Antioxidant Activity of Gamma-Irradiated Asparagus cochinchinensis (Asparagi radix) (Lour.) Merr. Extract and Inhibition Effect on Lipid Oxidation of Emulsion-Type Pork Sausage.

The objective of this study was to determine the antioxidant activity of gamma- irradiated Asparagus cochinchinensis (Asparagi radix) (Lour.) Merr. Extract (ARE) and its inhibition effect on food lipid oxidation using emulsion-type pork sausage as a model. ARE was prepared from dried Asparagi radix root and ARE solution (1.0 g/mL) was gamma-irradiated with designated doses at 5, 10, and 20 kGy. Antioxidant activity of ARE solution was determined by measuring 1,1-diphenyl-e-picrylhydrazyl (DPPH) and 2,2'-azino-bis (3-ethylbenzothiazoline-9-sulphonic acid) (ABTS) radicals. Activities of DPPH and ABTS radicals were decreased, whereas total phenolic contents increased after gamma irradiation with a dose dependence. Addition of gamma-irradiated ARE dose-dependently retarded lipid oxidation of emulsion-type pork sausage during storage at 4℃. These results indicated that gamma-irradiated ARE might have antioxidant activity more than non-irradiated ARE due to increase of the content of polyphenolic compounds by ionizing radiation.

EP2 Receptor Signaling Regulates Microglia Death.

The timely resolution of inflammation prevents continued tissue damage after an initial insult. In the brain, the death of activated microglia by apoptosis has been proposed as one mechanism to resolve brain inflammation. How microglial death is regulated after activation is still unclear. We reported that exposure to lipopolysaccharide (LPS) and interleukin (IL)-13 together initially activates and then kills rat microglia in culture by a mechanism dependent on cyclooxygenase-2 (COX-2). We show here that activation of the E prostanoid receptor 2 (EP2, PTGER2) for prostaglandin E2 mediates microglial death induced by LPS/IL-13, and that EP2 activation by agonist alone kills microglia. Both EP2 antagonists and reactive oxygen scavengers block microglial death induced by either LPS/IL-13 or EP2 activation. By contrast, the homeostatic induction of heme oxygenase 1 (Hmox1) by LPS/IL-13 or EP2 activation protects microglia. Both the Hmox1 inducer cobalt protoporphyrin and a compound that releases the Hmox1 product carbon monoxide (CO) attenuated microglial death produced by LPS/IL-13. Whereas CO reduced COX-2 protein expression, EP2 activation increased Hmox1 and COX-2 expression at both the mRNA and protein level. Interestingly, caspase-1 inhibition prevented microglial death induced by either LPS/IL-13 or low (but not high) concentrations of butaprost, suggestive of a predominantly pyroptotic mode of death. Butaprost also caused the expression of activated caspase-3 in microglia, pointing to apoptosis. These results indicate that EP2 activation, which initially promotes microglial activation, later causes delayed death of activated microglia, potentially contributing to the resolution phase of neuroinflammation.

Therapeutic window for cyclooxygenase-2 related anti-inflammatory therapy after status epilepticus.

As a prominent inflammatory effector of cyclooxygenase-2 (COX-2), prostaglandin E2 (PGE2) mediates brain inflammation and injury in many chronic central nervous system (CNS) conditions including seizures and epilepsy, largely through its receptor subtype EP2. However, EP2 receptor activation might also be neuroprotective in models of excitotoxicity and ischemia. These seemingly incongruent observations expose the delicacy of immune and inflammatory signaling in the brain; thus the therapeutic window for quelling neuroinflammation might vary with injury type and target molecule. Here, we identify a therapeutic window for EP2 antagonism to reduce delayed mortality and functional morbidity after status epilepticus (SE) in mice. Importantly, treatment must be delayed relative to SE onset to be effective, a finding that could be explained by the time-course of COX-2 induction after SE and compound pharmacokinetics. A large number of inflammatory mediators were upregulated in hippocampus after SE with COX-2 and IL-1ß temporally leading many others. Thus, EP2 antagonism represents a novel anti-inflammatory strategy to treat SE with a tightly-regulated therapeutic window.

Lead optimization studies of cinnamic amide EP2 antagonists.

