Molecular events involved in up-regulating human Na+-independent neutral amino acid transporter LAT1 during T-cell activation

We investigated the regulation of system-L amino acid transporter (LAT1) during T-cell activation. In quiescent T-cells, L-leucine transport is mediated mainly by the system-L amino acid transport system and is increased significantly during T-cell activation by PMA and ionomycin. In quiescent T-cells, the LAT1 protein was heterocomplexed with 4F2 heavy chain (4F2hc) in the plasma membrane. During T-cell activation, the amounts of 4F2hc and LAT1 heterocomplex were significantly elevated compared with those in quiescent T-cells. In addition, by Northern-blot analysis, these increments were found to be due to elevated levels of LAT1 and 4F2hc mRNA. Transient expression of constructs comprising various LAT1 gene promoter fragments, which contained all three of the GC boxes, was sufficient for promoting luciferase expression in Jurkat T-cells, but the promoter of the LAT1 gene did not respond to PMA and ionomycin. Similar observations were observed in the human 4F2hc gene promoter. In nuclear run-on assay, the LAT1 and 4F2hc genes were actively transcribed even in quiescent T-cells, but the low levels of both transcripts were shown to be the result of a block to transcription elongation within the exon 1 intron 1 regions. These findings indicated that a removal of the block to mRNA elongation stimulates the induction of system-L amino acid transporter gene transcripts (LAT1 and 4F2hc) in activated T-cells.