Zhiqin Fan

Neuronal MCP-1 Mediates Microglia Recruitment and Neurodegeneration Induced by the Mild Impairment of Oxidative Metabolism

Chemokines are implicated in the neuroinflammation of several chronic neurodegenerative disorders. However, the precise role of chemokines in neurodegeneration is unknown. Thiamine deficiency (TD) causes abnormal oxidative metabolism in the brain as well as a well‐defined microglia activation and neurodegeneration in the submedial thalamus nucleus (SmTN), which are common features of neurodegenerative diseases. We evaluated the role of chemokines in neurodegeneration and the underlying mechanism in a TD model. Among the chemokines examined, TD selectively induced neuronal expression of monocyte chemoattractant protein‐1 (MCP‐1) in the SmTN prior to microglia activation and neurodegeneration. The conditioned medium collected from TD‐induced neurons caused microglia activation. With a neuron/microglia co‐culture system, we showed that MCP‐1‐induced neurotoxicity required the presence of microglia, and exogenous MCP‐1 was able to activate microglia and stimulated microglia to produce cytokines. A MCP‐1 neutralizing antibody inhibited MCP‐1‐induced microglia activation and neuronal death in culture and in the thalamus. MCP‐1 knockout mice were resistant to TD‐induced neuronal death in SmTN. TD selectively induced the accumulation of reactive oxygen species in neurons, and antioxidants blocked TD‐induced MCP‐1 expression. Together, our results indicated an induction of neuronal MCP‐1 during mild impairment of oxidative metabolism caused by microglia recruitment/activation, which exacerbated neurodegeneration.

Thiamine deficiency induces endoplasmic reticulum stress in neurons.

Thiamine (vitamin B1) deficiency (TD) causes region selective neuronal loss in the brain; it has been used to model neurodegeneration that accompanies mild impairment of oxidative metabolism. The mechanisms for TD-induced neurodegeneration remain incompletely elucidated. Inhibition of protein glycosylation, perturbation of calcium homeostasis and reduction of disulfide bonds provoke the accumulation of unfolded proteins in the endoplasmic reticulum (ER), and cause ER stress. Recently, ER stress has been implicated in a number of neurodegenerative models. We demonstrated here that TD up-regulated several markers of ER stress, such as glucose-regulated protein (GRP) 78, growth arrest and DNA-damage inducible protein or C/EBP-homologus protein (GADD153/Chop), phosphorylation of eIF2alpha and cleavage of caspase-12 in the cerebellum and the thalamus of mice. Furthermore, ultrastructural analysis by electron microscopic study revealed an abnormality in ER structure. To establish an in vitro model of TD in neurons, we treated cultured cerebellar granule neurons (CGNs) with amprolium, a potent inhibitor of thiamine transport. Exposure to amprolium caused apoptosis and the generation of reactive oxygen species in CGNs. Similar to the observation in vivo, TD up-regulated markers for ER stress. Treatment of a selective inhibitor of caspase-12 significantly alleviated amprolium-induced death of CGNs. Thus, ER stress may play a role in TD-induced brain damage.

MMP-2 mediates ethanol-induced invasion of mammary epithelial cells over-expressing ErbB2

Ethanol is a tumor promoter and may enhance the metastasis of breast cancer. We have previously demonstrated that over‐expression of ErbB2 promoted ethanol‐mediated invasion of mammary epithelial cells and breast cancer cells. However, the underlying cellular/molecular mechanisms remain unknown. By gelatin zymography, we showed that over‐expression of ErbB2 increased the production of matrix metalloproteinase‐2 (MMP‐2) and MMP‐9 in human mammary epithelial cells (HB2). Transient or stable transfection of ErbB2 cDNA to HB2 cells upregulated the transcripts and the activity of the MMP‐2/‐9 gene promoter; the upregulation of MMP‐2/‐9 expression was mediated by p38 mitogen‐activated protein kinase (p38 MAPK) and phosphatidylinositol 3‐kinase (PI3K). Although ethanol, at physiologically relevant concentrations (100–400 mg/dl), did not affect the production of MMP‐2/‐9, it activated MMP‐2 in HB2 cells over‐expressing ErbB2 (HB2ErbB2), but not HB2 cells; it enhanced the cleavage of proform MMP‐2 (72 kDa) to an active form (62 kDa). The activation was dependent on c‐jun N‐terminal kinases (JNKs) and reactive oxygen species (ROS). On the other hand, ethanol affected neither the expression nor the activation of MMP‐9. Selective inhibitors of MMP‐2 (SB‐3CT and OA‐Hy) and antioxidants significantly inhibited ethanol‐stimulated invasion of HB2ErbB2 cells. Furthermore, knocking down MMP‐2 by small interference RNA also induced a partial blockage on ethanol‐promoted invasion of HB2ErbB2 cells. Thus, ethanol‐stimulated invasion of cells over‐expressing ErbB2 was mediated, at least partially, by MMP‐2 activation.

