Modulation of ER stress and apoptosis by endoplasmic reticulum calcium leakviatranslocon during unfolded protein response: involvement of GRP78

Abstract

The endoplasmic reticulum (ER) is involved in many cellular functions, including protein folding and Ca(2+) homeostasis. The ability of cells to respond to the ER stress is critical for cell survival, and disruption in such regulation can lead to apoptosis. ER stress is accompanied by alterations in Ca(2+) homeostasis, and the ER Ca(2+) store depletion by itself can induce ER stress and apoptosis. Despite that, the ER Ca(2+) leak channels activated in response to the ER stress remain poorly characterized. Here we demonstrate that ER Ca(2+) depletion during the ER stress occurs via translocon, the ER protein complex involved in translation. Numerous ER stress inducers stimulate the ER Ca(2+) leak that can be prevented by translocon inhibitor, anisomycin. Expression of GRP78, an ER stress marker, increased following treatment with puromycin (a translocon opener) and was suppressed by anisomycin, confirming a primary role of translocon in ER stress induction. Inhibition of ER store depletion by anisomycin significantly reduces apoptosis stimulated by the ER stress inducers. We suggest that translocon opening is physiologically modulated by GRP78, particularly during the ER stress. The ability to modulate the ER Ca(2+) permeability and subsequent ER stress can lead to development of a novel therapeutic approach.

Co-chaperone Specificity in Gating of the Polypeptide Conducting Channel in the Membrane of the Human Endoplasmic Reticulum

“…Based on the observations that the SEC61A1Y344H mutation causes the unfolded protein response and apoptosis in a diabetes mouse model and that the same mutation destroys the BiP binding site and leads to increased ER Ca 2ϩ leakage, we proposed that the role of BiP in limiting Ca 2ϩ leakage from the ER at the level of the Sec61 complex contributes to the connection between ER protein misfolding and apoptosis; misfolding polypeptides sequester BiP (in the absence of BiP, Sec61 complexes become leaky for Ca 2ϩ ), and Ca 2ϩ transmission to mitochondria triggers apoptosis. This view was already partially confirmed (53). Furthermore, this notion is consistent with the observations that apoptosis can also be induced in HeLa cells by an inhibitor of CaM (70) and that BIP overexpression can protect cells from ER stressassociated cell death (71).…”

“…Both anti-hexahistidine and anti-BiP antibodies precipitated BiP and co-immunoprecipitated Sec61␣, whereas the negative control antibodies failed to do so. Thus, BiP and Sec61 complex are associated with each other under normal growth conditions, as had been observed previously (53).…”

“…Furthermore, it was observed in vitro as well as in intact cells that the ER lumenal Hsp70-type molecular chaperone, BiP, is involved in limiting ER Ca 2ϩ leakage at the level of the Sec61 complex (8,9,16,17,53,60) (Fig. 1).…”

Asbestos-induced Disruption of Calcium Homeostasis Induces Endoplasmic Reticulum Stress in Macrophages

“…Furthermore, an inhibitor of translocon Ca 2ϩ leak, anisomycin, has been shown to alter the ER stress response in certain cell types (28). To assess if anisomycin modulates Ca 2ϩ flux in macrophages, Fura2-AMloaded cells were pretreated with anisomycin before chrysotile exposure.…”

Enhanced endoplasmic reticulum SERCA activity by overexpression of hepatic stimulator substance gene prevents hepatic cells from ER stress-induced apoptosis

“…The potential reason for this observation is that the extensive accumulation of fatty acids within the hepatocytes could provoke the peroxidation of unsaturated fatty acids, producing large amounts of harmful substances, such as free radical molecules and ROS, which disrupt the structures and functions of the endomembrane system and also trigger the occurrence of ER stress. ER calcium disequilibrium is considered to be one of the initial and pivotal events of ER stress-mediated cell death (18). ROS produced in combination with peroxidation is able to attack the sarco-endoplasmic reticulum Ca 2ϩ -ATPase (SERCA) directly.…”