Prostanoid receptor EP2 can play a proinflammatory role, exacerbating disease pathology in a variety of central nervous system and peripheral diseases. A highly selective EP2 antagonist could be useful as a drug to mitigate the inflammatory consequences of EP2 activation. We recently identified a cinnamic amide class of EP2 antagonists. The lead compound in this class (5d) displays anti-inflammatory and neuroprotective actions. However, this compound exhibited moderate selectivity to EP2 over the DP1 prostanoid receptor (∼10-fold) and low aqueous solubility. We now report compounds that display up to 180-fold selectivity against DP1 and up to 9-fold higher aqueous solubility than our previous lead. The newly developed compounds also display higher selectivity against EP4 and IP receptors and a comparable plasma pharmacokinetics. Thus, these compounds are useful for proof of concept studies in a variety of models where EP2 activation is playing a deleterious role.

Cyclooxygenase-2 in epilepsy.

Epilepsy is one of the more prevalent neurologic disorders in the world, affecting approximately 50 million people of different ages and backgrounds. Epileptic seizures propagating through both lobes of the forebrain can have permanent debilitating effects on a patient's cognitive and somatosensory brain functions. Epilepsy, defined by the sporadic occurrence of spontaneous recurrent seizures (SRS), is often accompanied by inflammation of the brain. Pronounced increases in the expression of key inflammatory mediators (e.g., interleukin -1ß [IL-1ß], tumor necrosis factor alpha [TNFα], cyclooxygenase-2 [COX-2], and C-X-C motif chemokine 10 [CXCL10]) after seizures may cause secondary damage in the brain and increase the likelihood of repetitive seizures. The COX-2 enzyme is induced rapidly during seizures. The increased level of COX-2 in specific areas of the epileptic brain can help to identify regions of seizure-induced brain inflammation. A good deal of effort has been expended to determine whether COX-2 inhibition might be neuroprotective and represent an adjunct therapeutic strategy along with antiepileptic drugs used to treat epilepsy. However, the effectiveness of COX-2 inhibitors on epilepsy animal models appears to depend on the timing of administration. With all of the effort placed on making use of COX-2 inhibitors as therapeutic agents for the treatment of epilepsy, inflammation, and neurodegenerative diseases there has yet to be a selective and potent COX-2 inhibitor that has shown a clear therapeutic outcome with acceptable side effects.

Impaired inflammatory responses in murine Lrrk2-knockdown brain microglia.

LRRK2, a Parkinson's disease associated gene, is highly expressed in microglia in addition to neurons; however, its function in microglia has not been evaluated. Using Lrrk2 knockdown (Lrrk2-KD) murine microglia prepared by lentiviral-mediated transfer of Lrrk2-specific small inhibitory hairpin RNA (shRNA), we found that Lrrk2 deficiency attenuated lipopolysaccharide (LPS)-induced mRNA and/or protein expression of inducible nitric oxide synthase, TNF-α, IL-1ß and IL-6. LPS-induced phosphorylation of p38 mitogen-activated protein kinase and stimulation of NF-κB-responsive luciferase reporter activity was also decreased in Lrrk2-KD cells. Interestingly, the decrease in NF-κB transcriptional activity measured by luciferase assays appeared to reflect increased binding of the inhibitory NF-κB homodimer, p50/p50, to DNA. In LPS-responsive HEK293T cells, overexpression of the human LRRK2 pathologic, kinase-active mutant G2019S increased basal and LPS-induced levels of phosphorylated p38 and JNK, whereas wild-type and other pathologic (R1441C and G2385R) or artificial kinase-dead (D1994A) LRRK2 mutants either enhanced or did not change basal and LPS-induced p38 and JNK phosphorylation levels. However, wild-type LRRK2 and all LRRK2 mutant variants equally enhanced NF-κB transcriptional activity. Taken together, these results suggest that LRRK2 is a positive regulator of inflammation in murine microglia, and LRRK2 mutations may alter the microenvironment of the brain to favor neuroinflammation.

Among γ-secretase substrates Notch1 alone is sufficient to block neurogenesis but does not confer self-renewal properties to neural stem cells.

