Biochimica et Biophysica Acta 1823 (2012) 335–347 Contents lists available at SciVerse ScienceDirect Biochimica et Biophysica Acta journal homepage: www.elsevier.com/locate/bbamcr Bcl2 at the endoplasmic reticulum protects against a Bax/Bak-independent paraptosis-like cell death pathway initiated via p20Bap31 Hannah M. Heath-Engel, Bing Wang, Gordon C. Shore ⁎ Department of Biochemistry and Goodman Cancer Research Center, McGill University, Montréal QC Canada H3G 1Y6 article info abstract Article history: Bap31 is an integral ER membrane protein which functions as an escort factor in the sorting of newly synthesized Received 1 August 2011 membrane proteins within the endoplasmic reticulum (ER). During apoptosis signaling, Bap31 is subject to early Received in revised form 29 November 2011 cleavage by initiator caspase-8. The resulting p20Bap31 (p20) fragment has been shown to initiate proapoptotic Accepted 30 November 2011 ER-mitochondria Ca2+ transmission, and to exert dominant negative (DN) effects on ER protein trafficking. We Available online 8 December 2011 now report that ectopic expression of p20 in E1A/DNp53-transformed baby mouse kidney epithelial cells initiates a non-apoptotic form of cell death with paraptosis-like morphology. This pathway was characterized by an early Keywords: 2+ 2+ Bap31 rise in ER Ca stores and massive dilation of the ER/nuclear envelope, dependent on intact ER Ca stores. Ablation Bcl-2 of the Bax/Bak genes had no effect on these ER/nuclear envelope transformations, and delayed but did not prevent Apoptosis cell death. ER-restricted expression of Bcl2 in the absence of Bax/Bak, however, delayed both ER/nuclear envelope Paraptosis dilation and cell death. This prosurvival role of Bcl2 at the ER thus extended beyond inhibition of Bax/Bak, and cor- Vacuolization related with its ability to lower ER Ca2+ stores. Furthermore, these results indicate that ER restricted Bcl2 is capable Calcium of antagonizing not only apoptosis, but also a non-apoptotic, Bax/Bak independent, paraptosis-like form of cell death. © 2011 Elsevier B.V. All rights reserved. 1. Introduction p20 can also exert dominant negative (DN) interference with the protein transport functions of full-length Bap31 [1,4,9]. The influence Bap31, an endoplasmic reticulum (ER) localized polytopic trans- of Bap31 or p20 in a particular cellular background, however, is likely membrane (TM) protein, plays a variety of roles in trafficking and dependent on the cargo protein under study, the relative levels of quality control within the secretory system. Bap31 has been shown Bap31/p20 interacting partners, and the cell death competence of to function in ER retention/retrieval, cell surface export, survival/ the system. stabilization and ER associated degradation (ERAD) of select TM pro- We have previously shown that p20 provides a proapoptotic signal via teins [1–11]. Consistent with these disparate effects on ER protein mobilization of ER Ca2+ stores. Ca2+ released from the ER is subsequently trafficking is our recent finding that Bap31 associates with the taken up by the mitochondria, leading to organelle fission. This alteration Sec61 preprotein translocon at the ER, interacts with nascent client in mitochondrial morphology, which is likely accompanied by cristae proteins as they emerge from the translocon, and escorts client pro- remodeling, is thought to act as a “sensitizing signal” for release of the teins to ER complexes which determine their subsequent fate [9]. proapoptotic factor cytochrome c, allowing efficient mobilization of this In addition to its role in protein trafficking, Bap31 also provides molecule following mitochondrial outer membrane permeabilization resistance to cytoplasmic events associated with membrane frag- (MOMP) [15,27]. Under physiological conditions, where apoptosis typi- mentation in response to select apoptotic stimuli [12–14].Anumber cally involves the induction of multiple pro-death proteins, Bap31 cleav- of apoptotic pathways involve cleavage of Bap31 by caspase-8; an age likely functions primarily as an amplification or sensitization signal, event which both abrogates its antiapoptotic function and generates with another (mitochondrial) signal being directly responsible for a membrane-embedded proapoptotic fragment, p20Bap31, or p20 MOMP. Nevertheless, prolonged ectopic expression of p20 does eventual- [1,12,15–26]. Of note, ectopic expression of p20 was shown to in- ly lead to cell death [15]. duce apoptosis even in the absence of endogenous Bap31 [15]. Apoptosis associated with mitochondrial release of cytochrome c is, in most cases, regulated by the Bcl2 family of proteins [28]. The Bcl2 family consists of both pro- and anti-apoptotic members, where the proapoptotic multidomain Bcl2 proteins (Bax/Bak), act to ⁎ Corresponding author at: Department of Biochemistry, McIntyre Medical Sciences permeabilize the outer mitochondrial membrane, leading to release Building, Room 915D, McGill University, Montréal QC Canada, H3G 1Y6. Tel.: +1 514 of intermembrane