Thiamine deficiency increases β-secretase activity and accumulation of β-amyloid peptides

Thiamine pyrophosphate (TPP) and the activities of thiamine-dependent enzymes are reduced in Alzheimers disease (AD) patients. In this study, we analyzed the relationship between thiamine deficiency (TD) and amyloid precursor protein (APP) processing in both cellular and animal models of TD. In SH-SY5Y neuroblastoma cells overexpressing APP, TD promoted maturation of β-site APP cleaving enzyme 1 (BACE1) and increased β-secretase activity which resulted in elevated levels of β-amyloid (Aβ) as well as β-secretase cleaved C-terminal fragment (β-CTF). An inhibitor of β-secretase efficiently reduced TD-induced up-regulation of Aβ and β-CTF. Importantly, thiamine supplementation reversed the TD-induced alterations. Furthermore, TD treatment caused a significant accumulation of reactive oxygen species (ROS); antioxidants suppressed ROS production and maturation of BACE1, as well as TD-induced Aβ accumulation. On the other hand, exogenous Aβ(1-40) enhanced TD-induced production of ROS. A study on mice indicated that TD also caused Aβ accumulation in the brain, which was reversed by thiamine supplementation. Taken together, our study suggests that TD could enhance Aβ generation by promoting β-secretase activity, and the accumulation of Aβ subsequently exacerbated TD-induced oxidative stress.

Activation of double-stranded RNA-activated protein kinase by mild impairment of oxidative metabolism in neurons.

Thiamine (vitamin B1) deficiency (TD) causes mild and chronic impairment of oxidative metabolism and induces neuronal death in specific brain regions. The mechanisms underlying TD‐induced cell death, however, remain unclear. The double‐stranded RNA‐activated protein kinase (PKR), has been well known for its anti‐viral function. Upon activation by viral infection or double‐stranded RNA, PKR phosphorylates its substrate, the α‐subunit of eukaryotic initiation factor‐2 (eIF2α), leading to inhibition of translation. In response to various cellular stresses, PKR can also be stimulated by its protein activators, or its mouse homologue, PKR activator (RAX). We demonstrated that TD in mice induced phosphorylation of PKR at Thr446 and Thr451 and phosphorylation of eIF2α at Ser51 in the cerebellum and the thalamus. TD caused phosphorylation of PKR and eIF2α, as well as nuclear translocation of PKR in primary cultures of cerebellar granule neurons. PKR phosphorylation is necessary for its nuclear translocation because TD failed to induce nuclear translocation of a T446A/T451A PKR mutant. Both PKR inhibitor and dominant‐negative PKR mutant protected cerebellar granule neurons against TD‐induced cell death. TD promoted the association between RAX and PKR. Antioxidant vitamin E dramatically decreased the RAX/PKR association and ameliorated TD‐induced cell death. Our results indicate that TD‐induced neuronal death is at least partially mediated by the activation of PKR.

