Nonlinear relationship between ER Ca 2+ depletion versus induction of the unfolded protein response, autophagy inhibition, and cell death

• Published in: Cell calcium (Edinburgh) 2018-12, Vol.76, p.48
• Main Authors: Szalai, Paula, Parys, Jan B, Bultynck, Geert, Christensen, Søren Brøgger, Nissen, Poul, Møller, Jesper V, Engedal, Nikolai
• Format: Article
• Language: English
• Subjects: Autophagy - drug effects; Calcium - deficiency; Calcium - metabolism; Cell Death - drug effects; Cell Survival - drug effects; Cells, Cultured; Dose-Response Relationship, Drug; Endoplasmic Reticulum - drug effects; Endoplasmic Reticulum - metabolism; Humans; Molecular Conformation; PC-3 Cells; Sarcoplasmic Reticulum Calcium-Transporting ATPases - antagonists & inhibitors; Sarcoplasmic Reticulum Calcium-Transporting ATPases - metabolism; Structure-Activity Relationship; Thapsigargin - pharmacology; Unfolded Protein Response - drug effects; Unfolded Protein Response - physiology
• ISSN: 1532-1991
• DOI: 10.1016/j.ceca.2018.09.005

Endoplasmic reticulum (ER) Ca depletion activates the unfolded protein response (UPR), inhibits bulk autophagy and eventually induces cell death in mammalian cells. However, the extent and duration of ER Ca depletion required is unknown. We instigated a detailed study in two different cell lines, using sarco/endoplasmic reticulum Ca -ATPase (SERCA) inhibitors to gradually reduce ER Ca levels in a controlled manner. Remarkably, UPR induction (as assessed by expression analyses of UPR-regulated proteins) and autophagy inhibition (as assessed by analyses of effects on starvation-induced bulk autophagy) required substantially higher drug concentrations than those needed to strongly decrease total ER Ca levels. In fact, even when ER Ca levels were so low that we could hardly detect any release of Ca upon challenge with ER Ca purging agents, UPR was not induced, and starvation-induced bulk autophagy was still fully supported. Moreover, although we observed reduced cell proliferation at this very low level of ER Ca , cells could tolerate prolonged periods (days) without succumbing to cell death. Addition of increasing concentrations of extracellular EGTA also gradually depleted the ER of Ca , and, as with the SERCA inhibitors, EGTA-induced activation of UPR and cell death required higher EGTA concentrations than those needed to strongly reduce ER Ca levels. We conclude that ER Ca depletion-induced effects on UPR, autophagy and cell death require either an extreme general depletion of ER Ca levels, or Ca depletion in areas of the ER that have a higher resistance to Ca drainage than the bulk of the ER.