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The apoptosis inhibitor ARC alleviates the ER stress response to promote β-cell survival.
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PMID:  22933109     Owner:  NLM     Status:  MEDLINE    
Abstract/OtherAbstract:
Type 2 diabetes involves insulin resistance and β-cell failure leading to inadequate insulin secretion. An important component of β-cell failure is cell loss by apoptosis. Apoptosis repressor with caspase recruitment domain (ARC) is an inhibitor of apoptosis that is expressed in cardiac and skeletal myocytes and neurons. ARC possesses the unusual property of antagonizing both the extrinsic (death receptor) and intrinsic (mitochondria/endoplasmic reticulum [ER]) cell death pathways. Here we report that ARC protein is abundant in cells of the endocrine pancreas, including >99.5% of mouse and 73% of human β-cells. Using genetic gain- and loss-of-function approaches, our data demonstrate that ARC inhibits β-cell apoptosis elicited by multiple inducers of cell death, including ER stressors tunicamycin, thapsigargin, and physiological concentrations of palmitate. Unexpectedly, ARC diminishes the ER stress response, acting distal to protein kinase RNA-like ER kinase (PERK) and inositol-requiring protein 1α, to suppress C/EBP homologous protein (CHOP) induction. Depletion of ARC in isolated islets augments palmitate-induced apoptosis, which is dramatically rescued by deletion of CHOP. These data demonstrate that ARC is a previously unrecognized inhibitor of apoptosis in β-cells and that its protective effects are mediated through suppression of the ER stress response pathway.
Authors:
Wendy M McKimpson; Jeremy Weinberger; Lech Czerski; Min Zheng; Michael T Crow; Jeffrey E Pessin; Streamson C Chua; Richard N Kitsis
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Publication Detail:
Type:  Journal Article; Research Support, N.I.H., Extramural; Research Support, Non-U.S. Gov't     Date:  2012-08-29
Journal Detail:
Title:  Diabetes     Volume:  62     ISSN:  1939-327X     ISO Abbreviation:  Diabetes     Publication Date:  2013 Jan 
Date Detail:
Created Date:  2012-12-21     Completed Date:  2013-03-08     Revised Date:  2014-07-14    
Medline Journal Info:
Nlm Unique ID:  0372763     Medline TA:  Diabetes     Country:  United States    
Other Details:
Languages:  eng     Pagination:  183-93     Citation Subset:  AIM; IM    
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MeSH Terms
Descriptor/Qualifier:
Animals
Apoptosis
Apoptosis Regulatory Proteins / physiology*
Calcium / metabolism
Cell Line, Tumor
Cell Survival
Endoplasmic Reticulum Stress*
Humans
Insulin-Secreting Cells / physiology*
Mice
Muscle Proteins / physiology*
Transcription Factor CHOP / physiology
Grant Support
ID/Acronym/Agency:
5P30-CA-013330-39/CA/NCI NIH HHS; 5P60-DK-020541-34/DK/NIDDK NIH HHS; 5R01-DK-063306-10/DK/NIDDK NIH HHS; 5R01-HL-060665-14/HL/NHLBI NIH HHS; 5R37-DK-033823-31/DK/NIDDK NIH HHS; 5T32-GM-007491-35/GM/NIGMS NIH HHS; P30 CA013330/CA/NCI NIH HHS; P30 DK026687/DK/NIDDK NIH HHS
Chemical
Reg. No./Substance:
0/Apoptosis Regulatory Proteins; 0/DDIT3 protein, human; 0/Muscle Proteins; 0/NOL3 protein, human; 0/Nol3 protein, mouse; 147336-12-7/Transcription Factor CHOP; SY7Q814VUP/Calcium
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From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine

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Journal Information
Journal ID (nlm-ta): Diabetes
Journal ID (iso-abbrev): Diabetes
Journal ID (hwp): diabetes
Journal ID (pmc): diabetes
Journal ID (publisher-id): Diabetes
ISSN: 0012-1797
ISSN: 1939-327X
Publisher: American Diabetes Association
Article Information
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© 2013 by the American Diabetes Association.
creative-commons:
Received Day: 19 Month: 4 Year: 2012
Accepted Day: 30 Month: 7 Year: 2012
Print publication date: Month: 1 Year: 2013
Electronic publication date: Day: 13 Month: 12 Year: 2012
Volume: 62 Issue: 1
First Page: 183 Last Page: 193
PubMed Id: 22933109
ID: 3526036
Publisher Id: 0504
DOI: 10.2337/db12-0504

The Apoptosis Inhibitor ARC Alleviates the ER Stress Response to Promote β-Cell Survival
Wendy M. McKimpson123
Jeremy Weinberger123
Lech Czerski123
Min Zheng123
Michael T. Crow4
Jeffrey E. Pessin156
Streamson C. Chua, Jr.167
Richard N. Kitsis12368
1Department of Medicine, Albert Einstein College of Medicine, Bronx, New York
2Department of Cell Biology, Albert Einstein College of Medicine, Bronx, New York
3Wilf Family Cardiovascular Research Institute, Albert Einstein College of Medicine, Bronx, New York
4Division of Pulmonary and Critical Care Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland
5Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York
6Diabetes Research Institute, Albert Einstein College of Medicine, Bronx, New York
7Department of Neuroscience, Albert Einstein College of Medicine, Bronx, New York
8Albert Einstein Cancer Center, Albert Einstein College of Medicine, Bronx, New York
Correspondence: Corresponding author: Richard N. Kitsis, richard.kitsis@einstein.yu.edu.

Hyperglycemia in type 2 diabetes is mediated by insulin resistance and β-cell failure, the latter leading to inadequate insulin secretion relative to the degree of insulin resistance. β-Cell failure results from dysfunction of these cells and decreases in their numbers, a significant portion of which is attributable to cell death (reviewed in 1,2) (3). Multiple studies have demonstrated a strong correlation between β-cell apoptosis and type 2 diabetes in humans (4,5).

