A. A. Astakhova

Regulation of peroxisome proliferator‐activated receptors (PPAR) α and ‐γ of rat brain astrocytes in the course of activation by toll‐like receptor agonists

Peroxisome proliferator‐activated receptors (PPAR)‐α and ‐γ in astrocytes play important roles in inflammatory brain pathologies. Understanding the regulation of both activity and expression levels of PPARs is an important neuroscience issue. Toll‐like receptor (TLR) agonists are inflammatory stimuli that could modulate PPAR, but the mechanisms of their control in astrocytes are poorly understood. In the present study, we report that lipopolysaccharide, peptidoglycan, and flagellin, which are agonists of TLR4, TLR1/2, and TLR5, respectively, exert time‐ and nuclear factor kappa‐light‐chain‐enhancer of activated B cells‐dependent suppression of mRNA, protein and activity of PPARα and PPARγ. In naïve astrocytes, PPARα and PPARγ mRNA have short turnover time (half‐life about 30 min for PPARα, 75 min for PPARγ) with a nearly two‐fold stabilization after TLR‐activation. p38 inhibition abolished TLR‐induced stabilization. The levels of PPARα and PPARγ mRNA, and protein and DNA‐binding activity could be modified using c‐Jun N‐terminal Kinase and p38 inhibitors. In addition, the expression levels of both PPARα and PPARγ isotypes were induced after inhibition of protein synthesis. This induction signifies participation of additional regulatory proteins with short life‐time. They are p38‐sensitive for PPARα and c‐Jun N‐terminal Kinase‐sensitive for PPARγ. Thus, PPARα and PPARγ are regulated in astrocytes on mRNA and protein levels, mRNA stability, and DNA‐binding activity during TLR‐mediated responses. Astrocytes have the triad of PPARα, PPARβ/δ, and PPARγ in regulation of proinflammatory responses. Activation of Toll‐like receptors (TLR) leads to PPARβ/δ overexpression, PPARα and PPARγ suppression via TLR/NF‐κB pathway on mRNA, protein and activity levels. Mitogen‐activated protein kinases (MAPK) p38 and JNK are involved in regulation of PPAR expression. p38 MAPK plays a special role in stabilization of PPAR mRNA.

Rosiglitazone as a Modulator of TLR4 and TLR3 Signaling Pathways in Rat Primary Neurons and Astrocytes

An antidiabetic drug of the thiazolidinedione class, rosiglitazone (RG) demonstrates anti-inflammatory properties in various brain pathologies. The mechanism of RG action in brain cells is not fully known. To unravel mechanisms of RG modulation of toll-like receptor (TLR) signaling pathways, we compare primary rat neuron and astrocyte cultures stimulated with the TLR4 agonist lipopolysaccharide (LPS) and the TLR3 agonist poly I:C (PIC). Both TLR agonists induced tumor necrosis factor (TNFα) release in astrocytes, but not in neurons. Neurons and astrocytes released interleukin-10 (IL-10) and prostaglandin E2 (PGE2) in response to LPS and PIC. RG decreased TLR-stimulated TNFα release in astrocytes as well as potentiated IL-10 and PGE2 release in both astrocytes and neurons. RG induced phosphorylation of p38 and JNK MAPK (mitogen-activated protein kinase) in neurons. The results reveal new role of RG as a modulator of resolution of neuroinflammation.

Regulation of Cyclooxygenase 2 Expression by Agonists of PPAR Nuclear Receptors in the Model of Endotoxin Tolerance in Astrocytes

Endotoxin tolerance (ET) represents a state of an altered immune response induced by multiple stimulations of a cell, a tissue, or an organism with lipopolysaccharide. Characteristics of ET include downregulation of induction of proinflammatory genes (TNFα, IL6, and others) and enhancement of induction of antiinflammatory genes (IL10, TGFβ). ET generally has protective functions; nevertheless, it might result in a state of innate immune deficiency and cause negative outcomes. A current issue is the search for the mechanisms controlling the level of inflammation in the course of endotoxin tolerance. In this work, we investigated the change in cyclooxygenase 2 (Cox2) expression in the model of endotoxin tolerance in astrocytes and analyzed the possibility of regulating this process applying nuclear receptor PPAR agonists. Our results indicate that: 1) endotoxin tolerance can be induced in astrocytes and results in TNFα and Cox2 mRNA induction decrease upon secondary stimulation; 2) tolerance is revealed on the level of TNFα release and Cox2 protein expression; 3) PPAR agonists GW7647, L-165041, and rosiglitazone control Cox2 mRNA expression levels under conditions of endotoxin tolerance. In particular, rosiglitazone (a PPARγ agonist) induces Cox2 mRNA expression, while GW7647 (a PPARα agonist) and L-165041 (a PPARβ agonist) suppress the expression. Our results demonstrate that Cox2 can be upand downregulated during endotoxin tolerance in astrocytes, and PPAR agonists might be effective for controlling this target under conditions of multiple proinflammatory stimulations of brain tissues with endotoxin.

