Identification of a plasma membrane glutamine transporter from the rat hepatoma cell line H4-IIE-C3

Glutamine is taken up into the rat hepatoma cell line H4-IIE-C3 by a Na⁺-dependent transport system which is specific for glutamine, alanine, serine, cysteine and asparagine and does not tolerate substitution of Na⁺ by Li⁺. Glutamine transport was relatively weakly inhibited by a 50-fold excess of leucine and was not inhibited by phenylalanine or N-methyl aminoisobutyrate. These general properties are characteristic of the recently identified ASCT/B⁰ family of transporters. Using a reverse transcriptase PCR-based homology cloning approach, we have characterized a cDNA for a novel member of this transporter family (H4-ASCT2) from H4-IIE-C3 cells. The cDNA encodes a 551-amino acid protein which exhibits similarities of between 75 and 85% with ASCT/B⁰ transporters previously cloned from other sources. When expressed in Xenopus oocytes, this transporter catalyses Na⁺-dependent glutamine uptake with characteristics very similar to those of glutamine uptake into the H4-IIE-C3 cells. This newly characterized transporter possesses a number of amino acid sequence differences from ASCT2 clones recently isolated from rat astroglial cells and from normal rat liver. In particular, the loop region between transmembrane helices 3 and 4 from H4-ASCT2 shares less than 60% sequence similarity with ASCT2 from rat liver; furthermore, there are some 25 single amino acid substitutions elsewhere in the H4-ASCT2 sequence compared with that from rat liver. Thus enhanced glutamine uptake in rat hepatoma cells is mediated by the expression of a novel ASCT/B⁰ transporter isoform rather than by increased expression of the ASCT2 mRNA found in normal rat liver.