Apoptosis is mediated by an extrinsic pathway that uses cell surface receptors and an intrinsic pathway involving the mitochondria and endoplasmic reticulum (ER) (reviewed in 6,7). The extrinsic pathway is triggered by specialized death ligands that stimulate the assembly of a multiprotein complex termed the death inducing signaling complex (DISC). The intrinsic pathway is activated by a wider spectrum of stimuli, including metabolic, oxidative, and proteotoxic stress, and triggers permeabilization of the outer mitochondrial membrane, an event regulated by Bcl-2 proteins. Extrinsic and intrinsic pathways converge to activate caspases, a class of cysteinyl proteases, which cleave multiple cellular proteins to kill the cell.

Apoptosis in type 2 diabetes was first demonstrated in islets from Zucker diabetic rats, and treatment of those islets with free fatty acids exacerbated cell death (8). Free fatty acids also induced cell death in nondiabetic human islets, which was inhibited by broad-spectrum caspase inhibitors (4). Postmortem human pancreata exhibited a threefold increase in islet cell apoptosis associated with a 63% reduction in β-cell volume in obese patients with type 2 diabetes compared with obese nondiabetic control subjects (5).

Involvement of the extrinsic pathway was demonstrated by β-cell–specific deletion of procaspase-8, which protected mice from diet-induced islet-cell apoptosis, hyperglycemia, and impaired glucose tolerance (9). The intrinsic pathway also plays an important role, as evidenced by multiple studies showing that overexpression of antiapoptotic Bcl-2 proteins in INS1E β-cells, isolated islets, and β-cells of transgenic mice inhibited apoptosis elicited by the free fatty acid palmitate and the ER stressor thapsigargin (10,11). Although these studies establish a role for apoptosis in the pathogenesis of type 2 diabetes, the underlying pathways remain incompletely understood.

Apoptosis repressor with caspase recruitment domain (ARC) is a cell death inhibitor that is expressed in cardiac and skeletal myocytes and some neurons (12,13). ARC is unusual in its antagonism of both intrinsic and extrinsic death pathways (14). The extrinsic pathway is inhibited through direct interactions of ARC with components of the DISC that prevent DISC assembly (13,14). ARC inhibits the intrinsic pathway by direct binding to Bax, a proapoptotic Bcl-2 protein, preventing Bax conformational activation and translocation to the mitochondria (14,15).

In this study, we discovered that abundant ARC resides in the mouse and human endocrine pancreas and protects β-cells against stresses relevant to type 2 diabetes. Surprisingly, inhibition of β-cell death in this context involves a novel effect of ARC on the ER stress response pathway.


RESEARCH DESIGN AND METHODS
Cell culture and treatments.

Mouse insulinoma MIN6 cells (provided by Dr. Peter Arvan, University of Michigan, Ann Arbor, MI) were cultured as described (16), except for the addition of 140 μmol/L β-mercaptoethanol to the media. βTC-tet cells (provided by Dr. Norman Fleischer, Albert Einstein College of Medicine, Bronx, NY) were used as described (17). MIN6 cells with stable expression of ARC were generated by transduction with a retrovirus encoding human ARC containing a 3′ HA-tag (18). Empty vector was used as the corresponding control. Stable knockdown of ARC in MIN6 cells was carried out using lentiviruses encoding short hairpin (sh)RNAs corresponding to the coding region or 3′ untranslated region of mouse ARC from Sigma-Aldrich (St. Louis, MO). Numbers of the shRNAs in Fig. 4 correspond to the last two digits of The RNAi Consortium (TRC) number. Scrambled shRNA was used as the control. In all experiments, populations of stable transductants were studied. Cells were plated at a density of 5 × 104 cells/cm2 and treated with the specified concentrations of thapsigargin, tunicamycin, staurosporine, or palmitate for the specified times. Palmitate was dissolved in culture media that had been supplemented with physiologic levels of fatty acid–free BSA (600 μmol/L) at 60°C overnight, and was filtered before use in treatments.

Islet isolation.

Isolated mouse islets were collected as previously described (19). Briefly, islets were handpicked after mouse pancreata were digested with collagenase and were separated via gradient centrifugation using Histopaque from Sigma-Aldrich (St. Louis, MO). Islets were placed on matrigel-coated plates and allowed to recover in RPMI media (Invitrogen, Grand Island, NY) supplemented with 5.5 mmol/L glucose and 10% (v/v) FBS for 18 h. After 2-h serum starvation, islets were transduced with adenovirus encoding scrambled shRNA or ARC shRNA (20) at 4 × 106 pfu/islet. Palmitate treatment was initiated 24 h after addition of the adenovirus.

Mice.

These studies used male 8–12-week-old wild-type C57Bl/6 mice and C/EBP homologous protein knockout (CHOP−/−) mice (B6.129S-Ddit3tm1Dron/J), back-bred at least five generations onto a C57Bl/6 background from Jackson Laboratories (Bar Harbor, ME). Mouse maintenance and all experimental procedures were approved by the Albert Einstein College of Medicine Institute for Animal Studies.

Immunoblotting.