Notch signaling pathway enhances neural stem cell characters and regulates cell fate decisions during neural development. Interestingly, besides Notch, other γ-secretase substrates such as APP, LRP2, and ErbB4 have also proven to have biological functions in neural development. We designed a unique experimental setting, combining gain-of- (expression of Notch intracellular domain, NICD) and loss-of-function (γ-secretase inhibition) methods, and were able to examine the function of Notch alone by excluding the activity of other γ-secretase substrates. Here, we show that the frequency and size of neurospheres generated from embryonic neural stem cells (NSCs) significantly decreased by 62.7% and 37.2%, respectively, in the presence of γ-secretase inhibitor even when NICD was expressed. Under the condition of differentiation, however, the γ-secretase inhibitor treatment did not influence the promotion of astrogenesis at the expense of neurogenesis by NICD. These results indicate that other γ-secretase substrate(s) along with Notch are important in the maintenance of the stemness of NSCs, but that Notch alone can sufficiently inhibit neurogenesis without the action of the other γ-secretase substrates during differentiation.

Retinoic acid enhances prostaglandin E2 production through increased expression of cyclooxygenase-2 and microsomal prostaglandin E synthase-1 in rat brain microglia.

Retinoic acid (RA) is a well-known antiinflammatory agent. In this study, we show that RA has a dual effect on cyclooxygenase-2 (COX-2) expression in inflammatory activated microglia, the resident brain macrophages. After treatment of microglia with LPS or thrombin, COX-2 expression was induced in two phases, specifically, an initial increase at about 12 hr after stimulation followed by a decrease, and another increase at about 48-72 hr. However, PGE(2) and 15d-PGJ(2) were detected at about 12 hr, and the levels continuously increased thereafter. Interestingly, all-trans retinoic acid (ATRA) suppressed the expression of early-phase COX-2 but augmented late-phase COX-2 and inhibited iNOS in the whole time sequence. ATRA enhanced PGE(2) production but had little effect on 15d-PGJ(2). Moreover, ATRA selectively up-regulated the expression of a PGE(2) synthase, mPGES-1, but had little effect on the PGD(2) synthase, H-PGDS. The results collectively suggest that ATRA modulates microglial responses to inflammatory stimulators, particularly at the late phase, via enhancement of COX-2 expression and PGE(2) production.

Resident microglia die and infiltrated neutrophils and monocytes become major inflammatory cells in lipopolysaccharide-injected brain.

Generally, it has been accepted that microglia play important roles in brain inflammation. However, recently several studies suggested possible infiltration of blood neutrophils and monocytes into the brain. To understand contribution of microglia and blood inflammatory cells to brain inflammation, the behavior of microglia, neutrophils, and monocytes was investigated in LPS (lipopolysaccharide)-injected substantia nigra pars compacta, cortex, and hippocampus of normal and/or leukopenic rats using specific markers of neutrophils (myeloperoxidase, MPO), and microglia and monocytes (ionized calcium binding adaptor molecule-1, Iba-1), as well as a general marker for these inflammatory cells (CD11b). CD11b-immunopositive (CD11b(+)) cells and Iba-1(+) cells displayed similar behavior in intact and LPS-injected brain at 6 h after the injection. Interestingly, however, CD11b(+) cells and Iba-1(+) cells displayed significantly different behavior at 12 h: Iba-1(+) cells disappeared while CD11b(+) cells became round in shape. We found that CD11b/Iba-1-double positive (CD11b(+)/Iba-1(+)) ramified microglia died within 6 h after LPS injection. The round CD11b(+) cells detected at 12 h were MPO(+). These CD11b(+)/MPO(+) cells were not found in leukopenic rats, suggestive of neutrophil infiltration. MPO(+) neutrophils expressed inducible nitric oxide synthase, interleukin-1beta, cyclooxygenase-2, and monocyte chemoattractant protein-1, but died within 18 h. CD11b(+) cells detected at 24 h appeared to be infiltrated monocytes, since these cells were once labeled with Iba-1 and were not found in leukopenic rats. Furthermore, transplanted monocytes were detectable in LPS-injected brain. These results suggest that at least a part of neutrophils and monocytes could have been misinterpreted as activated microglia in inflamed brain.

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.

Interleukin-13 enhances cyclooxygenase-2 expression in activated rat brain microglia: implications for death of activated microglia.

