Astrocyte TLR4 activation induces a proinflammatory environment through the interplay between MyD88‐dependent NFκB signaling, MAPK, and Jak1/Stat1 pathways

Abstract

There is increasing evidence that astrocytes play important roles in immune regulation in the brain. Astrocytes express toll-like receptors (TLR) and build up responses to innate immune triggers by releasing proinflammatory molecules. We investigate signaling pathways and released molecules after astrocyte TLR4 activation. Purified rodent brain astrocyte cultures were treated with the TLR4 activator bacterial lipopolysaccharide (LPS). Tools used to interfere with this system include small interference RNA, inhibitory drugs, and MyD88 or Stat1 deficient mice. LPS induced early activation of the transcription factor NFκB, through the MyD88 adaptor, and expression of TNF-α, VCAM-1, IL-15, and IL-27. LPS also induced delayed Jak1/Stat1 activation, which was MyD88-independent but was not mediated by IFN-β. Jak1/Stat1 activation induced the expression of negative cytokine regulator SOCS-1 and CXCL10 chemokine (IP-10). Mitogen-activated protein kinases (MAPK) were also involved in TLR4 signaling in a MyD88-independent fashion. p38 exerted a strong influence on LPS-induced gene expression by regulating the phosphorylation of Stat1 and the transcriptional activity of NFκB, while JNK regulated the Jak1/Stat1 pathway, and ERK1/2 controlled the expression of Egr-1 and influenced MyD88-dependent MMP-9 expression. Interplay between these signals was evidenced by the increased induction of MMP-9 in Stat1-deficient cells challenged with LPS, suggesting that Stat1 negatively regulates the expression of MMP-9 induced by LPS. Therefore, astrocytes are responsive to TLR4 activation by inducing a complex set of cell-dependent molecular reactions mediated by NFκB, MAPK and Jak1/Stat1 signaling pathways. Here we identified cross-talking signals generating a proinflammatory environment that will modulate the response of surrounding cells.

Induction of COX-2 Enzyme and Down-regulation of COX-1 Expression by Lipopolysaccharide (LPS) Control Prostaglandin E2 Production in Astrocytes

“…Rat astrocyte cultures contained only 2.01 Ϯ 1.68% of contaminating microglia cells, as reported (21). Purified mouse astrocyte cultures also contained very little microglia, as estimated by immunofluorescence and by examining the expression of CD11b mRNA (see below for description of these methods).…”

“…Cell Cultures-Glial cell cultures enriched in astrocytes were prepared from the cerebral cortex of 1-2-day-old rats or mice as described previously (21,23), with minor modifications. In brief, cells were maintained at 37°C in a humidified atmosphere of 5% CO 2 , 95% air in culture medium (DMEM (Invitrogen) for rat astrocytes, and DMEM/F-12 nutrient (1:1) (Invitrogen) for mouse astrocytes).…”

Astrocytes in the tempest of multiple sclerosis

“…Furthermore, astrocytic iNOS has been demonstrated in MS lesions, and it is recognized as an important pathogenic feature of MS [82]. [57,61,81,[83][84][85][86][87]. Thus, astrocytes could be involved in the complexity of cytokine functions in MS pathogenesis [88].…”

Phenotype-dependent alteration of pathways and networks reveals a pure synergistic mechanism for compounds treating mouse cerebral ischemia

“…Moreover, JU may act on more than 10 pathways, which may contribute to the pure synergistic mechanism (Supplement Table 2). Some of these new pathways were involved in inflammatory immunity (IL-18 Signaling, Actin Cytoskeleton Signaling, Glucocorticoid Receptor Signaling, Lymphotoxin B Receptor Signaling, and IL-15 Signaling [53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68] [69][70][71][72][73][74][75][76][77] . Therefore, we propose that the pure synergistic mechanism is focused on inflammation, immune responses, apoptosis, and nervous system development ( Figure 6).…”