Antiinflammatory effect of rosiglitazone via modulation of mRNA stability of interleukin 10 and cyclooxygenase 2 in astrocytes

Investigation of molecular mechanisms of proinflammatory stimuli signaling in astrocytes is important for understanding their role in pathogenesis of central nervous system diseases as well as in functioning of the innate immunity system in non-immune cells. Here we show that lipopolysaccharide (LPS) stimulation of primary rat astrocytes led to conventional inflammatory response: increase in both proinflammatory (tumor necrosis factor, TNFα; prostaglandin E2, PGE2) and antiinflammatory marker (interleukin 10, IL-10) levels. The protein level of cyclooxygenase 2 (COX-2) was also increased. Rosiglitazone strengthened LPS-induced mRNA expression of COX-2 and IL-10 but not TNFα. Rosiglitazone is an agonist of nuclear receptor PPARγ, but its impact on IL-10 expression was not influenced by a PPARγ antagonist, GW9662, suggesting PPARγ-independent effect of rosiglitazone. The degradation of mRNA is one of the steps of inflammation regulation and might be affected by small molecules. In experiments with actinomycin D, we found that mRNA half-lives of IL-10, COX-2, and TNFα in naive astrocytes were 70, 44, and 19 min, respectively. LPS stimulation caused 2-fold increase in IL-10 and COX-2 mRNA decay rates, whereas addition of rosiglitazone restored them to the initial level. TNFα decay rate was not changed by these stimulations. This suggests that mRNA decay rate could be regulated by small molecules. Moreover, rosiglitazone could be used as a substance stimulating the resolution of inflammation without influence on proinflammatory signals. These results open new perspectives in the search for inflammation resolution modulators.

Resolution of inflammation and mood disorders

Relationship between mood disorders and inflammation is now well-documented, although molecular mechanisms are not understood. Previously mostly pro-inflammatory cytokines of immune system (IL-6, TNF, etc.) were taken into account. However, recent understanding of resolution of inflammation as an active process drew attention to mediators of resolution, which include both proteins and ω-3 and ω-6 polyunsaturated fatty acids derivatives (resolvins, cyclopentenone prostaglandins, etc.). This review takes into account new data on resolution of inflammation and action of mediators of resolution in models of depression. New facts and ideas about mechanisms of chronic inflammation onset are considered in relation to mood disorders. Basic control mechanisms of inflammation at the cellular level and the role of resolution substances in regulation of depression and other mood disorders are discussed. Signaling systems of innate immunity located in non-immune cells and their ability to generate substances that affect an onset of depression are reviewed. A novel hypothesis of depression as a type of abnormal resolution is proposed.

The effect of dehydroepiandrosterone on inflammatory response of astroglial cells

An increased interest in neuroinflammation is conditioned by its involvement in various pathological processes in the brain. Astrocytes play an important role in neuroinflammation, participating in its regulation, throwing out a large number of signaling molecules. Steroid compounds, actively produced by astrocytes, are of interest with regards to the regulation of inflammatory processes in the central nervous system. In the present work the effect of dehydroepiandrosterone (DHEA) on astroglial cells (cultured primary rat astrocytes) in a model of inflammation was studied. The inflammatory response was stimulated with lipopolysaccharide (LPS). Expression levels of pro-inflammatory factor TNFα, antinflammatory interleukin IL-10, and both pro- and antiinflammatory protein COX-2 were measured. The expression of IL-10, COX-2, and TNFα mRNA was determined by real-time PCR, COX-2 protein level by immunoblotting method, TNFα and IL-10 release by enzyme immunoassay. The effect of short-term (30 min) and long-term (24 h) exposure to DHEA was evaluated. It was shown that DHEA potentiates LPS-stimulated (1) increase in the IL-10 mRNA level; (2) IL-10 release; (3) does not affect TNFα level, and (4) exerts a weak pulsating bidirectional effect on COX-2. Using trilostane, an inhibitor of 3β-hydroxysteroid dehydrogenase, a key enzyme of DHEA metabolism, it was shown that DHEA metabolites make the main contribution to its effect. Thus, DHEA is of interest as a stimulant of anti-inflammatory processes in the brain.

Cryosynthesis and Properties of Dehydroepiandrosterone Hormone Nanoparticles

Nanoparticles of the steroidal hormone dihydroepiandrosterone (DHEA) were prepared by cryosynthesis technology, which allowed starting DHEA particles of size (100 ± 50) μm to be converted to nanoparticles of size (100 ± 20) nm. The particle sizes were determined by optical, electron transmission, and scanning atomic-force microscopy. HPLC with mass-spectrometric detection of starting and modified DHEA samples showed that the hormone molecular structure did not change during cryosynthesis of the nanoparticles. A comparison of the cytotoxicities of the samples on C6 glial cell culture showed that modified DHEA was less toxic. The results indicated that cryosynthesis technology could be used effectively to prepare nanoparticles of steroidal hormones.

