Microglia Activation and Anti-inflammatory Regulation in Alzheimer’s Disease

Inflammatory regulators, including endogenous anti-inflammatory systems, can down-regulate inflammation thus providing negative feedback. Chronic inflammation can result from imbalance between levels of inflammatory mediators and regulators during immune responses. As a consequence, there are heightened inflammatory responses and irreversible tissue damage associated with many age-related chronic diseases. Alzheimer’s disease (AD) brain is marked by prominent inflammatory features, in which microglial activation is the driving force for the elaboration of an inflammatory cascade. How the regulation of inflammation loses its effectiveness during AD pathogenesis remains largely unclear. In this article, we will first review current knowledge of microglial activation and its association with AD pathology. We then discuss four examples of anti-inflammatory systems that could play a role in regulating microglial activation: CD200/CD200 receptor, vitamin D receptor, peroxisome proliferator-activated receptors, and soluble receptor for advanced glycation end products. Through this, we hope to illustrate the diverse aspects of inflammatory regulatory systems in brain and neurodegenerative diseases such as AD. We also propose the importance of neuronal defense systems, because they are part of the integral inflammatory and anti-inflammatory systems. Augmenting the anti-inflammatory defenses of neurons can be included in the strategy for restoration of balanced immune responses during aging and neurodegenerative diseases.     

Cytokines in the pathogenesis of rheumatoid arthritis

Cytokines regulate a broad range of inflammatory processes that are implicated in the pathogenesis of rheumatoid arthritis. In rheumatoid joints, it is well known that an imbalance between pro- and anti-inflammatory cytokine activities favours the induction of autoimmunity, chronic inflammation and thereby joint damage. However, it remains less clear how cytokines are organized within a hierarchical regulatory network, and therefore which cytokines may be the best targets for clinical intervention a priori. Here, we discuss the crucial effector function of cytokines in the immunological processes that are central to the pathogenesis of rheumatoid arthritis.     

CD154: the atherosclerotic risk factor in rheumatoid arthritis?

Atherosclerosis, now regarded as a chronic inflammatory disease of the arterial wall, and its clinical manifestations have increasingly been associated with rheumatoid arthritis (RA), supporting the notion that autoimmune diseases and vascular disorders share common etiological features. Indeed, evidence pertaining to this matter indicates that inflammation and its multiple components are the driving force behind the pathogenesis of these disorders. Interestingly, CD154 and its receptors have emerged as major players in the development of RA and atherosclerosis, which raises the possibility that this axis may represent an important biological link between both complications. Indeed, CD154 signaling elicits critical inflammatory responses that are common to the pathogenesis of both diseases. Here, we provide an overview of the traditional and disease-related interrelations between RA and vascular abnormalities, while focusing on CD154 as a potential mediator in the development of atherosclerotic events in RA patients.     

Oxidized Phospholipid OxPAPC Activates TRPA1 and Contributes to Chronic Inflammatory Pain in Mice

Oxidation products of the naturally occurring phospholipid 1-palmitoyl-2-arachidonoyl-sn-glycerol-3-phosphatidylcholine (PAPC), which are known as OxPAPC, accumulate in atherosclerotic lesions and at other sites of inflammation in conditions such as septic inflammation and acute lung injury to exert pro- or anti-inflammatory effects. It is currently unknown whether OxPAPC also contributes to inflammatory pain and peripheral neuronal excitability in these conditions. Here, we observed that OxPAPC dose-dependently and selectively activated human TRPA1 nociceptive ion channels expressed in HEK293 cells in vitro, without any effect on other TRP channels, including TRPV1, TRPV4 and TRPM8. OxPAPC agonist activity was dependent on essential cysteine and lysine residues within the N-terminus of the TRPA1 channel protein. OxPAPC activated calcium influx into a subset of mouse sensory neurons which were also sensitive to the TRPA1 agonist mustard oil. Neuronal OxPAPC responses were largely abolished in neurons isolated from TRPA1-deficient mice. Intraplantar injection of OxPAPC into the mouse hind paw induced acute pain and persistent mechanical hyperalgesia and this effect was attenuated by the TRPA1 inhibitor, HC-030031. More importantly, we found levels of OxPAPC to be significantly increased in inflamed tissue in a mouse model of chronic inflammatory pain, identified by the binding of an OxPAPC-specific antibody. These findings suggest that TRPA1 is a molecular target for OxPAPC and OxPAPC may contribute to chronic inflammatory pain through TRPA1 activation. Targeting against OxPAPC and TRPA1 signaling pathway may be promising in inflammatory pain treatment.     

