Regulation of astrocytic glutamate transporter expression by Akt: evidence for a selective transcriptional effect on the GLT‐1/EAAT2 subtype

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Abstract
In the nervous system, astrocytes express different ratios of the two glial glutamate transporters, glutamate transporter subtype 1 (GLT‐1) and glutamate/aspartate transporter (GLAST), but little is known about the signaling pathways that independently regulate their expression. Treatment with dibutyryl‐cAMP, epidermal growth factor (EGF) or other growth factors both induces expression of GLT‐1 and increases expression of GLAST in astrocyte cultures. The induction of GLT‐1 is correlated with morphological and biochemical changes that are consistent with astrocyte maturation. Pharmacological studies suggest that phosphatidylinositol 3‐kinase (PI‐3K) and the nuclear transcription factor‐κB (NF‐κB) may be involved in the induction of GLT‐1 expression. In several signaling systems Akt, also known as protein kinase B (PKB), functions downstream of PI‐3K. In these present studies we used lentiviral vectors engineered to express dominant‐negative (DN), constitutively active (CA), or null variants of Akt to study the possible involvement of Akt in the regulation of GLT‐1. Expression of DN‐Akt attenuated the EGF‐dependent induction of GLT‐1. Expression of CA‐Akt caused a dose‐ and time‐dependent increase in GLT‐1 protein, increased GLT‐1 mRNA levels, increased dihydrokainate‐sensitive (presumably GLT‐1 mediated) transport activity, and caused a change in astrocyte morphology to a more stellate shape, but had no effect on GLAST protein levels. Finally, the expression of CA‐Akt increased the expression of a reporter construct containing a putative promoter fragment from the human homolog of GLT‐1, called EAAT2. From these studies, we conclude that Akt induces the expression of GLT‐1 through increased transcription and that Akt can regulate GLT‐1 expression without increasing GLAST expression in astrocytes.