Regulation of astrocytic glutamate transporter expression by Akt: evidence for a selective transcriptional effect on the GLT-1/EAAT2 subtype.
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-kappaB (NF-kappaB) 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.
Li
Li-Bin
LB
Department of Pediatrics, University of Pennsylvania, Philadelphia, Pensylvania 19104-4318, USA.
Toan
Shuy Vang
SV
Zelenaia
Olga
O
Watson
Deborah J
DJ
Wolfe
John H
JH
Rothstein
Jeffrey D
JD
Robinson
Michael B
MB
D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism* / Q000821:virology; D015153:Blotting, Western / Q000379:methods; D003994:Bucladesine / Q000494:pharmacology; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000166:cytology; D004347:Drug Interactions; D004815:Epidermal Growth Factor / Q000494:pharmacology; D027342:Excitatory Amino Acid Transporter 2 / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology*; D005904:Glial Fibrillary Acidic Protein / Q000378:metabolism; D018698:Glutamic Acid / Q000493:pharmacokinetics; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism; D000071679:Glycogen Synthase Kinase 3 beta; D049452:Green Fluorescent Proteins / Q000378:metabolism; D007150:Immunohistochemistry / Q000379:methods; D016086:Lentivirus / Q000502:physiology; D051058:Oncogene Protein v-akt / Q000502:physiology*; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D013997:Time Factors; D014162:Transfection / Q000379:methods; D014316:Tritium / Q000493:pharmacokinetics
D027342:Excitatory Amino Acid Transporter 2; D005904:Glial Fibrillary Acidic Protein; D012333:RNA, Messenger; C493395:Slc1a2 protein, rat; D014316:Tritium; D049452:Green Fluorescent Proteins; D018698:Glutamic Acid; D004815:Epidermal Growth Factor; D003994:Bucladesine; D000071679:Glycogen Synthase Kinase 3 beta; D051058:Oncogene Protein v-akt; D038362:Glycogen Synthase Kinase 3