General Model of Inflammation

Dysfunctions in the immune system, due to genetics, disease or environmental factors, can cause bacterial colonization and chronic inflammation. In cystic fibrosis and chronic obstructive pulmonary disease, respiratory infections can initiate inflammation of the airway. We propose a system of nonlinear ordinary differential equations to describe interactions between macrophages, both inflammatory and anti-inflammatory cytokines, and bacteria. Small changes in parameters governing inflammatory cytokine production and macrophage sensitivity to cytokines result in dramatically different model behaviors. When the immune system is functioning properly, a non-aggressive pathogen will not provide a sufficient trigger to initiate chronic inflammation, however, in disease positive feedback of the inflammatory cytokine can induce chronic inflammation even after a bacterial infection has been resolved. In addition, if the macrophage population is more sensitive to inflammatory cytokines small perturbations initiated by bacteria will also lead to chronic inflammation. We have found nonaggressive bacteria are able to initiate chronic inflammation and propose why anti-inflammatory cytokine therapy may not be effective in resolving this inflammation.     

Osteocalcin does not influence acute or chronic inflammation in human vascular cells

Some human observational studies have suggested an anti-inflammatory role of osteocalcin (OCN). An inflammatory protocol using interferon-γ and tumor necrosis factor-α (10 ng/ml) was employed to examine the acute (24 hr) and chronic (144 hr) effects of uncarboxylated OCN (ucOCN) in commercial, primary, subcultured human aortic endothelial cells (HAEC), and human smooth muscle cells (HASMCs). The inflammatory protocol increased phosphorylation of intracellular signaling proteins (CREB, JNK, p38, ERK, AKT, STAT3, STAT5) and increased secretion of adhesion markers (vascular cell adhesion molecule-1, intracellular adhesion molecule-1, monocyte chemoattractant protein-1) and proinflammatory cytokines (interleukin-6 [IL-6], IL-8). After acute inflammation, there were no additive or reductive effects of ucOCN in either cell type. Following chronic inflammation, ucOCN did not affect cell responses, nor did it appear to have any pro- or anti-inflammatory effects when administered acutely or chronically on its own in either cell type. Additionally, ucOCN did not affect lipopolysaccharide (LPS)-induced acute inflammation in HAECs or HASMCs. The findings of this study do not support a causal role for OCN within the models of vascular inflammation chosen. Further confirmatory studies are warranted.     

T-cell cytokine induction of BMP-2 regulates human mesenchymal stromal cell differentiation and mineralization

How T-cells, attracted to local sites of inflammation in arthritides, affect heterotopic ossification is presently unknown. Here, we tested the hypothesis that T-cell cytokines play a role in the differentiation of human mesenchymal stromal cells (HMSC) into the osteoblast phenotype by inducing autologous BMP-2, providing a possible mechanism for heterotopic ossification. HMSC from multiple donor bones were treated with either activated T-cell conditioned medium (ACTTCM) or physiological concentrations of the major inflammatory cytokines, TNF-alpha, TGF-beta, IFN-gamma, and IL-17 (TTII), individually or in combinations. ACTTCM induced BMP-2 protein in a time-dependent manner over a 48 h period and alkaline phosphatase (AlkP) within 7 days. In combination, TTII, like ACTTCM, induced AlkP and synergistically induced BMP-2 protein. Either individually, or in combinations of up to three, the T-cell cytokines failed to induce BMP-2 above control levels while a combination of all four cytokines synergistically induced BMP-2 10-fold as assessed by ELISA. TTII induced mineralized matrix as effectively as dexamethasone. Inhibition of p38 MAPK completely inhibited TTII-induced BMP-2 production and matrix mineralization. Real time RT-PCR analysis demonstrated a striking early (within 4 h) increase in BMP-2 gene expression by TTII, which was suppressed by p38 MAP kinase inhibition. In localized chronic inflammatory diseases, T-cell cytokines released at localized sites of inflammation may be the driving force for differentiation of local mesenchymal stromal cells into the osteoblast phenotype thereby playing a significant role in the heterotopic ossification observed in these diseases.     

