Nonlinear relationship between ER Ca2+ depletion versus induction of the unfolded protein response, autophagy inhibition, and cell death. (December 2018)

• Record Type: Journal Article
• Authors: Szalai, Paula; Parys, Jan B.; Bultynck, Geert; Christensen, Søren Brøgger; Nissen, Poul; Møller, Jesper V.; Engedal, Nikolai
• Is Part Of: Cell calcium. Volume 76(2018)
• Page Start: 48
• Page End: 61
• Publication Date: 2018-12
• Subjects: AUC area under the curve -- EpoTg thapsigargin epoxide -- ER endoplasmic reticulum -- LDH lactate dehydrogenase -- PI propidium iodide -- SERCA sarco/endoplasmic reticulum Ca2+-ATPase -- SOCE store-operated calcium entry -- Tg thapsigargin -- UPR unfolded protein response; ER Ca2+depletion -- Unfolded protein response -- Thapsigargin -- SERCA -- Cell death -- Autophagy; Calcium -- Metabolism -- Periodicals; Vertebrates -- Physiology -- Periodicals; Calcium -- Physiological effect -- Periodicals; Cell physiology -- Periodicals; Calcium in the body -- Periodicals; 572.516
• DOI: 10.1016/j.ceca.2018.09.005
• Languages: English
• ISSNs: 0143-4160

Abstract

Graphical abstract: Highlights: Depletion of the bulk of ER Ca 2+ does not induce the unfolded protein response. Cells can tolerate sustained depletion of ∼ all releasable ER Ca 2+ for several days. Depletion of the bulk of ER Ca 2+ does not limit bulk autophagy. Partial depletion of ER Ca 2+ is sufficient to reduce cell proliferation. Cytotoxic effects require extreme ER Ca 2+ depletion or ER subdomain Ca 2+ depletion. Abstract: Endoplasmic reticulum (ER) Ca 2+ 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 2+ depletion required is unknown. We instigated a detailed study in two different cell lines, using sarco/endoplasmic reticulum Ca 2+ -ATPase (SERCA) inhibitors to gradually reduce ER Ca 2+ 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 2+ levels. In fact, even when ER Ca 2+ levels were so low that we could hardly detect any release of Ca 2+ upon challenge with ER Ca 2+ 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 2+, 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 2+, 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 2+ levels. We conclude that ER Ca 2+ depletion-induced effects on UPR, autophagy and cell death require either an extreme general depletion of ER Ca 2+ levels, or Ca 2+ depletion in areas of the ER that have a higher resistance to Ca 2+ drainage than the bulk of the ER.