Ischemic Preconditioning Reveals that GLT1/EAAT2 Glutamate Transporter is a Novel PPARγ Target Gene Involved in Neuroprotection

Abstract

Excessive levels of extracellular glutamate in the nervous system are excitotoxic and lead to neuronal death. Glutamate transport, mainly by glutamate transporter GLT1/EAAT2, is the only mechanism for maintaining extracellular glutamate concentrations below excitotoxic levels in the central nervous system. We recently showed that neuroprotection after experimental ischemic preconditioning (IPC) involves, at least partly, the upregulation of the GLT1/EAAT2 glutamate transporter in astrocytes, but the mechanisms were unknown. Thus, we decided to explore whether activation of the nuclear receptor peroxisome proliferator-activated receptor (PPAR) gamma, known for its antidiabetic and antiinflammatory properties, is involved in glutamate transport. First, we found that the PPARgamma antagonist T0070907 inhibits both IPC-induced tolerance and reduction of glutamate release after lethal oxygen-glucose deprivation (OGD) (70.1%+/-3.4% versus 97.7%+/-5.2% of OGD-induced lactate dehydrogenase (LDH) release and 61.8%+/-5.9% versus 85.9%+/-7.9% of OGD-induced glutamate release in IPC and IPC+T0070907 1 mumol/L, respectively, n=6 to 12, P<0.05), as well as IPC-induced astrocytic GLT-1 overexpression. IPC also caused an increase in nuclear PPARgamma transcriptional activity in neurons and astrocytes (122.1%+/-8.1% and 158.6%+/-22.6% of control PPARgamma transcriptional activity, n=6, P<0.05). Second, the PPARgamma agonist rosiglitazone increased both GLT-1/EAAT2 mRNA and protein expression and [(3)H]glutamate uptake, and reduced OGD-induced cell death and glutamate release (76.3%+/-7.9% and 65.5%+/-15.1% of OGD-induced LDH and glutamate release in rosiglitazone 1 mumol/l, respectively, n=6 to 12, P<0.05). Finally, we have identified six putative PPAR response elements (PPREs) in the GLT1/EAAT2 promoter and, consistently, rosiglitazone increased fourfold GLT1/EAAT2 promoter activity. All these data show that the GLT1/EAAT2 glutamate transporter is a target gene of PPARgamma leading to neuroprotection by increasing glutamate uptake.

Synthesis of Lipoxin A4by 5-Lipoxygenase Mediates PPARγ-Dependent, Neuroprotective Effects of Rosiglitazone in Experimental Stroke

“…Our results show that 1) rosiglitazone-induced 5-LO expression and subsequent lipoxin synthesis mediate most of the neuroprotective effects of rosiglitazone in experimental stroke as well as its late PPAR␥ agonistic effect and 2) that LXA 4 is neuroprotective by acting, at least partly, as a PPAR␥ agonist. Several groups have previously reported that PPAR␥ agonists are neuroprotective in brain ischemia (Pereira et al, 2005(Pereira et al, , 2006 Tureyen et al, 2007), due mainly to anti-inflammatory and anti-oxidant mechanisms; anti-excitotoxic actions could also be involved, either at the presynaptic level (Romera et al, 2007) or postsynaptically (X. . Intriguingly, using cDNA microarray analysis, we found a robust expression of 5-LO mRNA after rosiglitazone administration, concomitant to neuroprotection (Pereira et al, 2006).…”

Activation of NR2A Receptors Induces Ischemic Tolerance through CREB Signaling

“…In addition, glutamate excitotoxicity is important in the pathophysiology of ischemic brain injury, which induces massive release of glutamate into extracellular space. Ischemic preconditioning might induce tolerance either by lowering excessive glutamate release or by increasing glutamate uptake (Romera et al, 2007). Activation of glutamate receptors causes calcium influx, activates calcium/CaM kinase, PI3K-Akt pathway, and Ras-MAPK cascade (Soriano et al, 2006;Wang et al, 2007), and in turn converge on CREB activation and upregulation of its target genes.…”

Amyotrophic Lateral Sclerosis: Role of the Canonical Wnt/Beta- Catenin Pathway and PPAR Gamma

“…PPAR gamma has been shown to have antiinflammatory and neuroprotective effects [27,28]. Astrocytic GLT1/EAAT2 gene is a target of PPAR gamma, leading to neuroprotection by increasing the glutamate uptake [29]. PPAR gamma is a direct transcriptional modulator of the pyruvate carboxylase gene [30].…”