Preventative oral methylthioadenosine is anti-inflammatory and reduces DSS-induced colitis in mice

Methylthioadenosine (MTA) is a precursor of the methionine salvage pathway and has been shown to have anti-inflammatory properties in various models of acute and chronic inflammation. However, the anti-inflammatory properties of MTA in models of intestinal inflammation are not defined. We hypothesized that orally administered MTA would be bioavailable and reduce morbidity associated with experimental colitis. We examined clinical, histological, and molecular markers of disease in mice provided oral MTA before (preventative) or after (therapy) the induction of colitis with 3% dextran sulfate sodium (DSS). We found a reduction in disease activity, weight loss, myeloperoxidase activity, and histological damage in mice given preventative MTA compared with DSS alone. We also found that equivalent supplementation with methionine could not reproduce the anti-inflammatory effects of MTA, and that MTA had no detectable adverse effects in control or DSS mice. Expression microarray analysis of colonic tissue showed several dominant pathways related to inflammatory cytokines/chemokines and extracellular matrix remodeling were upregulation by DSS and suppressed in MTA-supplemented mice. MTA is rapidly absorbed in the gastrointestinal tract and disseminated throughout the body, based on a time course analysis of an oral bolus of MTA. This effect is transient, with MTA levels falling to near baseline within 90 min in most organs. Moreover, MTA did not lead to increased blood or tissue methionine levels, suggesting that its effects are specific. However, MTA provided limited therapeutic benefit when administered after the onset of colitis. Our results show that oral MTA supplementation is a safe and effective strategy to prevent inflammation and tissue injury associated with DSS colitis in mice. Additional studies in chronic inflammatory models are necessary to determine if MTA is a safe and beneficial option for the maintenance of remission in human inflammatory bowel disease.     

CD163 and its role in inflammation

Mononuclear phagocytes represent a heterogeneous population of cells with individual subpopulations exerting different pro- or anti-inflammatory functions. CD163 is a monocyte/macrophage specific marker expressed predominantly on cells which possess strong anti-inflammatory potential. The expression of CD163 is strongly induced by anti-inflammatory mediators such as glucocorticoids and interleukin-10, while being inhibited by pro-inflammatory mediators such as interferon-gamma. CD163-expressing mononuclear phagocytes, as well as soluble CD163, may both take part in downregulating an inflammatory response. It seems, therefore, that CD163 may be an interesting target for therapeutic modulation of the inflammatory response.     

LL-37 attenuates inflammatory impairment via mTOR signaling-dependent mitochondrial protection

The human cationic antimicrobial protein LL-37 is a multifunctional host defense peptide with a wide range of immunomodulatory activities. Previous work has shown that LL-37 exerts both pro- and anti-inflammatory effects. The role of mitochondria in the skin inflammatory effects of LL-37 has not been well studied. Therefore, our aim was to investigate the immunomodulatory effect of LL-37 in HaCaT cells and to delineate the underlying mechanisms related to mitochondrial function. Immunohistochemistry results from tissue microarrays showed strong cytoplasmic LL-37 staining in inflammatory cells in chronic dermatic inflammation. Using exogenous LL-37 stimulation and LL-37 knockdown and overexpression, LL-37 was demonstrated to dramatically reduce the mRNA levels and protein secretion of inflammatory cytokines including IL-6, IL-8, IL-1α and tumor necrosis factor-α (TNF-α), which are induced by lipopolysaccharides (LPS). The anti-inflammatory effects of LL-37 are dependent upon its ability to increase mitochondrial biogenesis and to maintain mitochondrial homeostasis. Furthermore, we observed that LL-37 enhances the LPS-induced phosphorylation of extracellular signal-regulated kinase (ERK1/2) and mammalian target of rapamycin (mTOR). The mTOR inhibitor rapamycin can neutralize the protective effects of LL-37 on mitochondria. In conclusion, these results suggest that high LL-37 expression levels correlate with chronic skin inflammation; mitochondrial dysfunction occurs in HaCaT cells during inflammation; and LL-37 attenuates inflammatory impairment by stimulating mitochondrial biogenesis and protecting mitochondrial function, which are dependent upon mTOR signaling. These findings provide new insights into targeting mitochondria with LL-37 to prevent skin inflammatory reactions.     

