The unfolded protein response (UPR) is an adaptive signaling pathway utilized to sense and alleviate the stress of protein folding in the endoplasmic reticulum (ER). In mammals, the UPR is mediated through three proximal sensors PERK/PEK, IRE1, and ATF6. PERK/PEK is a protein kinase that phosphorylates the alpha subunit of eukaryotic translation initiation factor 2 to inhibit protein synthesis. Activation of IRE1 induces splicing of XBP1 mRNA to produce a potent transcription factor. ATF6 is a transmembrane transcription factor that is activated by cleavage upon ER stress. We show that in Caenorhabditis elegans, deletion of either ire-1 or xbp-1 is synthetically lethal with deletion of either atf-6 or pek-1, both producing a developmental arrest at larval stage 2. Therefore, in C. elegans, atf-6 acts synergistically with pek-1 to complement the developmental requirement for ire-1 and xbp-1. Microarray analysis identified inducible UPR (i-UPR) genes, as well as numerous constitutive UPR (c-UPR) genes that require the ER stress transducers during normal development. Although ire-1 and xbp-1 together regulate transcription of most i-UPR genes, they are each required for expression of nonoverlapping sets of c-UPR genes, suggesting that they have distinct functions. Intriguingly, C. elegans atf-6 regulates few i-UPR genes following ER stress, but is required for the expression of many c-UPR genes, indicating its importance during development and homeostasis. In contrast, pek-1 is required for induction of approximately 23% of i-UPR genes but is dispensable for the c-UPR. As pek-1 and atf-6 mainly act through sets of nonoverlapping targets that are different from ire-1 and xbp-1 targets, at least two coordinated responses are required to alleviate ER stress by distinct mechanisms. Finally, our array study identified the liver-specific transcription factor CREBh as a novel UPR gene conserved during metazoan evolution.
The endoplasmic reticulum (ER) is an intracellular organelle where proteins fold and assemble prior to transport to the cell surface. The ER contains a finely tuned quality control apparatus to ensure that improperly folded proteins are retained in the ER lumen. A variety of physiological demands, environmental perturbations, and pathological conditions compromise protein folding in the ER and lead to the accumulation of unfolded proteins. The unfolded protein response (UPR) is an evolutionarily conserved intracellular adaptive signaling pathway that alleviates protein-folding defects in the ER. The unfolded protein signal is transmitted from the ER to the nucleus by three pathways involving the proteins ATF-6, PEK-1, and IRE-1/XBP-1. However, it is not known how these three pathways coordinate downstream transcriptional activation to mediate either cell adaptation or cell death. The authors have studied the nematode Caenorhabditis elegans to present a comprehensive genetic and gene expression analysis of the three UPR pathways. The findings demonstrate that the UPR regulates the expression of hundreds of genes in the presence, as well as the absence, of ER stress in a manner that is more complex and diverse than previously known.