Dennis Y. Chuang

Magnolia polyphenols attenuate oxidative and inflammatory responses in neurons and microglial cells

BackgroundThe bark of magnolia has been used in Oriental medicine to treat a variety of remedies, including some neurological disorders. Magnolol (Mag) and honokiol (Hon) are isomers of polyphenolic compounds from the bark of Magnolia officinalis, and have been identified as major active components exhibiting anti-oxidative, anti-inflammatory, and neuroprotective effects. In this study, we investigate the ability of these isomers to suppress oxidative stress in neurons stimulated by the ionotropic glutamate receptor agonist N-methyl-D-aspartate (NMDA) and oxidative and inflammatory responses in microglial cells activated by interferon-γ (IFNγ) and lipopolysaccharide (LPS). We also attempt to elucidate the mechanism and signaling pathways involved in cytokine-induced production of reactive oxygen species (ROS) in microglial cells.MethodsDihydroethidium (DHE) was used to assay superoxide production in neurons, while CM-H2DCF-DA was used to test for ROS production in murine (BV-2) and rat (HAPI) immortalized microglial cells. NADPH oxidase inhibitors (for example, diphenyleneiodonium (DPI), AEBSF, and apocynin) and immunocytochemistry targeting p47phox and gp91phox were used to assess the involvement of NADPH oxidase. Western blotting was used to assess iNOS and ERK1/2 expression, and the Griess reaction protocol was employed to determine nitric oxide (NO) concentration.ResultsExposure of Hon and Mag (1–10 μM) to neurons for 24 h did not alter neuronal viability, but both compounds (10 μM) inhibited NMDA-stimulated superoxide production, a pathway known to involve NADPH oxidase. In microglial cells, Hon and Mag inhibited IFNγ±LPS-induced iNOS expression, NO, and ROS production. Studies with inhibitors and immunocytochemical assay further demonstrated the important role of IFNγ activating the NADPH oxidase through the p-ERK-dependent pathway. Hon and, to a lesser extent, Mag inhibited IFNγ-induced p-ERK1/2 and its downstream pathway for ROS and NO production.ConclusionThis study highlights the important role of NADPH oxidase in mediating oxidative stress in neurons and microglial cells and has unveiled the role of IFNγ in stimulating the MAPK/ERK1/2 signaling pathway for activation of NADPH oxidase in microglial cells. Hon and Mag offer anti-oxidative or anti-inflammatory effects, at least in part, through suppressing IFNγ-induced p-ERK1/2 and its downstream pathway.

Quercetin Attenuates Inflammatory Responses in BV-2 Microglial Cells: Role of MAPKs on the Nrf2 Pathway and Induction of Heme Oxygenase-1

A large group of flavonoids found in fruits and vegetables have been suggested to elicit health benefits due mainly to their anti-oxidative and anti-inflammatory properties. Recent studies with immune cells have demonstrated inhibition of these inflammatory responses through down-regulation of the pro-inflammatory pathway involving NF-κB and up-regulation of the anti-oxidative pathway involving Nrf2. In the present study, the murine BV-2 microglial cells were used to compare anti-inflammatory activity of quercetin and cyanidin, two flavonoids differing by their alpha, beta keto carbonyl group. Quercetin was 10 folds more potent than cyanidin in inhibition of lipopolysaccharide (LPS)-induced NO production as well as stimulation of Nrf2-induced heme-oxygenase-1 (HO-1) protein expression. In addition, quercetin demonstrated enhanced ability to stimulate HO-1 protein expression when cells were treated with LPS. In an attempt to unveil mechanism(s) for quercetin to enhance Nrf2/HO-1 activity under endotoxic stress, results pointed to an increase in phospho-p38MAPK expression upon addition of quercetin to LPS. In addition, pharmacological inhibitors for phospho-p38MAPK and MEK1/2 for ERK1/2 further showed that these MAPKs target different sites of the Nrf2 pathway that regulates HO-1 expression. However, inhibition of LPS-induced NO by quercetin was not fully reversed by TinPPIX, a specific inhibitor for HO-1 activity. Taken together, results suggest an important role of quercetin to regulate inflammatory responses in microglial cells and its ability to upregulate HO-1 against endotoxic stress through involvement of MAPKs.

