Hyung Hwan Baik

Activation of adenosine A3 receptor suppresses lipopolysaccharide-induced TNF-α production through inhibition of PI 3-kinase/Akt and NF-κB activation in murine BV2 microglial cells

Adenosine is an endogenous nucleoside that regulates many processes, including inflammatory responses, through activation of its receptors. Adenosine receptors have been reported to be expressed in microglia, which are major immune cells of brain, yet little is known about the role of adenosine receptors in microglial cytokine production. Thus, we investigated the effect of adenosine and adenosine A3 receptor ligands on LPS-induced tumor necrosis factor (TNF-alpha) production and its molecular mechanism in mouse BV2 microglial cells. Adenosine and Cl-IB-MECA, a specific adenosine A3 receptor agonist, suppressed LPS-induced TNF-alpha protein and mRNA levels. Moreover, MRS1523, a selective A3 receptor antagonist, blocked suppressive effects of both adenosine and Cl-IB-MECA on TNF-alpha. We further examined the effect of adenosine on signaling molecules, such as PI 3-kinase, Akt, p38, ERK1/2, and NF-kappaB, which are involved in the regulation of inflammatory responses. Adenosine inhibited LPS-induced phosphatidylinositol (PI) 3-kinase activation and Akt phosphorylation, whereas it had no effect on the phosphorylation of p38 and ERK1/2. We also found that adenosine as well as Cl-IB-MECA inhibited LPS-induced NF-kappaB DNA binding and luciferase reporter activity. Taken together, these results suggest that adenosine A3 receptor activation suppresses TNF-alpha production by inhibiting PI 3-kinase/Akt and NF-kappaB activation in LPS-treated BV2 microglial cells.

5-Aminoimidazole-4-carboxamide-ribonucleoside enhances oxidative stress-induced apoptosis through activation of nuclear factor-κB in mouse Neuro 2a neuroblastoma cells

AMP-activated protein kinase (AMPK) was recently suggested to have a pro-apoptotic effect although its primary function is believed to mediate cellular adaptation to metabolic stresses. Here, we investigated the effect of the AMPK activator 5-aminoimidazole-4-carboxamide-ribonucleoside (AICAR) on oxidative stress-induced apoptosis using mouse Neuro 2a neuroblastoma cells. H2O2-induced apoptosis was increased by AMPK activation, either with AICAR pretreatment or with overexpression of active AMPK. AICAR also induced nuclear factor-kappaB (NF-kappaB) activation along with activation of p38 mitogen-activated protein kinase and c-Jun N-terminal kinase. Correlation between NF-kappaB activation and the AICAR-enhanced apoptotic cell death was observed. In addition, NF-kappaB inhibitor SN50 prevented the augmented cell death by AICAR. Thus, our data suggest that NF-kappaB mediates the pro-apoptotic effect of AICAR.

Melatonin protects nigral dopaminergic neurons from 1-methyl-4-phenylpyridinium (MPP+) neurotoxicity in rats

In the present study, the in vivo neuroprotective effects of melatonin, as an antioxidant, were assessed in Sprague-Dawley rats with a unilateral lesion of substantia nigra (SN) caused by a stereotaxic injection of neurotoxin, 1-methyl-4-phenylpyridinium (MPP+). When expressed as a percentage ratio of lesioned to intact side, increased lipid peroxidation product (malondialdehyde, MDA, 117% of control) and decreased tyrosine hydroxylase (TH) enzyme activity (60% of control) in SN were observed 4 h after MPP+ lesion. In contrast, however, melatonin treatment prevented MPP+ neurotoxicity by the almost complete recovery of MDA (99% of control) and TH levels (96% of control), indicating the potent antioxidative effects of melatonin. In addition, further reduction of TH enzyme activity (52% of control) was seen 1 week after MPP+ infusion. Continuous (twice a day for 5 days), not acute (4 h) treatment with melatonin produced the partial, but not statistically significant, recovery of TH enzyme activity (71% of control), when sacrificed 1 week after MPP+ lesion. Taken together, the present results support the hypothesis that melatonin may provide the useful therapeutic strategies for the treatment of oxidative stress-induced neurodegenerative disease such as Parkinsons disease (PD).