Cultured cells and isolated islets were lysed in 50 mmol/L Tris (pH 8.0), 150 mmol/L NaCl, 1% (v/v) Triton X-100, 100 µg/mL phenyl methanesulfonyl fluoride, and 1 µg/mL aprotinin, after which cells were sonicated and isolated islets rotated for 30 min at 4°C. Mouse heart and liver tissue were flash-frozen in liquid nitrogen, lysed in RIPA buffer, and homogenized. Immunoblotting was performed with rabbit polyclonal antisera against ARC (1:4,000, Cayman Chemical, Ann Arbor, MI), active caspase-3 (1:500, Cell Signaling Technology, Danvers, MA), poly(ADP-ribose) polymerase (PARP; 1:1,000, Cell Signaling Technology), caspase-8 (1:1,000, Cell Signaling Technology), eukaryotic initiation factor 2 α-subunit (eIF2α; 1:1,000, Cell Signaling Technology), p-eIF2α (1:1,000, Cell Signaling Technology), and protein kinase RNA-like ER kinase (PERK; 1:1,000, Cell Signaling Technology), GADD 153 (CHOP; 1:100, Santa Cruz Biotechnology, Santa Cruz, CA), and a mouse monoclonal antibody against α-tubulin (1:20,000, Sigma-Aldrich). Primary antibodies were incubated overnight at 4°C in 5% milk and 0.1% (v/v) Tween-20 in PBS. IRDye 800 and 680 secondary antibodies (1:4,000, LI-COR, Lincoln, NE) of the corresponding species were used for detection of the primary antibodies using LI-COR Odyssey and quantified using ImageJ software (National Institutes of Health, Bethesda, MD).

Immunofluorescence.

Mouse pancreatic tissue was fixed in 10% neutral-buffered formalin and embedded in paraffin. Tissue was sectioned at 5 μm, deparaffinized, and antigen retrieval performed using Antigen Unmasking Solution (Vector Laboratories, Burlingame, CA). Islets were fixed in 4% paraformaldehyde, embedded in Tissue Tek OCT compound (Electron Microscopy Sciences, Hatfield, PA), flash-frozen on dry ice, and sectioned at 5 μm. Cells were fixed in 4% paraformaldehyde and permeabilized in 0.1% (v/v) Triton X-100. After blocking for 1 h in 10% goat or donkey serum, samples were incubated with primary antibodies diluted in blocking solution. Primary antibodies used were active caspase-3 (1:200, Cell Signaling Technology), ARC (1:400, Cayman Chemical), insulin (1:100, Abcam, Cambridge, MA), glucagon (1:2,000, Abcam), somatostatin (1:50, Santa Cruz Biotechnology), and pancreatic polypeptide (1:200, Millipore, Billerica, MA). For immunofluorescence, primary antibodies were detected using Alexa Fluor 488, 568, and 647 secondary antibodies (1:1,000, Invitrogen, Grand Island, NY) of the corresponding species. Slides were coverslipped using VECTASHIELD HardSet Mounting Medium with DAPI (Vector Laboratories) to counterstain nuclei. For immunohistochemistry, nuclei were counterstained with methyl green as described (21). All images were collected using an Axio Observer .Z1 microscope (Zeiss, Thornwood, NY).

Cell death assays.

For the live/dead assay, propidium iodide (PI) and Hoechst 33342 (Invitrogen) were directly added to media. Cells were incubated for 5 min in the dark at 37°C and immediately imaged. Three random fields (1,000–2,000 cells) were scored. Terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) was performed using the Fluorescein or TMR red In Situ Cell Death Detection Kit (Roche Applied Science, Indianapolis, IN) according to manufacturer’s protocol. Six random fields (1,000–4,000 cells) were scored. When TMR red was used, it was displayed as pseudocolor green. Annexin V staining was performed using the Annexin V-FITC Apoptosis Detection Kit (Abcam) according to manufacturer’s protocol. Three random fields (1,000–3,000 cells) were scored.

X-box binding protein 1 (XBP1) splicing assay.

RNA was isolated from cells using TRIzol reagent (Ambion, Grand Island, NY). Oligo(dT)20 primers were used to produce cDNA with SuperScript III First-Strand Synthesis System (Invitrogen). RT-PCR was performed using the following primer pairs: mouse XBP1 sense 5′-GAA CCA GGA GTT AAG AAC ACG-3′ and mouse XBP1 antisense 5′-AGG CAA CAG TGT CAG AGT CC-3′. Samples were then separated on a 3% (w/v) agarose gel.

Statistics.

Data are presented as mean ± SEM. The two-tailed Student t test was used to compare two groups. ANOVA, followed by a Tukey post hoc test, was used for multiple comparisons. GraphPad Prism 5 software (GraphPad, La Jolla, CA) was used to calculate statistics. P < 0.05 was considered significant.


RESULTS
ARC is abundant in mouse and human pancreas.

The expression of ARC was originally reported to be restricted to cardiac and skeletal myocytes and some neurons (12,13), and more recent studies have uncovered that ARC is also highly induced in multiple cancers (18,2225). Because tissues contain a heterogeneous mixture of cells, some of which perform unique functions, we hypothesized that ARC expression may have been overlooked in studies that examined homogenates of whole organs. Accordingly, we screened for ARC expression in subpopulations of cells thought not to express ARC. Unexpectedly, we found that cells of the mouse (Fig. 1A) and human (Fig. 2D) endocrine pancreas contain high levels of ARC. Moreover, ARC abundance in isolated islets is roughly equivalent to that in heart tissue (Fig. 1B). In contrast, ARC was not detectable in the exocrine pancreas (Fig. 1A), explaining previous studies reporting its absence in the pancreas (13). ARC levels in isolated islets from wild-type mice were similar to those from ob/ob mice (Supplementary Fig. 1D). These data demonstrate that ARC protein is abundant in cells of the islet.

To determine which cells within islets express ARC, we immunostained mouse and human pancreatic tissue. ARC is expressed primarily in β-cells and is present in >99.5% of these cells in the mouse (Fig. 2A and B). It is predominantly localized to the cytoplasm because ARC immunofluorescence overlaps with insulin but not with the nuclear DAPI staining (Fig. 2A and C). Abundant ARC was also detected in various mouse β-cell lines (Fig. 1C). In addition, ARC is found in 30% of mouse α-cells, 16% of pancreatic polypeptide (PP) cells, and in 67% of δ-cells, the latter sharing a common progenitor cell with β-cells (26) (Fig. 2B). Moreover, in the human, ARC is present in 73% of β-cells and in 64% of α-cells (Fig. 2E). These data indicate that ARC is abundant in the endocrine pancreas, predominantly in β-cells.