Brain inflammation has recently attracted widespread interest because it is a risk factor for the onset and progression of brain diseases. In this study, we report that cyclooxygenase-2 (COX-2) plays a key role in the resolution of brain inflammation by inducing the death of microglia. We previously reported that IL-13, an anti-inflammatory cytokine, induced the death of activated microglia. These results revealed that IL-13 significantly enhanced COX-2 expression and production of PGE(2) and 15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)) in LPS-treated microglia. Two other anti-inflammatory cytokines, IL-10 and TGF-beta, neither induced microglial death nor enhanced COX-2 expression or PGE(2) or 15d-PGJ(2) production. Therefore, we hypothesized that the effect of IL-13 on COX-2 expression may be linked to death of activated microglia. We found that COX-2 inhibitors (celecoxib and NS398) suppressed the death of microglia induced by a combination of LPS and IL-13 and that exogenous addition of PGE(2) and 15d-PGJ(2) induced microglial death. Agonists of EP2 (butaprost) and peroxisome proliferator-activated receptor gamma (ciglitazone) mimicked the effect of PGE(2) and 15d-PGJ(2), and an EP2 antagonist (AH6809) and a peroxisome proliferator-activated receptor gamma antagonist (GW9662) suppressed microglial death induced by LPS in combination with IL-13. In addition, IL-13 potentiated LPS-induced activation of JNK, and the JNK inhibitor SP600125 suppressed the enhancement of COX-2 expression and attenuated microglial death. Taken together, these results suggest that IL-13 enhanced COX-2 expression in LPS-treated microglia through the enhancement of JNK activation. Furthermore, COX-2 products, PGE(2) and 15d-PGJ(2), caused microglial death, which terminates brain inflammation.

Astrocytes induce hemeoxygenase-1 expression in microglia: a feasible mechanism for preventing excessive brain inflammation.

Microglia are the major inflammatory cells in the brain, in which microglial inflammatory responses are modulated by interactions with other brain cells. Here, we show that astrocytes, the most abundant cells in the brain, can secrete one or more factors capable of modulating microglial activation by regulating the microglial levels of reactive oxygen species (ROS). Treatment of microglia with astrocyte culture-conditioned media (ACM) increased the expression level and activity of hemeoxygenase-1 (HO-1). ACM also induced nuclear translocation of the nuclear factor E2-related factor 2 transcription factor, increased the binding activity of the antioxidant response element (ARE), and enhanced HO-1 promoter activity in an ARE-dependent manner. Furthermore, treatment with ACM suppressed interferon-gamma (IFN-gamma)-induced ROS production, leading to reduced inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) release. In agreement with these results, mimickers of HO-1 products, such as bilirubin, ferrous iron, and a carbon monoxide-releasing molecule, reduced IFN-gamma-induced iNOS expression and/or NO release. Finally, we found that the active component(s) in ACM was heat labile and smaller than 3 kDa. Together, these results suggest that astrocytes could cooperate with microglia to prevent excessive inflammatory responses in the brain by regulating microglial expression of HO-1 and production of ROS.

Anti-inflammatory roles of retinoic acid in rat brain astrocytes: Suppression of interferon-gamma-induced JAK/STAT phosphorylation.

The anti-inflammatory effect of retinoic acid (RA) has been investigated for several decades. However, the underlying mechanisms responsible for this effect are largely unknown. In this study, we demonstrate that 9-cis-RA (cRA) and all-trans-RA (tRA) inhibit interferon-gamma (IFN-gamma)-induced inflammatory responses in astrocytes. In primary cultured rat brain astrocytes and C6 astroglioma cells, both cRA and tRA decreased IFN-gamma-induced expression of interferon regulatory factor-1. Both RA isoforms also reduced IFN-gamma-induced activation of signal transducers and activators of transcription (STAT)1, STAT3, Janus kinase (JAK)1, and JAK2. This inhibitory effect was significant when cells were pre-treated with RA prior to IFN-gamma. Furthermore, the effect of pre-treated RA was abolished in the presence of cycloheximide, indicating a requirement for de novo protein synthesis. Suppressors of cytokine signaling (SOCS), which are negative regulators of the JAK/STAT pathway, may be candidate mediators of the anti-inflammatory function of RA. Both cRA and tRA induced SOCS3 mRNA expression. These results suggest that RA induces an anti-inflammatory effect by suppressing the activation of the JAK/STAT pathway in IFN-gamma-treated astrocytes. SOCS3 may be at least one of the mechanisms that mediate the anti-inflammatory roles of RA.

Protein kinase A mediates microglial activation induced by plasminogen and gangliosides.

