Regulation of microglial cell function by corticosteroids and disruption by organotins

Abstract

Microglia cells are the resident brain macrophages regulating in the initiation and maintenance of neuroinflammation. Chronic or exacerbated activation of microglia can contribute to neurodegenerative diseases. In the present thesis, mechanism of endogenous corticosteroids and xenobiotics on microglia function were investigated using the mouse BV-2 microlgia cell line. Corticosteroids are potent modulators of inflammation and mediate their effects by binding to mineralocorticoid receptors (MR) and glucocorticoid receptors (GR). MR and GR are suggested to regulate microglia activation and suppression, respectively. We showed that GR and MR differentially regulate on nuclear factor kappaB (NF-?B) activation and neuroinflammatory parameters, including interleukin-6 (IL-6) and tumor necrosis factor-a (TNF-a). 11s-hydroxysteroid dehydrogenase type 1 (11s-HSD1), converting inactive 11-dehydrocorticosterone to active corticosterone, is involved in the action of GR and MR. Both 11-dehydrocorticosterone and corticosterone showed biphasic effects with low/moderate concentrations potentiating IL-6 and TNF-a expression and NF-?B activation through MR. At high concentrations, corticosteroids suppressed these mediators through GR. We also showed that the silane AB110873, indentified by a MR pharmacophore, stimulates mitochondrial reactive oxygen species (ROS) generation and the production of the IL-6 by activating MR. Metabotropic glutamate receptor 5 (mGluR5) has been documented to modulate microglia function. Microglial activation induced by (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), a mGluR5 agonist, has been shown to decrease microglia activation and release of associated pro-inflammatory factors. Additionally, oxidative stress and inflammatory response of microglia cells are associated with AMP-dependent protein kinase (AMPK) and calcium-mediated signaling. Here, we investigated the relationship between oxidative stress and inflammation and AMPK and calcium-mediated pathways in the antagonism of mGluR5 with 2-methyl-6-(phenylethynyl)-pyridine (MPEP). MPEP significantly increased oxidative stress parameters and inflammatory mediators in a concentration-dependent manner. MPEP reduced ATP production and changed the phosphorylation state of AMPK. MPEP increased the elevation of free intracellular Ca2+([Ca2+]i) from endoplasmic reticulum (ER) through IP3 receptor. ER stress markers were induced by MPEP and blocked by a chemical chaperone (4-phenyl butyric acid, 4-PBA) and a calcium chelator (BAPTA-AM). AMPK activation abolished and inhibition potentiated ER stress induced by MPEP. The effect of MPEP on phospholipase C (PLC)-associated pathways was also investigated. A PLC inhibitor (U73122), and a Gi protein inhibitor (pertussis toxin, PTX) blocked MPEP-induced increase of [Ca2+]i. MPEP also significantly increased PLC activity. Furthermore, AICAR, BAPTA-AM, U73122, and PTX prevented oxidative stress and inflammatory response induced by MPEP. Excessive and chronic exposure to organotin compounds has been associated with neurotoxicity and neuroinflammation. Dibutyltin (DBT) is an organotin widely used as a stabilizer in polyvinyl chloride plastics. In the present study, we found that DBT promotes mitochondrial oxidative stress and induces ATP depletion, leading to AMPK activation in a time-dependent manner. DBT induced concentration-dependent increases in IL-6, NADPH oxidase-2 (NOX-2), TNF-? expression. NOX-2 inhibitor, apocynin inhibited the action of DBT, not only on IL-6 up-regulation but also an intracellular ROS production. DBT induced the nuclear translocation of NF-?B and NF-?B inhibitor Cay10512 blocked IL-6 expression induced by DBT. Furthermore, we showed a role for phosphatidylinositol 3-kinase (PI3K)/ protein kinase B (Akt), extracellular signal-regulated protein kinase1/2 (ERK1/2), p38 mitogen-activated protein kinase (p38 MAPK), c-Jun NH(2)-terminal kinase (JNK), protein kinase C (PKC), PLC, and [Ca2+]i in the DBT-mediated toxicity. Trimethyltin (TMT) is an organotin with potent neurotoxic effects, characterized by neuronal destruction and neuroinflammation, which involves microglia activation as a consequence of neuronal damaged. In the present study, we found that TMT induces the expression of IL-6 and inducible nitric oxide synthase (iNOS). Cay10512 blocked TMT-induced translocation of NF-?B into nucleus. PD98059 and SB20190, inhibitors of ERK1/2 and p38 MAPK, respectively, inhibited the ability of TMT to induce IL-6 and iNOS expression. The proinflammatory action of TMT was substantially enhanced by low/moderate corticosterone and 11-dehydrocorticosterone but suppressed by dexamethasone. Spironolactone suppressed the effects of TMT and the potentiation by corticosterone on proinflammatory mediator expression. Similarly, the potentiation of corticosterone was inhibited by PD98059 and SB20190. In conclusion, a tightly coordinated activity of GR and MR regulates the NF-?B pathway and the control of inflammatory mediators in microglia cells. The balance of GR and MR activity is locally modulated by the action of 11s-HSD1, which is upregulated by pro-inflammatory mediators. This study highlights the role of mGluR5 antagonism in mediating oxidative stress, ER stress and inflammation in microglial cells. The calcium dependent pathways are mediated through Gi protein-coupled receptors, PLC, and PI3 receptors. AMPK also may play a role in the regulation of mGluR5 by disturbing the energy balance. TMT and corticosterone influence the same signaling pathways to exert inflammatory responses. TMT also directly disturbed the local corticosteroid balance.