ARC regulates cell death in β-cells.

To determine whether ARC plays a functional role in β-cells, we used gain- and loss-of-function approaches. Stable retroviral transduction of ARC in MIN6 cells (Fig. 3A) inhibited cell death triggered by the general apoptosis inducer staurosporine. This was indicated by changes in the abundance of active caspases-3 and -8, cleavage of the caspase substrate PARP, and loss of plasma membrane integrity (PI staining), the latter a readout of overall death in cultured cells (Fig. 3B–C, F). Because of metabolic disturbances, Ca2+ abnormalities, and proteotoxic stress resulting from increased insulin production, ER stress is a key component of type 2 diabetes (2731) and an initiator of β-cell death (32,33). Accordingly, we tested whether ARC inhibits ER stress-induced apoptosis in these cells. Cell death triggered by the ER stressors thapsigargin (an inhibitor of sarcoplasmic/endoplasmic reticulum Ca2+ATPase [SERCA], which mediates Ca2+ reuptake into the ER) and tunicamycin (an inhibitor of N-linked glycosylation, which is required for proper folding of proteins) was suppressed by ARC overexpression (Fig. 3D, E, G, and H). Of note, levels of endogenous ARC were not affected by any of these death inducers (Supplementary Fig. 1A–C).

Conversely, knockdown of ARC was carried out using RNAi. Five independent shRNAs were tested for their abilities to knockdown ARC after stable transduction into MIN6 cells. Compared with scrambled shRNA, ARC shRNAs 14 and 15 were most effective, with 55% and 70% knockdown, respectively (Fig. 4A). To define the importance of endogenous levels of ARC in regulating ER stress-mediated cell death, we used shRNA 15. Depletion of ARC resulted in two- to fourfold increases in cell death induced by thapsigargin (Fig. 4). This was demonstrated by activation of caspases-3 and -8 and PARP cleavage (Fig. 4C and D). In addition, there were increases in the percentage of cells exhibiting TUNEL, active caspase-3 immunofluorescence, annexin V, and PI entry (Fig. 4B and E–G). These data demonstrate that endogenous levels of ARC protect β-cells against ER stress-induced cell death.

ARC regulates cell death in a pathophysiological context.

ER stress is a critical component in the pathogenesis of type 2 diabetes (27,28). The free fatty acid palmitate, which models this stress, has been shown to induce β-cell death (34,35). Accordingly, we assessed whether ARC inhibits cell death in response to this pathophysiological stimulus. Palmitate at 1.2 and 2.4 mmol/L concentrations induced β-cell apoptosis, which was dramatically reduced by ARC overexpression (Fig. 5A and B, Supplementary Fig. 2). In addition, ARC overexpression continued to prevent cell death even at late time points (Supplementary Fig. 3). Conversely, knockdown of ARC potentiated palmitate-induced apoptosis in a concentration- and time-dependent manner (Fig. 5C–E). These data indicate that ARC regulates β-cell death elicited by a pathophysiologically relevant stimulus of ER stress.

ARC alleviates ER stress.

ER stress has been linked to β-cell death (32,33), but the precise functional and mechanistic relationships between these processes remain unclear. The major role of the ER stress response is to restore ER homeostasis, including refolding of misfolded proteins (reviewed in 3638). Upstream mediators include the kinases PERK and inositol-requiring protein 1α (IRE1α), the combined actions of which include attenuating translation and activating a complex transcriptional response that promotes protein refolding. However, when the ER stress response is prolonged, apoptosis can result (reviewed in 39,40). Although inhibition of cell death by ARC has traditionally been thought to be mediated solely through its antagonism of the central death machinery (14,41), we considered the possibility that ARC also regulates the ER stress response itself.

Using thapsigargin to stimulate the ER stress response, we observed that overexpression of ARC does not affect PERK activation (as indicated by phosphorylation of its downstream target eIF2α) or IRE1α activation, as indicated by XBP1 splicing (Fig. 6A and B). In contrast, ARC overexpression inhibited upregulation of the transcription factor CHOP, an important initiator of apoptosis in response to ER stress that has been implicated in type 2 diabetes (42,43) (Fig. 6C). This effect was even more marked when CHOP induction was elicited by the physiological ER stressor palmitate (Fig. 6D). These data indicate that ARC acts distal to activation of PERK and IRE1α to inhibit the ER stress response.

Endogenous ARC suppresses apoptosis in isolated islets through CHOP.

To determine whether endogenous levels of ARC are important in suppressing palmitate-induced apoptosis in the context of islet tissue, we used isolated islets. Transduction of islets with adenovirus encoding ARC shRNA knocked down ARC levels in ∼74% of cells (Fig. 7A and B). Although treatment with palmitate (1.2 mmol/L) did not result in significant cell death in islets transduced with scrambled shRNA, knockdown of ARC augmented apoptosis more than fivefold (Fig. 7A and C). Moreover, deletion of CHOP rescued the incremental cell death resulting from ARC knockdown (Fig. 7D and F, Supplementary Fig. 4) without influencing ARC levels (Fig. 7D and E, Supplementary Fig. 4). These data indicate that endogenous levels of ARC are critical in suppressing palmitate-induced cell death in islets and that this inhibition is mediated through CHOP.