Inflammation in Alzheimer disease: driving force, bystander or beneficial response?

Alzheimer disease is a progressive dementia with unknown etiology that affects a growing number of the aging population. Increased expression of inflammatory mediators in postmortem brains of people with Alzheimer disease has been reported, and epidemiological studies link the use of anti-inflammatory drugs with reduced risk for the disorder. On the initial basis of this kind of evidence, inflammation has been proposed as a possible cause or driving force of Alzheimer disease. If true, this could have important implications for the development of new treatments. Alternatively, inflammation could simply be a byproduct of the disease process and may not substantially alter its course. Or components of the inflammatory response might even be beneficial and slow the disease. To address these possibilities, we need to determine whether inflammation in Alzheimer disease is an early event, whether it is genetically linked with the disease and whether manipulation of inflammatory pathways changes the course of the pathology. Although there is still little evidence that inflammation triggers or promotes Alzheimer disease, increasing evidence from mouse models suggests that certain inflammatory mediators are potent drivers of the disease. Related factors, on the other hand, elicit beneficial responses and can reduce disease.     

Mechanistic simulations of inflammation: current state and future prospects

Inflammation is a normal, robust physiological process. It can also be viewed as a complex system that senses and attempts to resolve homeostatic perturbations initiated from within the body (for example, in autoimmune disease) or from the outside (for example, in infections). Virtually all acute and chronic diseases are either driven or modulated by inflammation. The complex interplay between beneficial and harmful arms of the inflammatory response may underlie the lack of fully effective therapies for many diseases. Mathematical modeling is emerging as a frontline tool for understanding the complexity of the inflammatory response. A series of articles in this issue highlights various modeling approaches to inflammation in the larger context of health and disease, from intracellular signaling to whole-animal physiology. Here we discuss the state of this emerging field. We note several common features of inflammation models, as well as challenges and prospects for future studies.     

Nitric oxide in gastrointestinal epithelial cell carcinogenesis: linking inflammation to oncogenesis

Chronic inflammation of gastrointestinal tissues is a well-recognized risk factor for the development of epithelial cell-derived malignancies. Although the inflammatory mediators linking chronic inflammation to carcinogenesis are numerous, current information suggests that nitric oxide (NO) contributes to carcinogenesis during chronic inflammation. Inducible nitric oxide synthase (iNOS), expressed by both macrophages and epithelial cells during inflammation, generates the bioreactive molecule NO. In addition to causing DNA lesions, NO can directly interact with proteins by nitrosylation and nitosation reactions. The consequences of protein damage by NO appear to be procarcinogenic. For example, NO inhibits DNA repair enzymes such as human 8-oxodeoxyguanosine DNA glycosylase 1 and blocks apoptosis via nitrosylation of caspases. These cellular events permit DNA damage to accumulate, which is required for the numerous mutations necessary for development of invasive cancer. NO also promotes cancer progression by functioning as an angiogenesis factor. Strategies to inhibit NO generation during chronic inflammation or to scavenge reactive nitrogen species may prove useful in decreasing the risk of cancer development in chronic inflammatory gastrointestinal diseases.     

