A review of the mammalian unfolded protein response

Abstract

Proteins requiring post-translational modifications such as N-linked glycosylation are processed in the endoplasmic reticulum (ER). A diverse array of cellular stresses can lead to dysfunction of the ER and ultimately to an imbalance between protein-folding capacity and protein-folding load. Cells monitor protein folding by an inbuilt quality control system involving both the ER and the Golgi apparatus. Unfolded or misfolded proteins are tagged for degradation via ER-associated degradation (ERAD) or sent back through the folding cycle. Continued accumulation of incorrectly folded proteins can also trigger the unfolded protein response (UPR). In mammalian cells, UPR is a complex signaling program mediated by three ER transmembrane receptors: activating transcription factor 6 (ATF6), inositol requiring kinase 1 (IRE1) and double-stranded RNA-activated protein kinase (PKR)-like endoplasmic reticulum kinase (PERK). UPR performs three functions, adaptation, alarm, and apoptosis. During adaptation, the UPR tries to reestablish folding homeostasis by inducing the expression of chaperones that enhance protein folding. Simultaneously, global translation is attenuated to reduce the ER folding load while the degradation rate of unfolded proteins is increased. If these steps fail, the UPR induces a cellular alarm and mitochondrial mediated apoptosis program. UPR malfunctions have been associated with a wide range of disease states including tumor progression, diabetes, as well as immune and inflammatory disorders. This review describes recent advances in understanding the molecular structure of UPR in mammalian cells, its functional role in cellular stress, and its pathophysiology.

Fig. 1

The calnexin/calreticulin protein folding cycle. Yellow circles denote glucose groups and while blue circles denote mannose groups. After entering the ER lumen, glucosidase I and II remove two glucose groups. The monoglucosylated glycoprotein then interacts with calnexin/calreticulin. These chaperones interact with the thiol-disulphide oxidoreductase ERp57. Cleavage of the last glucose residue by glucosidase II leads to the release of the chaperones. At this time, the protein could have either folded and left the ER or it could have attained an incorrect state. The incorrectly folded proteins are then the substrates of UDP glucose:glycoprotein glucosyltransferase, which puts a glucose residue back to the incorrectly folded protein. This enables the protein to spend some more time in folding in the ER. If the protein fails to fold in a repeated number of cycles, the mannose residue is removed by α-1,2-mannosidase I. This enables the protein to be recognized by ER-degradation-enhancing 1,2-mannosidase-like protein (EDEM). This targets the unfolded proteins for ER-associated degradation (ERAD).

Fig. 2

An array of cellular stressors can perturb the folding environment in the endoplasmic reticulum (ER) leading to unfolded or misfolded protein. In response to the folding imbalance, cells initiate the cytoprotective unfolded protein response (UPR). A: The problem of unfolded or misfolded proteins in the ER is addressed by increasing the folding capacity through the up-regulation of the expression of chaperone proteins, attenuating translation by regulating eIF2α, and promoting the degradation of misfolded proteins through ER-associated degradation (ERAD). If UPR is unable to restore the folding balance, ER stress will eventually lead to apoptotic cell-death. B: The three signal transduction pathways mediating the unfolded protein response in higher eukaryotes. First, the PRKR-like ER kinase (PERK) pathway is initiated after BiP dissociation from PERK. While PERK transduces both pro-and anti-apoptotic signals, its main function is translation attenuation through the phosphorylation of eIF2α. Next, the activating transcription factor 6 (ATF6) pathway is activated following BiP dissociation. ATF6 induces the expression of chaperones e.g., BiP as well as apoptosis effectors such as CHOP. Lastly, the inositol-requiring kinase 1 (IRE1) pathway is activated following BiP dissociation from IRE1. Activated IRE1 has both an endoribonuclease and a serine-threonine kinase activity that drive can pro-apoptotic signals.