DISCUSSION

These experiments reveal the unexpected presence of ARC in pancreatic islets and a novel role for ARC in modulating the ER stress response and inhibiting β-cell death. Under normal conditions, the expression of ARC was previously thought to be restricted to cardiac and skeletal myocytes and neurons. Accordingly, ARC has never been studied in the context of diabetes. Surprisingly, we found that ARC is as abundant in the endocrine pancreas as it is in the heart. Although expressed in variable percentages of cell types in islets, almost all mouse β-cells and a high proportion of human β-cells contain ARC. On the basis of studies in other cell types (21,41,44,45), the abundance of ARC in the β-cell may be mediated by both transcriptional and posttranslational mechanisms.

Although ARC has been shown to be a potent inhibitor of apoptosis in other systems, cell death is often regulated in a cell type- and stimulus-specific manner; therefore, we investigated the role of ARC in β-cell apoptosis. Overexpression of ARC inhibited cell death elicited by generic cell death stimuli and ER stressors, including palmitate, a free fatty acid relevant to the pathogenesis of type 2 diabetes. Conversely, knockdown experiments demonstrated the importance of endogenous levels of ARC in protecting against these death signals in the β-cell. These effects were demonstrated using multiple parameters indicative of apoptosis assessed at a variety of time points.

In addition to its known effects on the two central apoptosis pathways, a new finding in this study is that ARC modulates cell death by alleviating the ER stress response. This pathway plays an important adaptive role in helping the cell to resolve various stresses. When insults are of overwhelming magnitude or prolonged duration, however, this pathway is no longer sufficient to compensate, sometimes triggering cell death. This duality of the ER stress response is attributable, in part, to effectors such as CHOP, which are capable of signaling cellular adaptation and also demise. Although the mechanisms that control the transition from adaptation to death are currently not understood, our data indicate that ARC acts downstream of PERK and IRE1α and requires CHOP to ameliorate the ER stress response and inhibit β-cell death.

We observed that ER stress activates caspase-8 in MIN6 cells, an effect suppressed by ARC overexpression and exacerbated by ARC knockdown. These observations are consistent with a previous study in melanoma cells (46). It remains unclear, however, whether caspase-8 is modulated through known actions of ARC at the DISC or whether these effects of ARC involve a pool of caspase-8 known to reside at the ER (47). Interestingly, β-cell–specific deletion of procaspase-8 in mice ameliorates high fat diet–induced diabetes (9), a significant component of which involves ER stress.

This study raises a number of interesting avenues for future investigation. One relates to the mechanisms by which ARC suppresses CHOP. This potentially includes effects on CHOP transcription, mRNA or protein stability, posttranslational modifications, and nuclear localization. Moreover, indirect mechanisms may also be involved. Second, given known effects of ARC in regulating mitochondrial events in cell death, another important question is whether bidirectional ER–mitochondrial crosstalk plays a role in effects of ARC on the ER stress response. Third, and perhaps most interesting, is the possibility that ARC functions as a survival factor for β-cells exposed to the noxious milieu of type 2 diabetes in vivo. Experiments using ARC gain- and loss-of-function mice will be needed to test this possibility. In addition, because ARC is also expressed in skeletal muscle and brain, there remains the possibility that ARC modulates other aspects in the pathogenesis of type 2 diabetes.


Notes

fn1This article contains Supplementary Data online at http://diabetes.diabetesjournals.org/lookup/suppl/doi:10.2337/db12-0504/-/DC1.

ACKNOWLEDGMENTS

This work was supported by grants from the National Institutes of Health (5R01-HL-060665-14 to R.N.K., 5R01-DK-063306-10 to S.C.C., 5R37-DK-033823-31 to J.E.P., 5P60-DK-020541-34 to J.E.P., R.N.K., S.C.C., and L.C., 5P30-CA-013330-39 to R.N.K., and 5T32-GM-007491-35 to W.M.Mc.K.) and from the American Heart Association (AHA 09GRANT2290168 to M.T.C.). R.N.K. is supported by the Dr. Gerald and Myra Dorris Chair in Cardiovascular Disease.

No potential conflicts of interest relative to this article were reported.

W.M.Mc.K. conceived experiments, researched data, analyzed experiments, contributed to discussion, and wrote the manuscript. J.W. researched data and contributed to discussion. L.C. and M.Z. researched data. M.C.T. contributed reagents and reviewed and edited the manuscript. J.E.P. and S.C.C. conceived experiments, contributed to discussion, and reviewed and edited the manuscript. R.N.K. conceived and analyzed experiments, contributed to discussion, and wrote the manuscript. R.N.K. is the guarantor of this work and, as such, had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Parts of this study were presented in abstract form at the Islet Biology Keystone Meeting, Whistler, BC, Canada, 12–17 April 2010, and at the Gordon Research Conference: Stress Proteins in Growth, Development & Disease, Lucca (Barga), Italy, 17–22 July 2011.

The authors are most grateful to the Wilf Family for their ongoing generosity and support. The authors would also like to acknowledge Dr. Christina Medina-Ramirez and Dr. Melissa James (Albert Einstein College of Medicine, Bronx, NY) for their help in generating the ARC and ARC shRNA viruses.