Role of cytosolic phospholipase A2 in oxidative and inflammatory signaling pathways in different cell types in the central nervous system.

Phospholipases A2 (PLA2s) are important enzymes for the metabolism of fatty acids in membrane phospholipids. Among the three major classes of PLA2s in the mammalian system, the group IV calcium-dependent cytosolic PLA2 alpha (cPLA2α) has received the most attention because it is widely expressed in nearly all mammalian cells and its active participation in cell metabolism. Besides Ca2+ binding to its C2 domain, this enzyme can undergo a number of cell-specific post-translational modifications, including phosphorylation by protein kinases, S-nitrosylation through interaction with nitric oxide (NO), as well as interaction with other proteins and lipid molecules. Hydrolysis of phospholipids by cPLA2 yields two important lipid mediators, arachidonic acid (AA) and lysophospholipids. While AA is known to serve as a substrate for cyclooxygenases and lipoxygenases, which are enzymes for the synthesis of eicosanoids and leukotrienes, lysophospholipids are known to possess detergent-like properties capable of altering microdomains of cell membranes. An important feature of cPLA2 is its link to cell surface receptors that stimulate signaling pathways associated with activation of protein kinases and production of reactive oxygen species (ROS). In the central nervous system (CNS), cPLA2 activation has been implicated in neuronal excitation, synaptic secretion, apoptosis, cell-cell interaction, cognitive and behavioral function, oxidative-nitrosative stress, and inflammatory responses that underline the pathogenesis of a number of neurodegenerative diseases. However, the types of extracellular agonists that target intracellular signaling pathways leading to cPLA2 activation among different cell types and under different physiological and pathological conditions have not been investigated in detail. In this review, special emphasis is given to metabolic events linking cPLA2 to activation in neurons, astrocytes, microglial cells, and cerebrovascular cells. Understanding the molecular mechanism(s) for regulation of this enzyme is deemed important in the development of new therapeutic targets for the treatment and prevention of neurodegenerative diseases.

Cytosolic phospholipase A2 plays a crucial role in ROS/NO signaling during microglial activation through the lipoxygenase pathway

