D. V. Chistyakov

Regulation of peroxisome proliferator‐activated receptor β/δ expression and activity levels by toll‐like receptor agonists and MAP kinase inhibitors in rat astrocytes

Peroxisome proliferator‐activated receptor β/δ (PPARβ/δ) is a potential regulator of neuroinflammation. Toll‐like receptors (TLR) are innate immunity‐related receptors of inflammatory stimuli. In the present report, we evaluate the molecular mechanisms of regulation of mRNA, protein, and transcriptional activity levels of PPARβ/δ by agonists of TLR4, TLR1/2, and TLR5, using lipopolysaccharide (LPS), peptidoglycan, and flagellin, respectively. We found that these stimuli increase the PPARβ/δ levels in astrocytes. Expression and activity of PPARβ/δ are separately regulated by inhibitors of p38, MEK1/2, extracellular signal‐regulated kinases 1/2, and c‐Jun N‐terminal Kinase mitogen‐activated protein kinases. The LPS‐induced kinetics of PPARβ/δ expression is similar to that of the proinflammatory gene cyclooxygenase 2. Moreover, for both genes the expression depends on nuclear factor kappa‐light‐chain‐enhancer of activated B cells and p38, and is induced after inhibition of protein synthesis. The up‐regulation of the expression after inhibition of protein synthesis signifies the participation of a labile protein in regulation of PPARβ/δ expression. In contrast to cyclooxygenase 2, the cycloheximide‐sensitive PPARβ/δ expression was not responsive to nuclear factor kappa‐light‐chain‐enhancer of activated B cells inhibition. Measurements of PPARβ/δ mRNA stability showed that the PPARβ/δ mRNA levels are regulated post‐transcriptionally. We found that in LPS‐stimulated astrocytes, the half‐life of PPARβ/δ mRNA was 50 min. Thus, we demonstrate that PPARβ/δ expression and activity are regulated in TLR agonist‐stimulated astrocytes by mechanisms that are widely used for regulation of proinflammatory genes.

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.

Astrocytes synthesize primary and cyclopentenone prostaglandins that are negative regulators of their proliferation

Recently, the modulation of cellular inflammatory responses via endogenous regulators became a major focus of medically relevant investigations. Prostaglandins (PGs) are attractive regulatory molecules, but their synthesis and mechanisms of action in brain cells are still unclear. Astrocytes are involved in manifestation of neuropathology and their proliferation is an important part of astrogliosis, a cellular neuroinflammatory response. The aims of our study were to measure synthesis of PGs by astrocytes, and evaluate their influence on proliferation in combination with addition of inflammatory pathway inhibitors. With UPLC-MS/MS analysis we detected primary PGs (1410 ± 36 pg/mg PGE2, 344 ± 24 PGD2) and cyclopentenone PGs (cyPGs) (87 ± 17 15d-PGJ2, 308 ± 23 PGA2) in the extracellular medium after 24-h lipopolysaccharide (LPS) stimulation of astrocytes. PGs reduced astrocytic proliferation with the following order of potencies (measured as inhibition at 20 μM): most potent 15d-PGJ2 (90%) and PGA2 (80%), > PGD2 (40%) > 15d-PGA2 (20%) > PGE2 (5%), the least potent. However, PGF2α and 2-cyclopenten-1-one, and ciglitazone and rosiglitazone (synthetic agonists of PPARγ) had no effect. Combinations of cyPGs with SC-560 or NS-398 (specific anti-inflammatory inhibitors of cyclooxygenase-1 and -2, respectively) were not effective; while GW9662 (PPARγ antagonist) or MK-741 (inhibitor of multidrug resistance protein-1, MRP1, and CysLT1 receptors) amplified the inhibitory effect of PGA2 and 15d-PGJ2. Although concentrations of individual PGs and cyPGs are low, all of them, as well as primary PGs suppress proliferation. Thus, the effects are potentially additive, and activated PGs synthesis suppresses proliferation in astrocytes.

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.

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.

Rosiglitazone as a regulator of innate immunity in a cell model of hyperglycemia

Epidemiological studies have shown that severe inflammatory responses occur in patients with hyperglycemia. The molecular nature of these changes is currently under intense investigations. A central role of nuclear receptors PPAR has been shown in the regulation of metabolic changes associated with hyperglycemia, a selective agonist of nuclear receptor PPARγ rosiglitazone is used as a hypoglycemic drug. Rosiglitazone is known to have anti-inflammatory effects, but its properties as an anti-inflammatory drug in hyperglycemic conditions have not been studied. This was an aim of our work. We used a human cell culture model of hyperglycemia: HeLa cells incubated in the conditions of 25 mM glucose for 3 days. Control cells were incubated with 5 mM glucose. The cells were stimulated with lipopolysaccharide (LPS) that is known to trigger innate immune response through activation of Toll-like receptor 4 and influence mRNA expression levels of three of PPAR (α, β/δ, γ) isotypes as well as cyclooxygenase (COX-1 and COX-2). We have shown that under hyperglycemic conditions expression levels of PPARα and PPARβ/δ decreased almost twofold, expression level of COX-2 also decreased, while expression levels of COX-1 and PPARγ remained unchanged compared to those under normal glucose concentration. LPS administration in control cells leads to a 1.5–2.5-fold stimulation of expression of COX-2 and PPAR isotypes. In contrast, under hyperglycemia, LPS exhibited no effect on expression of COX-2 and the PPAR isotypes, which indicates potential mechanisms of hyperglycemia-related alterations in innate immunity. Rosiglitazone, an agonist of PPARγ, decreased expression level of PPARβ/δ and abolished the effect of LPS under hyperglycemia. Rosiglitazone also reduced expression level of COX-1 and COX-2, which indicates on the agonist possible role as an anti-inflammatory agent under high glucose concentrations. These data broaden applicability of rosiglitazone as an anti-inflammatory agent in hyperglycemic conditions.

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.