REFERENCES
1. Prentki M,Nolan CJ. Islet beta cell failure in type 2 diabetes. J Clin InvestYear: 2006;116:1802–181216823478
2. Poitout V,Robertson RP. Minireview: Secondary beta-cell failure in type 2 diabetes—a convergence of glucotoxicity and lipotoxicity. EndocrinologyYear: 2002;143:339–34211796484
3. Pick A,Clark J,Kubstrup C,et al. Role of apoptosis in failure of beta-cell mass compensation for insulin resistance and beta-cell defects in the male Zucker diabetic fatty rat. DiabetesYear: 1998;47:358–3649519740
4. Lupi R,Dotta F,Marselli L,et al. Prolonged exposure to free fatty acids has cytostatic and pro-apoptotic effects on human pancreatic islets: evidence that beta-cell death is caspase mediated, partially dependent on ceramide pathway, and Bcl-2 regulated. DiabetesYear: 2002;51:1437–144211978640
5. Butler AE,Janson J,Bonner-Weir S,Ritzel R,Rizza RA,Butler PC. Beta-cell deficit and increased beta-cell apoptosis in humans with type 2 diabetes. DiabetesYear: 2003;52:102–11012502499
6. Danial NN,Korsmeyer SJ. Cell death: critical control points. CellYear: 2004;116:205–21914744432
7. Tait SW,Green DR. Mitochondria and cell death: outer membrane permeabilization and beyond. Nat Rev Mol Cell BiolYear: 2010;11:621–63220683470
8. Shimabukuro M,Zhou YT,Levi M,Unger RH. Fatty acid-induced beta cell apoptosis: a link between obesity and diabetes. Proc Natl Acad Sci U S AYear: 1998;95:2498–25029482914
9. Liadis N,Salmena L,Kwan E,et al. Distinct in vivo roles of caspase-8 in beta-cells in physiological and diabetes models. DiabetesYear: 2007;56:2302–231117563067
10. Allagnat F,Cunha D,Moore F,Vanderwinden JM,Eizirik DL,Cardozo AK. Mcl-1 downregulation by pro-inflammatory cytokines and palmitate is an early event contributing to β-cell apoptosis. Cell Death DifferYear: 2011;18:328–33720798690
11. Zhou YP,Pena JC,Roe MW,et al. Overexpression of Bcl-x(L) in beta-cells prevents cell death but impairs mitochondrial signal for insulin secretion. Am J Physiol Endocrinol MetabYear: 2000;278:E340–E35110662719
12. Geertman R,McMahon A,Sabban EL. Cloning and characterization of cDNAs for novel proteins with glutamic acid-proline dipeptide tandem repeats. Biochim Biophys ActaYear: 1996;1306:147–1528634331
13. Koseki T,Inohara N,Chen S,Núñez G. ARC, an inhibitor of apoptosis expressed in skeletal muscle and heart that interacts selectively with caspases. Proc Natl Acad Sci U S AYear: 1998;95:5156–51609560245
14. Nam YJ,Mani K,Ashton AW,et al. Inhibition of both the extrinsic and intrinsic death pathways through nonhomotypic death-fold interactions. Mol CellYear: 2004;15:901–91215383280
15. Gustafsson AB,Tsai JG,Logue SE,Crow MT,Gottlieb RA. Apoptosis repressor with caspase recruitment domain protects against cell death by interfering with Bax activation. J Biol ChemYear: 2004;279:21233–2123815004034
16. Khoo C,Yang J,Rajpal G,et al. Endoplasmic reticulum oxidoreductin-1-like β (ERO1lβ) regulates susceptibility to endoplasmic reticulum stress and is induced by insulin flux in β-cells. EndocrinologyYear: 2011;152:2599–260821540283
17. Fleischer N,Chen C,Surana M,et al. Functional analysis of a conditionally transformed pancreatic beta-cell line. DiabetesYear: 1998;47:1419–14259726230
18. Medina-Ramirez CM,Goswami S,Smirnova T,et al. Apoptosis inhibitor ARC promotes breast tumorigenesis, metastasis, and chemoresistance. Cancer ResYear: 2011;71:7705–771522037876
19. Dokmanovic-Chouinard M,Chung WK,Chevre JC,et al. Positional cloning of “Lisch-Like”, a candidate modifier of susceptibility to type 2 diabetes in mice. PLoS GenetYear: 2008;4:e100013718654634
20. Zaiman AL,Damico R,Thoms-Chesley A,et al. A critical role for the protein apoptosis repressor with caspase recruitment domain in hypoxia-induced pulmonary hypertension. CirculationYear: 2011;124:2533–254222082675
21. Wu L,Nam YJ,Kung G,Crow MT,Kitsis RN. Induction of the apoptosis inhibitor ARC by Ras in human cancers. J Biol ChemYear: 2010;285:19235–1924520392691
22. Mercier I,Vuolo M,Madan R,et al. ARC, an apoptosis suppressor limited to terminally differentiated cells, is induced in human breast cancer and confers chemo- and radiation-resistance. Cell Death DifferYear: 2005;12:682–68615861191
23. Mercier I,Vuolo M,Jasmin JF,et al. ARC (apoptosis repressor with caspase recruitment domain) is a novel marker of human colon cancer. Cell CycleYear: 2008;7:1640–164718469522
24. Wang M,Qanungo S,Crow MT,Watanabe M,Nieminen AL. Apoptosis repressor with caspase recruitment domain (ARC) is expressed in cancer cells and localizes to nuclei. FEBS LettYear: 2005;579:2411–241515848180
25. Carter BZ,Qiu YH,Zhang N,et al. Expression of ARC (apoptosis repressor with caspase recruitment domain), an antiapoptotic protein, is strongly prognostic in AML. BloodYear: 2011;117:780–78721041716
26. Sosa-Pineda B,Chowdhury K,Torres M,Oliver G,Gruss P. The Pax4 gene is essential for differentiation of insulin-producing beta cells in the mammalian pancreas. NatureYear: 1997;386:399–4029121556
27. Ozcan U,Cao Q,Yilmaz E,et al. Endoplasmic reticulum stress links obesity, insulin action, and type 2 diabetes. ScienceYear: 2004;306:457–46115486293
28. Ozcan U,Yilmaz E,Ozcan L,et al. Chemical chaperones reduce ER stress and restore glucose homeostasis in a mouse model of type 2 diabetes. ScienceYear: 2006;313:1137–114016931765
29. Luciani DS,Gwiazda KS,Yang TL,et al. Roles of IP3R and RyR Ca2+ channels in endoplasmic reticulum stress and beta-cell death. DiabetesYear: 2009;58:422–43219033399
30. Park SW,Zhou Y,Lee J,Lee J,Ozcan U. Sarco(endo)plasmic reticulum Ca2+-ATPase 2b is a major regulator of endoplasmic reticulum stress and glucose homeostasis in obesity. Proc Natl Acad Sci U S AYear: 2010;107:19320–1932520974941
31. Fu S,Yang L,Li P,et al. Aberrant lipid metabolism disrupts calcium homeostasis causing liver endoplasmic reticulum stress in obesity. NatureYear: 2011;473:528–53121532591
32. Laybutt DR,Preston AM,Akerfeldt MC,et al. Endoplasmic reticulum stress contributes to beta cell apoptosis in type 2 diabetes. DiabetologiaYear: 2007;50:752–76317268797
33. Marchetti P,Bugliani M,Lupi R,et al. The endoplasmic reticulum in pancreatic beta cells of type 2 diabetes patients. DiabetologiaYear: 2007;50:2486–249417906960
34. Karaskov E,Scott C,Zhang L,Teodoro T,Ravazzola M,Volchuk A. Chronic palmitate but not oleate exposure induces endoplasmic reticulum stress, which may contribute to INS-1 pancreatic beta-cell apoptosis. EndocrinologyYear: 2006;147:3398–340716601139
35. Cunha DA,Hekerman P,Ladrière L,et al. Initiation and execution of lipotoxic ER stress in pancreatic beta-cells. J Cell SciYear: 2008;121:2308–231818559892
36. Scheuner D,Kaufman RJ. The unfolded protein response: a pathway that links insulin demand with beta-cell failure and diabetes. Endocr RevYear: 2008;29:317–33318436705
37. Walter P,Ron D. The unfolded protein response: from stress pathway to homeostatic regulation. ScienceYear: 2011;334:1081–108622116877
38. Cnop M,Foufelle F,Velloso LA. Endoplasmic reticulum stress, obesity and diabetes. Trends Mol MedYear: 2012;18:59–6821889406
39. Tabas I,Ron D. Integrating the mechanisms of apoptosis induced by endoplasmic reticulum stress. Nat Cell BiolYear: 2011;13:184–19021364565
40. Shore GC,Papa FR,Oakes SA. Signaling cell death from the endoplasmic reticulum stress response. Curr Opin Cell BiolYear: 2011;23:143–14921146390
41. Foo RS,Nam YJ,Ostreicher MJ,et al. Regulation of p53 tetramerization and nuclear export by ARC. Proc Natl Acad Sci U S AYear: 2007;104:20826–2083118087040
42. Zinszner H,Kuroda M,Wang X,et al. CHOP is implicated in programmed cell death in response to impaired function of the endoplasmic reticulum. Genes DevYear: 1998;12:982–9959531536
43. Song B,Scheuner D,Ron D,Pennathur S,Kaufman RJ. Chop deletion reduces oxidative stress, improves beta cell function, and promotes cell survival in multiple mouse models of diabetes. J Clin InvestYear: 2008;118:3378–338918776938
44. Foo RS,Chan LK,Kitsis RN,Bennett MR. Ubiquitination and degradation of the anti-apoptotic protein ARC by MDM2. J Biol ChemYear: 2007;282:5529–553517142834
45. Nam YJ,Mani K,Wu L,et al. The apoptosis inhibitor ARC undergoes ubiquitin-proteasomal-mediated degradation in response to death stimuli: identification of a degradation-resistant mutant. J Biol ChemYear: 2007;282:5522–552817142452
46. Chen LH,Jiang CC,Watts R,et al. Inhibition of endoplasmic reticulum stress-induced apoptosis of melanoma cells by the ARC protein. Cancer ResYear: 2008;68:834–84218245485
47. Breckenridge DG,Nguyen M,Kuppig S,Reth M,Shore GC. The procaspase-8 isoform, procaspase-8L, recruited to the BAP31 complex at the endoplasmic reticulum. Proc Natl Acad Sci U S AYear: 2002;99:4331–433611917123

