Upregulation of glutamate transporter GLT-1 by mTOR-Akt-NF-кB cascade in astrocytic oxygen-glucose deprivation.

ABSTRACT

Excessive extracellular glutamate leads to neuronal death in central nervous system. Excitatory glutamate transporter subtype 2 (GLT-1) carries bulk of glutamate reuptake in cerebral ischemia. Although GLT-1 expression fluctuates during the period of ischemia, little is known about its regulatory mechanism. Here we show an up-regulation of GLT-1 via mammalian target of rapamycin (mTOR)-Akt-nuclear factor-кB (NF-кB) signaling cascade in oxygen glucose deprivation (OGD). We found that brief rapamycin treatment significantly increased GLT-1 expression in cultured astrocytes. Rapamycin increased phosphorylation of raptor at Ser792 and decreased phosphorylation of rictor at Thr1135, suggesting that both mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) are involved in GLT-1 expression. This conclusion was further confirmed by raptor and rictor disruption experiments. Akt was activated by mTORC1 inhibition and required for GLT-1 expression because triciribine, a specific inhibitor of Akt, blocked the increase of GLT-1 expression. mTOR-Akt cascade then activated NF-кB and increased кB-motif-binding phosphoprotein (KBBP) expression and GLT-1 transcription. We next demonstrated that mTOR-Akt-NF-кB cascade was activated in OGD and subsequently caused the upregulation of GLT-1. Supporting evidence included (1) inhibition of Akt or NF-кB occluded OGD-induced GLT-1 upregulation; (2) Raptor knock-down plus OGD did not add to the increase of GLT-1 expression; (3) Intact mTORC2 was required for GLT-1 enhancement. In summary, our data first showed that mTOR-Akt-NF-кB cascade played critical roles to up-regulate GLT-1 in OGD. This signaling cascade may work to promote glutamate uptake in brain ischemia and neurodegenerative diseases.