In response to accumulation of unfolded proteins in the endoplasmic reticulum (ER), a homoeostatic response, termed the unfolded protein response (UPR), is activated in all eukaryotic cells. The UPR involves only transcriptional regulation in yeast, and approx. 6% of all yeast genes, encoding not only proteins to augment the folding capacity in the ER, but also proteins working at various stages of secretion, are induced by ER stress [Travers, Patil, Wodicka, Lockhart, Weissman and Walter (2000) Cell (Cambridge, Mass.) 101, 249–258]. In the present study, we conducted microarray analysis of HeLa cells, although our analysis covered only a small fraction of the human genome. A great majority of human ER stress-inducible genes (approx. 1% of 1800 genes examined) were classified into two groups. One group consisted of genes encoding ER-resident molecular chaperones and folding enzymes, and these genes were directly regulated by the ER-membrane-bound transcription factor activating transcription factor (ATF) 6. The ER-membrane-bound protein kinase double-stranded RNA-activated protein kinase-like ER kinase (PERK)-mediated signalling pathway appeared to be responsible for induction of the remaining genes, which are not involved in secretion, but may be important after cellular recovery from ER stress. In higher eukaryotes, the PERK-mediated translational-attenuation system is known to operate in concert with the transcriptional-induction system. Thus we propose that mammalian cells have evolved a strategy to cope with ER stress different from that of yeast cells.
Abbreviations used: AARE, amino acid-response element; Asn-S, asparagine synthetase; ATF, activating transcription factor; bZIP, basic leucine zipper; CHOP, CCAAT/enhancer-binding protein homologous protein; DMEM, Dulbecco's modified Eagle's medium; DOX, doxycyclin; DRAL, down-regulated in Rhabdomyasarcoma LIM protein; ECL® (Amersham Biosciences), enhanced chemiluminescence; eIF2α, eukaryotic translation initiation factor 2α; ER, endoplasmic reticulum; EDEM, ER-degradation-enhancing α-mannosidase-like protein; ERp, ER protein; ERSE, ER stress-response element; FBS, fetal bovine serum; FHL2, four and a half LIM domains 2; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GCN2, general control non-derepressible-2; GRP, glucose-regulated protein; HMG-CoA-S, 3-hydroxy-3-methylglutaryl-CoA synthetase; pATF6α(N) and pATF6β(N), active nuclear forms of ATF6α and ATF6β respectively; pATF6α(P) and pATF6β(P), ER-membrane-bound precursor forms of ATF6α and ATF6β respectively; PERK, double-stranded RNA-activated protein kinase-like ER kinase; PAX6, paired box gene 6; SCAMP, secretory carrier membrane protein; SEC, secretion; SERCA2, sarcoplasmic/ER Ca2+-ATPase 2; Trp-tRNA-S, tryptophanyl-tRNA synthetase; UPR, unfolded protein response; UPRE, UPR element; VPS, vacuolar protein sorting; XBP, X-box-binding protein.