide rings of the diendoperoxide. Furthermore, the diendoperoxide is an efficient substrate for the hematopoietic type of prostaglandin D synthase resulting in formation of HKD(2), equivalent to the enzymatic transformation of PGH(2) to PGD(2). HKD(2) and HKE(2) were formed in human blood leukocytes activated with bacterial lipopolysaccharide and calcium ionophore A23187, and biosynthesis was blocked by inhibitors of 5-LOX or COX-2. HKD(2) and HKE(2) stimulated migration and tubulogenesis of microvascular endothelial cells, implicating a proangiogenic role of the hemiketals in inflammatory sites that involve expression of 5-LOX and COX-2. Identification of the highly oxygenated hemiketal eicosanoids provides evidence for a previously unrecognized biosynthetic cross-over of the 5-LOX and COX-2 pathways.