Abstract
The glial glutamate transporter GLT-1 may be the predominant Na+-dependent glutamate transporter in forebrain. Expression of GLT-1 correlates with astrocyte maturation in vivo and increases during synaptogenesis. In astrocyte cultures, GLT-1 expression parallels differentiation induced by cAMP analogs or by coculturing with neurons. Molecule(s) secreted by neuronal cultures contribute to this induction of GLT-1, but little is known about the signaling pathways mediating this regulation. In the present study, we determined whether growth factors previously implicated in astrocyte differentiation regulate GLT-1 expression. Of the six growth factors tested, two [epidermal growth factor (EGF) and transforming growth factor-α] induced expression of GLT-1 protein in cultured astrocytes. Induction of GLT-1 protein was accompanied by an increase in mRNA and in the V max for Na+-dependent glutamate transport activity. The effects of dibutyryl-cAMP and EGF were additive but were independently blocked by inhibitors of protein kinase A or protein tyrosine kinases, respectively. The induction of GLT-1 in both EGF- and dibutyryl-cAMP-treated astrocytes was blocked by inhibitors targeting phosphatidylinositol 3-kinase (PI3K) or the nuclear transcription factor-κB. Furthermore, transient transfection of astrocyte cultures with a constitutively active PI3K construct was sufficient to induce expression of GLT-1. These data suggest that independent but converging pathways mediate expression of GLT-1. Although an EGF receptor-specific antagonist did not block the effects of neuron-conditioned medium, the induction of GLT-1 by neuron-conditioned medium was completely abolished by inhibition of PI3K or nuclear factor-κB. EGF also increased expression of GLT-1 in spinal cord organotypic cultures. Together, these data suggest that activation of specific signaling pathways with EGF-like molecules may provide a novel approach for limiting excitotoxic brain injury.
Footnotes
- Received November 11, 1999.
- Accepted January 3, 2000.
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Send reprint requests to: Dr. Michael B. Robinson, 502N Abramson Pediatric Research Building, 34th and Civic Center Blvd., Philadelphia, PA 19104-4318. E-mail: Robinson{at}pharm.med.upenn.edu
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↵1 O.Z. and B.D.S. contributed equally to the present study.
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This study was supported by Grants NS29868 and HD26979 (to M.B.R), NS33958 (to J.D.R.), NS36465 (to M.B.R. and J.D.R.), and NS34017 (to J.B.G.).
- The American Society for Pharmacology and Experimental Therapeutics
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