Oxidized phospholipids as modulators of inflammation in atherosclerosis

PURPOSE OF REVIEW: This review will summarize recent evidence demonstrating that biologically active phospholipid oxidation products modulate inflammatory reactions. RECENT FINDINGS: Structural identification of new biologically active oxidized phospholipids and the finding that they can also be formed at inflammatory sites other than the atherosclerotic lesion have expanded the potential role of these compounds in inflammation beyond atherogenesis. Various signaling pathways are induced by oxidized phospholipids, leading to the expression of inflammatory genes by mechanisms that differ from those mediated by the classic inflammatory agonists tumor necrosis factor or lipopolysaccharide. Furthermore, oxidized phospholipids can bind to pattern recognition molecules and thus potently influence inflammation and immune responses during host defense. SUMMARY: During inflammatory processes biologically active lipid oxidation products accumulate that modulate the inflammatory process and may determine the fate and outcome of the body's reaction in acute inflammation during host defense. Oxidized phospholipids may induce and propagate chronic inflammatory processes; however, evidence is accumulating that cells and tissues respond towards these oxidatively formed stress signals also by activation of anti-inflammatory, cytoprotective reactions.     

MKP-1 coordinates ordered macrophage-phenotype transitions essential for stem cell-dependent tissue repair

Re-establishing tissue homoeostasis in response to injury requires infiltration of inflammatory cells and activation of resident stem cells. However, full tissue recovery also requires that the inflammation is resolved. While it is known that disturbing the interactions between inflammatory cells and tissue resident cells prevents successful healing, the molecular mechanisms underlying the paracrine interactions between these cell types are practically unknown. Here, and in a recent study, we provide mechanistic evidence that macrophages control stem cell-dependent tissue repair. In particular, we found that the temporal spacing of the pro- to anti-inflammatory macrophage polarization switch is controlled by the balance of p38 MAPK (termed here p38) and the MAPK phosphatase MKP-1 during the muscle healing process. Moreover, we demonstrate a new function for MKP-1-regulated p38 signaling in deactivating macrophages during inflammation resolution after injury. Specifically, at advanced stages of regeneration, MKP-1 loss caused an unscheduled “exhaustion-like” state in muscle macrophages, in which neither pro- nor anti-inflammatory cytokines are expressed despite persistent tissue damage, leading to dysregulated reparation by the tissue stem cells. Mechanistically, we demonstrate that p38 and MKP-1 control the AKT pathway through a miR-21-dependent PTEN regulation. Importantly, both genetic and pharmacological interference with the individual components of this pathway restored inflammation-dependent tissue homeostasis in MKP-1-deficient mice and delayed inflammation resolution and tissue repair dysregulation in wild-type mice. Because the process of tolerance to bacterial infection involves a progressive attenuation of pro-inflammatory gene expression, we discuss here the potential similarities between the mechanisms underlying inflammation resolution during tissue repair and those controlling endotoxin tolerance.     

Curcumin as an anti-inflammatory agent: Implications to radiotherapy and chemotherapy

Cancer is the second cause of death worldwide. Chemotherapy and radiotherapy are the most common modalities for the treatment of cancer. Experimental studies have shown that inflammation plays a central role in tumor resistance and the incidence of several side effects following both chemotherapy and radiotherapy. Inflammation resulting from radiotherapy and chemotherapy is responsible for adverse events such as dermatitis, mucositis, pneumonitis, fibrosis, and bone marrow toxicity. Chronic inflammation may also lead to the development of second cancer during years after treatment. A number of anti-inflammatory drugs such as nonsteroidal anti-inflammatory agents have been proposed to alleviate chronic inflammatory reactions after radiotherapy or chemotherapy. Curcumin is a well-documented herbal anti-inflammatory agents. Studies have proposed that curcumin can help management of inflammation during and after radiotherapy and chemotherapy. Curcumin targets various inflammatory mediators such as cyclooxygenase-2, inducible nitric oxide synthase, and nuclear factor κB (NF-κB), thereby attenuating the release of proinflammatory and profibrotic cytokines, and suppressing chronic production of free radicals, which culminates in the amelioration of tissue toxicity. Through modulation of NF-κB and its downstream signaling cascade, curcumin can also reduce angiogenesis, tumor growth, and metastasis. Low toxicity of curcumin is linked to its cytoprotective effects in normal tissues. This protective action along with the capacity of this phytochemical to sensitize tumor cells to radiotherapy and chemotherapy makes it a potential candidate for use as an adjuvant in cancer therapy. There is also evidence from clinical trials suggesting the potential utility of curcumin for acute inflammatory reactions during radiotherapy such as dermatitis and mucositis.     