Overexpression of Glycogen Synthase Kinase 3β Sensitizes Neuronal Cells to Ethanol Toxicity

The developing central nervous system (CNS) is particularly susceptible to ethanol toxicity. The loss of neurons underlies many of the behavioral deficits observed in fetal alcohol spectrum disorders (FASD). The mechanisms of ethanol‐induced neuronal loss, however, remain incompletely elucidated. We demonstrated that glycogen synthase kinase 3β (GSK3β), a multifunctional serine/threonine kinase, was involved in ethanol neurotoxicity. The activity of GSK3β is negatively regulated by its phosphorylation at serine 9 (Ser9). Ethanol induced dephosphorylation of GSK3β at Ser9 and the activation of Bax as well as caspase‐3 in the developing mouse brain. These ethanol‐induced alterations were ameliorated by the pretreatment of a GSK3β inhibitor, lithium. To determine the role of GSK3β in ethanol neurotoxicity, we overexpressed wild‐type (WT), S9A mutant or dominant‐negative (DN) mutant GSK3β in a neuronal cell line (SK‐N‐MC). Ethanol only modestly reduced the viability of parental SK‐N‐MC cells but drastically induced caspase‐3 activation and apoptosis in cells overexpressing WT or S9A GSK3β, indicating that the high levels of GSK3β or the active form of GSK3β increased cellular sensitivity to ethanol. Contrarily, overexpression of DN GSK3β conferred resistance to ethanol toxicity. Lithium and other specific GSK3β inhibitors abolished the hypersensitivity to ethanol caused by WT or S9A overexpression. Bax, a proapoptotic protein, is a substrate of GSK3β. Cells overexpressing WT or S9A GSK3β were much more sensitive to ethanol‐induced Bax activation than parental SK‐N‐MC cells. Our results indicate that GSK3β may be a mediator of ethanol neurotoxicity, and its expression status in a cell may determine ethanol vulnerability.

Ethanol Induces Endoplasmic Reticulum Stress in the Developing Brain

BACKGROUND Ethanol exposure during brain development causes profound damages to the central nervous system (CNS). The underlying cellular/molecular mechanisms remain unclear. The endoplasmic reticulum (ER) is involved in posttranslational protein processing and transport. The accumulation of unfolded or misfolded proteins in the ER lumen triggers ER stress, which is characterized by translational attenuation, synthesis of ER chaperone proteins, and activation of transcription factors. Sustained ER stress ultimately leads to cell death. ER stress is implicated in various neurodegenerative processes. METHODS Using a third trimester equivalent mouse model of ethanol exposure, we tested the hypothesis that ethanol induces ER stress in the developing brain. Seven-day-old C57BL/6 mice were acutely exposed to ethanol by subcutaneous injection and the expression of ER stress-inducible proteins (ERSIPs) and signaling pathways associated with ER stress were examined. RESULTS Ethanol exposure significantly increased the expression of ERSIPs and activated signaling pathways associated with ER stress; these include ATF6, CHOP/GADD153, GRP78, and mesencephalic astrocyte-derived neurotrophic factor as well as the phosphorylation of IRE1α, eIF2α, PERK, and PKR. The ethanol-induced increase in ERSIPs occurred within 4 hours of ethanol injection, and levels of some ERSIPs remained elevated after 24 hours of ethanol exposure. Ethanol-induced increase in phosphorylated eIF2α, caspase-12, and CHOP was distributed in neurons of specific areas of the cerebral cortex, hippocampus, and thalamus. CONCLUSIONS Our finding indicates that ethanol induces ER stress in immature neurons, providing novel insight into ethanols detrimental effect on the developing CNS.

GSK3β and endoplasmic reticulum stress mediate rotenone-induced death of SK-N-MC neuroblastoma cells