Melatonin stimulates glucose transport via insulin receptor substrate-1/phosphatidylinositol 3-kinase pathway in C2C12 murine skeletal muscle cells

Abstract:  The prevalence of diabetes has exponentially increased in recent decades due to environmental factors such as nocturnal lifestyle and aging, both of which influence the amount of melatonin produced in the pineal gland. The present study investigated the effect of melatonin on signaling pathways of glucose transport in C2C12 mouse skeletal muscle cells. Intriguingly, treatment of C2C12 cells with melatonin (1 nm) stimulated glucose uptake twofold increase. Melatonin‐stimulated glucose transport was inhibited with co‐treatment with the melatonin receptor antagonist luzindole. Furthermore, treatment of stably over‐expressed melatonin receptor type 2B containing C2C12 myotubes with melatonin amplified glucose transport c. 13‐fold. Melatonin also increased the phosphorylation level of insulin receptor substrate‐1 (IRS‐1) and the activity of phosphoinositide 3‐kinase (PI‐3‐kinase). However, 3′,5′‐cyclic adenosine monophosphate‐activated protein kinase (AMPK), another important glucose transport stimulatory mediator via an insulin‐independent pathway, was not influenced by melatonin treatment. Activity of p38 mitogen‐activated protein kinase (MAPK), a downstream mediator of AMPK, was also not changed by melatonin. In addition, melatonin increased the expression level of forkhead box A2, which was recently discovered to regulate fatty acid oxidation and to be inhibited by insulin. In summary, melatonin stimulates glucose transport to skeletal muscle cells via IRS‐1/PI‐3‐kinase pathway, which implies, at the molecular level, its role in glucose homeostasis and possibly in diabetes. Additionally, exposure to light at night and aging, both of which lower endogenous melatonin levels may contribute to the incidence and/or development of diabetes.

Tetrandrine cytotoxicity and its dual effect on oxidative stress-induced apoptosis through modulating cellular redox states in Neuro 2a mouse neuroblastoma cells

Tetrandrine (TET), a plant alkaloid, is known primarily as a non-selective Ca(2+) channel blocker. On the contrary to the cytoprotective effect on ischemia/reperfusion injury, TET has also been reported to cause cytotoxicity. In this study, we wished to understand the apparently disparate effects of this potential drug and thus investigated molecular mechanisms on proliferation and apoptosis and its effect on oxidative stress-induced apoptosis in Neuro 2a mouse neuroblastoma cells. We showed that TET, at high concentrations, induced cell cycle arrest and apoptosis through oxidative stress with following observations. Firstly, 10 microM TET elevated the reactive oxygen species (ROS) level and accordingly depleted glutathione (GSH) content. Secondly, pretreatment with antioxidants (NAC or GSH) protected cells from TET-induced apoptosis. We also demonstrated that treatment with 10 microM TET caused not only induction of p53, p21(waf1), and Bax, but also nuclear translocation of p53 and hypo-phosphorylation of pRb concurrently. Our important finding is that the concentration-dependent dual effect of TET, either inhibiting or promoting cell death induced by H(2)O(2) was observed, probably through regulating redox balance, which was well reflected on the GSH content in each condition. Besides, inhibition of Ca(2+) influx protected cells from H(2)O(2)-induced apoptosis even in the presence of 10 microM TET. Taken together, our data suggest that TET regulation of cellular redox states may play a major role in its dual action of cytotoxicity and cytoprotection.

Fluoxetine increases the nitric oxide production via nuclear factor kappa B-mediated pathway in BV2 murine microglial cells

A body of recent evidence implicates that antidepressants affect the inflammatory response and immune system. The present study is focused on the effects of the most widely used antidepressant agent, fluoxetine on the production of nitric oxide (NO) in BV2 microglial cells. In this study, we observed interesting result that NO production was increased by fluoxetine. The mRNA level of nitric oxide synthase (iNos, Nos2) by RT-PCR was also stimulated by fluoxetine. We next conducted electophoretic mobility shift assay (EMSA) to determine the DNA binding activity of nuclear factor kappa B (Nfkappab), an important upstream modulator for Nos2 expression, to find that fluoxetine increased DNA binding activity of Nfkappab. By Western blot analysis, phosphorylation levels of p38 mitogen-activated protein kinase (p38 Mapk, Mapk14) and extracellular signal-related kinase (Erk)1/2 Mapk, upstream signaling mediators of Nfkappab were found to be increased by fluoxetine. In addition, the mRNA expressions of other proinflammatory cytokines, interleukin 6 (Il6) and tumor necrosis factor alpha (Tnfalpha) were examined. The expressions of both Il6 and Tnfalpha by fluoxetine treatment were similar to those of Nos2 and Nfkappab. Taken together, our results show that fluoxetine stimulates NO production via Nfkappab-mediated pathway in BV2 cells.

