Functional expression and adaptive regulation of Na+ -dependent neutral amino acid transporter SNAT2/ATA2 in normal human astrocytes under amino acid starved condition

We reported here the functional characteristics of Na+ -dependent neutral amino acid transport system A in normal human astrocytes and its adaptive regulation, a process in which amino acid starvation induces the transport activity. Reverse transcription-PCR revealed that the system A transporter subtype, SNAT2/ATA2, is only expressed in these cells. The other two known system A transporter subtypes, SNAT1/ATA1 and SNAT4/ATA3, could not be detected. Na+ -dependent uptake of alpha-(methylamino)isobutyric acid, a specific model substrate for system A, was pH-sensitive and saturable with a Michaelis-Menten constant of 0.22 +/- 0.03 mM. Exposures of human astrocytes to amino acid-free medium increased the system A activity and the steady-state levels of SNAT2/ATA2 mRNA in an exposure time-dependent manner. This stimulatory effect was attenuated significantly by actinomycin D, an inhibitor of RNA synthesis, and cycloheximide, an inhibitor of protein synthesis. Taken collectively, these data show that chronic exposure (6 h) of the cells to the amino acid-free medium increases the system A activity most likely by enhancing de novo synthesis of the transporter protein and consequently increasing the density of the transporter protein in the plasma membrane.