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

Abstract

Little is known concerning the expression of amino acid transporters during intestinal epithelial cell differentiation. The transport mechanism ofl-glutamate and its regulation during the differentiation process were investigated using the human intestinal Caco-2 cell line. Kinetic studies demonstrated the presence of a single, high-affinity,d-aspartate-sensitive l-glutamate transport system in both confluent and fully differentiated Caco-2 cells. This transport was clearly Na+ dependent, with a Hill coefficient of 2.9 ± 0.3, suggesting a 3 Na+-to-1 glutamate stoichiometry and corresponding to the well-characterized XA,G − system. The excitatory amino acid transporter (EAAT)1 transcript was consistently expressed in the Caco-2 cell line, whereas the epithelial and neuronal EAAT3 transporter was barely detected. In contrast with systems B0 and y+, which have previously been reported to be downregulated when Caco-2 cells stop proliferating, l-glutamate transport capacity was found to increase steadily between day 8 and day 17. This increase was correlated with the level of EAAT1 mRNA, which might reflect an increase in EAAT1 gene transcription and/or stabilization of the EAAT1 transcript.