EAAT1 is involved in transport of L-glutamate during differentiation of the Caco-2 cell line

Little is known concerning the expression of amino acid transporters during intestinal epithelial cell differentiation. The transport mechanism of L-g lutamate and its regulation during the differentiation process were investi gated using the human intestinal Caco-2 cell line. Kinetic studies demonstr ated the presence of a single, high-affinity, D-aspartate-sensitive L-gluta mate transport system in both confluent and fully differentiated Caco-2 cel ls. This transport was clearly Na+ dependent, with a Hill coefficient of 2. 9 +/- 0.3, suggesting a 3 Na+-to-1 glutamate stoichiometry and correspondin g to the well-characterized X-A,G(-) system. The excitatory amino acid tran sporter (EAAT)1 transcript was consistently expressed in the Caco-2 cell li ne, whereas the epithelial and neuronal EAAT3 transporter was barely detect ed. In contrast with systems B-0 and y(+), which have previously been repor ted to be downregulated when Caco-2 cells stop proliferating, L-glutamate t ransport capacity was found to increase steadily between day 8 and day 17. This increase was correlated with the level of EAAT1 mRNA, which might refl ect an increase in EAAT1 gene transcription and/or stabilization of the EAA T1 transcript.