Valproic acid induces the EAAT3 glutamate transporter in human oligodendroglioma cells

Introduction: Glutamate transport has never been studied in human oligodendroglioma cells, although the “neuronal” transporter EAAT3 is highly expressed in Oligodendrocyte Precursor Cells. On the other hand, transcriptional regulation of EAAT3 is still incompletely characterized and EAAT3 role in oligodendrocytic differentiation unknown. Conflicting evidence has been reported on the effects of HDAC inhibitors, such as valproic acid (VPA), on myelinating cells and glutamate transporters. Methods: Gene expression has been studied with RT-PCR and Western Blot. EAAT3 activity has been determined from the initial influx of the glutamate analogue D-aspartate. Results: EAAT3 expression and activity, barely detectable in human oligodendroglioma Hs683 cells under control conditions, are markedly induced by valproic acid (VPA). EAAT3 induction is detected at both mRNA (3x) and protein levels (10x after 72h of treatment). Under the same conditions, VPA does not change EAAT3 expression in human U373 glioblastoma cells. Transport activity is also increased by VPA and exhibits typical EAAT3 features, such as stimulation by phorbols and insensitivity to dihydrokainate and UCPH. While no saturable glutamate transport is detectable in control Hs683 cells, VPA-treated cells exhibit a high affinity transport activity (Km 7μM), the Vmax of which is increased by phorbol. EAAT3 mRNA induction is associated with a two-fold increase of PDGFRA mRNA, a marker of early Oligodendrocyte Precursor Cells, while CNP, TUBB3 and GFAP mRNAs remain unchanged. Conclusions: The VPA-dependent induction of the glutamate transporter EAAT3 in human oligodendroglioma cells may represent an indicator of early commitment to oligodendrocytic differentiation.