In the injured brain, microglia is known to be activated and produce proinflammatory mediators such as interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha) and inducible nitric oxide synthase (iNOS). We investigated the role of protein kinase A (PKA) in microglial activation by both plasminogen and gangliosides in rat primary microglia and in the BV2 immortalized murine microglial cell line. Both plasminogen and gangliosides induced IL-1beta, TNF-alpha and iNOS mRNA expression, and that this expression was inhibited by the addition of the PKA inhibitors, KT5720 and H89. Both plasminogen and gangliosides activated PKA and increased the DNA binding activity of the cAMP response element- binding protein (CREB). Furthermore, KT5720 and H89 reduced the DNA binding activities of CREB and NF-kappaB in plasminogen-treated cells. These results suggest that PKA plays an important role in plasminogen and gangliosides- induced microglial activation.

Thrombin induces expression of cytokine-induced SH2 protein (CIS) in rat brain astrocytes: involvement of phospholipase A2, cyclooxygenase, and lipoxygenase.

Previously we have reported that thrombin induces inflammatory mediators in brain glial cells (Ryu et al. 2000. J Biol Chem 275:29955). In the present study, we found that thrombin induced a negative regulator of a cytokine signaling molecule, cytokine-induced SH2 protein (CIS), in rat brain astrocytes. In response to thrombin, CIS expression was increased at both the mRNA and protein levels. Although STAT5 is known to regulate CIS expression, thrombin did not activate STAT5, and inhibitors of JAK2 (AG490) and JAK3 (WHI-P97 and WHI-P154) had little effect on thrombin-induced CIS expression. In contrast, cytosolic phospholipase A(2) (cPLA(2)), cyclooxygenase (COX), and lipoxygenase (LO) play a role in CIS expression, since inhibitors of cPLA(2), cyclooxygenase (COX), and LO significantly reduced CIS expression. Reactive oxygen species (ROS) scavengers (N-acetyl-cysteine [NAC] and trolox) reduced thrombin-induced CIS expression, and inhibitors of COX and LO reduced ROS produced by thrombin. Furthermore, prostaglandin E(2) (PGE(2)) and leukotriene B(4) (LTB(4)), products of COX and LO, respectively, potentiated thrombin-induced CIS expression, indicating that ROS, and PGE(2) and LTB(4) generated by COX and LO, mediate CIS expression. Since interferon-gamma (IFN-gamma)-induced GAS-luciferase activity and tyrosine phosphorylation of STAT1 and STAT3 were lower in CIS-transfected cells compared to control vector-transfected cells, CIS could have anti-inflammatory activity. These data suggest that thrombin-stimulation of ROS and prostaglandin and leukotriene production via the cPLA(2), COX and LO pathways results in CIS expression. More importantly, CIS expression may be a negative feedback mechanism that prevents prolonged inflammatory responses.

Gangliosides activate microglia via protein kinase C and NADPH oxidase.

Microglia, the major immune effector cells in the central nervous system, are activated when the brain suffers injury. A number of studies indicate that gangliosides activate microglia. However, the signaling mechanisms involved in microglial activation are not yet to be elucidated. Our results show that gangliosides induce the expression of interleukin (IL)-1beta, tumor necrosis factor-alpha (TNF-alpha), and inducible nitric oxide synthase (iNOS) in rat brain microglia and BV2 murine microglia via protein kinase C (PKC) and NADPH oxidase. Expression of IL-1beta, TNF-alpha, and iNOS in ganglioside-treated cells was significantly reduced in the presence of inhibitors of PKC (GF109203X, Go6976, Ro31-8220, and rottlerin) and NADPH oxidase (diphenyleneiodonium chloride [DPI]). In response to gangliosides, PKC-alpha, betaII, and delta and NADPH oxidase p67(phox) translocated from the cytosol to the membrane. ROS generation was also activated within 5 min of ganglioside treatment. Ganglioside-induced ROS generation was blocked by PKC inhibitors. Furthermore, ganglioside-induced activation of NF-kappaB, an essential transcription factor that mediates the expression of IL-1beta, TNF-alpha, and iNOS, was reduced in the presence of GF109203X and DPI. Our results collectively suggest that gangliosides activate microglia via PKC and NADPH oxidase, which regulate activation of NF-kappaB.

