Role of Intracellular Calcium in Proteasome Inhibitor-Induced Endoplasmic Reticulum Stress, Autophagy, and Cell Death
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Proteasome inhibition induces endoplasmic reticulum (ER) stress and compensatory autophagy to relieve ER stress. Disturbance of intracellular calcium homeostasis can lead to ER stress and alter the autophagy process. It has been suggested that inhibition of the proteasome disrupts intracellular calcium homeostasis. However, it is unknown if intracellular calcium affects proteasome inhibitor-induced ER stress and autophagy.
Human colon cancer HCT116 Bax positive and negative cell lines were treated with MG132, a proteasome inhibitor. BAPTA-AM, a cell permeable free calcium chelator, was used to modulate intracellular calcium levels. Autophagy and cell death were determined by fluorescence microscopy and immunoblot analysis.
MG132 increased intracellular calcium levels in HCT116 cells, which was suppressed by BAPTA-AM. MG132 suppressed proteasome activity independent of Bax and intracellular calcium levels in HCT116 cells. BAPTA-AM inhibited MG132-induced cellular vacuolization and ER stress, but not apoptosis. MG132 induced autophagy with normal autophagosome-lysosome fusion. BAPTA-AM seemed not to affect autophagosome-lysosome fusion in MG132-treated cells but further enhanced MG132-induced LC3-II levels and GFP-LC3 puncta formation, which was likely via impaired lysosome function.
Blocking intracellular calcium by BAPTA-AM relieved MG132-induced ER stress, but it was unable to rescue MG132-induced apoptosis, which was likely due to impaired autophagic degradation.
KEY WORDSautophagy cell death ER stress intracellular calcium proteasome inhibitor
1,2-bis-(o-aminophenoxy)-ethane-N,N,N′,N′-tetraacetic acid tetraacetoxymethyl esteris
calcium phosphate precipitates
Eukaryotic Translation Initiation Factor 2-alpha
c-Jun N-terminal Kinase
microtubule-associated protein 1 light chain 3
mouse embryonic fibroblasts
Protein Kinase RNA-like Endoplasmic Reticulum Kinase
Unfolded Protein Response
Ubiquitin Proteasome System
ACKNOWLEDGMENTS AND DISCLOSURES
Jessica A. Williams and Yifeng Hou contribute equally to this work. The research work in W.X Ding’s lab was supported in part by the NIAAA funds R01 AA020518-01 and National Center for Research Resources (5P20RR021940-07). J. A. Williams was supported by the “Training Program in Environmental Toxicology” [grant 5T32 ES007079] from the National Institute of Environmental Health Sciences. Y.F. Hou was supported by the National Natural Science Foundation of China (# 81072165) and the Shanghai Science and Technology Committee (# 09PJ1402700). The authors are indebted to Dr. Bert Vogelstein (Johns Hopkins University) and Lin Zhang (University of Pittsburgh) for the HCT116 Bax-positive and Bax-negative cell lines.
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