BackgroundOxidative stress and inflammation are important factors contributing to the pathophysiology of numerous neurological disorders, including Alzheimer’s disease, Parkinson’s disease, acute stroke, and infections of the brain. There is well-established evidence that proinflammatory cytokines and glutamate, as well as reactive oxygen species (ROS) and nitric oxide (NO), are produced upon microglia activation, and these are important factors contributing to inflammatory responses and cytotoxic damage to surrounding neurons and neighboring cells. Microglial cells express relatively high levels of cytosolic phospholipase A2 (cPLA2), an enzyme known to regulate membrane phospholipid homeostasis and release of arachidonic acid (AA) for synthesis of eicosanoids. The goal for this study is to elucidate the role of cPLA2IV in mediating the oxidative and inflammatory responses in microglial cells.MethodsExperiments involved primary microglia cells isolated from transgenic mice deficient in cPLA2α or iPLA2β, as well as murine immortalized BV-2 microglial cells. Inhibitors of cPLA2/iPLA2/cyclooxygenase (COX)/lipoxygenase (LOX) were used in BV-2 microglial cell line. siRNA transfection was employed to knockdown cPLA2 expression in BV-2 cells. Griess reaction protocol was used to determine NO concentration, and CM-H2DCF-DA was used to detect ROS production in primary microglia and BV-2 cells. WST-1 assay was used to assess cell viability. Western blotting was used to assess protein expression levels. Immunocytochemical staining for phalloidin against F-actin was used to demonstrate cell morphology.ResultsIn both primary and BV-2 microglial cells, stimulation with lipopolysaccharide (LPS) or interferon gamma (IFNγ) resulted in a time-dependent increase in phosphorylation of cPLA2 together with ERK1/2. In BV-2 cells, LPS- and IFNγ-induced ROS and NO production was inhibited by arachidonyl trifluoromethyl ketone (AACOCF3) and pyrrophenone as well as RNA interference, but not BEL, suggesting a link between cPLA2, and not iPLA2, on LPS/IFNγ-induced nitrosative and oxidative stress in microglial cells. Primary microglial cells isolated from cPLA2α-deficient mice generated significantly less NO and ROS as compared with the wild-type mice. Microglia isolated from iPLA2β-deficient mice did not show a decrease in LPS-induced NO and ROS production. LPS/IFNγ induced morphological changes in primary microglia, and these changes were mitigated by AACOCF3. Interestingly, despite that LPS and IFNγ induced an increase in phospho-cPLA2 and prostaglandin E2 (PGE2) release, LPS- and IFNγ-induced NO and ROS production were not altered by the COX-1/2 inhibitor but were suppressed by the LOX-12 and LOX-15 inhibitors instead.ConclusionsIn summary, the results in this study demonstrated the role of cPLA2 in microglial activation with metabolic links to oxidative and inflammatory responses, and this was in part regulated by the AA metabolic pathways, namely the LOXs. Further studies with targeted inhibition of cPLA2/LOX in microglia during neuroinflammatory conditions can be valuable to investigate the therapeutic potential in ameliorating neurological disease pathology.

Sutherlandia frutescens ethanol extracts inhibit oxidative stress and inflammatory responses in neurons and microglial cells.

Sutherlandia frutescens (L.) R.Br. (SF) is a medicinal plant indigenous to southern Africa and used in folk and contemporary remedies for stress, chronic diseases, cancer, and HIV/AIDS. While previous studies have focused on physiological effects of SF on cellular and systemic abnormalities associated with these diseases, little is known about its effects in the brain and immune cells in the central nervous system. Results of this study indicate that ethanol extracts of SF (SF-E) suppress NMDA-induced reactive oxygen species (ROS) production in neurons, and LPS- and IFNγ-induced ROS and nitric oxide (NO) production in microglial cells. SF-E’s action on microglial cells appears to be mediated through inhibition of the IFNγ-induced p-ERK1/2 signaling pathway which is central to regulating a number of intracellular metabolic processes including enhancing STAT1α phosphorylation and filopodia formation. The involvement of SF in these pathways suggests the potential for novel therapeutics for stress and prevention, and/or treatment of HIV/AIDS as well as other inflammatory diseases in the brain.

Integrating Cytosolic Phospholipase A2 with Oxidative/Nitrosative Signaling Pathways in Neurons: A Novel Therapeutic Strategy for AD

The pathophysiology of Alzheimers disease (AD) is comprised of complex metabolic abnormalities in different cell types in the brain. To date, there are not yet effective drugs that can completely inhibit the pathophysiological event, and efforts have been devoted to prevent or minimize the progression of this disease. Much attention has focused on studies to understand aberrant functions of the ionotropic glutamate receptors, perturbation of calcium homeostasis, and toxic effects of oligomeric amyloid beta peptides (Aβ) which results in production of reactive oxygen and nitrogen species and signaling pathways, leading to mitochondrial dysfunction and synaptic impairments. Aberrant phospholipase A2 (PLA2) activity has been implicated to play a role in the pathogenesis of many neurodegenerative diseases, including AD. However, mechanisms for their modes of action and their roles in the oxidative and nitrosative signaling pathways have not been firmly established. In this article, we review recent studies providing a metabolic link between cytosolic PLA2 (cPLA2) and neuronal excitation due to stimulation of ionotropic glutamate receptors and toxic Aβ peptides. The requirements for Ca2+ binding together with its posttranslational modifications by protein kinases and possible by the redox-based S-nitrosylation, provide strong support for a dynamic role of cPLA2 in serving multiple functions to neurons and glial cells under abnormal physiological and pathological conditions. Therefore, understanding mechanisms for cPLA2 in the oxidative and nitrosative pathways in neurons will allow the development of novel therapeutic targets to mitigate the detrimental effects of AD.

