TY - JOUR

T1 - Nonlinear relationship between ER Ca2+ depletion versus induction of the unfolded protein response, autophagy inhibition, and cell death

AU - Szalai, Paula

AU - Parys, Jan B.

AU - Bultynckb, Geert

AU - Christensen, Søren Brøgger

AU - Nissen, Poul

AU - Møller, Jesper Vuust

AU - Engedal, Nikolai

PY - 2018/12

Y1 - 2018/12

N2 - Endoplasmic reticulum (ER) Ca2+ depletion activates the unfolded protein response (UPR), inhibits bulk autophagyand eventually induces cell death in mammalian cells. However, the extent and duration of ER Ca2+depletion required is unknown. We instigated a detailed study in two different cell lines, using sarco/endoplasmicreticulum Ca2+-ATPase (SERCA) inhibitors to gradually reduce ER Ca2+ levels in a controlledmanner. Remarkably, UPR induction (as assessed by expression analyses of UPR-regulated proteins) and autophagyinhibition (as assessed by analyses of effects on starvation-induced bulk autophagy) required substantiallyhigher drug concentrations than those needed to strongly decrease total ER Ca2+ levels. In fact, evenwhen ER Ca2+ levels were so low that we could hardly detect any release of Ca2+ upon challenge with ER Ca2+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 Ca2+, cells could tolerateprolonged periods (days) without succumbing to cell death. Addition of increasing concentrations of extracellularEGTA also gradually depleted the ER of Ca2+, and, as with the SERCA inhibitors, EGTA-induced activationof UPR and cell death required higher EGTA concentrations than those needed to strongly reduce ER Ca2+levels. We conclude that ER Ca2+ depletion-induced effects on UPR, autophagy and cell death require either anextreme general depletion of ER Ca2+ levels, or Ca2+ depletion in areas of the ER that have a higher resistance toCa2+ drainage than the bulk of the ER.

AB - Endoplasmic reticulum (ER) Ca2+ depletion activates the unfolded protein response (UPR), inhibits bulk autophagyand eventually induces cell death in mammalian cells. However, the extent and duration of ER Ca2+depletion required is unknown. We instigated a detailed study in two different cell lines, using sarco/endoplasmicreticulum Ca2+-ATPase (SERCA) inhibitors to gradually reduce ER Ca2+ levels in a controlledmanner. Remarkably, UPR induction (as assessed by expression analyses of UPR-regulated proteins) and autophagyinhibition (as assessed by analyses of effects on starvation-induced bulk autophagy) required substantiallyhigher drug concentrations than those needed to strongly decrease total ER Ca2+ levels. In fact, evenwhen ER Ca2+ levels were so low that we could hardly detect any release of Ca2+ upon challenge with ER Ca2+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 Ca2+, cells could tolerateprolonged periods (days) without succumbing to cell death. Addition of increasing concentrations of extracellularEGTA also gradually depleted the ER of Ca2+, and, as with the SERCA inhibitors, EGTA-induced activationof UPR and cell death required higher EGTA concentrations than those needed to strongly reduce ER Ca2+levels. We conclude that ER Ca2+ depletion-induced effects on UPR, autophagy and cell death require either anextreme general depletion of ER Ca2+ levels, or Ca2+ depletion in areas of the ER that have a higher resistance toCa2+ drainage than the bulk of the ER.

KW - Faculty of Health and Medical Sciences

KW - Endoplasmic reticulum

KW - Calcium

KW - Unfolded Protein Response

KW - Thapsigargin

KW - Cell Death

KW - Autophagy

U2 - 10.1016/j.ceca.2018.09.005

DO - 10.1016/j.ceca.2018.09.005

M3 - Journal article

C2 - 30261424

VL - 76

SP - 48

EP - 61

JO - Cell Calcium

JF - Cell Calcium

SN - 0143-4160

M1 - https://doi.org/10.1016/j.ceca.2018.09.005

ER -