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

ABSTRACT Control of decay of mRNA containing the adenine‐uridine rich elements (AREs) is an important post‐transcriptional mechanism involved in the regulation of inflammatory gene expression. Two widely recognized proteins in this machinery are HuR (human antigen R) – a protein that stabilizes ARE‐containing mRNA and TTP (tristetraprolin) – a protein that shortens half‐lives of ARE‐containing mRNA. Although HuR and TTP regulation mechanisms have been well studied in cells of hematopoietic origin, there are no respective data in astrocytes, cells of ectodermal origin which play an important role in neuroinflammation. Therefore we evaluated the existence of TTP and HuR in primary astrocytes and characterized the features of their regulation after stimulation by the proinflammatory stimuli thrombin, ATP, and agonists of TLR4, TLR2. All proinflammatory stimuli increased levels of TTP mRNA, but not HuR mRNA. Transcripts of both HuR and TTP underwent stabilization upon lipopolysaccharide (LPS) treatment, measured with the actinomycin D protocol. This effect was abolished by treatment with SB203580, an inhibitor of p38 MAPK. Both TTP and HuR transcripts were sensitive to modulation by anisomycin and cycloheximide. LPS induced translocation of HuR protein from nucleus to cytoplasm. TTP is localized in the cytosolic fraction and localization is not sensitive to LPS treatment. Our data for the first time reveal specificity of regulation of ARE‐binding proteins in astrocytes. We propose possibilities to manipulate brain inflammatory processes via post‐transcription regulatory steps in astrocytes. HighlightsTTP (tristetraprolin) and HuR (human antigen R) are expressed in astrocytes.Thrombin, ATP, LPS, peptidoglycan increase TTP, but not HuR mRNA level in astrocytes.HuR and TTP transcripts are stabilized after LPS treatment via p38 MAPK pathway.HuR protein is translocated from nucleus to cytoplasm upon LPS treatment.TTP protein expression does not correlate with transcript levels.

Sex-Mediated Differences in LPS Induced Alterations of TNFα, IL-10 Expression, and Prostaglandin Synthesis in Primary Astrocytes

Although many neurological and psychiatric disorders reveal clear sex-dependent variations, the molecular mechanism of this process is not clear enough. Astrocytes are involved in the response of neural tissue to injury and inflammation, produce steroid hormones, and sense steroid presence. To explore the hypothesis that astrocytes may participate in sex-mediated differences of inflammatory responses, we have examined whether male and female primary rat astrocytes show different responses to lipopolysaccharide (LPS) as a toll-like receptor 4 (TLR4) agonist. Levels of mRNA and proteins of tumor necrosis factor alpha (TNFα), interleukin-10 (IL-10), and cyclooxygenase (COX)-2 were assessed using qPCR, immunoblotting, and ELISA. UPLC-MS/MS was used to detect prostaglandins (PGs). LPS stimulation resulted in different levels of cytokine production; more TNFα and less IL-10 were produced in female cells compared with male astrocytes. Although the levels of the COX-2 expression were not altered, LPS significantly induced the synthesis of PGs with notable sex-related differences. PGE2 and PGD2 were less and 6-keto-PGF1α was more upregulated in female astrocytes, and TXB2 had similar levels in cells obtained from males and females. Trilostane, an inhibitor of 3β-Hydroxysteroid dehydrogenase (3β-HSD), inhibited the LPS-induced TNFα production and the release of PGE2, PGD2, and 6-keto-PGF1α in female astrocytes. Thus, male and female astrocytes differentially respond to inflammatory challenges on the level of production of cytokines and steroid hormones. Sex-mediated differences in pro- and anti-inflammatory responses should be taken into consideration for the effective treatment of disorders with neuroinflammation.

Regulation of cyclooxygenase 2 mRNA degradation by rosiglitazone in C6 glioma cells in the presence of inflammation inductors

It has recently become clear that regulation of mRNA stability plays an important role in the development of cell responses to stimulation of toll-like receptors during inflammation. This discovery motivated a search for low molecular weight substances modulating the stability of mRNA encoding proteins involved in inflammatory responses. In this work, regulation of the cyclooxygenase 2 (COX-2) expression by rosiglitazone–a promising drug for regulation of inflammation–was studied on rat glioma cell line C6. Inflammatory response was induced by lipopolysaccaride (LPS). The concentration of prostaglandin E2 (PGE2) was measured by ELISA, the level of COX-2 mRNA was determined by real-time PCR. It was shown that treatment with LPS caused a 6-fold increase in the PGE2 synthesis, which correlated with an increase in the COX-2 mRNA expression. Rosiglitazone induced a 2-fold decrease of the LPS-stimulated PGE2 release and reduced the levels of COX-2 transcripts. To explore the molecular mechanisms of the rosiglitazone effect, we estimated the stability of COX-2 mRNA. Cells were incubated in the presence of LPS for 1 h, and then de novo mRNA transcription was blocked with actinomycin D. The levels of mRNA were determined at various time points. Treatment with rosiglitazone was carried out for 30 min before the LPS addition. The COX-2 mRNA half-life in native cells was found to be 75 min. LPS stimulation slowed down the mRNA degradation so that its half-life time was 120 min, and the treatment with rosiglitazone restored this process back to the normal level. The results suggest that rosiglitazone regulates the stability of COX-2 mRNA. This opens up new perspectives for therapeutic applications of this drug.