γ-aminobutyric acid transporter (BGT-1) expressed in human astrocytoma U373 MG cells: Pharmacological and molecular characterization and phorbol ester-induced inhibition

The properties of a transport system specific for γ‐aminobutyric acid (GABA) expressed in human U373 MG astrocytoma cells were examined. The uptake of [3H]GABA was dependent on both extracellular Na+ and Cl– ions and was inhibited by (±)‐nipecotic acid, guvacine, and β‐alanine, with a pharmacological profile corresponding to that reported for the human homologue of the GABA/betaine transporter (BGT‐1). Accordingly, [3H]GABA uptake was also inhibited by betaine, and reverse transcriptase‐polymerase chain reaction (RT‐PCR) analysis of total RNA from U373 MG cells with specific BGT‐1 primers resulted in the amplification of a 440 bp fragment that was further characterized by restriction analysis and sequencing. In addition, Western blot analysis with anti‐BGT‐1 antiserum revealed the presence of a characteristic 60 kDa band. The primary structure of the human BGT‐1 protein predicts two putative phosphorylation sites for the Ca2+/diacylglicerol‐dependent protein kinase (PKC), and treatment of U373 MG cells with the PKC activator phorbol 12‐myristate‐13‐acetate (TPA) led to a concentration‐ and time‐dependent decrease in [3H]GABA uptake. The maximal effect was detected at 2 hr of incubation, to disappear after 4 hr. TPA‐induced reduction in [3H]GABA uptake was reversed by preincubation with staurosporine. Taken together, these results indicate that U373 MG cells express a GABA transporter of the BGT‐1 subtype whose function is regulated by phosphorylation events through PKC. © 2002 Wiley‐Liss, Inc.