Docosahexaenoic acid (DHA): An essential nutrient and a nutraceutical for brain health and diseases

Docosahexaenoic acid (DHA), a polyunsaturated fatty acid (PUFA) enriched in phospholipids in the brain and retina, is known to play multi-functional roles in brain health and diseases. While arachidonic acid (AA) is released from membrane phospholipids by cytosolic phospholipase A2 (cPLA2), DHA is linked to action of the Ca2+-independent iPLA2. DHA undergoes enzymatic conversion by 15-lipoxygenase (Alox 15) to form oxylipins including resolvins and neuroprotectins, which are powerful lipid mediators. DHA can also undergo non-enzymatic conversion by reacting with oxygen free radicals (ROS), which cause the production of 4-hydoxyhexenal (4-HHE), an aldehyde derivative which can form adducts with DNA, proteins and lipids. In studies with both animal models and humans, there is evidence that inadequate intake of maternal n-3 PUFA may lead to aberrant development and function of the central nervous system (CNS). What is less certain is whether consumption of n-3 PUFA is important in maintaining brain health throughout ones life span. Evidence mostly from non-human studies suggests that DHA intake above normal nutritional requirements might modify the risk/course of a number of diseases of the brain. This concept has fueled much of the present interest in DHA research, in particular, in attempts to delineate mechanisms whereby DHA may serve as a nutraceutical and confer neuroprotective effects. Current studies have revealed ability for the oxylipins to regulation of cell redox homeostasis through the Nuclear factor (erythroid-derived 2)-like 2/Antioxidant response element (Nrf2/ARE) anti-oxidant pathway, and impact signaling pathways associated with neurotransmitters, and modulation of neuronal functions involving brain-derived neurotropic factor (BDNF). This review is aimed at describing recent studies elaborating these mechanisms with special regard to aging and Alzheimers disease, autism spectrum disorder, schizophrenia, traumatic brain injury, and stroke.

Inhibition of microglial activation by elderberry extracts and its phenolic components

AIMS Elderberry (Sambucus spp.) is one of the oldest medicinal plants noted for its cardiovascular, anti-inflammatory, and immune-stimulatory properties. In this study, we investigated the anti-inflammatory and anti-oxidant effects of the American elderberry (Sambucus nigra subsp. canadensis) pomace as well as some of the anthocyanins (cyanidin chloride and cyanidin 3-O-glucoside) and flavonols (quercetin and rutin) in bv-2 mouse microglial cells. MAIN METHODS The bv-2 cells were pretreated with elderberry pomace (extracted with ethanol or ethyl acetate) or its anthocyanins and flavonols and stimulated by either lipopolysaccharide (LPS) or interferon-γ (IFNγ). Reactive oxygen species (ROS) and nitric oxide (NO) production (indicating oxidative stress and inflammatory response) were measured using the ROS detection reagent DCF-DA and the Griess reaction, respectively. KEY FINDINGS Analysis of total monomeric anthocyanin (as cyanidin 3-O-glucoside equivalents) indicated five-fold higher amount in the freeze-dried ethanol extract as compared to that of the oven-dried extract; anthocyanin was not detected in the ethyl acetate extracts. Elderberry ethanol extracts (freeze-dried or oven-dried) showed higher anti-oxidant activities and better ability to inhibit LPS or IFNγ-induced NO production as compared with the ethyl acetate extracts. The phenolic compounds strongly inhibited LPS or IFNγ-induced ROS production, but except for quercetin, they were relatively poor in inhibiting NO production. SIGNIFICANCE These results demonstrated differences in anti-oxidative and anti-inflammatory effects of elderberry extracts depending on solvents used. Results further identified quercetin as the most active component in suppressing oxidative stress and inflammatory responses on microglial cells.

