In vitro experiments conducted
with human cell lineages showed an increase in cell survival when IRE1α activity is sustained for longer periods. This suggests that the IRE1/XBP-1 axis of the UPR pathway might balance the decision between life and death towards the anti-apoptotic side [37]. In contrast with IRE1, prolonged activation of PERK diminishes cell survival and this effect is associated with increased levels of CHOP mRNA and apoptosis markers, such as poly ADP-ribose polymerase [38]. Altogether, these results suggest that IRE1α and PERK have opposite roles on cell survival during ER stress. Several physiological events alter ER homeostasis, activating the UPR pathway: calcium unbalance, diminished glucose levels, tissue ischaemia, viral infections, and mutations that disturb protein folding. The ER lumen is an oxidative environment that is rich in calcium Palbociclib clinical trial and provides the ideal conditions for formation
of disulphide bond and proper protein folding. Depletion of calcium storages interferes with the functioning of chaperones BiP and calnexin [39, 40], inhibits glycosylation by enzyme UDP-glucose:glycoprotein U0126 chemical structure glucosyltransferase (UGGT), and diminishes the interaction of calreticulin and calnexin with misfolded proteins [41]. All these lead to improper folding of proteins, resulting in ER stress and activation of UPR. Diminished glucose levels activate the UPR pathway because folding and assembly of proteins require large amounts of energy. Besides, glycosylation of some proteins is a crucial step for their proper folding. An oligosaccharide chain (GlcNA2Man9Glc3) is added to nascent proteins. Misfolded proteins are held within the ER lumen by calnexin/calreticulin for re-glycosylation by the enzyme UGGT [42]. During ischaemia, the diminished blood flow results in local hypoglycemia leading to
accumulation of misfolded proteins within the ER mafosfamide through a similar mechanism [43, 44]. Viruses contribute to acute ER protein overload, leading to ER stress and consequent activation of the UPR pathway. Viruses also cause an increase in the metabolic rate from usage of the cellular machinery, resulting in higher usage of ATP and temporary depletion of glucose, altogether activating the UPR pathway [45–48]. Certain mutations that prevent the protein chain to fold in the most stable conformation also result in ER stress. Misfolded/unfolded proteins tend to associate and form aggregates that are toxic for the cell and/or result in premature degradation of these proteins via proteasome. Several neurodegenerative diseases have been associated to accumulation of misfolded proteins, such as Parkinson, Alzheimer, and Huntington [49–51]. Terminal differentiation of B lymphocytes into plasma cells also activates the UPR pathway and this activation is associated with the latter cells demand for increased levels of immunoglobulin synthesis and expansion of the ER to accommodate the immunoglobulin overload.