Figures

[Figure ID: F1]
FIG. 1. 

ARC protein in mouse pancreas. A: Immunohistochemistry of pancreas stained for ARC (brown). Nuclei counterstained with methyl green. Two fields are shown. Scale bar: 100 µm. Representative of three independent experiments. B: Immunoblot for ARC in the indicated tissues with quantification. Representative of two experiments using tissue from different mice. C: Immunoblot for ARC in the indicated β-cell lines. Representative of three independent experiments.



[Figure ID: F2]
FIG. 2. 

Mouse and human pancreatic tissue demonstrating ARC protein in islet cell types. A: Immunostaining for ARC and cell type–specific markers in mouse with β-cells, α-cells, pancreatic polypeptide (PP) cells, and δ-cells indicated by insulin, glucagon, PP, and somatostatin respectively. B: Respective quantification. Cells in 5–10 islets in one pancreas section × three mice were scored (2,000 cells). Scoring repeated by a second individual blinded to the results. C: High-magnification immunostaining for ARC in mouse. D and E: Analysis of ARC expression with cell type–specific markers in human. Cells in three islets in one pancreas section of one human sample were scored (500 cells). Scale bar: 50 µm. Comparisons made to β-cells only: ***P < 0.001 for α-cells, ###P < 0.001 for PP cells, ††P < 0.01 for δ-cells.



[Figure ID: F3]
FIG. 3. 

ARC overexpression protects β-cells. A: ARC after stable retroviral transduction of MIN6 cells with the indicated plasmids. Control is empty vector. Quantification represents three independent experiments. B: Representative images of PI-stained (red) unfixed ARC-overexpressing cells after treatment with staurosporine (Stauro, 1 μmol/L). Nuclei counterstained with DAPI (blue). C–E: Quantification of PI staining of ARC-overexpressing cells after treatment with Stauro, thapsigargin (TG, 1 μmol/L), and tunicamycin (Tun, 10 ng/µL), respectively. Three fields were scored for each condition. ARC vs. vector: *P < 0.05 at 12 h, #P < 0.05 at 24 h. FH: Immunoblot analysis of the indicated proteins after treatment of ARC-overexpressing cells with Stauro, TG, and Tun. The arrow denotes full-length PARP and the arrowhead denotes cleaved (cl) PARP. Representative of three independent experiments for Stauro and TG and of two experiments for Tun.



