Thrombin induces suppressor of cytokine signaling 3 expression in brain microglia via protein kinase Cdelta activation

Astrocytes, Microglia, and Parkinson's Disease

Astrocytes and microglia support well-being and well-function of the brain through diverse functions in both intact and injured brain. For example, astrocytes maintain homeostasis of microenvironment of the brain through up-taking ions and neurotransmitters, and provide growth factors and metabolites for neurons, etc. Microglia keep surveying surroundings, and remove abnormal synapses or respond to injury by isolating injury sites and expressing inflammatory cytokines. Therefore, their loss and/or functional alteration may be directly linked to brain diseases. Since Parkinson's disease (PD)-related genes are expressed in astrocytes and microglia, mutations of these genes may alter the functions of these cells, thereby contributing to disease onset and progression. Here, we review the roles of astrocytes and microglia in intact and injured brain, and discuss how PD genes regulate their functions.

The potential role of neuroinflammation and transcription factors in Parkinson disease

Parkinson disease (PD) is a neurodegenerative disorder characterized by dopaminergic neurons affected by inflammatory processes. Post-mortem analyses of brain and cerebrospinal fluid from PD patients show the accumulation of proinflammatory cytokines, confirming an ongoing neuroinflammation in the affected brain regions. These inflammatory mediators may activate transcription factors—notably nuclear factor κB, Ying-Yang 1 (YY1), fibroblast growth factor 20 (FGF20), and mammalian target of rapamycin (mTOR)—which then regulate downstream signaling pathways that in turn promote death of dopaminergic neurons through death domain-containing receptors. Dopaminergic neurons are vulnerable to oxidative stress and inflammatory attack. An increased level of inducible nitric oxide synthase observed in the substantia nigra and striatum of PD patients suggests that both cytokine—and chemokine-induced toxicity and inflammation lead to oxidative stress that contributes to degeneration of dopaminergic neurons and to disease progression. Lipopolysaccharide activation of microglia in the proximity of dopaminergic neurons in the substantia nigra causes their degeneration, and this appears to be a selective vulnerability of dopaminergic neurons to inflammation. In this review, we will look at the role of various transcription factors and signaling pathways in the development of PD.

Thrombin Augments LPS-Induced Human Endometrial Endothelial Cell Inflammation via PAR1 Activation

PROBLEM

Risk factors for preterm birth include placental abruption, giving rise to excessive decidual thrombin, and intrauterine bacterial infection. Human endometrial endothelial cells (HEECs) express Toll-like receptors (TLRs), and infection-derived agonists trigger HEECs to generate specific inflammatory responses. As thrombin, in addition to inducing coagulation, can contribute to inflammation, its effect on HEEC inflammatory responses to the TLR4 agonist, bacterial lipopolysaccharide (LPS), was investigated.

METHOD OF STUDY

HEECs were pre-treated with or without thrombin or specific protease-activated receptor (PAR) agonists, followed by treatment with or without LPS. Supernatants were measured for cytokines and chemokines by ELISA and multiplex analysis.

RESULTS

Thrombin significantly and synergistically augmented LPS-induced HEEC secretion of interleukin (IL)-6, IL-8, granulocyte colony-stimulating factor (G-CSF), and growth-regulated oncogene-alpha (GRO-α), and significantly augmented monocyte chemotactic protein (MCP)-1, tumor necrosis factor-alpha (TNF-α), and vascular endothelial growth factor (VEGF) secretion additively. Similar to thrombin, a PAR1 agonist synergistically augmented the LPS-induced HEEC secretion of inflammatory IL-6, IL-8, G-CSF, and GRO-α.

CONCLUSION

Thrombin, via PAR1 activation, synergistically augments LPS-induced HEEC production of chemokines involved in immune cell recruitment and survival, suggesting a mechanism by which intrauterine abruption and bacterial infection may together be associated with an aggravated uterine inflammatory response.

Naringenin Suppresses Neuroinflammatory Responses Through Inducing Suppressor of Cytokine Signaling 3 Expression

Accumulating evidence suggests that neuroinflammation is closely associated with the pathogenesis of neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease. The hallmark of neuroinflammation is considered to be microglial activation in the central nervous system (CNS). Activated microglia release pro-inflammatory cytokines which cause neuroinflammation and progressive neuronal cell death. Therefore, inhibition of microglial activation is considered an important strategy in the development of neuroprotective strategy. Naringenin, a flavonoid found in citrus fruits and tomatoes, has been reported to have anti-oxidant, anti-cancer, and anti-inflammatory properties. However, the mechanism of its beneficial anti-inflammatory effects in the CNS is poorly understood. In this study, we demonstrated that naringenin inhibites the release of nitric oxide (NO), the expression of inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2), as well as pro-inflammatory cytokines in microglial cells. Treatment of naringenin also induced suppressors of cytokine signaling (SOCS)-3 expression in microglia. The SOCS-3 expression and anti-inflammatory effects of naringenin were found to be regulated by adenosine monophosphate-activated protein kinase α (AMPKα) and protein kinase C δ (PKCδ). Besides, naringenin exerted protective property against neurotoxicity caused by LPS-induced microglial activation. Our findings suggest that naringenin-inhibited iNOS and COX-2 expression is mediated by SOCS-3 activation through AMPKα and PKCδ signaling pathways. In a mouse model, naringenin also showed significant protective effects on microglial activation and improved motor coordination function as well. Therefore, naringenin that involves in anti-neuroinflammatory responses and neuroprotection might be a potential agent for treatment of inflammation-associated disorders.