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

Microglia (brain macrophages) are activated upon brain damage. In this study, we demonstrated that thrombin, a pro-inflammatory stimulator of microglia, induced expression of suppressors of cytokine signaling (SOCS) in microglia. RT-PCR analysis and Northern blot analysis showed that thrombin induced SOCS3 mRNA expression. Further experiments indicated SOCS3 expression was not affected by cycloheximide, indicating thrombin directly stimulated SOCS3 transcript expression without de novo protein synthesis. We investigated whether PKCdelta played a role in thrombin-stimulated SOCS3 expression. We found that thrombin activated PKCdelta, and the specific inhibitor of PKCdelta, rottlerin, significantly suppressed thrombin-stimulated SOCS3 expression. In thrombin-pretreated cells, microglial activation-induced by another inflammatory stimulator, lipopolysaccharide, was attenuated compared to that in non-pretreated cells. These results suggest thrombin induce not only proinflammatory mediators but also negative feedback regulators of inflammation, SOCS, which prevent prolonged inflammatory reactions in microglia.

Microglia expressing interleukin-13 undergo cell death and contribute to neuronal survival in vivo.

How to minimize brain inflammation is pathophysiologically important, since inflammation induced by microglial activation can exacerbate brain damage. In the present report, we show that injection of lipopolysaccharide (LPS) into the rat cortex led to increased levels of interleukin-13 (IL-13) and to IL-13 immunoreactivity, followed by the substantial loss of microglia at 3 days post-LPS. IL-13 levels in LPS-injected cortex reached a peak at 12 h post-injection, remained elevated at 24 h, and returned to basal levels at day 4. In parallel, IL-13 immunoreactivity was detected as early as 12 h post-LPS and maintained up to 24 h; it disappeared at 4 days. Surprisingly, IL-13 immunoreactivity was detected exclusively in microglia, but not in neurons or astrocytes. Following treatment with LPS in vitro, IL-13 expression was also induced in microglia in the presence of neurons, but not in the presence of astrocytes or in cultured pure microglia alone. In experiments designed to determine the involvement of IL-13 in microglia cell death, IL-13-neutralizing antibodies significantly increased survival of activated microglia at 3 days post-LPS. Consistent with these results, the expression of inducible nitric oxide synthase (iNOS) and tumor necrosis factor-alpha (TNF-alpha) was sustained in activated microglia and neuronal cell death was consequently increased. Taken together, the present study is the first to demonstrate the endogenous expression of IL-13 in LPS-activated microglia in vivo, and to demonstrate that neurons may be required for IL-13 expression in microglia. Our data strongly suggest that IL-13 may control brain inflammation by inducing the death of activated microglia in vivo, resulting in an enhancement of neuronal survival.

Wortmannin enhances lipopolysaccharide-induced inducible nitric oxide synthase expression in microglia in the presence of astrocytes in rats.

Microglia, the primary inflammatory cells in the brain, are activated upon brain injury. Activated microglia produce nitric oxide (NO), a major toxin to neuronal cells. It has been reported that astrocytes inhibit microglial activation. In this study, we found that wortmannin, a natural inhibitor of phosphatidylinositol 3-kinase, significantly increased lipopolysaccharide (LPS)-induced NO release and inducible nitric oxide synthase (iNOS) expression in microglia in the presence but not in the absence of astrocytes. In response to LPS even in the presence of wortmannin, iNOS immunoreactivity was detected in microglia but not in astrocytes. These results suggest that astrocytes could regulate microglia-mediated brain inflammation by inhibiting microglial NO release/iNOS expression via a wortmannin-sensitive mechanism.

Interleukin-13 and -4 induce death of activated microglia.

When the brain suffers injury, microglia migrate to the damaged sites and become activated. These activated microglia are not detected several days later and the mechanisms underlying their disappearance are not well characterized. In this study, we demonstrate that interleukin (IL)-13, an anti-inflammatory cytokine, selectively induces cell death of activated microglia in vitro. Cell death was detected 4 days after the coaddition of IL-13 with any one of the microglial activators, lipopolysaccharide (LPS), ganglioside, or thrombin. This cell death occurred in a time-dependent manner. LPS, ganglioside, thrombin, or IL-13 alone did not induce cell death. Among anti-inflammatory cytokines, IL-4 mimicked the effect of IL-13, while TGF-beta did not. Cells treated with IL-13 plus LPS, or IL-13 plus ganglioside, showed the characteristics of apoptosis when analyzed by electron microscopy and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling staining. Electron micrographs also showed microglia engulfing neighboring dead cells. We propose that IL-13 and IL-4 induce death of activated microglia, and that this process is important for prevention of chronic inflammation that can cause tissue damage.