proapoptotic factors, including cytochrome c. The 398 7282; fax: +1 514 398 7384. E-mail addresses: [email protected] (H.M. Heath-Engel), antiapoptotic Bcl2 proteins act, both directly and indirectly, to inhibit [email protected] (B. Wang), [email protected] (G.C. Shore). Bax/Bak. The proapoptotic “BH3-only” Bcl2 proteins either inhibit the 0167-4889/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.bbamcr.2011.11.020 336 H.M. Heath-Engel et al. / Biochimica et Biophysica Acta 1823 (2012) 335–347 antiapoptotic family members, or activate Bax/Bak [29,30]. Although 2.3. Measurement of cell death, caspase activity and ER Ca2+ stores Bcl2 family members have been best studied with respect to their role at the mitochondria, they are also present at the ER, where Cells were infected with the stated adenoviral expression constructs, they act, at least in part, to regulate ER Ca2+ stores and/or signaling and both adherent and floating cells were collected at the indicated [31–33]. times after infection. Cell death was determined through FACS analysis Induction of Ca2+ release from the ER and ensuing apoptosis follow- of propidium iodide (PI) uptake. Cells were washed once with PBS, and ing p20 expression is likely regulated by Bcl2 proteins, as cell death was resuspended in PBS supplemented with 10 mg/ml propidium iodide preceded by Bax activation, and could be inhibited by overexpression of (PI). Uptake of PI was determined by analysis with a Becton Dickinson either wild-type or ER-restricted Bcl2 (Bcl2b5) [15]. Additional support FACScan Flow Cytometer. Samples were gated based on forward and for the potential involvement of Bcl2 proteins in this pathway was pro- side scatter, and the percentage of PI positive cells was determined vided by our study of the ER localized BH3-only protein Bik. Bik initiates based on intensity of fluorescence detected in the FL2 channel. Where a pathway similar to that seen for p20, and, in this system, both ER Ca2+ indicated, infections were performed in the presence of ABT-737 release and downstream mitochondrial events were shown to involve (1 μM, GeminX) or zVAD-fmk (10 μM, MP Biomedicals). Caspase-3/7 Bax/Bak, and could be inhibited by either Bcl2 or Bcl2b5 [34,35]. activity was determined by measurement of DEVDase activity in In our attempts to further explore the roles of Bax and Bak in the 25 μg of cell lysate, as per manufacturer's instructions (Upstate Bio- p20-induced cell death pathway, we used wild type (WT) and Bax/ technology). Thapsigargin (TG) releasable ER Ca2+ stores were de- Bak double knockout (DKO) baby mouse kidney (BMK) immortalized termined using Fura2-AM (Molecular Probes), as previously epithelial cell lines, transformed by E1A and DN p53 [36]. We found, described [15,34]. unexpectedly, that expression of p20 led to initiation of a non- apoptotic, paraptosis-like cell death pathway, in both WT and Bax/ 2.4. Immunofluorescence Bak DKO BMK cells. This p20-initiated pathway was characterized 2+ by an early rise, as opposed to release, of ER Ca stores, as well as Cells were seeded on glass coverslips and, where indicated, an early and dramatic dilation of the ER/nuclear envelope (NE). infected with AdHA-p20 or transfected with HA-p20, HA-A4, Flag- Thus the response to p20 expression can be dramatically different, Bap31 or Flag-crBap31 expression vectors. Following fixation in 4% dependent on cellular context. paraformaldehyde (PFA, Polysciences Inc), permeabilization and double Of particular importance, both p20-initiated cell death and ER dilation label immunofluorescence staining were conducted as previously de- fi could be signi cantly delayed by overexpression of Bcl2b5 in the Bax/Bak scribed [15]. Coverslips were mounted using ProLong Gold Antifade Re- DKO BMK cells. Bcl2 has been previously shown to function in a prosur- agent (Invitrogen), and images obtained using a Zeiss Axiovert 200 fi vival capacity through inhibition of Bax/Bak, and our ndings therefore Inverted Microscope. Primary antibodies used were: mouse anti-HA represent evidence of a novel, Bax/Bak independent, prosurvival role (Babco), mouse anti-Flag M2 (Sigma), goat anti-lactate dehydroge- for this protein at the ER. nase (LDH, Chemicon), and polyclonal rabbit anti-calnexin (kindly provided by Dr. J. Bergeron, McGill University, Montreal, QC). Sec- ondary antibodies used were: Alexa Fluor 488 or 594 conjugated 2. Materials and methods goat anti-rabbit, goat anti-mouse, or rabbit anti-goat (Molecular Probes). 2.1. Cell lines and culture conditions E1A/DNp53-transformed WT and Bax/Bak DKO BMK cells were kindly 2.5. Light and electron microscopy provided by Dr. Eileen White, Rutgers University, New Brunswick, NJ [36]. Phase contrast images were acquired using a Zeiss Axiovert 25 Cells were cultured at 37 °C in a humidified 5% CO2 atmosphere, in RPMI 1640 media (Gibco) supplemented with 10% heat-inactivated FBS, light-microscope, in combination with a Sony Cybershot DSC-S75 2 10 mM HEPES and 1 mM NaPyruvate. Cells stably expressing HA- camera.