Glutamine transport in C6 glioma cells: substrate specificity and modulation in a glutamine deprived culture medium

A previous study has shown that glutamine (Gln) uptake in C6 cells grown in a standard medium containing 2 mM Gln, is predominantly mediated by a sodium-dependent system that is inhibited by ASC system substrates alanine (Ala), serine (Ser), cysteine (Cys) and threonine (Thr), shows pH sensitivity and partial tolerance to substitution of Na+ by Li+, features compatible with system ASCT2 that is strongly expressed in cultured astrocytes. The uptake was not inhibited by the model system A substrate alpha-(methylamino)isobutyric acid (MeAiB), and glycine (Gly) or proline (Pro), indicating that the substrate-regulated system A as defined by routine criteria is relatively inactive in these cells (Dolinska et al., 2000). In this study we compared the uptake of radiolabeled Gln and a model ASC substrate -Thr in cells grown to the same density in Gln-containing and Gln-deprived media. Cells grown in the absence of Gln showed a reduced activity of system ASC-mediated Gln uptake, and the system lost tolerance for Li+ and became somewhat more resistant to lowering pH of the medium. In contrast to cultured astrocytes deprived of Gln, the overall Gln uptake activity in C6 cells adapted to grow in a medium without Gln was lower than in cells grown in a Gln containing medium, and the uptake by system A remained inactive. C6 cells cultured both in the presence and absence of Gln expressed ASCT2 mRNA, indicating that system ASCT2-mediated Gln uptake is modulated at a posttranscriptional level. In contrast to Gln uptake, Thr uptake was more active in cells cultured in the absence of Gln and showed neither pH dependence nor lithium tolerance in either medium, which is typical of an uptake mediated by the widespread ASCT1 isoform of system ASC. In C6 cells grown in the presence or absence of Gln alike, approximately 20% of the sodium-dependent Gln uptake was resistant to MeAiB+Thr, indicating contribution of system N. The N system-mediated uptake in C6 cells grown in the absence, but not in the presence of Gln was not inhibited by glutamate (Glu) that conforms to the characteristics of the glial N system variant, SN1.