Plasma Membrane Ca2+-ATPase Overexpression Depletes Both Mitochondrial and Endoplasmic Reticulum Ca2+ Stores and Triggers Apoptosis in Insulin-secreting BRIN-BD11 Cells

Lin Jiang,Florent Allagnat,Evrard Nguidjoe,Adama Kamagate,Nathalie Pachera,Jean-Marie Vanderwinden,Marisa Brini,Ernesto Carafoli,Décio L Eizirik,Alessandra K Cardozo,André Herchuelz

doi:10.1074/jbc.m110.116681

Abstract

Ca(2+) may trigger apoptosis in β-cells. Hence, the control of intracellular Ca(2+) may represent a potential approach to prevent β-cell apoptosis in diabetes. Our objective was to investigate the effect and mechanism of action of plasma membrane Ca(2+)-ATPase (PMCA) overexpression on Ca(2+)-regulated apoptosis in clonal β-cells. Clonal β-cells (BRIN-BD11) were examined for the effect of PMCA overexpression on cytosolic and mitochondrial [Ca(2+)] using a combination of aequorins with different Ca(2+) affinities and on the ER and mitochondrial pathways of apoptosis. β-cell stimulation generated microdomains of high [Ca(2+)] in the cytosol and subcellular heterogeneities in [Ca(2+)] among mitochondria. Overexpression of PMCA decreased [Ca(2+)] in the cytosol, the ER, and the mitochondria and activated the IRE1α-XBP1s but inhibited the PRKR-like ER kinase-eIF2α and the ATF6-BiP pathways of the ER-unfolded protein response. Increased Bax/Bcl-2 expression ratio was observed in PMCA overexpressing β-cells. This was followed by Bax translocation to the mitochondria with subsequent cytochrome c release, opening of the permeability transition pore, and apoptosis. In conclusion, clonal β-cell stimulation generates microdomains of high [Ca(2+)] in the cytosol and subcellular heterogeneities in [Ca(2+)] among mitochondria. PMCA overexpression depletes intracellular [Ca(2+)] stores and, despite a decrease in mitochondrial [Ca(2+)], induces apoptosis through the mitochondrial pathway. These data open the way to new strategies to control cellular Ca(2+) homeostasis that could decrease β-cell apoptosis in diabetes.

Highlights

Apoptosis, or programmed cell death, plays a major role in the maintenance of tissue homeostasis
The permeability transition pore can be activated by high Ca2ϩ concentrations in the mitochondrial matrix, and a prolonged opening of the permeability transition pore is followed by swelling of the mitochondrial matrix, rupture of the outer mitochondrial membrane (OMM), loss of the membrane potential (⌬⌿), and the release of cytochrome c [8]
We showed that overexpression of Na/Ca exchanger in an insulin-secreting cell line (BRIN-BD11; 22) depleted ER Ca2ϩ stores, leading to ER stress and apoptosis

Summary

Introduction

Programmed cell death, plays a major role in the maintenance of tissue homeostasis. In view of the putative proapoptotic role of Ca2ϩ, the control of intracellular Ca2ϩ may represent a potential approach to prevent or enhance apoptosis and to treat diseases characterized by either an increased rate of apoptosis (e.g. type 1 diabetes, neurodegenerative disorders, viral infections, etc.) or by a decrease rate of cell death (e.g. cancers), respectively. This could be done by overexpressing or down-regulating key mechanisms responsible for Ca2ϩ extrusion from cells, namely the Na/Ca exchanger and the plasma membrane Ca2ϩ-ATPase (PMCA; 12, 13), which have been both implicated in cell death (14 –17). We showed that overexpression of Na/Ca exchanger in an insulin-secreting cell line (BRIN-BD11; 22) depleted ER Ca2ϩ stores, leading to ER stress and apoptosis

Objectives

Results

Conclusion