Transcriptional induction of the human asparagine synthetase gene during the unfolded protein response does not require the ATF6 and IRE1/XBP1 arms of the pathway

Abstract

The UPR (unfolded protein response) pathway comprises three signalling cascades mediated by the ER (endoplasmic reticulum) stress-sensor proteins PERK [PKR (double-stranded RNA-activated protein kinase)-like ER kinase], IRE1 (inositol-requiring kinase 1) and ATF6 (activating transcription factor 6). The present study shows that ASNS (asparagine synthetase) transcription activity was up-regulated in HepG2 cells treated with the UPR activators thapsigargin and tunicamycin. ChIP (chromatin immunoprecipitation) analysis demonstrated that during ER stress, ATF4, ATF3 and C/EBPβ (CCAAT/enhancer-binding protein β) bind to the ASNS proximal promoter region that includes the genomic sequences NSRE (nutrient-sensing response element)-1 and NSRE-2, previously implicated by mutagenesis in UPR activation. Consistent with increased ASNS transcription, ChIP analysis also demonstrated that UPR signalling resulted in enhanced recruitment of general transcription factors, including RNA Pol II (polymerase II), to the ASNS promoter. The ASNS gene is also activated by the AAR (amino acid response) pathway following amino acid deprivation of tissue or cells. Immunoblot analysis of HepG2 cells demonstrated that simultaneous activation of the AAR and UPR pathways did not further increase the ASNS or ATF4 protein abundance when compared with triggering either pathway alone. In addition, siRNA (small interfering RNA)-mediated knockdown of XBP1 (X-box-binding protein 1), ATF6α or ATF6β expression did not affect ASNS transcription, whereas siRNA against ATF4 suppressed ASNS transcription during UPR activation. Collectively, these results indicate that the PERK/p-eIF2α (phosphorylated eukaryotic initiation factor 2α)/ATF4 signalling cascade is the only arm of the UPR that is responsible for ASNS transcriptional induction during ER stress. Consequently, ASNS NSRE-1 and NSRE-2, in addition to ERSE (ER stress response element)-I, ERSE-II and the mUPRE (mammalian UPR element), function as mammalian ER-stress-responsive sequences.