Dietary Sutherlandia and elderberry mitigate cerebral ischemia-induced neuronal damage and attenuate p47phox and phospho-ERK1/2 expression in microglial cells.

Sutherlandia (Sutherlandia frutescens) and elderberry (Sambucus spp.) are used to promote health and for treatment of a number of ailments. Although studies with cultured cells have demonstrated antioxidative and anti-inflammatory properties of these botanicals, little is known about their ability to mitigate brain injury. In this study, C57BL/6 J male mice were fed AIN93G diets without or with Sutherlandia or American elderberry for 2 months prior to a 30-min global cerebral ischemia induced by occlusion of the bilateral common carotid arteries (BCCAs), followed by reperfusion for 3 days. Accelerating rotarod assessment at 24 h after BCCA occlusion showed amelioration of sensorimotor impairment in the mice fed the supplemented diets as compared with the ischemic mice fed the control diet. Quantitative digital pathology assessment of brain slides stained with cresyl violet at 3 days after ischemia/reperfusion (I/R) revealed significant reduction in neuronal cell death in both dietary groups. Immunohistochemical staining for ionized calcium-binding adapter molecule-1 demonstrated pronounced activation of microglia in the hippocampus and striatum in the ischemic brains 3 days after I/R, and microglial activation was significantly reduced in animals fed supplemented diets. Mitigation of microglial activation by the supplements was further supported by the decrease in expression of p47phox, a cytosolic subunit of NADPH oxidase, and phospho-ERK1/2, a mitogen-activated protein kinase known to mediate a number of cytoplasmic processes including oxidative stress and neuroinflammatory responses. These results demonstrate neuroprotective effect of Sutherlandia and American elderberry botanicals against oxidative and inflammatory responses to cerebral I/R.

Botanical Polyphenols Mitigate Microglial Activation and Microglia-Induced Neurotoxicity: Role of Cytosolic Phospholipase A2

Microglia play a significant role in the generation and propagation of oxidative/nitrosative stress, and are the basis of neuroinflammatory responses in the central nervous system. Upon stimulation by endotoxins such as lipopolysaccharides (LPS), these cells release pro-inflammatory factors which can exert harmful effects on surrounding neurons, leading to secondary neuronal damage and cell death. Our previous studies demonstrated the effects of botanical polyphenols to mitigate inflammatory responses induced by LPS, and highlighted an important role for cytosolic phospholipase A2 (cPLA2) upstream of the pro-inflammatory pathways (Chuang et al. in J Neuroinflammation 12(1):199, 2015. doi:10.1186/s12974-015-0419-0). In this study, we investigate the action of botanical compounds and assess whether suppression of cPLA2 in microglia is involved in the neurotoxic effects on neurons. Differentiated SH-SY5Y neuroblastoma cells were used to test the neurotoxicity of conditioned medium from stimulated microglial cells, and WST-1 assay was used to assess for the cell viability of SH-SY5Y cells. Botanicals such as quercetin and honokiol (but not cyanidin-3-O-glucoside, 3CG) were effective in inhibiting LPS-induced nitric oxide (NO) production and phosphorylation of cPLA2. Conditioned medium from BV-2 cells stimulated with LPS or IFNγ caused neurotoxicity to SH-SY5Y cells. Decrease in cell viability could be ameliorated by pharmacological inhibitors for cPLA2 as well as by down-regulating cPLA2 with siRNA. Botanicals effective in inhibition of LPS-induced NO and cPLA2 phosphorylation were also effective in ameliorating microglial-induced neurotoxicity. Results demonstrated cytotoxic factors from activated microglial cells to cause damaging effects to neurons and potential use of botanical polyphenols to ameliorate the neurotoxic effects.