Suppression of miR-155 Expression in IFN-γ-Treated Astrocytes and Microglia by DJ-1: A Possible Mechanism for Maintaining SOCS1 Expression

Previously, we reported that DJ-1, encoded by a Parkinson's disease (PD)-associated gene, inhibits expression of proinflammatory mediators in interferon-gamma (IFN-γ)-treated astrocytes and microglia through inhibition of STAT1 activation. Here, using microglia and astrocytes cultured from wild-type (WT) and DJ-1-knockout (KO) mouse brains, we examined how DJ-1 regulates suppressor of cytokine signaling 1 (SOCS1), a negative feedback regulator of STAT1 (signal transducer and activator of transcription) that is also induced by STAT1. We found that IFN-γ significantly increased SOCS1 mRNA expression in WT microglia and astrocytes, but not in KO cells, although STAT1 was highly activated in these latter cells. We further found that SOCS mRNA stability was decreased in DJ-1-KO cells, an effect that appeared to be mediated by the microRNA, miR-155. IFN-γ increased the levels of miR-155 in DJ-1-KO cells but not in WT cells. In addition, an miR-155 inhibitor rescued SOCS1 expression and decreased STAT1 activation in DJ-1-KO cells. Taken together, these results suggest that DJ-1 efficiently regulates inflammation by maintaining SOCS1 expression through regulation of miR-155 levels, even under conditions in which STAT1 activation is decreased.

Multiple mechanisms that prevent excessive brain inflammation

Inflammation of the injured brain has a double-edged effect. Inflammation protects the brain from infection, but it aggravates injury. Furthermore, brain inflammation is considered a risk factor for neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Emerging evidence supports the activation of negative regulatory mechanisms during this process to prevent prolonged and extensive inflammation. The inflammatory stimulators themselves or products of inflammatory cells may induce the expression of negative feedback regulators, such as suppressor of cytokine signaling (SOCS)-family proteins, antioxidant enzymes, and antiinflammatory cytokines. Furthermore, death of activated microglia (major inflammatory cells in the brain) may regulate brain inflammation. Astrocytes, the most abundant cells in the brain, may also act in preventing microglial overactivation. Therefore, we propose that the extent and duration of brain inflammation is tightly regulated through the cooperation of multiple mechanisms to maximize antipathogenic effects and minimize tissue damage.

Unraveling thrombin's true microglia-activating potential: markedly disparate profiles of pharmaceutical-grade and commercial-grade thrombin preparations

Microglia are the resident immune cells of the CNS. Brain injury triggers microglial activation, leading to proliferation, changes in antigenic profile, NO production and cytokine release. It is widely believed that serum factors inundating the injured tissue can prompt this activation, leading to long-term phenotypic changes. We and others have recently reported that commercial-grade preparations of thrombin, a serine protease known for its central function in blood coagulation, activate microglial cells. Recent findings, however, have called into question the involvement of thrombin itself in the induction of microglial cytokine release and led us to systematically re-investigate the ability of the protease to induce a broad spectrum of microglial activation parameters. We used a pharmaceutical-grade recombinant human thrombin (rh-thr) and compared it with a commercial-grade plasma-derived bovine thrombin (pb-thr) preparation that has been used extensively in the literature, including in our own earlier report. We investigated the effect of these two thrombin preparations on proliferation, NO production, interleukin-6 and tumour necrosis factor-alpha release, intracellular calcium signaling and cell surface expression of CD95 (Fas) and CD40. Pb-thr induced robust responses in all variables tested. In contrast, rh-thr triggered calcium signals and induced small but significant changes in the expression of cell surface antigens, but had no effect on proliferation, NO production or cytokine release. Control studies assured equivalent thrombin potencies and excluded both species-specific effects and endotoxin (lipopolysaccharide) contamination as possible causes of the disparity. Our results indicate a substantially more restricted role for thrombin itself in microglial activation than previously appreciated, but point to several potentially important co-stimulatory effects. In addition, these results suggest that previous studies examining thrombin's activation of microglia should be cautiously re-interpreted.

Lymphoid gene expression as a predictor of risk of secondary brain tumors

Gene expression profiles are tissue-specific but may also reflect germ-line-driven expression patterns across tissue types. Previously, using a targeted pharmacologic approach, we identified germ-line polymorphisms in a single gene (thiopurine methyltransferase) associated with the risk of irradiation- and chemotherapy-induced secondary brain tumors in children with acute lymphoblastic leukemia (ALL). To identify additional candidate genetic risk factors, in identically treated patients, we compared the gene expression profiles of diagnostic ALL blasts of those who did develop irradiation-associated brain tumors (n = 9) with the profiles from those who did not (n = 33). Weighted rank regression was used to identify 33 probe sets associated with the time-dependent development of brain tumors; k-means clustering (k = 2) identified 2 groups that differed significantly in cumulative incidence of brain tumors (P = 0.012). Permutation analysis was used to estimate the probability (P = 0.18) of obtaining 2 such clusters by chance. Linear discriminant analysis (time-independent categorization of outcome) was used to identify 70 probe sets whose expression differentiated between the 2 groups of patients. Permutation analyses (n = 1,000) was used to estimate the probability of selecting these probe sets by chance (P = 0.055). Five probe sets were in common between the time-independent and time-dependent methods. The distinguishing genes are involved in neural growth (FGFR1) and in nuclear trafficking (HNRPL, KPNB1). These data suggest that gene expression profiling from accessible tissues may identify targets involved in therapy-related malignancies in unrelated tissues.