A possible role of microglia-derived nitric oxide by lipopolysaccharide in activation of astroglial pentose-phosphate pathway via the Keap1/Nrf2 system

Abstract

BackgroundToll-like receptor 4 (TLR4) plays a pivotal role in the pathophysiology of stroke-induced inflammation. Both astroglia and microglia express TLR4, and endogenous ligands produced in the ischemic brain induce inflammatory responses. Reactive oxygen species (ROS), nitric oxide (NO), and inflammatory cytokines produced by TLR4 activation play harmful roles in neuronal damage after stroke. Although astroglia exhibit pro-inflammatory responses upon TLR4 stimulation by lipopolysaccharide (LPS), they may also play cytoprotective roles via the activation of the pentose phosphate pathway (PPP), reducing oxidative stress by glutathione peroxidase. We investigated the mechanisms by which astroglia reduce oxidative stress via the activation of PPP, using TLR4 stimulation and hypoxia in concert with microglia.MethodsIn vitro experiments were performed using cells prepared from Sprague–Dawley rats. Coexisting microglia in the astroglial culture were chemically eliminated using l-leucine methyl ester (LME). Cells were exposed to LPS (0.01 μg/mL) or hypoxia (1 % O2) for 12–15 h. PPP activity was measured using [1-14C]glucose and [6-14C]glucose. ROS and NO production were measured using 2′,7′-dichlorodihydrofluorescein diacetate and diaminofluorescein-FM diacetate, respectively. The involvement of nuclear factor-erythroid-2-related factor 2 (Nrf2), a cardinal transcriptional factor under stress conditions that regulates glucose 6-phosphate dehydrogenase, the rate-limiting enzyme of PPP, was evaluated using immunohistochemistry.ResultsCultured astroglia exposed to LPS elicited 20 % increases in PPP flux, and these actions of astroglia appeared to involve Nrf2. However, the chemical depletion of coexisting microglia eliminated both increases in PPP and astroglial nuclear translocation of Nrf2. LPS induced ROS and NO production in the astroglial culture containing microglia but not in the microglia-depleted astroglial culture. LPS enhanced astroglial ROS production after glutathione depletion. U0126, an upstream inhibitor of mitogen-activated protein kinase, eliminated LPS-induced NO production, whereas ROS production was unaffected. U0126 also eliminated LPS-induced PPP activation in astroglial–microglial culture, indicating that microglia-derived NO mediated astroglial PPP activation. Hypoxia induced astroglial PPP activation independent of the microglia–NO pathway. Elimination of ROS and NO production by sulforaphane, a natural Nrf2 activator, confirmed the astroglial protective mechanism.ConclusionsAstroglia in concert with microglia may play a cytoprotective role for countering oxidative stress in stroke.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-016-0564-0) contains supplementary material, which is available to authorized users.

Metabolic compartmentalization between astroglia and neurons in physiological and pathophysiological conditions of the neurovascular unit

“…Of note, the functional activation of neurons increases glucose utilization locally, and both neurons and astroglia contribute to glucose consumption. We have focused on the roles of glucose metabolism, especially glycolysis in astroglia, from the perspective of their supportive aspects in the protection of neurons against oxidative stress . Neuronal energy production is dependent on mitochondrial oxidative metabolism, which produces small amounts of reactive oxygen species (ROS).…”

“…Neuronal energy production is dependent on mitochondrial oxidative metabolism, which produces small amounts of reactive oxygen species (ROS). Oxidative stress has been implicated in the pathogenesis of numerous neurological disorders (i.e., stroke, ALS, Parkinson's disease) as well as normal aging of the brain . The glutathione system acts as an intrinsic protective mechanism.…”

“…Neuro‐inflammation plays a cardinal role in the pathogenesis of stroke . During the early phase of stroke, ischemic cell damage makes neurons release various kinds of molecules, such as damage‐associated molecular pattern, which, in turn, act as Toll‐like receptor 4 (TLR4) ligands.…”

Efficacy of Lycium barbarum polysaccharide and synergism with paclitaxel/cisplatin in ovarian cancer in mice

“…The detoxifying enzymes, glutathione synthetase and peroxiredoxins activated and released by the Keap1/Nrf2 pathway, protect cells against oxidative stress and maintain homeostasis [13][14][15]. This study demonstrated that LBP upregulated the mRNA and protein levels of Keap1, Nrf2 and HO-1 in liver and kidney in the co-CH group.…”

“…Long-term chemotherapy causes oxidative damage, which reduces the levels and capacities of antioxidant enzymes. The resultant oxidative stress may decrease immunity and enhance adverse chemotherapy reactions [13][14][15]. The Kelch-like ECH-associated protein 1 (Keap1)/nuclear factor-erythroid-2-related factor 2 (Nrf2) system, which is the master regulator of the anti-stress response, induces the activities of various enzymes via Nrf2-mediated transcriptional regulation under stress conditions [16].…”