Regulation of GLYT1 and xCT in human intestinal cells by ATF4 but not XBP1 or ATF6

The glycine transporter GLYT1 and the cystine-glutamate exchanger xCT have been identified as targets of the eIF2α stress response pathway and the transcription factor ATF4. To determine the role of these transporters in the intestinal response to stress we have examined regulation of expression of GLYT1 and xCT in the human colon carcinoma cell line Caco-2 during ER-stress and knockdown of the key transcription factors, ATF4, X-box-binding protein 1 (XBP1) and ATF6, involved in the unfolded protein response. Cells exposed to tunicamycin, a stimulator of ER-stress, showed greatly (8–10 fold) increased expression of GLYT1 and xCT mRNAs, measured by quantitative PCR, but little change in expression of the peptide transporter PepT1 mRNA. Expression of markers of ER-stress, DNA-damage inducible transcript 3 (DDIT3, also known as CHOP) and glucose-regulated protein-78 (GRP-78) was also greatly increased. siRNA knockdown of ATF4 resulted in decreased expression of GLYT1 and xCT mRNAs but had no effect on PepT1. Knockdown of XBP1 or ATF6 did not alter expression of any gene. These data suggest that GLYT1 and xCT are specific targets of ATF4 in intestinal cells. We suggest that increased expression of these transporters induced by stress leads to increased uptake of glycine and cystine, required for glutathione synthesis, and thus to increased intracellular antioxidant capacity.