Lipid mediator networks and leukocyte transmigration

In intact tissues, vascular endothelial cells lie anatomically positioned as the central coordinator of inflammation. Endothelia communicate with underlying cells (e.g. smooth muscle, fibroblasts, epithelia) in ways that both coordinate leukocyte trafficking, and control the composition of the inflammatory microenvironment. Such coordination occurs through both direct communication (e.g. cell adhesion) as well as via soluble mediators liberated at sites of inflammation (e.g. chemokines, cytokines, lipids). Locally generated mediators bind to surface receptors, and mediate both physiologic and pathophysiologic functional responses. Important in this regard, both endothelial and subendothelial cell populations express enzymes capable of utilizing arachidonic acid substrates to generate bioactive lipid mediators (e.g. lipoxygenases, cyclooxygenases). Such lipid mediators can signal via autocrine or paracrine pathways and, depending on the tissue microenvironment, can convey a pro- or anti-inflammatory message. This review will highlight recent studies characterizing inflammatory responses to lipid mediators liberated at sites of inflammation, with a particular emphasis on neutrophil (polymorphonuclear leukocyte or PMN) trafficking.     

Effects of tepoxalin, a dual inhibitor of cyclooxygenase/5-lipoxygenase, on events associated with NSAID-induced gastrointestinal inflammation

Prostaglandins and thromboxanes are products of arachidonic acid metabolism via the cyclooxygenase (CO) enzyme and are responsible for the pain and swelling common to sites of inflammation. Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit the production of these substances and are used in the treatment of inflammatory diseases such as arthritis. However, one of the major side-effects of NSAID therapy is gastric ulceration. It is possible that inhibition of prostaglandin production and a related increase in the formation of leukotrienes via the 5-lipoxygenase (5-LO) enzymatic pathway are responsible for attracting inflammatory cells, causing local sites of inflammation and producing ulceration. To determine the effects of 5-LO inhibition on this hypothesis, studies were performed in rats to evaluate the effects of tepoxalin, a dual CO/LO inhibitor on leukotriene B₄ levels in gastric mucosa and neutrophil adhesion in mesenteric venules. In rats, chronic oral administration of an NSAID, indomethacin (2 mg/kg daily over 4 days), resulted in 40% mortality, accompanied by intestinal adhesions and perforations when evaluated 24 h after the fourth dose of drug. Additionally, neutrophil adhesion was increased in the mesenteric venules and cell infiltration was evident in the mesenteric interstitium. These gastrointestinal side-effects were inhibited in a separate group of rats administered tepoxalin (20 mg/kg, p.o) 30 min prior to each daily indomethacin treatment. Further studies were performed to determine tepoxalin's effects on early events associated with NSAID-induced gastrointestinal inflammation, including neutrophil adhesion, lipid peroxide generation and LTB₄ production. Indomethacin (100 mg/kg, p.o.) produced elevated levels of LTB₄ in rat gastric mucosa 90 min after administration. Additionally, neutrophil adhesion in mesenteric venules was increased at this dose and with the administration of another NSAID, naproxen. No generation of lipid peroxides was evident in the gastric mucosa at this timepoint. Tepoxalin (up to 400 mg/kg, p.o.) did not have an effects on gastric mucosal LTB₄ generation and lipid peroxide levels. A decrease in neutrophil adhesion was observed at the highest dose. In another study, pretreatment with tepoxalin (ED₅₀=7.5 mg/kg, p.o.) or the selective 5-LO inhibitor zileuton (100 mg/kg, p.o.) prevented the increases in gastric mucosal LTB₄ levels and neutrophil adhesion induced by indomethacin (100 mg/kg, p.o.). These data suggest that LO inhibition may play a vital role in the prevention of NSAID-induced gastric inflammation, providing insight into the lack of ulcerogenicity with tepoxalin and new approaches to anti-inflammatory therapy which may prevent gastric side effects.