Manganese induces inducible nitric oxide synthase (iNOS) expression via activation of both MAP kinase and PI3K/Akt pathways in BV2 microglial cells

It is well documented that manganese neurotoxicity induces clinical symptoms similar to those of idiopathic Parkinsons disease. Although microglial cytotoxic mediator-induced neurotoxicity is suggested, the mechanism by which manganese up-regulates cytotoxic mediator, such as nitric oxide (NO), remains poorly understood. Therefore, in this study, we investigated the mechanism of manganese on induction of iNOS in microglial cells. iNOS promoter/luciferase assay revealed that manganese (500 (M) regulated the iNOS expression at the transcriptional level. Immunoblot analysis also revealed that phosphorylation levels of ERK, JNK MAPKs and Akt (PKB, PI 3-kinase downstream effector), were increased. Both protein and mRNA levels of iNOS expression were abrogated by specific inhibitors, SP600125 (JNK inhibitor, 20 microM), PD98059 (ERKs inhibitor, 50 microM), or LY294002 (PI 3-kinase inhibitor, 20 microM), but not by SB203580 (20 microM), a p38 specific inhibitor. These data lead to the conclusion that manganese regulates the iNOS expression at the transcriptional level in BV2 microglial cells and the increased iNOS protein expression is mediated via both JNK-ERK MAPK and PI3K/Akt signaling pathways, but not via p38 MAPK pathway. Increased iNOS protein level was also found in RAW264.7 murine macrophage cells.

Fluoxetine prevents LPS-induced degeneration of nigral dopaminergic neurons by inhibiting microglia-mediated oxidative stress.

Lipopolysaccharide (LPS)-induced microglial activation causes degeneration of nigral dopaminergic (DA) neurons. Here, we examined whether fluoxetine prevents LPS-induced degeneration of DA in the rat substantia nigra (SN) in vivo. Seven days after LPS injection into the SN, immunostaining for tyrosine hydroxylase (TH) revealed a significant loss of nigral DA neurons. Parallel activation of microglia (visualized by OX-42 and ED1 immunohistochemistry), production of reactive oxygen species (ROS) (assessed by hydroethidine histochemistry), and degeneration of nigral DA neurons were also observed in the SN. Western blot analyses and double-label immunohistochemistry showed an increase in the expression of inducible nitric oxide synthase (iNOS) within activated microglia. LPS also induced translocation of p67(phox), the cytosolic component of NADPH oxidase, to the membrane of SN microglia, indicating activation of NADPH oxidase. The LPS-induced loss of nigral DA neurons was partially inhibited by fluoxetine, and the observed neuroprotective effects were associated with fluoxetine-mediated suppression of microglial NADPH oxidase activation and iNOS upregulation, and decreased ROS generation and oxidative stress. These results suggest that fluoxetine and analogs thereof may be beneficial for the treatment of neurodegenerative diseases, such as PD, that are associated with microglia-derived oxidative damage.

Melatonin protects 6-OHDA-induced neuronal death of nigrostriatal dopaminergic system

IN VIVO neuroprotective effects of melatonin on the nigrostriatal dopaminergic system in rats unilateral 6-hydroxydopamine (6-OHDA) lesions were tested. Two weeks after lesioning the dopamine receptor agonist, apomorphine produced rotational asymmetry. In contrast, melatonin treatment significantly reduced the motor deficit following apomorphine challenge. Analysis by tyrosine hydroxylase (TH) immunocytochemistry revealed the loss of cell bodies in the substantia nigra (SN) and absence of terminals in the dorsolateral striatum ipsilaterally. Melatonin treatment also resulted in the survival of dopaminergic neurons in SN and TH-immuoreactive terminals in the dorsolateral striatum. These behavioral and histochemical results may indicate a neuroprotective action of melatonin and suggest a potential pharmacological role in the treatment of Parkinsons disease.

Inhibition of ROS-induced p38MAPK and ERK activation in microglia by acupuncture relieves neuropathic pain after spinal cord injury in rats

Acupuncture (AP) is currently used worldwide to relieve pain. However, little is known about its mechanisms of action. We found that after spinal cord injury (SCI), AP inhibited the production of superoxide anion (O(2)·), which acted as a modulator for microglial activation, and the analgesic effect of AP was attributed to its anti-microglial activating action. Direct injection of a ROS scavenger inhibited SCI-induced NP. After contusion injury which induces the below-level neuropathic pain (NP), Shuigou and Yanglingquan acupoints were applied. AP relieved mechanical allodynia and thermal hyperalgesia, while vehicle and simulated AP did not. AP also decreased the proportion of activated microglia, and inhibited both p38MAPK and ERK activation in microglia at the L4-5. Also, the level of prostaglandin E(2) (PGE2), which is produced via ERK signaling and mediates the below-level pain through PGE2 receptor, was reduced by AP. Injection of p38MAPK or ERK inhibitors attenuated NP and decreased PGE2 production. Furthermore, ROS produced after injury-induced p38MAPK and ERK activation in microglia, and mediated mechanical allodynia and thermal hyperalgesia, which were inhibited by AP or a ROS scavenger. AP also inhibited the expression of inflammatory mediators. Therefore, our results suggest that the analgesic effect of AP may be partly mediated by inhibiting ROS-induced microglial activation and inflammatory responses after SCI and provide the possibility that AP can be used effectively as a non-pharmacological intervention for SCI-induced chronic NP in patients.