Rotenone, an environmental toxin that inhibits mitochondrial complex I, has been used to induce experimental Parkinsonism in animals and cell cultures. We investigated the mechanism underlying rotenone-induced death of SK-N-MC neuroblastoma cells. Rotenone-induced cell death preceded intracellular accumulation of reactive oxygen species, and antioxidants failed to protect cells, indicating that oxidative stress was minimally involved in rotenone-induced death of SK-N-MC cells. Glycogen synthase kinase 3beta (GSK3beta), a multifunctional serine/threonine kinase, has been implicated in the pathogenesis of neurodegeneration. We showed that rotenone activated GSK3beta by enhancing its phosphorylation at tyrosine 216 while inhibiting phosphorylation at serine 9. Inhibitors of GSK3beta and dominant negative (kinase deficient) GSK3beta partially protected SK-N-MC cells against rotenone cytotoxicity. Rotenone also induced endoplasmic reticulum (ER) stress which was evident by an increase in phosphorylation of PERK, PKR, and eIF2alpha as well as the expression of GRP78. Rotenone had a modest effect on the expression of CHOP. An eIF2alpha siRNA significantly reduced rotenone cytotoxicity. ER stress was experimentally induced by tunicamycin and thapsigargin, but tunicamycin/thapsigargin did not activate GSK3beta in SK-N-MC cells. Down-regulation of eIF2alpha also offered partial protection against rotenone cytotoxicity. Combined treatment of GSK3beta inhibitors and eIF2alpha siRNA provided much greater protection than either treatment alone. Taken together, the results suggest that GSK3beta activation and ER stress contribute separately to rotenone cytotoxicity.

Cyanidin-3-Glucoside Ameliorates Ethanol Neurotoxicity in the Developing Brain

Ethanol exposure induces neurodegeneration in the developing central nervous system (CNS). Fetal alcohol spectrum disorders (FASD) are caused by ethanol exposure during pregnancy and are the most common nonhereditary cause of mental retardation. It is important to identify agents that provide neuroprotection against ethanol neurotoxicity. Multiple mechanisms have been proposed for ethanol‐induced neurodegeneration, and oxidative stress is one of the most important mechanisms. Recent evidence indicates that glycogen synthase kinase 3β (GSK3β) is a potential mediator of ethanol‐mediated neuronal death. Cyanidin‐3‐glucoside (C3G), a member of the anthocyanin family, is a potent natural antioxidant. Our previous study suggested that C3G inhibited GSK3β activity in neurons. Using a third trimester equivalent mouse model of ethanol exposure, we tested the hypothesis that C3G can ameliorate ethanol‐induced neuronal death in the developing brain. Intraperitoneal injection of C3G reduced ethanol‐meditated caspase‐3 activation, neurodegeneration, and microglial activation in the cerebral cortex of 7‐day‐old mice. C3G blocked ethanol‐mediated GSK3β activation by inducing phosphorylation at serine 9 while reducing the phosphorylation at tyrosine 216. C3G also inhibited ethanol‐stimulated expression of malondialdehyde (MDA) and p47phox, indicating that C3G alleviated ethanol‐induced oxidative stress. These results provide important insight into the therapeutic potential of C3G.

Brain-derived neurotrophic factor suppresses tunicamycin-induced upregulation of CHOP in neurons

The accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) lumen triggers ER stress. ER stress initiates a number of specific compensatory signaling pathways including unfolded protein response (UPR). UPR is characterized by translational attenuation, synthesis of ER chaperone proteins such as glucose‐regulated protein of 78 kDa (GRP78, also known as Bip), and transcriptional induction, which includes the activation of transcription factors such as activating transcriptional factor 6 (ATF6) and C/EBP homologous protein (CHOP, also known as growth arrest and DNA damage‐inducible gene 153 [GADD153]). Sustained ER stress ultimately leads to cell death. ER functions are believed to be impaired in various neurodegenerative diseases, as well as in some acute disorders of the brain. Brain‐derived neurotrophic factor (BDNF), a member of the neurotrophin family, functions as a neuroprotective agent and rescues neurons from various insults. The molecular mechanisms underlying BDNF neuroprotection, however, remain to be elucidated. We showed that CHOP partially mediated ER stress‐induced neuronal death. BDNF suppressed ER stress‐induced upregulation/ nuclear translocation of CHOP. The transcription of CHOP is regulated by ATF4, ATF6, and XBP1; BDNF selectively blocked the ATF6/CHOP pathway. Furthermore, BDNF inhibited the induction of death receptor 5 (DR5), a transcriptional target of CHOP. Our study thus suggests that suppression of CHOP activation may contribute to BDNF‐mediated neuroprotection during ER stress responses.