[Figure ID: F4]
FIG. 4. 

ARC knockdown potentiates cell death. A: ARC levels after stable lentiviral transduction of MIN6 cells with the indicated shRNAs with corresponding quantification. The control scrambled (SCR) shRNA is represented by the letter C. B: Quantification of PI staining of unfixed ARC-knockdown cells after thapsigargin (TG, 1 μmol/L) treatment. Three fields were scored for each condition. ARC shRNA vs. SCR shRNA: *P < 0.05 at 12 h, #P < 0.05 at 24 h. C and D: Immunoblot analysis of the indicated proteins in ARC-knockdown cells treated with TG and corresponding quantification. The arrow denotes full-length PARP and the arrowhead denotes cleaved (cl)-PARP. Representative of three independent experiments. ARC shRNA vs. SCR shRNA: *P < 0.05 at 6 h, **P < 0.01 at 6 h, ###P < 0.001 at 12 h. E: TUNEL (green) in ARC-knockdown cells treated with TG with quantification. F: Immunofluorescence for cl (active) caspase-3 (green) in ARC-knockdown cells treated with TG with quantification. G: Immunofluorescence for annexin V (green) in ARC-knockdown cells treated with TG with quantification. Nuclei counterstained with DAPI (blue). Scale bar: 100 µm. Six fields scored for each condition. *P < 0.05 for ARC shRNA vs. SCR shRNA at 16 h. ***P < 0.001 for ARC shRNA vs. SCR shRNA at 16 h.



[Figure ID: F5]
FIG. 5. 

ARC inhibits palmitate-induced cell death. A: Immunoblot of the indicated proteins in ARC-overexpressing cells treated with the indicated concentrations of palmitate (Palm) for 3 days (cl, cleaved). Quantification is available in Supplementary Fig. 2 (omitted because of space constraints). B: TUNEL analysis of ARC-overexpressing cells treated with Palm at 3 days. Six fields were scored for each condition. ARC vs. vector: ***P < 0.001 at 1.2 mmol/L Palm, ###P < 0.001 at 2.4 mmol/L Palm. C: PARP cleavage and active caspase-3 in ARC-knockdown cells treated with Palm at the indicated concentrations and times. D: Immunoblot of the indicated proteins in ARC-knockdown cells treated with the indicated concentrations of Palm for 2 days. E: TUNEL analysis of ARC-knockdown cells treated with Palm for 2 days. Six fields were scored for each condition. ARC shRNA vs. scrambled (SCR) shRNA: *P < 0.05 at 1.2 mmol/L Palm, ###P < 0.001 at 2.4 mmol/L Palm. The arrow denotes full-length PARP and the arrowhead denotes cl-PARP. Immunoblots in A, C, and D are representative of three independent experiments.



[Figure ID: F6]
FIG. 6. 

ARC alleviates ER stress. A: Markers of the unfolded protein response in ARC-overexpressing cells treated with thapsigargin (TG, 1 μmol/L). Representative of four independent experiments. B: RT-PCR for XBP1 splicing in ARC-overexpressing cells treated with TG with quantification of spliced (S) XBP1 over total XBP1 (unspliced [U] + S). Representative of three independent experiments. C: CHOP levels in ARC-overexpressing cells treated with TG with quantification. Representative of five independent experiments. **P < 0.01 for ARC vs. vector at 12 h. D: CHOP levels in ARC-overexpressing cells treated with palmitate (Palm) at the indicated concentrations and times. Representative of three independent experiments at each palmitate concentration.



[Figure ID: F7]
FIG. 7. 

Deletion of CHOP inhibits augmentation of palmitate-induced cell death elicited by ARC knockdown in isolated islets. A: Palmitate (Palm)-induced apoptosis is augmented by ARC knockdown. TUNEL (green), ARC immunostaining (red), and DAPI counterstain (blue) in isolated islets transduced with adenovirus expressing scrambled (SCR) or ARC shRNA, with or without Palm treatment. The arrows denote TUNEL-positive cells within islets. Only cells clearly within islets were scored. B: Quantification of the percentage of ARC-positive cells in A shows that ARC knockdown is not affected by Palm treatment. All cells in 10 islets were scored for each condition (400–800 cells). ARC shRNA vs. SCR shRNA: ***P < 0.001 control condition, ###P < 0.001 Palm treatment. C: Quantification of TUNEL staining in A. ARC shRNA vs. SCR shRNA: ***P < 0.001 Palm treatment. AC are representative of two independent experiments. D: Rescue of apoptosis by deletion of CHOP. TUNEL (green), ARC immunostaining (red), and DAPI counterstain (blue) in Palm-treated isolated islets from wild-type or CHOP−/− mice transduced with adenovirus expressing SCR or ARC shRNA. The arrows denote TUNEL-positive cells within islets. E: Quantification of percentage of ARC-positive cells in D show ARC knockdown is not affected by deletion of CHOP. All cells in 20 islets were scored for each condition (900–2,200 cells). ARC shRNA vs. SCR shRNA: ***P < 0.001 wild-type islets, ###P < 0.001 CHOP−/− islets. F: Quantification of percentage of TUNEL-positive cells in D. ARC shRNA vs. SCR shRNA: ***P < 0.001 wild-type. CHOP−/− vs. wild-type: ###P < 0.001 ARC shRNA. DF are representative of three independent experiments (two others shown in Supplementary Fig. 4). Palm treatment was for 4 days in all panels. Scale bar: 50 µm.



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